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Statistical Study of Solar White-light Flares and Comparisons with Superflares on Solar-type Stars  

Kosuke Namekata   Submitted: 2017-11-04 04:30

Recently, many superflares on solar-type stars have been discovered as white-light flares (WLFs). The statistical study found a correlation between their energies (E) and durations (τ): τ ∝ E0.39 (Maehara et al. 2015, EP& S, 67, 59), similar to those of solar hard/soft X-ray flares: τ ∝ E0.2-0.33. This indicates a universal mechanism of energy release on solar and stellar flares, i.e., magnetic reconnection. We here carried out a statistical research on 50 solar WLFs observed with SDO/HMI and examined the correlation between the energies and durations. As a result, the E-τ relation on solar WLFs (τ ∝ E0.38) is quite similar to that on stellar superflares (τ ∝ E0.39). However, the durations of stellar superflares are one order of magnitude shorter than those expected from solar WLFs. We present the following two interpretations for the discrepancy. (1) In solar flares, the cooling timescale of WLFs may be longer than the reconnection one, and the decay time of solar WLFs can be elongated by the cooling effect. (2) The distribution can be understood by applying a scaling law (τ ∝ E1/3B-5/3) derived from the magnetic reconnection theory. In this case, the observed superflares are expected to have 2-4 times stronger magnetic field strength than solar flares.

Authors: Kosuke Namekata, Takahito Sakaue, Kyoko Watanabe, Ayumi Asai, Hiroyuki Maehara, Yuta Notsu, Shota Notsu, Satoshi Honda, Takako Ishii, Kai Ikuta, Daisaku Nogami, Kazunari Shibata
Projects: GOES X-rays,Hinode/SOT,RHESSI,SDO-AIA,SDO-HMI

Publication Status: accepted for publication in ApJ
Last Modified: 2017-11-06 11:34
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Validation of a Scaling Law for the Coronal Magnetic Field Strengths and Loop Lengths of Solar and Stellar Flares  

Kosuke Namekata   Submitted: 2016-10-31 19:51

Shibata & Yokoyama (1999, 2002) proposed a method of estimating the coronal magnetic field strengths (B) and magnetic loop lengths (L) of solar and stellar flares, on the basis of magnetohydrodynamic simulations of the magnetic reconnection model. Using the scaling law provided by Shibata & Yokoyama (1999, 2002), B and L are obtained as functions of the emission measure (EM=n^2L^3) and temperature (T) at the flare peak. Here, n is the coronal electron density of the flares. This scaling law enables the estimation of B and L for unresolved stellar flares from the observable physical quantities EM and T, which is helpful for studying stellar surface activities. To apply this scaling law to stellar flares, we discuss its validity for spatially resolved solar flares. EM and T were calculated from GOES soft X-ray flux data, and B and L are theoretically estimated using the scaling law. For the same flare events, B and L were also observationally estimated with images taken by Solar Dynamics Observatory (SDO)/ Helioseismic and Magnetic Imager (HMI) Magnetogram and Atmospheric Imaging Assembly (AIA) 94Å pass band. As expected, a positive correlation was found between the theoretically and observationally estimated values. We interpret this result as indirect evidence that flares are caused by magnetic reconnection. Moreover, this analysis makes us confident in the validity of applying this scaling law to stellar flares as well as solar flares.

Authors: Kosuke Namekata, Takahito Sakaue, Kyoko Watanabe, Ayumi Asai, Kazunari Shibata
Projects: GOES X-rays,SDO-AIA,SDO-HMI

Publication Status: Accepted for publication in PASJ
Last Modified: 2016-11-02 08:41
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Statistical Study of Solar White-light Flares and Comparisons with Superflares on Solar-type Stars
Validation of a Scaling Law for the Coronal Magnetic Field Strengths and Loop Lengths of Solar and Stellar Flares

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