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Article Not peer-reviewed version
SISA: The First Extreme-Ultraviolet Solar
Integral Field Spectrograph Using
Slicers
Ariadna Calcines Rosario * , Hamish Andrew Sinclair Reid , Sarah Matthews , Frédéric Auchère ,
Alain Jody Corso , Giulio Del Zanna , Jaroslav Dudik , Samuel Gissot , Laura Hayes , Graham Stewart Kerr ,
Christian Kintziger , Sophie Musset , Vanessa Polito , Daniel F. Ryan , David Orozco Suárez
Posted Date: 29 November 2023
doi: 10.20944/preprints202311.1854.v1
Keywords: EUV Spectroscopy; solar IFS; EUV slicers; image slicers; solar space mission; particle
acceleration
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Article
SISA: The First Extreme-Ultraviolet Solar Integral
Field Spectrograph Using Slicers
Ariadna Calcines Rosario 1,*, Hamish A. S. Reid 2, Sarah Matthews 2, Frederic Auchère 3,
Alain Jody Corso 4, Giulio Del Zanna 5, Jaroslav Dudík 6, Samuel Gissot 7,
Laura A. Hayes 8, Graham S. Kerr 9,10 , Christian Kintziger 11 , Sophie Musset 8,
Vanessa Polito 12,13 , Daniel F. Ryan 14 and David Orozco Suárez 15,16
1Durham University, Centre for Advanced Instrumentation, Department of Physics, Durham, UK.
2University College London, Mullard Space Science Laboratory, Holmbury Hill Rd, Dorking RH5 6NT,
United Kingdom
3Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405, Orsay, France
4National Research Council of Italy, Institute for Photonics and Nanotechnologies, via Trasea 7, 35131,
Padova, Italy
5Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road,
Cambridge CB3 0WA, United Kingdom
6Astronomical Institute of the Czech Academy of Sciences, Friˇcova 298, 251 65 Ondˇrejov, Czech republic
7Royal Observatory of Belgium, Ringlaan -3- Av. Circulaire, 1180 Brussels, Belgium
8European Space Agency, ESTEC, Keplerlaan 1 - 2201 AZ, Noordwijk, The Netherlands
9NASA Goddard Space Flight Center, Heliophysics Science Division, Code 671, Greenbelt, MD 20771, USA
10 Department of Physics, Catholic University of America, Washington DC 20064, USA
11 Centre Spatial de Liège, University of Liège (ULiège) – STAR Institute, Liège, Belgium
12 Lockheed Martin Solar and Astrophysics Laboratory, Building 252, 3251 Hanover Street, Palo Alto, CA
94304, USA
13 Department of Physics, Oregon State University, Corvallis, OR 97333, USA
14 University of Sciences and Arts Northwest Switzerland (FHNW), Bahnhofstrasse 6, Windisch 5210,
Switzerland
15 Spanish Space Solar Physics Consortium (S3PC)
16 Instituto de Astrofísica de Andalucía (IAA-CSIC), Granada, Spain
*Correspondence: ariadna.calcines@durham.ac.uk
Abstract:
Particle acceleration, and the thermalisation of energetic particles, are fundamental
processes across the universe. Whilst the Sun is an excellent object to study this phenomenon,
since it is the most energetic particle accelerator in the Solar System, this phenomenon arises in many
other astrophysical objects, such as active galactic nuclei, black holes, neutron stars, gamma ray
bursts, solar and stellar coronae, accretion disks and planetary magnetospheres. Observations in the
Extreme Ultraviolet (EUV) are essential for these studies but can only be made from space. Current
spectrographs operating in the EUV use an entrance slit and cover the required field of view using
a scanning mechanism. This results in a relatively slow image cadence on the order of minutes to
capture inherently rapid and transient processes, and/or in the spectrograph slit ‘missing the action’.
The application of image slicers for EUV integral field spectrographs is therefore revolutionary. The
development of this technology will enable observations of EUV spectra from an entire 2D field of
view in seconds, over two orders of magnitude faster than what is currently possible. The Spectral
Imaging of the Solar Atmosphere (SISA) instrument is the first integral field spectrograph proposed
for observations at
∼
180 Å combining the image slicer technology and curved diffraction gratings
in a highly efficient and compact layout, while providing important spectroscopic diagnostics for
characterization of solar coronal and flare plasmas. SISA’s characteristics, main challenges and the
on-going activities to enable the image slicer technology for EUV applications are presented in this
paper.
Keywords:
EUV spectroscopy; solar IFS; EUV slicers; image slicers; solar space mission; particle
acceleration
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© 2023 by the author(s). Distributed under a Creative Commons CC BY license.
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1. The Need for Solar Integral Field Spectrometers in the Extreme Ultraviolet
Particle acceleration is a fundamental process arising in many astrophysical objects including
active galactic nuclei, black holes, neutron stars,
γ
ray bursts, accretion disks, solar and stellar coronae,
and planetary magnetospheres.
In our solar system, the Sun is the most energetic particle accelerator. Its proximity makes it a
unique laboratory with which to explore the physics of energetic particles, which has wider importance
in understanding astrophysical particle acceleration more generally. However, despite its importance
and past observations, the physics underlying solar particle acceleration remains poorly understood.
For the study of solar particle acceleration and the evolution of solar eruptive events (solar flares and
coronal mass ejections; CMEs), it is useful to observe in the Extreme Ultraviolet (EUV) regime, since
this wavelength range contains a number of emission lines that allow the corona and transition region
to be probed and whose profiles allow the presence of non-Maxwellian particle distributions to be
diagnosed. Additionally, the plasma response following the thermalisation of these energetic particles
can be dramatic, with resultant heating, ionisation, mass flows, and turbulence, all of which can be
diagnosed with EUV spectroscopic observations. The plasma in the solar corona is typically fully
ionised because of the megaKelvin plasma temperatures. Such hot plasma contains ions that emit
brightly in EUV wavelengths (λ∼100 −1200 Å).
Measurements in the EUV can be used to characterise the temperature, density, elemental
composition and the turbulent state of the plasma in the upper solar atmosphere, for both the quiet and
active Sun. Plasma density can be diagnosed through the observation of density sensitive line ratios
in all coronal features, from the quiet Sun and coronal holes to active regions, flares, jets and coronal
mass ejections. Measurements of densities in the corona are fundamental for understanding how the
corona is heated. Moreover, they are key for understanding how the solar corona transitions into the
solar wind. Measuring electron densities during transient processes like solar flares also allows us
to understand the timescales of ionisation processes that occur during the conversion of magnetic to
kinetic energy (which in the standard solar flare model occurs following magnetic reconnection).
The traditional method for performing solar imaging spectroscopy in the EUV [e.g.
1
,
2
] has
been to use a slit spectrometer and a scanning mechanism to cover a larger field of view. Whilst
‘sit-and-stare’ observations can be produced within seconds for a strongly emitting EUV line from one
location, the traditional slit scanning spectrometer takes minutes to create a spectrum across a 2D
spatial region of the Sun depending on the desired field of view. Using integral field spectroscopy
(IFS), all spectra of a 2D field of view will be obtained simultaneously, reducing the integration time by
over two orders of magnitude and without the requirement for any rastering mechanism. We also
note that a novel solar ultraviolet IFS concept will be used for the first time during the Solar eruptioN
Integral Field Spectrograph (SNIFS) NASA sounding rocket launch in Spring 2024 [3,4].
