W. L. Oliver

A climatology of F region gravity wave propagation over the middle and upper atmosphere radar

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere radar, we have been able to track the passage of gravity waves and measure their propagation characteristics. Here we develop a climatology of wave propagation based on the observation of 58 daytime experiments conducted during 1986–1994. The thermosphere seems to be continuously swept by waves detectable by an incoherent scatter radar. These waves generally come for hours on end from a consistent or slowly varying direction, which can be any direction on a given day. Statistically, the waves show a moderate preference for southward travel, with this preference being reduced or shifted to southeastward travel during disturbed times. On average, the horizontal phase trace speed remains near 240 m/s for all periods inspected (40–130 min). This speed matches the behavior expected for lossless waves with 150–200 km vertical wavelength. We find small variability in this relation for different times of day, seasons, solar and magnetic conditions, and directions of wave travel, though waves on disturbed days seem to travel moderately faster on solar minimum mornings.

A climatology of middle and upper atmosphere radar observations of thermospheric winds

Shigaraki middle and upper atmosphere (MU) radar observations of horizontal thermospheric winds in the magnetic meridian plane over the period September 1986 to September 1996 are reported as climatological averages in the form of time-of-day variations for several combinations of seasonal and solar activity conditions and are compared with winds predicted by the horizontal wind model (HWM) and with winds measured at Saint Santin and Millstone Hill. The dominant feature of the MU wind behavior is its mean diurnal variation of northward flow by day and southward flow by night, with the nighttime wind smoothly approaching and receding from a midnight maximum, while the daytime wind tends to show two peaks, a strong one in the early daylight hours and a weak one in the afternoon-evening. HWM shows the same unimodal nighttime and bimodal daytime behavior, but the HWM pattern is shifted about 2 hours later in time. The amplitude of the diurnal harmonic decreases from 78 m/s at solar minimum to 45 m/s at solar maximum, while HWM shows a corresponding increase from 53 to 62 m/s. The diurnal amplitude is remarkably stable with season but is superposed on a steady wind of 41 m/s southward in summer, 15 m/s northward in winter, and midway between these limits at the equinoxes. HWM shows a symmetric pattern of 30 m/s southward in summer and 30 m/s northward in winter. Ion drag appears to be the main regulator of wind speed, and the seasonal wind patterns have a profound effect on the seasonal behavior of the ionosphere.

Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere (MU) radar, we have been able to track the passage of gravity waves and measure the horizontal electron density gradients that they produce. Our previous work on this topic, on one day of observations, verified that the gradient structure and fluctuations were consistent with a gravity wave source. The present work (1) looks at several days of observation to confirm the routine existence of these gradient structures; (2) identifies individual wave packets; (3) finds evidence that long trains of gravity wave fluctuations are composed of several juxtaposed wave packets of one or two cycles each rather than a single wave of many cycles; (4) finds that during quiet periods strong waves are present only during the daytime, but during disturbed periods they are present day and night; and (5) notes several other characterizations.

The height of the maximum ionospheric electron density over the MU radar

Ionospheric F-2-layer peak height h(m)F(2) variations, as measured over 1986-1995 by the MU radar (34.85 degrees N, 136.1 degrees E) and as calculated with a theoretical model, are discussed. The diurnal variations of the measured peak height for different seasons and levels of solar activity are compared with those estimated from ionosonde M3000F(2) and IRI predictions. Also given are the measured ion drift velocities and meridional neutral winds needed to understand the dynamic behavior of the F-2-layer. It is found that: (1) h(m)F(2) is generally higher during periods of the solar maximum than during periods of the solar minimum, and higher in summer than in winter; (2) for the solar maximum, h(m)F(2) drops markedly in the morning and in the afternoon, while, for the solar minimum, the h(m)F(2) minimum occurs in the morning during summer and usually in the afternoon during winter. In general, the measured h(m)F(2) is well reproduced by our model when we use the observed drift velocities and plasma temperatures as inputs. Our modeling study shows that the neutral wind contributes strongly to the diurnal variation of h(m)F(2) in winter by lowering the ionization layer by day, particularly for the solar maximum; it also helps to enlarge the daynight difference of h(m)F(2) in summer. The northward electromagnetic drifts that usually cancel the neutral wind effect have only a minor effect for the location of the MU radar. Other features of the observed h(m)F(2) variations, e.g., the solar maximum-minimum difference, the summer-winter difference, and the morning and afternoon drops, are explained by the basic processes of O+ production, loss and diffusion, as influenced by the atomic oxygen concentration and neutral and plasma temperatures

F-region seasonal behavior as measured by the MU radar

Abstract We present MU radar ionospheric incoherent scatter results from the period December 1986 to August 1988. We extend our previous 3-month model of the ion vector velocity to a full year and determine seasonal variations. We also present solstitial and equinoctial patterns of electron-density and plasma-temperature behavior. Compared with our previous 3-month model, the full-year velocity model is largely diurnal in nature, the higher harmonic being very variable and tending to average out as noise over a long period. The seasonal behavior of the electron density may be explained largely in terms of the behaviors of the neutral composition and of the timing of the diurnal change of direction of the neutral wind with respect to the time of sunset. According to the neutral composition a high-altitude O+ layer and a low-altitude molecular-ion layer compete for density predominance. The molecular-ion layer often becomes stronger during summer, and also during times of strong magnetic disturbance. The mean equinox daytime F-region electron and ion temperatures (Te and Ti), and particularly their ratio ( T r = T e T i ), is controlled largely by the electron density, with other specific time-of-day effects. At night Tr= 1, and at sunrise Tr > 1 at all F-region altitudes measured. During midday we have two cases. For a peak F-layer density of about 106 cm−3 or greater, Tr > 1 below 300 km altitude and Tr = 1 above 300 km altitude; for lower electron densities Tr > 1 at all F-region altitudes. Near sunset Tr generally increases as the electron density decreases while photoionization is continuing.

