Engineering topics

Greenwald, R. A.

Publications and source records attributed to Greenwald, R. A..

At least 19 records

NASA NTRS2012

Large-scale Observations of a Subauroral Polarization Stream by Midlatitude SuperDARN Radars: Instantaneous Longitudinal Velocity Variations

We present simultaneous measurements of flow velocities inside a subauroral polarization stream (SAPS) made by six midlatitude high-frequency SuperDARN radars. The instantaneous observations cover three hours of universal time and six hours of magnetic local time (MLT). From velocity variations across the field-of-view of the radars we infer the local 2D flow direction at three different longitudes. We find that the local flow direction inside the SAPS channel is remarkably constant over the course of the event. The flow speed, however, shows significant temporal and spatial variations. After correcting for the radar look direction we are able to accurately determine the dependence of the SAPS velocity on magnetic local time. We find that the SAPS velocity variation with magnetic local time is best described by an exponential function. The average velocity at 00 MLT was 1.2 km/s and it decreased with a spatial e-folding scale of two hours of MLT toward the dawn sector. We speculate that the longitudinal distribution of pressure gradients in the ring current is responsible for this dependence and find these observations in good agreement with results from ring current models. Using TEC measurements we find that the high westward velocities of the SAPS are - as expected - located in a region of low TEC values, indicating low ionospheric conductivities.

Clausen, L. B. N.↗

NASA NTRS2004

Cusp and LLBL as Sources of the Isolated Dayside Auroral Feature During Northward IMF

An intense dayside proton aurora was observed by Imager for Magnetopause-to- Aurora Global Exploration Far Ultra-Violet imager (IMAGE FUV) for an extensive period of northward interplanetary magnetic field (IMF) on 17 and 18 September 2000. This aurora partially coincided with the auroral oval and intruded farther poleward into the polar cap, and it showed longitudinal motions in response to IMF By variation. Intense magnetosheath-like electron and ion precipitations have been simultaneously detected by Defense Meteorological Satellite Program (DMSP) above the poleward portion of the high-latitude dayside aurora. They resemble the typical plasmas observed in the low-altitude cusp. However, less intense electrons and more energetic ions were detected over the equatonvard part of the aurora. These plasmas are closer to the low-latitude boundary layer (LLBL) plasmas. Under strongly northward IMF, global ionospheric convection derived from Super Dual Auroral Radar Network (SuperDARN) radar measurements showed a four-cell pattern with sunward convection in the middle of the dayside polar cap and the dayside aurora corresponded to two different convection cells. This result further supports two source regions for the aurora. The cusp proton aurora is on open magnetic field lines convecting sunward whereas the LLBL proton aurora is on closed field lines convecting antisunward. These IMAGE, DMSP, and SuperDARN observations reveal the structure and dynamics of the aurora and provide strong evidence for magnetic merging occurring at the high-latitude magnetopause poleward from the cusp. This merging process was very likely quasi-stationary.

Chang, S.-W.↗

NASA NTRS2002

Evolution of Ionospheric Convection during a Double Transpolar Arc Phenomenon on February 11, 1999

An evolution of ionospheric convection was studied for a double transpolar arc phenomenon on February 11, 1999. While one transpolar arc split from the auroral oval in the morning sector and drifted duskward, another arc appeared in the evening sector. The convection was investigated with three velocity data sets: E B drift velocities from the ASTRID-2 satellite; Ion Driftmeter data from the DMSP satellites; and Doppler-shift data from the Super-DARN radars. We inferred convection cells from these data sets and found that the number of convection cells changed from three to four as the dominance of IMF changed from a negative By to a positive Bz. Our result suggests that the ionospheric convection that has been so far discussed for various conditions of IMF may be applied even to the cases accompanied by transpolar arcs.

Narita, Y.↗

NASA NTRS1999

Global Dynamics of Dayside Auroral Precipitation in Conjunction with Solar Wind Pressure Pulses

Global observation of the dayside auroral region by the Ultraviolet Imager (UVI) during transient solar wind pressure pulse events on October 1, 1997 has revealed unusual features in the auroral precipitation. The auroral arc structure on the dayside, possibly connected with the LLBL, split into 2 arc structures; one moving poleward and fading over a 5 min period, and the other stationary or slightly shifted equatorward (by changes in the x component). The y component was large and positive, and the z component was small and negative. The splitting of the arc structure extended from 9 to 15 MLT and was concurrent with an enhancement of the convection in the cusp region identified by SuperDARN observations. The convection reversal on the morningside was adjacent to and poleward of the weak lower latitude band of precipitation. The sensitivity of the UVI instrument enabled observation of arc structures down to about 0.2 erg electron energy flux, as confirmed by comparison with particle measurements from the FAST satellite for other dayside events. Removal of the spacecraft wobble by PIXON image reconstruction restored the original resolution of the UVI of about 40 km from apogee. This event is being analyzed in connection with a larger study of global dynamics of dayside energy and momentum transfer related to changes in IMF conditions using UVI images in conjunction with observations from FAST and SuperDARN.

