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We report direct observations of a fast magnetosonic forward--reverse shock pair observed by Solar Orbiter on March 8, 2022 at the short heliocentric distance of 0.5 au. The structure, sharing some features with fully-formed stream interaction regions (SIRs), is due to the interaction between two successive coronal mass ejections (CMEs), never previously observed to give rise to a forward--reverse shock pair. The scenario is supported by remote observations from the STEREO-A coronographs, where two candidate eruptions compatible with the in-situ signatures have been found. In the interaction region, we find enhanced energetic particle activity, strong non-radial flow deflections and evidence of magnetic reconnection. At 1~au, well radially-aligned \textit{Wind} observations reveal a complex event, with characteristic observational signatures of both SIR and CME--CME interaction, thus demonstrating the importance of investigating the complex dynamics governing solar eruptive phenomena., Comment: Accepted in The Astrophysical Journal Letters

Simultaneous in situ measurements of coronal mass ejections (CMEs), including both plasma and magnetic field, by two spacecraft in radial alignment have been extremely rare. Here, we report on one such CME measured by Solar Orbiter (SolO) and Wind on 2021 November 3--5, while the spacecraft were radially separated by a heliocentric distance of 0.13 au and angularly by only 2.2{\deg}. We focus on the magnetic cloud (MC) part of the CME. We find notable changes in the R and N magnetic field components and in the speed profiles inside the MC between SolO and Wind. We observe a greater speed at the spacecraft further away from the Sun without any clear compression signatures. Since spacecraft are close to each other and computing fast magnetosonic wave speed inside the MC we rule out temporal evolution as the reason on the observed differences suggesting that spatial variations over 2.2{\deg} of the MC structure are at the heart of the observed discrepancies. Moreover, using shock properties at SolO, we forecast an arrival time 2h30 too late for a shock that is just 5h31 away hours from Wind. Predicting the north-south component of the magnetic field at Wind from SolO measurements leads to a relative error of 55 %. These results show that even angular separations as low as 2.2{\deg} (or 0.03 au in arclength) between spacecraft can have a large impact on the observed CME properties, rising up the issue of the resolutions of current CME models and potentially affecting our forecasting capabilities.

In situ measurements from spacecraft typically provide a time series at a single location through coronal mass ejections (CMEs) and they have been one of the main methods to investigate CMEs. CME properties derived from these in situ measurements are affected by temporal changes that occur as the CME passes over the spacecraft, such as radial expansion and ageing, as well as spatial variations within a CME. This study uses multi-spacecraft measurements of the same CME at close separations to investigate both the spatial variability (how different a CME profile is when probed by two spacecraft close to each other) and the so-called ageing effect (the effect of the time evolution on in situ properties). We compile a database of 19 events from the past four decades measured by two spacecraft with a radial separation <0.2 au and an angular separation <10{\deg}. We find that the average magnetic field strength measured by the two spacecraft differs by 18% of the typical average value, which highlights non-negligible spatial or temporal variations. For one particular event, measurements taken by the two spacecraft allow us to quantify and significantly reduce the ageing effect to estimate the asymmetry of the magnetic field strength profile. This study reveals that single-spacecraft time series near 1 au can be strongly affected by ageing and that correcting for self-similar expansion does not capture the whole ageing effect., Comment: Accepted for publication at ApJ. 18 pages, 6 figures, 2 tables

The ISWAT clusters H1+H2 have a focus on interplanetary space and its characteristics, especially on the large-scale co-rotating and transient structures impacting Earth. SIRs, generated by the interaction between high-speed solar wind originating in large-scale open coronal magnetic fields and slower solar wind from closed magnetic fields, are regions of compressed plasma and magnetic field followed by high-speed streams that recur at the ca. 27 day solar rotation period. Short-term reconfigurations of the lower coronal magnetic field generate flare emissions and provide the energy to accelerate enormous amounts of magnetised plasma and particles in the form of CMEs into interplanetary space. The dynamic interplay between these phenomena changes the configuration of interplanetary space on various temporal and spatial scales which in turn influences the propagation of individual structures. While considerable efforts have been made to model the solar wind, we outline the limitations arising from the rather large uncertainties in parameters inferred from observations that make reliable predictions of the structures impacting Earth difficult. Moreover, the increased complexity of interplanetary space as solar activity rises in cycle 25 is likely to pose a challenge to these models. Combining observational and modeling expertise will extend our knowledge of the relationship between these different phenomena and the underlying physical processes, leading to improved models and scientific understanding and more-reliable space-weather forecasting. The current paper summarizes the efforts and progress achieved in recent years, identifies open questions, and gives an outlook for the next 5-10 years. It acts as basis for updating the existing COSPAR roadmap by Schrijver+ (2015), as well as providing a useful and practical guide for peer-users and the next generation of space weather scientists., Comment: Accepted for publication in Advances in Space Research

