fl122.zarro01 Posted: 23-Apr-93 Updated: 26-Nov-93, 09-Mar-94, 27-Nov-94 Events specified: N/A
Collaborators: D. Zarro (GSFC, zarro@umbra.gsfc.nasa.gov) and J. T. Marska (NRL, mariska@aspen.nrl.navy.mil)
Motivation: Different flare heating and transport models predict different evolutions of the hard and soft X-ray emission during the early phases of a solar flare. In particular, in the thick target model, one would expect there to be a change in the hard X-ray spectrum with time at the lowest energies in response to the movement of plasma up the magnetic field lines from the location where the most energetic electrons deposit their energy. This study will use high spectral resolution BATSE data from GRO in combination with Yohkoh BCS and SXT data to study the evolution of the hard and soft X-ray emissions early in flares.
Required observations/Analysis Techniques: We will be analyzing Yohkoh BCS and SXT data for several flares observed simultaneously by GRO/Compton. One such event that was well-observed by both spacecraft occurred at 0910 UT on 1992 September 6. This event shows evidence for blueshifted Ca XIX emission that is temporally correlated with the hard X-ray burst emission detected by the BATSE Large Area Detectors (LADs). By comparing the evolution of the soft X-ray blueshift amplitude with the slope of the hard X-ray spectrum above 25 kev, we intend to discriminate between different flare transport models such as the thick-target and and electric-field acceleration models. For example, in the latter model, the soft X-ray blueshift is expected to correlate with the appearance of a high-energy spectral break due to a field-aligned electric field component.
Update 27-Nov-94
The work described in this proposal is complete and will be published in ApJ in February 1995. See update of 9-Mar-94 for abstract.
Update 09-Mar-94
The following paper was submitted to Astrophysical Journal Letters. The paper describes the results of interpreting hard and soft X-ray observations of a solar flare using the DC-electric field model. The next step in our analysis is to analyze additional flares that were observed jointly by Yohkoh and CGRO. We will concentrate on the spectral evolution of soft and hard X-rays and compare with predictions made by the nonthermal thick-target and runaway acceleration models. A proposal will be submitted to the Cycle 4 CGRO Guest Investigator program in order to secure additional BATSE hard X-ray spectral data.
TESTING THE DC-ELECTRIC FIELD MODEL IN A SOLAR FLARE OBSERVED BY YOHKOH AND THE COMPTON GAMMA-RAY OBSERVATORY
D.M. ZARRO, (Applied Research Corp., Solar Data Analysis Center, Code 682 NASA/GSFC, Greenbelt, MD 20771)
J.T. MARISKA, (Naval Research Laboratory, Code 7673 Washington, DC 20375)
B.R. DENNIS (Laboratory for Astronomy and Solar Physics, Code 682.2 NASA/GSFC, Greenbelt, MD 20771)
ABSTRACT
We apply a DC-electric field model to the analysis of soft and hard X-ray observations of a solar flare observed by Yohkoh and the Compton Gamma-Ray Observatory on 1992 September 6. The flare was observed in Ca XIX by the Yohkoh Bragg Crystal Spectrometer (BCS) and simultaneously in hard X-rays by the CGRO Burst and Transient Spectrometer Experiment (BATSE). A strong stationary component of Ca XIX emission was present at the start of impulsive hard X-ray emission indicating an extended phase of heating prior to the production of energetic nonthermal electrons. We interpret the preflare Ca XIX emission as a signature of Joule heating by field-aligned currents. We relate the temporal variation of impulsive hard X-ray emission to the rate of runaway electron acceleration by the DC-electric field associated with the current. We find that: (1) the initial increase in hard X-ray emission is consistent with a DC-electric field strength that increased from a preflare value of E ~ 10^(-5) volts cm^(-1) to a peak value of E ~ 9 x 10^(-5) volts cm^(-1) and which remained constant during the impulsive phase; and (2) the decrease in hard X-ray emission after flare maximum is consistent with a decrease in the number of runaway electrons due to an increase in coronal density produced by chromospheric evaporation. The increased density quenches the runaway process by enhancing collisional thermalization of electrons. We conclude that the DC-electric model provides a viable tool for studying the energetics and dynamics of solar flares. Our application of the DC-electric field model provides a new diagnostic for measuring coronal electric field strength and its evolution throughout a flare.
Update 26-Nov-93
We have been analyzing solar flares that show evidence for strong stationary Ca~XIX emission at the start of impulsive hard X-rays. A good example is an M3.3 event that occurred at 0857~UT on 1992~September~6 in active region AR~7270. We focussed on this event because it was observed in Ca~XIX by Yohkoh BCS and simultaneously in hard X-rays by the BATSE spectroscopy detectors on Compton GRO. A strong stationary component of Ca~XIX emission and a much weaker blueshifted component were detected during the early onset phase of impulsive hard X-ray emission.
We have performed spectral synthesis to derive a temperature $T \approx 10 \times 10^6$~K and an emission measure $EM \approx 7 \times 10^{47}$~cm$^{-3}$ for the coronal plasma at the onset of hard X-rays. We have also examined simultaneous Yohkoh SXT Be-filter images to infer the geometry of this event. These images imply a simple loop structure for the main soft X-ray source, with a characteristic cross-sectional area of $A \sim 10^{17}$~cm$^2$ and a half-length $L \sim 10^9$~cm. From the soft X-ray data we have derived the variation of electron density in the flare.
The detection of strong stationary soft X-ray emission before the peak of hard X-rays cannot be explained easily by the standard thick-target driven chromospheric evaporation model. In particular, the latter model predicts that a significant stationary component will not develop until after there has been sufficient heating and evaporation by nonthermal electrons. To explain these observations, we are investigating a DC-electric field model in which preflare thermal Ca~XIX emission is produced by current heating dominating early in the flare, and nonthermal HXR emission is produced by electron runaway acceleration dominating during the impulsive phase. Application of this model to the above flare observations has allowed us to derive the strength and temporal variation of the electric field. In particular, we find that the soft and hard X-ray time variations can be be explained by an electric field strength that increases from below .01 volts~cm$^{-1} during the preflare to a constant value of 0.1 volts~cm$^{-1}$ at the peak of hard X-rays. The next step in our analysis is to compare the hard X-ray spectrum that is produced by electric-field runaway acceleration with high resolution spectral observations from BATSE.
We have presented preliminary results of our study at the Yohkoh symposium: "A New Look at the Sun", that was held in Kofu, Japan. We are presently preparing a paper for submission to Ap.J. letters, and are examining additional flares for which we have joint Yohkoh/GRO observations.