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Magnetic reconnection is a critically important process in defining the dynamics and energy transport within plasma environments. In near‐Earth space we may track where and when reconnection occurs by identifying associated coherent magnetic structures. On a global scale these structures facilitate the flow of mass and magnetic flux into, within, and out of the magnetospheric system, whilst contributing to local plasma heating. In the Earth's magnetotail there are two similar structures we identify in this work: magnetic flux ropes and loops. We present a robust, automated and model independent method by which encounters with such structures may be identified using the Magnetospheric Multiscale (MMS) mission. The magnetic structures are first identified through their magnetic field signatures at a single spacecraft (MMS1), including checks on the local minimum variance coordinate system. Next, the local curvature of the magnetic field is evaluated with all four MMS spacecraft. Finally, the plasma conditions are checked to ensure that the interpretation is fully self‐consistent. We evaluate the data obtained by MMS between 2017 and 2022. In total we find 181 self‐consistent magnetic flux ropes and 263 magnetic loops, which fit an exponentially decaying size distribution with a scale size comparable to the ion gyroradius (∼0.23 RE/1,400 km). If we remove the requirements on the plasma properties of the structure, we locate 648 potential magnetic flux ropes and 1,073 magnetic loops. The magnetic structures are preferentially observed in the pre‐midnight region of the magnetotail, with most identifications occurring beyond 20 RE. All catalogs are provided to the community. Key Points: Automated, model independent method presented to identify ion‐scale magnetotail magnetic flux ropes and loop‐like plasmoidsWe provide comprehensive catalogs of magnetotail structures observed by MMS between 2017 and 2022 (Smith et al., 2023)Structures show a strong dawn‐dusk asymmetry: predominantly observed pre‐midnight, and follow an exponentially decaying size distribution [ABSTRACT FROM AUTHOR]

In the space plasma environments, there are a variety of electromagnetic structures, including current sheets, magnetic holes and flux ropes, for which scales range from fluid down to ion or even electron kinetic scales. Our understanding of these structures, especially their electromagnetic field profiles and particle distributions, has been greatly improved due to the recent improvement in spacecraft technologies and enhancement in the observational database. To fully achieve the three-dimensional configurations of these structures, it is insufficient to use only the observations but also the theoretical models and numerical simulations. This paper briefly reviews the general framework of constructing theoretical models over these structures, which are self-consistent equilibrium solutions of Vlasov-Maxwell equations. In this framework, to ensure the satisfaction of the equilibrium Vlasov equation, particle distribution is expressed as a function of the particle invariant's motion. The moments of the particle distributions are then substituted into the Maxwell equations to solve the profiles of electromagnetic fields, which are further used to determine the particle phase space densities that can be compared directly to spacecraft observations. In recent years, this framework has been widely applied in the modeling and reconstruction of electromagnetic structures, demonstrating the ubiquitous presence of kinetic equilibrium structures in space plasmas. Moreover, the self-consistent models can be used as initial conditions of kinetic simulations. Finally, we discuss the open questions and future directions in studying kinetic structures in space plasmas.

This paper presents the magnetospheric multiscale (MMS) observations of three flux ropes (FRs) sequentially in a reconnection exhaust at Earth's magnetopause. Three active X‐lines, which are responsible for the formation of these FRs, were also detected by MMS, providing strong evidence that the FRs were generated by multiple X‐line reconnection. We find that the core field inside the FRs considerably affects the Hall current in the adjacent X‐lines. The intense core field of the FRs deflected the electron outflow jet, forming the electron‐scale Hall magnetic field on the magnetosheath side. Additionally, an active secondary reconnection was observed at the center of one FR. This secondary reconnection split the FR into two smaller FRs and produced sharp density gradients and intense fluctuations of electric field and current within the FR. These observations advance the understanding of the formation and evolution of FRs during multiple X‐line reconnection at magnetopause. Plain Language Summary: Flux transfer event (FTE) is generally explained as the flux rope (FR) generated by multiple X‐line reconnection at the dayside magnetopause. Although the theory and simulations have shown that the multiple active X‐lines and FRs can be produced simultaneously by the tearing instability in the magnetopause current sheet, there is no in‐situ evidence to directly support this scenario yet. Using high‐resolution measurements from the magnetospheric multiscale mission, we present a case study that all active X‐lines expected at the edges of three successive FRs were observed. These observations confirm that reconnection could simultaneously occur at multiple sites to produce the FRs at Earth's magnetopause. Interestingly, the electron‐scale Hall magnetic field was observed at the magnetosheath side in this multiple X‐line reconnection. This is due to the electron outflow jet deflected into the magnetosheath side by the strong core field of FR. Additionally, an ongoing secondary reconnection was observed at the center of one FR. Intense waves and current perturbations were observed in a smaller FR which was produced by this secondary reconnection. Key Points: Ongoing multiple X‐line reconnection generates a series of flux ropes at the magnetopauseElectron‐scale Hall magnetic fields are formed due to the high deflection of electron jets caused by the core field of the flux ropeSecondary reconnection within the flux rope split the flux rope into smaller flux ropes and produces intense electromagnetic fluctuations [ABSTRACT FROM AUTHOR]

High-resolution global magnetohydrodynamics (MHD) simulations include both meso- and global-scale processes occurring at the magnetopause, which interact to determine the time-dependent orientation of the day-side x-line (DXL). This study demonstrates that the global orientation of the DXL in GAMERA global MHD simulations varies on a time scale of minutes during steady southward interplanetary magnetic field conditions. This behavior manifests in observational data when reconnection outflows indicate that the direction to the x-line is opposite to the prediction from a steady-state model of the reconnection location. Because steady-state models of the DXL do not capture dynamics that are independent of solar wind variations, particularly surface waves and flux transfer events, they represent a time-averaged state of the system.