2. SISA Science in the SPARK Framework
SISA is envisioned as one component of a suite of complementary instruments that together form
the The Solar Particle Acceleration Radiation and Kinetics (SPARK) mission concept, along with the
Large Imaging Spectrometer for Solar Accelerated Nuclei (LISSAN) and the Focusing Optics X-ray
Solar Imager (FOXSI) instruments both of which target high-energy x-ray and
γ
-ray emission. SPARK
is designed to address the open questions regarding particle acceleration on the Sun.
SPARK will attack the problem of particle acceleration through a powerful combination of
γ
-ray,
X-ray and EUV imaging and spectroscopy at high spectral, spatial, and temporal resolutions. In
addition to helping in addressing the physics of particle acceleration, SISA shall provide critical
measurements of plasma conditions (magnetic fields, temperatures, densities, and elemental
composition) in the various coronal and flare structures, both pre- and post-reconnection. As these
measurements are integral to understanding how the plasma responds to particle acceleration, and
critical for understanding of how and when flares occur, the SISA EUV integral field spectrometer
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is an crucial addition to the SPARK mission. The significant advantage of SISA with respect to
previous EUV spectrographs is that it will enable these crucial measurements in an integral field
regime, i.e., spectroscopic measurements simultaneously over a wide field of view. Below, we briefly
discuss how this instrument contributes to the key science goals to be addressed by the SPARK mission:
•How does impulsive energy release accelerate particles in the solar atmosphere?
SISA shall provide several measurements of accelerated particles in EUV, both electrons through
line ratios (see Section 3.2) and ions through line profiles (see Section 3.3). The integral field
spectroscopy of SISA will offer insights as to where and when the particles are accelerated,
while the fast cadence will reveal how long they persist at a given location. Measuring the
spatial distribution of the accelerated particles, and their relationship to magnetic field (see
Section 3.1) and field-aligned emission structures, will also offer insights as to the conditions
required for particle acceleration, both in solar flares and active-region corona. The fast-cadence
SISA observations with multiple hot lines (Table 1) shall also clarify the relationship of particle
acceleration to plasma heating. Finally, since the emission line profiles reflect the line-of-sight
distribution of ion velocities, from zero to very high velocities, they provide key information
about both the high-energy particles simultaneously with the low-energy end of the distribution,
not accessible with either LISSAN or FOXSI instruments.
•How is impulsively released energy transported and dissipated in the solar atmosphere?
Once heated, the hot flare plasma evolves rapidly on short timescales depending on the
conditions. The hot flare lines observed by SISA cover some of the largest available temperatures
via EUV line spectroscopy (Table 1) and are thus favorable for characterizing the hot flare plasma
and its evolution. Typically, the hot plasma is observed first above the chromospheric footpoints
in the form of localized bright kernels [e.g.,
5
,
6
], from which the flare loops are filled. Indications
however exist that these kernels located within bright flare ribbons can already be pre-heated
by electron beams [
7
,
8
]. These kernels have been long observed to move along ribbons [e.g.,
9
–
11
], which have been identified as a consequence of 3D slipping reconnection [e.g.,
12
–
18
],
where the field lines do not reconnect in an X-point, but slip (slide) past each other as they
mutually exchange their connectivities [see, e.g.,
19
–
21
]. The existence of this process implies
that the location of the energy deposition into the lower solar atmosphere changes with time as
the slipping reconnection proceeds. In the past, it has been extremely difficult to identify the
spectroscopic signatures of this process due to the slit not being able (or designed) to track a
particular moving (slipping) kernel. Using sit-and-stare observations Li and Zhang
[15]
showed
that the slipping reconnection is likely be related to periodic changes of Si IV spectral line
intensities accompanied by enhanced redshifts, as well as increased nonthermal widths, as
individual kernels moved through the location of the slit. Recently, Lörinˇcík et al.
[18]
detected
extremely short-lived blueshifts (upflows), lasting only seconds, and reaching about 50 km s
−1
in chromospheric and transition-region lines, at the leading edges of the slipping kernels. The
authors argued that such detection can be a matter of luck with slit spectrographs, as the slit
has to be in the right place at the right time. The IFS provided by SISA shall be enormously
helpful in this regard, as it can image the entire flare region and allow us to identify how the
flaring atmosphere undergoing slipping reconnection responds at short temporal cadences, as
well as enable tracking the spectral evolution of individual moving kernels as they slip along
flare ribbons. In addition, the SISA thermal coverage would help in establishing whether the
blueshifts at the leading edges of ribbons or individual kernels are related to evaporation of hot
flare plasma.
The fastest upflow velocities of the evaporating hot flaring plasma filling coronal loops are
detected in the hottest flare lines [
5
], while the ‘cooler’ (
T<∼
1 MK) lines show downflows at the
same location, implying multi-directional flows that could occur due to rapid plasma evolution.
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The fast SISA cadence and the integral field spectroscopy, coupled with the available plasma
diagnostics (Section 3) will provide key information on the thermal evolution of the heated
plasma. For example, diagnostics of electron density coupled with the measured timescales of
plasma cooling will allow for discerning whether the plasma is in ionization equilibrium, and at
what times. Meanwhile, the emission line profiles will provide information on the presence and
role of turbulence. Mass flows and the thermodynamic evolution of the atmosphere determined
via spectroscopy can reveal much about the energy input to the lower atmosphere during flares,
especially when combined with state-of-the-art modelling. For example, flare observations from
the Interface Region Imaging Spectrograph [IRIS;
2
,
22
] have been used in tandem with flare
loop models to interrogate our understanding of flare processes [
23
,
24
]. A major model-data
discrepancy is the duration of the flare gradual phase, with models under-predicting the cooling
time by an order of magnitude. SISA observations that provide plasma diagnostics over the full
field of view, with high cadence, will help illuminate the source of the continued heating or energy
input. Measurements of nonthermal line widths from lines formed at different temperatures,
will inform us about the roles of turbulence in suppressing thermal conduction [e.g.
25
], and the
potential role of Alfvén waves in flares [e.g. 26,27].
SISA measurements will provide key diagnostics of processes that occur within flares on short
timescales. For example, a key open question in solar flare energy release is what drives
"bursty" pulsations and oscillatory signatures observed in flare emission, known as quasi-periodic
pulsations (QPPs) [see
28
–
30
, for reviews]. QPPs and other oscillatory behaviour observed in
flares have timescales ranging from sub-seconds to minutes, and are identified across the entire
electromagnetic spectrum from radio, EUV [
31
], X-rays [
32
] and even
γ
-rays [
33
], essentially
encompassing all aspects of the flaring process. The exact nature and underlying physical
mechanism for the generation of these pulsations remains highly debated. It is suggested that they
may be related to magnetohydrodynamic oscillations in/near the flare site, or possibly connected
to the intermittent or time-dependent magnetic reconnection itself. Observational limitations
to date of temporal cadences, spatial resolution, and saturation issues with EUV imagers have
limited our ability to observational identify the locations of the emission modulations and
constrain the suggested models - both of which are directly linked to energy release and transport
in solar flares. Some work has aimed to identify the spatial locations of the modulations [e.g.
