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1.
We present an analysis of the evolution of the thermal flare plasma during the 14 July 2000, 10 UT, Bastille Day flare event,
using spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and TRACE. The spatial structure of this double-ribbon flare consists of a curved arcade with some 100 post-flare
loops which brighten up in a sequential manner from highly-sheared low-lying to less-sheared higher-lying bipolar loops. We
reconstruct an instrument-combined, average differential emission measure distribution dEM(T)/dT that ranges from T=1 MK to 40 MK and peaks at T
0=10.9 MK. We find that the time profiles of the different instrument fluxes peak sequentially over 7 minutes with decreasing
temperatures from T≈30 MK to 1 MK, indicating the systematic cooling of the flare plasma. From these temperature-dependent relative peak times
t
peak(T) we reconstruct the average plasma cooling function T(t) for loops observed near the flare peak time, and find that their temperature decrease is initially controlled by conductive
cooling during the first 188 s, T(t)∼[1+(t/τcond)]−2/7, and then by radiative cooling during the next 592 s, T(t)∼[1−(t/τrad)]3/5. From the radiative cooling phase we infer an average electron density of n
e=4.2×1011 cm−3, which implies a filling factor near 100% for the brightest observed 23 loops with diameters of ∼1.8 Mm that appear simultaneously
over the flare peak time and are fully resolved with TRACE. We reproduce the time delays and fluxes of the observed time profiles
near the flare peak self-consistently with a forward-fitting method of a fully analytical model. The total integrated thermal
energy of this flare amounts to E
thermal=2.6×1031 erg.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014257826116 相似文献
2.
We present our results of high temporal resolution spectroscopic observation and study in Hα, Ca II, and He I lines for the 2B/M1.9 confined disk flare on September 9, 2001, combining with GOES soft X-ray (SXR) and Yohkoh hard X-ray (HXR) observations. Apparent redshifted and red-asymmetric profiles were observed in the initial phase. The redshift
lasted until the late phase. The derived velocity depends on both the spectral line and the used method. The redshift velocities
computed from the line centers of the observed emission profiles (υ0) are of the order of 10 km s−1 both inside and outside the streak area. However, the velocities determined from the excess profiles by the bisector method
(υ) are larger in the streak (18–50 km s−1). Both υ and the red full widths (RFWs) derived from the excess profiles show temporal variations similar to the HXR light-curve
in the streak area. Moreover, the Hα line wings of nonthermal characteristics, the redshift velocities, and the lifetime of
impulsive broadening suggest that the streak is related to nonthermal electron bombardment. Spectral simulations reveal that
we cannot reproduce the observed profiles in the three lines simultaneously with a set of parameters, indicating that the
flare atmosphere was not homogeneous along the line-of-sight. Most of the observed Hα profiles showed a ‘flat-top’ structure,
implying the flare plasma was optically thick for this line. The electron temperatures (Te) deduced from the line-center intensity of the three lines are similar and estimated to be higher than 7200 K. The obvious
central reversal of the Hα profiles due to absorption of materials in the impulsive phase lasted more than 2 min. However,
the far blue wings of the Ca II profiles in the impulsive phase showed low-intensity emission, which is suggestive of the
existence of large turbulence or macroscopic motion (> 50 km s−1), which is inconsistent with the current flare model. 相似文献
3.
