Interferometric radar observations of filamented structures due to plasma instabilities and their relation to dynamic auroral rays

Abstract

Abstract. Several explanations have been proposed for Naturally Enhanced ion-acoustic Echoes observed at midand high-latitude Incoherent Scatter observatories. A decisive measure for distinguishing between these explanations is whether or not simultaneously observed up-and downshifted enhancement occur simultaneously, or if they are the result of temporal and/or spatial averaging.The EISCAT Svalbard Radar has two antennas in the same radar system, which can be used as an interferometer when pointed parallel. In observations from 17 January 2002, between 06:46:10 and 06:46:30 UT, we used this possibility, in combination with direct sampling of the received signals, to yield measurements of "naturally enhanced ion-acoustic echoes" with sufficiently high resolution to resolve such averaging, if any. For the first time, radar interferometry has been employed to estimate the sizes of coherent structures. The observations were coordinated with an image intensified video camera with a narrow field of view. Together, this forms the initial study on the causal relationships between enhanced echoes and fine structure in the auroral activity on sub-kilometer, sub-second scales.The results confirm that the enhanced echoes originate from very localised regions (∼300 m perpendicular to the magnetic field at 500 km altitude) with varying range distribution, and with high time variability (≈200 ms). The corresponding increase in scattering cross section, up to 50 dB above incoherent scattering, eliminates theoretical explanations based on marginal stability. The simultaneously observed up-and down-shifted enhanced shoulders, when caused by sufficiently narrow structures to be detected by the interferometer technique, originate predominantly from the same volume. These results have significant impact on theories attempting to explain the enhancements, in particular it is found that the ion-electron two-stream mechanism favoured by many authors is an unlikely candidate to explain the observations. The video data has helped establish a clearCorrespondence to: T. Grydeland (tom.grydeland@phys.uit.no) correlation between the enhanced echoes and auroral activity, on sub-second time scales, showing a threshold connection between the auroral intensity and the triggering of the radar enhancements. It appears that the up-and down-shifted enhanced echoes correlate with fine auroral structures in different ways.

Phase calibration of the EISCAT Svalbard Radar interferometer using optical satellite signatures

“…After initial studies into a connection between corresponding signatures seen on the nightside and aurora Rietveld et al, 1991; Correspondence to: J. M. Sullivan (jms@phys.soton.ac.uk) Cabrit et al, 1996), recent combined radar and optical studies of these filamentary enhancements, termed "Naturally Enhanced Ion-Acoustic Lines" or NEIALs, have identified a link with the occurrence of thin ray structure of similar scale size in the cusp region (Grydeland et al, 2004;Blixt et al, 2005). Enhanced echoes have been observed frequently at the ESR (Buchert et al, 1999;Sedgemore-Schulthess et al, 1999) and a review summarising the theories and observations of NEIALs from various observatories has been published by Sedgemore-Schulthess and St.-Maurice (2001).…”

Software Radar signal processing

“…This part of the figure corresponds to Fig. 3 by Grydeland et al (2004). In this case, four points are added for the zero lag, five for the first lag, etc.…”

“…12, corresponding to Fig. 3 by Grydeland et al (2004). figure. During decoding, the contribution from all other ranges is eliminated, and the resulting lag estimates end up in the positions indicated by the shaded circles.…”

“…figure. During decoding, the contribution from all other ranges is eliminated, and the resulting lag estimates end up in the positions indicated by the shaded circles. A description of the interferometric technique, including estimation of scattering structure size is found in Grydeland et al (2004).…”

“…These were used for the processing of interferometric observations of naturally enhanced ion-acoustic echoes at the EISCAT Svalbard Radar (Grydeland et al, 2003(Grydeland et al, , 2004). An example of interferometric results from the EISCAT Svalbard Radar system is shown in Fig.…”