15
,
31
,
34
–
36
], although they are limited temporal cadences, and the spatially diagnostics; for
example only spatially observing along the slit position. In order to correctly identify the
mechanism producing QPPs, characteristics of the temporal, spatial and spectral properties of
pulsations and their relationships across energy ranges and temperatures are required. The SISA
EUV measurements with high temporal cadence, and its ability to perfrom imaging spectroscopy
of the flaring region (rather than just over the slit) will allow us to observe rapid changes in
the flaring regions such as these pulsations, with information regarding where and at what
temperature they originate in the flare structure.
Finally, though there is unambiguous evidence for the presence of nonthermal particles in flares,
other mechanisms may also act to transport liberated magnetic energy. High frequency Alfvén
waves have been proposed as a means of transporting energy the magnetic reconnection site to
the lower atmosphere and heating it [e.g.
37
–
39
]. Modelling has revealed that those waves do
indeed heat the chromosphere, and drive explosive evaporation into the corona [
40
–
42
]. While
it is likely that MHD waves are produced during flares, which are fundamentally a large scale
change in the corona’s magnetic field, the proportion of energy that manifests itself in the form
of waves compared to energetic particles is not known. SISA’s capability to measure the coronal
magnetic field before, during and after a flare will provide crucial information regarding field
perturbations, and together with density diagnostics the Alfvén speed, which will help to provide
estimates of the Poynting flux carried by MHD waves. Furthermore, nonthermal broadening of
ions will also help constrain the Poynting flux [see discussion in, e.g., 26].
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•What are the physical low-corona origins of space weather events?
Both Coronal Mass Ejections (CMEs) and jets inject magnetic field and plasma into the
heliosphere, where they disturb the solar wind flow. Although previous studies of CME source
regions have provided details about the physical processes taking place once a CME has initiated,
many questions remain regarding their initiation and similarities with smaller scale flux rope
eruptions within solar jets. Observing the initiation of both CMEs and jets spectroscopically is
quite difficult with current or planned instrumentation for several reasons, including the use
of a single slit, long exposure times, or limited diagnostics and temperature coverage provided
by the available spectral lines. SISA will capture these processes at cadences down to a few
seconds from every pixel within its entire field of view. This will allow us to identify the
locations, spectral properties, plasma conditions, and thus the mechanisms behind the processes
of CME and jet initiations. In larger eruptive events, SISA will be able to capture the entirety of
precursor phase of the associated flare. The spatial localization of the precursors with respect
to the subsequent flare and eruption allows for identification of the CME initiation mechanism,
whether by tether-cutting, ideal MHD instability, or breakout [see
43
] for every flare observed.
The high-temperature lines observed by SISA (Table 1) will provide information on the plasma
properties during the onset of eruptive events, including the possibly constant, isothermal
10–15 MK onset temperatures detected by broad-band X-ray instrumentation [
44
]. These lines
will also allow the quantification of plasma heating (via temperature and density measurements)
as well as turbulence (via line broadening) in the precursor phase.
•How is the corona above active regions heated?
It is currently thought that the solar corona is heated by individual impulsive "nanoflare" events
that release small amounts of energy at either high or low frequencies [e.g.,
45
–
49
]. A key
prediction of such impulsive energy release is the existence of small amounts of hot 5–10MK
plasma, which is difficult to detect spectroscopically with current instrumentation [
50
–
55
]. The
spectral range of SISA contains several hot lines (Table 1) that will provide stringent constraints
on the amount of plasma reaching 10MK temperatures. The Fe XVII and Ni XVII lines (Table
1) shall provide additional constraints. SISA will also work in tandem with the HXR and SXR
observations to constrain the high-
T
component of the impulsive energy release by nanoflares.
Another key observable of impulsive energy release is that the plasma should at least temporarily
be out of thermal equilibrium, showing either presence of accelerated particles, out-of-ionization
equilibrium plasma [e.g.,
56
–
59
], or both. The coronal Fe XI lines formed in both quiet Sun
and active regions offer such diagnostics for electrons [
60
,
61
] while the line profiles of multiple
ionization stages shall provide information on ion velocity distribution [
62
–
65
]. Furthermore,
temporal evolution of line intensities of multiple ionization stages obtained in high cadences in
combination with electron density diagnostics (Section 3.7) shall provide information on both the
presence of energy release events and constraints on the presence of non-equilibrium ionization
for multiple coronal structures at the same time, a feat not possible with current or planned
instrumentation. Finally, it will become possible for the first time to tie all these measurements to
the measurements of the underlying magnetic field (Section 3.1), thus allowing for discerning
whether there are different heating mechanisms for different magnetic structures within the
active and quiescent solar corona.
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Table 1.
A selection of SISA diagnostic spectral lines. The approximate formation temperature (log
T
[K]) is shown, as well as the radiances for an AR core and an M2-class flare (in erg cm
−2
s
−1
sr
−1
).
Numbers in brackets are DN s−1per 1′′ SISA pixel.
Ion λ(Å) log T AR core M2 flare Notes
Ni XV 178.89 6.5 15 1.2×102
Fe XXI 178.90 7.1 - 4.8×102(145) ** Flare Ne
Fe XI 179.76 6.1 130 (40) 9.0×102(276) ** Ne
Fe XXIII 180.04 7.2 - 8.8×102(270) * Flare
Fe XI 180.40 6.1 1.4×103(429) 3.1×103(950) ** (bl Fe X)
Ca XV 181.90 6.5 - 4.2×102(156) *** Ne
Fe XI 182.17 6.1 120 (47) 1.1×103(429) *** Ne
Ca XV 182.86 6.5 - 6.4×102(310) *** Ne
Fe X184.54 6.0 220 (175) 5.5×102(439) *** Coronal B
Fe XI 184.79 6.1 76 (64) 5.0×102(424) *** Ne
Fe XII 186.89 6.2 600 (395) 2790 (1836) (bl) ** Ne
Fe XXI 187.93 7.1 - 3.8×103(1172) *** Flare (bl Ar XIV)
Ar XIV 187.96 6.5 21 3.4×102** Ne, FIP
Fe XI 188.22 6.1 710 (160) 1.6×103(362) ***
Ar XV 221.15 6.5 73 (13) 3.4×103(584) *** FIP
Fe XXIII 221.34 7.2 - 5.6×102(100) Flare
SXII 221.43 6.4 90 (16) 4.1×102(75) ** FIP
Fe XV 233.87 6.5 260 (247) 3.0×103(2850) ** Ne
Fe IX 241.74 5.9 120 (189) 120 (189) *** Ne
Fe XXI 242.05 7.1 - 4.1×103(6540) *** Flare Ne
He II 243.03
Ar XIV 243.75 6.4 43 5.3×102FIP (bl)
Fe XV 243.79 6.5 880 (1502) 6.5×103(11095) (bl Ar XIV)
Fe IX 244.91 5.9 80 (141) 2.3×102(406) *** Ne
Fe XXI 246.95 7.1 - 6.5×102(1205) *** Flare
Fe XXII 247.19 7.1 - 8.0×103(14887) *** Flare
Ar XIII 248.68 6.2 7 (13) 1.1×102(209) ** FIP
Ni XVII 249.19 6.6 840 (1598) 9.3×103(17690) *** Flare
Fe XII 249.39 6.2 65 (124) 2.4×102(457)
Fe XVI 251.06 6.5 650 (1234) 1.0×104(18979)
Fe XIII 251.95 6.2 460 (864) 1.7×103(3194)
Fe XIV 252.20 6.4 280 (524) 1.8×103(3367)
Fe XXII 253.17 7.1 - 3.9×103(7177) *** Flare
Fe XVII 254.88 6.6 - 3.5×103(6140) *** Flare
Fe XXIV 255.11 7.3 - 4.8×104(83656) *** Flare
He II 256.3 (bl)
SXIII 256.68 6.5 1.1×103(1797) 8.6×103(14055) ** FIP
Fe X257.26 6.0 140 (222) 150 (238) *** coronal B
Fe XIV 257.39 6.4 420 (663) 2.1×103(3317)
Fe XI 257.55 6.1 80 (125) 2.3×102(360) ** Te,NMED
Si X258.37 6.2 420 (628) 1.5×103(2243) Ne
SX259.50 6.2 53 (74) 1.8×102(251) *** FIP
Si X261.06 6.2 140 (175) 4.2×102(525) Ne
Fe XVI 262.98 6.5 1.1×103(1148) 1.7×104(17754)
Fe XXIII 263.77 7.2 - 2.1×104(20192) *** Flare
SX264.2 6.2 77 (71) 257 (237) *** FIP
3. SISA EUV Measurements and Diagnostics
Based on these scientific cases, two spectral windows have been defined for SISA:
λ= [
178
−
184
]
Å and
λ=
246
−
258 Å. There are numerous EUV spectral lines originating from a range of
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species that together allow a wide temperature coverage from transition region through to coronal
and flare plasma. Many density sensitive line pairs are included within these windows, including at
T>
10 MK, as are a pair of Fe Xtransitions whose ratio is proportional to the coronal magnetic field
strength. Via the inclusion of Fe XI the electron temperature and presence of non-Maxwellian electron
distributions can be probed. There are also transitions of S and Ar to measure the FIP bias, relative to
Fe lines, providing vital composition information.
