Abstract and Figures

The Particle Telescope (PATE) of FORESAIL-1 mission is described. FORESAIL-1 is a CubeSat mission to polar Low Earth Orbit. Its scientific objectives are to characterize electron precipitation from the radiation belts and to observe energetic neutral atoms (ENAs) originating from the Sun during the strongest solar flares. For that purpose, the 3-unit CubeSat carries a particle telescope that measures energetic electrons in the nominal energy range of 80–800 keV in seven energy channels and energetic protons at 0.3–10 MeV in ten channels. In addition, particles penetrating the whole telescope at higher energies will be measured in three channels: one >800 keV electron channel, two integral proton channels at >10 MeV energies. The instrument contains two telescopes at right angles to each other, one measuring along the spin axis of the spacecraft and one perpendicular to it. During a spin period (nominally 15 s), the rotating telescope will, thus, deliver angular distributions of protons and electrons, at 11.25-degree clock-angle resolution, which enables one to accurately determine the pitch-angle distribution and separate the trapped and precipitating particles. During the last part of the mission, the rotation axis will be accurately pointed toward the Sun, enabling the measurement of the energetic hydrogen from that direction. Using the geomagnetic field as a filter and comparing the rates observed by the two telescopes, the instrument can observe the solar ENA flux for events similar to the only one so far observed in December 2006. We present the Geant4-simulated energy and angular response functions of the telescope and assess its sensitivity showing that they are adequate to address the scientific objectives of the mission.
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Particle Telescope aboard FORESAIL-1: simulated
performance
Philipp Oleynika, Rami Vainioa, Hannu-Pekka Hedmanb, Arttu
Punkkinena, Risto Punkkinenb, Lassi Salomaab, Tero Sänttib, Jarno
Tuominenb,f, Pasi Virtanena, Alexandre Bosserc, Pekka Janhunend, Emilia
Kilpuae, Minna Palmrothe,d, Jaan Praksc, Andris Slavinskisc, Syed R. U.
Kakakhelb, Juhani Peltonena, Juha Plosilab, Jani Tammib, Hannu
Tenhunenb, Tomi Westerlundb
aDepartment of Physics and Astronomy, University of Turku, 20500 Turku, Finland
bDepartment of Future Technologies, University of Turku, 20500 Turku, Finland
cSchool of Electrical Engineering, Aalto University, 02150 Espoo, Finland
dFinnish Meteorological Institute, Erik Palménin aukio 1, 00560 Helsinki, Finland
eDepartment of Physics, University of Helsinki, Yliopistonkatu 4, 00100 Helsinki,
Finland
fTurku University of Applied Sciences, Joukahaisenkatu 3, 20520 Turku, Finland
Abstract
The Particle Telescope (PATE) of FORESAIL-1 mission is described. FORESAIL-
1 is a CubeSat mission to polar Low Earth Orbit. Its scientific objectives
are to characterize electron precipitation from the radiation belts and to ob-
serve energetic neutral atoms (ENAs) originating from the Sun during the
strongest solar flares. For that purpose, the 3-unit CubeSat carries a particle
Email addresses: philipp.oleynik@utu.fi (Philipp Oleynik),
rami.vainio@utu.fi (Rami Vainio), hannu-pekka.hedman@utu.fi (Hannu-Pekka
Hedman), arjupu@utu.fi (Arttu Punkkinen), rpunk@utu.fi (Risto Punkkinen),
laolsal@utu.fi (Lassi Salomaa), teansa@utu.fi (Tero Säntti),
Jarno.Tuominen@turkuamk.fi (Jarno Tuominen), pakavir@utu.fi (Pasi Virtanen),
alexandre.bosser@aalto.fi (Alexandre Bosser), Pekka.Janhunen@fmi.fi (Pekka
Janhunen), emilia.kilpua@helsinki.fi (Emilia Kilpua),
minna.palmroth@helsinki.fi (Minna Palmroth), jaan.praks@aalto.fi (Jaan Praks),
andris.slavinskis@aalto.fi (Andris Slavinskis), srukak@utu.fi (Syed R. U.
Kakakhel), juhpe@utu.fi (Juhani Peltonen), juplos@utu.fi (Juha Plosila),
jasata@utu.fi (Jani Tammi), hannu@kth.se (Hannu Tenhunen), tovewe@utu.fi (Tomi
Westerlund)
Accepted to Advances in Space Research December 2, 2019
telescope that measures energetic electrons in the nominal energy range of
80–800 keV in seven energy channels and energetic protons at 0.3–10 MeV
in ten channels. In addition, particles penetrating the whole telescope at
higher energies will be measured in three channels: one >800 keV electron
channel, two integral proton channels at >10 MeV energies. The instrument
contains two telescopes at right angles to each other, one measuring along
the spin axis of the spacecraft and one perpendicular to it. During a spin
period (nominally 15 s), the rotating telescope will, thus, deliver angular dis-
tributions of protons and electrons, at 11.25-degree clock-angle resolution,
which enables one to accurately determine the pitch-angle distribution and
separate the trapped and precipitating particles. During the last part of
the mission, the rotation axis will be accurately pointed toward the Sun,
enabling the measurement of the energetic hydrogen from that direction.
