Flare Footpoints Research Articles

Traces of the Dynamic Current Sheet during a Solar Flare

High-cadence and high-resolution time sequences of far Halpha off-band images provide a unique tool to study the evolution of the fine structure of flare kernels. The fine structure contains important information on flare topology and the triggering mechanism. In this Letter, we concentrate on the rapid changes of the relative positions of two conjugate flare footpoints. In order to carry out this study with the highest physical precision, we use r(c) = Sigmar(j)I(j)/SigmaI(j) (I-j is the Halpha brightness at r(j)) to compute the centroid of an Halpha bright kernel region caused by solar flares. Using this, we probe the fine temporal structures connected to the distance between the centroids of two conjugate kernels of an M2.3 flare. The flare, which occurred on 2002 September 9 in NOAA Active Region 0105, was observed at Big Bear Solar Observatory at the far off-band center wavelength of Halpha - 1.3 Angstrom, with a cadence of similar to40 ms. The flare was also observed by RHESSI. The time profile of the separation distance shows an excellent anticorrelation to that of the hard X-ray (HXR) emissions in 25-50 keV, which exhibit a number of separate spikes ( the linear Pearson correlation coefficient is found to be similar to-0.83). The separation between the two centroids decreases at the rising periods of four HXR spikes, then it increases after the peak time of the flare to show the expected separation motion. The most obvious decreasing, which occurred during the first HXR peak, was confirmed by corresponding images. This implies that during the impulsive phases, the energy transported from the corona is deposited increasingly inwardly between the two kernels. This new and perhaps surprising tendency for the energy deposition can be explained as being caused by current sheet pinch motions, which, at the same time, enhance the magnetic energy reconnection rate to produce the observed HXR spikes.

H Dimmings Associated with the X1.6 Flare and Halo Coronal Mass Ejection on 2001 October 19

As the optical counterpart of coronal dimmings, we present the first direct evidence of Halpha associated with the X1.6 flare, filament eruption, and coronal mass ejection (CME) that occurred on 2001 October 19. As two main Halpha flare ribbons parallel to the erupting filament slowly separated, two other ribbons first quickly expanded near the feet of the filament, then their interiors substantially dimmed and became darker than the preflare brightness. This formed two dimming regions with sharp bright edges. The two Halpha were different from the flare nimbus and the Halpha darkenings found by Ellison, McKenna, & Reid and Neidig et al., respectively, but similar to the coronal double dimmings studied by Sterling & Hudson. The coronal of the event were also observed in 171 Angstrom images from the Transition Region and Coronal Explorer (TRACE). The 171 Angstrom and Halpha showed very similar evolutions and occurred at the same two sites with opposite magnetic polarities. They formed in the course of the filament eruption, and after the eruption, some 171 Angstrom loops of the postflare arcade developed to connect their inner edges while some loops with footpoints near their outer edges disappeared. Observations from the Solar and Heliospheric Observatory Extreme-ultraviolet Imaging Telescope (SOHO/EIT) showed more extended associated with the eruption. Only a small portion of EIT was cospatial with the Halpha dimmings, while the major other portion was associated with the disappeared 171 Angstrom loops. These joint observations are interpreted by using the flux rope model of CMEs, in which the Halpha represent the evacuated feet of the erupted flux rope. The mass supply of the halo CME could be coming from both the erupted flux rope itself and the lost material close to the flare footpoint regions as shown by Halpha dimmings.

Motion of Flare Footpoint Emission and Inferred Electric Field in Reconnecting Current Sheets

