Search

A set of tabulated functions called ‘Hough Mode Extensions’ (HMEs), which represent numerical extensions of classical Hough modes into the viscous regime of the thermosphere, are used to least-squares fit a climatological data base of tidal measurements. The data base consists of monthly average vertical profiles of semidiurnal amplitudes and phases at 17 radar sites accessing some part of the 80–150 km height region. The radars are distributed between 78 S and 70 N latitude, and each one provides measurements of one or more of the following: eastward wind, southward wind, perturbation temperature. As a result of the fitting process, a single complex normalizing coefficient is derived for each month and for each of the four HMEs, designated (2,2), (2,3), (2,4) and (2,5) after their classical Hough function designations. Once the complex coefficients are derived, reconstruction by weighted superposition of the HMEs results in globally continuous specifications of semidiurnal horizontal and vertical wind, temperature, pressure, and density throughout the 80–150 km height region. The tidal variations in density, in particular, provide greater accuracy for several aerospace applications. The methodology developed here can also be utilized to derive tidal lower boundary conditions for Thermospheric General Circulation Models (TGCMs), or as a basis for future empirical model development. Comparisons are also made with HME coefficients and global tidal fields from the Forbes and Vial [(1989) J. atmos. terr. Phys. 51 , 649] numerical tidal model.

Seasonally averaged quiet-day F region ionospheric E × B drift observations from the Millstone Hill, St. Santin, Arecibo, and Jicamarca incoherent scatter radars are used to produce a model of the middle and low-latitude electric field for solar minimum conditions. A function similar to an electrostatic potential is fitted to the data to provide model values continuous in latitude, longitude, time of day, and day of the year. This model is intended to serve as a reference standard for applications requiring global knowledge of the mean electric field or requiring information at some location removed from the observing radars. This article contains supplementary material.

From January 1971 to March 1973, five satellites of the Navy Navigation Series (Transit) in polar orbits at 1000 km altitude were tracked from Millstone Hill to secure differential-Doppler measurements of N-S gradients in the ionosphere. In all, over 2000 passes were observed, encompassing all local times and seasons. All of the differential-Doppler records have been scanned to secure statistical information on the incidence, wavelength, amplitude, and location of the traveling ionospheric disturbances detected. Most of the TID's had wavelengths in the range of 150–350 km and were seen to the south of the station, suggesting that they were traveling from north to south. The incidence peaked at 60% of all passes at 0800 local time and dropped to less than 5% for a few hours before sunrise. The seasonal variation is less clear-cut, but the incidence seems to be increased in winter and the equinoxes and reduced in summer. This may be simply a consequence of the dependence of the detectability on total content. While the character of the fluctuations and a north-to-south motion would be consistent with a source in the auroral ionosphere, there seems to be no increase in incidence during magnetically disturbed times. Moreover, in summer the waves are seen equally to the north and the south of the station. It seems possible that there may be many sources for these waves distributed in latitude and that the observed locations are controlled by the filtering effects of F region winds.

On decrit les observations de la region F a moyenne latitude, aux environs de L=4, le jour. On a utilise une methode de Doppler differentielle de mesures au sol. Cette methode a permis de separer les effets en temps local et temps universel

Incoherent scatter measurements using multiple-pulse techniques were performed at Arecibo to study the E region with a height resolution of 2.4 km. These simultaneous measurements of ion temperature, ion-neutral collision frequency, and electron density, as well as horizontal and vertical components of ion drift velocity, have allowed comparison between theories of sporadic-E and experiment. Sporadic-E layers were consistently observed to descend along the curve where the vertical ion drift velocity (Vz) was zero and its gradient ∂ Vz/∂z was negative, as predicted by redistribution theory. Assuming no diffusion, the calculated effective recombination coefficient at the layer peak is more than an order of magnitude too small for NO+ or O2+, and varies with height in a manner which is unacceptable for a recombination coefficient. This indicates that the layer is composed of long-lived ions (probably metallic), and that the peak layer density is controlled by diffusion rather than recombination. By ignoring recombination, the diffusion coefficient was estimated from the layer thickness and the vertical gradient in the vertical ion velocity. The results were compared with independent values calculated from the measurements of ion temperature and ion-neutral collision frequency. The two estimates were in good agreement over the range 110 to 120 km, showing that electron densities in sporadic-E layers are primarily determined by molecular diffusion.

