R. Pradipta

VLF wave génération by amplitude-modulated HF heater waves at Gakona, Alaska

[1] Experiments conducted at Gakona, Alaska, using the intensity-modulated HF heating waves to interact with electrojet currents for the generation of VLF waves, are reported. An unexpected large increasing rate from 4 to 8 kHz in the frequency dependency of the VLF radiation intensity was observed. The peak value at 8 kHz was intense (about 7.5 dB above that of the 2 kHz signal used as a marker) and the wave intensity from 5 to 17 kHz appeared to be abnormally high (i.e., stronger than that at 2 kHz). In the experiments, we also observed the enhancement of spread-E irregularities at electrojet current altitudes due to the amplitude-modulated heater wave. These results and theoretical analyses suggest that temporally modulated electrojet currents mix with heater wave-excited density irregularities to form whistler mode currents, which generate VLF whistler waves directly with much larger intensities and better directivity than a Hertzian dipole can.

Generation of ionospheric ducts by the HAARP HF heater

We report an investigation of ionospheric ducts having the shape of large plasma sheets, generated by vertically transmitted HAARP HF heater waves in several experiments conducted in Gakona, Alaska. Theory predicts that O-mode heater wave-created ionospheric ducts form parallel-plate waveguides within the meridional plane, and those generated by the X-mode heater waves are orthogonal to the meridional plane. Our theoretical prediction is supported by measurements of ionosonde data (namely ionograms), range–time–intensity (RTI) plots of UHF and HF backscatter radars, as well as magnetometer data analyses. When these plasma sheets experienced E×B drifts, they were intercepted by the HAARP UHF radar and seen as slanted stripes in the RTI plots. This striking feature was also observed in our earlier experiments using the Arecibo UHF radar.

Space plasma disturbances caused by NAU-launched whistler waves

Radio signals from Naval (NAU) transmitter in Puerto Rico can interact effectively with naturally occurring or HF heater wave-induced large-scale ionospheric irregularities, allowing them to propagate as whistler-modes in the ionosphere and to the inner radiation belts. NAU-generated whistler-modes have intensities sufficient to parametrically excite lower hybrid waves and ten-meter and meter-scale ionospheric irregularities over Arecibo. Subsequent heating of electrons and ions by the lower hybrid waves yield a sequence of ionospheric plasma effects such as airglow, short-scale density depletion and plasma line enhancements in a range of altitudes which far exceed that caused by the HF heater. Furthermore, they can interact with trapped energetic electrons in inner radiation belts at L=1.35 and trigger precipitation of electrons into the lower ionosphere. We suggest that disturbances in the ionosphere above NAU caused by whistler-mode signals can significantly affect heater-induced perturbations and partially explain unique results obtained at other heater sites.

Controlled study of acoustic gravity waves (AGW) generated by anomalous heat sources

We investigate high power radio wave-induced acoustic gravity waves (AGWs) at Gakona, Alaska, using High-frequency Active Aurora Research Program (HAARP) heating facility (i.e., HF heater) and extensive diagnostic instruments. This work is aimed at controlled study of space plasma turbulence, triggered by acoustic gravity waves originating from anomalous heat sources, as observed in our earlier experiments at Arecibo, Puerto Rico [R. Pradipta, MS Thesis, MIT, 2007]. HF heater operated in CW O-mode can heat ionospheric plasmas effectively to yield depleted magnetic flux tube as rising plasma bubbles [Lee et al., Geophys. Res. Lett., 1998]. Two processes responsible for the depletion of magnetic flux tube are (1) thermal expansion and (2) chemical reactions caused by heated ions. The depleted plasmas create large density gradients that can augment spread F processes via generalized Rayleigh-Taylor instabilities [Lee et al., Geophys. Res. Lett., 1999]. It is thus expected that the temperature of neutral particles in the heated ionospheric region can be increased. Such heat source in the neutral atmosphere may potentially generate acoustic gravity waves (AGWs) in the form of traveling ionospheric plasma disturbances (TIPDs). We should point out that these TIPDs have features distinctively different from ExB drifts of HF wave-induced large-scale non-propagating plasma structures. Moreover, it is noted in our recent study of naturally-occurring AGW-induced TIDs that only large-scale AGWs can propagate upward to reach higher altitudes. Thus, in our Gakona experiments we select optimum heating schemes for HF wave-induced AGWs that can be distinguished from the naturally occurring ones. The generation and propagation of AGWs are monitored by Modular UHF Ionospheric Radar (MUIR), digisonde, and GPS/LEO satellites.

Excitation of large-scale plasma sheets and micropulsations by injected high power radio waves

We have conducted several experiments to investigate the simultaneous generation of large plasma sheets and micropulsations by injected high power radio waves via thermal filamentation instabilities [Cohen et al., Phys. Scrip., 2010]. These large plasma sheets generated by HF heater have different configurations, depending upon the polarizations (i.e., O- or X-mode) of the heater waves. It is expected that O-mode heater wave-created parallel-plate waveguides within the meridional plane, and those generated by the X-mode heater waves are orthogonal to the meridional plane. [Lee et al., Geophys. Res Lett., 1998]. One striking feature of thermal filamentation instabilities is the simultaneous excitation of sheet-like plasma density fluctuations (δn) and geomagnetic field fluctuations (δB). The physics can be simply understood as follows. The differential joule heating, resulting from the interactions of HF heater waves and excited high frequency sidebands, yields a thermal pressure force on electrons. Thermal pressure force (denoted by fT) leads to a fT × B0 drift motion of electrons and, consequently, induces a net current perpendicular to both the background magnetic field B0 and the wave vector k of the excited plasma density irregularities. Therefore, magnetic field fluctuations (δB) associated with micropulsations are excited along the background magnetic field (B0 designated as the z-axis) simultaneously with the density irregularities in both O- and X-mode heating processes. The excited magnetic field fluctuations (δB) have three components designated as dδBD, dδBH, and dδBZ. Based on above explanation of the simultaneous excitations of dδn and dδB, we can expect that dδBD and dδBZ (or dδBH and dδBZ) will be highly correlated in O-mode (or X-mode) heating experiments. Our theoretical predictions are confirmed by GPS satellite measurements, range-time-intensity (RTI) plots of UHF and HF backscatter radars, ionosonde data, as well as magnetometer data analyses. As these plasma sheets experienced E·B drifts, they were intercepted by the HAARP UHF radar and seen as slanted stripes in the RTI plots, as also seen in our earlier Arecibo experiments. Furthermore, based on the GPS satellite measurements, we infer that kilometer-scale plasma sheets can be generated by vertically injected O-mode heater waves.

Excitation and diagnosis of cascading Langmuir waves in ionospheric plasmas at Gakona, Alaska

Ionospheric plasma heating experiments were conducted at Gakona, Alaska to investigate cascading spectra of Langmuir wave turbulence, excited by parametric instabilities diagnosed by Modular UHF Ionospheric Radar (MUIR). This work is aimed at testing the recent theory of Kuo and Lee (2005 J. Geophys. Res. 110 A01309) that addresses how the cascade of Langmuir waves can distribute spatially via the resonant and non-resonant decay processes. The non-resonant cascade proceeds at the location where parametric decay instability (PDI) or oscillating two-stream instability (OTSI) is excited and severely hampered by the frequency mismatch effect. By contrast, the resonant cascade, which takes place at lower matching heights, has to overcome the propagation loss of the Langmuir pump waves in each cascade step. Our experimental results have corroborated these predictions about the generation of cascading Langmuir waves by the HAARP heater.