SA51A-01
Transient Dynamics of Secondary Radiation From a HF Pumped Magnetised Space Plasma
In order to systematically analyse the transient wave and radiation processes that are excited by a HF radio wave injected into a magnetised space plasma, we have measured the secondary radiation, or stimulated electromagnetic emission (SEE), from a preconditioned ionosphere. The ionosphere was heated using a pump duty cycle of 200 ms (180 ms on and 20 ms off) and 100~ms (80 ms on and 20 ms off) for various pump frequencies near the fifth harmonic of the electron gyro frequency. The results presented here show that within the first ten milliseconds after pump turn-on, frequency downshifted structures of the SEE exhibit an overshoot with a maximum at tm≈ 8 ms, whereas upshifted spectral components do not exhibit this feature. The relative magnitude of the overshoot is strongly dependent on the frequency offset of the pump from the fifth harmonic of the electron gyro frequency. A new, blue-shifted transient frequency component is identified. This component is upshifted from the pump by 14 kHz < Δ f <50 kHz and exists only for the first ten milliseconds after pump turn-on. On a slower time scale we analyse the amplitude modulation, or "ringings", of the pump, (in the Russian literature also known as "spikes"). The ringings have a frequency fR≈ 17 Hz and we show that this phenomenon is also present in the SEE. Furthermore, the ringings in the SEE are almost perfectly synchronised to the ringings of the pump.
SA51A-02
Effects of Lower Hybrid Turbulence During Injections of HF Waves at the Magnetic Zenith
Recent optical and ionospheric radar observations during HF heating experiments at the High-frequency Active Auroral Research Program (HAARP) facility are analyzed. The observations indicate that both the thermal and parametric instabilities can coexist at magnetic zenith [Mishin et al., GRL, 32, L23106, doi:10.1029/2005GL023864, 2005] and that up to three mechanisms of electron acceleration can be acting, depending on the pump frequency relative to the electron gyro-harmonic [Kosch et al., JGR, 2007 (in press)]. Clear evidence is found for the presence of lower hybrid (LH) waves. With regards to HF heating, LH-waves have mainly been discussed in association with the downshifted maximum in the SEE spectrum. However, it is well known that nonlinear coupling of LH waves can result in electron heating and magnetic field-aligned acceleration. We focus on this aspect for the case when the dynamics of low-frequency LH waves are dominated by the lower hybrid collapse, which threshold energy density is quite low and is surely exceeded in the conditions in question. In the course of collapse, the longitudinal and transverse dimensions of cavities decrease, while the density variation increases. Ultimately, the LH-wave energy in collapsing cavities is absorbed by cold-plasma particles via Landau or transit-time damping. This produces suprathermal tails in the field-aligned electron distribution and in the transverse ion distribution. The density of accelerated electrons can be determined by assuming that resonant particles carry away all of the energy pumped into collapsing cavities. We have shown that this mechanism is effective in the energy range below 10 eV, and can thus contribute to the red- and green-line emissions. Another observable outcome of LH-cavities is coherent scattering of HF radio waves, which has likely been observed by the SuperDARN radar.
SA51A-03
Artificial Generation of ULF Waves in the Pc1 Range Using the HAARP Heater
Artificial generation of ULF waves in the Pc1 frequency range by modulation of the auroral electrojet current using the HAARP heater is reported. The results complement previous experiments using the EISCAT heater (Maul et al., Ann. Geophys. 8, 765, 1990) by including the .1-.7 Hz frequency range that was not covered in previous experiments. The experiments were performed during the period 4/ 27-5/6, 2006. The artificial ULF signals were recorded by a pair of induction magnetometers aligned with NS and EW directions. The measurement site, 12 km northwest of the heating facility, was directly below the modified ionosphere region. The measured amplitudes of the artificial ULF signals were in the range of 10 fT to several pT and comparable in amplitude and polarization to the ones generated by VLF modulation at 1 kHz, interspersed with the ULF modulation. In addition to coherent narrowband ULF waves intensification of the broadband noise between .5-5 Hz by more than 20 dB correlated with HAARP turn-on was observed on the evening of May 5, potentially indicating triggering or amplification of the Alfven- Ion Cyclotron instability in the magnetosphere. In addition to the experimental results the paper will present a theoretical analysis for generating magnetosonic waves by modulated collisionless F- region heating and injecting them efficiently into the Alfvenic duct. The potential for such a source in resolving the long standing issues related to the Alfvenic duct properties (attenuation, group and phase velocities, preferred travel directions, day-night variability, frequency cut-off, etc) will be discussed along with planned experimental tests using the completed HAARP heater at 3.6 MW.
SA51A-04
Some new diagnostic methods for the active HF heating in the D-region ionosphere
According to present models the EISCAT Heater Facility is capable of increasing the electron temperature by a factor of ten in the D-region ionosphere. However, this maximum effect has not been verified by any radar observation yet. This is mostly due to lack of appropriate incoherent scatter data analysis package for the heated D region. Here we introduce a new nonlinear MCMC fitting of the theoretical autocorrelation function to detected lag profiles. This data analysis method is applied for the previous Finnish Eiscat campaign in November 2006, where three dedicated heater and radar experiments were carried out in order to quantify the active heating effect. The standard Eiscat ARC-D program and a newly designed SIPPI experiment, which is based on the direct sampling of the backscattered signal, were used in turns for the VHF radar. All the three experiments were successful technically and in terms of geophysical conditions. Preliminary results are shown in this presentation.
