Global auroral pictures from the Defense Meterological Satellite Program (DMSP) satellites are presented to show, for the first time, occurrences of large amplitude undulations on the equatorward boundary of the diffuse aurora in the afternoon-evening sector. The crest-to-trough amplitude of these waveforms ranges from about 40 to 400 km and the wavelength varies from about 200 to 900 km. The undulations are seen in one case to extend over 3000 km along the equatorward boundary. Auroral images from successive DMSP passes suggest that this phenomenon lasts for about 0.5 to 3.5 hours. In each of the four cases observed, the undulations occur during a geomagnetic storm interval near the peak development of the storm time ring current. In all instances, auroral pictures displaying the undulations show simultaneous substorm patterns of discrete auroras. In one instance when simultaneous electron (50 eV to 20 keV) measurements from DMSP satellite are available, the electron spectra near the diffuse auroral equatorward boundary resemble power law spectra, and the scale length for the density gradient at the boundary is determined to be about 12 km. Weak electron precipitation is also found equatorward of the diffuse aurora and the associated electron spectra frequently show a secondary population with peak fluxes at 1 -5 keV. The observed undulations are interpreted as surface waves propagating on the inner edge of the plasma sheet, and possible plasma instabilities responsible for it are briefly discussed.

Observations of polar cap sun‐aligned arcs obtained with the auroral scanning photometer on Isis 2 for the period 1971 to 1975 are examined. A 2∶1 asymmetry was found in the occurrence frequency between the morning and evening sectors of the polar cap. Sun‐aligned arcs were observed on only 0.6% of polar cap passes and occurred most frequently during periods of low magnetic activity (Kp and AE). Moreover, for all cases observed during times for which interplanetary magnetic field data were available, the field was directed northward. Although the intensity along any single arc varied considerably, it was found that the 5577 Å/3914 Å intensity ratio remained constant. Examination of particle data and the observed intensity ratios indicate that the arcs are excited by low‐energy (⩽1 keV) electron fluxes.

Sun‐aligned arcs are long and sometimes narrow optical structures which are oriented in the sun‐earth direction and which occur in the polar cap, generally during intervals of low magnetic activity. Their appearance, as seen by the DMSP and ISIS‐2 satellite photometers, is examined with respect to the north‐south orientation of the interplanetary magnetic field. In the DMSP pictures it is found that they appear when the IMF is directed northward. In 16 out of 18 unambiguous observations of sun‐aligned arcs by ISIS‐2, the IMF was directed northward; for the other two no IMF data was available. These results provide evidence for a strong correlation between the northward direction of the IMF and the occurrence of sun‐aligned arcs in the northern polar cap.

The spatial distribution of energetic particles in the distant magnetotail is examined by using identical sets of two complementary energetic particle experiments aboard the IMP 7 and 8 spacecraft. The combined IMP 7 and 8 data set provides nearly 10 years of satellite observation. The quantitative distributions of energetic electrons in two energy ranges (30 keV ≤Ee ≤100 keV, 220 keV ≤Ee ≤2.5 MeV) and energetic protons (50 keV ≤Ep ≤220 keV, 290 keV ≤Ep ≤500 keV) are presented in terms of occurrence frequency and the average particle flux intensity over an 80 RE × 70 RE plane perpendicular to the sun‐earth line. The θ‐shaped particle distribution over the solar magnetospheric YZ plane in the antisolar direction is clearly demonstrated at all energies and for both species. The ‘O’ part of the θ corresponds to the magnetosheath region, and the cross bar is the average plasma sheet configuration. A distinct, species dependent, dawn‐dusk asymmetry in particle distribution is observed in the plasma sheet and to a lesser extent also in the magnetosheath. Energetic electrons are observed most often in the morningside of the plasma sheet and protons in the evening side. The degree of asymmetry differs for electrons and protons and is energy dependent. Geomagnetic activity affects the particle distribution substantially. At high geomagnetic activity, the occurrence frequency increases by about a factor of 2 and the average flux intensity is enhanced by approximately one order of magnitude.

Aurora on the dayside of the earth observed under especially favorable viewing conditions by the scanning auroral photometer on the ISIS‐2 satellite show several distinctive features. (1) Usually, but not always, a gap or discontinuity in the 5577Å aurora is present through the 10.5 ‐ 12.5 MLT sector, in the same region where the 6300Å aurora typically reaches a maximum. Weak 3914Å emission smoothly fills this gap in most of the passes. (2) A persistent auroral intensity enhancement occurs between 14 and 16 MLT, which is probably related to a statistically deduced maximum in particle fluxes observed in this sector. (3) Aurora in the morning sector at these wavelengths is wider and the poleward boundary is more irregular than in the afternoon.