Solar flares and Kelvin-Helmholtz instabilities: A parameter survey

Hard X-ray (HXR) sources are frequently observed near the top of solar flare loops, and the emission is widely ascribed to bremsstrahlung. We here revisit an alternative scenario which stresses the importance of inverse Compton processes and the Kelvin- Helmholtz instability (KHI) proposed by Fang et al. (2016). This scenario adds a novel ingredient to the standard flare model, where evaporation flows from flare-impacted chromospheric foot-points interact with each other near the loop top and produce turbulence via KHI. The turbulence can act as a trapping region and as an efficient accelerator to provide energetic electrons, which scatter soft X-ray (SXR) photons to HXR photons via the inverse Compton mechanism. This paper focuses on the trigger of the KHI and the resulting turbulence in this new scenario. We perform a parameter survey to investigate the necessary ingredients to obtain KHI through interaction of chromospheric evaporation flows. When turbulence is produced in the loop apex, an index of -5/3 can be found in the spectra of velocity and magnetic field fluctuations. The KHI development and the generation of turbulence are controlled by the amount of energy deposited in the chromospheric foot-points and the time scale of its energy deposition, but typical values for M class flares show the KHI development routinely. Asymmetry of energy deposition determines the location where the turbulence is produced, and the synthesized SXR light curve shows a clear periodic signal related to the sloshing motion of the vortex pattern created by the KHI.Comment: 12 pages, 14 figure

Exploring self-consistent 2.5 D flare simulations with MPI-AMRVAC

Context. Multi-dimensional solar flare simulations have not yet included detailed analysis of the lower atmospheric responses such as down-flowing chromospheric compressions and chromospheric evaporation processes. Aims. We present an analysis of multi-dimensional flare simulations, including analysis of chromospheric up-flows and down-flows that provide important groundwork for comparing 1D and multi-dimensional models. Methods. We follow the evolution of an MHD standard solar flare model including electron beams, where localized anomalous resistivity initiates magnetic reconnection. We vary the background magnetic field strength, to produce simulations that cover a large span of observationally reported solar flare strengths. Chromospheric energy fluxes, and energy density maps are used to analyse the transport of energy from the corona to the lower atmosphere, and the resultant evolution of the flare. Quantities traced along 1D field-lines allow for detailed comparison with 1D evaporation models.Comment: Accepted to A&

MHD turbulence formation in solar flares: 3D simulation and synthetic observations

Turbulent plasma motion is common in the universe, and invoked in solar flares to drive effective acceleration leading to high energy electrons. Unresolved mass motions are frequently detected in flares from extreme ultraviolet (EUV) observations, which are often regarded as turbulence. However, how this plasma turbulence forms during the flare is still largely a mystery. Here we successfully reproduce observed turbulence in our 3D magnetohydrodynamic simulation where the magnetic reconnection process is included. The turbulence forms as a result of an intricate non-linear interaction between the reconnection outflows and the magnetic arcades below the reconnection site, in which the shear-flow driven Kelvin-Helmholtz Instability (KHI) plays a key role for generating turbulent vortices. The turbulence is produced above high density flare loops, and then propagates to chromospheric footpoints along the magnetic field as Alfvenic perturbations. High turbulent velocities above 200 km s^-1 can be found around the termination shock, while the low atmosphere reaches turbulent velocities of 10 km s^-1 at a layer where the number density is about 10^11 cm^-3. The turbulent region with maximum non-thermal velocity coincides with the region where the observed high-energy electrons are concentrated, demonstrating the potential role of turbulence in acceleration. Synthetic views in EUV and fitted Hinode-EIS spectra show excellent agreement with observational results. An energy analysis demonstrates that more than 10% of the reconnection downflow kinetic energy can be converted to turbulent energy via KHI

Particle Trapping and Acceleration in Turbulent Post-flare Coronal Loops

We present a study of energetic-electron trapping and acceleration in the Kelvin-Helmholtz-induced magnetohydrodynamic (MHD) turbulence of post-flare loops in the solar corona. Using the particle-tracing capabilities of MPI-AMRVAC 3.0, we evolve ensembles of test electrons (i.e. without feedback to the underlying MHD) inside the turbulent looptop, using the guiding-center approximation. With the MHD looptop model of Ruan et al. 2018, we investigate the relation between turbulence and particle trapping inside the looptop structure, showing that better-developed turbulent cascades result in more efficient trapping primarily due to mirror effects. We then quantify the electron acceleration in the time-evolving MHD turbulence, and find that ideal-MHD processes inside the looptop can produce nonthermal particle spectra from an initial Maxwellian distribution. Electrons in this turbulence are preferentially accelerated by mirror effects in the direction perpendicular to the local magnetic field while remaining confined within small regions of space between magnetic islands. Assuming dominance of Bremsstrahlung radiation mechanisms, we employ the resulting information from accelerated electrons (combined with the MHD background) to construct HXR spectra of the post-flare loop that include nonthermal-particle contributions. Our results pave the way to constructing more realistic simulations of radiative coronal structure for comparison with current and future observations.Comment: Accepted in MNRA

