Temporal power spectra of the horizontal velocity of the solar photosphere

TL;DR

This study analyzes the temporal power spectra of horizontal velocities in the solar photosphere using Hinode data, revealing a double power law shape and providing insights into photospheric convection relevant to coronal heating.

Contribution

First derivation of k-omega diagrams for photospheric horizontal velocity, offering new insights into the temporal evolution of solar convection.

Findings

Power spectra show a double power law shape with a bend at 4.7 mHz.

High frequency power law index is -2.4, low frequency index is -0.6.

Root mean square horizontal speed is about 1.1 km/s.

Abstract

We have derived the temporal power spectra of the horizontal velocity of the solar photosphere. The data sets for 14 quiet regions observed with the Gband filter of Hinode/SOT are analyzed to measure the temporal fluctuation of the horizontal velocity by using the local correlation tracking (LCT) method. Among the high resolution (~0.2") and seeing-free data sets of Hinode/SOT, we selected the observations whose duration is longer than 70 minutes and cadence is about 30 s. The so-called k-{\omega} diagrams of the photospheric horizontal velocity are derived for the first time to investigate the temporal evolution of convection. The power spectra derived from k-omega diagrams typically have a double power law shape bent over at a frequency of 4.7 mHz. The power law index in the high frequency range is -2.4 while the power law index in the low frequency range is -0.6. The root mean square of the horizontal speed is about 1.1 km/s when we use a tracer size of 0.4" in LCT method. Autocorrelation functions of intensity fluctuation, horizontal velocity, and its spatial derivatives are also derived in order to measure the correlation time of the stochastic photospheric motion. Since one of possible energy sources of the coronal heating is the photospheric convection, the power spectra derived in the present study will be of high value to quantitatively justify various coronal heating models.