J. Bjordal

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.

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.

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.

Energy deposition rates by charged particles

Abstract A summary of measurements of the precipitation of electrons and positive ions (in the keV-MeV range) detected aboard eight rockets launched within the Energy Budget Campaign from northern Scandinavia is given, together with corresponding satellite data. In some cases strong temporal variations of the downgoing integral fluxes were observed. The fluxes provide the background for the calculated ion production rates and altitude profiles of the energy deposition into the atmosphere at different levels of geomagnetic disturbance and cosmic noise absorption. The derived ion production rates by energetic particles are compared to other night-time ionisation sources.

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.

On the extension of auroral-zone X-ray microbursts

Abstract Simultaneous observations of auroral-zone X-ray microbursts made by two balloon-borne radiation detectors at a separation of about 300 km are compared with previous observations of the same kind, and used to estimate the size of the region of electron precipitation in such events. As both balloons were at nearly the same geomagnetic latitude, only the E-W extension of microburst events could be deduced. This was found to be approximately 200 km, which corresponds to about 3500 km if projected along the magnetic lines of force to the geomagnetic equatorial plane.

Magnetospheric and ionospheric response to a substorm: Geotail HEP‐LD and Polar PIXIE observations

The High Energy Particle - Low energy particle Detector experiment (HEP-LD) on board the Geotail spacecraft and the Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the Polar satellite have been used to examine a substorm event. On December 10, 1996, around 1700 UT, a substorm event with two onsets took place. The event occurred during a weak magnetic storm that started on December 9. Several of the classical substorm features were observed during the event: reconnection and neutral-line formation in the near-Earth geomagnetic tail, injection of energetic particles at geosynchronous orbit, and particle precipitation into the ionosphere. Magnetic field line mapping of the energetic precipitation area into the geomagnetic tail shows that the substorm development on ground is closely correlated with topological changes in the near-Earth tail. In the first onset, mainly soft electrons are involved, with only a transient energetic precipitation delayed relative to the onset. The second onset about 30 min later includes both soft and energetic electrons. The source regions of both onsets are found to be located near the earthward edge of the plasma sheet, while the source region of the transient energetic precipitation during the first onset is in the distant tail. Magnetic reconnection occurs sporadically and burst-like before the onsets. Both onsets appear to be triggered by northward turning of the interplanetary magnetic field. The study also demonstrates that the concept of pseudobreakups should be used with care unless global observations are available.

X-RAY IMAGING OF THE AURORA

Abstract X-ray imaging has proved to be a powerful technique for remote mapping of the precipitation of energetic electrons, providing spatial, temporal and spectral information on the precipitating electrons both at day and night. The strengths and weaknesses of the X-ray imaging technique compared to auroral imaging in the visible and ultra-violet spectral ranges are discussed. A short review is given of the development in this field, including some future plans. Some of the results obtained from auroral X-ray studies are discussed.

Global X-ray emission during an isolated substorm — a case study

Abstract The polar ionospheric X-ray imaging experiment (PIXIE) and the UV 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 UV emissions. While the UV images in the Lyman–Birge–Hopfield-long band used in this study respond to the total electron energy flux which is usually dominated by low-energy electrons (