SA21C-0362 0800h
High latitude structuring of plasma patches: present and future
Results from the three-dimensional nonlinear simulations of the gradient drift instability to study structuring in high latitude plasma patches are presented. The high-resolution simulation data have a dynamic range that allows us to make detailed comparisons with the observed density and velocity fluctuation spectra. We use the measured ion density and horizontal velocity data from the Dynamic Explorer 2 (DE 2) spacecraft examined by Kivanc and Heelis [1997] to compare them with our simulations. We have demonstrated that the existence of the mesoscale structures on the edges and inside a patch is due to the nonlinear evolution of the gradient drift instability, which occurs initially on the trailing edge and penetrates through the entire patch. The magnitudes of the density and electric field fluctuations and their spectra were studied and compared with the simultaneous density and electric field spectra obtained from the noon-midnight and dawn-dusk orbit data of the DE 2 satellite [Basu et al., 1990]. The observed asymmetry in the simulated electric field spectra in these two directions was demonstrated. The spectral slope for the simulated density fluctuations is steeper when compared with the spectral index from the observations [Basu et al., 1990]. We will discuss the future directions for our simulations of mesoscale structures in high-latitude plasma patches.
SA21C-0363 0800h
3-D Neutral Polar Wind Model
The high-latitude polar-cap region is an extremely dynamic region with many interesting phenomena associated with it, including: the ion and neutral polar winds, plasma patches, polar holes, etc. In order to better understand the coupling between the ionosphere and magnetosphere via the high-latitudes a new 3-D hydrodynamic model has been developed that solves the continuity, momentum, and simplified energy equations for five species (H$^{+}$, O$^{+}$, H$_{s}$, O$_{s}$, and e$^{-}$). The early studies conducted using a 1-D version of the model show a high outflow flux of neutral hydrogen in the polar cap, which is on the order of 10$^{9}$ cm$^{-2}$s$^{-1}$ at 1000 km. The new 3-D model also shows a substantial outflow flux of neutral hydrogen, on the order of 10$^{9}$ cm$^{-2}$s$^{-1}$, over the whole polar cap region. The neutral oxygen, on the other hand, has a relatively small, or negative, outflow flux. However, the new 3-D model shows that there is a substantial vertical flux at 1000 km, which is on the order of 10$^{9}$ cm$^{-2}$s$^{-1}$. This suggests a continual flow of hydrogen ions and neutrals into the magnetosphere from the ionosphere, while the oxygen ions and neutrals need some form of energization, such as auroral energization, frictional heating, or some other source, in order to escape into the magnetosphere. With the inclusion of particle precipitation and magnetospheric electric fields, the 3-D model indicates an increased outflow of ions in areas of increased auroral heating, and that the neutrals produced in charge exchange reactions flow in all directions due to the convection electric fields.
SA21C-0364 0800h
Globally Simulated Poynting Flux Into Earth's Ionosphere as a First Step to Simulating Auroral Ion Outflow
The LFM and TING codes numerically calculate and describe the properties of the earth's magnetosphere and ionosphere, respectively. Coupling in the spatial gap between TING and LFM has been implemented electrodynamically, but currently, no mass transfer occurs between the two computational domains and the exclusive source of the magnetospheric plasma population is the solar wind. In reality, some of the magnetosphere's plasma may come from an outflow of heavy ionospheric ions that are energized during geomagnetic storms. One of the primary modes of outflow evidently occurs when the topside ionospheric plasma is energized by magnetic field oscillations in the ultralow frequency range below and up to the oxygen gyrofrequency. These transverse oscillations are carried by Alfvän waves that originate in the outer magnetosphere via a dynamo action resulting from the motional electric field of the solar wind plasma. In order to determine the influence of Alfvän waves on ion outflow, Poynting flux diagnostics for the inner boundary of the LFM magnetosphere have been developed. Results for the Poynting flux at the magnetosphere's inner boundary are shown to vary greatly with sampling rates, data filtering techniques, and designation of the background magnetic field. Poynting flux statistical studies have been performed using in situ satellite measurements. For example, Keiling et al. used data from the Polar satellite, Gary et al. used data from DE 2, and Korth et al. used data from Iridium and SuperDARN. The purpose of this research is to evaluate the Poynting flux into the ionosphere from LFM. It will be compared with the Poynting flux magnitudes and morphology obtained via satellite and radar measurements by Keiling, Gary, and Korth, in order to quantify the energy available to facilitate auroral ion outflow, and eventually develop a mass transfer coupling module between the LFM and TING simulations. 1. Keiling, A., et al., The Global Morphology of Wave Poynting Flux: Powering the Aurora, Science 299, 383, 2003. 2. Gary, J., et al., Summary of field-aligned Poynting flux observations from DE 2, Geophysical Research Letters 22, 1861, 1995. 3. Korth, H., et al., Intercomparison of ionospheric electrodynamics from the Iridium constellation with global MHD simulations, Journal of Geophysical Research 109, A07303, 2004.
