Thomas E. Moore, Richard E. Denton, Barbara L. Giles, Huimin Fu, Adolfo F. Viñas, Justin H. Lee, Mats André, N. Kitamura, Daniel B. Graham, Benoit Lavraud, Stephen A. Fuselier, O. Le Contel, James L. Burch, Alfonso Salinas, D. J. Gershman, Yu. V. Khotyaintsev, Enrique A. Navarro, Sarah K. Vines, Jorge A. Portí, Robert Allen, Wangping Li, Sergio Toledo-Redondo, Drew Turner, Scott A. Boardsen, Department of Electromagnetism and Electronics [Murcia], Universidad de Murcia, The Aerospace Corporation, Johns Hopkins University Applied Physics Laboratory [Laurel, MD] (APL), Swedish Institute of Space Physics [Uppsala] (IRF), NASA Goddard Space Flight Center (GSFC), Southwest Research Institute [San Antonio] (SwRI), Department of Physics and Astronomy [Hanover], Dartmouth College [Hanover], School of Space and Environment [Beijing], Beihang University (BUAA), Department of Earth and Planetary Sciences [TITECH Tokyo], Tokyo Institute of Technology [Tokyo] (TITECH), Institut de recherche en astrophysique et planétologie (IRAP), Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Centre National de la Recherche Scientifique (CNRS), Laboratoire de Physique des Plasmas (LPP), Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-École polytechnique (X)-Sorbonne Université (SU)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS), National Space Science Center [Beijing] (NSSC), Chinese Academy of Sciences [Beijing] (CAS), University of Valencia, Universidad de Granada = University of Granada (UGR), Institut national des sciences de l'Univers (INSU - CNRS)-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Observatoire Midi-Pyrénées (OMP), Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Centre National de la Recherche Scientifique (CNRS), Department of Applied Physics, School of Science, University of Granada, 18071 Granada, Spain, University of Granada, and University of Granada [Granada]
STR acknowledges support from the ISSI international team Cold plasma of ionospheric origin in the Earth's magnetosphere and of the Ministry of Economy and Competitiveness (MINECO) of Spain (grant FIS2017-90102-R). Research at IRAP was supported by CNRS, CNES and the University of Toulouse. JHL and DLT acknowledge support from NASA Grant 80NSSC18K1378. RED was supported by NASA grants 80NSSC19K070 and 80NSSC19K0254. MA was supported by SNSA Grant 56/18. SKV and RCA acknowledge support from NASA Grant 80NSSC19K0270. Work performed by MMS team members is supported by NASA contract NNG04EB99C., We report observations of the ion dynamics inside an Alfvén branch wave that propagates near the reconnecting dayside magnetopause. The measured frequency, wave normal angle and polarization are consistent with the predictions of a dispersion solver. The magnetospheric plasma contains hot protons (keV), cold protons (eV), plus some heavy ions. While the cold protons follow the magnetic field fluctuations and remain frozen-in, the hot protons are at the limit of magnetization. The cold protons exchange energy back and forth, adiabatically, with the wave fields. The cold proton velocity fluctuations contribute to balance the Hall term fluctuations in Ohm's law, and the wave E field has small ellipticity and right-handed polarization. The dispersion solver indicates that increasing the cold proton density facilitates propagation and amplification of these waves at oblique angles, as for the observed wave., ISSI international team Cold plasma of ionospheric origin in the Earth's magnetosphere, Ministry of Economy and Competitiveness (MINECO) of Spain FIS2017-90102-R, Centre National de la Recherche Scientifique (CNRS), European Commission, Centre National D'etudes Spatiales, University of Toulouse, National Aeronautics & Space Administration (NASA) 80NSSC18K1378 80NSSC19K070 80NSSC19K0254 80NSSC19K0270 NNG04EB99C, SNSA 56/18