Fine‐scale characteristics of interplanetary sector boundaries

Abstract

This investigation has studied the structure of the interplanetary sector boundaries observed by Helios 1 within sector transition regions during the time interval from December 1974 to April 1975. A sector transition region is the region of variable magnetic field magnitude and direction observed in or near the ecliptic plane between magnetic sectors in the interplanetary magnetic field (IMF). Such regions are found generally to be complex in character, consisting of intermediate (nonspiral) average field orientations in some cases, as well as a number of large‐angle (120° ≤ω ≤180°) directional discontinuities (DD's) on the fine scale (time scales <1 h). Such DD's are found to be more similar to tangential than rotational discontinuities, to be oriented on average more nearly perpendicular than parallel to the ecliptic plane (the mean tilt is ∼58°), to be accompanied usually by a large dip (≳80%) in B (≡|B|), and, with a most probable thickness of 3 × 104 km, significantly thicker structures on average (by a factor of 10 or more) than ordinary DD's previously studied in the IMF. It is hypothesized that the observed structures represent multiple traversals of the global heliospheric current sheet caused by local fluctuations in the position of the sheet. There is evidence that such fluctuations are sometimes produced by wavelike motions or surface corrugations of scale length 0.05–0.1 AU, superimposed on the large‐scale structure. The observed steep inclinations could be produced by a combination of current sheet warping at the sun, a global wave structure, and the observed small‐scale fluctuations. No radial distance dependence is found for the current sheet properties studied.