The Heliospheric Current Sheet: A Review

Abstract:

The earliest in-ecliptic magnetic field measurements revealed the "Sector Structure", regions surrounding the Sun with opposite polarities either Sunward or Outward. The "sector Boundaries" separating these regions were considered to be thin current sheets. A thicker plasma sheet surrounded the boundaries where the solar wind speed and field magnitude reached a minimum and the density a maximum. The first out-of-ecliptic measurements found that the sector structure disappeared demonstrating that the boundary was a single warped current sheet rotating with the Sun and separating coronal magnetic fields from the north and south hemispheres, effectively the heliospheric magnetic equator. The largest current structure in and beyond the heliosphere, it is variable on time scales from the most rapid magnetic variations to the solar cycle. Scientific interest has continued: connection to the solar -coronal magnetic field, possible reconnection of the oppositely- directed current sheet fields,"closure" of the sheet current at higher latitudes, origin of multiple current sheet crossings, relation to the plasma sheet and to Heat Flux Reversals and Drop-outs, changing inclination of the current sheet over the solar cycle, relation to solar wind Co-rotating Interaction and Rarefaction regions, transport of solar energetic particles and galactic cosmic rays and comparison with magnetospheric current sheets.