D. L. Hysell

JULIA radar studies of equatorial spread F

This paper presents coherent scatter radar observations of plasma irregularities in the equatorial F region ionosphere made with the JULIA (Jicamarca unattended long-term studies of the ionosphere and atmosphere) radar and discusses model equations and simulations pertinent to their interpretation. The data set shows that bottom-type and bottomside scattering layers are prevalent throughout solar minimum equinox. Bottom-type layers are undifferentiated and relatively weak layers confined to a narrow range of altitudes falling within regions of the ionosphere driven by the E region dynamo. These layers are often precursors to large-scale radar plumes as well as to bottomside layers, which are broader, more structured, and more intense layers than the bottom-type variety. Bottomside layers are the source of intermediate-scale plasma depletions that can ascend and penetrate through to the topside. Fluid simulations of the collisional interchange instability suggest that dynamo theory explains why one kind of layer emerges at a given time and altitude.

Long term studies of equatorial spread F using the JULIA radar at Jicamarca

Jicamarca unattended long term investigations of the ionosphere and atmosphere radar observations of equatorial spread F (ESF) plasma irregularities made between August 1996 and April 2000 are analyzed statistically. Interpretation of the data is simplified by adopting a taxonomy of echo types which distinguishes between bottom-type, bottomside, topside, and post-midnight irregularities. The data reveal patterns in the occurrence of ESF in the Peruvian sector that are functions of season, solar flux, and geomagnetic activity. We confirm earlier work by Fejer et al. (J. Geophys. Res. 104 (1999) 19,859) showing that the quiet-time climatology of the irregularities is strongly influenced by the climatology of the zonal ionospheric electric field. Under magnetically quiet conditions, increasing solar flux implies greater pre-reversal enhancement amplitudes and, consequently, irregularity appearances at earlier times, higher initial altitudes, and higher peak altitudes. Since the post-reversal westward background electric field also grows stronger with increasing solar flux, spread F events also decay earlier in solar maximum than in solar minimum. Variation in ESF occurrence during geomagnetically active periods is consistent with systematic variations in the electric field associated with the disturbance dynamo and prompt penetration described by Fejer and Scherliess (J. Geophys. Res. 102 (1997) 24,047) and Scherliess and Fejer (J. Geophys. Res. 102 (1997) 24,037). Quiet-time variability in the zonal electric field contributes significantly to variability in ESF occurrence. However, no correlation is found between the occurrence of strong ESF and the time history of the zonal electric field prior to sunset.

An overview and synthesis of plasma irregularities in equatorial spread F

Abstract A unified picture of plasma irregularities in equatorial spread F is developed from the analysis of satellite, sounding rocket, and coherent scatter radar observations. The coherent scatter data are analyzed using a new in-beam radar imaging technique that permits direct comparison between radar data, in situ data, and computer simulations of the irregularities. Three varieties of irregularities, all produced by ionospheric interchange instabilities, are found to occur. Thin bottom-type layers are composed of waves with primary transverse wavelengths less than about 1 km and with significant parallel wavenumbers. These exist on magnetic flux tubes controlled by the E region dynamo and drift westward in the postsunset ionosphere. A nonlocal analysis is used to calculate their linear growth rate. When the F region dynamo takes control of the flux tube, bottomside irregularities can emerge. These are more robust irregularities with longer primary wavelengths and which exhibit greater vertical development. Nonlinear analyses explain the appearance of steepened structures in rocket observations and solitary waves in satellite observations of bottomside layers. The one-dimensional spectra of these irregularities obey power laws but are anisotropic and have variable spectral indices and spectral breaks. Very strong polarization electric fields can eject large regions of deeply depleted plasma through the F peak and form topside irregularities. Theoretical calculations supported by satellite data show that ion inertia may become important for topside irregularities. The one-dimensional spectra of irregularities in the inertial regime obey a k −5/3 power law, but strong plasma inhomogeneity implies that Kolmogorov weak turbulence is not the explanation. Topside depletions are shown to bifurcate and also to pinch off from the bottomside.

