Geomagnetic storm sudden-commencement rise times

Abstract

Adopting the view that the sudden commencement of a geomagnetic storm (SC) is the result of the impact on the geomagnetic field of an abrupt solar-plasma front, the form of the SC observed on the surface of the earth is investigated. A model is constructed to represent the shape of the geomagnetic field boundary as perturbed by the solar plasma. Calculations (carried out in the equatorial plane for simplicity) show that, regardless of how abrupt may be the impact of a solar-plasma front on the geomagnetic field, the variation in hydromagnetic transit times from different positions on the boundary, down to a point on the surface of the earth, yields SC rise times of several minutes at ground level. These times are in agreement with the observed SC rise times. 2715

Rise time of geomagnetic sudden commencements —Statistical analysis of ground geomagnetic data—

“…Nishida (1966) listed the followings as mechanisms which may determine the SC rise time; (e) The broadening of the wave front during the passage through the magnetosphere due to multi-reflection. Dessler et al (1960) tried to explain dT in terms of a combination of the mechanisms (a) and (b) above and calculated a build-up time of SC amplitude as 1-6 min. Yokouchi (1953) analyzed SCs observed at Kakioka (geomag.…”

Magnetospheric ULF Wave Phenomena Stimulated by SSC.

“…Most part of this wave energy is scattered in the magnetosphere, but some reaches the Earth. The ground signal is gradually build up with a time, governed by the propagation time of hydromagnetic disturbance from different parts of the magnetopause and the time of sweeping the dayside magnetosphere by a shock (Dessler et al, 1960). From our experimental data we can not rule out whether a decrease of H-component after a main peak is related to relaxation of the magnetopause or to a positive impulse of two-pulse structure (Araki, 1977;Araki et al, 1985).…”

Propagation of hydromagnetic waves in magnetosphere

“…This is the equa ti on whi c h Francis, Gre e n, and Dessler [1959] derived mathematically and used for their calculation of the tran sit time for the ray, and which Dessler, Francis, and Parker [1960] used for their two dimensional r ay tracing.…”

“…For altitudes below th e bottom limit of the above distribution (15,000 km geocen tri c distance), we base our model on that given by Dessler, Francis, and Parker [1960] , but to ens ure co ntinuity of the electron de nsity an appropriate smoothing was made. In so doing, th e region below 15,000 km was divided into two regions and in each region the electron density was ex pressed in a power series.…”