Balloon-Borne X-Ray Observations of
Relativistic Electron Precipitation


This web page shows several figures from posters that were presented at the 1997 and 1998 American Geophysical Union meetings.

The 1997 AGU abstract summarizes observations by the X-ray imager and the 1998 AGU abstract proposes a model to explain these observations.

A Geophysical Research Letters paper also describes the preliminary observations.

Introduction

The INTERBOA96 (Balloon Observations of Aurora) campaign launched three stratospheric balloons from Kiruna, Sweden, in August 1996. Launch times were chosen to coordinate with geomagnetic activity and ISTP satellite positions.

Balloon-borne instruments included the X-ray imager, X-ray spectrometers, magnetometers and electric field probes.

On August 20, 1996 at 1530 UT (1815 MLT, L ~ 5.7), the balloon-borne X-ray instruments observed a burst of X-rays with energies > 1 MeV which are best accounted for by atmospheric bremsstrahlung from mono-energetic 1.7 MeV precipitating electrons.

The geophysical condition was quiet, with Kp ~ 2. A small substorm was observed ~24 minutes before the X-ray event by the Polar UVI and by the LANL geosynchronous satellites.

Observations

Figure 1

 Figure 1. The measured count rate from the UW X-ray imager for 20-120 keV X-rays, over the entire flight and during the REP event. The countrate shows large variations with periodicity of about 150 seconds and smaller variations with periodicity of about 10 seconds.

Figure 2

 Figure 2. 80-second images from the X-ray imager show no significant spatial structure, indicating that variations in flux are a temporal effect rather than a spatial one. The flux brightens uniformly across the field of view (50 km at 60 km altitude) rather than moving in and out of the image.

Figure 3

 Figure 3. X-ray energy spectra taken by the Berkeley germanium spectrometer. Note the significant flux of X-rays at MeV energies. The spectra harden slightly after 15:42 UT. The solid lines show the fit obtained from bremsstrahlung calculations with a model input of mono-energetic 1.7 MeV X-rays [Foat, et al., First Detection of a Terrestrial MeV X-ray Burst, Geophys. Res. Letters, 25, 1998].

A Possible Explanation

Figure 4

 Figure 4. The X-ray observations and coincident observations of a nightside substorm can be explained by interaction of relativistic electrons with electromagnetic ion-cyclotron waves. The wave growth is driven by hot (~50 keV) protons drifting from the substorm injection into regions of high-density such as the plasmasphere or detached plasma regions. Calculations of wave growth and resonant electron energies show that this model provides a reasonable explanation for the observed energies, spectral characteristics and spatial localization of the event. This cartoon illustrates the main features of the model.

Geophysics Main Page |  Space Physics | Balloon Experiments

Kirsten Lorentzen / 4 December 1998 / kirsten@geophys.washington.edu