Ermanno Pietropaolo

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.

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.

Geomagnetic field variations at low and high latitude during the January 10–11, 1997 magnetic cloud

On Jan. 10–11, 1997 a wide magnetic cloud reached the Earth triggering intense geomagnetic activity. Observations performed at low and very high latitude show that the same features appear simultaneously in correspondence to different changes in the solar wind conditions. In particular, highly polarized modes are simultaneously observed at the same discrete frequencies after the passage of the high density solar wind region following the cloud. SIs and ULF waves polarization are also examined in a wide latitudinal and longitudinal extent.

Radial evolution of outward and inward Alfvénic fluctuations in the solar wind: A comparison between equatorial and polar observations by Ulysses

Ulysses measurements done during the ecliptic phase of the mission are used to investigate the radial evolution of outward and inward Alfvenic fluctuations at hourly scale in near-equatorial solar wind. This analysis has been stimulated by a recent study on Alfvenic turbulence in polar wind, showing that at hourly scale different radial regimes develop at different distances. In the present analysis a total of 30 time intervals, characterized by highly Alfvenic fluctuations, are examined, for a total duration of 2558 hours. They are quite uniformly distributed from 1.2 to 5.2 AU along the ecliptic trajectory of Ulysses. The results clearly indicate that in the investigated radial range the energy per unit mass of the dominant outward propagating fluctuations declines, for increasing distance, with approximately the same rate observed for inward fluctuations. In other words, the ratio of inward to outward fluctuation energy roughly remains the same in the examined region. Moreover, the gradient does not vary appreciably with radial distance. These features indicate that between 1 and 5 AU the Alfvenic fluctuations have a quite different behavior in polar and in near-equatorial solar wind. All this should imply a different role, in the two kinds of wind, of the mechanisms expected to be active in driving the Alfvenic turbulence evolution at hourly scale. Our results, combined with previous observations by Helios spacecraft, also suggest the likely presence of solar cycle effects.

Information entropy in solar atmospheric fields - I. Intensity photospheric structures

The existence of a quasi-regular pattern in solar photospheric convective fields is an open question. In the present work, this problem is quantitatively approached by means of the normalised information entropy measure H � (r) as introduced by Van Siclen (1997), which reports on the information content at different scales. Images were acquired at the THEMIS telescope of the European Northern Observatory by the IPM observing mode, and at the Richard B. Dunn Solar Telescope of the National Solar Observatory. The evaluation of H � (r) in the case of photospheric intensity binarized images shows the presence of maxima which are evidence of different prominent scales in the photospheric pattern. The relative positions of these maxima defines an ordering scale ∼1.6 Mm in both instantaneous and average images. This is read as the evidence of a spatio-temporal organization in the evolution of convective pattern. The emergence of an ordering scale is discussed in the framework of pattern formation in random systems and in connection with the findings of previous works. By averaging images with time, an increase of the information content characterized by a coherence time of ∼1 h is observed in the range of scales from 5.0 Mm to 10.0 Mm.

An analysis of the vertical photospheric velocity field as observed by THEMIS

We propose the application of Proper Orthogonal Decomposition (POD) analysis to the photospheric vertical velocity field obtained through the data acquired by the THEMIS telescope, to recover a proper optimal basis of functions. As first results we found that four modes, which are energetically dominant, are nearly sufficient to reconstruct both the convective field and the field of the “5-min” oscillations.

RADIAL EVOLUTION OF THE INTERMITTENCY OF DENSITY FLUCTUATIONS IN THE FAST SOLAR WIND

We study the radial evolution of the intermittency of density fluctuations in the fast solar wind. The study is performed by analyzing the plasma density measurements provided by Helios 2 in the inner heliosphere between 0.3 and 0.9 AU. The analysis is carried out by means of a complete set of diagnostic tools, including the flatness factor at different timescales to estimate intermittency, the Kolmogorov-Smirnov test to estimate the degree of intermittency, and the Fourier transform to estimate the power spectral densities of these fluctuations. Density fluctuations within the fast wind are rather intermittent and their level of intermittency, together with the amplitude of intermittent events, decreases with the distance from the Sun, at odds with the intermittency of both magnetic field and all other plasma parameters. Furthermore, the intermittent events are strongly correlated, exhibiting temporal clustering. This indicates that the mechanism underlying their generation departs from a time-varying Poisson process. A remarkable, qualitative similarity with the behavior of plasma density fluctuations obtained from a numerical study of the nonlinear evolution of parametric instability in the solar wind supports the idea that this mechanism has an important role in governing density fluctuations in the inner heliosphere.