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Flare-Associated Magnetic Field Changes Observed with HMI

Flare-Associated Magnetic Field Changes Observed with HMI. Permanent changes in photospheric magnetic fields have been observed to occur during solar flares (e.g., Sudol & Harvey 2005, Wang & Liu 2010, Petrie & Sudol 2010).

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Flare-Associated Magnetic Field Changes Observed with HMI

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  1. Flare-Associated Magnetic Field Changes Observed with HMI Permanent changes in photospheric magnetic fields have been observed to occur during solar flares (e.g., Sudol & Harvey 2005, Wang & Liu 2010, Petrie & Sudol 2010). For the X2.2 flare in AR 11158, we present preliminary results regarding the timescales over which these field changes occur. by Brian T. Welsch & George H. Fisher Space Sciences Lab, UC-Berkeley

  2. Topic 1: What is the time scale over which flare-associated field changes occur? Background: • Magnetic fields tend to become more horizontal during a flare (Hudson 2000; Hudson,Fisher, & Welsch 2008; Wang et al., ApJ716 L195, 2010). • These changes δB imply a radial Lorentz force (e.g., Hudson, Fisher, & Welsch 2008), • This force could be related to CME acceleration (Fisher, Bercik, Welsch, & Hudson 2012).

  3. This X2.2 flare resulted in a peak upward force of 4x1022 dynes acting on the outer atmosphere (and an equal &opposite downward force on the solar interior).

  4. This “Lorentz impulse” has implications for observations of CME momenta. • Acting over δt, the impulse increases CME radial velocity from zero to δvr: • Escape velocity ve = (2GM/R)1/2 = 620 km/sec is the min. ballistic CME speed, so this enables a testable prediction:

  5. A key unknown is the time scale of field changes, which should match the duration δtof the Lorentz impulse. • Typical area-integrated forces are δFr~1022dynes. • Assumingδt~103 sec gives Mmax~1018g --- and all CMEs fall well below this constraint! • Wang, Liu, & Wang (ApJ757 L5, 2012) note that if the duration of Lorentz impulses δt is ~10 sec, then CME momenta are consistent with Lorentz impulses. Is this 10 sec. time scale realistic?

  6. Data: HMI’s cameras record filtergrams with rapid cadence. • Each of 2 cameras records an image every 3.75 sec. • Tunable filter for 6 wavelengths (WL) • Internal labels: 465, 467, 469, 471, 473, 475 • Measures 6 polarization states (PS) • Internal labels: • 258/258 (LCP/RCP) • 250-253 (I,Q,U,V) • Front cam: BLOS (LCP – RCP)* 6 WL repeats in 45 sec. • Side cam: Bfull-Stokes cycle is 90 sec (4 PS *6 WL) Schou et al., Sol. Phys. 275 229 (2012)

  7. Timing of filtergrams are magnetic observations with both high cadence and highdutycycle (to catch flares). • Single-WL LCP or RCP series have 45 sec. resolution. • Intensity variationsδIassociated with δB can be identified in Stokes’ V in individual WL series. • Time shift between WLs series are multiples of 7.5 sec. • Hence, comparisons in timing between WL series can determine timing of field changes to within 7.5 sec!

  8. Single-WL Stokes V intensity I(x,y) clearly shows magnetic structure at a given time.

  9. Intensity changes δI(x,y,t) in single-WL Stokes V clearly shows structure associated with flare-related field changes, δB.

  10. Sudol & Harvey (2005) (and subsequent researchers) fitted time profiles of sudden changes in Bi(t) in single pixels via: Bi(t) = a + bt + c {1 + 2 π-1 tan-1[n(t – t0)]} • Bi represents one component of B(x0,y0,t) • a, b give background level + linear evolution in magnetic field • c gives the amplitude of the field change • t0is center of interval of change • π n-1parametrizes time scale of field change • We also investigated fitting tanh[n(t – t0)], w/similar results.

  11. We also fitted time profiles of pixels in single-WL images this way, but also included a term prop. to t2. • Our time series was 90 min. long, w/flare at midpoint. • Like Sudol & Harvey, we used a Levenberg-Markwardt fitting algorithm.

  12. Among single-WL fits, the distribution of fitted timescales is consistent with field changes over a few sec.

  13. Future Work: Next, we must compare fitted δt & t0in individual pixels between different WL series. • If changes are step-wise at the 45-sec. resolution of each WL series ==> δt = (π n-1)<< 45 sec. • Some WL might “catch” field at midpoint of change δB/2, implying δt = (π n-1)~ 45 sec. • Hence, differences in fits for δtcan constrain field changes. • Perhaps changes in different pixels have different physical timescales? • (Are fits to t0 robust, in the sense of consistent with time lags between observations in each WL?)

  14. Summary • Flares can cause changes in the photospheric magnetic field δBresulting in an upward “Lorentz impulse” δFron the outer solar atmosphere. • Observable predition: the Lorentz impulse should impart momentum to CMEs; these quantities should be related. • The duration δtover whichthe Lorentzimpulse acts on the CME is unknown; this should directly affect the CME momentum. • Measurements of CME momenta have been interpreted to imply impulse timescales of ~10sec. • Single-WL HMI filtergramsw/ 45 sec. cadence are consistent with changes on this short time scale. • Comparing filtergrams in different WLs should enable resolving time scales of a few seconds --- but we haven’t investigated this yet!

  15. Acknowledgements This work was supported by the NASA Heliophysics Theory Program (NNX11AJ65G), the NASA LWS TR&T Program (NNX11AQ56G), and the NSF AGS program (AGS 1048318).

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