The frontier between Small-scale bipoles and Ephemeral Regions in the solar photosphere: Emergence and Decay of an Intermediate-scale bipole observed with IMaX/SUNRISE

TL;DR

This study investigates the emergence and decay of an intermediate-scale magnetic bipole in the solar photosphere, revealing dynamic behavior, flux evolution, and decay mechanisms bridging small-scale bipoles and ephemeral regions.

Contribution

It provides detailed observations of an intermediate-scale bipole's evolution, highlighting its dynamic properties and decay process, bridging the understanding between small bipoles and larger ephemeral regions.

Findings

Bipole footpoints separate at 3.5 km/s reaching 4.5 Mm.

Flux growth rate is 2.6 x 10^15 Mx/s, decay rate is an order of magnitude smaller.

Decay involves breakup, cancellation, and a diffusive process with a diffusion constant of 8 x 10^2 km^2/s.

Abstract

We report on the photospheric evolution of an intermediate-scale (~4 Mm footpoint separation) magnetic bipole, from emergence to decay, observed in the quiet Sun at high spatial 0".3 and temporal (33 s) resolution. The observations were acquired by the IMaX imaging magnetograph during the first science flight of the Sunrise balloon-borne solar observatory. The bipole flux content is 6 x 10^17 Mx, representing a structure bridging the gap between granular scale bipoles and the smaller ephemeral regions. Footpoints separate at a speed of 3.5 km s-1 and reach a maximum distance of 4.5 Mm before the field dissolves. The evolution of the bipole is revealed to be very dynamic: we found a proper motion of the bipole axis and detected a change of the azimuth angle of 90{\deg} in 300 seconds. The overall morphology and behaviour are in agreement with previous analyses of bipolar structures emerging at granular scale, but we also found several similarities with emerging flux structures at larger scale. The flux growth rate is 2.6 x 15 Mx s-1, while the mean decay rate is one order of magnitude smaller. We describe in some detail the decay phase of the bipole footpoints which includes break up into smaller structures, interaction with pre-existing fields leading to cancellation but appears to be dominated by an as-yet unidentified diffusive process that removes most of the flux with an exponential flux decay curve. The diffusion constant (8 x 10^2 km^2 s-1) associated with this decay is similar to the values used to describe the large scale diffusion in flux transport models.