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1.
Zongjun Ning 《Solar physics》2011,273(1):81-92
We explore the speed distributions of X-ray source motions after the start of chromospheric evaporation in two Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) flares. First, we make CLEAN images at 15 energy bands with a 12 second integration window; then, we outline a flaring
loop geometry to cover the looptop and footpoint sources as much as possible. Consistent with the previous steps, we find
converging motion of the double footpoint sources along the flaring loop in these two events. This motion is dependent on
the energy band and time and is typically seen at 3 – 25 keV, indicating a chromospheric evaporation origin. The speed distributions
at various energy bands are measured for the 10 September 2002 flare, which exhibits a separation-to-mergence motion pattern
well correlated with the rising-to-decay phases at 50 – 100 keV. 相似文献
2.
A simple model is presented to account for theYohkoh flare observations of Feldmanet al. (1994), and Masuda (1994). Electrons accelerated by the flare are assumed to encounter the dense, small regions observed by Feldmanet al. at the tops of impulsively flaring coronal magnetic loops. The values of electron density and volume inferred by Feldmanet al. imply that these dense regions present an intermediate thick-thin target to the energised electrons. Specifically, they present a thick (thin) target to electrons with energy much less (greater) thanE
c
, where 15 keV <E
c
< 40 keV. The electrons are either stopped at the loop top or precipitate down the field lines of the loop to the footpoints. Collisional losses of the electrons at the loop top produce the heating observed by Feldmanet al. and also some hard X-rays. It is argued that this is the mechanism for the loop-top hard X-ray sources observed in limb flares by Masuda. Adopting a simple model for the energy losses of electrons traversing the dense region and the ambient loop plasma, hard X-ray spectra are derived for the loop-top source, the footpoint sources and the region between the loop top and footpoints. These spectra are compared with the observations of Masuda. The model spectra are found to qualitatively agree with the data, and in particular account for the observed steepening of the loop-top and footpoint spectra between 14 and 53 keV and the relative brightnesses of the loop-top and footpoint sources. 相似文献
3.
K. Kobayashi S. Tsuneta T. Tamura K. Kumagai Y. Katsukawa M. Kubo Y. Sakamoto N. Kohara T. Yamagami Y. Saito K. Mori 《Solar physics》2008,250(2):431-441
Spectroscopic observation of solar flares in the hard X-ray energy range, particularly the 20 ∼ 100 keV region, is an invaluable
tool for investigating the flare mechanism. This paper describes the design and performance of a balloon-borne hard X-ray
spectrometer using CdTe detectors developed for solar flare observation. The instrument is a small balloon payload (gondola
weight 70 kg) with sixteen 10×10×0.5 mm CdTe detectors, designed for a 1-day flight at 41 km altitude. It observes in an energy
range of 20−120 keV and has an energy resolution of 3 keV at 60 keV. The second flight on 24 May 2002 succeeded in observing
a class M1.1 flare. 相似文献
4.
The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) X-ray data base (February 2002 – May 2006) has been searched to find solar flares with weak thermal components and
flat photon spectra. Using a regularized inversion technique, we determine the mean electron flux distribution from count
spectra for a selection of events with flat photon spectra in the 15 – 20 keV energy range. Such spectral behavior is expected
for photon spectra either affected by photospheric albedo or produced by electron spectra with an absence of electrons in
a given energy range (e.g., a low-energy cutoff in the mean electron spectra of nonthemal particles). We have found 18 cases that exhibit a statistically
significant local minimum (a dip) in the range of 13 – 19 keV. The positions and spectral indices of events with low-energy
cutoff indicate that such features are likely to be the result of photospheric albedo. It is shown that if the isotropic albedo
correction is applied, all low-energy cutoffs in the mean electron spectrum are removed, and hence the low-energy cutoffs
in the mean electron spectrum of solar flares above ∼ 12 keV cannot be viewed as real features. If low-energy cutoffs exist
in the mean electron spectra, their energies should be less than ∼ 12 keV. 相似文献
5.
