Dynamics and Electrodynamics I Posters
Presiding: M Oppenheim, Center for Space Physics, Boston University; G Crowley, Southwest Research Institute
SA21A-01 0830h
Measurement of TID and Gravity Wave Parameters Using An HF Doppler System
The manifestation of atmospheric gravity waves (AGWs) in the ionosphere is called a traveling ionospheric disturbance (TID). TIDs can be thought of as traveling corrugations in the ionosphere, and as such can seriously affect HF radio communications and surveillance systems. They may indirectly play a greater role in disrupting communications by triggering the growth of ionospheric instabilities, resulting in scintillation of radio signals. It is therefore of great interest to monitor TIDs on a routine basis, and to correlate their properties with other phenomena. In this paper, we present data from a unique radio technique for measuring TID properties such as their spectrum, and their spectrally resolved propagation characteristics. One of the most sensitive methods for detecting transient changes in the ionosphere is the HF Doppler technique operating in the 3-10 MHz band. HF Doppler systems have advantages over all other techniques for the measurement of TID characteristics. They are more amenable to analysis than data from ionosonde chains, and their time resolution (30 sec) is much higher than that of ionosondes . Unlike total electron content (TEC) methods, which respond to height-integrated TID effects, the HF Doppler radar responds to TIDs at the altitude of the radio reflection point. Finally, HF Doppler systems have low power consumption, so that both spatial and temporal resolution can be maintained for many days without the costs that would be associated with an incoherent-scatter radar. SwRI recently designed, built and deployed an HF Doppler sounding system in Texas, to investigate TIDs. The TIDDBIT radar consisted of three transmitters (Austin, Uvalde and St. Hedwig) and a receiver in San Antonio, Texas. Using a cross-spectral analysis technique, TID speeds and azimuths were obtained for each wave frequency. We provide a synoptic survey of the TID characteristics observed over Texas during January-March 2002. The Doppler system provides an accurate measure of both the TID and AGW periods, and the TID velocities are also an accurate estimate of the underlying gravity wave horizontal and vertical trace velocities. Such a system could be usefully deployed to monitor the properties of bottomside F-region undulations that are thought to trigger ionospheric instabilities in low latitude regions.
SA21A-02 0830h
Implications of Auroral Electron Precipitation During the JOULE Experiment
Four sounding rockets for the JOULE mission were launched three minutes apart during a substorm event on 27 March 2003 from Poker Flat Research Range in Fairbanks, Alaska. Two rockets carried scientific instruments for in-situ measurements of phenomena in the auroral region. The other two rockets carried chemical payloads suitable for measuring the neutral winds. This work focuses on a calculation of the electrojet current using two independent methods, with good agreement: (1) In-situ determination of the electric fields and charge-carrier mobilities, and (2) Ground-based measurements of the electrojet current. Implications for calculations of Joule heating will be discussed.
SA21A-03 0830h
Observations of Neutral Wind Profiles in the Auroral Oval During Two Substorm Events
Two sounding rocket experiments, CODA and JOULE, were carried out at Poker Flat Research Range, Alaska, on February 21, 2002, and March 27, 2003, respectively. Three chemical tracer rockets in CODA and one in JOULE were launched successfully as part of the experiments. The trimethyl aluminum (TMA) trails were released on the upleg and downleg portion of the flight between approximately 80 and 180 km altitude thus provided measurements of the horizontal neutral wind profiles under different conditions. The CODA and JOULE launches were both characterized by significant gradients in the plasma drifts, and thus the forcing, although CODA occurred during a substorm that followed a long period of quiet conditions while the JOULE launches occurred after many hours of active conditions. In CODA, the neutral wind had two peaks, namely at 95 to 100km with ~110m s-1 southwestward and 100 to 105km with ~140 m s-1 northeastward, while in JOULE the largest winds were at 115 to 120 km altitude with speeds of ~220m s-1 southwestward. Typical accuracies are 5-10 m s-1 over the altitude range covered by the releases. Large shears were found in both experiment, but at a lower altitude of 95 to 105 km in CODA, as compared to 110 to 115km in JOULE. The tip of the wind vector rotated clockwise, nearly tracing a circle in CODA, and counterclockwise but with a more linear polarization in JOULE. The elongated wind hodograph observed in the JOULE experiment is a characteristic of more active conditions when the flow is dominated by plasma forcing. The NCAR TIME-GCM model was used to simulate the conditions of the JOULE experiment. The results show some structure that is similar to the observed winds, but in general, the model winds are much smaller in magnitude and have much weaker gradients. The talk focuses on a description of the geophysical conditions in the two experiments, on the rocket wind measurements, and on the comparison of the wind profiles under quiet and disturbed condition with the TIME-GCM model simulation results.
