Evidence of widespread hot plasma in a non-flaring coronal active region from Hinode/XRT

Abstract

Nanoflares, short and intense heat pulses within spatially unresolved
magnetic strands, are now considered a leading candidate to solve the coronal
heating problem. However, the frequent occurrence of nanoflares requires that
flare-hot plasma be present in the corona at all times. Its detection has
proved elusive until now, in part because the intensities are predicted to be
very faint. Here we report on the analysis of an active region observed with
five filters by Hinode/XRT in November 2006. We have used the filter ratio
method to derive maps of temperature and emission measure both in soft and hard
ratios. These maps are approximate in that the plasma is assumed to be
isothermal along each line-of-sight. Nonetheless, the hardest available ratio
reveals the clear presence of plasma around 10 MK. To obtain more detailed
information about the plasma properties, we have performed Monte Carlo
simulations assuming a variety of non-isothermal emission measure distributions
along the lines-of-sight. We find that the observed filter ratios imply
bi-modal distributions consisting of a strong cool (log T ~ 6.3-6.5) component
and a weaker (few percent) and hotter (6.6 < log T < 7.2) component. The data
are consistent with bi-modal distributions along all lines of sight, i.e.,
throughout the active region. We also find that the isothermal temperature
inferred from a filter ratio depends sensitively on the precise temperature of
the cool component. A slight shift of this component can cause the hot
component to be obscured in a hard ratio measurement. Consequently, temperature
maps made in hard and soft ratios tend to be anti-correlated. We conclude that
this observation supports the presence of widespread nanoflaring activity in
the active region.

Full text

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arXiv:0904.0878v1 [astro-ph.SR] 6 Apr 2009
Evidence of widespread hot plasma in a non-flaring coronal active region
from Hinode/XRT
Fabio Reale1
Dipartimento di Scienze Fisiche & Astronomiche, Universit` a di Palermo, Sezione di Astronomia,
Piazza del Parlamento 1, 90134 Palermo, Italy
Paola Testa
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
James A. Klimchuk
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
Susanna Parenti
Royal Observatory of Belgium, 3 Circular Avenue, B-1180 Brussels, Belgium
ABSTRACT
Nanoflares, short and intense heat pulses within spatially unresolved magnetic
strands, are now considered a leading candidate to solve the coronal heating prob-
lem. However, the frequent occurrence of nanoflares requires that flare-hot plasma be
present in the corona at all times. Its detection has proved elusive until now, in part
because the intensities are predicted to be very faint. Here we report on the analysis
of an active region observed with five filters by Hinode/XRT in November 2006. We
have used the filter ratio method to derive maps of temperature and emission mea-
sure both in soft and hard ratios. These maps are approximate in that the plasma is
assumed to be isothermal along each line-of-sight. Nonetheless, the hardest available
ratio reveals the clear presence of plasma around 10 MK. To obtain more detailed in-
formation about the plasma properties, we have performed Monte Carlo simulations
assuming a variety of non-isothermal emission measure distributions along the lines-
of-sight. We find that the observed filter ratios imply bi-modal distributions consisting
of a strong cool (logT ∼ 6.3−6.5) component and a weaker (few percent) and hotter
(6.6 < logT < 7.2) component. The data are consistent with bi-modal distribu-
tions along all lines of sight, i.e., throughout the active region. We also find that the
1INAF - Osservatorio Astronomico di Palermo “G.S. Vaiana”, Piazza del Parlamento 1, 90134 Palermo, Italy

