Heliospheric Plasma and Current Sheet Structure
N. U. Crooker
Center for Space Physics, Boston University, Boston, MA 02215, USA
Abstract. Recent analyses using suprathermal electrons as sensors of true magnetic polarity indicate that the steady-state
concepts of the heliospheric current sheet and its encasing plasma sheet often break down at solar wind time scales of less
than a day owing to transient and turbulent effects. These create magnetic configurations with localized current and
plasma sheets and sometimes cause the current to split off from the surface separating fields of true opposite polarity.
first realized by Kahler and Lin [4, 5] using higherenergy (E > 2 keV) electrons. They found the
following HCS dichotomy: Sometimes in situ field
reversals occur without true polarity reversals, and
sometimes true polarity reversals occur without in situ
field reversals. While the former are easily understood
in terms of fields locally turned back on themselves,
creating localized current sheets, the meaning of the
latter has remained elusive. This paper reports on
initial progress in understanding both aspects of the
dichotomy in terms of fields turned back on themselves
in mesoscale transient outflows. The reported insights
derive from an attempt to identify the HPS at true
polarity reversals, as described in the next section.
INTRODUCTION
The heliospheric community generally agrees on
the following definitions for the topics of this paper.
The heliospheric current sheet (HCS) is a surface that
separates magnetic field lines of opposite polarity. It
forms as the solar wind draws the Sun's dipolar field
lines into space and constitutes the heliomagnetic
equatorial plane. The heliospheric plasma sheet (HPS)
is a high-density layer that ensheathes the HCS.
While these definitions certainly hold at the global
scale, they describe steady state conditions and often do
not apply at the mesoscale on downward, where
transient outflows and turbulence create complications
[1].
Mesoscale here means of the scale of
interplanetary coronal mass ejections (ICMEs), which
cover solar wind time scales of about 1 day. Moreover,
regarding the heliospheric plasma sheet, there is as yet
no consensus on its diagnostic parameters and scale
size. This paper reviews the problems of understanding
the mesoscale and smaller-scale structure of the HCS
and HPS and reports on progress resulting from use of
suprathermal electrons as a tool for sensing true
magnetic polarity.
HELIOSPHERIC PLASMA SHEET
Several parameters with variations over a range of
scale sizes have been offered as diagnostic signatures
of the heliospheric plasma sheet. These are reviewed
below, after which follows a discussion of the
variability of the smallest-scale signatures and a
summary on HPS usage.
Borrini et al. [6] and Gosling et al. [7] first identified the heliospheric plasma sheet, though not by that
name, using superposed epoch analysis. They interpreted the region of high density surrounding the HCS
as the heliospheric extension of the coronal streamer
belt. The sharp density (n) peak derived in the analysis
coincided with a broad, days-long proton temperature
(T) depression. Together these parameters yield low
entropy, since entropy is proportional to T/nγ-1, where γ
is a constant (the ratio of specific heats). Thus low
entropy can be used as a diagnostic parameter.
Burlaga et al. [8] pointed out that the increase in
entropy at stream interfaces at the leading edge of highspeed streams is followed by a corresponding decrease
on the trailing edges and, citing a number of forerunner
SUPRATHERMAL ELECTRONS
AS SENSORS OF TRUE POLARITY
Because suprathermal (E > 80 eV) electrons
continually carry heat flux away from the Sun, their
direction relative to the magnetic field gives the true
polarity of any given field line as it leaves the Sun,
even if the field line locally turns back on itself in the
heliosphere [2, 3]. Electrons stream parallel to the field
on lines with away polarity and antiparallel to the field
on lines with toward polarity.
The usefulness of electrons as polarity sensors was
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
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93
as expected at the HCS. The first of the three reversals
coincides with a sharp spike in β in the bottom panel,
an excellent example of the kind of HPS identified by
Winterhalter et al. [13]. What complicates the pattern
is the lack of comparable spikes at the remaining
reversals and the occurrence of comparable β spikes
elsewhere. The second reversal coincides with a
broader, less intense peak in β and the third with
essentially no β signature. Some of the β spikes in the
first half of the plot align with φB reversals that lack
electron signatures, indicating crossings of localized
current sheets, as mentioned at the end of the previous
section.
studies, called the resulting low-entropy region
between streams the "heliospheric plasma sheet."