A five-pulse technique was implemented for the 430 MHz incoherent scatter radar at Arecibo Observatory (18.3/sup 0/N) to explore the detailed thermal structure of the E region from 105 to 130 km with an altitude resolution of 3 km. Five days of measurements in Sept-Oct 1970 showed long-period temperature fluctuations having a downward phase progression. The temperature oscillations are interpreted as manifestations of a semidiurnal tide which is quite stable over a 12-day period, together with a superimposed spectrum of shorter-period gravity waves which are randomly phased from day to day. The semidiurnal tide increased to a maximum amplitude of 17 percent of the mean temperature near 115 km and decreased above this altitude as dissipative effects became important. The vertical wavelength, deduced from the altitude variation of semidiurnal tidal phase, showed a smooth increase from about 20 km at an altitude of 109 km to about 50 km at an altitude of 127 km. No ready interpretation of the observed tidal characteristics was possible in terms of present theories for the semidiurnal tide. Altitude profiles of mean daytime temperature and ion-neutral collision frequency were also obtained from the measurements. The mean temperature gradient between 115 and 130 km wasmore » 15 K/km, which is somewhat larger than that given by current atmospheric models.« less

Troughs in the auroral ionosphere have been observed in satellite and radar data since 1965. One explanation for the appearance of these troughs is the stagnation theory of trough formation. According to this model, magnetic flux tubes are convected from the dayside to the nightside across the polar cap as a result of the impressed magnetospheric electric field. These tubes return to the dayside by subsequently drifting westward or eastward. Tubes drifting westward must travel faster than earth rotation if they are to reemerge in sunlit regions, and some fail to do so, allowing the ionization in them to fall to very low levels as it recombines. The trough is the locus of flux tubes which spend a long time in darkness due to the competing effects of the convection electric field and earth rotation. Commencing in 1979, a new observing technique was implemented at Millstone Hill in which the 150-ft antenna is moved slowly under computer control as data collection proceeds. The electron density results obtained with these experiments show that when a trough does form there is a great variation in when and where it is seen and how long it is present. On some nights the trough does not form at all. Furthermore, the appearance of the trough in the data often seems to occur in regions where the electric field is very large, with the ion drift velocity often far exceeding the speed of earth rotation. None of this behavior is entirely consistent with the stagnation theory of trough formation. An alternative explanation is to suppose that troughs are formed in regions of very strong convection (>1 km/s) as a result of the increase in the rate of charge transfer between O+ and O that occurs when the ions are driven through the neutral gas at high speed. According to such a view, “troughs” would be created in localized regions during substorms. The troughs might be created in regions of strong westward flow in the dusk sector. They then could migrate south and eastward depending upon the effects of convection and corotation. In an effort to test this picture we have begun conducting observations through 360° azimuth to see if the ionosphere to the northeast of the radar is the same as that seen later to the northwest. Results are presented for December 21–22, 1980. These do exhibit significant differences, tending to confirm that the trough is not a persistent extended E-W feature.

On 26 February 1979, the Millstone Hill incoherent scatter radar was operated to observe the F-region over Canada along the path of the total eclipse of the sun. A continuous scanning mode was used with the radar elevation fixed at 4° and the azimuth swept continuously from 350° to 298°. Each scan required 20 min to complete and useful results were obtained at ranges up to 2992 km. The path of totality crossed the center of the region swept by the radar beam. The F-region electron density, ion temperature, electron temperature and ion line-of-sight drift were measured. Electric field components have been extracted from the radar line-of-sight component of the ion drift by assuming that the electric field may be represented by a quasi-static two-dimensional potential with the potential assumed constant along geomagnetic field lines. Electron density decreases were observed in association with the eclipse. At 300 km an ∼50% decrease occurred near the region of totality, but the variations were small above 450 km. However, apparently unrelated to the eclipse, very large electric fields developed in the region under view. Somewhat later a well defined trough formed north of Λ = 70°, whose equatorward edge is extremely sharp and appears to be approaching the radar with time. Large values of the electron and ion temperatures were observed in the trough and the trough formed where the drift was westwards and exceeded 1000 m s−1. It is suggested that the observed through may be due to an increase in the rate of the charge transfer reaction O+ + N2 → NO+ + N in the presence of a very large electric field impressed from the magnetosphere in association with a large magnetic disturbance.

The Millstone Hill radar, Westford, Massachusetts (Λ=56°), has been upgraded for observations in support of the international magnetosphere study (IMS) by means of the addition of a 150-ft diameter fully steerable antenna. This antenna allows measurements of F region ion drifts, from which F region electric fields can be derived, using the existing UHF (68-cm wavelength) radar. The paper describes measurements of the electric field in the auroral zone over the interval 60° ≤Λ ≤75°. The electric field is obtained by means of a least mean squares fit between the drifts obtained in four beam directions and a smoothly varying model with 204 degrees of freedom. Results are presented for 11 moderately quiet days in 1978 and these are compared with the models published by Heppner (1977). All but one of the days exhibit a simple two-cell convection pattern. Two days appear reasonably consistent with the pattern expected when the IMF is toward the sun (symmetric cells) and six when the IMF is away (dusk cell larger). The Harang discontinuity is identifiable in all these cases in extending from 22 to 01 LT. The agreement between the observations and the Heppner (1977) models can be improved by moving the center of the dawn cell into the nightside by perhaps as much as 3 hours in the case of the asymmetric pattern. Two of the 11 days fit neither pattern in that a feature corresponding to the Harang discontinuity cannot be identified. The size and speed of the flow in the convection cells appear to increase with Kp (and/or decreasing Dst) without any obvious change in the direction of the flow (except in one instance near noon). We find no evidence for a permanent narrow ‘throat’ region in the dayside cleft through which all the flux tubes convect, though it seems possible that this behavior may manifest itself during disturbed periods (Kp >3). At other times, the Heppner (1977) models seem to describe our observations on the dayside quite adequately.

