Wednesday, January 16, 2013

Papers: Buoyant Magnetic Loops Generated by Global Convective Dynamo Action

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Find in the arXiv

  • Nelson, Nicholas J., Brown, Benjamin P., Brun, Allan Sacha, Miesch, Mark S., & Toomre, Juri, 2013, "Buoyant magnetic loops generated by global convective dynamo action", Solar Physics, 289, 441
This paper looks in greater detail at a large sample of buoyant loops from case S3 - a solar-like convective dynamo simulation. Below are three different views on a single buoyant loop - (a) looks south along the rotation axis, (b) looks radially inward, and (c) looks west along the axis of the loop.

Previous work has focused on small numbers of loops which were identified by visually inspecting 3D renderings of magnetic field line. Using an automated pattern recognition algorithm, I was able to locate 150+ buoyant loops in a systematic search of this simulation. The process is extremely data-intensive so I was only able to to a complete search for one magnetic activity cycle, however this provides enough loops to provide a statistical look at the properties of these loops. Here's a time-latitude map of the longitudinally-averaged magnetic field strength (red shows positive polarity, blue shows negative polarity) with wreaths of opposite polarity in each hemisphere. The southern wreath is clearly much stronger than the northern one. Over-laid are the times and latitudinal locations of each of the 138 buoyant magnetic loops located in this activity cycle, with red squares showing positive polarity loops and green diamonds showing negative polarity loops.

With a large sample of loops, we can compare properties of our simulated loops with observed properties of solar active regions.  For example, we find that our loops show similar latitudinal tilts to those proscribed by Joy's Law. We can also compare the twist of the loops with previous simulations and observations. Previous simulations have indicated that loops must have a certain amount of negative (left-handed) twist or they will break apart and dissipate as they rise. Observations show a wide variety of levels of twist at the solar surface, but a preference for left-handed twists. Our simulations show a slight preference for negative twists, but a wide variety of levels of twist are seen.

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