X. Sun¹, J. T Hoeksema¹, Y. Liu¹, M. Kazachenko² & R. Chen^1,3
1 W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305
² Space Sciences Laboratory, University of California, Berkeley, CA 94720
³ Department of Physics, Stanford University, Stanford, CA 94305

The solar active region (AR) photospheric magnetic field evolves rapidly during major eruptive events, suggesting appreciable feedback from the corona¹. One type of evolution features step-like, permanent jumps that we call “magnetic imprints”. The line-of-sight (LoS) field (B_l) has been shown to evolve with a time scale of a few minutes and magnitude of up to a few hundred Gauss. Routine HMI vector observations with 12-minute cadence have confirmed the speculation that changes occur mostly in the horizontal (B_h) rather than the radial field component (B_r). Another type of evolution, featuring more transient changes observed in flare ribbons, is thought to be an artifact induced by flare-altered line profiles.

Figure 1 | Comparison of 135-s cadence HMI vector data with the routine 720-s data. (a) A snapshot of AR 11158 at 2011-02-15T01:11:20UT. (b) Two-dimensional distribution of measured total field strength B, in log scale. Weaker field in the 135-s data (B₁₃₅) peaks about 50 G higher than the 720 s data (B₇₂₀), indicating higher noise. (c) Distribution of the difference in field strength for strong-field regions (B > 300 G). The histogram is centered at about 0 with a HWHM of 25 G.

We created a new high-cadence vector magnetogram dataset for HMI that is suited to investigate these phenomena. The 135-second cadence series (hmi.B_135s) has the same format as the routine 720-s magnetograms, and is processed with nearly identical pipeline options². The corresponding Stokes profile data series is also available (hmi.S_135s). As illustrated by Fig. 1, the noise level in the 135-s field strength B is about 50 G higher than the 720 s data; measurements in strong-field regions (B > 300 G) agree well with the routine data. The highest possible HMI cadence further improved to 90 s when HMI switched to the “Mod-L” observing scheme in April 2016 (ref. 3); the 90-s dataset is still under development.

We used the 135-s dataset to investigate both magnetic imprints and transients in an archetypical event, SOL2011-02-15T01:56, in AR 11158 (ref. 2). The new dataset allows us to perform quantitative temporal analysis on sequences of single-pixel vector measurements⁴. Our main conclusions are as follows.

1. B_h exhibits permanent, step-like changes with a time scale of several minutes, whereas B_r varies less.
2. B_h near the main polarity inversion line increases significantly during the earlier phase of the associated flare, whereas B_h in the periphery decreases at later times with smaller magnitudes.
3. Transient artifacts coincide with enhanced flare emission, where the Stokes profiles are no longer adequately modeled under standard settings, and the inferred magnetic field becomes unreliable.

Figure 2 | Magnetic imprint and transient changes seen in one example pixel in a flare ribbon. (a) From top to bottom, temporal evolution of Stokes I₀ (Δλ = +192 Å) normalized by quiet Sun continuum I_C, horizontal field B_h, radial field B_r, and the formal uncertainty of field strength from the inversion σ_B, respectively. Symbols are measurements; green curves are a step-like function (for B_h and B_r, ref. 4) or third-order polynomial fit (for σ_B). Red symbols are transient signals, as determined by our empirical method. Vertical gray band shows the GOES flare time; vertical dotted line shows flare peak. (b) Stokes profiles (I, Q, U, V) at two instances, normalized by I_C. Profiles near the flare peak (red) are distorted.

Fig. 2 show one example pixel where both types of magnetic field change occur, and we are able to distinguish them using a new empirical scheme². These results corroborate previous findings, and remove certain ambiguities that arise from LoS-only or lower-cadence vector observations.

The new dataset will be useful for studying sunquakes and for data-driven coronal field modeling, amongst other topics². High-cadence vector magnetograms will be processed for selected intervals of significant activity, or by request to the HMI team. The first release covers about 30 events and 290 hours, most of which feature M- or X-class flares. Refs. 2 and 5 provide further information on the dataset.

References

[1] Wang, H. & Liu, C. 2015, RAA, 15, 145
[2] Sun, X., Hoeksema, J. T., Liu, Y., Kazachenko, M. & Chen. R. 2017, ApJ, 839, 67 (arXiv:1702.07338)
[3] Liu, Y., Baldner, C., Bogart, R., et al. 2016, HMI Sci. Nuggets, #56 (http://hmi.stanford.edu/hminuggets/?p=1596)
[4] Sudol, J. J. & Harvey, J. W. 2005, ApJ, 633, 647
[5] “HMI High-Cadence Vector Magnetograms” (http://jsoc.stanford.edu/data/hmi/highcad/)