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Ion scale wave events or "wave storms" in the solar wind are characterised byenhancements in magnetic field fluctuations as well as coherent magnetic fieldpolarisation signatures at or around the local ion cyclotron frequencies. Inthis paper we study in detail one such wave event from Parker Solar Probe's(PSP) fourth encounter, consisting of an initial period of left-handed (LH)polarisation abruptly transitioning to a strong period of right-handed (RH)polarisation, accompanied by clear core-beam structure in both the alpha andproton velocity distribution functions. A linear stability analysis shows thatthe LH polarised waves are anti-Sunward propagating Alfv\'en/ion cyclotron(A/IC) waves primarily driven by a proton cyclotron instability in the protoncore population, and the RH polarised waves are anti-Sunward propagating fastmagnetosonic/whistler (FM/W) waves driven by a firehose-like instability in thesecondary alpha beam population. The abrupt transition from LH to RH is causedby a drop in the proton core temperature anisotropy. We find very goodagreement between the frequencies and polarisations of the unstable wave modesas predicted by linear theory and those observed in the magnetic field spectra.Given the ubiquity of ion scale wave signatures observed by PSP, this workgives insight into which exact instabilities may be active and mediating energytransfer in wave-particle interactions in the inner heliosphere, as well ashighlighting the role a secondary alpha population may play as a rarelyconsidered source of free energy available for producing wave activity.

Proton and Alpha Driven Instabilities in an Ion Cyclotron Wave Event