Effects of Ion Temperature Anisotropy on Ion-scale Waves’ Generation in the Near-Sun Solar Wind: Parker Solar Probe Observations

The ion-scale electromagnetic waves are observed frequently within 0.3 au. Their generation and dissipation driven by wave–particle interaction are very important energy transfer processes in the weak collision corona and solar wind and may be one of the important factors driving the evolution of ion velocity distribution functions therein. In this Letter, we statistically analyze the possible effects of ion temperature anisotropy on the generation of the observed ion-scale waves within 0.3 au. The statistical results indicate that the ion-scale waves’ occurrence rate is proportional to ion temperature anisotropy. Moreover, the high occurrence rate of left-handed (LH) waves is closely related to EMIC and firehose instabilities driven by ion temperature anisotropy. However, we only find the close connection between right-handed (RH) waves’ high occurrence rate with the proton firehose instability. Besides, for LH waves in the regions with Tp⊥/Tp∥ > 1, their high occurrence rate is usually accompanied by the higher ion temperature (Tp, Tα), larger ion temperature ratio (Tα/Tp), and weaker collision effect. The RH waves are usually accompanied by a relatively lower ion temperature, smaller ion temperature ratio, and stronger collision effect than the LH waves. This Letter suggests that the ion temperature anisotropy is one of the important energy sources for generating ion-scale waves within 0.3 au.