Plasma-beta Modulated Characteristics of Magnetohydrodynamic Waves around the Heliospheric Current Sheet

The magnetohydrodynamics (MHD) wave modes in the heliospheric current sheet (HCS) and the associated heliospheric plasma sheet (HPS) have not been comprehensively investigated in the literature. Based on a frequency-related identification approach, the properties of MHD waves are investigated during 154 HCS crossings observed by the Wind spacecraft from 1995 to 2013. Statistically, the incidence of MHD waves around HCS/HPS is found to be modulated by the plasma β within the HPS: (1) β > 5, both Alfvén and slow waves obviously decay within the HPS, with the occurrence rate (OR) decreasing from 60% and 20% in the upstream/downstream to 41% and 14% in the HPS vicinity, respectively; (2) 1 < β ≤ 5, the OR of Alfvén waves (AWs) remains nearly stable. However, more slow waves are generated after the HCS crossing, with OR increasing from 13% in the upstream/downstream to 22%; (3) β ≤ 1, the OR of Alfvén and slow waves remains at ∼58% and 20% during the entire crossing, in spite of some irregular fluctuations. The results for the HCS without a clear HPS are similar to the situations of a low β HPS. The parametric decay instability of AWs is proposed as being responsible for the more slow waves generated in the moderate β HPS, and some indirect observational clues are also given.