Ronald F. Woodman

Average vertical and zonal F region plasma drifts over Jicamarca

The seasonal averages of the equatorial F region vertical and zonal plasma drifts are determined using extensive incoherent scatter radar observations from Jicamarca during 1968-1988. The late afternoon and nighttime vertical and zonal drifts are strongly dependent on the 10.7-cm solar flux. The authors show that the evening prereversal enhancement of vertical drifts increases linearly with solar flux during equinox but tends to saturate for large fluxes during southern hemisphere winter. They examine in detail, for the first time, the seasonal variation of the zonal plasma drifts and their dependence on solar flux and magnetic activity. The seasonal effects on the zonal drifts are most pronounced in the midnight-morning sector. The nighttime eastward drifts increase with solar flux for all seasons but decrease slightly with magnetic activity. The daytime westward drifts are essentially independent of season, solar cycle, and magnetic activity.

Radar Observations of Winds and Turbulence in the Stratosphere and Mesosphere

Abstract A technique for the observation of radar echoes from stratospheric and mesospheric heights has been developed at the Jicamarca Radio Observatory. Signals are detected at the altitude ranges between 10–35 km and from 55–85 km with powers from many to several tens of decibels above noise level. The three most important frequency spectrum characteristics-power, Doppler shift and spectrum width-are observed in real time. The power levels as well as the spectral width are explained in terms of turbulent layers, with a thickness of the order of 100 m, in regions with a positive potential temperature or electron density vertical gradients. Continuous wind velocity records are obtained with a precision of the order of 0.02–0.2 m sec−1 for the vertical component and 0.20–2 m sec−1 for the horizontal, with a time resolution of the order of 1 min. The highest precisions are obtained at stratosphere heights. Fluctuations in velocity in the mesosphere are observed at the shortest gravity wave periods with amp...

Mid-latitude E region field-aligned irregularities observed with the MU radar

Fine structures E region field-aligned irregularities were observed on June 24–25, 1989, with the MU radar at Shigaraki, Japan (34.9°N, 136.1°E; geomagnetic latitude 25.0°N). The 3.2-m scale irregularities were observed with the MU radar in five main beam directions, each of which was nearly perpendicular to the geomagnetic field at 100 km altitude. Doppler spectra were obtained every 20 s with a range resolution of 600 m. Field-perpendicular echoes appeared from 2130 to 2330 LT and from 0400 to 1100 LT, times that correspond to postsunset and postsunrise period in the E region. A preliminary examination of the Doppler spectra indicates spectral widths of 50–120 m s−1 and the mean Doppler velocities are well below the ion acoustic speed. These spectral characteristics are consistent with those obtained in the equatorial and auroral electrojets, and have been attributed to the gradient drift instability. The echoes observed during the postsunset and postsunrise periods showed quite different morphologies in the time-height distribution. For this reason, they are classified into two types, ‘continuous’ and ‘quasi-periodic.’ The appearance of the ‘continuous’ echoes was mainly continuous in time and situated between 90 and 100 km altitude during the postsunrise period. The appearance of the ‘quasi-periodic’ echoes was intermittent with periods of 5–10 min and situated above 100 km altitude during the postsunset period. The quasi-periodic echoes showed phase propagation toward the radar, while the averaged mean Doppler velocity was away from the radar. By measuring the time delays in echo regions from five directions, an apparent westward motion (approximately 120 m s−1) of the irregularity regions was estimated.

Gravity wave modulation of gradient drift instabilities in mid‐latitude sporadic E irregularities

Recent E region VHF backscatter echoes observed by the MU radar at mid-latitudes show quasi-periodic striations with a fairly constant range vs. time tilt in a RTI display. These features are explained in terms of gravity waves with frequencies close to the Brunt-Vaisala frequency which modulate the shape of sporadic E layers. The conditions of instability, when the magnetic field has a significant dip angle, is revised. Differing from previous work, we argue that conditions of local gradient drift instability are not sufficient and one has to consider the integrated properties of each magnetic filed tube. Stratified sporadic E layers are stable using this new criteria, unless they are distorted to produce unstable integrated gradients. Gravity waves with phase fronts parallel to the magnetic dip angle are capable of producing such distortion, imposing its own temporal and spatial periodicity on the echoes.

A new approach in incoherent scatter F region E × B drift measurements at Jicamarca

Since 1996 incoherent scatter F region plasma drift measurements at Jicamarca have been implemented using a new signal processing approach replacing the traditional pulse-to-pulse correlation method. The new method, based on Doppler spectrum estimation and nonlinear least squares fitting to model spectra obtained from incoherent scatter theory, improves the instrumental sensitivity remarkably under low signal-to-noise conditions. With the new method it has become possible to obtain very high quality drifts data at nearly all hours of the day throughout most F region heights. Altitudinal smoothing of the drifts data to reduce measurement noise is no longer necessary, and studies of the height variations of drifts can be performed with much greater certainty than before. Small-amplitude gravity wave oscillations have been detected at F region heights and a vortical circulation of the F region plasma has been observed in the post sunset period.