Magnetic flux ropes are structures that are common in the corona of the sun and presumably all stars. They can be thought of as the building blocks of solar structures. They have been observed in Earth's magnetotail and near Mars and Venus. When multiple flux ropes are present magnetic field line reconnection, which converts magnetic energy to other forms, can occur when they collide. The structure of multiple magnetic ropes, the interactions between multiple ropes, and their topological properties such as helicity and writhing have been studied theoretically and in laboratory experiments. Here, we report on spiky potential and magnetic fields associated with the ropes. We show that the potential structures are chaotic for a range of their temporal half-widths and the probability density function (PDF) of their widths resembles the statistical distribution of crumpled paper. The spatial structure of the magnetic spikes is revealed using a correlation counting method. Computer simulation suggests that the potential structures are the nonlinear end result of an instability involving relative drift between ions and electrons.

Observations of magnetic clouds, within interplanetary coronal mass ejections (ICMEs), are often well described by flux rope models. Most of these assume either a cylindrical or toroidal geometry. In some cases, these models are also capable of accounting for non‐axisymmetric cross‐sections but they generally all assume axial invariance. It can be expected that any ICME, and its flux rope, will be deformed along its axis due to influences such as the solar wind. In this work, we aim to develop a writhed analytical magnetic flux rope model which would allow us to analytically describe a flux rope structure with varying curvature and torsion so that we are no longer constrained to a cylindrical or toroidal geometry. In this first iteration of our model we will solely focus on a circular cross‐section of constant size. We describe our flux rope geometry in terms of a parametrized flux rope axis and a parallel transport frame. We derive expressions for the axial and poloidal magnetic field components under the assumption that the total axial magnetic flux is conserved. We find an entire class of possible solutions, which differ by the choice of integration constants, and present the results for a specific example. In general, we find that the twist of the magnetic field locally changes when the geometry deviates from a cylinder or torus. This new approach also allows us to generate completely new types of in situ magnetic field profiles which strongly deviate from those generated by cylindrical or toroidal models. Key Points: We develop an analytical model that can be used to describe writhed flux ropes by describing the flux rope axis as a general space curveWe show how this model can be implemented numerically in terms of quadratic splines and configured for an arbitrary twist distributionWe find that the field lines resulting from our model have lower twist per unit length than would be expected from a toroidal approximation [ABSTRACT FROM AUTHOR]

Poleward moving auroral forms (PMAFs), the most common dayside auroral structure, images the process of energy and particle transport from the solar wind to the magnetosphere via magnetic reconnection. This form of aurora moves not only poleward, but also in the azimuthal (dawn‐dusk) direction. However, the underlying mechanism responsible for the azimuthal motion of PMAFs are not thoroughly investigated. Here based on all‐sky imager observations in the southern hemisphere, we statistically determine that the azimuthal motion of PMAFs is highly dependent on the dawn‐dusk component of the interplanetary magnetic field (IMF By). Using three‐dimensional (3‐D) global hybrid simulations, we demonstrate that IMF By controls the azimuthal motion of PMAFs through plasma flow derived from magnetic tension of magnetopause flux ropes. Our study, by focusing on the azimuthal motion of PMAFs, allows a better understanding of space weather, especially from a 3‐D perspective. Plain Language Summary: Aurora is caused by energetic particles precipitating to the ionosphere along magnetic field lines. The aurora monitors the precipitation of energetic particles in the ionosphere that can give variable disturbances to space weather and impacts on telecommunications, electric power, satellite navigation, and other systems. Therefore, aurora is not only visually spectacular but also important for science and technology. Among many auroral phenomena, poleward moving auroral form (PMAF) is the most common dayside auroral structure, and it images the process of energy and particle transport from the solar wind to the magnetosphere and the ionosphere via magnetic reconnection. PMAFs have been reported to have significant azimuthal (dawn‐dusk) propagations. However, the underlying mechanism responsible for the azimuthal motion of PMAFs are not thoroughly investigated. Thus, in this letter, by using all‐sky‐imager observations at the Automatic Geophysical Observatory P1 station, we show that the azimuthal motion of PMAFs is highly dependent on the dawn‐dusk component of the interplanetary magnetic field (IMF By). Using 3‐D global hybrid simulations, we demonstrate that IMF By controls the azimuthal motion of PMAFs through plasma flow derived from magnetic tension. Key Points: Based on all‐sky imager observations, the azimuthal motion of poleward moving auroral forms are statistically studiedUsing three‐dimensional global hybrid simulations, the azimuthal motion of flux ropes is studiedInterplanetary magnetic field controls aurora's azimuthal motion throughflow derived from magnetic tension of magnetopause flux rope [ABSTRACT FROM AUTHOR]

Sheath regions of interplanetary coronal mass ejections (ICMEs) are formed when the upstream solar wind is deflected and compressed due to the propagation and expansion of the ICME. Small-scale flux ropes found in the solar wind can thus be swept into ICME-driven sheath regions. They may also be generated locally within the sheaths through a range of processes. This work applies wavelet analysis to obtain the normalized reduced magnetic helicity, normalized cross helicity, and normalized residual energy, and uses them to identify small-scale flux ropes and Alfvén waves in 55 ICME-driven sheath regions observed by the Wind spacecraft in the near-Earth solar wind. Their occurrence is investigated separately for three different frequency ranges between 10–2 − 10–4 Hz. We find that small scale flux ropes are more common in ICME sheaths than in the upstream wind, implying that they are at least to some extent actively generated in the sheath and not just compressed from the upstream wind. Alfvén waves occur more evenly in the upstream wind and in the sheath. This study also reveals that while the highest frequency (smallest scale) flux ropes occur relatively evenly across the sheath, the lower frequency (largest scale) flux ropes peak near the ICME leading edge. This suggests that they could have different physical origins, and that processes near the ICME leading edge are important for generating the larger scale population.

We report, in detail, an optimization approach for fitting a three-dimensional (3D) magnetic cloud (MC) model to in situ spacecraft measurements. The model, dubbed the Freidberg solution, encompasses 3D spatial variations in a generally cylindrical geometry, as derived from a linear force-free formulation. The approach involves a least-squares minimization implementation with uncertainty estimates from magnetic field measurements. We present one case study of the MC event on 22 May 2007 to illustrate the method and demonstrate the satisfying result of the minimum reduced χ 2 ≲ 1 , obtained from the Solar and TErrestrial RElations Observatory (STEREO) Behind spacecraft measurements. In addition, since the Advanced Composition Explorer (ACE) spacecraft at Earth crossed the STEREO Behind solution domain with an appropriate separation distance, the result from the optimally fitted Freidberg solution along the ACE spacecraft path is compared with the actual measurements of magnetic field components. A correlation coefficient of 0.89 is obtained between the two sets of data. [ABSTRACT FROM AUTHOR]

We investigate here the magnetic properties of a large-scale magnetic flux rope related to a coronal mass ejection (CME) that erupted from the Sun on September 12, 2014 and produced a well-defined flux rope in interplanetary space on September 14–15, 2014. We apply a fully data-driven and time-dependent magnetofrictional method (TMFM) using Solar Dynamics Observatory (SDO) magnetograms as the lower boundary condition. The simulation self-consistently produces a coherent flux rope and its ejection from the simulation domain. This paper describes the identification of the flux rope from the simulation data and defining its key parameters (e.g., twist and magnetic flux). We define the axial magnetic flux of the flux rope and the magnetic field time series from at the apex and at different distances from the apex of the flux rope. Our analysis shows that TMFM yields axial magnetic flux values that are in agreement with several observational proxies. The extracted magnetic field time series do not match well with in-situ components in direct comparison presumably due to interplanetary evolution and northward propagation of the CME. The study emphasizes also that magnetic field time-series are strongly dependent on how the flux rope is intercepted which presents a challenge for space weather forecasting.

Recent accumulation of a critical mass of observational material from different spacecraft complete with the enhanced abilities of numerical methods have led to a boom of studies revealing the high complexity of processes occurring in the heliosphere. Views on the solar wind filling the interplanetary medium have dramatically developed from the beginning of the space era. A 2-D picture of the freely expanding solar corona and non-interacting solar wind structures described as planar or spherically-symmetric objects has dominated for decades. Meanwhile, the scientific community gradually moved to a modern understanding of the importance of the 3-D nature of heliospheric processes and their studies via MHD/kinetic simulations, as well as observations of large-scale flows and streams both in situ and remotely, in white light and/or via interplanetary scintillations. The new 3-D approach has provided an opportunity to understand the dynamics of heliospheric structures and processes that could not even be imagined before within the 2-D paradigm. In this review, we highlight a piece of the puzzle, showing the evolution of views on processes related to current sheets, plasmoids, blobs and flux ropes of various scales and origins in the heliosphere. The first part of the review focuses on introducing these plasma structures, discussing their key properties, and paying special attention to their observations in different space plasmas. [ABSTRACT FROM AUTHOR]

