Yakutsk, Russian Federation
Yakutsk, Yakutsk, Russian Federation
Yakutsk, Yakutsk, Russian Federation
Yakutsk, Yakutsk, Russian Federation
employee
Yakutsk, Yakutsk, Russian Federation
The observable anisotropy of cosmic rays has first been decomposed into zonal harmonics and components of vector and tensor anisotropy. We examine Forbush decreases in cosmic rays that occurred in November 2001 and November 2004. It is shown that at the beginning of a Forbush decrease an antisunward convective current of cosmic rays predominates; and during the recovery phase, a sunward diffusive current of particles along the interplanetary magnetic field dominates. During the phase of intensity drop, short-time decreases in the second zonal harmonic take place. These decreases occur with abrupt changes of the interplanetary magnetic field intensity and solar wind speed. During the passage of large-scale solar wind disturbances, the tensor anisotropy behaves in a complicated way. To explain its behavior, a further detailed investigation is required.
cosmic rays, tensor anisotropy, Forbush decrease, coronal mass ejection
NATURE OF ANISOTROPY
An anisotropic flow of cosmic rays (CR), which is registered by ground-based detectors, is originated first of all from dynamic processes in the solar wind, which affect CR through the interplanetary magnetic field (IMF).
Notice that CR anisotropy also occurs outside the helio-sphere, but for the particles registered by ground-based CR detectors, the solar wind is the main modulation factor. The observed anisotropy can be presented by the first two spherical harmonics expressing themselves as diurnal variations due to the Earth rotation. In the angular coordinate system, with the polar axis parallel to the axis of the Earth rotation, the first spherical harmonic is presented as a component R11 . This component can be represented as a vector and is called vector anisotropy. R11 reflects a direct motion or current of CR. The current has a convective or diffusive origin. The convective current appears when CR move together with the solar wind magnetic field; and the diffusive current, due to CR gradients and their scattering by magnetic irregularities. In the steady state of the interplanetary medium, the convective current is directed away from the Sun; and the diffusive current, toward the Sun along IMF lines. In general, the diurnal anisotropy shows the moment of maximum intensity about 18 LT.
The tensor anisotropy components R22 and R12 in the above angular coordinate system appear as semidiurnal and antisymmetric diurnal variations. The reason for the observed anisotropy is a deficit of particles moving along field lines in loop structures of IMF. In average over the course of a long period, this component is presented, on the 12-hour dial, as a vector directed to 3 LT. The same anisotropy should appear in the presence of the heliolatitudinal CR gradient with minimum intensity near Earth’s heliolatitude. A deviation of magnetic tubes from the Archimedian spiral leads to a change of the R22 direction. The component R12 usually appears in the region of heliolatitudinal shift of the CR current. If the convective current shifts, then R12 is maximum at 12 or 0 LT. If there is a heliolatitudinal shift of the diffusive current along the field lines, the variation is maximum at 9 or 21 LT. In more complicated cases, when the convective and diffusive currents shift simultaneously and when field tubes are deviated, the tensor anisotropy behaves in a complicated way.
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