High resolution measurements and modeling of auroral hydrogen emission line profiles

Abstract

Abstract. Measurements in the visible wavelength range at high spectral resolution (1.3Å) have been made at Longyearbyen, Svalbard (15.8 E,78.2 N) during an interval of intense proton precipitation. The shape and Doppler shift of hydrogen Balmer beta line profiles have been compared with model line profiles, using as input ion energy spectra from almost coincident passes of the FAST and DMSP spacecraft. The comparison shows that the simulation contains the important physical processes that produce the profiles, and confirms that measured changes in the shape and peak wavelength of the hydrogen profiles are the result of changing energy input. This combination of high resolution measurements with modeling provides a method of estimating the incoming energy and changes in flux of precipitating protons over Svalbard, for given energy and pitch-angle distributions. Whereas for electron precipitation, information on the incident particles is derived from brightness and brightness ratios which require at least two spectral windows, for proton precipitation the Doppler profile of resulting hydrogen emission is directly related to the energy and energy flux of the incident energetic protons and can be used to gather information about the source region. As well as the expected Doppler shift to shorter wavelengths, the measured profiles have a significant red-shifted component, the result of upward flowing emitting hydrogen atoms.

TRANS4: a new coupled electron/proton transport code – comparison to observations above Svalbard using ESR, DMSP and optical measurements

“…This blue shift corresponds to a parallel velocity of 127 km s −1 , which is very slow in comparison to Doppler shifts commonly measured in proton events, reaching velocities up to three times as large (e.g. Søraas et al, 1974;Lanchester et al, 2003). This first result is consistent with the spectral shifts recorded from the ground in NyAlesund.…”

“…Including redistributions at high energies can have a dramatic effect and may change the position of the peak in the case of high-energy proton precipitation. This was already emphasized in Lanchester et al (2003) in the case of H β , and we can now extend these conclusions to H α .…”

“…3d), which indicates that when choosing a wide angular redistribution due to collisions, computed Doppler shifts can become consistent with the measurements. Compared to magnetic mirroring, collisional angular redistributions play the major role at least at low Doppler shifts in the creation of the observed spectrum, a conclusion that was previously put forward to ac- count for H β lines (Lanchester et al, 2003). In this case, the line can become completely symmetric with a second peak appearing at longer wavelengths near 6565Å.…”

“…Little variation of the auroral brightness with latitude has been observed, except in the daytime polar cleft where profiles usually exhibit narrower shapes . Other instruments such as the High Throughput Imaging Echelle Spectrograph (Hi-TIES) have recently been installed at Svalbard (Chakrabarti et al, 2001;Lanchester et al, 2003). HiTIES is aimed at examining the H β line in the magnetic zenith with a spectral resolution of 1.3Å, allowing long-duration exposures and a very good signal-to-noise ratio.…”

First ground-based optical analysis of H<sub>β</sub> Doppler profiles close to local noon in the cusp

“…Such imaging spectrographs are currently the optimal instruments for high spectral resolution measurements of auroral hydrogen lines (Baumgardner et al, 1993). The High Throughput Imaging Echelle Spectrograph (HiTIES), the main instrument on the Spectrographic Imaging Facility (SIF) in Nordlysstasjonen on Svalbard is well documented (Chakrabarti et al, 2001;McWhirter et al, 2003;Lanchester et al, 2003;Galand et al, 2004). Nordlysstasjonen is located at geographic 78.2 N and 15.8 E, which corresponds to a local noon at 10:56:48 UT.…”