Solar cycle‐dependent north‐south field configurations observed in solar wind interaction regions

Abstract

This study investigates the structure of the normal or ‘north‐south’ field component BN in the ‘interaction regions’ of the interplanetary magnetic field (IMF). An interaction region corresponds to the region of interaction between a solar wind stream of lower velocity and the higher‐velocity stream overtaking it. A computer program was developed to locate interaction regions by testing for the enhancement in field magnitude known to be characteristic of these regions. The results of our statistical analysis of Pioneer 10 and 11 IMF data taken in 1972–1973 indicated a systematic configuration of enhanced southward BN which reversed to enhanced northward BN in the rear portion of the interaction regions. Our superposed epoch analyses of BN in the interaction regions in data taken with other spacecraft from 1964 through 1973 indicated a similar systematic configuration of BN. In pre‐1970 data the leading portion of the interaction regions had enhanced northward BN, on the average. The BN configuration reversed phase near the beginning of 1970 as did the dipolar‐related dominant polarity effect found by Rosenberg and Coleman in 1969. The evidence indicates that the BN configuration in corotating interaction regions is related to the interplanetary magnetic field being parallel to the global heliospheric current sheet near it and as far as a quarter (1.5 days at 1 AU) of a typical sector length away from it. Our results indicate that two thirds of interaction regions occur in the first quarter of a sector. The parallelism of the field to the current sheet implies that the field is southward following a positive to negative sector crossing, or vice versa, both above and below the solar equatorial plane for the phase of the solar‐dipolar cycle beginning in 1970. When the azimuthal component BT reverses or tends to reverse direction, due to azimuthal stress, preceding the plasma interface of the interaction region, BN reverses from a southward to a northward direction, so that the field remains parallel to the global current sheet. The components BT and BN are also enhanced by compression in the interaction region. Preliminary interpretations in terms of the warped current sheet are discussed for the dependence of BN on heliographic latitude found by Coleman and Rosenberg in 1971 and the effect on atmospheric vorticity of a sector crossing passing the earth found by Wilcox and Svalgaard and their colleagues in 1974. The characteristics of the interaction region found in this study define in detail the structure of the classic event in the solar wind that is closely related to interplanetary‐magnetospheric interactions. In turn, knowledge of this structure may allow investigators to finally choose between the many competing interaction models.