Longitude Zone Research Articles

True height calculation from ionograms

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 (p1 ~ 1.5) at long scalelengths (> 1km) and steep slope (p2 ~−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.

A relationship between polar magnetic substorms, ionospheric height rises and the occurrence of spread- F

Statistical analyses performed for ionosonde stations (at mid- and low-latitudes) in a longitude zone near the 75°W (geographic) meridian, suggest that increases in the ionospheric parameter h′ F are related to polar-magnetic-substorm onset times as recorded by a magnetogram station in the same longitude zone as the ionosonde stations. The height increases are recorded for several hours after the substorm onset times, and seem to trigger the onset of ionospheric spread- F. One individual event which was analyzed, showed a variation with latitude of the time of onset of the height increase suggesting the propagation towards the equator of an ionospheric disturbance with a speed of about 740 m s −1. On this particular occasion the onset of spread- F followed the height-rise onset at most of the stations used in the analysis. These effects were clearly discernable at the geomagnetic equator.

The signature of H+ in the equatorial anomaly: An empirical model

The task of developing empirical models of the thermal ion composition is complicated by the presence of both short‐term temporal and localized spatial variations in the constituents. The requirements of a detailed numerical representation of variable composition patterns such as those detected in the zone of the equatorial anomaly (EA) may exceed either the statistical data available or the numerical modeling resolution, or both. As an illustrative example a correlative test of a parameterized spherical harmonic expansion model of Ogo 6 dayside measurements of n(H+) is performed, particular characteristics of the observations being used as a basis for comparison. Whereas a review of other research confirms that the Ogo 6 observations of EA variability in longitude and local time are generally consistent with independent experimental and theoretical results, the model reproduces the observed broad‐scale longitudinal behavior but falls short of representing some details of these features. Specifically, the model depicts the persistent prominence of the EA development in the eastern Asian longitude zone relative to the eastern American longitude zone, as Well as the longitudinal shift in the hemispheric asymmetry in EA crests, although the amplitudes of some observed features are not precisely modeled. These results emphasize (1) the limitation in statistical coverage from a single satellite for identifying complex localized and short‐term ionospheric variations and (2) the need for integrating dynamic parameters, e.g., neutral wind and electric field‐ion drift terms, to produce a hybrid empirical‐analytical model.

Low latitude features of annual and semiannual periodicities in NmF2 in American and Indian zones

The monthly median NmF2 values during the high sunspot period 1958 from a number of low latitude stations in the American zone are subjected to harmonic analysis. The results are presented as a function of local time and are compared with those obtained earlier from the Indian zone. The gross features in the two zones are found to be identical. The semiannual periodicity is shown to have maxima in April and October at all stations for all hours of the day and this is attributed to similar changes in thermospheric atomic oxygen concentration controlling the production of ionization. The annual maximum exhibits systematic changes in the amplitude and phase as a function of latitude and local time. One of the main features is a north-south asymmetry in the amplitude of the annual variation which can also be attributed to larger concentration of atomic oxygen in January than that in July. Further, a geomagnetic control is seen to exist on the annual variation at both the eastern and western longitude zones. This is explainable in terms of plasma exchange between magnetically conjugate latitudes.

Low latitude features of annual and semiannual periodicities in NmF2 in American and Indian zones

The monthly median NmF2 values during the high sunspot period 1958 from a number of low latitude stations in the American zone are subjected to harmonic analysis. The results are presented as a function of local time and are compared with those obtained earlier from the Indian zone. The gross features in the two zones are found to be identical. The semiannual periodicity is shown to have maxima in April and October at all stations for all hours of the day and this is attributed to similar changes in thermospheric atomic oxygen concentration controlling the production of ionization. The annual maximum exhibits systematic changes in the amplitude and phase as a function of latitude and local time. One of the main features is a north-south asymmetry in the amplitude of the annual variation which can also be attributed to larger concentration of atomic oxygen in January than that in July. Further, a geomagnetic control is seen to exist on the annual variation at both the eastern and western longitude zones. This is explainable in terms of plasma exchange between magnetically conjugate latitudes.

Traveling planetary-scale waves in the E-region

An analysis of E-region minimum virtual height data obtained from a network of ionosonde stations in the northern hemisphere at mid-latitudes shows the presence of long period (10–15 days) planetary-scale fluctuations for the six-month period, October 1970–March 1971. A lag correlation analysis of the data between stations reveals a slow westward drift of these waves moving on the average 17° of longitude per day during the first three months of this period. The longitudinal fluctuations of the higher latitude data are shown to be consistent with a wave one type of variation. From the peak intensity of the apparent wave one pattern on 1 January 1971, and from the cessation of movement of these waves after December, it is concluded that the 1970/71 major mid-winter stratospheric/mesospheric disturbance reached to at least the lower E-region and preceded the peak of the stratospheric warming at 30 mb by about 5 days. The planetary-scale fluctuations observed in the virtual height data are also shown to be well correlated with harmonic one of a zonal Fourier analysis of lower stratospheric radiance data from the SIRS instrument on the Nimbus IV weather satellite. The peak positive correlations with small lag are generally found to occur when the longitude of the radiance data is within approximately 45° of the higher latitude ionosonde stations. The ionospheric data from most of the lower latitude stations are found to be negatively correlated with the radiance data interpolated to within 45° of their longitude zone. These observed correlations are interpreted in terms of planetary-scale temperature changes in the lower E-region.

