Robert C. Carlos

Observations of acoustic-gravity waves in the thermosphere following Space Shuttle ascents

Abstract Using an ionospheric Doppler sounder at Havelock, North Carolina, we observed upper atmospheric waves generated by three ascents of the Space Shuttle during 1990–1991. The exhaust plumes initial explosion and subsequent buoyant rise apparently launch acoustic and buoyancy waves, respectively. The bouyancy waves observed close (

A study of apparent ionospheric motions associated with multiple traveling ionospheric disturbances

Abstract The apparent undulatory motion of the ionospheric HF reflecting surface is studied using a method based on the relationship between angle-of-arrival and Doppler. The conditions are mid-latitude, daytime, F -region, winter/spring/summer. We usually find that multiple traveling ionospheric disturbances are active simultaneously, so that the HF Doppler evolves in a highly irregular and non-sinusoidal manner. We study the influence of this multiple-wave overlap on the apparent propagation velocity measured quasi-instantaneously. The apparent propagation velocity retains the key features observed by others, namely dominant propagation toward the E/SE, and virtually no propagation toward the W/NW. However, the apparent instantaneous velocity varies widely in both magnitude and azimuth over times as short as 500s. This is explained in terms of multiple-wave overlap.

The Los Alamos beacon receiver array

Describes radio receivers that monitor transmissions from beacons on geosynchronous satellites. The receivers can detect ionospheric perturbations of a 300-3000 s period in the electron density integrated from beacon to receiver, for amplitudes as low as (1-2)/spl times/10/sup 13/ m/sup -2/. Data are used in studies of atmospheric acoustic and acoustic-gravity waves. >

Coherent-array HF Doppler sounding of traveling ionospheric disturbances: I. Basic technique

Abstract We present an introduction to the use of phase-coherent, multi-receiver HF Doppier sounding arrays for measuring the horizontal velocity of traveling ionospheric disturbances (TIDs). The point of departure is the theorem of Pfister (1971, J. atmos. terr. Phys . 33 , 999) relating ray Doppler to ray zenith angle for a monostatic full reflection sounder. Retaining the simple model of a specular, smooth ionospheric reflector which is deformed by a propagating undulation, we first generalize the theorem to bistatic sounding geometry and then include the effects of amplitude in addition to phase. Next, these results are cast into an algorithm for treating multi-receiver phase sounders containing many diverse baselines, in order to obtain an accurate and unambiguous solution in the plane of wave slowness (inverse of velocity). The point spread function of this solution is controlled by process bandwidth and by array geometry. We illustrate the coherent-array approach using data from an eight receiver array during passage of a TID.

Total-electron-content signatures of plasmaspheric motions

Measurements of fluctuations in total electron content made with a radio interferometer reveal weak but frequent outbursts of total-electron-content disturbances, with trace azimuths roughly westward, with trace speeds up to ∼2 km/s, and with temporal periods in the range 100–500 seconds. Statistical tests of the dataset suggest that these disturbances are due to drift of plasmaspheric irregularities past the radio lines-of-sight. The phenomenon is likely to provide a new passive tracer of zonal convection in the inner magnetosphere.

The Los Alamos microwave interferometer

The authors describe a multi-antenna microwave receiver system that monitors an unmodulated beacon transmission from a geosynchronous satellite. The system interferometrically measures temporal fluctuations in tropospheric differential path length, which include fluctuations in precipitable water vapor, over 100to 400-meter baseline lengths. Over 300 s, the system root mean square error (rms) noise is 0.01 radian. These observations will facilitate studies of air parcel motion as the means by which the causative, phase-corrupting atmospheric inhomogeneities drift over the array. The resulting data will be useful for studies of convective boundary layer turbulence, a region difficult to fully access.

