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The cold ions, which are generally “invisible” to most instruments, have strong impacts on plasma wave and magnetic reconnection. Under particular situations, these cold ions could be accelerated and thus become detectable. In this study, we statistically investigated the properties of background cold ions based on Van Allen Probe observations. The cold ions could often be detected near the dusk sector, and a clear dawn‐dusk asymmetry is observed for all ion species with higher density at the dusk side, showing plasmaspheric plume‐like structures. Similar to the cold electrons, cold proton ions show a clear boundary of plasmapause with its location moving toward the Earth as geomagnetic activity increases. Furthermore, the percentage of oxygen increases, and the percentage of protons decreases as geomagnetic activity increases whereas the helium composition is generally small. Our results provide important information on ion compositions for the understanding of cold‐plasma dynamics in the inner magnetosphere. Plain Language Summary The cold ions play an important role in magnetospheric dynamics since they are the source of thermal plasma and they could affect the magnetic reconnection and wave generation. However, the main population of cold ions is difficult to measure due to their low energy and spacecraft charging. Magnetospheric convection and/or induced electric field could increase the energy of cold ions sufficiently above the spacecraft potential so that these ions can be detected by particle instrument. In this study, we investigate the properties of background cold ions when the total ion density is comparable to the background electron density. We found the cold ion could often be measured near the dusk sector and a clear dawn‐dusk asymmetry is observed for all ion species. Similar to the cold electrons, cold protons also show a clear boundary of plasmapause with its location moving toward the Earth as geomagnetic activity increases. Furthermore, the percentage of oxygen ions increases, and the percentage of protons decreases as geomagnetic activity increases whereas the percentage of helium ions is generally small. Our results provide important information on cold ion density for the study of wave‐particle interactions and magnetic reconnection in the Earth's magnetosphere. Key Points We statistically analyzed the cold ion densities and compositions based on Van Allen Probe observations The density above L = 3 decreases as geomagnetic activity increases for all three ion species, suggesting the shrinking of plasmasphere The percentage of cold oxygen ions increases as geomagnetic activity increases

The asymmetric distribution of Kelvin‐Helmholtz (KH) instability at Venusian ionopause is investigated using a multifluid model. Results show that KH instability distributes asymmetrically, preferentially developing in the −E (anti‐parallel to the interplanetary electric field) hemisphere. In the magnetosheath, solar wind H+ ions deflect toward both ±E hemispheres. Within the ionosphere, however, ionospheric ions are accelerated in the +E direction by the convective electric field. In addition, the ionopause is elevated in the −E hemisphere. These effects lead to a stronger velocity shear at the ionopause of the −E hemispheric, thereby promoting the KH growth there. The magnetic field variations in the KH region correlate with the H+ density but anti‐correlate with ionospheric ion density. The period of KH instability gradually decreases as it propagates downstream. During its evolution, plasma clouds form and take away ionospheric (especially O+) ions effectively, resulting in considerable ion escape.

Based on high‐resolution measurements from NASA's Magnetospheric Multiscale mission (MMS), we present the first direct observation of an ion diffusion region (IDR) with high number density O+ ions within dayside magnetopause reconnection during the May 2024 superstorm. The O+ ion density reaches a high value of ∼3.3 cm−3. It helps study heavy‐ion dynamics in dayside magnetopause reconnection. In the vicinity of IDR, O+ ions exhibit distinct acceleration to 300 km/s along the normal direction caused by the enhanced Hall electric field (|EN|max ≈ 80 mV/m). The distorted ion velocity distributions reveal the complex energization processes in the IDR. Crucially, these O+ ion dynamics can reduce reconnection rate by ∼10.3%–25.3%, providing the result that heavy‐ion can substantially alter magnetopause reconnection physics during the superstorm. This study advances our understanding of magnetopause reconnection by demonstrating that storm‐enhanced O+ populations modify the structure of diffusion regions, particle energization, and reconnection rate.

This letter provides a new physical insight into the fast ion effects on turbulence in plasmas having a high fast ion fraction and peaked fast ion density profile. We elucidate turbulence stabilization mechanisms by fast ions that result in internal transport barrier formation in the fast ion regulated enhancement mode plasma. Both linear and nonlinear gyrokinetic simulations show that the dominant turbulence suppression mechanisms are the dilution effects. In particular, we find that turbulence can be sufficiently suppressed solely by an inverted main ion density gradient due to fast ions, for the first time. New physical findings reported here improve our understanding of fast ion effects on turbulence, essential for fusion energy production where . Moreover, they will open up a new methodology to control plasma turbulence applicable to a wide range of plasma confinement regimes.

