J. Stadsnes

Auroral electron distributions derived from combined UV and X-ray emissions

The Polar Ionospheric X-ray Imaging Experiment and the Ultraviolet Imager on board the Polar satellite provide the first simultaneous global scale views of the electron precipitation over a wide range of electron energies. By combining the results from these two remote sensing techniques we have developed a method to derive the electron energy distributions that reproduce the true electron spectra from 1 to 100 keV and that can be used to calculate the energy flux in the energy range from 100 eV to 100 keV. The electron energy spectra obtained by remote sensing techniques in three 5-min time intervals on July 9 and July 31, 1997, are compared with the spectra measured by low-altitude satellites in the conjugate hemisphere. In the energy range from 90 eV to 30 keV the derived energy flux is found to be 1.03±0.6 of the measured energy fluxes. The method enables us to present 5-min time-averaged global maps of precipitating electron energy fluxes with a spatial resolution of ∼700 km. The study shows that the combination of UV and X-ray cameras on a polar orbiting spacecraft enables comprehensive monitoring of the global energy deposition from precipitating electrons over the energy range that is most important for magnetosphere-ionosphere coupling.

Global‐scale electron precipitation features seen in UV and X rays during substorms

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) and the ultraviolet imager (UVI) onboard the Polar satellite have provided the first simultaneous global-scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung and the auroral ultraviolet (UV) emissions. While the UV images respond to the total electron energy flux, which is usually dominated by electron energies below 10 keV, the PIXIE, 9.9–19.7 keV X-ray images used in this study respond only to electrons of energy above 10 keV. Previous studies by ground-based, balloon, and space observations have indicated that the patterns of energetic electron precipitation differ significantly from those found in the visible and the UV auroral oval. Because of the lack of global imaging of the energetic electron precipitation, one has not been able to establish a complete picture. In this study the development of the electron precipitation during the different phases of magnetospheric substorms is examined. Comparisons are made between the precipitation patterns of the high-energy (PIXIE) and low-energy (UVI) electron populations, correlated with ground-based observations and geosynchronous satellite data. We focus on one specific common feature in the energetic precipitation seen in almost every isolated substorm observed by PIXIE during 1996 and which differs significantly from what is seen in the UV images. Delayed relative to substorm onsets, we observe a localized maximum of X-ray emission at 5–9 magnetic local time. By identifying the location of the injection region and determining the substorm onset time it is found that this maximum most probably is caused by electrons injected in the midnight sector drifting (i.e., gradient and curvature drift) into a region in the dawnside magnetosphere where some mechanism effectively scatters the electrons into the loss cone.

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.

Cause of the localized maximum of X‐ray emission in the morning sector: A comparison with electron measurements

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the Polar satellite has provided the first global scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung. While other remote sensing techniques like ultraviolet (UV) and visible imaging sense emissions that are dominantly produced by low-energy electrons ( 2-10 keV electrons by wave-particle interaction into the loss cone is the main mechanism for this precipitation.

Energetic electron precipitation and the NO abundance in the upper atmosphere: A direct comparison during a geomagnetic storm

[1] Nitric oxide (NO) densities at heights between 96 and 150 km in the Earth’s upper atmosphere are directly compared with the energy deposition from precipitating energetic electrons. The comparisons are done for the beginning of a geomagnetic storm event on 2 May 1998. The electron energy is derived from X-ray bremsstrahlung observations from the Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the Polar spacecraft. Measurements of the NO density are performed by the Student Nitric Oxide Explorer (SNOE) on the dayside by measuring airglow spectral features of the NO g-band. Since a significant part of the electron precipitation takes place during the night, and considering the long lifetime of NO, we have accumulated the X-ray data in geographical boxes. This enables us to follow the development of the total energy deposition over a specific area during the night and morning hours. In agreement with theoretical predictions we find an increase in NO at higher latitudes due to electron precipitation. At 106 km altitude, which is found to be the average altitude of the peak values of both NO intensity and precipitating electron energy, 83% of the NO density is produced by electron precipitation. At this altitude we find that the electron precipitation results in the production of 8 NO molecules per keV deposited energy. The comparison of the data also shows effects of a horizontal neutral wind. Above 100 km the peak in NO density is displaced equatorward of the peak in electron energy deposition. INDEX TERMS: 2716 Magnetospheric Physics: Energetic particles, precipitating; 2736 Magnetospheric Physics: Magnetosphere/ ionosphere interactions; 2437 Ionosphere: Ionospheric dynamics; 2407 Ionosphere: Auroral ionosphere (2704); 2455 Ionosphere: Particle precipitation; KEYWORDS: PIXIE, SNOE, nitric oxide, electron precipitation

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.

Simultaneous observations of magnetotail reconnection and bright X-ray aurora on 2 October 2002

We present simultaneous Cluster and Polar X-ray and UVI observations on 2 October2002, when Cluster observed a magnetic reconnection diffusion region at Xgse = 16.6 Re. At the same time a bright auroral feature appeared at the footpoint of themagnetic field line connecting the ionosphere and the diffusion region. However, wefound that the electrons measured in the diffusion region by Cluster were notsufficiently accelerated by the reconnection process to produce the aurora X-ray fluxesmeasured by Polar. The DMSP F14 passed over the intense X-ray spot and showed thatthe X rays (and the fainter UV) were produced by electrons accelerated through a 30 kV potential drop. The coincidence in time and the fact that this inverted-V is veryclose to the open-closed field line boundary suggest that the inverted-V structure areproduced by flow shears that could be related to the reconnection process.

Statistical pitch angle properties of substorm‐injected electron clouds and their relation to dawnside energetic electron precipitation

[1] Using the existing large database of geosynchronous orbit particle measurements from Los Alamos instruments, statistical properties of substorm-injected electron clouds are investigated, with special focus on the pitch angle distribution (PAD) of the electrons. The electron distributions at 6.6 RE do in general show some anisotropy, and their PADs are probably caused by the combined influence of drift orbits, different for each energy and pitch angle, and pitch angle diffusion due to waves. The statistical results of this paper indicate that the PADs during intervals of increased electron flux at energies greater than 10 keV from midnight till noon are dominated by pitch angle diffusion by interaction with waves. The strength of the pitch angle diffusion is seen to initially limit the growth of anisotropy from differential drift speeds and orbits and later on to increase the anisotropy when the diffusion strength decreases. After local noon we find evidence that pitch angle diffusion is no longer important and the PADs are evolving due to differential drift effects.

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.

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.