Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsh51d..03p&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SH51D-03
Physics
[2159] Interplanetary Physics / Plasma Waves And Turbulence
Scientific paper
In high-speed streams, measurements of the normalized magnetic helicity as a function of the angle between the local mean magnetic field and the solar wind flow direction clearly reveal the existence of two distinct populations of electromagnetic fluctuations at proton kinetic scales. One population of fluctuations is observed at oblique angles, centered about an angle of 90 degrees, and a separate population at angles near zero degrees. The oblique waves are characterized by a right hand sense of polarization in the spacecraft frame and are consistent with kinetic Alfven waves which are expected, on theoretical grounds, to be generated by MHD turbulence as the anisotropic energy cascade makes a transition from MHD scales to kinetic scales. The parallel waves exhibit a left hand sense of polarization in the spacecraft frame and their average power is of the same order of magnitude or less than that of the oblique waves. Such parallel waves are not expected to be generated directly by MHD turbulence; however, they may be generated by kinetic instabilities and may coexist with the turbulence. The data suggest that the signals observed in the parallel direction are electromagnetic ion cyclotron waves propagating predominantly away from the sun or whistler waves propagating predominantly toward the sun along the local mean magnetic field, or both. Unfortunately, using magnetic field data alone it is difficult to determine whether these waves are left or right-hand polarized in the plasma frame which is necessary to uniquely identify the waves. The observed parallel propagating waves are believed to be generated in-situ through proton pressure anisotropy instabilities. Interestingly, the EMIC and whistler (firehose) instabilities can explain the predominantly unidirectional propagation of the observed waves when the differential streaming of alpha particles in fast wind is taken into account. The alpha-proton differential streaming along the magnetic field induces an asymmetry in the growth rates of the unstable waves for propagation parallel and anti-parallel to the streaming velocity such that when T⊥p > T||p, the EMIC instability preferentially generates ion cyclotron waves propagating away from the sun and, when T⊥p < T||p, the firehose instability preferentially generates whistler waves propagating toward the sun. For plasma conditions typical of high speed streams at 1 AU, both of these instabilities are characterized by the properties kc/ωpp ˜ 1, ω ˜ Ωp, ω/k ˜ vA and Δk/k ˜ 0.5, which are all consistent with solar wind observations. These instabilities provide the most natural explanation for the observed parallel waves.
No associations
LandOfFree
Solar wind fluctuations at kinetic scales: Existence of distinct populations of perpendicular and parallel propagating electromagnetic waves near the proton gyro-radius scale in high speed wind does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Solar wind fluctuations at kinetic scales: Existence of distinct populations of perpendicular and parallel propagating electromagnetic waves near the proton gyro-radius scale in high speed wind, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Solar wind fluctuations at kinetic scales: Existence of distinct populations of perpendicular and parallel propagating electromagnetic waves near the proton gyro-radius scale in high speed wind will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-881036