SA42A-01 INVITED
Pulsed Artificial Electrojet Generation
Traditional techniques for generating low frequency signals in the ULF/ELF range (.1-100 Hz) and rely on ground based Horizontal Electric Dipole (HED) antennas. It is, furthermore, well known that a Vertical Electric Dipole (VED) is by more than 50 dB more efficient than a HED with the same dipole current moment. However, the prohibitively long length of VED antennas in the ELF/ULF range coupled with voltage limitations due to corona discharge in the atmosphere make them totally impracticable. In this paper we discuss a novel concept, inspired by the physics of the equatorial electrojet, that allows for the conversion of a ground based HED to a VED in the E-region of the equatorial ionosphere with current moment comparable to the driving HED. The paper focuses in locations near the dip-equator, where the earth's magnetic is in predominantly in the horizontal direction. The horizontal electric field associated with a pulsed HED drives a large Hall current in the ionospheric E-region, resulting in a vertical current. It is shown that the pulsed vertical current in the altitude range 80-130 km, driven by a horizontal electric field of, approximately, .1 mV/m at 100 km altitude, is of the order of kA. This results in a pulsed VED larger than 106 A-m. Such a pulsed VED will drive ELF/ULF pulses with amplitude in excess of .1 nT at a lateral range larger than few hundred kilometers. This is by three orders of magnitude larger than the one expected by a HED with comparable current moment. The paper will conclude with the description of a sneak-through technique that allows for creating pulsed electric fields in the ionosphere much larger than expected from steady state oscillatory HED antennas.
SA42A-02 INVITED
Active ELF/VLF Wave Injection Experiments with HAARP and VLF Transmitters
Active wave-injection experiments constitute an excellent tool for investigation of the mechanisms and effects
of magnetospheric wave-particle interactions, primarily because of the fact that the known properties of the
injected signals allow quantitative interpretation of wave amplification, emission triggering, and resultant
precipitation of energetic radiation belt electrons. One of the important tools available for such
experimentation is the HAARP HF heating facility, which allows for the generation of ELF/VLF waveforms with
intricately detailed frequency-time formats, to investigate properties of the cyclotron resonant growth such
as rapid exponential growth, saturation, and entrainment. Observations of the amplified and triggered
signals can be made on the ground, in the conjugate hemisphere and in the hemisphere of HAARP, as well
as on satellites such as DEMETER. Another tool for VLF wave-injection experiments is the U.S. Navy VLF
transmitters operating in the 18-24 kHz range, which can be pulsed with predetermined on/off formats to
facilitate detection and identification of electron precipitation produced by the pitch angle scattering of
electrons near the equatorial plane. The precipitating electrons can be observed on satellites (e.g.,
DEMETER), as well as via the associated subionospheric VLF perturbation signatures. In this paper, we
review recent results from active ELF/VLF wave-injection experiments, conducted both with HAARP and with
VLF transmitters.
http://nova.stanford.edu/~vlf/
SA42A-03 INVITED
Electron and Ion-Molecule Kinetics Effects in HF-Modified Ionosphere
The electron and ion-molecule kinetics effects are usually disregarded in modeling non-equilibrium ionospheric plasma subjected to strong heating. This presentation aims to describe these effects during HF modification experiments. First, as long as electron heating is collisional, the population of thermal electrons at >2 eV is reduced due to excitation of mainly nitrogen vibrational states. As a result, the electron cooling rate, a key parameter in the thermal energy balance, decreases relative to that in Maxwellian plasma. Other outcomes are the decrease of the rates of excitation of the red-line emission and Landau damping. The latter leaves the background suprathermal electron population (photoelectrons in the F region and secondary electrons in precipitation-produced E layers) as the main candidate for acceleration by HF-excited plasma turbulence. In the course of heating, nitrogen and molecular oxygen vibrational states are excited by the energized electron population. This leads to significant changes in the rate coefficients and quantum chemical yields of the basic ion-molecule reactions depending on vibrational temperatures. As a result, while the density of the hot plasma is slowly depleted, the green-to-red ratio and intensity of HF-induced airglow increases. The latter is consistent with the observations of the gradual evolution of the airglow during injections HF O-mode waves at the magnetic zenith.
SA42A-04 INVITED
Ionospheric modification by radio waves: An overview and novel applications
High-power high-frequency radio waves, when beamed into the Earth's ionosphere, can heat the plasma by particle collisions in the D-layer or generate wave-plasma resonances in the F-layer. These basic phenomena have been used in many research applications. In the D-layer, ionospheric currents can be modulated through conductance modification to produce artificial ULF and VLF waves, which propagate allowing magnetospheric research. In the mesopause, PMSE can be modified allowing dusty plasma research. In the F-layer, wave-plasma interactions generate a variety of artificially stimulated phenomena, such as (1) magnetic field-aligned plasma irregularities linked to anomalous radio wave absorption, (2) stimulated electromagnetic emissions linked to upper-hybrid resonance, (3) optical emissions linked to electron acceleration and collisions with neutrals, and (4) Langmuir turbulence linked to enhanced radar backscatter. These phenomena are reviewed. In addition, some novel applications of ionospheric heaters will be presented, including HF radar sounding of the magnetosphere, the production of E-region optical emissions, and measurements of D-region electron temperature for controlled PMSE research.
