Relation Between Polar Plumes and Fine Structure in the
Solar Wind from Ulysses High-Latitude Observations
Yohei Yamauchi∗ , Steven T. Suess∗ and Takashi Sakurai†
†
∗
NASA/Marshall Space Flight Center, SD 50, Huntsville, AL 35812, U.S.A.
National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
Abstract. Ulysses observations showed that pressure balance structures (PBSs) are a common feature in the high-latitude and
high-speed solar winds near the solar minimum. PBSs have been hypothesized to be remnants of coronal plumes and to be
related to network activity such as magnetic reconnection in the photosphere. This suggests that information on the magnetic
structure of PBSs would help to study the relation between PBSs and polar plumes. We have investigated the magnetic
structures of the 104 PBSs by applying a minimum variance analysis to Ulysses/Magnetometer data and by examining the
pitch-angle distribution of energetic electrons measured with Ulysses/SWOOPS. We found that PBSs have relatively more
tangential discontinuities rather than rotational from the minimum variance analysis and there is no difference between PBSs
observed in north and south polar regions. From the analysis of energetic electron data, most PBSs also show local bidirectional electron flux or isotropic pitch-angle distribution expected in plasmoids or, less often, the distribution expected in
association with current-sheet structures. This suggests the hypothesis that PBSs are generated due by network activity such
as magnetic reconnection at the base of polar plumes.
INTRODUCTION
OBSERVATIONS AND ANALYSIS
Pressure balance structures (PBSs) are intervals in which
changes in the plasma and magnetic pressures balance
one another while total pressure remains constant, and
they permeate the high-speed solar wind [1]. Many observations and theoretical studies have suggested that
PBSs are the interplanetary remnant of coronal plumes,
since they have similar properties and plumes are common features in the solar corona, in particular at solar
activity minimum [1, 2, 3, 4, 5, 6].
Given the inherent magnetic structure of plumes [7],
it is logical to suppose that information on the magnetic structure of PBSs might be helpful in investigating the relation between PBSs and plumes in more detail. Previous studies imply that the formation of plumes
is related to network activity such as magnetic reconnection, thus PBSs could have magnetic features associated with that activity (e.g., plasmoids or current
sheets). We investigated what kind of magnetic structure PBSs have by analyzing magnetic field data from
the Ulysses/Magnetometer in north and south polar regions with a minimum variance analysis (MVA) [8]. We
also examined whether bi-directional electron flux exists
near magnetic discontinuities in PBSs using energetic
electron data from Ulysses/SWOOPS measurements. We
report results and discuss the possible relation between
PBSs and polar plumes.
Ulysses Observations
We identify PBSs using 1-hour averaged plasma data
from Ulysses/SWOOPS [9], and 1-hour averaged magnetic data from Ulysses/Magnetometer [10] with the criteria of PBSs shown in [11]. We use 2-sec magnetic data
for the MVA analysis. These data were taken in the north
and south polar regions above 50◦ latitude. The details of
the observations are shown in Table 1.
For the heat flux carrying energetic electrons,
we use data from the electrostatic analyzer of
Ulysses/SWOOPS. The electrostatic analyzer has
20 energy channels which are centered at a given energy
from 1.69 to 814 eV with a width of about 12% of its
energy. In this paper, we focus on the pitch-angle data
in two energy channels at 84 and 116 eV, because the
electrons in this energy range are directly associated
with those of the million-degree solar corona. It should
be noted that the electron data is of lower temporal
resolution and precision than the magnetometer data so
we regard our results here as supporting the magnetic
field analysis rather than superseding or supplanting that
analysis.
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
255
TABLE 1.
