Dynamics of the nighttime thermosphere at Arecibo

Incoherent scatter radar observations of the nighttime F layer at Arecibo, Puerto Rico, are used to determine the O+ diffusion velocity at different altitudes. Further analysis allows evaluation of the neutral wind and the ion-drag force in the direction of the magnetic meridian. The local acceleration of the meridional neutral wind is also determined. The possibility of evaluating the viscous force using incoherent scatter radar data was also investigated. Preliminary results indicate that, at certain times and at altitudes above about 350 km, viscous drag may be a significant term in the neutral equation of motion. Combining these results allows height profiles of the meridional pressure gradient to be deduced. The pressure gradients thus derived is compared with that determined from measurements of the horizontal temperature gradient and that given by the MSIS model atmosphere

Temporal, seasonal, and solar cycle variations of the topside proton concentration at Arecibo

An estimate of the topside proton concentration can be obtained by analyzing the incoherent scatter radar autocorrelation function from that region. Our observations at Arecibo, Puerto Rico indicate that the H+ to O+ composition transition height often occurs as low as 450 km during winter nights, near solar minimum. In the summer, the nighttime transition height is only slightly higher, near 600 km. This height is also affected by the level of magnetic activity. Maximum concentrations of H+ occur near 04:00 LT, with a minimum about twelve hours later. The observations of topside H+ were acquired during the past six years as part of the Regular World Day experiments, and from studies of the low latitude hydrogen geocorona. In particular, we will emphasize the results obtained during the solstice and equinox periods of the Thermospheric Mapping Study.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26936/1/0000502.pd

Observations and Interpretation of Gravity Wave Induced Fluctuations in the O I (557.7 nm) Airglow

Observations of fluctuations in the intensity and temperature of the O I (557.7 nm) airglow taken at Arecibo in 1989 are reported and interpreted on the assumption that they are caused by gravity waves propagating through the emission layer. The data give the magnitude of Krassovsky\u27s ratio as 3.5 ± 2.2, at periods between about 5 and 10 hours. Comparison with theory shows that the gravity waves responsible for the measured airglow variations must have long wavelengths of several thousand kilometers. The observed phases of Krassovsky\u27s ratio are in good agreement with theoretically predicted values at the long wavelengths and large periods for about half the cases. In the other cases, observed phases are near −180°, suggesting that the waves responsible for the airglow fluctuations have experienced strong reflections in the emission layer. The observations emphasize the importance of knowing the full altitude profiles of temperature and winds for extraction of wave information from the airglow fluctuations

Plasma dynamics in the night-time F-region at Arecibo

Incoherent scatter radar observations of the night-time F-layer at Arecibo, Puerto Rico, are used to determine horizontal gradients in peak electron density and the F-layer height. At certain times of night, in both the summer and winter, the horizontal gradients in these parameters may be significant. The continuity equation for O+ is used to examine the sources and sinks of plasma at the peak of the night-time F-layer. In summer, before midnight, the F-layer is sufficiently elevated that chemical recombination at the peak is very slow. At these times we find that the local peak electron density may be significantly influenced by horizontal (predominantly eastward) advection or convergence of plasma. In winter, horizontal advection is usually less important than plasma convergence and chemical loss.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25614/1/0000162.pd

Horizontal thermal structure of the mesosphere from observations of OH(8-3) band emissions

Two computerized tilting-filter photometers and a programmable dual axis mirror system have been used to produce maps of OH rotational temperature and intensity. Each map consists of a square array of 121 sky positions. Significant horizontal structure is not generally observed in mesospheric OH(8-3) rotational temperature at Arecibo. However, there is evidence for the occasional occurence of a thermal wave just after evening twilight.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24225/1/0000484.pd