A joint alert campaign was organized during the month of October 1980 by the incoherent scatter radars in the American sector: namely, Jicamarca, Arecibo, Millstone Hill, and Chatanika. The campaign, which met with success, was designed to study the behavior of the ionospheric electric field as a function of latitude during magnetically active conditions. The Arecibo data in this campaign support present and previous observations at Jicamarca that suggest that when the convection E field suddenly decreases, the Alfven layer shielding field becomes unbalanced and penetrates the plasmasphere. While this type of observation is reasonably convincing, others are more difficult to categorize. We suggest that, beside the high-latitude electric fields, time-varying auroral conductivity models will have to be considered in order to understand the morphology of the low-latitude E field disturbances. We present the first correlation analysis and determination of the amplitude ratio of the disturbed zonal electric field at 30° geometric latitude (Arecibo) to the field at 0° (Jicamarca). Other highlights of the paper are a discussion of DP2, which may help clarify the controversy surrounding it, and a discussion of the sensitivity of low- and mid-latitude radars to disturbances of magnetospheric origin. We show that this sensitivity maximizes at the magnetic equator.

The eastward component of the electric field was measured simultaneously at ionospheric heights and in the equatorial plane of the magnetosphere by using the incoherent scatter radar at Millstone Hill and whistler data from Siple, Antarctica, respectively. The locations of the two data sets correspond to approximately magnetically conjugate areas within the outer plasmaphere, and thus a unique opportunity arises for testing the electric field mapping hypothesis and for intercomparing the measurement techniques. The whistler results are averaged for L = 3.5-4.7, while the radar data correspond to L = 4.4. The measurements represent time averages over about 20 minutes. There is excellent correlation during most of the 12 hour period of simultaneous data on 10 July 1978. The period was one of moderate disturbance (Kp = 2-3). A 500 gamma substorm developed around 0830 UT (0330 LT). During this substorm, both of the electric field measurements showed a large westward perturbation, in agreement with previous observations. The meridional component of the electric field was also measured at Millstone Hill. Taken together, these data indicate an enhanced flow with components inward and towards dawn during the substorm. Except for a time near magnetic midnight the magnitude of the zonal field scales from the ionosphere to the equator in direct ratio with the spreading of magnetic field lines in the IGRF 1975 model. The whistler data clearly show that the measurements were obtained inside the plasmasphere. The middle latitude substorm perturbations appeared to be related to a reconfiguration of the magnetospheric current system.

The Atmospheric Explorer-C (AE-C) satellite passed almost directly over the Millstone Hill incoherent scatter radar station on 14 February 1974 and passed within the near vicinity of the station on 15 February 1974. Measurements of ionospheric and atmospheric properties were made simultaneously by the incoherent scatter radar and the AE-C satellite instruments. The incoherent scatter radar measured vertical profiles of the electron and ion temperatures and electron density and these data were used to derive a neutral gas temperature profile. The AE-C satellite measured the electron and ion densities and electron and ion temperatures, neutral gas composition, solar EUV flux, photoelectron spectra, the 6300 A volume emission rate profile and the distribution of NO along the satellite path. These simultaneous measurements provide a consistent set of data to examine current F-region theory in the daytime ionosphere. We used a time-dependent coupled model of the ionospheric E- and F-region to calculate the ionospheric properties over Millstone Hill at the times of the AE-C crossings and then compared the calculated structure to the observed structure. The results show good agreement between the incoherent scatter radar measurements and the model calculations. There is also good agreement among satellite and incoherent scatter radar measurements and model calculations for the altitude of the satellite crossing, 161 km. The satellite measurements along the orbital path, however, reveal considerable horizontal gradients in the measured ionospheric properties.

From January 1971 to March 1973, the Millstone Hill L-band satellite tracking radar was employed in a study of ionospheric refraction. Satellites of the Navy Navigation Series were tracked smoothly by radar (at 1295 MHz) by commanding the 84-ft diameter antenna to follow the predicted path of the satellite across the sky, and allowing any measured angular errors slowly to close out the offsets. Simultaneously, the apparent position of the satellite was sensed passively by receiving (on the same antenna) the UHF beacon signals. Ionospheric refraction manifested itself during the rising and setting portions of the path as a displacement between the UHF and L-band apparent positions. In addition, quasi-sinusoidal fluctuations in the displacement were seen at intervals along thepath that are believed to be caused by traveling ionospheric disturbances. On 40% of all passes, these waves were present with wavelengths typically in the range 30–60 km. These TIDs were seen equally frequently at all local times and seasons. They did not give rise to significant fluctuations of total electron content along the line of sight to the satellite, as measured simultaneously by a differential-Doppler experiment. This suggests that they are confined largely to the lower thermosphere and manifest themselves in tilting the lower surface of the ionosphere where the density gradients are largest. The source of these TIDs is not known and they appear to have been little studied heretofore.

