The Effects of Substorm Injection of Energetic Electrons and Enhanced Solar Wind Ram Pressure on Whistler-Mode Chorus Waves: A Statistical Study

Whistler-mode chorus waves in the inner magnetosphere are typically excited by an electron temperature anisotropy. The anisotropy can be driven by two sources: particle injections from the tail (such as during substorms) and the solar wind ram pressure on the dayside magnetosphere. Based on 5 years of data from Van Allen Probe A, we have separately studied the effects of substorm injection of energetic ∼10–100 keV electrons and their gradient and curvature drifts ((Formula presented.)) and enhanced solar wind ram pressure ((Formula presented.)) on the generation of whistler-mode chorus waves. We use time-modified (Formula presented.) and (Formula presented.) indices to take into account time delays. We find that during the period of large (Formula presented.) but small (Formula presented.), chorus waves are mainly observed in the midnight through dawn to noon sectors ((Formula presented.) MLT (Formula presented.) 13) near the magnetic equator (|MLAT| < 10°) at L = 4.5–6.5. With an increase in (Formula presented.) both the chorus occurrence rates and the wave amplitudes increase. While under the condition of enhanced (Formula presented.) but small (Formula presented.), chorus waves are preferentially detected on the dayside ((Formula presented.) MLT (Formula presented.) 14) in a wide range of latitudes (|MLAT| < 20°) at outer L-shells (L = 5.5–6.5). With the increase of (Formula presented.), chorus occurrence rates also increase, while the amplitude remains relatively constant. Our study supports the two mechanisms for chorus excitation in the Earth’s magnetosphere and provides a better understanding of the global distribution and properties of chorus waves.