M. J. Kosch

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

High‐latitude HF‐induced airglow displaced equatorwards of the pump beam

HF-induced airglow at 630 nm was observed by the Digital All-sky Imager, located near Skibotn in Norway, at F-region altitudes above the EISCAT HF facility near Tromso on 21 February 1999. The transmitter was operated in a 4-min on, 4-min off sequence at 4.04 MHz O-mode with the beam pointing vertically. The airglow reached a peak intensity of about 100 R above background and appeared equatorward of the HF beam’s projection on the reflection altitude, which was obtained from ionograms. Generally, the region of maximum airglow was displaced towards the magnetic field line (zenith angle = 12.8° S) passing through the HF facility. This is a unique feature of these observations. From mid-latitude studies, such airglow is thought to be excited either by electrons energised to several eV by plasma turbulence, or by thermal electron temperature enhancement. Such localisation towards the magnetic field is unexpected for both mechanisms of airglow generation and suggests this feature may be important at high latitudes.

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.

Coordinated optical and radar observations of ionospheric pumping for a frequency pass through the second electron gyroharmonic at HAARP

On 4 February 2005, the High-frequency Active Auroral Research Program (HAARP) facility was operated in O and X mode while pointing into the magnetic zenith to produce artificial optical emissions in the ionospheric F layer. The pump frequency was set to 2.85 MHz to ensure passing through the second electron gyroharmonic of the decaying ionosphere. Optical recordings at 557.7 and 630 nm were performed simultaneously with the side-viewing high frequency (HF) and colocated ultra high frequency (UHF) ionospheric radars. No X-mode effects were found. For O-mode pumping, when passing from below to above the second gyroharmonic frequency, the optical intensity shows a distinct increase when the plasma frequency passes through the second electron gyroharmonic, while the UHF backscatter changes from persistent to overshoot in character. The optical intensity decreases when pump wave reflection ceases, dropping to zero when upper-hybrid resonance ceases. The HF radar backscatter increases when the upper-hybrid resonance frequency passes from below to above the second gyroharmonic frequency. These observations are consistent with the coexistence of the parametric decay and thermal parametric instabilities above the second gyroharmonic. The combined optical and radar data provide evidence that up to three electron-acceleration mechanisms are acting, sometimes simultaneously, depending on the pump frequency relative to the second gyroharmonic. In addition, we provide the first evidence of lower-hybrid waves in HF radar centerline data and show that the parametric decay instability producing Langmuir waves can be stimulated in the magnetic zenith at high latitudes despite the pump wave not reaching the nominal frequency-matching height.

First tristatic studies of meso‐scale ion‐neutral dynamics and energetics in the high‐latitude upper atmosphere using collocated FPIs and EISCAT radar

A unique experiment was undertaken during the nights of 27 and 28 February 2003. Tristatic Fabry-Perot Interferometer (FPI) measurements of the upper thermosphere were co-located with tristatic EISCAT radar measurements of the ionosphere. Tristatic measurements should remove assumptions of uniform wind fields and ion drifts, and zero vertical winds. The FPIs are located close to the 3 radars of the EISCAT configuration in northern Scandinavia. Initial studies indicate that the thermosphere is more dynamic and responsive to ionospheric forcing than expected. Mesoscale variations are observed on the scales of tens of kilometers and minutes. The magnitude of the upper thermosphere neutral wind dynamo field is on average 50% of the magnetospheric electric field and contributes an average magnitude of 41% of in-situ Joule heating. The relative orientations of the 2 dynamo field vectors produce a standard deviation of ±65% in the contribution of the neutral wind dynamo.

Spatiotemporal evolution of radio wave pump-induced ionospheric phenomena near the fourth electron gyroharmonic

On 12 November 2001, the European Incoherent Scatter (EISCAT) high-frequency (HF) radio wave transmitter facility, operating in O-mode at 5.423 MHz with 550 MW effective radiated power, produced artificial optical rings which appeared immediately at transmitter turn-on and collapsed into blobs after ∼60 s while descending in altitude. A similar descent in altitude was observed in the EISCAT ultra high frequency (UHF) ion line enhancements. Likewise, the stimulated electromagnetic emission (SEE) spectra changed as the pump frequency approached the fourth electron gyroharmonic due to pump-induced variations in electron concentration. Optical recordings were made from Skibotn at 630.0 and 557.7 nm and from Ramfjord in white light. The altitude of the initial optical ring and steady state blob has been estimated by triangulation. The evolution in altitude of the optical emissions, ion line enhancements, and SEE spectra all show a similar morphology but are generally not at exactly the same height. Typically, the optical height is close to and a few kilometers below that of the radar backscatter but sometimes above it, both of which are above the SEE generation altitude. There is evidence that upper hybrid (UH) waves, which propagate perpendicular to the magnetic field line, and Langmuir (L) waves, which propagate parallel to the magnetic field line, act simultaneously to accelerate electrons even in the steady state.

