Competing Influences of Earthward Convection and Azimuthal Drift Loss on the Pitch Angle Distribution of Energetic Electrons

Utilizing the multi‐point observations by Van Allen Probe A, GOES 13 and 15, we analyzed the competing influences of earthward convection and azimuthal drift loss on the pitch angle distributions of energetic electrons during the simultaneous increases in solar wind flow velocity and pressure. The increase in solar wind speed amplifies the dawn‐dusk convection electric field and causes the earthward transport of energetic electrons, and meanwhile the enhancement of solar wind dynamic pressure causes the inward displacement of dayside magnetopause and triggers the azimuthal drift loss of energetic electrons. The earthward convection of low‐energy electrons (<60 keV) is much faster than their azimuthal drift loss at most pitch angles, and the fast earthward convections make the butterfly‐like electron pitch angle distributions formed early become pancake‐like distributions. The 60–530 keV electrons maintain the butterfly‐like pitch angle distributions during the earthward convections, whereas the high‐energy electrons above 530 keV are not transported to the low‐L shells because of fast drift loss in the high‐L source region. The competition between the earthward convection and the azimuthal drift loss finally determines the pitch angle distributions of energetic electrons near the trapping boundary during the increases in solar wind flow speed and pressure. Previous studies mainly focus on the butterfly‐like electron pitch angle distributions caused by magnetopause shadowing (drift loss) during the enhancement of solar wind dynamic pressure. However, it is not clear how the electron pitch angle distributions respond to the simultaneous increases in solar wind flow speed and pressure. Here, we found that the pitch angle distributions of different‐energy electrons display different responses to the simultaneous increases in solar wind flow speed and density/pressure. The different electron pitch angle distributions are due to different competitions between the electron drift loss and earthward convection. These results improve our understanding to the formation mechanisms of different electron pitch angle distributions during the simultaneous increases in solar wind flow speed and pressure. Increases in solar wind flow speed and dynamic pressure cause the earthward convection and azimuthal drift loss of energetic electronsThe fast earthward convections of low‐energy electrons (<60 keV) flatten their butterfly‐like pitch angle distributions formed earlyThe energetic electrons above 60 keV maintain butterfly‐like distributions because of the fast azimuthal drift loss in high‐L source region Increases in solar wind flow speed and dynamic pressure cause the earthward convection and azimuthal drift loss of energetic electrons The fast earthward convections of low‐energy electrons (<60 keV) flatten their butterfly‐like pitch angle distributions formed early The energetic electrons above 60 keV maintain butterfly‐like distributions because of the fast azimuthal drift loss in high‐L source region