Mapping Field-Aligned Currents as a Function of the Interplanetary Electric Field

Abstract:

Magnetometer measurements from both the Oersted and CHAMP satellite have been combined together to create a database that is used to construct maps of the polar, magnetic field-aligned currents (FAC). As these satellite data span the time range of years 1999 to 2005 for Oersted and 2001 to 2010 for CHAMP, this large quantity is useful to investigate how the currents change under various conditions. This presentation will focus on the magnitude of the total current as a function of the interplanetary electric field (IEF), the product of the interplanetary magnetic field (IMF) and solar wind velocity. This topic is of particular interest due to the numerous prior studies in the past, both experimental and theoretical, about the behavior of the polar cap electric potentials. Specifically, the electric potentials increase and then level off to a saturated value as the IMF and IEF increase in magnitude. The majority of the theoretical explanations for this saturation involve the "Region 1" currents.

What has been found in this investigation is that the total FAC is highly linear as a function of the IEF, up to values of 8 mV/m, for all IMF clock angles. This is more than double the value of the IEF where the electric potentials exhibit the roll-off toward saturation. The linear slope does vary as a function of dipole tilt angle, which correlates with season; the summer hemisphere has a larger slope and higher currents than the winter hemisphere. In addition to looking at the total FAC values, the totals within the Regions 0, 1, and 2 have also been extracted from the FAC patterns, and it has been found that the linearity of the response curve is maintained. Above IEF values of 8 mV/m, corresponding to IMF magnitudes around 20 nT, the volume of data is very low, resulting in much uncertainty in the FAC values that are obtained, although there is some evidence that the total FAC does begin to saturate above 8 mV/m.