Cluster observations of ELF/VLF signals generated by modulated heating of the lower ionosphere with the HAARP HF transmitter

M Platino,J G Trotignon,U S Inan,J L Rauch,T F Bell,J Pickett,E J Kennedy,P Canu

doi:10.5194/angeo-22-2643-2004

Abstract

Abstract. It is now well known that amplitude modulated HF transmissions into the ionosphere can be used to generate ELF/VLF signals using the so-called "electrojet antenna". Although most observations of the generated ELF/VLF signals have been made on the ground, several low and high-altitude satellite observations have also been reported (James et al., 1990). One of the important unknowns in the physics of ELF/VLF wave generation by ionospheric heating is the volume of the magnetosphere illuminated by the ELF/VLF waves. In an attempt to investigate this question further, ground-satellite conjunction experiments have recently been conducted using the four Cluster satellites and the HF heater of the High-Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska. Being located on largely closed field lines at L≈4.9, HAARP is currently also being used for ground-to-ground type of ELF/VLF wave-injection experiments, and will be increasingly used for this purpose as it is now being upgraded for higher power operation. In this paper, we describe the HAARP installation and present recent results of the HAARP-Cluster experiments. We give an overview of the detected ELF/VLF signals at Cluster, and a possible explanation of the spectral signature detected, as well as the determination of the location of the point of injection of the HAARP ELF/VLF signals into the magnetosphere using ray tracing.

Highlights

To radiate radio waves efficiently requires antennas whose dimensions are of the order of the radio wavelength being emitted
The present paper reports recent observations on the Cluster spacecraft of electromagnetic ELF/very low frequencies (VLF) waves produced through ELF/VLF modulation of auroral electrojet currents
The signal was detected for at least 15 min, and the natural ELF/VLF emissions that are usually present in these regions of the magnetosphere were not present, providing a much better environment for identification of the High-Frequency Active Auroral Research Program (HAARP) ELF/VLF signals

Summary

Introduction

To radiate radio waves efficiently requires antennas whose dimensions are of the order of the radio wavelength being emitted. March 2003 and 11 May 2003, in two cases of which (26 January 2003 and 11 May 2003) detectable ELF/VLF signals were observed on one or more of the Cluster spacecraft. We performed ray-tracing calculations to determine the propagation paths and to estimate the time delays of the HAARP VLF signals received on Cluster.

Results

Conclusion