An incoherent scatter radar experiment involving the transmission of pairs of short pulses at various spacings was employed at Millstone Hill to study the daytime thermal structure of the lower thermosphere. An altitude range from 110 to 130 km was covered with a height resolution of 6 km. The daily temperature variation exhibited oscillations having dominant periods in the range 6–14 hr and amplitudes as large as 28 per cent of the mean temperature. These waves possessed a downward phase velocity indicating an energy source at lower heights. A spectral analysis carried out on measurements gathered on successive days provided sufficient frequency resolution to resolve the different tidal frequencies and indicated the 12-hr period as the main component. Interpreting the measured oscillations as being primarily a manifestation of the solar semidiurnal tide, the observed phase and vertical wavelength are in agreement with theoretical predictions for the thermal (2, 4) tidal mode propagating from the lower atmosphere. On average, the semidiurnal wave amplitude is about 40 K with a time of maximum near 0700 hr LT at 115 km and is damped above that height. Large variability in amplitude and phase are found from one observing period to another. The height gradient of the average daytime temperature near 120 km is about 15 K/km. This value is larger than given by some theoretical treatments but is consistent when the energy carried into the region by the semidiurnal tide is allowed for.

The Arecibo Observatory recently began a regular program of coordinated tidal observations. Arecibo directly measures the ion velocity vector, the electron and ion temperatures, and the electron density over the 100–500 km region for this program. In addition, the ion neutral collision frequency can be inferred below 120 km. From these measurements neutral temperatures, the southward component of the neutral wind, and the eastward wind component below 130 km can be inferred. The height resolution is 2.3 km below 200 km, and 35 km above, while the time resolution is about 1 1 2 h for velocities and 30 min for temperatures. This paper describes the Arecibo measurements, and the method of analysis of the inferred neutral winds and temperatures for tidal components. Results for the initial observing period in August 1974 are presented.

Incoherent scatter measurements of temperature in the lower thermosphere (100–125 km) at Millstone Hill (42°N) and St. Santin (45°N) are analyzed to determine the influence of thermal tides propagating from the lower atmosphere. The average temperatures from several years of measurements exhibit a 12-hour oscillation having a downward phase progression with height. The characteristics of this oscillation are found to be in excellent agreement at the two mid-latitude stations. The average amplitude reaches a maximum of 11% of the mean at a height of 116 km and a local time of 6.6 hours, the vertical wavelength increasing from 40 km at 105 km to 64 km at 120 km. From a comparison with theoretical predictions we conclude that this oscillation is predominantly the solar (2, 4) tidal mode; however, the rate of damping above 116 km is larger than that given by dissipative theory. The results also reveal a significant variability in the amplitude and phase of the tidal oscillation from one observing period to the other. Seasonal temperature averages indicate a smaller amplitude in winter than in summer or at equinoxes because of a larger phase variability in winter. An empirical model of the mid-latitude temperature structure in the lower thermosphere based on the radar measurements is also presented.

Ion velocity and temperature measurements at Arecibo for two winter days were analyzed to determine the large-scale dynamical structure of the E region. Southward neutral winds were derived directly from the ion velocity, while the southward electric field was obtained with a reasonable extrapolation of the measured ion-neutral collision frequency. Eastward winds were estimated after assuming a model eastward electric field taken from previous F region measurements at Arecibo. The main results are as follows: Large oscillations in the ion drifts, neutral winds, and temperatures were present on the days of observation. The oscillations possessed a characteristic downward phase progression and dominant periods that were near semidiurnal. Wind and temperature amplitudes were 40--100 m/s and 30degree-60degreeK, respectively. Vertical wavelengths for a semidiurnal period were about 50 km. The calculated southward electric fields are typical of daytime F region values at Arecibo, -1 to -2 mV/m. The observed semidiurnal amplitudes, phases, and vertical wavelengths of the wind and temperature oscillations do not closely match the predictions of classical tidal theory or the theory of Lindzen and Hong which includes dissipation and background winds. (AIP)

