Height variation of magnetic field and plasma flows in isolated bright points

Abstract

The expansion with height of the solar photospheric magnetic field and the plasma flows is investigated for three isolated bright points (BPs). The BPs were observed simultaneously with 3 instruments at the GREGOR telescope: (1) filtergrams in the blue with HiFI, (2) imaging spectroscopy of Na I D2 at 5890 A with GFPI, and (3) slit spectropolarimetry with GRIS. Inversions were carried out for the Si I 10827 A Stokes profiles. BPs are identified in Ca II H and blue continuum filtergrams. They are also detected in the blue wing of Na I D2 and Si I 10827 A, as well as in Ca I 10839 A line-core images. We carried out two studies to validate the expansion of the magnetic field with height. First, we compare the photospheric Stokes V signals of two different spectral lines that are sensitive to different optical depths (Ca I vs. Si I). The area at which the Stokes V signal is significantly large is almost three times larger for the Si I line - sensitive to higher layers - than for the Ca I one. Second, the inferred line-of-sight (LOS) magnetic fields at two optical depths (log tau = -1.0 vs. -2.5) from the Si I line reveal spatially broader fields in the higher layer, up to 51% more extensive in one of the BPs. The dynamics of BPs are tracked along the Na I D2 and Si I lines. The inferred flows from Na I D2 Doppler shifts are slow in BPs (< 1 km/s). The Si I line shows intriguing Stokes profiles with important asymmetries. The analysis of these profiles unveils the presence of two components, a fast and a slow one, within the same resolution element. The faster one, with a smaller filling factor of 0.3, exhibits LOS velocities of about 6 km/s. The slower component is slightly blueshifted. The present work provides observational evidence for the expansion of the magnetic field with height. Moreover, fast flows are likely present in BPs but are often hidden because of observational limitations.