Longitude Zone Research Articles

First results from ground-based daytime optical investigation of the development of the equatorial ionization anomaly

A meridional scanning OI 630.0-nm dayglow photometer was operated from Ahmedabad (17.2°N dip lat.) scanning a region towards the south in the upper atmosphere extending over ~5° in latitude from 10.2°N to 15.2°N dip latitude. From the spatial and temporal variabilities of the dayglow intensity in the scanning region we show for the first time, evidence for the passage of the crest of the equatorial ionization anomaly (EIA) in the daytime by means of a ground-based optical technique. The relationship between the daytime eastward electric field over the dip equator in the same longitude zone as inferred from the equatorial electrojet strength and the evolutionary pattern of EIA is clearly demonstrated. The latter as inferred from the dayglow measurements is shown to be consistent with our present understanding of the electrodynamical processes in the equatorial region. The present results reveal the potential of this ground-based optical technique for the investigation of ionospheric/thermospheric phenomena with unprecedented spatial and temporal resolution.

Radar wind profiler observations of solar semidiurnal atmospheric tides

Semidiurnal solar tides in the mid‐latitude troposphere are investigated using harmonic analysis of 404 MHz radar profiler wind data obtained from a wide longitude zone in the U.S. The tides are apparent above a 1000‐m‐deep surface layer and increase in amplitude with height, attaining speeds of 0.5–0.7 m/s at 5–7 km. Observed wind characteristics agree well with tidal characteristics obtained with a dynamical model driven by observed global semidiurnal horizontal pressure gradients.

Magnetospheric Contribution to Low Latitude Sq Variation.

Non-regular features noticed in the quiet-time diurnal variation of H component on 11 December, 1990 (Ap = 2) and 12 December, 1990 (Ap = 7) in comparison with the average quiet day variations at a chain of equatorial, low and mid-latitude locations in the Indian longitude zone are studied. Stations away from the equatorial electrojet influence exhibited a minimum in the H field around 1600 LT followed by a secondary peak at 2100 LT. The globally observed anomalously enhanced variation in the “H” component at low and mid latitude is attributed to the effect of field aligned current driven by the magnetospheric plasma convection even during relatively quiet intervals. Causative mechanism to offset this peculiar feature, at equatorial locations is also discussed.

Secular variation in the geographic location of the dip equator

The geographic location of the dip equator on the globe has been determined for several epochs between 1550 and 1990 A.D. It is shown that two regions near 165°E and 285°E are devoid of migratory changes whereas between 300 and 0°E there is a large northward movement. The period between 1945 and 1990 is marked by a nearly linear change in the latitude of the dip equator while earlier epochs show more complex trends. Consistent with the earlier observations of the movement of the magnetic center towards or away from the centre of the earth, before and after 1800 A.D., the secular curves in certain longitude zones indicate a clear change in pattern. The observed secular variations in the vertical cornponent at several low latitude stations are generally in agreement with the direction of the movement of the dip equator. Examples of unexpected behaviour are highlighted. The eccentric dipole model appears to be adequate to delineate the dip equator for any epoch.

Submicron aerosols over Indian Ocean: some meteorological characteristics

The concentrations of submicron aerosols in the size range 10 −7 to 10 −5 cm, also called Aitken nuclei (AN) were measured over the Indian Ocean enroute India-Antarctica-India within the 10°E–70°E longitude zone from about 10°N to 70°S latitude on board MV Thuleland during the period from November 26, 1986 to March 18, 1987 as part of the scientific activities on the Sixth Indian Antarctic Expedition. Our analyses showed that only in about 25% of the cases, AN count fell below 1000 cm −3. Throughout the tropical trade wind region, the concentrations of AN were relatively stable with an average of about 3000 cm −3 (medians of ∼2600 and ∼1700 cm −3 in Northern and Southern Hemispheres, respectively). Large AN concentrations were found to be associated with higher sea surface temperatures and stronger surface winds in this region. In contrast, the scatter of single observations was found to be remarkable over South Indian Ocean and in Antarctic waters. The average AN concentration over the Indian Ocean to the south of 30°S was of the order of 1500 cm −3. No definite correlation could be established between large AN concentration and sea surface temperature, wind speed or wave height. Period with very low concentrations were, however, associated with clear sky conditions and calm winds or light breeze. Many events of sudden short-lived but large increase in AN concentrations were observed over the south Indian Ocean and in Antarctic waters and these were always associated with the approach of frontal systems. It is likely that particle production by bursting bubbles and sea spray as well as photochemical reactions and gas-to-particle conversions play important role in the observed high concentration of AN over South Indian Ocean.