Table 1lists the spectral lines of interest, along with notes regarding the diagnostics they provide.
There is some overlap with the EUV spectral lines observed by Hinode/EUV Imaging Spectrograph
[EIS;
1
], but with higher sensitivity, cadence, and simulataneous spatial coverage [for more details see,
e.g.
66
]. Whilst EIS requires an estimated 50 minutes to scan a 100
′′
region with its 2
′′
slit and 60 s
exposures to obtain enough signal for coronal magnetic field measurements, SISA would obtain the
same observations within a few seconds.
Once they thermalise, the energetic particles that carry flare energy from the corona to the
transition region and chromosphere, heats the local plasma. The intense heating subsequently leads to
ionisation and drives mass flows: so-called chromospheric evaporation and condensation. Spectral
lines from species forming at different temperatures are very useful in diagnosing the conditions of the
plasma before, during, and after energy deposition. They can reveal gradients in density, bulk velocity
and temperature. We have typically been restricted to observations in single locations, whether that be
at the flare footpoints, along the loops, or at looptops. However, to understand the full story of where
flare energy is deposited and how the plasma responds, simultaneous observations, at high-cadence,
of the full flaring structure would be transformative.
3.1. Magnetic Field Measurements
The solar coronal magnetic field can be measured via the recently identified Fe X
184.54 /257.26 Å line intensity ratio [see, e.g.
67
–
69
]. While the former line is an allowed transition, the
latter, Fe X257.26Å, is a blend of several intercombination transitions. In the presence of the external
magnetic field, a new decay channel to the ground configuration 3s
2
3p
5 2
P
2/2
occurs due to mixing
of the upper metastable levels 3s
2
3p
4
3d
4
D
7/2
and 3s
2
3p
4
3d
4
D
5/2
[
70
]. This magnetically-induced
transition (MIT) increases the intensity of the 257.26Å line, and allows for measurements of coronal
magnetic field. This has already been done using the solar EUV spectra measured by Hinode/EIS,
revealing presence of magnetic fields of up to several hundred Gauss in a C2-class flare [
70
] as well as
in the solar coronal loops [71].
However, making such measurements routine using slit spectroscopy (as in the case of
Hinode/EIS) is difficult, as the slit only samples a small portion of the solar atmosphere. In addition,
the Fe Xline at 256.26 Å is relatively weak, requiring long exposure times with previous instruments
such as EIS. Finally, the diagnostic ratio is also sensitive to a range of parameters, as the electron
density and temperature, so additional diagnostic lines are needed. They are provided by SISA, as
described below.
The SISA instrument shall allow for routine measurements of both Fe Xlines in a wide field
of view and at orders of magnitude faster cadence, providing revolutionary potential for magnetic
field measurements both before, during, and after the reconnection process; that is, from the active
region corona pre-reconnection to the post-reconnection flare loops. Such measurements will allow
for the first time to provide a direct measurement of the magnetic energy released during the flare
reconnection.
3.2. Electron temperature and nonthermal diagnostics
To measure the result of transport effects on accelerated electrons, the nonthermal electron
spectrum must be characterised at multiple locations in the flare structure, namely near the acceleration
region, along the flare loops, and near the chromospheric footpoints.
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SISA’s wavelength range contains lines of multiple Fe ions (see Table 1) from Fe IX formed at
upper transition region, Fe X, Fe X I, Fe XIV, and Fe XVI formed in the corona of quiet Sun and active
regions, to multiple flare lines of Fe XXI, Fe XXII, and Fe XXIV. Such wide temperature coverage
shall allow for characterization of the ionization temperature of the plasma, and in particular the
coronal structures participating in the reconnection process, from the pre-reconnection corona to
post-reconnection flare loops.
In addition, the EUV lines observed by SISA allow to assess if nonthermal electrons are present in
certain cases. The ratios of Fe XI line intensities at 182.17 /257.55 Å 182.17/ 257.77Å are sensitive to
high-energy electrons at temperatures above 1MK [
60
,
72
]. The Fe XI lines at 257.55 Å and 257.77 Å are
located in direct vicinity of the magnetically-sensitive Fe Xline at 257.26 Å. We note that since
the temperature and power-law tail parameters are free parameters of any electron distribution,
diagnostics of the high-energy electrons always need to be performed simultaneously with diagnostics
of temperature [60,72, and references therein].
3.3. Ion temperature and nonthermal diagnostics
Profiles of any emission line changes shape if energetic ions are present. This effect is detectable
through the presence of enhanced line wings, which correspond to high-energy tails in ion velocity
distribution via the Doppler effect. Such high-energy ions were detected in the last decade in a limited
amount of cases, mostly in flares [
62
–
65
]. The most significant enhancements of the wings were found
to occur at about twice the line FWHM [see Figure 1 of
64
]. This means that in principle, any emission
line with strong enough signal-to-noise in the far wings can be used for diagnostics of accelerated ions,
independently of the temperature at which the line is formed. Thus, the wide temperature coverage of
the solar corona provided by SISA fortuitously allows for wide diagnostics of energetic ions. However,
the diagnostics are complicated by the fact that the emission lines are usually broadened during the
impulsive phase of flares – a well known fact for decades [see Section 3.1 of [
73
] and also [
63
,
65
,
74
,
75
].
Since the broadening is indicative of the presence of turbulence, the wide spatial coverage of SISA shall
enable the study of spatial correlations between the occurrence of turbulence and ion acceleration. We
note that the two mechanisms can in principle be linked, with turbulence directly leading to particle
acceleration in some cases [see, e.g., 76].
