Nighttime Variations Research Articles

Thermospheric dynamics investigations with very high resolution spectrometers

Since 1972 high resolution Fabry-Perot spectrometers have been used at Fritz Peak Observatory (39.9 degrees N, 105.5 degrees W), Colorado to measure the nighttime variation of thermospheric temperatures and winds from the line profiles and Doppler shifts of the OI 15,867 K (630.0-nm) line emission in the nightglow. With the aid of these measurements we have defined the nighttime variation of winds and temperatures at F-layer heights for the various seasons of the year during geomagnetic quiet periods. During geomagnetic storm periods deviation in the nighttime variation of the winds and temperatures from those determined during geomagnetic quiet conditions have been shown to occur. In addition, measurements made during geomagnetic disturbed conditions have shown the existence of large-scale thermospheric waves generated at high latitudes by impulsive auroral heating events that are observed to propagate equatorward. The nighttime winds and temperatures measured from Fritz Peak Observatory have been used in various heoretical models of global thermospheric dynamics to infer the global circulation patterns, temperature structure, and thermospheric response to geomagnetic activity. By requiring agreement between the calculated and measured winds and temperatures over Fritz Peak Observatory, the over-all magnitude of the thermospheric high latitude heat source due to auroral processes has been inferred for both geomagnetic quiet and disturbed conditions. This energy source has been shown to be related to dissipation of the ring current energy in the high latitude ionosphere. The results of various geophysical studies using Fritz Peak Observatory data and theoretical model calculation are summarized.

Nighttime thermospheric winds at high latitudes

Nighttime thermospheric winds have been measured from Ester Dome, near College, Alaska (64.8°N, 147.8°W), using the high‐resolution Fabry‐Perot interferometer of the University of Michigan Airglow Observatory. The winds are determined from the Doppler shifts of the (0I) 15867 K (6300 Å) line emission. Measurements obtained during geomagnetic quiet times in January and February 1972 are averaged to obtain the characteristic nighttime behavior of both the zonal and the meridional neutral wind components at F‐region heights. Ion drifts during this period were measured by the Chatanika incoherent scatter radar facility. The measurements of the average nighttime zonal and meridional neutral wind components and of the average ion drift components are used with a three‐dimensional dynamic model of the neutral thermosphere to determine the effectiveness of various forces in controlling the winds at F‐region heights in the high‐latitude thermosphere. A least squares fit of the measured and calculated nighttime variations of the zonal and meridional winds, using the ion drag determined from the radar measurements, gives the Fourier coefficients of latitudinal and longitudinal pressure gradients that are necessary to drive the observed wind pattern. The results show that the ion drag force is essentially balanced by the pressure force in the meridional direction. However, in the zonal direction the pressure force does not balance the ion drag force. The pressure gradients, derived in terms of exospheric temperature variations, show a different nighttime variation from that predicted by the MSIS model for that latitude. The derived zonal winds are westward in the early evening hours, opposite of the winds calculated from a dynamic model that uses the pressure forces determined from the MSIS semiempirical model and ion drag.

Effect of the presence of a conducting channel between India and Sri Lanka Island on the features of the equatorial electrojet

Summary. Transient geomagnetic variations like SS&, Bays, Sq and stormtime variations show anomalously large Z amplitudes at the three permanent magnetic observatories in India under the equatorial electrojet. Our earlier studies have shown that these anomalies cannot be explained in terms of the usual coastal effect. Another unique feature of this area is the small equatorial enhancement of all magnetic fluctuations, which still remains to be completely understood. It is also noticed that Z/H ratio at Annamalainagar is very large for night-time variations but becomes insignificant for day-time fluctuations, whereas no such large difference is seen at Trivandrum, the other coastal station, All the above features have been explained here by introducing the presence of a conducting channel in the lower crust or upper mantle between India and Sri Lanka Island. The anomalies of the equatorial electrojet in the Indian region thus do not necessitate the conducting surface of the mantle to be deeper in this area as has been suggested by earlier workers. A less conducting mantle in the Indian region otherwise could be difficult to incorporate in the present theories of mantle convection when this area is known to be tectonically active. The presence of the conducting channel is further confirmed by analogue model experiments of Papamastorakis. We further observe that the equatorial electrojet does have an associated internal part in the Indian region.

Night-time variation of short period fluctuations (2–15 min) in the oxygen green line

Measurements of short period fluctuations in the oxygen green line intensity at the 100 km level show a systematic variation during the night with a minimum energy around midnight. This result when compared with other results obtained by different methods leads to a possible interpretation linked to the effect of the terminator.