HARRA-MURNION L. K. CULHANE J. L. HUDSON H. S. FUJIWARA T. KATO T. STERLING A. C. 《Solar physics》1997,171(1):103-122
We analyse the physical characteristics of a C5.7 class flare which was observed on 27 September, 1993 using data from the soft X-ray telescope (SXT), the Bragg crystal spectrometer (BCS), and the hard X-ray telescope (HXT) on Yohkoh. The flare takes the form of a simple loop which is much brighter at one of its footpoints than anywhere else for a period of 2 min. During this time there is an increase in the soft X-ray fluxes, and a corresponding peak in hard X-rays. The parameters derived from the hard X-ray and soft X-ray spectra and images are assumed to be from the footpoint. This flare showed two peaks in the non-thermal velocity, the first one simultaneous with the footpoint brightening. The non-thermal velocity corresponding to these first few minutes is unusually large – by a factor of 80%, 68%, and 26% relative to the second peak in the Fexxv, Caxix, and Sxv channels respectively. 相似文献
6.
The spatial and spectral behaviors of two solar flares observed by the Nobeyama Radioheliograph (NoRH) on 24 August 2002 and
22 August 2005 are explored. They were observed with a single loop-top source and double footpoint sources at the beginning,
then with looplike structures for the rest of the event. NoRH has high spatial and temporal resolution at the two frequencies
of 17 and 34 GHz where a nonthermal radio source is often optically thin. Such capabilities give us an opportunity to study
the spatial and spectral behaviors of different microwave sources. The 24 August 2002 flare displayed a soft – hard – soft
(SHS) spectral pattern in the rising – peak – decay phases at 34 GHz, which was also observed for the spectral behavior of
both loop-top and footpoint sources. In contrast, the 22 August 2005 flare showed a soft – hard – harder (SHH) spectral pattern
for its both loop-top and footpoint sources. It is interesting that this event showed a harder spectrum in the early rising
phase. We found a positive correlation between the spectral index and microwave flux in both the loop-top source and the footpoint
sources in both events. The conclusions drawn from the flux index could apply to the electron index as well, because of their
simple linear relationship under the assumption of nonthermal gyrosynchrotron mechanism. Such a property of spatial and spectral
behaviors of microwave sources gives an observational constraint on the electron acceleration mechanism and electron propagation. 相似文献
7.
The `ribbons' of two-ribbon flares show complicated patterns reflecting the linkages of coronal magnetic field lines through
the lower solar atmosphere. We describe the morphology of the EUV ribbons of the July 14, 2000 flare, as seen in SOHO, TRACE,
and Yohkoh data, from this point of view. A successful co-alignment of the TRACE, SOHO/MDI and Yohkoh/HXT data has allowed us to locate the EUV ribbon positions on the underlying field to within ∼ 2′′, and thus to investigate
the relationship between the ribbons and the field, and also the sites of electron precipitation. We have also made a determination
of the longitudinal magnetic flux involved in the flare reconnection event, an important parameter in flare energetic considerations.
There are several respects in which the observations differ from what would be expected in the commonly-adopted models for
flares. Firstly, the flare ribbons differ in fine structure from the (line-of-sight) magnetic field patterns underlying them,
apparently propagating through regions of very weak and probably mixed polarity. Secondly, the ribbons split or bifurcate.
Thirdly, the amount of line-of-sight flux passed over by the ribbons in the negative and positive fields is not equal. Fourthly,
the strongest hard X-ray sources are observed to originate in stronger field regions. Based on a comparison between HXT and
EUV time-profiles we suggest that emission in the EUV ribbons is caused by electron bombardment of the lower atmosphere, supporting
the hypothesis that flare ribbons map out the chromospheric footpoints of magnetic field lines newly linked by reconnection.
