SA11A-0286
Investigating the thermospheric response to solar flares
The Global Ionosphere Thermosphere Model (GITM) is used to simulate the response of the thermospheric density during several different solar flare events. Data from the SEE instrument onboard TIMED is used in GITM to specify the solar EUV flux. The model results run with and without high-latitude drivers are compared to mass density data from the Champ satellite in order to isolate and quantify the contribution from the excess solar energy. In addition, we will investigate the speed and uniformity of propagation of gravity waves created by the solar flares.
SA11A-0287
Analytical Expressions for Temperature Profiles in Model Atmospheres
The altitudinal variation of temperature is traditionally given in tabular form in most neutral atmospheric models. In the homosphere (troposphere, stratosphere and mesosphere), only one table is employed, which is indicative of the average conditions over the entire globe (latitude and longitude; land and water), day (hours) and year (seasons). In the thermosphere, on the other hand, many tables are used corresponding to different exospheric temperatures ranging from 600K to 2000K. It is assumed that the average temperature represents a steady-state condition governed by the time-independent thermal diffusion equation in the vertical dimension, which is nothing but a form of Fick's law in that dimension. It is found that: (1) In the troposphere, the temperature profile is best described by an exponential profile; (2) In the stratosphere-mesosphere region, a Normal distribution adequately fits the temperature profile; and (3) In the thermosphere-exosphere region, a modified inverted exponential profile best describes the observed temperature variation with altitude. By employing a least-squares fit between a simple exponential model and the observed temperature profiles above 120 km, the average exospheric temperature is estimated to be 1350K.
SA11A-0288
Precision Satellite Orbit Derived Total Density
This research uses precision satellite orbits from the Challenging Minisatellite Payload (CHAMP) satellite to produce a new data source for upper atmospheric density and changes that occur on time scales less than a day. The precision orbit derived density is compared to CHAMP accelerometer derived density to determine the accuracy of using precision orbit derived density. In addition, the precision orbit derived densities are used to examine density variations that have been observed with accelerometer data to see if they are observable. Currently highly accurate density data is available from three satellites with accelerometers and much lower accuracy data is available from hundreds of satellites for which two-line element sets are available from the Air Force. This paper explores a new data source that is more accurate and has better temporal resolution than the two-line element sets, and provides better spatial coverage than satellites with accelerometers. This data source will be valuable for studying atmospheric phenomena over short periods, for long term studies of the atmosphere, and for validating and improving complex coupled models that include neutral density. The objective of this research is to validate the use of precision orbit data to derive thermospheric density data sets using data from CHAMP. The technique for estimating density is optimal orbit determination. The estimates are optimal in the least squares or minimum variance sense. Precision orbit data from CHAMP is used as measurements in a sequential measurement processing and filtering scheme, which is a type of Kalman filter. The atmospheric density is estimated as a correction to an atmospheric model. Two corrections are applied to the model. The first is a baseline correction that is derived from historical measurements. The second is a dynamic correction based on current conditions and using the measurements.
SA11A-0289
Thermospheric Neutral Density Cooling Determined from a 40-Year Satellite Drag Database
Satellite drag measurements have proven a valuable tool for detecting long-term changes in thermospheric density. An extensive density database derived for the period 1970 – 2000 indicated an average decline in thermospheric density of about 1.7 percent per decade at 400 km altitude. Our database has been extended to fully cover the period 1966-2006. In addition subsets of data have been extended back to solar minimum in 1964 and ahead to the beginning of solar minimum in 2007. We therefore can examine trends for five solar minima. All data are generated from actual radar tracking observations to form precise orbit and drag/density data with improved accuracy and one-day resolution. Satellites with relatively high eccentricities were used to achieve long lifetimes and relatively localized latitude and local time resolution. Data are compared to the Jacchia 1970 and NRLMSIS empirical models. The model representations have also been enhanced by replacing their climatological average semiannual variations with realistic values derived for each satellite. By significantly reducing model errors, this procedure permits accurate representation of thermospheric cooling as a function of solar flux. Trends are also deduced for the first time in terms of thermospheric temperature changes. The derived thermospheric cooling is further analyzed as a function of altitude (250-550 km), latitude, season, local time, solar flux and geomagnetic activity.
