Change of properties related to characteristic ion length scales across four types of interplanetary shocks
Abstrakt
The mechanism of energy dissipation within collisionless plasmas has long been discussed. The interaction between the interplanetary (IP) shock and solar wind has been studied for the understanding of this mechanism.
In the power spectra of the magnetic field in the solar wind, the steepening the spectral slope, often called spectral break, is observed. This break has been considered as a threshold between inertial and kinetic ranges of the plasma, and thus, a transition between different energy dissipating processes.
These processes can be associated mainly with two characteristic ion length scales - ion inertial length and ion gyroradius. The relation of these length scales to the spectral break and its change across magnetohydrodynamic (MHD) shocks were estimated in terms of four different types of IP shocks (fast forwards, fast reverse, slow forwards and slow reverse).
Data of the magnetic field, ion bulk speed, ion thermal speed and ion number density from WIND at 1 AU were used, and continuous wavelet transform (CWT) for the estimation of the magnetic field power spectra was employed. Spectral breaks were determined by fitting 2-segment piecewise linear function around the expected break position in log-log space.
For the fast shocks, the length scale corresponding to the spectral break can be comparable to ion inertial length in the low plasma beta regime (β1) the spectral break length scale converges towards the gyroradius and deviates from the inertial length. This result is consistent with the previous studies, and a similar result can be obtained for the slow shocks.
Two main distinctive differences for slow IP shocks, however, were found: (1) the smaller level of power enhancement of the magnetic field fluctuations and (2) the increase of the plasma beta across the shock. Normalization of power spectra with respect to the spectral break length scale in up and downstream of IP shock facilitates the quantitative analysis of the fluctuation energy evolution across the shock and the dominant energy dissipating mechanisms.
This analysis entails the following open question: whether the IP shocks increase the turbulent energy and how much it increases, or whether they only "re-scale" the spectral parameters.