Large scale structures

The structure of the solar corona determines the global topology of the solar wind. The solar corona, however, changes strongly with solar activity. During solar minimum conditions, the corona is dominated by the polar coronal holes, from which fast solar wind escapes along open magnetic field lines, with opposite polarity in both hemispheres.

In the equatorial regions one finds the heliospheric current sheet that separates the hemispheres with opposite polarity; the solar wind speed is much lower in its vicinity. This is illustrated in the accompanying overview plot. At solar maximum, the topology is much more complicated and time-varying, with coronal holes occurring at all latitudes.

Solar Wind Overview

In the coronal holes, open magnetic field lines extend out into interplanetary space. Electrons and ions can escape along these field lines. While the plasma in the lower corona is dominated by collisions, it becomes collisionless farther outward, due to the expansion with increasing heliocentric distance. We have developed a kinetic model that describes the accompanying changes in the electron velocity distribution function.

In a different study we examine the large scale structure of the heliospheric current sheet. We do so by means of a hydrodynamic simulation, where we use Wind data (made available by R. Lepping and K. Ogilvie, Nasa GSFC) as boundary conditions at 1 AU, and by means of a comparative study of the sector boundaries observed by Wind and by Ulysses (made available by R. Forsyth and D. Reisenfeld) near the aphelion of the latter early 1998.

Heliospheric current sheet shape
The figure shows a qualitative reconstruction of the shape of the heliospheric current sheet between 1 and 5 AU somewhere during this radial alignment time period. The constant solar rotation rate and the essentially constant radial solar wind outflow produce the characteristic Parker spiral signature.

 

 

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