Whistler mode waves driven by anisotropic electrons in the Martian magnetosphere

We present MAVEN observations of narrowband electromagnetic waves at frequencies between the local electron cyclotron and lower hybrid frequencies in the Martian magnetosphere. Although Phobos-2 first measured bursts of electric field waves below the electron cyclotron frequency, the lack of magnetic field wave data hindered definitive identification of the wave mode and generation mechanisms. We analyze electric and magnetic field wave spectra obtained by MAVEN and demonstrate that the observed narrowband waves have properties consistent with whistler mode. We utilize electron measurements to compute linear growth rates of whistler mode and show that cyclotron resonance with anisotropic electrons on open and closed field lines can produce the observed narrowband features. Additionally, a statistical survey reveals that the narrowband wave events and anisotropic electrons have similar spatial distributions, providing further support for this scenario. Interestingly, some of the observed waves display complex frequency-time structures that resemble whistler-mode chorus waves in the terrestrial inner magnetosphere, including discrete elements of rising tones and two bands above and below half the electron cyclotron frequency. Whistler mode waves driven by anisotropic electrons, previously observed in intrinsic magnetospheres and at unmagnetized airless bodies, are now confirmed by MAVEN at Mars.