Model predictions of the occurrence of non-Maxwellian plasmas, and analysis of their effects on EISCAT data

The recent identification of non-thermal plasmas using EISCAT data has been made possible by their occurrence during large, short-lived flow bursts. For steady, yet rapid, ion convection the only available signature is the shape of the spectrum, which is unreliable because it is open to distortion by noise and sampling uncertainty and can be mimicked by other phenomena. Nevertheless, spectral shape does give an indication of the presence of non-thermal plasma, and the characteristic shape has been observed for long periods (of the order of an hour or more) in some experiments. To evaluate this type of event properly one needs to compare it to what would be expected theoretically. Predictions have been made using the coupled thermosphere-ionosphere model developed at University College London and the University of Sheffield to show where and when non-Maxwellian plasmas would be expected in the auroral zone. Geometrical and other factors then govern whether these are detectable by radar. The results are applicable to any incoherent scatter radar in this area, but the work presented here concentrates on predictions with regard to experiments on the EISCAT facility

Particle precipitations during NEIAL events : simultaneous ground based observations at Svalbard

In this paper we present Naturally Enhanced Ion Acoustic Lines (NEIALs) observed with the EISCAT Svalbard Radar (ESR). For the first time, long sequences of NEIALs are recorded, with more than 50 events within an hour, ranging from 6.4 to 140 s in duration. The events took place from ~08:45 to 10:00 UT, 22 January 2004. We combine ESR data with observations of optical aurora by a meridian scanning photometer at wavelengths 557.7, 630.0, 427.8, and 844.6 nm, as well as records from a magnetometer and an imaging riometer. The large numbers of observed NEIALs together with these additional observations, enable us to characterise the particle precipitation during the NEIAL events. We find that the intensities in all optical lines studied must be above a certain level for the NEIALs to appear. We also find that the soft particle precipitation is associated with the down-shifted shoulder in the incoherent scatter spectrum, and that harder precipitation may play a role in the enhancement of the up-shifted shoulder. The minimum energy flux during NEIAL events found in this study was ~3.5 mW/m2 and minimum characteristic energy around 50 eV

Variability of dayside high latitude convection associated with a sequence of auroral transients

10 second resolution ionospheric convection data covering the invariant latitude range from 71° to 76°, obtained by using the EISCAT UHF and VHF radars, are combined with optical data from Ny Ålesund during a sequence of auroral transients in the post-noon sector (∼ 15 MLT). Satellite observations of polar cap convection patterns suggest negative BZ and BY components of the interplanetary magnetic field. Burst-like enhancements of westward (sunward) post-noon convection were accompanied by eastward moving auroral forms at higher latitudes, above the convection reversal boundary. In this case the background convection was weak, whereas the integrated potential drop across the radar field-of-view associated with the westward flow bursts was typically ∼ 20-35 kV. The auroral phenomenon consists of a series of similar events with a mean repetition period of 8 min. A close correlation between the auroral activity and convection enhancements in the cleft ionosphere is demonstrated

Spectral imaging of proton aurora and twilight at Tromsø, Norway

An imaging Echelle spectrograph designed for high-resolution studies of selected spectral features located in the visible spectrum was deployed from November 2001 until April 2003 in Tromsø, Norway. For moderately disturbed magnetic conditions, Tromsø is located on the equatorial edge of the evening auroral oval for several hours. Energetic protons are frequently the dominant particle energy source in this region. For this experiment, four spectral windows were selected, each around different emission features: H? (656.3 nm), H? (486.1 nm), N2+1NG 427.8 nm, and OI 777.4 nm. The 8° long slit of the spectrograph was centered on the magnetic zenith. This instrument provided simultaneous, high-resolution (~0.1 nm) spectra of H? and H? emissions, which offers a unique opportunity to investigate the H? to H? Balmer decrement in proton aurora. Information on the cloud cover and on the overall auroral activity was provided by a large field of view (70°) conventional imaging spectrograph that spans the 350–800 nm spectral range. In this paper we describe both instruments and demonstrate their capabilities for the study of the H Balmer emissions in twilight and during auroral activity. Our high-resolution spectra taken in twilight could be used to observe the variability of the geocoronal component over time and to compare the derived variability with midlatitude sites. We conclude that the 0.1 nm spectral resolution is sufficient to identify and take into account contaminating OH and N2 1PG features in H? emission profiles. Comparison of H? Doppler profiles observed at different locations (Tromsø, Poker Flat, Svalbard) in proton aurora is presented. Lummerzheim and Galand [2001] find that the shape of the violet wing of the Balmer profile is a more suitable indicator of the mean energy of the incident protons than the Doppler shift of the peak. Numerous uncertainties in measured and modeled H? and H? line profiles preclude using the Balmer decrement as an indicator of the precipitating proton flux