Moving Magnetic Features around Sunspots

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Abstract

We study statistical properties of small-scale magnetic features around sunspots using time sequences of high-resolution magnetograms of eight sunspots made with the Michelson Doppler Imager (MDI) on board SOHO. Flow maps around the spots are also derived from cross-correlation analysis of MDI continuum or TRACE white light and used for comparison of photospheric flow patterns with the tracks of moving magnetic features. An automated algorithm to find and track unipolar concentrations of magnetic field was developed. Depending on the velocity, size, and distance from the spot, a selected subset of all concentrations can be identified as moving magnetic features (MMFs). Our method finds 4-24 MMFs per hour around the spots, with higher counts for larger sunspots. After being first detected, the MMFs have an average flux content = 2.5 × 1018 Mx. Their average lifetime is about 1 hr, but it takes a concentration only tmax = 25 minutes to reach its maximum flux content of about = 6.1 × 1018 Mx. MMFs are found to transport a net flux out of a spot at a rate of (0.4-6.2) × 1019 Mx hr-1: if sunspots were to decay only by outflowing MMFs, it would take a sunspot of 1022 Mx one to several weeks to completely disassemble. The MMFs have an initial velocity of = 1.8 km s-1, faster than the average moat flow. Before merging into the moat region or surrounding network, they travel a distance = 3.5 Mm. The tracks of the individual MMFs correlate with the direction of local plasma flows and sometimes display a spokelike pattern around the sunspots. We find an average initial size = 1.7 Mm2, but the distribution of sizes suggests features with a diameter of only 600-1000 km, which would not be recognized by our algorithm. Comparison of a cotemporal, cospatial magnetogram made with the Swedish Vacuum Solar Telescope (SVST) on La Palma (with 12 times the spatial resolution) indicates that unipolar magnetic fluxes in the MDI magnetogram may be comprised of smaller elements with both polarities.

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