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Register a new userEnergy budget of plasma motions, heating, and electron acceleration in a three-loop solar flare
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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.
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We use 2D particle-in-cell (PIC) plasma simulations to study electron acceleration by electron temperature anisotropy instabilities, assuming magnetic fields ($B$), electron densities ($n_e$) and temperatures ($T_e$) typical of the top of contracting magnetic loops in solar flares. We focus on the long-term effect of $T_{e,perp} > T_{e,parallel}$ instabilities by driving the anisotropy growth during the whole simulation time ($T_{e,perp}$ and $T_{e,parallel}$ are the temperatures perpendicular and parallel to the field). This is achieved by imposing a shear velocity, which amplifies the field due to magnetic flux freezing, making $T_{e,perp} > T_{e,parallel}$ due to electron magnetic moment conservation. We use the initial conditions: $T_e sim 52$ MK, and $B$ and $n_e$ such that the ratio between the electron cyclotron and plasma frequencies $omega_{ce}/omega_{pe}=0.53$. When the anisotropy becomes large enough, oblique, quasi-electrostatic (OQES) modes grow, efficiently scattering the electrons and limiting their anisotropy. After that, when $B$ has grown by a factor $sim 2-3$ (corresponding to $omega_{ce}/omega_{pe}sim 1.2-1.5$), the unstable modes become dominated by parallel, electromagnetic z (PEMZ) modes. In contrast to the OQES dominated regime, the scattering by PEMZ modes is highly inelastic, producing significant electron acceleration. When the field has grown by a final factor $sim 4$, the electron energy spectrum shows a nonthermal tail that resembles a power-law of index $sim$ 2.9, plus a high-energy bump reaching $sim 300$ keV. Our results suggest a critical role played by $omega_{ce}/omega_{pe}$ and $T_e$ in determining the efficiency of electron acceleration by temperature anisotropy instabilities in solar flares.
We present the study of SOL2015-03-15 M1.2 flare, revealing acceleration of electrons and plasma heating in the sheared twisted magnetic structure in the polarity inversion line (PIL). The scope is to make the analysis of nonthermal electrons dynamics and plasma heating in the highly stressed magnetic loops interacting in the PIL by using X-ray, microwave, ultraviolet, and optical observations. It is found that the most probable scenario for the energy release in the PIL is the tether-cutting magnetic reconnection between the low-lying (3 Mm above the photosphere) magnetic loops within a twisted magnetic flux rope. Energetic electrons with the hardest spectrum were appeared at the onset of plasma heating up to the super-hot temperature of 40 MK. These electrons are localized in a thin magnetic channel with width of around 0.5 Mm with high average magnetic field of about 1200 G. The plasma beta in the super-hot region is less than 0.01. The estimated density of accelerated electrons is about 10^9 cm^-3 that is much less than the super-hot plasma density. The energy density flux of non-thermal electrons is estimated up to 3x10^12 ergs cm^-2s^-1 that is much higher than in the currently available radiative hydrodynamic models. These results revealed that one need to develop new self-consisting flare models reproducing 3D magnetic reconnection in the PIL with strong magnetic field, spatial filamentation of energy release, formation of high energy density populations of nonthermal electrons and appearance of the super-hot plasma.
Plasma turbulence is thought to be associated with various physical processes involved in solar flares, including magnetic reconnection, particle acceleration and transport. Using Ramaty High Energy Solar Spectroscopic Imager ({it RHESSI}) observations and the X-ray visibility analysis, we determine the spatial and spectral distributions of energetic electrons for a flare (GOES M3.7 class, April 14, 2002 23$:$55 UT), which was previously found to be consistent with a reconnection scenario. It is demonstrated that because of the high density plasma in the loop, electrons have to be continuously accelerated about the loop apex of length $sim 2times 10^9$cm and width $sim 7times 10^8$cm. Energy dependent transport of tens of keV electrons is observed to occur both along and across the guiding magnetic field of the loop. We show that the cross-field transport is consistent with the presence of magnetic turbulence in the loop, where electrons are accelerated, and estimate the magnitude of the field line diffusion coefficient for different phases of the flare. The energy density of magnetic fluctuations is calculated for given magnetic field correlation lengths and is larger than the energy density of the non-thermal electrons. The level of magnetic fluctuations peaks when the largest number of electrons is accelerated and is below detectability or absent at the decay phase. These hard X-ray observations provide the first observational evidence that magnetic turbulence governs the evolution of energetic electrons in a dense flaring loop and is suggestive of their turbulent acceleration.
Particle acceleration is one of the most significant features that are ubiquitous among space and cosmic plasmas. It is most prominent during flares in the case of the Sun, with which huge amount of electromagnetic radiation and high-energy particles are expelled into the interplanetary space through acceleration of plasma particles in the corona. Though it has been well understood that energies of flares are supplied by the mechanism called magnetic reconnection based on the observations in X-rays and EUV with space telescopes, where and how in the flaring magnetic field plasmas are accelerated has remained unknown due to the low plasma density in the flaring corona. We here report the first observational identification of the energetic non-thermal electrons around the point of the ongoing magnetic reconnection (X-point); with the location of the X-point identified by soft X-ray imagery and the localized presence of non-thermal electrons identified from imaging-spectroscopic data at two microwave frequencies. Considering the existence of the reconnection outflows that carries both plasma particles and magnetic fields out from the X-point, our identified non-thermal microwave emissions around the X-point indicate that the electrons are accelerated around the reconnection X-point. Additionally, the plasma around the X-point was also thermally heated up to 10 MK. The estimated reconnection rate of this event is ~0.017.
We studied a circular-ribbon flare, SOL2014-12-17T04:51, with emphasis on its thermal evolution as determined by the Differential Emission Measure (DEM) inversion analysis of the extreme ultraviolet (EUV) images of the Atmospheric Imaging Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO). Both temperature and emission measure start to rise much earlier than the flare, along with an eruption and formation of a hot halo over the fan structure. In the main flare phase, another set of ribbons forms inside the circular ribbon, and expands as expected for ribbons at the footpoints of a postflare arcade. An additional heating event further extends the decay phase, which is also characteristic of some eruptive flares. The basic magnetic configuration appears to be a fan-spine topology, rooted in a minority-polarity patch surrounded by majority-polarity flux. We suggest that reconnection at the null point begins well before the impulsive phase, when the null is distorted into a breakout current sheet, and that both flare and breakout reconnection are necessary in order to explain the subsequent local thermal evolution and the eruptive activities in this confined magnetic structure. Using local DEMs, we found a postflare temperature increase inside the fan surface, indicating that the so-called EUV late phase is due to continued heating in the flare loops.
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