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Solar Wind Helium Abundance and the Minimum Speed of the Solar Wind

Solar Wind Helium Abundance and the Minimum Speed of the Solar Wind. Justin C. Kasper 1 and Michael L. Stevens 2 1 Harvard Smithsonian Center for Astrophysics 2 Boston University. 07 June 2007. Question. We have an abundance of information about thermal H, He++ in the ecliptic at 1 AU.

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Solar Wind Helium Abundance and the Minimum Speed of the Solar Wind

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  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 abundanceof 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

  5. Aellig et al 2001 The solar wind helium abundance He/H from 1960-2000 increases with solar activity • Ogilvie and Hirshberg (1974); Feldman et al (1978) • Why does He/H vary over the solar cycle?

  6. (Wang, 1993) 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

  7. The link between speed and He/H (2 of 3) Expansion evolves with activity Solar minimum corona Solar maximum corona

  8. The link between speed and He/H (3 of 3) Regulation of Coulomb drag • 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 Helium does not experience the Parker mechanism directly Instead Coulomb coupling to accelerated hydrogen is needed 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? Histogram of He/H in 300-320 km/s solar wind

  11. Six-Month Periodicity • Fit Carrington averages with sinusoid • Six month periodicity • Maxima in He/H as Earth leaves heliographic equatorial plane • Consider: • Significance • Offset A0 • Amplitude A1 Carrington rotation averages of He/H in two narrow speed windows during solar minimum 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 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 275-500 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 275-500 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? 300-320 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

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