SA21C-0365 0800h
Numerical Modeling of the Dynamics and Composition of a Nighttime Intermediate Layer
A NASA sounding rocket launched from Wallops Island, VA (37.84 N, 75.48 W) on 1 July 2003 at 2:50 EST made the first in-situ measurement of the relative concentrations of Fe+, Mg+, O2+, and NO+ while making simultaneous measurements electric fields within what we have identified as a nighttime intermediate layer below 140 km altitude. This payload was preceded by two TMA chemical tracer releases at 12:19 and 1:41 EST and followed by another at 3:07 EST. The measured wind, composition, and electric field profiles have been used as inputs into a time-dependent numerical model in order to asses their roles in the dynamics and evolution of the layer.
SA21C-0366 0800h
Coordinated Observation of Large-scale TIDs by SuperDARN and GEONET
We present results of the multi-event analysis of the relationship between mid-latitude large-scale traveling ionospheric disturbances (LSTIDs) observed by the GPS receiver network in Japan (GEONET), which have a period of 30 to 120 minutes and longer than 1000 km wavelengths, and the high-latitude ionospheric disturbances observed by the SuperDARN radars. Among the mid-latitude LSTID events observed by GEONET between July 2002 and December 2003, we found 5 examples where the Alaskan SuperDARN radars observed enough amounts of ground scatter echoes. For all of these events the radar data showed long-period (1 to 1.5 hours) oscillation in the Doppler velocities, probably corresponding to the vertical oscillation of the ionosphere, about 2-hours prior to the observation of mid-latitude LSTIDs. This is consistent with the general recognition that LSTIDs are ionospheric manifestations of large-scale gravity waves excited by the energy input from the magnetosphere to the auroral ionosphere. The time delays between these high and middle latitude LSTIDs are consistent with the difference in latitude and the observed propagation speed. Detailed discussion on the propagation and damping mechanism of LSTID will be presented.
SA21C-0367 0800h
On the Sensitivity of Total Electron Content (TEC) to Upper Atmospheric/Ionospheric Parameters
The total electron content (TEC) is a key ionospheric parameter for various space weather applications. Over the past decade an extensive amount of TEC measurements has become available from both space- and ground-based observations, and these measurements have established the general morphology of the global TEC distributions. In particular, the TOPEX TEC measurements have shown strong longitudinal variations of TEC in addition to the observed day-to-day variabilities. To better understand the observed TEC variations and to better guide its modeling, we have studied the sensitivity of quiet-time TEC to key atmospheric and ionospheric parameters such as the neutral densities, neutral wind, neutral and plasma temperatures, topside flux (plasmaspheric flux), and the $O$-$O^+$ collision frequency. These parameters are often only roughly known and can cause large uncertainties in model results. For this study, we have developed a mid-latitude ionospheric model, which solves the momentum and continuity equations for the $O^+$ density and a simplified set of equations for the $H^+$ density. To obtain TEC, the calculated ion densities are integrated from the bottom altitude (100 km) to the altitude of the TOPEX satellite (1336 km). Our study shows that during daytime the neutral wind and temperature variations have dominant effects on TEC. In particular, the zonal component of the neutral winds can have a large effect on TEC in the southern hemisphere where the declination angle is large. During nighttime, all of the above-mentioned parameters can have significant effects on the TEC (to 1336 km) morphology, except for the exospheric and plasma temperatures, which show very small effects on the nighttime TEC since their contributions to the peak and the topside ionosphere mostly counterbalance each other.
SA21C-0368 0800h
High-latitude Momentum Force Balance in the Lower Thermosphere : Dependence on the Interplanetary Magnetic Field
At high-latitude lower thermospheric winds are forced primarily by non-uniform solar heating, atmospheric tides and other waves coming from below, and energy and momentum sources associated with magnetosphere-ionosphere coupling, such as the magnetospheric convective electric field and the precipitation of energetic electrons and ions into the auroral regions. To understand the physical processes that control the high-latitude lower thermospheric dynamics, we quantify the forces that mainly contribute to maintaining the high-latitude lower thermospheric wind system, and we also examine the resulting momentum force balance with the aid of the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM). Momentum forcing is statistically analyzed in geomagnetic coordinates, and its behavior with respect to the magnitude and orientation of the interplanetary magnetic field (IMF) is further examined in this study. Differences between winds and forces for different IMF orientations are analyzed.
