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Motion of 3-6 keV Nonthermal Sources Along the Legs of a Flare Loop  

Linhui Sui   Submitted: 2006-05-31 13:03

Observations of nonthermal X-ray sources are critical to studying electron acceleration and transport in solar flares. Strong thermal emission radiated from the pre-heated plasma before the flare impulsive phase often makes it difficult to detect low-energy X-ray sources that are produced by relatively low-energy nonthermal electrons. Knowledge of the distribution of these low-energy nonthermal electrons is particularly important in determining the total nonthermal electron energy in solar flares. We report on an 'early impulsive flare' in which impulsive hard X-ray emission was seen early in the flare before the soft X-ray emission had risen significantly, indicating limited plasma pre-heating. Early in the flare, RHESSI < 25 keV images show coronal sources that moved first downward and then upwards along the legs of a flare loop. In particular, the 3-6 keV source appeared as a single coronal source at the start of the flare, and then it envolved into two coronal sources moving down along the two legs of the loop. After nearly reaching the two footpoints at the hard X-ray peak, the two sources moved back up to the looptop again. RHESSI images and light curves all indicate that nonthermal emission dominated at energies as low as 3-6 keV. We suggest that the evolution of both the spectral index and the low-energy cutoff of the injected electron distribution could result in the accelerated electrons reaching a lower altitude along the legs of the dense flare loop and hence result in the observed downward and upward motions of the nonthermal sources.

Authors: Sui, L., Holman, G. D., and Dennis, B. R.
Projects: RHESSI

Publication Status: ApJ, Letter (in press)
Last Modified: 2006-05-31 16:14
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Observation of Loop Connectivity Change in a Solar Flare Triggered by Loop-Loop Interaction  

Linhui Sui   Submitted: 2005-09-30 08:37

Magnetic reconnection between two adjacent magnetic loops or arcades has been a longstanding model for solar flares. However, the observations supporting this quadrupolar flare model are mainly post-reconnection observations. We present a set of flare observations in EUV, X-rays and Há which shows that a pre-flare loop changed its connectivity soon after the start of an M1 flare on June 2, 2002. A large arcade of loops seen in EUV expanded slowly at ~6 km s-1 about 3 min before the start of the flare. Another smaller EUV loop expanded rapidly at ~100 km s-1 at the start of the flare. After ~20 s, the smaller loop then reconnected with magnetic loops within the arcade, resulting in two new post-reconnection loops. The newly-formed, smaller EUV loop, with a long cusp at the looptop, was also observed in X-rays. The newly-formed, larger loop, visible in EUV and Há, with a twisted structure and a jagged looptop, expanded outwards at ~490 km s-1. The 6-12 keV image during the first of two hard X-ray peaks showed the two footpoints of the newly-formed, smaller loop and a separate coronal source which was located much higher than the looptop seen later in the same energy band. We speculate that the coronal X-ray source was at or near the initial reconnecting point of the two interacting loops. All these observations indicate: (1) the loop-loop interaction was the trigger of the flare; (2) new loop systems do result from the magnetic reconnection of interacting loops; and (3) the cusp-shaped loops often seen in soft X-rays and EUV can be the product of the interaction of two loops.

Authors: Sui, L., Holman, G. D., Dennis, B. R.
Projects: RHESSI,Soho-MDI,TRACE

Publication Status: submitted to ApJ
Last Modified: 2005-09-30 08:37
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Multiwavelength Analysis of a Solar Flare on 2002 April 15  

Linhui Sui   Submitted: 2005-09-30 08:28

We carried out a multiwavelength analysis of the solar limb flare on 2002 April 15. The observations all indicate that the flare occurred in an active region with an asymmetric dipole magnetic configuration. The earlier conclusion that magnetic reconnection is occurring in a large-scale current sheet in this flare is further supported by these observations: (1) Several blob-like sources, seen in RHESSI 12-25 keV X-ray images later in the flare, appeared along a line above the flare loops. These indicate the continued presence of the current sheet and are likely to be magnetic islands in the stretched sheet produced by the tearing-mode instability. (2) A cusp-like structure is seen in NoRH 34 GHz microwave images around the time of the peak flare emission. We quantitatively demonstrate that the X-ray emitting thermal plasma seen with RHESSI had a higher temperature than the microwave emitting plasma seen with NoRH. Since the radio data preferentially see cooler thermal plasma, this result is consistent with the picture in which energy release occurs at progressively greater heights and the hard X-rays see hot new loops while the radio sees older cooling loops. The kinetic energy of the coronal mass ejection (CME) associated with this flare was found to be about one order of magnitude less than both the thermal energy in the hot plasma and the nonthermal energy carried by the accelerated electrons in the flare, as deduced from the RHESSI observations. This contrasts with the higher CME kinetic energies typically deduced for large flares.

Authors: Sui, L, Holman, G. D., White, S. M., Zhang, J.
Projects: RHESSI,Soho-MDI,Soho-LASCO,TRACE

Publication Status: ApJ, in press (Issue of Novermber 10)
Last Modified: 2005-09-30 08:28
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Determination of Low-Energy Cutoffs and Total Energy of Nonthermal Electrons in a Solar Flare on 2002 April 15  

Linhui Sui   Submitted: 2005-03-09 08:28

The determination of the low-energy cutoff to the spectrum of accelerated electrons is decisive for the estimation of the total nonthermal energy in solar flares. Because thermal bremsstrahlung dominates the low-energy part of flare X-ray spectra, this cutoff energy is difficult to determine with spectral fitting alone. We have used a new method that combines spatial, spectral, and temporal analysis to determine the cut-off energy for the M1.2 flare observed with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on 2002 April 15. A low-energy cutoff of 24~(pm2)~keV is required to ensure that the assumed thermal emissions always dominate over nonthermal emissions at low energies (< 20 keV) and that the spectral fitting results are consistent with the RHESSI light curves and images. With this cut-off energy, we obtain a total nonthermal energy in electrons of 1.6~(pm1) imes1030 ergs that is comparable to the peak energy in the thermal plasma, estimated from RHESSI observations to be 6 (pm0.6) imes1029 ergs assuming a filling factor of one.

