Flux cancellation associated with flux emergence on the Sun
T. Magara, S. K. Antiochos, C. R. DeVore, and M. G. Linton
We study the mechanism for flux cancellation that occurs in the photosphere
when a twisted flux tube emerges from below the photosphere. We use a
three-dimensional, flux emergence MHD simulation in which magnetic field
evolves through a highly stratified atmosphere extending from the
subphotosphere to the corona. We find that different mechanisms work for
flux cancellation at different stages of flux emergence. When the flux tube
starts to emerge into the photosphere, Omega-shaped field lines
(Omega-loops) appear on the surface to form a bipolar region. As emergence
further proceeds, U-shaped field lines (U-loops) originally distributed at
the lower part of the flux tube appear on the surface, and the emergence of
these U-loops causes flux cancellation at the neutral line in the bipolar
region. At the late stage of the simulation the dipped part of emerging
U-loops is vertically stretched by strong downflows, which produces a deep
dip with a strong current enhancement inside it. This might lead to
magnetic reconnection occurring at this dip, followed by the submergence of
reconnected Omega-loops which is another mechanism for flux cancellation.
Correspondence
Tetsuya Magara, UCB/NRL (Univ. of California Berkeley/Naval Research Lab.)
presentation
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