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Observations of a radio-quiet solar preflare by A. Benz et al.*

17 Oct 2017, 11:05 UTC
Observations of a radio-quiet solar preflare  by A. Benz et al.*
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A substantial fraction of the flare energy (as observed e.g. in bolometric luminosity) first appears in non-thermal electrons and ions (Emslie et al. 2012). It implies that the particles are efficiently accelerated to a non-Maxwellian velocity distribution. Most of the non-thermal electrons have energies in the 10 – 30 keV range, but some are accelerated beyond 100 keV and radiate gyro-synchrotron radio emission. Nearly all of the flare energy is subsequently thermalized and a fraction of some 20% is finally radiated in soft X-rays (SXR).
The ratio between gyro-synchrotron emission and thermal SXRs in solar and stellar flares was found to be stable over seven orders of magnitude (Benz and Güdel, 1994). The two emissions are related as
\[
\log (L_X/L_R)= 15.5 ± 0.5,
\]
where $L_X$ [erg s$^{-1}$] is the luminosity in the SXR band, $L_R$ [erg s$^{-1}$ Hz$^{-1}$] is the radio luminosity density at 8 – 9 GHz, and ± indicates the width of the observed distribution (see Figure 1).

Figure 1. Comparison of flare peak SXR and 3.6 cm (gyro-synchrotron) luminosities. Upper limits derived from these preflare observations are shown by red arrows. Two peaks in the early main phase are indicated with a little red x, ...

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