Dominance of 2-Minute Oscillations near the Alfv\'en Surface

TL;DR

This study analyzes Parker Solar Probe data to reveal that 2-minute oscillations, likely originating from the solar atmosphere, dominate near the Alfvén surface and are associated with outward-propagating Alfvén waves, shedding light on solar wind heating.

Contribution

It provides the first direct evidence linking 2-minute oscillations in the solar wind to their solar atmospheric origins through comprehensive spectral and Doppler analyses.

Findings

2-minute oscillations dominate near the Alfvén surface.

Oscillations are primarily outward-propagating, spherically polarized Alfvén waves.

Wave frequency in the spacecraft frame maps to the solar base frequency.

Abstract

Alfv\'en waves, considered one of the primary candidates for heating and accelerating the fast solar wind, are ubiquitous in spacecraft observations, yet their origin remains elusive. In this study, we analyze data from the first 19 encounters of the Parker Solar Probe (PSP) and report dominance of 2-minute oscillations near the Alfv\'en surface. The frequency-rectified trace magnetic power spectral density (PSD) of these oscillations indicates that the fluctuation energy is concentrated around 2 minutes for the ``youngest'' solar wind. Further analysis using wavelet spectrograms reveals that these oscillations primarily consist of outward-propagating, spherically polarized Alfv\'en wave bursts. Through Doppler analysis, we show that the wave frequency observed in the spacecraft frame can be mapped directly to the launch frequency at the base of the corona, where previous studies have identified a distinct peak around 2 minutes ($\sim 8$ mHz) in the spectrum of swaying motions of coronal structures observed by SDO AIA. These findings strongly suggest that the Alfv\'en waves originate from the solar atmosphere. Furthermore, statistical analysis of the PSD deformation beyond the Alfv\'en surface supports the idea of dynamic formation of the otherwise absent $1/f$ range in the solar wind turbulence spectrum.