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In Situ Observations of the Formation of Periodic Collisionless Plasma Shocks from Fast Mode Waves
Lican Shan, Aimin Du, Bruce Tsurutani, Yasong Ge, Quanming Lu, Christian Mazelle, Can Huang, Karl-Heinz Glassmeier, Pierre Henri
To cite this version:
Lican Shan, Aimin Du, Bruce Tsurutani, Yasong Ge, Quanming Lu, et al.. In Situ Observations of the
Formation of Periodic Collisionless Plasma Shocks from Fast Mode Waves. The Astrophysical journal
letters, Bristol : IOP Publishing, 2020, 888 (2), pp.L17. �10.3847/2041-8213/ab5db3�. �insu-02514825�
In Situ Observations of the Formation of Periodic Collisionless Plasma Shocks from Fast Mode Waves
Lican Shan 1,2,10 , Aimin Du 1,2,3,10 , Bruce T. Tsurutani 4 , Yasong S. Ge 1,2,3 , Quanming Lu 5,6 , Christian Mazelle 7 , Can Huang 1,2 , Karl-Heinz Glassmeier 8 , and Pierre Henri 9
1
Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, Peopleʼs Republic of China ysge@mail.iggcas.ac.cn
2
Innovation Academy for Earth Science, CAS, Beijing, 100029, Peopleʼs Republic of China
3
College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, 100049, People ʼ s Republic of China
4
Independent Scholar, Pasadena, California, 91109, USA
5
CAS Key Lab of Geospace Environment, University of Science and Technology of China, Hefei, 230026, People ʼ s Republic of China
6
CAS Center for Excellence in Comparative Planetology, People ʼ s Republic of China
7
Institut de Recherche en Astrophysique et Planétologie, University Paul Sabatier, CNRS, Toulouse, F-31400, France
8
Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstrae 3, D-38116, Braunschweig, Germany
9
Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, UMR 7328 CNRS, Université d’Orléans, F-45100, Orléans, France Received 2019 November 20; accepted 2019 November 27; published 2020 January 9
Abstract
It has been long theorized, but not directly observed, that low-frequency magnetosonic plasma waves can steepen and form shocks. We show an example of small-amplitude, sinusoidal magnetosonic waves at the proton gyrofrequency upstream of the Martian bow shock. We hypothesize that these waves are produced by an ion beam instability associated with the ionization of hydrogen atoms by charge exchange with solar wind protons, solar photoionization, and / or electron impact ionization. As the waves are convected toward the planet by solar wind fl ow, the wave amplitude grows due to additional free energy put into the system by further ion beam particles.
Finally, the steepened waves form shocks. Because of their development, the shocks are periodic with the separation at the proton gyroperiod. These observations lead to the conclusion that newborn ions may play a crucial role in the formation process of some collisionless plasma shocks in astrophysical and space plasmas.
Unified Astronomy Thesaurus concepts: Shocks ( 2086 ) ; Pickup ions ( 1239 ) ; Plasma physics ( 2089 )
1. Introduction
Collisionless plasma shocks ( CPSs ) are believed to be abundant in astrophysical plasmas. They occur when a super-magnetosonic streaming plasma encounters a blunt obstacle with a magnetic fi eld, a planetary or cometary ionosphere, or a slowly moving or stagnant plasma. CPSs are responsible for the acceleration of energetic (∼ TeV ) cosmic rays ( Aharonian et al. 2004 ) and solar fl are energetic particles ( Tsurutani & Lin 1985; Reames 1999;
Turner et al. 2018 ) . Nevertheless, how CPSs form in the vast universe is still one of the biggest mysteries in astrophysics.
Presently, one well-accepted theory is that small-amplitude, low- frequency sinusoidal waves are excited by a backstreaming ion beam through a background plasma ( Kennel & Petschek 1966 ) . As the instability continues to grow, the amplitudes of the sinusoidal waves increase. Nonlinear forces and the addition of more free energy input cause the waves to steepen ( Treumann 2009 ) . In view of magnetohydrodynamics theory, when further steepening occurs, the waves will ultimately break until the shock dissipation and dispersion happen, which limits or balances the nonlinear process ( Haerendel & Paschmann 1982; Kennel et al. 1985; Treumann &
Baumjohann 1997 ) . These waves evolve into phase-steepened, large-amplitude waves, and fi nally to CPSs that are both dispersive and dissipative. However, in situ measurements of CPSs that have evolved from linear waves have not yet been observed.
2. Observations
In this Letter, we report in situ observations of an entire formation sequence of the periodic plasma shocks by the Mars
Atmosphere and Volatile Evolution (MAVEN) instrumentation, as the spacecraft travels through the solar wind, the Martian exosphere, and ionosphere ( see Figure 1 ) . The developed shocks are intriguing because their original waves are excited by newborn ions when we analyze the wave properties. The newborn ions are created by photoionization, charge exchange, and electron impact ionization ( Rahmati et al. 2017 ) of the escaped neutral atoms of the Martian exosphere. The newly formed ions are an ion beam in the solar wind plasma frame and become the source of plasma instabilities for the sinusoidal seed electromagnetic waves, and ultimately for the shocks. This provides the possibility of studying the shock formation process behind some astrophysical and space plasma scenarios.
The full view of the various stages of sinusoidal wave evolution during this event on 2015 February 12 are demonstrated by the magnetic fi eld variations in four time intervals marked by different color bars in Figure 2 ( a ) : quasi-sinusoidal low-frequency waves ( QSLFWs ) , nonsinusoidal phase-steepened waves ( NPSWs ) , fast mode periodic shocks ( FMPSs ) , and magnetosonic collisionless shocks ( MCSs ) . Individual stage of the wave evolution have been observed at Mars ( Dubinin & Fraenz 2016; Collinson et al.
2018; Halekas et al. 2019 ) . The interplanetary magnetic fi eld ( Figure 2 ( a )) is measured by the Magnetometer ( MAG ) instrument ( Connerney et al. 2015 ) . We calculated the average fi eld with a low-pass fi lter from 0 to 0.04 Hz during the interval from 01:25 to 01:32 UT. The average fi eld has a small inclination (θ
Bx∼ 34 °) from the x -axis of Mars Solar Orbital ( MSO ) coordinates and a constant magnitude. Within this quiet solar wind, we identify an interval of the QSLFW with the average magnetic magnitude of
∼ 3.03 ± 0.34 nT ( Figure 2 ( b )) . The frequency of the observed waves ( 0.041 Hz ) is very close the local proton gyrofrequency
The Astrophysical Journal Letters, 888:L17 ( 5pp ) , 2020 January 10 https: // doi.org / 10.3847 / 2041-8213 / ab5db3
© 2020 The Author(s). Published by the American Astronomical Society.
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