A systematic motion of Hα kernels during solar flares can be regarded as the chromospheric signature of progressive magnetic reconnection in the corona, in that the magnetic field lines swept through by the kernel motion are those connected to the diffusion region at the reconnection point. In this paper, we present high-cadence and high-resolution Hα-1.3 A observations of an impulsive flare that exhibits a systematic kernel motion and relate them to the reconnecting current sheet (RCS) in the corona. Through analyses of X-ray and microwave observations, we further examine the role of the macroscopic electric field inside the RCS in accelerating electrons. We measure the velocity of the kernel motion to be 20-100 km s-1. This is used together with the longitudinal magnetic field to infer an electric field as high as 90 V cm-1 at the flare maximum. This event shows a special magnetic field configuration and motion pattern of Hα kernels, in that a light bridge divides a flare kernel into two parts that move in different manners: one moving into the stronger magnetic field and the other moving along the isogauss contour of the longitudinal magnetic field. The temporal variation of the electric field inferred from the former type of kernel motion is found to be correlated with 20-85 keV hard X-ray light curves during the rise of the major impulsive phase. This would support the scenario of magnetic energy release via current dissipation inside the RCS, along with the hypothesis of the DC electric field acceleration of X-ray-emitting electrons below 100 keV. However, there is no good temporal correlation between the hard X-ray emission and the inferred electric field from the other motion pattern. Furthermore, the microwave emission, which supposedly comes from higher energy electrons, shows a time profile and electron spectrum that differs from those of the X-ray bursts. We conclude that either the two-dimensional magnetic reconnection theory related to the Hα kernel motion is applicable to only some part of the flare region due to its special magnetic geometry, or the electron acceleration is dominated by other mechanisms depending on the electron energy.

Ultraviolet Flare Ribbon Brightenings and the Onset of Hard X-Ray Emission

The broadband UV images taken by the Transition Region and Coronal Explorer (TRACE) provide a unique opportunity to observe transition region and chromospheric emission from flare footpoints at high spatial and temporal resolution. In this Letter, we present comparisons of UV flare footpoint evolution with hard X-ray light curves from the Yohkoh Hard X-Ray Telescope and the Compton Gamma Ray Observatory Burst and Transient Source Experiment (BATSE). The nine events analyzed in this Letter all show evidence for flare footpoint brightenings that precede the onset of the hard X-ray emission. Regression analysis between TRACE UV data taken at very high cadence (2-3 s) and hard X-ray light curves, however, shows that the initial hard X-ray burst is positively correlated only with footpoints that show no pre-hard X-ray activity. This indicates that energy release during the preflare and impulsive phases of the flare is occurring on different loops.

Analysis of evolution of NOAA 7912 active region on 19 October 1995

An analysis of a minor (B3.2) flare related to an X-ray jet in the reversed polarity NOAA 7912 active region is presented, focusing on various kinds of activity observed in the H line with the MSDP instrument. Using complementary Yohkoh soft X-ray observations and a Kitt Peak magnetic eld map we study the influence of a rather low and graduate energy release on laments, brils and chromospheric brightenings and their relevant coronal fea- tures. We nd that this small flare aected the entire AR and was associated with strong up- and downflows along remote brils and laments, which appear to be connected by large-scale loops. It is shown that at least one of the observed brightenings was caused by downflow of the mat- ter, thus, by the kinematic heating of the material. The lament in the vicinity of the flare footpoints changed the most: it broke into two parts, temporarily rose to higher altitudes inside the AR and even erupted outside of the AR, re-forming only three hours later. We show that even a minor flare can lead to important morphological and dynamical changes in an active region.

Theoretical Study of the Spatially Resolved Soft X‐Ray versus Hard X‐Ray Relationship in Solar Flares

We have carried out a series of calculations of the hard (>30 keV) and soft (1-3 keV) X-ray time histories for various parts of a solar flare loop using hydrodynamic simulations of the electron-heated flare atmosphere. We have considered two typical cases: the strongly evaporated flare in which the injected energetic electron flux is large and the initial atmospheric density is small, and the weakly evaporated flare in which the electron flux is small and the initial atmospheric density is large. The spatially unresolved Neupert effect that the time of the soft X-ray time history is proportional to the hard X-ray time history holds for these models; however, in both cases, we find that the footpoint soft X-ray time history is not proportional to the hard X-ray time history at the same location. The model cannot explain recent observations in which a correlation between the soft and hard X-ray time histories (and not a derivative relation) at the flare footpoint was observed. Furthermore, the loop-top soft X-ray source decays much earlier than indicated by observations. In order for these model predictions to be consistent with observations, the footpoint soft X-ray source may have to be nonthermal, excited by low-energy electrons. Also, continuous heating of the plasma at the top of the loop is required in addition to that from energetic electrons.