10 research outputs found
The efficiency of electron acceleration during the impulsive phase of a solar flare
Solar flares are known to be prolific electron accelerators, yet identifying
the mechanism(s) for such efficient electron acceleration in solar flare (and
similar astrophysical settings) presents a major challenge. This is due in part
to a lack of observational constraints related to conditions in the primary
acceleration region itself. Accelerated electrons with energies above
20~keV are revealed by hard X-ray (HXR) bremsstrahlung emission, while
accelerated electrons with even higher energies manifest themselves through
radio gyrosynchrotron emission. Here we show, for a well-observed flare on
2017~September~10, that a combination of \emph{RHESSI} hard X-ray and and
SDO/AIA EUV observations provides a robust estimate of the fraction of the
ambient electron population that is accelerated at a given time, with an upper
limit of on the number density of nonthermal (~keV)
electrons, expressed as a fraction of the number density of ambient protons in
the same volume. This upper limit is about two orders of magnitude lower than
previously inferred from microwave observations of the same event. Our results
strongly indicate that the fraction of accelerated electrons in the coronal
region at any given time is relatively small, but also that the overall
duration of the HXR emission requires a steady resupply of electrons to the
acceleration site. Simultaneous measurements of the instantaneous accelerated
electron number density and the associated specific electron acceleration rate
provide key constraints for a quantitative study of the mechanisms leading to
electron acceleration in magnetic reconnection events.Comment: 5 figures, 10 page
Spatiotemporal energy partitioning in a nonthermally dominated two-loop solar flare
Solar flares show remarkable variety in the energy partitioning between thermal and nonthermal components. Those with a prominent nonthermal component but only a modest thermal one are particularly well suited for study of the direct effect of the nonthermal electrons on plasma heating. Here, we analyze such a well-observed, impulsive single-spike nonthermal event, a solar flare SOL2013-11-05T035054, where the plasma heating can be entirely attributed to the energy losses of these impulsively accelerated electrons. Evolution of the energy budget of thermal and nonthermal components during the flare is analyzed using X-ray, microwave, and EUV observations and three-dimensional modeling. The results suggest that (i) the flare geometry is consistent with a two-loop morphology and the magnetic energy is likely released due to interaction between these two loops; (ii) the released magnetic energy is converted to the nonthermal energy of accelerated electrons only, which is subsequently converted to the thermal energy of the plasma; (iii) the energy is partitioned in these two flaring loops in comparable amounts; (iv) one of these flaring loops remained relatively tenuous but rather hot, while the other remained relatively cool but denser than the first. Therefore, this solar flare demonstrates an extreme efficiency of conversion of the free magnetic energy to the nonthermal energy of particle acceleration and the flow of energy into two loops from the nonthermal component to the thermal one with negligible direct heating
Cold Solar Flares I. Microwave Domain
We identify a set of ~100 "cold" solar flares and perform a statistical
analysis of them in the microwave range. Cold flares are characterized by a
weak thermal response relative to nonthermal emission. This work is a follow up
of a previous statistical study of cold flares, which focused on hard X-ray
emission to quantify the flare nonthermal component. Here we focus on the
microwave emission. The thermal response is represented by the soft X-ray
emission measured by the GOES X-ray sensors. We obtain spectral parameters of
the flare gyrosynchrotron emission and investigate patterns of the temporal
evolution. The main results of the previous statistical study are confirmed: as
compared to a "mean" flare, the cold flares have shorter durations, higher
spectral peak frequencies, and harder spectral indices above the spectral peak.
Nonetheless, there are some cold flares with moderate and low peak frequencies.
In a majority of cold flares, we find evidence suggesting the presence of the
Razin effect in the microwave spectra, indicative of rather dense flaring
loops. We discuss the results in the context of electron acceleration
efficiency
On the origin of 140 GHz emission from the 4 July 2012 solar flare
The sub-THz event observed on the 4 July 2012 with the Bauman Moscow State
Technical University Radio Telescope RT-7.5 at 93 and 140~GHz as well as
Kislovodsk and Mets\"ahovi radio telescopes, Radio Solar Telescope Network
(RSTN), GOES, RHESSI, and SDO orbital stations is analyzed. The spectral flux
between 93 and 140 GHz has been observed increasing with frequency. On the
basis of the SDO/AIA data the differential emission measure has been
calculated. It is shown that the thermal coronal plasma with the temperature
above 0.5~MK cannot be responsible for the observed sub-THz flare emission. The
non-thermal gyrosynchrotron mechanism can be responsible for the microwave
emission near ~GHz but the observed millimeter spectral characteristics are
likely to be produced by the thermal bremsstrahlung emission from plasma with a
temperature of about 0.1~MK.Comment: 18 pages, 6 figure
Data-constrained 3D Modeling of a Solar Flare Evolution: Acceleration, Transport, Heating, and Energy Budget
