Journal of Geophysical Research: Space Physics最新文献

{"title":"Investigating Rotation and Anisotropic Ablation of Small Meteoroids and Their Effects on Head Echo Plasma Formation via Computational Techniques","authors":"T. Hedges,&nbsp;J. C. Ferguson,&nbsp;N. Lee,&nbsp;S. Elschot,&nbsp;G. Sugar,&nbsp;M. M. Oppenheim","doi":"10.1029/2025JA033883","DOIUrl":"https://doi.org/10.1029/2025JA033883","url":null,"abstract":"<p>High-power large-aperture radar instruments observe numerous meteor head echoes per minute. Head echoes result from reflections of radio waves from plasma surrounding meteoroids as they enter Earth's atmosphere. Knowledge of the spatial distribution of electrons in this plasma is essential to determining the mass loss rate of the meteor as a function of its measured radar cross-section. Prior work applies theoretical and computational methods to determine the electron density distribution, but assumes the meteoroid emits neutral particles uniformly across its surface. In this paper, a numerical surface ablation model demonstrates that meteoroid mass loss may occur preferentially in the direction facing the oncoming atmosphere. Specifically, meteoroid mass loss becomes proportional to the frontal surface area facing the freestream atmosphere in the limit of high Biot number, but remains isotropic in the limit of low Biot number. Meteoroid rotation has a small effect on the direction of ejected mass, but the effect is insignificant compared to variation in meteoroid properties that affect the Biot number. This result informs our computational meteor plasma model, in which we compare the effect of meteoroid vaporization on the plasma distribution in the limits of low versus high Biot number. The resulting electron density profiles demonstrate order-of-magnitude agreement between each other, with peak difference of 70% immediately upstream of the meteoroid. This implies that the directional distribution of vaporizing neutrals likely does not significantly influence head echo observations, lending credence to existing work that assumes isotropic ablation.</p>","PeriodicalId":15894,"journal":{"name":"Journal of Geophysical Research: Space Physics","volume":"130 6","pages":""},"PeriodicalIF":2.6,"publicationDate":"2025-06-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144367216","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Lidar Observations of Thermosphere-Ionospheric Ca+ at Mohe (122.3°E, 53.5°N)","authors":"Jing Jiao,&nbsp;Fang Wu,&nbsp;Lifang Du,&nbsp;Guotao Yang,&nbsp;Haoran Zheng,&nbsp;Xiaofei Wu","doi":"10.1029/2025JA033813","DOIUrl":"https://doi.org/10.1029/2025JA033813","url":null,"abstract":"<p>The ionosphere is an open area, affected by solar activity or geomagnetic activity, and affected by the lower atmosphere. Moreover, previous studies believed that metallic iron ions were a necessary condition for the formation of ionospheric irregularities, but so far there are no favorable observations to support it. In this paper, we use a Ca⁺ lidar at Mohe to explore the relationship between metal ions and ionospheric irregularities. The reason for choosing Mohe Station is that this region is at the northernmost point of China and is on the edge of the sub-aurora zone during the peak year of solar activity. Mohe thermosphere and ionosphere Ca⁺ (TICa⁺) is associated with Spread F or may be related with magnetic storms. The TICa⁺ generally occurred before dawn. In addition, the density of main Ca⁺ layer (80–120 km) in the autumn months is low, and the upper TICa⁺ (120–300 km) event density is also low with an order of 10 cm⁻³. However, during magnetic storm periods, the density of TICa⁺ increased with an order of 100 cm⁻³, which is comparable to or higher than summer results.</p>","PeriodicalId":15894,"journal":{"name":"Journal of Geophysical Research: Space Physics","volume":"130 6","pages":""},"PeriodicalIF":2.6,"publicationDate":"2025-06-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144339599","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Spatial Distribution and Geomagnetic Dependence of Radiation Belt Electron Reversed Energy Spectrum","authors":"Jiaming Li,&nbsp;Yuequn Lou,&nbsp;Xudong Gu,&nbsp;Binbin Ni,&nbsp;Qi Zhu,&nbsp;Xin Ma,&nbsp;Shuqin Chen","doi":"10.1029/2025JA033738","DOIUrl":"https://doi.org/10.1029/2025JA033738","url":null,"abstract":"<p>Using high-quality electron measurements from Van Allen Probes during October 2013 and March 2019, this study investigates the spatial distribution and geomagnetic dependence of the electron reversed energy spectrum in the Earth's radiation belts. The reversed energy spectrum is primarily observed within the L-shell range of ∼2.6–5.2, with peak occurrence