Effects of magnetic field configuration on the day-night asymmetry of chorus occurrence rate: A numerical study

Effects of magnetic field configuration on the day-night asymmetry of chorus occurrence rate are investigated using a recently developed hybrid code. Previous observational studies show that dayside chorus occurs in regions where the background magnetic field is nearly homogeneous; these observations suggest that the magnetic field configuration might play a significant role in the dayside chorus excitation. In this work, we use a hybrid code to numerically study the threshold conditions of hot electron distributions required to excite chorus waves for different background magnetic field inhomogeneities. Our numerical experiments demonstrate that in a magnetic field with smaller inhomogeneity, smaller temperature anisotropy and smaller number density of hot electrons are needed to excite chorus waves. This conclusion could help explain the higher occurrence rate of chorus waves on the dayside compared with that on the nightside especially in quiet times. Key PointsThe nearly uniform magnetic field is important to the dayside chorus excitationA small magnetic field inhomogeneity lowers the threshold free energy