Predicting the strength of the ring current

Abstract

The Dessler-Parker-Sckopke relationship states that the fractional depression in the magnetic field averaged over the surface of a non-conducting Earth is 2 3 of the ratio of the energy of the charged particles trapped in the Earth's magnetic field to the energy of that magnetic field above the surface of the Earth. When the interior conductivity of the Earth is included in the calculation, a moderate storm with a 100 nT depression corresponds to a ring current whose total energy is 2.8 × 10 15J. Nevertheless deducing the ring current strength from ground magnetograms is not quite as simple as it would seem from the DPS relationship. The ring current is often not symmetric and the observing stations are not uniformly situated, nor are there enough of them. Accurate baselines are required and the strength of the quiet day ring current is unknown. Despite these uncertainties, disturbances in the Dst index can be predicted quite well when accurate measurements of the solar wind and interplanetary magnetic field that strike the magnetosphere are available, by use of the formula of Burton et al. (1975). In this model the field on the surface of the Earth has two components: the magnetopause current flowing on the surface of the magnetopause and the ring current in the equatorial regions. The magnetopause current is taken to be proportional to the square root of the solar wind dynamic pressure. The ring current is energized by the solar wind east-west electric field and de-energized through an exponential decay of the energy into the upper atmosphere. This simple formula works quite well despite the fact that the solar wind input to the magnetosphere must be controlled by additional factors such as the dynamic pressure of the solar wind and the electrical conductivity of the ionosphere. Improvements are needed in order to increase the advance warning time for solar wind conditions reaching the Earth. To achieve such an improvement requires multipoint measurements near the Earth-sun line. These measurements should be both continuous and significantly closer to the Sun. Such measurements are technically feasible today and affordable using solar-sailing “sentinels” carrying a small payload consisting of a solar wind analyzer and magnetometer, and possibly energetic particles.