F. Honary

First tomographic estimate of volume distribution of HF‐pump enhanced airglow emission

This report presents the first estimates of the three-dimensional volume emission rate of enhanced O(1D) 6300 A airglow caused by HF radio wave pumping in the ionosphere. Images of the excitation show how the initially speckled spatial structure of excitation changes to a simpler shape with a smaller region that contains most of the excitation. A region of enhanced airglow was imaged by three stations in the Auroral Large Imaging System (ALIS) in northern Scandinavia. These images allowed for a tomography-like inversion of the volume emission of the airglow. The altitude of maximum emission was found to be around 235 ± 5 km with typical horizontal and vertical scale sizes of 20 km. The shape of the O(1D) excitation rate varied from flatish to elongated along the magnetic field. The altitude of maximum emission is found to be approximately 10 km below the altitude of the enhanced ion line and 15 km above the altitude of maximum electron temperature. Comparisons of the measured altitude and temporal variations of the 6300 A emission with modelled emission caused by O(1D) excitation from the high energy tail of a Maxwellian electron distribution show significant deviations. The 6300 A emission from excitation of the high energy tail is about a factor of 4 too large compared with what is observed. This shows that the source of O(1D) excitation is electrons from a “sub-thermal” distribution function, i.e. the electron distribution is Maxwellian at low energies and at energies above 1.96 eV there is a depletion.

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.

GPS scintillation in the high arctic associated with an auroral arc

A rapid signal-fading event produced by diffractive scintillations was observed around 0123 UT on 8 November 2004 by three closely sited (less than 250 m apart) GPS scintillation receivers in northern Norway. The entire duration of the event was about 10 s and was recorded by all three receivers. Intense, short duration events such as these are not clearly observable in the 1-min scintillation index (S4) because they do not necessarily last for the entire minute. In spite of their short duration they can cause a receiver to lose lock because of their intensity. The geomagnetic conditions were disturbed at this time with the interplanetary magnetic field southward for a period of several hours. Magnetometers from the IMAGE network in Scandinavia showed evidence of a 2000 nT substorm. The GPS measurements are compared with all-sky camera (ASC) data to show that the signal fades can be attributed to the GPS ray paths crossing electron density structures associated with the aurora. The ASC images reveal moving auroral structures at the same time as the GPS signals show movement of the ionospheric regions causing fading. The results indicate that at high latitudes low-elevation GPS signals can suffer sudden fading due to E-region auroral events. This is the first time that a direct connection has been established between the loss of lock on a GPS receiver and diffractive fading caused by auroral precipitation.

Ionospheric plasma response to HF radio waves operating at frequencies close to the third harmonic of the electron gyrofrequency

Experimental results concerning European incoherent scatter observations of heater-induced electron temperature enhancements, anomalous absorption of low-power HF probe waves, and the spectrum of stimulated electromagnetic emission (SEE) in the sidebands of a high-power HF electromagnetic wave are presented. For the experiments reported in this paper, an O mode pump wave was transmitted vertically into the F region above Tromso, Norway, while the injected frequency was varied in small steps around the third harmonic of the electron gyrofrequency. Systematic variations with pump frequency were observed in the data obtained from all three diagnostics. Measurements of anomalous absorption, the downshifted maximum (DM) spectral feature, and heater-induced electron temperature enhancements all exhibited broad minima as the heater frequency approached the third harmonic of the electron gyrofrequency. In addition, the signal strength of the HF probe wave measured during heater off periods is also reduced at these and higher heater frequencies. The experimental findings suggest that at heater frequencies in the vicinity of the third gyroharmonic, small-scale field-aligned irregularities are not excited, whereas very small scale irregularities, of the order of a few electron cyclotron radii, which are responsible for the production of fast electrons, may be generated. The observed reduction in the diagnostic signal strength is then attributed to the ionized patches produced by these energetic electrons.

Anomalous absorption during artificial modification at harmonics of the electron gyrofrequency

Observations of the anomalous absorption of low-power HF probe waves caused by the action of a high-power HF ordinary mode radio wave are presented. At pump frequencies in the vicinity of harmonics of the electron gyrofrequency the anomalous absorption measured on the low-power probe waves exhibit local minima. Also at these frequencies, large-scale changes deduced from phase measurements of the low-power probe waves and heater self-absorption are reduced, whereas the growth time of the anomalous absorption is increased. The electron gyrofrequency estimated from the frequency of the absorption minimum compares favorably with that derived from the International Geomagnetic Reference Field (IGRF) magnetic field model.

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.

