Flare-Associated Magnetic Field Changes Observed with HMI

by Brian T. Welsch & George H. FisherSpace Sciences Lab, UC-Berkeley

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.

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., ApJ 716 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).

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).

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:

A key unknown is the time scale of field changes, which should match the duration δt of the Lorentz impulse.

• Typical area-integrated forces are δFr ~1022 dynes.

• Assuming δt ~103 sec gives Mmax ~1018 g --- and all CMEs fall well below this constraint!

• Wang, Liu, & Wang (ApJ 757 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?

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: B full-Stokes cycle is 90 sec (4 PS *6 WL)

Schou et al., Sol. Phys. 275 229 (2012)

Timing of filtergrams are magnetic observations with both high cadence and high duty cycle (to catch flares).

• Single-WL LCP or RCP series have 45 sec. resolution.

• Intensity variations δI associated 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!

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

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

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• t0 is center of interval of change• π n-1 parametrizes time scale of field change

• We also investigated fitting tanh[n(t – t0)], w/similar results.

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.

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

Future Work: Next, we must compare fitted δt & t0 in 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 δt can 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?)

Summary

• Flares can cause changes in the photospheric magnetic field δB resulting in an upward “Lorentz impulse” δFr on the outer solar atmosphere.

• Observable predition: the Lorentz impulse should impart momentum to CMEs; these quantities should be related.

• The duration δt over which the Lorentz impulse 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 ~10 sec.

• Single-WL HMI filtergrams w/ 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!

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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).