Electron Trapping in Solar Flares explored with YOHKOH and COMPTON

fl242.aschwanden04
Posted:  16-Dec-96
Updated: 27-Jan-97 
Events specified: N/A

Markus Aschwanden (UMd) Takeo Kosugi (NOAO) Hugh Hudson (ISAS) Masanori Nishio (Nobeyama) Kentaro Yaji (Nobeyama)

It has been recently established that the time sturctures of hard X-rays consist of 2 components: (1) pulsed (sub-second) HXRs show energy-dependent time delays that are consistent with electron time-of-flight differences and are thus produced by directly precipitating electrons, and (2) the smoothly-varying HXR flux shows energy-dependent time delays that are consistent with electron collisional deflection times and are thus related to trapped-plus-precipitating electrons.

Based on this framework we propose the following studies:

measure trap densities in terms of the collisional trap model with

   Compton GRO and compare them with flare loop densities inferred from 
   the SXR emission measure determined from YOHKOH/SXT.
     decompose the HXR time profiles measured by CGRO into pulsed and smooth     
   component to infer information on the magnetic mirror ratio and 
   critical loss-cone angle.
    
 modeling of the asymmetric HXR flux at conjugate HXR footpoints (using     
   Yohkoh/HXT images) based on the magnetic mirror ratios inferred from
   the ratio of trapped/directly precipitating flux.
    
 modeling of coronal versus chromospheric thick-target HXR emission     
   based on the energy-dependent ratio of loop transit time to energy loss
   time 
    
 infer constraints on the location of the acceleration region from     
   the measured mirror ratio and extrapolated photospheric magnetic field
   at the HXR double footpoint sources.
    
 modeling of gyrosynchrotron emission (in microwaves) using trapped

particle distribution parameters inferred from CGRO delays and 17 GHz microwave maps obtained from Nobeyama.

Update 27-Jan-97

I submitted this proposal to the Yohkoh DUCs on October 25, which was then accepted and posted on December 9. In the meantime I submitted a first paper related to this proposal to ApJ, on November 20, from which I like to report the abstract below. This work completes the first point of the 6-point plan outlined in the proposal. The full manuscript is accessible by anonymous ftp:

Instructions to ftp from UMd site:

        %ftp ftp.astro.umd.edu          or  %ftp 129.2.163.100
        Name: anonymous
        Password: your@email.address
        ftp>cd /pub/markus
        ftp>get dens_paper.ps           (1.25 Mbytes)
        ftp>quit

THE ASTROPHYSICAL JOURNAL, submitted, 1996 Nov 20

Electron Trapping Times and Trap Densities in Solar Flare Loops Measured with Compton and Yohkoh

Markus J. Aschwanden, Robert M. Bynum, Takeo Kosugi, Hugh S. Hudson, Richard A. Schwartz

Abstract:

We measure energy-dependent time delays of <20-200 keV hard X-ray (HXR) emission from 78 flares simultaneously observed with the Compton Gamma Ray Observatory (CGRO) and Yohkoh. Fast time structures (<1 s) are filtered out, because their time delays have been identified in terms of electron time-of-flight (TOF) differences from directly precipitating electrons (Aschwanden et al. 1995a; 1996b). For the smooth HXR flux we find systematic time delays in the range of t_S=t(50 keV)-t(200 keV) ~ -(1...10) s, with a sign opposite to TOF delays, i.e. the high-energy HXRs lag the low-energy HXRs.

We interpret these time delays of the "smooth HXR flux" in terms of electron trapping and fit a model of the collisional deflection time t^Defl(E) ~ E^3/2 / n_e to the observed HXR delays to infer electron densities n_e^Trap in the trap. Independently we determine the electron density n_e^SXR in flare loops from soft X-ray (SXR) peak emission measures EM=int[ n_e^2 dh], using loop width (w) measurements to estimate the column depth dh ~ w. Comparing the two independent density measurements in HXR and SXR we find a mean ratio of q_e=n_e^Trap/n_e^SXR ~ 1, with a typical deviation by a factor of ~2 in both directions. This may be interpreted that the SXR-bright flare loops have generally a higher density than the trapping loops (when q_e < 1), but also have often filling factors smaller than unity (when q_e > 1). The measurements provide comprehensive evidence that electron trapping in solar flares is governed in the "weak diffusion limit", i.e. that the trapping time corresponds to the collisional deflection time, while pitch-angle scattering by resonant waves seems not to be dominant in the 20-200 keV energy range. The measurements do not support a second-step acceleration scenario for energies <200 keV.