Parker Solar Probe Observations of Alfvénic Waves and Ion-cyclotron Waves in a Small-scale Flux Rope

Abstract

Abstract Small-scale flux ropes (SFRs) are common in the interplanetary environment. However, previous identification procedures generally discard SFRs with medium and high Alfvénicity, which are thought to be Alfvénic waves or Alfvénic structures. This paper first identifies an SFR event with medium Alfvénicity in the inner heliosphere (at ∼0.2 au) using Parker Solar Probe measurements. We find Alfvénic waves that arise inside SFR based on high correlations between the magnetic field and velocity fluctuations. We also observe quasi-monochromatic electromagnetic waves with frequencies f that are usually larger than the local proton cyclotron frequency at the leading and trailing edges of this SFR. These waves are well explained by the outward-propagating ion-cyclotron waves, which have wave frequencies ∼0.03–0.3 Hz and wavelengths ∼60–2000 km in the plasma frame. Moreover, we show that the power spectral density of the magnetic field in SFR middle region follows the power-law distribution, where the spectral index changes from −1.5 (f ≲ 1 Hz) to −3.3 (f ≳ 1 Hz). These findings would motivate developing an automated program to identify SFRs with medium and high Alfvénicity from Alfvénic waves/structures.