Interplanetary Coronal Mass Ejections (ICMEs) originate from the eruption of complex magnetic structures occurring in our star's atmosphere. Determining the general properties of ICMEs and the physical processes at the heart of their interactions with the solar wind is a hard task, in particular using only unidimensional in situ profiles. Thus, these phenomena are still not well understood. In this study we simulate the propagation of a set of flux ropes in order to understand some of the physical processes occurring during the propagation of an ICME such as their growth or their rotation. We present simulations of the propagation of a set of flux ropes in a simplified solar wind. We consider different magnetic field strengths and sizes at the initiation of the eruption, and characterize their influence on the properties of the flux ropes during their propagation. We use the 3D MHD module of the PLUTO code on an Adaptive Mesh Refinement grid. The evolution of the magnetic field of the flux rope during the propagation matches evolution law deduced from in situ observations. We also simulate in situ profiles that spacecraft would have measured at the Earth, and we compare with the results of statistical studies. We find a good match between simulated in situ profiles and typical profiles obtained in these studies. During their propagation, flux ropes interact with the magnetic field of the wind but still show realistic signatures of ICMEs when analyzed with synthetic satellite crossings. We also show that flux ropes with different shapes and orientations can lead to similar unidimensional crossings. This warrants some care when extracting magnetic topology of ICMEs using unidimensional crossings., Comment: Accepted for publication A&A. 14 pages, 9 figures, 3 tables

The aim of this white paper is to briefly summarize some of the outstanding gaps in the observations and modeling of stellar flares, CMEs, and exoplanetary space weather, and to discuss how the theoretical and computational tools and methods that have been developed in heliophysics can play a critical role in meeting these challenges. The maturity of data-inspired and data-constrained modeling of the Sun-to-Earth space weather chain provides a natural starting point for the development of new, multidisciplinary research and applications to other stars and their exoplanetary systems. Here we present recommendations for future solar CME research to further advance stellar flare and CME studies. These recommendations will require institutional and funding agency support for both fundamental research (e.g. theoretical considerations and idealized eruptive flare/CME numerical modeling) and applied research (e.g. data inspired/constrained modeling and estimating exoplanetary space weather impacts). In short, we recommend continued and expanded support for: (1.) Theoretical and numerical studies of CME initiation and low coronal evolution, including confinement of "failed" eruptions; (2.) Systematic analyses of Sun-as-a-star observations to develop and improve stellar CME detection techniques and alternatives; (3.) Improvements in data-inspired and data-constrained MHD modeling of solar CMEs and their application to stellar systems; and (4.) Encouraging comprehensive solar--stellar research collaborations and conferences through new interdisciplinary and multi-agency/division funding mechanisms., Comment: 9 pages, 5 figures, white paper submitted to the Heliophysics 2024--2033 Decadal Survey

The solar wind, when measured close to 1 au, is found to flow mostly radially outward. There are, however, periods when the flow makes angles up to 15$^\circ$ away from the radial direction, both in the east-west and north-south directions. Stream interaction regions (SIRs) are a common cause of east-west flow deflections. Coronal mass ejections (CMEs) may be associated with non-radial flows in at least two different ways: 1) the deflection of the solar wind in the sheath region, especially close to the magnetic ejecta front boundary, may result in large non-radial flows, 2) the expansion of the magnetic ejecta may include a non-radial component which should be easily measured when the ejecta is crossed away from its central axis. In this work, we first present general statistics of non-radial solar wind flows as measured by STEREO/PLASTIC throughout the first 13 years of the mission, focusing on solar cycle variation. We then focus on the larger deflection flow angles and determine that most of these are associated with SIRs near solar minimum and with CMEs near solar maximum. However, we find no clear evidence of strongly deflected flows, as would be expected if large deflections around the magnetic ejecta or ejecta with elliptical cross-sections with large eccentricities are common. We use these results to develop a better understanding of CME expansion and the nature of magnetic ejecta, and point to shortcomings in our understanding of CMEs., Comment: Accepted to ApJ

We present a comprehensive statistical analysis of 106 sheath regions driven by coronal mass ejections (CMEs) and measured near 1 AU. Using data from the STEREO probes, this extended analysis focuses on two discrete categorizations. In the first categorization, we investigate how the generic features of sheaths change with their potential formation mechanisms (propagation and expansion sheaths), namely, their associations with magnetic ejectas (MEs) which are primarily expanding or propagating in the solar wind. We find propagation sheaths to be denser and driven by stronger MEs, whereas expansion sheaths are faster. Exploring the temporal profiles of these sheaths with a superposed epoch technique, we observe that most of the magnetic field and plasma signatures are more elevated in propagation sheaths relative to expansion sheaths. The second categorization is based on speed variations across sheaths. Employing linear least squares regression, we categorize four distinct speed profiles of the sheath plasma. We find that the associated shock properties and solar cycle phase do not impact the occurrence of such variations. Our results also highlight that the properties of the driving MEs are a major source of variability in the sheath properties. Through logistic regression, we conclude that the magnetic field strength and the ME speed in the frame of the solar wind are likely drivers of these speed variations.