The results of an experiment to study the strongly interacting particles in air shower cores are presented. The experiment used 64 large plastic scintillators in three different close configurations, two Wilson cloud chambers, about 600 G-M counters and 100 tons of lead. It is shown that there is no unique structure function for the strongly interacting particles which will fit all showers. Rather, showers can be classed as single-core or multi-core. The multi-core showers can have various numbers of cores from 2 upwards. Single-cored showers appear to be due to proton primaries; multi-cored showers to be due to primaries of A.≥2. There is some evidence that deuterons form about 5% of the hydrogen nuclei in the cosmic radiation at total energies of ∼ 4. 1014 eV. It seems likely that the only shower of size > 107 particles whose core hit the shielded scintillators was due to a very heavy nucleus, possibly iron. Above a shower size of 106 particles we have observed 47 multi-cored showers and 7 single-cored showers. Between sizes of 105 and 106 particles we have seen 68 multi-cored showers and 85 single-cored showers. It has proved possible with this apparatus not only to study individual showers in detail, but also individual nuclear interactions in these showers. The methods of doing this are explained and discussed. In contrast with much emulsion work the surviving baryon can often be easily identified and its momentum determined. It is found that the elasticity in individual nucleon-air nucleus collisions varies from 0.3 to 0.7 with a median value of 0.54. A method of obtaining the average value of the elasticity for energies of 1012 to 1016 eV is described. In ∼ 400 individual mesons studied none were found with transverse momenta in the region ≥ 4 GeV/c and all had values consistent with a mean of 0.4 GeV/c. Interactions were identified at heights above the array varying from 25 metres to 4 kilometres. This is consistent with an interaction mean free path of protons in air of ∼ 90 g/cm2. This value is confirmed by an independent measurement of the total number of collisions suffered by 14 proton primaries. The results are compared with previous experiments and their implications discussed.

The response of plastic scintillators to the flux of particles occurring in cosmic ray air showers has been measured at various distances from the center of showers of various sizes. The number of ``scintillator particles'' S obtained by dividing the observed pulse height by the mode of the pulse‐height distribution due to single fast muons has been compared with the number of incident ionizing particles G seen by Wilson cloud chambers and Geiger counters. The ratio S/G is always greater than unity. It increases with increasing number of charged particles per square meter, being 1.1 for 100 scintillator particles per m2 and 4.0 for 10 000 scintillator particles per m2. It varies with radial distance from the shower center and with shower size, being 1.2 on average at 30 m from the center of showers of ∼105 particles and greater than 4 near the center of large showers. The increase in S/G is partly due to cascading of the electromagnetic component within the plastic scintillator but mostly to nuclear interactions produced in the scintillator by strongly interacting particles.

Values of ion temperature (Ti) and ion-neutral collision frequency (ν) in the daytime E region have been obtained from radar Thomson scatter measurements at the Arecibo Ionospheric Observatory. These quantities were deduced from the shape of the autocorrelation function of the backscattered signal that was measured with a height resolution of 15 km using a double-pulse technique. Usually the daily variation of Ti at 115 and 130 km exhibited wave-like structure that must also have been present in the neutral atmosphere because the ion and neutral constituents have the same temperature at these altitudes. The observed temperature oscillations are interpreted as a manifestation of the reversible adiabatic heating accompanying long-period internal gravity waves in the neutral atmosphere. A simple harmonic analysis of the temperatures for a number of days gives dominant wave periods of from 7 to 13 hours and fractional amplitudes as large as 22%. On the reversible heating explanation, the observed amplitudes of the temperature waves imply average horizontal wind amplitudes of 60 to 70 m/sec. A downward phase progression (corresponding to upward energy propagation) is evident with an average vertical wavelength near 60 km. A significant wave-like variation is also detected in some of the ion-neutral collision frequency results at 115 km. The period is similar to that of the corresponding temperature wave, but the fractional amplitude is larger, and there is an appreciable phase difference. The phase and amplitude of the variation in the neutral concentration, as predicted from the collision frequency variation using the polarization ion-neutral interaction, does not agree with that deduced from the temperature variation using simple internal gravity wave theory with upward energy propagation. The resolution of this discrepancy may depend on the removal of Some of the idealizations in the gravity wave theory or the inclusion of additional ion-neutral interaction terms.

The results of rocket and ground-based radar Thomson scatter (incoherent scatter) measurements of electron densities and temperatures obtained at the same time and location are presented. Two-electrode spherical electrostatic analyzers were used in the rocket experiments. This report describes the experimental techniques and compares the experimental results and theoretical predictions. Good agreement is found between the results obtained on two daytime rocket flights and the radar measurements over the altitude range of comparison, 100 to 180 km. The rocket electron temperature measurements are consistently 10 to 15% higher than the radar data. The electron densities agree within 25%, which is within the experimental error of the two techniques. Comparison of measured electron temperatures with values calculated from the solution of the heat conduction equation by using methods developed by Dalgarno et al. shows good agreement below 160 km. Between 200 and 350 km the measured temperatures are 200° to 300° higher than predicted. Electron cooling rates were calculated from the experimental results for the relevant cooling mechanisms and were compared with the theoretical heat input to the electron gas. The results imply that the differences between theoretical and experimental temperatures above 200 km are primarily due to an underestimation of the heat input by a factor of 3 to 5.

The cosmic-ray differential density spectrum in the range 3 to 10000 p/m2 has been obtained, using Wilson cloud chambers, at sea level and at 2285 m a.s.l. In neither case can the whole spectrum be fitted by a single power law. Using a three-parameter fit the sea-level spectrum is best approximated by two power laws of exponents − 2.491±0.036 and −3.42±0.32, respectively, joining at 560± 180 particles per square metre. At 2285 m a.s.l. the three best parameters are − 2.431 ±0.015, − 4.8−0.8+1.3 and 6000−800+1000 particles per square metre. The change with altitude supports the hypothesis that the change in exponent is due to a steepening of the primary cosmic-ray energy spectrum.

