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1.
Rajmal Jain Vishal Joshi Yoichiro Hanaoka T. Sakurai Nipa Upadhyay 《Journal of Astrophysics and Astronomy》2006,27(2-3):339-346
We present a study of 10 microflares observed in 4–30 keV by SOXS mission simultaneously with Hα observations made at NAOJ,
Japan during the interval between February and August 2004. The X-ray and Hα light curves showed that the lifetime of microflares
varies between 4 and 25 min. We found that the X-ray emission in all microflares under study in the dynamic energy range of
4–30 keV can be fitted by thermal plus non-thermal components. The thermal spectrum appeared to start from almost 4 keV, low
level discriminator (LLD) of both Si and CZT detectors, however it ends below 8 keV. We also observed the Fe line complex
features at 6.7 keV in some microflares and attempted to fit this line by isothermal temperature assumption. The temperature
of isothermal plasma of microflares varies in the range between 8.6 and 10.1 MK while emission measure between 0.5 and 2x1049 cm-3. Non-thermal (NT) emission appeared in the energy range 7–15 keV with exponent -6.8 ≤γ≤-4.8. Our study of microflares that had occurred on 25 February 2004 showed that sometimes a given active region produces
recurrent microflare activity of a similar nature. We concluded from X-ray and simultaneous Hα observations that the microflares
are perhaps the result of the interaction of low lying loops. It appears that the electrons that accelerated during reconnection
heat the ambient coronal plasma as well as interact with material while moving down along the loops and thereby produce Hα
bright kernels. 相似文献
2.
Open-shutter RHESSI observations of 3–15 keV X-rays are found to exhibit active-region transient brightenings and microflares
at a rate of a least 10 per hour occurring even during the periods of lowest solar activity so far in the mission. A thermal
component fitted by temperatures of 6–14 MK dominates from 3 keV to about 9 keV, but can be traced up to 14 keV in some cases,
and has an average duration of 131(±103) s at 7–8 keV. The duration increases with decreasing photon energy. The peak count
rate defined by cross-correlation is delayed at low energies. The temperature peaks early in the event and then decreases,
whereas the emission measure increases throughout the event. The properties are consistent with thermal conduction dominating
the evolution. In some of the bigger events, a second component was found in the 11–14 keV range extending down to 8 keV in
some cases. The duration is typically 3 times shorter and ends near the peak time of the thermal component consistent with
the Neupert effect of regular flares. Therefore the second component is suggested to be of non-thermal origin, presumably
causing the beam-driven evaporation of the first component. The two components can be separated and analyzed in detail for
the first time. Low-keV measurements allow a reliable estimate of the energy input by microflares necessary to assess their
relevance for coronal heating.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022496515506 相似文献
3.
We studied the evolution of two small flares (GOES class C2 and C1) that developed in the same active region with different
morphological characteristics: one is extended and the other is compact. We analyzed the accuracy and the consistency of different
algorithms implemented in Reuven Ramaty High-Energy Spectroscopic Imager (RHESSI) software to reconstruct the image of the
emitting sources, for energies between 3 and 12 keV. We found that all tested algorithms give consistent results for the peak
position, while the other parameters can differ at most by a factor 2. Pixon and Forward-fit generally converge to similar results but Pixon is more reliable for reconstructing a complex source. We investigated the spectral characteristics of the two flares during
their evolution in the 3–25 keV energy band. We found that a single thermal model of the photon spectrum is inadequate to
fit the observations and we needed to add either a non-thermal model or a hot thermal one. The non-thermal and the double
thermal fits are comparable. If we assume a non-thermal model, the non-thermal energy is always higher than the thermal one.
Only during the very final decay phase a single thermal model fits the observed spectrum fairly well. 相似文献
4.
