High time‐resolution measurements of trapped and precipitated >30‐keV electrons from the low‐altitude polar‐orbiting satellite Ogo 6 in the postmidnight‐to‐dayside region of the auroral zone are presented. It is found that precipitation usually occurs in brief bursts of a few seconds duration or less, with normally a very abrupt transition between no precipitation and isotropic (full loss cone) precipitation. Thus if pitch angle scattering is the precipitation mechanism, it seems to be either very strong or nearly nonexistent, and intermediate‐strength pitch angle scattering modes do not usually seem to represent a stable precipitation regime in this part of the auroral zone. Sustained precipitation in time and/or space is found only at very high flux levels, whilst the duration as well as the frequency of the burstlike precipitation events observed at lower flux levels apparently decreases with decreasing flux of trapped electrons. This is consistent with theories which predict the existence of a critical flux level at which the electron population becomes unstable and strong pitch angle scattering takes place, though there remains the question of how the burstlike precipitation is generated. The action of a delayed negative feedback mechanism is suggested. The burstlike character of the precipitation is of great importance for the average lifetime of electrons drifting in the geomagnetic field, as it reduces the average loss rate to a level which obviates the need for any additional acceleration in the morning‐to‐dayside region to explain the ability of such electrons to gradient drift most of the way around the auroral zone.

A search through 2-3 months of data from Ogo 6 has revealed about 10 cases of field-aligned precipitation of electrons at energies greater than 30 keV. Brief descriptions are given of the four most spectacular of these events, in which the ratio between precipitated and trapped fluxes reached about 100 in one case. Preliminary indications are that such events occur mainly in the evening and midnight sectors and at high geomagnetic latitudes (usually at or above the trapping boundary for electrons with energies greater than 30 keV).

The morphology of energetic (≥30 keV) electron precipitation during the growth phase of magnetospheric substorms has been investigated using measurements of auroral-zone bremsstrahlung X-rays obtained from multiple balloon flights and supplementing riometer recordings. Growth-phase precipitation typically starts about one hour before the onset of a negative magnetic bay and occurs in a limited region parallel to the auroral oval around local midnight. The precipitation is first observed in the northern part of the auroral zone and moves southwards with a speed of 5–10 km/min. To the north this precipitation therefore ceases well before bay onset whereas a continuous transition from ‘prebay’ precipitation to bay-associated precipitation takes place in the south. A decrease in the intensity or at least a levelling off may occur some minutes before bay onset. The southward movement of the precipitation region is associated with a similar movement of a weak ionospheric current system. The events studied were all associated with a southward-pointing interplanetary magnetic field and with growth-phase conditions in the magnetotail. It is suggested that growth-phase precipitation originates from the ‘horns’ of the plasma sheet. The equatorward motion of the precipitation is then a consequence of an expansion of the polar cap, a thinning of the plasma sheet, and an equatorward motion of its inner edge. It is also suggested that this precipitation provides a stabilization of the outer boundaries of the plasma sheet by restricting the ionospheric mobility of the bordering field lines through enhanced conductivity.

Multiple balloon recordings of bremsstrahlung X-rays supported by recordings of cosmic noise absorption have been used to study in detail energetic (≥30 keV) electron precipitation events occurring near local midnight at the onset of the expansion phase of magnetospheric substorms. This type of precipitation occurs during the first 5–10 min after bay onset and can usually be distinguished from the subsequent bay-associated precipitation by its characteristic time structure, variation in energy spectrum, and higher intensities. During this same interval, the poleward border of the precipitation region moves rapidly towards higher latitudes with speeds of typically 1–2 km/s, whereas the equatorward border seems to move slowly towards lower latitudes. The northward expansion starts just poleward of the lowest latitude reached during the slow equatorward motion of the preceding growth-phase precipitation. The previous narrow precipitation region may thus expand to as much as 10° of invariant latitude within a few minutes. Within the expanding region there are additional intrinsic temporal variations. As the flux of precipitating electrons tends to be most intense and most energetic near the poleward border, recordings made northward of the latitude where the poleward motion started tend to give the appearance of an impulsive precipitation event. The bay-onset precipitation starts abruptly at the onset of Pi 2 magnetic pulsations. Associated with these pulsations there are modulations of the flux of precipitating electrons. An intensified westward electrojet appears to have its center in the equatorward part of the precipitation region. It is suggested that the poleward expansion is associated with the expansion of the plasma sheet earthward of a newly formed X-type neutral line, and is caused by a sudden enhancement of field-line re-connection across the neutral sheet. The intense, more energetic electron precipitation at the poleward border of the precipitation region then takes place along the outer border of the expanding plasma sheet.

