  Electromagnetic ion cyclotron (EMIC) waves are important for Earth's inner
magnetosphere as they can effectively drive relativistic electron losses to the
atmosphere and energetic (ring current) ion scattering and isotropization. EMIC
waves are generated by transversely anisotropic ion populations around the
equatorial source region, and for typical magnetospheric conditions this almost
always produces field-aligned waves. For many specific occasions, however,
oblique EMIC waves are observed, and such obliquity has been commonly
attributed to the wave off-equatorial propagation in curved dipole magnetic
fields. In this study, we report that very oblique EMIC waves can be directly
generated at the equatorial source region. Using THEMIS spacecraft observations
at the dawn flank, we show that such oblique wave generation is possible in the
presence of a field-aligned thermal ion population, likely of ionospheric
origin, which can reduce Landau damping of oblique EMIC waves and cyclotron
generation of field-aligned waves. This generation mechanism underlines the
importance of magnetosphere-ionosphere coupling processes in controlling wave
characteristics in the inner magnetosphere.