Direct measurements of nighttime thermospheric winds and temperatures, 3. Monthly variations during solar minimum

We used a high-resolution Fabry-Perot spectrometer at Fritz Peak Observatory (39.9°N, 105.5°W) to determine the nighttime variation of thermospheric temperatures and winds from the line profiles and Doppler shifts of the O I 15,867-K (6300 A) line emission. With data obtained during the period April 1975 through March 1976 we examined the variation of the nighttime neutral gas temperature, zonal winds, and meridional winds at F layer heights throughout the year during solar cycle minimum. We compare our observed temperatures to predictions made by the MSIS (mass spectrometer and incoherent scatter) empirical model of neutral temperature and composition in the thermosphere. The calculated nighttime variation of thermospheric temperature is about 100°–150°K lower than measurements for summer and equinox months and about 50°–75°K lower than measurements for winter months. We compare the observed winds to the zonal and meridional winds, which we calculate from a three-dimensional semiempirical dynamic model of the thermosphere by using the pressure gradients specified by the MSIS model. The calculated nighttime winds are in general agreement with observed winds; both reveal a systematic monthly variation during geomagnetic quiet times. The nighttime equatorward meridional winds are greater in summer than in winter. The zonal winds are eastward throughout most of the night during winter; however, as the season progresses toward summer, a transition develops in the early morning hours from eastward to westward winds. This transition occurs earlier in the night as summer approaches. We use model predictions of the diurnal variations of winds and temperatures to calculate diurnal averages of the zonal and meridional wind components over Fritz Peak Observatory. The seasonal variation of these zonal mean values is presented.

F-region temperatures at Malvern and S. Santin

The incoherent scatter technique has been used to measure ionospheric temperatures above S. Santin, France since 1966 and above Malvern, U.K. between 1968 and 1975. Whereas the French data have all been obtained with a C.W. radar system, observations at Malvern were made with a monostatic pulsed radar until 1972 and mainly with a multistatic C.W. radar from 1971. In this paper we compare simultaneous C.W. data from Malvern and S. Santin on a diurnal and seasonal basis, finding overall similarity but considerable difference in detail. The seasonal variation of the daytime mean electron temperature is compared with measurements at Millstone Hill and found to exhibit similar features. Nighttime mean temperatures at Malvern and S. Santin show a distinct semi-annual variation with maximum temperatures near each equinox. This is quite different from the nighttime variation at Millstone Hill and is explained in terms of magnetic conjugate point geometry. The mean difference between daytime temperatures at Malvern and S. Santin is not related to magnetic disturbance, but a strong correlation exists between ion temperature at each station and the solar 10.7 cm flux.

DIURNAL AND REGIONAL CHANGES OF RELATIONSHIP BETWEEN AIR POLLUTION AND METEOROLOGICAL CONDITIONS

An attempt was made to clarify the atmospheric conditions associated with high SO2 pollution, and the diurnal changes of the relation between atmospheric conditions and air pollution were discussed regarding its regional difference in a multiple-source situation. Mean hourly SO2 measurements at nine stations in Sendai (Fig. 1) were analyzed by a statistical method. In order to evaluate not only the factors having effects upon the day-to-day variation in concentration but also the diurnal and regional change patterns of the factors, daily SO2 data were examined at each hour by using Factor Analysis. Contributions of main factors, that is, “first-factor” and “second-factor”, are shown in Fig. 3. Because of high contribution throughout the day, the day-to-day variation in the suburban area is generally explained by the “first-factor”. On the other hand, in the built-up area, a major source area, the variation in the daytime is mainly connected with the “second-factor”, though the night-time variation is caused by the “first-factor”. In the built-up area, therefore, the atmospheric condition which causes air pollution at night is considered to be different from that which causes it in the daytime. High concentrations resulting from the “first-factor” occur in the suburban area through-out the day and in the built-up area at night under a migratory anticyclone, whereas those from the “second-factor” occur in the daytime in the built-up area under windy conditions when winter-monsoon prevails (Fig. 4). Fig. 5 shows mean concentrations classified by wind speed at two selected stations. When it is windy, the concentration at Station-4 in the built-up area increases extremely by day, but it decreases by night. On the other hand, the concentration at Station-1 in the suburban area increases with decreasing wind speed throughout the day. According to Fig. 6, showing the high-pollution appearance classified both by wind speed and lapse rate (between Station-A (H: 38m) and Station-B (H: 220m)), high concentrations at night tend to occur under calm and stable conditions at the both stations. Therefore, the “first-factor” corresponds to the meteorological conditions for stagnation of pollutants, while the “second-factor” corresponds to conditions for “gale-pollution”. Pollutants are generally much diluted by the strong wind, and concentration decreases with the distance from the sources. However, strong wind tends to be accompanied with extraodinary high concentrations due to exposure of undiluted plume in the vicinity of active sources, where the effect of strong wind is in opposition to its diluting action. Thus, the relationship between concentration and meteorological condition not only varies from place to place in a small area but also changes diurnally.

