Ejection Events Research Articles

Toroidal modified Miller-Turner CME model in EUHFORIA: Validation and comparison with flux rope and spheromak

Context. Rising concerns about the impact of space-weather-related disruptions demand modelling and reliable forecasting of coronal mass ejection (CME) impacts. Aims. In this study, we demonstrate the application of the modified Miller-Turner (mMT) model implemented within EUropean Heliospheric FORecasting Information Asset (EUHFORIA) in forecasting the geo-effectiveness of observed coronal mass ejection (CME) events in the heliosphere. Our goal is to develop a model that not only has a global geometry, in order to improve overall forecasting, but is also fast enough for operational space-weather forecasting. Methods. We test the original full torus implementation and introduce a new three-fourths Torus version called the Horseshoe CME model. This new model has a more realistic CME geometry, and overcomes the inaccuracies of the full torus geometry. We constrain the torus geometrical and magnetic field parameters using observed signatures of the CMEs before, during, and after the eruption. We perform EUHFORIA simulations for two validation cases – the isolated CME event of 12 July 2012 and the CME–CME interaction event of 8–10 September 2014. We performed an assessment of the model’s capability to predict the most important Bz component using the advanced dynamic time-warping (DTW) technique. Results. The Horseshoe model predictions of CME arrival time and geo-effectiveness for both validation events compare well with the observations and are weighed against the results obtained with the spheromak and FRi3D models, which were already available in EUHFORIA. Conclusions. The runtime of the Horseshoe model simulations is close to that of the spheromak model, which is suitable for operational space weather forecasting. However, the capability of the magnetic field prediction at 1 AU of the Horseshoe model is close to that of the FRi3D model. In addition, we demonstrate that the Horseshoe CME model can be used for simulating successive CMEs in EUHFORIA, overcoming a limitation of the FRi3D model.

A Data-driven Magnetohydrodynamic Simulation of the 2011 February 15 Coronal Mass Ejection from Active Region NOAA 11158

We present a boundary data-driven magnetohydrodynamic (MHD) simulation of the 2011 February 15 coronal mass ejection (CME) event of Active Region NOAA 11158. The simulation is driven at the lower boundary with an electric field derived from the normal magnetic field and the vertical electric current measured from the Solar Dynamics Observatory (SDO) Helioseismic Magnetic Imager vector magnetograms. The simulation shows the buildup of a pre-eruption coronal magnetic field that is close to the nonlinear force-free field extrapolation, and it subsequently develops multiple eruptions. The sheared/twisted field lines of the pre-eruption magnetic field show qualitative agreement with the brightening loops in the SDO Atmospheric Imaging Assembly (AIA) hot passband images. We find that the eruption is initiated by the tether-cutting reconnection in a highly sheared field above the central polarity inversion line and a magnetic flux rope with dipped field lines forms during the eruption. The modeled erupting magnetic field evolves to develop a complex structure containing two distinct flux ropes and produces an outgoing double-shell feature consistent with the Solar Terrestrial Relations Observatory B/Extreme UltraViolet Imager observation of the CME. The foot points of the erupting field lines are found to correspond well with the dimming regions seen in the SDO/AIA observation of the event. These agreements suggest that the derived electric field is a promising way to drive MHD simulations to establish the realistic pre-eruption coronal field based on the observed vertical electric current and model its subsequent dynamic eruption.

Elucidation of spatiotemporal structures from high-resolution blowing-snow observations

Abstract. Systematic observations were conducted to investigate the spatiotemporal structures of blowing snow. Along a line perpendicular to the dominant wind direction on the lee side of a flat field, 15 snow particle counters (SPCs) and ultrasonic anemometers (USAs) were placed 1.5 m apart. Data were recorded at high frequencies of 100 kHz for SPCs and 1 kHz for USAs. The horizontal mass flux distributions, representing the spatiotemporal variability of blowing snow, exhibited non-uniformity in both time and space and manifested periodic changes akin to snow waves. Additionally, the presence of “snow snakes”, meandering near the snow surface, was observed. Quadrant analysis revealed predominant snow fluxes in quadrants Q1 (u′>0, w′>0) and Q4 (u′>0, w′<0). However, a more detailed parametric curve analysis indicated the existence of ejection events in Q2 (u′<0, w′>0) before snow waves and in front of snow snakes, shifting to Q1 and Q4 afterward, implying the consideration of both top-down and bottom-up mechanisms for burst sweep events.

