Bruno Bavassano

Identifying intermittency events in the solar wind

Abstract Many complex physical systems in Nature are characterized by intermittency. For these systems, energy at a given scale is not evenly distributed in space and any variable affected by intermittency alternates strong activity to quiescence. Within interplanetary space context, solar wind parameters are highly intermittent. Although the influence of this phenomenon on the scaling of solar wind fluctuations has been evaluated using existing intermittency models, it has never been possible to single out intermittent structures within a given time series until new methods, based on the use of wavelets, have been recently adopted. This new approach to the study of intermittency allows to begin a characterization of those events contributing to solar wind intermittency. Our first results on a single case study of magnetic field intermittency showed that this event was located at the border between two adjacent interplanetary regions mainly characterized by different total pressure and bulk velocity, possibly the border between two adjacent flux-tubes.

On the evolution of outward and inward Alfvénic fluctuations in the polar wind

Plasma and magnetic field measurements by Ulysses are used to investigate the radial evolution of hourly-scale Alfvenic fluctuations in the polar wind. The data span from 1.4 to 4.3 AU in heliocentric distance. Different radial regimes at different distances emerge. Inside about 2.5 AU the large outward traveling fluctuations decrease faster, in terms of energy per unit mass, than the small inward ones. This is in agreement with previous low-latitude observations inside 1 AU within the trailing edge of fast streams. As a result of this different gradient the ratio of inward to outward fluctuation energy rises to about 0.5 near 2.5 AU. Beyond this distance the radial gradient of the inward fluctuations becomes increasingly steeper, while that of the outward ones does not vary appreciably. A state is quickly reached where both populations decline at almost the same rate. These results on the behavior of outward and inward Alfvenic fluctuations are new and represent a constraint for models of turbulence evolution in steadily expanding flows like the polar wind. Finally, an extrapolation to regions near the Sun would suggest that Alfvenic fluctuations at hourly scale should not play a relevant role in solar wind heating and acceleration. Obviously, this last conclusion may be invalidated by non-WKB effects and by compressive and dissipative processess close to the Sun.

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

Solar wind plasma and magnetic field measurements by Ulysses have been used to study magnetohydrodynamic turbulence in different heliospheric regions. Four intervals of six solar rotations have been analyzed. Two of them are on the ecliptic around 2 and 5 AU, respectively, one is at midlatitude near 5 AU, and the last one is at high latitude around 3 AU. Conditions on the ecliptic are those typical of high solar activity periods. The midlatitude interval is characterized by very strong gradients in the wind speed, due to an intermittent appearance of the wind coming from the polar coronal hole. In the high-latitude interval, fully inside the polar wind, the speed is steadily high. We investigated at three different scales (1, 4, and 12 hours) the level of correlation between velocity and magnetic field fluctuations, as given by the normalized cross-helicity, and the sharing of the fluctuation energy between its kinetic and magnetic component, as measured by the normalized residual energy. The observations on the ecliptic, while confirming previous findings based on Voyagers data, clearly indicate that the normalized cross-helicity is well different from zero also at distances as large as 5 AU. The midlatitude turbulence, when compared to that at low and high heliographic latitudes, appears much more evolved, with a remarkably lower normalized cross-helicity (in absolute value). This unambiguously highlights that processes at velocity gradients are an important factor in the turbulence evolution. For all the analyzed intervals the residual energy values indicate an imbalance in favor of magnetic fluctuations, in agreement with previous results. The strongest imbalance is observed for the high-latitude sample, where the turbulence is comparatively the least evolved. This is a quite unexpected result, probably related to the presence of interstellar pickup ion populations. In conclusion, our analysis indicates that (1) velocity gradients play a dominant role in driving the turbulence evolution in the solar wind and (2) pickup ion effects might be significant.

Effects of intermittency on interplanetary velocity and magnetic field fluctuations anisotropy

Intermittency is a well-established feature of interplanetary MHD fluctuations and, we show the effect of intermittency on the radial evolution of solar wind velocity and magnetic field fluctuations anisotropy. On one hand we confirm results obtained by previous investigations which showed that magnetic fluctuation anisotropy increases with distance and, on the other hand, we prove that much of this trend is due to intermittency. Once intermittency has been reduced, thanks to a technique based on wavelet transform for the identification of the intermittent events, the radial trend vanishes. Similar analysis performed on velocity fluctuations, showed that intermittency although altering the anisotropy, does not markedly change its radial trend.

Heliospheric plasma sheet and coronal streamers

Helios 2 measurements of solar wind plasma and magnetic field are used to investigate the structure of the heliospheric plasma sheet between 0.3 and 1 AU. In agreement with previous observations at 1 AU, the plasma sheet thickness is much larger than that of the embedded current sheet. The plasma sheet appears surrounded by a density halo, a region of slightly raised density. High-time resolution data show that decreases in relative helium abundance coincide with the plasma sheet boundaries, reinforcing the notion that the solar wind within the plasma sheet is of a different nature (with different solar origins) than that outside it. Although radio occultation measurements of the corona were not available at the time of the Helios data, a synthesis of recent results on coronal streamers shows that there is a remarkable similarity between their major features and those of plasma sheets, demonstrating that the coronal counterpart of the plasma sheet is the stalk of the coronal streamer. These measurements also suggest that the density halo seen in the Helios data is associated with the radial extension of the boundaries of the streamer observed in the extended corona before the streamer narrows to a stalk.

