Correlation between decimetric radio emission and hard X-rays in solar flares

Abstract

Aims. The emission of decimetric flare radiation, in particular narrowband spikes and pulsations, is generally considered to originate in accelerated, non-thermal particles. On the other hand, non-thermal hard X-rays are also understood to be products of this acceleration. Do radio emission and hard X-ray signatures originate from the same acceleration process? A strong correlation between the light curves in the radio and HXR ranges may help answer this question. Methods. The delay between the radio and hard X-ray emission was determined by cross-correlation. The time profiles of X-ray and radio emission include a wide range of energies and frequencies. Thus, correlation is not simply a yes/no question, but must be systematically searched for in various ranges. The high spectral resolution of RHESSI ensured that it was possible to carefully choose the energy range, excluding thermal emission. The broad bandwidth of Phoenix-2 allowed the selection of any emission in the full decimetre range. The energy range and duration in hard X-rays, and the frequency range in radio spectrograms were chosen to optimize the correlation. The cross-correlation coefficient was then analyzed by a Gaussian fitting method. Results. The measured delays have a distribution of FWHM 4.9 s and 4.7 s for pulsations and spikes, respectively, evaluated from such a Gaussian fitting method. The mean delay for pulsations was found to be −1.4 ± 0.9 s (minus indicates that hard X-ray emission comes first), and for narrowband spikes to −2.5 ± 2.5 s. There are broad wings in the distribution, which we interpret as chance coincidences. The delays do not depend on centre frequency, cross-correlation coefficient, duration of the correlating sequence, and position on the disk. However, we find an increase in the delay for the spikes with GOES magnitude (peak soft X-ray emission) of the flare and with peak hard X-ray flux. This was not the case for pulsations. Conclusions. In contrast to previous reports, the average delays for all pulsations and all spike groups are consistent with zero. Thus, correlated decimetric pulsations and spikes are, on average, concomitant with non-thermal X-rays.

Flare Observations

“…Correlation of coherent radio emissions with hard X-rays (or gyrosynchrotron emission) has been reported for spikes and pulsations (Slottje 1978;Benz and Kane 1986;Aschwanden et al 1990;Kliem et al 2000;Dabrowski and Benz 2009). If these coherent radiations are emitted at the second harmonic of the plasma frequency-the usual assumption-the density of the source region is in the range from 10 9 cm −3 to a few times 10 10 cm −3 .…”

Multiwavelength Study of a Solar Eruption from AR NOAA 11112 I. Flux Emergence, Sunspot Rotation and Triggering of a Solar Flare

“…8 and 3×10 10 cm 3 or a magnetic field between 100 and 1000 G (assuming second harmonic emission is present), which are typically expected in flare acceleration regions (Dabrowski and Benz, 2009). Figure 2 displays the Hα images of the active region before (08:52:19 UT) and after (22:30:35 UT) the flare observed respectively at Big Bear Solar Observatory (BBSO) and with the Solar Magnetic Activity Research Telescope (SMART) (Ueno et al, 2004).…”

Millisecond Radio Spikes in the Decimetric Band

“…The intense emission of decimetric radiation is generally considered to originate from nonthermal electron velocity distributions. These distributions can be unstable to maser emission or plasma waves that couple coherently with radio waves (Dabrowski and Benz, 2009).…”

“…By contrast, only 2% of all hard X-ray (HXR) events are associated with dmspikes (Aschwanden and Guedel, 1992). The emission of the dm-spikes is often correlated in time with the HXR emission recorded during impulsive phases of solar flares (Benz, 1986;Aschwanden and Guedel, 1992;Dabrowski and Benz, 2009). Unfortunately, due to an insufficient temporal resolution of the presently available X-ray observations, it is not possible to investigate temporal correlations of the individual radio spikes with short-term changes of the HXR flux.…”