Effects of artificially modified ionospheres on HF propagation: Negative Ion Cation Release Experiment 2 and CRRES Coqui experiments

We report the results of measurements obtained in conjunction with a series of high-altitude chemical release experiments of effects of artificially modified ionospheres upon high-frequency, ionospherically reflected radio paths. Computer simulations indicate that under optimum conditions, ionospheric modifications induced by chemical releases could perturb or even disrupt a communication channel; our experiments corroborate this but also indicate that it is very difficult to actualize such disruptions. Our experiments have shown that an ionospheric depletion, in which the electron density hole forms a huge radio frequency lens, generates new modes which, however, do not significantly affect a communications system. Under optimum path geometry a signal strength decrease of 10 dB or more is possible for several tens of minutes. Enhancements, such as those produced by barium releases, act as reflecting mirrors that can create a large shadow zone on the ground and block off significant amounts of energy. We measured signal strength decreases of up to 20 dB.{copyright} 1997 American Geophysical Union

Smoothness of the HF virtual reflector in the quiet, daytime mid-latitude F-region

Abstract Traveling ionospheric disturbances (TIDs) modulate the amplitude and phase of received F -region HF skywave signals. This TID-generated complex modulation usually masks the weaker effect of fine-scale ionospheric irregularities, complicating application of phase-screen analyses to HF array data. We employ knowledge of the instantaneous TID trace velocity, measured by compact-array velocimetry, in separating the effects of TIDs from those of finer-scale irregularities in the same data set. A classical phase-screen analysis is then applied to the suitably transformed data. We find that during normal conditions the irregularities consist simply of weak random tilts (a few milliradians r.m.s.), on scales exceeding 1 km. On the other hand, application of phase-screen analysis directly to the untransformed data, containing TID effects, would give erroneous conclusions that the random tilts are significantly stronger and on shorter scales. These results confirm a long-standing critique of full-reflection irregularities drift measurements: the effect of TIDs largely dominates that of drifting irregularities, making use of the latter (as a marker of mean drift) difficult or impossible.

Coherent-array HF Doppier sounding of traveling ionospheric disturbances: II. Assessment of errors

Abstract We examine errors which are likely to occur in the compact-array HF sounding technique for determining the velocity of traveling ionospheric disturbances. The errors arise from ionospheric irregularities (either large-scale specular facets, or fine-scale scatterers) and from the presence of multiple disturbance undulations propagating simultaneously with different velocities. The errors are either systematic (that is, not causing internal inconsistencies within the array velocimetry data) or random (that is, causing quantifiable inconsistencies between alternative velocity estimates furnished by the array). The random errors can be directly assessed using array data, as is illustrated for actual data from a 16-channel array.

Observations of precipitable water vapor fluctuations in convective boundary layer via microwave interferometry

At microwave frequencies, each centimeter of precipitable water vapor (PWV) causes about 6.45 cm of extra electrical path length relative to the “dry” air. The fluctuations of the water vapor dominate the changes of the effective path length through the atmosphere in a relatively short time period of a few hours. In this paper we describe a microwave interferometer developed for water vapor investigations and present the observation results. The interferometer consists of 10 antennas along two orthogonal 400-m arms that form many baselines (antenna pairs) ranging from 100 to 400 m. All the antennas receive a common CW signal (11.7 GHz) from a geostationary television satellite, and phase differences between pairs of antennas are measured. The phase differences reflect the column-integrated water vapor differences from the top of the atmosphere to the spatially separated antennas at the ground. The interferometric, baseline-differential measurements allow us to study the statistical properties of the PWV fluctuations, as well as the turbulent activity of the convective boundary layer (CBL). Structure function analysis of the interferometer measurements shows good agreement with results obtained from the Very Large Array (VLA) and with a theoretical model developed for radio astronomical very long baseline interferometry (VLBI), reported previously by other investigators. The diurnally varying structure constant correlates remarkably well with the combination of the latent and sensible heat fluxes measured simultaneously from a 10-m meteorological tower. The average drift velocity of the PWV over the interferometer was also derived from the measurements. The derived velocity agrees well during the morning hours with the wind measured by an anemometer at the center of the interferometer.