SSC‐excited pulsations recorded near noon on GEOS 2 and on the ground (CDAW 6)

Abstract

The SSC occurring on March 22, 1979, at 0826 UT had an unusually sharp onset in Scandinavia, in Middle Europe and in experiments on the geostationary satellite GEOS 2, which was near noon, local magnetic time. The ground magnetometer stations showed a small preimpulse which started ∼5 s before the main impulse. Both impulses needed ∼2 s to “propagate” from ground stations at L = 6.3–4.6. Search coil magnetometers indicate a very small precursor in northern Finland (L ∼ 4.4–6.0) which started ∼15–20 s before the main impulse. This small precursor also occurred close to the time of the SSC onset at GEOS 2. We interpret this precursor as an effect of precipitating electrons changing the ionospheric conductivity in a localized region. The main impulse triggered damped magnetic pulsations (Psc) with periods near 160 s and 50 s visible in northern Scandinavia and the electric field detector on GEOS 2. Furthermore, the magnetic field and the energetic ions at GEOS observed pulsations with periods near 80 s, but these could only be observed at the northernmost ground stations. There are several indications that the first three harmonics of standing hydromagnetic waves are detected. They may correspond to periodic oscillations of the subsolar point or eigenperiods of the SSC‐excited fast mode (compressional cavity resonance). The tentatively identified second harmonic wave (period ∼80 s) is indicative of a bounce resonance of ring current protons. Inside the plasmasphere the dominant period of a superimposed Psc 4 event increased with latitude for the H component indicating several toroidal eigenoscillations.

Magnetospheric ULF Wave Phenomena Stimulated by SSC.

“…This apparent propagation of the MI's is directed towards the equator in both the hemispheres. The similar effect of equatorward propagation of ssc onset at sub-auroral latitudes was reported by Wedeken et al (1986). It should be kept in mind that the observed time delays between peaks of the H components may correspond either to the equatorward propagation effect or to the increase of the rise time while approaching the equator.…”

“…Wedeken et al (1986) really revealed that the pre-impulse in the D component runs 5 sec ahead of MI, and is probably related to a burst of precipitating electrons. However, the observed pre-impulse of ssc had an amplitude much less than a typical PI.…”

“…As a result, a wave front may impinge the ionosphere nearly simultaneously on a global scale. The 20 s delay between ssc onset at the geostationary orbit and at the ground observed by Wedeken et al (1986) corresponds to the apparent super-Alfvenic propagation. This fact could be interpreted as an indication on a diffractive distortion of MHD wave front in the magnetosphere.…”

Global Structure of Low-Latitude and Equatorial Geomagnetic Pulsations Associated with Storm Sudden Commencements.

“…They found that Psc 2.3's are generally observed at middle and low latitudes, while Psc 4's and Psc 5's are observed in the subauroral and auroral regions. Recently, Wedeken et al (1986) showed through a case study of an SSC with a number ofmagnetometers and the GEOS 2 satellite that the Psc had a complex spatial variation of spectral structure with a small value (m -0-4) of the azimuthal wavenumber, some features of which were consistent with the coupled global mode theory Southwood, 1985, 1986). Further, *Now at Ishikawajima System Technology Co., Ltd., Shinagawa-ku, Tokyo 141, Japan.…”

External and internal sources of low-frequency MHD waves in the magnetosphere - A review.

“…This surface impulse also can be coupled into the inner magnetosphere through the field line resonance mechanism, that was analyzed using simultaneous satellite observations of plasma drift velocity and magnetic field observations (cf. KOKUBUN, 1983;KUWASHIMA and FUKUNISHI, 1985;NAGANO and ARAKI, 1986;WEDEKEN et al, 1986).…”