Forbush Effects and their Connection to the Solar, Interplanetary and Geomagnetic Phenomena

A.V. Belov

Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation Russian Academy of Sciences (IZMIRAN), 142190 Troitsk, Moscow region, Russia

Forbush decrease (or, in more wide sense, Forbush effect) -it is a storm in cosmic rays, which is a direct consequence of heliospheric storm and is very often observed simultaneously with a geomagnetic storm. All these three kinds of perturbations: disturbances in the solar wind, magnetosphere and cosmic rays are closely interrelated and caused by the same active processes on the Sun. It is natural and useful to investigate them in common.

Such an investigation in given work presented, is based on the characteristics of cosmic rays with rigidity of 10 GV. They are derived from the worldwide neutron monitor network data by means of the global survey method and are combined together with relevant information into the data base on Forbush effects and large interplanetary disturbances. At present this data base includes >5500 events and is permanently extended.

Characteristics of Forbush-effect observable on the Earth are defined by conditions in the large-scale changing heliospheric region, however they are closely connected also with local conditions in a point of observation, in particular, with speed of a solar wind and intensity of an interplanetary magnetic field.

The Earth's magnetosphere and cosmic rays are completely different physical objects. It is not surprisingly that their responses on the interplanetary disturbance are different. Nevertheless, between an Ap-index and magnitude of Forbush-effect the certain connection, on the average, is being revealed. Deviations from this interrelation can be used for diagnostics of interplanetary disturbances.

The most noticeable moments at development of greater interplanetary environment disturbances, creating largest Forbush-effects, are arrival of a interplanetary shock wave, and also an entrance into a magnetic cloud and an exit from it. These moments are special points also in development of Forbush-effects.

Cosmic ray variations during the earliest period of development of interplanetary disturbance before its arrival to the Earth are especially important. Forbush-effect as heliospheric phenomenon, begins simultaneously with an output of disturbance in interplanetary space and much more earlier, than a magnetic storm. Influence of a solar wind disturbance on cosmic rays is felt on big enough distance from it. Thus angular and pitch-angular distribution of high energy charged particles, in particular, can be reconstructed. It is possible to separate such phenomenon from groundlevel observations of cosmic rays long before the beginning of a magnetic storm and to use this in forecasting purposes.

Besides the above mentioned, the following questions are discussed in the report: