Observations of random and quasi-periodic scintillations at southern mid-latitudes over a solar cycle

An extended period (1973–1985) of recording of random and Fresnel type quasi-periodic (QP) scintillations in southern mid-latitudes, using satellite beacon transmissions at a frequency of 150 MHz, has provided some new information on the morphology of scintillation-producing irregularities.It has become evident that a pronounced daytime increase of the random type of scintillations in the southern winter (at 1200–1600 LT) occurs throughout the solar cycle and becomes a distinct daytime maximum during the years of sunspot minimum. Scintillations are most intense in the pre-midnight period in the southern summer (2000–2400 LT). There is a gradual decline in scintillation activity by about 40% from the period of sunspot maximum to the period of sunspot minimum. It appears that a specific type of sporadic-E, so-called constant height Es (Esc), is responsible for daytime scintillation activity in winter. Night-time scintillations are strongly correlated with the presence of the range-spread type of spread-F, but not so with the frequency-spread type.There are two peaks in the occurrence of QP scintillations, predominantly in the southern summer: in the late morning (0800–1000 LT) and in the pre-midnight period (2000–2200 LT). The daytime QP scintillations occur mainly polewards of the station, whereas the night-time scintillations are recorded predominantly equatorwards. There is a distinct increase in the occurrence number of QP scintillations with a decrease in the sunspot number.     

Electron density near the plasmapause measured over one year by GEOS-2: a statistical analysis

This paper presents the results of a statistical analysis made under the 1 h average regime from the electron density data obtained over one year by the relaxation sounder on board the satellite GEOS-2 of ESA. The electron density diurnal variations, monthly and annually averaged, are sorted out. A comparison between monthly averaged, daily electron density profiles obtained over all the year has revealed the existence of a seasonal variation of plasma density in the equatorial region, the electron density being larger, on average, in summer than in winter. This seasonal variation is superimposed on the variation related to geomagnetic activity. The asymmetry of the Earth's internal magnetic field is mentioned as being a potential candidate for explaining this seasonal modulation. The annually averaged, daily profile is given. It is found to reach a maximum at 1600 LT, which indicates a plasmaspheric bulge in the predusk, rather than dusk, sector. This is found to be significantly earlier than the average LT location of the stagnation point (≳ 1800 LT) inferred from previous empirical studies for comparable geomagnetic activity levels. This latter feature is interpreted as resulting from the asymmetry of the individual daily density profiles with respect to their maximum, which has been previously reported, and whose effect, after averaging individual daily profiles having their maxima distributed around dusk, is to shift the maximum of the average profile toward the predusk sector.     

Anomalous fading and crystalline structure: Studies on individual chondrules from the same parent body

Plagioclase feldspar is the major luminescent mineral in meteorites. Thermoluminescence (TL) characteristics, peak temperature (T_m), full width at half maximum (FWHM), ratio of high (HT) to low temperature (LT) peak, and TL sensitivity (TL/dose/mass) to an extent reflect degree of crystallinity of the mineral. The present study explores and establishes a correlation between quantum mechanical anomalous (athermal) fading and structural state by examining TL of individual chondrules. Chondrules were separated using freeze-thaw technique from a single fragment of Dhajala meteorite. The results show large variation in T_m (155?230°C), FWHM (80?210°C) and HT/LT (0.07–0.47) and seem to be positively correlated. TL sensitivity (ranging from 14 to 554 counts/s/Gy/mg) decreases with increasing T_m and FWHM. Large variations in TL parameters (Tm, FWHM, HT/LT, and Sensitivty) suggest that individual chondrules had different degree of crystallization. Thermal annealing experiments suggest that comparatively ordered form of feldspar can be converted to a disordered form by annealing the sample at high temperatures (1000°C) for long time (10 hr) in vacuum (1 mbar pressure) condition and rapidly cooling it. Measured anomalous fading suggest that fading rate increases as the crystal form changes from an ordered state to a disordered state. However, the fading rate becomes nearly negligible for the most disordered feldspars.     

