Electromagnetic cyclotron waves in the dayside subsolar outer magnetosphere generated by enhanced solar wind pressure: EMIC wave coherency

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.