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.

Automated summary

Whistler-mode chorus waves in the inner magnetosphere are typically excited by an electron temperature anisotropy. The effects of particle injections from the tail (during substorms) and solar wind ram pressure on chorus wave generation have been separately studied. Chorus waves are mainly observed in the midnight through dawn to noon sectors near the magnetic equator during the period of large particle injections but small solar wind ram pressure. Chorus waves are preferentially detected on the dayside in a wide range of latitudes during the period of enhanced solar wind ram pressure but small particle injections.