Dynamic Magnetography of the SOL2017-09-04T202800 M5.5 Solar Flare Using Microwave Imaging Spectroscopy

We use multi-frequency imaging of the SOL2017-09-04T202800 M5.5 solar flare from the Expanded Owens Valley Solar Array (EOVSA) to perform spatially-resolved microwave imaging spectroscopy, which provides dynamic measurements of the magnetic field strength in the flaring region on timescales of order 1 s. The M5.5 impulsive peak in the flare is followed about 15 minutes later by a longer-duration gradual peak that was weak in soft X-rays and extreme ultraviolet wavelengths, yet reached higher hard X-ray (HXR) energies as revealed by RHESSI, and was also about 20 times stronger in microwaves than the impulsive peak. In contrast to the impulsive phase source, which was a relatively small and weakly-emitting loop-like source, in the delayed peak the microwave and nonthermal HXR sources are characterized by two simple footpoint sources of an extremely large loop with a high mirror ratio and strong trapping. The event was associated with a coronal mass ejection (CME) that interacted with an earlier, slower and more massive CME, and was also associated with a solar energetic particle (SEP) event seen at Earth. We use the EOVSA spatially resolved spectral data in the 3-18 GHz microwave range to produce parameter map movies by fitting the microwave spectrum at each position and time with a multi-parameter fit assuming gyrosynchrotron emission from a homogeneous source. The fitted parameters provide not only magnetic field strength, but also angle of the field to the line of sight, thermal component temperature and density, and nonthermal electron density and powerlaw index. In a companion talk (Nita et al. in this session) we use this parameter map information as input to 3D forward-fit modeling with GX Simulator.