Component Of Geomagnetic Field Research Articles

Long period variations of the geomagnetic field

The spectrum of fluctuations of the geomagnetic field extends from periods of < 10 years to periods of 106 years. Although cycles of constant period appear to be absent, definite segments of the spectrum may be associated with particular phenomena in the core. Secular variation with periods of 10 to 103 years is associated with changes in the nondipole field. Changes in the orientation and intensity of the main dipole contribute to the spectrum at about 104 years. At periods of 105 to 106 years, there may be a contribution from a random walk of the rotation axis. The more recent reversals of the polarity of the main dipole occur at characteristic intervals of about 106 years. The asymmetry of the nondipole component of the present geomagnetic field also appears in the results of new paleomagnetic studies of secular variation in the eastern Pacific basin. Two models for interpreting these results are presented, one based on lateral changes in the electrical conductivity of the lower mantle, and the other on the control of thermal convection patterns in the core by lateral temperature differences in the lower mantle.

Recovery times of geomagnetic storms

Using the hourly values of the horizontal component of geomagnetic field at Alibag, India, the recovery times of several geomagnetic storms occuring during the period 1947-59 are evaluated. Relation between average recovery time and sunspot activity is studied. Bough parallelism is observed. However, recovery characteristics are found to be very complex on several occasions and vary widely from storm to storm even in the same year. Theoretical implications of these peculiarities are discussed.

Analysis of Present Geomagnetic Field for Comparison with Paleomagnetic Results

Both the dipole and nondipole components of the present geomagnetic field are analyzed by calculating the orientation of hypothetical geocentric dipoles which, if acting alone, would produce the present geomagnetic field as observed at points on a grid covering the earth at 10° intervals. The dispersion in direction of these virtual geomagnetic poles due to the nondipole component of thee geomagnetic field (1) is larger in the southern hemisphere than in the northern; (2) increases with increase in latitude; (3) at all latitudes is of the same order of magnitude as the dispersion due to a wobble of several tens of degrees of the main geocentric dipole; (4) is anomalously small over a region of continental dimensions in the central Pacific ocean. The parameters describing these variations of poles are transformed to parameters describing variations of the geomagnetic field for comparison with paleomagnetic data.

Recovery of Cosmic-Ray Intensities after the “Forbush-Type Decrease”

By use of Chree's superposed-epoch method, an investigation was made of the recoveries of cosmic-ray intensities, horizontal components of geomagnetic field and sunspot areas following remarkable Forbush-type decrease. Cosmic-ray intsnsities and horizontal components of the geomagnetic field show recovery curves which resemble each other, but the curve of sunspot areas remains as a lower level than those of the former two. Cosmic-ray intensities show an exponential recovery until the 26th day, with 14.7 days mean life. On the other hand, sunspot areas show such reoovery until only the llth day, with 7.3 days mean life. Can the recovery-curve of logarithms of cosmicray intensity decrements, there exists a knee at the 4th . day. It is proposed that this knee corresponds to the escape of the earth from the corpuscular stream ejected from the sun. (auth) With reference to solsr magnetic observstions, it is proposed that the sun has a magnetic field of dipole character such that the intensity is about one gauss in the polar latitude regions and its polarity is inverse of the earth's. If this field results from a magnetic dipole that the sun may have, the value of this dipole moment is estimated as about 5 xmore » l0/sup 22/ gauss/cm/sup 3/. On the assumption that the magnetic field from this dipole spreads to the outer space surrounding the sun, the orbits of solar cosmic particles were calculated. lt is concluded that a solar dipole magnetic field does not exist between the sun and the earth since the existence of this field can not explain cosmic-ray increases associated with large solar flares. Therefore, although the magnetic field extending from the polar regions into outer space is regarded as closing each other in this spate, the features of this field may be remarkably different from the dipole character. A probable configuration in the interplanetary space of the solar general magnetic field is introduced and discussed. (auth)« less

Ionospheric Drift in the F2 Region near the Magnetic Equator

IT has been suggested recently1 that the drift of ionization in the F2 region of the ionosphere, known to exist from measurements made at middle latitudes2, is caused directly by the electric field associated with the S system of currents in the ‘dynamo’ region of the atmosphere. The presence of these currents was originally invoked to explain the daily variations in the geomagnetic elements; recent ionospheric analysis3,4 and direct experiments using rockets have shown that this dynamo region corresponds in height with the ionospheric E region. According to Martyn, magnetic lines of force linking the E and F regions represent directions of high electric conductivity and can be considered as equipotentials. Thus a horizontal north–south electric field in the E region can give rise to a vertical component of electric field in the F2 region which is greatest at the magnetic equator, where it may exceed by several times the horizontal E region field. If this field exists it must give rise to a horizontal drift of ionization in the F region in an east–west direction. Martyn5 has proposed on theoretical grounds that, except in the immediate vicinity of the magnetic equator, the eastward drift velocity resulting from the diurnal potential in the dynamo region should be given approximately by where θ is the geomagnetic co-latitude of the point in the F region, ϕ is longitude measured positively eastward from the midnight meridian and H is the vertical geomagnetic field component. This equation predicts for high and middle latitudes a drift toward the east by day and to the west by night, which is in agreement with observations. At geomagnetic latitudes less than 35°, however, a phase reversal of this drift should take place, and near the magnetic equator the westward drift by day should reach large values of the order of 200 m./sec.

Extrapolation of geomagnetic field components along a radius from the center of the Earth

An approximate method of extrapolating geomagnetic field components is presented in which a very small amount of data is required to effect the continuation. From intensity values at two points lying on a common radius from the center of the Earth, an attenuation factor is obtained which is used in an exponential formula to extrapolate to additional collinear points.

Vertical extrapolation of geomagnetic field components

In the region external to sources of magnetism, both the divergence and the curl of the magnetic intensity are zero. From the corresponding analytic expressions, the derivatives in the vertical direction of each field component are obtained in terms of the values of the components on a surface surrounding the sources. A Taylor series is formed with these derivatives and then used to continue, either upward or downward, the respective components. The rapidity of the convergence depends on the complexity of the surface field and on the distance of the computed point from the surface.