EGUsphere EGUsphere EGUsphere EGUsphere Copernicus Publications Göttingen, Germany 10.5194/egusphere-2025-60 Ion beam instability model for the Mercury upstream waves Narita Yasuhito https://orcid.org/0000-0002-5332-8881 1 3 Schmid Daniel https://orcid.org/0000-0001-7818-4338 2 3 Motschmann Uwe 1 3 Institut für Theoretische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria These authors contributed equally to this work. 31 01 2025 2025 1 10 Copyright: © 2025 Yasuhito Narita et al. 2025 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://egusphere.copernicus.org/preprints/2025/egusphere-2025-60/ The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2025/egusphere-2025-60/egusphere-2025-60.pdf

An analytic model for the ion beam instability is constructed in view of application to the Mercury upstream waves. Our ion beam instability model determines the frequency and the wavenumber by equating the whistler dispersion relation with the beam resonance condition in favor of planetary foreshock wave excitation. By introducing the Doppler shift in the instability frequency, our model can derive the observer-frame relation of the resonance frequency to the beam velocity and the flow speed. The frequency relation serves as a useful diagnostic tool to the Mercury upstream wave studies in the upcoming BepiColombo observations.