Storm-time disturbance field and the computed equatorial D_(st) index

摘要

The geomagnetic disturbance field (D) is a composite of both universal time component (D_(st)) and the local time-dependent part (SD) of the field. The equatorial D_(st) index is an accepted measure to quantify the strength of the magnetic storm, defining the contribution' from the symmetric magnetospheric ring current. A new approach is made to compute the D_(st) index with a less cumbersome process using observed horizontal field intensity at low latitudes, which incorporates certain normalization factors for removing the part associated with the quiet-time ionospheric current and the elimination of the secular trend. Systematic correspondence between the conventional equatorial D_(st) index [D_(st(cv))] and the computed D_(st) [D_(st(cal))] is shown to exist. Since the spacing of stations used for the calculation of D_(st(cv)) is non-uniform, too large and far from ideal, an attempt is made to compute the D_(st) index by the inclusion of an additional station Alibag (lat.l8.38°N ; long.72.52°E) in the Indian longitude, covering a wide gap. Progressive change in the asymmetric component after the inclusion of the station Alibag in the computation of D_(st) is brought out from the analysis.