An interferometer experiment to explore the aspect angle dependence of stimulated electromagnetic emission spectra

Abstract

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 structureCorrespondence to: B. Isham (bisham@bc.inter.edu) 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.

Aspect sensitive E- and F-region SPEAR-enhanced incoherent backscatter observed by the EISCAT Svalbard radar

“…Djuth et al, 1994;Dhillon and Robinson, 2005), measurements of artificial airglow (e.g. Kosch et al, 2000;Pedersen et al, 2003;Kosch et al, 2004) and observations of stimulated electromagnetic emissions (Isham et al, 2005). These previous investigations highlighted the importance of many directions relative to the geomagnetic field and the study presented here again provides evidence of the apparent special nature of certain directions, including field-aligned.…”

“…Such aspect sensitive responses include those that have been observed in Langmuir turbulence , RF-induced descent of the interaction altitude (Djuth et al, 1994;Dhillon and Robinson, 2005), optical emissions (Kosch et al, 2000;Pedersen et al, 2003;Kosch et al, 2004), electron temperature increases (Rietveld et al, 2003) and stimulated electromagnetic emissions (SEE) (Isham et al, 2005). The field-aligned direction was found to be important in some of these studies, e.g.…”

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

“…These properties suggest that features in the SEE spectrum should also depend on geomagnetic aspect angle (Isham et al, 2005), a topic that is investigated in this report. SEE is also known to be sensitive to pump duty cycle, power, and to pumping by additional waves (Leyser, 2001).…”

“…This ionospheric modification experiment discussed here was carried out using the powerful HF transmitter at Ramfjordmoen near Tromsø, Norway (69.6 • N, 19.2 • E, L=6.2, magnetic dip angle 78 • ) and a multi-channel digital receiving system designed for high-resolution interferometric measurements of radio signals (Isham et al, 2005). A technical description of the HF transmitter facility and its parameters can be found in Rietveld et al (1993).…”

Stimulated electromagnetic emission polarization under different polarizations of pump waves

“…This interferometer has previously been used for the determination of the angle-of-arrival of SEE signals at the Sura (Nizhniy Novgorod, Russia), EISCAT (Tromsø, Norway) and SPEAR heating facilities (Isham et al, 2005;Tereshchenko et al, 2006Tereshchenko et al, , 2008Tereshchenko et al, , 2011. The HF interferometer consists of an HF receiver with a high dynamic range and a band width of 300 kHz, a digital system for the conversion and recording of the received signal, and HF and IF (intermediate frequency) notch filters.…”