Effects of a spoiler attachment on the wake flow of a bed-mounted horizontal pipe

This research investigated the spatial flow field of a pipe with a spoiler attachment and the modifications in the wake flow field of a bed-mounted horizontal pipe due to the attachment of a spoiler to the pipe for two different approach flow conditions. To attain the above-mentioned objective, the spatial evolution of the mean and turbulence flow properties in the flow field of the cylinder with an attached spoiler was determined. In addition, the mean flow and turbulence properties of a pipe without a spoiler attachment were compared with the properties of a pipe with an attached spoiler of height 0.2 D. For this purpose, two approach flow Reynolds numbers based on the pipe's diameter (D) and the free-stream velocity (U∞), Re∞ = 8850 and 11650, were considered. Three-dimensional (3D) velocity measurements were recorded with an acoustic Doppler velocimetry (ADV) between locations 4 D upstream (u/s) and 12 D downstream (d/s) of the pipe. It is observed that the length of the recirculation region is increased extensively due to the spoiler on the pipe from 6 D and 6.4 D to 10.7 D. The peak streamwise velocities for both Reynolds numbers are located near the free-water surface, as separated flows are deflected upward and result in enhanced free-stream velocities, which are higher than their approach flow free-stream velocities. In addition to that, the strength of the backflow is enhanced by 23% due to the attachment of the spoiler as compared to that of the pipe without the spoiler. The streamwise and vertical turbulence intensities are increased by 24% and 36%, respectively, and the wake flow field is found to be highly anisotropic. The streamwise turbulence intensities, Reynolds shear stress (RSS), turbulent kinetic energy (TKE), and turbulence indicator are attaining their peaks near the top level of the spoiler in the wake region. The triple velocity correlations indicate that the switching of sweeps and ejection events takes place near the top level of the spoiler. Therefore, it can be concluded that the spoiler attachment has increased the turbulence level in the wake region, and their highest magnitudes are located near the top level of the spoiler.

Turbulence characteristics of non-spherical isolated particles with different flatness on a rough bed surface

The transport of non-spherical particles on bed is significantly influenced by flow dynamics and particle shape. To study the impact of changes in the flatness of isolated bed particles on the surrounding flow characteristics, we prefabricated three isovolumetric particles with varying levels of flatness and analyzed the flow field around them using two-dimensional particle image velocimetry measurement methods. The results show that the presence of the particles significantly alters the turbulence parameters of the surrounding flow. Specifically, the influence extends up to 1D upstream of the particles, 0.5D above the particle tops, and throughout most of the downstream region, where D represents the equivalent particle diameter. The particles cause a reduction in Reynolds stress and turbulent kinetic energy in the near-bed upstream region, with a more pronounced effect observed for particles with lower flatness. Increased particle flatness is associated with a smaller difference in the longitudinal time-averaged velocity between upstream and downstream regions. In the wake region, particle flatness shows a negative correlation with Reynolds stress, longitudinal turbulence intensity, and turbulent kinetic energy, with this effect being more pronounced closer to the particles. Quadrant analysis indicates that ejection (Q2) and sweep (Q4) quadrant events remain dominant in the presence of particles, while particle flatness positively correlates with the frequency of outward (Q1) and inward (Q3) events in the wake region and negatively correlates with the frequency of Q2 and Q4 events. This study enhances our understanding of particle-fluid interactions, particularly the response relationship between non-spherical particles and turbulent mechanisms.

