Two Problems Related To Plasma Waves In Solar And Space Plasmas

This dissertation focuses on two important problems related to plasma waves in solar and space plasmas: studies for anomalous resistivity due to lower hybrid frequency turbulence in fast magnetic reconnection in magnetotail based on Cluster satellite data and the synthetic coronal heating mechanism on current sheets.The violation to the frozen-in condition in ideal magnetohydrodynamics (MHD) is fundamental for magnetic reconnection which is thought decisively important for fast energy release and conversion events in space, astrophysical and laboratory plasmas, such as solar flares, coronal mass ejections, magnetospheric storms and substorms, as well as sawtooth and disruptive instabilities in toroidal confined devices. Magnetic reconnection relies on the dissipation mechanism in a localized narrow zone called "diffusion" region, where the frozen-in condition is broken. Resistivity anomalously generated via the wave-particle interaction is thought to be able to provide sizable dissipation to explain violation of frozen-in condition and the sudden onset of fast magnetic reconnection.In this thesis, an intensive spectrum of turbulence peaked at lower-hybrid (LH) frequency near the X-point in a fast magnetic reconnection event is observed by the Cluster spacecraft in the magnetotail. The electric field power density around LH frequency is calculated in the thesis from observed spectrum to measure the resistivity anomalously generated by wave-particle interaction. It is found that, the turbulence at LH frequency induced anomalous resistivity is sufficient to trigger fast reconnection.On the other hand, one of the outstanding problems in solar physics is how to understand the multi-million degree temperature of the solar corona, and correspondingly the large energy flux lost from the corona on the order of～10~4Wm~2 in the active, and about 3×10~2 Wm~2 in the quiet regions. Thus, there should be a heating source to maintain the high temperature of the corona and balance the energy loss due to radiation, thermal diffusion and convection, as well as other ways. Based on whether the timescale of the motion of the footpoints is longer or shorter than the shear Alfvén transit time along the loop, the proposed heating mechanisms can be divided into two groups: direct current (DC) and alternating current (AC) heating. The DC heating is the dissipation of the magnetic energy in conventional Joule heating process. On the other hand, the AC heating, or wave heating, is thought to be caused by wave energy dissipation.In this dissertation, a synthetic heating model of solar coronal loops combining current sheet and wave heating mechanisms is proposed. The formation of singular current structures such as current sheets can be caused not only by magnetic reconnection and footpoint convection of coronal loops, but also Alfvén resonance induced by shear flows. On the other hand, with the Hall magnetohydrodynamics (MHD) effect, the kinetic Alfvén wave (KAW) can also be excited on such current structures. Therefore a synthetic energy dispassion process of both current and wave heating mechanisms on the current structures may lead to better understanding of the coronal heating problem.The contents of this thesis are arranged as follows:In Chapter 1, basic theory and physical problem of reconnection, coronal physics, and related plasma waves are introduced briefly; Chapter 2 describes the data from Cluster, and the related analysis methods used in this thesis; Chapter 3 presents the observation of the anomalous resistivity due to lower hybrid frequency turbulence in magnetotail during a reconnection event; In Chapter 4, a synthetic coronal heating model on current sheets is proposed. Then, a brief summary concludes the thesis in the last chapter.