Novel artificial optical annular structures in the high latitude ionosphere over EISCAT

The EISCAT low-gain HF facility has been used repeatedly to produce artificially stimulated optical emissions in the F-layer ionosphere over northern Scandinavia. On 12 November 2001, the high-gain HF facility was used for the first time. The pump beam zenith angle was moved in 3° steps along the north-south meridian from 3°N to 15°S, with one pump cycle per position. Only when pumping in the 9°S position were annular optical structures produced quite unexpectedly. The annuli were approximately centred on the pump beam but outside the −3 dB locus. The optical signature appears to form a cylinder, which was magnetic field-aligned, rising above the pump wave reflection altitude. The annulus always collapsed into the well-known optical blobs after ∼60 s, whilst descending many km in altitude. All other pump beam directions produced optical blobs only. The EISCAT UHF radar, which was scanning from 3° to 15°S zenith angle, shows that enhanced ion-line backscatter persisted throughout the pump on period and followed the morphology of the optical signature. These observations provide the first experimental evidence that Langmuir turbulence can accelerate electrons sufficiently to produce the optical emissions at high latitudes. Why the optical annulus forms, and for only one zenith angle, remains unexplained.

A new digital all-sky imager experiment for optical auroral studies in conjunction with the Scandinavian twin auroral radar experiment

Studies of the relationship between the optical aurorae and the ionospheric electric fields, as observed by the bi-static Scandinavian twin auroral coherent backscatter radar experiment (STARE) and the tri-static European incoherent backscatter radar facility (EISCAT), are to be undertaken in Scandinavia. For this purpose, an unmanned and fully automatic low-light-level television camera system, coupled to an all-sky lens, has been constructed. A personal computer controls all aspects of the instrument, operating it for all dark and moon-free periods. Monochrome optical data, usually at 557.7 nm, are pre-processed in real time at the recording site. The transformed images are stored digitally to magneto-optical disk with a temporal and spatial resolution directly compatible with the STARE radar data, thus making comparisons easy. Simultaneous TV recordings to tape may be made on a campaign basis. The camera has been calibrated for all gain settings, thereby permitting auroral images to be recalled in any ...

The equatorial electrojet during geomagnetic storms and substorms

The climatology of the equatorial electrojet during periods of enhanced geomagnetic activity is examined using long-term records of ground-based magnetometers in the Indian and Peruvian regions. Equatorial electrojet perturbations due to geomagnetic storms and substorms are evaluated using the disturbance storm time (Dst) index and auroral electrojet (AE) index, respectively. The response of the equatorial electrojet to rapid changes in the AE index indicates effects of both prompt penetration electric field and disturbance dynamo electric field, consistent with previous studies based on F region equatorial vertical plasma drift measurements at Jicamarca. The average response of the equatorial electrojet to geomagnetic storms (Dst<−50 nT) reveals persistent disturbances during the recovery phase, which can last for approximately 24 h after the Dst index reaches its minimum value. This “after-storm” effect is found to depend on the magnitude of the storm, solar EUV activity, season, and longitude.

Coherent radar estimates of average high-latitude ionospheric Joule heating

The Scandinavian Twin Auroral Radar Experiment (STARE) and Sweden and Britain Radar Experiment (SABRE) bistatic coherent radar systems have been employed to estimate the spatial and temporal variation of the ionospheric Joule heating in the combined geographic latitude range 63.8°-72.6° (corrected geomagnetic latitude 61.5°-69.3°) over Scandinavia. The 173 days of good observations with all four radars have been analyzed during the period 1982 to 1986 to estimate the average ionospheric electric field versus time and latitude. The AE dependent empirical model of ionospheric Pedersen conductivity by Spiro et al. (1982) has been used to calculate the Joule heating. The latitudinal and diurnal variation of Joule heating as well as the estimated mean hemispherical heating of 1.7 × 1011 W are in good agreement with earlier results. Average Joule heating was found to vary linearly with the AE, AU, and AL indices and as a second-order power law with Kp. The average Joule heating was also examined as a function of the direction and magnitude of the interplanetary magnetic field. It has been shown for the first time that the ionospheric electric field magnitude as well as the Joule heating increase with increasingly negative (southward) Bz .