Comparison of the On-disk Apparent Current Sheets with the Limb Ones

Based on observations from the Solar Dynamics Observatory (SDO) and the Solar and Heliospheric Observatory (SOHO), we investigate 30 apparent current sheets during 1999–2021, including 10 on-disk and 8 limb ones from the SDO, as well as 12 limb ones from the SOHO. Each on-disk current sheet is formed among an X-type configuration consisting of two sets of atmospheric structures, and each limb one is involved in a flare–coronal mass ejection event. During magnetic reconnection period, the on-disk apparent current sheet evolves from a bright point to an elongated line-like structure, and the structure becomes thin in the late stage of the reconnection. Subsequently, the plasma distribution within the current sheet manifests as a plasmoid chain. For the limb apparent current sheet, the length elongation is faster than that of the on-disk one, and the thinning process is also detected. Although the aspect ratios of the limb cases are comparable to the value for the occurrence of tearing mode instability from simulation research, no obvious plasmoid chain is detected within these limb current sheets, and the density distribution is locally uniform. We suggest that due to the rapid extension of limb cases, the tearing mode instability is very fast, resulting in the formation of tiny plasmoids that are smaller than the instrument resolution. Moreover, there is another possible scenario. The observed limb apparent current sheet is just a bright ray, and the actual current sheet is only a small segment of the ray.

Turbulent characteristics of momentum flux in the marine atmospheric boundary layer of North Bay of Bengal

We use a 16-month-long, 20 Hz wind data from a mooring deployed in the Bay of Bengal (BoB) to study the characteristics of turbulent wind stress (u′w′)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${u}{\\prime}{w}{\\prime})$$\\end{document} events in the marine atmospheric boundary layer (MABL). Quadrant analysis of the motion-corrected u′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${u}{\\prime}$$\\end{document} and w′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${w}{\\prime}$$\\end{document} suggests that sweep and ejections, representing downward stress transfer into the ocean, dominate the u′w′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${u}{\\prime}{w}{\\prime}$$\\end{document} (~ 140%). In comparison, outward and inward interactions representing an upward stress transfer into the atmosphere provide the counter-contribution (~ 40%). We found a wind speed (ws) dependency on stress transfer for ws > 3 m/s, while for low ws, the swell-dominated ocean state modulates the u′w′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${u}{\\prime}{w}{\\prime}$$\\end{document} with a significant reverse stress transfer into the atmosphere, especially during intermonsoon periods. It is found that for weak winds (ws\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ws$$\\end{document}< 3 m/s), the number of turbulent events (N) is less, but they frequently repeat with more considerable flux per event (f^)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\widehat{f})$$\\end{document}, with outward and inward interactions (sweeps and ejections) dominating during intermonsoon periods (monsoon periods). For medium to strong winds, sweeps and ejections dominate u′w′.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${u}{\\prime}{w}{\\prime}.$$\\end{document} Ejections are found to be the most efficient method of stress transfer in the BoB, contributing 80% of u′w′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${u}{\\prime}{w}{\\prime}$$\\end{document}, compared to sweeps contributing ~ 60% and interaction processes contributing ~ − 20% each to the u′w′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${u}{\\prime}{w}{\\prime}$$\\end{document}. Though the duration of sweep events is larger than ejections and with comparable flux energy per event (f^\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\widehat{f}$$\\end{document}), the larger number N of ejection events makes it the dominant stress transfer mechanism in the Bay in all seasons.

