Origin and dynamics of the heliospheric streamer belt and current sheet

Abstract

The broad high-density, low-temperature region around the thin heliospheric current sheet at solar minimum forms a relatively stable “streamer belt” associated with the slow wind flow. This region contains highly structured magnetic fields, with large rotations and discontinuities being common. This observational study examines the likely origins and dynamics of the interplanetary plasma and current sheets primarily using Helios data. The striking differences sometimes observed between the plasma on the two sides of the current sheet support the common interpretation that the plasma above and below the sheet comes from often very different regions located either side of the helmet streamer at the base of the slow wind flow. The entropy per proton in the streamer belt often shows sharply defined regions of strongly different plasma; this implies that the origin of the filamentary structure is in initial conditions near the Sun because dynamical evolution can only equalize entropy. The observed large relative density and other fluctuations may thus represent the conditions on flow tubes with different boundary conditions. The average entropy in the streamer belt increases by about the same factor as in the surrounding high-speed streams, indicating that this region is heated substantially, consistent with studies of the temperature evolution and with turbulence modeling. Compressive stream interaction regions are not preferentially heated (in the sense of an entropy increase) in the inner heliosphere. Strong anticorrelations between density and both temperature and magnetic field magnitude are observed within the streamer belt but not in the surrounding regions, and these become weaker as the flow moves outward. Both the smoother appearance of the entropy at greater heliocentric distance and simulation evidence support the view that the streamer belt region undergoes significant dynamical evolution. This evolution seems to also affect the current sheet because sector boundary crossings are observed to become more complex (more multiple crossings) with increasing heliocentric distance. The crossings are more complex in general near solar maximum, although the complexity still increases with radial distance.