Abstract:
Electromagnetic ion (proton) cyclotron (EMIC) waves and whistler mode chorus are
simultaneously detected in the Earth’s dayside subsolar outer magnetosphere. The observations were
made near the magnetic equator 3.1∘ –1.5∘ magnetic latitude at 1300 magnetic local time from L = 9.9
to 7.0. It is hypothesized that the solar wind external pressure caused preexisting energetic 10–100 keV
protons and electrons to be energized in the T⟂ component by betatron acceleration and the resultant
temperature anisotropy (T⟂ >T∥) formed led to the simultaneous generation of both EMIC (ion) and chorus
(electron) waves. The EMIC waves had maximum wave amplitudes of ∼6 nT in a ∼60 nT ambient field
B0. The observed EMIC wave amplitudes were about ∼10 times higher than the usually observed chorus
amplitudes (∼0.1–0.5 nT). The EMIC waves are found to be coherent to quasi-coherent in nature.
Calculations of relativistic ∼1–2 MeV electron pitch angle transport are made using the measured wave
amplitudes and wave packet lengths. Wave coherency was assumed. Calculations show that in a ∼25–50 ms
interaction with an EMIC wave packet, relativistic electron can be transported ∼27∘ in pitch. Assuming
dipole magnetic field lines for a L = 9 case, the cyclotron resonant interaction is terminated ∼±20∘ away
from the magnetic equator due to lack of resonance at higher latitudes. It is concluded that relativistic
electron anomalous cyclotron resonant interactions with coherent EMIC waves near the equatorial plane
is an excellent loss mechanism for these particles. It is also shown that E >1 MeV electrons cyclotron
resonating with coherent chorus is an unlikely mechanism for relativistic microbursts. Temporal structures
of ∼30 keV precipitating protons will be ∼2–3 s which will be measurable at the top of the ionosphere.