Study of solar activities associated with a Halo CME on 17 Feb 2023 event

In the present work, propagation of an earth directed fast and wide Coronal Mass Ejection event on 17 February 2023 is studied in detail. The complex magnetic configuration in the Active Region (AR) 13229 at N25E64 caused an intensive X2.3 flare with a peak at 19:38 UT. It is followed by a massive halo CME event observed in the LASCO C3 coronagraph with a linear speed of 930 km/s and shock speed of 1300 km/s. A low frequency Type II emission was detected in the frequency range 10 MHz – 180 kHz during 20:30 UT-04:45 UT on 18 Feb 2023 by space borne Wind/WAVES instrument. From the OMNI data, the IP shock and the ICME reached earth's magnetosphere on 20 Feb 2023. A fast forward type shock was observed using OMNI high resolution data. The IP shock and ICME affected the Galactic Cosmic ray (GCR) detection. This event caused large magnetic turbulences in sheath region caused a major geomagnetic storm (∼-100 nT).

Preferential enhancement of convective heat transfer over drag via near-wall turbulence manipulation using spanwise wall oscillations

This study investigates the manipulation of convective heat transfer through spanwise wall oscillations in a turbulent channel flow. Direct numerical simulations are performed at Reτ=180 and Pr=1.The primary focus of this work is to explore the heat transfer response to oscillation parameters that promote drag increase, a regime that has received limited attention. By adopting an extended oscillation period (T+=500) and amplitude (W+=30), which have been reported to enhance drag, a remarkable dissimilarity between momentum and heat transport emerges. Under these conditions, the convective heat transfer undergoes a substantial 15% intensification, while the drag increases by a comparatively moderate 7.7%, effectively breaking the Reynolds analogy. To elucidate the physical mechanisms responsible for this dissimilar behaviour, a comprehensive statistical analysis is conducted. The control effect on the near-wall streaks and the associated mixing of momentum and heat is investigated by examining the energy distribution across scales and wall-normal locations. This analysis provides valuable insights into the control’s impact on the turbulent structures. Furthermore, the correlation between wall-normal velocity fluctuations and both streamwise velocity and temperature fluctuations is scrutinized to understand the modification of sweep and ejection events, which drive the transport of momentum and heat. The Fukagata–Iwamoto–Kasagi (FIK) identity is employed to identify the contributing factors to the changes in drag and heat transfer. The analysis highlights the importance of the pressure term in the streamwise velocity equation and the linearity of the temperature equation. Further investigation is necessary to fully unravel the complex mechanisms governing the decoupling of heat and momentum transport. The results of this study underscore the potential of using unconventional spanwise wall oscillations parameters to preferentially enhance convective heat transfer while minimizing the associated drag penalty.

Time Evolution Images of the Hypergiant RW Cephei during the Rebrightening Phase Following the Great Dimming

Stars with initial masses larger than 8 M ⊙ undergo substantial mass loss through mechanisms that remain elusive. Unraveling the origins of this mass loss is important for comprehending the evolutionary path of these stars, the type of supernova explosion, and whether they become neutron stars or black hole remnants. In 2022 December, RW Cep experienced the Great Dimming in its visible brightness, presenting a unique opportunity to understand mass-loss mechanisms. Our previous observations of RW Cep from the CHARA Array, taken during the dimming phase, show a compelling asymmetry in the star images, with a darker zone on the west side of the star indicating the presence of dust in front of the star in our line of sight. Here, we present multiepoch observations from CHARA while the star rebrightened in 2023. We created images using three image reconstruction methods and an analytical model fit. Comparisons of images acquired during the dimming and rebrightening phases reveal remarkable differences. Specifically, the west side of RW Cep, initially obscured during the dimming phase, reappeared during the subsequent rebrightening phase, and the measured angular diameter became larger by 8%. We also observed image changes from epoch to epoch while the star is brightening, indicating the time evolution of dust in front of the star. We suggest that the dimming of RW Cep was a result of a recent surface mass ejection event, generating a dust cloud that partially obstructed the stellar photosphere.