MHD simulation of solar flare by applying analytical energetic fast electron model

&amp;lt;p&amp;gt;In order to study the evaporation of chromospheric plasma and the formation of hard X-ray (HXR) sources in solar flare events, we coupled an analytic energetic electron model with the multi-dimensional MHD simulation code MPI-AMRVAC. The transport of fast electrons accelerated in the flare looptop is governed by the test particle beam approach reported in Emslie et al. (1978), now used along individual field lines. Anomalous resistivity, thermal conduction, radiative losses and gravity are included in the MHD model. The reconnection process self-consistently leads to formation of a flare loop system and the evaporation of chromospheric plasma is naturally recovered. The non-thermal HXR emission is synthesized from the local fast electron spectra and local plasma density, and thermal bremsstrahlung soft X-ray (SXR) emission is synthesized based on local plasma density and temperature. We found that thermal conduction is &amp;amp;#160;an efficient way to trigger evaporation flows.&amp;amp;#160;We also found that the generation of a looptop HXR source is a result of fast electron trapping, as evidenced by the pitch angle evolution. By comparing the SXR flux and HXR flux, we found that a possible reason for the &amp;amp;#8220;Neupert effect&amp;amp;#8221; is that the increase of non-thermal and thermal energy follows the same tendency.&amp;lt;/p&amp;gt; </jats:p

Luminescence dynamics of Te doped CdS quantum dots at different doping levels

We have examined steady-state and time-resolved luminescence properties of CdS:Te quantum dots (QDs). The transient emission spectra have a red shift along the emission process. Using singular value decomposition and multiexponential decay analysis, the luminescence is found to originate from two distinct and parallel channels: band-edge excitonic emission and trapping state emission. With increasing amount of Te, the emission peaks of the QDs show an obvious red shift. Our experimental results suggest that CdS: Te quantum dots have tunable emission spectra and luminescence lifetimes which may have applications in chemical sensing, high throughput screening and other biotechnological applications

Conflicts in Implementing Environmental Flows for Small-Scale Hydropower Projects and Their Potential Solutions—A Case from Fujian Province, China

Releasing environmental flows is a valuable strategy for mitigating negative impacts of small-scale hydropower projects on river and riparian ecosystems. However, maintaining environmental flows has faced considerable resistance from different stakeholders, and previous studies have failed to appropriately investigate solutions. Here, online questionnaires and interviews were conducted among small-scale hydropower project owners, government administrators, and the public in Fujian Province, China. The results showed that the major hindrance to implementing environmental flows was the potential economic loss resulting from reductions in electricity production, stakeholders’ skepticism, technical difficulties, and a lack of the government supervision. Diversion-type projects pose the largest losses of electricity production after the release of environmental flows, and by adopting a 10% of mean annual flow as minimum target, most small-scale hydropower projects obtain low marginal profits without compensation. Here, we proposed an appropriate payment for ecosystem services by introducing an economic compensation program for different types of small-scale hydropower projects scaled by potential losses in electricity generation. Under such a scheme, economic losses from a reduction in electricity production are covered by the government, hydropower project owners, and electricity consumers. Our study offers recommendations for policymakers, officials, and researchers for conflict mitigation when implementing environmental flows.</jats:p

Luminescence dynamics of Te doped CdS quantum dots at different doping levels

Abstract We have examined steady-state and time-resolved luminescence properties of CdS:Te quantum dots (QDs). The transient emission spectra have a red shift along the emission process. Using singular value decomposition and multiexponential decay analysis, the luminescence is found to originate from two distinct and parallel channels: band-edge excitonic emission and trapping state emission. With increasing amount of Te, the emission peaks of the QDs show an obvious red shift. Our experimental results suggest that CdS:Te quantum dots have tunable emission spectra and luminescence lifetimes which may have applications in chemical sensing, high throughput screening and other biotechnological applications

A simulation of flare-driven coronal rain

&amp;lt;p&amp;gt;Coronal rains are cool materials (~10,000 K) that appear at hot corona. They are frequently observed in non-flaring loops of active regions and recently observed in flaring loops at gradual phases. Hot coronal loops (~10 MK) are often produced in flare events due to magnetic reconnection. The hot flare loops gradually recover to typical coronal temperature due to thermal conduction and radiative loss, during which condensation can happen due to thermal instability. Here we demonstrate how the rains formed in a flare loop with a two-and-a-half dimensional magnetohydrodynamic simulation. We simulate a flare event from pre-flare phase all the way to gradual phase and successfully reproduce coronal rains. We find that thermal conduction and radiative losses alternately dominate the cooling of the flare loop. We find that runaway cooling and rain formation also induce the appearance of dark post-flare loop systems, as observed in extreme ultraviolet (EUV) channels.&amp;lt;/p&amp;gt;</jats:p