Investigating the differential emission measure and energetics of microflares with combined SDO/AIA and RHESSI observations

Abstract

An important question in solar physics is whether solar microflares, the smallest currently observable flare events in X-rays, possess the same energetic properties as large flares. Recent surveys have suggested that microflares may be less efficient particle accelerators than large flares, and hence contribute less nonthermal energy, which may have implications for coronal heating mechanisms. We therefore explore the energetic properties of microflares by combining Extreme Ultraviolet (EUV) and X-ray measurements. We present forward-fitting differential emission measure (DEM) analysis of 10 microflares. The fitting is constrained by combining, for the first time, high temperature RHESSI observations and flux data from SDO/AIA. Two fitting models are tested for the DEM; a Gaussian distribution and a uniform DEM profile. A Gaussian fit proved unable to explain the observations for any of the studied microflares. However, 8 of 10 events studied were reasonably fit by a uniform DEM profile. Hence microflare plasma can be considered to be significantly multi-thermal, and may not be significantly peaked or contain resolvable fine structure, within the uncertainties of the observational instruments. The thermal and non-thermal energy is estimated for each microflare, comparing the energy budget with an isothermal plasma assumption. From the multithermal fits the minimum non-thermal energy content was found to average approximately 30% of the estimated thermal energy. By comparison, under an isothermal model the non-thermal and thermal energy estimates were generally comparable. Hence, multi-thermal plasma is an important consideration for solar microflares that substantially alters their thermal and non-thermal energy content.

arXiv comment13 pages, 10 Figures, 2 tables. Accepted for publication in the Astrophysical Journal

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• Simultaneous ALMA-Hinode-IRIS observations on footpoint signatures of a soft X-ray loop-like microflare
• Energy Partition in Four Confined Circular-Ribbon Flares
• Differential Emission Measure and Electron Distribution Function Reconstructed from RHESSI and SDO Observations
• The observational evidence that all microflares that accelerate electrons to high energies are rooted in sunspots
• Spectroscopic and Imaging Observations of Spatially Extended Magnetic Reconnection in the Splitting of a Solar Filament Structure
• Thermal and Non-Thermal Properties of Active Region Recurrent Coronal Jets
• Three-dimensional Magnetic and Thermodynamic Structures of Solar Microflares
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• Relationship between EUV microflares and small-scale magnetic fields in the quiet Sun
• Flare-accelerated Electrons in the Kappa Distribution from X-Ray Spectra with the Warm-Target Model
• Thermal Evolution of an Active Region through Quiet and Flaring Phases as Observed by NuSTAR XRT, and AIA
• Moving Plasma Structures and Possible Driving Mechanisms of Solar Microflares Observed with High-Resolution Coronal Imaging
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• Energy release in the solar atmosphere from a stream of infalling prominence debris