Simulation of ionospheric currents and geomagnetic field variations of Sq for different solar activity

Variations of ionospheric Sq electric currents and fields caused by changes in electric conductivity due to changes in solar activity are studied using the International Reference Ionosphere (IRI) model. Calculations are made for R (sunspot number) = 35 and 200 on the assumption of constant (1, −2)mode tidal winds. It is shown that electric fields grow when solar activity is high, because the ratio of the conductivity in the F-region to that in the E-region increases. Currents in the F-region become stronger than those in the E-region, and nocturnal currents are not negligible when solar activity becomes high. F-region currents also play an important role in the westward currents on the high latitude side of the current vortex. The calculated geomagneticH component at the equator has a depression around 1600 LT for R = 35, while it decreases smoothly from 1100 LT to 1900 LT for R = 200. This difference is consistent with the observed geomagnetic field variation. The ratio of total Sq currents obtained by our simulation is about 3.5, which is a little larger than is found in the observed results.     

On the nighttime increase of NmF2 at Jicamarca on 2–3 February 1965

The conditions of the nighttime increase in NmF2 at the geomagnetic equator are investigated. It is shown that, in an altitude region above 250 km, after 2100 LT, the increase is due to a downward electromagnetic drift when the compression of the plasma associated with the drift overcomes the chemical loss rate.     

Equatorial night-time spread-F characteristics observed from stations in the Western Pacific

A comparative study of the ionospheric spread-F observed at an equatorial station. Port Moresby (9°24.5′S, 147°9.9′E; magn. dip lat. 18°S). Papua New Guinea, with corresponding data from the East Asian stations during a period 27 May–19 June 1983, as well as the same periods for 1981 and 1982, is presented. The Port Moresby spread-F condition is particularly outstanding and persistent at night and the occurrence of spread-F between 23 LT and 05 LT near June solstice is very high, reaching 100 % of all observed nights. The latitudinal variation of nocturnal spread-F, its duration and connection with sunspot number are analyzed. The relationship between long-term variation of night-time spread-F occurrence and solar activity during June and December solstices from 1954 up to 1984 at a typical lower magnetic latitudinal station, Wuhan (30°32.7′N, 114°21.5′E; magn. dip lat. 26°N). is also investigated.     

Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data

The daily variations of the meridional wind at ±18° latitude have been obtained for summer and winter between 1977 and 1979 using the in situ measurements from the Atmosphere Explorer-E (AE-E) satellite. The AE-E altitude increased from about 250 to about 450 km during this period, with solar activity increasing simultaneously. Data are presented at three altitudes, around 270, 350 and 440 km. It was possible to average the data to obtain the 24 h variations of the meridional wind simultaneously at northern and southern latitudes and thereby study the seasonal variation of the meridional wind in the altitude range covered. Two features are found showing significant seasonal variation: (a) a late afternoon maximum of the poleward wind occurring only in winter at 1800 LT at all three altitudes; (b) a night-time maximum in the equatorward wind—the summer equatorward wind abating earlier (near 2130 LT) and more rapidly than the winter wind (after 2300 LT). Furthermore, in summer the night-time wind reaches higher amplitudes than in winter. The night-time feature is consistent with the observed seasonal variation of the equatorial midnight temperature maximum, which occurs at or before midnight in summer and after midnight in winter, showing a stronger maximum in summer. The observed night-time abatement and seasonal variations in the night-time winds are in harmony with ground based observations at 18° latitude (Arecibo). The time difference found between summer and winter abatements of the night-time equatorward wind are in large part due to a difference between the phases of the summer and winter diurnal (fundamental) components, and diurnal amplitudes are larger in summer than in winter at all threee altitudes. However, the higher harmonics play an important role, their amplitudes being roughly 50% of the diurnal and in some instances larger. The 24 h variation is mainly diurnal at all altitudes in both summer and winter, except in winter around 2700 km altitude where the semi- and ter-diurnal components are approximately equal to or larger than the diurnal.     

Geomagnetic field variations at low latitudes and ionospheric electric fields

The solar cycle, seasonal and daily variations of the geomagnetic H field at an equatorial station, Kodaikanal, and at a tropical latitude station, Alibag, are compared with corresponding variations of the E-region ionization densities. The solar cycle variation of the daily range of H at either of the stations is shown to be primarily contributed to by the corresponding variation of the electron density in the E-region of the ionosphere. The seasonal variation of the ΔH at equatorial stations, with maxima during equinoxes, is attributed primarily to the corresponding variation of the index of horizontal electric field in the E-region. The solar daily variation of ΔH at the equatorial station is attributed to the combined effects of the electron density with the maximum very close to noon and the index of electric field with the maximum around 1030 LT, the resulting current being maximum at about 1110 LT. These results are consistent with the ionosphere E-region electron horizontal velocity measurements at the equatorial electrojet station, Thumba in India.     

