Quiescent Current Sheets in the Solar Wind

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Quiescent Current Sheets in the Solar Wind and
the Origins of Slow Wind
Steve Suess (NSSTC) with Yuan-Kuen Ko
(NRL) Ruedi von Steiger (Intl. Space Sci. Inst.,
Bern) Ron Moore (NSSTC) A superposed epoch
study of composition near current sheets using
Ulysses (SWOOPS, MAG, SWICS) and ACE (SWEPAM,
MAG, SWICS) data.
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We have revisited the analysis of solar wind
composition near the current sheet using Ulysses
and ACE data. Today, I'm just going to show
Ulysses data.
Borrini et al. 1981, J. Geophys. Res., 86(A6),
4565-4573.
The approach is to do a superposed epoch analysis
of solar wind composition for several days on
either side of current sheet crossings.
Gosling, et al., JGR, v86(A7), 5438-5448, 1981.
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Look only at solar wind near current sheets
around sunspot minimum to minimize the
contribution of coronal mass ejections. We have
data from two minima with Ulysses (and one with
ACE).
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• The results for He/H The better data set (more
  current sheets, better instrument, two solar
  cycles, few coronal mass ejections) gives
• a narrower, deeper sharp minimum
• and, also,
• a broad, less deep reduction.
• relative to the earlier results.
• Why the difference?

The 1981 study result for He/H
Borrini et al., 1981 - IMP data
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• If we look at He/H over a 2-month interval (in
  which there were no wind speeds above 650 km/s),
  we see
• There are no narrow depletions.
• Depletions tend to last at least a few hours, and
  up to a several days.
• Current sheets usually lie at the edges of a
  depletions.
• There seems to be (at least) two slow solar wind
  states distinguished by high or low He/H.

This is typical for the He/H depletions
throughout the Ulysses and ACE data sets.
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• What questions do these results present?
• What is the reason for the narrow minimum in the
  superposed epoch analysis? -gt I will show it is
  an illusion due to the current sheets usually
  being at the edge of depletions.
• If the current sheet is usually at the edge, is
  it just inside the edge or is it just outside the
  edge?
• Where do the He/H depletions come from? (or,
  possibly, where do the He/H enhancements come
  from?) - 1. from mixing with adjacent coronal
  hole flow? - 2. or from the core? -3. or from the
  legs?

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First, why the current sheets must lie just
inside the edge of most low He/H plasma parcels.
Below is a cartoon showing what happens in a
superposed epoch analysis where the current sheet
is (1) in the middle of the He/H depletion, (2)
just outside the edge of the depletion, and (3)
just inside the depletion.

• In this exercise, just two current sheet
  intervals are superposed.
• The width and depth of the narrow minimum and the
  relative amplitudes of the broad and narrow
  minima imply
• Most He/H depletions must have a current sheet
  inside the edge (otherwise the narrow minimum
  would be smaller relative to the broad minimum
  and the background).
• The current sheet must be at the edge to within
  the resolution of our analysis (1-3 hours).
• The depletions must typically last a day or more.
  
• Only a few (5-10) of the depletions can lack a
  current sheet.
• Only a few (5-10) of the depletions can have
  the current sheet in the middle.

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Second, where do the He/H depletions come from?
(or, possibly, where do the He/H enhancements
come from?) - 1. from mixing with adjacent
coronal hole flow? - 2. or from the legs? - 3. or
from the core?

• What do we find regarding the source?
• By looking at Fe/O (FIP effect) and He/H, we find
  that there is no detectable admixture of coronal
  hole plasma. Source '1' is negligible.
• Variations in O/H and He/H are strongly
  correlated. Hence, He/H depletions come from
  where O/H is depleted in streamers. This would be
  the streamer core - '2'. Hence, the legs - '3'-
  do not supply the He/H depleted plasma.

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Our Hypothesis - to commonly produce current
sheets at the edge of He/H depletions It has
always been assumed that the flow speed out of
the legs on either side of streamers is the
same. Even a speed difference of 5 km/s, in an
ambient flowing at 100 km/s, would shear the
parcels, separating the two halves well inside
1 AU. In fact, the flow speed difference
across current sheets in the solar wind is
commonly observed to be a few km/s.
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What about those 5-10 of the cases in which the
current lies towards the middle of the He/H
depletion? - These are explained by those cases
in which the speed difference from one leg of the
streamer to the other is negligible.
What about those 5-10 of the cases in which
there is no current sheet associated with the
He/H depletion? - Y.-M. Wang has noted the
existence of 'coronal pseudostreamers' which
overlie twin loop arcades (left, below). These
streamers have no current sheet extending upward
from the cusp. Otherwise, the physics of the
confinement is identical to the classical
streamer. The streamers constitute about the
right percentage of all streamers to explain the
observations.
Wang, Y.-M., N. R. Sheeley, Jr., N. B. Rich
2007, Coronal Pseudostreamers, ApJ, 658,
1340-1348.
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• Summary
• The paradigm proposed by Gosling et al. (1981)
  and Borinni et al (1981) is flawed because the
  narrow depletion in He/H seen at the current
  sheet is solely a consequence of the superposed
  epoch analysis.
• Instead, there are transient He/H depletion
  distributed throughout slow wind, the majority of
  which have current sheets just inside one edge.
• The He/H depletions are a consequence of
  transient releases of plasma from the cores of
  streamers that are usually sheared by flow speed
  differences between the legs of the streamers.
• The remaining slow wind comes from quasi-steady
  and transient flow from the legs of the
  streamers.
• There is no mixing with adjacent coronal hole
  plasma.