The effects of magnetic storm phases on F-layer irregularities

The effects of magnetic storm phases on F-layer irregularities from auroral to equatorial latitudes in a nearly constant western longitude zone are presented by considering scintillation, spread F, and low-energy (< 12 eV) electron precipitation data for eight magnetic storms that occurred during the high solar flux period September–November 1981. In the equatorial region, F-layer irregularities can be inhibited during the main phase. In the high latitude region, F-layer irregularities are found to be generated at the auroral latitudes during the main phase. During the recovery phase when there are conditions of low magnetic activity, low auroral irregularities and low-energy electron precipitation crossing the subauroral regions, strong F-layer irregularities are found to be generated in the subauroral regions.

Comparative study of scintillations at the magnetic equator and at the crest region of the equatorial anomaly in Indian zone

Using simultaneous long-term observations of ionospheric scintillation at equator and anomaly crest region in the same longitude (Indian) zone comparative features of scintillation occurrence are brought out. The salient features are: (a) predominantly pre-midnight occurrence of scintillation at equator during winter and equinox seasons, (b) increase of pre-midnight scintillation occurrence with solar activity (c) shifting of occurrence peak during summer from post-midnight in low to pre-midnight in high solar activity periods (d) similarity of scintillation behaviour at these locations during winter and equinoxes but dissimilarity during summer. The solar activity response and magnetic effects indicate that the scintillations at the anomaly crest region in winter and equinox, particularly during high solar activity periods, are of equatorial origin while the summer events may be of local or mid-latitude origin.

Cyclotron resonance precipitation of energetic electrons from the inner magnetosphere

Cyclotron resonance between trapped energetic electrons and VLF waves produces pitch angle scattering which leads to electron precipitation in the inner magnetosphere. Previous experiments have shown that in the drift loss cone at L values from 1.5 to 1.8 the energy spectrum of the electrons above 50 ReV is often dominated by a single narrow peak. The center energy of this peak varies with L in a manner characteristic of cyclotron resonance between the electrons and monochromatic VLF waves in the vicinity of the geomagnetic equator. The source of the waves is probably VLF communication or navigation transmitters. We report here the results of a study of 680 occurrences of these peaks detected by the low altitude polar orbiting satellite S81‐1. The present data, from altitudes between 170 and 270 km, show the resonance peaks only in two restricted longitude zones centered at 110°E and 300°E; this result contrasts with the previous measurements at higher altitudes, which detected peaks over a wide range of longitudes. The majority of nighttime events were near transmitter locations where the cyclotron resonance condition was met by electrons with pitch angles near the edge of the loss cone. Detailed study of the events occurring near 110°E reveals a 27‐day periodicity in the frequency of occurrence; peaks are most often observed during minima in solar/geomagnetic activity. The peaks observed at nighttime in both longitude zones have a possible seasonal variation, being most frequent when it is wintertime at the site of the VLF transmitters responsible for the precipitation.

Indian programme on middle atmosphere: Some results

We present here an account of the very extensive programme on the middle atmosphere carried out in India since 1982, using three rocket ranges (Thumba, SHAR and Balasore), a high altitude balloon facility at Hyderabad, a Lidar at Thumba, a Laser Heterodyning system at Delhi, a Meteor Radar in Thumba, a network of UVB and multi-wavelength radiometers and a host of conventional ground based facilities scattered over the entire subcontinent spanning a range of latitudes from 8°N to 34°N and largely around the same longitude zone of 75°E. A special nature of the Indian effort is the emphasis on campaign mode operations, knitting special rocket and balloon efforts with more conventional ground based activities around specific themes. Major compaigns carried out included: (i) Indo-Soviet Ozone Intercomparison campaigns in 1983 and 1987 (ii) Aerosol campaign (iii) Ionization and conductivity campaigns (iv) Equatorial Wave Campaign (v) Antarctic Ozone Hole campaign in Dakshin Gangotri. A few of the more important findings are outlined.