Electron precipitation events on the morning side of the auroral zone have been surveyed by means of balloon measurements of X-ray bremstrahlung events made in Northern Scandinavia and by comparison of these with riometer measurements from stations in North America. The morning events seem to be a manifestation of isolated subatorms of medium activity level. A close correlation was found between the midnight and morning sectors, in particular when the energy spectral variations were carefully examined. The midnight precipitation pattern characterizes the source of energetic electrons giving rise to the morning precipitation. The development of the latter type of events is consistent with the drifting rain cloud model. Variable time delays between the Canadian and Scandinavian sectors may be attributed mainly to changes in the source location of electrons reaching Scandinavia. The possible role of magnetospheric electric fields and ionospheric cold plasma flow into the magnetosphere has also been considered.

Simultaneous balloon recordings of auroral-zone X-rays from precipitating electrons, covering a range of L-values from ≈5 to ≈7.5, are presented. The precipitation event was observed in the early morning sector (from about 0200 to 0500 local magnetic time), and was associated with a negative magnetic bay. Before the bay, precipitation associated with the growth phase of the substorm was observed at high L-values. After bay onset, precipitation was observed over the whole range of L-values covered, but with a delayed onset in the southern part of the precipitation region as compared with the onset of cosmic noise absorption in the local midnight sector. At high L-values the X-ray flux was completely unstructured and drizzle-like, both before and after bay onset. At low L-values, where precipitation occurred only after bay onset, the event was splash-like with X-ray bursts of typically 4–6 sec duration apparently rising out of the cosmic-ray background. The precipitation bursts had spatial extensions of 300–400 km. They were accompanied by weak magnetic impulses which were, both temporally and spatially, closely related to the X-ray bursts. The unstructured precipitation at high L-values was apparently associated with and extending along the auroral electrojet, presumably representing freshly accelerated particles. The highly structured and burst-like precipitation to the south seems to have come from a cloud of electrons drifting out from the acceleration region, from which wave-particle instabilities or some other mechanism caused electrons to be precipitated.

Energy spectral characteristics of auroral-zone X-ray events from electron precipitation are surveyed. The energy spectrum during morning SVA events is shown to vary in a systematic way: during the increasing phase of the SVA event softening of the X-ray flux occurs, but this is followed by a pronounced hardening. The softening phase is consistent with the energy-dependent effect of particle drift in the geomagnetic field thus supporting the drifting rain cloud model of morning electron precipitation events. The hardening phase may be attributed at least to some extent to energy-dependent effects of atmospheric and pitch-angle scattering processes.

Multiple balloon recordings of bremsstrahlung X-rays from a large scale auroral-zone electron precipitation event are presented. Additional riometer recordings show that it extended from noon, via dusk, to midnight. The X-ray observations show electron precipitation over a range of L-values from ∼− 5.5 to 7.5. This was briefly interrupted during a negative sudden impulse in the geomagnetic field. A close similarity between variations in the X-ray fluxes and locally recorded variations in the geomagnetic field was observed. Magnetic records from around the auroral zone suggest that the precipitation was related to an asymmetric magnetospheric convection system.

Simultaneous balloon measurements of bremsstrahlung X-rays from electron precipitation over Iceland and Scandinavia indicate that ⪆ 30 keV electron precipitation events in the geomagnetic midnight-to-morning sector extend for more than 2000 km in the west-east direction. Some events are spatially directly associated with the auroral electrojet, whereas others occur along the auroral zone, south of the electrojet. Precipitation of the former type seems to start almost simultaneously over the whole region studied, whereas precipitation of the latter type starts progressively later as one goes eastwards from the midnight sector. According to previous studies, precipitation along the electrojet is believed to be directly associated with the acceleration of electrons, whereas precipitation south of the electrojet comes from clouds of electrons drifting in the Earth's magnetic field. The large-scale observations presented here tend to confirm this model.

Auroral-zone electron precipitation during early morning hours (0200–0600 hr magnetic local time) has been analysed with the aid of X-ray measurements from northern Scandinavia together with recordings of geomagnetic variations and cosmic noise absorption (CNA). The electron precipitation can be divided in two parts: one occurring close to the location of the electrojet, the other, when the electrojet is far away or absent. The main features of these two types of precipitation distinctly resemble those found earlier in the midnight hours and in the late-morning (SVA-events), respectively. Both types of precipitation may occur simultaneously in the early morning hours. The SVA-type precipitation may extend to very early local times, and the midnight-type precipitation towards dawn. Fast pulsations of the X-ray intensity were found in both types. The midnight-type precipitation apparently stems directly from the acceleration process. The SVA-precipitation was observed to be delayed with respect to the break-up phase in the midnight sector and showed characteristic variations of the energy spectrum in a sense as to support the assumption that drifting electrons were the cause of this phenomenon. It is proposed to call the part characteristic for local times around midnight ‘direct precipitation’ and the SVA-like part ‘drift precipitation’.

Recordings of bremsstrahlung x rays from auroral-zone election precipitation events, obtained in July and August 1965 by means of balloon-borne detectors launched from sites in northern Norway are presented. An attempt is made to systematically classify these measurements. Companative studies between these and simultaneous measurements of related phenomena will be encouraged. (FR)