North-south Symmetry Research Articles

Circulation around a thin zonal island

Laboratory and numerical experiments are used to study flow of a uniform-density fluid on the β-plane around a thin zonally elongated island (or ridge segment in the abyss). This orientation is chosen specifically to highlight the roles of the zonal boundary layer dynamics in controlling the circulation around the island. There are examples of deep ocean topography that fall into this category which make the work directly applicable to oceanic flows. Linear theory for the transport around the island and the flow structure is based on a modification of the Island Rule (Pedlosky et al. 1997; Pratt & Pedlosky 1999). The linear solution gives a north–south symmetric flow around the island with novel features, including stagnation points which divide the zonal boundary layers into eastward and westward flowing zones, and a western boundary layer of vanishing length, and zonal jets. Laboratory experiments agree with the linear theory for small degrees of nonlinearity, as measured by the ratio of the inertial to Munk boundary layer scales. With increasing nonlinearity the north–south symmetry is broken. The southern stagnation point (for anticyclonic forcing) moves to the eastern tip of the island. The flow rounding the eastern tip from the northern side of the island now separates from the island. Time-dependence emerges and recirculation cells develop on the northern side of the island. Mean transport around the island is relatively unaffected by nonlinearity and given to within 20% by the modified Island Rule. Numerical solutions of the shallow water equations are in close agreement with the laboratory results. The transition from zonal to meridional island orientation occurs for island inclinations from zonal greater than about 20°.

The Dependence of Chromospheric Ca II K Network Cell Sizes on Solar Latitude

Calcium K line spectroheliograms obtained during the solar minimum phases at Kodaikanal between 1913–1974 have been used to study the network cell sizes. The autocorrelation is calculated for two-dimensional strips at 5° interval up to ±50° latitude. The average size of the network cells was found to have a dependence on solar latitude with a maximum variation of about 7%. The pattern shows an apparent north–south symmetry with two minima at about 20° N and S.

Optimum slope of a north-south aligned absorber plate from the north to the south poles

The analytical method developed by Nijegorodov et al. [ Renewable Energy, 4, 529–543 (1994)] in order to find the optimum slope of an absorber plate, variously oriented at any latitude, is applied to study the variation of daily optimum slope with latitude from the north to the south poles. For a clear, cloudless condition the daily optimum slopes are found to have symmetry features which reflect the symmetry of the motion of the sun and the north-south symmetry about the equator. Such symmetry features of the optimum slope are discussed. However, the actual optimum slope also depends on the local meteorological and sunshine conditions, due to which the symmetry features of optimum slope are not always exploitable. The effects of meteorological parameters such as temperature, pressure, relative humidity, visibility and cloud cover on the optimum slope are assessed. The method is applied to obtain the mean monthly optimum slopes for the locations of 12 synoptic stations in Botswana from which the climatic and sunshine data are available, and preliminary maps of mean monthly optimum slopes for Botswana for the months of June and December are presented.

Long-term behavior of latitudinal cosmic-ray gradients

We have used neutron monitor data covering a wide range of energy over a period of 22 years (1966–1987), as well as sea-level multidirectional meson telescope data from Nagoya to examine the latitude effect of solar diurnal vectors and its dependence on the polarity of interplanetary magnetic field (IMF). By sorting the daily cosmic-ray data according to whether the IMF is toward (T) or away (A) from the Sun, the annual mean solar diurnal variations (amplitude and phase) for the T and A days were determined separately. Results showed a northward-pointing latitudinal gradient from neutron monitors of the most northerly latitudes, and a predominant southward gradient at high southerly latitudes. The resultant latitudinal cosmic-ray gradients are the sum of two gradients: a north-south symmetry gradient (occurring in minimum and maximum solar activity years), and a north-south asymmetry gradient (occurring during different phases of solar activity cycles). The difference vector (T - A) between the solar diurnal vector for two groups was calculated, which represents a good indicator for the resultant perpendicular gradient relative to the Earth. This difference vector shows a considerable change in phase for detectors located in the northern hemisphere of the Earth. On the other hand, there exists much less change in phase for detectors located in the southern hemisphere.

