ANGEO Annales Geophysicae ANGEO 1432-0576 Copernicus GmbH Göttingen, Germany 10.5194/angeo-23-1585-2005 Artificial optical emissions at HAARP for pump frequencies near the third and second electron gyro-harmonic Kosch M. J. 1 2 3 Pedersen T. 1 Hughes J. 4 Marshall R. 5 Gerken E. 6 Senior A. 2 Sentman D. 4 McCarrick M. 7 Djuth F. T. 8 Space Vehicles Directorate, Air Force Research Laboratory, Hanscom AFB, Massachusetts, USA Communication Systems, Lancaster University, Lancaster, UK Honorary Research Fellow, University of Kwazulu-Natal, Durban, South Africa Geophysical Institute, University of Alaska, Fairbanks, USA STARLab, Stanford University, Stanford CA, USA Dept. Electrical & Computer Engineering, Cornell University, Ithaca NY, USA Advanced Power Technologies, Washington D.C., USA Geospace Research, El Segundo CA, USA 27 07 2005 23 5 1585 1592 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.ann-geophys.net/23/1585/2005/angeo-23-1585-2005.html The full text article is available as a PDF file from http://www.ann-geophys.net/23/1585/2005/angeo-23-1585-2005.pdf

High-power high-frequency radio waves beamed into the ionosphere cause plasma turbulence, which can accelerate electrons. These electrons collide with the F-layer neutral oxygen causing artificial optical emissions identical to natural aurora. Pumping at electron gyro-harmonic frequencies has special significance as many phenomena change their character. In particular, artificial optical emissions become strongly reduced for the third and higher gyro-harmonics. The High frequency Active Auroral Research Program (HAARP) facility is unique in that it can select a frequency near the second gyro-harmonic. On 25 February 2004, HAARP was operated near the third and passed through the second gyro-harmonic for the first time in a weakening ionosphere. Two novel observations are: firstly, a strong enhancement of the artificial optical emission intensity near the second gyro-harmonic, which is opposite to higher gyro-harmonics; secondly, the optical enhancement maximum occurs for frequencies just above the second gyro-harmonic. We provide the first experimental evidence for these effects, which have been predicted theoretically. In addition, irregular optical structures were created when the pump frequency was above the ionospheric critical frequency.<p><b>Keywords.</b> Active experiments – Auroral ionosphere – Wave-particle interactions