SA21C-0369 0800h
Modeling ionospheric electric fields and currents with combined magnetospheric and ionospheric sources
We describe a model of ionospheric electrodynamics that forms part of the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General-Circulation Model (NCAR TIE-GCM), and use it to examine some properties of ionosphere-magnetosphere electric fields and currents. Electrical coupling with the magnetosphere is represented by a combination of three possible effects: (1) a specified net field-aligned current into or out of conjugate points of both the northern and southern ionospheres; (2) a high-latitude reference electric field with an accompanying reference conductance that determines how strongly the actual electric field is pulled toward the reference field; and (3) region-2 field-aligned currents computed from the divergence of field-perpendicular magnetospheric currents which are related to the electric field through equivalent magnetospheric conductances. In addition, the ionospheric wind dynamo contributes to the electric fields and currents at all latitudes. We illustrate the influences of the different sources on electric fields and currents at high and low latitudes with a TIE-GCM simulation for solstice conditions, when the field-aligned currents are asymmetric between the two hemispheres.
SA21C-0370 0800h
2D Modeling of Geomagnetic Induction near Coastal Regions
Accurate calculation of the electric and magnetic fields induced in the Earth by time-varying ionospheric currents is an important step in computing the strength of geomagmatically induced currents (GICs) and in accurately inferring the strength and location of the source currents from ground-based magnetometers. The complexity of the Earth's conductivity adds to the difficulty of the problem, particularly near coastal regions where the conductivity of salt water is several orders of magnitude larger than that of the surrounding land mass. We demonstrate the use of a numerical technique known as the method of auxiliary sources (MAS) to calculate the induced electric and magnetic fields, resulting from a uniform source, near a 2D modeled coast. Dependence of the resulting fields on frequency, conductivity, and geometry are shown.
SA21C-0371 0800h
Preliminary Electron Density Reconstruction of April 2002 Geomagnetic Storms
Following the April 20, 2002 geomagnetic storm, the near Earth space environment experienced a fairly smooth recovery over April 21-22, 2002. The Dst index reached a minimum level of -148 nT at 0900 UTC on April 20, 2002. The Kp level of 7.33, reached between 0300 and 0700 UTC, indicated that the April 20, 2002 storm was a strong G3 geomagnetic storm. Following the peak at 0900 UTC, the Dst values returned to between -40 and -50 nT during April 21-22, 2002. Similarly, Kp returned to an undisturbed value of 2.67 around 2200-2400 UTC on April 20, and remained under 3 for most of April 21-22. Using the Ionospheric Data Assimilation Three Dimensional, an objective analysis algorithm, preliminary space weather maps of the electron density demonstrate the global ionospheric behavior during this storm recovery. These maps are generated from available electron content and electron density measurements.
SA21C-0372 0800h
Polar cap Artificial Auroras
It is well established that high-power high-frequency radio waves, when beamed into the F-layer ionosphere, accelerate electrons 1-2 orders of magnitude above thermal levels. These electrons collide with the neutral oxygen atoms and nitrogen molecules, which subsequently produce optical emissions identical to those in natural auroras. Artificial optical emissions are one of few methods available to directly detect energetic electrons in the ionosphere. The mechanism of acceleration remains under debate but there is evidence for turbulent upper-hybrid and Langmuir electrostatic waves. Artificial optical emissions have been observed at low latitudes (e.g. Arecibo in Puerto Rico), mid-latitudes (e.g. SURA in Russia), and at high latitudes (e.g. EISCAT in Norway and HAARP in Alaska). The electron accelerating mechanisms are sensitive to the magnetic field aspect angle to the pump beam, hence the need to reproduce the phenomenon at different latitudes. Here we report on the first attempt within the polar cap using the SPEAR facility on Svalbard, where the magnetic dip angle is only 8 degrees. Svalbard also has the unique situation of being under the cusp during the daytime whilst the ground is in total darkness during the winter months. In addition, Svalbard is the only location currently available where in-situ rocket measurements are possible within an artificial aurora. Plans for a future launch are discussed.
