Modeling the quiet Sun cell and network emission with ALMA

TL;DR

This study uses high-resolution ALMA observations at 1.26 mm and 3 mm to model the temperature structure of the quiet Sun's chromosphere, focusing on network and cell regions, and compares results with existing models.

Contribution

It expands previous models by including brightness temperature measurements of network and cell regions, and provides improved estimates of electron temperature variations and calibration of ALMA solar images.

Findings

Brightness temperature in Band 6 is ~440 K higher than previous estimates.

Electron temperature in network regions is lower than model predictions, while in cell interiors it is higher.

Temperature contrast between network and cell increases with height, indicating steeper temperature gradients.

Abstract

ALMA observations of the Sun at mm-$\lambda$ offer a unique opportunity to investigate the temperature structure of the solar chromosphere. In this article we expand our previous work on modeling the chromospheric temperature of the quiet Sun, by including measurements of the brightness temperature in the network and cell interiors, from high resolution ALMA images at 3 mm (Band 3) and 1.26 mm (Band 6). We also examine the absolute calibration of ALMA full-disk images. We suggest that the brightness temperature at the center of the solar disk in Band 6 is $\sim440$ K above the value recommended by White et al. (2017) and we give improved results for the electron temperature variation of the average quiet Sun with optical depth, as well as the derived spectrum at the center of the disk. We found that the electron temperature in the network is considerably lower than predicted by model F of Fontenla et al. (1993) and that of the cell interior considerably higher than predicted by model A. Depending upon the network/cell segregation scheme, the electron temperature difference between network and cell at $\tau=1$ (100 GHz) is from $\sim$660 to $\sim$1550 K, compared to $\sim$3280 K predicted by the models; similarly, the $T_e$ ratio is from $\sim$1.10, to 1.24, against $\sim$1.55 of the model prediction. We also found that the network/cell $T_e(\tau)$ curves diverge as $\tau$ decreases, indicating an increase of contrast with height and possibly a steeper temperature rise in the network than in the cell interior.