Magnetospheric convection induced by the positive and negativeZcomponents of the interplanetary magnetic field: Quantitative analysis using polar cap magnetic records

Abstract

The dependence of the polar cap magnetic disturbance on the polarity and magnitude of the Z component of the interplanetary magnetic field is investigated by regression analysis using hourly values. The Svalgaard-Mansurov effect has been eliminated by assuming a linear dependence on the Y component of the interplanetary field. It is shown that as the northward component of the interplanetary magnetic field increases, a characteristic current system appears in the polar cap. This current system is composed of two current vortices in the day side polar cap: one in the prenoon sector and the other in the afternoon sector. The current direction is antisunward in the central polar cap, suggesting that sunward plasma convection is induced in the polar cap. Current intensity is strongest at фM ∼ 84° around the noon meridian. We propose that the tail field lines are reconnected with the northward interplanetary field on the polar side of the day side polar cusp and, as a result, that plasma convection is induced which is closed within the high-latitude magnetosphere. On the other hand, when the interplanetary magnetic field is directed southward, a transpolar current sheet appears, covering the whole polar cap (фM ≥ 77.5°). The characteristics of this transpolar current sheet are as follows: (1) On the day side, especially around noon, the direction of the current is roughly consistent with the Hall current direction expected from the dawn-to-dusk electric field, while on the night side the direction of the current is considerably skewed from the noon-midnight meridian. Skewing of the current direction can be explained by the effect of the currents external to the ionosphere. (2) The strength of the current is almost linearly dependent on Bz when the interplanetary magnetic field is directed southward (θ < −45°). However, the current intensity is also a function of the magnitude of By, this being apparent when Bz ∼ 0. This indicates that the day side reconnection rate is a function of |By| as well as of Bz, and information is derived about the applicability of the three-dimensional reconnection model in the presence of finite |By|.