First results on daytime mesopause OH rotational temperatures using ground-based photometry from equatorial latitudes

Daytime mesospheric OH , rotational temperatures were estimated from the measurements of the relative intensities of the rotational lines in the same vibrational band, using the unique multiwavelength daytime photometer, from Tirunelveli (8.7°N; 77.8°E; 0.5°N dip latitude), an equatorial station in the Indian zone. On a couple of days when simultaneous measurements were available in the vicinity of the experimental site, the estimated temperatures compared well with those measured by the Wind Imaging Interferometer (WINDII) onboard the Upper Atmosphere Research Satellite (UARS). These results, the first of their kind, revealed: (1) large day-to-day variabilities; (2) oscillatory features with periodicities ranging from 0.5 to 6 h; and (3) large deviations of ±30–50 K, on occasions, from the MSIS-90 model temperatures. Comparisons have also been made with simultaneous measurements of neutral winds around mesopause altitudes using the Partial Reflection radar from the same site to look for any possible association of neutral dynamical parameters with the mesopause temperature variabilities. These results are presented and discussed.     

Equatorial ionization anomaly of the total electron content and equatorial electrojet of ground-based geomagnetic field strength

Measurements from ground-based receiver chains of the global positioning system (GPS) and magnetometers of the Circum-pan Pacific Magnetometer Network (CPMN) in the west Pacific region during 1999–2003 are examined. The ionospheric total electron content (TEC) derived from the GPS receivers is used to observe the strength, location, and occurrence time of the equatorial ionization anomaly (EIA) crests, which resulted from the equatorial plasma E×B drift fountain. The magnetic field strength of CPMN is employed to monitor the equatorial electrojet (EEJ), and to further estimate the effectiveness of the E×B drift to the EIA crests. Results show that the strength and location of the EIA crests are proportional to the EEJ strength.     

Effect of equatorial ionization anomaly on the occurrence of spread-F

The unique geometry of the geomagnetic field lines over the equatorial ionosphere coupled with the E–W electric field causes the equatorial ionization anomaly (EIA) and equatorial spread-F (ESF). lonosonde data obtained at a chain of four stations covering equator to anomaly crest region (0.3 to 33 °N dip) in the Indian sector are used to study the role of EIA and the associated processes on the occurrence of ESF. The study period pertains to the equinoctial months (March, April, September and October) of 1991. The ratios of critical frequency of F-layer (f₀F₂) and electron densities at an altitude of 270 km between Ahmedabad (33 °N dip) and Waltair (20 °N dip) are found to shoot up in the afternoon hours on spread-F days showing strengthening of the EIA in the afternoon hours. The study confirms the earlier conclusions made by Raghava Rao et al. and Alex et al. that a well-developed EIA is one of the conditions conducive for the generation of ESF. This study also shows that the location of the crest is also important in addition to the strength of the anomaly.     

An empirical relation of daytime equatorial total electron content with equatorial electrojet in the Indian zone

The variability of Total Electron Content (TEC) at Trivandrum, located within equatorial anomaly region at the dip equator, with respect to a reference level derived from the TEC measurements at Shimla, located outside the region has been studied during low solar activity period. Chapman function is assumed to hold good for regions outside the anomaly extent. It shows that the difference of total measured TEC at the equator from the derived reference is highly correlated with equatorial electrojet. The observations conform to the previous investigations and are interpreted in light of established relations. A stochastic relationship with electrojet is derived and validated.     

Typical disturbances of the daytime equatorial F region observed with a high-resolution HF radar

HF radar measurements were performed near the magnetic equator in Africa (Korhogo 9°2463N–5°3738W) during the International Equatorial Electrojet Year (1993–1994). The HF radar is a high–resolution zenithal radar. It gives ionograms, Doppler spectra and echo parameters at several frequencies simultaneously. This paper presents a comparative study of the daytime ionospheric structures observed during 3 days selected as representative of different magnetic conditions, given by magnetometer measurements. Broad Doppler spectra, large echo width, and amplitude fluctuations revealed small-scale instability processes up to the F-region peak. The height variations measured at different altitudes showed gravity waves and larger-scale disturbances related to solar daytime influence and equatorial electric fields. The possibility of retrieving the ionospheric electric fields from these Doppler or height variation measurements in the presence of the other possible equatorial ionospheric disturbances is discussed.     

Specific propagation of radiowaves from the Intercosmos-19 satellite in the region of the nighttime equatorial anomaly crest

It has been indicated how a complex ionogram of topside sounding near the outer slope of the winter southern crest of the equatorial anomaly, where a large NmF2, gradient and a deep hmF2, minimum are observed, is formed. The model latitudinal cross-sectio n of the ionosphere, used to perform trajectory calculations, has been constructed based on the corrected Intercosmos-19 data. The ray trajectories have been modeled using the method of characteristics. It has been indicated that a complex Intercosmos-19 ionogram is formed by an oblique reflection from the equatorial anomaly crest slope (the main trace) and by a strongly oblique reflection from the crest bottom as a result of the wave capture by a large-scale inhomogeneity (the additional trace).     