A calculation of ionospheric currents due to semidiurnal antisymmetric tides

The fully developed equivalent circuit method is used to calculate the ionospheric electric current flow due to the (2,3) and (2,5) tidal modes. The (2,5) mode currents are of negligibly small amplitude. The (2,3) current pattern has meridional currents in the same direction at a given local time in both north and south hemispheres. There is also sometimes a minimum in the zonal current around 10° dip latitude and the equatorial electrojet pattern may be different in different longitude zones. These features may be possible explanations of (1) the meridional current pattern observed at Saint‐Santin, (2) current flows on abnormal quiet days (AQDs), (3) the reverse equatorial electrojet, (4) the seasonal phase anomaly in the lunar geomagnetic tide, and (5) the “invasion” of one hemisphere's current pattern by that from the opposite hemisphere. The recent detection of antisymmetric semidiurnal winds just below the dynamo region further demonstrates the importance of these modes.

Giant longitude zones on the sun

Regularities in the longitudinal distribution of high-flare-activity active regions during 1964–1979 have been examined. The Fourier spectrum for each 30°-wide longitude strip in northern and southern hemispheres has been obtained. It is seen that there exist two giant longitude zones in each hemisphere and solar activity remains concentrated in one of the giant zones. It also appears that the leading edge of a giant zone is more active as compared to its following edge.

Some observations on the role of planetary waves in determining the spring time ozone distribution in the Antarctic

Ozone measurements from 1970 to 1984 from the Nimbus 4 backscattered ultraviolet (BUV) and the Nimbus 7 solar backscattered ultraviolet (SBUV) spectrometers show significant decrease in total ozone only after 1979. The downward trend is most apparent in October south of 70°S in the longitude zone 0° to 30°W where planetary wave activity is weak. Outside this longitude region, the trend in total Ozone is much smaller due to strong interannual variability of wave activity. This paper gives a phenomenological description of ozone depletion in the Antarctic region based on vertical advection and transient planetary waves.

Radio Emission from Starspots on RSCVn Binary HR1099

AbstractProperties of the microwave emission from HR1099 are examined in an attempt to determine whether the emission arises as gyro-synchrotron radiation from mildly relativistic electrons trapped in magnetic fields above starspots on the active K subgiant component. It is shown that radio curves do not exhibit a systematic variation in phase with the rotation rate, as one might expect for emission from a source situated above a long-lived starspot. However, there is some evidence that the radio flaring occurs at two preferred longitude zones. Whether these zones agree with starspot locations remains to be determined by light curve modelling. What we can say with confidence is that the measured spectral index of the microwave emission does not fit a simple gyro-synchrotron source model, such as that proposed to explain the observed reversal with frequency of the sense of circular polarization.

Modeling the low latitude ionospheric total electron content

The undisturbed ambient total electron content of the ionosphere in the equatorial region exhibits two characteristic features: 1. (i) a longitudinal behavior of the post-sunset variation of the ionization near the crests of the equatorial anomaly 2. (ii) an enhancement at lower latitudes following the post-sunset decay. During high solar activity periods the southern crest of the equatorial anomaly in the African longitude sector is characterized by a post-sunset maximum often exceeding the afternoon maximum. In the Indian and other longitude zones, the post-sunset peak is not so prominent. Instead, a ledge is obtained in the corresponding local time period. At lower magnetic latitudes, the ionization decays very rapidly around sunset, but an enhancement lasting 2–4 h is observed afterwards. Numerical solution of the plasma continuity equation, including the effects of ionization production by solar ultraviolet radiation, loss through charge exchange and transport by diffusion, electrodynamic drift and neutral wind, has been used to investigate the above two features. It is found that the pre-reversal peak of the E × B drift at the magnetic equator around sunset is the dominant mechanism responsible for the post-sunset behavior near the crests of the equatorial anomaly. The zonal wind causes an asymmetry of the total content in the northern and southern hemispheres. In African longitudes, where the magnetic declination is about 20°W, the southern crest is more developed at the expense of the northern counterpart. The north-south asymmetry is practically absent in the Asian sector, with its low (< 5°) declination angle. In the Pacific area, an easterly declination (about 9°E) results in a higher post-sunset ionization at the northern crest, although the asymmetry is less pronounced than that in the African zone. The night-time enhancement at lower latitudes has been found to be controlled by the post-sunset increase in the vertical drift, possibly also modulated by the neutral wind.

