ACE/Wind multispacecraft analysis of the magnetic correlation in the solar wind

Abstract

The propagation of galactic and solar cosmic rays in the solar wind (SW) can be strongly influenced by the SW fluctuations properties. Magnetohydrodynamic (MHD) scale fluctuations in the solar wind are usually highly anisotropic, and have also been found to exhibit different properties in regions of high and low solar wind speed. Previous studies analyzed the anisotropy properties of the solar wind magnetic fluctuations at scales of the order of (¤ ¦ © ¤ ¦ ¥) km (inertial range) from two-times/single-point (assuming the Taylor frozen-in hypothesis), and found that the fluctuations in the fast solar wind present a trend to having wave numbers with their parallel (to the mean magnetic field) larger than the perpendicular one, while the fluctuations in the slow wind present the opossite trend. In the present study we present a comparison of the self-magnetic correlation function in the solar wind between two-times/one-point observations (from a single spacecraft) and one-time/two-points observations (from simultaneous observations of two spacecrafts, observing the pure spatial structures). We compare also previous results of the anisotropy of the solar wind fluctuations, obtained from a single spacecraft, with our new multispacecraft analysis using combining observations of the spacecrafts ACE and Wind.

Full text

Page 1

30TH INTERNATIONAL COSMIC RAY CONFERENCE
ACE/Wind multispacecraft analysis of the magnetic correlation in the solar wind
S. DASSO
SMITH
Instituto de Astronom´ ıa y F´ ısica del Espacio (IAFE) and Departamento de F´ ısica, Facultad de Ciencias
Exactas y Naturales, Universidad de Buenos Aires and CONICET, Argentina.
Bartol Res. Institute, Department of Physics and Astronomy, University of Delaware, Newark, Delaware,
USA.
IGPP, UCLA, Los Angeles, California, USA.
Institute of Earth, Oceans and Space, University of New Hampshire, New Hampshire, USA.
?
, W.H. MATTHAEUS
, M.G. KIVELSON
?
, J.M. WEYGAND
?
, PIYANATE CHUYCHAI
?
, L.J. MILANO
?
, C.W.
?
?
?
?
?
?
dasso@df.uba.ar
Abstract:
influenced by the SW fluctuations properties. Magnetohydrodynamic (MHD) scale fluctuations in the
solar wind are usually highly anisotropic, and have also been found to exhibit different properties in
regions of high and low solar wind speed. Previous studies analyzed the anisotropy properties of the
solar wind magnetic fluctuations at scales of the order of (
times/single-point (assuming the Taylor frozen-in hypothesis), and found that the fluctuations in the fast
solar wind present a trend to having wave numbers with their parallel (to the mean magnetic field) larger
than the perpendicular one, while the fluctuations in the slow wind present the opossite trend. In the
present study we present a comparison of the self-magnetic correlation function in the solar wind between
two-times/one-point observations (from a single spacecraft) and one-time/two-points observations (from
simultaneous observations of two spacecrafts, observing the pure spatial structures). We compare also
previous results of the anisotropy of the solar wind fluctuations, obtained from a single spacecraft, with
our new multispacecraft analysis using combining observations of the spacecrafts ACE and Wind.
The propagation of galactic and solar cosmic rays in the solar wind (SW) can be strongly
????????????? ) km (inertial range) from two-
Introduction
Theoriesofscatteringofenergeticsolarparticlesin
the heliosphere [2] and solar modulation of galac-
tic cosmic rays [9] require knowledge of turbu-
lence parameters to express particle diffusion co-
efficients.
In particular several theories (as the quasi-linear
theory [3]) need the correlation of the turbu-
lent magnetic fields as an input that describe the
cosmic-rays transport
It is known that the presence of a uniform ’direct
current’(constantinspaceandtime)magneticfield
(
anisotropies for isotropic initial conditions (e.g.,
[7, 10]): high wavenumbers components develop