SA21C-0373 0800h
Generation and evolution of heater-induced density irregularities due to self-focusing instabilities in the ionosphere
Simulations of the generation and evolution of the ionospheric density irregularities due to self-focusing instabilities (SFI) are presented. A nonlinear, self-consistent development of the instabilities is studied with the 2D model of HF radio wave propagation in inhomogeneous magnetized plasmas. The full-wave model is coupled with the density and temperature equations. In these studies, the beam is incident vertically or tilted to the magnetic zenith direction. Our results demonstrate that irregularities generated and developed due to a local heating of the anisotropic ionospheric plasma are density depletions (striations) with a strong enhancement of electron temperature inside striations elongated along magnetic field lines. The reflection point of the ordinary wave is shifted upward with increasing density perturbations. During the evolution of the SFI, the formation of bunches of striations is observed. The number of 0.4-4 km bunch scale irregularities is increased when the beam is tilted to the magnetic zenith direction. The presented results are relevant to the experiments carried out at mid-latitude SURA HF heating facility, Russia and at high-latitude facility near Tromso, Norway
SA21C-0374 0800h
Evidence of Naturally Occurring Wave-Wave Interactions in the Polar Ionosphere and its Relation to Naturally Enhanced Ion Acoustic Lines
In this paper we present the first observational evidence of naturally occurring wave-wave interactions in the polar ionosphere, using data obtained with the EISCAT Svalbard Radar (ESR). The simultaneous and co-located observations of enhancements in the ion and plasma lines can only be explained in terms of wave-wave interactions taking place within the scattering volume. These measurements strengthen previous indications that Langmuir wave decay/turbulence triggered by a low energy electron beam associated with the aurora may be the source of Naturally Enhanced Ion Acoustic Lines (NEIAL) observed with incoherent scatter radars over the last decade. Our main objectives and results in this paper are: (1) To show that our measurements represent experimental proof that wave-wave interactions take place in the ionosphere naturally. (2) To investigate the possibility that this wave-wave interaction is causally related to the NEIAL.
SA21C-0375 0800h
Enhanced Aurora and Instabilities in Thin Ionization Layers
Thin layers of enhanced luminosity are commonly observed in auroral displays. There is a substantial body of evidence that connects these displays with thin, dense, heavy ion layers in the E-region. Based on the spectral characteristics of the enhanced layers, it is believed that the enhanced emissions result when wave-particle interactions heat ambient electrons to energies at or above the 17 eV ionization energy of $\rm N_2$. We investigate instabilities that could occur in dense, heavy ion layers in the presence of strong cross-field currents that accompany electron precipitation. We present analytical full-wave solutions and electrostatic particle simulations of the nonlinear development of the instability. The heavy ion layer increases the growth rate of the instability and heats ambient electrons into a suprathermal tail that could produce the enhanced emissions.
http://w3.pppl.gov/~jrj/ionization.html
SA21C-0376 0800h
Role of Cavitons in Electromagnetic Wave Interactions with Ionospheric plasmas
Cavitons, nonlinear density cavities created by localized enhanced electric fields, can explain the lowering of thresholds in parametric instabilities, the trapping of waves, accelerated electrons and optical emission. With the support of laboratory observations and computer modeling, it is postulated that energetic electrons are produced in density cavities or field-aligned density striations. Experimentally it is optimal to steer the EM wave along the earth's magnetic field ({\bf B}) to cause conversion of EM waves to localized ES waves that are trapped inside cavitons [Wong et al., 1981]. Electrons are accelerated by series of cavitons both along and across the earth's magnetic field. We will demonstrate that electrons are efficiently accelerated at the second harmonic of the electron cyclotron frequency $2f_{ce}$ due to finite larmor radius effects. This finding also explains the requirement of the optimal propagation direction along {\bf B}. Laboratory experiments and computer modeling have demonstrated the acceleration of electrons and ions by self-consistent enhanced ES fields. Optical, SEE and EM measurements of polarization in field experiments are presented to support our picture of cavitons responsible for such accelerations. Our model predicts strong elliptical polarization of electric fields with axes both parallel to and perpendicular to B in the waves reflected from the critical layer. Experiments on detection of such polarizations and the lowering of thresholds at frequencies near $2f_{ce}$ will be reported. Wong, A.Y. et al., JGR, 86(A9), pp 7718-32, 1981
SA21C-0377 0800h
ELF/VLF Waves Generated by an Artificially-Modulated Auroral Electrojet Above the HAARP HF Transmitter
Naturally-forming, global-scale currents, such as the polar electrojet current and the mid-latitude dynamo, have been used as current sources to generate electromagnetic waves in the Extremely Low Frequency (ELF) and Very Low Frequency (VLF) bands since the 1970's. While many short-duration experiments have been performed, no continuous multi-week campaign data sets have been published providing reliable statistics for ELF/VLF wave generation. In this paper, we summarize the experimental data resulting from multiple ELF/VLF wave generation campaigns conducted at the High-frequency Active Auroral Research Project (HAARP) HF transmitter in Gakona, Alaska. For one 14-day period in March, 2002, and one 24-day period in November, 2002, the HAARP HF transmitter broadcast ELF/VLF wave generation sequences for 10 hours per day, between 0400 and 1400 UT. Five different modulation frequencies broadcast separately using two HF carrier frequencies are examined at receivers located 36, 44, 147, and 155 km from the HAARP facility. Additionally, a continuous 24-hour transmission period is analyzed to compare day-time wave generation to night-time wave generation. Lastly, a power-ramping scheme was employed to investigate possible thresholding effects at the wave-generating altitude. Wave generation statistics are presented along with source-region property calculations performed using a simple model.