A Statistical Approach to a Better Understanding of the Conditions that Produce White-Light Enhancements in Solar Flares Observed by Hinode/SOT

Abstract:

After Hinode was launched in 2006, the observing program designed to catch rare white-light (WL) events has steadily been improved. So we now have a large enough number of WL events to make a statistical study.

 To understand the conditions that produce enhancements of WL in solar flares, we performed a statistical analysis of Hinode/SOT WL data. From the analysis of data obtained between January 2011 and August 2013, we found that the precipitation of large amounts of accelerated electrons into a compact area within a short time plays a key role in generating a WL event (Kitagawa et al., submitted to ApJ). And we also found that the coronal magnetic field strength in the flare region is one of the important factors that can distinguish between WL and non-WL (NWL) events.

 In this paper, we present the results derived from a new statistical study from the observing period extended to December 2014. In this study, we use not only the Hinode/SOT WL (G-band, Blue, Green, Red) data, but we also SDO/HMI continuum data. The total number of events is now about two times larger than that of Kitagawa’s previous study. We compared the white-light emission data with GOES soft X-rays, RHESSI hard X-rays and/or the strength of the photospheric magnetic fields, and investigated the relationships between many physical parameters. For example, from scatter plots of the start-to-peak time-scale in X-rays and the energy of nonthermal electrons, one can easily find a clear boundary between WL and NWL events. WL events show a relatively shorter time-scale among events with almost the same nonthermal energy. Most of the results in this study show the same characteristics as in the previous study, but they are more clearly seen thanks to the larger number of events.

 We also report some statistical results on WL and NWL events from Nobeyama Radio Heliograph (NoRH) and Nobeyama Radio Polarimeters (NoRP) data, which give information on the magnetic field of the flare loop.