more readily perpendicularto
lel to it.
??? ) in a turbulent MHD system develops spectral
?
? than those paral-
Of the various descriptions of anisotropic wave
and turbulence properties of the solar wind, the
so-called ’Maltese cross’ [5] illustrates level con-
tours of the magnetic self correlation which are
seen to have a cross-like pattern when plotted in
a 2D plane where one of the axes is parallel to the
magnetic field. There is a lobe along each axis. A
suggestive but oversimplified interpretation is the
presence of two components or populations: slab-
like fl uctuations (with wavevectors mainly parallel
to the mean field) and quasi-2D-like fl uctuations
(having mainly perpendicular wave vectors).
Unidirectionally propagating Alfv´ en waves corre-
spond to values of the normalized cross helicity
(
the propagation,whilehighlevelsofturbulenceare
usually accompaniedby a value of
(see, e.g., [11], and references therein). From an
??? ) equal to +1 or -1, depending of the sense of
??? close to zero
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MAGNETIC CORRELATION IN THE SOLAR WIND
analysis of 5 years of solar wind data (one minute
of time cadence) measured by the spacecraft ACE
on the Sun-Earth line at 1AU, [6] shown that
is roughly isotropic in the inertial range, a result
plausible with similar levels of turbulence for the
’slab-like’ population and for the ’quasi-2D-like’
one. Using the same sample of solar wind data,
[1] found that at scales of the order of (
km (i.e., the inertial range) the fl uctuations in the
fast (younger) solar wind present a trend to having
wavenumberswiththeirparallel(tothemeanmag-
netic field) largerthan theperpendicularone, while
the fl uctuationsintheslow (older)windpresentthe
oppositetrend; theseobservationssupporttheexis-
tence of an spectral transfer from wavevectors par-
allel to the perpendicular ones, during the evolu-
tion of the solar wind turbulence.
All these previous studies are based on single
spacecraft (SSC, using two-times/one-point) ob-
servations. Because the solar wind speed is much
larger than the local Alfv´ en/sound speeds, the spa-
tial correlation function can be measured in the di-
rection of the fl ow direction (i.e., the solar wind
fl uctuations are convected in the reference frame
of the spacecraft in a short time compared with the
characteristictimescale ofthevariationofthefl uc-
tuations).
Recently a few studies on the spatial dependence
of the magnetic correlation function in the so-
lar wind from two spacecrafts (TSC, using two-
point/single-time) were done [4, 8, 12] using data
from the Cluster fl eet. However, comparison be-
tween the previous studies (using the single space-
crafts) and the new approach have not been done
yet.
In the present work we present a comparison of
magnetic correlation functions in the solar wind
obtained from a SSC with those obtained from
TSC. We also present some preliminary results
from the analysis of the anisotropy of the mag-
netic fl uctuations at the inertial range scales (
km).
??
?????????????)
?
?????????????
Data analysis, results, and preliminary
conclusions
We analyze observations of the magnetic field
measured by the Advanced Composition Explorer
(ACE) and Wind spacecrafts, using the same sam-
ples (intervals of one day of duration, with a time
cadence of one minute) than those used by [4].
Thus, we analyze here the solar wind observations
that correspond to a distance of
Sun, and essentially on the ecliptic plane.
Our maingoalis to computetwo-pointcorrelations
of the form
?
1 AU from the
???????????????????! #"#?%$&???'?( )"#??*
Note that Equation 1 is the trace of the usual
two-point correlation tensor for the magnetic field,
where spatial and temporal translation symme-
tries were assumed.
represents the fl uctuating
magnetic field, and we will study the variance-
normalized correlations, with the normalization
factor as:
[6, 1, 4].For simplicity of notation, we omit
the “
were computed as described in [6, 1], while the
