Gamma-ray spectroscopy provides a wealth of information about acceler-ated particles in solar flares, as well as the ambient medium with which these energetic particles interact. The neutron capture line (2.223 MeV), the strongest in the solar gamma-ray spectrum, forms in the deep atmosphere. The energy of these photons can be reduced via Compton scattering. With the fully relativistic GEANT4 toolkit, we have carried out Monte Carlo simulations of the transport of a neutron capture line in solar flares, and applied them to the flare that occurred on 2005 January 20 (X7.1/2B), one of the most powerful gamma-ray flares observed by RHESSI during the 23rd solar cycle. By comparing the fitting results of different models with and without Compton scattering of the neutron capture line, we find that when including the Compton scattering for the neutron capture line, the observed gamma-ray spectrum can be reproduced by a population of accelerated particles with a very hard spectrum (s ≤ 2.3). The Compton effect of a 2.223 MeV line on the spectra is therefore proven to be significant, which influences the time evolution of the neutron capture line flux as well. The study also suggests that the mean vertical depth for neutron capture in hydrogen for this event is about 8 g cm 2 .
Using the visual inspection and base difference method and data from the X-ray Telescope (XRT) onboard Hinode and TRACE with improved spatial and temporal resolution, we selected 48 X-ray transient brightenings (XTBs) and 237 EUV transient brightenings (ETBs) to study the connection between these two types of transient brightenings (TBs). These ETBs and XTBs have smaller areas (8.42 Mm 2 and 36.3 Mm 2 , respectively, on average) and shorter durations (9.0 min and 6.9 min, respectively, on average) than previous studies. These XTBs show three types of morphological structure: point-like, single-loop and multiple-loop. We find only 20% of the ETBs have corresponding XTBs while the other 80% have no X-ray signatures at all. This is presumably due to the small amount of released energy, which is not enough to heat the plasma to coronal temperatures which produce X-ray emission rather than being due to the limitation of spatial resolution and temperature sensitivity of the X-ray instrument. These small ETBs may significantly contribute to the coronal heating.
Among the RHESSI flare samples,we concentrated on a kind of flare that presents two successive peaks(that is,it presents both an impulsive phase and a gradual phase)in 12-25 keV light curves.Taking the C1.4 flare on 2002 August 12 as an example,we studied the light curves,spectra,and images in detail.Making full use of the capabilities of RHESSI,we showed some evidence to support the expected causal relationship between these two peaks:the first peak is mainly nonthermal,while the second peak is mainly thermal;the energy carried by nonthermal electrons during the first peak seems to be comparable to the thermal energy of the second peak.The morphologies of X-ray images and their evolutions provide additional evidence for this causality.We conclude that two such peaks in the 12-25 keV light curve are good evidence for the chromospheric evaporation.However,the maximum time of the second peak is later than the end time of the first peak,suggesting that for some events,a modification of the traditional Neupert effect could be necessary by inclusion of a time delay,which might be partly related to the filling of the loop by evaporated material.
Anomalous resistivity is critical for triggering fast magnetic reconnection in the nearly collisionless coronal plasma.Its nonlinear dependence on bulk drift velocity is usually assumed in MHD simulations.However,the mechanism for the production of anomalous resistivity and its evolution is still an open question.We numerically solved the one dimension Vlasov equation with the typical solar coronal parameters and realistic mass ratios to infer the relationship between anomalous resistivity and bulk drift velocity of electrons in the reconnecting current sheets as well as its nonlinear characteristics.Our principal findings are summarized as follows:1) the relationship between the anomalous resistivity and bulk drift velocity of electrons relative to ions may be described as ηmax = 0.03724 vvde 5.702 Ω m for vd/ve in the range of 1.4-2.0 and ηmax = 0.8746 vvde 1.284 Ω m for vd/ve in the range of 2.5-4.5;2) if drift velocity is just slightly larger than the threshold of ion-acoustic instability,the anomalous resistivity due to the wave-particle interactions is enhanced by about five orders as compared with classic resistivity due to Coulomb collisions.With the increase of drift velocity from 1.4ve to 4.5ve,the anomalous resistivity continues to increase 100 times;3) in the rise phase of unstable waves,the anomalous resistivity has the same order as the one estimated from quasi-linear theory;after saturation of unstable waves,the anomalous resistivity decreases at least about one order as compared with its peak value;4) considering that the final velocity of electrons ejected out of the reconnecting current sheet(RCS) decreases with the distance from the neutral point in the neutral plane,the anomalous resistivity decreases with the distance from the neutral point,which is favorable for the Petschek-like reconnection to take place.