SA51A-05
Unique Artificial Optical Emission Experiments on the Second Electron Gyroharmonic in the Ionosphere over Alaska
In March of 2007 an "artificial aurora" ionospheric pumping experiment was executed by the HIPAS facility, pumping on the second electron gyro-harmonic at 2.85 MHz. Diagnostics included the Kodiak SuperDARN coherent scatter radar, the new Poker Flat AMISR incoherent scatter radar and a variety of optical instruments. The optical observations were for multiple wavelengths, primary O(1D) 630 nm and O(1S) 557.7 nm, corresponding to different threshold energies. A unique aspect is the first incoherent scatter measurements during an "artificial aurora" experiment on the second electron gyroharmonic. Measurements of the electron and ion temperature are important, in conjunction with optical measurements at multiple wavelengths, because the electron heating efficiency of various wave-plasma interactions can be estimated, e.g. parametric decay instabilities or the thermal parametric instability, resulting in either upper-hybrid, lower-hybrid or Langmuir waves.
SA51A-06
First Bi-Static SuperDARN Observations of ULF Waves in SPEAR Induced Backscatter
We present the first bi-static observations from the SuperDARN CUTLASS radars of naturally occurring ULF wave
activity in artificially generated backscatter at 78°N. The backscatter was generated through O mode
heating of the F-region ionosphere using the SPEAR high power system, which is located on Svalbard. By
merging line-of-sight velocity data from the 2 radars the true ionospheric velocity vector of the flows could be
determined and compared with the magnetic perturbations observed on ground magnetometer stations in the
vicinity of the heater patch. Two large scale ULF wave events are detailed in this paper. Comparisons of the
wave characteristics as observed in the ground magnetometer and radar data are made and discussed.
http:www.ion.le.ac.uk/spear/index.html
SA51A-07
Observations of Aspect Sensitive SPEAR-Induced Enhancements in Incoherent Scatter Spectra
RF-induced plasma instabilities excited by the Space Plasma Exploration by Active Radar (SPEAR) facility give rise to characteristic spectral enhancements in incoherent scatter spectra recorded by the EISCAT Svalbard Radar (ESR), which is collocated with SPEAR. The SPEAR-induced ion and plasma line enhancements are consistent with excitation of both the purely growing mode and the parametric decay instability. The aspect sensitivity of these enhancements provides valuable information regarding the physical processes that occur within the SPEAR-affected ionospheric patch. We present observations of spectral enhancements from several directions in the magnetic meridian plane, centred on field-aligned. These direction-dependent signatures demonstrate significant variability and help shed light on possible coupling between artificial field-aligned irregularities generated at the upper-hybrid height and SPEAR-induced instabilities excited near the reflection height for O-mode-polarized radio waves.
SA51A-08
The Spectral Characteristics of Coherent HF Radar Observations of Heater-induced Irregularities and Their Geophysical Influences.
The CUTLASS HF coherent radars provide important measurements of the perturbed ionosphere during heating
experiments utilising the SPEAR high power radar, located in the Svalbard archipelago, and the EISCAT heating
facility at Tromso, northern Norway. The heaters induce field-aligned irregularities (FAIs) which provide intense
targets in the CUTLASS field of view. Once the FAIs are created, CUTLASS can diagnose dynamic phenomena
occurring in the coupled magnetosphere-ionosphere system. As a result of their long decorrelation time, the
CUTLASS backscatter from the artificial FAIs exhibit low spectral widths and, as a result, provide very accurate
measurements of flow velocities.
It has been observed that on occasions when ULF waves influence the motion of the heater-induced
irregularities, the spectral characteristics associated with the scatter are modified. In particular, enhancements in
the spectral width are often observed when particle-driven waves are detected. A number of events will be
presented where this is the case and the possible mechanisms which may lead to these spectral enhancements
will be discussed.
http:www.ion.le.ac.uk/spear
SA51A-09
UHF and HF Radar Studies of Langmuir Turbulence Experiments at HAARP
High power HF transmitters induce a number of plasma instabilities in the interaction region of overdense ionospheric plasma. Radars such as SuperDARN have been used to study artificial field-aligned irregularities (AFAI) created by the high power HF radiowave at the HAARP Ionospheric Observatory, Gakona, AK. A new Modular UHF Ionospheric Radar (MUIR) sited at HAARP, may now be used to monitor changes in the Langmuir plasma waves detected in the UHF backscatter. We report the results from recent campaigns using these new facilities in coordinated and comprehensive studies of strong Langmuir turbulence (SLT). Among the effects observed and studied are: SLT spectra including the outshifted plasma line or free-mode, appearance of a short timescale ponderomotive overshoot effect, temporal evolution of SLT, dependence of SLT on growth or suppression of AFAI, dependence of AFAI and MUIR backscatter on HAARP pulselength and duty-cycle, aspect angle dependence of the intensity of the plasma line. In particular, we explore the observed magnetic-zenith effect of increased turbulence with the HF wave directed up the field line. Langmuir modes parallel to the geomagnetic field are proposed to explain other features in stimulated electromagnetic emissions (SEE). These plasma waves are theorized to play a key role in certain features of radio-induced aurora. Experimental results are then compared to previous high latitude experiments and predictions from recent modeling efforts.