Gas Dynamics in the Herbig Ae Star HD 163296 in 2024

We observed the Herbig Ae star HD 163296 over 9 nights in 2024. On 4 of the nights, the He i line at 1.083 μm exhibited a profile rarely seen in the star. Rather than the typical P Cygni profile indicating a net gas outflow, the profile consisted of a deep absorption line with weak emission peaks on either side. These seem to be consistent with either an outflowing bipolar jet, a circular ring of gas orbiting close to the star, and/or accretion columns connecting the inner disk to the star. Given the history of the ejection of Herbi-Haro object in the star’s bipolar jets, these observations should coincide with the very beginning of the ejection event. Coordinated observations with multiple observing techniques over the next decade are warranted.

Evaluating the Geoeffectiveness of Interplanetary Coronal Mass Ejections: Insights from a Support Vector Machine Approach with SHAP Value Analysis

In this study, we compiled a data set of 510 interplanetary coronal mass ejections (ICME) events from 1996–2023 and trained a radial basis function support vector machine (RBF-SVM) model to investigate the geoeffectiveness of ICMEs and its dependence on the solar wind conditions observed at 1 au. The model demonstrates high performance in classifying geomagnetic storm intensities at specific Disturbance Storm Time thresholds and evaluating the geoeffectiveness of ICMEs. The model’s output was assessed using precision, recall, F1 score, and true skill statistics (TSS), complemented by stratified k-folds cross-validation for robustness. At the −50 nT threshold, the model achieves precisions of 0.84 and 0.93, recalls of 0.94 and 0.82, and corresponding F1 scores of 0.89 and 0.87 for the categories separated by this threshold, respectively. Overall accuracy is noted at 0.88, with a TSS of 0.76. Despite challenges at the −100 nT threshold due to data set imbalance and limited samples, the model maintains an overall accuracy of 0.87, with a TSS of 0.69, demonstrating the model’s ability to effectively handle imbalanced data. Physical insights were gained through model explanation with a SHapley Additive exPlanations (SHAP) value analysis, pinpointing the role of the southward magnetic field component in triggering geomagnetic storms, as well as the critical impact of shock-ICME combinations in intensifying these storms. The effective application of an SVM model with SHAP value analysis offers a way to understand and predict the geoeffectiveness of ICMEs. It also underscores the capability of a relatively simple machine learning model in predicting space weather and revealing the underlying physical mechanisms.

Study on Thermal Runaway Behavior and Jet Characteristics of a 156 Ah Prismatic Ternary Lithium Battery

Ternary lithium batteries have been widely used in transportation and energy storage due to their high energy density and long cycle life. However, safety issues arising from thermal runaway (TR) need urgent resolution. Current research on thermal runaway in large-capacity ternary lithium batteries is limited, making the study of hazard indicators during the thermal runaway ejection process crucial. This study places a commercial 156 Ah prismatic battery (positive electrode material: Li(Ni0.8Mn0.1Co0.1)O2, negative electrode material: graphite) in a nitrogen-filled sealed container, triggering thermal runaway through lateral heating. The experimental results show that the battery’s maximum surface temperature can reach 851.8–943.7 °C, exceeding the melting point of aluminum. Temperature surge inflection points at the battery’s bottom and near the small side of the negative electrode coincide with the inflection point on the heated surface. The highest jet temperatures at three monitoring points 50 mm, 150 mm, and 250 mm above the safety valve are 356.9 °C, 302.7 °C, and 216.5 °C, respectively. Acoustic signals reveal two ejection events. The average gas production of the battery is 0.089 mol/Ah, and the jet undergoes three stages: ultra-fast ejection (2 s), rapid ejection (32 s), and slow ejection (47 s). Post-thermal runaway remnants indicate that grooves from internal jet impacts are mainly located at ±45° positions. This study provides valuable insights for the safety design of batteries and the suppression of thermal runaway propagation.

