Large scale modeling

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.

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. 

The figure below 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. Click here if you want to see the full movie (animated gif, about 7 Mb).

Ulysses observations of sector boundaries at aphelion
J. De Keyser, M. Roth, R. Forsyth, D. Reisenfeld 
J. Geophys. Res., 1999. Accepted

We study a sample of sector boundaries observed by Ulysses near its early 1998 aphelion at 5.4 AU. We relate these sector boundaries to solar wind structure seen by Wind at 1 AU, guided by a hydrodynamic simulation. For each Ulysses sector boundary we are able to identify a corresponding 1 AU sector boundary, except when strong transients are present. Sector boundaries appear embedded in complex plasma structures that generally are in a state of pressure balance.  Minimum variance analysis confirms the tangential discontinuity nature of the heliospheric current sheet and indicates that the current sheet tends to be inclined more steeply than at 1 AU. We discuss evidence for the non-planarity of the current sheet.  Magnetic field depressions are characteristic features of the sector boundaries. We attribute them to particles that are magnetically confined inside the current sheet and that carry the diamagnetic current responsible for the large magnetic field rotation. We also present a case where this confined plasma appears to be heated, presumably by wave-particle interactions. 
 
 
 
 

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