Solar radio emission is a mixture of various kinds of radiation, some of which are thermal while others are of non-thermal origin. The total solar radio emission is strongly variable and is dominated by different radiation types at different times.
Thermal radiation dominates as long as no strong flares are occurring. Three principal types of thermal radiation are observed (e.g., Gary and Hurford, 1994):
The third contribution is usually rather weak; the non-flaring solar radio emission is dominated by the first and second contributions. Although variable, the average radio luminosity at 3.6 cm wavelength from these contributions is log LR 10.80 – 11.15 [erg s–1 Hz–1] (Drake et al., 1993). The picture changes entirely during solar flares, when two new contributions dominate the solar radio output:
The discovery of solar analogs in radio waves (Güdel et al., 1994) was motivated by a correlation between radio and X-ray luminosities that holds for M-type MS stars, but also extends to RS CVn-type binaries and seems to hold similarly for solar flares (Güdel and Benz, 1993; Benz and Güdel, 1994). The following systematics apply:
Figure 23 shows a summary of normalized radio luminosities of solar analogs with different rotation periods. Like X-ray emission, the radio output decays with increasing rotation period (i.e., decreasing magnetic activity), although the decay is much steeper. The radio decay is reminiscent of the very steep decay of the harder portion of soft X-ray emission which is also an excellent tracer of magnetic activity. The steep decline of non-thermal radio emission therefore extends the trend observed above (Section 5.6): Radio emission originates from the most energetic electrons in the stellar atmosphere.
Figure 24 shows the radio luminosity as a function of the average coronal temperature of solar analogs; only radio upper limits are available for the intermediately active coronae with temperatures Tav 3 – 5 MK; a regression fit to the points including the upper limits therefore provides a lower limit to the slope:
(based on the APEC emission line code; Telleschi et al. 2005). This empirical relation shows again, and explicitly, that strong non-thermal radio emission develops only in very active stars with hot coronae. Because non-thermal electrons need to be replenished on short time scales, presumably in flare-like processes, a model in which the very hot plasma component is formed by the same flares is suggestive. Alternatively, non-thermal electron distributions may be accelerated continuously out of a Maxwellian distribution into a runaway tail by electric fields (Holman, 1986), a concept also proposed to operate in solar flares (Holman and Benka, 1992). Such mechanisms of course work best if very hot plasmas are present.
Other types of radio emission are too faint to be detected from solar analogs. Signatures for thermal gyroresonance emission have been found in an M dwarf (Güdel and Benz, 1989), and Drake et al. (1993) reported evidence for non-flaring radio emission probably related to chromospheric bremsstrahlung and some active-region gyroresonance emission in the nearby F-type subgiant Procyon; more systematic observations of solar analogs during all phases of their MS life will require more sensitive radio telescopes than hitherto available.
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