We studied the evolution of a small eruptive flare (GOES class C1) from its onset phase using multi-wavelength observations
that sample the flare atmosphere from the chromosphere to the corona. The main instruments involved were the Coronal Diagnostic
Spectrometer (CDS) aboard SOHO and facilities at the Dunn Solar Tower of the National Solar Observatory/Sacramento Peak. Transition
Region and Coronal Explorer (TRACE) together with Ramaty High-Energy Spectroscopic Imager (RHESSI) also provided images and
spectra for this flare. Hα and TRACE images display two loop systems that outline the pre-reconnection and post-reconnection magnetic field lines and
their topological changes revealing that we are dealing with an eruptive confined flare. RHESSI data do not record any detectable
emission at energies ≥25 keV, and the observed count spectrum can be well fitted with a thermal plus a non-thermal model of
the photon spectrum. A non-thermal electron flux F ≈ 5 × 1010 erg cm−2 s−1 is determined. The reconstructed images show a very compact source whose peak emission moves along the photospheric magnetic
inversion line during the flare. This is probably related to the motion of the reconnection site, hinting at an arcade of
small loops that brightens successively. The analysis of the chromospheric spectra (Ca II K, He I D3 and Hγ, acquired with a four-second temporal cadence) shows the presence of a downward velocity (between 10 and 20 km s−1) in a small region intersected by the spectrograph slit. The region is included in an area that, at the time of the maximum
X-ray emission, shows upward motions at transition region (TR) and coronal levels. For the He I 58.4 and O v 62.97 lines, we determine a velocity of ≈−40 km s−1 while for the Fe XIX 59.22 line a velocity of ≈−80 km s−1 is determined with a two-component fitting. The observations are discussed in the framework of available hydrodynamic simulations
and they are consistent with the scenario outlined by Fisher (1989). No explosive evaporation is expected for a non-thermal
electron beam of the observed characteristics, and no gentle evaporation is allowed without upward chromospheric motion. It
is suggested that the energy of non-thermal electrons can be dissipated to heat the high-density plasma, where possibly the
reconnection occurs. The consequent conductive flux drives the evaporation process in a regime that we can call sub-explosive. 相似文献
4.
We present SOHO/CDS observations taken during the gradual phase of the X17 flare that occurred on October 28, 2003. The CDS
data are supplemented with TRACE and ground-based observations. The spectral observations allow us to determine velocities
from the Doppler shifts measured in the flare loops and in the two ribbon kernels, one hour and a half after the flare peak.
Strong downflows (>70 km s−1) are observed along the loop legs at transition-region temperatures. The velocities are close to those expected for free
fall. Observations and results from a hydrodynamic simulation are consistent with the heating taking place for a short time
near the top of the arcade. Slight upflows are observed in the outer edges of the ribbons (<60 km s−1) in the EUV lines formed at log T < 6.3. These flows could correspond to the so-called “gentle evaporation.” At “flare” temperatures (Fe xix, log T = 6.9), no appreciable flows are observed. The observations are consistent with the general standard reconnection models
for two-ribbons flares. 相似文献
5.
H. S. Hudson T. N. Woods P. C. Chamberlin L. Fletcher G. Del Zanna L. Didkovsky N. Labrosse D. Graham 《Solar physics》2011,273(1):69-80
The Extreme-ultraviolet Variability Experiment (EVE; see Woods et al., 2009) obtains continuous EUV spectra of the Sun viewed as a star. Its primary objective is the characterization of solar
spectral irradiance, but its sensitivity and stability make it extremely interesting for observations of variability on time
scales down to the limit imposed by its basic 10 s sample interval. In this paper we characterize the Doppler sensitivity
of the EVE data. We find that the 30.4 nm line of He ii has a random Doppler error below 0.001 nm (1 pm, better than 10 km s−1 as a redshift), with ample stability to detect the orbital motion of its satellite, the Solar Dynamics Observatory (SDO). Solar flares also displace the spectrum, both because of Doppler shifts and because of EVE’s optical layout, which
(as with a slitless spectrograph) confuses position and wavelength. As a flare develops, the centroid of the line displays
variations that reflect Doppler shifts and therefore flare dynamics. For the impulsive phase of the flare SOL2010-06-12, we
find the line centroid to have a redshift of 16.8 ± 5.9 km s−1 relative to that of the flare gradual phase (statistical errors only). We find also that high-temperature lines, such as
Fe xxiv 19.2 nm, have well-determined Doppler components for major flares, with decreasing apparent blueshifts as expected from chromospheric
evaporation flows. 相似文献
6.
We present an analysis of hard X-ray imaging observations from one of the first solar flares observed with the Reuven Ramaty
High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February 2002. The data were obtained from the 22
February 2002, 11:06 UT flare, which occurred close to the northwest limb. Thanks to the high energy resolution of the germanium-cooled
hard X-ray detectors on RHESSI we can measure the flare source positions with a high accuracy as a function of energy. Using
a forward-fitting algorithm for image reconstruction, we find a systematic decrease in the altitudes of the source centroids
z(ε) as a function of increasing hard X-ray energy ε, as expected in the thick-target bremsstrahlung model of Brown. The altitude
of hard X-ray emission as a function of photon energy ε can be characterized by a power-law function in the ε=15–50 keV energy
range, viz., z(ε)≈2.3(ε/20 keV)−1.3 Mm. Based on a purely collisional 1-D thick-target model, this height dependence can be inverted into a chromospheric density
model n(z), as derived in Paper I, which follows the power-law function n
e(z)=1.25×1013(z/1 Mm)−2.5 cm−3. This density is comparable with models based on optical/UV spectrometry in the chromospheric height range of h≲1000 km, suggesting that the collisional thick-target model is a reasonable first approximation to hard X-ray footpoint sources.
