Fields and plasmas in the outer solar system

Abstract

The most significant information about fields and plasmas in the outer solar system, based on observations by Pioneer 10 and 11 investigations, is reviewed. The characteristic evolution of solar wind streams beyond 1 AU has been observed. The region within which the velocity increases continuously near 1 AU is replaced at larger distances by a thick interaction region with abrupt jumps in the solar wind speed at the leading and trailing edges. These abrupt increases, accompanied by corresponding jumps in the field magnitude and in the solar wind density and temperature, consist typically of a forward and a reverse shock. The existence of two distinct corotating regions, separated by sharp boundaries, is a characteristic feature of the interplanetary medium in the outer solar system. Within the interaction regions, compression effects are dominant and the field strength, plasma density, plasma temperature and the level of fluctuations are enhanced. Within the intervening quiet regions, rarefaction effects dominate and the field magnitude, solar wind density and fluctuation level are very low. These changes in the structure of interplanetary space have significant consequences for the many energetic particles propagating through the medium. The interaction regions control the access to the inner solar system of relativistic electrons from Jupiter's magnetosphere. The interaction regions and shocks appear to be associated with an acceleration of solar protons to MeV energies. Flare-generated shocks are observed to be propagating through the outer solar system with constant speed, implying that the previously recognized deceleration of flare shocks takes place principally near the Sun. Radial gradients in the solar wind and interplanetary field parameters have been determined. The solar wind speed is nearly constant between 1 and 5 AU with only a slight deceleration of =30 km s -~ on the average. The proton flux follows an r -2 dependence reasonably well, however, the proton density shows a larger departure from this dependence. The proton temperature decreases steadily from 1 to 5 AU and the solar wind protons are slightly hotter than anticipated for an adiabatic expansion. The radial component of the interplanetary field falls off like r -2 and, on the average, the magnitude and spiral angle also agree reasonably well with theory. However, there is evidence, principally within quiet regions, of a significant departure of the azimuthal field component and the field magnitude from simple theoretical models. Pioneer 11 has obtained information up to heliographic latitudes of 16 ~ Observations of the interplanetary sector structure show that the polarity of the field becomes gradually more positive, corresponding to outward-directed fields at the Sun, and at the highest latitudes the sector structure disappears. These results confirm a prior suspicion that magnetic sectors are associated with an interplanetary current sheet surrounding the Sun which is inclined slightly to the solar equator.

Large‐scale interplanetary magnetic fields: Voyager 1 and 2 observations between 1 AU and 9.5 AU

“…Voyager observations of B(R) between 1 AU and 5 AU have been discussed by Burlaga et al (1982). This paper extends that analysis to 1, 9.5 AU, and it complements the studies based on Pioneer 10 and 11 observations made beyond 1 AU during a different part of the solar cycle (Smith, 1974 and1979;Wolfe, 1979 and1977;and Rosenberg et al, 1978). These early measurements indicated that the large scale field could be described by Parker's spiral field model (Parker, 1963), which gives Bo = B p (R) = A(1 + R ) 1/2 /R 2 .…”

Differential measurement and model calculations of cosmic ray latitudinal gradient with respect to the heliospheric current sheet

“…Those features, which may have considerable and as yet unknown effects on our understanding of drift-dominated cosmic ray transport, are the effects of complicated current sheet topologies, variable solar wind speeds, and large-scale magnetic field intensity perturbations. For example, high-speed solar wind streams restructure the interplanetary medium by overtaking and interacting with slower speed solar wind as they propagate outward from the sun, generating corotating interaction regions (CIRs) [e.g., Smith and Wolfe, 1979] or merged Measurements of cosmic rays with kinetic energies greater than energy threshold. interaction regions [e.g., Burlaga et al, 1983;Burlaga, 1984].…”

STEREO Observations of Energetic Ions in Corotating Interaction Regions During the May 2007 Solar Events

“…The leading edge of the interaction region is characterized by an increase in plasma density, temperature and magnetic field intensity and the trailing edge by decrease in these quantities (Smith and Wolfe, 1979). The solar wind speed increases at both edges.…”