Magnetohydrodynamic simulations of the space weather in Proxima b: Habitability conditions and radio emission

Context.The habitability of exoplanets hosted by M dwarf stars dramatically depends on the space weather, where the magnetic and ram pressure of the stellar wind, and the exoplanet magnetic field are the three main players. These three parameters also likely drive the radio emission arising close to the planet.Aims.Our aim is to characterize the magneto-plasma environment and thus the habitability of the Earth-like planet Proxima b, which is inside the habitable zone of its host M dwarf star Proxima, when it is subject to average calm space weather conditions, and to more extreme space weather conditions, for example a coronal mass ejection (CME) event. We study the role of the stellar wind and planetary magnetic field, and their mutual orientation. We also determine the radio emission arising from the interaction between the stellar wind of Proxima and the magnetosphere of its planet Proxima b, which is relevant to guiding radio observations aimed at unveiling planets.Methods.We used the PLUTO code to run a set of 3D magneto-hydrodynamic simulations focused on the space weather around planet Proxima b. We considered both calm and space weather conditions for Proxima b, under three different scenarios: (a) Proxima b subject to calm space weather in a sub-Alfvénic regime, where the stellar wind magnetic pressure dominates over the wind’s ram pressure; (b) Proxima b subject to calm space weather in a super-Alfvénic regime, where the ram pressure of the wind dominates, and a bow shock is formed; and (c) Proxima b subject to a coronal mass ejection event, when the dynamical and magnetic pressure of the stellar wind from its host star are increased enormously for a short period of time.Results.We find that if Proxima b has a magnetic field similar to that of the Earth (Bp=B⊕≈ 0.32 G) or larger, the magnetopause standoff distance is large enough to shield the surface from the stellar wind for essentially any planetary tilt but the most extreme values (close to 90°) under a calm space weather. Even if Proxima b is subject to more extreme space weather conditions, for example a CME event from its host star, the planet is well shielded by an Earth-like magnetosphere (Bp≈B⊕;i≈ 23.5°), or if it has a tilt smaller than that of the Earth. Otherwise, the planetary magnetic field must be larger to shield the planet from particle precipitation on the surface. For calm space weather conditions, the radio emission caused by the day-side reconnection regions can be as high as 7×1019erg s−1in the super-Alfvénic regime, and is on average almost an order of magnitude larger than the radio emission in the sub-Alfvénic cases, due to the much larger contribution of the bow shock, which is not formed in the sub-Alfvénic regime. We also find that the energy dissipation at the bow shock is essentially independent of the angle between the planet’s magnetic dipole and the incident stellar wind flow. If Proxima b is subject to extreme space weather conditions, the radio emission is more than two orders of magnitude larger than when under calm space weather conditions. This result yields expectations for a direct detection (from Earth) in radio of giant planets in close-in orbits as they are expected to have magnetic fields large enough, so that their electron-cyclotron frequency exceeds the ionosphere cutoff.

Spectral Indices and Evidence of Wave–Wave Modulation in Observations of the Interplanetary Magnetic Field

We present wave and turbulence observations from the DSCOVR spacecraft during the 2017 September solar flare and coronal mass ejection (CME) events. On September 4–12, the spectral index within the magnetic field power spectral density inertial range was consistent with Kolmogorov −5/3. This is despite the 9 days being composed of a complex mix of different features, including solar flares, solar energetic particle events, and CMEs. When analyzing shorter time periods, the spectral index varies. For two days where there were consecutive CMEs, the index follows Kraichnan–Iroshinikov −3/2, while on two quiet days, it was a mixture of −1, −3/2, and −2. The inertial range spectral index taken over the entire 9 days hides or averages out spectral features that occur over shorter time periods. We use a more realistic estimate of the amount of Doppler shifting into the spacecraft frame to show that the break frequencies on most days were located close to the H+ cyclotron frequency. We present evidence of wave–wave modulation and suggest that lower-frequency waves in the solar wind can modulate the growth rates/propagation of ion cyclotron waves, providing a method to transfer energy in the solar wind to smaller scales. Furthermore, we suggest that the indices in the inertial range can be explained by combining containment due to wave generation/propagation and stochastic Brownian motion in the solar wind. When these two phenomena are equal, they combine to create a −3/2 index.