Solar flares are driven by the release of free magnetic energy and its conversion to other forms of energy—kinetic, thermal, and nonthermal. Quantification of partitions between these energy components and their evolution is needed to understand the solar flare phenomenon including nonthermal particle acceleration, transport, and escape as well as the thermal plasma heating and cooling. The challenge of remote-sensing diagnostics is that the data are taken with finite spatial resolution and suffer from line-of-sight (LOS) ambiguity including cases when different flaring loops overlap and project one over the other. Here, we address this challenge by devising a data-constrained evolving 3D model of a multiloop SOL2014-02-16T064620 solar flare of GOES class C1.5. Specifically, we employed a 3D magnetic model validated earlier for a single time frame and extended it to cover the entire flare evolution. For each time frame we adjusted the distributions of the thermal plasma and nonthermal electrons in the model so that the observables synthesized from the model matched the observations. Once the evolving model had been validated in this way, we computed and investigated the evolving energy components and other relevant parameters by integrating over the model volume. This approach removes the LOS ambiguity and permits us to disentangle contributions from the overlapping loops. It reveals new facets of electron acceleration and transport as well as of the heating and cooling of the flare plasma in 3D. We find signatures of substantial direct heating of the flare plasma not associated with the energy loss of nonthermal electrons
The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare
Solar flares are known to be prolific electron accelerators, yet identifying the mechanism(s) for such efficient electron acceleration in solar flare (and similar astrophysical settings) presents a major challenge. This is due in part to a lack of observational constraints related to conditions in the primary acceleration region itself. Accelerated electrons with energies above ∼20 keV are revealed by hard X-ray (HXR) bremsstrahlung emission, while accelerated electrons with even higher energies manifest themselves through radio gyrosynchrotron emission. Here, we show, for a well-observed flare on 2017 September 10, that a combination of RHESSI HXR and and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV observations provides a robust estimate of the fraction of the ambient electron population that is accelerated at a given time, with an upper limit of ≲10 ^−2 on the number density of nonthermal (≥20 keV) electrons, expressed as a fraction of the number density of ambient protons in the same volume. This upper limit is about 2 orders of magnitude lower than previously inferred from microwave observations of the same event. Our results strongly indicate that the fraction of accelerated electrons in the coronal region at any given time is relatively small but also that the overall duration of the HXR emission requires a steady resupply of electrons to the acceleration site. Simultaneous measurements of the instantaneous accelerated electron number density and the associated specific electron acceleration rate provide key constraints for a quantitative study of the mechanisms leading to electron acceleration in magnetic reconnection events
Energy budget of plasma motions, heating, and electron acceleration in a three-loop solar flare
Non-potential magnetic energy promptly released in solar flares is converted
to other forms of energy. This may include nonthermal energy of
flare-accelerated particles, thermal energy of heated flaring plasma, and
kinetic energy of eruptions, jets, up/down flows, and stochastic (turbulent)
plasma motions. The processes or parameters governing partitioning of the
released energy between these components is an open question. How these
components are distributed between distinct flaring loops and what controls
these spatial distributions is also unclear. Here, based on multi-wavelength
data and 3D modeling, we quantify the energy partitioning and spatial
distribution in the well observed SOL2014-02-16T064620 solar flare of class
C1.5. Nonthermal emissions of this flare displayed a simple impulsive
single-spike light curves lasting about 20\,s. In contrast, the thermal
emission demonstrated at least three distinct heating episodes, only one of
which was associated with the nonthermal component. The flare was accompanied
by up and down flows and substantial turbulent velocities. The results of our
analysis suggest that (i) the flare occurs in a multi-loop system that included
at least three distinct flux tubes; (ii) the released magnetic energy is
divided unevenly between the thermal and nonthermal components in these loops;
(iii) only one of these three flaring loops contains an energetically important
amount of nonthermal electrons, while two other loops remain thermal; (iv) the
amounts of direct plasma heating and that due to nonthermal electron loss are
comparable; (v) the kinetic energy in the flare footpoints constitute only a
minor fraction compared with the thermal and nonthermal energies.Comment: ApJ accepted. 19 pages, 16 figure
AtLAST Science Overview Report
International audienceSubmillimeter and millimeter wavelengths provide a unique view of the Universe, from the gas and dust that fills and surrounds galaxies to the chromosphere of our own Sun. Current single-dish facilities have presented a tantalising view of the brightest (sub-)mm sources, and interferometers have provided the exquisite resolution necessary to analyse the details in small fields, but there are still many open questions that cannot be answered with current facilities. In this report we summarise the science that is guiding the design of the Atacama Large Aperture Submillimeter Telescope (AtLAST). We demonstrate how tranformational advances in topics including star formation in high redshift galaxies, the diffuse circumgalactic medium, Galactic ecology, cometary compositions and solar flares motivate the need for a 50m, single-dish telescope with a 1-2 degree field of view and a new generation of highly multiplexed continuum and spectral cameras. AtLAST will have the resolution to drastically lower the confusion limit compared to current single-dish facilities, whilst also being able to rapidly map large areas of the sky and detect extended, diffuse structures. Its high sensitivity and large field of view will open up the field of submillimeter transient science by increasing the probability of serendipitous detections. Finally, the science cases listed here motivate the need for a highly flexible operations model capable of short observations of individual targets, large surveys, monitoring programmes, target of opportunity observations and coordinated observations with other observatories. AtLAST aims to be a sustainable, upgradeable, multipurpose facility that will deliver orders of magnitude increases in sensitivity and mapping speeds over current and planned submillimeter observatories