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Kinetic Structure of an Interplanetary Shock Observed at Two Heliocentric Longitudes.

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Title: Kinetic Structure of an Interplanetary Shock Observed at Two Heliocentric Longitudes.
Authors: Boldú, J. J.1,2 (AUTHOR) jordi.boldu@irfu.se, Graham, D. B.1 (AUTHOR), Morooka, M.1 (AUTHOR), André, M.1 (AUTHOR), Khotyaintsev, Yu. V.1,2 (AUTHOR), Dimmock, A.1 (AUTHOR), Lalti, A.3 (AUTHOR), Píša, D.4 (AUTHOR), Souček, J.4 (AUTHOR), Maksimovic, M.5 (AUTHOR), Louarn, P.6 (AUTHOR), Fedorov, A.6 (AUTHOR), Owen, C. J.7 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. Mar2026, Vol. 131 Issue 3, p1-14. 14p.
Subject Terms: Ion acoustic waves, Energy dissipation, Kinetic theory of matter, Magnetohydrodynamic waves, Collisionless plasmas, Solar wind
Company/Entity: Magnetospheric Multiscale Mission (U.S.)
Abstract: Collisionless shocks convert bulk flow energy into heat, electromagnetic fields, and non‐thermal particle populations. Recent studies suggest that downstream magnetic oscillations could play an important role in ion‐scale energy dissipation at low‐Mach‐number shocks; however, the specific shock and plasma parameters involved remain poorly understood. Interplanetary (IP) shocks, often characterized by low Mach numbers, provide an excellent opportunity for investigating these kinetic dissipation mechanisms. We demonstrate, using observations of an IP shock from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) missions, supported by test‐particle simulations, that gyrating protons generate the downstream magnetic oscillations. We found bursts of ion‐acoustic waves at the troughs and crests of the magnetic oscillations, suggesting their energy source is related to proton gyration. Comparing MMS and SolO observations, we conclude that the upstream flow speed to ion thermal speed ratio and magnetic compression ratio are key parameters controlling the ion kinetic behavior that produces downstream magnetic oscillations. Plain Language Summary: Collisionless shocks are characterized by abrupt changes in plasma parameters. In the low‐collisionality environment of the solar wind, the energy dissipation necessary for the shocks to exist is typically provided by processes involving the electromagnetic fields. Recent research suggests that fluctuations in the magnetic field downstream of the shock transition layer are a key feature of energy dissipation at low‐Mach‐number shocks. These oscillations have been attributed to gyrating α $\alpha $‐particles at Earth's bow shock. However, observational evidence of these oscillations at interplanetary (IP) shocks has not been conclusively established. We analyze an IP shock using data from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) missions and combined them with test‐particle simulations. We identify key kinetic features associated with the IP shock, including downstream magnetic oscillations, reflection of protons and α $\alpha $‐particles, and periodic bursts of ion‐acoustic waves (IAWS). Our findings indicate that downstream magnetic oscillations and IAWs are closely linked to the gyration of protons as they transverse the shock, suggesting that ion‐scale dissipation is driven by the proton kinetic behavior. Additionally, the differences between the measurements from MMS and SolO arise from distinct solar wind conditions at both locations, specifically the upstream bulk velocity and the shock's magnetic compression ratio. Key Points: We observe a low Alfvén‐Mach‐number interplanetary shock with a similar shock normal angle at two heliocentric longitudes ∼13° ${}^{\circ}$ apartWe identify key kinetic features, including downstream magnetic oscillations, ion‐acoustic waves, and proton and alpha reflectionIon gyro‐motion differs significantly between the two downstream regions, producing distinct magnetic profiles and ion‐acoustic activity [ABSTRACT FROM AUTHOR]
Copyright of Journal of Geophysical Research. Space Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Kinetic Structure of an Interplanetary Shock Observed at Two Heliocentric Longitudes.
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  Data: <searchLink fieldCode="AR" term="%22Boldú%2C+J%2E+J%2E%22">Boldú, J. J.</searchLink><relatesTo>1,2</relatesTo> (AUTHOR)<i> jordi.boldu@irfu.se</i><br /><searchLink fieldCode="AR" term="%22Graham%2C+D%2E+B%2E%22">Graham, D. B.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Morooka%2C+M%2E%22">Morooka, M.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22André%2C+M%2E%22">André, M.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Khotyaintsev%2C+Yu%2E+V%2E%22">Khotyaintsev, Yu. V.</searchLink><relatesTo>1,2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Dimmock%2C+A%2E%22">Dimmock, A.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Lalti%2C+A%2E%22">Lalti, A.</searchLink><relatesTo>3</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Píša%2C+D%2E%22">Píša, D.</searchLink><relatesTo>4</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Souček%2C+J%2E%22">Souček, J.</searchLink><relatesTo>4</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Maksimovic%2C+M%2E%22">Maksimovic, M.</searchLink><relatesTo>5</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Louarn%2C+P%2E%22">Louarn, P.</searchLink><relatesTo>6</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Fedorov%2C+A%2E%22">Fedorov, A.</searchLink><relatesTo>6</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Owen%2C+C%2E+J%2E%22">Owen, C. J.</searchLink><relatesTo>7</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Journal+of+Geophysical+Research%2E+Space+Physics%22">Journal of Geophysical Research. Space Physics</searchLink>. Mar2026, Vol. 131 Issue 3, p1-14. 14p.