A new study of Magnetospheric Multiscale Mission (MMS) measurements in the Earth's magnetotail, presented by Zhong et al. (2020, https://doi.org/10.1029/2019GL085141) finds that enhanced energetic electrons in a reconnection‐generated ion‐scale flux rope in the magnetotail are due to the adiabatic processes of betatron and Fermi (curvature drift) acceleration. These mechanisms act, respectively, on the perpendicular and parallel components of the distribution function so that the role of these mechanisms may be distinctly identified. This study presents a useful template for studying in situ electron energization and contributes significantly to the ongoing discussion of the role of mesoscale structure in reconnection‐driven particle acceleration throughout the heliosphere. Plain Language Summary: Magnetic reconnection is an explosive plasma process thought to play a key role in generating high‐energy particles in space plasmas. Recent observations and simulations have revealed that coherent magnetic structures known as flux ropes are likely to be important in the acceleration process. However, while energetic particles have often been observed in flux ropes, direct identification of the acceleration mechanism(s) was difficult due to the limited resolution of the measurements. A new study by Zhong et al. (2020) uses high‐resolution Magnetospheric Multiscale Mission (MMS) measurements of a flux rope in the Earth's magnetotail in order to identify the accelerationmechanisms at play and link them to "smoking gun" signatures in the electron distribution functions. This study highlights the value of high‐resolution magnetospheric observations and points to exciting opportunities for future in situ investigations of reconnection‐driven particle energization. Key Points: A new study by Zhong et al. (2020) uses high‐resolution MMS measurements to identify acceleration mechanisms in a magnetotail flux ropeThis study highlights the role of mesoscale structure in reconnection‐driven plasma energization [ABSTRACT FROM AUTHOR]

The magnetic fields of interplanetary coronal mass ejections (ICMEs), which originate close to the Sun in the form of a flux rope, determine their geoeffectiveness. Therefore, robust flux rope‐based models of CMEs are required to perform magnetohydrodynamic (MHD) simulations aimed at space weather predictions. We propose a modified spheromak model and demonstrate its applicability to CME simulations. In this model, such properties of a simulated CME as the poloidal and toroidal magnetic fluxes, and the helicity sign can be controlled with a set of input parameters. We propose a robust technique for introducing CMEs with an appropriate speed into a background, MHD solution describing the solar wind in the inner heliosphere. Through a parametric study, we find that the speed of a CME is much more dependent on its poloidal flux than on the toroidal flux. We also show that the CME speed increases with its total energy, giving us control over its initial speed. We further demonstrate the applicability of this model to simulations of CME‐CME collisions. Finally, we use this model to simulate the 12 July 2012 CME and compare the plasma properties at 1 AU with observations. The predicted CME properties agree reasonably with observational data. Key Points: CME simulations can be used for predictionThe magnetic and kinematic properties of CMEs must be constrainedModified Spheromak model can robustly simulate CMEs in inner heliosphere [ABSTRACT FROM AUTHOR]

Previous work at Titan presented a set of 85 flux ropes detected during Cassini flybys of Titan from 2005 to 2017. In that study a force‐free model was used to determine the radii and axial magnetic field of the flux ropes. In this work we apply non‐force‐free models. The non‐force‐free model shows an improvement in the number of flux ropes that can be fitted with a model, along with improved uncertainties and χ2 values. A number of asymmetries and features in the magnetometer data cannot be reproduced by either model; therefore, we deform the force‐free model to show that small deformations can replicate these features. One such deformation is to use an elliptical cross section, which replicates a plateau in magnetic field strength along with asymmetries on either side of the center of the flux ropes. Additionally, we explore the properties of bending a flux rope, where we find that minimum variance analysis becomes increasingly degenerate with bending, along with a slight bend causing the switching of the axial field direction from intermediate to maximum variance direction. We conclude that the flux ropes at Titan show aspects of developing flux ropes, compared to other planetary bodies, which exhibit more agreement to the force‐free assumptions of mature flux ropes. Key Points: Various models are fitted to 85 flux ropes that were detected in Titan's ionosphereA non‐force‐free model is a more suitable fit statistically compared to a force‐free model, implying that flux ropes are dynamic at TitanVarious deformations that act on a flux rope are explored, and in some cases, an elliptical cross section is more appropriate than circular [ABSTRACT FROM AUTHOR]

Titan's magnetic environment is a dynamic and unique place. We detect 85 flux ropes during all the Cassini flybys of Titan from 2005–2017. Analysis describing the location of flux ropes in Titan's ionosphere as well as where Titan is in Saturn's magnetosphere shows that the flux ropes are more often found when Titan is in the noon sector of Saturn's magnetosphere. A secondary peak of occurrence is found in the postmidnight area, where it is expected that Saturn's magnetosphere is highly dynamic. We also find that the flux rope occurrence is correlated with the average electron density profile of Titan's ionosphere. A force‐free model is utilized to estimate the radii and core magnetic field strength of the flux ropes. We find a large range of radii from 150–500 km with a small number of very large flux ropes (500–1,000 km) and a range of core field strengths of 1–15 nT, again with some much larger values(20–40 nT). The model also shows that more flux ropes are right‐handed than left‐handed in twist; however, we are unable to determine if there is a physical reason or if this is due to an observer bias. Additionally, we evaluate the goodness of fit for the model in each instance and conclude that, on average, the flux ropes may be better represented by a non‐force‐free model. Key Points: Eighty‐five flux ropes are detected in Titan's ionosphere using all of Cassini's flybys of TitanFlux ropes are present when Titan is in dynamic areas of Saturn's magnetosphereFitting a force‐free flux rope model leads to the conclusion that on average they are unlikely to be force free [ABSTRACT FROM AUTHOR]

