Estimates of Densities and Filling Factors from a Cooling Time Analysis of Solar Microflares Observed with RHESSI

Abstract

We use more than 4,500 microflares from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) microflare data set (Christe et al., 2008, Ap. J., 677, 1385) to estimate electron densities and volumetric filling factors of microflare loops using a cooling time analysis. We show that if the filling factor is assumed to be unity, the calculated conductive cooling times are much shorter than the observed flare decay times, which in turn are much shorter than the calculated radiative cooling times. This is likely unphysical, but the contradic- tion can be resolved by assuming the radiative and conductive cooling times are comparable, which is valid when the flare loop temperature is a maximum and when external heating can be ignored. We find that resultant radiative and con- ductive cooling times are comparable to observed decay times, which has been used as an assumption in some previous studies. The inferred electron densities have a mean value of 10^11.6 cm^-3 and filling factors have a mean of 10^-3.7. The filling factors are lower and densities are higher than previous estimates for large flares, but are similar to those found for two microflares by Moore et al. (Ap. J., 526, 505, 1999).

arXiv commentPublished in Ap. J

References in corpus (13)

• Impulsive phase flare energy transport by large-scale Alfven waves and the electron acceleration problem
• Highly Efficient Modeling of Dynamic Coronal Loops
• RHESSI Microflare Statistics II. X-ray Imaging, Spectroscopy & Energy Distributions
• The Origin of the Solar Flare Waiting-Time Distribution
• RHESSI Line and Continuum Observations of Super-hot Flare Plasma
• Inter-occurrence Times in the Bak-Tang-Wiesenfeld Sandpile Model: A Comparison with the Turbulent Statistics of Solar Flares
• Scaling laws of solar and stellar flares
• Evolution of the Loop-Top Source of Solar Flares--Heating and Cooling Processes
• Dynamics and evolution of an eruptive flare
• From Solar and Stellar Flares to Coronal Heating: Theory and Observations of How Magnetic Reconnection Regulates Coronal Conditions
• The sub-arcsecond hard X-ray structure of loop footpoints in a solar flare
• Parameters of the Magnetic Flux inside Coronal Holes
• Transition Region Emission and Energy Input to Thermal Plasma During the Impulsive Phase of Solar Flares

Cited by in corpus (4)

• Investigating the differential emission measure and energetics of microflares with combined SDO/AIA and RHESSI observations
• Understanding breaks in solar flares x-ray spectra: Evaluation of a co-spatial return-current model
• $K2$ Ultracool Dwarfs Survey. VI. White light superflares observed on an L5 dwarf and flare rates of L dwarfs
• Disentangling auroral, cloud and magnetic spot driven variability in three early L-dwarfs with HST/WFC3