Ulysses was launched in October 1990, and its Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) has provided more than 13 years
of uninterrupted observations of solar X-ray flare activity. Due to the large variation of the relative solar latitude and
longitude of the spacecraft orbit with respect to the Earth, the perspective of the GRB instrument often differed significantly
from that of X-ray instruments on Earth-orbiting satellites. During extended periods the GRB experiment made direct observations
of flares on the hidden face of the Sun, providing a unique record of events not visible to other instruments. The small detector
area of GRB and its optimization for very high counting rates minimized the effects of pulse pile-up. We interpret the spectra,
time histories, and occurrence distribution patterns of GRB data in terms of “thermal feed-through”, the confusion of thermal
soft X-rays and non-thermal hard X-rays. This effect is a systematic problem for scintillation-counter spectrometers observing
the solar hard X-ray spectrum. This paper provides a definitive catalog of the Ulysses X-ray flare observations and discusses various features of this unique database. For the equivalent GOES range X2 – X25,
we find a power-law fit for the (differential) occurrence frequency at >25 keV with slope −1.61±0.04, with no evidence for
a downturn at the highest event magnitudes (for the relatively small sample of such events available in this study). If the
nine most intense events are excluded because of concerns about the effects of pulse pile-up, the slope steepens to −1.75±0.08. 相似文献
5.
R. K. Manchanda 《Journal of Astrophysics and Astronomy》2001,22(2-3):145-154
Spectral measurement of Mkn 421 were made in the hard X-ray energy band of 20–200 keV using a high sensitivity, large area
scintillation counter telescope on November 21, 2000 and these coincided with the onset of an active X-ray phase as seen in
the ASM counting rates on board RXTE. The observed spectrum can not be fitted to a single power law similar to the PDS data
of BeppoSAX. The data can be fitted both by a two component power-law function or a combination of an exponential function
with a power law component at the high energies above 80 keV. We identify these components with those arising from the synchrotron
self compton and the high energy power-law tail arising from the upgrading of the thermal photons due to multiple Compton
scattering a la Cyg X-1. A comparison with the earlier data clearly suggests a spectral variability in the hard X-ray spectrum
of the source. We propose a continuously flaring geometry for the source as the underlying mechanism for energy release. 相似文献
6.
In an effort to examine the relationship between flare flux and corresponding CME mass, we temporally and spatially correlate
all X-ray flares and CMEs in the LASCO and GOES archives from 1996 to 2006. We cross-reference 6733 CMEs having well-measured
masses against 12 050 X-ray flares having position information as determined from their optical counterparts. For a given
flare, we search in time for CMEs which occur 10 – 80 minutes afterward, and we further require the flare and CME to occur
within ± 45° in position angle on the solar disk. There are 826 CME/flare pairs which fit these criteria. Comparing the flare
fluxes with CME masses of these paired events, we find CME mass increases with flare flux, following an approximately log-linear,
broken relationship: in the limit of lower flare fluxes, log (CME mass)∝0.68×log (flare flux), and in the limit of higher
flare fluxes, log (CME mass)∝0.33×log (flare flux). We show that this broken power-law, and in particular the flatter slope
at higher flare fluxes, may be due to an observational bias against CMEs associated with the most energetic flares: halo CMEs.
Correcting for this bias yields a single power-law relationship of the form log (CME mass)∝0.70×log (flare flux). This function
describes the relationship between CME mass and flare flux over at least 3 dex in flare flux, from ≈ 10−7 – 10−4 W m−2. 相似文献
7.
We present the analysis of a compact flare that occurred on 26 February 2002 at 10:26 UT, seen by both RHESSI and TRACE. The
size of the nearly circular hard X-ray source is determined to be 5.6 (±0.8)′′, using different methods. The power-law distribution
of non-thermal photons is observed to extend down to 10 keV without flattening, and to soften with increasing distance from
the flare kernel. The former indicates that the energy of the precipitating flare electron population is larger than previously
estimated: it amounts to 2.6 (±0.8)×1030 erg above 10 keV, assuming thick-target emission. The thermal energy content of the soft X-ray source (isothermal temperature
of 20.8 (±0.9) MK) and its radiated power were derived from the thermal emission at low energies. TRACE has observed a low-temperature
ejection in the form of a constricted bubble, which is interpreted as a reconnection jet. Its initial energy of motion is
estimated. Using data from both satellites, an energy budget for this flare is derived. The kinetic energy of the jet bulk
motion and the thermal and radiated energies of the flare kernel were more than an order of magnitude smaller than the derived
electron beam energy. A movie is available on the CD-ROM accompanying this volume.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022478300679 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.
