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Impulsive behavior in solar soft X-radiationThe Yohkoh soft X-ray telescope has observed impulsive, thermal, soft X-ray emission at the footpoints of magnetic loops during solar flares. The soft X-ray (thermal) time profiles at the footpoints closely match the hard X-ray (nonthermal) time profiles, directly demonstrating the heating of the lower solar atmosphere on short timescales during the interval of nonthermal energy release. This phenomenon is the rule, rather than the exception, occurring in the majority of flares that we have examined with the Yohkoh data. We illustrate the impulsive behavior with data from the major flare of 1992 January 26. For this flare, the soft X-ray peak times matched the hard X-ray peak times within the time resolution of the soft X-ray measurements (about 10 s), and the soft and hard X-ray locations match within the resolution of the hard X-ray imager. The impulsive soft X-ray emission clearly has a thermal spectral signature, but not at the high temperature of a 'superhot' source. We conclude that the impulsive soft X-ray emission comes from material heated by precipitating electrons at loop footpoints and evaporating from the deeper atmosphere into the flaring flux tube.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Hudson, H. S.
(University of Hawaii, Honolulu, HI United States)
Strong, K. T.
(Lockheed Palo Alto Research Laboratory, Palo Alto, CA United States)
Dennis, B. R.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Zarro, D.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Inda, M.
(University of Tokyo Tokyo, Japan)
Kosugi, T.
(National Astronomical Observatory of Japan Tokyo, Japan)
Sakao, T.
(National Astronomical Observatory of Japan Tokyo, Japan)
Date Acquired
August 16, 2013
Publication Date
February 10, 1994
Publication Information
Publication: Astrophysical Journal, Part 2 - Letters
Volume: 422
Issue: 1
ISSN: 7004-637X
Subject Category
Solar Physics
Accession Number
Funding Number(s)
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