We describe the interpretation of our observations within the standard model, and the implications for the distribution of
magnetic fields in this active region. 相似文献
8.
Tracking of TRACE Ultraviolet Flare Footpoints 总被引：1，自引：0，他引：1
Solar flares produce bright, compact sources of UV emission in the lower atmosphere, identified as flare footpoints. Observed at high time cadence with the Transition Region and Coronal Explorer, groups of UV footpoints define flare `ribbons' which move as the flare progresses. We have developed a procedure to track individual bright kernels within flare ribbons, enabling us to study the motion of these sites of excitation through the solar chromosphere. We have applied this to a flare observed by TRACE in the 1600 Å passband at 2-s cadence. In this event, the footpoints have an average speed of 15 km s–1, with a superposed random `meandering' component, consistent with the footpoint magnetic field being anchored around the edges of granular cells. Examining the brightness as a function of time, we find that the timing of peaks in brightness is significantly correlated with the timing of peaks in the product of the footpoint speed with the line-of-sight magnetic field strength at the footpoint location; in other words with a measure of the coronal reconnection rate. 相似文献
9.
We describe the instrumental corrections which have to be incorporated for reliable correction and deconvolution of images obtained in the 16–22 keV and 22–30 keV energy bands of the Hard X-Ray Imaging Spectrometer (HXIS) aboard the Solar Maximum Mission (SMM). These corrections include amplifier gain and collimator hole size variations across the field of view, amplifier/filter efficiency, variation in effective collimator hole size and angular response with photon energy, dead-time, and hard X-ray plate transmission. We also emphasise the substantial Poisson noise in these energy bands, and describe the maximum entropy deconvolution/correction routine we have developed to establish the spatial structure which can be reliably inferred from HXIS data.
Next we discuss the results of application of our routine to the three impulsive flare phases reported by Duijveman et al. (1982) as exhibiting hard X-ray ‘footpoints’, namely 1980, April 10, May 21, and November 5. Our main conclusions are:
-
(1)
Maximum entropy smoothing and Poisson noise data perturbations do not remove the main footpoint features in 16–30 keV nor change their basic morphology. However the results emphasise the asymmetry in footpoint size in the May 21 flare and confirm its possible presence in April 10. They also reveal the 3rd weak distant footpoint in the May 21 flare at an earlier time than found by Duijveman et al.
When the 16–22 and 22–30 keV bands are analysed separately, however, it is found that the footpoints are much less visible above noise in the harder band - i.e. the footpoint spectra are steep. In the April 10 and November 5 flares they are steeper than either the spectrum of intervening pixels or the spectrum at higher energies measured for the whole flare by the SMM Hard X-Ray Burst Spectrometer (HXRBS).
-
(2)
The footpoint contrast with surroundings is less than found by Duijveman et al., despite image deconvolution, because of the maximum entropy smoothing of noise.
-
(3)
The 16–30keV HXIS footpoint fluxes in the three flares are respectively 28%, 17%, and 15% of the simultaneous HXRBS flare power-law spectrum extrapolated into this energy range.
-
(4)
Where Poisson noise is taken into account we find, by cross-correlating pixel count rates, that footpoint synchronism was either not provable at all, or substantially less close than reported by Duijveman et al.
Next we considered the implications of these results for models of the footpoint emission. Contrary to Duijveman et al. we do not consider the HXIS ‘footpoint’ data as supporting a conventional thick target beam interpretation since:
-
(A)
The footpoint photon (and electron) fluxes are much less than expected from HXRBS extrapolation. This result casts doubt on recent models of chromospheric heating by electron beams which usually assume all of the HXRBS emission to come from HXIS footpoints.
-
(B)
The footpoint spectra for the April 10 and November 5 flares are much softer than the HXRBS spectrum and than the spectrum of intervening pixels, contrary to thick target predictions.
-
(C)
Contrary to Duijveman et al. footpoint synchronism does not demand an unreasonable Alfvén speed and so does not require non-thermal particles.
