Alfvén waves, alpha particles, and pickup ions in the solar wind

Abstract

Past studies of the properties of Alfvtn waves in the solar wind have indicated that (1) the amplitude of the velocity 'fluctuations is almost always smaller than expected on the basis of the amplitude of the field fluctuations, even when the anisotropy of the plasma is taken into account, and (2) the alpha particles often do not participate in the wave motions because they "surf" on the waves carried by the proton fluid. Ulysses data are used to demonstrate that (1) the discrepancy between the velocity and field fluctuations is greater at high heliographic latitudes than in the ecliptic plane, and (2) the alphas do participate in the waves, being either in phase or out of phase with the proton motions depending on whether the differential flow speed between the alphas and protons is greater than or less than the "observed" wave speed, Vwave = õV Bo/õB, as determined from the ratio of amplitudes of velocity and magnetic fluctuations. The possibility that the modification of Alfvtn wave propagation speed is due to pressure anisotropies resulting from pickup ion distributions is investigated. 339O

Observed constraint on proton‐proton relative velocities in the solar wind

“…But, candidates we have considered for mimicking outward-going Alfvdn waves seem problematic as explanations. If the reduced Alfv•n speeds as indicated by Goldstein et al [1995] are correct, then many of the points in Figure 2 Finally, we note that there is an important difference between the present results and the results of Marsch and Livi [1987]. We find from Ulysses that Vo/VA is almost always observed to be less than 2, and thus to statistically well satisfy the constraint corresponding to the instability threshold conditions.…”

“…An additional issue is the actual propagation speed of Alfv•n waves in the solar wind: is it significantly modified by processes not in our theoretical model? Goldstein et al [1995] estimated that known sources (crosses in their Fig. 4) would reduce the Alfv•n speed by from 5 to 15% at high latitudes from 1.5 to 3 AU; the contribution of the two parallel proton beams is implicitly included in our theoretical calculation.…”

Cross‐helicity and residual energy in solar wind turbulence: Radial evolution and latitudinal dependence in the region from 1 to 5 AU

“…In the light of all these arguments'we then propose that the large imbalance toward the magnetic energy found in the high-latitude interval is due to the inadequacy of the F = i assumption. In that region the presence of pickup ions has the effect of lowering F to values appreciably lesser than 1, in the line indicated by Goldstein et al [1995a], and hence of increasing the values of erR. The discrepancy of our result at high latitude can thus be canceled out.…”

“…Finally, the high-latitude region (around 3 AU) offers a sample of turbulence typical of the polar solar wind. As shown by several studies (for instance, see Goldstein et al [1995a, hi, Hotbury et al [1995, and Smith et al [1995]), the polar turbulence appears significantly less evolved than that observed near the ecliptic.…”

“…This may be a good approximation at low latitudes in the inner heliosphere, but there is no reason to expect that such approach be valid for all regions of the heliosphere. As a matter of fact, it has been recently indicated by Goldstein et al [1995a] that at high latitudes beyond about 2 AU contributions from interstellar pickup ions might lead to values of F appreciably lesser than 1. This occurs if the total parallel pressure plus the contribution from relative streamings is greater than the total perpendicular pressure (see equation (1) Incidentally, we would note that an increase of the total mass density p (e.g., for inclusion of contributions from heavy ions) goes in the direction of a decrease of < b 2 > too.…”