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

📝 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.

💡 Analysis

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.

📄 Content

arXiv:0901.3591v1 [astro-ph.SR] 23 Jan 2009 Imaging Spectroscopy on Preflare Coronal Nonthermal Sources Associated with the 2002 July 23 Flare Ayumi Asai1, Hiroshi Nakajima1, Masumi Shimojo1, Takaaki Yokoyama2, Satoshi Masuda3, and S¨am Krucker4 asai@nro.nao.ac.jp ABSTRACT We present a detailed examination on the coronal nonthermal emissions dur- ing the preflare phase of the X4.8 flare that occurred on 2002 July 23. The mi- crowave (17 GHz and 34 GHz) data obtained with Nobeyama Radioheliograph, at Nobeyama Solar Radio Observatory and the hard X-ray (HXR) data taken with 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 emis- sion 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 elec- tron spectral indices (∼4.7) is the same value as that from microwaves (∼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 × 109 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 (∼6.7) is much different, while the gap of the number density of the accelerated electrons is somewhat reduced. 1 Nobeyama Solar Radio Observatory, National Astronomical Observatory of Japan, Minamimaki, Mi- namisaku, Nagano, 384-1305, JAPAN 2 Department of Earth and Planetary Science, University of Tokyo, Hongo, Bunkyo, Tokyo, 113-0033, JAPAN 3 Solar-Terrestrial Environment Laboratory, Nagoya University, Chikusa, Nagoya, Aichi, 464-8601, JAPAN 4 Space Sciences Laboratory, University of California, Berkeley, CA94720, USA – 2 – Subject headings: acceleration of particles — Sun: corona — Sun: flares — Sun: radio radiation — Sun: X-rays, gamma rays 1. Introduction Nonthermal emissions from accelerated particles are often observed in hard X-rays (HXRs), γ-rays, and microwaves at the beginning of a solar flare. Although these nonthermal emissions are undoubtedly associated with intense energy release processes, the mechanisms to accelerate particles are still unclear, and they have been one of the most important and the most difficult problems in solar physics (see reviews by, e.g., Aschwanden 2002). The HXR nonthermal emission is well explained with the bremsstrahlung emission, which is emitted by the nonthermal electrons with energies E ≳20 keV. In the microwave range, on the other hand, the gyrosynchrotron emission is the most promising nonthermal emission. The microwave-emitting electrons have relatively higher energies, such as sub-relativistic to relativistic energy. Although the emission mechanisms and electron energies are totally different, HXR and microwave emissions have shown a lot of similarities, especially in the lightcurves (e.g., Kundu 1961). The similarities have been thought to be evidences that microwave-emitting electrons are accelerated with the same mechanism as that for HXR- emitting electrons. On the other hand, it has been also reported that an electron energy spectral index derived from HXR emissions is often larger (softer) than that derived from microwave emissions (e.g., Silva et al. 1997), and the temporal behaviors of the spectral indices are totally different between HXR and microwave. These suggest a possibility that spectra of nonthermal electrons have a bend, and that the high energy electrons that emit microwaves are accelerated more efficiently than the HXR-emitting electrons. To explain the gap of spectral indices, several models have been suggested. For example, Somov & Kosugi (1997) suggested that collapsing magnetic trap works efficiently for particle acceleration of higher energy electrons. Alternatively, Silva et al. (1997) suggested that we should take into account the transport mechanism of accelerated electrons, such as magnetic trapping, since the nature of the emission mechanisms is different for each emission range. In HXRs, nonthermal emissions from footpoints of flare loops are dominant, and these are described with “thick-target” bremsstrahlung emission model. However, the thick-target model only gives us a spectrum of the injected electron flux from a HXR emission spectrum at rather lower energy. Therefore, the conversion from the injected electron flux to the number of the nonthermal electrons is needed to compare it with the number of the microwave- emitting electrons at higher energy, and it requires another estimation such as the deflection – 3 – time of precipitating electrons. If the deflection time has some dependences on electron

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