| Magnetic reconnection is an essential and common phenomenon in space plasmas and laboratory plasmas,with the topology change of magnetic field lines,associated with remarkable energy exchange.It is believed that magnetic reconnection is of virtual importance in accelerating and heating of the charged particles and in driving multi-scale instabilities.Observations indicate that wave perturbations may be generated during reconnection,such as the macroscale MHD and the microscale kinetic perturbations,among which one of the most important electromagnetic modes is the Alfven wave,which plays essential roles in the coupling of magnetosphere and ionosphere.However,Alfven wave cannot explain the particle acceleration in the parallel direction during the coupling of the magnetosphere and ionosphere during substorms due to the lack of parallel component of electric field.Satellite observations also report the existence of the particles flowing at a super-Alfvenic bulk speed.This indicates that there is a mechanism that can break the Alfven speed limit.Investigations on ion kinetic effects indicate that Alfven waves will be modified as the kinetic Alfven waves(KAWs)once there are perpendicular perturbations at the scale of ion Larmor radius.KAWs can propagate along the magnetic field,associated with parallel electric field and parallel Poynting flux.It is believed that KAWs maybe are responsible for such as the super-Alfvenic particle flows,the parallel acceleration of charged particles,and the aurora acceleration during substorms.Consequently,the kinetic modification on Alfven waves is one of the most important topics in the research of magnetic reconnection.Moreover,observations also indicate that the bursty bulk flows and the reconnection with different spatial isolated X lines are the main course of the energy transport in the magnetotail.Thus,examinations of reconnection with various X line lengths are also important.Observations have shown the existence of KAWs,which are believed related to magnetic reconnection.However,numerical simulations on the generation and characteristics,the propagation and damping,and the energy transport of KAWs during reconnection,in particular during three-dimensional(3D)reconnection,are still poor.In addition,whether KAWs generated during reconnection play important roles in particle acceleration and heating as well as the heating mechanism is yet to be studied.The hybrid model,in which the ions are treated as full particles while electrons are regarded as a massless fluid,can effectively depict the physics at the scale of the ion Larmor radius within a much larger spatial scale.In this paper,we have investigated magnetic reconnection with various X line lengths and guide field strengths,in order to systematically study the generation,the features of wave structures,the propagation and damping,the process of energy transport,and the roles in ion acceleration and heating of the KAWs.The main results are generalized as follows.1.KAWs in reconnection with single infinite X lineIn the infinite X line reconnection,it is found that the perturbations are 2D like both with and without a guide field.Perturbations of KAWs,which are generated from the X line and propagate along the magnetic field lines,are found to exist in the whole bulge region,with a large perpendicular wave number.Base on the analyses and discussions on the characteristics,propagation speed,and polarization relation,we identify such perturbations as the KAWs.Under a zero guide field,there is a quadrupole out-of-plane magnetic field perturbation as well as a quasi-steady whistler structure with zero group velocity near the ion diffusion region.Both the KAWs and the whistler structure result from the X line and they co-exist in different regions during reconnection.Under a finite guide field,neither the Hall quadrupole out-of-plane magnetic field nor the whistler structures is seen near the ion diffusion region.Moreover,we have also investigated the energy flux carried by the KAWs.It is found that the KAWs own 7.5%-8%of the released magnetic energy without a guide field while 18.6%of the released magnetic energy with a finite guide field.Consequently,the finite guide can effectively facilitate more powerful local KAW perturbations.2.KAWs in reconnection with a single finite X line and with multiple X linesThe perturbations generated during reconnection are 3D in nature with a finite X line length and finite guide field strength.It is found that perturbations with the feature of the KAWs exist around the downstream separatrices near the bulge region.Such perturbations are proven to be KAWs through the analysis on the wave number,the propagation speed,and the dispersion relation.We have studied the damping of the KAWs and found a damping rate being about-0.0035--0.0062,under which KAWs generated during tail reconnection can propagate into the ionosphere carrying significant energy flux.Moreover,we have also investigated the effect of the guide field with five different strengths.It is found that the perturbation structures inside the reconnection layer are different with a zero or a moderate guide field.Besides,it is also found that the energy ratio between the wave energy and the released magnetic energy is larger with a moderate guide field than that with a small guide field.Furthermore,we have also simulated the multiple X line reconnection,in which the perturbation structures associated with each X line are very similar to the single finite X line case.Thus,it is reasonable to apply the analysis and results of the single finite X line case to the multiple X line case.In addition,we have found the consistency in the polarization,spectral density,energy transport,and ion heating rate between the KAWs in our simulations and satellite observations.3.Ion acceleration and heating due to KAWsIn the simulation,we have investigated the ion velocity distributions in different regions along the propagation path of the wave.As the wave approaching,an accelerated beam forms in the parallel direction with ion velocities departing from Maxwellian distributions.In the parallel direction,the bulk speed of the accelerated beam is very close to the phase speed of the wave.In addition,we have found through the velocity distributions that ion heating takes place in both the parallel and the perpendicular directions.Once the ions are influenced by the waves,they are picked up and then accelerated.In the early time,the cyclotron or sub-cyclotron resonance condition may be satisfied because the parallel velocities of the ions are small.Thus,stochastic ion heating are seen in the perpendicular direction.In the later time,as the parallel velocities of the ions being close to the phase speed of the wave,the ions are trapped by the wave in the minimum field potential.As such,the ions are heated in the parallel direction through Landau resonance,while the perpendicular stochastic heating is weak.For the whole ion population,the parallel and perpendicular heating appear in the same time because there are particles that are accelerated by the KAW pulse continuously generated from the X line.Moreover,beam-plasma interaction will also take place after the generation of the accelerated ion beam.The beam-plasma interaction first causes an excitation of instability at the expense of the particle energy subsequently followed by plasma heating due to further wave-particle interactions.In general,there are several evidences indicating that the ion heating is due to the KAWs.First,the parallel velocity of the accelerated ions is approximately equal to the phase speed of the wave.Second,the damping rate of the wave and the hearing rate of the ions are consistent with each other.Third,coherent variations are found between the perpendicular ion temperature and the characteristic electric and magnetic perturbations of the KAWs.Through the above results,we have found that the KAWs exist in the whole bulge region with a large perpendicular wave number and propagate with a super-Alfvenic speed along the field lines,resulting in the acceleration and heating of ions through different mechanisms.These results will help to explain some of the energy transport processes associated with KAWs,such as the aurora brightening during substorms. |
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