Properties of solar plage from a spatially coupled inversion of Hinode SP data

Abstract

Aims. The properties of magnetic fields forming an extended plage region in AR 10953 were investigated. Methods. Stokes spectra of the Fe I line pair at 6302 Å recorded by the spectropolarimeter aboard the Hinode satellite were inverted using the SPINOR code. The code performed a 2D spatially coupled inversion on the Stokes spectra, allowing the retrieval of gradients in optical depth within the atmosphere of each pixel, whilst accounting for the effects of the instrument's PSF. Consequently, no magnetic filling factor was needed.Results. The inversion results reveal that plage is composed of magnetic flux concentrations (MFCs) with typical field strengths of 1520 G at log(τ) = −0.9 and inclinations of 10 • −15 • . The MFCs expand by forming magnetic canopies composed of weaker and more inclined magnetic fields. The expansion and average temperature stratification of isolated MFCs can be approximated well with an empirical plage thin flux tube model. The highest temperatures of MFCs are located at their edges in all log(τ) layers. Whilst the plasma inside MFCs is nearly at rest, each is surrounded by a ring of downflows of on average 2.4 km s −1 at log(τ) = 0 and peak velocities of up to 10 km s −1 , which are supersonic. The downflow ring of an MFC weakens and shifts outwards with height, tracing the MFC's expansion. Such downflow rings often harbour magnetic patches of opposite polarity to that of the main MFC with typical field strengths below 300 G at log(τ) = 0. These opposite polarity patches are situated beneath the canopy of their main MFC. We found evidence of a strong broadening of the Stokes profiles in MFCs and particularly in the downflow rings surrounding MFCs (expressed by a microturbulence in the inversion). This indicates the presence of strong unresolved velocities. Larger magnetic structures such as sunspots cause the field of nearby MFCs to be more inclined.

The Heating of Solar Coronal Loops by Alfvén Wave Turbulence

“…At the merging height B 0 (z m ) ≈ 176.6 G, slightly lower than the flux density B z at the base of the photosphere because of the overall expansion of the loop with height. We assume that at the base of the photosphere the flux tubes have widths w 0 = 400 km, which is consistent with the observed sizes of magnetic flux concentrations in plage regions (e.g., Buehler et al 2015). The magnetic flux of each tube is Φ = w 2 0 B 0 (0) = 2.4 × 10 18 Mx.…”

“…The flux tubes are modeled using the zeroth order thin-tube approximation, which is not very accurate at larger heights in the photosphere (Yelles Chaouche et al 2009). The merging of flux tubes is described in a simple way (see Figure 1), which does not accurately represent the complex 3D magnetic structure of a plage region (Buehler et al 2015). Potential-field modeling suggests that the transverse velocities in the chromosphere may be enhanced by the presence of magnetic nulls at the merging height (van Ballegooijen et al 1998;van Ballegooijen & Hasan 2003), but the present model does not include such null points.…”

Three-dimensional non-LTE radiative transfer effects in Fe i lines

“…The strongest downflow is found at the boundary of the FS where the magnetic field direction is reversed (probably due to the strong downflow pulling the weak magnetic field downwards). Such reversed polarity fields surrounding magnetic elements were detected observationally by Zayer et al (1989), Narayan (2011), Bühler (2014, Bühler et al (2015).…”

“…The significantly different behavior between the idealized thin-tube model analyzed in Paper I and the MHD simulations considered here introduces a caveat for the uncritical use of the thin-tube model (Defouw 1976), which has been widely and very successfully used. Apart from being invaluable for studies of the thermal structure (Spruit 1976) and instabilities (e.g., Spruit 1979) of magnetic elements as well as for investigations about the propagation of MHD waves in these elements (e.g., Roberts & Webb 1979), the thin-tube has turned out to describe a number of observations of polarized spectra rather well (e.g., Zayer et al 1989;Bruls & Solanki 1995).…”

“…For simplicity in the following we refer to the elongated magnetic elements as flux sheets, or FS, and to the rounder ones as flux tubes, or FT. Only few areas are found with negative vertical fields and the field strength there is much lower than that in the magnetic elements. They are mostly located at the boundaries between intergranular and granular areas where the downflows are strongest (see Bühler et al 2015;Bühler 2014). …”