Using the geomagnetic field as a filter and comparing the rates observed by
the two telescopes, the instrument can observe the solar ENA flux for events
similar to the only one so far observed in December 2006. We present the
Geant4-simulated energy and angular response functions of the telescope and
assess its sensitivity showing that they are adequate to address the scientific
objectives of the mission.
Keywords: Radiation belts, Electron precipitation, Solar energetic
particles, CubeSats
1. Introduction
The FORESAIL-1 mission (Palmroth et al., 2019) is designed for signifi-
cant scientific advancements regarding the near-Earth radiation environment.
Its Particle Telescope (PATE) will target both of the two main components
of the Earth’s high-energy radiation environment, namely the Van Allen ra-
diation belts and Solar Energetic Particles (SEPs). Van Allen belts consist of
highly energetic electrons and protons accelerated and lost by various phys-
ical processes in the inner magnetosphere, while SEPs are accelerated by
solar eruptions in the corona, after which they propagate quickly (in tens of
minutes) from Sun to Earth. The crucial measurements of PATE are related
to defining pitch-angle and energy signatures of electrons precipitating from
the radiation belts as a function of magnetic local time (MLT) and mea-
suring solar Energetic Neutral Atoms (ENAs). Both measurements involve
aspects that have not been previously investigated. The energy-dependent
2
pitch angle spectra has not been measured before by an instrument carried
by a nanosatellite and there are no systematic observation of solar ENA.
2. The FORESAIL-1 Mission
2.1. Scientific objectives and requirements
The key science objectives for PATE are (1) to give significant new insight
on processes that scatter energetic electrons and protons from the Earth’s
radiation belts into the upper atmosphere as a function of level of magneto-
spheric activity and solar wind forcing conditions and (2) to quantify the how
coronal suprathermal ion populations affect SEP production and the energy
budget of solar eruptions.
Measuring reliably the precipitating electron population is of crucial im-
portance for radiation belt research. The belts consist of a relatively stable
inner belt of energetic protons and of a highly dynamic outer belt of ener-
getic electrons (Summers et al., 2012). The outer belt extends from L3
outward, where Ldescribes the distance in Earth radii where the magnetic
field lines of the geomagnetic field cross the Earth’s magnetic equator, as
measured from the center of the Earth. The most intense activity occurs
between 4< L < 5, i.e., in the "heart of the belts" (e.g., Reeves et al., 2013).
The magnetic field lines at these L-values map to the geomagnetic latitudes
of about 60–65. One key challenge in the radiation belt research is how the
relativistic electrons (from 600–700 keV up to tens of MeVs) gain their ener-
gies, are transported inward and outward in the belts and finally lost, either
at the dayside magnetopause (magnetopause shadowing) or precipitate to the
upper atmosphere through local pitch angle scattering due to wave-particle
interactions. To understand their dynamics the whole range of energies from
a few tens of keV to relativistic energies has to be covered (Jaynes et al.,
2015; Baker et al., 2018). One of the performance targets for PATE is thus
to cover as large energy range as possible, and the range between 80 and 800
keV taken as the requirement.
A crucial requirement for studying precipitation is obtaining pitch-angle
resolved measurements. Electrons precipitate at the altitude of about 100 km
if their pitch angle is small enough, i.e., they have enough parallel energy so
that they do not mirror before hitting the upper atmosphere. The local loss
cone width varies along the field line increasing towards the Earth. Under
dipole approximation on spherical polar Low Earth Orbits, the local loss-
3
cone boundary is rather independent on L, near 57for 500 km altitude and
near 47for 820 km.
Precipitation characteristics from the belts are expected to vary according
to the type of the large-scale solar wind transients (coronal mass ejections and
their shocks and sheaths, slow–fast solar wind stream interaction regions and
fast streams) that will interact with the magnetosphere (Hietala et al., 2014;
Kilpua et al., 2015). Therefore, to quantify how precipitation characteristics
vary depending on the solar wind forcing details and geomagnetic activity
requires extended measurements, optimally at least for six months. This is
long enough time to capture different solar wind structures influencing the
Earth, since, e.g., at the ascending solar cycle phase there are on average a
1-2 stream interaction regions and coronal mass ejections detected per month
(e.g., Kilpua et al., 2017; Richardson, 2018).
