Modulational instability of fast magnetosonic waves emitted from a pinching current sheet

We investigate how fast magnetosonic waves can be produced from a pinching current sheet, by using 3-D MHD code. We show that after magnetic pinch of the current sheet due to pressure imbalance, the current sheet begins to expand by an excess of plasma pressure at the center of the current sheet. During the expansion phase, strong fast magnetosonic waves can be created at the steep region of the density gradient and propagate away from the current sheet. It is shown that the fast magnetosonic waves become unstable against modulational instability, as found by Sakai (1983). After the emission of the fast magnetosonic waves, the current sheet will relax to a new equilibrium state, where the current sheet can be heated by adiabatic compression. The emission processes of the fast magnetosonic waves from the current sheet, as well as the modulational instability of these waves that can lead to effective plasma heating through the Landau damping of the slow waves, are important for an understanding of coronal heating and coronal transient brightening.     

Simulation studies of electron acceleration by ion ring distributions in solar flares

Using a 2 1/2-D fully relativistic electromagnetic particle-in-cell code (PIC) we have investigated a potential electron acceleration mechanism in solar flares. The free energy is provided by ions which have a ring velocity distribution about the magnetic field direction. Ion rings may be produced by perpendicular shocks, which could in turn be generated by the super-Alfvénic motion of magnetic flux tubes emerging from the photosphere or by coronal mass ejections (CMEs). Such ion distributions are known to be unstable to the generation of lower hybrid waves, which have phase velocities in excess of the electron thermal speed parallel to the field and can, therefore, resonantly accelerate electrons in that direction. The simulations show the transfer of perpendicular ion energy to energetic electrons via lower hybrid wave turbulence. With plausible ion ring velocities, the process can account for the observationally inferred fluxes and energies of non-thermal electrons during the impulsive phase of flares. Our results also show electrostatic wave generation close to the plasma frequency: we suggest that this is due to a bump-in-tail instability of the electron distribution.     

太阳大气中动力学阿尔文波的产生与加热研究进展

动力学阿尔文波是垂直波长接近离子回旋半径或者电子惯性长度的色散阿尔文波.由于波的尺度接近粒子的动力学尺度,动力学阿尔文波在太阳和空间等离子体加热、加速等能化现象中起重要作用.因此,动力学阿尔文波通常被认为是日冕加热的候选者.本研究工作深入、系统地调研了太阳大气中动力学阿尔文波的激发和耗散机制.基于日冕等离子体环境,介绍了几种常见的动力学阿尔文波激发机制:温度各向异性不稳定性、场向电流不稳定性、电子束流不稳定性、密度非均匀不稳定性以及共振模式转换.还介绍了太阳大气中动力学阿尔文波的耗散机制,并讨论了这些耗散机制对黑子加热、冕环加热以及冕羽加热的影响.不仅为认识太阳大气中动力学阿尔文波的驱动机制、动力学演化特征以及波粒相互作用提供合理的理论依据,而且有助于揭示日冕等离子体中能量储存和释放、粒子加热等能化现象的微观物理机制.     

太阳运动IV型射电爆发的相干辐射模型

为解释太阳运动IV型射电爆发的相干辐射机制提出一个理论模型．从耀斑中产生的高能电子，可以被扩展上升的太阳磁流管俘获．在磁流管顶部，这些高能电子的速度分布形成为类束流速度分布，激发束流等离子体的不稳定性，并且主要直接放大O模电磁波．不稳定性增长率敏锐地依赖了日冕等离子体参数，fpe／fce和射束温度Tb，这能定性解释在太阳运动IV型射电爆发中观测到的高亮温度和高偏振度，以及宽频谱的特性．     

Coronal loop heating by discrete Alfvén waves

We have modeled the solar coronal active loop heating by discrete Alfvén waves. Discrete Alfvén waves (DAW) are a new class of Alfvén waves which can be described by the two-fluid model with finite ion-cyclotron frequency, or the MHD model with plasma current along the magnetic field line as shown by Appert, Vaclavik, and Villar (1984). We have modeled the coronal loop as a semi-toroidal plasma with the major toroidal radius much larger than the plasma radius. We have shown that the absorption of discrete Alfvén waves by the plasma through viscosity can account for at least 30% of the coronal heating rate density of 10^–4 J m^–3 s^–1.     