Quasi-biennial oscillation in foF2 at the south crest of the equatorial anomaly

The aim of this paper is to report some periodicities observed in the ionospheric parameter foF2 measured at Tucuman (26.9°S; 65.4°W), station placed near the southern crest of the equatorial anomaly. For that, monthly medians of foF2 at several hours of LT for the period 1958–1987 are used. The data are run with Fast Fourier Transform (FFT). Data gaps (4–5 months) are filled by means of linear interpolation. Several periodicities are present. Besides the solar cycle dominant dependence (11 years), semi-annual, annual, five years and quasi-biennial periodicities are also observed. A marked quasi-biennial periodicity is observed at daytime and nighttime hours being their greater amplitude at local noon and midnight. Different mechanisms or combined effects possibly cause them. It is suggested that the solar activity by means of extreme ultraviolet radiation (EUV), which present a quasi-biennial oscillation (QBO) and it is responsible for the ionization, could be the dominant mechanism for the diurnal quasi-biennial periodicity of foF2. At night, since the photoionization by extreme ultraviolet radiation is not significant and the F2 layer is lower than during daytime (100 km) other mechanism may be operative for the quasi-biennial periodicity observed. Possibly the stratospheric QBO contributes to the modulation of the observed behaviour in foF2 at night. This result is preliminary because it needs to be extended to other stations so as to extract definite conclusions. Moreover, we cannot dismiss the possibility of a combined effect of both these mechanisms mainly at daytime and/or QBO influence of geomagnetic parameters.     

Characteristics of VHF scintillations in the equatorial anomaly crest region in India

The characteristics of ionospheric scintillations at Rajkot in the equatorial anomaly crest region in India are described for the years 1987–1991 by monitoring the 244-MHz transmission from the satellite FLEETSAT. This period covers the ascending phase of solar cycle 22. Scintillations occur predominantly in the pre-midnight period during equinoxes and winter seasons and in the post-midnight period during summer season. During equinoxes and winter, scintillation occurrence increases with solar activity, whilst in summer it is found to decrease with solar activity. Statistically, scintillation occurrence is suppressed by magnetic activity. The characteristics observed during winter and equinoxes are similar to those seen at the equatorial station, Trivandrum. This, coupled with the nature of the post-sunset equatorial F-region drift and h‘F variations, supports the view that at the anomaly crest station, scintillations are of equatorial origin during equinox and winter, whilst in summer they may be of mid-latitude type. The variations in scintillation intensity (in dB) with season and solar activity are also reported.     

Disturbances at F2-region heights of equatorial anomaly during geomagnetic storms

Neutral gas composition and ionospheric measurements taken by the Dynamic Explorer 2 satellite at F2-region heights during two geomagnetic storms are used to analyze the role of some possible physical mechanisms responsible for the changes of electron density at equatorial and low geomagnetic latitudes. The storms considered occurred on October 2, 1981 (storm 1) and July 13, 1982 (storm 2). During storm 1 (weak), vertical plasma drifts and equatorward storm-time winds operated increasing of the electron density at the trough of equatorial anomaly and the decreases at the crest region. During storm 2 (intense) changes of composition (increase of molecular nitrogen and atomic oxygen) played a fundamental role for the changes of electron density observed at low latitudes in summer hemisphere. It is concluded that different physical processes seem to have varying degrees of importance depending on the intensity of the storm.     

VHF ionospheric scintillations near the equatorial anomaly crest: solar and magnetic activity effects

The paper presents a study of solar and magnetic activity effects on VHF ionospheric scintillations recorded during three and half years at Bhopal, a station near the northern crest of the equatorial anomaly in India. During E- (equinox) and D- (winter) months, scintillations occur mainly in the pre-midnight period whereas during J- (summer) months their occurrence is larger in the post-midnight period. Very intense scintillations (>20 dB) mainly occur in the pre-midnight period, and in the post-midnight period, the scintillations are generally moderate (5–10 dB) or weak (<5 dB). The nocturnal scintillation occurrence decreases with the decrease in solar activity from 1989 to 1992. Monthly mean scintillation occurrence changes according to solar activity during E- and D-months but not so during J-months. The effects of magnetic activity on scintillations vary with season and, in general, inhibit the scintillation occurrence in the pre-midnight period and enhance it a little in the post-midnight period, especially after 0300 hours IST (Indian Standard Time). For most of the severe magnetic storms in which Dst goes below −125 nT and the recovery phase starts in the post-midnight to dawn local time sector, strong post-midnight scintillations, which sometimes extend for several hours beyond the local sunrise, are observed.     

