Browsing Geophysical Institute by Author "Chapman, Sydney"
A study of magnetic storms and aurorasAkasofu, Syun-Ichi; Chapman, Sydney (Geophysical Institute of the University of Alaska, 1961-03)New notations for magnetic disturbance fields are proposed, based on the theoretical consideration of the electric current systems by which they are produced. A typical magnetic storm begins suddenly when the onrush of the front of the solar gas is halted by the earth's magnetic field. This effect (DCF field) is most markedly observed as a sudden increase of the horizontal component of the earth's field (the storm sudden commencement, abbreviated to ssc)— like a step function. In many cases, however, the change of the field during the ssc is more complicated, and different at different places. Such a complexity superposed on the simple increase (DCF) is ascribed to a complicated current system generated in the polar ionosphere (DP current). It is found that the changes of electromagnetic conditions in the polar regions are communicated, without delay, to lower latitudes, even down to the equatorial regions. It is inferred that the equatorial jet is affected by such a change and produces the abnormal enhancement of ssc along the magnetic dip equator. From the extensive analysis of several magnetic storms that occurred during the IGY and IGC, it is suggested that the capture of the solar particles in the outer geomagnetic field occurs when irregularities (containing tangled magnetic fields and high energy protons) embedded in the solar stream, impinge on the earth.. Thus the development of a magnetic storm depends on the distribution of such irregularities in the stream. The motions and resulting currents and magnetic fields of such "trapped" solar particles are studied in detail for a special model. It is inferred that a large decrease (DR field) must follow the initial increase; it is ascribed to the ring current produced by such motion of solar protons oi energy of order 500 Kev. It is proposed that during the storm there appears a transient 'storm-time1 belt well outside the outer radiation belt. It is predicted that the earth's magnetic field is reversed in limited regions when the ring current is appreciably enhanced. This involves the formation of neutral lines there. These may be of two kinds, called X lines or 0 lines according as they are crossed or encircled by magnetic lines of force. These may be entirely separated or may be joined to form a loop, called an OX loop. It is shown that one of them, the X line, which is connected with the auroral ionosphere by the lines of force, could be the proximate source of th<e particles that produce the aurora polaris. By postulating the existence of such X-type neutral lines at about 6 earth radii, an explanation is obtained of the detailed morphology of the aurora. This includes the auroral zones and their changes, the nighttime peak occurrence of auroras, their thin ribbon-like structure and their multiplicity, their diffuse and active forms and the transition between them (break-up) the required electron and proton flux, and the ray and wavy structures. Among the most important phenomena associated with the sudden change of the aurora from the diffuse to the active form are the simultaneous appearance of the auroral electrojet and the resulting polar magnetic disturbances (DP sub-storms). Several typical DP sub-storms are studied in detail. It is concluded that a westward auroral jet is produced by a southward electric field. It is shown that an instability of the sheetbeam issuing from along the X-type neutral line can produce a southward electric field of the required intensity. The southward electric field produces an eastward motion of the electrons in the ionosphere. This may be identified with the eastward motion of an active aurora and with the westward auroral electrojet. Besides such large changes- of the field, there often appear various quasi-sinusoidal changes of the field, much less intense. They are supposed to be hydromagnetic waves, some of which are generated in the outer atmosphere and propagated through the ionosphere, where a certain amount of their energy is dissipated. It is concluded however that Such a dissipation is not sufficient to produce any appreciable heating of the ionosphere.
A Study of the Morphology of Magnetic Storms Great Magnetic StormsSugiura, Masahisa; Chapman, Sydney (Geophysical Institute at the University of Alaska, 1958-08-31)Average characteristics are determined for 74 great magnetic storms with sudden commencements that occurred in 1902-1945. The storm field is resolved for different epochs of storm time into tv;o parts: (i) Dst, which is independent of local time, that is, of longitude A, relative to the sun, and (ii) DS, which depends on A . They are obtained, for each of the three magnetic elements, declination, horizontal force, and vertical force, at eight geomagnetic latitudes ranging from 80°N to 1°S. DS is harmonically analyzed; the first harmonic component is shown to be the main component of DS. The storm-time course of this component is compared with that of Dst; DS attains its maximum earlier and decays more rapidly. The results of the analysis of great storms are compared with those for weak and moderate storms that were reported previously. Some characteristics of Dst change with intensity. Except in magnitude, main characteristics of DS are independent of intensity.
A Study of the Morphology of Magnetic Storms: Moderate Magnetic StormsSugiura, Masahisa; Chapman, Sydney (Geophysical Institute at the University of Alaska, 1957-06-30)Some average characteristics are determined for 136 moderate magnetic storms with sudden commencements that occurred during the interval 1902-1945. The average storm field is resolved for different epochs of storm time st into Dst, independent of local time, that is, of longitude X , relative to the sun, and into DS, that depends on X , Part DS is expressed in terms of harmonic components with respect to X , and like Dst, the amplitudes and phases of these components, are functions of st and of geomagnetic latitude. They are determined, for each of the three magnetic elements, declination, horizontal force, and vertical force, at eight geomagnetic latitudes ranging from 80*N to 1°S. In the first, and main harmonic component of DS, its variations with respect to storm time differs notably from that of Dst: its maximum is attained earlier and its decay is more rapid. The storm -time changes of the smaller harmonic components of DS have been less fully determined. The average characteristics of moderate storms are compared with those of weak storms.