A study of Magnetic Bright Points in the Na I D1 line

TL;DR

This study investigates magnetic bright points in the Na I D1 line, revealing their association with strong downflows, a two-component heating process, and providing the first evidence of rapid downdrafts in these features.

Contribution

It presents the first observational evidence of rapid downdrafts in Na I D1 magnetic bright points and characterizes their intensity and magnetic flux expansion behaviors.

Findings

Na I D1 bright points are linked to strong downflows up to 7 km/s.

An intensity plateau at weak magnetic fields suggests a two-component heating process.

Magnetic flux tubes expand by approximately 76% at weak fields and 44% at stronger fields.

Abstract

High cadence, multiwavelength, optical observations of solar magnetic bright points, captured at disk center using the ROSA and IBIS imaging systems on the Dunn Solar Telescope, are presented. Magnetic bright points manifesting in the Na I D1 core are found to preferentially exist in regions containing strong downflows, in addition to co-spatial underlying photospheric magnetic field concentrations. Downdrafts within Na I D1 bright points exhibit speeds of up to 7km/s, with preferred structural symmetry in intensity, magnetic field and velocity profiles about the bright point center. Excess intensities associated with G-band and Ca II K observations of magnetic bright points reveal a power-law trend when plotted as a function of magnetic flux density. However, Na I D1 observations of the same magnetic features indicate an intensity plateau at weak magnetic field strengths below ~150G, suggesting the presence of a two-component heating process; one which is primarily acoustic, the other predominantly magnetic. We suggest that this finding is related to the physical expansion of magnetic flux tubes, with weak field strengths (~50G) expanding by ~76%, compared to a ~44% expansion when higher field strengths (~150G) are present. These observations provide the first experimental evidence of rapid downdrafts in Na I D1 magnetic bright points, and reveal the nature of a previously unresolved intensity plateau associated with these structures.