The dominant wave modes that can cause pitch angle scattering of ra-
diation belt electrons and their subsequent precipitation vary considerably
according to magnetic local time. For example, electromagnetic ion cyclotron
(EMIC) waves are typically observed on the dusk side of the magnetosphere
and they primarily scatter relativistic electrons (Meredith et al., 2003; Sum-
mers and Thorne, 2003; Usanova et al., 2014). Very Low Frequency (VLF)
chorus waves in turn occur predominantly on the dawnside. They can scat-
ter low energy electrons (Lam et al., 2010) or, when having large amplitudes,
lead to so-called microburst precipitation of relativistic electrons (Thorne
et al., 2005; Douma et al., 2017). Distribution of hiss waves may also be
highly asymmetric depending on the shape of the plasmasphere where they
occur. Hiss can scatter electrons of wide range of energies, although at high-
est energies the process is slow (Bortnik et al., 2008). It is thus important
that PATE will capture electrons precipitating from different regions of the
magnetosphere, i.e., originating from wave-particle interactions with different
wave modes. This requires that the orbit will drift in MLT.
The nature of wave-particle interactions and the orbital speed of the
spacecraft puts constraints also on the time resolution of the measurements.
The electrons can be scattering locally very fast, even in time-scales of micro-
seconds in a case on large-amplitude chorus waves through non-linear inter-
actions. Most interactions occur however from about seconds to minutes
time-scales upward. The requirement for PATE to resolve most of the loss
processes is that at least three bins in pitch-angle should be measured every
15 seconds in each electron energy channel of the instrument.
The second key science objective of PATE relates to measuring solar
4
ENA flux. This is a crucial measurement to understand better acceleration
of SEPs and the energy budget of solar eruptions. The efficiency at which
SEPs are accelerated by shock waves of coronal mass ejections (CMEs) de-
pends critically on the presence of suprathermal ions in the corona (Desai
and Giacalone, 2016); the large densities of surpathermal ions leads to more
intense waves in the corona which increases the SEP acceleration rates by
the CME-driven shock waves due to strengthening of particle scattering (e.g.,
Vainio and Laitinen, 2007; Afanasiev et al., 2015). These ions are however
trapped by the waves and cannot thus be observed by remote sensing ob-
servations, and thus their characteristics and consequences remain poorly
understood. The measurements of ENA created by charge-exchange pro-
cesses between suprathermal ions and neutral atoms in the corona, however,
offer a viable way to study the suprathermal population. So far, only one
such event has been measured during an extreme solar eruption (Mewaldt
et al., 2009). The requirement for making successful ENA measurements is
to observe during the solar cycle phase when significant CMEs occur, i.e.,
outside solar minimum. A telescope in LEO will use the geomagnetic field
as a filter of the neutral solar particle emission and needs to be pointed at
the Sun. Certainly, this method will be usable only at energies exceeding the
typical magnetospheric ring-current proton energies (above 300 keV).
2.2. Mission
FORESAIL-1 is the first in FORESAIL mission series developed by the
Finnish Centre of Excellence for Sustainable Space. In addition to the PATE
payload, it carries a plasma brake for lowering of the orbit (Iakubivskyi
et al., 2019), which is crucial to achieve the requirements of acquiring particle
measurements with a drifting MLT. The initial PATE demonstration will
take place in the original orbit for about four months with the satellite’s spin
plane aligned with the meridional plane to achieve scanning of the different
pitch angles by spacecraft spin. Likely, due to the popularity of such orbit
for piggyback CubeSat launches, it will be a Sun-synchronous orbit with the
altitude larger than 600 km. After the initial PATE demonstration, a 300-
m plasma-brake tether will be deployed and used for lowering the altitude
by 100 km which will cause the MLT to drift. Lowering the orbit with
the plasma brake will last for about six months with additional two months
required for satellite spin up and centrifugal tether deployment, as well as
reeling in the tether and spinning down the satellite to prepare for PATE
observations. During PATE nominal observations, the spin axis will point
5
towards the Sun which is not an inertially-fixed direction and, therefore,
cannot be maintained with a deployed tether. The nominal phase will last
for about four years.