OH emission intensity measurements during the 1969 NASA Airborne Auroral Expedition

Absolute intensity measurements of the (8, 6) OH band obtained during 10 flights of the December 1969 NASA Auroral Airborne Expedition are presented. Nightglow intensities higher by a factor of 2 than the usual values are recorded during flights 8, 14, and 15. The OH variations are compared with the evolution of the green line and O2(1 Delta g) emissions measured by other experimenters on board the aircraft. Before sunrise the twilight variations of OH down to a solar depression angle of 5 deg show a rapid decrease. A theoretical prediction of the OH, O I 5577 A, and O2(1 Delta g) emissions is evaluated by means of an extensive time-dependent oxygen-hydrogen model of the 25- to 150-km region. Twilight decrease of the OH emission is interpreted in terms of mesospheric ozone photodissociation. Nighttime variations of the emissions may be reproduced if modifications of the dynamic regime are introduced into the model.

Further examples of the nighttime variations of ELF signal strengths in Connecticut

A few extremely low frequency (ELF) horizontal magnetic field strength measurements have been taken in Connecticut during the past 5 years. These measurements were made to further investigate sunrise, daytime, sunset, nighttime, and seasonal ELF propagation variations. The transmission source for these 1.6-Mm-range measurements was the U.S. Navy ELF Wisconsin Test Facility. The principal results obtained from these measurements were that 1. (1) ELF night-time propagation is much more variable than daytime propagation, 2. (2) the ‘Halloween effect’ has been observed for 5 years in a row, and 3. (3) there may be as many as 80 nights each year when the average nighttime field strength will be approximately 3 dB lower than on the preceding or following nights.

Nighttime and sunrise variations of absorption

The average behavior of the MF A1 absorption data at night and sunrise times at Calcutta has been described with a view to understanding the maintenance of the nighttime absorbing region. The nighttime variation of absorption which attains a minimum around one hour after local midnight could easily be accounted for in terms of the scattered UV fluxes.A small decrease in absorption before sunrise is evidenced. The analysis using the partially reflecting Es echo suggests this is caused by a decrease in deviative loss at F‐region heights. The onset time for a further increase in absorption agrees very well with that observed from other techniques.

Nighttime Variations of Extremely Low Frequency (ELF) Signal Strengths in Connecticut

A limited amount of extremely low frequency (ELF) horizontal magnetic field strength measurements have been taken in Connecticut during the past 3 years. The transmission source for these 1.6 Mm range measurements was the U. S. Navy ELF Wisconsin Test Facility. The principal result obtained from these measurements is that there are considerable variations in the ELF nighttime propagation parameters.

Combined airglow and incoherent scatter observations as a technique for studying neutral atmospheric variations

Night airglow 6300 Å intensities and electron density altitude profiles observed at Arecibo have been combined with dissociative recombination theory to obtain information about the nighttime variation of F‐region N2 and O2 densities. The application of this technique is illustrated using data from two nights in March 1971. The gross nighttime variation shows reasonable similarity to the Jacchia [1970] model, and also follows the time variation of the measured exospheric temperature. However, on both nights there is evidence of a postmidnight enhancement of the O2/N2 density ratio associated with a rapid decrease in the height of the F layer.

Comments on the paper: Diurnal, annual and solar cycle variations of hydroxyl and sodium nightglow intensities in the Europe-Africa sector

Night-time variations of the OH nightglow intensity reported by Wiens and Weill are compared with the theoretical predictions of a number of models. The behaviour of this emission agrees better with the theoretical one for locations in the equatorial zone but becomes more variable and less predictable for mid-latitude stations. It is calculated that, as a result of an increase of the eddy diffusion coefficient K, the OH emission can deviate from the typical night-time variation and increase by a factor as high as 2 if K is multiplied by 10. It is suggested that the eddy diffusion coefficient in the upper atmosphere is lower and undergoes lower amplitude variations in the equatorial zone than in the mid-latitude regions.

The nighttime distribution of ozone in the low-latitude mesosphere

The intensity of stars at wavelengths in the Hartley continuum region of ozone has been monitored by the University of Wisconsin stellar photometers aboard the OAO-2 satellite during occultation of the star by the earth's atmosphere. These occultation data have been used to determine the ozone number density profile at the occultation tangent point. The nighttime ozone number density profile has a bulge in its vertical profile with a peak of 1 to 3×108 cm−3 at approximately 83 km and a minimum near 75 km. The ozone number density at high altitudes varies by as much as a factor of 4, but does not show any clear seasonal variation or nighttime variation. The retrieved ozone number density profiles define a data envelope that is compared with other nighttime observations of the ozone number density profile and also the results of theoretical models.