SA21A-04 0830h
Can Gravity Waves Seed Spread F?"
Evidence from backscatter radar (Hysell et al. 1990) shows that quasi-periodic "bubbles" or "Equatorial Spread F" (ESF) could be "seeded" by gravity waves having horizontal wavelengths of order100 km at altitudes of around 200km (the bottom of the F-Layer). In this paper two questions will be considered: (a) "Can such waves be generated by thunderstorms in the troposphere such that they can propagate to high altitudes?" and (b) " Can such waves sufficiently overcome viscous damping effects to propagate all the way up to 200 km altitudes? "Based upon simulations by Vadas and Fritts, 2004 and Alexander et al. 1995, it will be shown that mesoscale convective complexes, but not "squall lines", could indeed generate such waves, and that, in agreement with Vadas and Fritts, they could indeed reach a position to directly seed "Spread -F" irregularities. In any case, as will be shown, both types of such waves can certainly easily reach the E Region altitude of 100 km. In principle, therefore, they could seed Spread F through E-field perturbations that map up to the F region in the manner described by Prakash, 1999. It still remains to be experimentally demonstrated that gravity wave sources in the troposphere can trigger ESF.
SA21A-05 0830h
Validation of Nighttime LORAAS Instrument Measurements of the 1356-Ã… Emission
This study will focus on electron density profiles derived by inversion of ultraviolet limb scans observed over a period of several months by the Low Resolution Airglow and Aurora Spectrograph (LORAAS) instrument on the Advanced Research and Global Observing Satellite (ARGOS). The O+ density profile, which is approximately equal to the electron density profile in the F-region ionosphere, was determined by inverting the limb radiance profile of O I 1356 Ã… emission of atomic oxygen. This emission is produced purely by radiative recombination of O+ ions and electrons at night and has been shown to be a viable means of globally sensing the ionospheric state. The results of this study will be valuable for the Special Sensor Ultraviolet Limb Imager (SSULI) instruments flying on the satellites of the Defense Meteorological Satellite Program (DMSP). The first DMSP satellite to carry a SSULI was launched in October 2003, with more launches planned. The SSULI instruments are similar to the LORAAS instrument, which served as the SSULI prototype. Dymond et al. [Geophys. Res. Lett., Vol. 28, No. 5, 927-930, 2001] showed a preliminary validation of this technique covering a single day of observations. We will show a more extended validation of the retrieved peak electron density determined by a one-dimensional inversion of nighttime observations of the 1356-Ã… altitude profiles with nearly coincident ionosonde measurements.
SA21A-06 0830h
Particle albedo in proton aurora
A unique signature of proton precipitation in the auroral regions is the Doppler-shifted H emissions. These emissions have been widely observed from ground and space as a tracer of magnetospheric regions and processes and as a probe of particle energy input upon the high latitude regions. Spectroscopic observations of H emissions are required for any quantitative assessment of the incident proton flux. Without information on the spectral profile of the H emission, any quantitative analysis is limited by strong assumptions on the hardness of the precipitation. A spectral resolution of 0.2 nm or less over a spectral range of at least 8 nm is required for a suitable analysis of the H Balmer emissions. A particular feature of the spectral profile of H emissions is the presence of a red-shifted wing (for field-aligned viewing from the ground). Such a feature is induced by upgoing H atoms produced within the proton beam. Few proton/H atom transport comprehensive models include angular redistribution processes and to date no direct validation of the upward particle flux in proton aurora has been performed. The NASA FAST mission offers the unique opportunity to provide measurements of downward and upward proton fluxes at a high angular resolution at the top of the atmosphere. We have recently improved our proton transport model for a more detailed evaluation of the upward fluxes including an energy-dependent phase function for collisional angular redistribution. We will validate this new model using particle fluxes observed by the FAST satellite.
