Probing the Bi-Directional Magnetic Reconnection Outflow Region of An Eruptive Solar Flare with Microwave Spectroscopic Imaging

The newly commissioned Expanded Owens Valley Solar Array (EOVSA) obtained microwave spectroscopic imaging of the spectacular eruptive limb flare on 2017 September 10 in 2.5-18 GHz. During the early impulsive phase of the flare ( 15:54 UT), an elongated microwave source is shown to connect the top of the flare arcade to the bottom of the erupting magnetic flux rope. Multi-frequency images reveal that the source bifurcates into two parts: One is located at and above the hard X-ray looptop source, and another located behind the flux rope. They appear to follow closely with the bi-directional reconnection downflow and upflow region as inferred from extreme ultraviolet imaging data. The spatially resolved spectra of this microwave source show characteristics of gyrosynchrotron radiation, suggesting the presence of mildly relativistic (100s of keV to MeV) electrons throughout the bi-directional reconnection outflow region. Based on the microwave spectra, we derive spatially resolved magnetic field strength in the outflow region and compare with magnetohydrodynamics simulation results. We also constrain spatial distribution and temporal evolution of accelerated electrons in the region. Implications of these results on electron acceleration and transport will be discussed.