The effects of density inhomogeneities on the radio wave emission in electron beam plasmas

Type III radio bursts are radio emissions associated with solar flares. They are considered to be caused by electron beams travelling from the solar corona to the solar wind. Magnetic reconnection is a possible accelerator of electron beams in the course of solar flares since it causes unstable distribution functions and density inhomogeneities (cavities). The properties of radio emission by electron beams in an inhomogeneous environment are still poorly understood. We capture the nonlinear kinetic plasma processes of the generation of beam-related radio emissions in inhomogeneous plasmas by utilizing fully kinetic particle-in-cell code numerical simulations. Our model takes into account initial electron velocity distribution functions (EVDFs) as they are supposed to be created by magnetic reconnection. We focus our analysis on low-density regions with strong magnetic fields. The assumed EVDFs allow two distinct mechanisms of radio wave emissions: plasma emission due to wave–wave interactions and so-called electron cyclotron maser emission (ECME) due to direct wave–particle interactions. We investigate the effects of density inhomogeneities on the conversion of free energy from the electron beams into the energy of electrostatic and electromagnetic waves via plasma emission and ECME, as well as the frequency shift of electron resonances caused by perpendicular gradients in the beam EVDFs. Our most important finding is that the number of harmonics of Langmuir waves increases due to the presence of density inhomogeneities. The additional harmonics of Langmuir waves are generated by a coalescence of beam-generated Langmuir waves and their harmonics.

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Formation of non-thermal electron velocity distribution functions in kinetic magnetic reconnection

10.5194/egusphere-egu21-11164 ◽

2021 ◽

Author(s):

Xin Yao ◽

Patricio A. Muñoz ◽

Jörg Büchner

Keyword(s):

Magnetic Field ◽

Free Energy ◽

Field Strength ◽

Magnetic Reconnection ◽

Electron Beams ◽

Distribution Functions ◽

Electron Velocity ◽

Diffusion Region ◽

Electron Velocity Distribution ◽

Velocity Distribution Functions

<div> <div>Magnetic reconnection can convert magnetic energy into non-thermal particle energy in the form of electron beams. Those accelerated electrons can, in turn, cause radio emission in environments such as solar flares. The actual properties of those electron velocity distribution functions (EVDFs) generated by reconnection are still not well understood. In particular the properties that are relevant for the micro-instabilities responsible for radio emission. We aim thus at characterizing the electron distributions functions generated by 3D magnetic reconnection by means of fully kinetic particle-in-cell (PIC) code simulations. Our goal is to characterize the possible sources of free energy of the generated EVDFs in dependence on an external (guide) magnetic field strength. We find that: (1) electron beams with positive gradients in their parallel (to the local magnetic field direction) distribution functions are observed in both diffusion region (parallel crescent-shaped EVDFs) and separatrices (bump-on-tail EVDFs). These non-thermal EVDFs cause counterstreaming and bump-on-tail instabilities. These electrons are adiabatic and preferentially accelerated by a parallel electric field in regions where the magnetic moment is conserved. (2) electron beams with positive gradients in their perpendicular distribution functions are observed in regions with weak magnetic field strength near the current sheet midplane. The characteristic crescent-shaped EVDFs (in perpendicular velocity space) are observed in the diffusion region. These non-thermal EVDFs can cause electron cyclotron maser instabilities. These non-thermal electrons in perpendicular velocity space are mainly non-adiabatic. Their EVDFs are attributed to electrons experiencing an E&#215;B drift and meandering motion. (3) As the guide field strength increases, the number of locations in the current sheet with distributions functions featuring a perpendicular source of free energy significantly decreases.</div> </div>

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Radio signatures of shock-accelerated electron beams in the solar corona

Astronomy and Astrophysics ◽

10.1051/0004-6361/201730546 ◽

2018 ◽

Vol 609 ◽

pp. A41 ◽

Cited By ~ 5

Author(s):

G. Mann ◽

V. N. Melnik ◽

H. O. Rucker ◽

A. A. Konovalenko ◽

A. I. Brazhenko

Keyword(s):