Equatorial scintillations: advances since ISEA-6

Since the last equatorial aeronomy meeting in 1980, our understanding of the morphology of equatorial scintillations has advanced greatly due to more intensive observations at the equatorial anomaly locations in the different longitude zones. The unmistakable effect of the sunspot cycle in controlling irregularity belt width and electron concentration responsible for strong scintillation in the GHz range has been demonstrated. The fact that night-time F-region dynamics is an important factor in controlling the magnitude of scintillations has been recognized by interpreting scintillation observations in the light of realistic models of total electron content at various longitudes. A hypothesis based on the alignment of the solar terminator with the geomagnetic flux tubes as an indicator of enhanced scintillation occurrence and another based on the influence of a transequatorial thermospheric neutral wind have been postulated to describe the observed longitudinal variation. A distinct class of equatorial irregularities known as the bottomside sinusoidal (BSS) type has been identified. Unlike equatorial bubbles, these irregularities occur in very large patches, sometimes in excess of several thousand kilometers in the E-W direction and are associated with frequency spread on ionograms. Scintillations caused by such irregularities exist only in the VHF band, exhibit Fresnel oscillations in intensity spectra and are found to give rise to extremely long durations (~ several hours) of uninterrupted scintillations. These irregularities maximize during solstices, so that in the VHF range, scintillation morphology at an equatorial station is determined by considering occurrence characteristics of both bubble type and BSS type irregularities. The temporal structure of scintillations in relation to the in situ measurements of irregularity spatial structure within equatorial bubbles has been critically examined. A two-component irregularity spectrum with a shallow slope ( p 1 ~ 1.5) at long scalelengths (> 1km) and steep slope ( p 2 ~−3) at shorter scalelengths has been found in both vertical and horizontal spectra. Phase and intensity scintillation modelling was found to be consistent with this two-component irregularity spectrum. Finally, the information provided by the major experimental undertaking represented by Project Condor in the fields of night-time scintillations and zonal irregularity drifts with be briefly outlined.

Worldwide features of the strength of recurrent geomagnetic activity

Variation of the strength of recurrent geomagnetic activity, which occurs just before a sunspot minimum, with local time is studied for a network of observatories covering different latitude and longitude zones. For this purpose, hourly averages of horizontal intensity(H) for each UT hour for 173 days, which are totally free of disturbances due to solar transients, have been subjected to spectral analysis. Well-defined spectral peaks associated with periodicities of 28, 14 and 9 days were present in almost all the spectra. The pattern of daily variation of the strength of the 27-day signal changes from a diurnal one at low latitudes to a semi-diurnal one near the Sq focus and in this region, the 14-day signal appears to have an independent origin irrespective of the longitude zone. A study of 27-day oscillation in mean dailyH field also indicates that apart from ring current modulation, both Sq and electrojet fields also undergo 27-day oscillations during the declining phase of a solar cycle possibly through the ionospheric wind system.

Longitudinal structure of the F-region equatorial anomaly—a review

A review has been undertaken of various ground-based and satellite measurements of the equatorial anomaly. The particular methods employed affect the credibility of the results. Transit satellite measurements cannot give the true seasonal variations of the equatorial anomaly diurnal variations. Bottomside sounding is very much better in this respect but in this case there is the difficulty of obtaining sufficient data from stations situated closely in dip location near the magnetic equator and which are also confined to a narrow range of longitude. Considering various longitude zones, such as the American, African and East Asian, an attempt has been made to relate the observed differences of the forms of the equatorial anomaly, throughout the day and from season to season in these zones, with differences of (1) magnetic equator location relative to the geographic, (2) magnetic declination and (3) electric and magnetic fields in the vicinity of the magnetic equator. Attention has also been drawn to the very significant differences in equatorial anomaly development which may also occur over relative small changes in longitude (40°). These may possibly be due to the upward propagation of semi-diurnal tides or planetary waves from the lower atmosphere affecting the magnitude of the eastward electric field over the magnetic equator. It is proposed that further efforts should be made towards more careful comparison measurements of the equatorial anomaly, neutral winds and electric fields at varying longitudes. The provision of a sun synchronized satellite with transit periods of 1300–1500 LT and 0100–0300 LT would seem very desirable for ionization measurements.

