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...

Initial results of the electric and magnetic field observations from the DE‐2 satellite show a remarkably good correlation between the north‐south component of the electric field and the east‐west component of the magnetic field in many passes of the field‐aligned current regions. For a dayside cusp pass on August 15, 1981 the coefficient of correlation between these components was 0.996. A preliminary inspection of the available data from the first 6 months of the DE operation indicates that the similarity between the electric and magnetic field signatures of the field‐aligned currents is a commonly observed feature at all local times. We interpret this high correlation to be an indication that the closure of the field‐aligned current is essentially meridional. When the correlation between these components is not good, the closure current is likely to be flowing along the auroral belt. When the correlation between the electric and magnetic fields is high, it is possible to estimate the height‐integrated Pedersen conductivity from the observed field components.

High‐time‐resolution (0.5 s) measurements of the vehicle frame to ambient plasma potential were made with a 50‐m antenna experiment on the SCATHA satellite. Accurate measurements were limited to sunlight conditions or to low‐level eclipse charging periods. Significant variations in the spacecraft potential in sunlight occurred which depended on the orientation of the satellite with respect to the sun. Using the SCATHA data, statistical occurrence of charging at near‐geosynchronous orbit in daylight is studied. Charging greater than −10 V (in the negative sense) occurs only between 1900 LT and 0900 LT but at all altitudes and latitudes of the SCATHA orbit. High‐level (> −100 V) charging occurs only for magnetic activity, as measured by Kp, of 2+ or greater. Three “worst case” daylight charging event periods (−340 V to −740 V) show that the electron population that directly drives the vehicle potential on the SCATHA satellite has an energy typically greater than 30 keV. The vehicle potential (in the negative sense) is directly proportional to the electron flux carried by the population above 30 keV, although the linear regression coefficients change from case to case. The vehicle potential is insensitive to changes in the electron flux below 30 keV, even though the fluxes in this range are considerably more intense. To explain the data, we suggest that the low‐energy electron fluxes are essentially self‐balanced by the combination of their own secondary and backscattered emissions and that a substantial portion of the photoelectron emissions are returned to the satellite by the action of the surrounding magnetic field.

The s o l a r f l a r e of 7 J u l y 1966 produced a sudden commencement on t h e e a r t h a t 2102 UT on 8 J u l y . The OGO-3 s a t e l l i t e observed a sudden i n c r e a s e i n t h e magnetic f i e l d i n t h e m a g n e t o t a i l f o l l o w i n g t h e sudden commencement. Using t h e IMP-3 and Explorer 33 o b s e r v a t i o n s by Ness and Taylor of t h e i n t e r p l a n e t a r y shock t h a t caused t h e sudden commencement, t h e p r o p a g a t i o n o f t h e f i e l d i n c r e a s e from t h e f r o n t s i d e of t h e magneto- sphere t o t h e t a i l i s d i s c u s s e d . It i s shown t h a t t h e magnetosphericpropagation of t h i s p e r t u r b a t i o n toward t h e t a i l i s f a s t e r t h a n t h e p r o p a g a t i o n of the i n t e r p l a n e t a r y shock j u s t o u t s i d e t h e bow shock. Based on t h e o b s e r v a t i o n s by OGO-3, t h e ground o b s e r v a t i o n s , and t h o s e made i n i n t e r p l a n e t a r y space by IMP-3 and Explorer 33, we e s t i m a t e t h e i n t e rp l a n e t a r y shock speed t o be approximately 500 km/sec, w i t h an AlfvhnMach number f o r t h e shock about 1 . 2 . This speed i s c o n s i d e r a b l y lower t h a n t h a t deduced by Lazarus and Binsack from t h e i r plasma measurements on E x p l o r e r 33, namely, 700 km/sec. Conclusions drawn from t h e p r e s e n t s t u d y a r e : ( i ) t h a t t h e observed magnetic f i e l d i n c r e a s e i n t h e t a i l i s u n l i k e l y t o be due t o an increased l a t e r a l p r e s s u r e o f t h e post-shock s o l a r wind gas from t h e s i d e o f t h e t a i l , and ( i i ) t h a t t h e t r a n s f e r of a d d i t i o n a l p o l a r magnetic f l u x t o t h e t a i l due t o t h e i n c r e a s e i n t h e s o l a r wind p r e s s u r e on t h e f r o n t s i d e of t h e magnetosphere can account f o r t h e observed t a i l f i e l d i n c r e a s e .

Magnetic field data from the Goddard Space Flight Center magnetometer experiment on board Ogo 5 are analyzed by the minimum variance technique for two magnetopause crossings, believed to provide the best evidence presently available of magnetopause rotational discontinuities. Approximate agreement with predictions from MHD and first‐order orbit theory is found, but available low‐energy electron data suggest the presence of significant non‐MHD effects. The paper also illustrates an improved method for data interval selection, a new magnetopause hodogram representation, and the utility of data simulation.