H. Trefall

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.

Auroral-zone electron precipitation events observed before and at the onset of negative magnetic bays

Abstract Balloon recordings of bremsstrahlung X-raya from precipitating electrons are presented, showing impulsive X-ray events just at the onset of bay activity as well as weaker and smoother electron precipitation before the bay, the latter apparently being connected with a growth phase of the same substorm. The pre-bay events are generally very smoothly varying events, but show modulation effects associated with the occurrence of irregular magnetic pulsations. The impulsive precipitation events start very abruptly together with Pi 2 magnetic pulsations at bay onset, and also show modulation effects, so that maxima in the X-ray flux tend to accompany maxima in the H -component at auroral-zone stations. These X-ray pulsations have a shorter period than the simultaneous mid-latitude Pi 2, which we take to indicate that the impulsive precipitation events occur on field lines close to the inner edge of the neutral sheet.

On the morphology of auroral-zone X-ray events—I Dynamics of midnight events

Abstract Simultaneous multiple balloon measurements have been performed of X-ray bremsstrahlung from electrons ⩾30keV precipitated into the auroral zone during polar magnetic substorms. Observations in the midnight sector have shown that the precipitation region is normally narrow in the north-south direction, but has probably a large extension in the eastwest direction. It has been found that impulsive electron precipitation events frequently occur in the midnight sector near the onset of a negative bay. Indications of rapid poleward motion have been found for such events. During the growing phase of the negative bay, the precipitation region may move equatorward as often as poleward between L ⋍ 5 and L ⋍ 7 . Towards the end of the substorm the electron precipitation usually moves poleward to L ⋍ 7 or beyond. When magnetograms indicate the existence of an apparently well-defined electrojet, the precipitation region also seems to be well delimited, and its motions well correlated with simultaneous motions of the auroral electrojet. Following the initial impulsive precipitation, the energy spectrum of the precipitated electrons shows a gradual softening at least throughout the expanding phase of the substorm, irrespective of the direction of motion of the precipitation region. There seems to be a close agreement between the development of an X-ray substorm in the midnight sector and Akasofus picture of the dynamics of the auroral substorm.

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’.

Auroral-zone X-ray events and their relation to polar magnetic substorms

The general relationship between auroral-zone X-ray events produced by precipitating electrons, and magnetic bay activity, has been studied. Every X-ray event, irrespective of the local time at which it was observed, could be associated with a negative disturbance in the geomagnetic H-component in the midnight sector of the auroral zone. And in most cases, a polar magnetic substorm of global character could be identified. X-ray events observed in the local midnight sector are prompt with respect to the associated magnetic disturbance, but it has been found that those which occur later tend to be delayed, and those observed after 0600 hr LMT are always delayed. The average delay increases with increasing separation of the region of electron precipitation from the instantaneous midnight sector, which is interpreted as an effect of drift of electrons in the magnetosphere, from an acceleration region on the night side of the Earth. Reasons are given for assuming that the acceleration region is usually rather narrow in latitude, and may extend from about magnetic midnight to dawn.

Observations of large-scale coherent pulsating electron precipitation events in the auroral zone, accompanied by geomagnetic continuous pulsations

Abstract Through balloon observations of X-ray bremsstrahlung in the auroral zone pulsating electron precipitation events have been found that are coherent over areas of at least several hundred kilometers in both the N-S and E-W directions. During one such event, which was observed simultaneously from three widely separated balloons in 1964, the X-ray intensity peaks were found to be simultaneous within the accuracy of measurement, which was one second or less. The X-ray pulsations had periods between 65 and 100 sec, and lasted for more than two hours around local noon. This noon-time pulsating electron precipitation arose nearly out of the cosmic-ray background, and did not seem to be caused by modulation of a continuously existing electron flux. It was accompanied by geomagnetic continuous pulsations, but these did not always show that the same period as the X-ray variations. There are indications that the pulsating electron flux and the accompanying MHD-wave were related, and that both phenomena may have been generated in the same process through interaction of the solar wind with the sunward side of the magnetosphere. Two probably similar events were observed in 1965, also around local noon.

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.

SIMULTANEOUS BALLOON MEASUREMENTS OF AURORAL X RAYS DURING SLOWLY VARYING IONOSPHERIC ABSORPTION EVENTS.

Abstract In the present paper we report the results of simultaneous balloon measurements of auroral X-rays in northern Scandinavia during slowly varying ionospheric absorption events. It is found that during typical events the electron precipitation starts north of L = 6·5 and spreads southwards to at least L ≈ 4·5 with speeds of about 500 m/s. The events apparently have a large longitudinal extension with the southern border aligned almost geomagnetically east—west. After the expansion the electron precipitation seems to decay uniformly over the area which was covered during the expanding phase. Faster pulsations are often observed superimposed on the general trend of the events and microbursts are probably always present.

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.