Burton et al. [9] found the same entropy pattern and
showed that it matched the composition pattern that
distinguishes what was originally slow wind from what
was originally fast wind [10]. They thus confirmed that
although entropy is not a streamline constant, as it
would be if the solar wind expanded adiabatically, its
time variations still serve as markers of plasma origins.
In this context, if low entropy is treated as the
diagnostic HPS parameter, the HPS is a days-long
structure and becomes synonymous with what was
originally slow wind.
Sometimes the scale size of a low-entropy HPS is
shorter, on the order of several hours to a day. This
was particularly true for the solar maximum period
analyzed by Neugebauer et al. [11]. In addition, they
found low-entropy HPSs that lacked HCSs. These
occurred where flows from different solar sources
converged, as do flows from the two polar coronal
holes at the HCS, but in these cases the sources had the
same magnetic polarity. From this point of view the
HPS is a source boundary rather than a sheath for a
current sheet.
Based upon high-density signatures surrounding
the HCS in the inner heliosphere and their similarity to
the structure of coronal streamers obtained from radio
occultation measurements, Bavassano et al. [12]
described the HPS as an hours-long feature immersed
in a days-long halo. This view incorporates the largescale slow wind structure discussed above but focuses
on the shorter scale for the HPS itself.
Winterhalter et al. [13] drew the attention of the
community to the smallest-scale HPS in a paper
entitled "The Heliospheric Plasma Sheet." They
searched for HCS cases with the sharpest magnetic
field reversals and found them embedded in minuteslong spikes of β, their diagnostic parameter, where β is
the ratio of gas pressure to magnetic pressure. Followon studies with suprathermal electron data, however,
have shown that high-β HPSs can have transient
characteristics [14] and can occur at localized current
sheets as well as at the HCS [2, 15]. In this context the
general term "plasma sheets" seems more appropriate
than "HPSs," especially since high-β plasma sheets
seem to be more a property of current sheets in general
than a property specific to the HCS.
FIGURE 1. Time variations of magnetic longitude φB and β,
the ratio of total gas (electron plus ion) pressure to magnetic
pressure, calculated with data from the MFI and SWE instruments on the Wind spacecraft, courtesy of R. P. Lepping and
K. Ogilvie. The dashed lines mark true polarity reversals
apparent in electron data obtained from the 3DP instrument,
courtesy of R. Lin.
Some survey studies have begun to quantify the
variability in high-β plasma sheets apparent in Figure
1. This section reports on surveys that test current
sheets for plasma sheet occurrence, true polarity
reversals for plasma sheet occurrence, and plasma
sheets for current sheet and polarity reversal
occurrence.
A. D. Coleman [15], under the guidance of A. J.
Lazarus, searched for current sheets in magnetic field
data across which the field reversed polarity,
independent of whether the current sheets were the
HCS, with a true polarity reversal, or just localized
current sheets. He found that 74% were encased in
some type of high-β plasma sheet. Coupled with the
remarkable finding of Szabo et al. [16], discussed in the
next section, that most field reversals do not coincide
with true polarity reversals, one can conclude that highβ plasma sheets are common properties of current
sheets, most of which, however, are not the HCS.
Another survey, currently underway, is assessing
the probability of encountering a high-β plasma sheet at
the HCS in 35 successive crossings of the global sector
boundary late in the declining phase of the solar cycle,
when the sector structure was well-ordered. Prelimi-
High-β Plasma Sheet Variability
Figure 1 illustrates the variability encountered
when searching for the HPS as defined by high beta.
Three true polarity reversals identified in suprathermal
electron data are marked with dashed vertical lines.
They align with reversals in the longitude angle φB of
the magnetic field in the top panel, from the toward
sector to the away sector (hatched area) and back again,
94
appears to be the middle of a sector, are not isolated
islands of opposite polarity. Half result from fields
turned back on themselves, and the remaining half are
associated with interplanetary coronal mass ejections
(ICMEs). Further, Kahler et al. [19] found that fields
turned back on themselves constitute about 7% of data
collected in the ecliptic plane at 1 AU and that the
orientation of these fields borders on the ortho-Parkerspiral direction.
