|
|
 |
|
|
Ann. Geophys., 23, 1889-1908, 2005
www.ann-geophys.net/23/1889/2005/
© European Geosciences Union 2005
Magnetohydrodynamic waves within the medium separated by the plane shock wave or rotational discontinuity
A. A. Lubchich and I. V. Despirak
Polar Geophysical Institute, Kola Science Centre, Russian Academy of Sciences, 184200 Apatity, Russia
Abstract. Characteristics of small amplitude plane waves within the
medium separated by the plane discontinuity into two half spaces are analysed.
The approximation of the ideal one-fluid magnetohydrodynamics (MHD) is used.
The discontinuities with the nonzero mass flux across them are mainly
examined. These are fast or slow shock waves and rotational discontinuities.
The dispersion equation for MHD waves within each of half space is obtained
in the reference frame connected with the discontinuity surface. The
solution of this equation permits one to determine the wave vectors versus the
parameter cp, which is the phase velocity of surface discontinuity
oscillations. This value of cp is common for all MHD waves and
determined by an incident wave or by spontaneous oscillations of the
discontinuity surface. The main purpose of the study is a detailed analysis
of the dispersion equation solution. This analysis let us draw the following
conclusions. (I) For a given cp, ahead or behind a discontinuity at
most, one diverging wave can transform to a surface wave damping when moving away
from the discontinuity. The surface wave can be a fast one or, in rare
cases, a slow, magnetoacoustic one. The entropy and Alfvén waves always
remain in a usual homogeneous mode. (II) For certain values of cp and
parameters of the discontinuity behind the front of the fast shock wave,
there can be four slow magnetoacoustic waves, satisfying the dispersion
equation, and none of the fast magnetoacoustic waves. In this case, one of the
four slow magnetoacoustic waves is incident on the fast shock wave from the
side of a compressed medium. It is shown that its existence does not
contradict the conditions of the evolutionarity of MHD shock waves. The four
slow magnetoacoustic waves, satisfying the dispersion equation, can also
exist from either side of a slow shock wave or rotational discontinuity.
(III) The expressions determining the polarisation of the MHD waves are
derived in the reference frame connected with the discontinuity surface.
This form of presentation is much more convenient in investigating the
interaction of small perturbations with MHD discontinuities. It is shown
that the perturbations of the velocity and magnetic field associated with
the surface magnetoacoustic wave have the elliptic polarisation. Usually the
planes of polarisation for the perturbations of the velocity and magnetic
field are not coincident with each other.
Keywords. Space plasma physics (Discontinuities; Shock
waves) – Interplanetary physics (Discontinuities; Interplanetary
shocks) – Magnetospheric physics (Solar windmagnetosphere
interactions)
Full Article in PDF (757 KB) |
|
|
