Collisionless shock waves in plasmas are usually considered as stationary nonlinear waves that cause irreversible changes of plasma state. Quasiperpendicular shock front is well-structured. Ions are thought to be responsible for this structure formation. Electrons are supposed to move in the ion prescribed fields with only small feedback on the shock structure. The foot and overall front scale are determined by the typical ion length: drifting ion gyroradius. Ramp width, on the contrary, is believed to be determined by the wavelength of the whistler, phase standing just upstream of the ramp, and is supposed to be therefore intermediate between typical ion and electron scales. However, in the very beginning of the collisionless shock physics it was observed experimentally (Morse et al., 1971) and in computer simulations (Biscamp and Welter, 1972) that quasiperpendicular shock behavior can become nonstationary. Later it was hypothesized that nonstationary dynamics is typical for high-Mach-number shocks. Now it is clear that there exist several types of nonstationary effects. Both computer simulation and experimental observations have shown different manifestations of shock front variability, which differ by dimensionality and strength. In general, temporal variations result in spatially inhomogeneous multidimensional shock front structure. Relatively weak effects result in a "vibrating" shock front structure resembling that of a stationary shock with relatively small variations of the number of reflected ions and wave activity upstream of the shock, while strong effects may cause the shock front disruption and overturning. In this last case the shock front "disappears" and a "new" one is formed in the vicinity of the "old" front, this phenomenon is usually called a reformation. Several spacecraft programs, including multi-spacecraft missions like ISEE, AMPTE and Cluster provided an opportunity to measure spatial scales and characteristic times more carefully. These measurements evidenced that for High Mach number shocks characteristic ramp scale is of the order of several electron inertial lengths rather than intermediate scale between ions and electrons as it was thought. This leads to the conclusion that the ramp transition is determined by the combination of the dispersion and nonlinearity. These measurements allowed also observing unambiguous manifestations of shock front reformation. We shall discuss the relation between observations of small scale structure of the shock ramp and nonstationary dynamics.
|Original language||English GB|
|Title of host publication||38th COSPAR Scientific Assembly. Held 18-15 July 2010, in Bremen, Germany|
|State||Published - 2010|