Brittnacher, M.↗

NASA NTRS1995

HF radar signatures of the cusp and low-latitude boundary layer

Continuous ground-based observations of ionospheric and magnetospheric regions are critical to the Geospace Environmental Modeling (GEM) program. It is therefore important to establish clear intercalibrations between different ground-based instruments and satellites in order to clearly place the ground-based observations in context with the corresponding in situ satellite measurements. HF-radars operating at high latitudes are capable of observing very large spatial regions of the ionosphere on a nearly continuous basis. In this paper we report on an intercalibration study made using the Polar Anglo-American Conjugate Radar Experiment radars located at Goose Bay, Labrador, and Halley Station, Antarctica, and the Defense Meteorological Satellite Program (DMSP) satellites. The DMSP satellite data are used to provide clear identifications of the ionospheric cusp and the low-latitude boundary layer (LLBL). The radar data for eight cusp events and eight LLBL events have been examined in order to determine a radar signature of these ionospheric regions. This intercalibraion indicates that the cusp is always characterized by wide, complex Doppler power spectra, whereas the LLBL is usually found to have spectra dominated by a single component. The distribution of spectral widths in the cusp is of a generally Gaussian form with a peak at about 220 m/s. The distribution of spectral widths in the LLBL is more like an exponential distribution, with the peak of the distribution occurring at about 50 m/s. There are a few cases in the LLBL where the Doppler power spectra are strikingly similar to those observed in the cusp.

Baker, K. B.↗

NASA NTRS1995

Observations of IMF and seasonal effects in high-latitude convection

Strong interplanetary magnetic field (IMF) and seasonal effects in the convection of nightside ionospheric plasma are described. The findings are based on a statistical analysis of observations made with the Johns Hopkins University/ Applied Physics Lab (JHU/APL) HF radar located at Goose Bay, Labrador. For positive sign of the IMF dusk-dawn component, By greater than 0 the dawn cell is more crescent shaped and the dusk cell more round while for BY less than 0 these pairings of size and shape are reversed. The more extreme crescent /round cell dichotomy is obtained for BY greater than 0. The return flows associated with the crescent-shaped cell dominate at midnight MLT (magnetic local time); the reversal in the zonal velocity in the 67 deg-69 deg lambda (magnetic latitude) interval occurs 2.5 hr earlier in summer than in winter. The maximum effects are obtained on the nightside for the pairings By greater than 0, summer and BY less than 0, winter; the first produces the more structured cell in the morning, the second in the evening, and this cell dominates the return flow at midnight. The difference in the zonal flow reversals for these pairings exceeds 4 hr in MLT.

Ruohoniemi, J. M.↗

NASA NTRS1995

Estimating gravity wave parameters from oblique high-frequency backscatter: Modeling and analysis

A new technique for estimating electron density perturbation amplitudes of traveling ionospheric disturbances (TIDs), using HF radar data, is presented. TIDs are observed in HF radar data as enhancements of the ground-scattered power which propagate through the radar's field of view. These TIDs are the ionospheric manifestation of atmospheric acoustic-gravity waves. TID electron density perturbation amplitudes were estimated by simulating the radar returns, using HF ray tracing through a model ionosphere perturbed by a model gravity wave. The simulation determined the return power in the ground-scattered portion of the signal as a function of range, and this was compared to HF radar data from the Goose Bay HF radar at a time when evidence of gravity waves was present in the data. By varying the amplitude of the electron density perturbation in the model it was possible to estimate the perturbation of the actual wave. It was found that the perturbations that are observed by the Goose Bay HF radar are of the order of 20% to 35%. It was also found that the number of observable power enhancements, and the relative amplitudes of these enhancements, depended on the vertical thickness of the gravity wave's source region. From the simulations and observations it was estimated that the source region for the case presented here was approximately 20 km thick. In addition, the energy in the wave packet was calculated and compared to an estimate of the available energy in the source region. It was found that the wave energy was about 0.2% of the estimated available source region energy.