Magnetohydrodynamic study of shock waves in the current sheet of a solar coronal magnetic loop

A Lagrangian Remap (LareXd) Code is employed to investigate the shock wave formation in the current sheet of a solar coronal magnetic loop and its effect on the magnetic reconnection. We constructed the slow shock structure in the presence of viscosity and heat conduction parallel and perpendicular to the magnetic field and pairs of slow shocks propagate away from the central current sheet, the so-called diffusion region. Significant jumps in plasma density, pressure, velocity and magnetic field occur across the main shock while the temperature appears in the foreshock. In the presence of dissipative effects, the distinct jumps disappear and the shock profiles show smooth transition between the downstream and the upstream regions while the plasma density and the pressure show very narrow and a sharp decrease with time. These results can be applied to the heating of the solar corona, the structure of the magnetic reconnection and the solar wind.     

The Flare-Energy Distributions Generated by Kink-Unstable Ensembles of Zero-Net-Current Coronal Loops

It has been proposed that the million-degree temperature of the corona is due to the combined effect of barely detectable energy releases, called nanoflares, that occur throughout the solar atmosphere. Unfortunately, the nanoflare density and brightness implied by this hypothesis means that conclusive verification is beyond present observational abilities. Nevertheless, we investigate the plausibility of the nanoflare hypothesis by constructing a magnetohydrodynamic (MHD) model that can derive the energy of a nanoflare from the nature of an ideal kink instability. The set of energy-releasing instabilities is captured by an instability threshold for linear kink modes. Each point on the threshold is associated with a unique energy release; thus we can predict a distribution of nanoflare energies. When the linear instability threshold is crossed, the instability enters a nonlinear phase as it is driven by current sheet reconnection. As the ensuing flare erupts and declines, the field transitions to a lower energy state, which is modelled by relaxation theory; i.e., helicity is conserved and the ratio of current to field becomes invariant within the loop. We apply the model so that all the loops within an ensemble achieve instability followed by energy-releasing relaxation. The result is a nanoflare energy distribution. Furthermore, we produce different distributions by varying the loop aspect ratio, the nature of the path to instability taken by each loop and also the level of radial expansion that may accompany loop relaxation. The heating rate obtained is just sufficient for coronal heating. In addition, we also show that kink instability cannot be associated with a critical magnetic twist value for every point along the instability threshold.     

Heating of chromospheric magnetic features by direct current dissipation

It is shown that the direct current dissipation is very unlikely to be the heat source of the coronal loop, because it accompanies unacceptably high heating rate in the chromospheric portion of the loop. This also suggests that a rather weak current density can supply the heat to a small (R < 10⁷ cm) chromospheric magnetic features. A larger magnetic element may be heated by the direct current dissipation only if the current changes directions within a single element so that the generated magnetic field is sufficiently weak to insure MHD stability.     

Plasma waves caused by transient heat conduction in a coronal loop and anomalous resistivity

Numerical simulation is made of the transient heat conduction during local heating in a model coronal magnetic loop with an axial electric current. It is assumed that a segment near the top of the normal coronal loop is heated to above 10⁷ K by a sufficiently small heat input as compared with the total flare energy. A hump appears in the velocity distribution of electrons moving down the temperature gradient with speeds slightly below the thermal one. Consequently, electron plasma waves are excited. The high intensity of the waves persists in the upper region of the loop for more than a second until the termination of the simulation. The energy density of the plasma waves normalized with respect to thermal density is 10^–3.5 at maximum. A theoretical estimate gives an anomalous resistivity 5 orders of magnitude greater than an initial value. Based on the above result, we propose a model for impulsive loop flares.     

Steady heat conduction in coronal loop unstable against plasma instability

The Fokker-Planck equation is numerically solved to study the electron velocity distribution under steady heat conduction with an applied axial electric current in a model coronal loop.If the loop temperature is so high that the electron mean-free path is longer than the local temperature scale height along the loop, a velocity hump appears at about the local thermal electron velocity. The hump is attributed to cooler electrons moving up the temperature gradient to compensate for the runaway electrons moving down the gradient. If the ratio between the mean free path and temperature scale height is greater than about 2, negative absorption for the plasma waves can appear (waves grow). This effect is enhanced by the presence of axial electric current in the half of the coronal loop in which the electrons carrying the current are drifting up the temperature gradient. Thus, the plasma instability may occur in the coronal elementary magnetic flux tubes. Although the present paper is limited to show the critical condition and linear growth rate of the instability, the following scenarios may be inferred.If the flux tubes change from marginally stable to unstable against the plasma instability, due to an increase in the loop temperature, anomalous resistivity may suddenly appear because of the growth of plasma waves. Then a high axial electric field is induced that may accelerate particles. This could be the onset of impulsive loop flares.For a low electric current, if the loop temperature is sufficiently high to give the negative absorption for the plasma waves in a large part of the coronal loop, steady plasma turbulence may originate. This could be a source for the type I radio noise storm.     

