Asian Sector Research Articles

The latitudinal variation of the electron concentration in the topside ionosphere in winter

Data obtained in the American sector by means of Alouette I during the winter of 1963–1964 are used to investigate the latitudinal variation of the electron concentration at the F2-layer peak and at 1000 km. By day, the values of ƒ 0F2 appear to depend on geographic latitude rather than on magnetic dip, but at night the Earth's magnetic field controls the latitudinal variation of critical frequency. The daytime ƒ 0F2 values are enhanced at latitudes where the magnetic dip is 60°; this enhancement, which has been reported by previous authors, does not exist at 1000 km. The daytime high-latitude electron concentration values for 1000 km are extremely variable; particularly large values are observed during magnetically disturbed conditions. Ground-based ionosonde data have been used to show that the main high-latitude trough occurs at 57° invariant latitude in both the American and Asian sectors.

A theoretical study of the ionospheric F region equatorial anomaly—II. results in the American and Asian sectors

When observed noontime values of the maximum electron density, NMAX( F2), in the ionospheric F2 region are plotted as a function of magnetic latitude, a curve is produced which has two peaks, one on either side of the dip equator at ±16° dip latitude. This paper theoretically investigates the daily variation of this latitudinal distribution in NMAX( F2) (the so-called Appleton or equatorial anomaly) and specifically attempts to account for the longitudinal differences observed between the American and Asian sectors. In Part II, models of the neutral atmosphere, production, loss and diffusion rates, neutral wind, and electric field are described and the electron densities obtained by solving the continuity equation utilizing these models are presented. In each sector, the extent to which the equatorial anomaly's daily variation is affected by changes in the geomagnetic field configuration, neutral wind, and E × B drift is examined. It is found that development of the anomaly is most sensitive to the electric field model assumed, and that the observed differences at the magnetic equator between the American and Asian sectors could be accounted for by an upward E × B drift which commences an hour or two earlier in the Asian sector.

A theoretical study of the ionospheric F region equatorial anomaly—I. Theory

When observed noontime values of the maximum electron density, NMAX( F2), in the ionospheric F2 region are plotted as a function of magnetic latitude, a curve is produced which has two peaks, one on either side of the dip equator at ± 16° dip latitude. This paper theoretically investigates the daily variation of this latitudinal distribution of NMAX( F2) (the so-called Appleton or equatorial anomaly) and specifically attempts to account for the longitudinal differences observed between the American and Asian sectors. Part I outlines the theory involved in solving the time-dependent plasma continuity equation in which production, loss, and transport of ionization are taken into account, where the effects of neutral wind, ambipolar diffusion and E × B drift are included in the transport term. By describing the geomagnetic field in two equivalent ways, B = − ▽γ and B = ▽α × ▽β , where α, β and γ are known magnetic scalar potentials, the spherical r, θ and φ space coordinates of the continuity equation are transformed to coordinates which define directions parallel and perpendicular to the magnetic field thus putting the equation in a form suitable for numerical integration.

LARGE-SCALE FEATURES OF MONTHLY MEAN NORTHERN HEMISPHERE ANOMALY MAPS OF SEA-LEVEL PRESSURE

Spatial patterns of circulation variability over the Northern Hemisphere and their changes during the past 70 yr (1899–1969) are examined using eigenvector analyses of mean January and July sea-level pressure maps. The first several eigenvectors display variability associated with the major centers of action (the subpolar oceanic Lows, the subtropical oceanic Highs, the winter Siberian High, and the summer Asiatic Low). The pattern of the first eigenvector of January suggests that the intensity and latitudinal position of the major circulation features over the North Atlantic are associated with the intensity and position of the Aleutian Low over the North Pacific. The time series of the coefficients of the hemispheric eigenvectors are used to identify intervals of change in the hemispheric circulation associated with features on the scale of thc major centers of action. These time series provide a more general description of circulation change than that obtained from local or regional indices; but at the same time, they provide more detailed information than time series of hemispherically averaged indices. Two intervals of change in the large-scale hemispheric circulation are identified: the early to mid-1920s and the early to mid-1950s. Of the three periods separated by these two intervals of change, maximum contrast is noted between the first and third. For January, the strongest circulation features are found in the North Atlantic and European sectors in the first (earliest) period and in the North Pacific and Asian sectors in the third (latest) period.