Bristow, W. A.↗

NASA NTRS1993

The response of the high-latitude dayside ionosphere to an abrupt northward transition in the IMF

We examine the response of the high-latitude ionosphere in the prenoon sector to a northward turning of the IMF. The event was observed in the 11-13 UT interval on June 1, 1987, in the course of a multiday SUNDIAL campaign. The transition in the IMF was observed by the IMP-8 satellite which was located upstream of the earth at a distance of 36 Re. The ionospheric response in the 70-80 deg invariant latitude interval was monitored by two radars. The preexisting plasma convection observed by the radars exhibited large velocities (500-1000 m/s) and stable longterm trends, consistent with the inertial rotation of the convection pattern expected of the conditions then prevailing, B(z) less than 0, B(y) greater than 0. The plasma flow rapidly abated in response to the IMF transition. The electron density measurements made by radar in the meridional plane showed that the ionosphere had been rich in structure with the active deposition of ionization by particle precipitation. Subsequently it resembled an inactive, unstructured mid-latitude configuration. There was a dramatic decrease in the amount of backscatter observed by the HF radar. We analyze the times of transition in the various data sets and show that the ionosphere began to show the effects of the IMF transition about 2 min after its probable arrival at the magnetopause boundary.

Ruohoniemi, J. M.↗

NASA NTRS1993

Observations of an enhanced convection channel in the cusp ionosphere

The development of an enhanced convection channel within the Southern Hemisphere cusp ionosphere is studied on the basis of the combined PACE radar and DMSP satellite data sets. All data (solar wind and ionosphere) suggest that the event is a signature of enhanced magnetic merging, i.e., the ionospheric signature of a flux transfer event. It is suggested that such a signature arises from several minutes of continuous, enhanced merging rather than short-lived pulses of enhanced merging. The channel propagated westward at about 3 km/s and, when fully formed, its zonal dimension was comparable with the average width of the cusp. It is shown that the formation of polar patches is associated with the occurrence of convection channels.

Pinnock, M.↗

NASA NTRS1992

Spatial and temporal behavior of ULF pulsations observed by the Goose Bay HF radar

Techniques which allow the instantaneous amplitude and phase to be determined as functions of geomagnetic lattitude, longitude, and time are employed to carry out a detailed analysis of HF radar data of a ULF pulsation event in the postmidnight sector on January 11, 1989. Field line resonances with several different frequencies occur simultaneously at different latitudes. These can be associated with cavity mode frequencies of 1.3 mHz, 1.9 mHz, 2.7 mGz, and 3.3 mHz. These frequencies are constant to better than 10 percent over a local time period of nearly 4 hr. The field-aligned currents driven by the resonances can be as large as 5 micro-A/sq m at ionospheric heights. The data support a picture of modes driven by solar wind impulses.

Walker, A. D. M.↗

NASA NTRS1991

HF radar observations of Pc 5 field line resonances in the midnight/early morning MLT sector

The technique of F region sounding by HF coherent radar is applied to the study of field line resonances equatorward of a region of shear flow in the early morning sector. The motions were predominantly in the geomagnetic east-west direction, indicating north-south electric fields. These oscillations had pronounced peaks in their latitudinal power distribution. For the pulsation at 1.95 mHz, a latitudinal phase shift of 180 deg was observed across the peaks in all the look directions of the radar, and a longitudinal wavelength corresponding to an m value of 3 was obtained. For the 2.6-mHz pulsation, the phase shifts across the peaks had a variation with a look direction that indicated a significant longitudinal as well as latitudinal variation; for this activity, an m value of about 16 is estimated. These features are interpreted in terms of the field line resonance theory, and the possible sources of the pulsation energy are discussed.

Ruohoniemi, J. M.↗

NASA NTRS1991

The determination of time-stationary two-dimensional convection patterns with single-station radars

A critical examination of the accuracy of ionospheric vector velocity determinations, using realistic modeled flow patterns that are time-stationary but not spatially uniform, is presented. Under certain circumstances the actual and inferred flow fields are found to exhibit considerable discrepancy, sometimes not even agreeing in the sense of flow direction. It is shown that the natural curvature present in ionospheric convection on varying spatial scales can introduce significant error in the velocity estimate, particularly when the radius of curvature of the flow structure is less than or equal to the radar range to the scattering volume. It is argued that the ionospheric convection should be measured by bidirectional or multidirectional observations of a common ionospheric volume and that a synthesis of coherent and incoherent radar observations from different sites is preferable to multidirectional single-station observations using either radar alone.

