Within the known universe,more than 99%of all observable matter is plasma,a state often highly dynamic and far from thermal,as well as mechanical,equilibrium.In particular,for our own solar-terrestrial system,various plasma active phenomena frequently occur such as solar flares,coronal plasma heating,solar wind acceleration,and coronal mass ejections in the solar atmosphere;interplanetary magnetic clouds and collisionless shock waves in interplanetary space;and
In solar radiophysics,many theories for type Ⅲ bursts have been proposed during the past 60 years.Almost all these theories are based on the plasma hypothesis,which assumes that(i)the radiation is mainly generated by Langmuir waves via nonlinear processes and(ii)the radiation has frequencies close to the local plasma frequency and/or its second harmonic in the source region. We feel strongly that it is time to advocate an alternative approach without recourse to the plasma hypothesis.This brief discussion explains why.
Since their use in the study of charged particle motion in the 1960s,Euler potentials(α ,β)have been widely employed as magnetic field coordinates in both space plasma and fusion plasma studies.People related them to magnetic vector potential A via the relation A=α▽β subject to gauge condition A·B=0(B is the magnetic induction).For a given magnetic field,the Euler potentials are often constructed with the relation B·△ S=△α△βon a surface that crosses the field lines,where △ S is the area-element surrounding by two line-elements corresponding to the changes in α and β,then mapping the values of α and βalong field lines into space.In this short paper,we show that in the presence of field line-aligned currents,the mapping does not work and the orthogonality gauge condition is not satisfied.
Dispersive magnetohydrodynamic (MHD) waves with short-wavelength modification have an important role in transforming energy from waves into particles.In this paper,based on the two-fluid mode,a dispersion equation,including the short-wavelength effect,and its exact solution are presented.The outcome is responsible for the short-wavelength modification versions of the three ideal MHD modes (i.e.the fast,slow and Alfve'n).The results show that the fast and Alfve'n modes are modified considerably by the shortwavelength effect mainly in the quasi-parallel and quasi-perpendicular propagation directions,respectively,while the slow mode can be affected by the short-wavelength effect in all propagation directions.On the other hand,the dispersive modification occurs primarily in the finite-β regime of 0.001 < β < 1 for the fast mode and in the high-β regime of 0.1 < β < 10 for the slow mode.For the Alfve'n mode,the dispersive modification occurs from the low-β regime of β < 0.001 through the high-β regime of β > 1.
Based on a three-component description of partially ionized plasmas(i.e.,electrons,ions,and neutral atoms),effects of inelastic collisions between ions(neutrals)and electrons on Alfve′n waves(AWs)in a partially ionized plasma are studied.It is shown that for a fixed ionizability(i)or a fixed inelastic collision parameter(v,i.e.,the ratio of the inelastic to elastic collision frequency),the damping rate of AWs has a peak value round kz vA/min*1,where kz is the parallel wavenumber of AWs,vA is the Alfve′n velocity,and min is the elastic collision frequency between ions and neutrals.On the other hand,the damping rate of AWs decreases monotonously with the ionizabilityi for a fixed inelastic collision parameter,but has a peak value when the inelastic collision parameter varies for sufficiently small ionizability(i\0:1).For sufficiently large ionizability(i[0:1),it is found that the damping rate decreases with the inelastic collision parameter.The results may help us to understand the physics of AWs in partially ionized plasmas.
Electromagnetic ion cyclotron (EMIC) waves,particularly their generation and excitation mechanisms,have been a subject of wide interest because of their potential importance in ion acceleration and heating.In this work,the parameter-dependence of EMIC instabilities is investigated with a combined loss-cone and temperature anisotropy distribution for suprathermal ions.The calculation of the linear growth rate of EMIC waves with an arbitrary propagation angle is presented.The results show that the growth rates of EMIC waves propagating quasi-perpendicular to the ambient magnetic field increase as the loss-cone parameter α increases,whereas the growth rates of EMIC waves propagating quasi-parallel to the ambient magnetic field increase as the temperature anisotropy parameter AT increases.This indicates that the free energies associated with the loss-cone and temperature anisotropic distributions are primarily responsible for the excitation of the quasi-perpendicular and parallel propagating EMIC waves,respectively,and provides us with a more comprehensive understanding of excitation and generation mechanisms for EMIC waves in space plasmas.
