Effects of Waves on the Spectral Anisotropy of Transition Range in the Solar Wind Turbulence

In the transition range of the solar wind turbulence, the magnetic spectrum has been observed to be strongly anisotropic with respect to local mean field. However, the generation mechanism of the anisotropy remains not well understood. There are two typical types of waves existing in the transition range, including ion cyclotron waves (ICWs) and kinetic Alfven waves (KAWs) propagating in the directions parallel and perpendicular to magnetic field, respectively. In this work, we perform a statistical study on the effects of the waves on the spectral anisotropy of the transition range. We select 31 intervals from the measurements of Parker Solar Probe between 2018 and 2021. The magnetic helicity (sigma_m) diagnosis is applied on the magnetic field data at the frequency domain [0.1 Hz, 10 Hz], and the wavelet coefficients with sigma_m < -0.5 and sigma_m > 0.4 are considered as signals of ICWs and KAWs, respectively. We then remove them and find that the spectral anisotropy in the transition range becomes significantly weaker. Specifically, the spectra in the quasi-parallel direction statistically get shallower, and the average spectral index changes from -5.68±0.74 to -4.72±0.56. By contrast, the spectra in the perpendicular direction get slightly steeper, and the index changes from -3.63±0.34 to -3.95±0.41. Moreover, the anisotropic scaling in the transition range is found to be k_⊥ ~ k_∥^1.55±0.33. The new results about the magnetic field spectra after the removal of ICW and KAW will help to further understand the possible mechanisms that cause the spectral anisotropy in the transition range.