Magnetic signatures of the Langumuir-Slow Extraordinary mode: Theory, PSP data and Vlasov-Maxwell simulations

We present a statistical study of the Langumuir-Slow Extraordinary Mode, (sometimes called Z-mode) observed by PSP, that reveals the presence of magnetic signatures for few tens of events out of thousands. A magnetic signature for these modes was inferred in previous work by looking at the polarization of the electric field, but actual and clear measurements are a novelty of the search-coil magnetometer (SCM) onboard PSP. We theoretically link their appearance to the intensity of the wave electric field and to the decrease of the refractive index due mainly to density inhomogeneities. The behaviour of the beam plasma interaction that generates these wave modes has been previously studied in the presence of density inhomogeneities with numerical simulations only in the electrostatic approximation (e.g. Krafft et al. 2013, ApJ). In order to study the full effect of the density fluctuations with respect to the presence of the magnetic signatures and to test our theoretical approach, we simulate the beam-plasma interaction in the electromagnetic case. Our study makes use of the fully kinetic Vlasov-DArwin (ViDA) (Pezzi et al. 2019, JPP) numerical code which is specifically designed to investigate small scales with high accuracy.