Diagnostics From Three Rising Submillimeter Bursts

TL;DR

This study analyzes three rising submillimeter bursts in a super-active solar region, revealing differences in electron populations, spectral indices, and energy release mechanisms, with implications for understanding solar flare energetics.

Contribution

It introduces a calculation formula for energy released by ultrarelativistic electrons, including relativistic corrections, and compares energy outputs in THz and microwave sources during solar bursts.

Findings

Steeply rising THz spectra produced by high relativistic electrons with low-energy cutoff of 1 MeV.

Energy released by electrons in THz sources exceeds that in microwave sources due to strong GS radiation loss.

Total energy from electrons in THz sources is significantly larger than thermal energies from soft X-ray observations.

Abstract

In the paper we investigate three novel rising submillimeter (THz) bursts occurred sequentially in a super-Active Region NOAA 10486. The average rising rate of the flux density above 200 GHz is only 20 sfu/GHz (corresponding spectral index $\alpha$ of 1.6) for the THz spectral components of 2003 October 28 and November 4 bursts, while it can attain values of 235 sfu/GHz ($\alpha$=4.8) for 2003 November 2 burst. The steeply rising THz spectrum can be produced by a population of high relativistic electrons with a low-energy cutoff of 1 MeV , while it only requires a low-energy cutoff of 30 keV for the two slowly rising THz bursts, via gyrosynchrotron (GS) radiation based on our numerical simulations of burst spectra in the magnetic dipole field case. The electron density variation is much larger in the THz source than that in microwave (MW) one. It is interesting that the THz source radius decreased by 20--50$\%$ during the decay phase for the three events, but the MW one increased by 28$\%$ for the 2003 November 2 event. In the paper we will present a calculation formula of energy released by ultrarelativistic electrons, accounting the relativistic correction for the first time. We find that the energy released by energetic electrons in the THz source exceeds that in microwave one due to the strong GS radiation loss at THz range, although the modeled THz source area is 3--4 orders smaller than the modeled MW one. The total energies released by energetic electrons via the GS radiation in radio sources are estimated, respectively, to be $5.2\times10^{33}$, $3.9\times10^{33}$ and $3.7\times10^{32}$ erg for the October 28, November 2 and 4 bursts, which are 131, 76 and 4 times as large as the thermal energies of $2.9\times10^{31}$, $2.1\times10^{31}$ and $5.2\times10^{31}$ erg estimated from the soft x-ray GOES observations.