rates reaching ∼50% at <i>L</i> = ∼4. Occurrence rates are higher in the post-noon to midnight sectors and lower on the pre-dawn side. In terms of magnetic latitude (MLAT), the spectrum spans ∼−20°–20°, exhibiting south-north asymmetry, particularly in the noon and night regions. The characteristic energies defining the spectrum correspond to the flux minimum (Ev) and maximum (Ep), which typically range from ∼100 keV to ∼1 MeV and hundreds of keV–∼2 MeV, respectively, with both Ev and Ep decreasing as <i>L</i> increases. The spectrum is more frequently observed during geomagnetically quiet periods, with maximum occurrence rates exceeding 50%. However, as geomagnetic activity intensifies, the occurrence rates decrease significantly, and the favorable region contracts toward lower L-shells. Analysis of geomagnetic indices shows that the reversed energy spectrum is more strongly affected by the Dst index than the auroral electrojet (AE) index. This could suggest a more substantial influence of geomagnetic storms than the substorm activity on suppressing the electron reversed energy spectrum. These results improve our understanding of how radiation belt electron dynamics respond to geomagnetic disturbances, emphasizing the interplay between storms, substorms, and wave-particle interactions in shaping the evolution of the reversed electron energy spectrum.</p>","PeriodicalId":15894,"journal":{"name":"Journal of Geophysical Research: Space Physics","volume":"130 6","pages":""},"PeriodicalIF":2.6,"publicationDate":"2025-06-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144339372","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Interplanetary Hydrogen Observations of the Emirates Ultraviolet Spectrometer Onboard the Emirates Mars Mission","authors":"R. Susarla,&nbsp;J. Deighan,&nbsp;M. S. Chaffin,&nbsp;E. Quemerais,&nbsp;S. Jain,&nbsp;R. J. Lillis,&nbsp;G. Holsclaw,&nbsp;K. Chirakkil,&nbsp;D. Brain,&nbsp;Ed. Thiemann,&nbsp;F. Eparvier,&nbsp;F. Lootah,&nbsp;M. Gacesa,&nbsp;M. O. Fillingim,&nbsp;J. S. Evans,&nbsp;H. AlMazmi,&nbsp;H. AlMatroushi,&nbsp;M. R. El-Maarry","doi":"10.1029/2025JA033903","DOIUrl":"https://doi.org/10.1029/2025JA033903","url":null,"abstract":"<p>The Emirates Mars Ultraviolet Spectrometer (EMUS) aboard the Emirates Mars Mission (EMM) has been studying backscattered interplanetary hydrogen (interplanetary hydrogen (IPH)) Lyman emissions with dedicated observation strategies viz., U-OS3b and U-OS4b. These observation techniques involve looking away from Mars from the EMM's orbit. U-OS3b provides broader coverage of the Martian sky, while U-OS4b is designed to achieve high signal-to-noise measurements. Here we present analysis of the interplanetary hydrogen emission distribution across the sky, particularly at the Lyman-<span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mi>α</mi>\u0000 </mrow>\u0000 <annotation> $alpha $</annotation>\u0000 </semantics></math> and <span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mi>β</mi>\u0000 </mrow>\u0000 <annotation> $beta $</annotation>\u0000 </semantics></math> emission wavelengths. EMUS observes a well-known reduction in the brightness of interplanetary hydrogen as the observation angle increases across the sky from the hydrogen bulk flow upwind direction. Our modeled emission intensities indicate that when Mars is around aphelion, the observed emissions are dominated by IPH rather than Martian exospheric hydrogen. Our modeled intensities for H Ly-<span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mi>α</mi>\u0000 </mrow>\u0000 <annotation> $alpha $</annotation>\u0000 </semantics></math> emission are smaller by a factor of 1.6 compared to the observations, and they are consistent with the observations for H Ly-<span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mi>β</mi>\u0000 </mrow>\u0000 <annotation> $beta $</annotation>\u0000 </semantics></math> emission. The H Ly-<span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mi>α</mi>\u0000 </mrow>\u0000 <annotation> $alpha $</annotation>\u0000 </semantics></math> intensities measured by the Imaging Ultraviolet Spectrograph (IUVS) on board the Mars Atmosphere and Volatile EvolutioN mission were found to be 45% lower than those from EMUS measurements, likely owing to differences in the instruments' calibration factors. During the observation period, differences in the measured intensities between the Solar Wind ANisotropy (SWAN) instrument aboard the Solar and Heliospheric Observatory and EMUS were also found to vary. These discrepancies can be attributed not only to instrumental factors but also to the relative positions of Earth and Mars.