SA11A-0290
Evaluation of Trends in a CHAMP 2001-2005 Density Database
A database of 5 years of CHAMP accelerometer neutral density measurements was examined for long and short term bias trends. Techniques applied were the same as those used in detecting a long term negative slope in satellite drag density to model ratios. This trend was attributed to thermospheric cooling. The straight line fits were found by standard least square methods. The estimate of the variance of the slope and the probability that the slopes were not zero was found using a Monte-Carlo technique. The reason standard estimates are not applicable is that the data is highly correlated. The Monte-Carlo technique assumed that the data was approximately equally spaced (one day apart) with missing data points. Each value for the Monte-Carlo technique was found as follows. Burg's algorithm with 50 linear predictor coefficients was used to estimate the autocorrelation of the data minus the straight line. The autocorrelation estimates were then used to estimate the power spectral density (PSD). The square root of the PSD was used to weight a set of frequency estimates from a set of pseudo Gaussian random numbers. The inverse FFT of the result plus the straight line is a simulated data set. Any missing data were placed in the simulated data set and the slope of a straight line fit to the simulated data was estimated. As an addendum to this study, CHAMP data were evaluated to detect and quantify unmodeled latitudinal variations.
SA11A-0291
Using ULF Waves Artificially Excited by the SPEAR High Power Radar as Diagnostics for Probing the High Latitude Ionospheric Alfven Resonator
The SPEAR high power radar facility located in the vicinity of Longyearbyen on Spitsbergen has been utilised to artificially excite ULF waves through the modulated heating of the Polar electrojet. The high power radar beam interacts with the plasma in the D- and E-regions leading to oscillations in the local conductivity. This leads to the radiation of Alfven waves along field lines into the magnetosphere. It has been shown that a narrow flux tube can be tagged by the presence of these waves and field aligned plasma acceleration caused by the complex interaction of the artificial ULF waves with the upper boundary of the ionospheric Alfven resonator (IAR). Recent experiments with SPEAR have focussed on investigating the IAR, which acts a resonant cavity partially trapping ULF waves. These techniques are designed to stimulate one of the resonant harmonics of this cavity in order to improve the efficiency of the process which causes the electron acceleration at the upper boundary of the IAR. This interaction may be fundamentally important to the natural formation of aurora. Measurements of the SPEAR-induced ULF waves made by a set of induction coil magnetometers newly deployed on Spitsbergen will be presented. The observations will be discussed in the context of the ionospheric conditions required for the heating process to successfully excite ULF waves and the nature of the IAR cavity on these occasions. http://www.ion.le.ac.uk/spear
SA11A-0292
Geometric modulation: A new method of ELF/VLF wave generation with continuous HF heating of the auroral electrojet
ELF (300 – 3000 Hz) and VLF (3 – 30 kHz) radio waves are very difficult to generate with practical antennae, because of their extraordinarily long (10 – 1000 km) wavelengths, and the lossiness of the Earth's surface at these frequencies. In recent decades, ELF and VLF waves have been successfully generated via amplitude modulated HF (2-10 MHz) heating of the auroral electrojet. Through the temperature dependent conductivity of the lower ionospheric plasma, a patch of the auroral electrojet becomes a large radiating body in the presence of modulated heating. Of particular note have been facilities near Tromso, Norway, and more recently, the HAARP facility near Gakona, Alaska, each of which have successfully generated ELF/VLF waves with AM HF heating, and detected the generated signal as far as 4400 km away. In this paper, we introduce a new and more powerful method of ELF/VLF wave generation, geometric modulation, which involves steering the HF heating beam in a geometric pattern without modulating its power. Utilizing results obtained from the HAARP facility, a phased HF antenna array recently upgraded to 3.6 MW of radiated power, we show that geometric modulation can strengthen ELF/VLF generation by 4-10 dB. We explore the effect of different geometric configurations, sizes and shapes, and observe signal parameters such as amplitude, phase, and polarization, for both nearby and long distance observations. Observations are placed in the context of a realistic, quantitative physical model of the HF-ELF conversion process, along with a discussion of ELF propagation in the Earth-ionosphere waveguide.