SA21A-07 0830h
Effect of E-Region Ionospheric Electric Field on Meteor Plasma Trails
Meteoroids penetrating the Earth's ionosphere leave behind dense plasma trails. Electron density irregularities within these trails create a significant part of the total radar clutter from the E-region ionosphere. These irregularities are partially caused by destabilizing electric fields which develop in or near the plasma trail. To model the radar echoes for meteor and atmospheric diagnostics, we need to properly understand the underlying physical cause of these electric fields. In this talk, we will introduce a quantitative model of the meteor-induced polarization electric fields for fully 3-D trails in the presence of external magnetic and DC electric fields in the E region. For the cases when a sufficiently strong DC electric field perpendicular to the geomagnetic field or strong neutral winds exist, we have calculated the 3-D spatial distribution of the polarization electric field around the trail. This electric field may reach significant values (tens mV/m and more) and may excite instabilities that cause non-specular radar echoes. This may help explain non-specular echoes which persist for a long time after the meteoroid has gone [Chapin and Kudeki, JGR, 99, 8937 (1994)]. In this case, the near-trail electric field increases with the meteoroid size until, for sufficiently big meteoroids, the linear plasma density in the trail reaches 1015--1016 m-1, and the polarization electric field saturates. The additional polarization electric field may also result in strong heating of electrons which in turn may lead to a modified rate of plasma trail diffusion and an additional airglow. Combining our theory with radar observations of specular and non-specular echoes should yield useful information about meteor trails and the surrounding atmosphere.
SA21A-08 0830h
Spectral Studies of Farley-Buneman Waves in the Auroral E-Region from a New Generation of High-Resolution Simulations
In the auroral electrojet, strong ambient DC electric fields drive the Farley-Buneman instability that creates plasma density irregularities responsible for type 1 radar echoes. These irregularities have been studied experimentally and theoretically for five decades. In the last decade, numerical simulations became an important tool in exploring the nonlinear behavior of E-region instabilities. However, these simulations were limited to 2-D and meshes resolving only 4096 (64 by 64) modes. Today, taking advantage of modern, massively parallel, supercomputers, we can resolve over 262,144 (512 by 512) modes in 2-D or over a million modes in 3-D. In this paper, we describe the spectra of type 1 waves from these high-resolution simulations and how they relate to measurements of electrojet spectra made by radar and rockets. In 2-D, our simulator modeled electron dynamics with an adiabatic, inertial, fluid solver while resolving ions kinetically with a particle-in-cell method (PIC). We ran a set of simulations appropriate for the auroral E-region. In all cases, the phase velocity of the most energetic modes lies well below the linearly predicted phase velocity. We see that for short wavelengths (< 1m), the dominant mode maintains a roughly constant phase velocity as the angle with respect to the drift direction increases from 0° to nearly ± 90° while for longer wavelengths in the system (> 6m), the phase velocity shows more complex behavior. In 3-D, our latest generation simulations model both electrons and ions with kinetic algorithms. This allows us to explore thermal effects with great accuracy but requires us to resolve the system Debye length and electron gyrofrequency, limiting our total resolution. Nevertheless, we see similar spectral features similar to those described in the 2-D system and we observe coupling to modes with a small component parallel to the geomagnetic field. Finally, we measure wave driven electron heating, a phenomena clearly observed by radars. Work was supported by NSF Grants ATM- 0332354 and ATM-0442075. Essential computational support was provided by the SCV and CCS at Boston University.
