EISCAT

Statistical signature of active D-region HF heating in IRIS riometer data from 1994–2004

Kero, A. — Wed, 07 Mar 2007 23:00:00 GMT

Kero, A.; Rietveld, Michael T.; Enell, C.-F.; Ulich, Th.; Turunen, E.; Honary, F. H.
In this paper we study the effect of artificial HF heating on cosmic radio noise absorption in the D-region ionosphere. The effect has earlier been studied theoretically in idealised cases and without experimental verification. Here we present a 3-dimensional modelling of the effect, taking into account the directivity patterns of the vertical beam of the EISCAT Heater at Tromsø, Norway, and the intersecting beam of the IRIS imaging riometer at Kilpisjärvi, Finland. The heater-induced enhancement of cosmic radio noise absorption at the IRIS frequency (38.2 MHz) is estimated to be between 0.02 dB and 0.05 dB in the most representative model cases. However, a statistical study of IRIS data from a selected set of heating experiments carried out during the years 1994– 2004 shows that the median effect is between 0.002 dB and 0.004 dB, i.e. an order of magnitude less than theoretically predicted. This indicates that the actual HF heating effect at D-region altitudes is substantially overestimated by the present theory.

Probing of medium-scale traveling ionospheric disturbances using HF-induced scatter targets

Rietveld, Michael T. — Tue, 19 Sep 2006 22:00:00 GMT

Rietveld, Michael T.; Blagoveshchenskaya, N. F.; Borisova, T. D.; Kornienko, V. A.; Moskvin, I. V.; Frolov, V. L.; Uryadov, V. P.; Kagan, L. M.; Yampolski, Yu. M.; Galushko, V. L.; Koloskov, A. V.; Kasheev, S. B.; Zalizovski, A. V.; Vertogradov, G. G.; Vertogradov, V. G.; Kelley, M. C.
Experimental results from the Tromsø and Sura heating experiments at high and mid-latitudes are examined. It is shown that the combination of HF-induced target and bi-static HF Doppler radio scatter observations is a profitable method for probing medium-scale traveling ionospheric disturbances (TIDs) at high and mid-latitudes. HF ionospheric modification experiments provide a way of producing the HF-induced scatter target in a controlled manner at altitudes where the sensitivity to TIDs is highest. Bi-static HF Doppler radio scatter observations were carried out on the London– Tromsø–St. Petersburg path in the course of a Tromsø heating experiment on 16 November 2004 when the pump wave was reflected from an auroral Es-layer. During Sura heating experiments on 19 and 20 August 2004, when the HF pump wave was reflected from the F2 ionospheric layer, multiposition bi-static HF Doppler radio scatter observations were simultaneously performed at three reception points including St. Petersburg, Kharkov, and Rostov-on-Don. Ray tracing and Doppler shift simulations were made for all experiments. A computational technique has been developed allowing the reconstruction of the TID phase velocities from multi-position bi-static HF Doppler scatters. Parameters of medium-scale TIDs were found. In all experiments they were observed in the evening and pre-midnight hours. TIDs in the auroral E-region with periods of about 23 min were traveling southward at speeds of 210 m/s. TIDs in the mid-latitudinal F-region with periods from 20 to 45 min travelled at speeds between 40 and 150 m/s. During quiet magnetic conditions the waves were traveling in the north-east direction. In disturbed conditions the waves were moving in the south-west direction with higher speeds as compared with quiet conditions. Possible sources for the atmospheric gravity waves at middle and high latitudes are discussed.

A case study of a sporadic sodium layer observed by the ALOMAR Weber Na lidar

Nesse, H. — Tue, 27 May 2008 22:00:00 GMT

Nesse, H.; Rietveld, Michael T.; Heinrich, D.; Williams, B.; Hoppe, U.-P.; Stadsnes, J.; Singer, W.; Blum, U.; Sandanger, M. I.; Trondsen, E.
Several possible mechanisms for the production of sporadic sodium layers have been discussed in the literature, but none of them seem to explain all the accumulated observations. The hypotheses range from direct meteoric input, to energetic electron bombardment on meteoric smoke particles, to ion neutralization, to temperature dependent chemistry. The varied instrumentation located on Andøya and near Tromsø in Norway gives us an opportunity to test the different theories applied to high latitude sporadic sodium layers. We use the ALOMARWeber sodium lidar to monitor the appearance and characteristics of a sporadic sodium layer that was observed on 5 November 2005. We also monitor the temperature to test the hypotheses regarding a temperature dependent mechanism. The EISCAT Tromsø Dynasonde, the ALOMAR/UiO All-sky camera and the SKiYMET meteor radar on Andøya are used to test the suggested relationships of sporadic sodium layers and sporadic E-layers, electron precipitation, and meteor deposition during this event. We find that more than one candidate is eligible to explain our observation of the sporadic sodium layer.