In spite of these objections we also re-considered the constraints placed on the acceleration site conditions in a beam interpretation by return current stability and footpoint contrast in the summed 16–30 keV range. Using the smoothed maximum entropy contrast and taking explicit account of coronal thermal emission, we find maximum densities somewhat larger than Duijveman et al. estimated, and much higher maximum values of T e /T i .
Regarding thermal interpretations we found:
-
(a)
Models involving continuous production of short-lived hot kernels in the arch top with Maxwellian tail electrons escaping to the footpoints could explain the 16–30 keV contrast with a rather higher energetic efficiency than a pure beam model. However, whatever the temperature distribution of hot kernel production, the model predicts footpoints harder than the arch summit, contrary to HXIS data.
-
(b)
A model with hot kernels produced in one limb of an arch can explain the asymmetry in footpoint size observed in May 21, and probably April 10, and is energetically even more efficient than (a) but is also inconsistent with the spectral data.
-
(c)
Finally we point out that HXIS footpoint data may be consistent with a purely geometric interpretation in an almost uniform arch filled with hot plasma.
10.
Krucker Säm Christe Steven Lin R.P. Hurford Gordon J. Schwartz Richard A. 《Solar physics》2002,210(1-2):445-456
The excellent sensitivity, spectral and spatial resolution, and energy coverage down to 3 keV provided by the Reuven Ramaty
High-Energy Solar Spectroscopic Imager mission (RHESSI) allows for the first time the detailed study of the locations and
the spectra of solar microflares down to 3 keV. During a one-hour quiet interval (GOES soft X-ray level around B6) on 2 May,
1:40–2:40 UT, at least 7 microflares occurred with the largest peaking at A6 GOES level. The microflares are found to come
from 4 different active regions including one behind the west limb. At 7′′ resolution, some events show elongated sources,
while others are unresolved point sources. In the impulsive phase of the microflares, the spectra can generally be fitted
better with a thermal model plus power law above ∼ 6–7 keV than with a thermal only. The decay phase sometimes can be fitted
with a thermal only, but in some events, power-law emission is detected late in the event indicating particle acceleration
after the thermal peak of the event. The behind-the-limb microflare shows thermal emissions only, suggesting that the non-thermal
power law emission originates lower, in footpoints that are occulted. The power-law fits extend to below 7 keV with exponents
between −5 and −8, and imply a total non-thermal electron energy content between 1026–1027 erg. Except for the fact that the power-law indices are steeper than what is generally found in regular flares, the investigated
microflares show characteristics similar to large flares. Since the total energy in non-thermal electrons is very sensitive
to the value of the power law and the energy cutoff, these observations will give us better estimates of the total energy
input into the corona. (Note that color versions of figures are on the accompanying CD-ROM.)
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022404512780 相似文献
11.
In a sample of strong RHESSI M-class flares we have made a study of the relationship between the `hardness' of the HXR spectrum
and the intensity in the 30–50 keV energy range. In all events we find clear evidence for a `soft–hard–soft' pattern of correlation
between hardness and flux, on time scales as short as 10 s. We investigate whether or not this pattern is intrinsic to the
acceleration mechanism. The RHESSI images in this energy range are dominated by footpoint brightenings, and we have searched
for a correlation between footpoint separation velocity and spectral hardness, to be compared qualitatively with theoretical
flare models. We find quite systematic footpoint motions, and also note that episodes in which footpoint separation varies
rapidly often correspond with episodes of significant change in the flare spectral index, though not as the simplest flare
models would predict. We report also on one of our events, on 14 March 2002, which exhibits highly sheared HXR footpoint ribbons
extending over a scale of 100 arc sec. For this flare we find a correlation between footpoint motion and hard X-ray flux.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022479610710 相似文献
12.
