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Scale-dependence of magnetic helicity in the solar wind

Scale-dependence of magnetic helicity in the solar wind.  a ssume isotropy to get helicity spectrum. Axel Brandenburg (Nordita/Stockholm) Kandaswamy Subramanian (IUCAA, Pune) Andre Balogh (ISSI and Imperial) Melvyn Goldstein (NASA Goddard, Greenbelt). Kemel+12. K äpylä +12. Warnecke+11.

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Scale-dependence of magnetic helicity in the solar wind

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  1. Scale-dependence of magnetic helicity in the solar wind  assume isotropy to get helicity spectrum Axel Brandenburg (Nordita/Stockholm) Kandaswamy Subramanian (IUCAA, Pune) Andre Balogh (ISSI and Imperial) Melvyn Goldstein (NASA Goddard, Greenbelt) Kemel+12 Käpylä+12 Warnecke+11 Brandenburg+13

  2. Dynamo produces bi-helical field Magnetic helicity spectrum Southern hemisphere Pouquet, Frisch, & Leorat (1976)

  3. Helicity fluxes to alleviate catastrophic quenching Brandenburg (2005, ApJ) 1046Mx2/cycle

  4. Magnetic helicity flux • EMF and resistive terms still dominant • Fluxes import at large Rm ~ 1000 • Rm based on kf • Smaller by 2p

  5. Magnetic helicity flux Gauge-invariant in steady state! • EMF and resistive terms still dominant • Fluxes import at large Rm ~ 1000 • Rm based on kf • Smaller by 2p Del Sordo, Guerrero, Brandenburg (2013, MNRAS 429, 1686)

  6. Coronal mass ejections from helical structures This is how it looks like… Gibson et al. (2002)

  7. Helicity from solar wind Matthaeus et al. (1982) Measure correlation function In Fourier space, calculate magnetic energy and helicity spectra  Should be done with Ulysses data away from equatorial plane

  8. Measure 2-point correlation tensor u1 u2 Taylor hypothesis:

  9. Ulysses: scaling with distance Vector helium magnetometer 2 sec resolution 10 pT sensitivity (0.1 mG) * Fairly isotropic * Falls off faster than R-2 * Need to compensate before R averaging Power similar to US consumption Energy density similar to ISM

  10. Noisy helicity from Ulysses • Taylor hypothesis • Roundish spectra • Southern latitude with opposite sign • Positive H at large k Brandenburg, Subramanian, Balogh, Goldstein (2011, ApJ 734, 9)

  11. Bi-helical fields from Ulysses • Taylor hypothesis • Broad k bins • Southern latitude with opposite sign • Small/large distances • Positive H at large k • Break point with distance to larger k

  12. Latitudinal scaling and trend • Antisymmetric about equator • Decline toward minum

  13. Comparison Southern hemisphere • Field in solar wind is clearly bi-helical • ...but not as naively expected • Need to compare with direct and mean-field simulations • Recap of dynamo bi-helical fields

  14. Shell dynamos with ~CMEs Warnecke, Brandenburg, Mitra (2011, A&A, 534, A11) Strong fluctuations, but positive in north

  15. Dynamos with exterior  CMEs? Warnecke, Brandenburg, Mitra (2011, A&A, 534, A11)

  16. To carry negative flux: need positive gradient Brandenburg, Candelaresi, Chatterjee (2009, MNRAS 398, 1414) Sign reversal makes sense!

  17. Conclusions • Magnetic helicity measurable • High latitudes ( Ulysses) • Expect bi-helical • Bi-helical fields in dynamo & solar wind • + sign in wind by turbulent diffusion • also found in CME-like simulations

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