Summary of quantitative interpretation of IMAGE far ultraviolet auroral data

Abstract

Direct imaging of the magnetosphere by instruments on the IMAGE spacecraft is supplemented by simultaneous observations of the global aurora in three far ultraviolet (FUV) wavelength bands. The purpose of the multi-wavelength imaging is to study the global auroral particle and energy input from the magnetosphere into the atmosphere. This paper describes the method for quantitative interpretation of FUV measurements. The Wide-Band Imaging Camera (WIC) provides broad band ultraviolet images of the aurora with maximum spatial resolution by imaging the nitrogen lines and bands between 140 and 180 nm wavelength. The Spectrographic Imager (SI), a dual wavelength monochromatic instrument, images both Doppler-shifted Lyman-α emissions produced by precipitating protons, in the SI-12 channel and OI 135.6 nm emissions in the SI-13 channel. From the SI-12 Doppler shifted Lyman-α images it is possible to obtain the precipitating proton flux provided assumptions are made regarding the mean energy of the protons. Knowledge of the proton (flux and energy) component allows the calculation of the contribution produced by protons in the WIC and SI-13 instruments. Comparison of the corrected WIC and SI-13 signals provides a measure of the electron mean energy, which can then be used to determine the electron energy flux. To accomplish this, reliable emission modeling and instrument calibrations are required. In-flight calibration using early-type stars was used to validate the pre-flight laboratory calibrations and determine long-term trends in sensitivity. In general, very reasonable agreement is found between in-situ measurements and remote quantitative determinations.

Global observations of electromagnetic and particle energy flux for an event during northern winter with southward interplanetary magnetic field

“…The dual-channel SI images both Doppler-shifted Ly-α emissions, produced by precipitating protons, and OI 135.6 nm emissions in the SI-12 and SI-13 channels, respectively. The technique to estimate the particle energy flux from the multi-wavelength auroral images is described in detail by Frey et al (2003). In brief, we first estimate the precipitating proton energy flux from the luminosities in the Ly-α band and a model prediction for the proton mean energy (Hardy et al, 1989(Hardy et al, , 1991.…”

“…Comparison of the WIC and SI-13 signals provides a measure of the electron mean energy, which can then be used to determine the electron energy flux. These estimates are typically accurate to within a factor of two (Frey et al, 2003). The total particle precipitation energy flux is the sum of the proton and electron contributions.…”

Energetic electron precipitation during sawtooth injections

“…Subsequently electron mean energies and fluxes have been determined from the emission measurements (e.g. Frey et al, 2003). Satellite borne instruments also provide observations of the corresponding magnetospheric variations during the sawtooth event.…”

High-latitude electromagnetic and particle energy flux during an event with sustained strongly northward IMF

“…Finally, DMSP SSJ/4 particle data (Hardy et al, 1984) are used to provide additional confirmation of the precipitating particles and to estimate their characteristic energy. This characteristic energy is required to convert the auroral luminosity to particle energy flux (Frey et al, 2003).…”

“…We then estimate the electron flux from the FUV image results as follows. The conversion of the WIC count rate to electron energy flux is governed by the energy dependent transfer function from emission intensity to energy flux (Frey et al, 2003). Since the spectrum of the precipitating population is not determined by the WIC data, we use the DMSP F15 SSJ/4 particle data during the satellite's transit through the enhanced emission region to determine a characteristic electron energy.…”