</p>","PeriodicalId":15894,"journal":{"name":"Journal of Geophysical Research: Space Physics","volume":"130 6","pages":""},"PeriodicalIF":2.6,"publicationDate":"2025-06-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1029/2025JA033903","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144331991","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Parametric Regimes of Thin Current Sheets in Planetary Magnetospheres and Solar Wind","authors":"David S. Tonoian,&nbsp;Xiao-Jia Zhang,&nbsp;Anton Artemyev,&nbsp;Qianli Ma,&nbsp;Robert W. Ebert,&nbsp;Frederic Allegrini","doi":"10.1029/2025JA033942","DOIUrl":"https://doi.org/10.1029/2025JA033942","url":null,"abstract":"<p>Current sheets are quasi-1D layers of strong current density, which play a crucial role in storing magnetic field energy and subsequently releasing it through charged particle acceleration and plasma heating. They are observed in planetary magnetospheres and solar wind flows, where they are also known as solar wind discontinuities. Despite significant variations in plasma parameters across different magnetospheres and the solar wind, current sheet configurations can remain fundamentally similar. In this study, we analyze current sheets observed in various regions, including the near-Earth (within 30 Earth radii) and distant (50–200 Earth radii) magnetotail, Earth's dayside and nightside magnetosheath, the near-Earth solar wind, and Martian and Jovian magnetotails. We examine three key plasma parameters: the plasma beta (ratio of plasma to magnetic pressure), the Alfvénic Mach number (ratio of plasma bulk flow speed to Alfvén speed in the current sheet reference frame), and the ion to electron temperature ratio. Additionally, we investigate the kinetic, thermal, and magnetic field energy densities. Our cross-system analysis demonstrates that the same current sheet configuration can exist across a very wide parametric space spanning multiple orders of magnitude. We also highlight the distinct plasma environments of the Martian and Jovian magnetotails, characterized by large populations of heavy ions, emphasizing their significance in comparative magnetospheric studies.</p>","PeriodicalId":15894,"journal":{"name":"Journal of Geophysical Research: Space Physics","volume":"130 6","pages":""},"PeriodicalIF":2.6,"publicationDate":"2025-06-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144331999","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Joint Observations of Plasmapause Surface Waves and Giant Undulations in the Electron Aurora","authors":"Xin-Yu Ai,&nbsp;Jie Ren,&nbsp;Qiu-Gang Zong,&nbsp;Yi-Xin Hao,&nbsp;Zi-Jian Feng,&nbsp;Ting-Yan Xiang","doi":"10.1029/2025JA033878","DOIUrl":"https://doi.org/10.1029/2025JA033878","url":null,"abstract":"<p>After the onset of an intense storm-time substorm (the minimum AL <span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mo>∼</mo>\u0000 </mrow>\u0000 <annotation> ${sim} $</annotation>\u0000 </semantics></math>−2,400 nT) on 8 September 2017, both Van Allen Probes and ground stations observed large-amplitude plasmasphere surface waves (PSWs) in the duskside, which lasted for more than 3 hr. Van Allen Probe B observations show that PSWs caused spatial and temporal modulations of sub-keV electrons with bidirectional pitch-angle distributions and higher-frequency plasma waves outside the plasmapause at the L-shells of 3.5–5.5. These kinetic-scale waves include kinetic Alfvén waves (KAWs) with frequencies below 10 Hz, time domain structures with frequencies from tens to hundreds of Hz, and electrostatic electron cyclotron harmonic waves (ECHs) with frequencies of tens of kHz. The bidirectional pitch angles and flux enhancement of sub-keV electrons indicate that they originate from the ionospheric outflow and are susceptible to pitch-angle scattering by kinetic-scale plasma waves. Following Probe B with a time delay of about 2 hr, Probe A observed PSW-associated ULF waves inside the plasmasphere at L-shells of 5–6, which can cause the drift-bounce resonance of cold (<span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mo>&lt;</mo>\u0000 </mrow>\u0000 <annotation> ${&lt; } $</annotation>\u0000 </semantics></math>50 eV) electrons. Corresponding to the duration and location of PSWs, DMSP-F17 observed intense sub-keV electron precipitation and weak ion precipitation, which coincides with the observations of giant undulations (GUs) in the electron aurora. These space-ground joint observations provide new sights into understanding the complicated middle processes between magnetospheric PSWs and ionospheric GUs.