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Title: 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 non-thermal energy, which may have implications for coronal heating mechanisms. We therefore explore the energetic properties of microflares by combining 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 Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations and flux data from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (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 budgetmore » with an isothermal plasma assumption. From the multi-thermal 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.« less

Authors:
 [1];  [2]
  1. Physics Department, The Catholic University of America, Washington, DC 20664, USA. (United States)
  2. Solar Physics Laboratory, Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
Publication Date:
OSTI Identifier:
22365694
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 789; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATORS; APPROXIMATIONS; EMISSION; ENERGY ACCOUNTING; ENERGY BALANCE; EXTREME ULTRAVIOLET RADIATION; FINE STRUCTURE; GAMMA RADIATION; GAUSS FUNCTION; GRAY ENERGY; IMAGES; LIFE CYCLE ASSESSMENT; PLASMA; SUN; X RADIATION

Citation Formats

Inglis, A. R., and Christe, S. Investigating the differential emission measure and energetics of microflares with combined SDO/AIA and RHESSI observations. United States: N. p., 2014. Web. doi:10.1088/0004-637X/789/2/116.
Inglis, A. R., & Christe, S. Investigating the differential emission measure and energetics of microflares with combined SDO/AIA and RHESSI observations. United States. doi:10.1088/0004-637X/789/2/116.
Inglis, A. R., and Christe, S. Thu . "Investigating the differential emission measure and energetics of microflares with combined SDO/AIA and RHESSI observations". United States. doi:10.1088/0004-637X/789/2/116.
@article{osti_22365694,
title = {Investigating the differential emission measure and energetics of microflares with combined SDO/AIA and RHESSI observations},
author = {Inglis, A. R. and Christe, S.},
abstractNote = {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 non-thermal energy, which may have implications for coronal heating mechanisms. We therefore explore the energetic properties of microflares by combining 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 Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations and flux data from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (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 multi-thermal 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.},
doi = {10.1088/0004-637X/789/2/116},
journal = {Astrophysical Journal},
number = 2,
volume = 789,
place = {United States},
year = {Thu Jul 10 00:00:00 EDT 2014},
month = {Thu Jul 10 00:00:00 EDT 2014}
}
  • Differential emission measure (DEM) analysis is a major diagnostic tool for stellar atmospheres. However, both its derivation and its interpretation are notably difficult because of random and systematic errors, and the inverse nature of the problem. We use simulations with simple thermal distributions to investigate the inversion properties of SDO/AIA observations of the solar corona. This allows a systematic exploration of the parameter space, and using a statistical approach the respective probabilities of all the DEMs compatible with the uncertainties can be computed. Following this methodology, several important properties of the DEM inversion, including new limitations, can be derived andmore » presented in a very synthetic fashion. In this first paper, we describe the formalism and we focus on isothermal plasmas as building blocks to understand the more complex DEMs studied in the second paper. The behavior of the inversion of AIA data being thus quantified, and we provide new tools to properly interpret the DEM. We quantify the improvement of the isothermal inversion with six AIA bands compared to previous EUV imagers. The maximum temperature resolution of AIA is found to be 0.03 log T{sub e} , and we derive a rigorous test to quantify the compatibility of observations with the isothermal hypothesis. However, we demonstrate limitations in the ability of AIA alone to distinguish different physical conditions.« less
  • Differential emission measure (DEM) analysis is one of the most used diagnostic tools for solar and stellar coronae. Being an inverse problem, it has limitations due to the presence of random and systematic errors. We present in this series of papers an analysis of the robustness of the inversion in the case of SDO/AIA observations. We completely characterize the DEM inversion and its statistical properties, providing all the solutions consistent with the data along with their associated probabilities, and a test of the suitability of the assumed DEM model. While Paper I focused on isothermal conditions, we now consider multithermalmore » plasmas and investigate both isothermal and multithermal solutions. We demonstrate how the ambiguity between noises and multithermality fundamentally limits the temperature resolution of the inversion. We show that if the observed plasma is multithermal, isothermal solutions tend to cluster on a constant temperature whatever the number of passbands or spectral lines. The multithermal solutions are also found to be biased toward near-isothermal solutions around 1 MK. This is true even if the residuals support the chosen DEM model, possibly leading to erroneous conclusions on the observed plasma. We propose tools for identifying and quantifying the possible degeneracy of solutions, thus helping the interpretation of DEM inversion.« less
  • X-ray and extreme ultraviolet (EUV) observations are an important diagnostic of various plasma parameters of the solar atmosphere during solar flares. Soft X-ray and EUV observations often show coronal sources near the top of flaring loops, while hard X-ray emission is mostly observed from chromospheric footpoints. Combining RHESSI with simultaneous Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA) observations, it is possible for the first time to determine the density, temperature, and emission profile of the solar atmosphere over a wide range of heights during a flare, using two independent methods. Here we analyze a near limb event during the first ofmore » three hard X-ray peaks. The emission measure, temperature, and density of the coronal source is found using soft X-ray RHESSI images while the chromospheric density is determined using RHESSI visibility analysis of the hard X-ray footpoints. A regularized inversion technique is applied to AIA images of the flare to find the differential emission measure (DEM). Using DEM maps, we determine the emission and temperature structure of the loop, as well as the density, and compare it with RHESSI results. The soft X-ray and hard X-ray sources are spatially coincident with the top and bottom of the EUV loop, but the bulk of the EUV emission originates from a region without cospatial RHESSI emission. The temperature analysis along the loop indicates that the hottest plasma is found near the coronal loop-top source. The EUV observations suggest that the density in the loop legs increases with increasing height while the temperature remains constant within uncertainties.« less
  • We present a detailed description of the interrelation between the Type III radio bursts and energetic phenomena associated with the flare activities in active region AR11158 at 07:58 UT on 2011 February 15. The timing of the Type III radio burst measured by the radio wave experiment on Wind/WAVE and an array of ground-based radio telescopes coincided with an extreme-ultraviolet (EUV) jet and hard X-ray (HXR) emission observed by SDO/AIA and RHESSI, respectively. There is clear evidence that the EUV jet shares the same source region as the HXR emission. The temperature of the jet, as determined by multiwavelength measurementsmore » by Atmospheric Imaging Assembly, suggests that Type III emission is associated with hot, 7 MK, plasma at the jet's footpoint.« less
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