2.3. Satellite
PATE and plasma brake payloads are integrated within a three-unit
CubeSat. The satellite bus supplies the PATE with 2.5 W of nearly-
continuous power. The telescope’s duty cycle depends on areas of interest –
for example, covering the latitudes of the outer belt region is of utmost impor-
tance for the electron measurements, while operating in the low-background
low-latitude region outside of the South Atlantic Anomaly (SAA) will be im-
portant when monitoring the solar ENA flux. The telescope will not operate
during the telemetry downlink mode which has a duty cycle of 7% assuming
one ground station in Otaniemi, Finland. The PATE produces 1300 bit·s1
and the compression rate for simulated data is better than six. Resulting data
volume can be downlinked via the Ultra High Frequency (UHF) band which
provides >2 Mbytes per day. The plasma brake payload is 0.5 units in
size and, generally, requires less power and downlink than PATE, with the
exception that the tether deployment motor has a peak-power consumption
of 7 W. In order to deploy the tether, FORESAIL-1 is required to spin up
to 130 deg s1, which will provide enough angular momentum to deploy 11
m of the tether. The remaining angular momentum will be provided by the
plasma brake itself (Iakubivskyi et al., 2019).
3. Particle Telescope (PATE)
3.1. Instrument requirements and adopted overall design
FORESAIL-1/PATE (Fig. 1) consists of two telescopes with identical
stacks of silicon detectors below passive collimator structures. Telescope
1 (T1) is mounted perpendicular to the spin axis of the spacecraft, while
Telescope 2 (T2) is aligned with the spin axis. The collimators of T1 and T2
have different lengths. T1, aligned with the spacecraft Z axis, is longer than
T2 and provides a narrower field of view to allow the angular distribution to
be measured with 11.25 degree resolution. The instrument has a total mass
of 1.2 kg and an outer envelope of 94×94×140 mm3.
Each telescope consists of a stack (Fig. 2) of three silicon detectors – D1
(20 μm), D2 (350 μm) and D3 (350 μm) – capable of stopping particles in
6
Figure 1: A simplified mechanical model of PATE used for generating the geometry de-
scription file for the response simulations. Telescope T1 is the one viewed from the side
with the longer collimator and Telescope T2 is the shorter one viewed head-on.
7
Figure 2: Upper left: schematic PATE detector stack cross section. Lower left: The
mechanical assembly of the detector stack. Right: exploded detector stack of PATE.
the nominal energy range of the instrument (80–800 keV for electrons, 0.3–
10 MeV for hydrogen). In addition to the D detectors, the stack consists of
two anti-coincidence (AC) detectors. AC1 is an annular detector with a hole
in the centre limiting the aperture of the instrument rejecting background
from particles penetrating the passive material defining the nominal aperture
(collimator and mechanical structures hosting the stack). AC2 is a circular
detector placed at the bottom of the stack and signaling those particles that
penetrate the whole stack. The detector stack is covered from above with a
double Ni foil (2×0.5 μm) to prevent low-energy charged particles and soft
(.500 eV) photons from entering the detectors. Outer foil could be damaged
by micrometeoroid dust particles in orbit. We chose the double foil design
so that in case a tiny hole in the outer foil opens, the instrument continues
to operate normally.
8
Table 1: Properties of detector elements from top to bottom in the PATE stack. (ddenotes
nominal detector thickness)
ID d[μm] description
AC1 300 annular: central hole diameter 14.0 mm,
active area inner diameter 16.0 mm,
active area outer diameter 33.8 mm
D1 20 three segments: central area diameter 5.2 mm (C= 120 pF),
outer area diameter 16.4 mm (two equal segments, C= 480 pF)
D2 350 two segments: central area diameter 5.2 mm (C= 8.7pF),
outer area diameter 16.4 mm (C= 62 pF)
D3 350 circular: diameter 16.4 mm (C= 75 pF)
AC2 350 circular: diameter 33.8 mm (C= 280 pF)
3.2. PATE subsystems
The properties of the detectors are listed in Table 1. Detector segmenta-
tion is depicted in Fig. 3. Detectors are adhesively attached to their Printed
Circuit Boards (PCBs) hosting the resistors and capacitors needed for sup-
plying voltages for guard rings and for filtering. Detectors are bonded with a
short 25 μm Al wire. The detector PCBs are separated with approximately
1 mm thick spacers forming totally a pile of about 1 cm.
Thin coaxial cables are used to provide the voltages for the detectors and
bringing the detector signals to the preamplifiers locating in the preamplifier
board. Signals are then fed to the signal processing board where they are
first digitized at 14-bit accuracy and finally fed to the field programmable
gate array (FPGA) for final calculation and separation of the particle and
its energy range. A power supply board is generating the required voltages
from the battery voltage apart from the adjustable detector bias voltages,
which are generated on a separate board.
3.3. Signal path and particle classification algorithms
The scientific data path, implemented in the FPGA, consists of three
major components. These are a trapezoid filter, pulse detector and particle
classifier. Trapezoid filter and pulse detector have a main clock of 10 MHz
and the particle classifier has a main clock of 40 MHz.