Field‐Aligned Current Structures During the Terrestrial Magnetosphere's Transformation Into Alfvén Wings and Recovery

AbstractOn 24 April 2023, a Coronal Mass Ejection event caused the solar wind to become sub‐Alfvénic, leading to the development of an Alfvén Wing configuration in the Earth's magnetosphere. Alfvén Wings have previously been observed as cavities of low flow around moons in Jupiter's and Saturn's magnetospheres, but the observing spacecraft did not have the ability to directly measure the Alfvén Wings' current structures. Through in situ measurements made by the Magnetospheric Multiscale spacecraft, the 24 April event provides us with the first direct measurements of current structures during an Alfvén Wing configuration. These structures are observed to be significantly more anti‐field‐aligned and electron‐driven than the typical diamagnetic magnetopause current, indicating the disruption caused to the magnetosphere current system by the Alfvén Wing formation. The magnetopause current is then observed to recover more of its typical, perpendicular structure during the magnetosphere's recovery from the Alfvén Wing formation.

Hypersonic turbulent boundary layer over the windward side of a lifting body

In the present study, we performed direct numerical simulations for a hypersonic turbulent boundary layer over the windward side of a lifting body, the HyTRV model, at Mach number $6$ and attack angle 2 $^{\circ }$ to investigate the global and local turbulent features, and evaluate its difference from canonical turbulent boundary layers. By scrutinizing the instantaneous and averaged flow fields, we found that the transverse curvature on the windward side of the HyTRV model induces the transverse opposing pressure gradients that push the flow on both sides towards the windward symmetry plane, yielding significant effects of the azimuthal inhomogeneity and large-scale cross-stream circulations, moderate and azimuthal independent influences of adverse pressure gradient, and negligible impact of the mean flow three-dimensionality. Further inspecting the local turbulent statistics, we identified that the mean and fluctuating velocity become increasingly similar to the highly decelerated turbulent boundary layers over flat plates in that the mean velocity deficit is enhanced, and the outer layer Reynolds stresses are amplified as it approaches the windward symmetry plane, and prove to be self-similar under the scaling of Wei &amp; Knopp (J. Fluid Mech., vol. 958, 2023, A9) for adverse-pressure-gradient turbulent boundary layers. Conditionally averaged Reynolds stresses based on strong sweeping and ejection events demonstrated that the Kelvin–Helmholtz instability of the strong embedded shear layer induced by the large-scale cross-stream circulations is responsible for the turbulence amplification in the outer layer. The strong Reynolds analogy that relates the mean velocity and temperature was refined to incorporate the non-canonical effects, showing considerable improvements in the accuracy of such a formula. On the other hand, the temperature fluctuations are still transported passively, as indicated by their resemblance to the velocity. The conclusions obtained in the present study provide potentially profitable information for turbulent modelling modification for the accurate predictions of skin friction and wall heat transfer.

Characterizing turbulence structures in convective and neutral atmospheric boundary layers via Koopman mode decomposition and unsupervised clustering

The atmospheric boundary layer (ABL) is a highly turbulent geophysical flow, which has chaotic and often too complex dynamics to unravel from limited data. Characterizing coherent turbulence structures in complex ABL flows under various atmospheric regimes is not systematically well established yet. This study aims to bridge this gap using large eddy simulations (LESs), Koopman theory, and unsupervised classification techniques. To this end, eight LESs of different convective, neutral, and unsteady ABLs are conducted. As the ratio of buoyancy to shear production increases, the turbulence structures change from roll vortices to convective cells. The quadrant analysis indicated that as this ratio increases, the sweep and ejection events decrease, and inward/outward interactions increase. The Koopman mode decomposition (KMD) is then used to characterize their turbulence structures. Our results showed that KMD can reveal non-trivial modes of highly turbulent ABL flows (e.g., transverse to the mean flow direction) and can reconstruct the primary dynamics of ABLs even under unsteady conditions with only ∼5% of the modes. We attributed the detected modes to the imposed pressure gradient (shear), Coriolis (inertial oscillations), and buoyancy (convection) forces by conducting novel timescale and quadrant analyses. We then applied the convolutional neural network combined with the K-means clustering to group the Koopman modes. This approach is displacement and rotation invariant, which allows efficiently reducing the number of modes that describe the overall ABL dynamics. Our results provide new insights into the dynamics of ABLs and present a systematic data-driven method to characterize their complex spatiotemporal patterns.

