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Register a new userDynamical modulation of solar flare electron acceleration due to plasmoid-shock interactions in the looptop region
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A fast-mode shock can form in the front of reconnection outflows and has been suggested as a promising site for particle acceleration in solar flares. Recent development of magnetic reconnection has shown that numerous plasmoids can be produced in a large-scale current layer. Here we investigate the dynamical modulation of electron acceleration in the looptop region when plasmoids intermittently arrive at the shock by combining magnetohydrodynamics simulations with a particle kinetic model. As plasmoids interact with the shock, the looptop region exhibits various compressible structures that modulate the production of energetic electrons. The energetic electron population varies rapidly in both time and space. The number of 5$-$10 keV electrons correlates well with the area with compression, while that of $>$50 keV electrons shows good correlation with strong compression area but only moderate correlation with shock parameters. We further examine the impacts of the first plasmoid, which marks the transition from a quasi-steady shock front to a distorted and dynamical shock. The number of energetic electrons is reduced by $sim 20%$ at 15$-$25 keV and nearly 40% for 25$-$50 keV, while the number of 5$-$10 keV electrons increases. In addition, the electron energy spectrum above 10 keV evolves softer with time. We also find double or even multiple distinct sources can develop in the looptop region when the plasmoids move across the shock. Our simulations have strong implications to the interpretation of nonthermal looptop sources, as well as the commonly observed fast temporal variations in flare emissions, including the quasi-periodic pulsations.
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Solar flares - the most powerful explosions in the solar system - are also efficient particle accelerators, capable of energizing a large number of charged particles to relativistic speeds. A termination shock is often invoked in the standard model of solar flares as a possible driver for particle acceleration, yet its existence and role have remained controversial. We present observations of a solar flare termination shock and trace its morphology and dynamics using high-cadence radio imaging spectroscopy. We show that a disruption of the shock coincides with an abrupt reduction of the energetic electron population. The observed properties of the shock are well-reproduced by simulations. These results strongly suggest that a termination shock is responsible, at least in part, for accelerating energetic electrons in solar flares.
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.
The shock structure of a plasmoid in magnetic reconnection in low-beta plasmas is investigated by two-dimensional magnetohydrodynamic simulations. Using a high-accuracy code with unprecedented resolution, shocks, discontinuities, and their intersections are resolved and clarified. Contact discontinuities emanate from triple-shock intersection points, separating fluids of different origins. Shock-diamonds inside the plasmoid appear to decelerate a supersonic flow. New shock-diamonds and a slow expansion fan are found inside the Petschek outflow. A sufficient condition for the new shock-diamonds and the relevance to astrophysical jets are discussed.
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.
A common feature of electromagnetic emission from solar flares is the presence of intensity pulsations that vary as a function of time. Known as quasi-periodic pulsations (QPPs), these variations in flux appear to include periodic components and characteristic time-scales. Here, we analyse a GOES M3.7 class flare exhibiting pronounced QPPs across a broad band of wavelengths using imaging and time-series analysis. We identify QPPs in the timeseries of X-ray, low frequency radio and EUV wavelengths using wavelet analysis, and localise the region of the flare site from which the QPPs originate via X-ray and EUV imaging. It was found that the pulsations within the 171 .A, 1600 .A, soft X-ray (SXR), and hard X-ray (HXR) light curves yielded similar periods of $sim$122 s, $sim$131s, $sim$123 s, and $sim$137 s, respectively, indicating a common progenitor. The low frequency radio emission at 2.5 MHz contained a longer period of $sim$231 s. Imaging analysis indicates that the location of the X-ray and EUV pulsations originates from a HXR footpoint linked to a system of nearby open magnetic field lines. Our results suggest that intermittent particle acceleration, likely due to bursty magnetic reconnection, is responsible for the QPPs. The precipitating electrons accelerated towards the chromosphere produce the X-ray and EUV pulsations, while the escaping electrons result in low frequency radio pulses in the form of type III radio bursts. The modulation of the reconnection process, resulting in episodic particle acceleration, explains the presence of these QPPs across the entire spatial range of flaring emission.
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