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isothermal temperature inferred from a filter ratio depends sensitively on the precise
temperature of the cool component. A slight shift of this component can cause the
hot component to be obscured in a hard ratio measurement. Consequently, tempera-
ture maps made in hard and soft ratios tend to be anti-correlated. We conclude that
this observation supports the presence of widespread nanoflaring activity in the active
region.
Subject headings: Sun: corona - Sun: X-rays
1.Introduction
The solar corona, once thought to be heated in a quasi-steady fashion, has proven to be much
more difficult to explain than early observations suggested. Developements over the last few
years point to a different picture in which coronal loops and perhaps also the diffuse corona are
heated impulsively by nanoflares occurring within unresolved strands (Parker 1988; Cargill 1994;
Cargill & Klimchuk 1997; Klimchuk 2006; Warren et al. 2003; Parenti et al. 2006). Nanoflares
can explain a number of observations that are clearly inconsistent with static models, such as the
density excess and superhydrostatic scale height of ∼ 1 MK loops. Furthermore, the proposed
mechanisms for coronal heating predict that the heating should be impulsive when viewed prop-
erly from the perspective of individual magnetic flux strands (Klimchuk 2006). Despite these
successes, the existence of nanoflares is still under debate. No “smoking gun” has yet been found.
The strongest evidence in support of nanoflares would be the detection of very hot plasma
(Cargill 1995). Hydrodynamic simulations show that impulsively heated strands should reach
temperatures in excess of 5 MK. Much higher temperatures are possible depending on the energy
of the nanoflare. Until now, there has been littleevidence of such very hot plasma outsideof proper
flares. There are likely two reasons for this. First, few studies have explicitly looked for such ex-
treme temperatures during quiescent (nonflaring) conditions. Second, and more importantly, the
intensity of the hot emission is expected to be very weak. The reason is as follows. Tempera-
ture rises abruptly when a nanoflare occurs, but density is much slower to respond. By the time
chromospheric evaporation has raised the density significantly, the coronal plasma has cooled to
temperatures far below the peak value. Low densities combined with rapid initial cooling means
that the hot phase contributes relatively little to the total emission measure of a heating and cooling
event. Hydrodynamic simulations predict that the emission measure of the hottest plasma is 2-3
orders less than that of the dominant coronal plasma (Klimchuk et al. 2008). The emitted radiation
is correspondingly faint. Ionization nonequilibrium effects may diminish the hot component even
more (Reale & Orlando 2008; Bradshaw & Cargill 2006).

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Recent observations suggest that very hot plasma may indeed be present at low levels in
nonflaring active regions (Zhitnik et al. 2006; Urnov et al. 2007; Patsourakos & Klimchuk 2008;
McTiernan 2008; Schmelz et al. 2009). We have therefore undertaken a focused investigation to
determineas precisely as possibletheamountof very hotplasmain oneparticularactiveregionand
to evaluate whether it is consistent with nanoflare heating. We use wide-band multi-filter imaging
observations(Reale et al. 2007) from theX-Ray Telescope (XRT, Golub et al. (2007)) on board the
Hinode mission (Kosugi et al. (2007)). These observations have the advantage of a strong signal
compare to spectrometer observations of individual spectral lines. The disadvantage is that each
band is sensitive to a range of plasma temperatures. To distinguish hot from cool plasmas, we use
different filter combinations as described below.
The major problem is to screen out the dominant cooler emitting components which may
inhibit the detection of the minor hot component. Hinode/XRT is equipped with several wide-
band filters, which might fulfill this task. The filters with a band ranging down to relatively low
energies do not allow the detection of the hot plasma, because it is overwhelmed by the much
larger quantities of cooler plasma. At the other extreme, flare filters hardly detect flare-hot plasma
with very low emission measure. There is a third opportunity offered by the wide choice of filters:
filters of intermediate thickness which may be the right compromise between sensitivity level and
energy range.
2.The observation and preliminary analysis
Through its nine filters in the soft X-ray band the XRT is sensitive to the emission of plasma
in the temperature range 6.1 < log T < 7.5. The CCD camera has a 1 arcsec pixel and
the Full-Width-at-Half-Maximum (FWHM) of the Point Spread Function (PSF) is ≈ 0.8 arcsec
(Golub et al. 2007). We consider the same data as those already illustrated in Reale et al. (2007).
The 512×512 arcsec2field of view includes active region AR10923 observed close to the Sun cen-
ter on 12 November 2006. The filters used were Al poly (F1), C poly (F2), Be thin (F3), Be med
(F4) and Al med (F5), with exposure times of 0.26 s, 0.36 s, 1.44 s, 8.19 s and 16.38 s, respec-
tively. The selected dataset covers one hour, starting at 13:00 UT, and the time interval between
one exposure and the next in the same filter is about five minutes (12 images in each filter). This
active region was monitored by the XRT all along its passage from the east to the west limb. It
flared several times during this period. However, during the selected hour no major flare and no
significant rearrangement of the region morphology occurred (the average of the RMS intensity
deviation of the individual pixels is ≈ 12%), so that we can average over the whole hour. We used
the up-to-date standard XRT software to preprocess the data, including corrections for the read-
out signal, flat-field, CCD bias. The observed images were co-aligned with a cross-correlation