TSC correlations as described in [4].
In this preliminary work we compare
from SSC and MSC for the interval having the
minimum angle (
between ACE and Wind, and the Sun-Earth line
(to can compare the spatial correlation in the same
direction of the spatial lag from SSC and TSC),
which for the analyzed interval is
terval corresponds to the full day of Oct 4, 1999.
The separation distance between the ACE and
Wind is 199(
dius).
Figure 1 shows a comparison of
SSC (solid line), Wind-SSC (dashed line), and
both ACE/Wind-TSC (asterisk). The mean value
of the two SSC observations (
and
(the separation between ACE and Wind) re-
sulted 0.16, while
thatmeansthatthe
at spatial scales of
.
In order to analyze the anisotropy of
the full set of analyzed intervals according with
three angular channels for the angle (
the directionof the spatial lag, givenby the relative
positions between ACE and Wind, and the mean
magnetic field (
(1)
?
?,+?-).0/
?
?
?
?1???2?3$4?5*
, as done in
687:9:; ” label hereafter. The SSC correlations
?=<
obtained
> ) between the relative positions
>
?@?(A. This in-
??B??BC?ED??(F?Gkm, is the Earth ra-
?=<
using ACE-
?=<&HJI
BLKNM(M
I
?=<OHJI
BLKNM
I
M) at a separation of
??P?P
?
B
?=<
HJI
BRQOSUT
+?V
TSC-SSC
KXWNM
I
?
?4Y
ratio,
Z
?;
?
Z[???
?,B
?
?????km, is
?=<OWNM
I?\
?=<&M(M
I
?
?
??]
?=<, we split
^ ) between
_a` ):
?
`cb
^
?
?
Z?d
`,
e
?
`fb
^
?
?
d[?
`, and
D
d
`,b
^
?
?
P??
`.
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30TH INTERNATIONAL COSMIC RAY CONFERENCE
0500
separation distance, r [RE]
10001500
−0.2
0
0.2
0.4
0.6
0.8
1
Rb(r)/Rb(0)
Figure 1: Comparison of
(asterisk).
?=<
from ACE-SSC (solid line), Wind-SSC (dashed line), and ACE/Wind-TSC
050100 150200 250300350
0
0.2
0.4
0.6
0.8
1
θ ε [0,25] (o)
θ ε [40,50] (∆)
Rb(r)/Rb(0)
separation distance, r [RE]
θ ε [65,90] (*)
Figure 2: Observations of
tween the spatial lag direction and
?,<
from ACE/Wind-TSC for fast solar wind for different angular channels be-
_?` . Exponential fit is included to each angular channel (dashed line for
^
?, dotted line for
^
?, and solid line for
^
?).
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MAGNETIC CORRELATION IN THE SOLAR WIND
Because from SSC in previous studies there were
only a few intervals for fast solar wind, in this first
stage of the study, we select only faster solar wind
intervals (km/sec). Figure 2 shows
the normalized
wind and for different angular channels: symbol
?
MS
*
e
F
?
?=<, only from TSC for fast solar
7 to represent
field
intermediate angles respect to
to represent
is possible to observethat
lower than the value obtained for the other two di-
rections, a result consistent with a fast solar wind
having a more important population of ’slab-like’
fl uctuations than ’quasi-2D-like’ ones.
We also compute the correlations lengths (
(computed as in [6, 1]) to
the angular channels. We obtained
^
?(spatial lag parallel to the mean
to represent
_
` ), symbol
?
^
?(spatial lag at
_
` ), and symbol
?
^
?(spatial lag perpendicularto
_
` ). It
?=<
for
^
?is significantly
?
?)
?=<, considering each of
?
?
?
?
e
Y
G??
?????km,
Thus, our result is consistent with the correlation
scales obtainedpreviouslyforthemixedsolarwind
(all angular channels and all speeds) by [4] (
?
?
?
?F
Y
F??
?????km, and
?
?
?
?@G
Y?
?
?????km.
?
?
?
?????km).
We presentedherea preliminaryanalysis that com-
pares from SSC and TSC, and using only TSC
we analyzed the anisotropy of the fast solar wind
fl uctuations. We resist tothetemptationofdrawing
any major conclusion from the anisotropy study
because it is necessary to make an analysis of more
samples, ranging a larger angular set of bins. We
plantoextendourresearchandpublishourfinalre-
sults elsewhere. In the meantime, we believe that
the physics involved in this research project will
help to achieve a better understanding of the solar
wind fl uctuations and its infl uence on the cosmic
rays propagation in the heliosphere.
?=<
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