Accelerated Electrons Observed Down to NuSTAR Solar Microflare

Area of Science:

• Solar physics
• High-energy astrophysics
• Plasma physics

Background:

• Solar flares release energy and particles into space.
• Understanding particle acceleration in solar flares is crucial for space weather prediction.
• Small solar flares (microflares) are less understood than larger events.

Purpose of the Study:

• To detect and characterize nonthermal electron distributions in a small solar microflare.
• To investigate the energy deposition and plasma properties of microflares.
• To compare microflare characteristics with those of larger solar flares.

Main Methods:

• Observation of a GOES A5.7 class solar microflare using the Nuclear Spectroscopic Telescope Array (NuSTAR).
• Supporting observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI).
• Analysis using a thick-target model for accelerated electrons.

Main Results:

• First direct imaging detection of nonthermal hard X-rays from a solar microflare.
• The accelerated electron distribution has a spectral index of 6.3 ± 0.7, extending down to 6.5 keV.
• Energy deposition rate of ~2 × 10^27 erg s^-1, heating the flare loop to at least 10 MK.
• RHESSI emission is predominantly nonthermal, contrary to common assumptions.
• Nonthermal to thermal energy ratio is similar to large flares.

Conclusions:

• Small solar microflares exhibit characteristics, like coronal thick-target behavior, previously associated with larger flares.
• Nonthermal emission is significant in microflares, challenging existing spectroscopic assumptions for RHESSI.
• Coronal thick targets may be common in microflares, providing insights into particle acceleration across energy scales.
• Further observations are needed to understand particle acceleration and coronal heating mechanisms, including nanoflares.