Incoherent scatter radar measurements gathered since 1969 at Millstone Hill (42.6°N, 71.5°W) are used to study the seasonal variations of the neutral gas temperature, composition and wind velocity in the thermosphere. Between 110 and 120 km, the mean temperature is found to oscillate with a 20 K annual amplitude, 10 K semiannual amplitude, and maxima in July; the mean density oscillates with an annual amplitude of 13% of the mean and a semiannual amplitude of 6% of the mean with maxima in November. Similarly, the mean daily exospheric temperature possesses a 100 K amplitude variation with a maximum in summer; the semiannual component is only about 10 K. Moreover, a weak winter maximum characterizes the neutral density at 400 km, and the neutral wind at 300 km is found to blow northward for a longer period during the day in winter than in summer. The amplitude and phase of the daily semidiurnal tides in the lower thermosphere do not vary systematically with season but a large variability is observed from day to day, particularly in the phase of the oscillation during winter. In contrast, the semidiurnal harmonic of the exospheric temperature varies seasonally in amplitude and phase and reflects the controlling influence of the length of the day on the shape of the daily temperature oscillation. In the middle thermosphere, the neutral composition ratio O/N2 is larger in winter than summer, with a rapid transition at the equinox.

An array of 64 large plastic scintillators, 5 Wilson cloud chambers and about 500 G-M counters has been used to study the electronic structure of air-shower cores. It has been found that there is no unique structure function that will describe all cores. Cores can be classed as single, multiple or flat-topped. On average the single cores follow the Nishimura-Kamata distribution with the age parameter slightly greater than unity with the exception of the region within 20 cm of the core. Multiple-cored and flat-topped showers can be simulated by the summation of a number of spatially separated single cores. It is shown that the multiply cored showers cannot be due to cascades produced by the neutral pions from a single interaction but that the core structure is due to the nature of the primary particle. The central density of single-cored showers of a given total size is considerably higher on average than the central density of multiply cored showers of the same size. Up toN=106 the ratio of singly cored showers to others is, within the errors, the same as the ratio of the numbers of protons to other primaries (of the same total energy) at lower energies. AboveN=106 there is a great increase in the ratio of multiply cored to single-cored showers. This suggests that there is a change in the composition of cosmic radiation at about 3·1015 eV. The deficiency of protons above this energy could be due to a failure of the acceleration mechanism and/or to the escape of protons from the galactic disc. Both explanations seem to require that, in the vicinity of the sun, the interstellar magnetic fields should, generally, be considerably less than 10−5 G. Future experiments are discussed.

Radar Thomson scatter observations made at the Arecibo Ionospheric Observatory on August 10, 1966, are reported. Analysis of the ion and plasma line components of the Thomson scatter signal from the ionosphere provided two essentially independent methods of obtaining the electron-to-ion temperature ratio Te/Ti. In the first method, Te/Ti and Ti were deduced from the shapes of the autocorrelation functions of the ion component by using assumed values for the ion composition. In the second method values of Te/Ti were derived from the radar equation for Thomson scatter using electron concentrations from the plasma line observations and power measurements of the ion component. Altitude profiles of Te/Ti from these two methods were compared over the range 130 to 450 km and were found to be in excellent agreement. At 130 km Te/Ti was unity, and this result does not support the conclusion drawn from many midlatitude rocket observations that an E-region source of electron heating exists in addition to solar EUV. Above 130 km the electron temperature was always greater than the ion temperature for daytime conditions. A downward heat flux of 1.7±0.3×109 ev cm−2 sec−1 near local noon was inferred from the positive Te gradient at 400 km. The plasma line measurements provided accurate electron concentration (ne) profiles in close agreement with simultaneous ionosonde determinations. Composite profiles of Te/Ti, Ti, and ne are presented for different times throughout the day.

The Millstone Hill (42.6°N, 71.5°W) incoherent scatter radar, and the data analysis techniques used to determine temperature are described. Results for 10–15 August 1974 and 14–17 October 1975, are presented. Specific attention is given to the semidiural tide, which was found to have substantial differences in amplitude and phase in the two observing periods, with the amplitude being greater in October.

