arXiv subjects

Vincenzo Carbone

Publications and source records attributed to Vincenzo Carbone.

21 records · Page 2Linked to original sources

arXiv1999

Power Laws in Solar Flares: Self-Organized Criticality or Turbulence?

We study the time evolution of Solar Flares activity by looking at the statistics of quiescent times $τ_{L}$ between successive bursts. The analysis of 20 years of data reveals a power law distribution with exponent $α\simeq 2.4$ which is an indication of complex dynamics with long correlation times. The observed scaling behavior is in contradiction with the Self-Organized Criticality models of Solar Flares which predict Poisson-like statistics. Chaotic models, including the destabilization of the laminar phases and subsequent restabilization due to nonlinear dynamics, are able to reproduce the power law for the quiescent times. In the case of the more realistic Shell Model of MHD turbulence we are able to reproduce all the observed distributions.

chao-dyn↗

arXiv1999

Intermittency in the solar wind turbulence through probability distribution functions of fluctuations

Intermittency in fluid turbulence can be emphasized through the analysis of Probability Distribution Functions (PDF) for velocity fluctuations, which display a strong non-gaussian behavior at small scales. Castaing et al. (1990) have introduced the idea that this behavior can be represented, in the framework of a multiplicative cascade model, by a convolution of gaussians whose variances is distributed according to a log-normal distribution. In this letter we have tried to test this conjecture on the MHD solar wind turbulence by performing a fit of the PDF of the bulk speed and magnetic field intensity fluctuations calculated in the solar wind, with the model. This fit allows us to calculate a parameter depending on the scale, which represents the width of the log-normal distribution of the variances of the gaussians. The physical implications of the obtained values of the parameter as well as of its scaling law are finally discussed

physics.plasm-ph↗

arXiv1996

Multifractal structure of Ly$α$ clouds: An example with the spectrum of QSO 0055--26

Ly$α$ forests are usually associated with intergalactic gas clouds intercepting the quasar sight--line. Using as an example the Ly$α$ forest of QSO 0055--26 ($z_{em} = 3.66$), we show that the probability of observing a line at the velocity difference $Δv$ has an intermittent structure in the redshift space. On small scales (70 km~s$^{-1} \leq Δv \leq 1580$ km~s$^{-1}$) the signature of intermittency appears as a self--similar structure of spikes systematically visible at the same redshift on all scales. This behaviour can be interpreted as due to the presence of clustering of the Ly$α$ lines, which appear as singularities of the density of the probability measure. From a direct measurement of the generalized dimensions, we show that the intermittency can be described by a multifractal structure which is due to a Fourier phase correlation of the signal. The multifractal structure disappears at scales larger than $Δv \simeq 1580$ km~s$^{-1}$.

astro-ph↗