Alfvén Waves in the Interplanetary Medium

• Main Author: Belcher, John Winston
• Format: Others
• Published: 1971
• Online Access: https://thesis.library.caltech.edu/10569/1/Belcher_JW_1971.pdf; Belcher, John Winston (1971) Alfvén Waves in the Interplanetary Medium. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4A2M-VJ81. https://resolver.caltech.edu/CaltechTHESIS:11222017-095749232 <https://resolver.caltech.edu/CaltechTHESIS:11222017-095749232>

<p>A study of the wave properties of the microscale fluctuations (scale lengths of .01 a.u. and less) in the interplanetary medium is presented using plasma and magnetic field data from Mariner V (Venus 1967). The reduction procedure for the magnetic field data is summarized, and descriptions are given of the MIT plasma data and the merged plasma/field data tapes used in the analysis.</p> <p>Observationally, it is found that large amplitude, nonsinusoidal Alfvén waves propagating outward from the sun with a broad wavelength range from 10³ to 5 x 10⁶km dominate the microscale structure at least 50% of the time. The waves frequently have an energy density comparable both to the unperturbed magnetic field energy density and to the thermal energy density. The purest examples of the Alfvén waves are found in high velocity solar wind streams and on their trailing edges. The largest amplitude waves occur in the compression regions at the leading edges of high velocity streams where the velocity increases rapidly with time. In addition to being transverse to the average magnetic field direction, e_~B, the Alfvénic fluctuations generally exhibit a 10% partial polarization in the e_~Bxe_~R direction, where e_~R is a unit vector radially away from the sun. Presumably magnetoacoustic wave modes occur, but they have not been identified, and, if present, have a small average power of the order of 10% or less of that in the Alfvén mode.</p> <p>These observations are organized on the basis of a model of the solar wind velocity structure. Most of the Alfvén waves in the interplanetary medium seem likely to be the undamped remnants of waves generated at or near the sun. The high level of wave activity in high velocity, high temperature streams can be interpreted as evidence for the extensive heating of these streams by wave damping near the sun. The highest level of Alfvénic wave activity in the compression regions at the leading edges of high velocity streams may be due either to the amplification of ambient Alfvén waves in high velocity streams as they are swept into the compression regions or to the fresh generation of waves in these regions by the stream-stream collisions. The observed absence of the magnetoacoustic modes is evidence for their strong damping. The e_~Bxe_~R anisotropy is viewed as due to the partial conversion of the Alfvén waves to the damped magnetoacoustic modes as they convected away from the sun; this process continually transfers energy from the microscale field fluctuations to the thermalized solar wind plasma.</p> <p>The detailed behavior of the Alfvén waves and their effects on the dynamics of the expanding solar corona as they propagate and are convected out into interplanetary space is investigated in a simplified one fluid polytrope model of the solar wind. It is found that the inclusion of energy fluxes due to Alfvén waves at the base of the corona can result in significant changes in the large streaming properties of the solar wind.</p>