The UCSD solar X-ray instrument on the OSO-7 satellite observes X-ray bursts in the 2–300 keV range with 10.24 s time resolution. Spectra obtained from the proportional counter and scintillation counter are analyzed for the event of November 16, 1971, at 0519 UT in terms of thermal (exponential spectrum) and non-thermal (power law) components. The energy content of the approximately 20 × 106K thermal plasma increased with the 60 s duration hard X-ray burst which entirely preceded the 5 keV soft X-ray maximum. If the hard X-rays arise by thick target bremsstrahlung, the nonthermal electrons above 10 keV have sufficient energy to heat the thermally emitting plasma. In the thin target case the collisional energy transfer from non-thermal electrons suffices if the power law electron spectrum is extrapolated below 10 keV, or if the ambient plasma density exceeds 4 × 1010 cm–3.Formerly at UCSD. 相似文献
10.
R. K. Manchanda 《Journal of Astrophysics and Astronomy》2002,23(3-4):197-212
The attempts at unified model fitting to explain the spectral variations in Cyg X-3 suggest equally probable fits with a combination
of an absorbed blackbody and a separately absorbed power law with an exponential cut-off or a composite of absorbed free-free
emission with a power law hard X-ray component apart from the iron emission line. These seemingly ordinary but ad hoc mixtures
of simple X-ray emission mechanisms have a profound implication about the geometry of the X-ray source. While the first set
suggests a black-hole nature of the compact object, the second combination is consistent with a neutron star binary picture.
The spectral variability at hard X-ray energies above 30 keV can provide crucial input for the unified picture. In this paper,
we present spectral observations of Cyg X-3, made in our on-going survey of galactic and extragalactic X-ray sources in the
20–200 keV energy region, using Large Area Scintillation counter Experiment. The data show a clear power-law photon spectrum
of the form dN/dE ∼ E−2.8 in the 20 to 130 keV energy range. A comparison with earlier data suggests that the total number of X-ray photons in the
entire 2–500 keV energy band is conserved at all time for a given luminosity level irrespective of the state. We propose that
this behaviour can be explained by a simple geometry in which a thermal X-ray source is embedded in a hot plasma formed by
winds from the accretion disk within a cold shell. The high/soft and low/hard X-ray states of the source are simply the manifestation
of the extent of the surrounding scattering medium in which the seed photons are Comptonized and hot plasma can be maintained
by either the X-ray driven winds or the magneto-centrifugal winds. 相似文献
11.
A database combining information about solar proton enhancements (SPEs) near the Earth and soft X-ray flares (GOES measurements)
has been used for the study of different correlations through the period from 1975 to May 2006. The emphasis of this work
is on the treatment of peak-size distributions of SXR flares and SPEs. The frequency of SXR flares and solar proton events
(>10 and >100 MeV, respectively) for the past three solar cycles has been found to follow mainly a power-law distribution
over three to five orders of magnitude of fluxes, which is physically correct beyond the “sensitivity” problem with the smallest
peak values. The absence of significant spectral steepening in the domain of the highest peak values demonstrates that during
the period considered, lasting 30 years, the limit of the highest flare’s energy release has not yet been achieved. The power-law
exponents were found to be −2.19±0.04, −1.34±0.02, and −1.46±0.04, for the total SXR flare distribution and the total SPE
distributions (for both E
P>10 MeV and E
P>100 MeV), respectively. For SPEs associated with flares located to the West of 20° W, the exponents are −1.22±0.05 (E
P>10 MeV) and −1.26±0.03 (E
P>100 MeV). The size distribution for corresponding flares follows a power law with a slope of −1.29±0.12. Thus, X-ray and
proton fluxes produced in the same solar events have very similar distribution shapes. Moreover, the derived slopes are not
incompatible with a linear dependence between X-ray flare power and proton fluxes near the Earth. A similar statistical relation
is obtained independently from the direct comparison of the X-ray and proton fluxes. These all argue for a statistically significant
relationship between X-ray and proton emissions. 相似文献
12.
Some 15% of solar flares having a soft X-ray flux above GOES class C5 are reported to lack coherent radio emission in the
100 – 4000 MHz range (type I – V and decimetric emissions). A detailed study of 29 such events reveals that 22 (76%) of them
occurred at a radial distance of more than 800″ from the disk center, indicating that radio waves from the limb may be completely
absorbed in some flares. The remaining seven events have statistically significant trends to be weak in GOES class and to
have a softer non-thermal X-ray spectrum. All of the non-limb flares that were radio-quiet above 100 MHz were accompanied
by metric type III emission below 100 MHz. Out of 201 hard X-ray flares, there was no flare except near the limb (R>800″) without coherent radio emission in the entire meter and decimeter range. We suggest that flares above GOES class C5
generally emit coherent radio waves when observed radially above the source. 相似文献
13.