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• This leaves behind four very nice problems yet to
  study, two theoretical and two empirical.
• Theoretical problems
• The boundary layer flow at the current sheet in
  the sheared plasma parcel released from the core
  of the streamer.
• The instability (esp. the energetics) of the
  plasma release from the streamer.
• Empirical problems
• Our study was never designed to specifically
  study the He/H depletions themselves. It would be
  easy to design such a study.
• The thin boundary layer at the edge of the He/H
  depletions has been indirectly studied, but a
  focused study would be nice, especially if using
  the results of theoretical problem 1 above.

Winterhalter, D., E. J. Smith, M. E. Burton, N.
Murphy, D. J. McComas 1994, The heliospheric
plasma sheet, J. Geophys. Res., 99(A4),
6667-6680.
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The current sheet extends outward from here,
above the cusp. It is identified in the solar
wind by the magnetic field reversal.
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How the heliospheric current sheet looks near
solar sunspot minimum. Remember, it is the
surface separating regions of opposite magnetic
polarity in the solar wind. On the left is an
artist's rendition (Wilcox, 1980) of how the
heliospheric current sheet looks for a small tilt
of the magnetic dipole, out to 5 AU. On the
right is a computed heliospheric current sheet
out to 6 AU, for a 22.5 degree tilt, in a 400
km/s wind.
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This eruption creates a temporarily 'active
current sheet' where the reconnection heats the
plasma and deposits anomalously high ionization
state matter (e.g. Fe16).
Why we want to focus on 'quiescent current
sheets' and avoid CMEs.
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Slow wind, fast wind, and the heliospheric
current sheet
The sunspot minimum global view of the solar
wind and the corona. Ulysses 6 yr orbit 80o
inclination 1.34 AU perihelion 5.4 AU
aphelion Fast latitude scan takes 1yr.
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The previous study Superposed epoch results for
He at 74 isolated, well defined sector
boundaries in 1971-1978. This result was highly
variable if looking at individual years.
A reduction from 4.5 to 3.5 around (/- 1
day) the HCS, in this IMP 6, 7, and 8 data set.
Note The solar rotation period is 27 days. /-
20 days is more than one rotation.
Borrini et al., 1981 - IMP data
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Two months of solar wind data at Ulysses showing
the spiral angle (top), magnetic field amplitude
and N-S component (middle), and flow speed and
density (bottom) just before sunspot minimum in
2004. The field reversals are easy to identify.
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• What the He/H data shows
• The He/H depletions occur randomly throughout
  slow wind.
• He/H depletions are a few hours to a few days in
  width.
• There seems to be no narrow depletions in He/H
  centered on the current sheet.
• He/H depletions occur both with and without a
  current sheet. But, when there is a current
  sheet, it is generally at the edge of a
  depletion.
• High He/H intervals are also distributed
  randomly throughout slow wind.
• This implies (at least) two slow solar wind
  states, distinguished by He/H.

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At this point, sources 1) and 3) have been
eliminated for the He/H depletions, leaving
source 2). Now, what about the location of the
current sheets? And, what about the relationship
between the depletions and the current sheet?
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• To summarize
• Most He/H depletions must have a current sheet
  inside the edge (otherwise the narrow minimum
  would be smaller relative to the broad minimum
  and the background).
• The current sheet must be at the edge to within
  the resolution of our analysis (1-3 hours).
• The depletions must typically last a day or more.
  
• Only a few (5-10) of the depletions can lack a
  current sheet.
• Only a few (5-10) of the depletions can have
  the current sheet in the middle.

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Mixing (1) and the 'FIP Effect' Fast solar wind
has nearly photospheric abundances. He/H is
4.75 in fast wind. Slow solar wind is, on
average, depleted in high first ionization
potential elements (e.g., O). He/H is, on
average, much reduced in slow wind. The ratio of
densities Fe/O gives a measure of the FIP effect.
Although there are lots of fluctuations, there
is no evidence of mixing of fast wind into
slow. Hence, the high He/H plasma in slow wind
does not come from mixing with coronal hole
plasma.
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Do the enhancements/depletions come, then, from
the core, or the legs? SOHO/UVCS spectroscopic
measurements of abundances in the core of
streamers have shown that they are often (but not
always) depleted in O/H. On the right, it is
obvious that O/H and He/H depletions are strongly
correlated, while the absolute densities of O, H,
and He are not correlated with He/H. This shows
that He/H depletions arise in the same place as
O/H depletions. The strong implication is that
this is the core, where gravitational settling
takes place. It also implies that the occasional
UVCS observations failing to show O/H depletions
in streamer cores are a seeing problem.
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• The observations imply a source of the He/H
  depletions in the core.
• When the cusp is sharply pointed, the flow is
  confined be the equivalent of a magnetic pinch -
  notoriously poorly.

• In the core, ßgtgt1. So, an escaping parcel of
  plasma will carry a magnetic loop. This does not
  result in a current sheet at the edge of the
  parcel.
• What might cause the current sheet to lie, so
  often, at the edge of the parcel???

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What about those 5-10 of the cases in which
there is no current sheet associated with the
He/H depletion? Y.-M. Wang has noted the
existence of 'coronal pseudostreamers' which
overlie twin loop arcades (left, below). These
streamers have no current sheet extending upward
from the cusp. Otherwise, the physics of the
confinement is identical to the classical
streamer. The streamers constitute about the
right percentage of all streamers to explain the
observations.
After fig. 1 from Moore, R. N., A. Sterling
2007, The coronal-dimming footprint of a
streamer-puff coronal mass ejection confirmation
of the magnetic-arch-blowout scenario, ApJ, 661,
543-550.
Wang, Y.-M., N. R. Sheeley, Jr., N. B. Rich
2007, Coronal Pseudostreamers, ApJ, 658,
1340-1348.