Periodic Behavior and Stochastic Fluctuations of Solar Activity: Proper Orthogonal Decomposition Analysis

We use the Proper Orthogonal Decomposition (POD) to investigate the spatiotemporal features of the solar activity. Daily observation in the period 1949–1996 of the green coronal emission line at 530.3 nm are used as indicators of the activity behavior. We show that few POD modes suffice in describing both the space and time main periodicities. In particular, being affected by a strongly energetic stochastic behavior, daily data are described by five POD modes, while two POD modes are enough to describe the butterfly diagram in monthly averaged data. Apart from the basic period T₀ = 11 years, using daily data we found evidences for intercycle temporal periodicities.     

Coordinated Study on Solar Wind Turbulence During the Venus-Express, ACE and Ulysses Alignment of August 2007

At the end of August 2007, Venus, Earth and Ulysses were aligned within a few degrees. This unusual event gives the opportunity to attempt a coordinated study on the radial evolution of solar wind turbulence and coronal transients like CMEs between 0.7 and 1.4 AU. Interplanetary magnetic field data and moments of proton velocity distribution function such as density, speed and temperature are required for this programme and will be provided by ACE at Earth, Venus Express at Venus and Ulysses at 1.4 AU. This project has been recently proposed as a Coordinated Investigation Programme (CIP35) for the International Heliophysical Year.     

Scaling laws and coherent structures in the solar wind

The interplanetary medium is characterized by a very high Reynolds number and is pervaded by fluctuations providing information on a wide range of scales, from fractions of second up to the solar rotation period. In the past decade or so, turbulence in the solar wind has been used as a large wind tunnel to investigate scaling laws of turbulent fluctuations and multifractal models. Moreover, new interesting insights in the theory of turbulence have been derived from the point of view which considers a turbulent flow as a complex system, a sort of benchmark for the theory of dynamical systems. Important finding like the lack of a strict self-similarity of the fluctuations with the consequent nonapplicability of strict scale invariance, the strong anisotropy of velocity and magnetic field fluctuations, the clear lack of equipartition between magnetic and kinetic fluctuations all contributed to suggest the idea that interplanetary fluctuations could possibly be due to a mixture of propagating waves and static structures convected by the wind. In this paper we further discuss this point and bring new evidence about the fact that the presence of a background magnetic field introduces not only a symmetry breaking in interplanetary space but also organizes fluctuations about its large scale orientation.     

Spatio-Temporal Analysis of Photospheric Turbulent Velocity Fields Using the Proper Orthogonal Decomposition

The spatio-temporal dynamics of the solar photosphere are studied by performing a proper orthogonal decomposition (POD) of line-of-sight velocity fields computed from high-resolution data coming from the SOHO/MDI instrument. Using this technique, we are able to identify and characterize the different dynamical regimes acting in the system. All of the POD modes are characterized by two well-separated peaks in the frequency spectra. In particular, low-frequency oscillations, with frequencies in the range 20?–?130 μHz, dominate the most energetic POD modes (excluding solar rotation) and are characterized by spatial patterns with typical scales of about 3 Mm. Patterns with larger typical scales, of about 10 Mm, are dominated by p-mode oscillations at frequencies of about 3000 μHz. The p-mode properties found by POD are in agreement with those obtained with the classical Fourier analysis. The spatial properties of high-energy POD modes suggest the presence of a strong coupling between low-frequency modes and turbulent convection.     

The magnetohydrodynamic turbulent cascade in the ecliptic solar wind: Study of Ulysses data

The occurrence of a nonlinear turbulent energy cascade in solar wind plasma has been recently established through the observation of an exact law from spacecraft measurements. The main results obtained in the fast, polar wind measured by Ulysses spacecraft are reviewed here. In particular, the turbulent cascade is seen as the mean to provide the energy necessary for the local heating in the non-adiabatic expansion of the solar wind. The importance of the density fluctuations in enhancing the turbulent energy transport is also evidenced. The ecliptic wind data measured by Ulysses are studied here in the same framework. This has been done by separating fast and slow streams, in order to avoid mixing of different physical conditions. The results further support the need for separate analysis of the two types of wind.     

Topological changes of the photospheric magnetic field inside active regions: A prelude to flares?

The detection of magnetic field variations as a signature of flaring activity is one of the main goals in solar physics. Past efforts gave apparently no unambiguous observations of systematic changes. In the present study, we discuss recent results from observations that scaling laws of turbulent current helicity inside a given flaring active region change in response to large flares in that active region. Such changes can be related to the evolution of current structures by a simple geometrical argument, which has been tested using high Reynolds number direct numerical simulations of the MHD equations. Interpretation of the observed data within this picture indicates that the change in scaling behavior of the current helicity seems to be associated with a topological reorganization of the footpoint of the magnetic field loops, namely with the dissipation of small scales structures in turbulent media.