Dynamics and Electrodynamics II
Presiding: R Pfaff, NASA Goddard Space Flight Center; R L Bishop, Aerospace Corporation
SA23B-01 13:30h
Simultaneous Measurements of Lower E-region Nighttime Electrodynamics Gathered with Rockets at the Altair Radar During the EQUIS II Campaign at Kwajalein Atoll
In order to investigate the complex electrodynamics and neutral-plasma coupling inherent to the unstable nighttime E-region near the earth's magnetic equator, a series of rocket/radar experiments were conducted at Kwajalein Atoll (9.4 deg N, 167.5 deg E) near 5 degrees magnetic latitude in September, 2004, as part of the NASA EQUIS II Campaign. The rocket experiments consisted of two identical, instrumented payloads launched on separate nights with limited apogees so that the payloads "hovered" in the E region below 120 km. Each payload included vector DC and AC electric field detectors, a flux-gate DC magnetometer, a combined Langmuir probe/impedance probe to measure the absolute plasma density and its variations, multi-sensor ionization gauges to measure the neutral density and its variations, and spaced-electric field receivers to measure the wavelength and phase velocity of the unstable plasma waves. Separate rockets launched in conjunction with the instrumented rockets released TMA trails on the upleg and downleg between roughly 90-160 km that revealed the neutral wind and its velocity shear. These payloads also included a beacon experiment that provided an independent measure of the plasma density. In addition to the rocket experiments, coherent and incoherent scatter radar measurements at 160 MHz and 422 MHz were gathered with the fully steerable Altair radar and detected layers of backscatter echoes near 105 km that were both intense and sporadic in their appearance. The layers were narrow in their altitude extent, spatially modulated, and typically lasted for about 30 minutes. The payloads were launched into unstable layers on two nights, one prior to the pre-reversal enhancement (LT 20:00:45) and one near midnight (LT 23:43:53). The initial electric field and plasma density data reveal well-defined layers of waves between 95-105 km altitude with predominant wavelengths of 10's of meters and longer within regions that contained weak to moderate gradients in the plasma number density. The associated DC electric fields had relatively low amplitudes (~2-3 mV/m). We investigate whether the irregularities may result primarily from a variety of wind-driven instabilities, as compared to the traditional gradient-drift instability mechanism invoked to explain nighttime irregularities at the magnetic equator. An overview of the observations will be presented.
SA23B-02 13:45h
Nighttime E-region Electron Density Profiles Measured During the EQUIS II Campaign at Kwajalein Atoll
The EQUIS II nighttime E-region rocket and radar measurements were made in order to improve our understanding of the electrodynamics associated with density gradients, neutral wind shear, and enhanced electric fields that develop post-sunset in the near-equatorial region. Four rocket experiments were launched on two separate nights in September, 2004 from Kwajalein Atoll (9.4° N, 167.5° E), while simultaneous E-region radar observations were made with the ALTAIR radar. The focus of this presentation are the electron density profiles measured by two instrumented rockets as they passed through the unstable region on the upleg and downleg. Each rocket used two Langmuir probes and an impedance probe of a new design to measure both the absolute electron density and small-scale density fluctuations with spatial scales on the order of one meter. The impedance probe returned measurements from 7 kHz to 4 MHz, using a new design that excited the plasma using a pseudo-white-noise generator, allowing for an altitude resolution of approximately 40 meters. These impedance curves allow determination of the electron density from the identification of the upper hybrid frequency. In addition, evidence is presented that the impedance probe observed the lower-frequency "series" resonance which is dependent on the electron temperature. Data from the Langmuir probes, a beacon experiment, and the impedance probe are compared and the resulting density profiles are examined to estimate their contribution to the observed electric field irregularities via the gradient-drift and other instabilities.
SA23B-03 14:00h
Measurements of Neutral Density During the EQUIS II Nighttime Electrodynamics Missions
Results from the 2004 EQUIS II low-latitude sounding rocket campaign are reported. The two nighttime electrodynamics instrumented rockets each carried multiple-sensor ionization gauges (IGs) to investigate the structure of the neutral atmosphere associated with an active, unstable nighttime E-region ionosphere. These measurements are used to understand the plasma-neutral interactions in this environment and their role in the production of irregularities. The primary goals of the IG instrumentation are to characterize the neutral density profile (and thus the temperature profile), the variations in neutral density, and wind shears. These measurements are used to understand the roles played by gravity waves, turbulence, wind shear, and atmospheric instability in the production of the ionospheric irregularities. Preliminary analysis shows the presence of wind shears near the layers of waves observed by the rocket-borne electric field experiment.
