Imaging Spectroscopy on Preflare Coronal Nonthermal Sources Associated with the 2002 July 23 Flare

TL;DR

This study analyzes preflare coronal nonthermal emissions during the 2002 July 23 X4.8 solar flare, revealing electron acceleration to over 1 MeV and comparing microwave and HXR spectral properties to understand electron distributions.

Contribution

It provides the first detailed imaging spectroscopic analysis of preflare nonthermal sources in microwaves and HXRs, confirming electron acceleration and spectral characteristics during this phase.

Findings

Electrons are accelerated from tens of keV to over 1 MeV during preflare phase.

Electron spectral indices are consistent (~4.7) between microwaves and HXRs under thin-target assumptions.

Number density of microwave-emitting electrons exceeds that of HXR-emitting electrons unless low plasma density is assumed.

Abstract

We present a detailed examination on the coronal nonthermal emissions during the preflare phase of the X4.8 flare that occurred on 2002 July 23. The microwave (17 GHz and 34 GHz) data obtained with Nobeyama Radioheliograph, at Nobeyama Solar Radio Observatory and the hard X-ray (HXR) data taken with {\it Reuven Ramaty High Energy Solar Spectroscopic Imager} obviously showed nonthermal sources that are located above the flare loops during the preflare phase. We performed imaging spectroscopic analyses on the nonthermal emission sources both in microwaves and in HXRs, and confirmed that electrons are accelerated from several tens of keV to more than 1 MeV even in this phase. If we assume the thin-target model for the HXR emission source, the derived electron spectral indices ($\sim 4.7$) is the same value as that from microwaves ($\sim 4.7$) within the observational uncertainties, which implies that the distribution of the accelerated electrons follows a single power-law. The number density of the microwave-emitting electrons is, however, larger than that of the HXR-emitting electrons, unless we assume low ambient plasma density of about $1.0 \times 10^9$ cm$^{-3}$ for the HXR-emitting region. If we adopt the thick-target model for the HXR emission source, on the other hand, the electron spectral index ($\sim 6.7$) is much different, while the gap of the number density of the accelerated electrons is somewhat reduced.