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Recent studies have indicated that highly oblique lower band chorus could be excited by temperature anisotropy with a low-energy electron plateau. Here we present another excitation mechanism of highly oblique lower and upper band chorus by using the simultaneous observations and numerical modeling. During 23:00-24:00UT on 3 July 2016, Van Allen probe A observed chorus with the wave normal angle close to the resonance cone both in the lower and upper bands around L=5.0 and MLT=3. Enhanced flux and pitch angle anisotropy of energetic electrons were observed in the same spatial region. Calculations of chorus growth rates show that highly oblique chorus waves can be excited by the energetic electrons with a loss cone feature and distinct temperature anisotropy, particularly more efficient in the presence of a low-energy plateau. The temperature anisotropy for the oblique upper band is higher than that for the lower band.

Plain Language Summary Highly oblique chorus can provide efficient energy diffusion for the rapid acceleration of electrons. The electron pitch angle scattering rates driven by them could be stronger than those by quasi-parallel chorus with the same wave amplitudes. Recent studies have indicated that highly oblique lower band chorus could be excited by a low-energy electron beam or electron plateau. Here we present another excitation mechanism of highly oblique lower and upper band chorus by using the simultaneous observations and numerical modeling. A highly oblique chorus waves event was observed by Van Allen Probe A on 3 July 2016. The waves occurred in both lower and upper bands during 23:08-23:25UT and then only in the lower band during 23:25-24:00UT. We calculated chorus growth rates based on the observed data. At 23:18:11UT, the growth rate has peaks approximate to 1.3x10(-4) and 1.6x10(-4) at 0.32f(ce) and 0.64f(ce), respectively. At 23:54:07UT, the growth rate has a peak approximate to 1.2x10(-4) at 0.34f(ce) in the lower band and a peak below 1x10(-5) in the upper band. Our new results provide a reliable evidence that highly oblique chorus waves can be excited by the energetic electrons with a loss cone distribution and distinct temperature anisotropy.