Ground-based radar unveils natural electron Bernstein waves in the ionosphere of Earth

Electron Bernstein waves (EBWs) are electrostatic modes that propagate perpendicular to the magnetic field in magnetized plasmas, serving as a diagnostic tool for understanding plasma properties and the energetic particles that drive energy transport. However, their naturally occurring harmonic structures have long eluded ground-based radar detection due to their faint signals and the requirement for an observational geometry nearly perpendicular to the geomagnetic field. Here, we report high-resolution broadband spectral observations of naturally excited EBWs in Earth's ionosphere using the Sanya Incoherent Scatter Radar. Our measurements reveal a series of discrete, well-defined spectral lines corresponding to the first seven electron gyroharmonics, a result consistent with theoretical predictions. Our analysis suggests that these EBW modes, enhanced by suprathermal photoelectrons, become observable under near-perpendicular geometry, where Landau damping is reduced. The frequency structure of these harmonics acts as a precise diagnostic ruler that enables the retrieval of electron density and perpendicular electron temperature, parameters that are difficult to retrieve with ion line techniques under such geometric constraints. This work provides ground-based radar observations of naturally excited EBWs and shows their use for diagnosing plasma parameters in magnetized plasmas.