We developed a new fitting method of a toroidal model for the analysis of interplanetary flux ropes (IFRs). The pitch angle of the magnetic field at the IFR boundary [ α p ] is fixed to 90 ∘ in the toroidal model of Romashets and Vandas (Geophys. Res. Lett.30, 2065, 2003) (conventional method). To relax this condition for our method, the pitch angle was made one of the free parameters (generalized method). In this paper, we examine the difference between the results of these two toroidal IFR model fitting methods by applying them to magnetic-obstacle (MO) events observed by the Wind and Solar Terrestrial Relations Observatory (STEREO) spacecraft. We found that the generalized method gives an α p near 90 ∘ (within the range of 90 ∘ ± 30 ∘ ) for approximately 60% of events. Thus, the assumption in the conventional method that α p is 90 ∘ is valid for many cases. This also implies that many flanks of IFR have a magnetic field line nearly perpendicular to the IFR axis. We also found that the generalized method gives a similar normal direction of the torus plane and poloidal magnetic flux as the conventional method. However, the generalized method gives a toroidal magnetic flux that is significantly different from that given by the conventional method for a non-negligible number of events. Thus, we concluded that it is better to use the generalized method than the conventional method in order to get more accurate estimation of the toroidal magnetic flux of IFRs. [ABSTRACT FROM AUTHOR]

Interplanetary (IP) shocks are driven in the heliosphere by fast coronal ejecta, they can accelerate particles and are associated with solar energetic particle and energetic storm particle events. IP shocks can interact with structures in the solar wind and with magnetospheres. We show how the properties of an IP shock change when it interacts with a small‐scale flux rope‐like structure (FRLS). Data from Cluster, Wind, and ACE show that the spacecraft observed the shock‐FRLS interaction at different stages of evolution. Wind and ACE observed the FRLS at shock crossing; Cluster observed the FRLS downstream, after it had crossed the shock. The shock‐FRLS interaction changes shock geometry, affecting ion injection processes, energetic particles fluxes, and the upstream/downstream regions. While Wind and ACE observed a quasi‐perpendicular shock, Cluster crossed a quasi‐parallel shock and a foreshock with a variety of ion distributions. The FRLS modified the shock on scales of at least ∼ 10–20 RE. The complexity of the ion foreshock measured by Cluster is explained by the dynamics of the shock transitioning from quasi‐perpendicular to quasi‐parallel and the geometry of the magnetic field within the flux rope. Fluxes of particles with energy up to 125 keV are affected by the FRLS‐shock interaction, modulating the associated energetic storm particle event. The interaction of a FRLS with an IP shock has not been discussed before using multispacecraft observations. Interactions like this should occur often along the shock fronts; hence, they are important for a better understanding of shock structure, evolution, and particle acceleration. Key Points: Shock interaction with small flux ropes in the solar wind can change shock geometry and impact ion injection at the shockInterplanetary shock interaction with flux ropes can modify the upstream and downstream regionsFluxes of particles with energy up to 125 keV are affected by the FRLS‐shock interaction, modulating associated ESP events [ABSTRACT FROM AUTHOR]

Three‐dimensional global hybrid simulations and observations have shown that earthward‐moving flux ropes (FRs) can undergo magnetic reconnection (or re‐reconnection) with the near‐Earth dipole field to create dipolarization front (DF)‐like signatures that are immediately preceded by brief intervals of negative BZ. The simultaneous erosion of the southward BZ field at the leading edge of the FR and continuous reconnection of lobe magnetic flux at the X‐line tailward of the FR result in the asymmetric south‐north BZ signature in many earthward‐moving FRs and possibly DFs with negative BZ dips prior to their observation. In this study, we analyzed Magnetospheric MultiScale (MMS) observation of fields and plasma signatures associated with the encounter of an ion diffusion region ahead of an earthward‐moving FR on 3 August 2017. The signatures of this re‐reconnection event were (i) +/− BZ reversal, (ii) −/+ bipolar‐type quadrupolar Hall magnetic fields, (iii) northward super‐Alfvénic electron outflow jet of ~1,000–1,500 km/s, (iv) Hall electric field of ~15 mV/m, (v) intense currents of ~40–100 nA/m2, and (vi) J·E′ ~0.11 nW/m3. Our analysis suggests that the MMS spacecraft encounters the ion and electron diffusion regions but misses the X‐line. Our results are in good agreement with particle‐in‐cell simulations of Lu et al. (2016, https://doi.org/10.1002/2016JA022815). We computed a dimensionless reconnection rate of ~0.09 for this re‐reconnection event and through modeling, estimating that the FR would fully dissipate by −16.58 RE. We demonstrated pertubations in the high‐latitude ionospheric currents at the same time of the dissipation of earthward‐moving FRs using ground‐ and space‐based measurements. Key Points: Observations of the fields and plasma signatures associated with the encounter of a re‐reconnection X‐line and its ion diffusion regionOur observations indicate that the leading edge of earthward flux rope is being eroded through re‐reconnection with the geomagnetic fieldGround‐ and space‐based measurements show correlation between the dissipation process of earthward flux ropes and auroral substorm activity [ABSTRACT FROM AUTHOR]