Some further results on long term mesospheric and lower thermospheric wind observations by the Arecibo UHF radar

A second series of long term mesospheric and lower thermospheric wind observations was conducted at Arecibo (18.4°N, 66.8°W) between 6 and 20 March 1981 using the UHF Doppler radar, following the first observations in August 1980 (Hirota et al., 1983). Zonal and meridional wind velocities were measured during the morning (8–10 LT) and afternoon (13–15 LT) periods. The mean wind profile averaged over the entire observational period shows the predominance of the diurnal tide. The fluctuating wind vector rotates clockwise relative to height with a characteristic vertical scale of about 10 km. The phase difference inferred by a cross correlation analysis between morning and afternoon profiles indicates that the dominant period is about 20–30 h. This oscillation is discussed in relation to internal inertia-gravity waves observed by the same radar in the lower stratosphere. On the other hand, wind fluctuation with a vertical scale larger than 20 km shows a substantial day-to-day variation with a period of 5–8 days. This long period oscillation shows a good correlation with the global scale geopotential height anomalies at 1 mb (46–48 km) observed by the Tiros-N satellite at 20°N. Our evidence suggests that westward travelling planetary-scale waves with zonal wavenumber one may propagate up to the lower thermosphere.     

Post-midnight equatorial scintillation activity in relation to geomagnetic disturbances

VHF amplitude scintillation measurements made during the period April 1978 through December 1982 at Calcutta (23°N, 88.5°E; 32°N dip), situated near the northern crest of the Appleton Anomaly in the Indian sector, have been used to study the association of post-midnight (as well as post-sunrise) scintillations with the occurrences of the maximum negative excursion in the variation of the Earth's horizontal magnetic intensity. The post-midnight scintillation has been found to be related to the maximum negative excursion occurring in the 0000–0600 LT interval. No such relation is observed with the pre-midnight excursions. Scintillation with onset between 0000 and 0300 LT shows remarkable correspondence with the occurrence of negative excursion (18 out of 20 available cases). Magnetic conditions with Dst < −150 nT have been found to be most effective in producing the above scintillation activity. From the present observations, a threshold value of the maximum negative excursion of Dst for producing scintillation may be obtained, Dst < −75 nT being significantly associated with the post-midnight scintillation occurrences. The results are interpreted in terms of the reversal of the equatorial horizontal electric field, under magnetically disturbed conditions, due to a coupling of the high latitude and magnetospheric current systems with the equatorial electric field.     

Diurnal variations of equatorial electrojet parameters derived from POGO solstitial data

The POGO electrojet data have been analysed for the winter and summer solstitial seasons of the two years, 1968 and 1969, respectively. Our analysis yielded a very large number of values (about 432), each of the electrojet half width, w, its peak current intensity, J₀, and its total eastward current, I₊, at 0900–1400 LT in December, and at 1000–1500 LT in June solstitial seasons, respectively. The all-longitude daytime values of the parameters are 246 ± 48 km for w,216 ± 60 A km⁻¹ for J₀, and (58 ± 8) × 10³A for I₊, in December of 1968 and 218 ± 19 km for w, 187 ± 20 A km⁻¹ for J₀, and (45 ± 3) × 10³ A for I₊, in June of 1969, respectively. We therefore present a diurnal study covering the entire Earth, in which for the first time, morning data earlier than 1000 LT are incorporated in the analysis. This has enabled us to chart a clearer picture of the temporal variations of electrojet parameters at two different solstices. This shows that all of the three parameters vary substantially with local time, in such a manner that J₀ and I₊ attain maximal values around local noon, while w is a minimum then, and therefore confirms the finding of Agu and Onwumechili.     

Study of equatorial plasma bubble dynamics using GHz scintillation observations in the Indian sector

To study equatorial plasma bubble dynamics, telemetry signals (4 GHz) were recorded simultaneously from two geostationary satellites. INSAT-1B (74°E) and INSAT-1C (94°E) at Sikandarabad satellite Earth station (dip 42.0°) from January to December 1989 and at the Chenglepet satellite Earth station (dip 10.5°) during September–October 1989 along the same geomagnetic meridian. The characteristics and occurrence pattern of the scintillations suggest that these are equatorial plasma bubble induced events. Observations from the two satellites recorded simultaneously at each of these locations were utilized to estimate the east-west plasma bubble irregularity motion. Plasma bubble rise velocities over the magnetic equator were calculated from the systematic onset time differences observed between an equatorial and a low latitude station. The east-west plasma bubble velocity estimated at Sikandarabad, corresponding to 1200 km altitude in the equatorial plane, shows a night time variation pattern with a peak at around 2100 LT. The mean values over Chenglepet, which correspond to 400 km altitude, start decreasing right from 1900 LT and seem to be influenced by the plasma bubble rise velocities. The differences in magnitude observed between the present results and those reported elsewhere by other techniques are interpreted in terms of vertical shears in the plasma zonal flow over the equator. The near alignment of the two observing stations along a common geomagnetic meridian and the simultaneous use of two satellites located twenty degrees apart in longitude provided an excellent data base to study plasma bubble dynamics.     

