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Bounce Resonance Between ULF Waves and Electrons in the Dayside Outer Magnetosphere.

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Title: Bounce Resonance Between ULF Waves and Electrons in the Dayside Outer Magnetosphere.
Authors: Zhao, Zi‐He1 (AUTHOR), Li, Xing‐Yu2 (AUTHOR), Zong, Qiu‐Gang1,3 (AUTHOR) qgzong@pku.edu.cn, Liu, Zhi‐Yang4 (AUTHOR), Zhao, Xing‐Xin1 (AUTHOR), Sun, Yi‐Xin1,3 (AUTHOR), Hao, Yi‐Xin1 (AUTHOR), Pollock, Craig5 (AUTHOR), Russell, Christopher T.6 (AUTHOR), Ergun, Robert E.7 (AUTHOR), Lindqvist, Per‐Arne8 (AUTHOR)
Source: Geophysical Research Letters. 6/28/2026, Vol. 53 Issue 12, p1-11. 11p.
Subject Terms: *Electrons, *Magnetosphere, *Angles, *Energy transfer
Company/Entity: United States. National Aeronautics & Space Administration , Magnetospheric Multiscale Mission (U.S.)
Abstract: Resonant interactions between ultra‐low‐frequency (ULF) waves and particles are critical for energy transfer in the Earth's magnetosphere, and have been extensively investigated in the inner magnetosphere. However, resonant processes between ULF waves and electrons in the dayside outer magnetosphere remain largely unexplored. Using MMS data, we investigate their bounce resonance in the dayside outer magnetosphere. Quasi‐periodic oscillations of electron fluxes below 1 keV and ULF waves with periods comparable to the electron bounce periods are observed simultaneously. Test‐particle simulations demonstrate that off‐equatorial magnetic field minima can increase the number of resonant pitch angles for electrons at a given energy. Overlap of stripes corresponding to different resonant energies gives rise to observed 360° ${}^{\circ}$ phase shifts. Furthermore, off‐equatorial minima and temporal evolution of ULF waves broaden the conventional bounce resonance condition ω=Nωb $\omega =N{\omega }_{b}$, which links the harmonic type of ULF waves to the parity of N $N$. Plain Language Summary: In the Earth's magnetosphere, electric and magnetic fields can oscillate over timescales ranging from a few seconds to several minutes. These oscillations, known as ultra‐low‐frequency (ULF) waves, can transfer energy to charged particles through resonant interactions. In the dayside outer magnetosphere, where the geomagnetic field is compressed by the solar wind, these interactions differ from the traditional picture developed for more symmetric magnetic fields. We analyze an event observed by NASA's Magnetospheric Multiscale (MMS) mission and conduct numerical simulations to better understand this process. We find that solar wind compression of the geomagnetic field, together with changes in ULF waves over time, can broaden the conditions under which resonance occurs. These findings improve our understanding of how particles are accelerated in the Earth's magnetosphere. Key Points: Bounce resonance between ultra‐low‐frequency (ULF) waves and electrons at several hundred eV is observed by MMS in the dayside outer magnetosphereFor electrons at a given energy, off‐equatorial magnetic field minima significantly increase the number of resonant pitch anglesOff‐equatorial magnetic field minima and the temporal evolution of ULF waves broaden the conventional resonance condition [ABSTRACT FROM AUTHOR]
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  Data: Bounce Resonance Between ULF Waves and Electrons in the Dayside Outer Magnetosphere.
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  Data: <searchLink fieldCode="AR" term="%22Zhao%2C+Zi‐He%22">Zhao, Zi‐He</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Li%2C+Xing‐Yu%22">Li, Xing‐Yu</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Zong%2C+Qiu‐Gang%22">Zong, Qiu‐Gang</searchLink><relatesTo>1,3</relatesTo> (AUTHOR)<i> qgzong@pku.edu.cn</i><br /><searchLink fieldCode="AR" term="%22Liu%2C+Zhi‐Yang%22">Liu, Zhi‐Yang</searchLink><relatesTo>4</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Zhao%2C+Xing‐Xin%22">Zhao, Xing‐Xin</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Sun%2C+Yi‐Xin%22">Sun, Yi‐Xin</searchLink><relatesTo>1,3</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Hao%2C+Yi‐Xin%22">Hao, Yi‐Xin</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Pollock%2C+Craig%22">Pollock, Craig</searchLink><relatesTo>5</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Russell%2C+Christopher+T%2E%22">Russell, Christopher T.</searchLink><relatesTo>6</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Ergun%2C+Robert+E%2E%22">Ergun, Robert E.</searchLink><relatesTo>7</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Lindqvist%2C+Per‐Arne%22">Lindqvist, Per‐Arne</searchLink><relatesTo>8</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Geophysical+Research+Letters%22">Geophysical Research Letters</searchLink>. 6/28/2026, Vol. 53 Issue 12, p1-11. 11p.
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  Data: *<searchLink fieldCode="DE" term="%22Electrons%22">Electrons</searchLink><br />*<searchLink fieldCode="DE" term="%22Magnetosphere%22">Magnetosphere</searchLink><br />*<searchLink fieldCode="DE" term="%22Angles%22">Angles</searchLink><br />*<searchLink fieldCode="DE" term="%22Energy+transfer%22">Energy transfer</searchLink>
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  Label: Abstract
  Group: Ab
  Data: Resonant interactions between ultra‐low‐frequency (ULF) waves and particles are critical for energy transfer in the Earth's magnetosphere, and have been extensively investigated in the inner magnetosphere. However, resonant processes between ULF waves and electrons in the dayside outer magnetosphere remain largely unexplored. Using MMS data, we investigate their bounce resonance in the dayside outer magnetosphere. Quasi‐periodic oscillations of electron fluxes below 1 keV and ULF waves with periods comparable to the electron bounce periods are observed simultaneously. Test‐particle simulations demonstrate that off‐equatorial magnetic field minima can increase the number of resonant pitch angles for electrons at a given energy. Overlap of stripes corresponding to different resonant energies gives rise to observed 360° ${}^{\circ}$ phase shifts. Furthermore, off‐equatorial minima and temporal evolution of ULF waves broaden the conventional bounce resonance condition ω=Nωb $\omega =N{\omega }_{b}$, which links the harmonic type of ULF waves to the parity of N $N$. Plain Language Summary: In the Earth's magnetosphere, electric and magnetic fields can oscillate over timescales ranging from a few seconds to several minutes. These oscillations, known as ultra‐low‐frequency (ULF) waves, can transfer energy to charged particles through resonant interactions. In the dayside outer magnetosphere, where the geomagnetic field is compressed by the solar wind, these interactions differ from the traditional picture developed for more symmetric magnetic fields. We analyze an event observed by NASA's Magnetospheric Multiscale (MMS) mission and conduct numerical simulations to better understand this process. We find that solar wind compression of the geomagnetic field, together with changes in ULF waves over time, can broaden the conditions under which resonance occurs. These findings improve our understanding of how particles are accelerated in the Earth's magnetosphere. Key Points: Bounce resonance between ultra‐low‐frequency (ULF) waves and electrons at several hundred eV is observed by MMS in the dayside outer magnetosphereFor electrons at a given energy, off‐equatorial magnetic field minima significantly increase the number of resonant pitch anglesOff‐equatorial magnetic field minima and the temporal evolution of ULF waves broaden the conventional resonance condition [ABSTRACT FROM AUTHOR]
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        Value: 10.1029/2026GL122727
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        Text: English
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      – SubjectFull: Electrons
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      – SubjectFull: Magnetosphere
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