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  Data: <searchLink fieldCode="DE" term="%22Ion+acoustic+waves%22">Ion acoustic waves</searchLink><br /><searchLink fieldCode="DE" term="%22Energy+dissipation%22">Energy dissipation</searchLink><br /><searchLink fieldCode="DE" term="%22Kinetic+theory+of+matter%22">Kinetic theory of matter</searchLink><br /><searchLink fieldCode="DE" term="%22Magnetohydrodynamic+waves%22">Magnetohydrodynamic waves</searchLink><br /><searchLink fieldCode="DE" term="%22Collisionless+plasmas%22">Collisionless plasmas</searchLink><br /><searchLink fieldCode="DE" term="%22Solar+wind%22">Solar wind</searchLink>
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  Data: <searchLink fieldCode="DE" term="%22Magnetospheric+Multiscale+Mission+%28U%2ES%2E%29%22">Magnetospheric Multiscale Mission (U.S.)</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: Collisionless shocks convert bulk flow energy into heat, electromagnetic fields, and non‐thermal particle populations. Recent studies suggest that downstream magnetic oscillations could play an important role in ion‐scale energy dissipation at low‐Mach‐number shocks; however, the specific shock and plasma parameters involved remain poorly understood. Interplanetary (IP) shocks, often characterized by low Mach numbers, provide an excellent opportunity for investigating these kinetic dissipation mechanisms. We demonstrate, using observations of an IP shock from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) missions, supported by test‐particle simulations, that gyrating protons generate the downstream magnetic oscillations. We found bursts of ion‐acoustic waves at the troughs and crests of the magnetic oscillations, suggesting their energy source is related to proton gyration. Comparing MMS and SolO observations, we conclude that the upstream flow speed to ion thermal speed ratio and magnetic compression ratio are key parameters controlling the ion kinetic behavior that produces downstream magnetic oscillations. Plain Language Summary: Collisionless shocks are characterized by abrupt changes in plasma parameters. In the low‐collisionality environment of the solar wind, the energy dissipation necessary for the shocks to exist is typically provided by processes involving the electromagnetic fields. Recent research suggests that fluctuations in the magnetic field downstream of the shock transition layer are a key feature of energy dissipation at low‐Mach‐number shocks. These oscillations have been attributed to gyrating α $\alpha $‐particles at Earth's bow shock. However, observational evidence of these oscillations at interplanetary (IP) shocks has not been conclusively established. We analyze an IP shock using data from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) missions and combined them with test‐particle simulations. We identify key kinetic features associated with the IP shock, including downstream magnetic oscillations, reflection of protons and α $\alpha $‐particles, and periodic bursts of ion‐acoustic waves (IAWS). Our findings indicate that downstream magnetic oscillations and IAWs are closely linked to the gyration of protons as they transverse the shock, suggesting that ion‐scale dissipation is driven by the proton kinetic behavior. Additionally, the differences between the measurements from MMS and SolO arise from distinct solar wind conditions at both locations, specifically the upstream bulk velocity and the shock's magnetic compression ratio. Key Points: We observe a low Alfvén‐Mach‐number interplanetary shock with a similar shock normal angle at two heliocentric longitudes ∼13° ${}^{\circ}$ apartWe identify key kinetic features, including downstream magnetic oscillations, ion‐acoustic waves, and proton and alpha reflectionIon gyro‐motion differs significantly between the two downstream regions, producing distinct magnetic profiles and ion‐acoustic activity [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Journal of Geophysical Research. Space Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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      – Type: doi
        Value: 10.1029/2026JA035129
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      – Code: eng
        Text: English
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      Pagination:
        PageCount: 14
        StartPage: 1
    Subjects:
      – SubjectFull: Ion acoustic waves
        Type: general
      – SubjectFull: Energy dissipation
        Type: general
      – SubjectFull: Kinetic theory of matter
        Type: general
      – SubjectFull: Magnetohydrodynamic waves
        Type: general
      – SubjectFull: Collisionless plasmas
        Type: general
      – SubjectFull: Solar wind
        Type: general
      – SubjectFull: Magnetospheric Multiscale Mission (U.S.)
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      – TitleFull: Kinetic Structure of an Interplanetary Shock Observed at Two Heliocentric Longitudes.
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              Text: Mar2026
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