PFISR nightside observations of naturally enhanced ion acoustic lines, and their relation to boundary auroral features

Abstract

Abstract. We present results from a coordinated camera and radar study of the auroral ionosphere conducted during March of 2006 from Poker Flat, Alaska. The campaign was conducted to coincide with engineering tests of the first quarter installation of the Poker Flat Incoherent Scatter Radar (PFISR). On 31 March 2006, a moderately intense auroral arc, (∼10 kR at 557.7 nm), was located in the local magnetic zenith at Poker Flat. During this event the radar observed 7 distinct periods of abnormally large backscattered power from the F-region. These were only observed in the field-aligned radar beam, and radar spectra from these seven times show naturally enhanced ion-acoustic lines (NEIALs), the first observed with PFISR. These times corresponded to (a) when the polar cap boundary of the auroral oval passed through the magnetic zenith, and (b) when small-scale filamentary dark structures were visible in the magnetic zenith. The presence of both (a) and (b) was necessary for their occurrence. Soft electron precipitation occurs near the magnetic zenith during these same times. The electron density in the vicinity where NEIALs have been observed by previous studies is roughly between 5 and 30×10 10 m −3 . Broadband extremely low frequency (BBELF) wave activity is observed in situ by satellites and sounding rockets to occur with similar morphology, during active auroral conditions, associated with the poleward edge of the aurora and soft electron precipitation. The observations presented here suggest further investigation of the idea that NEIALs and BBELF wave activity are differently-observed aspects of the same wave phenomenon. If a connection between NEIALs and BBELF can be established with more data, this could provide a link between in situ measurements of downward current regions (DCRs) and dynamic aurora, and ground-based observations of dark auroral structures and NEIALs. Identification of in situ processes, namely wave activity, in ground-based signaCorrespondence to: R. G. Michell (rmichell@swri.edu) tures could have many implications. One specific example of interest is identifying and following the temporal and spatial evolution of regions of potential ion outflow over large spatial and temporal scales using ground-based optical observations.

Soliton-induced spectrally uniform ion line power enhancements at the ionospheric F region peak

“…48 events of spectral enhancements and their corresponding average profiles of plasma parameters remained. As suggested by the observations of Michell et al (2008Michell et al ( , 2009) and Michell and Samara (2010), there is a possible connection between auroral activity and the type of ion line enhancements presented here. The K index is a measure of magnetic disturbance and in order to quantify a possible relation to auroral activity, the K index was noted for each event.…”

“…NEIAL are often seen simultaneously over a height range of several 100 km (e.g., Rietveld et al, 1991) and have been observed up to 1900 km altitude (Ogawa et al, 2006). Michell et al (2008) reported observations of enhanced ion lines at low altitudes (250-350 km) made with the Poker Flat Incoherent Scatter Radar (PFISR). Using long uncoded pulses, their range resolution was limited to 75 km, which roughly coincided with the altitude range of the enhancements.…”

“…Michell et al (2008) suggested a possible connection to broad-band extremely low-frequency (BBELF) wave activity, based on the auroral morphology accompanying their observations. However, the enhancements reported by Michell et al (2008Michell et al ( , 2009) and Michell and Samara (2010), where both ion-acoustic shoulders and the spectral region between them are enhanced equally and simultaneously, can also be replicated by the recently published spectral-shape-conserving soliton-induced ion line enhancement model by Ekeberg et al (2010).…”

Enhanced EISCAT UHF backscatter during high-energy auroral electron precipitation

“…The altitude extent of regions with enhanced backscattered radar power is limited to about 2 km in our observations. While unresolved, the altitude extent has been reported to be ∼ 10 km (Isham et al, 2012) and 72 km (Akbari et al, 2012;Michell and Samara, 2010;Michell et al, 2008) in recent observations of a enhanced zero Doppler shift features. More common observations of NEIALs, where no zero Doppler shift feature is observed, typically extend over hundreds of kilometers and have been related to dynamic aurora exhibiting fine-scale structuring.…”

“…Among the models for NEIALs, the Langmuir turbulence model and the model proposed by Ekeberg et al (2010) the enhanced ion line shoulders (Michell et al, 2008;Michell and Samara, 2010;Ekeberg et al, 2010;Akbari et al, 2012), the enhanced radar spectra reported herein and those reported by Isham et al (2012) show a distinct zero Doppler shift feature. A distinct zero Doppler shift feature is only accounted for in LT models.…”