T. K. Yeoman

Ionospheric electron heating, optical emissions, and striations induced by powerful HF radio waves at high latitudes: Aspect angle dependence

radio-induced aurora showed that the enhancement caused by the HF radio waves also remained localized near the field-aligned position. Coherent HF radar backscatter also appeared strongest when the pump beam was pointed field-aligned. These results are similar to some Langmuir turbulence phenomena which also show a strong preference for excitation by HF rays launched in the field-aligned direction. The correlation of the position of largest temperature enhancement with the position of the radio-induced aurora suggests that a common mechanism, upper-hybrid wave turbulence, is responsible for both effects. Why the strongest heating effects occur for HF rays directed along the magnetic field is still unclear, but self-focusing on field-aligned striations is a candidate mechanism, and possibly ionospheric tilts may be important. INDEX TERMS: 2403 Ionosphere: Active experiments; 6929 Radio Science: Ionospheric physics (2409); 7839 Space Plasma Physics: Nonlinear phenomena; KEYWORDS: HF-heating, ionospheric modification, electron heating, EISCAT, striations, aspect angle

Reconnection in a rotation-dominated magnetosphere and its relation to Saturn's auroral dynamics

The first extended series of observations of Saturns auroral emissions, undertaken by the Hubble Space Telescope in January 2004 in conjunction with measurements of the upstream solar wind and interplanetary magnetic field (IMF) by the Cassini spacecraft, have revealed a strong auroral response to the interplanetary medium. Following the arrival of the forward shock of a corotating interaction region compression, bright auroras were first observed to expand significantly poleward in the dawn sector such that the area of the polar cap was much reduced, following which the auroral morphology evolved into a spiral structure around the pole. We propose that these auroral effects are produced by compression-induced reconnection of a significant fraction of the open flux present in Saturns open tail lobes, as has also been observed to occur at Earth, followed by subcorotation of the newly closed flux tubes in the outer magnetosphere region due to the action of the ionospheric torque. We show that the combined action of reconnection and rotation naturally gives rise to spiral structures on newly opened and newly closed field lines, the latter being in the same sense as observed in the auroral images. The magnetospheric corollary of the dynamic scenario outlined here is that corotating interaction region-induced magnetospheric compressions and tail collapses should be accompanied by hot plasma injection into the outer magnetosphere, first in the midnight and dawn sector, and second at increasing local times via noon and dusk. We discuss how this scenario leads to a strong correlation of auroral and related disturbances at Saturn with the dynamic pressure of the solar wind, rather than to a correlation with the north-south component of the IMF as observed at Earth, even though the underlying physics is similar, related to the transport of magnetic flux to and from the tail in the Dungey cycle.

Phase and spectral power of mid-latitude Pi2 pulsations: Evidence for a plasmaspheric cavity resonance

Abstract An examination has been made of the phase and spectral power of mid-latitude Pi2 pulsations measured on a ground array. The power spectra of pulsations observed on three longitudinally separated ground observatories show constant frequency spectral peaks over 5 1 2 h of M.L.T. Apparent longitudinal frequency variations appear to arise from changes in the relative sizes of spectral peaks, not in the frequencies of the peaks themselves. This behaviour is not that expected from a surface wave on the plasmapause. Consideration of the phase and power along a meridian shows 180° H-component phase shifts seemingly associated with the plasmapause. One or more spectral peak amplitude enhancements are often found equatorward of this position. This behaviour is compared with two plasmaspheric cavity resonance models: period estimates are obtained from a time of flight model and antinode positions are estimated from a 2-D Cartesian plasmasphere model. It is concluded that a plasmaspheric cavity resonance is the most likely mechanism for the mid-latitude Pi2 secondary amplitude maximum.

Pi2 pulsation polarization patterns on the U.K. sub-auroral magnetometer network (SAMNET)

Abstract An array of seven fluxgate magnetometers has been deployed covering mid-latitudes in western Europe. The array is configured to allow simultaneous observations of mid-latitude pulsation characteristics in both latitude and longitude. Here the two-dimensional variations in the horizontal polarization of Pi2 pulsations are related to the substorm current wedge position, deduced from the midtitude bay behaviour, and an estimated plasmapause position. Features of the polarization pattern which are not in agreement with the substorm current wedge model are found. These include unexpected polarization ellipse azimuths and longitudinal phase propagations, and latitudinal variations in polarization ellipse azimuths not associated with ellipticity reversals. An extension of previous travelling wave models to include low latitude effects is considered, which may explain at least some of these discrepancies.