</p>","PeriodicalId":15894,"journal":{"name":"Journal of Geophysical Research: Space Physics","volume":"130 6","pages":""},"PeriodicalIF":2.6,"publicationDate":"2025-06-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144332000","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Bipolar Reversal in the Off-Diagonal Ion Pressure Term in Collisionless Magnetic Reconnection","authors":"Kailai Wang,&nbsp;Lei Dai,&nbsp;Shan Wang,&nbsp;Yong Ren,&nbsp;Minghui Zhu,&nbsp;Chi Wang,&nbsp;Benoit Lavraud,&nbsp;C. Philippe Escoubet,&nbsp;James L. Burch","doi":"10.1029/2025JA034185","DOIUrl":"https://doi.org/10.1029/2025JA034185","url":null,"abstract":"<p>Magnetic reconnection is a fundamental process that converts magnetic energy into particle energy, with wide applications in space plasmas. A key manifestation of this energy conversion is the acceleration of fast ion outflows. However, ion processes and their associated signatures in energy conversion remains only partially understood in collisionless magnetic reconnection. In this study, we utilize statistical analyses of in-situ observations and particle-in-cell (PIC) simulations to identify a distinct signature in the off-diagonal component of the ion pressure tensor. This signature displays a bipolar reversal that correlates with ion outflows across the reconnection X-line. The bipolar signal originates from distorted ion velocity distributions during acceleration. These signals are confirmed by statistical in-situ evidence for the first time and captured by PIC simulations. PIC simulations further indicate the peak of the off-diagonal ion pressure is near the magnetic pileup region associated with the ion outflow. Trajectories of ions in the distorted velocity distributions are traced within the PIC simulations. Ions in the distorted distributions undergo partial cyclotron motion around the reconnected magnetic field (<i>B</i>_<i>z</i>) and acceleration by the reconnection electric fields. The observation of bipolar reversal in the off-diagonal ion pressure term indicate its spatial gradient, which could contribute to supporting ion-scale reconnection electric fields. These findings provide new insights into the fundamental features of energy conversion in collisionless magnetic reconnection.</p>","PeriodicalId":15894,"journal":{"name":"Journal of Geophysical Research: Space Physics","volume":"130 6","pages":""},"PeriodicalIF":2.6,"publicationDate":"2025-06-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144315038","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Determining the Subsolar Magnetopause Position Using CMEM","authors":"S. J. Wharton,&nbsp;J. A. Carter,&nbsp;S. Sembay,&nbsp;Y. Soobiah,&nbsp;A. M. Read,&nbsp;T. R. Sun","doi":"10.1029/2025JA033837","DOIUrl":"https://doi.org/10.1029/2025JA033837","url":null,"abstract":"<p>The wide field-of-view soft X-ray imager on the upcoming SMILE mission will revolutionize our understanding of magnetopause dynamics from observations of SWCX in the magnetosheath. Inferring the position of the 3D magnetopause boundary in the 2D images is challenging, but several methods have been developed to do this. One method generates images through a 3D emissivity model and adapts its parameters to achieve the best fit with real images. The subsolar magnetopause position is extracted from the fitted model. We show that the Wharton et al. (2025, https://doi.org/10.1029/2024ja033307), cusp and magnetosheath emissivity model can be used successfully for this purpose in a wide range of scenarios. The method works when varying the image resolution, the orientation of the imager around its pointing axis, the aim point of the imager, and for a range of solar wind densities. We also show the method works from a range of orbital positions with realistically constrained viewing geometries. Finally, we tested the method on ensembles of realistic X-ray counts images which contain noise and instrumental contributions, allowing us to determine the uncertainty in the subsolar magnetopause distance over a wide range of solar wind densities. We found that CMEM accurately determined the subsolar magnetopause distance whenever it lies within the field-of-view and the signal-to-noise ratio was sufficient to observe the magnetosheath. The standard deviation of the subsolar magnetopause distance was less than 0.25 <span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <msub>\u0000 <mi>R</mi>\u0000 <mi>E</mi>\u0000 </msub>\u0000 </mrow>\u0000 <annotation> ${mathrm{R}}_{E}$</annotation>\u0000 </semantics></math> whenever the subsolar magnetopause was visible.