Hydromagnetic interpretation of sudden commencements of geomagnetic storms

Abstract

A new hydromagnetic model for the sudden commencement (SC) of a magnetic storm is presented. The model is based on a new morphology of the SC field that was derived from an analysis of the characteristics of vector diagrams for the first few minutes of the SC field variation. The vector diagrams representing the locus of the end point of the total horizontal disturbance vector of the SC field were constructed from rapid-run magnetograms from stations all over the world for SC’s that occurred during a four year period beginning with the I.G.Y. The most characteristic feature of the SC field is the polarization of the field that is due to a combination of circularly and linearly polarized components. The variation of the SC field over the earth is described in terms of the variations with local time and latitude of the direction of polarization (i.e. clockwise or counterclockwise) and initial phase of the circularly polarized component and the ration of the amplitudes of the two components. The two polarized components of the SC field are identified as circularly polarized transverse and linearly polarized longitudinal hydromagnetic waves. The longitudinal wave is the immediate consequence of the impact of a solar plasma cloud on the magnetosphere; whereas the transverse wave is produced by a coupling with the longitudinal shock wave in the magnetosphere. The triply refracting nature of the plasma in the magnetosphere results in the production of three hydromagnetic modes by the SC disturbance; namely, ordinary and extraordinary transverse waves with opposite directions of circular polarization which propagate to high latitudes in the morning and evening hemispheres respectively, and longitudinal waves which propagate to the earth in low latitudes.