Two-dimensional evolutions of two flares of October 18, 1990 have been well observed in the Caii K line with a CCD camera at Norikura station of National Astronomical Observatory in Japan. There are two common characteristics for the flares: 3 - 5 min before the impulsive phase, the heating already begins at the footpoints of the flares, but no asymmetry in line emission has been detected. After the onset of the impulsive phase, Caii K line emission at the footpoints shows strong red asymmetry, with the maximum asymmetry occurring at the same time as the peak of the radio bursts. The maximum downward velocity is about 30 50 km s–1. For flare 1, blue and red asymmetries were observed in two sides of the footpoint area. They developed and attained a maximum nearly at the same time and the inferred Doppler velocities are comparable (30 40 km s–1). This implies that two mass jets started from a small region and ejected along a loop but in opposite directions with roughly equivalent momentum. A possible mechanism has been discussed. 相似文献
13.
The Yohkoh hard X-ray telescope (HXT) observed hard X-rays from the impulsive phase of a long-duration event (LDE) occurring on 14 July
2000. The Yohkoh soft X-ray telescope (SXT) and other instruments observed a large arcade, with width and length ∼30 000 km and ∼120 000 km,
respectively. In hard X-rays, for the first time, a two-ribbon structure was clearly observed in the energy range above 30 keV.
This result suggests that electrons are in fact accelerated in the whole system of this arcade, not merely in a particular
dominant loop. We analyzed the motions of bright kernels in the two hard X-ray ribbons in detail. Assuming these bright kernels
to be footpoints of newly reconnected loops, we infer from their motions that the loops reconnecting early are highly sheared,
while the loops reconnecting later are less sheared. We have also analyzed the hard X-ray spectra of the two ribbons independently.
At the outer edge of a ribbon, the spectrum tends to be harder than that in the inner edge. This suggests that higher-energy
electrons precipitate at the footpoints of outer loops and lower ones do at those of inner loops. We discuss what kind of
model can support this tendency. 相似文献
14.
A loop flare that occurred on 22 April 1993 near the disk center is examined using the Yohkoh Hard X-ray Telescope (HXT). We specifically looked into the faint early phase of the flare prior to the start of the strong impulsive phase. The pre-impulsive phase, though weak in intensity, is expected to contain essential clues to the mechanism of loop flares according to the causality principle, but it has not received attention previously, probably due to the insufficient dynamic range and cadence of observations by the instruments on earlier satellites. Observations with Yohkoh/HXT can clarify what occurs in this phase. This flare, like many other flares of this type, shows a relatively weak emission with a smooth and gradual increase during this pre-impulsive phase, followed by impulsive bursts, and then turns into a smooth decay phase without impulsive bursts. First, we found that the spectrum for the initial smooth rise part is consistent with a thin-thermal source at a temperature around 80 MK. Imaging of this phase in the HXT/L and M bands shows a single source between the footpoint sources that will come up in the impulsive phase following this phase, suggesting that this hyperhot source is located at a high part of the loop between the footpoints, since this flare takes a form of a loop. Furthermore, as we go up to the earliest times of the flare before this `hyperhot' source phase, two fainter sources are found near the footpoint sources that will appear later in the impulsive phase. The spectra of these sources at this earliest time of the flare, in contrast to the `hyperhot' source, cannot be determined from the HXT because the instrument was not in flare mode, and HXT/M1, M2, and H-band data are, unfortunately, not available at this very initial time. We can guess, however, that they are also of thermal character because the time profile is smooth without any spikes just as in the following `hyperhot' thermal phase, and in the post-impulsive `superhot' thermal phase coming up much later. These findings suggest that there is an important, and probably dynamic, early phase in loop flares that has been unnoticed in the still dark pre-impulsive phase, because the very early footpoint sources change into the loop top source in a matter of 20–30 s, comparable to the dynamic Alfvén time scale. Some implications of our new findings are discussed. 相似文献
15.
