Heliocentric distance dependence of the interplanetary magnetic field

Abstract

Recent and ongoing planetary missions have provided and are continuing to provide extensive observations of the variations of the interplanetary magnetic field (IMF) both in time and with heliocentric distance from the sun. Large time variations in both the IMF and its fluctuations are observed. These are produced predominantly by dynamical processes in the interplanetary medium associated with stream interactions. Magnetic field variations near the sun are propagated to greater heliocentric distances, a process also contributing to the observed variability of the IMF. Temporal variations on a time scale comparable to or less than the corotation period complicate attempts to deduce radial gradients of the field and its fluctuations from the various observations. However, recent measurements inward to 0.46 AU and outward to 5 AU suggest that the radial component of the field on average decreases approximately as r−2, as was predicted by Parker, while the azimuthal component decreases more rapidly than the r−1 dependence predicted by simple theory. Three sets of observations are consistent with an r−1.3 dependence for |Bϕ|. The temporal variability of solar wind speed is most likely the predominant contributor to this latter observational result. The long‐term average azimuthal component radial gradient is probably consistent with the Parker r−1 dependence when solar wind speed variations are taken into account. The observations of the normal component magnitude |Bθ| are roughly consistent with a heliocentric distance dependence of r1.4. The observed radial distance dependence of the total magnitude of the IMF is well described by the Parker formulation. There is observational evidence that amplitudes of fluctuations of the vector field with periods less than 1 day vary with heliocentric distance as approximately r3/2, in agreement with theoretical models by Whang and Hollweg. In relation to total field intensity, the amplitude of directional fluctuations is on average nearly constant with radial distance, at most decreasing weakly with increasing distance, although temporal variations are large. There is evidence that fluctuations in field intensity grow in relation to those in field direction with increasing distance. More observations are needed to confirm these conclusions. The number of directional discontinuities per unit time is observed to decrease with increasing distance from the sun. The apparent decrease may possibly be caused by geometric or selection effects. The relationship between fluctuations of the field and the corotating stream structure is still not understood in detail, and therefore the origins of the various mesoscale and microscale features are at present uncertain.