A theoretical study of the distribution of ionization in the high-latitude ionosphere and the plasmasphere: first results on the mid-latitude trough and the light-ion trough

A model of the O⁺ and H⁺ distributions in the plasmasphere and high-latitude ionosphere is described and first results are presented. The O⁺ and H⁺ continuity and momentum equations are solved from the F-region to the equatorial plane in the inner plasmasphere, and to an altitude of 1400 km in the outer plasmasphere and high-latitude ionosphere. Account is taken of high-latitude convection, departure from corotation inside the plasmasphere, and neutral air winds. The neutral air winds are consistent with the assumed convection pattern. For equinox and magnetically quiet conditions the calculations show that a mid-latitude trough in F-layer electron concentration is present from 1600 to 0600 LT and the trough may occur either inside or outside the plasmasphere. The movement of the trough in this period is from higher to lower latitudes and is in qualitative agreement with AE-C and ESRO-4 data. A light-ion trough feature is apparent in the H⁺ distribution in the topside ionosphere at all local times. During the day the upward H⁺ flow increases with latitude to produce the light-ion trough. At night the H⁺ trough may be directly produced by the occurrence of the mid-latitude O⁺ trough. The relationships between the position of the plasmapause and the trough are discussed. Also discussed are the influence of particle ionization in the auroral zone and the effect of the neutral air wind.     

Temporal variations of POGO equatorial electrojet parameters

An analysis of the POGO satellites observations of the magnetic field of the equatorial electroject for the September equinoctial months of the years 1967, 1968 and 1969, provided about 500 values each of the electrojet half-width w, its peak current intensity J₀, its total eastward current I₊ at 11, 12, 13, 14 and 15 h LT. The all-sector daytime average values of the parameters for the three years are 232 ± 47 A km⁻¹ for J₀, 234 ± 6 km for w and (55± 8)×10³ A for I₊.This first coverage of all sectors of the globe gives the first study of the diurnal variation of the total current and shows that all the three parameters vary substantially with local time; that w has minimal values around local noon; that J₀ has a pronounced peak around local noon as may be expected from the diurnal variation of H; and that I₊ has a broad maximum around 11 h and 12 h LT.     

Subsidiary maxima in late evening NmF2 at a low latitude station at solar maximum

At solar maximum during the late evening hours (2100–2400 LT), N_mF2 at Tahiti frequently does not decrease monotonically but exhibits temporary subsidiary maxima. Thus, in 1980, of 66 evening periods for which good data were available, 20 showed monotonie decreases but the remainder exhibited such subsidiary maxima. In summer the subsidiary maxima correspond to hmF2 significantly lower than the monotonie decreases. This lower hmF2 during subsidiary maxima corresponds to a weakening or reversal of the equatorward neutral wind, accompanied by an increase in the flux from the equatorial fountain. In winter the subsidiary maxima are fully accounted for by increases in the flux from the fountain effect, probably due to increases in the strength of the equatorial electrojet.     

Geomagnetically quiet day ionospheric currents over the Indian sector—III. Counter equatorial electrojet currents

Counter equatorial electrojet (CEJ) occurring at all hours from 0700 to 1700 h LT in the Indian sector have been studied. The percentage occurrence of morning and afternoon CEJ in each hour and each season are given and discussed. The first quantitative determinations of eight landmark parameters that depict the structures of hourly latitudinal profiles of CEJ current have been presented and discussed.These are the peak height integrated forward current density (or intensity) at the centre of the current, the peak return current intensity, the ratio of the peak return to the peak forward current intensity, the total forward current, the dip latitude of the current centre, the distance of the focus from the current centre, the distance of the peak return current intensity from the centre, and the latitudinal extent of the current. They reveal new and interesting features of diurnal and seasonal variations, and marked contrasts between morning and afternoon CEJ. Other evidences support our focal distance w of CEJ. Comparison with equatorial electrojet (EEJ) shows that five of the parameters are similar for EEJ and morning CEJ but substantially different for afternoon CEJ. The counter worldwide part of Sq currents have also been compared with CEJ.     

