The solar wind exhibits variations on various scale sizes. We have focussed on scales of the order of the particle gyroradii. This is the scale that corresponds to plasma discontinuities.

We have developed a kinetic model that describes the fine structure of tangential discontinuities; we have applied this to solar wind discontinuities at different latitudes, in slow and fast solar wind streams. We have also done a statistical study of both tangential and rotational discontinuities.

Theoretical plasma distributions consistent with Ulysses magnetic field observations in a solar wind tangential discontinuity
J. De Keyser, M. Roth, J. Lemaire, B. T. Tsurutani, C. M. Ho, C. M. Hammond
Solar Phys., 166, 415, 1996. 

The overall multi-layer structure of the magnetic field profile observed by ULYSSES across a broad solar wind tangential discontinuity can be reproduced fairly well by means of a kinetic model. Such a simulation provides complementary information about the velocity distribution functions, which are not always available from the plasma experiment due to the low time resolution inherent in plasma measurements. The success of such a simulation proves that the kinetic model can be used as a realistic basis for further studies of the structure and stability of solar wind tangential discontinuities. 

Solar wind velocity jumps across tangential discontinuities: Ulysses observations and kinetic interpretation
J. De Keyser, M. Roth, B. T. Tsurutani, C. M. Ho, J. L. Phillips
Astronomy and Astrophysics, 321, 945-959, 1997. 

Some tangential discontinuities (TDs) observed in the solar wind by Ulysses interface plasma regions that only differ in their bulk velocity and the orientation of the magnetic field; solar wind composition, density, temperature and magnetic field intensity are essentially the same in both regions. The influence of a plasma velocity jump across the TD on the magnetic field is investigated through the analysis and simulation of equilibrium plane TD configurations. Theoretical results obtained with a kinetic model are compared with Ulysses observations. It is concluded that (a) the theoretically predicted magnetic field profile agrees with the morphology of the observed profile, given the observed velocity jump, (b) solar wind transitions are essentially of mixed type, i.e. both ion and electron velocity distribution functions are non-Maxwellian inside the transition layer, (c) there are constraints on the orientation and magnitude of the velocity jump that can be supported across a single transition, (d) large magnetic field rotations correspond to wide transition layers, and (e) in addition to density and temperature inhomogeneities, variations in the bulk velocity are a major reason for the solar wind plasma to set up current-carrying boundary layers. 

Relative plasma velocity and magnetic field rotation at the heliospheric neutral sheet
J. De Keyser, M. Roth
Annales Geophysicae, Supplement III, 14, 1996

Under suitable IMF conditions the heliospheric neutral sheet may be regarded in a first approximation as a tangential discontinuity. We use a kinetic description of tangential discontinuities to derive theoretical predictions about the influence of the velocity difference between the solar wind above and below the sheet on the magnetic field rotation. These predictions are compared with actual satellite observations made at the neutral sheet.

Flow shear across solar wind discontinuities: WIND observations
J. De Keyser, M. Roth and A. Söding
Geophys. Res. Letters, 25(14), 2649-2652, 1998

We examine the tangential magnetic field and velocity shears across directional discontinuities (DDs) with significant change in magnetic field intensity observed by WIND in slow and fast solar wind streams. The magnetic field rotation sense in fast wind DDs is that predicted by theory for outward propagating rotational discontinuities (RDs), but flow shear magnitude and orientation do not always satisfy RD theory. Alternatively, DDs with small normal magnetic field can be regarded as tangential discontinuities (TDs); the observed shears imply that the length scale over which the proton velocity distribution changes at the discontinuity can be both smaller or larger than that of the electron distribution. The slow wind includes a larger fraction of DDs that disagree with RD theory. It is shown that the flow shear orientations allowed in a TD provide a continuous transition between the opposite orientations for RDs propagating along or against the magnetic field direction.

Author: J. De Keyser   Curator: J. De Keyser   Johan.DeKeyser@oma.be