FOXSI-2 Solar Microflares. II. Hard X-ray Imaging Spectroscopy and Flare Energetics

Area of Science:

• Solar physics
• Plasma astrophysics
• High-energy astrophysics

Background:

• Solar microflares are small-scale solar eruptions.
• Understanding energy release in microflares is crucial for solar physics.
• Previous observations lacked the resolution to study microflares in detail.

Purpose of the Study:

• To investigate energy release and transfer in sub-A class solar microflares.
• To utilize the advanced capabilities of the Focusing Optics X-ray Solar Imager-2 (FOXSI-2) for microflare observations.
• To compare microflare evolution with larger solar flares.

Main Methods:

• Spectral analysis of microflares using an optically thin isothermal plasma model.
• Utilizing hard X-ray (HXR) imaging spectroscopy with FOXSI-2 (4-20 keV).
• Combining FOXSI-2 data with contemporaneous observations from the Solar Dynamics Observatory's Atmospheric Imaging Assembly.

Main Results:

• Evidence for plasma heated to approximately 10 MK and emission measures down to ~10^44 cm^-3.
• Potential for nonthermal electrons to account for thermal energy in microflare 1, consistent with the thick-target model.
• Spatial and temporal complexity observed in sub-A class flares, challenging the single-burst energy model.

Conclusions:

• Microflares exhibit evolutionary complexity more akin to large solar flares than previously assumed.
• FOXSI-2 provides unprecedented sensitivity and angular resolution for studying microflares.
• These findings advance our understanding of energy dynamics in small-scale solar events.