K. Brønstad

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’.

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.

Morphology and fine time structure of an early-morning electron precipitation event

Abstract 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.

EAST--WEST MOVEMENTS OF PULSATING AURORAL-ZONE X-RAY EVENTS.

Abstract During simultaneous recordings by means of balloon-borne X-ray detectors, two cases of pulsating auroral-zone electron precipitation events have been observed, which were moving from east to west along the auroral zone. Both events were observed between 07.00 and 09.00 hr L.T. They moved with velocities of 120 and 150 km/min, and had periods of 300 and 500 sec, corresponding to wavelengths of about 600 and 1250 km respectively. Both events were superimposed upon the beginning phase of a more slowly varying electron precipitation event, and seem to represent modulation effects by some travelling hydromagnetic wave upon the main precipitation event.

Balloon observations of auroral-zone X-rays in conjugate regions

Abstract Conjugate aspects of auroral zone electron precipitation are examined using X-ray data obtained from balloon flights launched from conjugate sites, Reykjavik, Iceland and Syowa, Antarctica, in the winter of 1969. In spite of the close conjugacy of these sites, dissimilarities were noted between observations in the two hemispheres, especially for midnight events. This is attributed in part to balloon drift and motions of narrow precipitation regions. For a dayside event, close conjugate relations were found even though the X-ray detectors were out of conjugacy by approx. 700 km. Riometer data from this event suggest that the intensity of electron precipitation was significantly smaller in the southern hemisphere.

Large-scale auroral-zone electron precipitation event, briefly interrupted during a negative magnetic impulse

Abstract 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.

On the origin of electron precipitation in the auroral zone related to the onset of magnetospheric substorms

Abstract Observations of electron precipitation via X-ray measurements with balloon-borne instruments and electron measurements onboard the geostationary satellite GEOS-2 are used to analyse the relationship between electron precipitation and magnetospheric processes at the onset of a magnetospheric substorm. The observations are interpreted in terms of electron scattering and different acceleration processes at the outer boundary of the plasma sheet and simultaneous movements of its inner boundary.