A review of topside sounder studies of the equatorial ionosphere

The structure of the equatorial ionosphere as derived from topside sounder data is reviewed. Shortly after sunrise a single ionization crest occurs above the magnetic equator at all heights, but later during the day an equatorial anomaly develops in the form of an arch along which the ionization is enhanced and which lies along different magnetic field lines at different local times. The arch intersects the F2-layer peak at the latitudes where the well-known equatorial anomaly crests occur. The anomaly develops after about 0800 LMT in the Asian sector, but there is some evidence which suggests that the anomaly may start to develop several hours later in the American sector. The anomaly starts to decay after about 1500 LMT in the Asian sector, but only after about 2000 LMT in the American sector. The behavior during this decay phase, however, has not yet been fully established. During the night hours marked spread-F echoes are observed on the low-latitude topside sounder ionograms.

The F2-region equatorial anomaly during solstice periods at sunspot maximum

The diurnal development of the F2-region equatorial anomaly is studied for the African, American, and East Asian sectors during quiet days about the June 1958 and December 1958 solstices. It is found that marked asymmetries in the daytime anomaly can occur with the ƒ 0F2 crest on the winter side being larger than on the summer side, and h m F2 at the winter crest being about 100 km lower than at the summer crest. Such asymmetries are observed only in the American sector during the June solstice period, and in the African and East Asian sectors during the December solstice period. At these times the sub-solar point is far removed from the magnetic equator. The daytime anomaly in ƒ 0F2 shows little or no asymmetry in the American sector during the December period or the African and East Asian sectors during the June period, but in each case h m F2 at the winter crest is higher than at the summer crest. Furthermore, during the subsequent evening hours in these sectors a marked enhancement of the winter crest occurs and h m F2 at the winter crest becomes lower than that at the summer crest. These results are briefly discussed in terms of ionization transport by neutral winds, differences in electron temperature, and other phenomena believed to contribute to the observed asymmetries.

On the location of the effective equator for theFregion of the ionosphere

A method capable of precisely defining the position of the effective equator is outlined. The method is applied to the analysis of the noon values of ƒoF2 and post sunset h′F peak during the equinoctial months, March and September 1958. It is found that in the latitude zone of 40° to 70° only the dipole latitude has significant control on ƒoF2, whereas in the equatorial zone only the dip equator controls ƒoF2 significantly. The data analyzed showed minimum ƒoF2 at the dip equator in the African and American sectors; lack of southern stations in the Asian sector makes a definite statement about this region difficult. The analysis of the postsunset h′F peak reveals a dual control of equal weight by the dip equator and the dipole equator. The effective equator for this parameter and anomaly is therefore halfway between the dipole and the dip equators and is termed the mean magnetic equator. The h′F data analyzed showed a maximum at the mean magnetic equator in all sectors of the earth.

The F2-region equatorial anomaly in the African, American and East Asian sectors during sunspot maximum

A study has been made of the characteristics of the F2-region equatorial anomaly in the African, American and East Asian sectors during quiet days of the 1958 equinoxes, particular attention being given to its diurnal development. It is shown that the evening enhancement of the crests north and south of the magnetic equator occurs first on the side near to the geographic equator. An examination of the latitude variation of hmF2, derived from M(3000) F2 data, for the evening hours shows a marked asymmetry about the magnetic equator, the greater heights also occurring on the side near to the geographic equator. It is suggested that this sequence in the development of the crests, together with the height asymmetry, supports the hypothesis that they result from diffusion of ionization along magnetic lines of force from above the magnetic equator.