The digitized 14-bit wide data of each detector signal is filtered with the
trapezoid filter. This filter is symmetric and has a rise/fall time of 4 clock
cycles. The flattop duration of the filter is 3 clock cycles. The output of the
9
Figure 3: Segmentation of the PATE detectors.
filter is saturated to 14 bits in order to preserve uniform data width inside
the data path.
The output of the trapezoid filter is connected to the pulse detector, which
compares it to the set threshold value. If the output of the trapezoid filter
exceeds the threshold, a hit is detected. The height of the detected pulse is
monitored and the peak value of this pulse is sent to the particle classifier.
The peak value is obtained so, that as long as the output of the trapezoid
filter gets larger the output value is stored, but when the output remains the
same or gets smaller repeatedly for 3 times in a row, the logic ends the peak
search and forwards the last stored value forward.
The particle classifier uses the hit information and signal pulse heights
from all of the detector plates. The particle classifier converts the detected
pulse height values to energy scale and uses these energy-loss proxies to per-
form a classification process. The particle classifier consists of five different
sub-classifiers (denoted here as PC1 – PC5), and the correct sub-classifier is
selected based on the hit combination. The different combinations can be
seen from Fig. 4.
PC1 classifies particles that stop in the D1 detector. These particles are
classified into different proton bins based on the detected energies. PC2
classifies particles that only cause pulse to the D2 detector. These particles
10
Figure 4: Particle classifier valid hit combinations
are classified into different electron bins based on the detected energies. PC3
classifies particles that cause pulse in detectors D1 and D2. In this case
the detected energies of both detector plates are summed together and the
classification is done with this combined energy. PC3 can classify the particle
to be either electron or proton. PC4 classifies particles that cause pulse into
detector plates D2 and D3. PC4 has similar operation as PC3 but it can only
classify the particle to be an electron. PC5 classifies particles that penetrate
the whole detector stack. PC5 classifies the particle as proton or electron
based on the measured energies.
The instrument generates seven (ten) electron (proton) energy channels in
the nominal energy range through particle classifiers PC1 through PC4 and
three penetrating particle channels through PC5. Two proton penetrating
particle channels are integral channels with thresholds of 10 and 15 MeV
and the penetrating electron channel has response to >800 keV electrons
and >100 MeV protons. The measured energy limits of the channels are
log-spaced. The basic time resolution of the measurement is equal to the
rotation period of the satellite, nominally 15 seconds, which agrees with the
time resolution of the short telescope T2 pointed along the rotation axis.
The long telescope T1 scanning the sky will deliver the counts in 32 angular
sectors per each rotation thus giving the full pitch angle distribution every
spin period.
11
4. Simulation Models
4.1. Geant4 model
A realistic model of the PATE instrument (see Fig. 1) was created by
converting a 3-D mechanical model from mechanical engineering software
to a Geometry Description Markup Language (GDML) (Chytracek et al.,
2006) model which was used for simulations within the GEANT4 framework
(Agostinelli et al., 2003; Allison et al., 2006; Allison et al., 2016). We have
simplified the initial mechanical model by removing all thread structures in
order to make simulations more time effective. The electronic components
were also removed from the model for the same reason. The difference in
stopping power of the instrument structures due to the removed mass was
counted as negligible. The conversion done by the mechanical engineering
software coarsens the model, limiting it to tesselated surfaces. We have
replaced sensitive volumes of the instrument as well as passive areas of the
detectors with native volumes (cylinders, prisms) available in the GDML.
This way we remove uncertainties which could arise from approximation of
round shapes by tessellated ones.
The model was placed inside a Geant4 world of a cubic shape with di-
mensions of 30×30×30 cm3. A particle source was constructed according
to Greenwood (2002) as a sphere with a radius of 10 cm. Particle initial
positions were chosen randomly by a spherically uniform distribution. The
momentum of a particle has a direction calculated using the uniform Lam-
bertian angular distribution.
We used the following procedure to produce a vector for the particle
momentum. Let x∼ U(0,1) and y∼ U(0,1), and φ= 2πx and θ=
cos1(1 2y). Then, ~a = (r, θ, φ)is the initial particle position A(in spher-
ical coordinates), where ris a radius of the sphere, θand φare standard
spherical angles. Thus, ~p = (r, θ +π/2, φ)is a vector perpendicular to ~a
and has a direction which is tangential to the sphere. Then ~
a0=~a is a
vector pointing to the center of the sphere from the initial particle position
A. In the local spherical coordinate system at the point Athe vector ~
a0is
a local Z-axis, and ~p is a perpendicular axis. In order to obtain a correct
direction for the particle we determine its local spherical angles as follows.
