Diagnostics of Soft X-ray Emission from the Sun: Energetic Solar Flares and Gentle Quiescence

Abstract

The Sun is complex and enchanting. It produces all types of electromagnetic radiation and releases the energy needed to sustain life on Earth. The variability of the solar soft X-ray (SXR) emission can answer: (1) How plasma is transported within the solar atmosphere during solar flares? (2) What mechanisms could drive Quasi-Periodic Pulsations (QPP) signatures in solar flare radiation? (3) What are the characteristic temperatures of the quiescent Sun? During periods of solar activity, the dim and hot solar corona becomes a blazing ball emitting flaring phenomena. Solar flares are capable of releasing over 10^32 erg, radiating SXR light, and heating the ambient plasma to excess of 20 MK. The SXR inferred elemental abundance of low first-ionization potential (FIP) elements are observed to deviate from their nominal values during solar flares. The Miniature X-ray Solar Spectrometer CubeSat–1 (MinXSS-1) conducted solar SXR spectral observations of ~30 flares C to M class flares. The flares were spectrally examined to determine the time evolution of temperature, volume emission measure, and elemental abundances of Fe, Ca, Si, S, Mg, and Ar. The low FIP elements were depleted near the SXR peak which is consistent when lower atmospheric plasma fills the coronal loops through chromospheric evaporation. A key observational signature in flaring plasma is the presence of low-amplitude pulsations known as QPP. These small flux variations could be driven by periodic or "bursty" energy releases and place important constraints on the energy release processes and heating mechanisms. Using multithreaded hydrodynamic models we synthesized SXR and EUV emission to understand the generation of QPP signatures. Results showed that the successive reconnection of the flare loops could generate the ~26 second QPP observed in SXR. In the tranquility of the Sun, the solar corona has minimal flaring events. The MinXSS-2 CubeSat made solar daily quiescent SXR spectra observations and were analyzed to determine the time evolution of temperature, volume emission measure. The average isothermal temperature ranged from 1~2 MK being consistent with other SXR observations during the solar minimum.