3.4. Electron Density Diagnostics
Measuring the electron density is a fundamental requirement for any spectrometer. Knowing the
density is important for a wide range of issues, for example to estimate radiative losses of the plasma
and thus the energy input required not only during flares, but also for the active region corona. In
addition, electron density is critical for determining the ionization and recombination timescales in the
plasma, and thus whether the plasma is in ionization equilibrium. The EUV offers a wide range of
diagnostic possibilities, see the review by [69].
The SISA spectral range and sensitivity was also chosen to enable measurements of electron
densities from a wide range of lines, formed from coronal (e.g. Fe IX, Fe XI, Fe XV, Ca XV)
to flare temperatures (Fe XXI). Measurements of coronal densities have been carried out with
previous spectrometers, most notably Hinode EIS, but measurements at high temperatures are nearly
non-existent in the literature. The best diagnostics are in the soft X-rays around 140 Å, as discussed in
[
77
], using lines mainly from Fe XXI and Fe XX. Such measurements have been carried out for a limited
number of large X-class flares by [
78
] using SDO/EVE, an instrument without any spatial resolution.
To estimate the ionization and recombination times of the flaring plasma, it is essential to measure
the electron density at high temperatures with good spatial resolution. SISA has two excellent line
ratios from Fe XXI. One is 242.05Å /246.95Å, which is sensitive to high densities, above 10
12
cm
−3
.
The lines are strong and close. The other one is a ratio involving the weaker 178.9 Å line, which is
sensitive already to active region densities of 10
10
cm
−3
or more. One issue with the 178.9 Å line is that
it could be blended with a Ni XV line, depending on the relative amount of plasma at the formation
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temperatures of these ions. To correct for the Ni XV blend, two other density-sensitive Ni XV lines need
to be observed.
Figure 1provides a summary of the main diagnostic ratios for SISA, although other ones are
available. CHIANTI version 10 [
79
] was used. The ratios cover a wide range of temperatures and are
sensitive to a wide range of densities. Note that the Fe IX ratio is the best density diagnostic for the
solar corona, but has never been observed routinely. Also note that the Fe XXI ratio is one of the best
ones across all wavelengths, to measure low electron densities which are expected to be present during
the heating phase. Most other density diagnostics of flare lines, from the X-rays to the UV, are sensitive
above 1012 cm−3.
Figure 1. A selection of some important diagnostic spectral lines to measure electron densities.
3.5. Elemental Composition Diagnostics
Within the solar corona, there is a wide range of elemental abundances. As is the case for in-situ
measurements, the variations are mainly associated with the First Ionization Potential (FIP) of the
elements, indicating that the fractionation process that creates these variations occurs in the low
chromosphere, where neutrals become ionized. The ratio of a low-FIP to a high-FIP element tend to be
enhanced, compared to its photospheric value (the so-called FIP bias), although interesting cases of
so-called inverse FIP effect have been observed. There is ample literature, but many earlier studies
turned out to be incorrect, leading to a very confusing picture. A recent review of observational aspects
is given in [69].
In a nutshell, the quiescent corona has nearly photospheric abundances, at least up to 1 MK;
the hot 3 MK cores of active regions have an FIP bias of about 3–4; the cool active region loops
have a wide range of FIP biases. Surges and flaring loops have been known since the Skylab time
(1970’s) to have photospheric abundances, interpreted as the indication that the plasma filling the
loops evaporated from the chromosphere with a fast timescale such that the fractionation process did
not occur. Interestingly, recent X-ray observations of small flares of GOES B-class have shown for the
first time a rapid decrease of the FIP bias, followed by a gradual increase [
80
]. While the rapid decrease
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is expected to be caused by chromospheric evaporation, the gradual increase could be a signature of
Alfvén waves.
There is general consensus that the FIP effect is related to the strength and topology of the magnetic
field. A relatively simple hydrostatic model of the chromosphere put forward by M. Laming is able to
explain the main features of the FIP effect and its inverse [see the review and references in
81
]. The FIP
effect would be caused by the ponderomotive force of trapped Alfven waves in magnetically-closed
loops. Therefore, measuring the FIP effect and its variations during flares is important, as it could
provide a way to indirectly confirm for the presence of Alfven waves or at least indicate the origin
of the flaring plasma: if the flare loops have a FIP bias, then the plasma originated from closed loops
already present in active regions.
The diagnostics to measure the relative abundances are limited, however, as there are only few
ions formed at high temperatures from high-FIP elements. There are plenty of spectral lines from
low-FIP elements such as Si, Fe, Mg, Ni, Ca instead. The best high-FIP ones, exploited in the 1970’s
and 1980’s, are the H-like and He-like ions from oxygen, neon, argon and sulphur in the X-rays. Note
that sulphur as a FIP of 10 eV, but in remote-sensing observations varies as the other high-FIP elements.
Very few weak lines from high-FIP elements are present in the EUV and UV. A few very weak lines
from Ar XIV and Ar XIII have been observed with Hinode EIS, which also has stronger S X, S XII and
SXIII lines.
Across the whole EUV/UV, the best diagnostic lines are the strong Li-like doublets from Ar XVI
at 353.8, 389.0 Å and from S XIV at 417.7, 445.7 Å. After them, the best diagnostic is the strongest
(resonance) line from Ar XV at 221.15 Å, which is within the SISA spectral range. There are also
interesting Ar XIV lines as the the strong 243.75 Å line, blended with Fe XV, or the 187.9 Å line,
which becomes strong at high densities (but can also be blended), meaning that one would have to
measure the density independently. Other good options within the SISA range are the strong S XIII at
256.6 Å and the lower-temperature S Xlines, often used in previous Hinode EIS studies. Weak lines
from Ar XIII are also available. There are plenty of lines from iron to be able to estimate the FIP bias
measuring Ar/Fe and S/Fe in the 1–3 MK range.
3.6. Flare lines
There are several hot flare lines in the SISA spectral range. Close to Fe X257.26 Å are the strong
Fe XVII 254.9, Fe XXI 246.95 Fe XXII 247.19, 253.17, Ti XX 259.26, and Fe XXIV 255.1 Å lines. They span a
temperature range
log T= [
6.6
−
7.3
]
. Nearby, there is a strong Fe XXI line at 242.05Å which could be
used in combination with the 246.95 (or the 187.9 Å) to measure densities above 10
11
cm
−3
(Section
3.4). This is an important diagnostic.
The strong Fe XXIII intercombination line is located at longer wavelengths, 263.76 Å. It would be
an ideal electron temperature diagnostic in combination with the resonance line at 132 Å, if a second
order could be observed.
Close to the 184.5 Å, there are several weaker hot lines, which become observable for large flares.
Fe XXIII produces a line around 180. Å. There is a relatively strong Fe XXI line around 187.93 Å, however
this line for weak flares is blended with an Ar XIV 187.96 Å line which is strongly density-sensitive, so
careful deblending is needed. A weak Fe XXI around 178.9 Å is particularly important as it becomes
strong at high electron densities, so it is one of the best density diagnostics for flare plasma, able to
measure densities above 1010 cm−3.
