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S. T. Suess, NSSTC Colloquium 10 December 2008. Current Sheets in the Solar Wind and How Slow Wind Forms. Steve Suess (NSSTC) with Yuan-Kuen Ko (NRL) Ruedi von Steiger (Intl. Space Sci. Inst., Bern) Ron Moore (NSSTC) Outline: Introduction - slow solar wind & current sheets

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slide1

S. T. Suess, NSSTC Colloquium

10 December 2008

Current Sheets in the Solar Wind and

How Slow Wind Forms

  • Steve Suess (NSSTC)
  • with
  • Yuan-Kuen Ko (NRL)
  • Ruedi von Steiger (Intl. Space Sci. Inst., Bern)
  • Ron Moore (NSSTC)
  • Outline:
  • Introduction - slow solar wind & current sheets
  • Revisiting an old study
  • New results and the toppling of a paradigm
slide2

S. T. Suess, NSSTC Colloquium

10 December 2008

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.

slide3

S. T. Suess, NSSTC Colloquium

10 December 2008

The paradigm (1981-2008):

Gosling, et al., JGR, v86(A7), 5438-5448, 1981.

slide4

S. T. Suess, NSSTC Colloquium

10 December 2008

The current sheet extends outward from here, above the cusp. It is identified in the solar wind by the magnetic field reversal.

slide5

S. T. Suess, NSSTC Colloquium

10 December 2008

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.

slide6

S. T. Suess, NSSTC Colloquium

10 December 2008

By studying solar wind near the CS during years near sunspot minimum, the contribution of Coronal Mass Ejections (CMEs) can be minimized.

slide7

S. T. Suess, NSSTC Colloquium

10 December 2008

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.

slide8

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.

S. T. Suess, NSSTC Colloquium

10 December 2008

slide9

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

S. T. Suess, NSSTC Colloquium

10 December 2008

slide10

S. T. Suess, NSSTC Colloquium

10 December 2008

All Ulysses data, for well-defined current sheets through-out the sunspot cycle for more than two cycles.

Ulysses data for well-defined current sheets only near sunspot minima.

slide11

S. T. Suess, NSSTC Colloquium

10 December 2008

  • What is happening here? The better data shown above (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.
slide12

S. T. Suess, NSSTC Colloquium

10 December 2008

If we look at He/H throughout the 2-month interval shown before, we see:

There are no narrow depletions.

Depletions tend to last at least a few hours, and up to a several days.

When there is a clean current sheet, it is usually at the edge of a depletion.

slide13

S. T. Suess, NSSTC Colloquium

10 December 2008

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.

slide14

S. T. Suess, NSSTC Colloquium

10 December 2008

  • What questions do these results present?:
  • Why is the current sheet 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?
    • from mixing with adjacent coronal hole flow?
    • or from the legs?
    • from the core?
  • What are the implications?

Each of these questions can be quantitatively analyzed.

slide15

S. T. Suess, NSSTC Colloquium

10 December 2008

First, it's important to understand why the current sheet 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 sheets are super-imposed in the superposed epoch analysis.

slide16

S. T. Suess, NSSTC Colloquium

10 December 2008

Summarizing:

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.

(and, note that the 1981 paradigm has been demolished)

slide17

S. T. Suess, NSSTC Colloquium

10 December 2008

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.

slide18

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.

S. T. Suess, NSSTC Colloquium

10 December 2008

slide19

S. T. Suess, NSSTC Colloquium

10 December 2008

At this point, sources 1) and 3) have been eliminated for the He/H depletions, leaving source 2) - the core.

slide20

S. T. Suess, NSSTC Colloquium

10 December 2008

  • The observations imply a source of the He/H depletions in the core.
  • When the cusp is sharply pointed, the flow is confined by the equivalent of a magnetic pinch - notoriously poorly.
  • In the core, β>>1. 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???
slide21

S. T. Suess, NSSTC Colloquium

10 December 2008

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 a few km/s.

slide22

S. T. Suess, NSSTC Colloquium

10 December 2008

What about those 5-10% of the cases in which the current lies towards the middle of the He/H depletion?

- These may be explained by cases in which the speed difference from one leg of the streamer to the other is negligible.

slide23

S. T. Suess, NSSTC Colloquium

10 December 2008

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.

slide24

S. T. Suess, NSSTC Colloquium

10 December 2008

  • Summary:
  • The paradigm derived from the Gosling et al. (1981) and Borinni et al (1981) studies 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.
slide25

S. T. Suess, NSSTC Colloquium

10 December 2008

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.