Using data from the Extreme Ultraviolet (EUV) Spectroheliometer onSkylab, we study the empirical characteristics of the variable emission in active regions. These simultaneous multi-wavelength observations
clearly confirm that active regions consist of a complex of loops at different temperatures. The variable emission from this
complex has very well-defined properties that can be quantitatively summarized as follows: (1) It is localized predominantly
around the footpoints where it occurs at discrete locations. (2) The strongest variability does not necessarily coincide with
the most intense emission. (3) The fraction of the area of the footpoints,δn/N, that exhibits variable emission, varies by ±15% as a function of time, at any of the wavelengths measured. It also varies
very little from footpoint to footpoint. (4) This fractional variation is temperature dependent with a maximum around 105 K. (5) The ratio of the intensity of the variable to the average background emission, δI/Ī, also changes with temperature. In addition, we find that these distinctive characteristics persist even when flares occur
within the active region. 相似文献
16.
A series of solar flares was observed near the same location in NOAA active region 8996 on 18–20 May 2000. A detailed analysis of one of these flares is presented where the emitting structures in soft and hard X-rays, EUV, H, and radio at centimeter wavelengths are compared. Hard X-rays and radio emission were observed at two separate loop footpoints, while soft X-rays and EUV emission were observed mainly above the nearby positive polarity region. The flare was confined although the observed type III bursts at the time of the flare maximum indicate that some field lines were open to the corona. No flux emergence was evident but moving magnetic features were observed around the sunspot region and within the positive polarity (plage) region. We suggest that the flaring was due to loop–loop interactions over the positive polarity region, where accelerated electrons gained access to the two separate loop systems. The repeated radio flaring at the footpoint of one loop was visible because of the strong magnetic fields near the large sunspot region while at the footpoint of the other loop the electrons could precipitate and emit in hard X-rays. The simultaneous emission and fluctuations in radio and X-rays – in two different loop ends – further support the idea of a single acceleration site at the loop intersection. 相似文献
17.
The multi-wavelength analysis is performed on a flare on September 9, 2002 with data of Owens Valley Solar Arrays (OVSA),
Big Bear Solar Observatory (BBSO), Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and Extreme UV Imager Telescope
(EIT), and The Michelson Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory (SOHO). The radio sources
at 4.8 and 6.2 GHz located in the intersection of two flaring loops at 195 of SOHO/EIT respectively with two dipole magnetic
fields of SOHO/MDI, in which one EIT loop was coincident with an X-ray loop of RHESSI at 12–25 keV, and two Hαbright kernels a1 and a2 of BBSO, respectively at the two footpoints of this loop; the second EIT loop connected another two
Hαkernels b1 and b2 and radio sources at 7.8 and 8.2 GHz of OVSA. The maximum phase of microwave bursts was evidently later
than that of hard X-ray bursts and Hαkernels a1 and a2, but consistent with that of Hαkernels b1 and b2. Moreover, the flare may be triggered by the interaction of the two flaring loops, which is suggested by
the cross-correlation of radio, optical, and X-ray light curves of a common quasi-periodic oscillation in the rising phase,
as well as two peaks at about 7 and 9 GHz of the microwave spectra at the peak times of the oscillation, while the bi-directional
time delays at two reversal frequencies respectively at 7.8 and 9.4 GHz (similar to the peak frequencies of the microwave
spectra) may indicate two reconnection sites at different coronal levels. The microwave and hard X-ray footpoint sources located
in different EUV and optical loops may be explained by different magnetic field strength and the pitch angle distribution
of nonthermal electrons in these two loops. 相似文献
18.