SA11A-0293
A Lack of Electron Density Production During Long-Pulse Ionospheric HF Heating Experiments
It is by now well-known that modulated high frequency (HF) heating of the lower ionosphere in the presence of the auroral electrojet current system can efficiently produce electromagnetic waves in the extremely low frequency (ELF, 3-3000 Hz) and very low frequency (VLF, 3-30 kHz) bands. One active area of research addresses the improvement of the HF-to-ELF/VLF conversion efficiency. In this paper, we experimentally evaluate the hypothesis that minutes-long HF pulses (so-called pre-heating pulses) modify the ionospheric electron density, in turn enhancing the efficiency of ELF/VLF wave generation. Recent upgrades at the High-frequency Active Auroral Research Program (HAARP) HF transmitter in Gakona, Alaska allow for dual-beam operations. During a thirty-minute period on 2 August 2007, one beam of the HAARP HF transmitter generated ELF/VLF waves (at 1215 Hz and 2430 Hz) by modulated ionospheric heating at 4.5 MHz (X-mode), stepping the peak HF power in 15 distinct log-based steps. Simultaneously, the second beam of the HAARP HF transmitter continually heated the same patch of ionosphere at 3.25 MHz (CW, X-mode) for a period of 8 minutes. This 8-minute transmission block was followed by a 7-minute period without CW heating (i.e., the first beam continued to modulate at 3.25 MHz while the second beam was OFF). The experiment was repeated twice during the 30-minute window. ELF/VLF wave observations at a ground-based receiver indicate that the electron temperature change produced by HF heating dominates the ionospheric conductivity change even for minutes-long HF heating pulses. In the case presented, ionospheric conductivity variations on the minutes-long time-scale for electron density change are not detected. It thus appears that in at least some cases, the electron density changes produced by minutes- long HF heating pulses are insignificant compared to the electron temperature changes produced (at the altitude of ELF/VLF wave generation). Possible explanations for a lack of detectable electron density change are discussed.
SA11A-0294
Aspect angle dependency of the HF modification measured with MUIR at HAARP
In this paper we present results of height-resolved observations of F-region Langmuir turbulence measured with MUIR (Modular UHF Ionospheric Radar; 446 MHz) at HAARP (High frequency Active Auroral Research Program) in Alaska, USA. The scientific objective of this paper is to study aspect angle dependency of the HF modification. The best way to achieve the objective is the simultaneous multi-position measurement with the incoherent- scatter (IS) radar. However, general IS radars take, at least, a few seconds to change the radar beam position. MUIR is the best diagnostic tool for this study because it can change the beam direction every IPP (interpulse period) with the phased array system. We conducted two experiments at HAARP; 26 March 2006 and 31 July 2007. For the March 2006 experiment, three MUIR beam positions were selected: geographical vertical, up B (elevation angle = 75 degree), and midway between the two (elevation angle = 82 degree). This experiment was arranged for studying the aspect angle dependency of Langmuir oscillations associated with low HF duty cycle (1%: on/off = 0.1s / 9.9s). The radar-backscatter spectra with 10-ms time resolution were deduced at individual radar-beam positions. For the July 2007 experiment, nine MUIR beam positions around the up B position were selected under relatively high HF duty cycle (50%: on/off = 3min/3min). The presentation will report aspect angle dependencies of (1) the Langmuir oscillation development in the first 100 ms after HF turn-on using data taken during the March 2006 experiment and (2) persistency of the Langmuir oscillation using data taken during the August 2007 experiment.
SA11A-0295
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 cascade, collapse, 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, duty-cycle, and aspect angle. In particular, we explore the aspect angle effect of increased turbulence with the HF wave directed at small angles to 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 compared to previous high latitude experiments and predictions from recent modeling efforts.
SA11A-0296
The New Arecibo High Frequency Facility: Plans for the Science and the Facility
The large electric field that occurs when a high frequency (HF) high power radio wave reflects from an ionospheric layer stimulates several physical phenomena of interest in space and plasma physics. Their study requires diagnostics such as an incoherent scatter (IS) radar, optical imagers, photometers and spectrometers, and HF receivers. As the beginning of the construction phase of the new Arecibo HF facility approaches, it is time to review the science that can be best accomplished with the facility we can build and the diagnostics we have or can support. Since the closure of the Arecibo Islote HF facility less than a decade ago improvements in information storage and processing have transformed the way experiments proceed. It is now possible to store all the voltage samples from radar and radio experiments for off line analysis while also processing on line for monitoring purposes. Receivers are now digital and have vastly improved capabilities for handling the wide bandwidth high dynamic range signals that occur in these experiments. Improvements in computing capability have also had a huge impact on the studies of plasmas, enabling multidimensional simulations. Thus we expect theoretical work on plasma instabilities and, for example, the excitation of accelerated electrons to provide new opportunities for explaining the observations in greater detail. We will begin with a description of the planned facility and its characteristics, describing its strengths and weaknesses for these experiments. The new facility, unlike the Islote facility, will use the Arecibo main dish as its reflecting antenna. This is similar in idea to the original Arecibo HF facility from the 1970s, but the new facility will have about six times the transmitter power and will illuminate the dish using a wire grid sub-reflector. We will then describe the Arecibo diagnostics. These include the extraordinarily sensitive 430 MHz IS radar and a number of optical instruments. We also have the ability to host visitor instrumentation. Finally, we will present some results from the the Islote facility, and show what improvements will be possible with the new one.
