A statistical study of the spatial distribution of Co‐operative UK Twin Located Auroral Sounding System (CUTLASS) backscatter power during EISCAT heater beam‐sweeping experiments

H Shergill,R S Dhillon,T R Robinson,M Lester,T K Yeoman,S E Milan

doi:10.1029/2009ja014659

Abstract

High‐power electromagnetic waves can excite a variety of plasma instabilities in Earth's ionosphere. These lead to the growth of plasma waves and plasma density irregularities within the heated volume, including patches of small‐scale field‐aligned electron density irregularities. This paper reports a statistical study of intensity distributions in patches of these irregularities excited by the European Incoherent Scatter (EISCAT) heater during beam‐sweeping experiments. The irregularities were detected by the Co‐operative UK Twin Located Auroral Sounding System (CUTLASS) coherent scatter radar located in Finland. During these experiments the heater beam direction is steadily changed from northward to southward pointing. Comparisons are made between statistical parameters of CUTLASS backscatter power distributions and modeled heater beam power distributions provided by the EZNEC version 4 software. In general, good agreement between the statistical parameters and the modeled beam is observed, clearly indicating the direct causal connection between the heater beam and the irregularities, despite the sometimes seemingly unpredictable nature of unaveraged results. The results also give compelling evidence in support of the upper hybrid theory of irregularity excitation.

Highlights

[1] High‐power electromagnetic waves can excite a variety of plasma instabilities in Earth’s ionosphere
In one early experiment using the Co‐operative UK Twin Located Auroral Sounding System (CUTLASS) radar, Bond et al [1997] studied the form of the CUTLASS backscatter power distribution in patches of irregularities enhanced by the European Incoherent Scatter (EISCAT) heater, both in range and in azimuth, using static beam configurations
[67] Backscatter power profiles measured by the CUTLASS Finland radar during fast and slow heater beam‐sweeping experiments at the EISCAT facility have been statistically analyzed

Summary

Introduction

[2] The generation of field‐aligned irregularities (FAIs) in the ionospheric plasma density was one of the most unexpected results of early ionospheric modification experiments carried out at the midlatitude heating facility in Platteville, Colorado (40.2°N, 104.7°W). Considering equation (1) with the requirement for radio waves to reach an altitude hU for FAI enhancement to occur implies that if the heater beam transmission angle is great enough, U is not reached by the transmitted waves, and plasma density irregularities should not be enhanced there. Electron density profiles provided by the EISCAT UHF radar were used to estimate a value of the upper hybrid height, hU In these EISCAT UHF data a heater‐induced ionline overshoot occurs at the altitude where the plasma frequency is equal to the heater frequency, causing an apparent enhancement in the electron density [Jones et al, 1986; Rietveld et al, 2000]. The upper hybrid width was calculated to be approximately 130 km, using equations (3) and (4)

Statistical Relations

Analysis

CUTLASS Data

Discussion

Summary and Conclusions