1. Solar Wind Helium Abundance and the Minimum Speed of the Solar Wind
Justin C. Kasper1 and Michael L. Stevens2
1Harvard Smithsonian Center for Astrophysics
2Boston University
07 June 2007

2. Question
We have an abundance of information about thermal H, He++ in the ecliptic at 1 AU.
What can it teach us about the slow solar wind mechanism?

3. Outline
“Solar wind helium abundance as a function of speed and heliographic latitude: variation through a solar cycle”, Ap.J. 2007
Justin C. Kasper, Michael L. Stevens, Alan J. Lazarus (MIT)
John T. Steinberg, Los Alamos National Laboratory
Keith W. Ogilvie (NASA)
H and He measurements with the Faraday cup instruments on Wind
Review variation of He/H within heliosphere
Previous observations in the solar wind
This study: higher time and speed resolution over two solar cycles
New properties of He/H variation during solar minimum

4. Relative abundance of helium A brief review of He/H variation
He/H (ratio by number density) within the solar core is 10%
Matching observed radius and luminosity (Guzik & Cox, 1993)
Solar Interior ~ 8% within convection zone
Helium becomes singly ionized then neutral within ~ 0.02 Rs (Basu & Antia 1995).
Solar corona ~ 4.5%
Few direct measurements, but He/H ~ 4.5% (Lamming, 2004)
Transport of He from convection zone through transition region to corona is poorly understood
model H, H+, He, He+ and He+2 consistently (Hansteen et al 1994))
simulations must boost drag x10 to produce coronal He/H
Solar Wind 0-30%
He/H is the most variable quantity in the solar windHe/H is no more than 4.5% of quiet solar wind
He/H can rise to 30% in ejecta (50% of mass flux!)

5. The solar wind helium abundance
The solar wind helium abundance He/H from increases with solar activity
Aellig et al 2001
Ogilvie and Hirshberg (1974); Feldman et al (1978)
Why does He/H vary over the solar cycle?

6. The link between speed and He/H
The link between speed and He/H (1 of 3) Solar wind speed and expansion
Parker model of solar wind: supersonic acceleration in DeLaval nozzle
Modification: field expansion controls the nozzle
(Wang, 1993)

7. The link between speed and He/H
The link between speed and He/H (2 of 3) Expansion evolves with activity
Solar minimum corona
Solar maximum corona
As summarized in Linker, Cranmer talks

8. The link between speed and He/H (3 of 3) Regulation of Coulomb drag
Helium does not experience the Parker mechanism directly
Instead Coulomb coupling to accelerated hydrogen is needed
Early explanation for dependence on level of solar activity:
Solar wind near magnetic reversal, the heliospheric current sheet (HCS), has low He/H
Slow solar wind near HCS, and HCS more often in equatorial plane during solar minimum
Fast wind and CMEs, seen more at maximum, have high He/H
So how does He/H vary with speed?

9. Wind Faraday Cup Observations Challenge existing model
1995-present
250-day averages based on Aellig et al [2001]
He/H linear with speed during solar minimum
In agreement with expansion – drag argument
Ordering vanishes during solar maximum, why?
Lower overall He/H in present minimum
Updated version of Aellig et al [2001] Figure 2

10. Why does He/H modulation vanish?
Questions
Why does He/H modulation vanish?
What accounts for the width of the He/H distribution?
Improve resolution
Finer speed windows (25)
Carrington rotations
Transients
Remove shocks and coronal mass ejections (CMEs)
Histogram of He/H in
km/s solar wind

11. Six-Month Periodicity
Carrington rotation averages of He/H in two narrow speed windows during solar minimum
Fit Carrington averages with sinusoid
Six month periodicity
Maxima in He/H as Earth leaves heliographic equatorial plane
Consider:
Significance
Offset A0
Amplitude A1
He/H
latitude

12. Comparing latitude variation for the two cycles

13. Carrington rotation He/H averages
Lomb-Scargle spectral analysis
428 km/s
362 km/s

14. Six month periodicity Due to Earth’s motion relative to the equator
Radial field component at 2.5 Rsun surface
-Cranmer mentions footpoint latitudes map all over the place at solar max. Active regions track where it comes from at solar max
What changes with heliographic latitude during solar minimum: distance from the HCS, typical loop size and duration
Data provided by Wilcox Solar Observatory

15. Explanation for latitudinal gradient Long lived magnetic loops
Previous models have two flaws
Magnetic topology is simple – expanding field
Structures are stationary in time
New time dependent models introduce gravitational settling
Endev et al. (2005)
Helium settles to bottom of loops
Decreased He/H at loop top
TRACE

16. Dependence of A0 on speed
A0 is a linear function of V for speeds between km/s
This is not predicted analytically or numerically
Extrapolates to a “vanishing speed” of 265 km/s

17. Dependence of A0 on speed
A0 is a linear function of V for speeds between km/s
This is not predicted analytically or numerically
Extrapolates to a “vanishing speed” of 265 km/s
Histogram of distribution of solar wind speed at 1 AU over mission
Why does this correspond to lowest observed solar wind speeds?

18. Hypothesis: Helium in corona impedes solar wind escape and acceleration
Escaping hydrogen
and helium fluxes
Fraction of
helium in corona
Hansteen et al., ApJ, 1994

19. Minimum speed in present solar min

20. Two sources of slow solar wind?
km/s solar wind

21. Summary
Better understanding of He/H variation, but also identified physical tests for coronal and wind models
Findings:
Slow solar wind He/H is a strong function of solar cycle
6-month period variation of He/H- latitude and neutral line
Previous Minimum: periodicity present, small NS angle (~6 mo between crossings), thin source region
Present Minimum: periodicity absent, large NS angle, thick and structured source region
De-trended He/H is an extremely linear function of speed
Implications
More physics needed than just expansion factor to explain He/H variation
Two sources of slow wind (near field reversal and from active regions?)
He/H correlation with sunspot number 0.94 at low speeds
Helium in the corona places a “stranglehold” on solar wind