Sub-terahertz emission from solar flares: The plasma mechanism of chromospheric emission

We consider the plasma mechanism of sub-terahertz emission from solar flares and determine the conditions for its realization in the solar atmosphere. The source is assumed to be localized at the chromospheric footpoints of coronal magnetic loops, where the electron density should reach n ≈ 10¹⁵ cm^?3. This requires chromospheric heating at heights h ? 500 km to coronal temperatures, which provides a high degree of ionization needed for Langmuir frequencies ν _p ≈ 200–400 GHz and reduces the bremsstrahlung absorption of the sub-THz emission as it escapes from the source. The plasma wave excitation threshold for electron-ion collisions imposes a constraint on the lower density limit for energetic electrons in the source, n ₁ > 4 × 10⁹ cm^?3. The generation of emission at the plasma frequency harmonic ν ≈ 2ν _p rather than the fundamental tone turns out to be preferred. We show that the electron acceleration and plasma heating in the sub-THz emission source can be realized when the ballooning mode of the flute instability develops at the chromospheric footpoints of a flare loop. The flute instability leads to the penetration of external chromospheric plasma into the loop and causes the generation of an inductive electric field that efficiently accelerates the electrons and heats the chromosphere in situ. We show that the ultraviolet radiation from the heated chromosphere emerging in this case does not exceed the level observed during flares.     

Thermal instability in slab geometry in the presence of anisotropical thermal conduction

Prominences and filaments are thought to arise as a consequence of a magnetized plasma undergoing thermal instability. Therefore, the thermal stability of a magnetized plasma is investigated under coronal conditions. The equilibrium structure of the plasma is approximated by a 1-D slab configuration. This is investigated in thermal instability taking into account optically thin plasma radiation and anisotropic thermal conduction. The thermal conduction perpendicular to the magnetic field is taken to be small but non-zero.The classic rigid wall boundary conditions which are often applied in the literature, either directly on the plasma or indirectly through some other medium, are replaced by a more physical situation in which the plasma column is placed in a low-density background stretching towards infinity. Results for a uniform equilibrium structure indicate the major effect of this change is on the eigenfunctions rather than on the growth rate. Essentially, perpendicular thermal conduction introduces field-aligned fine structure. It is also shown that in the presence of perpendicular thermal conduction, thermal instability in a slab model is only possible if the inner plasma has the shortest thermal instability time scale.Research Assistant of the National Fund for Scientific Research (Belgium).     

Coronal heating by ion acoustic waves

A quantitative study is made based on the suggestion that coronal heating results from Landau damping of ion acoustic waves. It is shown that both ions and electrons are heated by nonlinear saturation of ion acoustic instability. Ions are heated more rapidly than the electrons.     

Coronal heating by Alfvén waves,II

I extend a previous paper which argued that Alfvén waves traveling up a large coronal loop may heat this loop at the top and increase its visibility. This heating is now evaluated more completely, taking into account the changes along the loop in field strength, gas density and flux of waves. The location and efficiency of the heating depend very non-linearly on the intensity of the waves, which allows rapid changes in the visibility of a loop. Observational and theoretical conditions for the applicability of the theory are summarized. Alfvén waves preferentially heat the upper portions of coronal helmets, but a measurable excess temperature on a loop requires somewhat implausibly high wave fluxes. Radiation losses from low-lying loops with strong magnetic fields cannot be explained without modifying the theory.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Intermittent heating in a model of solar coronal loops

X-ray and EUV observations of the solar corona reveal a very complex and dynamic environment where there are many examples of structures that are believed to outline the Sun's magnetic field. In this present study, the authors investigate the temporal response of the temperature, density and pressure of a solar coronal plasma contained within a magnetic loop to an intermittent heating source generated by Ohmic dissipation. The energy input is produced by a one-dimensional MHD flare model. This model is able to reproduce some of the statistical properties derived from X-ray flare observations. In particular the heat deposition consists of both a sub-flaring background and much larger, singular dissipative events. Two different heating profiles are investigated: (a) the spatial average of the square of the current along the loop and (b) the maximum of the square of the current along the loop. For case (a), the plasma parameters appear to respond more to the global variations in the heat deposition about its average value rather than to each specific event. For case (b), the plasma quantities are more intermittent in their evolution. In both cases the density response is the least bursty signal. It is found that the time-dependent energy input can maintain the plasma at typical coronal temperatures. Implications of these results upon the latest coronal observations are discussed.     

Time-dependent behaviour of a stable auroral red arc excited by an electric field

We examine the electric field hypothesis as a possible explanation of a stable auroral red arc. An electric field perpendicular to the geomagnetic field in the ionosphere heats the ambient F-region electrons and ions. Given large enough electric fields, the electrons can be heated sufficiently to excite the OI (¹D) term of atomic oxygen by electron impact, giving rise to the λ6300 emission characteristic of the red arc. The electron and ion heating rates are determined by the relative drift between the plasma and neutral gas.     