With RHESSI data from five solar flares taken from beginning to end,we investigate the power conversion factorμdefined as the ratio of the time derivative of total thermal energy(ERHESSI+Erad+Econd)and the kinetic power(PRHESSI)of nonthermal electrons.Here, ERHESSI is the computed energy contained in thermal plasmas traced by RHESSI SXRs.Other two contributions(Erad and Econd)to the total energy are the energies lost through radiation and conduction,both of which can be derived from the observational data.If both are not considered,μis only positive before the SXR maximum.However,we find that for each flare studiedμis positive over the whole duration of the soalr flare after taking into account both radiation and conduction.Mean values forμrange from 11.7% to 34.6%for these five events,indicating roughly that about this fraction of the known energy in nonthermal electrons is efficiently transformed into thermal energy from start to end.This fraction is traced by RHESSI SXR observations;the rest is lost.The bulk of the nonthermal energy could heat the plasma low in the atmosphere to drive mass flows(i.e.chromospheric evaporation).
With an extensive analysis,we study the temporal evolution of magnetic flux during three successive M-class flares in two adjacent active regions:NOAA 10039 and 10044.The primary data are full disk longitudinal magnetograms observed by SOHO/MDI.All three flares are observed to be accompanied by magnetic flux changes.The changes occurred immediately or within 1 ～ 10 minutes after the starting time of the flares,indicating that the changes are obvious consequences of the solar flares.Although changes in many points are intrinsic in magnetic flux,for some sites,it is caused by a rapid expansion motion of magnetic flux.For the second flare,the associated change is more gradual compared with the 'step-function' reported in literature.Furthermore,we use the data observed by the Imaging Vector Magnetograph(IVM) at Mees Solar Observatory to check possible line profile changes during the flares.The results from the IVM data confirm the flux changes obtained from the MDI data.A series of line profiles were obtained from the IVM's observations and analyzed for flux change sites.We find that the fluctuations in the width,depth and central wavelength of the lines are less than 5.0 even at the flare's core.No line profile change is observed during or after the flare.We conclude that the magnetic field changes associated with the three solar flares are not caused by flare emission.
We statistically study the properties of emerging flux regions(EFRs)and response of the upper solar atmosphere to the flux emergence using data from the Helioseismic and Magnetic Imager and the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory.Parameters including total emerged flux,flux growth rate,maximum area,duration of the emergence and separation speed of the opposite polarities are adopted to delineate the properties of EFRs.The response of the upper atmosphere is addressed by the response of the atmosphere at different wavelengths (and thus at different temperatures).According to our results,the total emerged fluxes are in the range of(0.44-11.2)×1019 Mx while the maximum area ranges from 17 to 182 arcsec2.The durations of the emergence are between 1 and 12 h,which are positively correlated to both the total emerged flux and the maximum area.The maximum distances between the opposite polarities are 7-25 arcsec and are also positively correlated to the duration.The separation speeds are from 0.05 to 1.08 km s-1,negatively correlated to the duration.The derived flux growth rates are(0.1-1.3)×1019 Mx h-1, which are positively correlated to the total emerging flux.The upper atmosphere first responds to the flux emergence in the 1600Achromospheric line,and then tens to hundreds of seconds later,in coronal lines,such as the 171(T=105.8 K)and 211(T=106.3 K)lines almost simultaneously,suggesting the successive heating of the atmosphere from the chromosphere to the corona.
We aim to investigate the influence of plasma instability on electron acceleration and heating near the neutral point of a turbulent reconnecting current sheet (RCS).Through numerically solving the one dimensional relativistic Vlasov equation with typical solar coronal parameters and a realistic mass ratio in the presence of a strong inductive electric field E0,we suggest that the wave-particle scattering may produce a flat electron flux spectrum from thermal to nonthermal electrons without a sudden low-energy cutoff in the acceleration region.The ratio between electron heating and acceleration decreases with the increase of the induced electric field.It is about one for E0=1 V cm-1 and one fourth for E0=10 V cm-1.The unstable waves excited by the beam plasma instability first accelerate the electrons,then trap these electrons from further acceleration by an induced electric field through wave-particle resonant interactions.
With the aim of studying the relationship between the relative motions of the loop-top (LT) source and footpoints (FPs) during the rising phase of solar flares, we give a detailed analysis of the X7.1 class flare that occurred on 2005 January 20. The flare was clearly observed by RHESSI, showing a distinct X-ray flaring loop with a bright LT source and two well-defined hard X-ray (HXR) FPs. In particular, we correct the projection effect for the positions of the FPs and magnetic polarity inversion line. We find that: (1) The LT source showed an obvious U-shaped trajectory. The source of the higher energy LT shows a faster downward/upward speed. (2) The evolution of FPs was temporally correlated with that of the LT source. The converging/separating motion of FPs corresponds to the downward/upward motion of the LT source. (3) The initial flare shear of this event is found to be nearly 50 degrees, and it has a fluctuating decrease throughout the contraction phase as well as the expansion phase. (4) Four peaks of the time profile of the unshearing rate are found to be temporally correlated with peaks in the HXR emission flux. This flare supports the overall contraction picture of flares: a descending motion of the LT source, in addition to converging and unshearing motion of FPs. All results indicate that the magnetic field was very highly sheared before the onset of the flare.
Tuan-Hui ZhouJun-Feng WangDong LiQi-Wu SongVictor MelnikovHai-Sheng Ji
