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 similar to 10-100 keV electrons and their gradient and curvature drifts (AE*) and enhanced solar wind ram pressure (P-d*) on the generation of whistler-mode chorus waves. We use time-modified AE* and P-d* indices to take into account time delays. We find that during the period of large AE* but small P-d*, chorus waves are mainly observed in the midnight through dawn to noon sectors (00 <= MLT <= 13) near the magnetic equator (vertical bar MLAT vertical bar < 10 degrees) at L = 4.5-6.5. With an increase in AE*, both the chorus occurrence rates and the wave amplitudes increase. While under the condition of enhanced P-d* but small AE*, chorus waves are preferentially detected on the dayside (07 <= MLT <= 14) in a wide range of latitudes (vertical bar MLAT vertical bar < 20 degrees) at outer L-shells (L = 5.5-6.5). With the increase of P-d*, 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.