The relationship between small-scale and large-scale ionospheric electron density irregularities generated by powerful HF electromagnetic waves at high latitudes

Tereshcenko, E.D. — Mon, 20 Nov 2006 23:00:00 GMT

Tereshcenko, E.D.; Rietveld, Michael T.; Brekke, Asgeir; Khudukon, B.Z.; Isham, B.; Hagfors, T.
Satellite radio beacons were used in June 2001 to probe the ionosphere modified by a radio beam produced by the EISCAT high-power, high-frequency (HF) transmitter located near Tromsø (Norway). Amplitude scintillations and variations of the phase of 150- and 400-MHz signals from Russian navigational satellites passing over the modified region were observed at three receiver sites. In several papers it has been stressed that in the polar ionosphere the thermal self-focusing on striations during ionospheric modification is the main mechanism resulting in the formation of large-scale (hundreds of meters to kilometers) nonlinear structures aligned along the geomagnetic field (magnetic zenith effect). It has also been claimed that the maximum effects caused by small-scale (tens of meters) irregularities detected in satellite signals are also observed in the direction parallel to the magnetic field. Contrary to those studies, the present paper shows that the maximum in amplitude scintillations does not correspond strictly to the magnetic zenith direction because high latitude drifts typically cause a considerable anisotropy of small-scale irregularities in a plane perpendicular to the geomagnetic field resulting in a deviation of the amplitude-scintillation peak relative to the minimum angle between the line-of-sight to the satellite and direction of the geomagnetic field lines. The variance of the logarithmic relative amplitude fluctuations is considered here, which is a useful quantity in such studies. The experimental values of the variance are compared with model calculations and good agreement has been found. It is also shown from the experimental data that in most of the satellite passes a variance maximum occurs at a minimum in the phase fluctuations indicating that the artificial excitation of large-scale irregularities is minimum when the excitation of small-scale irregularities is maximum.

Infrasound - the cause of strong Polar Mesosphere Winter Echoes?

Kirkwood, S. — Wed, 22 Mar 2006 23:00:00 GMT

Kirkwood, S.; Rietveld, Michael T.; Chilson, P.; Belova, E.; Häggström, I.; Singer, W.; Dalin, P.
The ESRAD 52-MHz and the EISCAT 224-MHz radars in northern Scandinavia observed thin layers of strongly enhanced radar echoes from the mesosphere (Polar Mesosphere Winter Echoes - PMWE) during a solar proton event in November 2004. Using the interferometric capabilities of ESRAD it was found that the scatterers responsible for PMWE show very high horizontal travel speeds, up to 500 ms-1 or more, and high aspect sensitivity, with echo arrival angles spread over as little as 0.3°. ESRAD also detected, on some occasions, discrete scattering regions moving across the field of view with periodicities of a few seconds. The very narrow, vertically directed beam of the more powerful EISCAT radar allowed measurements of the spectral widths of the radar echoes both inside the PMWE and from the background plasma above and below the PMWE. Spectral widths inside the PMWE were found to be indistinguishable from those from the background plasma. We propose that scatter from highly-damped ion-acoustic waves generated by partial reflection of infrasonic waves provides a reasonable explanation of the characteristics of the very strong PMWE reported here.

Multi-frequency HF radar measurements of artificial F-region field-aligned irregularities.

Senior, A. — Tue, 02 Nov 2004 23:00:00 GMT

Senior, A.; Rietveld, Michael T.; Borisov, N.D.; Kosch, M.J.; Yeoman, T.K.; Honary, F.
We present radar backscatter power measurements using the CUTLASS HF radar at Hankasalmi, Finland from F-region field-aligned irregularities induced by HF radio pumping with the EISCAT Heating facility. A novel radar operating mode is used in which the radar frequency is rapidly swept through a number of bands, making use of the varying ionospheric refraction to probe different heights within the heated region. We obtain height profiles of backscatter power which correspond to e-folding scale lengths of around 20km for the mean-square electron density perturbations for pump wave interaction heights in the region of 240-250km in daytime conditions. The results are in agreement with previous measurements made by other techniques. We discuss some problems with the method and suggest improvements for future experiments.

Comparison of high latitude electron density profiles obtained with the GPS radio occultation technique and EISCAT measurements.

Stolle, C. — Wed, 31 Dec 2003 23:00:00 GMT

Stolle, C.; Rietveld, Michael T.; Jakowski, N.; Schlegel, K:
To obtain a comprehensive view on high latitude processes by applying different observation techniques, the SIRCUS campaign was initiated in 2001/2002. This paper compares electron density profiles derived from CHAMP radio occultation data and those measured with the EISCAT facility. Since ionospheric profiling with the help of space-based received GPS is a relatively new technique, validations with established independent instruments are of crucial need. We present 28 profiling events for quasi-statistical analyses, which occurred during the SIRCUS campaigns and describe some of them in more detail. We found out that the majority of profile comparisons in electron density peak value and height, as well as in TEC, lie within the error ranges of the two methods. Differences in the ionospheric quantities do not necessarily occur when the locations of the occultation and of the radar site show considerable distances. Differences are more pronounced when the ionosphere is remarkably structured.