Model predictions of the diurnal variations of plasma convection velocities and electron densities in the high-latitude ionosphere were compared with Millstone Hill incoherent scatter radar observations for an equinox day on which there was moderate magnetic activity. On the observation day, three major morphological features were evident at 500 km, including a dayside high density region, a nightside mid-latitude trough, and a region of slightly enhanced densities in the auroral zone. Although the dayside high density region was due to solar EUV radiation, it was not symmetrical about local noon (1000-1900 LT sector) owing to the effect of horizontal transport. The nightside mid-latitude trough was the deepest, the widest, and reached its most equatorward latitude in the morning sector. The model was able to reproduce these two features quite accurately. In the dusk sector, the trough was filled in, and its latitudinal extent was restricted by a discrete auroral arc, a feature not included in the ionospheric model. Except for this arc region, the enhanced electron densities in the auroral zone were adequately described by the average precipitation fluxes used in the model. The observed plasma drift velocities were consistent with a two-cell, asymmetric convection pattern with enhanced flow in the dusk sector. Outside the polar cap, the fall-off of the magnetospheric potential with latitude was proportional to the inverse of the sine of colatitude to the fourth power. The convection pattern employed in the model included these features and had a 60 kV cross-polar-cap potential. Efforts to reproduce the observed behavior by using a larger cross-polar-cap potential (90 kV) or a symmetric pattern are also presented. These were generally less successful and demonstrate the sensitivity of the morphology of the F region at high latitudes to the convection process.

The Atmosphere Explorer-C satellite (AE-C) is uniquely suited for correlative studies with ground-based stations because its on-board propulsion system enables a desired ground station overflight condition to be maintained for a period of several weeks. It also provides the first low-altitude (below 260 km) comparison of satellite and incoherent scatter electron and ion temperatures. More than 40 comparisons of remote and in situ measurements were made by using data from AE-C and four incoherent scatter stations (Arecibo, Chatanika, Millstone Hill, and St. Santin). The results indicate very good agreement between satellite and ground measurements of the ion temperature, the average satellite retarding potential analyzer temperatures differing from the average incoherent scatter temperatures by −2% at St. Santin, +3% at Millstone Hill, and +2% at Arecibo. The electron temperatures also agree well, the average satellite temperatures exceeding the average incoherent scatter temperatures by 3% at St. Santin, 2% at Arecibo, and 11% at Millstone Hill. Several temperature comparisons were made between AE-C and Chatanika. In spite of the highly variable ionosphere often encountered at this high-latitude location, good agreement was obtained between the in situ and remote measurements of electron and ion temperatures. Longitudinal variations are found to be very important in the comparisons of electron temperature in some locations. The agreement between the electron temperatures is considerably better than that found in some earlier comparisons involving satellites at higher altitudes.

The Millstone Hill 440 MHz radar at Westford, Massachusetts, has been upgraded for studies of turbulence and winds in the troposphere and lower stratosphere. Useful data have been obtained up to 20 km altitude. The radar has been operated with a peak pulse power of 1–1.5 MW, a pulse length of 10 μs and a pulse repetition of 500 Hz. The 150-foot diameter steerable antenna has been used differently in three types of experiments, (1) Antenna azimuth scans with fixed elevation, (2) elevation scan with fixed azimuth, and (3) fixed antenna. Azimuth scan experiments can obtain both the horizontal and the vertical wind components. Antenna elevation scans have been used to investigate the horizontal structure of turbulence. The fixed antenna experiments provide maximum time resolution and are useful for studying the occurrence of waves. Waves in both the velocity and Cn2, the refractive index turbulence structure constant, have been frequently observed at heights near the tropopause and below. The waves have periods 10–20 min and have been interpreted as internal gravity waves. The largest amplitude waves have been found to occur (1–2 km) below the level of a velocity shear. Waves generally persist for several cycles. By using the antenna elevation scan method, the level of turbulence (as measured by the Cn2 value) has been measured at points separated by 10–20 km. Horizontal patches of turbulence have been found that appear to move with the wind.

The Millstone Hill radar, Westford, Massachusetts (Λ = 56°), has been upgraded for observations in support of the International Magnetospheric Study (IMS) by means of the addition of a 150-ft. diameter fully steerable antenna. This antenna allows measurements of F-region ion drifts, from which F-region electric fields can be derived, using the existing UHF (68-cm wavelength) radar. This paper describes analytical models of the mean electric field behavior in the auroral zone over the interval 60° 3o). Each model incorporates a polynomial description of intensity with invariant latitude and a sinusoidal harmonic variation in local time with 102 degrees of freedom for each of the perpendicular electric field components. Coefficients of this model are provided for numerical computation by users. Tabulations of these fields and a computer program to calculate fields and potentials are available on request from the authors. The results are compared with models published by Heppner [1977]. Each of these average field models is consistent with a two-cell convection pattern having antisunward plasma flow over the pole and return paths eastward and westward through the dawn and dusk meridians. The centers of the vortices are near the far limit of observation (Λ = 75°). The entire convection pattern appears to be rotated by as much as two hours toward earlier local times from the nominal noon-midnight over-the-pole flow direction, though this conclusion is influenced strongly by our least reliable (highest latitude) data. The dusk cell is dominant in size and strength; however, both the pattern rotation and cell dissimilarity lessen as Kp increases. The Harang discontinuity, which was recognizable on individual days, is largely absent in the average patterns, presumably as a result of the variability in its position. Nevertheless, we do see a residual, low-speed, late-evening protrusion of the dusk cell extending toward lower latitudes with increasing local time, reaching as far as 0300 LT for low Kp but only to midnight for high Kp. For low Kp, we observe a region of very small average flow from dawn to noon, apparently disconnected from the overall convection pattern. The dawn cell encroaches into this region as Kp increases, but the 1000–1200 LT sector maintains its low-speed character even for high Kp. The cross-polar-cap potential varies linearly with Kp as V = 10.06 + 14.44 Kp kV, but the smoothing in our observations may have caused this to be an underestimate. Penetration of auroral electric fields to mid-latitudes is most likely to occur and to be strongest in the evening sector.

