Evolution of ionospheric multicell convection during northward interplanetary magnetic field with |Bz/By| > 1

Abstract

During northward interplanetary magnetic field (IMF), it is generally believed that ionospheric convection appears as a four‐cell structure for |Bz/By| > 1 and as a distorted two‐cell structure for |Bz/By| < 1, where By and Bz are the Y and Z components of the IMF. In this paper we present Super Dual Auroral Radar Network (SuperDARN) observations of ionospheric convection in the Northern Hemisphere during northward IMF for |Bz/By| > 1 on November 11, 1998. We show in detail the evolution of the convection patterns as |Bz/By| changes. Nearly symmetric four‐cell convection, with two reverse cells in the polar cap and two normal cells at lower latitudes, occurs for |Bz/By| ≈ 7. The ionospheric flow associated with the reverse cells is closed almost completely on the dayside. A shifted four‐cell convection pattern, with the reverse cells shifted toward earlier magnetic local time (MLT) for negative By and toward later MLT for positive By, is observed for |Bz/By| ≈ 2.3. When |Bz/By| decreases to ∼1.7, the convection appears as a three‐cell pattern, with a single reverse cell focused near noon and two normal cells. The normal morning and afternoon cells are focused at quite high magnetic latitudes (between 76° and 80°); the spatial extent of the normal cells is 10°–15° or 1000–1500 km in the latitudinal direction. We also present Defense Meteorological Satellite Program (DMSP) satellite data which show sunward convection over the polar cap in the Southern Hemisphere at the same time as the Northern Hemisphere radar observations. We propose a new model of convection patterns during northward IMF for |Bz/By| > 1 and By < 0 on the basis of the combined observations. In the model the convection appears as a symmetric four‐cell structure for |Bz/By| ≥ 3, a shifted four‐cell structure for |Bz/By| = 2–3, and a three‐cell structure for |Bz/By| = 1–2.