At h≈1000–2500 km, the hard X-ray based density model, however, is more consistent with the `spicular extended-chromosphere model' inferred from radio sub-mm observations, than with standard models based on hydrostatic equilibrium. At coronal heights,
h≈2.5–12.4 Mm, the average flare loop density inferred from RHESSI is comparable with values from hydrodynamic simulations
of flare chromospheric evaporation, soft X-ray, and radio-based measurements, but below the upper limits set by filling-factor
insensitive iron line pairs. 相似文献
7.
A flare observed with the Hard X-Ray Imaging Spectrometer (HXIS) was studied during its rise to maximum temperature and X-ray emission rate. Two proximate flare loops, of lengths 2.8 × 109 cm and 1.1 × 1010 cm, rose to temperatures of 21.5 × 106 K and 30 × 106 K, respectively, in 30 s. Assuming equal heat flux F into each loop from a thermal source at the point where they met, we derive a simple relationship between temperature T and loop length
, which gives a loop temperture ratio of 0.68, in close agreement with the observed ratio of 0.72. The observations imply that heating in each loop was maintained by a thermal flux of 5 × 109 ergs cm-2 s-1. It is suggested that conductive heating adequately describes the rise and maximum phase emissions in the loops and that long flare loops reach higher temperatures than short loops during the impulsive phase because of an equipartition of energy between them at their point of interaction. 相似文献
8.
Rajmal Jain Arun Kumar Awasthi Arvind Singh Rajpurohit Markus J. Aschwanden 《Solar physics》2011,270(1):137-149
We report solar flare plasma to be multi-thermal in nature based on the theoretical model and study of the energy-dependent
timing of thermal emission in ten M-class flares. We employ high-resolution X-ray spectra observed by the Si detector of the
“Solar X-ray Spectrometer” (SOXS). The SOXS onboard the Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May
2003. Firstly we model the spectral evolution of the X-ray line and continuum emission flux F(ε) from the flare by integrating a series of isothermal plasma flux. We find that the multi-temperature integrated flux F(ε) is a power-law function of ε with a spectral index (γ)≈−4.65. Next, based on spectral-temporal evolution of the flares we find that the emission in the energy range E=4 – 15 keV is dominated by temperatures of T=12 – 50 MK, while the multi-thermal power-law DEM index (δ) varies in the range of −4.4 and −5.7. The temporal evolution of the X-ray flux F(ε,t) assuming a multi-temperature plasma governed by thermal conduction cooling reveals that the temperature-dependent cooling
time varies between 296 and 4640 s and the electron density (n
e) varies in the range of n
e=(1.77 – 29.3)×1010 cm−3. Employing temporal evolution technique in the current study as an alternative method for separating thermal from nonthermal
components in the energy spectra, we measure the break-energy point, ranging between 14 and 21±1.0 keV. 相似文献
9.
We report very high temporal and spectral resolution interferometric observations of some unusual solar radio bursts near
1420 MHz. These bursts were observed on 13 September 2005, 22 minutes after the peak of a GOES class X flare from the NOAA
region 10808. Our observations show 11 episodes of narrow-band intermittent emission within a span of ≈ 8 s. Each episode
shows a heavily frequency-modulated band of emission with a spectral slope of about −245.5 MHz s−1, comprising up to 8 individual blobs of emission and lasts for 10 – 15 ms. The blobs themselves have a spectral slope of
≈ 0 MHz s−1, are ≈ 200 – 250 kHz wide, appear every ≈ 400 kHz and last for ≈ 4 – 5 ms. These bursts show brightness temperatures in the
range 1012 K, which suggests a coherent emission mechanism. We believe these are the first high temporal and spectral resolution interferometric
observations of such rapid and narrow-bandwidth solar bursts close to 1420 MHz and present an analysis of their temporal and
spectral characteristics. 相似文献
10.
