Magnetic helicity Hm is one of the three quadratic invariants of the ideal MHD equations of motion. An important property is that it is a pseudoscalar and changes sign under coordinate inversion x→ -x. Then, Hm represents a measure of lack of mirror symmetry of the magnetic field or, in other words, it estimates the "knottedness" of magnetic field lines. A positive/negative Hm would indicate a right/left- hand sense of polarization. A standard analysis of reduced Hm, based on Fourier decomposition and performed in interplanetary space, would show that the handedness of the fluctuations in a given wave number is uncorrelated with the handedness of fluctuations at nearby wavenumbers. As a matter of fact, Hm would continuously fluctuate between positive and negative values throught the whole frequency range. However, this kind of analysis provides only part of the information needed to fully characterize the topology of magnetic field lines. A remarkable help comes out from a new technique, which uses a wavelet decomposition, able to unravel interesting features of the magnetic field fluctuations probably related to the filamentary structure of turbulence. Results from a recent Sun-Earth-Ulysses alignment and, more in general, statistical studies performed in fast and slow wind within the inner heliosphere will be discussed. Moreover, single case studies focussing on interplanetary flux ropes will be reported and possible implications with persistence and sharpness of observed energetic particles dropouts phenomenon at the Earth orbit will be addressed.

Spatial and Temporal Analysis of Magnetic Helicity in the Solar Wind

PIETROPAOLO, Ermanno;
2008-01-01

Abstract

Magnetic helicity Hm is one of the three quadratic invariants of the ideal MHD equations of motion. An important property is that it is a pseudoscalar and changes sign under coordinate inversion x→ -x. Then, Hm represents a measure of lack of mirror symmetry of the magnetic field or, in other words, it estimates the "knottedness" of magnetic field lines. A positive/negative Hm would indicate a right/left- hand sense of polarization. A standard analysis of reduced Hm, based on Fourier decomposition and performed in interplanetary space, would show that the handedness of the fluctuations in a given wave number is uncorrelated with the handedness of fluctuations at nearby wavenumbers. As a matter of fact, Hm would continuously fluctuate between positive and negative values throught the whole frequency range. However, this kind of analysis provides only part of the information needed to fully characterize the topology of magnetic field lines. A remarkable help comes out from a new technique, which uses a wavelet decomposition, able to unravel interesting features of the magnetic field fluctuations probably related to the filamentary structure of turbulence. Results from a recent Sun-Earth-Ulysses alignment and, more in general, statistical studies performed in fast and slow wind within the inner heliosphere will be discussed. Moreover, single case studies focussing on interplanetary flux ropes will be reported and possible implications with persistence and sharpness of observed energetic particles dropouts phenomenon at the Earth orbit will be addressed.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11697/30839
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