Browsing by Author "Romanelli, N."
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Item Upstream Ultra‐Low Frequency Waves Observed by MESSENGER's Magnetometer: Implications for Particle Acceleration at Mercury's Bow Shock(American Geophysical Union, 2020-04-09) Romanelli, N.; DiBraccio, G.; Gershman, D.; Le, G.; Mazelle, C.; Meziane, K.; Boardsen, S.; Slavin, J.; Raines, J.; Glass, A.; Espley, J.We perform the first statistical analysis of the main properties of waves observed in the 0.05–0.41 Hz frequency range in the Hermean foreshock by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Magnetometer. Although we find similar polarization properties to the “30 s” waves observed at the Earth's foreshock, the normalized wave amplitude (δB/|B₀|∼0.2) and occurrence rate (∼0.5%) are much smaller. This could be associated with relatively lower backstreaming proton fluxes, the smaller foreshock size and/or less stable solar wind (SW) conditions around Mercury. Furthermore, we estimate that the speed of resonant backstreaming protons in the SW reference frame (likely source for these waves) ranges between 0.95 and 2.6 times the SW speed. The closeness between this range and what is observed at other planetary foreshocks suggests that similar acceleration processes are responsible for this energetic population and might be present in the shocks of exoplanets.Item Variability of Upstream Proton Cyclotron Wave Properties and Occurrence at Mars observed by MAVEN(American Geophysical Union, 2020-12-11) Romeo, O. M.; Romanelli, N.; Espley, J. R.; Mazelle, C.; DiBraccio, G. A.; Gruesbeck, J. R.; Halekas, J. S.The presence of plasma waves upstream from the Martian bow shock, with frequencies near the local proton cyclotron frequency in the spacecraft frame, constitutes, in principle, an indirect signature for the existence of planetary protons from the ionization of Martian exospheric hydrogen. In this study, we determine the ‘proton cyclotron wave’ (PCW) occurrence rate between October 2014 through February 2020, based on Magnetometer (MAG) and Solar Wind Ion Analyzer (SWIA) measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We characterize its dependence on several wave and solar wind (SW) properties, and solar longitude ranges. We confirm a previously reported long‐term trend with more PCWs near perihelion, likely associated with changes in exospheric hydrogen density. Furthermore, we report for the first time a decrease in median PCW amplitude for each consecutive Martian perihelion. Such variability cannot be attributed to differences in the distribution of SW conditions. This trend could be associated with changes in solar inputs, foreshock effects, and asymmetries due to the SW convective electric field influencing newborn protons. In addition, we observe PCWs more frequently for low to intermediate interplanetary magnetic field (IMF) cone angles, slower SW speeds, and higher SW proton densities. The IMF cone angle preference likely results from the trade‐off between associated linear wave growth rates, wave saturation energies, and pick‐up proton densities. Moreover, the dependencies on SW speed and density indicate the importance of the characteristic SW transit timescale and the charge exchange process coupling SW protons with the hydrogen exosphere.