Interplanetary coronal mass ejections (ICMEs) are a significant feature of the heliospheric environment and the primary cause of adverse space weather at the Earth. ICME propagation and the evolution of ICME magnetic field structure during propagation are still not fully understood. We analyze the magnetic field structures of 18 ICME magnetic flux ropes observed by radially aligned spacecraft in the inner heliosphere. Similarity in the underlying flux rope structures is determined through the application of a simple technique that maps the magnetic field profile from one spacecraft to the other. In many cases, the flux ropes show very strong underlying similarities at the different spacecraft. The mapping technique reveals similarities that are not readily apparent in the unmapped data and is a useful tool when determining whether magnetic field time series observed at different spacecraft are associated with the same ICME. Lundquist fitting has been applied to the flux ropes, and the rope orientations have been determined; macroscale differences in the profiles at the aligned spacecraft may be ascribed to differences in flux rope orientation. Assuming that the same region of the ICME was observed by the aligned spacecraft in each case, the fitting indicates some weak tendency for the rope axes to reduce in inclination relative to the solar equatorial plane and to align with the solar east‐west direction with heliocentric distance. Plain Language Summary: Coronal mass ejections (CMEs) are large eruptions of magnetic field and plasma from the Sun. When they arrive at the Earth, these eruptions can cause significant damage to ground and orbital infrastructure; forecasting this "space weather" impact of CMEs at the Earth remains a difficult task. The impact of individual CMEs is largely dependent on their magnetic field configurations, and an important aspect of space weather forecasting is understanding how CME field configuration changes with distance from the Sun. We have analyzed the signatures of 18 CMEs observed by pairs of lined‐up spacecraft and show that their basic magnetic field structures often display significant self‐similarities, that is, they do not often show significant reordering of field features with heliocentric distance. This similarity points to the general usefulness of placing spacecraft between the Sun and Earth to act as early‐warning space weather monitors. CME signatures observed at such monitors would likely be similar to the signatures subsequently arriving at the Earth and could be used to produce space weather forecasts with longer lead times. Key Points: Eighteen interplanetary flux ropes observed by radially aligned spacecraft in the inner heliosphere have been examinedMany of the flux ropes showed significant self‐similarities in magnetic field structure at the aligned spacecraftMacroscale differences in the magnetic field profiles are consistent with the flux ropes displaying different axis orientations [ABSTRACT FROM AUTHOR]

Abstract: Three flux ropes associated with near‐Earth magnetotail reconnection are analyzed using Magnetospheric Multiscale observations. The flux ropes are Earthward propagating with sizes from ∼3 to 11 ion inertial lengths. Significantly different axial orientations are observed, suggesting spatiotemporal variability in the reconnection and/or flux rope dynamics. An electron‐scale vortex, associated with one of the most intense electric fields (E) in the event, is observed within one of the flux ropes. This E is predominantly perpendicular to the magnetic field (B); the electron vortex is frozen‐in with E × B drifting electrons carrying perpendicular current and causing a small‐scale magnetic enhancement. The vortex is ∼16 electron gyroradii in size perpendicular to B and potentially elongated parallel to B. The need to decouple the frozen‐in vortical motion from the surrounding plasma implies a parallel E at the structure's ends. The formation of frozen‐in electron vortices within reconnection‐generated flux ropes may have implications for particle acceleration. [ABSTRACT FROM AUTHOR]

Large-scale heliospheric flux ropes are observed by in situ instruments onboard heliospheric observatories as magnetic structures embedded in interplanetary coronal mass ejections (ICMEs). They are known as the interplanetary counterpart of the coronal mass ejections (CMEs), and the main drivers of geomagnetic activity. These structures can be reconstructed by fitting an axial symmetric flux rope model to the data. By using a circular-cylindrical analytical flux rope model, the 3-D reconstruction provides information about the heliospheric orientation, geometry, magnetic field at the flux rope center, and force distribution along the radius. Other quantities that can be derived are the magnetic flux and helicity. This model is constrained by the current density distribution along the tube. Thus, this paper aims to evaluate how different current density profiles change the physical and geometrical quantities. We selected two Earth-directed ICME events from the Wind ICME catalog (wind.gsfc.nasa.gov) and fit the model parameters to the data using two model-cases with different current density profiles. In general, the orientation, geometry, magnetic field strength, relative helicity, and size seem well-defined quantities. However, in the case of the magnetic flux and number of turns, the reconstructions are more sensitive to the current density constraint. This comparative analysis of the variation of the quantity will allow us to constrain not only this model but also the coronal models and the dynamical evolution of ICMEs in the solar wind by comparing the physical quantities. [ABSTRACT FROM AUTHOR]