Comments on the course of solar activity during the declining phase of solar cycle 20 (1970?74)

In the declining phase of solar cycle 20 (1970–74) three pulses of activity occurred and resulted in two well defined ‘stillstands’ in the smoothed means of sunspot, 2800 MHz, and calcium plage data. Marked diminutions in spot and 2800 MHz flux took place in 1970 and 1971, respectively, and were accompanied by concomitant decreases in flare-occurrence. Studies of the latitude distribution of spots and flares show the extent of the dominance of the northern hemisphere in cycle 20 and the marked phase shift between northern and southern hemispheres. In the years studied, the longitudes of centers of activity clustered in identifiable zones or hemispheres for relatively long intervals of time. From mid-1973 to mid-1974 the Sun had a relatively inactive hemisphere centered on ∼0° longitude. The relationship of certain well defined ‘coronal holes’ to this inactive hemisphere of the chromosphere is noted. The first two spot groups of the new cycle formed in November 1974 and January 1975 in the longitude zone associated with relatively high levels of old cycle activity, a repetition of the pattern observed in 1963–64.

The electrostatic field in the ionosphere

Theoretical calculations of the electrostatic field in the ionosphere are presented for different seasons and longitude zones. The corresponding current systems have been shown earlier to give good agreement with the geomagnetic Sq variations. The question of whether the electrostatic field is generated by winds in the ‘dynamo region’ or by other processes higher in the magnetosphere is evaluated in the light of recent observations. Several details of the electrostatic field variation, such as an increase near sunset, are noted.

On the Nature of the 41-Day Cycle of RR Lyrae

AbstractPhotoelectric observations obtained with the 24-in. telescope of the Konkoly Observatory from 1950 to 1972 show that the amplitudes of the changes of the 0.d57 light-curve in the course of the 41-day cycle undergo considerable cyclic variations. The length of the cycles varies between 3.8 and 4.8 yr. A new cycle always begins with a large phase-shift in the 41-day period; afterwards there is generally no further shift during one 4-yr cycle. Following Julia Balázs’ hypothesis, a tentative model is suggested: The 41-day cycle is the rotation period of the star. Magnetic fields concentrated in a limited longitude zone influence the 0.d57 pulsation in such a way that variations in light amplitude and phase arise as aspect effects of the 41-day rotation period. In this way, the 4-yr cycle can be interpreted as the magnetic cycle of the star. The location of the magnetic fields strongly differs in longitude from cycle to cycle causing the observed large phase shifts at the beginning of a new 4-yr cycle.

Quasi-biennial oscillation in the geomagnetic Sq field in the low latitude region

Power spectral studies of the geomagnetic Sq field under the electro jet (Kodaikanal) and outside it (Alibag, Honolulu, Kakioka and San Juan) indicate the existence of the 26month oscillation at Kodaikanal, but not at other stations. Determination of the quasi-biennial wave (QBW) by harmonic analysis after cutting out all other oscillations by using a numerical band pass filter, shows that the QBW has a larger amplitude under the equatorial electrojet than outside it, suggesting an association between the electrojet and the QBW. The change in the period of the QBW over Alibag from 19 months (May 1954–November 1955) to 23 months (November 1955–September 1967) and over Kakioka from 24 months (July 1961–June 1963) to 22 months (June 1962–March 1964) implies that the characteristics of the QBW are changing from one cycle to the next. The period of QBW changes from one longitude zone to another. The advance of phase in the order Alibag, San Juan, Honolulu and Kakioka, suggests a westward progression of the QBW. The amplitude of the wave is highest under the equatorial electrojet, decreasing with increase of distance from the magnetic equator up to 21° geomagnetic latitude. The amplitude is surprisingly large at San Juan, although it is close to the Sq focus.

North-South asymmetry of ionospheric storms—dependence on longitude and season

The study of ionospheric storms at pairs of north and south mid-latitude stations in different longitude zones, individually for each season, has shown asymmetry in their behaviour, most significantly in the Pacific zone, to some extent in the Asian and American zones. The decrease of ƒ 0F2 which is found to occur during geomagnetic storms in Summer months is conspicuously absent at Rarotonga. This suggests that ionospheric storm variations are greatly affected by horizontal and vertical movements of ionisation, which are controlled by the asymmetric magnetic field of the Earth.