We let u∼ U(0,1) and v∼ U(0,1) and define the local spherical angles as
φ0= 2πu and θ0= cos1(v). Since the distribution of the local φ0must be
uniform, it has rotational symmetry around the local Z-axis set by ~
a0~p.
Without loss of generality we can initialize the particle momentum vector as
12
~
m0=~
a0cos θ0+~p sin θ0. Then we rotate ~
m0around the ~
a0by φ0which yields
an inward pointing vector with the Lambertian angular distribution. The
vector rotation operation is generally complex, but this way we minimize
own code complexity by using functions of the Geant4 toolbox.
4.2. Signal path simulation model
To assess the performance of the signal processing, we also modeled the
different parts of the signal path. The trapezoid filter and pulse detector were
modeled and simulated using MATLAB and Simulink for signals generated
for generic detector capacitances of 100 and 500 pF. Later the whole data
path, including the particle classifier, was simulated using ModelSim. The
results will be reported in more detail in a later paper but they confirm that
the signals generated by the PATE detectors can be adequately analyzed by
the circuitry and also provide evidence that the particle classifiers work as
expected.
5. Results
5.1. Simulated energy response
PATE sensitivity for electrons starts at an energy slightly less than 80
keV. First six channels in the nominal energy range are nicely differential,
while the last channel (E7) in the nominal range appears to show a bit
more integral characteristics (see Figure 5). The spectrum is, however, well
resolved. A further integral channel of penetrating electrons will extend the
spectrum beyond the nominal range.
PATE effectively detects protons starting from an energy of 300 keV.
All ten proton channels are differential ones, mostly with boxcar-like energy
response curves but with high-energy side bands from energies >30 MeV,
where the passive collimator becomes transparent.
5.2. Simulated angular response
The angular sensitivity of the T2 telescope for electrons and protons is
depicted in Figs. 7 and 8, respectively. The instrument field of view is well
limited for particles inside the nominal energy range, but beyond that the
angular response becomes wider as the collimating structures become trans-
parent to the particles. The simulated response is very close to nominal
analytical approximation obtained from an effective detector area visible at
an angle from the axis, which allows one to estimate the 30% response level
13
Figure 5: The response curves of the PATE electron channels. Black solid lines indicate
the level, vivid color bars around it show a 3-σstatistical estimate of confidence intervals.
Figure 6: The response curves of the PATE proton channels. Black solid lines indicate the
level, vivid color bars around it show a 3-σstatistical estimate of confidence intervals.
14
Figure 7: Angular sensitivity of PATE short tube to electrons in electron channels E1 –
E7. A subplot illustrates the angular sensitivity profile in arbitrary units. A bright line
above the color map shows limits of angular sensitivity at a level of 0.3 from the maximum
for each energy.
for the longer telescope as 5.6 degrees in contrast to the value of 9.7 de-
grees obtained analytically for the short telescope. Thus, determining the
pitch angle distribution at 10 degree resolution, consistent with the sectored
intensities of telescope T1, is feasible.
5.3. Simulated data along the orbit
We have simulated orbital electron and proton intensities using the AP-8
and AE-8 (at 97.725% confidence level) trapped particle models implemented
in SPENVIS (Heynderickx et al., 2004) system on a 600 km sun-synchronous
polar orbit. The values in the models are tabulated as angle integrated
integral intensities J(E > Ej)in the units cm2s1. To get to differential
intensities I(E)in cm2sr1s1MeV1, we use
I(E
j) = 1
∆Ω
J(E >Ej)J(E > Ej+1)
Ej+1 Ej
,(1)
where E
j=pEjEj+1 is the channel logarithmic mid-point energy, ∆Ω =
4πcos αLC and αLC = 53.5is the adopted value for the local loss-cone bound-
ary pitch-angle at 600 km. The intensities are then folded with the response
15
Figure 8: Angular sensitivity of PATE short tube to protons in proton channels P1 –
P10. A subplot illustrates the angular sensitivity profile in arbitrary units. A bright line
above the color map shows limits of angular sensitivity at a level of 0.3 from the maximum
for each energy. High energy (>50 MeV) protons penetrate the detector assembly so
that they are registered from all directions. At energies above 30 MeV PATE starts to
be sensitive to protons incident outside the aperture, which pass through the instrument
casing and hit the D1 detector, only. At about 50 MeV the thickest part of the casing
and the collimator becomes transparent so that the D1 acts as a bare plate detector in
a certain solid angle range. Only those side penetrating particles which hit AC detectors
are discarded.
16
Figure 9: Simulated orbital count rates in lower proton channels with the highest count
rates among P1 – P10. The highest count rates occur during passes in the South Atlantic
Anomaly.
functions described above assuming a piece-wise power-law form between the
energy channels. This gives estimates for the counting rates in orbit due to
trapped radiation, when the telescope is sampling the trapped populations.