SA42A-05
Aeronomy With the new HF Ionospheric Modification Facility at Arecibo
The advent of the new Arecibo high-frequency (HF) modification facility opens up new opportunities for users to perform controlled aeronomy experiments at Arecibo Observatory. In this presentation, three such applications are described. These include studies of electron/ion energy balance and O-O+ charge exchange in the upper ionosphere/topside ionosphere, electron beam experiments that offer tests of energy loss processes, energy transport, and inelastic collision cross sections below about 250 km, and electron thermal balance in sporadic E, which yields information about collisional cross sections and/or background neutral density between approximately 110 km and 130 km. These experiments are conducted in the nighttime midlatitude ionosphere above Arecibo where there is no absorption of the HF en route to the E region and F region ionospheres, optical and radar diagnostics are available, and a smooth stratified F region profile persists. The HF investigations are referred to as controlled experiments because they yield reproducible results from one HF transmission to the next. Particular emphasis is placed on the controlled injection of suprathermal electrons because it allows electron thermal balance to be studied in greater detail in the F region than is achievable with daytime photoelectrons. Finally, we note that the incoherent scatter radar capability at Arecibo has improved greatly since the loss of the Arecibo HF facility in 1998 and that many new optical diagnostics are now available at the Observatory.
SA42A-06
Comprehensive Studies of Langmuir Turbulence Experiments at HAARP
We report the results from a recent series of campaigns employing the HAARP HF transmitter to generate and study strong Langmuir turbulence (SLT) in the interaction region of overdense ionospheric plasma. Diagnostics included the Modular UHF Ionospheric Radar (MUIR) sited at HAARP, the SuperDARN-Kodiak HF radar, and HF receivers to record stimulated electromagnetic emissions (SEE). Dependence of diagnostic signals on HAARP HF parameters, including pulselength, duty-cycle, and aspect angle were recorded. Short pulse, low duty cycle experiments demonstrate control of artificial field-aligned irregularities (AFAI). Among the effects observed and studied are: SLT spectra including cascade, collapse, and co-existence spectra and an outshifted plasma line. High time resolution studies of the temporal evolution of the plasma line reveal the appearance of an overshoot effect on ponderomotive timescales. Plasma line spectra exhibit a marked dependence on aspect angle including the discovery of a second region of strong turbulence displaced southward from the primary HF interaction region along the geomagnetic field line. Experimental results are compared to previous high latitude experiments and predictions from recent modeling efforts.
SA42A-07
Ground and Satellite Observations of ULF Waves Artificially Produced by HAARP
Modulated ionospheric heating at ULF frequencies using the HAARP heater was performed from April 28 to May 3, 2008 (http://www.haarp.alaska.edu). Simultaneous ground-based ULF measurements were made locally at Gakona, AK and at Lake Ozette, WA that is 2000 km away. The ground-based results showed that ULF amplitudes measured at Gakona are mostly proportional to the electrojet strength above HAARP, indicating electrojet modulation to be the source of the local ULF waves. However, the timing of ULF events recorded at Lake Ozette did not correlated with the electrojet strength at Gakona, indicating that modulation of F region pressure is the more likely source for distant ULF waves. These observations are consistent with the theoretical understanding that ULF waves generated by current modulation are shear Alfven waves propagating along the magnetic field line, thus at high latitude their observations are limited to the vicinity of the heated spot. On the other hand, propagation of ULF waves at significant lateral distances requires generation of magnetosonic waves since they are the only mode that propagates isotropically and can thus couple efficiently in the Alfvenic duct. In addition to ground-based observations, the DEMETER satellite also provided space measurements of the heating effects during its passes over HAARP. The DEMETER results showed direct detection of HAARP ULF waves at 0.1 Hz. Moreover, density dips were observed every time HAARP was operated at CW mode, which provides clear evidence of duct formation by direct HF heating at F peak. Details of these results will be presented at the meeting. We would like to acknowledge the support provided by the HAARP facility during our ULF experiments.
SA42A-08
Optimizing ELF/VLF generation via HF heating utilizing beam motion
ELF/VLF (300 Hz - 30 kHz) waves are difficult to generate with conventional antennae due to their extraordinary long wavelengths, and the good conductance of the Earth at these frequencies. Recently, ELF and VLF waves have been generated using HF (3-10 MHz) heating of the lower ionosphere, in the presence of natural currents such as the auroral electrojet, which modulates the ionospheric conductivity and therefore turns the lower ionosphere into a large radiating element. The recently upgraded HAARP facility, near Gakona Alaska, utilizes 3.6 MW of HF power, along with an unprecedented ability to steer the HF heating beam over a large area extremely rapidly. Since the completion of the upgrade in 2007, the first successful implementation of techniques such as geometric modulation [Cohen et al. 2008, Borisov et al. 1998], and beam painting [Papadopoulos et al. 1989] have occurred. These results have shown as much as 7-11 dB improvement in the signal strengths, as well as the first ability to direct ELF/VLF signals via an unprecedented ELF/VLF phased array. Here, we use a combination of experimental and theoretical investigations to discuss the optimization of ELF/VLF generation via HF heating, including the effect of HF and ELF frequency on the amplitude and the directional pattern for various generation techniques. The experimental observations occur over an array of receivers across Alaska. The theoretical formulation utilizes a 3D model of the HF heating and subsequent electron cooling processes, leading to spatial structure of modulated ionospheric conductivities, the results of which are input into a model of ELF/VLF propagation in the Earth-ionosphere waveguide.