SA21A-09 0830h
The Relationship Between the Enhanced Aurora and Thin, Dense Ionization Layers
Thin layers of enhanced luminosity are commonly observed during auroral displays. The enhanced luminosity occurs at altitudes where thin, dense, heavy ion layers are often observed 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 N2. 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 full-particle electrostatic simulations of the nonlinear development of the instability. We also investigate the role of ion-neutral and electron-neutral collisions on the instability. The dense, heavy ion layer increases the growth rate of the instability and heats ambient electrons into a suprathermal tail that could produce enhanced emissions.
http://w3.pppl.gov/~jrj/ionization.html
SA21A-10 0830h
Global Neutral Polar Wind Model
The polar wind is an outflow of thermal ions from high-latitudes along geomagnetic field lines that are connected to the interplanetary magnetic field (IMF), or on field lines that are stretched out in the anti-sunward direction forming the magnetotail. The ions in the polar wind are highly dependent on convection and their outflow dynamics are modified by the heat input of precipitating electrons in the auroral zone. Charge exchange occurs between the outflowing ions and the neutral thermal and geocoronal atoms, producing neutral stream particles. A new three-dimensional model of the neutral polar wind has been developed which takes into account the convecting ion polar wind and heating due to precipitating electrons in the auroral zone, along with charge exchange between the ion polar wind and the background thermal and geocoronal neutral atoms. Three-dimensional simulations for both northward and southward IMF conditions show that over the polar caps the neutral polar wind particles exhibit extremely large fluxes in both the horizontal and vertical directions, and to a fixed observer it would appear that the neutral polar wind particles are moving in all directions. The results also show that the heating that occurs during geomagnetic storms leads to a significant coupling between the ionosphere and magnetosphere via both ion and neutral particles. For northward IMF, sunward streams of neutrals are created as a result of the sunward plasma convection.
SA21A-11 0830h
Characteristics of Waves in the Upper Mesosphere from Ground-Based Airglow Measurements in the Northern High-Latitude
Terrestrial nightglow emissions in the near infrared region have been monitored from several ground-based optical observatories in the northern high latitude; Eureka (80.0 N, 85.9 W), and Resolute Bay (74.68 N, 94.90 W), Canada. Spectral analysis of the derived rotational temperatures yields the periodicities of the atmospheric waves that are known to propagate upward from the lower atmosphere and perturb the airglow brightness and the rotational temperatures in the upper atmosphere. Spectral features at various periods are found and investigated in relation to atmospheric tides and planetary waves. The commonly observed features are waves near the tidal periods, i.e. 6, 8, and 12 hour oscillations. The data also show fluctuations at longer periods of 4-day and quasi-16 day waves. Harmonic analysis is also performed to seasonal data sets to identify the amplitudes and phase information of the major oscillations over season.
SA21A-12 0830h
Upper thermal boundary layer of planetary atmosphere: an experience of developing a general theory
Any planetary atmosphere has an upper layer where molecular heat conduction contributes significantly to the energy balance. In that layer the heat from absorbed solar radiation is transferred down to the lower atmosphere where it is removed away by the longwave radiation of atmosphere. We call that layer an upper thermal boundary layer and propose a general model for it in terms of dimensionless variables and parameters. The values of the dimensionless parameters and relations between them were obtained for Earth and Mars by fitting the calculated temperature profile to the empirical one. A distinction in kind in the interrelations of the mechanisms that determine the thermal structure of the layer is revealed for these planets, which reflects the difference in the mixing ratios of carbon dioxide in the atmospheres of Earth and Mars to orders of magnitude. The model consistency and accuracy is examined by an attempt to retrieve approximately the mixing ratio of atomic oxygen in the thermosphere of Mars from the empirical temperature profile. The accelerometer data obtained by MGS spacecraft during aerobraking are used to derive an average temperature profile of dayside thermosphere of Mars in Northern hemisphere during spring and intermediate (from minimum to moderate) solar activity conditions.