Midnight Time Research Articles

The dynamics of equatorial irregularity patch formation, motion, and decay

Using scintillation observations from a series of equatorial propagation paths as well as backscatter and airglow data, the development, motion, and decay of equatorial irregularity patches have been studied. Assembling the results of earlier studies in the field with our observations, we find the following: the patch has limited east‐west dimensions with a minimum of 100 km. Several patches may be melded together to reach an extent of 1500 km. Its magnetic north‐south dimensions are often greater than 2000 km; the most intense irregularities (as evidenced by the Jicamarca radar at the dip equator) are from 225 to 450 km in altitude, although irregularities are found as high as 1000 km. The patch initially has a westward expansion following the solar terminator, then, maintaining its integrity, moves eastward. Evidence over a limited series of experiments suggests that premidnight patches are formed within 1½ hours after ionospheric sunset in the absence of special magnetic conditions. From Ascension Island (∼16°S dip latitude) the individual patches can be clearly distinguished. The decay of patches in the midnight time period was studied, pointing to a rapid decrease in scintillation intensity in this time period.

The temporal and spatial development of mid-latitude thermospheric electron temperature enhancements during a geomagnetic storm

Measurements are presented of thermospheric electron temperatures made by a Langmuir probe mounted on the polar-orbiting satellite Esro 1 before, during, and after the geomagnetic storm of October 31 to November 1, 1968. The data cover a complete time sequence over a period of 9 days while the satellite covered the altitude range 280–1500 km and remained in the noon/midnight local time zones. The data clearly show the development of enhanced temperatures at mid-latitudes associated with stable auroral red arcs in the midnight time zone, trace the latitude dependence of the heating during the progress of the storm, and show the persistence of the zone of elevated temperatures after the magnetic activity has declined. The heating of electrons in the noon time zone is not observed to follow the same pattern as that in the midnight time zone.

OGO-A magnetic field observations

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 regions are noted in terms of boundary instability, plasma entry, and the possible existence of secondary shocks in the transition region. A similar, but not identical, condition may exist in the evening twilight local time sector.