Using measurements from the Mars Atmosphere and Volatile EvolutioN mission, we investigate the densities of H+ (nH+${n}_{{\\mathrm{H}}^{+}}$ ), O+ (nO+${n}_{{\\mathrm{O}}^{+}}$ ), and O2+ (no2+${n}_{{\\mathrm{o}}_{2}^{+}}$ ), respectively, in the Martian magnetotail current sheet. We find that the current sheet when it is closer to the terminator than 0.75 Mars radii is mostly dominated by heavy ions ((nO++no2+${n}_{{\\mathrm{O}}^{+}}+{n}_{{\\mathrm{o}}_{2}^{+}}$ )>2 nH+${n}_{{\\mathrm{H}}^{+}}$ ), regardless of the variation of the upstream solar wind, but that it is sometimes dominated by H+ (nH+${n}_{{\\mathrm{H}}^{+}}$>2(nO++no2+${n}_{{\\mathrm{O}}^{+}}+{n}_{{\\mathrm{o}}_{2}^{+}}$ )) at downstream distances exceeding 0.75 Mars radii. The occurrence rate of the dominant H+ weakly increases (and that of the heavy ions decreases) with solar wind density and dynamic pressure. Our results suggest that solar wind protons could enter the Martian tail and may become the dominant ion species in the current sheet, particularly when the solar wind density or dynamic pressure is high. Plain Language Summary The current sheet of the Martian magnetotail is a major channel for the escape of planetary ions. The ion composition in the current sheet is essential to our understanding of this escape, as well as the magnetotail plasma dynamics. Our current knowledge, however, is poor. Based on the measurements of the ion density of different species in the current sheet from the Mars Atmosphere and Volatile EvolutioN spacecraft, we report that the current sheets we have surveyed are dominated by either the heavy ions from the planet or H+ (mostly) from the solar wind. We find that the downstream distance and the variation of the upstream solar wind are the two key factors that account for which ion species dominates in the tail current sheet. Key Points Current sheets are mostly dominated by heavy ions but are sometimes dominated by H+ at the downstream distance exceeding 0.75 Mars radii The occurrence rate of current sheets with dominant H+ (heavy ions) weakly increases (decreases) with solar wind density and dynamic pressure Our results suggest that the dominant H+ in the current sheet could originate from solar wind

The ion density and velocity measured by the advanced ionospheric probe (AIP) onboard FORMOSAT‐5 (F5) and ion velocity meter (IVM) onboard FORMOSAT‐7/COSMIC‐2 (F7C2), and the electron density assimilated by the global ionospheric specification (GIS) are employed to study the ionospheric plasma structure and dynamics during the 10 May 2024 Mother's Day storm (Dst −412 nT). The F5/AIP and F7C2/IVM display a large‐scale hole over the magnetic equator in the Atlantic Ocean area (−10° to 25°N, −60° to 20°E) during the local midnight period, with the minimum ion density of 1.7 × 104 #/cm3 and 1.6 × 103 #/cm3, respectively. In the hole area, F5/AIP and F7C2/IVM reveal upward ion velocities at 720 km altitude and downward ones at 550 km altitude, respectively, while GIS profiles show that the electron density yields the lower peak at ∼440 km and upper peak at ∼760 km altitude. This suggests that the downward and upward ion velocities result in the double‐peak feature.

We study the kinematics of the AS 209 disk using the J = 2–1 transitions of 12CO, 13CO, and C18O. We derive the radial, azimuthal, and vertical velocity of the gas, taking into account the lowered emission surface near the annular gap at ≃1.″7 (200 au) within which a candidate circumplanetary-disk-hosting planet has been reported previously. In 12CO and 13CO, we find a coherent upward flow arising from the gap. The upward gas flow is as fast as 150 m s−1 in the regions traced by 12CO emission, which corresponds to about 50% of the local sound speed or 6% of the local Keplerian speed. Such an upward gas flow is difficult to reconcile with an embedded planet alone. Instead, we propose that magnetically driven winds via ambipolar diffusion are triggered by the low gas density within the planet-carved gap, dominating the kinematics of the gap region. We estimate the ambipolar Elsässer number, Am, using the HCO+ column density as a proxy for ion density and find that Am is ∼0.1 at the radial location of the upward flow. This value is broadly consistent with the value at which numerical simulations find that ambipolar diffusion drives strong winds. We hypothesize that the activation of magnetically driven winds in a planet-carved gap can control the growth of the embedded planet. We provide a scaling relationship that describes the wind-regulated terminal mass: adopting parameters relevant to 100 au from a solar-mass star, we find that the wind-regulated terminal mass is about one Jupiter mass, which may help explain the dearth of directly imaged super-Jovian-mass planets.