North Pole
South Pole
Parameters of Ulysses high-latitude observations
Period
Latitude (deg)
Distance (AU)
May 11, 1995 – Jan 23, 1996
Jan 19, 1994 – Dec 21, 1994
50.0 – 80.5
-50.0 – -80.5
1.50 – 3.19
3.74 – 1.62
Analysis of magnetic data with MVA
e-
We have investigated the magnetic structures inside
of PBSs using 2-sec magnetic field data with an MVA,
a method to derive a normal vector, n, to the plane of
a magnetic directional discontinuity by computing the
minimum value of the standard deviation, σ MV A , of the
magnetic field component in that direction [8]
σ2MV A
1
=
N
N
X
2
[Bi · n − hBi · n] ,
e-
e-
e-
e-
e-
e-
e-
e-
e-
U
U
U
e-
e-
Current Sheet
(1)
θ
θ
θ
180
180
180
i=1
0
where Bi is i-th vectorPfield measurement and hBi is the
N
average vector, (1/N) i=1
Bi . Minimizing the value of
the standard deviation is equivalent to finding the smallest eigenvalue, λmin , of the covariant matrix although one
derives three eigenvalues (λmin , λint , λmax ) from Eq.(1),
where λmin < λint < λmax . In this paper, we use a cutoff ratio of the intermediate to the minimum eigenvalues
λint /λmin < 2 to define a discontinuity.
The parameters B · n/|B| and ∆|B|/|B| are used to determine whether each event is a rotational or tangential
discontinuity with the following event identification criteria [e.g., [12]]:
0
t
(b)
0
t
(c)
FIGURE 1. Relation between the magnetic structure and
electron pitch-angle distribution: Top panels show magnetic
structures which PBSs would have in three cases (a) plasmoid,
(b) current sheets, and (c) Alfvénic fluctuations. The Sun is
downward in this case. Bottom panels show the time variation
of electron pitch-angle distribution in PBSs which Ulysses
(“U”) would measure if the spacecraft passed through each
structure along the dash-dotted line.
parallel to B, respectively. Figure 1 shows three cases
of magnetic structures which PBSs might have and their
ideal influence on the electron pitch angle distributions;
(a) is the case of a plasmoid, (b) a current sheet, and
(c) a magnetic field bent by Alfvénic fluctuations. The
figure assumes that the Sun is downward and the magnetic field at polar regions is positive so that the electron
heat flux goes upward. In the first case, electron pitch angles show a bi-directional distribution at θ = 0◦ and 180◦
when Ulysses goes across the plasmoid. Or, the pitch
angles may be nearly isotropic, since Ulysses observes
the solar wind at far from the Sun (∼ 2 − 3AU) where
the scattering, for examples, due to the collisions among
circulating electrons or wave-particle interactions could
make the pitch-angle distribution isotropic rather than bidirectional. In the second case, the pitch angle θ changes
from 0◦ (180◦ ) to 180◦ (0◦ ), and then it goes back to 0◦
(180◦ ) as Ulysses crosses a pair of current sheets extending back to the Sun. In the last case, the pitch angle does
not change even though Ulysses observes magnetic polarity reversals. This is because the direction of electron
flux against local magnetic field never changes, i.e., the
pitch angle is a constant 0◦ or 180◦ .
Here, NDs are inconsistent with RDs or TDs, while EDs
could be either in principle.
Electron Pitch-Angle Distribution
We use energetic electron data to determine whether
PBSs contain magnetic structures like plasmoids or current sheets, because the electron pitch-angle distribution
depends on the configuration of the interplanetary magnetic field directly. The limitation with this is the low
temporal resolution of SWOOPS electron data (∼ 10 min
or more).
The pitch angle θ is defined as
B·v
v⊥
= tan−1 ,
|B||v|
vk
t
(a)
Rotational (RDs): B · n/|B| ≥ 0.4, ∆|B|/|B| < 0.2
Tangential (TDs): B · n/|B| < 0.4, ∆|B|/|B| ≥ 0.2
Either (EDs): B · n/|B| < 0.4, ∆|B|/|B| < 0.2
Neither (NDs): B · n/|B| ≥ 0.4, ∆|B|/|B| ≥ 0.2
θ = cos−1
e-
(2)
where B is the magnetic field vector, and v⊥ and vk are
components of solar wind velocity v perpendicular and
256
BDE
17
4
ISO
FIGURE 2. Scatter plot of the normal field component and
relative field magnitude across PBSs from Ulysses observations
in the north (•) and south (4) polar regions.
27
4
20
9
CS
AF
Unknown
10
13
FIGURE 3. Distribution of events identified as bi-directional
electron (BDE) flow, isotropic distribution (ISO), current sheet
(CS), Alfvénic fluctuations (AF), or unknown case in the case
of PBSs.