The 440-MHz radar at Millstone Hill (42.6°N, 71.5°W) was employed to examine the detailed evolution of turbulence structures in the tropostratosphere with a very fine Doppler resolution of 0.12 Hz, corresponding to 4 cm s−1 radial velocity resolution. The measurement technique involved directing the antenna downwind at a low elevation angle (15° or 20°) and processing the echos from a range where the background wind shear was high. Although no attempt was made to obtain good range resolution, a wide variety of turbulence structures were evident by virtue of their different Doppler shifts in the presence of a high wind shear. Using arguments based on the dominance of ‘shear broadening’ and the required velocity gradient to create instabilities, maximum layer thicknesses in the vicinity of 20–300 m were inferred. The implication of these observations to bulk transport of passive contaminants in the stratosphere is briefly discussed.

The Millstone Hill radar, Westford, Massachusetts (Λ = 56°), has been upgraded for observations in support of the international magnetosphere study by means of the addition of a 150-foot-diameter fully steerable antenna. This antenna allows measurements of F region ion drifts, from which F region electric fields can be derived, using the existing UHF (68-cm wavelength) radar. The paper describes attempts to measure the electric field in the auroral zone over the interval 60° ≤ Λ ≤ 75°. This effort entails making observations to the north of Millstone at very low elevation angles over periods of ≥24 hours. To secure estimates of both the E-W and the N-S components of the drifts, the measurements are made in two directions separated by ±15° from the magnetic meridian, and to achieve the desired coverage in latitude, measurements must be made at two elevation angles for each of these azimuths. The separation in time and space of the two components of the drift that are observed introduces a difficulty in interpreting the results. This has been overcome by matching the observations, in a least mean squares sense, by analytical functions describing the components of the electric field, each of which has 102 degrees of freedom. The results obtained thus represent the variation of the auroral electric fields over 24 hours local time smoothed with respect to both latitude and time. The utility of this approach is illustrated using sample results for one moderately quiet day, and the resolution and accuracy of the method are discussed.

Observations of F region electron density, electron and ion temperatures and ion drift velocity over the latitude interval 40° < Λ < 70° have been made from Millstone Hill by means of incoherent scatter radar. We conclude that the F region density trough that forms on the nightside of the earth may have more than one cause. Current computer models indicate that it can be produced as a consequence of the competing effects of convection and corotation that cause some flux tubes to remain for long periods in darkness in the premidnight sector, thereby allowing the ionisation in them to decay. Our observations show that troughs often are first seen in the late afternoon sector in regions of fast sunward convective flow and can move in latitude in association with substorms. We suggest that these troughs are produced by convection of low density regions from the nightside into daylight and the increased recombination rate that occurs in regions where the O+ ions are forced through the neutral air at high speed (∼1 km/s). Troughs created in the dusk sector by this mechanism during substorms can be left as the ionospheric signature of the substorm or “fossil” when the region of fast convective flow moves poleward following the substorm. These extended E-W troughs then are carried across the nightside of the earth by corotation. Relative to the surrounding regions, the trough changes little with time and finally is destroyed only when it is carried into sunlight on the dawnside.

Incoherent scatter observations of E and F region drifts over Millstone Hill (Λ=56°) were made routinely from May 1976 to November 1977 at monthly intervals. Some 667 hours of data gathered over this 19-month period has been analyzed to yield the F region polarization electric field in the magnetic south and magnetic east directions. In an effort to determine what part of the observed fields can be attributed to the penetration of auroral convection fields to the latitude of Millstone, these data have been used to construct an analytical model of these two field components having a time of day and Kp dependence (Wand, this issue). To guide the interpretation, mean convection fields also were constructed for Λ=60°, 65°, 70° and 75°, using data gathered between May 1978 and December 1978 from a separate series of experiments (Evans et al., 1979, 1980). The results suggest that, under quiet magnetic conditions, there is no significant penetration of the magnetospheric electric field to the latitude of Millstone at any time of day except possibly for the southward component of the field at night. During magnetically disturbed periods the incremental fields observed over Millstone exhibit all the gross characteristics associated with convection fields seen at higher latitudes. The results also indicate an eastward rotation of the convection pattern of about 4 hours in local time in going from quiet or moderate conditions (Kp ∼ 2+) at Λ=65° to disturbed conditions (Kp=5) at Λ=56°.