The equatorial ionospheric anomaly in electron content from solar minimum to solar maximum for South East Asia

Median hourly, electron content-latitude profiles obtained in South East Asia under solar minimum and maximum conditions have been used to establish seasonal and solar differences in the diurnal variations of the ionospheric equatorial anomaly (EIA). The seasonal changes have been mainly accounted for from a consideration of the daytime meridional wind, affecting the EIA diffusion of ionization from the magnetic equator down the magnetic field lines towards the crests. Depending upon the seasonal location of the subsolar point in relation to the magnetic equator diffusion rates were increased or decreased. This led to crest asymmetries at the solstices with (1) the winter crest enhanced in the morning (increased diffusion rate) and (2) the same crest decaying most rapidly in the late afternoon (faster recombination rate at lower ionospheric levels). Such asymmetries were also observed, to a lesser extent, at the equinoxes since the magnetic equator (located at about 9○N lat) does not coincide with the geographic equator. Another factor affecting the magnitude of a particular electron content crest was the proximity of the subsolar point, since this increased the local ionization production rate. Enhancements of the EIA took place around sunset, mainly during the equinoxes and more frequently at solar maximum, and also there was evidence of apparent EIA crest resurgences around 0300 LST for all seasons at solar maximum. The latter are thought to be associated with the commonly observed, post-midnight, ionization enhancements at midlatitudes, ionization being transported to low latitudes by an equatorward wind. The ratio increases in crest peak electron contents from solar minimum to maximum of 2.7 at the equinoxes, 2.0 at the northern summer solstice and 1.7 at northern winter solstice can be explained, only partly, by increases in the magnitude of the eastward electric field E overhead the magnetic equator affecting the [E×B] vertical drifts. The most important factor is the corresponding increase in ionization production rate due to the increase in solar radiation flux. The EIA crest asymmetries observed at solar maximum were less significant, and this is probably due to the corresponding increase in ionization densities leading to an increase of the retarding effect of ion-drag on the daytime meridional winds.     

Variability of total electron content near the crest of the equatorial anomaly during moderate geomagnetic storms

The ionospheric responses to a large number (116) of moderate (?50≥D_st>?100 nT) geomagnetic storms distributed over the period (1980–1990) are investigated using total electron content (TEC) data recorded at Calcutta (88.38°E, 22.58°N geographic, dip: 32°N). TEC perturbations exhibit a prominent dependence on the local times of main phase occurrence (MPO). The storms with MPO during daytime hours are more effective in producing larger deviations and smaller time delays for maximum positive deviations compared to those with nighttime MPO. Though the perturbations in the equinoctial and winter solstitial months more or less follow the reported climatology, remarkable deviations are detected for the summer solstitial storms. Depending on the local times of MPO, the sunrise enhancement in TEC is greatly perturbed. The TEC variability patterns are interpreted in terms of the storm time modifications of equatorial electric field, wind system and neutral composition.     

Impact of winter thermal condition of the Tibetan Plateau on the zonal wind anomaly over equatorial Pacific

Using the improved CCM1 dynamic climate model, the impact of abnormal heat source and sink over the Tibetan Plateau in winter on the abnormal zonal wind over the Pacific Ocean is studied in this paper. The following new-findings are obtained: (1) When the at mospheric cold source during January—March on the Tibetan Plateau gets intensified, an abnormal anticyclone around the Tibetan Plateau will appear in lower troposphere. Abnormal northerly wind at the coastal area of the mainland of China and an abnormal cyclone will appear on the West Pacific in the following months. Then, abnormal west wind will appear over the equator of the West Pacific and extends to the East Pacific. (2) When the atmospheric cold source during January—March over the Tibetan Plateau is unusually weak, an abnormal cyclone around the Tibetan Plateau will appear at lower levels first, then abnormal anticyclone will appear on the West Pacific and move to the south and result in abnormal easterly wind over the equator of the West Pacific, which will extend to the east. Furthermore, abnormal changes of zonal wind on equatorial Indian Ocean can be caused by the intensity change of atmospheric cold source in winter and early spring over the Tibetan Plateau.

