Large-Amplitude Alfvn Waves in the Interplanetary Medium'

An extensive study of the dynamic nonshock properties of the microscale fluctuations (scale lengths of 0.01 AU and less) in the interplanetary medium was made by using plasma and magnetic field data from Mariner 5 (Venus 1967). The observational results of the study are: (1) Large-amplitude, nonsinusoidal Alfven waves propagating outward from the sun with a broad wavelength range from 103 to 5×106 km dominate the microscale structure at least 50% of the time; the waves frequently have energy densities comparable both to the unperturbed magnetic field energy density and to the thermal energy density. (2) The purest examples of these outwardly propagating Alfven waves occur in high-velocity solar wind streams and on their trailing edges (where the velocity decreases slowly with time). In low-velocity regions Alfven waves are also outwardly propagating but usually have smaller amplitudes than in the fast streams and tend to be less pure in the sense that they are more strongly intermixed with structures of a non-Alfvenic and possibly static nature. (3) The largest amplitude Alfvenic fluctuations are found in the compression regions at the leading edges of high-velocity streams where the velocity increases rapidly with time; these regions may contain significant amounts of inwardly propagating or non-Alfvenic wave modes. (4) Power spectra of the interplanetary magnetic field in the frequency range from 1/(107 min) to 1/(25.2 sec) have frequency dependencies of ƒ−1.5 to ƒ−2.2; the spectra with slower falloffs tend to be associated with higher temperature regions. (5) The microscale magnetic field fluctuations have on the average a 5∶4∶1 power anisotropy in an orthogonal coodinate system whose axes are (eB×eR, eB× (eB ×eR), eB), where eB is a unit vector in the average field direction and eR is a unit vector radially away from the sun; this anisotropy tends to be strongest (6∶3∶1) in the compression regions at the leading edges of high-velocity streams. (6) 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 Alfven mode. These observations are organized on the basis of a model of the solar wind velocity structure. Most Alfven waves in the interplanetary medium are likely 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 Alfvenic wave activity in the compression regions at the leading edges of high-velocity streams may be due to either the amplification of ambient Alfven waves in high-velocity streams as they are swept into the compression regions or the fresh generation of waves in these regions by the stream-stream interactions. The observed absence of the magnetoacoustic modes is evidence for their strong damping. The eB×eR anisotropy is viewed as due to the partial conversion of the Alfven waves to the damped magnetoacoustic modes as they are convected away from the sun; this process continually transfers energy from the microscale field fluctuations to the thermalized solar wind plasma.