  Ion scale wave events or "wave storms" in the solar wind are characterised by
enhancements in magnetic field fluctuations as well as coherent magnetic field
polarisation signatures at or around the local ion cyclotron frequencies. In
this 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 and
proton velocity distribution functions. A linear stability analysis shows that
the LH polarised waves are anti-Sunward propagating Alfv\'en/ion cyclotron
(A/IC) waves primarily driven by a proton cyclotron instability in the proton
core population, and the RH polarised waves are anti-Sunward propagating fast
magnetosonic/whistler (FM/W) waves driven by a firehose-like instability in the
secondary alpha beam population. The abrupt transition from LH to RH is caused
by a drop in the proton core temperature anisotropy. We find very good
agreement between the frequencies and polarisations of the unstable wave modes
as predicted by linear theory and those observed in the magnetic field spectra.
Given the ubiquity of ion scale wave signatures observed by PSP, this work
gives insight into which exact instabilities may be active and mediating energy
transfer in wave-particle interactions in the inner heliosphere, as well as
highlighting the role a secondary alpha population may play as a rarely
considered source of free energy available for producing wave activity.
