Riometer

Abstract: Most measurements of cosmic radio noise absorption in the polar regions have been made with riometers using broad-beam (∼60 deg) antennas. Only limited information about the spatial structure and dynamics of the aurora can be obtained by this means. Recent trends in riometry have emphasized the use of multiple narrow-beam antennas operated in a fixed beam or one-dimensional scanning mode to examine smaller ionospheric regions. A further step in this direction has been the development of the imaging riometer for ionospheric studies (IRIS). This instrument provides a two-dimensional image of regions of enhanced cosmic noise absorption at 38.2 MHz with a spatial resolution as small as 20 km and time resolution of 1 s. The IRIS antenna is a 64-element dipole array, phased to produce 49 independent beams viewing an ionospheric area about 200 km square at 90 km altitude. One IRIS instrument has been operating at the South Pole station since January, 1988; a second instrument was recently installed at Sondre Stromfjord, Greenland. This paper presents a technical description of the IRIS system. The response of the IRIS to a sun-aligned absorption arc is illustrated and compared with that of a broad-beam riometer. An intense, localized structure that developed within the arc was observed by IRIS to propagate in the magnetic poleward direction with a speed of 1 km/s.

Abstract: Magnetic pulsations in the Pc1-Pc2 frequency range (0.1-5 Hz) are often observed on the ground and in the Earth's magnetosphere during the aftermath of geomagnetic storms. Numerous studies have suggested that they may play a role in reducing the fluxes of energetic ions in the ring current; more recent studies suggest they may interact parasitically with radiation belt electrons as well. We report here on observations during 2005 from search coil magnetometers and riometers installed at three Antarctic stations, Halley (-61.84 degrees magnetic latitude, MLAT), South Pole (-74.18 degrees MLAT), and McMurdo (-79.96 degrees MLAT), and from energetic ion detectors on the NOAA Polar-orbiting Operational Environment Satellites (POES). A superposed epoch analysis based on 13 magnetic storms between April and September 2005 as well as case studies confirm several earlier studies that show that narrowband Pc1-Pc2 waves are rarely if ever observed on the ground during the main and early recovery phases of magnetic storms. However, intense broadband Pi1-Pi2 ULF noise, accompanied by strong riometer absorption signatures, does occur during these times. As storm recovery progresses, the occurrence of Pc1-Pc2 waves increases, at first in the daytime and especially afternoon sectors but at essentially all local times later in the recovery phase (typically by days 3 or 4). During the early storm recovery phase the propagation of Pc1-Pc2 waves through the ionospheric waveguide to higher latitudes was more severely attenuated. These observations are consistent with suggestions that Pc1-Pc2 waves occurring during the early recovery phase of magnetic storms are generated in association with plasmaspheric plumes in the noon-to-dusk sector, and these observations provide additional evidence that the propagation of waves to ground stations is inhibited during the early phases of such storms. Analysis of 30- to 250-keV proton data from four POES satellites during the 24-27 August and 18-19 July 2005 storm intervals showed that the location of the inner edge of the ring current matched well with the plasmapause model of O'Brien and Moldwin (2003). However, the POES data showed no evidence of the consequences of electromagnetic ion cyclotron waves (localized proton precipitation) during main and early recovery phase. During later stages of the recovery phase, when such precipitation was observed, it was coincident with intense wave events at Halley, and it occurred at L shells near or up to 1 RE outside the modeled plasmapause but well equatorward of the isotropy boundary.

Abstract: The mobile SOUSY VHF (53.5 MHz) Radar has been operated on a campaign basis on the island of Andoya in Northern Norway (69°17′N, 16°01′E), with the aim of investigating the backseattering structures and dynamics of the polar middle atmosphere, since November 1983. During winter the occurrence of mesospheric echoes at altitudes from 50 to 90 km is strongly correlated with radio wave absorption events measured with a riometer on 32.5 MHz. Individual structures are observed for up to 8 hours and, in general, appear as thin layers separated by 3–5 km, with typical maximum signal-to-noise ratios (SNRs) of 5–25 dB. Many of these are related to variations in the background wind produced by long-period gravity waves and tidal period motions and, in as much, are similar to mesospheric layers at middle and low latitudes. In summer the echo region is largely restricted to a height interval from 75 to 95 km, with 80% of the echoes occurring in the 80- to 93-km height range, and on average, it has a distinct maximum in SNR near 86 km. In the 83- to 91-km height region, echoes are detected almost continuously, with a minimum occurrence rate of 85%. There is a low correlation (∼0.26) between echo strength and absorption. Many of the weaker echoing regions, those with SNRs up to 30 dB, are related to the background wind shear. Spectral analysis of time series of both SNR and zonal velocity for a period of 9 days reveals 54-, 24-, 12-, and 8-hour components, with the 12-hour component dominating. Strong bursts in backscattered power tend to occur in the late afternoon and early morning hours, so they coincide with the time of maximum westward velocity of the semidiurnal tide. This suggests that the bursts are due to an increase in the turbulent intensity produced by dynamical instability of this mode. Layers consistent with dynamically unstable upward propagating inertio-gravity waves are also often observed to move down through the 93- to 80-km height range. Thus dynamical instability of the semidiurnal tide and long-period gravity waves appears to play a major role in determining much of the structure observed in the backscatter region. However, the strongest summer polar mesopause echoes, those with peak SNRs greater than 30 dB, while showing modulation due to the background winds, often show no clear relation to the background wind shear. This supports the view that the production mechanism of the very strong SNRs associated with this layer is different in character from that of winter echoes observed in the polar mesosphere by VHF radars, and to those mesospheric layers observed at mid-latitudes using the same technique.

Abstract: As a result of the IGY riometer program, it has been found that the measurement of ionospheric absorption in arctic regions is a sensitive method of detecting low-energy cosmic rays associated with solar flares. The normal morphology of these events is described, and details are given of the 24 such events that have been detected in the period from May 1957 through July 1959. Two features have been noted: an apparent asymmetry in the distribution of cosmic-ray-producing flares across the solar disk; a pronounced degree of uniformity in the distribution of the radio-wave absorption over the terrestrial polar cap. These features are discussed, and tentative explanations are suggested.