     

The variation of ionospheric slab thickness over equatorial ionization area crest region

The GPS-derived total electron content (TEC) and NmF2 are measured at the Chung-Li ionosonde station (24.9°N, 121°E) in order to study the variations in slab thickness (τ) of the ionosphere at low-latitudes ionosphere during 1996–1999, corresponding to half of the 23rd solar cycle. This study presents the diurnal, seasonal, and solar flux variations in τ for different solar phases. The seasonal variations show that the average daily value is greater during summer and the reverse is true during equinox in the equatorial ionization anomaly (EIA) region. Moreover, the τ values are greater during the daytime (0800–1600 LT) and nighttime (2000–0400 LT) for summer and winter, respectively. The diurnal variation shows two abnormal peaks that appear during the pre-sunrise and post-sunset hours. The peak values decrease as the sunspot number increases particularly for the pre-sunrise peak. Furthermore, the variation in the F-peak height (hpF2) indicates that a thermospheric wind toward the equator leads to an increase in hpF2 and an enhancement in τ during the pre-sunrise period. Furthermore, the study shows the variations of τ values for different geophysical conditions such as the geomagnetic storm and earthquake. A comprehensive discussion about the relation between τ and the geophysical events is provided in the paper.     

Retrieval of volcanic ash particle size, mass and optical depth from a ground-based thermal infrared camera

Volcanoes can emit fine-sized ash particles (1–10 μm radii) into the atmosphere and if they reach the upper troposphere or lower stratosphere, these particles can have deleterious effects on the atmosphere and climate. If they remain within the lowest few kilometers of the atmosphere, the particles can lead to health effects in humans and animals and also affect vegetation. It is therefore of some interest to be able to measure the particle size distribution, mass and other optical properties of fine ash once suspended in the atmosphere. A new imaging camera working in the infrared region between 7–14 μm has been developed to detect and quantify volcanic ash. The camera uses passive infrared radiation measured in up to five spectral channels to discriminate ash from other atmospheric absorbers (e.g. water molecules) and a microphysical ash model is used to invert the measurements into three retrievable quantities: the particle size distribution, the infrared optical depth and the total mass of fine particles. In this study we describe the salient characteristics of the thermal infrared imaging camera and present the first retrievals from field studies at an erupting volcano. An automated ash alarm algorithm has been devised and tested and a quantitative ash retrieval scheme developed to infer particle sizes, infrared optical depths and mass in a developing ash column. The results suggest that the camera is a useful quantitative tool for monitoring volcanic particulates in the size range 1–10 μm and because it can operate during the night, it may be a very useful complement to other instruments (e.g. ultra-violet spectrometers) that only operate during daylight.     

Complex resistivity and radar investigation of building material: First results of field scale measurements

Moisture ingress is one of major damaging factors for masonry buildings. As the complex resistivity (CR) is sensitive to textural properties as well as to the pore fluid chemistry of wet porous media, its non-destructive application can provide helpful information for conservators. In a comprehensive laboratory study it has been shown that CR might even be able to distinguish between salt content and saturation degree in only one measurement. The combined use of electrical and electromagnetic measurement techniques in two field-scale flooding experiments has shown some unexpected differences. Possible reasons are discussed and it is shown that bringing together the information of both methods leads to a clearer picture.     

Predicted and measured electron density at 600 km altitude in the South American peak of the equatorial anomaly

Measurements of the electron density at 600 km altitude (N₆₀₀) were obtained with the Hinotori satellite launched by the Institute of Space and Astronautical Science of Japan. These measurements were used to check the validity of the International Reference Ionosphere (IRI) model in predicting the electron density at that altitude in the South American peak of the equatorial anomaly. The measurements correspond to the longitude zone from 285 to 369° and −15° geomagnetic latitude. To model the electron density at 600 km altitude, two cases were considered, namely (i) N₆₀₀ was calculated with the IRI model at 10° intervals within the corresponding longitudinal zone and mean values were obtained, and (ii) N₆₀₀ was calculated with the IRI using ionosonde data as input coefficients in the model. The data used for this study were measured almost simultaneously with the total electron content data used in a previous work. The results show good predictions at hours of minimum ionisation for the equinox and the December solstice. For the June solstice, the best agreement was obtained around noon. However, strong disagreements were observed in some cases such as the equinox at 15:00 LT, suggesting that there is a need to improve the modeled topside profile.     

On the possibility of coupling satellite and ground-based optical measurements in the region of pulsating auroras

The possibility of comparing in detail CLUSTER satellite data with data on pulsating spots registered with a TV camera at the Lovozero Observatory when the satellite crossed the magnetospheric region related to the camera’s field of view is discussed. The satellite ionospheric projections were calculated using the T89, T96, and T01 models. It was shown that the projection allows us to judge with confidence whether or not a satellite will find itself in a region of pulsating auroras when only the level of geomagnetic activity and the characteristics of the interplanetary medium are a priori known. When different models are used in the projection, the spread is not less than the characteristic dimensions of the pulsating spots and can be as high as 100 km. The corresponding satellite flight time is ∼4 min. Such a large spatial and time uncertainty does not allow us to compare in detail the satellite data with ground-based optical measurements without a priori information on, e.g., the character of precipitation above a spot, as has been done by other researchers in the case of auroral arcs. The situation becomes even more complex if a satellite is in the region of greatly stretched magnetic field lines.