RESULTS
We have identified 51 PBSs from Ulysses north-polar observations and 53 PBSs from south observations, respectively. Figure 2 shows the results of the MVA analysis.
The figure is a scatter plot of the normal field component and relative field magnitude across discontinuities
in PBSs. The averaged thickness of the discontinuities
is 52.7 ± 44.0 secs. Tsurutani and Smith[13] report that
the width of discontinuities at 5 AU is 5 − 10 times as
thick as that at 1AU which is typically ∼ 2 − 3 sec, and
the radial distance range of Ulysses observations in our
analysis period is 1.50 − 3.74 AU, Therefore, the averaged thickness of discontinuities we obtained is reasonable. In Figure 2, there is no difference between PBSs
observed in the north and south polar regions. The percentages that we find for RDs:TDs:EDs:NDs in PBSs is
6%:46%:47%:1%. Thus, there is a preponderance of TDs
relative to RDs. This indicates that the structures of discontinuities contained in PBSs appear to be like current
sheets or plasmoids.
Next, we have investigated energetic electron pitchangle distributions in 104 PBSs. Figure 3 gives the overall distribution of the number of events identified as
each case. The figure shows that 17 events were bidirectional electron (BDE) flow, 27 were isotropic distribution (ISO), 20 were current sheet (CS), and 10 were
Alfvénic fluctuations (AF). We also have thirteen unknown events due to the limited time resolution of the
pitch-angle data; there is only one pitch angle data point
in 10 to 40 minutes. The data is therefore subject to aliasing and some of our examples may thus be misleading.
This is the main reason why we have tried to examine a
large number of examples. Crossing areas of BDE and
ISO, BDE and CS, and ISO and CS in Figure 3 indicate
that it is sometimes hard to identify the events in the areas to one of two cases and this is probably the reason.
However, since 81 events are identified as BDE or ISO or
CS, we conclude that PBSs usually contain the magnetic
structure of plasmoids or current sheets. The results support the idea that PBSs are the remnants of polar plumes
and are created by magnetic reconnection due to network
activity in the photosphere of the Sun.
DISCUSSION
We have examined magnetic structures of 104 PBS with
a minimum variance analysis using magnetic field data
from Ulysses/MAG in the south polar regions in 1994
and north in 1995. We find that PBSs have tangential
discontinuities, in preference to rotational. Further, we
have examined energetic electron pitch-angle data taken
by Ulysses/SWOOPS and found that most PBSs show
the bi-directional or isotropic flux of plasmoids or flux
associated with current sheets.
257
REFERENCES
Thieme et al. [2, 3] found from Helios observations
that the angular size of PBSs is ∼ 2 degree (≈ 30000km
as projected onto the Sun), which is the same angular
size as network cells. They therefore suggested that PBSs
are associated with open magnetic flux tubes originating
from the network. Their definition of PBSs was when
the gas and magnetic pressures anticorrelate while total pressure is almost constant. Reisenfeld et al. [6] suggested that the solar origin for PBSs appear to be polar
plumes, since they found that the plasma beta and the helium abundance inside of PBSs correlate with each other
and since solar wind abundances are fixed in the chromosphere and transition region. Therefore, our PBS criteria
require the enhancement of the plasma beta and the helium abundance in PBSs as well, thus they are more restrictive and the PBSs we identified are a subset of those
by Thieme et al. [2, 3]. Since polar plumes are the part
of the structures on the network in the coronal holes and
since most PBSs apparently show electron flux associated with plasmoids or current sheets, this supports the
suggestion that PBSs are the remnants of the network activity (e.g., magnetic reconnection) at the base of polar
plumes. However, since plumes have a very small filling
factor (∼ 1-3%) in coronal holes, the possibility remains
PBSs also originate elsewhere from the network activity.
In the model of Yamauchi et al. [11], which was based
on the analysis of Ulysses magnetic data and previous
studies, a local loop created by magnetic reconnection
in a polar plume is expected to expand outward to the
corona and form a pair of current sheets. The current
sheets may reconnect with the surrounding flux and create plasmoids. The fraction of BDE and ISO is greater
than CS as shown in Figure 3. This indicates that plasmoids are formed preferentially. Therefore our results
suggest that the expanding loop is subject to magnetic reconnection before forming a pair of current sheets and a
plasmoid is created, although only some loops expand to
interplanetary space without the reconnection and form
the current sheets. This process could occur anywhere in
the network but plumes are known to have a favorable
magnetic field for this to happen [14].