SA11A-0297
Emission of ELF/VLF Waves by a Modulated Electrojet upwards into the Ionosphere and into the Earth-Ionosphere Waveguide
The modulation of the auroral electrojet by a ground-based ionosphere heating facility is calculated by solving a kinetic equation for electron distribution function. The resulting current is used to calculate the emission of ELF/VLF waves both into ionosphere and into the Earth-ionosphere waveguide. For this purpose, we developed a finite element method of calculation of electromagnetic field in a horizontally-stratified ionosphere filled with magnetized plasma, with arbitrary harmonically-varying current distribution and the direction of geomagnetic field. This method is proven to be stable against the loss of precision due to "swamping" of useful modes by evanescent waves. The electromagnetic field is calculated both in the Earth-Ionosphere waveguide (at arbitrary horizontal distance and direction) and in the ionosphere (as a whistler mode). The presented method requires less computational resources than traditional FDFD and FDTD methods. The calculated values are compared to ground and satellite observations of electrojet emissions from modulation by existing ionosphere heating facilities. In particular, we find that the emission of the whistler waves upward into the ionosphere is contained in a relatively narrow channel, even in the absence of ducting, due to a substantial horizontal size of the emitting region. We discuss the difference in the treatment of the heating of free electron by using the kinetic approach and the approach assuming a thermal (Maxwellian) distribution of electrons.
SA11A-0298
Wavelength dependence of the linear growth rate of the Es layer instability
It has recently been shown, by computation of the linear growth rate, that midlatitude sporadic-E (Es) layers should be subject to a large scale electrodynamic instability. This instability is a logical candidate to explain certain frontal structuring events, and polarization electric fields, which have been observed in Es layers by ionosondes, by coherent scatter radars, and by rockets. As such it provides an alternative, or complementary mechanism to that proposed by Larsen, of a Kelvin-Helmholtz instability in the Neutral wind, for explaining more or less the same set of observations. However, the original growth rate derivation assumed an infinitely thin Es layer, and therefore did not address the short wavelength cutoff. Also, the same derivation ignored the effects of F region loading, which is a significant wavelength dependent effect. Herein is given a generalized derivation that remedies both these short comings, and thereby allows a prediction for the wavelength dependence of the linear growth rate, as well as computations of various threshold conditions. The wavelength dependence of the linear growth rate is compared with observed periodicities, and the role of the zeroth order meridional wind is explored.
SA11A-0299
Storm-enhanced neutral wind dynamo effects on the Earth's magnetic field
The neutral wind dynamo driven currents in the E-region of the ionosphere are one of many sources contributing to the Earth's overall magnetic field. Characterisation of the various sources is a vital step in understanding the Earth's whole magnetic environment from the core to the magnetosphere. Storm activity increases the kinetic energy in the neutral atmosphere and the enhanced motion decays gradually. This has a knock-on effect on the magnetic field at the ground at low to mid-latitudes. We have used the Coupled Thermosphere Ionosphere Plasmasphere model (CTIP) to assess the impact of the neutral particle inertia on the magnetic field. Our motivation is to quantify the magnitude of this effect such that it can be accounted for in geomagnetic field modelling. Our initial results using an artificial storm indicated a small (of the order of 10 nT) and long-lived (a few days) elevation in the magnetic field originating from this source. This has been extended to analyse the model output during a real storm event.
SA11A-0300
Results From the Study of Solar and Geomagnetic Activities
Abstract Some intense geomagnetic storm activities during the past four solar cycles, 1957-2001 have been analyzed. It was discovered that these selected geomagnetic storm events analyzed, have stronger intensity during the maximum solar activity cycle and the intensity is weaker during the minimum solar activity. It is evident from our results that the yearly intense geomagnetic storm, strongly correlate with the 11-year sunspot cycle. The monthly variations of sunspots during the maximum and minimum solar activity depict no strong correlation between the two phases. It was suggested that most of these geomagnetic storms analyzed were associated with Coronal Mass Ejections (CMEs). It is also noted that variation of large storm events depicts a kind of variation which peaks around June and September for maximum solar activity and peaks around same June and October for minimum solar activity. It was concluded that solar and geomagnetic activities are very important factors in planning and managing space missions.
