W. Riedler

On the morphology of energetic (≥30keV) electron precipitation during the growth phase of magnetospheric substorms

Abstract 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.

On the morphology of auroral-zone X-ray events—II. Events during the early morning hours

Abstract 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’.

Potential observations of an electron-emitting rocket payload and other related plasma measurements

Abstract Observations of plasma effects due to an energetic electron beam near a rocket payload are summarized and an attempt is made to outline the basic processes in the plasma such as beam plasma discharge (BPD). The experiment had some unique features such as a high apogee and low background plasma density. The measurements covered most parameters relevant for the study of plasma effects, as well as payload potential, optical and radio emissions, energetic charged particles and plasma densities. Among the noteworthy observations are radio emissions up to VHP frequencies and very high payload potentials which even exceeded the gun voltage. The observed features are interpreted as BPD at low altitudes and as discharges in the E × B field at other heights.

On the morphology of energetic (≥30keV) electron precipitation at the onset of negative magnetic bays

Abstract 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 associated with a sudden commencement of a geomagnetic storm

Abstract Balloon observations of X-rays produced by precipitated electrons were made in the morning sector of the auroral zone at the time of the geomagnetic storm sudden commencement (SSC) of July 27, 1966. The impulsive precipitation event lasted 4 min, both the rise and fall times being nearly 1 min. On a shorter time scale a pronounced variation with a period of 1.8–1.9 sec existed, which occurred together with hm-emissions of the same period range. Besides the rapid fluctuations in the precipitation a 50-sec period was also present. The energy spectrum of the observed X-ray flux was rather steep, characterized by an e-folding energy of E0 = 18–22 keV. The SSC apparently triggered a polar magnetic substorm in the midnight sector of the auroral zone.

On the morphology of auroral-zone X-ray events—III. Large-scale observations in the midnight-to-morning sector

Abstract 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 Earths magnetic field. The large-scale observations presented here tend to confirm this model.

On the morphology of auroral-zone X-ray events—IV. Substorm-related electron precipitation in the local morning sector

Abstract 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.

Laboratory studies for beam plasma discharge experiments with rockets

Abstract Laboratory results of beam plasma discharge (BPD) (dimensions several metres) in a homogeneous atmosphere and a jet of neutral gas are presented. The beam was obtained by an electron gun of the same type as used in rocket experiments. The BPD parameters glow intensity, electromagnetic field and its spectrum, electron density as well as the flux of scattered electrons were measured. In addition, photographs of the BPD were taken. The threshold of BPD ignition was determined as a function of gas pressure and beam parameters. The initial stage of BPD formation as well as various types of oscillatory processes in the discharge were studied. A possible process of BPD initiation is presented.

Energy spectral variations during SVA precipitation events on the morning and day side of the auroral zone

Abstract 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.

Potential observations of an electron-emitting rocket payload

Abstract Direct and indirect potential measurements of a very high flying sounding rocket, which carried a powerful electron accelerator are reported. The observed payload potential during the pulses does not always follow the accelerator voltage. Observations after the pulses suggest a small negative payload potential which vanishes with time constants in the second-range.