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1. Particle-in-cell simulations – ion beam instabilities and the generation of Alfvén and whistler waves in low β plasma

   https://doi.org/10.1093/mnras/stad2897  

   Che, H.; Benz, A_O; Zank, G_P (September 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Ion beam-driven instabilities in a collisionless space plasma with low β, i.e. low plasma and magnetic pressure ratio, are investigated using particle-in-cell (PIC) simulations. Specifically, the effects of different ion drift velocities on the development of Buneman and resonant electromagnetic (EM) right-handed (RH) ion beam instabilities are studied. Our simulations reveal that both instabilities can be driven when the ion beam drift exceeds the theoretical thresholds. The Buneman instability, which is weakly triggered initially, dissipates only a small fraction of the kinetic energy of the ion beam while causing significant electron heating, owing to the small electron-ion mass ratio. However, we find that the ion beam-driven Buneman instability is quenched effectively by the resonant EM RH ion beam instability. Instead, the resonant EM RH ion beam instability dominates when the ion drift velocity is larger than the Alfvén speed, leading to the generation of RH Alfvén waves and RH whistler waves. We find that the intensity of Alfvén waves decreases with decrease of ion beam drift velocity, while the intensity of whistler waves increases. Our results provide new insights into the complex interplay between ion beams and plasma instabilities in low β collisionless space plasmas. 

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2. Isolated Proton Aurora Driven by EMIC Pc1 Wave: PWING, Swarm, and NOAA POES Multi‐Instrument Observations

   https://doi.org/10.1029/2021GL095090  

   Kim, Hyangpyo; Shiokawa, Kazuo; Park, Jaeheung; Miyoshi, Yoshizumi; Miyashita, Yukinaga; Stolle, Claudia; Connor, Hyunju Kim; Hwang, Junga; Buchert, Stephan; Kwon, Hyuck‐Jin; et al (September 2021, Geophysical Research Letters)

   Abstract We report the concurrent observations of F‐region plasma changes and field‐aligned currents (FACs) above isolated proton auroras (IPAs) associated with electromagnetic ion cyclotron Pc1 waves. Key events on March 19, 2020 and September 12, 2018 show that ground magnetometers and all‐sky imagers detected concurrent Pc1 wave and IPA, during which NOAA POES observed precipitating energetic protons. In the ionospheric F‐layer above the IPA zone, the Swarm satellites observed transverse Pc1 waves, which span wider latitudes than IPA. Around IPA, Swarm also detected the bipolar FAC and localized plasma density enhancement, which is occasionally surrounded by wide/shallow depletion. This indicates that wave‐induced proton precipitation contributes to the energy transfer from the magnetosphere to the ionosphere. 

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3. Field Line Resonances and Cavity Modes at Earth and Jupiter

   https://doi.org/10.3389/fspas.2022.913554  

   Lysak, Robert L. (June 2022, Frontiers in Astronomy and Space Sciences)

   Ultra-Low-Frequency (ULF) waves provide a means for the rapid propagation of energy and field-aligned current in planetary magnetospheres. At Earth, the ULF frequency range is usually defined as including waves with periods of 0.2–600 s; however, at Jupiter these waves can extend to periods of tens of minutes. In both magnetospheres, shear mode Alfvén waves can form field line resonances that exist between the ionospheres, with periods of a few minutes at Earth and a few tens of minutes at Jupiter. A major distinction between these two magnetospheres is in the density distribution. Earth has a dense ionosphere full of heavy ions, an extended, cold plasmasphere and a relatively low-density plasma sheet. In contrast, at Jupiter, the ionosphere is largely hydrogen (both in atomic form and in the H 3 + molecular ion), there is no appreciable plasmasphere and the plasma disk is dense and populated with heavy ions (largely sulfur and oxygen) originating at the moon Io and to some extent from other moons. As at Earth, the sharp Alfvén speed gradient above the ionosphere forms an ionospheric Alfvén resonator at Jupiter with periods of seconds. Furthermore, the high-latitude lobes at Jupiter have very low density and a resonant structure can be formed by waves bouncing between the ionosphere and the dense plasma disk. This structure leads to periods of tens of seconds. Finally, the dense Io plasma torus and plasma sheet provide conditions for compressional cavity modes to form in this region. Thus, the structure of the field line resonance modes is quite different at the two planets. Implications of these resonances on auroral particle acceleration will be discussed. 

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4. Mid‐Latitude Auroras and Energetic Particle Precipitation Occurred Unusually in a Moderate Magnetic Storm on 1 December 2023

   https://doi.org/10.1029/2024GL110764  

   Ma, Longxing; Yu, Yiqun; Ding, Xiaoqi; Liu, Xiyu; An, Depeng; Zhou, Chenlong; Cao, Jinbin; Shiokawa, Kazuo (September 2024, Geophysical Research Letters)

   Abstract Mid‐latitude auroras are conventionally generated during intense magnetic storms. However, mid‐latitude auroras were observed by naked eyes at Beijing China (39°N, 116°E) unusually during a moderate storm event on 1 December 2023 with the minimum Sym‐H index only −120 nT. This study combines conjugative in‐site and ground‐based observations to analyze the auroras and underlying physical processes. Results indicate that both electron and proton auroras appeared at low latitudes. Electron auroras predominantly arise from low‐energy electron precipitation, but proton auroras may be explained by energetic tens of keV proton precipitation. Pc1/EMIC waves are observed at low latitudes in the ionosphere, potentially accounting for mid‐latitude proton auroras. Downward field‐aligned currents (FACs) are also detected at low latitudes, producing significant magnetic perturbations. This study reveals the underlying ionospheric responses to the mid‐latitude auroras to understand potential reasons for observing aurora at such mid‐latitudes during a moderate storm. 

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5. MHD and Ion Kinetic Waves in Field-aligned Flows Observed by Parker Solar Probe

   https://doi.org/10.3847/1538-4357/ac28fb  

   Zhao, L.-L.; Zank, G. P.; He, J. S.; Telloni, D.; Adhikari, L.; Nakanotani, M.; Kasper, J. C.; Bale, S. D. (November 2021, The Astrophysical Journal)

   Abstract Parker Solar Probe (PSP) observed predominately Alfvénic fluctuations in the solar wind near the Sun where the magnetic field tends to be radially aligned. In this paper, two magnetic-field-aligned solar wind flow intervals during PSP’s first two orbits are analyzed. Observations of these intervals indicate strong signatures of parallel/antiparallel-propagating waves. We utilize multiple analysis techniques to extract the properties of the observed waves in both magnetohydrodynamic (MHD) and kinetic scales. At the MHD scale, outward-propagating Alfvén waves dominate both intervals, and outward-propagating fast magnetosonic waves present the second-largest contribution in the spectral energy density. At kinetic scales, we identify the circularly polarized plasma waves propagating near the proton gyrofrequency in both intervals. However, the sense of magnetic polarization in the spacecraft frame is observed to be opposite in the two intervals, although they both possess a sunward background magnetic field. The ion-scale plasma wave observed in the first interval can be either an inward-propagating ion cyclotron wave (ICW) or an outward-propagating fast-mode/whistler wave in the plasma frame, while in the second interval it can be explained as an outward ICW or inward fast-mode/whistler wave. The identification of the exact kinetic wave mode is more difficult to confirm owing to the limited plasma data resolution. The presence of ion-scale waves near the Sun suggests that ion cyclotron resonance may be one of the ubiquitous kinetic physical processes associated with small-scale magnetic fluctuations and kinetic instabilities in the inner heliosphere. 

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