SA11A-0301
Geophysical Effects of Solar Eclipse of 29 March 2006
Infrasonic, radio physical, electric and magnetic fields measurements were spent at geophysical station Mikhnevo of the Institute of Geospheres Dynamics of Russian Academy of Science located in 80 km from Moscow during a solar eclipse of 29 March 2006. During the maximum phase of the eclipse the atmosphere acoustic- gravity waves on frequency close to Brent-Vaisala frequency were excited. Global variations of the total electron content of the ionosphere were also registered. Revealing of these effects on a distance more than 2000 km from a zone of a total solar eclipse shows that perturbations of the ionosphere had global character and quickly extended on significant distances. Change of the geomagnetic field in the form of a positive bay have arise at the maximum phase of the eclipse. Change of the spectral density of the near-earth electric field variations of the frequencies of the order of 10 mHz were coincided with variations of the TEC of the ionosphere. Effects of the eclipse have been investigated also by a method of Doppler tilt sounding on short-wave radio paths crossing the trajectory of moon shadow movement under different angles. The character of a radio signals reaction reminds a blackout during solar x-ray flashes. So, the solar eclipse was accompanied by the ionosphere perturbations which could be caused both by direct effects of a change of the solar radiation, and by their imposing on the ionosphere electron density distribution above the Eastern hemisphere. Superposition of these effects has led to excitation of the ionosphere wave perturbations of different type and scale (thermosphere winds, acoustic-gravity waves) and to variations of the TEC of the ionosphere.
SA11A-0302
Investigation of Dust Charging Effects on Low-Frequency Ion Waves in Magnetized Dusty Plasmas
Dust charge fluctuations generated in a localized dust cloud expanding into background plasma across a magnetic field is considered in investigation. This configuration has important applications for dusty plasmas in space and in the laboratory. Under these circumstances, the charging time of the dust grains may be comparable to the timescales for development of fluctuations due to plasma instabilities. Therefore, dust charge fluctuations are expected to play an important role in plasma collective effects in this case. A two-dimensional numerical model that has been developed to study the evolution and dynamics of an expanding dust cloud released into plasma across the magnetic field. The background plasma electrons and ions are treated as a fluid whose density is reduced by dust charging. Fourier spectral methods with a predictor corrector time advance are used to temporally evolve the background plasma electron and ion equations. The dust is treated with a Particle-In-Cell (PIC) model in which the dust charge varies with time according to the standard dust charging model. Simulation results show that as the dust expands into the background plasma and charges up, a strongly sheared ambipolar electric field develops across the dust cloud boundary. Some of the processes that are also investigated include dust charging, reduction of the background plasma density due to dust charging. The consequences of the results of this investigation on understanding dust charge fluctuation effects in general are also discussed.
SA11A-0303 INVITED
Anti-ExB Vortex Formation in a Plasma Interacting with the Neutral Flow.
A vortex is observed in a cylindrical magnetized plasma. The characteristic feature of this vortex is that the direction of rotation is opposite to that of ExB drift, which is usually dominant mechanism for driving plasma rotation. This result suggests that there exists a force acting on the ion fluid and it exceeds the ambipolar electric field. To clarify the origin of anti-ExB rotation, detailed experiments have been carried out to find that the anti-ExB vortex always accompanies with a deep density hole in the background neutrals. The density depletion causes a flow of neutrals due to steep density gradient. When the ion energy is of the order or less than 1 eV, ions interact with neutrals through charge exchange collisions. Then, momentum possessed by the neutral flow can be transferred to the ions, and consequently an effective force against the ions arises. If this force is opposite to and dominates the electric field, the anti-ExB rotation is possible. To experimentally verify the above-mentioned mechanism, a direct observation of neutral flow is the key issue, for which we have been developing a laser induced fluorescence (LIF) spectroscopy system using a tunable diode laser. Since the neutral flow is expected to be very slow (several m/s), the spectrum of the laser should be very narrow, which is the reason why we use a tunable diode laser. We have observed the expected neutral flow with a velocity of 80m/s by using the newly developed LIF spectroscopy system. Our experimental result shows that the dynamical behavior of ions is drastically changed by the interaction with neutral flow. This mechanism provides a new route to vortex formation in magnetized plasmas. It is emphasized that charge exchange process plays an essential role in the transport of momentum.