Height-dependent velocity structure of photospheric convection in granules and intergranular lanes with Hinode/SOT

TL;DR

This study uses Hinode/SOT observations and bisector analysis to reveal the height-dependent velocity structure of solar granules and intergranular lanes, showing larger velocities than previous studies and insights into convective instability.

Contribution

It provides the first detailed observational velocity profiles of photospheric convection at different heights, highlighting the role of radiative cooling and pressure gradients.

Findings

Convective velocities decrease in granules as material rises.

Velocities increase in intergranular lanes as material descends.

Bisector analysis effectively reveals long-term convective dynamics.

Abstract

The solar photosphere is the visible surface of the Sun, where many bright granules, surrounded by narrow dark intergranular lanes, are observed everywhere. The granular pattern is a manifestation of convective motion at the photospheric level, but its velocity structure in the height direction is poorly understood observationally. Applying bisector analysis to a photospheric spectral line recorded by the Hinode Solar Optical Telescope, we derived the velocity structure of the convective motion in granular regions and intergranular lanes separately. The amplitude of motion of the convective material decreases from 0.65 to 0.40 km/s as the material rises in granules, whereas the amplitude of motion increases from 0.30 to 0.50 km/s as it descends in intergranular lanes. These values are significantly larger than those obtained in previous studies using bisector analysis. The acceleration of descending materials with depth is not predicted from the convectively stable condition in a stratified atmosphere. Such convective instability can be developed more efficiently by radiative cooling and/or a gas pressure gradient, which can control the dynamical behavior of convective material in intergranular lanes. Our analysis demonstrated that bisector analysis is a useful method for investigating the long-term dynamic behavior of convective material when a large number of pixels is available. In addition, one example is the temporal evolution of granular fragmentation, in which downflowing material develops gradually from a higher layer downward.