Radio studies of the southern hemisphere high-latitude ionosphere

Radio studies are crucial for the study of the high-latitude ionosphere and this paper is a brief review of the use of radio probing techniques being used in Antarctica. It is important to study the Antarctic ionosphere in order to understand conjugate effects and the result of differences in north-south symmetry such as the greater offset between the geographic and geomagnetic poles which occurs in the southern hemisphere. The main radio techniques being used in Antarctica are HF auroral radars, digital ionosondes and satellite beacon observations. Current developments of the SHARE HF radar system, installation of additional digital ionosondes, and the installation of GPS receivers which can provide TEC data, will all provide significant advances in our ability to study the Antarctic ionosphere. Details of the structure and convection of the ionosphere, and its response to magnetospheric phenomena, will all be studied in much more detail than before. Undoubtedly collaborative studies with conjugate northern hemisphere systems will have the highest priority but collaborative studies using the different Antarctic instruments will reveal new insights into phenomena such as polar arcs, patches and blobs. Advances in the use of digital ionosondes to study the ionosphere will also be important at lower latitudes. As an example, observations of storm effects at sub-auroral latitudes, obtained using a digital ionosonde operating as an oblique HF radar, are presented.

Electromagnetic interaction of the solar wind generator with the magnetosphere carried by field-aligned currents

Solar wind plasma flow creates convective motion in the geomagnetic tail. We can consider this motion as a generator of the large-scale Region 2 field-aligned currents. Region 1 currents are generated by the solar wind plasma. So in the equivalent electric circuit which is described using the assumption of finite field-aligned conductivity, the two generators (one localized near the magnetopause and the other near the inner edge of the current sheet) exist. Such an approach allows us to estimate field-aligned currents connecting the two hemispheres. These currents arise when the north-south symmetry of the magnetic or electric fields is violated. The variation of the equivalent circuit parameters allows us to describe the dynamic processes in the magnetosphere.

Precipitating and trapped ions and electrons observed at 840 km during the Great Magnetic Storm of February 1986

A detailed picture is presented of the equatorward boundaries of the auroral ovals at dawn, morning, dusk, and evening for the three most disturbed days of February 1986. North–south symmetry for the boundaries of keV particles was good, and the differences between the ion and electron boundaries agreed with statistics which show the ion edge slightly equatorward of the electron edge at dusk, with the reverse for dawn. The electron boundary was most equatorward of the ion boundary for morning. Best symmetry and least difference were for evening, the sector nearest the central plasma sheet. Ions with energies from thermal to several hundred electron volts penetrated inward to L = 1.2. Initial penetration was confined mainly, if not exclusively, to the dawn sector. The sudden appearance of low‐energy ions deep in the plasmasphere at dusk and evening after storm maximum suggests corotation from a plasmapause as low as L = 1.7 at dawn. Low‐energy electrons rarely advanced equatorward of the keV electrons. On the other hand, keV electrons occasionally were detected about the equator, apparently in relation to the inward convection of the radiation belt. Very energetic, MeV, particles occurred near 52° MLAT, mainly after storm maximum and often unsymmetrically, which may reflect a characteristic akin to the South Atlantic Anomaly.

Large‐scale characteristics of magnetospheric equatorial currents

We have analyzed the vector magnetic field data acquired with the AMPTE CCE satellite for prolonged periods of disturbed conditions (2 ≤ Kp ≤ 6, and −70 ≤ Dst ≤ −20 nT). A total of 300 orbits for 180 days during 1985 and 1986 were subjected to the present analysis. We have determined the characteristics of the intensities and flow directions of currents flowing parallel to and near the geomagnetic equatorial plane in the region between L = 4.0 and 8.8 RE. This analysis was done in a cylindrical magnetic coordinate system and by assuming north–south symmetry with respect to the geomagnetic equatorial plane. The characteristics determined from this study include the following: (1) The azimuthal currents flow westward and exhibit pronounced midnight‐noon asymmetries, with the nightside intensities greater than dayside intensities by a factor of 2 to 3. The enhanced nightside current intensities have localized relative minima between 2200 and 0200 MLT and L = 5.6 and 8.8 RE, resulting from the integrated effect of a number of substorm‐associated geomagnetic dipolarizations. (2) The inferred equatorial radial currents flow toward the Earth in the evening sector and away from the Earth in the morning sector throughout the region between L = 4.0 and 8.8 RE. (3) A net positive divergence (source) of equatorial current in the evening sector and a net negative divergence (sink) in the morning sector is inferred from the distribution of azimuthal and radial currents. The location, flow direction, and intensities of these divergent currents are almost identical with those of the region 2 Birkeland system. We conclude that the equatorial currents, primarily the westward ring current, and the region 2 Birkeland current system are closely connected.

