H. C. Carlson

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.

Magnetic zenith effect in ionospheric modifications

The theory of ionospheric modification for the beam of powerful radio emission directed along magnetic field lines is developed. Nonlinear process of beam self-focusing on striations is shown to determine strong amplification of heating and acceleration of plasma electrons. It results in a dramatic enhancement of optic emission from the magnetic zenith region in ionospheric F-layer. An excellent agreement between the theory and recent fundamental observations at HAARP facility (Alaska) [T. Pedersen et al., Geophys. Res. Lett. (2002), in press] is demonstrated.

Implications of the altitude of transient 630-nm dayside auroral emissions

The altitude from which transient 630-nm (“red line”) light is emitted in transient dayside auroral breakup events is discussed. Theoretically, the emissions should normally originate from approximately 250 to 550 km. Because the luminosity in dayside breakup events moves in a way that is consistent with newly opened field lines, they have been interpreted as the ionospheric signatures of transient reconnection at the dayside magnetopause. For this model the importance of these events for convection can be assessed from the rate of change of their area. The area derived from analysis of images from an all-sky camera and meridian scans from a photometer, however, depends on the square of the assumed emission altitude. From field line mapping, it is shown for both a westward and an eastward moving event, that the main 557.7-nm emission comes from the edge of the 630 nm transient, where a flux transfer event model would place the upward field-aligned current (on the poleward and equatorward edge, respectively). The observing geometry for the two cases presented is such that this is true, irrespective of the 630-nm emission altitude. From comparisons with the European incoherent scatter radar data for the westward (interplanetary magnetic field By > 0) event on January 12, 1988, the 630-nm emission appears to emanate from an altitude of 250 km, and to be accompanied by some 557.7-nm “green-line” emission. However, for a large, eastward moving event observed on January 9, 1989, there is evidence that the emission altitude is considerably greater and, in this case, the only 557.7-nm emission is that on the equatorward edge of the event, consistent with a higher altitude 630-nm excitation source. Assuming an emission altitude of 250 km for this event yields a reconnection voltage of >50 kV during the reconnection burst but a contribution to the convection voltage of >15 kV. However, from the motion of the event we infer that the luminosity peaks at an altitude in the range of 400 and 500 km, and for the top of this range the reconnection and average convection voltages would be increased to >200 kV and >60 kV, respectively. (These are all minimum estimates because the event extends in longitude beyond the field-of-view of the camera). Hence the higher-emission altitude has a highly significant implication, namely that the reconnection bursts which cause the dayside breakup events could explain most of the voltage placed across the magnetosphere and polar cap by the solar wind flow. Analysis of the plasma density and temperatures during the event on January 9, 1989, predicts the required thermal excitation of significant 630-nm intensities at altitudes of 400-500 km.

Continuous observation of cusp auroral dynamics in response to an IMF By polarity change

The IMF BY control of the motion pattern and the location of cusp auroral activity has been continuously monitored in a case when the BY component of the interplanetary magnetic field (IMF) went from −15 nT to +15 nT in the course of 20 minutes. This is the first time a direct observation of an IMF BY regulated shift in the longitudinal position of the auroral cusp has been documented. The IMF BY regulation of the cusp is regarded to be a unique signature of magnetopause reconnection. In this particular case the zonal shift of the cusp centre is estimated to be at least 3 hours in magnetic local time. The observations indicate that the cusp reconfigured within a few minutes after the BY polarity change imposed on the magnetopause.

Large airglow enhancements produced via wave‐plasma interactions in sporadic E

In the past there has been great interest in monitoring enhanced 557.7 nm O(¹S) emissions from the thermosphere in connection with high-power, high-frequency (HF) radio wave modification of the F region ionosphere. These emissions are considered to be evidence that the HF-modified electron distribution function is non-Maxwellian because a significant flux of ∼5–6 eV electrons is required to produce the airglow. The suprathermal tail is believed to develop as a result of nonlinear plasma processes. Past F region observations of 557.7 nm airglow at Arecibo Observatory, Puerto Rico have yielded only a few Rayleighs of enhanced emissions. Recently, airglow enhancements were monitored in sporadic E above Arecibo. Surprisingly, these experiments yielded ∼55 Rayleighs of enhanced 557.7 nm airglow and the first observations of emissions from the N2 first positive molecular bands. The observations imply that a large flux of energetic (5–10 eV) electrons is generated as part of the wave-plasma interaction in sporadic E.

Self-oscillations and bunching of striations in ionospheric modifications

Abstract Based on the theory of stationary striations created in ionospheric modifications by powerful radio waves, as developed by Gurevich et al. [Phys. Lett. A 206 (1995) 247], the existence of self-oscillations of striations and of a large scale nonlinear structuring in a modified region is predicted. The theory is shown to be in agreement with existing observational data. Direct in situ rocket measurements of the electron temperature inside the striations, analogous to the recent plasma density measurements [M.C. Kelley et al., J. Geophys. Res. 100 (1995) 17 367], are desirable.

Nonlinear structuring and southward shift of a strongly heated region in ionospheric modification

Abstract The theory of self-focusing on striations for the powerful radio wave beam propagating in the northern or midlatitude ionosphere is developed. It is demonstrated that focusing lead to formation of a large scale nonlinear structure aligned magnetic field. It consists of a number of closely packed solitons — bunches of striations. The anomalous absorption of a pump wave trapped in solitons lead to the strong ohmic heating of electrons in the focused region. The theory is in a good agreement with the results of recent observations of optic emission and temperature enhancement in northern ionospheric modification experiments.

Nonlinear structuring of the ionosphere modified by powerful radio waves at low latitudes

Abstract The problem of the nonlinear structuring of the modified ionosphere due to the self-focusing of the pump wave on the bunches of striations is investigated. Two main conditions of self-focusing are formulated: (1) propagation of the pump wave quite along the magnetic field for effective excitation of striations, and (2) trapping of the pump wave by large-scale irregularities. It is shown that both conditions can be easily satisfied for small inclination angles α of the magnetic field to the vertical. A detailed study of the low latitude case was performed using model calculations of the pump wave propagation. It is shown that at low latitudes self-focusing conditions also can be satisfied but mostly for the special form of the large-scale irregularities and mostly in the southern part of the pump wave beam. These results may reconcile apparent differences between radiowave and rocket probing of the irregularities.

Parametric decay of upper hybrid plasma waves trapped inside density irregularities in the ionosphere

Abstract The parametric decay of an upper hybrid (UH) wave into a lower hybrid (LH) wave and a downshifted UH wave inside inhomogeneous ionospheric striations, when both pump and decay UH modes are trapped, is investigated. The threshold and the growth rate of the instability are determined. The possibility of the existence of both main features in the usually observed spectrum of stimulated electromagnetic emission (SEE), downshifted maximum and long continuum tail, is demonstrated. The conditions of the excitation of the second and third decay modes are established.

Langmuir Turbulence in Ionospheric Plasma

A kinetic theory is developed for strong Langmuir turbulence in the region of the reflection of a high-power ordinary radiowave in ionospheric plasma. The structure and quantity of the cavitons that form in the stage of well-developed turbulence are determined. The acceleration of electrons is investigated, and it is found that the electron distribution function acquires a significant tail with an effective temperature Teff of 50 to 100 times the plasma temperature. The region occupied by fast electrons is hundreds of times thicker than the layer of Langmuir turbulence. The theoretical results are shown to correlate well with the observational data on the electron acceleration and plasma emission in ionospheric experiments.