The Transient Events of the Sun

Solar flares and Coronal Mass Ejections (CMEs) involve the sudden catastrophic release of magnetic energy stored and plasma in the Sun’s corona. This talk focuses on investigating the underlying processes of flares and CMEs using microwave and white-light observations respectively. In the flare studies, the spatial and spectral dynamics of the low-frequency (LF) microwave (MW), optically-thick gyrosynchrotron flare emission are discussed and for CMEs, the internal structure and reconstruction of the CME from the imaging observations of the Parker Solar Probe are discussed.

The first part of this talk addresses the high-resolution spectra and imaging of a set of microwave bursts observed by the Expanded Owens Valley Solar Array (EOVSA) in the 2.5-18 GHz frequency range with a 1-s time resolution. Five out of 12 bursts in a study display “flat” spectrum (spectral index, l 1.0) compared to that of a homogeneous source( (l ~2.9). These flat spectra at the low-frequencies (<10 GHz) can be defined as the emission from a spatially inhomogeneous source with a large area and/or with multiple emission components. Modeling based on inhomogeneity supports the conclusion that only multiple discrete sources can reproduce a flat spectrum, but not a single confined homogeneous source.

Unlike the usually observed flare emission from hard X-rays, high-frequency microwave, EUV, and H-alpha, that neatly fit the “standard solar model” from a simple, straightforward loop system/arcade, the low-frequency sources have shown an extended emission over the flaring active region and are spatially almost ten times as large as the other associated observations. These sources cannot be entirely explained by a standard two-dimensional model but with a “three-dimensional loop-loop interaction” scenario as observed from the contributions of multiple loop systems with different sizes. This leads to observational evidence of a more realistic flare model consisting of a multi-polar magnetic field configuration with the accelerated particles having large access over the flaring region through the overlying loops, where other wavelength emissions are almost invisible, presumably due to the collisionless conditions in these regions of low ambient density and magnetic field strength.

In the second part, the interior structure of a CME cavity imaged by the Wide-field Imager for Solar Probe (WISPR) heliospheric imager on board the Parker Solar Probe (PSP) during its seventh encounter is presented. This CME shows a complex three-part CME structure with a cavity consisting of non-concentric nested rings, lacking a clear front and bright core, which probably encompasses the helical magnetic flux rope (MFR) of the CME. The morphology of these nested density structures is examined, which can be interpreted as the magnetic field geometry and the three-dimensional projection of the flux rope seen through the ideal viewing angles of the observing instrument.