17:15 〜 19:15
[PEM11-P06] Statistical study about the characteristics of microflares observed with Nobeyama Radioheliograph (NoRH)
キーワード：フレア、マイクロ波、粒子加速
In solar flares, magnetic energy stored in the solar corona is released through magnetic reconnection and converted into various kinds of energy. Some amount of the released energy is used to accelerate charged particles. While several models for the acceleration mechanism have been proposed, it is not still understood well. In this study, to provide new insights into these unsolved issues, we focused on microflares, which have been less studied compared to large and medium-scale flares, and statistically examined their characteristics. High-energy particles accelerated by flares mainly emit microwaves and hard X-rays, but studies on microflares in microwave are less extensive than those in hard X-rays, and any statistical analysis has not yet been conducted. Therefore, we used microwave data observed with the Nobeyama Radioheliograph (NoRH), which has been shown in previous studies to have higher sensitivity than hard X-ray observational instruments like RHESSI, to detect and analyze microflares.
To reduce the background emission (the radiation from regions other than the flare area), we divided the full-sun images of NoRH and created macro-pixels. Furthermore, we developed a new algorithm for detecting microflares and applied it to each macro-pixel. To investigate the relationship between the non-thermal and thermal emissions of flares, we also analyzed soft X-ray data from the GOES satellite for the same dates as the detected microflares.
As a result of analyzing the data for two years (2012-2013), we successfully detected 418 new microflares. To compare the physical characteristics of the detected microflares with large and medium-scale flares, we used 2,623 events listed on the NoRH website as large and medium-scale flares. In total, we analyzed 3,041 flare events, covering approximately four orders of magnitude in 17 GHz peak flux. From this dataset, we examined the frequency distributions of various physical quantities and their correlations with 17 GHz peak flux for both large/medium-scale flares and microflares. As a result, we found that the duration, area, and 34 GHz peak flux of microflares decrease in a manner that extends the trends observed for large and medium-scale flares as the 17 GHz peak flux decreases. On the other hand, the power-law index derived from the ratio of 17 GHz to 34 GHz peak flux showed a trend where the spectrum becomes softer as the peak flux decreases, suggesting a lower efficiency of particle acceleration in microflares. While this appears to contradict the previous finding, it can be explained by differences in the magnetic field strength of the emission regions due to differences in spatial scale. Specifically, the energy of electrons emitting 17 GHz microwaves may differ depending on the magnetic field strength in the emitting region. To further verify this interpretation, a comprehensive analysis incorporating magnetic field data and multi-wavelength observations for individual microflares will be necessary.
To reduce the background emission (the radiation from regions other than the flare area), we divided the full-sun images of NoRH and created macro-pixels. Furthermore, we developed a new algorithm for detecting microflares and applied it to each macro-pixel. To investigate the relationship between the non-thermal and thermal emissions of flares, we also analyzed soft X-ray data from the GOES satellite for the same dates as the detected microflares.
As a result of analyzing the data for two years (2012-2013), we successfully detected 418 new microflares. To compare the physical characteristics of the detected microflares with large and medium-scale flares, we used 2,623 events listed on the NoRH website as large and medium-scale flares. In total, we analyzed 3,041 flare events, covering approximately four orders of magnitude in 17 GHz peak flux. From this dataset, we examined the frequency distributions of various physical quantities and their correlations with 17 GHz peak flux for both large/medium-scale flares and microflares. As a result, we found that the duration, area, and 34 GHz peak flux of microflares decrease in a manner that extends the trends observed for large and medium-scale flares as the 17 GHz peak flux decreases. On the other hand, the power-law index derived from the ratio of 17 GHz to 34 GHz peak flux showed a trend where the spectrum becomes softer as the peak flux decreases, suggesting a lower efficiency of particle acceleration in microflares. While this appears to contradict the previous finding, it can be explained by differences in the magnetic field strength of the emission regions due to differences in spatial scale. Specifically, the energy of electrons emitting 17 GHz microwaves may differ depending on the magnetic field strength in the emitting region. To further verify this interpretation, a comprehensive analysis incorporating magnetic field data and multi-wavelength observations for individual microflares will be necessary.