Computer Science
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997crlrv..43..717m&link_type=abstract
Review of the Communications Research Laboratory, vol. 43, p. 717
Computer Science
Solar Wind, Research, Magnetoacoustic Waves, Magnetohydrodynamic Waves, Data Acquisition, Geomagnetic Tail, Spectrum Analysis, Wave Propagation, Magnetic Fields, Ampte (Satellites), Explorer Satellites, Extremely Low Frequencies, Geosynchronous Orbits, Ground Stations, Magnetic Variations, Magnetometers, Magsat Satellites, Plasma Interactions, Time Response, Waveforms
Scientific paper
This study investigated two types of magnetospheric ULF pulsations, Pc 3 and Pi 3, using multipoint measurements of magnetic field from satellites and ground stations. It is important to clarify the properties of these pulsations if we are to understand the global response of the magnetosphere to changes in the solar wind plasma with different spatial and temporal scales. It is also important to investigate the propagation mechanisms of these pulsations in order to understand the general properties of MHD waves propagating in inhomogeneous plasmas. The Pc 3 and Pi 3 pulsations are chosen here to represent solar wind input to the magnetosphere at two extremes of the MHD temporal and spatial scales. Pc 3 pulsations represent the short-time-scale input, with wavelengths comparable to the inhomogeneity scale or the dimensions of the dayside magnetosphere. Pi 3 pulsations represent the large-time-scale input, with wavelengths far exceeding the dimension of the dayside magnetosphere, The first part of this work investigates the amplitude and phase structure of Pc 3 pulsations. Although these pulsations are observed at various latitudes, there had been no systematic studies of their phase and amplitude structure over a wide range of latitudes. Magnetic field data from the Western Pacific magnetometer array centered at 210 deg. geomagnetic longitude and from the GEOTAIL satellite are used here to reveal the latitudinal structure of a Pc 3 pulsation event that occurred on October 17-18, 1992. During this event, GEOTAIL was located in the dayside magnetosphere at a radial distance of approx. 8 RE and observed tailward-propagating fast-mode magnetosonic waves in the Pc 3 band. Pulsations observed at the ground stations, also on the dayside, exhibited dynamic spectra similar to those observed on board GEOTAIL. Spectral analyses of the ground magnetometer data reveal that the pulsations are often coherent from L approx. 1 to L approx. 6 and that the phase of the pulsations varies with L in a complex manner: at low-latitude stations (1, between 1.1 and 2.9), there is little phase. delay; at a high-latitude station (L = 5.5) the phase relative to that at low latitudes is approx. 180 deg. and at an equatorial station (L = 1.01) the phase relative to that at low latitudes is approx. 150 deg. The pulsation amplitude is greatest at the highest-latitude station of the magnetometer array, but two local maxima occur at L approx. 1 and at L approx. 2.1. MHD wave propagation effects in the magnetosphere are examined here in order to explain these observations. The second part of the present work explores the generation and propagation properties of Pi 3 pulsations, Compressional Pi 3 magnetic pulsations with irregular waveforms and periods greater than 150 s have been studied by using data from the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE/CCE) satellite and GOES 5 and 6 satellites in the dayside magnetosphere and comparing this data with signatures of geomagnetic field variations on the ground at a low latitude (L = 1.25). On the ground the pulsations appear in the horizontal component. A study of 17 such concurrent events during a 2-month period in 1986 reveals the following: (1) The peak-to-peak amplitudes in the magnetosphere ((delta)BT) and on the ground ((delta)H) are comparable and are between 0.5 and 7 nT. (2) On the ground the pulsations can be seen at all local times, even at midnight, while at geostationary orbit they are observed only on the dayside and have a clear amplitude maximum at noon. (3) The pulsations on the ground lag those observed by CCE near local noon, and the lag increases with increasing local time difference between CCF and the ground station. The time lag is 1-2 min longer when the ground station is on the nightside than when it is on the dayside. (4) The time lag between pulsations observed at geostationary orbit and near noon by CCE varies systematically with local time and is about 2 min per 6 hours of local time difference.
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