Abstract: Predicting the magnetic field within an Earth‐directed coronal mass ejection (CME) well before its arrival at Earth is one of the most important issues in space weather research. In this article, we compare the intrinsic flux rope type, that is, the CME orientation and handedness during eruption, with the in situ flux rope type for 20 CME events that have been uniquely linked from Sun to Earth through heliospheric imaging. Our study shows that the intrinsic flux rope type can be estimated for CMEs originating from different source regions using a combination of indirect proxies. We find that only 20% of the events studied match strictly between the intrinsic and in situ flux rope types. The percentage rises to 55% when intermediate cases (where the orientation at the Sun and/or in situ is close to 45°) are considered as a match. We also determine the change in the flux rope tilt angle between the Sun and Earth. For the majority of the cases, the rotation is several tens of degrees, while 35% of the events change by more than 90°. While occasionally the intrinsic flux rope type is a good proxy for the magnetic structure impacting Earth, our study highlights the importance of capturing the CME evolution for space weather forecasting purposes. Moreover, we emphasize that determination of the intrinsic flux rope type is a crucial input for CME forecasting models. [ABSTRACT FROM AUTHOR]

An automated method was applied to identify magnetotail flux rope encounters in MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) magnetometer data. The method identified significant deflections of the north-south component of the magnetic field coincident with enhancements in the total field or dawn-dusk component. Two hundred forty-eight flux ropes are identified that possess well-defined minimum variance analysis (MVA) coordinate systems, with clear rotations of the field. Approximately 30% can be well approximated by the cylindrically symmetric, linearly force-free model. Flux ropes are most common moving planetward, in the postmidnight sector. Observations are intermittent, with the majority (61%) of plasma sheet passages yielding no flux ropes; however, the peak rate of flux ropes during a reconnection episode is ∼5 min−1. Overall, the peak postmidnight rate is ∼0.25 min−1. Only 25% of flux ropes are observed in isolation. The radius of flux ropes is comparable to the ion inertial length within Mercury's magnetotail plasma sheet. No clear statistical separation is observed between tailward and planetward moving flux ropes, suggesting the near-Mercury neutral line (NMNL) is highly variable. Flux ropes are more likely to be observed if the preceding lobe field is enhanced over background levels. A very weak correlation is observed between the flux rope core field and the preceding lobe field orientation; a stronger relationship is found with the orientation of the field within the plasma sheet. The core field strength measured is ∼6 times stronger than the local dawn-dusk plasma sheet magnetic field. [ABSTRACT FROM AUTHOR]

A flux rope is a domain where a twisted magnetic field [ B] is concentrated; it can be described as the core of a singularity of the outer field or the outer vector potential [ A] (Kleman and Robbins in Solar Phys. 289, 1173, ). This latter case, occurring when the outer field is vanishing, is mathematically analysed for a straight infinite rope. Concepts from condensed-matter physics defect theory are used: the flux [Φ], measured as ∮ A⋅d s along any loop [ C] surrounding the rope, is a topological constant of the theory. A flux rope with a small outer magnetic field can be treated as a perturbation of the above. This theoretical framework allows for the use of classical configurations inside the core, e.g. the linear force-free field (LFFF) Lundquist model or the nonlinear (NLFFF) Gold-Hoyle model, but restricts the number of stable solutions: they are quantised into strata of increasing energies (an infinite number of strata in the first case, only one stratum in the second case); each stratum is defined by a number 2 πζ= b/ r, where b is the periodicity along the axis of the rope and r is its radius, and the rope is made of a continuous set of stable states. We also analyse the merging of identical flux ropes (belonging to the same stratum), with conservation of the relative magnetic helicity: they merge into a unique rope of the first stratum, with a considerable release of energy. These results might apply to ropes that nucleate in the convection zone and the photosphere, where the magnetic field outside the ropes is weak, and less accurately to the magnetic clouds (MCs) into which they evolve after coronal mass ejections (CMEs) are triggered. However, the lowest LFFF stratum and the unique NLFFF stratum, which numerically come close to each other in this analysis, match the spacecraft data remarkably well. [ABSTRACT FROM AUTHOR]

We present a study of the impact of the December 7, 2003 fast forward interplanetary (IP) shock on the distant tail of the Earth׳s magnetosphere. Using the data from the several spacecraft located in the solar wind/magnetosheath upstream to the Earth, we monitor a propagation of the IP shock from the L1 point to the magnetosphere. A behavior of the far magnetotail is inferred from the Wind observations at X GSM ≈ − 230 R E . Shortly after the shock arrival, Wind crossed consequentially southern and northern lobes and observed a flux rope and the tailward fast plasma flow (≈780 km/s) within the plasmasheet. Moreover, a change of the solar wind V Z component across the shock creates a huge kink of the tail magnetosphere that propagates down the tail with the IP shock. [ABSTRACT FROM AUTHOR]

