Temporal features of the refilling of a plasmaspheric flux tube

Abstract

Time-dependent hydrodynamic equations were solved in order to study the refilling of plasmaspheric flux tubes depleted during magnetic storms. Depending on the initial density profiles in the depleted flux tube, a variety of shock formations occur during the early stage (•-1 hour) of the refilling. After this early stage, an important feature of the flow is the formation of a pair of shocks, which propagate down, one in each hemisphere. The refilling occurs by the downward propagating shocks with speeds • 1.5 km/s and by a simultaneous increase in the plasma density between the two shocks. The time rates of this increase in the density are given for magnetic shells L = 4 and 6.6. These rates depend on the density (No) at the ionospheric boundaries. Depending on No, a fair agreement is found between the refilling rates obtained from our calculations for L = 6.6 and those determined from the GEOS 2 observations. The refilling of a flux tube with different densities in the conjugate ionospheres was also considered, and it was found that after the initial stage of about one hour, the filling occurs through the propagation of only one shock from the hemisphere of the weak ionospheric source to that of the strong one. An interesting feature of the temporal evolution of the density of the shocked plasma after the early stage is the persistence of fluctuations with time periods ranging from about 2 to 10 minutes and corresponding wavelengths ranging from about one to several thousand kilometers. These fluctuations appear to be sound waves. Prior to the shock formation, counterstreaming of the ions is expected to occur in the equatorial region. The single-fluid hydrodynamic model used in the present study is not suited to handle such multiple plasma streams. Therefore, some kinetic considerations for the formation of shocks are given. ResUlts from the particle simulations of counterstreaming plasma expansion show that a potential hill, which eventually slows down the ion streams, leads to the formation of a shock pair in the "equatorial plane." These shocks propagate in opposite directions in a manner analogous to that obtained with the hydrodynamic model of a geomagnetic flux tube. However, the kinetic calculations show that the shock formation occurs only when the electron temperature Te • 3 T/, where T/is the ion temperature. tions become valid. Banks et aL [ 1971] estimated that for the L = 5 flux tube the transition from supersonic to subsonic flow should occur about 22 hours after the filling process began. Schulz and Koons [1972] studied the stability of counter-streaming ion flows using a one-dimensional analysis based on the ion acoustic mode. They concluded that electrostatic shocks were not likely to form at the equator, but that ion trapping could occur via wave-particle interactions. Specifically, they predicted that the interaction between the ion streams occurs via an electrostatic instability until the equatorial density reaches about 20 cm -3. The electrostatic waves induce weak pitch angle : diffusion su...

Simulation of field-aligned H<sup>+</sup> and He<sup>+</sup> dynamics during late-stage plasmasphere refilling

“…This study differs from previous physics-based refilling simulations (Singh et al, 1986;Rasmussen and Schunk, 1988;Guiter and Gombosi, 1990;Tu et al, 2003;Singh and Horwitz, 1992, and references therein) in that both H + and He + are included, the source of plasmasphere ions is a comprehensive physics-based simulated ionosphere (see, e.g. Huba et al, 2000aHuba et al, ,b, 2005, refilling is initiated with the simulated loss of high-altitude plasma taking place over a period of hours (to simulate a storm), the complete ion dynamics along the direction of the field are computed for all ion species (i.e.…”

“…After the first 12 h, however, the results are quite similar, with quantitative differences being of order 20% (see Fig. 5b, Singh et al, 1986;Rasmussen and Schunk, 1988, Fig. 10).…”

“…For example, Rasmussen and Schunk (1988) used a twofluid version of the single-fluid Singh et al (1986) code to perform a two-fluid simulation of the same model refilling problem that was addressed by Singh et al (1986). As expected, results from the two studies differ during early stage refilling, when shock-like flows are evident.…”

“…Hydrodynamic models are differentiated between one-fluid models (Singh et al, 1986;Guiter and Gombosi, 1990) and two fluid models (Rasmussen and Schunk, 1988;Singh et al, 1994), with the latter having separate fluids to distinguish northward and southward interhemispheric flows. During the initial inflow stage, these models produce shocks or shock-like behavior, with the shocks being above the equator in the one-fluid models and near the conjugate end-points in the two-fluid models.…”

Recent Progress in Physics-Based Models of the Plasmasphere

“…The more recent of these models take into account photochemistry, recombination processes and production of various ions due to reactions with the neutral atmosphere. They are based on the transport equations for mass, momentum and energy for the various ions (Singh et al 1986;Rasmussen and Schunk 1988). Ion flows across magnetic field lines have been taken into account in several time dependent models and simulations for various given convection electric field models.…”