Simultaneous observations of ELF waves from an artificially modulated auroral electrojet in space and on the ground

Abstract

Radio waves at frequencies between 525 and 5850 Hz were observed simultaneously on the ground and in the VLF receiver of the ISIS 1 spacecraft during a flight over the Max‐Planck‐Institut für Aeronomie ionospheric heater near Tromsø, Norway, on December 9, 1981. The heater carrier frequency, 4.04 MHz, was amplitude modulated with a set of four frequencies: 525, 1725, 2925, and 4125 Hz. The satellite detected all these fundamental frequencies plus harmonics of the 525 Hz and 2925 Hz components, caused by the nonsinusoidal modulation of the carrier. Characteristics of the signal received along the satellite track are in approximate agreement with the results of three‐dimensional ray tracing applied to a model of the ionosphere around Tromsø. The time dispersion of the signals is best fitted with an ionospheric density model based on real time data with relatively low peak values. Observed signal levels have been transformed to power flux by using a plasma dipole theory and wave polarization information obtained from the three‐dimensional ray solutions. Through the use of a magnetoplasma dipole theory for the induced D/E region current, the observed levels are found to correspond to radiation created by effective dipole currents between about 1.0 and 100 A. The geometrical optics theory also predicts two solutions for rays reaching a given satellite point near the polar limit of the reception zone, and a northern limit corresponding to a caustic surface. The frequencies of signal fades observed in this region are also predicted by the geometrical optics theory, but the fade depths are not. Simultaneous ground observations correspond to source region currents of about 1 A or less. Also, the ELF waves observed on the ground have different harmonic signal ratios and much smaller fluctuations than those observed on ISIS.

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

“…There are several possible explanations for this variability, the two most important ones being the intensity of the electrojet and the ambient ELF/VLF wave activity level in the magnetosphere. Indeed, as noted in previous similar experiments (Lefeuvre et al, 1985;James et al, 1984;Inan and Helliwell, 1985), the intensity of the ELF/VLF radiation is expected to be higher when the intensity of the electrojet is higher. This condition imposes a degree of uncertainty for the experiment, because it then depends on external conditions that cannot be controlled.…”

“…In contrast to the many ground-based observations, space-based observations of HFgenerated ELF/VLF waves above the auroral electrojet are few in number. In this grouping we find observations concerning the heating facility of the Max-Planck Institut für Aeronomie (MPAe Heating) and the spacecraft AUREOL 3 (Lefeuvre et al, 1985), ISIS 1 (James et al, 1984) and DE 1 (Inan and Helliwell, 1985;James et al, 1990), all of them performed in December 1981. These experiments show that the modulated auroral electrojet can sometimes radiate detectable signals into the topside ionosphere and the magnetosphere.…”

“…1b) projection. The HF carrier frequency was chosen to provide maximum heating in the D-region of the ionosphere (James et al, 1984), taking into account the allocated frequencies at which HAARP can transmit at its highest power. In this case the chosen frequency was 3.3 MHz, but transmissions also took place at 5.8 MHz to test if there are differences in the emitted VLF radiation.…”

Resonant scattering of energetic electrons in the plasmasphere by monotonic whistler-mode waves artificially generated by ionospheric modification

“…Those observational results indicate that some of these artificial ELF/VLF waves generated by ionospheric modulation can propagate directly upwards and penetrate through the ionosphere into the overlying magnetosphere (James et al, 1984), and others propagate within the Earth-ionosphere waveguide and leak up progressively with each reflection off the ionosphere Platino et al, 2006). The wave intensity detected by the groundbased receiver or by the spacecraft instrument is generally at a magnitude of a few pT.…”

“…1, when ELF/VLF waves are generated as a result of periodic modulation of the ionospheric currents by HF heating (Ferraro et al, 1982;Barr and Stubbe, 1991;Inan et al, 2004;Platino et al, 2006;Piddyachiy et al, 2008), some waves propagate directly upwards and others propagate within the Earth-ionosphere waveguide and leak up progressively with each reflection off the ionosphere (James et al, 1984;Inan et al, 2004;Platino et al, 2006;Piddyachiy et al, 2008). Subsequently, these leaked waves propagate outwards and encounter energetic electrons that can potentially undergo resonant wave-particle interactions.…”