The data presented in figures 9 and 10 are counts from the short telescope T2
in those differential electron and proton channels that have the highest count-
ing rates. The channels exhibit numbers of counts enough to produce very
high-quality spectra for the trapped particles. The corresponding counting
rates are also in the range of well-resolved pulse counting.
For assessing the maximum number of particles observed by telescope
T1 in each solid-angle–energy bin we must further divide these numbers by
32 (the number of sectors), and 2.9 (the ratio of geometric factors of the
telescopes). That brings us up to values of a few hundred to a thousand per
bin for electrons, which are still adequate for producing accurate pitch-angle
distributions from the measurement by function-fitting techniques. During
quiet times between storms, time integration has to be imposed and sector
counts obtained during several revolutions must be averaged into the time-
averaged angular distributions. These distributions would be then available
as functions of energy, L and MLT.
The geometric factor of PATE is tuned so that it can reliably, without
saturation, measure the trapped electrons throughout its orbit. This means
that quiet time fluxes deep inside the loss cone will not produce meaningful
statistics at the 15 second time resolution of the instrument. However,
17
Figure 10: Simulated orbital count rates in lower electron channels with the highest count
rates among E1 – E7.
through its very good angular resolution, PATE can deliver the shape of the
pitch angle distribution at the edge of the loss cone and, thus, give us a
good measure of both the pitch-angle diffusion coefficient and the flux of the
precipitating particles. In addition, through time integration, the instrument
can produce also the angular distribution deeper inside the loss cone.
5.4. Solar ENA sensitivity
The solar ENA event observed in December 2006 (Mewaldt et al., 2009)
had a fluence spectrum that can be fitted (at energies above 1.5 MeV) with
the power law
dN
dE dA = 50 E
MeV2.46
MeV1cm2,(2)
where dN is the number of counts per area element, dA, and energy interval,
dE. Extrapolating that spectrum to the PATE low energy threshold of 300
keV, integrating over energy from the threshold to infinity and using the
effective area of detection of 1.5 cm2(the area of the central hole of the AC1
detector) gives about 300 ENAs per event. The duration of the December
2006 event was longer than one orbit of FORESAIL-1, but if PATE catches
the time period around the peak of a similar event as in 2006, the number
of counts per event could exceed a hundred. That would already provide a
signal well above background, as long as the magnetospheric activity is low
and the local ENA production or low-energy ions will not generate too many
18
counts. Note also that the Sun is a point source, and the well-collimated
aperture of PATE would not collect a large background from more isotropic
magnetospheric sources.
5.5. Contamination estimates
The thin D1 detector allows efficient separation of particle species. How-
ever, there are ways for cross-contamination of proton channels by electrons
and electron channels by protons.
The first type of contamination occurs in P1 in a narrow energy band
where a probability for electron to deposit enough energy in D1 to be above
the threshold is moderately high (Fig. 11). Given that PATE has separate
channels for these electrons, which in turn are not contaminated by protons
registered in the P1 channel, this contamination can be taken into account in
the analysis of measurements. For the ENA measurement, which has a low
signal level, data from regions with high electron fluxes must be excluded.
We estimate that the region between ±50in geomagnetic latitude, outside
the longitudes of SAA, is suitable for observing the solar hydrogen flux. That
gives us about 15–40% duty cycle for the ENA observation over the orbit,
depending on the orientation of the orbital plane with respect to the magnetic
axis and the solar direction.
The second type of contamination occurs when protons of energies around
1 MeV pass through passive areas of the D1 detector, thus yielding no sig-
nal there (Fig. 12). They are indistinguishable from electrons by logic of the
classifier, which puts them into lower electron channels. This type of contam-
ination can be mitigated in the same way as the first type, since the PATE
instrument has an independent dedicated channel for protons of relevant en-
ergies. Protons of energies higher than 30 MeV also contaminate electron
channels, but outside the SAA their fluxes are negligible. Thus, there are
two possibilities for cross-contamination of particle channels, which both can
be properly assessed during the data reduction phase.
Finally, energetic photons can also contaminate the measurement of the
sun-pointing telescope. The photon absorption yield at energies high enough
to trigger the P1 channel in the thin D1 (deposited energy exceeding 110
keV, thickness 20 μm) is extremely low. Soft X-rays (between about 0.5 and
5 keV), however, will pass the Ni foil system and be absorbed in the D1
detector and will lead to an increase of leakage current in the detector by
some tens of nA cm2during the most intense flares, which is not considered
to be a problem. A conservative estimate for the increase of shot noise in
19
Figure 11: Modeled geometric factors of contamination of proton channels P1 – P10 by
electrons. Electrons of energies 120 keV contaminate the P1 channel. Black solid lines
indicate the level, vivid color bars around it show a 3-σstatistical estimate of confidence
intervals. At higher energies color bars present upper limits for contamination. Note this
plot has different scale than Fig. 6 and 5.