Solar activity and recurrences in magnetic-field distribution

A study of the Mount Wilson magnetic-field synoptic chart material divided into latitude zones for the interval 1959–67, and a comparison of the data with sunspot groups have provided a better understanding of the structure of the background-field pattern and its relation to activity. The interaction of old and new fields within the pattern seems to result in long-lived sections of alternating polarity in both hemispheres. We postulate subsurface sources with rotation periods of about 27 days which produce active regions over a longitude zone of some tens of degrees. There is a tendency for the background-field features with strong fields to resist to some extent the shearing effects of differential rotation. A prediction is made concerning the nature of the interplanetary magnetic field above the ecliptic.

North-south asymmetry in storm time variation of f0F2

The study of Dst ( f 0 F2) variations at equatorial and subtropical stations at different longitude zones for 1951–1956, show that at equatorial stations of any in the zones the variation indicates an increase of f 0 F2 during the main phase. The northern subtropical stations in any of the zones indicate a decrease of f 0 F2 during the main phase. The southern subtropical stations show decreased f 0 F2 for Asian and African zones but at American and Pacific zones an increase of f 0 F2 during main phase of the storm is noted.

Sq current effects on the seasonal variations in the E-region of the ionosphere

The index n that describes the seasonal variation of the E-region critical frequency has been obtained for a large number of I.G.Y. stations. Charts are given that show the variation of this index with latitude and local tune, for three different longitude zones. The contours of constant n exhibit asymmetries in latitude, time and magnitude. These asymmetries correlate closely with corresponding asymmetries in the S q current system.

Long term variation in equatorial sporadic- E and the electrojet

It has been established by earlier workers that a narrow bond of strong current system called the electrojet existing at E-region heights near the equator is responsible for the equatorial sporadic- E. Since the electrojet shows a remarkable correlation with sunspot activity, it is to be expected that equatorial E s should also show a similar variation. A systematic study of the year to year variation in the critical frequencies of sporadic- E for the three equatorial stations belonging to three different longitude zones has been made in this investigation. It is observed that for some stations the maximum occurs about one year before sunspot maximum whereas for others it occurs 1 year after the sunspot maximum. An attempt has been made to explain these anomalies and some other peculiarities noted in this investigation.

Seasonal Variations in the Lower and Upper Atmosphere

ALTHOUGH the main temporal variations in the ionosphere show a very large measure of solar control, it has long been recognized that some ionospheric phenomena have features which strongly suggest a non-solar origin or control. One such example is the ‘winter anomaly’ in ionospheric absorption. Thus a recent survey1 snows that this phenomenon seems to have a definite regional character in that although on any one day it may extend over areas of at least 106 km2 its enhancement in one longitude zone appeared to correspond to a diminution in its intensity in another longitude zone. Recently there has been clear evidence that at least part of this ‘winter anomaly’ is to be associated with abnormal warmings in the stratosphere at levels of about 30 km. Convincing evidence for such a relationship has been found in some recent work by Shapley and Beynon at Aberystwyth (preceding communication). As part of a broader consideration of this problem we have examined the seasonal variation in various parameters at different levels in the atmosphere and these are briefly reported below.

‘Winter Anomaly’ in Ionospheric Absorption and Stratospheric Warmings

IT has long been known that in middle latitudes the ionospheric absorption of radio waves shows a distinct ‘winter anomaly’. This takes the form of a very marked day-to-day variability coupled, on many days, with an excessively large absorption to the extent that on days of ‘winter anomaly’ the absorption is often considerably larger than that found on a normal summer day. It has been established that this abnormal winter absorption is due to certain changes in the lower ionosphere—in the D and lower E region—but the precise cause of this phenomenon has not yet been identified. Investigations of the winter anomaly have established certain facts, such as that it is only weakly correlated with solar or magnetic activity and that it occurs more or less simultaneously over large areas. At the same time there appears to be a peculiar inverse relationship between its incidence in different longitudes in that a period of marked winter anomaly in one longitude zone seems to be associated with an absence of the phenomenon in another longitude sector. In a recent paper, Bossolasco and Elena1 have produced evidence for a correlation between the winter anomaly in ionospheric absorption and temperature in the upper stratosphere.

The Columbus geoid

The author gives a summary of the preparatory work, collection of the needed gravity material, reduction method used, the gravity anomaly maps of different types needed, and the details of the method of computation of the geoid. Using the available materials, 6679 squares of 1° X 1°, four times larger than was available in 1928, several new gravity formulas have been derived. The globe was divided into 36 longitude zones of 10°. A diagram and table give an idea of the corrections to the International Gravity Formula obtained from different zones. When all gravity observations made in the oceans are available, the equatorial value of gravity may get a small negative correction.The geoid in Europe, where Tanni already had rather good material, will remain nearly unchanged, but the geoid in America, in the Atlantic, and in India changes materially as a result of additional gravity observations, particularly at sea. Sketches of portions of the geoid are given. The astro‐geodetically computed geoid of Europe, by Bomford, is compared with the Columbus geoid.