Geomagnetic results from Sabhawala, Yangi-Bazar and Alma-Ata in relation to the asthenosphere beneath the Pamir-Himalaya

An attempt is made to study the properties of the Mohorovičić discontinuity and the asthenosphere beneath the Pamir-Himalaya from the geomagnetic results obtained at Sabhawala (Dehra Dun), Yangi-Bazar (Tashkent) and Alma-Ata observatories, situated on its southern and northern flanks, in a narrow longitude zone. Medium-sized induction arrows (Wiese vectors) for night-time “bay” events at all the three stations are found to point northward. The northward orientation at Sabhawala indicates a subsurface concentration of induced currents in a step structure at the Moho to the south of Sabhawala and higher electrical conductivity in the Aravalli-Cambay region, where high heat flow and gravity values have also been reported, supporting an elevated asthenosphere, as against the one beneath the western Himalaya. The arrows at Yangi-Bazar and Alma-Ata also show an east-west current channelling in another step structure at the Moho to the south of these stations and north of the Pamir and the Tien Shan mountains. The Sq variations of the H, D and Z components at the three stations, lying close to the northern Sq ionospherics current focus, reveal the characteristic features, without reflecting any unusual asthenospheric conditions in the region. It is concluded that the orogenesis of the Pamir-Himalaya mountain system in terms of plate tectonics or block uplift is different from that of the Cordilleran and Andean mountains in North and South America respectively, where subsurface conductive structures have been delineated from geomagnetic observations by earlier workers.

Ion drifts observed at Malvern: evidence of ion drag and substorm induced electric fields

Day-time ion drift observations, made at Malvern (52.1°N, 2.3°W) using an incoherent scatter radar in quadristatic configuration during the period October 1972 to October 1974 are analysed. Out of the seventeen days' observations considered here nine are magnetically quiet days and eight are magnetically disturbed days. A significant negative correlation was observed between v ∥ (field aligned ion drift) and v TN (crossfield drift in meridional plane) on all the days, indicating that ion drag is important even on magnetically disturbed days. Drifts along the field line v ∥ ' ( v ∥ after removing the ion drag component) showed less day-to-day variability as compared to v ⊥ N and v ⊥ E where v ⊥ E is the crossfield drift in the zonal plane. The value of v ∥ 'is ~20 ms −1 and it is found to be mostly downwards. Crossfield drifts ( v ⊥ N , v ⊥ E ) showed larger day-to-day variability indicating the same variability for their driving electric fields. Values of v ⊥ E are found to be larger than those of v ⊥ N indicating stronger north-south electric fields. The large day-to-day variability in crossfield drifts suggest that even on quiet days there is a significant contribution from electric fields of magnetospheric origin. A major substorm event on 18 September 1974 is accompanied by a substantial reduction in eastward drift ( v ⊥ e ) though change in v ⊥ N is comparatively smaller. Similar changes are noticed for other substorm events occurring in after noon hours in European longitude zone. These results confirm that substorm electric fields penetrate down to the latitude of Malvern.

Association of equatorial counter-electrojets with geomagnetic changes in low and middle latitudes in the Indian region.

Choosing quiet days in 1964 wnen counter-electrojets occurred at the equatorial station Trivandrum in India, the H, X and Y variations were examined at northern locations in the same longitude zone. It was noticed that, in summer (May, June, July, August) nothing abnormal happened at the low and middle latitudes. But in winter and equinoxes, strong equatorial counter-electrojets were associated with larger amplitudes of H, X and Y at low and middle latitudes. Thus, equatorial counter-electrojets may be related to intensifications of the winter hemisphere Sq current systems. A preliminary analysis for Huancayo at equator and Fuquene at a low latitude in the American zone did not reveal any similar consistent relationship. This needs furthur investigation.