THE FORMATION AND DISINTEGRATION OF MAGNETIC BRIGHT POINTS OBSERVED BYSUNRISE/IMaX

Abstract

The evolution of the physical parameters of magnetic bright points (MBPs) located in the quiet Sun (mainly in the interwork) during their lifetime is studied. First, we concentrate on the detailed description of the magnetic field evolution of three MBPs. This reveals that individual features follow different, generally complex, and rather dynamic scenarios of evolution. Next, we apply statistical methods on roughly 200 observed MBP evolutionary tracks. MBPs are found to be formed by the strengthening of an equipartition field patch, which initially exhibits a moderate downflow. During the evolution, strong downdrafts with an average velocity of 2.4 km s −1 set in. These flows, taken together with the concurrent strengthening of the field, suggest that we are witnessing the occurrence of convective collapses in these features, although only 30% of them reach kG field strengths. This fraction might turn out to be larger when the new 4 m class solar telescopes are operational as observations of MBPs with current state of the art instrumentation could still be suffering from resolution limitations. Finally, when the bright point disappears (although the magnetic field often continues to exist) the magnetic field strength has dropped to the equipartition level and is generally somewhat weaker than at the beginning of the MBP's evolution. Also, only relatively weak downflows are found on average at this stage of the evolution. Only 16% of the features display upflows at the time that the field weakens, or the MBP disappears. This speaks either for a very fast evolving dynamic process at the end of the lifetime, which could not be temporally resolved, or against strong upflows as the cause of the weakening of the field of these magnetic elements, as has been proposed based on simulation results. It is noteworthy that in about 10% of the cases, we observe in the vicinity of the downflows small-scale strong (exceeding 2 km s −1 ) intergranular upflows related spatially and temporally to these downflows. The paper is complemented by a detailed discussion of aspects regarding the applied methods, the complementary literature, and in depth analysis of parameters like magnetic field strength and velocity distributions. An important difference to magnetic elements and associated bright structures in active region plage is that most of the quiet Sun bright points display significant downflows over a large fraction of their lifetime (i.e., in more than 46% of time instances/measurements they show downflows exceeding 1 km s −1 ).

Long-term trends of magnetic bright points

“…Therefore this spectral region is often chosen when identifying them (Muller & Roudier 1984;Steiner et al 2001;Schüssler et al 2003). Recent studies, based on observations and computer simulations, have investigated the temporal evolution from the formation until the disintegration of single features, which occur over timescales of just a few minutes (Bodnárová et al 2013;Hewitt et al 2014;Utz et al 2014).…”

P-mode induced convective collapse in vertical expanding magnetic flux tubes?

“…Thus after another minute, downflows of about 2.5 km/s should have evolved. Such downflows are indeed observed as well as they evolve in such time-scales (e.g., Utz et al 2014).…”

Revisiting the building blocks of solar magnetic fields by GREGOR

“…Indeed, this pore disappeared on (or before) September 30. Figure 5 depicts the pore on September 28 as well as the smallest yet detectable solar magnetic features -so called MBPs (see, e.g., Utz et al 2014;Kuckein 2019). Magneto-convective features can be seen in the centre of the pore.…”