Thermospheric tides during thermosphere mapping study periods

Neutral exospheric temperatures at 53[deg], 43[deg] and 33[deg] latitude from Millstone Hill steerable-antenna Thomson scatter measurements, and at 19[deg] latitude from the Arecibo Observatory, obtained during three Thermosphere Mapping Study (TMS) coordinated campaign intervals during 1984 and 1985, are analyzed for diurnal and semidiurnal tidal components. The resulting amplitude and phase latitudinal structures are compared with numerical simulations. The observed semidiurnal tidal components are thought to be significantly affected by tidal waves propagating upwards from below the thermosphere during these solar minimum periods. We speculate that current inadequacies in specifying F-region plasma densities and mean zonal winds at lower altitudes within the simulation model may account for certain discrepancies between observations and theory.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26931/1/0000497.pd

Climatologies of nighttime upper thermospheric winds measured by ground‐based Fabry‐Perot interferometers during geomagnetically quiet conditions: 1. Local time, latitudinal, seasonal, and solar cycle dependence

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94633/1/jgra18559.pd

Measurements of the E region neutral wind field

The neutral E-region wind field was measured at Calgary, Canada (51[deg]N, 114[deg]W) during 75 nights in 1982. Observations of the Doppler shift of the 5577-A emission line of atomic oxygen using a Fabry-Perot interferometer were converted to horizontal wind vectors. From the analysis of the data, four categories of wind characteristics were identified. In order of increasing magnetic activity these categories are (a) wind field mostly variable in space and time, (b) predominantly equatorward flow throughout the night, (c) predominantly poleward flow throughout the night and (d) north-westward flow before midnight and southward after midnight. The wind magnitude was also variable and on some disturbed days exceeded 200 m s-1.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25723/1/0000280.pd

Ground-based measurements of O1D and the H2O production rate from comets

Ground-based high spectral resolution measurements of Comet Halley in the 6300.3 A spectral region establish this technique as a powerful and low cost method for determining cometary H2O production rates. A model on the line profile to be expected from the O1D coma emission indicates that ground-based observations with fields of view on the comet of about 1 x 105 km measure profiles characteristic of O1D from the H2O parent. As the field of view increases, the signature of more energetic O1D ejected by CO2 and CO photolysis may become evident in the wings of the 6300.3 A line profile. The width of the 6300.3 A O1D profile is found to be equal to the ejection speed of O1D following H2O photolysis for fields of view to 1 x 106 km. Halley observations show that cometary NH2 (0,8,0) emissions do not contaminate the cometary O1D signature when the spectral resolution is 2O production rates determined by scaling the measured O1D production rates according to the H2O photolysis branching ratio are in good agreement with other measurements. Minor upgrades in the tracking and spectral resolution capabilities of observatories now dedicated to measurements of the terrestrial airglow would be useful for expansion of the high resolution cometary O1D data base - to include comets that come with less fanfare than did Halley.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28149/1/0000601.pd

Simultaneous optical observations of long-period gravity waves during AIDA '89

Ground-based optical instrumentation supported the AIDA '89 wind measurement comparisons by describing the gravity waves affecting the 80-100 km altitude region during clear dark hours over Puerto Rico. This study tabulates the characteristics of gravity waves with fractional column emission rate amplitudes up to 30% and with periods greater than 45 min as seen in the O2 airglow layer by MORTI, a sensor of O2 rotational temperature and column emission rate in twelve look directions. Data from seven other sensors operating at Guanica and the Arecibo Observatory are then compared with the MORTI data to check the consistency of the entire data set with the wave parameters, primarily velocities, deduced from MORTI. Nine nights of visually distinct crests and troughs were found, one of which was dominated by an evanescent wave and the rest by internal waves. The nights of 5/6 April and 4/5 May 1989 were selected for multi-sensor comparisons. The comparisons showed substantial agreement between the MORTI characterizations and the observations by others, and most differences were attributed to complexities introduced by higher frequency components with shorter coherence distances. Nightly summaries of the O2 rotational temperature and column emission rate are also given.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30949/1/0000621.pd