3.7. Departures from ionization equilibrium
Rapid heating or cooling of plasma can lead to departures from ionization equilibrium; a state
when the plasma ionic composition does not reflect its electron temperature [e.g.,
56
,
82
–
85
]. This
transient ionization in turn can affect other diagnostics [e.g.,
86
]. For a review of transient ionization
conditions, see [59].
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Conditions favorable for occurrence of transient ionization are expected to arise in solar flares,
where the energetic particles travel at fractions of the speed of light, and thus swiftly deposit large
amounts of energy when encountering the dense solar chromosphere. This is so despite the electron
densities ion flares are relatively high, leading to decrease of the timescales for collisional ionization
and recombination [
84
], which are in flare plasma of the order of seconds or tens of seconds. Since the
plasma evolves rapidly following the energy deposit, detecting transient ionization conditions requires
fast cadence. Previous diagnostics of transient ionization [e.g.,
82
,
83
] focused on X-ray spectra, which
were obtained with little or no spatial resolution and required scans across the wavelength range,
while the detection of transient ionization from EUV was hampered by the twin problems of slit and
throughput/cadence. For the first time, SISA will offer the spatial coverage, resolution, cadence, and
lines from multiple ionization stages of iron (see Table 1), all required to detect signatures of transient
ionization in rapidly evolving plasma along newly created flare loops.
3.8. Predicted signal in the main SISA lines
Given the previous requirements, namely provide spectral line diagnostics to measure magnetic
fields, electron temperatures and densities, departures from ionization equilibrium in flaring plasma,
non-Maxwellian electron distributions, variations in the FIP bias, and transient ionization, we designed
a few possible multilayers to cover the key lines, as described below. Having selected one, we have
then calculated the signal in the SISA lines.
As the target is to observe active regions and flares, two simulations have been carried out, one
for the quiescent AR core, and one for an M2-class flare. For the AR core the predictions are based
on an AR core loop observed by Hinode, see [
87
]. The Del Zanna (2013) coronal abundances are
used, with an electron density of 2
×
10
9
cm
−3
. For the M2 flare the predictions are based on an
M2-class flare observed by Hinode EIS. The Same set of abundances is used, but with electron density
of 2
×
10
10
cm
−3
. In this case a full DEM analysis was carried out (Del Zanna et al. 2023, in prep).
CHIANTI version 10 [
79
] was used. Table 1 shows the radiances of a few selected SISA lines and the
corresponding data numbers per second per 1" pixel. It is clear that most lines are observable with
exposures of 1 s.
4. Image slicer technology in the EUV regime
Among the Integral Field Unit (IFU) alternatives available to perform IFS, image slicers are the
optimum for space: they are very compact with no moving mechanisms, lightweight, highly efficient
and all parts are made of the same material.
As shown in Figure 2, an image slicer is an optical system composed typically of two or three
arrays of mirrors, placed at the telescope’s image focal plane. The first array (slicer mirror array)
is composed of multiple narrow mirrors with a rectangular shape distributed such that the short
dimension (width) corresponds to the spectral direction and the long dimension (length) is associated
to the spatial direction. Each one of the slicer mirrors have a different tilt angle around the
X
and
Y
axes. Thus, each mirror reflects only a slice of the field of view, defined by its effective surface, in a
different direction. The other arrays of mirrors are used to control the intermediate and exit pupils,
re-image the slices of the field and reorganise them generating the spectrograph entrance slit. They
can also produce a magnification, if required, enabling the perfect coupling between telescope and
instrument, and adjusting the slit width to meet the desired resolution. Other advantages include no
focal-ratio degradation and no polarisation effects.
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Figure 2.
Conceptual sketch of an image slicer. The slicer mirror array divides the image generated by
the telescope into slices that are re-imaged and reorganised by other mirror arrays, redistributing the
2D field of view into the spectrograph entrance slit.
Image slicers have been used for ground-based telescopes in the last few decades, operating at
different spectral ranges from the infrared to UV/visible, for a wide variety of science cases. Some
examples are shown in Figure 3and include: VLT CUBES [
88
], BlueMUSE [
89
] and MUSE [
90
];
GTC FRIDA [
91
]; GNIRS [
92
] at Gemini North Telescope or ELT HARMONI [
93
] and METIS [
94
]
for night-time observation. For solar physics, image slicers were first proposed for the integral field
spectrograph [
95
,
96
] of the European Solar Telescope, which would observe in the optical. A prototype
of this image slicer is operative at GREGOR solar telescope [
97
], which upgraded GRIS from a long-slit
spectrograph to an integral field spectrograph, and observes in the infrared. No EUV solar IFS
instrument has flown, though the SNIFS rocket mission, scheduled for a 2024 launch, will observe
solar Ly αusing IFS [3].
There are also spaceborne image slicers on-board the James Webb Space Telescope in NIRSpec
[
98
] and MIRI [
99
] operating at infrared wavelengths. At much shorter wavelengths, INFUSE [
100
]
was the first IFS in the Far Ultraviolet (FUV), while SISA [
101
] will be the first IFS in the Extreme
Ultraviolet, applying the image slicer technology at the shortest wavelengths ever proposed.
Finally, a different approach to the 2D coverage problem has been adopted by the Multi-slit Solar
Explorer [
102
–
104
, MUSE;], a NASA Heliophysics Medium-class Explorer with scheduled launch
in 2027. Thanks to its 35-slit design, MUSE will provide high-cadence (
<
20 s or faster for flares),
sub-arcsecond resolution spectroscopic rasters over an active region size, targeting 4 isolated, strong
spectral lines (Fe IX 171 Å at 0.7 MK, Fe XV 284 Å at 2 MK, Fe XIX at 108 Å at 10 MK, and Fe XXI 108 Å
at 12 MK). With its unique capabilities, MUSE will provide new crucial discoveries for flare science
[
104
]. SISA would build upon the science legacy of MUSE by providing high-cadence 2D imaging
from a larger set of spectral lines that offer rich diagnostic potential.
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Figure 3.
SISA will break new ground both technologically by observing using image slicers in the
EUV, and scientifically by providing high cadence 2D spectra of various solar phenomenon. Currently
operating image slicers based IFS operating at different spectral ranges for solar and night-time
ground-based telescopes and space missions observe at longer wavelengths than SISA.
At the shorter wavelengths of the EUV regime that SISA will explore, the application of image
slicers presents additional challenges [
101
], like: narrower slicer mirrors for high resolution solar
observations, surface roughness, and coating. A slicer width of 15
µ
m has been defined for SISA, which
is narrower than the slicer mirror width that is currently achievable both for glass and metallic slicer
mirrors. This width is even more difficult to achieve on curved substrates (powered slicer mirrors),
whose use would enable the reduction of the number of optical components to maximise throughput.
At these short wavelengths the surface roughness is a key factor to minimise stray light that could
potentially compromise the instrument sensitivity.
These technological challenges of the image slicer technology for the EUV are being addressed
and developed in the UK by a collaboration between Durham University and University College
London (UCL) within two on-going projects: MINOS and LUCES. MINOS (Manufacturing of Image
slicer Novel technology for Space), funded by Durham University, explores the state-of-the-art of
glass slicers producing a slicer demonstrator with the thinnest width currently achievable on spherical
substrates of 70
µ
m and enhanced surface roughness below 1 nm RMS. This demonstrator is being
manufactured by Bertin Winlight, in France.