We present simple analytic models which predict the peak X-ray emission measure and temperature attained in flares in which
the chromospheric evaporation process takes place either in a single ‘monolithic’ loop or in a loop consisting of several
filaments that are created successively as the energy release process proceeds in time. As possible mechanisms driving chromospheric
evaporation we consider both classical heat conduction from the loop top and non-thermal electron beams. The model predictions
are tested for a set of 18 well studied RHESSI microflares. The results suggest beam driven evaporation in filamented loops
as being capable of accounting for the observed emission measures and temperatures though there are issues with the very high
beam densities needed. On the other hand, estimates of the emission measures achieved by conductive evaporation which are
derived by using the Rosner – Tucker – Vaiana (RTV) scaling law are much larger than the observed ones. Possible reasons for
this discrepancy are discussed. 相似文献
14.
The NOAA active region (AR) 11029 was a small but highly active sunspot region which produced 73 GOES soft X-ray flares during its transit of the disk in late October 2009. The flares appear to show a departure from the well-known power law frequency-size distribution. Specifically, too few GOES C-class and no M-class flares were observed by comparison with a power law distribution (Wheatland, Astrophys. J. 710, 1324, 2010). This was conjectured to be due to the region having insufficient magnetic energy to power the missing large events. We construct nonlinear force-free extrapolations of the coronal magnetic field of AR 11029 using data taken on 24 October by the SOLIS Vector SpectroMagnetograph (SOLIS/VSM) and data taken on 27 October by the Hinode Solar Optical Telescope SpectroPolarimeter (Hinode/SP). Force-free modeling with photospheric magnetogram data encounters problems, because the magnetogram data are inconsistent with a force-free model. We employ a recently developed “self-consistency” procedure which addresses this problem and accommodates uncertainties in the boundary data (Wheatland and Régnier, Astrophys. J. 700, L88, 2009). We calculate the total energy and free energy of the self-consistent solution, which provides a model for the coronal magnetic field of the active region. The free energy of the region was found to be ≈?4×1029?erg on 24 October and ≈?7×1031?erg on 27 October. An order of magnitude scaling between RHESSI non-thermal energy and GOES peak X-ray flux is established from a sample of flares from the literature and is used to estimate flare energies from the observed GOES peak X-ray flux. Based on the scaling, we conclude that the estimated free energy of AR 11029 on 27 October when the flaring rate peaked was sufficient to power M-class or X-class flares; hence, the modeling does not appear to support the hypothesis that the absence of large flares is due to the region having limited energy. 相似文献
15.
Hard X-ray lightcurves, spectrograms, images, and spectra of three medium-sized flares observed by the Reuven Ramaty High-Energy
Solar Spectroscopic Imager (RHESSI) are presented. Imaging spectroscopy of the 20 February 2002, 11:06 UT flare at 10′′ spatial
resolution, comparable to the best previous hard X-ray imaging from Yohkoh, shows two footpoints with an ∼ 8 s delay of peak emission between footpoints. Subsequent imaging at le4′′ shows three sources consistent with two separate loops and simultaneous brightening in connected footpoints. Imaging for
the simple two footpoint flare of 2 June 2002 also shows simultaneous footpoint brightening. The more complex 17 March 2002
flare shows at least four different sources during the main peak of the event, and it is difficult to clearly demonstrate
simultaneous brightening of connected footpoints. Non-thermal power laws are observed down to ∼ 12–13 keV without flattening
in all these events, indicating the energy content in energetic electrons may be significantly greater than previously estimated
from assumed 25 keV low energy cutoff. Simultaneously brightening footpoints show similar spectra, at least in the three flares
investigated. Double-power-law spectra with a relatively sharp break are often observed.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022469902940 相似文献
16.
