Geomagnetic Variations of Cosmic Rays

In this chapter we shall deal with the CR variations caused by variations of the geomagnetic field. Changes in this field may be connected with processes in the earth's interior (for instance, the well-known phenomenon of the earth's magnetic field inversions, moving of magnetic poles, geomagnetic secular variations), or with electrical currents in the earth's ionosphere and magnetosphere, controlled mostly by solar activity.What are the direct influences of each of these field variations on CRs and particularly on the cutoff rigidities and the asymptotic directions of incidence? Of course, the influence of the secular geomagnetic field variations due to internal sources may be determined by the trajectory-tracing method described in Chapter 3 (if the long-term time-variation of the space distribution of the geomagnetic field is known). The change of the planetary distribution of CR cutoff rigidities in the present epoch of direct CR continuous measurements by a NM network was considered in detail in Section 3.8. The experimental and theoretical data showing that there are long-term (thousands of years) variations of CR intensity due to variations of the interior sources of the field, will be considered in Sections 6.2–6.5.

For sources in the earth's magnetosphere are very important detail information on radiation belts and electrical currents in the magnetosphere and their connection with the processes in the ionosphere (Sections 6.6–6.13). These currents change the magnetic field and, correspondingly, CR cutoff rigidities. In order to take into account the geomagnetic field variations due to ring currents in the magnetosphere, it is important to compute not only the variation of the vertical cutoff rigidity in the real field in the presence of a thin ring current, but also to estimate the change of threshold rigidities for particles incident under various zenith angles, and to consider the widening of the current in latitude, and to estimate the effect of volume currents. Also, the influence of the confinement and asymmetry of the geomagnetic field by the action of the solar-wind plasma must be taken into account. These current systems and changes of the magnetosphere also influence the asymptotic directions and the acceptance cones of the particles. The observed CR variations arising from changes in the geomagnetic field caused by exterior sources are considered in Section 6.14. In Sections 6.15 and 6.16 we discuss the experimental data and the possible nature of the so-called local solar-daily and lunar-daily CR variations, respectively. The first type of CR anisotropy possibly due to an asymmetry of the magnetosphere, and second type may be connected with the tidal phenomenon in the earth's magnetosphere but which may also be spurious, due to a complex amplitudephase modulation (with a period of about 27 days) of the solar CR anisotropy. The observations in the high-latitude region show considerable anomalies in the cutoff rigidities. Here the influence of the tail of the earth's magnetosphere on the CR, which should be strongly felt in the high-latitude region, offers a promising explanation (see Section 6.17).