6.3 The effect of compressive phenomena on Alfv´enic correlations

6.3 The effect of compressive phenomena on Alfvénic correlations

A lack of $dV - dB$ correlation does not strictly indicate a lack of Alfvénic fluctuations since a superposition of both outward and inward oriented fluctuations of the same amplitude would produce a very low correlation as well. In addition, the rather complicated scenario we observe at the base of the corona, where complicated kinetic and magnetic phenomena contribute to the birth of the wind, suggest that the imprints of such a structured corona is carried away by the wind during its expansion. At this point, we would expect that solar wind fluctuations would not solely due to the ubiquitous Alfvénic and other MHD propagating modes but also to an underlying structure convected by the wind, not necessarily characterized by Alfvén-like correlations. Moreover, dynamical interactions between fast and slow wind, built up during the expansion, contribute to increase the compressibility of the medium.

It has been suggested that disturbances of the mean magnetic field intensity and plasma density act destructively on $dV - dB$ correlation. Bruno and Bavassano (1993 ) analyzed the loss of the Alfvénic character of interplanetary fluctuations in the inner heliosphere within the low frequency part of the Alfvénic range, i.e., between $2$ and $10 h$ . In Figure 71 , from their work, shows the wind speed profile, $sc$ , the correlation coefficients, phase and coherence for the three components (see Appendix 13.2.1), the angle between magnetic field and velocity minimum variance directions, and the heliocentric distance. Magnetic field sectors were rectified (see Appendix 13.3) and magnetic field and velocity components were rotated into the magnetic field minimum variance reference system (see Appendix 15). Although the three components behave in a similar way, the most Alfvénic ones are the two components $Y$ and $Z$ transverse to the minimum variance component $X$ . As a matter of fact, for an Alfvén mode we would expect high $dV - dB$ correlation, a phase close to zero for outward waves and a high coherence. Moreover, it is rather clear that the most Alfvénic intervals are located within the trailing edges of high velocity streams. However, as the radial distance increases, the Alfvénic character of the fluctuations decreases and the angle $Qbv$ increases. The same authors found that high values of $Qbv$ are associated with low values of $sc$ and correspond to the most compressive intervals. They concluded that the depletion of the Alfvénic character of the fluctuations, within the hourly frequency range, might be driven by the interaction with static structures or magnetosonic perturbations able to modify the homogeneity of the background medium on spatial scales comparable to the wavelength of the Alfvénic fluctuations. A successive paper by Klein et al. (1993 ) showed that the $dV - dB$ decoupling increases with the plasma $b$ , suggesting that in regions where the local magnetic field is less relevant, compressive events play a major role in this phenomenon.

Figure 71:

Wind speed profile $V$ and $|sc|V$ are shown in the top panel. The lower three panels refer to correlation coefficient, phase angle and coherence for the three components of $dV$ and $dB$ fluctuations, respectively. The successive panel indicates the value of the angle between magnetic field and velocity fluctuations minimum variance directions. The bottom panel refers to the heliocentric distance (adopted from Bruno and Bavassano, 1993 ).