20
Figure 12: Modeled geometric factors of contamination of electron channels E1 – E7 by
protons. The narrow peak at about 1 MeV is caused by protons penetrating passive areas
of the D1 silicon plate. These protons leave no measurable signal in the D1 detector and
are counted as electrons. Protons with energies above 100 MeV, which occur in South
Atlantic Anomaly, would contaminate electron channels quite noticeably, but the mission
objectives lie outside this region. Color scheme and scale are the same as in Fig. 11.
21
the D1 channel is about 10 keV, which is also tolerable. However, triggering
the E1 channel in D2 (deposited energy exceeding 50 keV, thickness 350
μm) by hard X-rays is plausible. Accounting for photoabsorption, only, the
absorption coefficient in Si at 50 keV is about 0.45 cm1, so the probability
of being detected reaches almost 2% for such photons. A photon at about
100 keV has the mass-energy-absorption coefficient levels to about 0.07 cm1,
making the probability of depositing a detectable amount of energy in D2 to
be very small for photons beyond 100 keV energies. Large flares will, thus,
probably generate detectable fluxes in E1 and (possibly even in E2) channel,
as is commonly observed in similar instruments, but not cause any problems
for the detector.
5.6. Calibration plan
The ground calibration of the instrument will be performed using radioac-
tive sources and accelerator facilities. The energy responses of individual
detectors along with the signal path can be tested and calibrated with al-
pha, beta and gamma sources. The whole flight model will be further tested
in energetic electron beam in Turku University hospital and in the RADEF
cyclotron facility of the University of Jyväskylä, Finland. We will calibrate
the on axis response of PATE with a dense array of beam energies and the
angular response with some selected energies. Finally, after launch we will
perform in-flight calibration campaigns obtaining full pulse height data from
the orbital environment including the outer belt crossings and SAA. These
data will then be used to tune and validate the Geant4 model presented in
this paper. The validated model will then be used for producing the final
response functions of PATE.
6. Summary and Conclusions
We have presented the simulated response of FORESAIL-1 / PATE to
energetic electrons and protons and shown that its energy and angular re-
sponse fulfill the requirements set by its target to observe the precipitating
electron spectrum and energetic protons in very well defined energy channels
in the nominal range of particle energies. Proton contamination in electron
channels was shown to be negligible for the measurement of electrons, and
electron contamination to proton channels can be handled well through data
cleaning. The sensitivity of the instrument to electrons seems tuned to the
22
expected fluxes with a reasonable safety margin. Finally, the target of ob-
serving solar ENAs with PATE looks feasible, provided that large events like
the December 2006 flare can be detected in action, while PATE is in the
low-latitude region outside the South Atlantic anomaly with low background
from magnetospheric particle sources.
Acknowledgements. This work was performed in the framework of the Finnish
Centre of Excellence in Research of Sustainable Space (FORESAIL) funded
by the Academy of Finland (grant numbers 312351, 312390, 312358, 312357,
and 312356). We gratefully also acknowledge the Academy of Finland grants
309937 and 309939. Computations necessary for the presented modeling were
conducted on the Pleione cluster at the University of Turku.
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This paper is republished in Acta Astronautica as "Coulomb drag propulsion experiments of ESTCube-2 and FORESAIL-1": https://www.researchgate.net/publication/337626758_Coulomb_drag_propulsion_experiments_of_ESTCube-2_and_FORESAIL-1 // Here we present the preliminary mission design for the ESTCube-2 three-unit CubeSat. Its main mission is to test Coulomb drag propulsion. Coulomb drag can be used in Low-Earth Orbit by deploying and charging a tether that is used to brake the orbital velocity of the satellite and reduce its orbital altitude. To test this concept, ESTCube-2 will deploy and charge a 300 m tether. Such a tether could deorbit ESTCube-2 from the altitude of 700 km to 500 km in half a year. Other payloads that are being considered for the ESTCube-2 satellite are an Earth observation camera, a C-band communications system and an experimental laser communication system. ESTCube-2 in-orbit demonstration platform will also be designed for other electric solar wind sail experiments outside of the influence of Earth’s magnetic field. The satellite bus will be integrated into one system that could also be reused for different types of missions. The integrated system is developed to maximise the space for payloads on a nanosatellite. This paper presents the payloads and system design of ESTCube-2.
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