Global X-ray emission during an isolated substorm — a case study
Introduction
The International Solar Terrestrial Physics (ISTP) program provides a unique opportunity to study the global substorm. Combining satellite monitoring, ground-based measurements, and remote sensing techniques such as visible, ultraviolet (UV) and X-ray imaging, one might be able to establish a comprehensive picture of the substorm development in the entire energy range of precipitating electrons taking part in the global substorm (Robinson and Vondrak, 1994). The first global schematics of the auroral substorm were based on statistical studies of data from a large number of all-sky camera stations (Akasofu, 1964, Akasofu, 1968, Feldstein and Starkov, 1967). Many of the large-scale features from these schematics have been confirmed by global UV imagers and visible imagers. As UV and visible emissions are proportional to the total electron energy flux which is usually dominated by electron energies below 10 keV, the global UV and visible images mainly display the patterns of the low-energy electron precipitation. Visible imagers are also restricted to image only the night side aurora due to contamination by sunlight.
Until recently, global imaging of the energetic electron precipitation has not been available. Our knowledge of how the precipitating energetic electrons behave during substorms has been based on measurements of cosmic radio noise absorption (riometer) (Hartz and Brice, 1967, Jelly and Brice, 1967, Berkey et al., 1974), X-ray measurements from balloon campaigns (Bjordal et al., 1971, Sletten et al., 1971, Kangas et al., 1975), particle measurements in space (McDiarmid et al., 1975, Hardy et al., 1985) and X-ray measurements from low-altitude satellites (Imhof et al., 1980, Chenette et al., 1992). The polar ionospheric X-ray imaging experiment (PIXIE) (Imhof et al., 1995) onboard the Polar satellite is the first true two-dimensional imaging instrument developed to measure the global X-ray emission simultaneously. As the X rays are produced by high-energy electrons interacting with the contents of the ionosphere, PIXIE provides the ability of studying both the spatial and temporal patterns of the global energetic electron precipitation during substorms.
From several statistical studies based on satellite measurements (McDiarmid et al., 1975, Hardy et al., 1985), riometer measurements (Hartz and Brice, 1967, Jelly and Brice, 1967) and global images in UV (Liou et al., 1997) and X rays (Petrinec et al., 1998) there are found to exist two maximum regions of energetic precipitation but three maxima in the softer precipitation (<1 keV). McDiarmid et al., 1975, Hardy et al., 1985 studied electron measurements in the energy range from tens of eV up to tens of keV, while Jelly and Brice, 1967, Berkey et al., 1974 studied absorption of cosmic radio noise, which is sensitive to electrons of energies from 10 to 100 keV. Focusing on the energetic precipitation all these studies found the first and most intense maximum to be situated around midnight and to be related to the injection of fresh electrons. They found another maximum to be located between dawn and noon, most probably related to the drifting electrons. However, by focusing on electron precipitation at lower energies (<1 keV) there is found to exist an additional maximum in the postnoon region (McDiarmid et al., 1975, Liou et al., 1997) where an almost complete lack of X-ray emission is observed (Petrinec et al., 1998). All of these studies, except for the study of Berkey et al. (1974), were based on adding all the observed precipitation during all kind of geomagnetic activity, and provide no information on the temporal behaviour of single substorms. However, a statistical study of 14 isolated substorms during 1996 (Østgaard et al., 1999b) using PIXIE and UVI data from the Polar satellite combined with ground- based measurements and data from geosynchronous satellites have confirmed many of these global characteristics.
To further investigate the global features of both soft and energetic precipitation during isolated substorms we will present an isolated substorm from 1997 where both the low-energy and high-energy range X rays were detected by PIXIE. Particle measurements obtained in the injection region in the magnetotail and at geosynchronous orbit are examined. Electron measurements from low-altitude satellite passes through the region of the localized maximum in the morning sector are used to compare calculated X rays from electron spectra with the directly measured X rays.
