Modeling the contributions of ring, tail, and magnetopause currents to the corrected Dst index

Abstract

We present a new semiempirical model describing the contributions of the ring, tail, and magnetopause currents to the Dcx index. We use the isotropic boundary (IB) location of energetic particles measured by the NOAA/POES satellites, as a proxy for the tail current strength. Using local linear regression, we derive the model parameters and their functional dependencies on solar wind and interplanetary magnetic field parameters and on IB latitude. The model gives the ring, tail, and magnetopause current contributions for the whole time interval 1999–2007, performing roughly equally well during all activity levels. We find that the coefficient of proportionality between the square root of solar wind pressure and the magnetopause current contribution is larger than in earlier estimates. Ring current decay time is found to decrease with increasing solar wind electric field and dynamic pressure. We estimate the average quiet time level of the combined ring and tail (magnetopause) current contributions to Dcx to be roughly −7 nT (+13 nT). The average tail current contribution is found to be about 34% of the Dcx index, which is somewhat larger than previous estimates based on smaller‐intensity storms. For individual storms the tail current contribution can reach up to −160 nT (about 40%–60% of the pressure corrected Dcx). The present model agrees well with earlier results for individual storms based on detailed dynamical models of the magnetosphere. Our work demonstrates that the different current contributions to Dcx during both active and quiet time intervals can be reliably estimated using solar wind observations and isotropic boundary location.