Waves and irregularities in the solar wind

This review is not meant to be for the specialist who already knows the subject area but rather for the nonspecialist who wants a simple physical picture. Therefore some simple calculations are given to show why waves and irregularities in the solar wind are of importance in obtaining a physical understanding of the dynamical processes occurring in the interplanetary medium. The basic physics of divers phenomena observed in the heliosphere, resulting because of the presence of waves in the solar wind, is spelled out in some detail. The most fundamental point made in this review is that a considerable body of phenomena occurring in the solar wind are either caused directly or modified significantly by the turbulent waves that are always present in the solar wind. Spacecraft data indicate the essentially continuous presence of strong magnetic irregularities in the solar wind, which, apparently, are predominantly Alfvén waves being convected outward from the sun by the solar wind. The bulk dynamical properties of the wind are changed substantively by the waves and irregularities that the wind carries, this change leading, in particular, to higher solar wind speeds than occur in the absence of the waves. Also the observed spread of the ‘frozen‐in’ field lines in the solar wind can be traced back to the random wanderings of the photospheric ‘feet’ of field lines emanating from the sun. The observed variations in neutron monitor intensities and the low‐energy particle fluctuations observed on Explorer 34 are accountable in terms of a ‘slab’ of material containing an irregular frozen‐in magnetic field being convected past an observer. This field causes irregular fluctuations in the number of charged particles crossing the slab that are picked up by a detector. The detected irregularities are related to the irregularities in the ‘slab,’ which also are measurable. The agreement is surprisingly good. The presence of magnetic irregularities and waves in the solar wind also causes scattering of cosmic ray particles as they journey inward through the solar system from interstellar space. This leads to a modulation of the intensity of the cosmic rays seen at earth. The relevant diffusion coefficients, conventionally used to describe this process, are expressible in terms of both the power spectra of the waves present in the solar wind and the charged particle parameters, such as speed, charge, and pitch angle. Although the main physical ideas incorporated into this process seem to be reasonably well understood, there remain an embarrassingly large number of unanswered questions, concerning the details.