1. McComas, D. J., G. W. Hoogeveen, J. T. Gosling, J. L.
Phillips, M. Neugebauer, A. Balogh, and R. Forsyth, Ulysses
observations of pressure-balance structures in the polar solar
wind, Astron. and Astrophys., 316, 368-373, 1996.
2. Thieme, K. M., E. Marsch, and R. Schwenn, Relationship
between structures in the solar wind and their source regions
in the corona, in Proceedings Sixth International Solar Wind
Conference, edited by V. J. Pizzo, T. E. Holzer, and D. G.
Sime, pp.317-321, Tech. Note NCAR/TN-306 + Proc., Natl.
Center for Atoms. Res., Boulder, Colo., 1988.
3. Thieme,K. M., E. Marsch, and R. Schwenn, Special
structures in high speed streams as signatures of fine
structures in coronal holes, Ann. Geophysi., 8, 713-724,
1990.
4. Velli, M., S. R. Habbal, and R. Esser, Coronal plumes and
fine scale structure in high speed solar wind streams, Space
Sco. Rev., 70, 391, 1994.
5. Casalbuoni, S., L. Del Zanna, S. R. Habbal, and M. Velli,
Coronal plumes and the expansion of pressure-balanced
structures in the fast solar wind, J. Geophys. Res., 104,
9947-9961, 1999.
6. Reisenfeld, D. B., D. J. McComas, and J. T. Steinberg,
Evidence of a solar origin for pressure balance structures
in the high-latitude solar wind, Geophys. Res. Lett., 26,
1805-1808, 1999.
7. Suess, S. T., A.-H. Wang, I. Cnseru, G. Poletto, and S. T.
Wu, The geometric spreading of coronal plumes and coronal
holes, Solar Phys., 180, 231-246, 1998.
8. Sonnerup, B. U. Ö., and L. J. Cahill, Magnetopause
structure and attitude from Explorer 12 observations, J.
Geophys. Res., 72, 171-183, 1967.
9. Bame, S. J., D. J. McComas, B. L. Barraclough, J. L.
Phillips, K. J. Sofaly, J. C. Chavez, B. E. Goldstein, and
R. K. Sakurai, The Ulysses solar wind plasma experiment,
Astron. and Astrophys., Suppl. Ser., 92, 237-265, 1992.
10. Balogh, A., T. J. Beek, R. J. Forsyth, P. C. Hedgecock,
R. J. Marquedant, E. J. Smith, D. J. Southwood, and B. T.
Tsurutani, The magnetic field investigation on the Ulysses
mission: Instrumentation and preliminary scientific results,
Astron. and Astrophys., Suppl. Ser., 92, 221-236, 1992.
11. Yamauchi, Y., S. T. Suess, and T. Sakurai, Relation
between pressure balance structures and polar plumes from
Ulysses high latitude observations, Geophys. Res. Lett., in
press, 2002.
12. Neugebauer, M., D. R. Clay, B. E. Goldstein, B. T.
Tsurutani, and D. Zwickl, A reexamination of rotational and
tangential discontinuities in the solar wind, J. Geophys. Res.,
89, 5395-5408, 1984.
13. Tsurutani, B. T., and E. J. Smith, Interplanetary
discontinuities: temporal variations and the radial gradient
from 1 to 8.5 AU, J. Geophys. Res., 84, 2773-2787, 1979.
14. Wang, Y.-M., Network activity and the evaporative
formation of polar plumes, Astrophys J., 501, L145-L150,
1998.
ACKNOWLEDGMENTS
This work was performed while Y. Yamauchi held a National Research Council NASA/MSFC Research Associateship. We are grateful to A. Balogh and G. H. Jones
for preparation of 2-sec magnetometer data. We also
thank J. T. Gosling and R. Skoug for preparing electron pitch-angle data. STS received support from the
Ulysses/SWOOPS experiment.
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