Dayside Region Research Articles

Observational evidence for a boundary layer source of dayside region 1 field‐aligned currents

The addition of magnetic field measurements to the Atmosphere Explorer C data base has enabled us to examine the relations between field‐aligned currents (FAC), energetic particles, and plasma convection in the high‐latitude ionosphere. We observe that in general, magnetic field perturbations extend throughout the regions of gradients in the plasma convection, particularly equatorward of the plasma convection reversal. A study of 13 dayside polar passes shows that the region 1 FAC's (Iijima and Potemra, 1976a) are observed to span the convection reversal and extend an average of 0.5° invariant latitude poleward of the reversal. Region 2 currents are observed entirely on sunward convecting plasma. Simultaneous observations of low‐energy electron data (200 ev → 25 keV) indicate that the poleward limit of the region 1 FAC's is nearly coincident with the high‐latitude limit of 1‐keV plasma sheet electrons. These observations are consistent with a model in which the dayside region 1 FAC's have a source that is at least partially due to a ‘viscouslike” interaction on closed field lines in the low‐latitude magnetospheric boundary layer.

A new ULF‐modulated electrostatic wave detected in the extremely low frequency range onboard Geos

A new type of magnetospheric electrostatic wave, discovered onboard the GEOS 1 satellite, is described. These waves have a wide spectrum, extending between the lower hybrid frequency and the ion plasma frequency. Their main characteristic is their modulation in amplitude at a ULF frequency which is of the order of the helium gyrofrequency. The corresponding electromagnetic ULF waves are detected simultaneously in all cases (26) but one. These ULF‐associated electrostatic waves (10 <E <200 µVm−1 Hz−1/2) are observed in the dayside region of the outer magnetosphere (L >6) at all geomagnetic latitudes sampled by GEOS 1 (0° <λm <+30°). At least two factors seem to control the existence of these waves: the ULF wave power has to be stronger than 0.02 γ² Hz−1, and the cold plasma density has to be lower than 25 cm−3. Some possible mechanisms for explaining their generation are discussed.

Sunward flow in Jupiter's magnetosheath

The position of Voyager crossings of Jupiter's bow shock show a dependence on solar wind pressure to the −1/3 power. This dependence is used to calculate typical bow shock speeds of 50 km/s from Voyager solar wind plasma data. Since the bow shock and magnetopause move approximately in unison in response to solar wind pressure changes, the resulting movement of the magnetosheath at a sizeable fraction of the solar wind speed leads to reversed, sunward flow in large portions of the dayside region when the boundaries are expanding. Voyager 1 plasma data show evidence of such reversed flow.

The solar terrestrial event of 14–21 December 1971: The pattern of 6300 Å emission over the polar cap

An exceptional solar terrestrial event was initiated by the ejection of plasma from the Sun on 14 December 1971 and was followed by a spectacular pattern of soft particle precipitation into the polar cap, which evolved in a slowly changing sequence until 21 December. The storm was characterized by high proton fluxes in space and near the Earth, an extended interval of northward B z , a highly developed ring current and widespread polar cap particle fluxes. These varied from a dayside “butterfly pattern” early in the event, to highly structured Sun-aligned polar cap patterns late in recovery. A number of polar cap ground-based measurements were compiled and are reported upon. The storm seems reminiscent of the great red auroras of the IGY and some common features are noted. The 6300 Å emission is shown to result largely from direct excitation by low energy electrons, of a few hundred eV. Since the emission covered the Earth's high-latitude dayside region nearly to the invariant pole it indicates a magnetospheric topology that permits entry of low energy plasma over this region or of mechanisms which allow the generation or penetration of the plasma.

Magnetic field variations in the polar region during magnetically quiet periods and interplanetary magnetic fields

The concepts of near-pole magnetic field variations during magnetically quiet periods are explored, with special emphasis on the relationships of these variations to interplanetary magnetic field components. Methods are proposed for relating the variations which have been observed to the fields from the various sources, based on a thorough selection of reference levels. We assume that the field variations in the summer polar cap during magnetically quiet periods consist of the following components: (i) the middle-latitude Sqvariation extended to the polar region; (ii) the DPC(By) single-cell current system with a polar electrojet in day-side cusp latitudes; (iii) the DMC(Bz) two-cell current system of magnetospheric convection, in the form of a homogeneous current sheet in the polar cap towards the sun, with return currents through lower latitudes; (iv) the DPC(Bz) single-cell counterclockwise current system with a focus in the day-side cusp region. Quantitative relations between the near-pole variation intensities and the value and sign of the IMF azimuthal component, with a 1 hr time resolution, have been obtained and used to suggest ways of diagnosing the interplanetary magnetic field on the basis of ground observations.

The dayside of the plasmasphere

The concentrations of H(+) ions in the dayside region of the plasmasphere, measured from March 1968 through February 1969 by the Lockheed light-ion mass spectrometer aboard the OGO 5 satellite, are presented and analyzed. The position of the plasmapause on the dayside appears to be determined by the level of magnetic activity present during the previous corotation of the dayside sector through the formative nightside region. Observations of the buildup of H(+) density versus local time following magnetic storms indicate that H(+) ions flow from the dayside ionosphere into the plasmasphere and plasma trough. Plasmapause density profiles in the afternoon-dusk sector show the effects of the dayside filling from the ionosphere. In addition, several of the dayside profiles display a steep drop in the H(+) density of about a factor of 10 inside the plasmapause position.