Abstract
Abstract. Observations of naturally enhanced ion acoustic lines (NEIALs) taken with the Poker Flat Incoherent Scatter Radar (PFISR) using a mode with very high time resolution are presented. The auroral event took place over Poker Flat, Alaska on 8 February 2007 at 09:35 UT (~22:00 MLT), and the radar data are complemented by common-volume high-resolution auroral imaging. The NEIALs occurred during only one of the standard 15-s integration periods. The raw data of this time show very intermittent NEIALs which occur only during a few very short time intervals (≤1 s) within the 15-s period. The time sampling of the raw data, ~19 ms on average, allows study of the time development of the NEIALs, though there are indications that even finer time resolution would be of interest. The analysis is based on the assumption that the NEIAL returns are the result of Bragg scattering from ion-acoustic waves that have been enhanced significantly above thermal levels. The spectra of the raw data indicate that although the up- and down-shifted shoulders can both become enhanced at the same time, (within 19 ms), they are most often enhanced individually. The overall power in the up-and down-shifted shoulders is approximately equal throughout the event, with the exception of one time, when very large up-shifted power was observed with no corresponding down-shifted power. This indicates that during the 480 μs pulse, the strongly enhanced ion-acoustic waves were only traveling downward and not upward. The exact time that the NEIALs occurred was when the radar beam was on the boundary of a fast-moving (~10 km/s), bright auroral structure, as seen in the high resolution auroral imaging of the magnetic zenith. When viewed with high time resolution, the occurrence of NEIALs is associated with rapid changes in auroral luminosity within the radar field of view due to fast-moving auroral fine structures.
Highlights
The origin of naturally enhanced ion acoustic lines (NEIALs) in incoherent scatter radar (ISR) data has long been a mystery
We argue that NEIALs appear to exhibit characteristics of coherent backscatter in the localization and strength of the returned echo
Continued observations of NEIALs with high time resolution radar measurements in conjunction with high-resolution images of the aurora are needed in order to continue to investigate the auroral plasma physics involved in their generation
Summary
The origin of naturally enhanced ion acoustic lines (NEIALs) in incoherent scatter radar (ISR) data has long been a mystery. It was noted by Grydeland et al (2003, 2004) that the occurrence of NEIALs can show significant variation between adjacent five- or ten-second integration periods, and the processes involved must be occurring on shorter timescales It was demonstrated by Grydeland et al (2003) that the enhanced returns were coming from a small region of space (a few hundred meters perpendicular to the magnetic field) and occurring for very short times (sub-second), indicating that the returns were several orders of magnitude stronger than ordinary ISR returns and, could be detected with much shorter integration times. One of the main goals of studying the raw data NEIAL returns is to gain insight into their generation mechanisms
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