Large-scale resonant modification of the polar ionosphere by electromagnetic waves

Abstract

Large-scale density modification was optimally achieved by electromagnetic waves whose frequency matched the plasma frequency at a height where the ionospheric density profile was flat. The density in this region was dramatically clamped during the morning when it normally increases from solar ionization. Electromagnetic wave propagation in the polar magnetic field geometry and strongly resonantly enhanced electrostatic fields over a large region of constant density account for the observation.PACS 52.25.Sw, 52.70.Gw We report the observation of large-scale modification of the polar ionosphere by electromagnetic (EM) waves at modest power density (0.1 <po< 1 mW/m 2 ). Maximum density perturbations are observed when the excitation frequency matches the plasma frequency of the location in the density profile where the gradient is nearly zero. This observed sensitive dependence on the ionospheric density and its gradient indicates that the plasma resonance and the enhancement of the incident field control the dynamic modification process. The nearly vertical polar magnetic field geometry and the vertical density profile made it possible for a large cone of EM rays to reach the resonant height to create the large-scale perturbation. The long energy confinement time (=10 3 sec, as determined mainly by the recombination process 1 ) allows the incident energy absorbed by the ionosphere to be accumulated and a large region to be affected in the course of time. A consequence of the present experiment is the controlled coupling to large scale natural phenomena.The observation reported here is significantly different from previous experiments 2 in the location and extent of the perturbation at resonance, the magnetic field geometry, the deliberate choice of matching conditions with and without solar uv ionization, and the diagnostics.The experiments were performed at the newly completed HIPAS Observatory, located near Fairbanks, AK (geographic coordinates: latitude 64° 52'22" N, longitude 146°50'06" W). The antenna was radiating EM waves vertically upward with 0-mode polarization (E vector rotating in a direction counter to the electron cyclotron gyration). This mode was chosen to avoid strong absorption in the lower ionosphere and to produce a highly enhanced oscillating electric field component along the vertical density gradient which is nearly parallel to the magnetic field. The frequency is 3.349 MHz and the beam width is 19° FWHM. The maximum radiated power from the antenna array is 1 MW, which is equivalent to 80 MW radiated isotropically at this fre-quency. The calculated power density is 0.2 mW/m 2 with an oscillating Eo field of 0.3 V/m at an altitude of 200 km before taking into consideration the enhancement of the E field as a result of plasma collective effects or absorption at lower heights (D and E regions). When directed vertically, the angular spread in the main beam covers a range of angles (6° to 32°) with respect to the Earth's magnetic field, which is inclined at an angle of 13° with respect to the ...

Powerful electromagnetic waves for active environmental research in geospace

“…Large‐scale electron density modifications have been observed at auroral latitudes in experiments at the HIPAS facility during conditions when the pump frequency was near the maximum ionospheric plasma frequency (critical frequency) where the vertical gradient in the density profile is nearly zero [ Wong et al , 1989]. The experiments were conducted during mornings when the ionospheric plasma density normally slowly increases as a result of the increasing ionizing solar radiation.…”

“…The experimental results for pumping near the critical frequency were compared to particle‐in‐cell simulations [ Wong et al , 1989]. The simulations showed that after about 150 electron plasma periods, multiple density cavitons of roughly 10% depth had formed.…”

Experimental study of double resonance parametric excitations in the ionosphere

“…When the pump frequency (--2.85 MHz) matches plasma frequency at the bottom of the F layer, the electric field has been resonantly enhanced and a strong interaction takes place [Wong et al, 1989].…”

“…Since the power of seven antennas at the center of the heating region is about three times stronger than that of four antemlas, it is expected that the density modulation in the 1-7 configuration will be more pronounced than that of the 4-4 configuration. A larger density modulation at the resonance height has been observed at HIPAS when all eight transmitters were turned on [Wong et al, 1989].…”