LUCES (Looking Up image slicers optimum Capabilities in the EUV for Space) is funded by the
Enabling Technology Programme of the UK Space Agency and will produce multiple powered slicer
mirror demonstrators in metal using different widths to determine the narrowest achievable width
with diamond machining techniques at Durham University, and on different substrate to compare
surface roughness. These projects will increase the technology readiness level (TRL) of the image slicer
technology for EUV applications.
5. SISA instrument proposal
5.1. Specifications
The Spectral Imager of the Solar Atmosphere (SISA) is the first EUV imaging spectrometer using
image slicer technology in the
λ= [
180
−
260
]
Å range. SISA will observe a threshold field of view
(FoV) of 100
′′ ×
100
′′
(with a goal of 100
′′ ×
250
′′
), obtaining spectra of the whole 2D field of view in
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less than 3 seconds for high signal objects (e.g. flares) and less than 30 seconds for low signal ones. This
field of view will be observed in two spectral windows simultaneously, Window 1
λ= [
178
−
184
]
Å,
and Window 2
λ= [
246
−
258
]
Å, with a spectral resolving power (
R
) of
R= [
3560
−
5160
]
, a spatial
resolution of <3′′, and a spectral resolution of 0.05 Å Full Width at Half Maximum (FWHM).
The defined spatial resolution is required to satisfactorily resolve flare loops and ribbon/footpoint
sources, including newly reconnected ones, whilst the spectral resolution is required to resolve the
main diagnostic lines and measure nonthermal line profiles. The FoV of 100
′′ ×
100
′′
can capture the
core of large X-class flares, the entirety of smaller flares, or a portion of an active region. A high signal
temporal resolution (Cadence 1) of 3 s will capture much of the dynamics of many flares, whilst being
two orders of magnitude faster than previously observed for 2D EUV imaging spectroscopy. A low
signal temporal resolution (Cadence 2) of 30 seconds will capture the slower dynamics present in the
active regions or quiet Sun areas.
The two spectral ranges are required to measure the parameters of both 1 MK plasma and
the much hotter 15 MK plasma that appears during flares, and include lines sensitive to the
coronal magnetic field strength, electron temperature/non-Maxwellian electron distributions, a
wide range of lines to measure electron densities from coronal (e.g. Fe IX, Fe XI, Fe XII, Fe XV, Ca
XV) to flare temperatures (Fe XXI), and the FIP bias. The specifications for SISA are presented in Table 2.
Table 2. Specifications for the instrument SISA.
Field of View 100′′ ×100′′
Spectral Window 1 178 −184 Å
Spectral Window 2 246 −258 Å
Spectral resolution 0.05 Å FWHM
Spatial resolution <3′′
Spectral Resolving Power (R) 3560 −5160
Temporal resolution (high signal) <3 seconds
Temporal resolution (low signal) <30 seconds
5.2. Layout
The solar EUV photon flux is very low, demanding a sensitive instrument. SISA combines the
surfaces of the Integral Field Unit (IFU) with those of the spectrograph in a compact and highly
efficient layout that uses only an array of powered slicer mirrors and an array of curved diffraction
gratings as shown in Figure 4. This layout is based on a design presented by Calcines et al. 2018
[
105
] for applications for ultra-compact integral field spectrograph and is inspired in IGIS [
106
], which
first proposed the combination of the IFU and the spectrograph components. SISA does not have an
entrance slit, it covers a 2D field of view without any scanning mechanisms obtaining all spectra of a
2D region of the Sun within one exposure.
Two subsystems are identified: the telescope and the integral field spectrograph, with a total of
three surfaces. To optimise efficiency, the telescope is composed of one off-axis parabolic mirror, with
an aperture of 200 mm diameter and an effective focal length of 3 metres. At its focal plane, the array
of slicer mirrors, each one of them with different orientations, divide the image of the field into thin
slices of 15
µ
m. To cover a field of view of 100
′′ ×
100
′′
, a hundred slicer mirrors are needed, with
rectangular shape and whose dimensions are 3.64 mm length by 15
µ
m width. The slicer mirror array
decomposes the incoming beam into as many sub-beams as the number of slicer mirrors, generating a
pupil for each one of them. At the pupil position an array of curved gratings will disperse the light
and focus the beams at their focal length. Each grating will produce the spectrum associated with a
slice of the field. The different orientations for the gratings are defined to distribute the spectra on the
detectors. These orientations are fixed, with no moving mechanisms.
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The design of SISA includes different trade-off analyses, such as the definition of the telescope
effective focal length versus resolution and slicer mirror width. For the dispersive element, two options
were considered: a larger diffraction grating versus an array of gratings, one per slice. Whilst a larger
grating would provide higher spectral resolving power, curved gratings present aberrations for off-axis
fields. The use of an array facilitates the organisation of the spectra on the detectors. This is currently
the selected option.
Figure 4.
SISA’s conceptual layout (left) and view of the spectra per slice generated by each curved
grating (right).
5.3. Components
As mentioned in the previous section, SISA is composed of two subsystems: (1) the telescope
and (2) the instrument. A door protection mechanism will be installed at the front of the instrument.
This door will be used to provide safety against acoustic vibration and venting during the launch.
It will also protect the instrument from contamination during assembly, integration and verification
(AIV), launch and in-orbit, and control the venting and reduce air flow during on-ground purging
and launch. CSL is considered as the future supplier for the door based on their previous experience
designing the door for the Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter.
The aperture of the telescope (200 mm) was selected to optimise throughput with respect to
existing EUV instruments. The same supplier of the METIS/Solar Orbiter mirrors is considered,
TOPTEC, Institute of Plasma Physics of the Czech Academy of Sciences with TRL 9. The manufacturing
feasibility of mirrors with apertures up to 300 mm has been confirmed.
The slicer mirror technology presents technology readiness level (TRL) 9 in the infrared. For the
EUV this technology is currently under development. MINOS and LUCES projects will raise the TRL
in the EUV to TRL 4. Although the slicers are the component with the lowest TRL for this spectral
range, its application is a game-changer for EUV solar spectroscopy. The on-going developments
within the UK in this field, together with a plan in place to achieve TRL 6 within the next several years
will enable this technology to be considered for the next generation of solar space missions.
Curved holographic gratings have already been used for other instruments like EIS and the
upcoming EUV High-Throughput Spectroscopic Telescope [EUVST
107
], with TRL 9. For the gratings
for SISA, IAS in France is currently considered as the future lead institution. The possibility of using
metallic curved gratings made on the same material as the slicer mirrors will also be investigated.
The SISA instrument requires a large-size image sensor with high EUV sensitivity. The baseline
will be to use back-illuminated CMOS radiation-hardened active-pixel image sensor (APS), developed
by CSL/ROB (Belgium). Compared to CCD technology, CMOS-APS detectors do not require a
mechanical shutter, have a greater radiation tolerance (low charge transfer), low power consumption,
high speed and dynamic range, with non destructive readout for low readout noise. Additional
features include random addressing, windowing and anti-blooming. SISA’s detectors must be cooled
to a temperature lower than -40 C (ideally around -60 C), with a stability of ±5 C.
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5.4. Efficiency
The efficiency of the instrument will be limited by two important factors: (1) the possibility of
using only two optical components for the integral field spectrograph and only one for the telescope
based on the performance on optical quality and (2) the coating.
