OGO-A magnetic field observations

Abstract

This paper summarizes the new findings that have come from the intial study of the OGO‐A fluxgate magnetometer measurements between 4 and 24.5 RE (earth radii). These include the following: (a) A model magnetic field profile of the cross‐sectional structure of the bow shock is derived in terms of the sharpness of the interface, the rise time, and the total time interval occupied by a field pile‐up at the shock. Using a simple model to derive the velocity of shock movements, these times are converted to three thickness dimensions roughly of the order < 20, 70, and 250 km, which emphasizes the need for strict definition of the meaning of ‘thickness’ in collisionless shock theories, (b) Superimposed on the average shock structure, (a) above, two classes of field oscillations are frequently observed: coherent circularly polarized waves with frequencies typically between 0.5 and 1.5 cps in the satellite reference frame, and higher frequency fluctuations, >7 cps, which are unresolved by the measurements and whose identity is not known. The coherent oscillation is identified as being in the whistler mode, exists in the form of wave packets, and usually shows a sharp upper frequency cut‐off in power spectrum analysis, (c) A series of bow shock crossings during the main phase of the April 18, 1965, magnetic storm occur at an abnormally large distance from the earth principally as a consequence of the strong, 20–27 γ, interplanetary field that lowers the Alfvén Mach number to 1.5. The transition region magnetic field adjacent to the shock interface is exceptionally stable in contrast to a number of theoretical predictions and the typical shocks observed at high Mach numbers, (d) The magnetopause in the sunward hemisphere is most typically observed as a smooth transition over a dimension comparable to the ion cyclotron radius, (e) The correlation of negative bay onsets in the auroral belt with OGO‐A observations on the night side of the earth supports more general morphological arguments that the onset originates within the closed magnetosphere or auroral ionosphere and is not dependent on being triggered by a sudden change in the solar wind plasma or field. The view is advanced that the onset results from short‐circuiting effects in the ionosphere. (f) At middle latitudes between 5 and 10 RE near the midnight time sector the total field intensity is found to be considerably stronger than predicted by existing field models. This is believed to be caused by high plasma pressures near the equator at similar distances in the same time sector, (g) Near the magnetopause within the local time sector 4h30m‐6h30m and geomagnetic latitudes ±15° the magnetospheric field intensity is generally found to be ≤Bt, the field intensity in the adjacent transition region. This condition and the behavior of the field gradient within the magnetosphere leads to the conclusion that a β ≥ 1 condition must persist over this sector of the outer magnetosphere beyond 11 RE. The consequences of the magnetopause being a boundary between two high β regio...

Waves in the Vicinity of the Magnetopause

“…In fact, the magnetosheath field is often e qual to or greater thar. the magnetosphere field (Heppner et al, 1967) . This more usual situation is displayed in Figure 1 On certain days the interplanetary field may happen to be oriented such that the magnetosheath is aligned with the magnetosphere field.…”

Bow shock associated waves observed in the far upstream interplanetary medium

“…Frank (1968) has observed similar proton events at greater distances from the shock on higher apogee spacecraft. Heppner (1967) has reported small amplitude waves in the magnetic field in close proximity to the bow shock at frequencies predominantly near 1 hz and sometimes greater than 7 hz. Greenstadt et al (1968) have used a searchcoil magnetometer on the Vela spacecraft near the shock to detect disturbances with -4 -periods 20-60 seconds in a plane perpendicular to the spacecraft spin axis.…”

Magnetotail changes in relation to the solar wind magnetic field and magnetospheric substorms

“…Magnetic field observations between 10 Re and 40 Re behind the earth have shown two main kinds of variation of the tail magnetic field namely, slow increase or decrease of the field at a rate of some gammas per hour, and rapid increase or decrease of the field at a rate which can be as high as several gammas per minute (Anderson and Ness 1966, Heppner et al, 1967, Fairfield and Ness 1970, Camidge and Rostoker 1970, Brody and Holzer 1970. The typical substorm signature in the tail is a slow increase of the field followed by a rapid decrease.…”