2D solar wind speeds from 6 to 26 solar radii in solar cycle 24 by using Fourier filtering

TL;DR

This study develops a Fourier filtering method to measure 2D solar wind speeds from 6 to 26 solar radii using SOHO/LASCO C3 data, revealing cycle-dependent spatial structures and radial evolution consistent with Parker's model.

Contribution

The paper introduces a novel Fourier filtering technique to accurately determine 2D solar wind speeds over a wide radial range and timescales, enhancing understanding of solar wind acceleration.

Findings

Solar wind speeds are relatively uniform azimuthally during solar maximum.

Bimodal speed distribution appears during solar minimum, especially around 2009.

Radial evolution of speeds aligns with Parker's solar wind solution.

Abstract

Measurement of the solar wind speed near the Sun is important for understanding the acceleration mechanism of the solar wind. In this study, we determine 2D solar wind speeds from 6 to 26 solar radii by applying Fourier motion filters to \textit{SOHO}/LASCO C3 movies observed from 1999 to 2010. Our method successfully reproduces the original flow speeds in the artificially generated data as well as streamer blobs. We measure 2D solar wind speeds from 1-day to 1-year timescales and their variation in solar cycle 24. We find that the solar wind speeds at timescales longer than a month in the solar maximum period are relatively uniform in the azimuthal direction, while they are clearly bimodal in the minimum period, as expected from the \textit{Ulysses} observations and IPS reconstruction. The bimodal structure appears at around 2006, becomes most distinctive in 2009, and abruptly disappears in 2010. The radial evolution of the solar wind speeds resembles the Parker's solar wind solution.