Solar Corona ◽

Electron Beams ◽

Radio Spectrum ◽

Plasma Emission ◽

Radio Bursts ◽

Thermal Electron ◽

Langmuir Waves ◽

Energetic Electrons ◽

Fine Structures ◽

Theoretical Results

Context. The Sun’s activity can appear in terms of radio bursts. In the frequency range 8−33 MHz the radio telescope URAN-2 observed special fine structures appearing as a chain of stripes of enhanced radio emission in the dynamic radio spectrum. The chain drifts slowly from 26 to 23 MHz within 4 min. The individual structures consist of a “head” at the high-frequency edge and a “tail” rapidly drifting from the “head” to lower frequencies over an extent of ≈10 MHz within 8 s. Since they resemble the well-known “herring bones” in type II radio bursts, they are interpreted as shock accelerated electron beams. Aims. The electron beams generating these fine structures are considered to be produced by shock drift acceleration (SDA). The beam electrons excite Langmuir waves which are converted into radio waves by nonlinear wave-plasma processes. That is called plasma emission. The aim of this paper is to link the radio spectral data of these fine structures to the theoretical results in order to gain a better understanding of the generation of energetic electrons by shocks in the solar corona. Methods. Adopting SDA for generating energetic electrons, the accelerated electrons establish a beam-like velocity distribution. Plasma emission requires the excitation of Langmuir waves, which is efficient if the velocity of the beam electrons exceeds a few times thermal electron speed. That is the case if the angle between the shock normal and the upstream magnetic field is nearly perpendicular. Hence, the Rankine-Hugoniot relationships, which describe the shock transition in the framework of magnetohydrodynamics, are evaluated for the special case of nearly perpendicular shocks under coronal circumstances. Results. The radio data deduced from the dynamic radio spectrum can be related in the best way to the theoretical results, if the electron beams, which generate these fine structures, are generated via SDA at an almost perpendicular shock, which is traveling nearly horizontally to the surface of the Sun.

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Magnetic reconnection in solar flares

Uspekhi Fizicheskih Nauk ◽

10.3367/ufnr.0180.201009j.0997 ◽

2010 ◽

Vol 180 (9) ◽

pp. 997 ◽

Cited By ~ 5

Author(s):

B.V. Somov

Keyword(s):

Magnetic Reconnection ◽

Solar Flares

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Alfven resonances, forced magnetic reconnection and model of solar flares

Plasma Physics and Controlled Fusion ◽

10.1088/0741-3335/45/6/308 ◽

2003 ◽

Vol 45 (6) ◽

pp. 949-955

Author(s):

C Uberoi

Keyword(s):

Magnetic Reconnection ◽

Solar Flares ◽

Forced Magnetic Reconnection

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Electron beam relaxation in inhomogeneous plasmas

Annales Geophysicae ◽

10.5194/angeo-31-1379-2013 ◽

2013 ◽

Vol 31 (8) ◽

pp. 1379-1385 ◽

Cited By ~ 5

Author(s):

A. Voshchepynets ◽

V. Krasnoselskikh

Keyword(s):

Electron Beam ◽

Beam Energy ◽

Electron Beams ◽

Inhomogeneous Plasma ◽

Density Fluctuations ◽

Langmuir Waves ◽

Wave Trapping ◽

Beam Plasma ◽

Beam Plasma Instability ◽

Accelerated Particles

Abstract. In this work, we studied the effects of background plasma density fluctuations on the relaxation of electron beams. For the study, we assumed that the level of fluctuations was so high that the majority of Langmuir waves generated as a result of beam-plasma instability were trapped inside density depletions. The system can be considered as a good model for describing beam-plasma interactions in the solar wind. Here we show that due to the effect of wave trapping, beam relaxation slows significantly. As a result, the length of relaxation for the electron beam in such an inhomogeneous plasma is much longer than in a homogeneous plasma. Additionally, for sufficiently narrow beams, the process of relaxation is accompanied by transformation of significant part of the beam kinetic energy to energy of accelerated particles. They form the tail of the distribution and can carry up to 50% of the initial beam energy flux.

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Response of chromospheric lines to different periodic non-thermal electron beams

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316000429 ◽

2015 ◽

Vol 11 (S320) ◽

pp. 239-242

Author(s):

Jianxia Cheng ◽

Mingde Ding

Keyword(s):

Electron Beam ◽

Solar Flares ◽

Local Thermodynamic Equilibrium ◽

Response Times ◽

Electron Beams ◽

Lower Atmosphere ◽

Mass Motion ◽

Thermal Electron ◽

Hydrodynamic Simulations ◽

Beam Injection

AbstractSolar flares produce radiations in very broad wavelengths. Spectra can supply us abundant information about the local plasma, such as temperature, density, mass motion and so on. Strong chromospheric lines, like the most studied Hα and Ca II 8542 Å lines are formed under conditions of departures from local thermodynamic equilibrium in the lower atmosphere subject to flare heating. Understanding how these lines are formed is very useful for us to correctly interpret the observations. In this paper, we try to figure out the response of chromospheric lines heated by different periodic non-thermal electron beams. Our results are based on radiative hydrodynamic simulations. We vary the periods of electron beam injection from 1.25 s to 20 s. We compare the response times to different heating parameters. Possible explanations are discussed.

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