Using RHESSI and some auxiliary observations we examine possible connections between the spatial and temporal structure of
nonthermal hard X-ray (HXR) emission sources from the two-ribbon flares of 29 May 2003 and 19 January 2005. In each of these
events quasi-periodic pulsations (QPP) with time period of 1 – 3 minutes are evident in both hard X rays and microwaves. The
sources of nonthermal HXR emission are situated mainly at the footpoints of the flare arcade loops observed by TRACE and the
SOHO/EIT instrument in the EUV range. At least one of the sources moves systematically during and after the QPP phase in each
flare. The sources move predominantly parallel to the magnetic inversion line during the 29 May flare and along flare ribbons
during the QPP phase of both flares. By contrast, the sources start to show movement perpendicular to the flare ribbons with
velocity comparable to that along the ribbons’ movement after the QPP phase. The sources of each pulse are localized in distinct
parts of the ribbon during the QPP phase. The measured velocity of the sources and the estimated energy release rate do not
correlate well with the flux of the HXR emission calculated from these sources. The sources of microwaves and thermal HXRs
are situated near the apex of the flare loop arcade and are not stationary either. Almost all of the QPP as well as some pulses
of nonthermal HXR emission during the post-QPP phase reveal soft – hard – soft spectral behavior, indicating separate acts
of electron acceleration and injection. In our opinion at least two different flare scenarios based on the Nakariakov et al. (2006, Astron. Astrophys.
452, 343) model and on the idea of current-carrying loop coalescence are suitable for interpreting the observations. However,
it is currently not possible to choose between them owing to observational limitations. 相似文献
19.
A detailed analysis of characteristics of coronal mass ejections and flares associated with deca-hectometer wavelength type-II radio bursts (DH-CMEs and DH-flares) observed in the period 1997–2008 is presented. A sample of 62 limb events is divided into two populations known as after-flare CMEs (AF-CMEs) and before-flare CMEs (BF-CMEs) based on the relative timing of the flare and CME onsets. On average, AF-CMEs (1589 km s−1) have more speed than the BF-CMEs (1226 km s−1) and the difference between mean values are highly significant (P∼2%). The average CME nose height at the time of type-II start is at larger distance for AF-CMEs than the BF-CMEs (4.89 and 3.84 R o, respectively). We found a good anti-correlation for accelerating (R a=−0.89) and decelerating (R d=−0.78) AF-CMEs. In the case of decelerating BF-CMEs, the correlation seems to be similar to that for decelerating AF-CMEs (R d=−0.83). The number of decelerating AF-CMEs is 51% only; where as, the number of decelerating BF-CMEs is 83%. The flares associated with BF-CMEs have shorter rise and decay times than flares related to AF-CMEs. We found statistically significant differences between the two sets of associated DH-type-II bursts characteristics: starting frequency (P∼4%), drift rate (P∼1%), and ending frequency (P∼6%). The delay time analysis of DH-type-II start and flare onset times shows that the time lags are longer in AF-CME events than in BF-CME events (P≪1%). From the above results, the AF-CMEs which are associated with DH-type-II bursts are found to be more energetic, associated with long duration flares and DH-type-IIs of lower ending frequencies. 相似文献
20.
We present the first in-depth statistical survey of flare source heights observed by RHESSI. Flares were found using a flare-finding
algorithm designed to search the 6 – 10 keV count-rate when RHESSI’s full sensitivity was available in order to find the smallest
events (Christe et al. in Astrophys. J.
677, 1385, 2008). Between March 2002 and March 2007, a total of 25 006 events were found. Source locations were determined in the 4 – 10 keV,
10 – 15 keV, and 15 – 30 keV energy ranges for each event. In order to extract the height distribution from the observed projected
source positions, a forward-fit model was developed with an assumed source height distribution where height is measured from
the photosphere. We find that the best flare height distribution is given by g(h)∝exp (−h/λ) where λ=6.1±0.3 Mm is the scale height. A power-law height distribution with a negative power-law index, γ=3.1±0.1 is also consistent with the data. Interpreted as thermal loop-top sources, these heights are compared to loops generated
by a potential-field model (PFSS). The measured flare heights distribution are found to be much steeper than the potential-field
loop height distribution, which may be a signature of the flare energization process. 相似文献