Hybrid code simulations of whistler mode chorus waves in one- and two-dimensinoal dipole coordinates

Abstract:

We simulate whistler mode chorus waves using a hybrid code with a dipole field in both one and two dimensions. Simulations in two dimensional Cartesian coordinates are also performed and compared with full particle simulations. There are four species in the simulations, hot ring current electrons initialized with a bi-Maxwellian distribution with A_hot=T_&perp,hot/T_||,hot-1>0, warm electrons, cold inertialess fluid electrons and parotons as an immobile background. The density of the hot population is a small fraction of the total plasma density. The warm electrons are used to make the dispersion relation of whistler waves in our model more accurate. Comparison between the dispersion relation of our model and other dispersion relations shows that our model is more accurate for lower frequency whistlers than for higher frequency whistlers. Simulations in 2-D Cartesian coordinates agree very well with a full particle code. In the one dimensional simulations along the dipole magnetic field, the predicted frequency and wave number are observed. Rising tones are observed in the 1/14 scale simulations that have larger than realistic inhomogeneity. However, in the full scale 1-D simulation in a dipole field, the waves are more broad-band and do not exhibit rising tones. In the 2-D simulations, the waves are generated with propagation approximately parallel to the background magnetic field. However, the wave fronts become oblique as they propagate to higher latitudes. Simulations with different plasma density profiles across L-shell are performed to study the effect of the background density on whistler propagation. With constant total density n_e or total density that decreased moderately with respect to L, the waves refract toward larger L. With a steeper gradient of total density in L, the refraction is inward toward smeller L.