Simultaneous THEMIS observations in the near-tail portion of the inner and outer plasma sheet flux tubes at substorm onset

We analyzed the measurements made by two Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes in ideal observational conditions (quiet background, near midnight, inside the substorm current wedge) during two distinct isolated substorm onsets, with probe P2 measuring the inner plasma sheet at ∼8 Re and P1 near the plasma sheet–lobe interface at 11–12 Re. The earliest onset-related strong perturbations were observed by P1; they include the increase of both B z (dipolarization) and E y (a few mV/m) as well as the simultaneous drop in total pressure, indicating the unloading process. This was also accompanied by fast inward plasma motion (up to 100 km/s, toward the neutral sheet) and fast plasma sheet thinning while the poleward auroral expansion was in progress in the conjugate ionosphere. These perturbations were followed after 6–8 min by the rapid expansion of the already heated plasma sheet. While in the adjacent lobe during this thinning phase, probe P1 continued to observe intense flux transfer toward the sheet center plane. The inner probe observed intense dipolarization and inward plasma injection but with a smaller flux transfer and starting 1–2 min after the perturbations at P1, supporting the conclusion that onset instability took place tailward of 12 Re. We also demonstrate the global MHD simulations to show that a nontrivial combination of dipolarization and the plasma sheet thinning may be observed simultaneously in the outmost part of the dipolelike region during a sudden increase of the reconnection rate at the nearby active X line, staying a few Re from the observation point. These observations provide constraints for the choice of substorm onset mechanism and indicate near-Earth magnetic reconnection as the most probable source process.

Key features of >30 keV electron precipitation during high speed solar wind streams: A superposed epoch analysis

We present an epoch analysis of energetic (>30 keV) electron precipitation during 173 high speed solar wind streams (HSS) using riometer observations of cosmic noise absorption (CNA) as a proxy for the precipitation. The arrival of the co-rotating interaction region (CIR) prior to stream onset, elevates the precipitation which then peaks some 12 h after stream arrival. Precipitation continues for several days following the HSS arrival. The MLT distribution of CNA is generally consistent with the statistical pattern explained via the substorm process, though the statistical deep minimum of CNA/precipitation does change during the HSS suggesting increased precipitation in the 15–20 MLT sector. The level of precipitation is strongly controlled by the average state of the IMF BZ component on the day prior to the arrival of the stream interface. An average negative IMF BZ will produce higher CNA across all L-shells and MLT, up to 100% higher than an average positive IMF BZ.

High-resolution maps of the characteristic energy of precipitating auroral particles

For the first time we produce high-resolution maps of the characteristic energy of precipitating electrons from ground-based instrumentation in the auroral zone over northern Scandinavia. This is done by combining intensity-calibrated optical data at 557.7 nm from the Digital All-Sky Imager (DASI) with auroral absorption images from the Imaging Riometer for Ionospheric Studies (IRIS). Energy maps are produced with high temporal (10 s) and spatial (10 km) resolution within a common geographic area of 240 × 240 km. Both IRIS and DASI have the European Incoherent Scatter (EISCAT) radar within their common field of view. EISCAT is capable of making accurate measurements of the electron density height profile which, with the assistance of an atmospheric model, are inverted into equivalent energy spectra of the flux of precipitating electrons. However, incoherent scatter radars generally have a very small field of view (<1°), making studies of the energy spectrum of the precipitating particles over a wide field of view impractical. Since IRIS and DASI are sensitive to high- and medium-energy electrons, respectively, EISCAT data are used to calibrate the characteristic energy of the precipitating particles for an assumed energy spectrum against a combination of IRIS and DASI data. This empirical calibration is then used throughout the common field of view of IRIS and DASI. An initial study illustrates the spatial relationship between the different energy ranges during a substorm onset and illustrates a new way to interpret auroral phenomena.

Invited Article: Digital beam-forming imaging riometer systems

The design and operation of a new generation of digital imaging riometer systems developed by Lancaster University are presented. In the heart of the digital imaging riometer is a field-programmable gate array (FPGA), which is used for the digital signal processing and digital beam forming, completely replacing the analog Butler matrices which have been used in previous designs. The reconfigurable nature of the FPGA has been exploited to produce tools for remote system testing and diagnosis which have proven extremely useful for operation in remote locations such as the Arctic and Antarctic. Different FPGA programs enable different instrument configurations, including a 4 × 4 antenna filled array (producing 4 × 4 beams), an 8 × 8 antenna filled array (producing 7 × 7 beams), and a Mills cross system utilizing 63 antennas producing 556 usable beams. The concept of using a Mills cross antenna array for riometry has been successfully demonstrated for the first time. The digital beam forming has been validated by comparing the received signal power from cosmic radio sources with results predicted from the theoretical beam radiation pattern. The performances of four digital imaging riometer systems are compared against each other and a traditional imaging riometer utilizing analog Butler matrices. The comparison shows that digital imaging riometer systems, with independent receivers for each antenna, can obtain much better measurement precision for filled arrays or much higher spatial resolution for the Mills cross configuration when compared to existing imaging riometer systems.