Simultaneous observations of the cusp in optical, DMSP and HF radar data

A favourable conjunction of HF coherent radar backscatter, meridian scanning photometer data and an overflight of the DMSP F13 spacecraft has enabled the study of the ionospheric signature of the cusp with these three important techniques simultaneously. Strong HF backscatter power, poleward-moving red line auroral forms and latitude-dispersed ion precipitation features are all observed to be collocated. The precipitation of ions in the 0.1–2 keV energy range is found to be very closely associated with the production of the F region irregularities detected by the HF radar.

High-latitude pump-induced optical emissions for frequencies close to the third electron gyro-harmonic

It has been long established that high-power O-mode HF pumping of the ionosphere can produce artificial optical emissions. 630 nm O(1D) photons are produced by pump-accelerated electrons colliding with the F-layer neutral oxygen. However, the mechanism for artificial electron acceleration remains unclear. Competing theories include Langmuir and upper-hybrid turbulence. Pump-induced HF coherent radar backscatter power is closely linked with upper-hybrid turbulence, both of which are known to reduce when pumping on an electron gyro-harmonic frequency. On 3 November 2000, the EISCAT HF facility was systematically stepped in frequency through the 3rd gyro-harmonic. A significant reduction in the artificial optical intensity coincides with that of CUTLASS radar backscatter power. This is conclusive proof that upper-hybrid turbulence is intimately linked to the mechanism for high-latitude pump-induced aurora, at least for 630 nm photons and the steady state.

A comparison of midlatitude Pi 2 pulsations and geostationary orbit particle injections as substorm indicators

Both the injection of energetic particles at geostationary orbit and ground magnetic observations of Pi 2 wave activity are characteristic indicators of the onset of the substorm expansion phase. Occurrence statistics for the appearance of electron and proton particle injection at three geostationary spacecraft and for the detection of midlatitude magnetic Pi 2 pulsations in a 3-hour local time sector have been compiled from 240 hours of data. Throughout this interval a signature was detected on one or more of the instruments on average every 65 min. It is demonstrated that the detection of geostationary orbit particle injections and the detection of ground-based Pi 2 pulsations are correlated at a very high significance level, and that both appear to be effective substorm indicators. However, a small percentage of events (∼10% in each case) may be identified as a Pi 2 event but not as an injection event or viceversa, without any obvious explanation, such as the local time of the observing instrumentation. A number of possible explanations for the discrepancies between the two data sets are discussed.

An evaluation of range accuracy in the Super Dual Auroral Radar Network over-the-horizon HF radar systems

The authors thank the director and staff of EISCAT for the operation of the Tromso heater facility. EISCAT is an international facility funded collaboratively by the research councils of Finland (SA), France (CNRS), the Federal Republic of Germany (MPG), Japan (NIPR), Norway (NFR), Sweden (NFR), and the United Kingdom (PPARC). The SuperDARN Hankasalmi and Pykkvib•er HF radars are deployed and operated by the University of Leicester and funded by the PPARC (grant PPA/R/R/1997/00256), the Finnish Meteorological Institute, and the Swedish Institute for Space Physics. D.M.W. is supported on PPARC grant PPA/G/O/1997/000254.

The dayside auroral zone as a hard target for coherent HF radars

Observations from the CUTLASS Finland coherent HF radar on 23 February 1996 are employed to demonstrate that changes in propagation mode from 1/2F to 1 1/2F and back again, determined from elevation angle measurements, do not significantly alter the ranges over which ionospheric backscatter is observed. This indicates that the latitudinal extent of backscatter in the dayside auroral oval and cusp region correspond to the boundaries of geophysical processes, as opposed to limits in the illumination of the F region ionosphere by the radar. Hence, the HF radar technique is confirmed as an excellent diagnostic of the cusp and other dayside regions.

The influence of the IMF By component on the location of pulsed flows in the dayside ionosphere observed by an HF radar

A study has been performed on the effect of the By and Bz components of the Interplanetary Magnetic Field (IMF), as measured by the WIND satellite, on the occurrence of pulsed ionospheric flows as observed by the CUTLASS Finland radar. These flows have been suggested as being created at the ionospheric footprint of newly reconnected field lines. The statistical location of the pulsed flows is strongly dependent on the IMF By component, moving to the pre-noon sector during intervals of positive By and into the post-noon sector for intervals of negative By. These results directly confirm previous predictions, demonstrating that during intervals of flux transfer at the magnetopause, pulsed ionospheric convective flows are initiated. In the northern hemisphere these flows are shifted about noon in the opposite sense to the IMF By component, consistent with the average ionospheric convection pattern.