Day-to-day variation of equivalent Sq current system during March 11-26, 1970.

The instantaneous equivalent current system of Sq is obtained at every 2 hours from March 11 to 26, 1970 under the assumption of north-south symmetry. Trends in H- and Z-components due to UT dependent disturbance fields are corrected by using the Dst index. It is found that UT variation in the intensity and location of external current vortex is similar to that of September, 1964, though the intensity itself is about twice. Day-to-day variation of Sq is examined by subtracting the UT mean values. The residual patterns are quite different day by day, but on some days they resemble currents generated by semi-diurnal tidal winds, and on other days they seem to be caused by the field-aligned currents in high latutudes. The latter are shown to be correlated with the AE index.

On Symmetric Stability and Instability of Zonal Mean Flows Near the Equator

Abstract Obserations of longitudinally-averaged zonal flows in the atmosphere and ocean tend to display north–south symmetry about the equator, with a characteristic wind maximum or minimum, and therefore little horizontal wind shear locally near the equator. It is shown that this configuration is required for balanced flow on a sphere to be inertially stable. If dissipation can be neglected, any horizontal wind shear at the equator will cause inertial instability to develop. effectively eliminating the horizontal shear. It follows that the potential vorticity (q) must vanish at the equator for the symmetric circulation to be stable; and that it must increase to the north and decrease to the south. Balanced cross-equatorial flow can occur only if there is a north–south gradient in the torque or the diabatic heating at the equator. These conclusions are obtained under the assumption of a balanced zonal flow; i.e., acceleration and dissipation are explicitly neglected in the meridional momentum equation. Th...

Interpretation of the apparent north-south asymmetry and fluctuations of galactic rotation

A model of the apparent north-south asymmetry of the rotation curve of the Galaxy and of the asymmetry appearing for the rotation of its outskirts has been presented, in terms of interaction redshifts. Fluctuations in the rotation curve for neutral hydrogen are discussed and interpreted as expanding motions in the arms. The expansion of the disk of the Galaxy which has been suggested previously as an explanation for the asymmetry has been shown to meet some serious difficulties. A remarkable north-south symmetry both in the spatial distribution and in the kinematics of neutral hydrogen in the subsolar region follows from the present model.

Investigations of gram quantities of Atlantic and Pacific surface particulates

Particulates amounting to 0.1–2.0 g efficiently collected from large volumes of Atlantic and Pacific surface waters have been analyzed for carbonate, opal, quartz and several natural and man-made radioisotopes. The concentrations of particles range between 10 and 600 μg/kg. In the equatorial regions particle concentrations are low and similar in both the oceans. At higher latitudes (>30°N or S), the Atlantic waters, however, have higher concentrations of particles compared to those in the Pacific. The latitudinal distribution exhibits a north-south symmetry with higher concentrations in the 30°–60° belt. Based on the particulate abundance for CaCO 3 and opal and their sedimentation, we have estimated their production and in-situ integrated dissolution rates for a few regions. Radioisotopes having different source functions, namely 14C and 239Pu injected due to nuclear weapon tests, 234Th, 230Th and 228Th produced in-situ in seawater, 232Th which derives primarily from land, 210Pb introduced via wet precipitations and 226Ra introduced through diffusion from deep-sea sediments have been measured in the particulates. The relative enrichment factors for these nuclides in particles vary as Th ⋍ Pu > Pb > Ra. The atmospheric bomb fallout pattern is discernible in the surface particulates; the 239Pu concentration increases with latitude in both the hemispheres; however, the values are about a factor of two lower in the southern hemisphere. The distribution pattern of radioisotopes is found to be complex, even for 234Th whose source function in the oceans is uniform. In view of the differences in the source functions it becomes possible to delineate the principal geochemical/geophysical processes which determine the concentrations of these nuclides in surface waters.