Interplanetary sector polarity effects are examined, over a 29-year period, in the annual variation of occurrence frequency of days from quietest to extremely disturbed conditions. Sector polarity dependent and independent components of frequencies are derived as a function of season. Observed annual variation in the occurrence frequencies, separately for A and T polarities, for most of the groups, are in fair agreement with the Russell-McPherron model. However, evidence has been found for two separate mechanisms responsible for the semiannual variation in geomagnetic activity as suggested independently by Svalgaard and Berthelier. In most of the groups, the polarity-independent variation conforms to the findings of McIntosh. Likewise the component dependent on the polarity also conforms, for most of the groups, to Russell-McPherron model. Activity ranges which are exceptions to these are identified and discussed. From the annual variation of polarity-dependent component for the group of extremely quiet days, which is consistent with the R-M model, it is shown that the index A_p of activity is sensitive even at extremely low levels of disturbance.

View full abstract

A plasma computer experiment by the clouds-in-cells method is presented for the study of the evolutionary process of the reconnection of magnetic field lines, where initially there is a plane current sheet, i. e. antiparallel magnetic field lines. A plasma flow from outside toward the magnetic neutral sheet will cause the reconnection. The model is two-dimensional and positive and negative charges have the same mass. It is shown that rapid reconnection can easily take place, though the solution is obtained only for the vicinity of the resulting magnetic neutral point. The diffusion rate of the magnetic field at the neutral point is self-adjustable to a given reconnection rate, so that the reconnection rate can take any value up to the local Alfvén velocity immediately outside the field reversal region.

View full abstract

Using data from 15 observatories at a wide distribution of latitude locations in North and South America the spectra of geomagnetic field variations having periods from 5min to 4hr were examined for six days of 1965 when the Ap index values were 3 ol less. At high latitudes, where the maximum amplitudes occurred more poleward on low Ap days than on high Ap days, the ratio of the short to long period amplitudes was higher than at other latitudes. For all spectral samples of the data, a minimum in activity was found near 30° geomagnetic latitude and a dayside enhancement was found at the equator. At all latitudes most amplitudes were larger during the local daytime hours and smallest at the predawn hours. East-west component amplitudes were about the same size as the north-south component amplitudes averaged over the day yet regular diurnal variations were found in the ratio of these two components. On the average there was a 10 to 20 percent reduction of the east-west field component in the region of the ionospheric equatorial and auroral electrojet currents. The vertical component of field was about 40 percent less than the horizontal field. This reduction seemed to be accounted for by an effective crustal conductivity of about 2×10^-3mhos/meter extending to an equivalent depth of about 300km. The general characteristics of the quiet-time spectra seemed to match the projection of the changes in active-day spectra to low Ap values. Empirical relationships between the spectral features and the Ap index are provided to allow some prediction of geomagnetic amplitude levels as a function of period, time, and location on magnetically quiet days.

View full abstract

A study of the secular variation in the geomagnetic elements H, Z, F and D at Colaba (Bombay)-Alibag for the period 1848-1973, has been made. Fifth degree polynomials are fitted to the data of annual mean values of H, Z and F, and third degree to D, and the residuals discussed. The trends are also examined at the six Indian observatories using the data for 1960-1974.
The increasing trend of Z at Alibag is found to decrease from about 1937, while that of H and F from 1965 at 20-30nT/year, it being of the same order at Sabhawala and Hyderabad but smaller at the equatorial stations, particularly for Z component. The westward annual change in D swings eastward again around 1965 at all the Indian stations. This reversal of the secular variation trend in India after 1965 emerges as an important regional feature connected with a southward migration of the dip equator in India from 1968.
The secular change in D at Alibag (+0.4′/year) is somewhat anomalous in the sense that it is reduced as compared to Hyderabad and other stations (+1.6′/year), probably due to the local magnetic anomaly of the Deccan lavas, and calls for detailed investigations.

View full abstract