Perhaps the most surprising result to come from
studies using suprathermal electrons is the finding of
Szabo et al. [16], mentioned in the previous section,
that not only some but most field reversals lack true
polarity reversals, reflecting the first aspect of the HCS
dichotomy discussed in the second section. This
finding strongly impacts conclusions drawn from
earlier studies of the fine structure of the HCS, which
treated every field reversal as an HCS crossing [e.g.,
20, 21]. One might now conclude that the HCS is
much simpler than previously thought. This conclusion
would be premature, however, because it fails to take
into account the second aspect of the HCS dichotomy,
that not all true polarity reversals constitute HCS
crossings. Sometimes the boundary between fields of
true opposite polarity separates from the HCS, leaving
patches of it current-free. As discussed below, what
turns out to be simple is not the HCS but the polarity
reversal boundary.
nary results indicate that even under these optimal
conditions the steady-state concept of a high-β-plasmasheet-encased HCS applies, at most, about half the
time.
A complementary survey covering a similar time
period is assessing the probability that a pronounced
high-β structure (hourly average of proton β > 10) is an
HPS. Preliminary results indicate that most contain
field reversals without true polarity reversals,
supporting the idea that high-β plasma sheets are more
closely associated with localized current sheets than
with the HCS.
High-β plasma sheets fall into the larger category
of pressure balance structures, which cover a wide
range of scale sizes [17]. In the range of minutes up to
a few hours, a preliminary log-log histogram of the
durations of events with proton β > 1 shows a straightline fall-off, consistent with turbulent cascades from
convected plasma structures of solar origin [18]. Thus
the high degree of variability in high-β plasma sheets is
consistent with a strong presence of transient and
turbulent structures.
What is the HPS?
The heliospheric community has yet to settle on
what is meant by "heliospheric plasma sheet." As
reviewed above, the term has been applied to structures
with solar wind time scales ranging from minutes to
days and with diagnostic parameters ranging from high
density to low entropy to high β. Its meaning is
commonly understood only in the most general sense as
a sheath for the HCS, and at short time scales it often is
not even that, either because it is missing or because
the field reversal embedded in it is often not a true
polarity reversal. Moreover, as a source boundary, it
need not even contain a current sheet. Logically, since
"HPS" is a steady-state concept, the term should apply
to something that is always there, as proposed for the
global-scale low-entropy regions between high-speed
streams. But usage, not logic, prevails, and the term
"slow wind" has already gained wide usage for that
plasma regime. Current HPS usage seems to favor the
small-scale high-β structures, but this may change in
view of the increasing evidence of their variability and
association with localized current sheets. In the long
run, owing to these ambiguities, the term "HPS" may
disappear from our vocabulary.
Mismatched Field and Polarity Reversals
In the search for high-β HPSs at successive sector
boundary crossings reported in the previous section,
one reason so few were found is that some cases were
dismissed for lack of field reversals at the true polarity
reversals, reflecting the second aspect of the HCS
dichotomy. Figure 2 gives two examples.
As in Figure 1, the dashed vertical lines in Figure 2
mark the true polarity reversals identified in the
electron data. In contrast to Figure 1, however, they
are not aligned with clear field reversals. In the top
panel, there is no change at all in φB at the polarity
reversal. The field stays in the away sector while the
electron data give an incontrovertible signature of
toward polarity. After about 8 hours, φB dips into the
toward sector twice but remains primarily in the away
sector until a total of 21 hours after the true polarity
reversal. At that point φB clearly returns to the toward
sector and stays there. Since the electron data continue
to indicate toward polarity after that point, the two data
sets are back in agreement after a 21-hour mismatch.
The second panel of Figure 2 shows a similar
pattern, this time with a 15-hour mismatch. In this case
the field actually reverses at the true polarity reversal
but then quickly returns to the away sector, although
HELIOSPHERIC CURRENT SHEET
Recent progress in understanding the structure of
the heliospheric current sheet has been made primarily
through the use of suprathermal electrons as polarity
sensors. For example, Kahler et al. [3] showed that
intrasector field reversals, those occurring in what
95
FIGURE 2. Same as top panel of Figure 1. Ticks mark hours, and double-headed arrows mark intervals between true polarity
reversals and in situ field reversals.
just barely. It hovers for hours around the orthoParker-spiral direction, as discussed above in general
for fields that turn back on themselves [19], before
moving back to the Parker spiral direction and then
clearly reversing 15 hours after the true polarity
reversal. Strictly speaking this case does not have a
polarity reversal without a field reversal; but, in the
larger context, the briefness of the return to the toward
sector, the clear field reversal much later, and the
similarity not only to the case in the top panel but to
several other cases during the first half of 1995 argue
for its inclusion as a case of mismatched field and
polarity reversals. A complete analysis of these events
will be published elsewhere.
separates from the polarity reversal boundary and passes
around the coil.