Pankaj Kumar Ablishek K. Srivastava B. Filippov R. Erdélyi Wahab Uddin 《Solar physics》2011,272(2):301-317
We present the multiwavelength observations of a flux rope that was trying to erupt from NOAA AR 11045 and the associated
M-class solar flare on 12 February 2010 using space-based and ground-based observations from TRACE, STEREO, SOHO/MDI, Hinode/XRT, and BBSO. While the flux rope was rising from the active region, an M1.1/2F class flare was triggered near one of its
footpoints. We suggest that the flare triggering was due to the reconnection of a rising flux rope with the surrounding low-lying
magnetic loops. The flux rope reached a projected height of ≈0.15R
⊙ with a speed of ≈90 km s−1 while the soft X-ray flux enhanced gradually during its rise. The flux rope was suppressed by an overlying field, and the
filled plasma moved towards the negative polarity field to the west of its activation site. We found the first observational
evidence of the initial suppression of a flux rope due to a remnant filament visible both at chromospheric and coronal temperatures
that evolved a couple of days earlier at the same location in the active region. SOHO/MDI magnetograms show the emergence
of a bipole ≈12 h prior to the flare initiation. The emerged negative polarity moved towards the flux rope activation site,
and flare triggering near the photospheric polarity inversion line (PIL) took place. The motion of the negative polarity region
towards the PIL helped in the build-up of magnetic energy at the flare and flux rope activation site. This study provides
unique observational evidence of a rising flux rope that failed to erupt due to a remnant filament and overlying magnetic
field, as well as associated triggering of an M-class flare. 相似文献
11.
A model for the high-frequency (20–2400 Hz) quasi-periodic oscillations (QPOs) of magnetars based on the representation of
coronal magnetic loops as equivalent electric RLC circuits is proposed. The observed periods of the QPOs and their high Q-factor (Q ≈ 104–105) are explained. It follows from the model that the QPOs can be excited not only in the tail of a flare but also before the
main pulse. The parameters of the QPO source at the “ringing tail” stage of the flare from SGR 1806–20 on December 27, 2004,
have been estimated: electric current I ≈ 3 × 1019 A, minimum magnetic field strength B
min ≈ 1013 G, and electron density n ≈ 2 × 1016 cm−3. 相似文献
12.
We analyze a special kind of temporal fine structure in microwave radio emission for the 25 August 1999 solar flare observed
by the PMO spectrometer over the range of 4.5 – 7.5 GHz. This flare displays continuum emission after a group of reverse-slope
type III bursts around 6 GHz. High-resolution dynamic spectra reveal three evolving emission lines (EELs) following the type
III group. They are characterized by isolated, narrow, and continuous emission strips, which display frequency fluctuations
with time. Their frequency-drift rates are between −2 and 3 GHz s−1. Distinct from the EELs at lower frequencies, three EELs have a very short duration of a few seconds. They show an average
bandwidth of Δf≈330 MHz and a relative bandwidth of Δf/f≈0.057. This is the first time that this kind of fine structure has been observed around 6 GHz. 相似文献
13.
Recent observations with EUV imaging instruments such as SOHO/EIT and TRACE have shown evidence for flare-like processes at
the bottom end of the energy scale, in the range of E
th≈1024–1027 erg. Here we compare these EUV nanoflares with soft X-ray microflares and hard X-ray flares across the entire energy range.
From the observations we establish empirical scaling laws for the flare loop length, L(T)∼T, the electron density, n
e(T)∼T
2, from which we derive scaling laws for the loop pressure, p(T)∼T
3, and the thermal energy, E
th∼T
6. Extrapolating these scaling laws into the picoflare regime we find that the pressure conditions in the chromosphere constrain a height level for flare loop footpoints, which scales
with h
eq(T)∼T
−0.5. Based on this chromospheric pressure limit we predict a lower cutoff of flare loop sizes at L
∖min≲5 Mm and flare energies E
∖min≲1024 erg. We show evidence for such a rollover in the flare energy size distribution from recent TRACE EUV data. Based on this
energy cutoff imposed by the chromospheric boundary condition we find that the energy content of the heated plasma observed
in EUV, SXR, and HXR flares is insufficient (by 2–3 orders of magnitude) to account for coronal heating. 相似文献
14.