VHF radar and rocket observations of equatorial spread F on Kwajalein

VHF radar data from the Summer 1990 Equatorial Spread F campaign on Kwajalein are presented. The Cornell 50 MHz portable radar interferometer (CUPRI) operated concurrently with the Altair UHF incoherent scatter radar throughout July and August and supported two sounding rocket flights on July 30 and August 2. This experiment provided the first opportunity to simultaneously diagnose equatorial spread F using the three prime experimental techniques: VHF/UHF coherent scatter, incoherent scatter, and in situ probe measurements of electric field and density fluctuations. The intensity of the coherent echoes observed was consistent with typical Jicamarca spread F observations, but chains of periodic, large-scale plasma upwellings were observed more often and for much longer durations on Kwajalein than have been seen over Peru. CUPRI also measured Doppler frequencies in one upwelling corresponding to 1200-m/s plasma drift velocities. This measurement agrees with recent observations of supersonic drift rates at the magnetic equator by spacecraft. Near the most active localized plasma upwellings, interferometer data reveal that the zonal drift rate of plasma irregularities can vary sharply in space, as one would expect for two-dimensional incompressible flow. We introduce a semiempirical model of the three-dimensional spectrum of F region irregularities that is consistent with the one-dimensional spectra of density fluctuations observed by sounding rockets and with the axial ratio of irregularities determined recently. Normalized to data from one of the rocket flights on Kwajalein, the model predicts the 3-m scattering cross-section measured by CUPRI to within a few decibels.

Radar imaging of equatorial F region irregularities with maximum entropy interferometry

Radar interferometry with multiple collinear antenna baselines has been performed at Jicamarca, near Lima, Peru, and high-resolution images of plasma irregularities in equatorial spread F have been constructed from the data. The images represent the brightness distribution of the radar backscatter, which is the Fourier transform of the visibility or spatial cross correlation of the scattered radar signal on the ground. The visibility data are noisy and incompletely sampled, and we have used the maximum entropy method to perform the Fourier transform, making optimal use of the data available. Using the current antenna configuration at Jicamarca, the radar imaging technique is able to resolve structures within the radar beam that are just a few kilometers wide. By animating sequences of images, we can observe the evolution of ionospheric irregularities without confusing spatial and temporal variations.

JULIA radar studies of electric fields in the equatorial electrojet

First results from the JULIA radar at Jicamarca are presented. These include coherent scatter observations of the equatorial electrojet and of 150-km echoes. Interferometry is used to measure the zonal drift rate of kilometer scale waves in the electrojet as functions of altitude. A technique for estimating the background zonal electric field from the interferometry data is described. The electric field estimates can be calibrated against the Doppler speed of the 150-km echoes when the latter are present. The kilometer-scale wave drifts sometimes exhibit large-amplitude, periodic height variations with vertical wavelengths of about 10 km. These signatures are reminiscent of the wind profiles measured with chemical release techniques in the lower thermosphere during the Guara campaign.

Simulations of plasma clouds in the midlatitude E region ionosphere with implications for type I and type II quasiperiodic echoes.

The electrodynamics of clouds of enhanced plasma density in the the postsunset midlatitude E region ionosphere are simulated in three dimensions. Such clouds become polarized in the presence of a background electric field, as would be imposed by the F region dynamo. If the clouds are elongated so that their major and minor axes in the horizontal plane are much larger and smaller than about 1 km, respectively, the polarization electric field can become an order of magnitude larger than the applied field. Elongated depressions or ripples in planar layers also become polarized but to a lesser degree. Electric fields and Hall drifts sufficiently large to excite Farley Buneman instabilities can be produced, particularly when neutral wind forcing is considered in addition to background electric fields. The plasma clouds are also unstable to a collisional drift instability capable of generating large-scale primary waves. A linear, local dispersion relation for this instability is derived. The primary waves are presumed to be capable of generating small-scale irregularities through mode coupling and plasma turbulence. Polarized plasma clouds drifting through the radar scattering volume may account for many of the characteristics of type I and type II quasiperiodic echoes.