What kind of structure can account for mismatched
field and true polarity reversals? It must have fields
turned back themselves and thus be transient. Figure 3
shows an example. A field line with away polarity
coils back on itself so that the top portion of the coil
locally points toward the Sun. It is thus parallel to the
adjacent field lines with toward polarity so that no
current I is required in that section of the polarity
reversal boundary. On the other side of the coiled field
line lie uncoiled field lines with the same away polarity
but opposite direction to the top of the coil. Hence I
must flow there to accommodate the local field
reversal. The net effect is that the HCS is locally
displaced from the polarity reversal boundary.
Figure 3 thus demonstrates that not all field
reversals without true polarity reversals are necessarily
signatures of localized current sheets. Those that pair
with polarity reversals that lack field reversals, a
pairing that is required for current continuity, must be
treated as the HCS. These must then be subtracted
from the larger pool of field reversals without true
polarity reversals before one can address the question
of the degree to which the HCS is simpler than
previously thought.
For clarity Figure 3 depicts all fields aligned with
the Parker spiral, more like the fields across the true
polarity reversal in the top panel of Figure 2 than in the
bottom panel. The hovering about the ortho-Parkerspiral direction in the bottom panel, characteristic of
fields turned back on themselves, might be a signature
of interchange reconnection in the Fisk model of
footpoint circulation [22, N. Schwadron, private
communication], a possibility that will be considered in
a future publication.
The configuration in Figure 3 is only one of
several that can explain mismatched field and polarity
reversals. For example, the coiled field line could be
extended into a series of singly coiled field lines or into
a flux rope, in which case the signature would result
FIGURE 3. Magnetic configuration in which the reversal
between field lines with toward (black) and away (gray)
polarity is displaced from the local magnetic field reversal
owing to a field line with away polarity that coils back on
itself. The current I (gray band) in the HCS consequently
96
from a skimming trajectory. A trajectory through the
heart of a flux rope inclined to the ecliptic plane and
attached to the Sun at only one end will also give a
signature of mismatched reversals [23]. All the field
lines in a flux rope have a single polarity so that any
true polarity change will be at its boundary. The field
reversal, on the other hand, takes place gradually as a
smooth rotation across the rope. In the case of a flux
rope, the current of the HCS is distributed throughout
the structure. Finally, when considered in three
dimensions, mismatched reversals can result from
interchange reconnection in the legs of ICMEs [24].
2.
3.
4.
5.
6.
7.
Heliospheric Polarity Reversal Sheet
8.
While at the global scale the HCS can be
considered as a boundary between field lines of
opposite polarity, at the mesoscale this is not always
true. Sometimes the current separates from that
boundary, and sometimes the current extends well
beyond it. The current flows where it must in order to
satisfy the given magnetic field configuration, which
can be complicated in transient structures. Discussing
the noncoincidence of the HCS and the boundary
across which the polarity of the magnetic field reverses
is facilitated with the introduction of a new term, the
"heliospheric polarity reversal sheet" or HPRS. Unlike
the HCS, the HPRS is a true surface or sheet. It
separates field lines which have either one polarity or
the other, depending upon their direction as they leave
the Sun. It is the sector boundary in the truest sense of
that term, but unfortunately "sector boundary" is often
used in a more general way. The familiar global view
of the HCS as a twirling ballerina skirt [25] applies to
the HPRS on all scales.
9.
10.
11.
12.
13.
14.
15.
16.
17.
ACKNOWLEDGMENTS
18.
19.
Preliminary results have been reported from
ongoing research projects with collaborators S. W.
Kahler, D. E. Larsen, and L. Kepko and students C.
Huang and S. Lamassa. This research was supported
by the National Science Foundation under grant ATM011970 and by the National Aeronautics and Space
Administration under grant NAG5-10856.
20.
21.
22.
23.
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