The singularities of an irrotational magnetic field are lines of electric current. This property derives from the relationship between vector fields and the topology of the underlying three-space and allows for a definition of cosmic field flux tubes and flux ropes as cores (in the sense of the physics of defects) of helical singularities. When applied to force-free flux ropes, and assuming current conservation, an interesting feature is the quantization of the radii, pitches, and helicities. One expects similar quantization effects in the general case. In the special case when the total electric current vanishes, a force-free rope embedded in a medium devoid of magnetic field is nonetheless topologically stable, because it is the core of a singularity of the vector potential. Magnetic merging is also briefly discussed in the same framework. [ABSTRACT FROM AUTHOR]

A hyperbolic flux-tube configuration containing a null point below the flux rope is considered as a pre-eruptive state of coronal mass ejections that start simultaneously with flares. We demonstrate that this configuration is unstable and cannot exist for a long time in the solar corona. The inference follows from general equilibrium conditions and from analyzing simple models of the flux-rope equilibrium. A direct consequence of the stable flux-rope equilibrium in the corona are separatrices in the horizontal-field distribution in the chromosphere. They can be recognized as specific 'herring-bone structures' in a chromospheric fibril pattern. [ABSTRACT FROM AUTHOR]

Abstract: Using minimum variance analysis of the circular mapping data from the Mars Global Surveyor (MGS) spacecraft during four selected weeks of observation, we identify 360 magnetic field structures in the Martian topside ionosphere with characteristic signatures of flux ropes. Physical parameters including size, peak field strength, helicity, orientation, and external conditions at the time of each observation are compiled for the events in each population. We observe that Martian flux ropes typically have a peak field amplitude of ∼15nT and a diameter of ∼80–100km assuming they are stationary. Flux ropes tend to be aligned approximately parallel to the planetary surface, and perpendicular to the direction from which the solar wind flows. They are more frequently observed during times of low solar wind pressure, but do not show a clear preference for a particular Interplanetary Magnetic Field (IMF) draping direction. Flux rope characteristics of peak field amplitude, diameter, and helicity vary with solar zenith angle. Amplitudes tend to be higher during periods of high solar wind pressure. The events are sorted into three populations based on the location at which they were observed, possibly corresponding to distinct formation mechanisms. Flux ropes observed in eclipse tend to have smaller peak amplitudes and are larger than those observed in sunlight, and are less likely to be oriented parallel to the planetary surface. Proximity to crustal fields does not appear to influence the characteristics of flux ropes observed at the 400km spacecraft altitude. The frequent observation of flux rope structures near Mars in a variety of locations suggests that the low-altitude plasma environment is quite dynamic, with magnetic shear playing a prominent role in determining magnetic field structure near the planet. [Copyright &y& Elsevier]

Abstract: Interplanetary coronal mass ejections (ICMEs) and their subset, magnetic clouds (MCs), are important manifestations of solar activity which have substantial impact on the geomagnetic field. We re-analyze events already identified in Wind and Voyager 2 data and estimate changes of their geometry along the path from the Sun. The analysis is based on the thickness of the sheath between a shock and a particular ICME or MC which is proportional to the apparent curvature radius of ICMEs/MCs. We have found that this apparent radius of curvature increases with the Mach number and this effect is attributed to the larger deformation of the fast ICME/MC. Further, the relative sheath thickness that is proportional to the flux rope oblateness decreases with the magnetic field intensity inside the ICME/MC and increases with the heliospheric distance. [Copyright &y& Elsevier]

Abstract: Magnetic clouds (MCs) observed by Wind during the 1995–2003 years and listed in Lepping et al. [2006. A summary of WIND magnetic clouds for years 1995–2003: model-fitted parameters, associated errors and classifications. Ann. Geophys. 24(1), 215–245] are fitted using force-free cylindrical flux rope models. The cloud parameters yielded by a static model are compared with those obtained from a modified model that includes magnetic cloud expansion. The deviations of these fits from observations are quantified using 1h averages of measured parameters by Wind. In the case of the static MC model, the deviations between fitted and observed magnetic fields increase with the magnetic strength and with the MC radius. The comparison of both the static and the dynamic models reveals that the modified model with expansion provides better fits in the range of low expansion speeds , whereas the results are not conclusive for larger expansion speeds. The dynamic model provides better fits than static one in of investigated MCs. We conclude that the expansion of magnetic clouds plays an important role in the MC formation and propagation but a further progress requires determination of cloud parameters from a model with expansion self-consistently involved. [Copyright &y& Elsevier]

We consider some general aspects of twisted magnetic flux ropes (TFR), which are thought to play a fundamental role in the structure and dynamics of large scale eruptive events. We first discuss the possibility to show the presence of a TFR in a pre-eruptive configuration by using a model along with observational informations provided by a vector magnetograph. Then we present, in the framework of a generic model in which the coronal field is driven into an evolution by changes imposed at the photospheric level, several mechanisms which may lead to the formation and the disruption of a TFR, including the development of a MHD instability, and we discuss the issues of the energy and helicity contents of an erupting configuration. Finally we report some results of a recent and more ambitious approach to the physics of TFRs in which one tries to describe in a consistent way their rising through the convection zone, their emergence through the photosphere, and their subsequent evolution in the corona. [ABSTRACT FROM PUBLISHER]