Browsing by Author "Isham, B."
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Isham, B.; Rietveld, Michael T.; Hagfors, T.; Belyey, Vasyl; Khudukon, B.; La Hoz, Cesar; Yurik, R.Yu; Brekke, Asgeir; Tereshchenko, E.D.; Grill, M.; Heinselman, C. (Journal article; Peer reviewed, 31-Jan-2005)[+][-]
Abstract: When the Earth's ionosphere is irradiated by a radiofrequency (RF) electromagnetic wave of sufficiently high power density and tuned to match a natural E- or F-region plasma frequency, ionospheric magnetoionic wave modes may be excited and may generate RF electromagnetic sideband waves via nonlinear interactions. These secondary emissions, which may then escape from the ionosphere, have been termed stimulated electromagnetic emission or SEE. The frequency spectra of this radiation has been studied extensively, and a number of characteristic spectral features have been identified and in some cases related to particular plasma processes. The separation in frequency between the RF pump and the harmonics of the local electron gyrofrequency is critical in determining the amount of anomalous absorption suffered by the pump wave and the spectral properties of the stimulated sidebands. The pump can excite electrostatic waves which do not propagate away but can in some cases be observed via radio-wave scattering from the electron density fluctuations associated with them. These enhanced density fluctuations are created by processes commonly referred to as upper-hybrid and Langmuir turbulence. Langmuir turbulence has been the subject of 930-MHz scattering observations with antenna scanning through several pre-selected angles between the geographic and geomagnetic zenith directions, and a preference for pointing angles between the Spitze angle and geomagnetic field-aligned was identified. Other phenomena, such as the generation of enhanced electron temperatures and artificial aurora, have more recently been shown to have special behavior at similar angles, near but apparently not quite at field-aligned. In view of this evidence for angular structure in several pump-induced effects, in light of the rich variety of SEE phenomena strongly dependent on the geomagnetic field via the frequency interval between the pump and the gyrofrequency harmonics, and in view of the not yet understood but complex relationship between electrostatic fluctuations and SEE, it is of interest to investigate experimentally whether a similar angular structure is present in the various spectral features of the SEE signals and to compare the results with radar and other observations of RF-pump-induced effects. To this end we describe a simple two-element radio interferometer designed to search for aspect angle dependence of SEE features. We present an example of the initial data produced by this system, and draw preliminary conclusions based on the example data. URI: http://hdl.handle.net/10037/565 File(s) in this item: 1
article.pdf (5.513Mb) -
Tereshchenko, E.D.; Rietveld, Michael T.; Yurik, R.Yu; Belyey, Vasyl; Khudukon, B.Z.; Brekke, Asgeir; Isham, B.; Hagfors, T.; Grill, M. (Journal article; Peer reviewed, 09-Aug-2006)[+][-]
Abstract: 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. URI: http://hdl.handle.net/10037/588 File(s) in this item: 1
article.pdf (450.9Kb) -
Rietveld, Michael T.; Gustavsson, B.; Sergienko, T.; Grydeland, Tom; La Hoz, Cesar; Kosch, M.J.; Brändström, B.U.E.; Leyser, T.B.; Isham, B.; Gallop, P.; Aso, T.; Ejiri, M.; Steen, A.; Kaila, K.; Jussila, J.; Holma, H. (Journal article; Peer reviewed, 28-Jul-2005)[+][-]
Abstract: The shape of the electron energy distribution has long been a central question in the field of highfrequency radio-induced optical emission experiments. This report presents estimates of the electron energy distribution function, fe(E), from 0 to 60 eV, based on optical multiwavelength (6300, 5577, 8446, 4278 A° ) data and 930-MHz incoherent scatter radar measurements of ion temperature, electron temperature and electron concentration. According to our estimate, the electron energy distribution has a depression at around 2 eV, probably caused by electron excitation of vibrational states in N2, and a high energy tail that is clearly supra-thermal. The temporal evolution of the emissions indicates that the electron temperature still plays an important role in providing electrons with energies close to 2 eV. At the higher energies the electron energy distribution has a nonthermal tail. URI: http://hdl.handle.net/10037/553 File(s) in this item: 1
article.pdf (2.174Mb) -
Tereshcenko, E.D.; Rietveld, Michael T.; Brekke, Asgeir; Khudukon, B.Z.; Isham, B.; Hagfors, T. (Journal article; Peer reviewed, 21-Nov-2006)[+][-]
Abstract: 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. URI: http://hdl.handle.net/10037/912 File(s) in this item: 1
article.pdf (435.8Kb)
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