The nocturnal intermediate layer over South Georgia : solar and magnetic influence on occurrence

Ionograms from South Georgia (54°S, 37°W; L = 1.9) are used to investigate the influence of magnetic and solar activity upon the occurrence of the nocturnal intermediate layer. The diurnal variation in occurrence of this layer exhibits a peak at about 2000 LT and a subsidiary peak after midnight. It is shown that magnetic activity has no significant influence on the behaviour before midnight, but is positively correlated with the size of the post-midnight peak. The effect of varying solar activity is to introduce a local time shift (42 min for a change in mean solar activity from 80 to 130× 10⁻²² Wm⁻²Hz⁻¹)inthediurnal variation without otherwise changing the overall morphology of the layer significantly. Most of the features of the intermediate layer before midnight can be explained by a wind shear mechanism associated with the solar semi-diurnal tide. Some possible causes for the post-midnight observations are considered, but no firm conclusions can be made.     

Simultaneous ground-based observations of the plasmapause and the F-region mid-latitude trough

The plasmapause and the mid-latitude ionospheric trough have been observed simultaneously from two Antarctic stations, Halley and Faraday, during five winter nights covering a range of geomagnetic disturbance conditions. The equatorial radius of the plasmapause was measured using whistlers recorded at Halley, whilst the poleward edge of the trough was located from ionospheric soundings at one or other of the stations.Before midnight the trough was well poleward of the plasmapause (by 1–2 L) when first observed (typically at ~21 LT), but then moved rapidly equatorwards. After local magnetic midnight the two features were roughly coincident, and in general moved slowly to lower L-shells with increasing local time. At no time were there simultaneous and identical movements of the two features, suggesting a lack of coupling between them. Agreement of the observations with statistical studies and models was fair, given the considerable variability among the five cases studied. For the geomagnetically quieter nights the trough data fit the Spiro model predictions, whereas in the most disturbed case, agreement is better with the Quegan et al. model. The latter model predicts a difference in L between the two features which would fit the data better if shifted 1–2 h later in local time.     

Solar activity effects on equatorial plasma bubble zonal velocity and its latitude gradient as measured by airglow scanning photometers

Using a new mode of scanning 630-nm photometer operation the zonal velocities of ionospheric plasma depletions were measured over Cachoeira Paulista in Brasil in two east-west planes tilted 30°N and 30° S with respect to zenith. The measurements cover a time period of approximately 2 years, from January 1988 to January 1990, a period marked by significant increase in solar activity of the ongoing cycle. The results have permitted a rather detailed evaluation of the local time and latitude variations in the zonal plasma bubble velocity as a function of solar activity. Although the mean trend in the velocity local time variation is a decrease from early evening to post-midnight hours, a strong tendency for velocity peaks is observed near 21 LT and midnight. The velocities as well as their height (latitude) gradients show perceivable increases with solar activity represented as sunspot numbers. The present results are compared with the ambient plasma velocities measured using the Jicamarca radar by Fejer el al. (1985), J. Geophys. Res. 90, 12249, with that measured on board the DE 2 satellite on the equatorial latitudes by Coley and Heelis (1989), J. geophys. Res. 94, 6751, and with various theoretical calculations, in an attempt to bring out the salient features of the plasma dynamics of the equatorial ionosphere.     

Experimental production of bending and radial flake fractures and implications for lithic technologies

Bend and radially broken flake tools have been identified in Paleolithic and Paleoindian assemblages, and their presence raises important questions. Were these breaks intentionally produced to serve as tool edges or were broken flakes simply scavenged? More importantly, can we distinguish between intentionally produced breaks and those produced incidentally? Experimental archaeology can help answer these questions. In this paper, three sets of experimentally produced bend and radial flake breaks were compared. Flakes were intentionally broken by percussion, and these breaks were compared to those produced during bifacial core reduction and by flake trampling. The presence of point of impact markers, near ninety degree break angles, and an assemblage with high percentages of bend and radial breaks distinguish intentional fracture from incidental fractures produced during bifacial reduction. High percentages of radial breaks distinguish intentional fracture from trampling. Finally, it may not be possible to identify intentional breaks in a bifacial reduction assemblage severely affected by flake-on-flake trampling.