Dependence of Geoeffective Interplanetary Parameters on Heliographic Longitude: A Case Study for March 1979

We inquire into how much information content is lost in using interplanetary field and plasma parameters to predict the Dst index from monitors in the inner heliosphere (distance R < 1 AU) and displaced from the Sun-Earth line. To this end, we selected a period in March, 1979, during which 2 major geomagnetic storms (Dst < -100 nT) occurred. Data are from ISEE 3, in orbit around the L1 Lagrangian point, and from Helios 2 situated at R ~ 0.8 AU and at a substantial separation from the Sun-Earth line are used. Scaling factors with R are the same as those in Lindsay et al. (1999), irrespective of interplanetary configuration. Previous investigations have recorded good overall agreement between the measured Dst and that predicted from Burton et al.'s formula (1975). However, there are occasional large discrepancies even when input interplanetary parameters are from probes near the Sun-Earth line. In computing the Dst in this study we shall therefore contrast predictions using two approaches. One is based on Burton et al.'s formula; the other is based on a numerical simulation with our kinetic ring current model [3] driven by a magnetopsheric electric field derived from the Weimer [2001] model. The twofold aim of the work is thus to answer the two questions: (1) which part of the Dst is still predicted from off the Sun-Earth line and what are the corresponding interplanetary configurations?; (2) Is prediction improved when the model of Weimer (2001) is used?. This work is supported by NASA Living with a Star grant NAG5-10883. 1. Lindsay, G. M., et al., JGR, 104, 10,335, 1999. 2. Burton, R. K., et al., JGR, 80, 4204, 1975. 3. Jordanova, V. K., et al., JGR, 103, 79, 1998. 4. Weimer, D. R., JGR, 106, 407, 2001.