ELF wave generation in the ionosphere using pulse modulated HF heating. Initial tests of a technique for increasing ELF wave generation efficiency.

Barr, R. — Thu, 31 Dec 1998 23:00:00 GMT

Barr, R.; Rietveld, Michael T.; Stubbe, P.
This paper describes the results of a preliminary study to determine the effective heating and cooling time constants of ionospheric currents in a simulated modulated HF heating, `beam painting' configuration. It has been found that even and odd harmonics of the fundamental ELF wave used to amplitude modulate the HF heater are sourced from different regions of the ionosphere which support significantly different heating and cooling time constants. The fundamental frequency and its odd harmonics are sourced in a region of the ionosphere where the heating and cooling time constants are about equal. The even harmonics on the other hand are sourced from regions of the ionosphere characterised by ratios of cooling to heating time constant greater than ten. It is thought that the even harmonics are sourced in the lower ionosphere (around 65 km) where the currents are much smaller than at the higher altitudes around 78 km where the currents at the fundamental frequency and odd harmonics maximise.

Observations of plasma density structures in association with the passage of traveling convection vortices and the occurrence of large plasma jets.

Van Eyken, A.P. — Thu, 31 Dec 1998 23:00:00 GMT

Van Eyken, A.P.; Valladares, C.E.; Alcaydé, D.; Rodriguez, J.V.; Ruohoniemi, J.M.
We report important results of the first campaign specially designed to observe the formation and the initial convection of polar cap patches. The principal instrumentation used in the experiments comprised the EISCAT, the Sondrestrom, and the Super DARN network of radars. The experiment was conducted on February 18, 1996 and was complemented with additional sensors such as the Greenland chain of magnetometers and the WIND and IMP-8 satellites. Two different types of events were seen on this day, and in both events the Sondrestrom radar registered the formation and evolution of large-scale density structures. The first event consisted of the passage of traveling convection vortices (TCV). The other event occurred in association with the development of large plasma jets (LPJ) embedded in the sunward convection part of the dusk cell. TCVs were measured, principally, with the magnetometers located in Greenland, but were also confirmed by the line-of-sight velocities from the Sondrestrom and SuperDARN radars. We found that when the magnetic perturbations associated with the TCVs were larger than 100 nT, then a section of the high-latitude plasma density was eroded by a factor of 2. We suggest that the number density reduction was caused by an enhancement in the O+ recombination due to an elevated Ti, which was produced by the much higher frictional heating inside the vortex. The large plasma jets had a considerable (>1000 km) longitudinal extension and were 200-300 km in width. They were seen principally with the Sondrestrom, and SuperDARN radars. Enhanced ion temperature (Ti) was also observed by the Sondrestrom and EISCAT radars. These channels of high Ti were exactly collocated with the LPJs and some of them with regions of eroded plasma number density. We suggest that the LPJs bring less dense plasma from later local times. However, the recent time history of the plasma flow is important to define the depth of the density depletion. Systematic changes in the latitudinal location and in the intensity of the LPJs were observed in the 2 min time resolution data of the SuperDARN radars. The effect of the abrupt changes in the LPJs location is to create regions containing dayside plasma almost detached from the rest of the oval density. One of these density features was seen by the Sondrestrom radar at 1542 UT. The data presented here suggest that two plasma structuring mechanisms (TCVs and LPJs) can act tens of minutes apart to produce higher levels of density structures in the near noon F-region ionosphere.

Directional features of the downshifted peak observed in HF-induced stimulated electromagnetic emission spectra obtained using an interferometer.

Tereshchenko, E.D. — Tue, 08 Aug 2006 22:00:00 GMT

Tereshchenko, E.D.; Rietveld, Michael T.; Yurik, R.Yu; Belyey, Vasyl; Khudukon, B.Z.; Brekke, Asgeir; Isham, B.; Hagfors, T.; Grill, M.
A high frequency (HF) ionospheric modification experiment was carried out between 25 September and 8 October 2004, using the EISCAT HF transmitter located near Tromsø, Norway. During this experiment the spectra of the stimulated HF sideband waves (stimulated electromagnetic emission or SEE) induced by the HF pump were observed using an interferometer consisting of three spaced receiving antennas with baselines both along and perpendicular to the meridian, and a multi-channel coherent receiver, installed in the vicinity of the HF facility. The transmitter operated at 4040kHz and its antenna beam was scanned to angles of 0°, 7°, 14°, and 21° south from vertical, pausing 4min at each position. This paper focuses on features of the downshifted peak (DP) emission, which has not been as thoroughly studied as many of the other SEE spectral features observable within the EISCAT pump frequency range. It was found that the signal-weighted direction of the DP source region remained within 5° of magnetic zenith as the HF beam was tilted between 0 and 21° south of vertical.