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6.06.5 7.0 7.58.08.5
log T
1
10
100
Response p.u.EM x Exp.Time
Al_poly
C_poly
Be_thin
Be_med
Al_med
Fig. 1.— Temperature response function of the 5 filters used in the XRT observation. To compen-
sate for the different exposure times, the responses are multiplied by the respective total exposure
times.
technique.
Each XRT filter has a different response to plasma temperature, described by a function
GFj(T), where j indicates the filter order number. Thinner filters F1 and F2 are more sensitive to
the emission of cooler plasma than thicker filters F4 and F5. Images of the same region in different
filters provide information about the temperature of the emitting plasma. If the emitting plasma
is isothermal and optically thin along the line of sight, the ratio between signals in thick and thin
filters is a function of the temperature only. Reale et al. (2007) devised a special filter ratio from a
combination of all available filters (Combined Improved Filter Ratio, CIFR), which optimizes for
the signal-to-noise ratio and reveals a very fine thermal structure of the active region.
The analysis presented here considers also several standard ratios of singlefilters, and we take
an up-to-date filter temperature calibration with a special fine tuning to make the analysis entirely
self-consistent (see Appendix). Fig. 1 shows the temperature response functions of the filters. Five
available filters allow to define 10 different ratios of single filters. We will exclude the ratio F2/F1
from our analysis because the dependence on temperature is double-valued over a large range. We
are then left with 9 relevant ratios. The 6 ratios with F2 and F1 in the denominator, and also CIFR,
are able to diagnose plasmas down to logT ≈ 6, i.e. relatively cool plasma; the two ratios with
F3 in the denominator down to logT ≈ 6.2; and the ratio F4/F5 down to logT ≈ 6.3 − 6.4. An
important point is that the coolest plasma detectable by the last ratio is about twice as hot as the

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coolest plasma detectable by the first two ratios. The F4/F5 ratio is a monotonic function of logT
in the range 6.4 ≤ logT ≤ 7.5. We have set minimum acceptance thresholds to compute the
ratios. All pixels with signal below 200 DN/s for F1, 100 DN/s for F2, 30 DN/s for F3, 2 DN/s for
F4 and 10 DN/s for CIFR have been masked out.
3.Analysis and results
3.1.Data analysis
Fig. 2 shows temperature and emission measure maps obtained with CIFR (left column) and
with the hard (F4/F5) filter ratio. To improvethe signal-to-noiseratio, the F4/F5 maps are obtained
after binning over boxes of 4×4 pixels. We have estimated the uncertainties on temperature and
emission measure in each pixel from the photon counts derived from the DN in each filter, with a
conversion factor obtained assuming an average spectrum at central temperature about logT = 6.7
(the conversion factor does not change much in the range 6.5 ≤ logT ≤ 7). Error propagation to
temperature and emission measure has been applied according to Klimchuk & Gary (1995). The
uncertainties are globally consistent with the average root mean square fluctuations of the tem-
perature and emission measure values of the pixels surrounding a given pixel (Reale & Ciaravella
2006). The average errors in temperature and emission measure obtained from CIFR in each pixel
are around 0.004 (∼ 1%) and 0.015 in the log, respectively, in regions of high signal (e.g. region
SH in Fig.4), and 0.008 and 0.03 in regions of low signal (e.g. region HH in Fig.4). The errors
from F4/F5 in each 4×4 pixel box are around 0.05 and 0.2, respectively, in regions of high signal,
and 0.1 and 0.25 in regions of low signal.
The difference in resolution and detail definition is immediately apparent: the softer ratio
maps show much better-defined structuring. While the soft ratio detects plasma mostly in the
range 6.3 < logT < 6.6, the hard ratio appears to detect hotter components to logT > 7. In
particular we find hot plasma at the boundary of dense structures and in an extensive region on the
center left of the active region, where the emission measure is instead low. We also notice that the
temperature maps do not look similar, while the emission measure maps to some extent do, e.g.
we find high emission measure in the same zones of the active region.
Therefore we find different thermal conditions using different filter ratios. In particular, rela-
tively hot regions in the CIFR and other soft filter ratios appear as relatively cool in the hard filter
ratio. This is not trivially expected and deserves further investigation, that we will show later.
Our interest is also to exploit the multi-band imaging observation to obtain quantitative in-
formation about the thermal distribution of the plasma both along the line of sight and across the
active region.