Authors: Sui, L, Holman, G. D., Dennis, B. R.
Projects: RHESSI

Publication Status: ApJ (in press)
Last Modified: 2005-03-09 08:28
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Modeling Solar Flare Hard X-ray Images and Spectra Observed with RHESSI  

Linhui Sui   Submitted: 2005-02-02 11:48

Observations obtained with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) of a flare on February 20, 2002 indicate a hard X-ray (HXR) coronal source at or near the top of a flare loop (called a HXR looptop source). The existence of the HXR looptop source suggests that magnetic reconnection, which is believed to power flares, occurs above the loop. In order to explain this HXR looptop source, I created a steady-state particle transport model, in which high-energy electrons are continuously injected at the top of a semicircular flare loop. Based on the simulation results, I find that the model predictions are consistent with the RHESSI observations in many respects, but the spectrum of the looptop source obtained from the model is steeper than that from the RHESSI data. This suggests that, instead of being accelerated above the loop as generally believed, the particles might be accelerated in the looptop itself. RHESSI observations of three other homologous flares that occurred between April 14 and 16, 2002, provide strong evidence for the presence of a large-scale current sheet above a flare loop, which is the basis of standard flare models. The most convincing finding is the presence of the temperature distribution of a separate coronal source above the flare loops: the hotter part of the coronal source was located lower in altitude than the cooler part. Together with the fact that the hotter flare loops are higher than the cooler loops, the observations support the existence of a large-scale current sheet between the top of the flare loops and the coronal source above. Blob-like sources along a line above the loop in the decay phase of the April 15, 2002, flare, which are suggestive of magnetic islands initiated by the tearing-mode instability, and the observation of a cusp structure in microwaves, further support the presence of the current sheet. The observations of the three homologous flares reveal two other features which are beyond the predictions of the standard flare models: the downward motion of flare loops in the early impulsive phase of each flare, and an initially stationary coronal source above the loops. These features are believed to be related to the formation and development of a current sheet. In particular, the downward loop motion seems to be a common phenomenon in flares, suggesting the necessity for modifications to the existing standard flare models. Finally, thanks to the broad energy coverage of the RHESSI spectra, a low-energy cutoff of 28~(pm2)~keV in the nonthermal electron distribution was determined for the April 15, 2002, flare. As a result, the energy carried by the nonthermal electrons is found to be comparable to the thermal energy of the flare, but one order of magnitude larger than the kinetic energy of the associated coronal mass ejection. The method used to deduce the electron low-energy cutoff will be useful in the analyses of similar events.

Authors: Linhui Sui
Projects: RHESSI

Publication Status: Doctoral Dissertation (Catholic University of America), 2005, NASA Technical Memorandum (2005-212776)
Last Modified: 2005-02-02 11:48
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Evidence for the Formation of a Large-Scale Current Sheet in a Solar Flare  

Linhui Sui   Submitted: 2003-09-09 12:23

We present X-ray evidence for the formation of a large-scale current sheet in a flare observed by the Ramaty High Energy Solar Spectroscopic Imager on 2002 April 15. The flare occurred on the northwest limb, showing a cusp-shaped flare loop in the rise phase. When the impulsive rise in hard X-rays (>25 keV) began, the cusp part of the coronal source separated from the underlying flare loop and remained stationary for about 2 minutes. During this time the underlying flare loops shrank at ~9 km s-1. The temperature of the underlying loops increased towards higher altitudes, while the temperature of the coronal source increased towards lower altitudes. These results indicate that a current sheet formed between the top of the flare loops and the coronal source during the early impulsive phase. After the hard X-ray peak, the flare loops grew outward at ~8 km s-1, and the coronal source moved outward at ~300 km s-1, indicating an upward expansion of the current sheet. About 30 minutes later, post-flare loops seen in the SOHO Extreme Ultraviolet Imaging Telescope 195 A passband rose at ~10 km s-1. A large coronal loop-like structure, observed by the SOHO Large Angle and Spectrometric Coronagraph C2 and C3 detectors, also propagated outward at ~300 km s-1. These observations are all consistent with the continued expansion of the current sheet.

Authors: Sui, L, Holman, G. D.
Projects: RHESSI

Publication Status: ApJ Letter (in press, issue of Oct. 20)
Last Modified: 2003-09-09 12:25
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Abstracts by Author
Motion of 3-6 keV Nonthermal Sources Along the Legs of a Flare Loop
Observation of Loop Connectivity Change in a Solar Flare Triggered by Loop-Loop Interaction
Multiwavelength Analysis of a Solar Flare on 2002 April 15
Determination of Low-Energy Cutoffs and Total Energy of Nonthermal Electrons in a Solar Flare on 2002 April 15
Modeling Solar Flare Hard X-ray Images and Spectra Observed with RHESSI
Evidence for the Formation of a Large-Scale Current Sheet in a Solar Flare

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