Ground and Space-Based Measurement of Rocket Engine Burns in the Ionosphere

On-orbit firings of both liquid and solid rocket motors provide localized disturbances to the plasma in the upper atmosphere. Large amounts of energy are deposited to ionosphere in the form of expanding exhaust vapors which change the composition and flow velocity. Charge exchange between the neutral exhaust molecules and the background ions (mainly O+) yields energetic ion beams. The rapidly moving pickup ions excite plasma instabilities and yield optical emissions after dissociative recombination with ambient electrons. Line-of-sight techniques for remote measurements rocket burn effects include direct observation of plume optical emissions with ground and satellite cameras, and plume scatter with UHF and higher frequency radars. Long range detection with HF radars is possible if the burns occur in the dense part of the ionosphere. The exhaust vapors initiate plasma turbulence in the ionosphere that can scatter HF radar waves launched from ground transmitters. Solid rocket motors provide particulates that become charged in the ionosphere and may excite dusty plasma instabilities. Hypersonic exhaust flow impacting the ionospheric plasma launches a low-frequency, electromagnetic pulse that is detectable using satellites with electric field booms. If the exhaust cloud itself passes over a satellite, in situ detectors measure increased ion-acoustic wave turbulence, enhanced neutral and plasma densities, elevated ion temperatures, and magnetic field perturbations. All of these techniques can be used for long range observations of plumes in the ionosphere. To demonstrate such long range measurements, several experiments were conducted by the Naval Research Laboratory including the Charged Aerosol Release Experiment, the Shuttle Ionospheric Modification with Pulsed Localized Exhaust experiments, and the Shuttle Exhaust Ionospheric Turbulence Experiments.

Three‐dimensional numerical simulation of equatorial F region plasma irregularities with bottomside shear flow

[1] A three-dimensional numerical simulation of plasma density irregularities in the postsunset equatorial F region ionosphere leading to equatorial spread F (ESF) is described. The simulation advances the plasma number density and electrostatic potential forward in time by enforcing the constraints of quasi-neutrality and momentum conservation. The magnetic field lines are not modeled as equipotentials. Simulations are performed for cases with no background winds, with no background electric field or gravity, and with winds, a background electric field, and gravity all working in concert. The first run produced generalized Rayleigh Taylor (GRT) instability, and the second produced collisional shear instability (CSI). The combined run produced an instability which developed into an intense ESF event more quickly and with more realistic characteristics than the other two. Simulation results are compared with incoherent and coherent scatter radar data from the magnetic equator. A number of signature ESF characteristics are shown to be reproduced by the simulation.

High‐resolution radar observations of daytime kilometer‐scale wave structure in the equatorial electrojet

Using a new antenna configuration and new data processing hardware, the authors probed the electrojet with unprecedented spatial resolution in January 1991 at Jicamarca. The altitude resolution was 125 m, and the east-west antenna beamwidth was about 0.6[degrees], or about 1 km at electrojet altitudes. The high spatial resolution allowed them to observe, at certain altitudes and times, pronounced wavelike structures with vertical wavelengths of the order of 1 km and periods of tens of seconds. These observations closely resemble the predictions of recent nonlocal, nonlinear numerical simulations of large-scale turbulence in the equatorial electrojet. The comparison demonstrated the validity of the theory and confirmed the idea that the Doppler power spectra of type 2 echoes at 50 MHz (due to 3-m plasma waves generated by the gradient-drift instability and a nonlinear cascade of energy from long-wavelength modes to short) are controlled primarily by the large-scale dynamics and not by the nonlinear local growth and decay rates of the 3-m waves themselves. 16 refs., 7 figs.