Electron beam propagation in plasmas

Electron beam propagation in a fully ionized plasma has been studied using a one-dimensional particle simulation model. The electrostatic results show the essential features of beam-plasma interactions. It is found that the return currents are enhanced by the beam-plasma instability which accelerates ambient plasmas. The results also show the heating of ambient plasmas and the trapping of plasmas due to the locally generated electric field. The electromagnetic simulations show much the same results as the electrostatic simulations do. We discuss the results in context of the heating of coronal plasma during solar flares.     

How To Use Magnetic Field Information For Coronal Loop Identification

The structure of the solar corona is dominated by the magnetic field because the magnetic pressure is about four orders of magnitude higher than the plasma pressure. Due to the high conductivity the emitting coronal plasma (visible, e.g., in SOHO/EIT) outlines the magnetic field lines. The gradient of the emitting plasma structures is significantly lower parallel to the magnetic field lines than in the perpendicular direction. Consequently information regarding the coronal magnetic field can be used for the interpretation of coronal plasma structures. We extrapolate the coronal magnetic field from photospheric magnetic field measurements into the corona. The extrapolation method depends on assumptions regarding coronal currents, e.g., potential fields (current-free) or force-free fields (current parallel to magnetic field). As a next step we project the reconstructed 3D magnetic field lines on an EIT-image and compare with the emitting plasma structures. Coronal loops are identified as closed magnetic field lines with a high emissivity in EIT and a small gradient of the emissivity along the magnetic field.     

Solar plasma structuring due to two-dimensional pinch effect over the photosphere

We study the behavior of the solar plasma over the photosphere in the zone of contact of oppositely directed magnetic fields. A special technique of numerical simulation is used, which allows passing to the class of generalized functions as soon as the solution loses smoothness. An initial-value problem is solved for the self-consistent nonlinear system of equations of collisional magneto-gas-dynamics under the assumption that the distribution of physical quantities is two-dimensional and the plasma has an initial temperature of 50 000 degrees. It is assumed that the magnetic field lines are straight, the physical quantities are constant along them, and the resulting fluid velocity is perpendicular to the magnetic field. It is shown that a pinch effect develops under such conditions, which gives rise to much more diverse effects in a natural ambient medium than in a laboratory plasma. The pinch effect produces narrow, variously directed jets of matter (including those going beyond the zone of contact of the fields), forms cross-shaped patterns in the distribution of the magnetic field, velocity and density, and gives rise to specific temperature nonuniformities. In the center of the contact zone, the plasma temperature increases (we terminate the computations when it doubles). The jet velocity can exceed 20 km/s.     

A model of impulsive loop flares caused by anomalous heat conduction

Numerical simulation is made of the impulsive loop flare caused by transient heat conduction along the loop with an applied axial electric current.It is assumed that a segment near the top of the coronal loop is heated to above 10⁷ K by a heat input that is small compared with the total flare energy, which is given by the magnetic energy of the initial current. Due to the heat conduction, a hump appears in the velocity distribution of electrons, which may excite electron plasma waves with a sufficiently high intensity to cause an anomalous resistivity, as shown theoretically in a previous paper. In that paper, an effect of the plasma waves on the dynamics of electrons was taken into account consistently, but an anomalous heating due to an ohmic dissipation of the initial current under the anomalous resistivity was not taken into account.The aim of the present study is to study the subsequent dynamics of the heated gas caused by the anomalous heating, but in order to avoid an unpractically long computation time, the energy density of the plasma waves is estimated by the energy density of electrons in the velocity hump, without taking into account the effect of the plasma waves consistently in the dynamics of the electrons.The initial current starts to decay gradually by an ohmic dissipation under the anomalous resistivity occurring near the top of the loop to heat this region more. The enhanced heat conduction causes the velocity humps in a wider location. Consequently, the anomalous heating continues and spreads in a self-generating way even after the end of the initial minor heating. Thus the temperature near the loop top becomes above 10⁸ K and the high-temperature region spreads in both directions along the loop with such a high speed as (2–3) × 10⁴ km s^–1, which is nearly equal to the speed of flux-limited heat conduction. On the other hand, induced electric field estimated from the anomalous resistivity is 3.3 × 10⁷ V at the termination of the present simulation, under the modest initial current of 1.5 A m^–2.X-ray emissions expected from the present model loop, show three sources, two footpoints with unequal brightness and a coronal source expanding along the loop in both directions.