fl241.aschwanden03 Posted: 12-Oct-96 Updated: 10-Nov-96 Events specified: N/A
Authors: Markus J. Aschwanden (Univ.Maryland) Arnold O. Benz (ETH Zurich) Saku Tsuneta (NAO Japan)
Abstract: We propose to analyse all solar flares simultaneously observed with Yohkoh/SXT and the radio spectrometer Phoenix of ETH Zurich. The broadband (0.1-3.0 GHz) capability of Phoenix allows us to probe electron beams over a range of plasma frequencies that cover electron densities from 10^8 to 10^11 cm^-3. By analyzing emission measure (EM) maps of flare loops imaged by Yohkoh/SXT we plan to determine the width w of flare loops, and to infer their electron densities n_e from the relation EM = n_e^2 * w. The motivation of this study is to compare the electron density in SXR-bright flare loops with that of the acceleration site, as inferred from the plasma frequency at the separatrix between upward and downward propoagating electron beams detected in broadband radio spectra. This test should allow us to decide whether the acceleration site is located inside or outside of the SXR-bright flare loop. The latter possiblity could be consistent with acceleration sites in the cusp region, as suggested in magnetic reconnection models. Additionally, we plan also to investigate the plasma frequency at high-frequency cutoffs of decimetric radio bursts, which provides a direct electron density measurment at the interface of chromospheric evaporation fronts (Aschwanden & Benz 1995, ApJ 438, p.997).
Update 10-Nov-96
Paper submitted to THE ASTROPHYSICAL JOURNAL, 1996 September 4
Title: Electron Densities in Solar Flare Loops, Chromospheric Evaporation Upflows, and Acceleration Sites
Authors:
Markus J. Aschwanden
Department of Astronomy, University of Maryland,
College Park, MD 20742;
e-mail: markus@astro.umd.edu
Arnold O. Benz
Institute of Astronomy, ETHZ,
CH-8092 Zurich, Switzerland;
e-mail: benz@astro.phys.ethz.ch}
Saku Tsuneta
Institute of Astronomy,
University of Tokyo,
Mitaka, Tokyo 181, Japan
Abstract:
We compare electron densities measured at three different locations in solar flares: (1) in soft X-ray (SXR)-bright flare loops, (2) in chromospheric evaporation upflows, (3) and in particle acceleration sites.
(1) The electron density in SXR-bright flare loops is determined from the SXR emission measure EM=n_e^2 dh and the geometric loop diameter from Yohkoh/SXT maps. SXR densities are found in the range of n_e^SXR= 0.2-2.5 10^11 cm^-3.
(2) The electron density in chromospheric evaporation upflows is inferred from plasma frequency cutoffs of decimetric radio bursts detected with the 0.1-3 GHz spectrometer Phoenix of ETH Zurich. These measurements yield electron densities of n_e^upflow = 0.3-11 10^10 cm^-3 (corresponding to plasma frequencies of 0.5-3.0 GHz).
(3) The electron density in acceleration sites is inferred from the plasma frequency at the separatrix between upward (type III bursts) and downward (reverse-drift bursts) accelerated electron beams, detected with Phoenix in a range of n_e^acc= 0.6-10 10^9 cm^-3 (corresponding to plasma frequencies of 200-900 MHz).
The comparison of these density measurements, obtained from 44 flare episodes (during 14 different flares), demonstrate the compatibility of flare plasma density diagnostics with SXR and radio methods. The density in the upflowing plasma is found to be somewhat lower than in the filled loops, having ratios in a range of n_e^upflow/n_e^SXR = 0.02-1.3. The acceleration sites are found to have a much lower density than the SXR-bright flare loops, i.e. n_e^acc/n_e^SXR = 0.005-0.13, and thus, must be physically displaced from the SXR-bright flare loops. The recently established scaling law between electron time-of-flight distances l' and loop half lengths s, i.e. l'/s= 1.4 +/- 0.3 (Aschwanden et al. 1996b), together with the observed simultaneity of HXR footpoint emission (Sakao 1994), suggest that the centroid of the acceleration region is located above the SXR-bright flare loop, as envisioned in cusp geometries (e.g. in magnetic reconnection models).