Multiple chemical tracers finally unveil the intricate NGC 1333 IRAS 4A outflow system. FAUST XVI

ABSTRACT The exploration of outflows in protobinary systems presents a challenging yet crucial endeavour, offering valuable insights into the dynamic interplay between protostars and their evolution. In this study, we examine the morphology and dynamics of jets and outflows within the IRAS 4A protobinary system. This analysis is based on ALMA observations of SiO(5–4), H2CO(30, 3–20, 3), and HDCO(41, 4–31, 3) with a spatial resolution of ∼150 au. Leveraging an astrochemical approach involving the use of diverse tracers beyond traditional ones has enabled the identification of novel features and a comprehensive understanding of the broader outflow dynamics. Our analysis reveals the presence of two jets in the redshifted emission, emanating from IRAS 4A1 and IRAS 4A2, respectively. Furthermore, we identify four distinct outflows in the region for the first time, with each protostar, 4A1 and 4A2, contributing to two of them. We characterize the morphology and orientation of each outflow, challenging previous suggestions of bends in their trajectories. The outflow cavities of IRAS 4A1 exhibit extensions of 10 and 13 arcsec with position angles (PA) of 0° and -12°, respectively, while those of IRAS 4A2 are more extended, spanning 18 and 25 arcsec with PAs of 29° and 26°. We propose that the misalignment of the cavities is due to a jet precession in each protostar, a notion supported by the observation that the more extended cavities of the same source exhibit lower velocities, indicating they may stem from older ejection events.

Dynamics and potential origins of decimeter-sized particles around comet 67P/Churyumov-Gerasimenko

Context. One of the primary goals of the European Space Agency’s Rosetta mission to comet 67P/Churyumov-Gerasimenko was to investigate the mechanisms responsible for cometary activity. Aims. Our aim is to learn more about the ejection process of large refractory material by studying the dynamics of decimeter-sized dust particles in the coma of 67P and estimating their potential source regions. Methods. We algorithmically tracked thousands of individual particles through four OSIRIS/NAC image sequences of 67P’s near-nucleus coma. We then traced concentrated particle groups back to the nucleus surface, and estimated their potential source regions, size distributions, and projected dynamical parameters. Finally, we compared the observed activity to dust coma simulations. Results. We traced back 409 decimeter-sized particles to four suspected source regions. The regions strongly overlap and are mostly confined to the Khonsu-Atum-Anubis area. The activity may be linked to rugged terrain, and the erosion of fine dust and the ejection of large boulders may be mutually exclusive. Power-law indices fitted to the particle size–frequency distributions range from 3.4 ± 0.3 to 3.8 ± 0.4. Gas drag fits to the radial particle accelerations provide an estimate for the local gas production rates (Qg = 3.6 × 10−5 kg s−1 m−2), which is several times higher than our model predictions based on purely insolation-driven water ice sublimation. Our observational results and our modeling results both reveal that our particles were likely ejected with substantial nonzero initial velocities of around 0.5–0.6 m s−1. Conclusions. Our findings strongly suggest that the observed ejection of decimeter-sized particles cannot be explained by water ice sublimation and favorable illumination conditions alone. Instead, the local structures and compositions of the source regions likely play a major role. In line with current ejection models of decimeter-sized particles, we deem an overabundance of CO2 ice and its sublimation to be the most probable driver. In addition, because of the significant initial velocities, we suspect the ejection events to be considerably more energetic than gradual liftoffs.