Fabio Lepreti

Observation of Inertial Energy Cascade in Interplanetary Space Plasma

Direct evidence for the presence of an inertial energy cascade, the most characteristic signature of hydromagnetic turbulence (MHD), is observed in the solar wind by the Ulysses spacecraft. After a brief rederivation of the equivalent of Yagloms law for MHD turbulence, a linear relation is indeed observed for the scaling of mixed third-order structure functions involving Elsässer variables. This experimental result firmly establishes the turbulent character of low-frequency velocity and magnetic field fluctuations in the solar wind plasma.

Solar Flare Waiting Time Distribution: Varying-Rate Poisson or Lévy Function?

The waiting time distribution for solar flares has been considered. Using Geostationary Operational Environmental Satellite X-ray data, we show that a local Poisson hypothesis for solar flares is not consistent with observations, while the observed distribution is well reproduced by a Levy function, which displays asymptotically a power-law tail. The origin of the observed waiting time distribution is then attributed to the fact that the physical process underlying solar flares is statistically self-similar in time and is characterized by a certain amount of memory.

Position and velocity space diffusion of test particles in stochastic electromagnetic fields

The two-dimensional diffusive dynamics of test particles in a random electromagnetic field is studied. The synthetic electromagnetic fluctuations are generated through randomly placed magnetised clouds oscillating with a frequency ω. We investigate the mean square displacements of particles in both position and velocity spaces. As ω increases the particles undergo standard (Brownian-like) motion, anomalous diffusion and ballistic motion in position space. Although in general the diffusion properties in velocity space are not trivially related to those in position space, we find that energization is present only when particles display anomalous diffusion in position space. The anomalous character of the diffusion is only in the non-standard values of the scaling exponents while the process is Gaussian.

Evidence of Shock-driven Turbulence in the Solar Chromosphere

We study the acoustic properties of the solar chromosphere in the high-frequency regime using a time sequence of velocity measurements in the chromospheric Ca II 854.2 nm line taken with the Interferometric Bidimensional Spectrometer (IBIS). We concentrate on quiet-Sun behavior, apply Fourier analysis, and characterize the observations in terms of the probability density functions (PDFs) of velocity increments. We confirm the presence of significant oscillatory fluctuation power above the cutoff frequency and find that it obeys a power-law distribution with frequency up to our 25 mHz Nyquist limit. The chromospheric PDFs are non-Gaussian and asymmetric, and they differ among the network, fibril, and internetwork regions. This suggests that the chromospheric high-frequency power is not simply the result of short-period waves propagating upward from the photosphere but rather is the signature of turbulence generated within the chromosphere from shock oscillations near the cutoff frequency. The presence of this pervasive and broad spectrum of motions in the chromosphere is likely to have implications for the excitation of coronal loop oscillations.

A Simple Model for the Solar Cycle

A simple nonlinear model which describes the 11-year solar cycle can be derived from the usual α−ω dynamo theory in the form of a Van der Pol equation. Solar activity displays also small-scale inter-cycle persistent stochastic oscillations with a Hurst exponent of the order of H≃0.76±0.01. The results obtained from the Van der Pol oscillator superimposed on a fractional Brownian motion which describes the stochastic fluctuations are presented.

TURBULENT PAIR DISPERSION OF PHOTOSPHERIC BRIGHT POINTS

Observations of solar granulation obtained with the New Solar Telescope of Big Bear Solar Observatory are used to study the turbulent pair dispersion of photospheric bright points in a quiet-Sun area, a coronal hole, and an active region plage. In all the three magnetic environments, it is found that the pair mean-squared separation Δ2(t) follows a power-law timescaling Δ2(t) ~ t η in the range 10 s t 400 s. The power-law index is found to be η 1.5 for all the three investigated regions. It is shown that these results can be explained in the same framework as the classical Batchelor theory, under the hypothesis that the observed range of timescales corresponds to a non-asymptotic regime in which the photospheric bright points keep the memory of their initial separations.

Proper Orthogonal Decomposition of Solar Photospheric Motions

The spatiotemporal dynamics of the solar photosphere is studied by performing a proper orthogonal decomposition (POD) of line of sight velocity fields computed from high resolution data coming from the MDI/SOHO instrument. Using this technique, we are able to identify and characterize the different dynamical regimes acting in the system. Low-frequency oscillations, with frequencies in the range 20-130 microHz, 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 approximately 10 Mm, are associated to p-modes oscillations at frequencies of about 3000 microHz.

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.

SIGN SINGULARITY AND FLARES IN SOLAR ACTIVE REGION NOAA 11158

Solar Active Region NOAA 11158 has hosted a number of strong flares, including one X2.2 event. The complexity of current density and current helicity are studied through cancellation analysis of their sign-singular measure, which features power-law scaling. Spectral analysis is also performed, revealing the presence of two separate scaling ranges with different spectral index. The time evolution of parameters is discussed. Sudden changes of the cancellation exponents at the time of large flares?and the presence of correlation with Extreme-Ultra-Violet and X-ray flux?suggest that eruption of large flares can be linked to the small-scale properties of the current structures.

Recurrent flares in active region NOAA 11283

The authors wish to thank the referee for his/her very useful comments and suggestions, which led to a sounder version of the article. This research work has received funding from the European Commissions Seventh Framework Programme under the grant agreements No. 284461 (eHEROES project), No. 312495 (SOLARNET project), No. 606862 (F-Chroma project). This research work is partly supported by the Italian MIUR-PRIN grant 2012P2HRCR on The active Sun and its effects on Space and Earth climate and by Space Weather Italian COmmunity (SWICO) Research Program. The research by the KSU astronomy unit – A.E. and A.S.K. – was supported by King Saud University, Deanship of Scientific Research, College of Science Research Center.