Lin R. P. Curtis D. W. Primbsch J. H. Harvey P. R. Levedahl W. K. Smith D. M. Pelling R. M. Duttweiler F. Hurley K. 《Solar physics》1987,113(1-2):333-345
We describe a balloon payload designed to study the processes of energy release, particle acceleration, and heating of the active corona, in hard X-ray microflares and normal flares. An array of liquid nitrogen-cooled germanium detectors together with large area phoswich scintillation detectors provide the highest sensitivity (∼500 cm2) and energy resolution (≤0.7 keV) ever achieved for solar hard X-ray (∼15–600 keV) measurements. These detectors were flown in February 1987 from Australia on a long duration RAdiation COntrolled balloON (RACOON) flight (LDBF) which provided 12 days of observations before cutdown in Brazil. The payload includes solar cells for power, pointing and navigation sensors, a microprocessor controlled data system with VCR tape storage, and transmitters for GOES and ARGOS spacecraft. This successful flight illustrates the potential of LDBF's for solar flare studies.
相似文献17.
The flattening at the low energy end of the hard X-ray (HXR) photon spectrum of solar flares was generally thought to be due to a cutoff of nonthermal electrons in flares. However, some authors have suggested that inverse Compton scattering (i.e., the albedo effect) or certain other reaction of flare photons with the lower atmosphere can also lead to the flattening. This paper adopts the method of deriving the cutoff proposed by Gan et al. [12–14], and makes a statistical analysis on 100 flares observed by the satellite Ramaty High Energy Solar Spectroscopy Imager (RHESSI) in 2002–2005. We found that after the albedo correction, the HXR photon spectra of 18 flares can be fitted with single powerlaw spectra, and those of 80 flares, with double power-law spectra. Besides, 21 flares can be directly interpreted with a single power-law electron spectrum plus a low energy cutoff. The range of the low energy cutoff is 20–50 keV and the mean value is approximately 30 keV. Some other possible interpretations are also investigated. 相似文献
18.
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
19.
Details of the discovery (in February 2004) and results of subsequent (in 2004–2009) INTEGRAL observations of the transient
X-ray burster IGR J17380-3749 (IGR J17379-3747) are presented. Over the period of its observations, the INTEGRAL observatory
recorded two hard X-ray flares and one type I X-ray burst from the source, which allowed the nature of IGR J17380-3749 to
be determined. The burster radiation spectrum during the flares was hard—a power law with a photon index α = 1.8–2.0 or bremsstrahlung corresponding to a plasma with a temperature kT = 90–140 keV. The spectral shape at the flare peaks turned out to be the same, despite a more than twofold difference in
flux (the peak flux recorded in the energy range 18–100 keV reached ∼20 mCrab). The upper limit on the flux from the source
in its quiescent (off) state in the range of 18–40 keV was 0.15 mCrab (3σ). 相似文献
20.
In the years 2002 – 2005, 38 groups of the reverse drift bursts (RDBs) were observed in the 0.8 – 4.5 GHz frequency range
by the Ondřejov radiospectrograph. In 21 cases, which were observed at the times of the RHESSI observations, spatial structure,
positional changes, and spectra of X-ray sources during RDB observations are studied in detail. First, based on the frequency
drift and the spatial structure of the associated X-ray source, the events are classified as: (a) fast drifting RDBs with
a compact X-ray source, (b) fast drifting RDBs with a multiple X-ray source (FM), and slowly drifting RDBs. Then, the spectra
of X-ray sources at the times of RDBs are analyzed. It is found that most fast drifting RDBs (16 of 17 cases) are associated
with the spectra having a distinct power-law (non-thermal) component. In contrast, the X-ray spectra associated with the slowly drifting RDBs are predominantly purely
thermal (in three out of four cases; in the 26 July 2004, case the X-ray spectrum is thermal and high temperature, with non-thermal
component). Two special cases of RDBs observed during the 28 October 2003, and 23 July 2004, flares are added for comparison.
The most frequent events are those with fast drifting RDBs, a compact short-lasting X-ray sources, and a power-law X-ray spectrum.
The individual reverse drift bursts (∼1 s duration) do not show a clear temporal association with individual peaks of hard
X-ray bursts. During slowly drifting RDBs the shape of the associated X-ray source changed or expanded. Among them the most
interesting one was observed in 26 July 2004, when the very slowly drifting RDBs (+40 MHz s−1) were associated with an X-ray loop-like source continuously elongating in the southwest direction. In the most cases the
model of RDBs with electron beams is compatible with the observations, but in flares on 26 July 2004, and 28 October 2003,
the RDBs are probably generated by some other type of an agent; we propose here a thermal conduction front. 相似文献