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Space Plasma Simulations and Models
  Impulsive menetration mechanism
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Impulsive Penetration Mechanism
The theory of the Impulsive Penetration mechanism has been formulated by Lemaire (1977) and Lemaire and Roth (1978) and describes a non-stationary process taking place at the magnetopause. It is argued that plasma elements with an excess momentum, due to an increased density or velocity, can penetrate across the magnetopause and move inside the magnetosphere where it becomes a plasma irregularity. Observations in the the low latitude boundary layer (LLBL) made by Lundin and Dubinin (1985) and later all by Lundin et al. (2003) confirmed the propagation of this type of irregularities inside the magnetosphere.
In the IP scenario the density inside a penetrating plasmoid gradually becomes smaller while it moves inward across closed geomagnetic field lines. This is a direct consequence of the expansion of the volume of the plasmoid due to its spreading along field lines as shown below. The boundaries of the plasma element penetrated by geomagnetic field lines are the seat of parallel electric fields and/or weak double layers. When the plasmoid penetrates through the magnetopause and enters the magnetosphere the external kinetic plasma pressure decreases. While the plasmoid penetrates inside magnetosphere, its boundaries propagate downward toward the ionosphere along field lines of the cusps/clefts. The weak-double layers can be treated as an electrostatic perturbation propagating along geomagnetic field lines with a velocity of the order of the Alfven velocity or less (Drell et al., 1965, VanZeeland et al., 2001). Since B decreases with altitude along geomagnetic field lines much faster than the ion density, the upper most edge of the penetrating plasmoid has a smaller downward (Alfven) speed than its parts at lower altitudes. Thus the field-aligned volume of the plasmoid increases: it expands faster and faster while it drifts earthward into the magnetosphere. The expansion of plasma blobs into a background plasma has been simulated numerically by Mason (1971) and studied in the laboratory by Dimonte and Wiley (1991) and VanZeeland et al. (2001).
Ignoring possible losses of mass by erosion, the field-aligned expansion of the intruding plasma element necessarily produces a decrease of the plasma density. All parcels of the penetrating plasma blob located at smaller L-values have experienced this spreading effect for a longer period of time than the trailing parts at larger L. Thus at larger distances from Earth the volume has expanded significantly less than at the front edge. More details on this qualitative description of the density gradient inside a moving plasmoid may be found in Echim and Lemaire (2002). A review on the numerical and laboratory simulations of the impulsive penetration mechanism has been published by Echim and Lemaire (2000). A model for the decoupling parallel electric field at the edges of an intruding plasma element has been constructed by Echim (2004).
Cross section in the plane xOy of an impulsively penetrating plasma blob. Due to the differential drifts (grad-B and polarization) ions and electrons accumulate at the edges of the plasmoid. These surface charges are indicated by the + and - signs. These are the sources of the electric field, E, which drives the plasma inside the magnetosphere with the bulk velocity UE = E x B/B2. Adiabatic and non-adiabatic braking mechanisms decrease the polarization electric field and the bulk velocity. The advancement of the frontside of the blob stops when and where the energy of convection of the intruding plasmoid has been completely converted into thermal gyromotion and Joule heating of the coupled ionosphere at the polar cusp/cleft latitudes
Three types of possible orbits of the MAGION-4 (M4) and INTERBALL-TAIL (IT) pair of satellites. Orbit 1 is threading radially the plasma blob; a positive density gradient in the Ox direction was observed by Sibeck et al. (2000) between both spacecraft along such types of orbits. Orbit 2 is close to an "isodensity" surface inside the plasmoid; this kind of orbits is not the most favorable to detect density gradients in the radial direction. Orbit 3 illustrates a proper positioning of the spacecraft such that a negative gradient may be evidenced in the radial direction although the plasmoid is not (yet) detached from the magnetopause

Dimonte, G. and L.G. Wiley, Dynamics of exploding plasmas in a magnetic field. Phys. Rev. Lett., 67, 1755-1758, 1991
Drell, S.D., Foly, H.M., and Ruderman, M.A., Drag and propulsion of large satellites in the ionosphere: an Alfv�n propulsion engine in space. J. Geophys. Res., 70, 3131-3145, 1965
Echim, M., and Lemaire, J. , Laboratory and numerical simulations of the impulsive penetration mechanism. Space Sci. Rev., 92, 565-601, 2000
Echim, M. and J. Lemaire, Positive density gradients at the magnetopause: interpretation in the framework of the impulsive penetration mechanism, Journal of Atmospheric and Solar-Terrestrial Physics, 64, 2019-2028, 2002
Echim, M., Kinetic investigation of the impulsive penetration mechanism of 2D plasma elements into the Earth's magnetosphere, Ph. D. Thesis, Universite Chatolique de Louvain, 2004
Lemaire, J., Impulsive penetration of filamentary plasma elements into the magnetospheres of the Earth and Jupiter. Planet. Space Sci., 25, 887-890, 1977.
Lemaire, J., and Roth, M., Penetration of solar wind plasma elements into the magnetosphere. J. Atmos. Terr. Phys., 40, 331-335, 1978.
Lundin, R. and Dubinin, E.M., Solar wind energy transfer regions inside the dayside magnetopause: accelerated heavy ions as tracers for MHD-processes in the dayside boundary layer. Planet. Space Sci., 33, 891, 1985.
Lundin, R., Sauvaud, J.-A., Reme, H. et al., Evidence for impulsive solar wind plasma penetration through the dayside magnetopause, Ann. Geophys., 21, 457-472, 2003
Mason, R.J., Computer simulation of ion-acoustic shocks: the diaphram problem. Phys. Fluids 14, 1943, 1971
Sibeck, D.G., Prech, L., Safrankova, J. and Nemecek, Z., Two-point measurements of the magnetopause: Interball observations, J. Geophys. Res., 105, 237-244, 2000 VanZeeland, M., Gekelman, W., Vincena, S., Dimonte, G., 2001. Production of Alfven waves by a rapidly expanding plasma. Proceeedings of ISSS-6, Copernicus Gesellschaft.
Contact: Dr. Marius Mihai Echim
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