E. A. Gerken

Telescopic imaging of sprites

Telescopic images of sprites show a wide variety of generally vertical but also slanted fine structure, including branching tree-like shapes and well defined but isolated columns, with transverse spatial scales ranging from tens of meters to a few hundred meters at ∼60–85 km altitude. Simultaneous analysis of radio atmospheric and lightning data indicates that specific columnar regions are selectively excited by successive discharges.

Magnetic zenith enhancement of HF radio-induced airglow production at HAARP

[1] Airglow production at various beam positions relative to the magnetic field was investigated as part of an optics campaign at HAARP in February 2002. Strong emissions up to several hundred Rayleigh at 630.0 nm and more than 50 R at 557.7 nm were produced in a small spot approximately 6° in diameter located near the magnetic zenith when the transmitter beam was directed up the magnetic field. This effect was observed hundreds of times over a wide range of frequencies and ionospheric conditions. The spot at HAARP appears on average just equatorward of the nominal magnetic field direction, deflects somewhat toward the beam center when the beam is scanned, and varies slightly in size with transmitter frequency. Red-to-green ratios as low as 3 were observed, with both wavelengths showing significant onset delay. Identifiable enhancements in red-line emission were produced down to 2 MW ERP in a power ramp experiment. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 2403 Ionosphere: Active experiments; 2483 Ionosphere: Wave/particle interactions; 2487 Ionosphere: Wave propagation (6934); 2494 Ionosphere: Instruments and techniques. Citation: Pedersen, T. R., M. McCarrick, E. Gerken, C. Selcher, D. Sentman, H. C. Carlson, and A. Gurevich, Magnetic zenith enhancement of HF radio-induced airglow production at HAARP, Geophys. Res. Lett., 30(4), 1169, doi:10.1029/2002GL016096, 2003.

Observations of decameter-scale morphologies in sprites

Abstract Using telescopic imaging it is observed that sprite morphologies such as upward branching, downward branching, beading, and columns can assume a wide variety of shapes, sizes, and time scales. Beads at the base of columniform sprites can glow for over 100 ms while slowly drifting upward. Faint positive streamers are observed at the base of angel sprites and immediately prior to bright sprite events. A transition region between streamer formation and diffuse glow is observed at ∼80 km altitude. Telescopic images containing these features are presented for a number of cases.

Creation of visible artificial optical emissions in the aurora by high-power radio waves

Generation of artificial light in the sky by means of high-power radio waves interacting with the ionospheric plasma has been envisaged since the early days of radio exploration of the upper atmosphere, with proposed applications ranging from regional night-time street lighting to atmospheric measurements. Weak optical emissions have been produced for decades in such ionospheric ‘heating’ experiments, where they serve as key indicators of electron acceleration, thermal heating, and other effects of incompletely understood wave–particle interactions in the plasma under conditions difficult to replicate in the laboratory. The extremely low intensities produced previously have, however, required sensitive instrumentation for detection, preventing applications beyond scientific research. Here we report observations of radio-induced optical emissions bright enough to be seen by the naked eye, and produced not in the quiet mid-latitude ionosphere, but in the midst of a pulsating natural aurora. This may open the door to visual applications of ionospheric heating technology or provide a way to probe the dynamics of the natural aurora and magnetosphere.

Impact of vibrational excitation on ionospheric parameters and artificial airglow during HF heating in the F region

[1] Vibrational excitation by the impact of electrons on molecular nitrogen in the energy range of 1.8-3.2 eV is well known. Electrons heated by high-power HF radio waves can effectively lose energy due to this vibrational excitation, which, in turn, creates a sharp energy barrier. Electrons must overcome this barrier in order to reach higher energies. A model for this vibrational barrier has been developed, and deviations of the electron energy distribution from a Maxwellian distribution have been estimated under different conditions. The depletion of electrons with energy higher than 2 eV decreases the excitation rate of the optical emissions (for example, 630.0 nm airglow) observed during HF heating. We show that the vibrational barrier can explain why the 630 nm airglow reported by Gustavsson et al. [2001] was only slightly enhanced during HF heating, in spite of the measured electron temperature being higher than 3000 K. In some cases, vibrationally excited molecules can accumulate in the ionosphere due to slow deactivation. These species increase the recombination coefficient, which, in turn, may result in the decrease of electron density. This effect can also contribute to the saturation of the optical emission strength as a function of heater power, as reported by Pedersen et al. [2003].

Imaging of ionospheric density structures and plasma drifts using artificial illumination by high-power radio waves

Artificial aurora from energetic electron fluxes may be generated by powerful radio waves beamed into the ionosphere. This radio induced aurora (RIA) provides optical diagnostics for measurements of field aligned irregularities, artificial plasma cavities, and plasma drifts in the E and F layer ionosphere. High-power radio waves coupled with low-light-level optical sensors provide unique measurements of the upper atmosphere.