IMF Bx and By dependencies of the polar cap auroral distribution for northward IMF orientation inferred from observations at Vostok station

Abstract

The effects of the IMF radial ( B x ) and azimuthal ( B y ) components on the distribution of polar cap arcs are examined using all-sky camera data from Vostok station for the winter months of 1977–1985. We conclude that three factors control the character of the aurora distribution: the type of the sector structure, the IMF radial component, and the IMF azimuthal component. Based on the experimental results, the following scheme for the auroral distribution in the northern and southern polar caps for different signs of B x and B y is put forward. The ‘garden hose’ structure ( B x > 0, B y < 0 or B x < 0, B y > 0) produces symmetric auroral distributions in the morning and evening sectors of both the northern and southern polar caps; the ‘orthogonal garden hose’ structure ( B x > 0, B y > 0 or B x < 0, B y < 0) is evidently inefficient in the production of aurorae. The B x component determines the intensity of aurorae in that polar cap where geomagnetic field lines are in the opposite direction to the IMF ( B x < 0 in the case of the northern cap, and B x > 0 for the southern cap) and produces the daytime auroral belt poleward of the auroral oval and parallel to it. The B y component affects the auroral appearance in the morning or evening sectors of the polar cap, depending on its sign, and acts asymmetrically in the opposite polar cap. The appropriate patterns of plasma filament distributions in the high-latitude tail lobes are proposed. The characteristics of auroral movements affected by the B y component (such as the direction and speed of the arc motion and the magnitude of displacements) are examined.