For the simultaneous observation of the two spectral windows, a dual-band multi-layer coating is
being designed for SISA at the National Research Council of Italy, Padova. Considering the required
operational spectral bands for SISA, the most efficient coatings in terms of efficiency were identified to
be the three-material multilayer Al/Mo/B
4
C or, alternatively, the Al/Mo/SiC [
108
]. Both coatings
show very good long-term stability, and in the case of Al/Mo/SiC its reliability has been proven
through its successful use in the Solar Orbiter/EUI instrument. The multiband response required for
SISA was obtained by using a design consisting of a superposition of two periodic multilayer stacks
separated by two or three buffer layers. The coatings optimization was performed by using a genetic
algorithm, following the procedure previously described in [
109
]. Figure 5shows the three options
obtained with an Al/Mo/B
4
C multilayer, among which Option 3 is the preferred alternative due to
it offering the broadest response, with signal near 220 Å. Similar results can be obtained with the
Al/Mo/SiC multilayer.
Figure 5. Reflectivities for the three multi-layer coatings under design for SISA.
A preliminary estimation of the throughput has been performed. The effective area for SISA is
represented in Figure 6(in black), compared to Hinode/EIS (in blue). To calculate the effective area
we have considered the following components: (1) a front Al filter, the same used for SDO/AIA, but
with an improved mesh transmission of 95%, compared to the AIA one (85%). Such improved mesh
has already flown on the Hi-C sounding rockets. The AIA filters have shown little degradation at the
SISA wavelengths, probably thanks to their carbon layer, but alternative coatings will be considered.
(2) Two mirror reflectivities (one for the primary, one for the slicer mirrors) using the multilayer option
3; (3) A third reflectivity using the same multilayer for the grating, with an efficiency of 35%, the same
measured for the Hinode EIS grating in the long wavelength channel; and (4) The same quantum
efficiency of the EIS CCD, 0.85. A secondary focal plane filter has not been included to maximise
throughput, although a rotating wheel of redundant focal plane filters should be included.
The significant improvement over the EIS effective areas is due to the slightly larger aperture (20
cm instead of 15), and the fact that a single multilayer is used (the EIS aperture is sectioned with two
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different multilayers). Also, the multilayer has been designed to have maximum reflectance close to
the main diagnostic lines around 250 Å.
Figure 6.
Preliminary esimates of SISA’s effective area (black), compared to Hinode/EIS’ short
wavelength (SW) and long wavelength (LW) channels (blue).
6. Conclusions
Integral Field Spectroscopy is one of the most powerful techniques applied in astronomy. The
ability to obtain the spectra of a 2D field of view simultaneously reduces the integration time by over
two orders of magnitude. Despite this great advantage, this technique has not been used in the Extreme
Ultraviolet regime, which can only be observed from space, due to the limitations and challenges for
the existing Integral Field Unit alternatives. The image slicer technology seems to be the optimum
choice, offering a compact, robust, highly efficient solution with no moving mechanisms. The SPARK
(Solar Particle Acceleration Radiation and Kinetics) mission proposal led by the UK (UCL) includes the
first ever integral field spectrograph in the EUV: SISA. SISA consists of a three mirror system including
the telescope and the instrument.
A preliminary study has identified optimal spectral ranges to allow many plasma diagnostics,
some of which are not available to current (Hinode EIS) or future EUV instruments (EUVST, MUSE).
With a relatively small aperture of 20 cm diameter, a single multilayer and the slicers, we expect to
perform the types of measurements that Hinode EIS has been producing about 100 times faster.
Although the image slicer technology currently presents low TRL at EUV wavelengths, the UK
(Durham University and UCL) have some on-going research and development projects that will soon
enable this technology for the next generation of space missions, including solar physics and all fields
of astronomy.
SISA, currently under design study, promises to be a game changer in our understanding of our
star, the Sun.
Author Contributions:
The paper writing was led by Ariadna Calcines. The paper was based upon the SPARK
mission concept that was coordinated by Hamish A. S. Reid, Sophie Musset and Daniel F. Ryan. All other
authors contributed by providing scientific expertise or instrumentation expertise in the development of the SISA
instrument concept and/or the paper itself. The selection of the SISA diagnostic lines in Table 1 was led by G.D.Z.
All authors have read and agreed to the published version of the manuscript.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 29 November 2023 doi:10.20944/preprints202311.1854.v1
18 of 25
Funding:
HASR and SAM were funded by the UK Science, Technology and Facilities Council (STFC) under
the consolidated grant ST/W001004/1. GDZ acknowledges support from STFC via the consolidated grants to
the atomic astrophysics group at DAMTP, University of Cambridge (ST/P000665/1. and ST/T000481/1). JD
acknowledges the Czech National Science Foundation, Grant No. GACR 22-07155S, as well as institutional
support RWO:67985815 from the Czech Academy of Sciences. GSK acknowledges financial support from NASA’s
Early Career Investigator Program (Grant# NASA 80NSSC21K0460). DOS acknowledges financial support from
the grants AEI/MCIN/10.13039/501100011033/(RTI2018-096886-C5, PID2021-125325OB-C5, PCI2022-135009-2)
and ERDF “A way of making Europe” and “Center of Excellence Severo Ochoa” award to IAA-CSIC
(CEX2021-001131-S). LAH is supported by an ESA Research Fellowship.
Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable
527 to this article.
Acknowledgments: JD wishes to acknowledge useful discussions with E. Dzifˇcáková.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript:
AIV Alignment Integration and Verification
CCD Charge-coupled Device
CME Coronal Mass Ejection
CUBES Cassegrain U-Band Efficient Spectrograph
EIS EUV Imaging Spectrometer onboard Hinode
ELT Extremely Large Telescope
EUV Extreme Ultraviolet
EUVST Extreme Ultraviolet High-Throughput Spectroscopic Telescope
EVE EUV Variability Experiment onboard SDO
FIP First Ionisation Potential
FOXSI Focusing Optics X-ray Solar Imager
FRIDA inFRared Imager and Dissector for Adaptive optics
Full Width Half Maximum
FUV Far Ultraviolet
FWHM Full Width Half Maximum
GOES Geostationary Operational Environmental Satellite
GNIRS Gemini Near-InfraRed Spectrograph
GRIS Gregor Infrared Spectrograph
GTC Gran Telescopio Canarias
HARMONI High Angular Resolution Monolithic Optical
and Near-infrared Integral field spectrograph
HXR Hard X-ray
IFS Integral Field Spectrograph
IFU Integral Field Unit
INFUSE INtegral Field Ultraviolet Spectroscopic Experiment
LISSAN Large Imaging Spectrometer for Solar Accelerated Nuclei
LUCES Looking Up image slicer optimum Capabilities in the EUV for Space
METIS Mid-Infrared E-ELT Imager and Spectrograph
MHD Magnetohydrodynamic
MINOS Manufacturing of Image slicer NOvel technology for Space
MIRI Mid-Infrared Instrument
MUSE Multi-Slit Solar Explorer
QPP Quasi-Periodic Pulsations
SDO Solar Dynamics Observatory
SISA Spectral Imager of the Solar Atmosphere
SNIFS Solar eruptioN Integral Field Spectrograph
SPARK Solar Particle Acceleration, Radiation and Kinetics mission
SXR Soft X-ray
TRL Technology Readiness Level
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19 of 25
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