Spectra and Field Lines in the Nonlinear Heliosphere

Dr. D. Aaron Roberts
Lab. for Extraterrestrial Physics
NASA Goddard Space Flight Center

Spatial inhomogeneity and temporal variability near the Sun produce a highly nonuniform solar wind. Spacecraft studies show that this nonlinear medium is evolving and producing, for example, shocked stream interaction regions, turbulent cascades, and possible vortex streets. It has now become possible to study this evolution using 3-D MHD simulations in spherical geometry, so that a wide variety of effects can be included. The simulations now reproduce much of the observed behavior, such as strong spectral evolution near current sheets and shear layers and the formation of the expected interaction regions. New insights include the realization that nonlinear effects do not turn wave vectors in the way required by some simple models of the fluctuations, and that the observed two-dimensional correlation function for interplanetary fluctuations can be reproduced by sheared waves in an expanding geometry. On the global scale, the simulations have revealed that it is difficult to keep the two heliospheric magnetic sectors separated, and that loop-like ``connection" fields should be common. This conclusion is supported by Helios data at heliospheric current sheet crossings. The effects causing the connection field may be important for other phenomena, such as where rigidly rotating coronal holes are sheared by differential rotation.