Middle and upper atmosphere radar observations of ionospheric electric fields

Middle and upper atmosphere (MU) radar observations of electric fields over the period September 1986 to January 1991 are reported in the form of time-of-day variations for several different seasonal and solar activity cases. The perpendicular north velocity has on average a smooth and distinct northward excursion from 0600-1500 LT followed by a period of constant southward velocity until midnight and then a slow linear change to merge with the daytime northward excursion period. This excursion feature is small in summer and enhanced in winter but, seasonally averaged, has only small solar activity variation. The individual seasonal averages show afternoon/evening excursion behavior apparently tied to the time of sunset. The perpendicular east velocity is characterized by a major eastward excursion in the evening and a minor eastward excursion in the morning, with a tendency to level off or form a slight additional local eastward excursion after midnight. The evening excursion is enhanced during winter and at high solar activity, and the postmidnight excursion is enhanced during summer. The morning excursion has no comparable counterpart in existing electric field models or data averages from Arecibo or Jicamarca. We suggest that this may be a semidiurnal component that evidences enhanced propagation of the semidiurnal tide at the location of the MU radar. The maximum daily amplitudes and the time-of-day and solar activity trends at the Arecibo and MU radar locations have strong resemblances, but the seasonal trends in the MU data, the Arecibo data, and existing radar models disagree strongly.

Solar cycle variations of the thermospheric meridional wind over Japan derived from measurements of h m F 2

This paper describes an analysis of the meridional equivalent neutral wind for geomagnetically quiet conditions as determined from data collected at the Kokubunji ionosonde station (35.7N, 139.5E), Japan over one solar cycle (1981–1991). The wind is derived from the altitude of peak F2 layer electron density using the Field Line Interhemispheric Plasma (FLIP) model. For low and moderate solar activity the wind is poleward in the daytime and equatorward in the nighttime. For high solar activity the wind is weak and almost always poleward throughout the day. In winter the local time of peak poleward velocity occurs in the afternoon for low solar activity but in the night for high solar activity. In summer the peak poleward wind occurs in the morning for all levels of solar activity. The diurnal amplitude decreases with increasing solar activity. It also reaches its maximum around the solstices and its minimum around the equinoxes. The mean wind is larger at solar maximum than solar minimum, except for the year 1984. The daily-mean wind is smallest in summer and largest in winter. Qualitatively, the meridional wind at Asian midlatitudes has characteristics similar to those seen in other sectors (such as Millstone Hill, Boulder, Wallops Island, King George Island, and Saint-Santin), but the details in behavior are different and warrant further investigation.

Middle and upper atmosphere radar observations of the dispersion relation for ionospheric gravity waves

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere radar, we have been able to track the passage of gravity waves and measure their propagation characteristics. Here we investigate the dispersion relation for these waves. This relation varies widely from day to day, even on two successive magnetically extremely quiet days. On average, however, we find that the wave horizontal phase trace speed is about 200 m/s for periods from 60 to 130 min with little variation with solar or magnetic activity. There is some indication that the speed increases slightly from equinox to winter.

Measurements of ionospheric and thermospheric temperatures and densities with the middle and upper atmosphere radar

The first incoherent scatter radar local time/seasonal/solar cycle averages of F region electron, ion, and neutral temperatures for Asian longitudes are presented. These measurements were made with the four-pulse experiment by the middle and upper atmosphere (MU) radar in Japan over the period August 1986 to April 1990. The neutral temperature and density results are compared with MSIS-86 model predictions. We divide our data into low and high solar activity levels, four seasons, and 24 one-hour times bins. While the seasonal and solar activity behavior of the ionospheric density, and of the anticorrelation of the electron temperature with this density, shows many aspects in common with those at other locations, a few particular differences are evident. At low solar activity the height of the F layer varies from 240 km during the day to 320 km at night, regardless of season. At high solar activity the maximum density of the F layer varies little with season. These facts seem to indicate little seasonal change in F region neutral atmospheric composition at this geographic location. At solar maximum the F layer is established at a higher altitude than at low solar activity, and the summer F layer in particular is formed at such a high altitude that its decay at night is very slow, its diurnal variation is hence weak, and the density remains so high at sunrise that little sunrise effect in the electron temperature is produced. The neutral temperatures in the F region have the same basic diurnal pattern as does the MSIS-1986 model temperatures, but the radar temperatures are consistently lower by an amount ranging up to 160 K for summer conditions at high solar activity. The measured and model temperatures are closest in the autumn and winter. The neutral density results are of low quality and do not provide any evidence of need to modify the MSIS model densities.

Solar EUV flux, exospheric temperature and thermospheric wind inferred from incoherent scatter measurements of the electron density profile at Millstone Hill and Shigaraki

[1] We explore a method for inferring solar EUV flux, atmospheric composition and wind using ionospheric electron density Ne profile measurements. Incoherent scatter radar data from Millstone Hill and Shigaraki measured on October 5, 1989 are assimilated into a theoretical model whose driving forces, solar EUV flux, exosphere temperature Tex, and meridional wind, are adjustable. Adjustments are made to give best match between the model Ne profile and the data. The derived Tex values, found to be low near noon at Millstone and high in the afternoon at Shigaraki, are essentially those required to give the [O]/[N-2] ratio necessary to fit the data. Our inferred EUV fluxes for the two sites are similar. Our technique of using profile data may resolve the ambiguity in deriving EUV and [O]/[N-2] from electron-density measurements.