Ionospheric convection inferred from interplanetary magnetic field‐dependent Birkeland currents

Abstract

It is fairly well‐established that sunward convection of the ionosphere exists in the central portion of the polar cap when the interplanetary magnetic field (IMF) is northward. What is unclear, however, is whether the sunward convection is created by additional cells within the polar cap or by a distortion of the normal two‐cell pattern. Computer simulations of ionospheric convection were conducted by combining empirical models of Birkeland currents with a model of ionospheric conductivity to study IMF‐dependent convection characteristics. Birkeland currents for three IMF configurations were available: (1) Bz southward, (2) Bz weakly northward, and (3) Bz more strongly northward. In all cases the currents used represented conditions in the northern polar cap of a negative IMF By component. Two different conclusions are possible given the limited dayside measurements of Birkeland currents. The first is that as the IMF turns northward, the morning cell becomes slightly elongated in roughly the sunward direction, and the evening cell begins to wrap around the morning cell by extending into the morning sector. This process continues as the IMF becomes more strongly northward, until what was originally the evening cell becomes severely distorted, almost completely surrounding the morning cell. What was originally the morning cell shifts into the polar cap and it is this cell along with the distorted evening cell which provide for sunward flow in the polar cap. With a different distribution of Birkeland currents on the dayside, it is also possible that when the IMF is strongly northward, a three‐cell convection pattern exists. However, this is different from the expected four‐cell pattern since the low‐latitude cell in the morning sector is missing.