We report the first evidence of artificial ionospheric plasmas reaching sufficient density to sustain interaction with a high‐power HF pump beam produced by the 3.6 MW High‐Frequency Active Auroral Program (HAARP) transmitter in Gakona, Alaska. The HF‐driven ionization process is initiated near the 2nd electron gyroharmonic at 220 km altitude in the ionospheric F region. Once the artificial plasma reaches sufficient density to support interaction with the transmitter beam it rapidly descends as an ionization wave to ∼150 km altitude. Although these initial artificial layers appear to be dynamic and highly structured, this new ability to produce significant artificial plasma in the upper atmosphere opens the door to a new regime in ionospheric radio wave propagation where transmitter‐produced plasmas dominate over the natural ionospheric plasma and may eventually be employed as active components of communications, radar, and other systems.

[1] Stimulated electromagnetic emissions (SEEs) are secondary radiation produced during active space experiments in which the ionosphere is actively heated with high power high frequency (HF) ground-based radio transmitters. Recently, there has been significant interest in ion gyro-harmonic structuring the SEE spectrum due to the potential for new diagnostic information available such as electron acceleration and creation of artificial ionization layers. These relatively recently discovered gyro-harmonic spectral features have almost exclusively been studied when the transmitting frequency is near the second electron gyro-harmonic frequency. The first extensive systematic experimental investigations of the possibility of these spectral features for third electron gyro-harmonic heating are provided here. Discrete spectral features shifted from the transmit frequency ordered by harmonics of the ion gyro-frequency were observed for third electron gyro-harmonic heating for the first time at a recent campaign at the High Frequency Active Auroral Research Program (HAARP) facility. These features were also closely correlated with a broader band feature at a larger frequency shift from the transmit frequency known as the downshifted peak (DP). The power threshold of these spectral features was measured, as well as their behavior with heater beam angle, and proximity of the transmit frequency to the third electron gyro-harmonic frequency. Comparisons were also made with similar spectral features observed during second electron gyro-harmonic heating during the same campaign. A theoretical model is provided that interprets these spectral features as resulting from parametric decay instabilities in which the pump field ultimately decays into high frequency upper hybrid/electron Bernstein and low frequency neutralized ion Bernstein IB and/or obliquely propagating ion acoustic waves at the upper hybrid interaction altitude. Coordinated optical and SEE observations were carried out in order to provide a better understanding of electron acceleration and precipitation processes. Optical emissions were observed associated with SEE gyro-harmonic features for pump heating near the second electron gyro-harmonic during the campaign. The observations affirm strong correlation between the gyro-structures and the pump-induced optical emissions.

We have used all‐sky imaging to relate different types of auroral oval disturbances to large‐scale traveling ionospheric disturbances (LSTIDs). We selected eight nights with good all‐sky imaging and Global Positioning System total electron content coverage, including five non–storm time periods with isolated initiations of geomagnetic activity and three storm main phase periods with continuous activity. Periods with LSTIDs generally started and stopped with initiation and cessation of activity. We found evidence that individual LSTIDs often show 1‐1 correspondence with identifiable auroral disturbances, disturbances either being related to a substorm onset or to auroral streamers without a substorm. Since substorm ground magnetic depressions are directly related to the electric fields and electron precipitation of auroral streamers, we hypothesize that streamers may be the primary drivers of individual nightside LSTIDs with or without a substorm. Additionally, we found evidence that (1) LSTIDs detection is more likely near the longitude range of the initiating disturbance than further away, (2) the orientation of LSTID phase fronts depends on location relative to disturbance longitude, and (3) disturbance ionospheric current and magnetic latitude may influence whether a given disturbance leads to a detectable LSTID. Numerous LSTIDs (10 to 12 over 7‐ to 8‐hr periods) were detected during southward interplanetary magnetic field periods of coronal mass ejection storm main phases, the vast majority reflecting streamers in the absence of substorms. Less LSTIDs were seen during the one examined high‐speed‐stream storm. We have also found evidence that omega band disturbances may drive interesting TIDs that are distinct from the LSTIDs driven by the substorm and streamer disturbances.