Section snippets
Observations and interpretation
To examine the temporal behaviour of the energetic electron precipitation during substorms and particularly the localized maximum seen in the morning sector delayed relative to the substorm onset, we have looked for an isolated substorm when PIXIE was detecting both the low-energy X rays (≈3–10 keV) and the high-energy range (≈10–20 keV). Due to a problem with the high-voltage supply in the front chamber of the PIXIE instrument, which measures the low-energy X rays, the front chamber had to be
Conclusions
By examining an isolated substorm from 4 September 1997, we have found many of the same characteristics as reported in the statistical study of Østgaard et al. (1999b).
- 1.
Growth phase signatures of directly driven precipitation in the postnoon and dusk sector are not seen by PIXIE but clearly seen in the UV substorm, indicating mainly soft precipitation.
- 2.
The substorm onset is seen simultaneously by UVI and PIXIE and correlates fairly well with the dipolarization and injection signatures seen in the
Acknowledgments
This study was supported by the Norwegian Research Council (NFR) and by the National Aeronautics and Space Administration under contract NAS5-30372 at the Lockheed-Martin Advanced Technology Center, contract NAS5-30369 at the Aerospace Corporation and contract NAG5-3170 at the University of Washington. We want to thank D. Evans for providing particle measurements from the NOAA-12 satellite and F. Rich at Space Physics Model Branch for providing data from the SSJ/4 particle detectors onboard the
References (30)
The development of the auroral substorm
Planetary and Space Science
(1964)- et al.
A synoptic investigation of particle precipitation dynamics for 60 substorms in IQSY(1964–65) and IASY(1969)
Planetary and Space Science
(1974) - et al.
On the morphology of auroral-zone X-ray events, I, Dynamics of midnight events
Journal of Atmospheric and Terrestrial Physics
(1971) - et al.
Dynamics of auroral belt and polar geomagnetic disturbances
Planetary and Space Science
(1967) - et al.
The general pattern of auroral particle precipitation
Planetary and Space Science
(1967) - et al.
On the morphology of auroral-zone X-ray events, IV, Substorm-related electron precipitation in the local morning sector
Journal of Atmospheric and Terrestrial Physics
(1975) - et al.
Auroral-zone X-ray events and their relation to polar magnetic substorms
Journal of Atmospheric and Terrestrial Physics
(1971) Polar and Magnetospheric Substorms
(1968)- et al.
Global spectroscopy and imaging of atmospheric X-ray bremsstrahlung: instrumentation and initial results from the PEM/AXIS instrument aboard the Upper Atmosphere Research Satellite
- et al.
A statistical model of auroral electron precipitation
Journal of Geophysical Research
(1985)
Satellite X-ray mappings of sporadic auroral zone electron precipitation events in the local dusk sector
Journal of Geophysical Research
The polar ionospheric X-ray imaging experiment (PIXIE)
Space Science Review
Simultaneous observations of energetic electrons and dawnside chorus in geosynchronous orbit
Journal of Geophysical Research
Changes in Van Allen radiation associated with polar substorms
Journal of Geophysical Research
Cited by (7)
Characteristics of Energetic Electron Precipitation Estimated from Simulated Bremsstrahlung X-ray Distributions
2019, Journal of Geophysical Research: Space PhysicsAuroral Energy Flux Distribution Over the Nightside Auroral Oval Observed by the DMSP F16/SSUSI: Seasonal, Geomagnetic, and Solar Activity Dependences
2018, Journal of Geophysical Research: Space PhysicsPlasmas and energetic processes in the geomagnetosphere: Volume II: Plasmas/magnetic and current sheets, reconnections, particle acceleration, and substorms
2016, Plasmas and Energetic Processes in the Geomagnetosphere: Volume II: Plasmas/Magnetic and Current Sheets, Reconnections, Particle Acceleration, and SubstormsContribution of proton precipitation to space-based auroral FUV observations
2004, Journal of Geophysical Research: Space PhysicsComparisons of thermospheric high-latitude nitric oxide observations from SNOE and global auroral X-ray bremsstrahlung observations from PIXIE
2003, Journal of Geophysical Research: Space PhysicsImaging riometer observations on energetic electron precipitation at SANAE IV, Antarctica
2002, Journal of Geophysical Research: Space Physics