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1. Nonlinear Ion-acoustic Waves, Ion Holes, and Electron Holes in the Near-Sun Solar Wind

   https://doi.org/10.3847/1538-4357/abed52  

   Mozer, F. S.; Bonnell, J. W.; Hanson, E. L. M.; Gasque, L. C.; Vasko, I. Y. (April 2021, The Astrophysical Journal)

   Abstract Nonlinear ion-acoustic waves, ion holes, and electron holes have been observed on the Parker Solar Probe at a heliocentric distance of 35 solar radii. These time domain structures contain millisecond duration electric field spikes of several mV m⁻¹. They are observed inside or at boundaries of switchbacks in the background magnetic field. Their presence in switchbacks indicates that both electron- and ion-streaming electrostatic instabilities occur there to thermalize electron and ion beams. 

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2. Multisatellite Observations of Ion Holes in the Earth's Plasma Sheet

   https://doi.org/10.1029/2022GL097919  

   Wang, R.; Vasko, I. Y.; Artemyev, A. V.; Holley, L. C.; Kamaletdinov, S. R.; Lotekar, A.; Mozer, F. S. (April 2022, Geophysical Research Letters)

   Abstract We present the first observations of electrostatic solitary waves with electrostatic potential of negative polarity around a fast plasma flow in the Earth's plasma sheet. The solitary waves are observed aboard four Magnetospheric Multiscale spacecraft, which allowed accurately estimating solitary wave properties. Based on a data set of 153 solitary waves, we show that they are locally one‐dimensional Debye‐scale structures with amplitudes up to 20% of local electron temperature and they propagate at plasma frame speeds ranging from a tenth to a few ion‐acoustic speeds at arbitrary angles to the local magnetic field. The solitary waves are associated with multi‐component proton distributions and their velocities are around those of a beam‐like proton population. We argue that the solitary waves are ion holes, nonlinear structures produced by ion‐streaming instabilities, and conclude that once ions are not magnetized, ion holes can propagate oblique to local magnetic field. 

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3. A Case Study of Nonresonant Mode 3‐s ULF Waves Observed by MMS

   https://doi.org/10.1029/2020JA028557  

   Wang, Shan; Chen, Li‐Jen; Ng, Jonathan; Bessho, Naoki; Le, Guan; Fung, Shing F.; Gershman, Daniel J.; Giles, Barbara L. (November 2020, Journal of Geophysical Research: Space Physics)

   Abstract The nature of the 3‐s ultralow frequency (ULF) wave in the Earth's foreshock region and the associated wave‐particle interaction are not yet well understood. We investigate the 3‐s ULF waves using Magnetospheric Multiscale (MMS) observations. By combining the plasma rest frame wave properties obtained from multiple methods with the instability analysis based on the velocity distribution in the linear wave stage, the ULF wave is determined to be due to the ion/ion nonresonant mode instability. The interaction between the wave and ions is analyzed using the phase relationship between the transverse wave fields and ion velocities and using the longitudinal momentum equation. During the stage when ULF waves have sinusoidal waveforms up to |dB|/|B₀| ~ 3, wheredBis the wave magnetic field andB₀is the background magnetic field, the wave electric fields perpendicular toB₀do negative work to solar wind ions; alongB₀, a longitudinal electric field develops, but theV × Bforce is stronger and leads to solar wind ion deceleration. During the same wave stage, the backstreaming beam ions gain energy from the transverse wave fields and get deceleration alongB₀by the longitudinal electric field. The ULF wave leads to electron heating, preferentially in the direction perpendicular to the local magnetic field. Secondary waves are generated within the ULF waveforms, including whistler waves near half of the electron cyclotron frequency, high‐frequency electrostatic waves, and magnetosonic whistler waves. The work improves the understanding of the nature of 3‐s ULF waves and the associated wave‐particle interaction. 

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4. Langmuir-Slow Extraordinary Mode Magnetic Signature Observations with Parker Solar Probe

   https://doi.org/10.3847/1538-4357/ac4e85  

   Larosa, A.; Dudok de Wit, T.; Krasnoselskikh, V.; Bale, S. D.; Agapitov, O.; Bonnell, J.; Froment, C.; Goetz, K.; Harvey, P.; Halekas, J.; et al (March 2022, The Astrophysical Journal)

   Abstract Radio emission from interplanetary shocks, planetary foreshocks, and some solar flares occurs in the so-called “plasma emission” framework. The generally accepted scenario begins with electrostatic Langmuir waves that are driven by a suprathermal electron beam on the Landau resonance. These Langmuir waves then mode-convert to freely propagating electromagnetic emissions at the local plasma frequency f pe and/or its harmonic 2 f pe . However, the details of the physics of mode conversion are unclear, and so far the magnetic component of the plasma waves has not been definitively measured. Several spacecraft have measured quasi-monochromatic Langmuir or slow extraordinary modes (sometimes called z -modes) in the solar wind. These coherent waves are expected to have a weak magnetic component, which has never been observed in an unambiguous way. Here we report on the direct measurement of the magnetic signature of these waves using the Search Coil Magnetometer sensor of the Parker Solar Probe/FIELDS instrument. Using simulations of wave propagation in an inhomogeneous plasma, we show that the appearance of the magnetic component of the slow extraordinary mode is highly influenced by the presence of density inhomogeneities that occasionally cause the refractive index to drop to low values where the wave has strong electromagnetic properties. 

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5. Cascading parametric decay coupling between whistler and ion acoustic waves: Darwin particle-in-cell simulations

   https://doi.org/10.3389/fspas.2022.1007240  

   Karbashewski, Scott; Sydora, Richard D.; Agapitov, Oleksiy V. (November 2022, Frontiers in Astronomy and Space Sciences)

   We present the results of numerical studies of the whistler wave parametric decay instability in the system with the suppressed Landau damping of ion acoustic waves (IAWs) based on the self-consistent Darwin particle-in-cell (PIC) model. It has been demonstrated that a monochromatic whistler wave launched along the background magnetic field couples to a counter-propagating whistler mode and co-propagating ion acoustic mode. The coupling of the electromagnetic mode to the electrostatic mode is guided by a ponderomotive force that forms spatio-temporal beat patterns in the longitudinal electric field generated by the counter-propagating whistler and the pump whistler wave. The threshold amplitude for the instability is determined to be δB w / B 0 = 0.028 and agrees with a prediction for the ion decay instability: δB w / B 0 = 0.042 based on the linear kinetic damping rates, and δB w / B 0 = 0.030 based on the simulation derived damping rates. Increasing the amplitude of the pump whistler wave, the secondary and tertiary decay thresholds are reached, and cascading parametric decay from the daughter whistler modes is observed. At the largest amplitude ( δB w / B 0 ∼ 0.1) the primary IAW evolves into a short-lived and highly nonlinear structure. The observed dependence of the IAW growth rate on the pump wave amplitude agrees with the expected trend; however, quantitatively, the growth rate of the IAW is larger than expected from theoretical predictions. We discuss the relevant space regimes where the instability could be observed and extensions to the parametric coupling of whistler waves with the electron acoustic wave (EAW). 

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