Freeman, M. P.↗

NASA NTRS1991

An ionospheric signature of possible enhanced magnetic field merging on the dayside magnetopause

The high temporal and spatial resolution of the Polar Anglo-American Conjugate Experiment radar data have permitted the evolution and formation of a transient plasma velocity signature in the cusp ionosphere to be studied. The signature satisfies a number of the predictions of theoretical models that require the formation of vortices. A much weaker enhancement in velocities is shown in the Northern Hemisphere, which is consistent with the vector superposition arguments presented by Crooker (1979) for the antiparallel merging model.

Pinnock, M.↗

NASA NTRS1990

Simultaneous HF-radar and DMSP observations of the cusp

The Geospace Environment Modeling (GEM) Program is directed toward modeling the coupled solar wind/magnetosphere/ionosphere system. The inter-calibration of ground-based observations of the ionosphere and satellite observations has been identified as an essential step in tying together the data to produce a global picture of geospace. On October 10, 1988 the DMSP-F9 satellite passed through the Southern Hemisphere cusp while a coheret scatter HF-radar was observing 10-m scale irregularities present in the ionosphere. The combined data indicate that these irregularities were being generated in the cusp, and that the cusp was a region of greater than normal electric field turbulence. The radar data indicate that the cusp was colocated with the region where the ionospheric convection rotated from sunward to anti-sunward with increasing latitude. These observations provide an unambiguous case where simultaneous satellite and ground-based observations of the cusp can be compared.

Baker, K. B.↗

NASA NTRS1990

Simultaneous conjugate observations of dynamic variations in high-latitude dayside convection due to changes in IMF By

Data from two conjugate HF radars currently operating at Goose Bay (Labrador) and the Halley Station (Antarctica), obtained for a single 45-min period about local noon on April 22, 1988, were used to study the near-instantaneous conjugate two-dimensional patterns of plasma convection in the vicinity of the cusp. In particular, the response of these plasma convection patterns to changes in the By component of the IMF was examined. Results indicate that, under quasi-stationary IMF conditions, the conjugate convection patterns are quite similar to the synthesized patterns of Heppner and Maynard (1987) and that the patterns respond rapidly to changes in the IMF By component. Results also show that transitions between convection states begin to occur within minutes of the time that an IMF state change is incident on the magnetospheric boundary, and that the convection reconfigurations expand poleward, completely filling the field of view of an HF radar within 6 min of the time of onset.

Greenwald, R. A.↗

NASA NTRS1989

Mapping high-latitude plasma convection with coherent HF radars

Several methods developed for mapping high-latitude plasma convection with a high-latitude HF radar are described, which utilize coherent backscatter from electron density irregularities at F-region altitudes to observe convective plasma motion. Several examples of two-dimensional convection-velocity maps are presented, showing instances of L-shell-aligned flow in the dusk sector, the reversal of convection near magnetic midnight, and counterstreaming in the dayside cleft.

Ruohoniemi, J. M.↗

NASA NTRS1988

Coherent HF radar backscatter from small-scale irregularities in the dusk sector of the subauroral ionosphere

This paper describes the characteristics of backscatter from decameter-scale ionospheric plasma density irregualarities, observed with an impressive regularity by the Goose Bay (Labrador) high-frequency (HF) radar in the dusk sector of the winter ionosphere, and discusses the relation of the scatter to the midlatitude trough. It is shown that this dusk scatter can be readily distinguished from other types of late afternoon/early evening scatter by the extreme equatorward position of its source region and by the low values of its associated radar Doppler velocities (not above 200 m/s) and spectral widths (not more than 200 m/s). A comparison of the radar observations with nearly simultaneous particle precipitation data obtained with the DMSP F6 satellite demonstrated that the source region of the backscatter lies within the subauroral ionosphere. It is shown that the characteristics of dusk scatter are compatible with the Spiro et al. (1978) model of the electrodynamics of the midlatitude trough.

Ruohoniemi, J. M.↗

NASA NTRS1980

Comparative rocket observations of ionospheric electric fields in the auroral oval

The ion drift technique of measuring ionospheric electric fields is compared to two other simultaneous measurements. Rocket measurements in the evening auroral oval are checked against a dual probe, also in situ, and the ground based STARE auroral radar. The technique is explained thoroughly as well as tested for its dependence on mass. Two evening auroral oval conditions were observed from Andoya, Norway in January and February 1977. The first flight, 18:1005, measured electric fields over a quiet pre-midnight discrete arc. An interesting plasma convection reversal was observed poleward of the arc. The subsequent 18:1004, samples a breakup phase aurora nearer local midnight.

Zanetti, L. J., Jr.↗