Juno's flyby of Ganymede revealed ion composition in its vicinity with the Jovian Auroral Distributions Experiment–Ion (JADE‐I) instrument. Throughout this flyby, we derive species‐resolved ion density and velocity moments by decomposing the time‐of‐flight data into contributions from individual ion species using species‐dependent fits. At the sub‐Jovian flank magnetopause—a region previously linked to reconnection by previous studies—Juno encountered a strong field‐aligned ion jet. Its direction and magnitude are consistent with Hall‐mediated flank magnetopause reconnection at Ganymede. As reconnection‐accelerated electrons have been associated with Ganymede's polar aurora, the persistence of auroral emission suggests reconnection, and associated ion acceleration may occur along an extended X‐line. These results imply reconnection at Ganymede can act not only as a localized driver of ion jets, but also as a distributed pathway for ion and neutral loss. Given the appropriate reconnection geometry, such a mechanism is likely operating at a broad range of magnetized astrophysical bodies immersed within plasma.

The recently discovered asteroid 2016 HO3 is the most stable quasi-satellite of our Earth. Several missions to 2016 HO3 have been proposed, including the Tianwen-2 mission of China. Here we study the solar wind interaction with 2016 HO3 with three-dimensional particle-in-cell simulations. It is found that the sunlit surface can be positively charged to more than +10 V, and the shadowed surface is negatively charged to lower than −30 V. The typical electric field on the sunlit surface is about 2 V m−1 but can increase up to 20 V m−1 near the terminator. There is a plasma wake behind 2016 HO3 with a reduced plasma density. Normally, the ion density can be reduced to about 0.3 of the solar wind density at 100 m downstream from 2016 HO3, and the plasma wake is confined by a Mach cone with a cone angle of about 6.°5. In addition, we find that both the solar wind parameters and the secondary electron emission can affect the surface charging, which, in return, changes the wake structure.

We report the preliminary inter-satellite comparisons of the in situ ion density measurements by the ion velocity meter (IVM) onboard FORMOSAT-7/COSMIC-2 (F7/C2) and Ionospheric Connection Explorer (ICON) missions, during the solar minimum period of December 2019 to November 2020. The initial comparisons reveal identical diurnal, seasonal, and latitude/longitude variations in the two ion-density measurements, with F7/C2 consistently yielding stronger values than ICON, which could partly result from the difference in their orbit altitudes. The diurnal variation in the equatorial region did not show any effect of pre-reversal enhancement (PRE) during 2019–2020. The daytime plasma distributions show larger ion densities over a narrow latitudinal belt around the geomagnetic equator in all seasons, and the low-latitude densities reveal signatures of hemispherical asymmetry, with larger values occurring in the summer hemisphere. The observations also reveal distinct wavenumber-4 longitudinal modulation, which is most prominent in equinox and becomes less distinguishable during December solstice months. The simultaneous observations from F7/C2 IVM and ICON IVM also provide opportunities to study the spatial configuration and time evolution of ionospheric irregularities in the equatorial and low latitude regions. The F7/C2 and ICON simultaneously observed the equatorial plasma bubbles (EPBs) occurring around Taiwan on 18 October 2020, and the observations are consistent with each other. The EPBs were also observed by an all-sky imager located in Taiwan, comparing the satellite observations.Key pointsF7/C2 IVM shows similar patterns of diurnal, seasonal and latitude/longitude variations of ion density to ICON IVM but with stronger magnitudes.Distinct latitudinal and longitudinal variations of plasma distributions along seasons were observed during 2019-2020.Simultaneous observations by the multi-satellite constellation of F7/C2 and ICON and all-sky imager provide opportunity to monitor evolutions of EPBs.