A model is presented to explain the radial evolution of the power spectra of interplanetary Alfvénic fluctuations found by Bavassano et al. (1982a) based on the magnetic field data of Helios 1 and 2. It is assumed in this model that Alfvénic fluctuations represent an asymmetric state of MHD turbulence in which most of the fluctuations are in the Alfvénic wave mode propagating outward from the sun, with a small part of the fluctuations in the wave mode propagating inward. There is weak nonlinear interaction between the two modes. It is also assumed that the turbulence will not become dissociated from the sources that create the waves propagating inward and that the weak nonlinear interaction will remain between 0.3 AU and 1 AU. The nonlinear interaction results in an energy cascading process. Both the effects of the cascading process and the effects of the slow variation of the solar wind on the outward propagating waves determine the radial evolution of the power spectra. Starting with the magnetohydrodynamic equations and deriving the equations governing fluctuations and correlation moments, we finally get an equation which describes the power spectrum. We present an analytic solution. The radial evolution of the power spectra of the fluctuations given by the analytical solution, from 0.29 AU to 0.87 AU, is in agreement with the observation of Helios 1 and 2 in the following aspects: (1) The spectral slope increases (in absolute value) for the frequency range lower than 2.5×10−3 Hz while the slope remains almost unchanged for the frequency range higher than 10−1.5 Hz. (2) The radial gradient of the power spectrum densities increases with increasing frequency. The dissipation length remains nearly unchanged for the frequency range f ≥ 10−2 Hz. (3) The radial variation of 〈b²〉 is approximately r−3.56. (4) The ratio of 〈b²〉/Bo² is nearly a constant.
Solar eruptions are spectacular magnetic explosions in the Sun's corona and how they are initiated remains unclear. Prevailing theories often rely on special magnetic topologies which, however, may not generally exist in the pre-eruption source region of corona. Here using fully three-dimensional magnetohydrodynamic simulations with high accuracy, we show that solar eruption can be initiated in a single bipolar configuration with no additional special topology. Through photospheric shearing motion alone, an electric current sheet forms in the highly sheared core field of the magnetic arcade during its quasi-static evolution. Once magnetic reconnection sets in, the whole arcade is expelled impulsively, forming a fast-expanding twisted flux rope with a highly turbulent reconnecting region underneath. The simplicity and efficacy of this scenario argue strongly for its fundamental importance in the initiation of solar eruptions.
[1] Based on a statistical analysis of the boundary physical states of 80 magnetic clouds reported in the literature from the years 1969 to 2001, we suggest a new identification of the magnetic cloud boundary by describing it as front and tail boundary layers (BLs) formed through the interaction between the magnetic cloud and the ambient medium. In our identification the outer boundary of the layer often displays the properties of magnetic reconnection, which could be characterized by a ''three-high state'' (relatively high proton temperature, high proton density, and high plasma beta) and the corresponding magnetic signatures (the intensity drop and the abrupt azimuthal changes, Áf $ 180°, and latitudinal changes, Áq $ 90°, in the magnetic field). The inner boundary of the layer exhibits a ''three-low state'' (relatively low proton temperature, low proton density, and low plasma b) and separates the magnetic cloud body, which has not basically been affected by the interactions, from the boundary layers. The front boundary layer could be associated with the outer loops of CMEs and its average time scale is 1.7 hours; the tail boundary layer seems not be a filament and its average time scale is 3.1 hours. The distribution function of magnetic fluctuations in the boundary layer is significantly different from those in the ambient solar wind and the cloud body itself. The preliminary numerical simulation in principle confirms this new identification and could qualitatively explain most of the observations of the cloud boundary. This work could help partly overcome some inconsistencies in identifying the boundaries of magnetic clouds.
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