</p>","PeriodicalId":15894,"journal":{"name":"Journal of Geophysical Research: Space Physics","volume":"130 6","pages":""},"PeriodicalIF":2.6,"publicationDate":"2025-06-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1029/2025JA033837","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144323573","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Geomagnetic Disturbances and Midlatitude Airglow During the 20 December 2015 Magnetospheric Storm","authors":"R. A. Marchuk,&nbsp;V. V. Mishin,&nbsp;Yu. V. Penskikh,&nbsp;Yu. Yu. Klibanova,&nbsp;A. V. Mikhalev","doi":"10.1029/2025JA033979","DOIUrl":"https://doi.org/10.1029/2025JA033979","url":null,"abstract":"<p>We report on the novel features of stormtime, midlatitude PiB/PiC geomagnetic pulsations, ionospheric and field-aligned currents, and oxygen, O¹S and O¹D, emissions at 557.7 and 630.0 nm, respectively. Those were observed during the main phase of the 20 December 2015 storm with significant variations of the solar wind dynamic pressure, <i>P</i>_<i>d</i>, and interplanetary magnetic field <i>B</i>_<i>z</i>. Intensifications of the shortest period, <i>Т</i> &lt; 10 s, Pi1B pulsations and oxygen emissions marked the substorm onsets. The distinct characteristic of the strong substorm (SME = 2,910 nT) was the presence of bay-like geomagnetic variations with the X and Z components with the opposite signs in the northern and southern sections of the IMAGE chain near 18 MLT. Using the magnetogram inversion technique of our institute, we obtained the MLT-MLAT distribution (map) of equivalent and field aligned currents revealing an additional westward electrojet to the north of the eastward current. We have shown that such a current system provides the observed distribution of geomagnetic variations along the 18 MLT meridian during the 20 December 2015 strong substorm, including at the high-latitude half of the IMAGE chain of stations. These conclusions were clearly validated by our simple current system model. We also revealed a localized geomagnetic event during the SME decrease interval between two substorm activations, when the magnitudes of the H/X geomagnetic component, PiB/PiC pulsations, and oxygen emissions at mid latitudes were more than twice greater than during the strong substorm.</p>","PeriodicalId":15894,"journal":{"name":"Journal of Geophysical Research: Space Physics","volume":"130 6","pages":""},"PeriodicalIF":2.6,"publicationDate":"2025-06-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144315065","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Multifluid Equations for MHD","authors":"J. G. Lyon,&nbsp;V. G. Merkin,&nbsp;K. A. Sorathia,&nbsp;M. J. Wiltberger","doi":"10.1029/2025JA033884","DOIUrl":"https://doi.org/10.1029/2025JA033884","url":null,"abstract":"<p>A set of magnetohydrodynamic equations is developed for multiple ion species in the limit of small Larmor radius. The derivation proceeds to explicitly calculate the Lorentz force resulting from the small ion drift velocities. The net effect is that in this limit all the ions move perpendicular to the magnetic field with the same <span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mrow>\u0000 <mi>E</mi>\u0000 <mo>×</mo>\u0000 <mi>B</mi>\u0000 </mrow>\u0000 </mrow>\u0000 <annotation> $mathbf{E}times mathbf{B}$</annotation>\u0000 </semantics></math> speed, but are free-streaming relative to one another in the parallel direction. The ions couple one to another in the parallel direction from changes in the magnetic field direction, leading to a collision-like term that, however, maintains a constant total kinetic energy. The equations governing parallel (centrifugal) acceleration, which may be an important process for ionospheric outflow, are then derived. The dispersion relation for a two-species, isotropic fluid with arbitrary mass and charge fractions, as well as electron pressure, is derived and the resulting wave modes are analyzed. A planar Alfvén mode separates from other, generally compressible, modes. It becomes unstable when the ram pressure of the streaming exceeds the firehose limit. The remaining modes satisfy a sixth order equation in the phase speed when counterstreaming and electron pressure are allowed. Without counterstreaming, three stable modes always exist, with two counter-propagating waves each, regardless of the presence of electron pressure. For the streaming case there are three modes, with two asymmetrically propagating waves each, whose behavior can be quite complicated, especially near the firehose instability limit. An example global magnetosphere simulation of a geomagnetic storm is presented using the derived multifluid formalism.</p>","PeriodicalId":15894,"journal":{"name":"Journal of Geophysical Research: Space Physics","volume":"130 6","pages":""},"PeriodicalIF":2.6,"publicationDate":"2025-06-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1029/2025JA033884","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144323572","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}