Showing papers by "Iain M. Reid published in 2017"

Response of neutral mesospheric density to geomagnetic forcing

Abstract: We report an analysis of the neutral mesosphere density response to geomagnetic activity from January 2016 to February 2017 over Antarctica. Neutral mesospheric densities from 85-95 km are derived using data from the Davis meteor radar (68.5°S, 77.9°E) and the Microwave Limb Sounder (MLS) on the Aura satellite. Spectral and Morlet wavelet analyses indicate that a prominent oscillation with a periodicity of 13.5-day is observed in the mesospheric density during the declining phase of solar cycle 24 and is associated with variations in solar wind high-speed streams and recurrent geomagnetic activity. The periodic oscillation in density shows a strong anti-correlation with periodic changes in the auroral electrojet (AE) index. These results indicate that a significant decrease in neutral mesospheric density as the geomagnetic activity enhances.

First observation of mesosphere response to the solar wind high-speed streams

Abstract: We present a first analysis of 9 and 6.75-day periodic oscillations observed in the neutral mesospheric density in 2005 and 2006. Mesospheric densities near 90 km are derived using data from the Davis meteor radar (68.5°S, 77.9°E; magnetic latitude, 74.6°S), Antarctica. Spectral analysis indicates that the pronounced periodicities of 9 and 6.75 days observed in the mesosphere densities are associated with variations in solar wind high-speed streams and recurrent geomagnetic activity. Neutral mesospheric winds and temperatures, simultaneously measured by the Davis meteor radar, also exhibit 9- and 6.75-day periodicities. A Morlet wavelet analysis shows that the time evolution of the 9- and 6.75-day oscillations in the neutral mesosphere densities and winds are similar to those in the solar wind and in planetary magnetic activity index, Kp in 2005 and 2006. These results demonstrate a direct coupling between Sun's Corona (upper atmosphere) and the Earth's mesosphere.

Interferometer angle-of-arrival determination using precalculated phases

Abstract: A method has been developed to determine the angle-of-arrival (AoA) of incident radiation using pre-computed lookup tables. The phase difference between two receiving antennas can be used to infer AoA as measured from the pair baseline, but there will be more than one possible solution for antenna spacings greater than or equal to half a wavelength. Larger spacings are preferable to minimize mutual coupling of elements in the receive array and to decrease the relative uncertainty in measured phase difference. We present a solution that uses all unique antenna pairs to determine probabilities for all possible azimuth and zenith values. Prior to analysis, the expected phase differences for all AOAs are calculated for each antenna pair. For a received signal, histograms of possible AoAs for each antenna pair phase difference are extracted and added to produce a two dimensional probability density array that will maximize at the true value of the AoA. A benefit of this method is that all possible antenna pairs are utilized, rather than the restriction to specific pairs along baselines used by some interferometer algorithms. Numerical simulations indicate that performance of the suggested algorithm exceeds that of existing methods, with the benefit of additional flexibility in antenna placement. Meteor radar data has been used to test this method against existing methods, with excellent agreement between the two approaches. This method of AoA determination will allow the construction of low-cost interferometric direction finding arrays with different layouts, including construction of difficult terrain and three-dimensional antenna arrangements.

Seasonal MLT-region nightglow intensities, temperatures, and emission heights at a Southern Hemisphere midlatitude site

Abstract: . We consider 5 years of spectrometer measurements of OH(6–2) and O2(0–1) airglow emission intensities and temperatures made near Adelaide, Australia (35° S, 138° E), between September 2001 and August 2006 and compare them with measurements of the same parameters from at the same site using an airglow imager, with the intensities of the OH(8–3) and O(1S) emissions made with a filter photometer, and with 2 years of Aura MLS (Microwave Limb Sounder) v3.3 temperatures and 4.5 years of TIMED SABER (Thermosphere Ionosphere Mesosphere Energetics and Dynamics Sounding of the Atmosphere using Broadband Emission Radiometry) v2.0 temperatures for the same site. We also consider whether we can recover the actual emission heights from the intercomparison of the ground-based and satellite observations. We find a significant improvement in the correlation between the spectrometer OH and SABER temperatures by interpolating the latter to constant density surfaces determined using a meteor radar.

Observations and modeling of traveling ionospheric disturbance signatures from an Australian network of oblique angle-of-arrival sounders

Abstract: An Australian network of oblique angle-of-arrival (AoA) ionosondes was installed as part of the ELOISE experimental campaign in September 2015, aimed at an improved understanding of the spatial and temporal structure of traveling ionospheric disturbances (TIDs) at mid-latitudes. In this paper, the array design and signal processing for the AoA sounder is described, along with a sample of results showing typical disturbance signatures. Realistic parameterized models of electron density perturbations, along with geometric ray tracing, were used to synthesize the effects of medium to large scale TIDs on the sounder observables and aid in classifying the measurements.

Mesospheric gravity wave momentum flux estimation using hybrid Doppler interferometry

Abstract: . Mesospheric gravity wave (GW) momentum flux estimates using data from multibeam Buckland Park MF radar (34.6° S, 138.5° E) experiments (conducted from July 1997 to June 1998) are presented. On transmission, five Doppler beams were symmetrically steered about the zenith (one zenith beam and four off-zenith beams in the cardinal directions). The received beams were analysed with hybrid Doppler interferometry (HDI) (Holdsworth and Reid, 1998), principally to determine the radial velocities of the effective scattering centres illuminated by the radar. The methodology of Thorsen et al. (1997), later re-introduced by Hocking (2005) and since extensively applied to meteor radar returns, was used to estimate components of Reynolds stress due to propagating GWs and/or turbulence in the radar resolution volume. Physically reasonable momentum flux estimates are derived from the Reynolds stress components, which are also verified using a simple radar model incorporating GW-induced wind perturbations. On the basis of these results, we recommend the intercomparison of momentum flux estimates between co-located meteor radars and vertical-beam interferometric MF radars. It is envisaged that such intercomparisons will assist with the clarification of recent concerns (e.g. Vincent et al., 2010) of the accuracy of the meteor radar technique.