Collision-free Magnetic Reconnection In The Associated Low Frequency Electromagnetic Activity

The magnetic reconnection is arguably the most important plasma transport and energy conversion process in space physical plasma. Magnetic reconnection is involved in formation and ejection of coronal mass ejecta and plays a role in coronal heating and facilitates the entry of solar wind plasma and electromagnetic energy into magnetosphere. In the magnetosphere proper, magnetic reconnection converts energy stroed in the magnetotail lobes to plasma internal and kinetic energy. The magnetic reconnection constitutes a fundmental and ubiquitous element for Sun-Earth connected sytem. In the past several decades, the magnetic reconnection has developed from large scale MHD theory into small scale collisionless theory, and the reconnection rate increased largly. In this paper, the lower electromagnetic wave instabilities are studied by including collisionless item MHD model in collisonless current sheet, and the wave activity in reconnection event are studied by Cluster data. The main results are followed.1. Theroy study1)In collisionless current sheet, whistler instabitly is observed. The guiding field has greatly affected the instability in current sheet. a) If the disturbed propagation included the component in the direction of non-uniform, the guiding field disappeared or the guiding field smaller, the waves is stable. The relation between the 2-dimensional disturbed propagation and guiding filed is very close. When the guiding was larger than the edge of Harries magntic field, the wave is instable. b)The 3-dimensional ditrubed propagation wave instability is enhanced with the enhanced guiding field. In the middle plane of current sheet, the frequency of wave is increased from low-frequncy hybrid mode to whistler mode. But in the middle plane of current sheet no guiding field no instability exists. c)Hall effect di have no effect on wave mode, but it can make the wave instable more.2)By computing of the 2-dimensional and 3-component collisionless and incompressible MHD modle, whistler is instable too. In this modle the disturbed propagation includes the component of direct of magnetic and density non-uniform. Oblique whistler waves are instable. The growth reate is larger for the whistler wave propagating towards the neutral sheet(k_zd_i< 0 than away form the neutral sheet(k_zd_i>0).3)The disturbed propagation vector include the component of magnetic field and density non-uniform direction, and there exist whistler instability away from current sheet middle plane.2. Observation and analysis1) On August 21, 2002, the collisionless reconnection event and whistler were observed. a)Prior to the southward turning of magnetospheric magnetic field, the weak whistler waves were firstly observed. The burst of whistler waves appeared about 30s earlier than the southward turning. During the whole high-speed flow and south magnetic field event, the whistler was observed. b)In addition, as reconnection proceeded, the wave frequency became higher and higher, and the polarization became stronger and stronger. When tailward flows reached the maximum, the wave frequency got closer to the electron cyclotron frequency. c)When whistler was observed, the parallel and anti-parallel energe electron beam were observed simultaneously. The close relation between whistler waves and energetic electrons during the reconnection process, strongly suggest that the whistler waves are generated and/or enhanced by bursts of counter streaming energetic electron beams. This kind of parallel and anti-parallel directions electrons may exist in the reconnection regions.2)During the period of 2001-2003 year, there are 14 magnetotail reconnection events by Cluster. There are 10 collisionless reconnection events. In these 10 collisionless reconnection events, there are 8 event that observed wave activities prior to the turning of Bz component. a)In these 14 events, there are 10 events that observed Hall magntic field polarity. 8 events observed wave activites prior to the Bz component turning southward more than 30s. For most wave activities observed prior to the Bz component turning southward included right-hand polarization and quasi- parallel propagation whistler mode. b)Hall magnetic field and whistler observed in the reconnection synchronously. The wave frequency became higher and higher, and the power-spectral densities of magnetic field became stronger and stronger with the enhancing tailward ion flow. When tailward flows reached the maximum, the wave frequency got closer to the electron cyclotron frequency.