The Relationship Between Solar Radio and Hard X-ray Emission

Abstract

This review discusses the complementary relationship between radio and hard X-ray observations of the Sun using primarily results from the era of the Reuven Ramaty High Energy Solar Spectroscopic Imager satellite. A primary focus of joint radio and hard X-ray studies of solar flares uses observations of nonthermal gyrosynchrotron emission at radio wavelengths and bremsstrahlung hard X-rays to study the properties of electrons accelerated in the main flare site, since it is well established that these two emissions show very similar temporal behavior. A quantitative prescription is given for comparing the electron energy distributions derived separately from the two wavelength ranges: this is an important application with the potential for measuring the magnetic field strength in the flaring region, and reveals significant differences between the electrons in different energy ranges. Examples of the use of simultaneous data from the two wavelength ranges to derive physical conditions are then discussed, including the case of microflares, and the comparison of images at radio and hard X-ray wavelengths is presented. There have been puzzling results obtained from observations of solar flares at millimeter and submillimeter wavelengths, and the comparison of these results with corresponding hard X-ray data is presented. Finally, the review discusses the association of hard X-ray releases with radio emission at decimeter and meter wavelengths, which is dominated by plasma emission (at lower frequencies) and electron cyclotron maser emission (at higher frequencies), both coherent emission mechanisms that require small numbers of energetic electrons. These comparisons show broad general associations but detailed correspondence remains more elusive.

Validation of the Coronal Thick Target Source Model

“…We have mentioned that a similar flattening was reported earlier based on the microwave-to-X-ray slope comparisons (Silva et al 2000;White et al 2011). In the previous cases, however, the radio-derived and X-ray-derived spectral indices of nonthermal electrons were obtained using one or another approximation, e.g., from the thick-target fit of the footpoint X-ray emission or a simplified analytical Dulk-Marsh formula for the GS spectrum, which necessarily involves some level of Figure 2); (e): [30,50,70,90]% contours of the OVSA 5.6 GHz emission (white; same as in Figure 2) on top of the computed microwave 5.6 GHz image; (f): model HXR emission at 15 keV (blue) on top of the model microwave image at 5.6 GHz.…”

“…However, we quickly find that it is not possible to obtain a good fit to the microwave spectrum with such a steep electron energy spectrum, irrespective of other model assumptions. Unavoidably, we have to assume the existence of a spectral break up above some energy, to a lower spectral index of about δ n2 ≈3.5 to fit the high-frequency microwave spectral slope, which may not be surprising given that a number of previous radio-to-X-ray comparisons (see, e.g., Silva et al 2000;White et al 2011, and references therein) concluded that the radio-derived spectra of the nonthermal electrons are often flatter than the X-rayderived ones.…”

“…It is well established that considering the X-ray and microwave data together can often improve the flare diagnostics greatly (see, e.g., Bastian et al 2007;Fleishman et al 2011White et al 2011). To perform a joint study of the X-ray and microwave data on the coronal thick-target sources requires a combination of reasonably complementary data sets, which would allow quantitative modeling of both X-ray and microwave emissions and their quantitative comparison with observations.…”

Sources of Quasi-periodic Pulses in the Flare of 18 August 2012

“…The measured value is in the bounds of the estimates δ MW = γ X + 1.5 and δ MW = γ X − 0.5 for the radio-emitting electrons depending upon whether we assume thick target or thin-target approximation for the X-ray source (e.g. White, Benz, and Christe, 2011).…”

Impulsive radio and hard X-ray emission from an M-class flare

“…The impulsive radio and X-ray emissions are produced by energetic electrons via the gyro-synchrotron and bremsstrahlung processes, respectively White et al 2011). The eastern source is partially self-absorbed at 17 GHz, which means that the 17 GHz flux density of the impulsive emission component derived in the previous section should be treated as a lower limit for gyro-synchrotron emission since the 34 GHz flux density of the eastern source (which is subtracted from the 17 GHz flux density) is slightly higher than that at 17 GHz.…”