Plasma density enhancement and generation of spatially periodic irregularities in the ionospheric E‐region by powerful HF waves

Kuo, Spencer; Koretzky, E.; Lee, M. C.

doi:10.1029/1999gl900115

Cited by 5 publications

(10 citation statements)

References 13 publications

(12 reference statements)

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“…Moreover, the record of the riometer shows a continuous increase of the D-region absorption in the experiment period. These correlations lead to the conclusion that a nonlinear thermal instability [Kuo et al, 1999a] was excited in the 27 July afternoon experiment. As discussed in section 4.3, the thermal instability produces periodic density irregularities distributed parallel to the geomagnetic field.…”

Section: Summary and Discussionmentioning

confidence: 97%

“…Such temperature dependence provides a feedback channel to the heating, leading to the excitation of a thermal instability and the subsequent nonlinear evolution of the produced density perturbation after exceeding a threshold. The spatial distribution of the periodic density irregularities is parallel to, rather than perpendicular to, the geomagnetic field [Kuo et al, 1999a], which for Gakona, AK is 14 o off the zenith. The combination of the increasing D region absorption to weaken the ionosonde signals and the scattering of the ionosonde signals by the produced periodic density irregularities causes the disappearance of ionogram echoes over a wideband.…”

Section: Background Conditions and Experimentsmentioning

confidence: 99%

“…There are several mechanisms which can generate density irregularities by the HF heaters [Kuo and Schmidt, 1983;Lee and Kuo, 1983]. A likely one is the nonlinear thermal instability [Kuo et al, 1999a], which produces periodic density irregularities in the D and lower E regions of the ionosphere. Both elastic and inelastic electron-neutral collision frequencies are electron temperature dependent and modified by the HF heating.…”

Section: Background Conditions and Experimentsmentioning

confidence: 99%

“…During this period, wideband disappearance of the O-mode ionosonde echoes also occurred. It was shown that thermal instability [Kuo et al, 1999a] excited by the HF heaters in the D and lower E region could evolve nonlinearly to form periodic density irregularities having a broad spatial spectrum to scatter O-mode ionosonde signals over a wideband. These density irregularities could also couple to the temporally modulated electrojet (background current) to produce a whistler mode current and the associated whistler (VLF) wave at the beat frequency [Kuo et al, 1999b;Kuo and Koretzky, 2001].…”

Section: Generation Of Vlf Wavesmentioning

confidence: 99%

“…where the up-propagating and down-propagating whistler wave fields, E Wu (z) and E Wd (z), are determined, respectively, to be (20) [34] The sinc function in N(k w ) imposes a band (k w À p/Δz, k w + p/Δz); since the periodic density irregularities generated/ enhanced by the HF heater have a broad spectrum [Kuo et al, 1999a], n(k) may be approximated to be n(k w ) in this band and N(AEk w ) ffi [n(AEk w )/Δz] sinc(pz 0 /Δz).…”

Section: Solving Equation (13)mentioning

confidence: 99%

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Observation and theory of whistler wave generation by high‐power HF waves

et al. 2013

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[1] Summer midnight and afternoon VLF wave generation comparison experiments were conducted on 25 July and 27 July 2011, respectively, using two CW HF X-mode waves with 11 VLF frequency differences from 2 to 21.5 kHz. The background magnetic variations were at comparable levels. During the afternoon experiment, the D region absorption was significant and increasing. The number of the ionogram echoes decreased during the afternoon experiment. VLF signals were detected from 2 to 7.6 kHz in both experiments, showing an inverse frequency dependence of intensity, although signal intensity (except at 5.5 kHz) detected during the midnight experiment was stronger than the corresponding afternoon intensity before observing a decrease of the number of ionogram echoes. However, VLF signals from 11.5 to 21.5 (except at 19.6 kHz) were also generated in the afternoon experiment concurrent with a decrease of the O-mode ionosonde echoes from 2 to 4 MHz. The concurrence of a decrease of the afternoon ionogram echoes, the unexpected generation of VLF waves at higher frequencies, and the increasing D region absorption throughout the experiment may be explained by the generation of large-scale density irregularities, which scatter the ionosonde signals as well as couple with the modulated electrojet to generate whistler waves. A theoretical formulation of the coupling mechanism for the whistler wave generation is presented.

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Section: Summary and Discussionmentioning

confidence: 97%

Section: Background Conditions and Experimentsmentioning

confidence: 99%

Section: Background Conditions and Experimentsmentioning

confidence: 99%

Section: Generation Of Vlf Wavesmentioning

confidence: 99%

Section: Solving Equation (13)mentioning

confidence: 99%

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Observation and theory of whistler wave generation by high‐power HF waves

et al. 2013

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A new mechanism of whistler wave generation by amplitude modulated HF waves in the polar electrojet

Kuo¹,

Koretzky²,

Lee³

1999

Geophysical Research Letters

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Abstract. It is shown that a spatially distributed mode current of whistler waves, oscillating at the modulation frequency of the ground-transmitted powerful HF wave, can be induced in the Eregion of the lfigh-latitude ionosphere through the coupling between HF wave-modulated electrojet current and induced density irregularities. As shown in Kuo et. al. [1999], these density irregularities generated by the I-IF wave via a thermal instability vary periodically along the geomagnetic field. This current produces wlfisfler waves directly rather than through an antelma radiation process. Tiffs mechanism generates whistler waves in the VLF range (3-30 KHz) wi.'th reduced harmonic components. The frequencies of produced whistler waves depend on the polarization, frequency, power, and modulation scheme of the I-IF wave. It is predicted that whistler waves at frequencies around 25 KHz can be most favorably excited, awaiting the corroboration of future ionospheric heating experiments.

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VLF wave generation by amplitude‐modulated HF heater waves at Gakona, Alaska

Kuo¹,

Wu²,

Pradipta

et al. 2008

Geophysical Research Letters

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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.

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Plasma density enhancement and generation of spatially periodic irregularities in the ionospheric E‐region by powerful HF waves

Cited by 5 publications

References 13 publications

Observation and theory of whistler wave generation by high‐power HF waves

Observation and theory of whistler wave generation by high‐power HF waves

A new mechanism of whistler wave generation by amplitude modulated HF waves in the polar electrojet

VLF wave generation by amplitude‐modulated HF heater waves at Gakona, Alaska

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