Coherent radar imaging: Signal processing and statistical properties

The recently developed technique for imaging radar scattering irregularities has opened a great scientific potential for ionospheric and atmospheric coherent radars. These images are obtained by processing the diffraction pattern of the backscattered electromagnetic field at a finite number of sampling points on the ground. In this paper, we review the mathematical relationship between the statistical covariance of these samples, ( †), and that of the radiating object field to be imaged, ( †), in a self-contained and comprehensive way. It is shown that these matrices are related in a linear way by ( †) = aM(FF†)M†a*, where M is a discrete Fourier transform operator and a is a matrix operator representing the discrete and limited sampling of the field. The image, or brightness distribution, is the diagonal of (FF†). The equation can be linearly inverted only in special cases. In most cases, inversion algorithms which make use of a priori information or maximum entropy constraints must be used. A naive (biased) “image” can be estimated in a manner analogous to an optical camera by simply applying an inverse DFT operator to the sampled field and evaluating the average power of the elements of the resulting vector . Such a transformation can be obtained either digitally or in an analog way. For the latter we can use a Butler matrix consisting of properly interconnected transmission lines. The case of radar targets in the near field is included as a new contribution. This case involves an additional matrix operator b, which is an analog of an optical lens used to compensate for the curvature of the phase fronts of the backscattered field. This “focusing” can be done after the statistics have been obtained. The formalism is derived for brightness distributions representing total powers. However, the derived expressions have been extended to include “color” images for each of the frequency components of the sampled time series. The frequency filtering is achieved by estimating spectra and cross spectra of the sample time series, in lieu of the power and cross correlations used in the derivation.

VHF radar interferometry measurements of vertical velocity and the effect of tilted refractivity surfaces on standard Doppler measurements

At VHF wavelengths, aspect sensitivity may result in an apparent beam direction that is off vertical even for a nominally vertically pointing beam direction if the refractivity surfaces responsible for the scatter are tilted with respect to the horizontal plane. Middle and upper atmosphere radar measurements obtained by using the system in a standard multireceiver configuration typical for radar interferometry (RI) and spaced antenna measurements have been analyzed for evidence of such effects. The analysis is based on the linear variation of the cross-spectral phase as a function of the radial velocity in the frequency domain for the RI cross spectra. True-vertical velocity estimates are obtained by using the fact that the phase difference between two antennas should be equal to zero when the echoes are being received from the vertical direction. The tilt angles of the refractivity surfaces were obtained from the phase of the cross-correlation function at zero lag, and the radial velocity in that direction was determined from the cross spectra. The results indicate that the vertical velocity derived from standard Doppler analyses is actually the velocity perpendicular to the refractivity surfaces and thus can be biased by the projection of the horizontal wind along the effective pointing direction.

First observations of polar mesosphere summer echoes in Antarctica

A 25-kW peak power 50-MHz radar was installed at the Peruvian base on King George Island, Antarctica (62°S), in early 1993. A search for polar mesospheric summer echoes (PMSEs) was made during late January and early February of the first year of operation with negative results. These results have been reported in the literature [Balsley et al., 1993; 1995]. We report here results obtained during the austral summer of the second year (1994) of operation. Observations during the second year were begun earlier, i.e., closer to the austral summer solstice. PMSEs were observed during this period, albeit the echoes were much weaker than what one would expect based on earlier Poker Flat radar results at a comparable latitude (65°N) in the Northern Hemisphere. A large and measurable asymmetry in PMSE strength in the two hemispheres therefore exists. We explain this asymmetry by postulating a difference in summer mesopause temperatures between the two hemispheres of ∼7.5 K. This difference has been estimated using an empirical relationship between the variations of the Poker Flat PMSE power as a function of temperature given by the mass spectrometer incoherent scatter extended (MSISE-90) model.

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.

On the lack of southern hemisphere polar mesosphere summer echoes

We report VHF radar observations of the southern high-latitude mesopause region using wind profilers that were installed recently on King George Island, Antarctica, and Ushuaia, Argentina. Briefly, our observations, which were made during January and February 1993, show almost no evidence of so-called polar mesosphere summer echoes, or PMSE. Since these echoes are a predominant feature of the northern high-latitude mesosphere in summer, their absence in the southern hemisphere is both surprising and intriguing. In this paper we present evidence demonstrating the virtual absence of the echoes and demonstrate that our systems were capable of detecting them had they been present. We also outline some of the consequences of this intriguing result, which are supported by observed hemispheric differences in polar mesospheric clouds, mesospheric temperatures, upper atmospheric gravity wave activity, and mean circulation patterns.