Universität zu Köln

von Papen, M., Saur, J. and Alexandrova, O. (2014). Turbulent magnetic field fluctuations in Saturn's magnetosphere. J. Geophys. Res-Space Phys., 119 (4). S. 2797 - 2819. WASHINGTON: AMER GEOPHYSICAL UNION. ISSN 2169-9402

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Abstract

We analyze the statistical properties of magnetic field fluctuations measured by the Cassini spacecraft inside Saturn's magnetosphere. We introduce Saturn's magnetosphere as a new laboratory for plasma turbulence, where background magnetic field is strong (1nT<B(0)75nT), fluctuations are weak (<B/B>=0.07), and the ion plasma (i) is smaller than one. We also show that the dissipation of these magnetic field fluctuations has important implications for the magnetosphere. In a case study of the second orbit of Cassini around Saturn we show that at MHD scales, the spectra and the nature of fluctuations are characterized by large-scale nonstationary processes. The spectral slope varies between -0.8 and -1.7. At higher frequencies we observe a steeper spectrum with nearly constant power law exponent. A spectral break correlating with ion scales separates the two frequency ranges. We carry out a statistical study of high-frequency range fluctuations using the first seven orbits of Cassini. We find that the energy density of raw frequency spectra depends on radial distance from Saturn and thermal and magnetic pressures. However, normalized spectra depend only on ion plasma (i). Closer to Saturn the spectral slope is about -2.3 and for radial distances r>9R(s) the average slope is -2.6. The fluctuations have probability density functions with increasingly non-Gaussian tails and a power law increase of flatness with frequency, which indicates intermittency. We estimate the total energy flux contained in the turbulent cascade as 60-100GW, which is on the same order of magnitude as needed to heat an adiabatically expanding plasma to the temperatures measured in Saturn's magnetosphere.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code von Papen, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Saur, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Alexandrova, O. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-441092
DOI:	10.1002/2013JA019542
Journal or Publication Title:	J. Geophys. Res-Space Phys.
Volume:	119
Number:	4
Page Range:	S. 2797 - 2819
Date:	2014
Publisher:	AMER GEOPHYSICAL UNION
Place of Publication:	WASHINGTON
ISSN:	2169-9402
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language SOLAR-WIND TURBULENCE; MAGNETOHYDRODYNAMIC TURBULENCE; MHD TURBULENCE; ASTROPHYSICAL GYROKINETICS; CLUSTER OBSERVATIONS; DISSIPATION RANGE; SCALING LAWS; ALFVEN WAVES; E-RING; PLASMA Multiple languages Astronomy & Astrophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/44109