Magnetic fields due to sea tides in the english channel

The M 2 (12.42-h period) magnetic field variation present at night in the United Kingdom can be interpreted in terms of sea tides, the main contribution being from the Atlantic Ocean. Osgood et al. (1970) observed a difference between the M 2-variations in the total magnetic field ( F) variations at Sidmouth (0.8 km from the coast) and Exeter (16 km from the coast) of amplitude 0.8 γ, which must be due to local effects. Analysis of the phase of this difference between Exeter and Sidmouth has shown that it cannot be due entirely to tidally induced electric current systems confined to the English Channel. Measurements with two three-component magnetometers have shown that the night-time variations in the north-south ( H) component of 12.42-h period are significantly different at Sidmouth and Exeter. The difference in H is in phase with the expected electric potential difference across Devon due to the difference in the phases of the sea tides in the English Channel and the Bristol Channel. The results of Donato and Rosser (1973) on micropulsations suggest the existence of a region of high conductivity under Exeter. If this connects the English Channel to the Bristol Channel, a current will flow beneath Exeter giving a difference in the M 2-variations in H at Exeter and Sidmouth. This difference in H dominates the M 2-variations in the total magnetic field ( F) variations at Exeter and Sidmouth measured by Osgood et al. (1970).

The solar and lunar daily geomagnetic variation of declination at Trevandrum, 1853?1869

The eye readings of easterly declination at Trevandrum observatory have been analysed to determine solar and lunar daily vatiations. Hourly readings during February 1853 to February 1865 are analysed by Chapman-Miller method. The 8 values read at unequidistant day-time hours during March 1865 to December 1869 are analysed to compute the amplitude and phase of the semimonthly wave at each of the hours separately. The semi-monthly lunar wave obtained from the hourly readings shows a considerable night-time variation. A brief discussion of the results is also given.

Steadiness of average incoming long wave radiation

Analysis of the pyrgeometer data on the incoming long wave sky radiation available in India shows that while there is variation in the mean value from month to month, the variation from hour to hour in a night and from year to year for the same month is quite small. The variations of the incoming long wave radiation run closely parallel to the variations in the precipitable water content of the atmosphere.
 
 As pyrgeometer observations are not available outside India, the net radiation data between the hours 2200 to 0300 LAT for a number of stations in Asian and European U.S.S.R., Canada, Australia, England and Europe are examined for the year 1966.From this study it is inferred that the night time variation in sky radiation is quite small even in other parts of the globe.

Nighttime variations ofF-region electron density profiles at Puerto Rico: 3. Seasonal and solar cycle variations in temperature and vertical drift velocity deduced from ionospheric data

A method developed in Part 2 (Shimazaki, 1966) to calculate nighttime variations in the ‘effective temperature’ and vertical drift velocity from the F-region electron density profiles obtained from radio soundings has been applied to observational data of various seasons and years at Puerto Rico. Seasonal and solar cycle variations in temperature calculated in this study are reasonable, but differ quantitatively from those obtained from satellite drag data. Our result indicates that the summer temperature is higher than the winter one by ∼100°K for the same solar activity, while the annual average temperature decreases by 200 ∼ 300°K with the decrease of sunspot activity from 1959 to 1961. There is no systematic seasonal and solar cycle variation in the prevailing and semidiurnal component of variations in the vertical drift velocity except that the downward prevailing component is appreciably larger in winter than in summer in years of higher sunspot activity. In its nocturnal variations, the temperature tends to rise before sunrise; the onset time and the magnitude of the increase depend upon the season. It is suggested, on the basis of backscatter observations of Te, Ti and , that the predawn increase of the ‘effective temperature’ derived in this paper represents a departure from the atmospheric temperature.

Nighttime variation of ionospheric winds over Barbados, West Indies

The use of gun-launched projectiles to produce luminous trails above 90 km by chemical release has shown sufficient economy to allow a large number of measurements. On four occasions, six trails have been produced on the same night so that the nighttime variation of ionospheric winds can be studied. These trails did not show the Rosenberg rotating wind vector phenomena common to higher latitudes. Meridional wind contours showed a well-determined descent during a night although the zonal wind contours showed no such behavior. By averaging the large wind fluctuations at fixed altitude throughout a night, meaningful steady-state components have been obtained that show seasonal variations. For over half the altitudes and nights considered, large-amplitude sinusoidal variations were observed.

NighttimeF2layer at the equator

The behavior of the nighttime equatorial F2 layer under the influence of ambipolar diffusion and an electrodynamic drift has been studied theoretically. It is shown that there exists a maximum in the nighttime variation of the electron number density at constant height and that this maximum depends mainly on the nature of an electrodynamic drift. The effect due to ambipolar diffusion is comparatively small, and the rate of decay of ionization is affected to an appreciable degree by an increase in the magnitude of the drift. The results lead us to believe that, apart from the thermal contraction of the atmosphere, an electromagnetic drift may be an important factor in the formation of the maxima observed in electron profiles at a constant height.