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SUMMARY:[SH147] Geometry of solar wind magnetic turbulence at 1 AU
DTSTART;VALUE=DATE-TIME:20170718T080000Z
DTEND;VALUE=DATE-TIME:20170718T081500Z
DTSTAMP;VALUE=DATE-TIME:20210804T201707Z
UID:indico-contribution-679@cern.ch
DESCRIPTION:Speakers: BURGER\, Renier (North-West University)\nAb initio c
osmic-ray modulation models require turbulence spectra as input for the di
ffusion tensor. If one assumes a composite slab/2D structure for the turbu
lence\, the so-called slab ratio\, which is the ratio of the energy densit
y in the slab spectrum compared with the total energy density is required.
Bieber et al. (1996\, JGR\, 101) describe two methods for calculating thi
s ratio\; Saur and Bieber (1999\, JGR\, 104) add a third method. In this p
aper we present a preliminary study of the slab ratio for both the inertia
l- and the energy range of magnetic turbulence observed at 1 AU\, using AC
E data for 1998 to 2015. We use only the so-called ratio test of Bieber et
al.\, which is based on fitting data to the ratio of the two theoretical
spectrum components perpendicular to the mean magnetic field\, denoted by
Pyy/Pxx. Here the y-direction is perpendicular to the plane containing the
mean magnetic field and the radial direction\, and the x-direction is in
that plane. Saur and Bieber tested both the radial and the mean field dire
ction as candidate symmetry axes for turbulence in the energy range\; here
we test this only for the slab component but for both the energy- and the
inertial range. For the inertial range we find an average slab fraction o
f 0.28 for the whole data set\, which is in good agreement with values rep
orted in other studies using the ratio test. Our analysis suggests that th
e slab turbulence in this high-frequency regime is symmetric with respect
to the mean magnetic field\, and not the radial direction. However\, for t
he energy range\, we find that at low frequencies the radial direction is
the symmetry axis for the slab turbulence\, in agreement with Saur and Bie
ber. Moreover\, we find an average slab fraction of 0.36 for the energy ra
nge\, which is in exact agreement with the result reported by these author
s. We also show that the slab fraction can vary by as much as a factor of
four during the course of a solar activity cycle. This implies that the pa
rallel mean free path could vary by the same factor. In non-linear theorie
s for perpendicular diffusion\, this would also affect the perpendicular m
ean free path\, and most probably also the drift scale. The consequences o
f a slab fraction that is different for the energy- and for the inertial r
ange\, and that varies with solar activity\, have yet to be studied in ab
initio modulation models. This work is based on research supported by Sout
h African National Research Foundation Grants 80824\, 93592 and 96478.\n\n
https://indico.snu.ac.kr:443/indico/event/15/session/4/contribution/679
LOCATION:BEXCO Room C(108)
URL:https://indico.snu.ac.kr:443/indico/event/15/session/4/contribution/67
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