Search

Van Allen Probes observations during the 17 March 2015 major geomagnetic storm strongly suggest that VLF transmitter-induced waves play an important role in sculpting the earthward extent of outer zone MeV electrons. A magnetically confined bubble of very low frequency (VLF) wave emissions of terrestrial, human-produced origin surrounds the Earth. The outer limit of the VLF bubble closely matches the position of an apparent barrier to the inward extent of multi-MeV radiation belt electrons near 2.8 Earth radii. When the VLF transmitter signals extend beyond the eroded plasmapause, electron loss processes set up near the outer extent of the VLF bubble create an earthward limit to the region of local acceleration near L = 2.8 as MeV electrons are scattered into the atmospheric loss cone., United States. National Aeronautics and Space Administration (contract NAS5-01072)

Observations of airglow at 630 nm (red line) and 557.7 nm (green line) during HF modification experiments at the High Frequency Active Auroral Research program (HAARP) heating facility are analyzed. We propose a theoretical framework for understanding the generation of Langmuir and ion acoustic waves during magnetic zenith injections. We show that observations of HF-induced airglow in an underdense ionosphere as well as a decrease in the height of the emitting volume are consistent with this scenario., Pub. in Annales Geophysicae v23 p47-53, 2005.

We analyze airglow induced by intense radio waves at the High-frequency Active Auroral Research Program (HAARP) facility for pump frequencies f 0 near the second and third electron gyroharmonic (f c ). While passing through the double resonance in the weakening ionosphere during heating at f 0 ∼ 2f c , airglow intensities rose abruptly and remained elevated during four consecutive pulses. During this period both the thermal and parametric instabilities coexisted. Subsequently, the thermal parametric instability was hampered. The effectiveness of heating near the second gyroharmonic is ascribed to specific dispersion characteristics near the double resonance. The observations suggest that Langmuir waves participate in cyclotron acceleration when the local plasma and second gyroharmonic frequencies match. With f 0 ∼ 3f c the airglow terminated due to the suppression of cyclotron acceleration and parametric decay instability.

Observations of airglow at 630 nm (red line) and 557.7 nm (green line) during the February 2002 campaign at the High Frequency Active Auroral Research Program (HAARP) heating facility are analyzed. We find that during injections toward magnetic zenith (MZ) the green and red lines gain ^5 R within ^1 s and ^20 R within ^10 s, respectively. We term this period the onset of the HF-induced airglow. A model of the onset at magnetic zenith is developed. It accounts for background photoelectrons and dissociative recombination of O(+)2. It is shown that heating and acceleration of background electrons dominate the airglow onset. We propose a scenario for the generation of strong Langmuir turbulence for injections outside the Spitze region, including magnetic zenith., Pub. in Journal of Beophysical Research, v109, A02305,p1-10 2004.

We report on subauroral plasma density troughs observed by Defense Meteorological Satellite Program (DMSP) satellites during the major magnetic storm of 6 April 2000. The troughs were embedded within strongly irregular, greater than or equal to 500-km wide subauroral convection streams, composed of two distinctive parts. The poleward part coincided with strong wave structures, energetic (ring current) ion precipitations, and enhanced vertical ion flows. One or several narrow density decreases were present here. The equatorward part was marked by a density trough coincident with a local maximum of the electron temperature ^7000-9000 K. Here the ion temperatures and downward vertical ion velocities were ^2000 K and <20 m/s, respectively. The mean convection velocity was typically less than or equal to 500 m/s. We develop analytical approximations for the rate coefficients of the charge exchange reactions based on recent laboratory experiments. These have been used in local modeling of the equatorward F peak density depletions. Topside steady-state density profiles were evaluated assuming diffusive equilibrium in a given Te profile. A scenario for the transition of an initial density-height profile to a flat profile is described. Substantial agreement between the modeling results and DMSP observations indicates that the vibrational mechanism contributes significantly to the formation of high-Te-related density troughs., Pub. in Geophysical Research, v109 pA10301-1/A10301-9, 2004.

We report on oscillations in electric (deltaEy) and magnetic (deltaBz) fields and plasma density (deltaNi) observed by Defense Meteorological Satellite Program (DMSP) satellites within fast subauroral convection streams in the evening sector during the magnetic storm of 6 November 2001. There are two types of wave phenomena. The first and more common is characterized by electromagnetic and plasma density variations that have the same frequency range of ^0.15 Hz in the spacecraft frame of reference. The second is characterized by large-amplitude plasma and field oscillations over a broader range of frequencies ^O.1 to 0.3 Hz. In this case the perturbation densities and fields appear to have different frequency responses. In this and other magnetic storms, strong waves are associated with the precipitation of ^30 keV ions. Ratios of deltaEy/deltaBz indicate encounters with mixtures of electromagnetic (in part Alfvenic) and electrostatic modes. Poynting vectors associated with the oscillations can be directed either into or out of the ionosphere. The density perturbations appear to be extended east-west corrugations in the plasma flow streams with north-south wavelengths of ^50 km. The deltaEy and deltaNi variations were anticorrelated, as required for current conservation. Our analysis shows that Alfvenic perturbations arc consistent with expected effects of irregular potential distribution around ionospheric density irregularities mapped to the magnetosphere. Inertial currents act to generate mesoscale field-aligned currents carried by Alfven waves, as was previously discussed with regards to auroral arcs formation. We suggest that deltaNi irregularities observed by DMSP satellites in the evening sector began as striated plasma patches in the polar cap that convected to subauroral latitudes.

Experimental results from ionospheric HF pumping experiments in the nightside auroral E region above Tromsø are reported. We found intriguing evidence that a modification of the ionosphere-magnetosphere coupling, due to the effects of powerful HF waves beamed into an auroral sporadic E layer, can lead to a local intensification of the auroral activity. Summarizing multi-instrument observations during two consecutive nights, one can distinguish the following peculiarities related to this auroral activation: a modification of the auroral arc and its breakup above Tromsø, local changes in the horizontal currents above Tromsø, a burst-like increase of the electron density and temperature, a large increase in the ion temperature in a wide altitude range and in the north-south component of the electric field, distinctive features in dynamic HF radio scatter Doppler spectra, pump-induced electron precipitation, and substorm activation exactly above Tromsø. We discuss the modification of the ionosphere-magnetosphere coupling in terms of the excitation of a turbulent Alfvén boundary layer between the base of the ionosphere (∼100 km) and the level of sharp increase of the Alfvén velocity (at heights up to one Earth radius), and the formation of a local magnetospheric current system. The results suggest that a possible triggering of local auroral activation requires specific geophysical conditions.