G. Trottet J.-P. Raulin G. Giménez de Castro T. Lüthi A. Caspi C. H. Mandrini M. L. Luoni P. Kaufmann 《Solar physics》2011,273(2):339-361
Solar flares observed in the 200 – 400 GHz radio domain may exhibit a slowly varying and time-extended component which follows
a short (few minutes) impulsive phase and can last for a few tens of minutes to more than one hour. The few examples discussed
in the literature indicate that such long-lasting submillimeter emission is most likely thermal bremsstrahlung. We present
a detailed analysis of the time-extended phase of the 27 October 2003 (M6.7) flare, combining 1 – 345 GHz total-flux radio
measurements with X-ray, EUV, and Hα observations. We find that the time-extended radio emission is, as expected, radiated
by thermal bremsstrahlung. Up to 230 GHz, it is entirely produced in the corona by hot and cool materials at 7 – 16 MK and
1 – 3 MK, respectively. At 345 GHz, there is an additional contribution from chromospheric material at a few 104 K. These results, which may also apply to other millimeter–submillimeter radio events, are not consistent with the expectations
from standard semiempirical models of the chromosphere and transition region during flares, which predict observable radio
emission from the chromosphere at all frequencies where the corona is transparent. 相似文献
15.
We present the results of our infrared observations of WR 140 (=V1687 Cyg) in 2001–2010. Analysis of the observations has
shown that the J brightness at maximum increased near the periastron by about 0
m
.3; the M brightness increased by ∼2
m
in less than 50 days. The minimum J brightness and the minimum L and M brightnesses were observed 550–600 and 1300–1400 days after the maximum, respectively. The JHKLM brightness minimum was observed in the range of orbital phases 0.7–0.9. The parameters of the primary O5 component of the
binary have been estimated to be the following: R(O5) ≈ 24.7R
⊙, L(O5) ≈ 8 × 105
L
⊙, and M
bol(O5) ≈ −10
m
. At the infrared brightness minimum, T
g ∼ 820–880 K, R
g ≈ 2.6 × 105
R
⊙, the optical depth of the shell at 3.5 μm is ∼5.3 × 10−6, and its mass is ≈1.4 × 10−8
M
⊙. At the maximum, the corresponding parameters are ∼1300 K, 8.6 × 104
R
⊙, ∼2 × 10−4, and ∼6 × 10−8
M
⊙; the mean rate of dust inflow (condensation) into the dust structure is ∼3.3 × 10−8
M
⊙ yr−1. The mean escape velocity of the shell from the heating source is ∼103 km s−1 and the mean dispersal rate of the shell is ∼1.1 × 10−8
M
⊙ yr−1. 相似文献
16.
The flare plasma temperature calculated from GOES-11 (1.5–12.4 and 3.1–24.8 keV) data is compared with the solar nonthermal
fluxes in various energy ranges in the December 6, 2006 event. Particle acceleration and plasma heating episodes took place
in the pre-flare and impulsive phases; a hard (ACS SPI > 150 keV) X-ray emission was observed 5 min before the onset of the GOES X-ray flare and was not accompanied by a temperature
rise. A close correlation has been found between the flare plasma temperature and the hard X-ray intensity. The temperature
delayed by 0.4 min turned out to be directly proportional to the logarithm of the ACS SPI count rate within the first 3 min
of the impulsive phase. This shows that the accelerated electrons responsible for the X-ray emission were the main plasma
heating source in the pre-flare and impulsive phases. The correlation between the temperature and the hard X-ray intensity
disappears after the observation of a resonance peak at a frequency of 245 MHz. Significant electron fluxes may no longer
be able to effectively heat the expanding plasma when its density in the interaction region reaches ∼109 cm−3. The observations of the July 23, 2002 and December 5, 2006 events confirm the trends found. 相似文献
17.
