Masahisa Sugiura

A magnetospheric field model incorporating the OGO 3 and 5 magnetic field observations

Abstract A magnetospheric field model is presented in which the usually assumed toroidal ring current is replaced by a circular disk current of finite thickness that extends from the tail to geocentric distances less than 3R E . The drastic departure of this model from the concept of the conventional ring current lies in that the current is continuous from the tail to the inner magnetosphere. This conceptual change was required to account for the recent results of analysis of the OGO 3 and 5 magnetic field observations. In the present model the cross-tail current flows along circular arcs concentric with the Earth and completes circuit via surface currents on the magnetopause. Apart from these return currents in the tail magnetopause, Meads (1964) model is used for the field from the magnetopause current. The difference scalar field, ΔB, defined as the difference between the scalar field calculated from the present model and the magnitude of the dipole field is found to be in gross agreement with the observed ΔB (i.e. the observed scalar field minus a scalar reference geomagnetic field). An updated version of the ΔB contours from the OGO 3 and 5 observations, which is used for the comparison, is presented in this paper. Significant differences in details exist, however, between the model and the observed results. These differences will provide a guide for making modifications in the equatorial current system in future models.

Geomagnetic field variations caused by changes in the quiet-time solar wind pressure

Abstract Solar wind data from the Vela 2A and 2B satellites are compared with hourly D st values for the period July 1964–July 1965. During periods of positive D st on magnetically quiet days the square root of the solar wind pressure is found to be statistically linearly correlated to the hourly D st values. This relation is in accordance with the theoretical expectation from the pressure balance at the magnetopause between the solar wind plasma and the Earths magnetic field. A constant of proportionality relating the two quantities is statistically determined. The average value for the proportionality constant agrees favorably with the theoretical value given by the Mead-Beard magnetosphere model.

Dayside pulsation events near the plasmapause

Four dayside pulsation events were observed from DE-l near the equatorial plasmapause region on 23, 24 August, 30 September and 10 October 1982. The corresponding ground data from Siple for three of them are presented and compared to the DE-l data. The concept of field line resonance is applied to interpret the standing wave structure of the events. Most of the events exhibit complicated wave forms with transverse and compressional components seen simultaneously on the same field shell. Long period transverse waves (> 100s) were often seen as fundamental mode oscillations of magnetic shells and had superimposed on them short period waves (20–40 s) which are the higher harmonics excited simultaneously with the fundamental mode. For the long period waves a possible energy source is the Kelvin-Helmholtz instability at the magnetopause, while the radially polarized Pc 4 seen on 10 October was attributed to bounce resonance excitation. “Wave bursts” (short intervals of 5–10 or more cycles) were seen in almost all frequency ranges. The brief duration may be due to temporal or spatial variation of wave sources. A clear tendency of continuously decreasing wave period with decreasing L value was seen in many cases. This is attributed to the broad band frequency of the wave source which excites individual magnetic shells oscillating at their own eigenfrequencies.

Magnetic Field Observations in High β Regions of the Magnetosphere

For a description of the distortions of the geomagnetic field in the magnetosphere ΔB provides a useful measure. Figure 1 shows approximate equi-ΔB contours for the two 90° sectors centered at the noon and midnight meridian half planes, based on a preliminary sampling of a much greater body of OGO 3 and 5 rubidium magnetometer data.

Pi 3 magnetic pulsations associated with substorms

Abstract Using magnetic data from the North American IMS network at high latitudes, Pi 3 pulsations are analysed for a period of 4 1 2 continuously-disturbed days. The data were obtained from 13 stations in the Alaska and Fort Churchill meridional chains and in the east-west chain along the auroral zone. In the past, Pi 3 pulsations associated with substorms have been classified into two sub-categories, Pi p and Ps 6. However, we find that Pi 3s which have longer periods than Pi p and which are different from Ps 6 are more commonly observed than these two special types. Power spectra, coherence and phase differences are compared among the stations. Results show that noticeable differences for latitudinal dependence of period and amplitude exist among midnight, morning and late-evening Pi 3 pulsations. Results for Pi 3 occurring near midnight indicate that the periods at which the power spectral density is a maximum are longest, and the amplitude largest, near the center of the westward auroral electrojet. On the other hand, for Pi 3 pulsations occurring in the morning, the periods at which the power spectral density is a maximum are longest, and the amplitude largest, near the poleward edge of the westward electrojet. Furthermore, for Pi 3 pulsations occurring in the late evening, their periods are longer and their amplitudes larger near both the Harang discontinuity and the poleward edge of the westward electrojet than near its center. Correlations between pairs of adjoining stations are better in the polar cap than at auroral latitudes. It is also found from hodograms that the sense of polarization often varies from one station to another for the same event, and that the time duration in which the same rotational sense is maintained is shorter near midnight than in the morning and late evening. It is suggested that the source regions of the morning and late-evening Pi 3s lie on the electrojet boundaries; that is at the Harang discontinuity (in the evening) and at the poleward edge of the westward electrojet (in the morning and evening). The generation of midnight Pi 3 pulsations, centered at a location within the westward auroral electrojet appears to be associated directly with the generation of that electrojet.