A. Senior

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.

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.

Variations in cutoff latitude during selected solar energetic proton events

Proton count rate measurements were obtained from the polar orbiting environmental satellites POES15 and POES16 during three solar proton events in 2001. From these data the invariant latitude of the cutoff in proton flux was determined at four local times. The satellites sample the edge of the northern polar cap at approximately 0300, 0830, 1230, and 1800 hours local time, covering the early morning, late morning, noon, and evening. During the event of 2 April the cutoff latitude for 35–70 MeV protons was almost constant, showing no more than 1.5° variation with local time. CME impacts occurred during the 24 September and 4 November events, the effects of which were to reduce the cutoff latitudes by between 5° and 8°. The equatorward displacement of the cutoff latitude was found to be strongly correlated with the magnetic storm indices (D st and SYM-H). Cutoff latitude prediction formulae are defined. Simulations using the Tsyganenko (2002) model of the magnetosphere are consistent with observations to within 2σ during quiet conditions and weak disturbances.

A study of aspect angle effects in the E‐region irregularity velocity using multi‐point electric field measurements

E-region irregularity velocity measured by the STARE Norway VHF radar is considered as a function of magnetic aspect angle α and EISCAT-derived electron drift velocity at 7 locations with α between 0.38° and 2.64°. It is shown that the irregularity line-of-sight (l-o-s) velocity normalized to the electron velocity component V e0 comp exhibits a decrease with an increasing aspect angle for V e0 comp exceeding 500 m/s. The rate of velocity decrease is greater than those reported previously and is close to that predicted by the linear theory of electrojet irregularities without assuming anomalously large collision frequencies.

New Capabilities of the Upgraded EISCAT High-Power HF Facility

The high-power HF (High Frequency) facility (commonly known as Heating) near Tromso, Norway, which is an essential part of the EISCAT (European Incoherent SCATter) Scientific Association, has been upgraded in certain key areas in recent years. It is one of only four similar facilities in the world operating at present. An updated description of the facility is given, together with scientific motivation and some results. The main high-power parts such as transmitters, feed-system and antennas remain essentially the same as built in the late 1970s. The improvements are in the areas of radio frequency waveform generation, computer control and monitoring. In particular, fast stepping in frequency is now possible, an important aspect in examining features close to harmonics of the electron gyrofrequency. One antenna array has been modified to allow reception to implement an HF radar mode for mesospheric and magnetospheric probing. More realistic modelling of the antenna gain gives improved estimates of the total effective radiated power for both wanted and unwanted circular polarizations. Results are presented using these new capabilities, but their full scientific potential has yet to be achieved.

Statistical relationships between cosmic radio noise absorption and ionospheric electrical conductances in the auroral zone

Statistical models expressing the Hall and Pedersen conductances and their ratio as functions of cosmic noise absorption (CNA) are derived for five intervals of magnetic local time. The models are based on simultaneous measurements of electron densities from the EISCAT UHF radar at Tromso (69.6 N, 19.2 E) and absorption from the imaging riometer at Kilpisjarvi (69.1 N, 20.8 E). The Hall conductance and the conductance ratio are found to be rather strongly related to CNA, whereas the Pedersen conductance is less so. The Hall conductance-CNA relationship is strongly dependent on magnetic local time. These results are interpreted as being the consequence of the particular sensitivity of CNA to the typical energy of electron precipitation, the latter changing as a function of local time as the electrons drift around the Earth. The models are compared to a previous study which did not use simultaneous measurements or take into account the local time dependence. There is a significant difference between that study and the results presented here.

The thresholds of ionospheric plasma instabilities pumped by high‐frequency radio waves at EISCAT

We test the existing theories regarding the thresholds for the parametric decay instability (PDI), the oscillating two-steam instability (OTSI), and the thermal parametric instability (TPI) using the European Incoherent Scatter (EISCAT) facilitys ionospheric heater. In these processes, the pump wave can couple to various electrostatic waves in the F layer ionosphere, which can be observed using the EISCAT UHF radar (PDI and OTSI) or by HF radar (TPI). On 19 October 2012, the heater power was stepped from ∼0.5 MW to ∼100 MW effective radiated power in seven steps using a 1 min on, 1 min off cycle. We use an electric field model, taking into account D region absorption, to compare theory with our observations. In all three cases, we find good agreement. In addition, the growth of striations formed during the TPI causes anomalous absorption of the heater wave, which we observe as decreased UHF ion line and plasma line backscatter power. We show evidence that heating for a prolonged period of time reduces the UHF ion line intensity throughout the experiment.

First modulation of high‐frequency polar mesospheric summer echoes by radio heating of the ionosphere

The first high-frequency (HF, 8 MHz) observations of the modulation of polar mesospheric summer echoes (PMSE) by artificial radio heating of the ionosphere are presented and compared to observations at 224 MHz and model predictions. The experiments were performed at the European Incoherent Scatter facility in northern Norway. It is shown that model results are in qualitative and partial quantitative agreement with the observations, supporting the prediction that with certain ranges of ice particle radii and concentration, PMSE at HF radar wavelengths can be enhanced by heating due to the dominance of dust charging over plasma diffusion.

Combined ground-based optical support for the aurora (DELTA) sounding rocket campaign

The Japan Aerospace Exploration Agency (JAXA) DELTA rocket experiment, successfully launched from Andøya at 0033 UT on December 13, 2004, supported by ground based optical instruments, primarily 2 Fabry- Perot Interferometers (FPIs) located at Skibotn, Norway (69.3°N, 20.4°E) and the KEOPS Site, Esrange, Kiruna, Sweden (67.8°N, 20.4°E). Both these instruments sampled the 557.7 nm lower thermosphere atomic oxygen emission and provided neutral temperatures and line-of-sight wind velocities, with deduced vector wind patterns over each site. All sky cameras allow contextual auroral information to be acquired. The proximity of the sites provided overlapping fields of view, adjacent to the trajectory of the DELTA rocket. This allowed independent verification of the absolute temperatures in the relatively quiet conditions early in the night, especially important given the context provided by co-located EISCAT ion temperature measurements which allow investigation of the likely emission altitude of the passive FPI measurements. The results demonstrate that this altitude changes from 120 km pre-midnight to 115 km post-midnight. Within this large scale context the results from the FPIs also demonstrate smaller scale structure in neutral temperatures, winds and intensities consistent with localised heating. These results present a challenge to the representation of thermospheric variability for the existing models of the region.

First observation of the anomalous electric field in the topside ionosphere by ionospheric modification over EISCAT

We have developed an active ground-based technique to estimate the steady state field-aligned anomalous electric field (E*) in the topside ionosphere, up to ~600 km, using the European Incoherent Scatter (EISCAT) ionospheric modification facility and UHF incoherent scatter radar. When pumping the ionosphere with high-power high-frequency radio waves, the F region electron temperature is significantly raised, increasing the plasma pressure gradient in the topside ionosphere, resulting in ion upflow along the magnetic field line. We estimate E* using a modified ion momentum equation and the Mass Spectrometer Incoherent Scatter model. From an experiment on 23 October 2013, E* points downward with an average amplitude of ~1.6 μV/m, becoming weaker at higher altitudes. The mechanism for anomalous resistivity is thought to be low-frequency ion acoustic waves generated by the pump-induced flux of suprathermal electrons. These high-energy electrons are produced near the pump wave reflection altitude by plasma resonance and also result in observed artificially induced optical emissions.