V. V. Grechnev V. G. Kurt I. M. Chertok A. M. Uralov H. Nakajima A. T. Altyntsev A. V. Belov B. Yu. Yushkov S. N. Kuznetsov L. K. Kashapova N. S. Meshalkina N. P. Prestage 《Solar physics》2008,252(1):149-177
The famous extreme solar and particle event of 20 January 2005 is analyzed from two perspectives. Firstly, using multi-spectral
data, we study temporal, spectral, and spatial features of the main phase of the flare, when the strongest emissions from
microwaves up to 200 MeV gamma-rays were observed. Secondly, we relate our results to a long-standing controversy on the origin
of solar energetic particles (SEP) arriving at Earth, i.e., acceleration in flares, or shocks ahead of coronal mass ejections (CMEs). Our analysis shows that all electromagnetic emissions
from microwaves up to 2.22 MeV line gamma-rays during the main flare phase originated within a compact structure located just
above sunspot umbrae. In particular, a huge (≈ 105 sfu) radio burst with a high frequency maximum at 30 GHz was observed, indicating the presence of a large number of energetic
electrons in very strong magnetic fields. Thus, protons and electrons responsible for various flare emissions during its main
phase were accelerated within the magnetic field of the active region. The leading, impulsive parts of the ground-level enhancement
(GLE), and highest-energy gamma-rays identified with π
0-decay emission, are similar and closely correspond in time. The origin of the π
0-decay gamma-rays is argued to be the same as that of lower-energy emissions, although this is not proven. On the other hand,
we estimate the sky-plane speed of the CME to be 2 000 – 2 600 km s−1, i.e., high, but of the same order as preceding non-GLE-related CMEs from the same active region. Hence, the flare itself rather
than the CME appears to determine the extreme nature of this event. We therefore conclude that the acceleration, at least,
to sub-relativistic energies, of electrons and protons, responsible for both the major flare emissions and the leading spike
of SEP/GLE by 07 UT, are likely to have occurred nearly simultaneously within the flare region. However, our analysis does
not rule out a probable contribution from particles accelerated in the CME-driven shock for the leading GLE spike, which seemed
to dominate at later stages of the SEP event.
S.N. Kuznetsov deceased 17 May 2007. 相似文献
18.
We analyze multiple-wavelength observations of a two-ribbon flare exhibiting apparent expansion motion of the flare ribbons
in the lower atmosphere and rising motion of X-ray emission at the top of newly-formed flare loops. We evaluate magnetic reconnection
rate in terms of V
r
B
r by measuring the ribbon-expansion velocity (V
r) and the chromospheric magnetic field (B
r) swept by the ribbons. We also measure the velocity (V
t) of the apparent rising motion of the loop-top X-ray source, and estimate the mean magnetic field (B
t) at the top of newly-formed flare loops using the relation 〈V
t
B
t〉≈〈V
r
B
r〉, namely, conservation of reconnection flux along flare loops. For this flare, B
t is found to be 120 and 60 G, respectively, during two emission peaks five minutes apart in the impulsive phase. An estimate
of the magnetic field in flare loops is also achieved by analyzing the microwave and hard X-ray spectral observations, yielding
B=250 and 120 G at the two emission peaks, respectively. The measured B from the microwave spectrum is an appropriately-weighted value of magnetic field from the loop top to the loop leg. Therefore,
the two methods to evaluate coronal magnetic field in flaring loops produce fully-consistent results in this event. 相似文献
19.
An X17 class (GOES soft X-ray) two-ribbon solar flare on October 28, 2003 is analyzed in order to determine the relationship
between the timing of the impulsive phase of the flare and the magnetic shear change in the flaring region. EUV observations
made by the Transition Region and Coronal Explorer (TRACE) show a clear decrease in the shear of the flare footpoints during the flare. The shear change stopped in the middle
of the impulsive phase. The observations are interpreted in terms of the splitting of the sheared envelope field of the greatly
sheared core rope during the early phase of the flare. We have also investigated the temporal correlation between the EUV
emission from the brightenings observed by TRACE and the hard X-ray (HXR) emission (E > 150 keV) observed by the anticoincidence system (ACS) of the spectrometer SPI on board the ESA INTEGRAL satellite. The
correlation between these two emissions is very good, and the HXR sources (RHESSI) late in the flare are located within the
two EUV ribbons. These observations are favorable to the explanation that the EUV brightenings mainly result from direct bombardment
of the atmosphere by the energetic particles accelerated at the reconnection site, as does the HXR emission. However, if there
is a high temperature (T > 20 MK) HXR source close to the loop top, a contribution of thermal conduction to the EUV brightenings cannot be ruled out. 相似文献