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Title: Measures of Scale-dependent Alfvénicity in the First PSP Solar Encounter
Authors: T. N. Parashar
M. L. Goldstein
M. L. Goldstein
B. A. Maruca
W. H. Matthaeus
D. Ruffolo
R. Bandyopadhyay
R. Chhiber
R. Chhiber
A. Chasapis
R. Qudsi
D. Vech
D. Vech
D. A. Roberts
S. D. Bale
S. D. Bale
S. D. Bale
J. W. Bonnell
T. Dudok De Wit
K. Goetz
P. R. Harvey
R. J. MacDowall
D. Malaspina
M. Pulupa
J. C. Kasper
J. C. Kasper
K. E. Korreck
A. W. Case
M. Stevens
P. Whittlesey
D. Larson
R. Livi
M. Velli
N. Raouafi
Universite d'Orleans
University of Minnesota Twin Cities
Space Sciences Laboratory at UC Berkeley
University of California, Los Angeles
University of Michigan, Ann Arbor
University of California, Berkeley
University of Maryland, Baltimore County
Imperial College London
Mahidol University
Smithsonian Astrophysical Observatory
NASA Goddard Space Flight Center
Johns Hopkins University
The Bartol Research Institute
University of Colorado Boulder
Keywords: Earth and Planetary Sciences;Physics and Astronomy
Issue Date: 1-Feb-2020
Citation: Astrophysical Journal, Supplement Series. Vol.246, No.2 (2020)
Abstract: © 2020. The American Astronomical Society. All rights reserved.. The solar wind shows periods of highly Alfvénic activity, where velocity fluctuations and magnetic fluctuations are aligned or antialigned with each other. It is generally agreed that solar wind plasma velocity and magnetic field fluctuations observed by the Parker Solar Probe (PSP) during the first encounter are mostly highly Alfvénic. However, quantitative measures of Alfvénicity are needed to understand how the characterization of these fluctuations compares with standard measures from prior missions in the inner and outer heliosphere, in fast wind and slow wind, and at high and low latitudes. To investigate this issue, we employ several measures to quantify the extent of Alfvénicity - the Alfvén ratio r A, the normalized cross helicity σ c , the normalized residual energy σ r , and the cosine of angle between velocity and magnetic fluctuations . We show that despite the overall impression that the Alfvénicity is large in the solar wind sampled by PSP during the first encounter, during some intervals the cross helicity starts decreasing at very large scales. These length scales (often >1000d i ) are well inside inertial range, and therefore, the suppression of cross helicity at these scales cannot be attributed to kinetic physics. This drop at large scales could potentially be explained by large scale shears present in the inner heliosphere sampled by PSP. In some cases, despite the cross helicity being constant down to the noise floor, the residual energy decreases with scale in the inertial range. These results suggest that it is important to consider all these measures to quantify Alfvénicity.
ISSN: 00670049
Appears in Collections:Scopus 2020

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