Anisotropy and minimum variance of magnetohydrodynamic fluctuations in the inner heliosphere

In this paper we examine the anisotropy, minimum variance, and related distinguishing plasma parameters of small-scale fluctuations occurring in the solar wind. We use Helios 2 data taken at solar minimum, at a time when high- and low-speed streams are clearly distinguished and a separation of the characteristics of fluctuations from disparate solar sources is facilitated. We find that while variance directions of fluctuations are generally aligned with the mean magnetic field in regions of high speed and relatively low plasma β, they are more three-dimensional or isotropic in low-speed, high-β intervals. In our analysis period, these latter regions are generally the trailing edges of high-speed streams and slow-flow-containing current sheets. In these low-speed intervals, we find a tendency for large proton density fluctuations to be associated with a preference for fluctuation variance directions to be three-dimensional and also a tendency for field and velocity fluctuations to decouple. In the past it has been emphasized that in high-speed streams, fluctuations are initially outwardly propagating and Alfvenic or two-dimensional in k space and that this Alfvenicity is destroyed by the production of inwardly traveling waves as the flow evolves. It has been suggested that this admixture of waves is produced in a turbulent cascade initiated by stream shears. Here we suggest that this picture is incomplete, that in addition to inwardly propagating plane waves, compressive waves, convected pressure balances, or other density fluctuations can generate or scatter the original spectrum and produce the observed scattering and decoupling of fluctuation directions. Additionally, while we do not dispute the supposition that the long-wavelength free energy in stream-stream interactions can initiate a turbulent cascade in the wind, the role of compressive fluctuations in altering the high-frequency spectrum cannot be ruled out.

Heating the Solar Wind by a Magnetohydrodynamic Turbulent Energy Cascade

Solar wind plasma is known to cool down more slowly while it is blown away from the Sun than expected from an adiabatic spherical expansion. Some source of heating is thus needed to explain the observed temperature radial profile. The presence of a nonlinear turbulent magnetohydrodynamic energy cascade has been recently observed in solar wind plasma. This provides for the first time a direct estimation of the turbulent energy transfer rate, which can contribute to the in situ heating of the wind. The value of such contribution is shown to represent an important fraction (from 5% to 100%) of the total heating, and is strongly correlated with the wind temperature.

Alfvénic turbulence in the polar wind: A statistical study on cross helicity and residual energy variations

A study of MHD turbulence properties in the polar wind has been performed with Ulysses data. The parameters under examination are the normalized cross helicity and the normalized residual energy. The correlation coefficient between velocity and magnetic field fluctuation vectors has also been computed. Both southern and northern phases of full immersion in the high-latitude fast wind have been examined. Observations during the short low-latitude phase around the Ulysses perihelion have been used as a term of comparison. Our results indicate that in the region up to about 2 AU the turbulence evolution leads to a decrease of cross helicity and Alfvenic correlation when distance increases. Farther out no appreciable variation is observed. It is concluded that in the high-latitude heliosphere the MHD turbulence maintains a clear Alfvenic character even at large distances. As regards the residual energy, a clear imbalance in favor of the magnetic fluctuations is everywhere present. This imbalance becomes even more pronounced for increasing distance inside 2 AU, due to turbulence evolution. Other effects lead to an inversion of this trend in the outer region. It is noteworthy that the polar turbulence does not appear too different from that observed inside the trailing edge of low-latitude fast streams.

On the probability distribution function of small-scale interplanetary magnetic field fluctuations

In spite of a large number of papers dedicated to the study of MHD turbulence in the solar wind there are still some simple questions which have never been sufficiently addressed, such as: a) Do we really know how the magnetic field vector orientation fluctuates in space? b) What are the statistics followed by the orientation of the vector itself? c) Do the statistics change as the wind expands into the interplanetary space? A better understanding of these points can help us to better characterize the nature of interplanetary fluctuations and can provide useful hints to investigators who try to numerically simulate MHD turbulence. This work follows a recent paper presented by some of the authors which shows that these fluctuations might resemble a sort of random walk governed by Truncated Levy Flight statistics. However, the limited statistics used in that paper did not allow for final conclusions but only speculative hypotheses. In this work we aim to address the same problem using more robust statistics which, on the one hand, forces us not to consider velocity fluctuations but, on the other hand, allows us to establish the nature of the governing statistics of magnetic fluctuations with more confidence. In addition, we show how features similar to those found in the present statistical analysis for the fast speed streams of solar wind are qualitatively recovered in numerical simulations of the parametric instability. This might offer an alternative viewpoint for interpreting the questions raised above.

Scaling of density fluctuations with Mach number and density‐temperature anticorrelations in the inner heliosphere

We present observations of field and plasma fluctuations of duration of a few hours or less of spacecraft time frame as observed by Helios 2 in the inner heliosphere. Two distinct turbulent plasma regimes are found. In the regions between high-speed streams, small-amplitude density fluctuations show an excellent correlation with plasma frame Mach number (M), and they are often anticorrelated with small-scale proton temperature fluctuations. In high-speed flows, however, density fluctuations have several scales, both M and M2, and density and temperature fluctuations are more positively correlated. Our observations cast in a new light the distinctly different nature of high-speed flows from coronal holes and flows from other equatorial regions. They provide a characterization of small-scale fluctuations in the inner heliosphere, and we interpret the results in terms of the nearly incompressible fluid description of the solar wind (Zank and Matthaeus, 1990). Additionally, we compare our results with Voyager observations in the outer heliosphere (Matthaeus et al., 1991).