SA23B-04 14:15h
Investigation of ionospheric precursors leading to spread F during the EQUIS II campaign on Kwajalein.
A sounding rocket investigation of bottom-type scattering layers, shear flow, and the factors which may precondition the equatorial F region ionosphere for postsunset instability was carried out in August, 2004 from the Roi Namur range during the EQUIS II campaign on Kwajalein Atoll. Identical experiments were performed on August 7 and 15, each comprised of the launch of an instrumented payload which measured plasma number density and vector electric field profiles along with two chemical release payloads. The latter deployed TMA trails from which vector neutral wind profiles in three locations could be deduced. Ground-based support was provided by the Altair radar, a dual-frequency radar capable of measuring both coherent and incoherent scatter. The purpose of the experiment was 1) to understand and quantify the vertical shear in the zonal plasma drift that occurs each day around sunset, 2) to understand the nature of the bottom-type scattering layers that inhabit westward-drifting strata in the bottomside F region and that serve as precursors for fully developed spread F, and 3) to understand the influence of shear flow on the postsunset ionosphere. Ground-based and in situ data confirm the presence of strong shear in the flow proceeding the emergence of spread F irregularities in both experiments. The bottom-type layers that formed exhibited properties consistent with horizontal wind-driven gradient drift instabilities growing in an inhomogeneous bottomside. Regular 30-50 km structuring similar to what has been observed recently at Jicamarca was also evident in the layers. This structuring was repeated in the spread F depletions that finally appeared. The structuring of the bottom-type layers therefore served as a telltale of the spread F irregularities to come. Moreover, the structuring may have been produced by shear instabilities as described recently by Hysell and Kudeki [2004]. Numerical modeling of the causes of the shear as well as its effects on stability may therefore point the way to a spread F forecast strategy.
http://landau.geo.cornell.edu
SA23B-05 14:30h
Observations of Neutral and Electron Structure in the Equatorial Mesosphere by Rocket and Radar During EQUIS-2/LEMMA
The NASA EQUIS-2 rocket campaign was conducted in August-September 2004 from USAKA/RTS on Roi-Namur, Marshall Islands (9 N, 168 E). One part of the program was dedicated to neutral and plasma density fluctuations in the equatorial mesosphere and lower thermosphere. The objective was to detect layers of small scale structures that can be associated with mesospheric VHF radar echoes observed frequently at equatorial and low latitudes. One instrumented rocket and three passive falling spheres were launched successfully on September 20, 2004, supported by ALTAIR UHF radar observing incoherent backscatter from ~85-700 km. We give an overview of the investigation and present first results that include the comparison of in situ and radar electron density profiles, neutral temperature and wind structure, and characteristics of neutral and electron fluctuation layers.
SA23B-06 14:45h
An investigation of possible coupling between the passage of a tropical storm and the local ionosphere
Tropical Storm Odette was the first storm since 1887 to form in the Caribbean during the month of December. It formed at approximately 15A N and 73A W on December 4, 2003 and proceeded to travel in a northeasterly direction over the next 4 days before dissipating. During that period, Odette passed within 600 km north of Puerto Rico. Data obtained from the Arecibo Observatory (AO), ionosondes at AO and Ramey, a microbarograph operated by Penn. State University, and limited co-located GPS occultations from the CHAMP satellite provide information on the state of the atmosphere extending from the ground to ionospheric altitudes. Observations made by AO on the nights of December 5-6 and 7-8 show behavior that may be attributable to the tropospheric disturbance. On the night of December 5-6, AO measured the plasma density in the E and F-regions using a vertically directed beam. Significant and continuous altitude modulations of the sporadic-E layer were observed over a seven-hour period. During the day of Dec. 7, Odette made its closest approach to the island. The microbarograph showed significant high frequency fluctuations indicating the generation of gravity waves. The following night AO measured F region plasma densities and drift velocities nearly coincident with the tropical storm track. Large drift velocity variations, ranging from 10 to 80 m/s, are evident in all three plasma drift components over the entire observational period. The zonal drift was atypical with magnitudes significantly greater than that observed at AO during similar magnetic and solar conditions. During the Dec. 7-8 observation period, the Ramey ionosonde showed a range-spreading event also indicative of a disturbed ionosphere. This presentation takes a closer look at the low altitude and ionospheric conditions in order to characterize possible coupling between the two regions.