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

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?

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

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!)

The solar wind helium abundance The solar wind helium abundance He/H from 1960-2000 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?

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)

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

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?

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

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 300-320 km/s solar wind

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

Comparing latitude variation for the two cycles

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

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

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

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

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?

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

Minimum speed in present solar min

Two sources of slow solar wind? 300-320 km/s solar wind

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