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

Abstract

Aims. The expansion with height of the solar photospheric magnetic field and the plasma flows is investigated for three isolated bright points (BPs). Methods. The BPs were observed simultaneously with three different instruments attached to the 1.5-meter GREGOR telescope: (1) filtergrams of Ca ii H and blue continuum (4505 Å) with the HiFI, (2) imaging spectroscopy of the Na i D 2 line at 5890 Å with the GFPI, and (3) slit spectropolarimetry in the 1 µm spectral range with the GRIS. Spectral-line inversions were carried out for the Si i 10827 Å Stokes profiles.Results. Bright points are identified in the Ca ii H and blue continuum filtergrams. Moreover, they are also detected in the blue wing of the Na i D 2 and Si i 10827 Å lines, as well as in the Ca i 10839 Å line-core images. We carried out two studies to validate the expansion of the magnetic field with height. On the one hand, 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. On the other hand, the inferred line-of-sight (LOS) magnetic fields at two optical depths (log τ = −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 D 2 and Si i lines. The inferred flows from Na i D 2 Doppler shifts are rather slow in BPs ( 1 km s −1 ). However, 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 −1 . The slower component is slightly blueshifted. Conclusions. 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 sometimes hidden because of observational limitations.

Capabilities of bisector analysis of the Si I 10 827 Å line for estimating line-of-sight velocities in the quiet Sun

“…Rezaei & Beck 2015), sunspots and pores (e.g. Xu et al 2010;Joshi 2014), flares (Judge et al 2014;Kuckein et al 2015), or independently, for example, in small-scale magnetic bright points (Kuckein 2019) or sunspots (e.g. Rezaei et al 2012;Felipe et al 2016;Orozco Suárez et al 2017).…”

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.…”

Observational evidence for two-component distributions describing solar magnetic bright points

“…lines formed in the chromosphere. Furthermore, the Na i D 1 and D 2 lines can also be used to identify MBPs, as well as the photospheric Si i 10827 Å and Ca i 10839 Å lines (Jess et al 2010;Kuckein 2019). Wiehr & Bovelet (2009) found a preferred location of these structures (that they called magnetic inter-granular structures) at the footpoints of dark Hα fibrils, which are known to delineate the network boundaries.…”