We examine 188 coronal mass ejections (CMEs) measured by the twin STEREO spacecraft during 2007-2016 to investigate the generic features of the CME sheath and the magnetic ejecta (ME) and dependencies of average physical parameters of the sheath on the ME. We classify the MEs into three categories, focusing on whether a ME drives both a shock and sheath, or only a sheath, or neither, near 1 AU. We also reevaluate our initial classification through an automated algorithm and visual inspection. We observe that even for leading edge speeds greater than 500 km/s, 1 out of 4 MEs do not drive shocks near 1 AU. MEs driving both shocks and sheaths are the fastest and propagate in high magnetosonic solar wind, whereas MEs driving only sheaths are the slowest and propagate in low magnetosonic solar wind. Our statistical and superposed epoch analyses indicate that all physical parameters are more enhanced in the sheath regions following shocks than in sheaths without shocks. However, differences within sheaths become statistically less significant for similar driving MEs. We also find that the radial thickness of ME-driven sheaths apparently has no clear linear correlation with the speed profile and associated Mach numbers of the driver.

To make progress on the open questions on CME/CIR propagation, their interactions and the role and nature of the ambient solar wind, we need spatially resolved coverage of the inner heliosphere -- both in-situ and (critically) imaging -- at temporal scales matching the evolutionary timescales of these phenomena (tens of minutes to hours), and from multiple vantage points. The polar vantage is uniquely beneficial because of the wide coverage and unique perspective it provides. The ultimate goal is to achieve full $4\pi$ coverage of the solar surface and atmosphere by 2050., Comment: 3 pages, 2 figures, 1 table White Paper submitted to the Heliophysics 2050 Workshop

The radial expansion of coronal mass ejections (CMEs) is known to occur from remote observations; from the variation of their properties with radial distance; and from local in situ plasma measurements showing a decreasing speed profile throughout the magnetic ejecta (ME). However, little is known on how local measurements compare to global measurements of expansion. Here, we present results from the analysis of 42 CMEs measured in the inner heliosphere by two spacecraft in radial conjunction. The magnetic field decrease with distance provides a measure of their global expansion. Near 1 au, the decrease in their bulk speed provides a measure of their local expansion. We find that these two measures have little relation with each other. We also investigate the relation between characteristics of CME expansion and CME properties. We find that the expansion depends on the initial magnetic field strength inside the ME, but not significantly on the magnetic field inside the ME measured near 1 au. This is an indirect evidence that CME expansion in the innermost heliosphere is driven by the high magnetic pressure inside the ME, while by the time the MEs reach 1 au, they are expanding due to the decrease in the solar wind dynamic pressure with distance. We also determine the evolution of the ME tangential and normal magnetic field components with distance, revealing significant deviations as compared to the expectations from force-free field configurations as well as some evidence that the front half of MEs expand at a faster rate than the back half., Comment: 14 pages, accepted to ApJ

Our knowledge of the properties of Coronal Mass Ejections (CMEs) in the inner heliosphere is constrained by the relative lack of plasma observations between Sun and 1 AU. In this work, we present a comprehensive catalog of 47 CMEs measured in situ measurements by two or more radially aligned spacecraft (MESSENGER, Venus Express, STEREO or Wind/ACE). We estimate the CME impact speeds at Mercury and Venus using a drag-based model and present an average propagation profile of CMEs (speed and deceleration/acceleration) in the inner heliosphere. We find that CME deceleration continues past Mercury's orbit but most of the deceleration occurs between the Sun and Mercury. We examine the exponential decrease of the maximum magnetic field strength in the CME with heliocentric distance using two approaches: a modified statistical method and analysis from individual conjunction events. Findings from both the approaches are on average consistent with previous studies but show significant event-to-event variability. We also find the expansion of the CME sheath to be well fit by a linear function. However, we observe the average sheath duration and its increase to be fairly independent of the initial CME speed, contradicting commonly held knowledge that slower CMEs drive larger sheaths. We also present an analysis of the 3 November 2011 CME observed in longitudinal conjunction between MESSENGER, Venus Express, and STEREO-B focusing on the expansion of the CME and its correlation with the exponential fall-off of the maximum magnetic field strength in the ejecta.

Using in situ measurements and remote-sensing observations, we study a coronal mass ejection (CME) that left the Sun on 9 July 2013 and impacted both Mercury and Earth while the planets were in radial alignment (within $3^\circ$). The CME had an initial speed as measured by coronagraphs of 580 $\pm$ 20 km s$^{-1}$, an inferred speed at Mercury of 580 $\pm$ 30 km s$^{-1}$ and a measured maximum speed at Earth of 530 km s$^{-1}$, indicating that it did not decelerate substantially in the inner heliosphere. The magnetic field measurements made by MESSENGER and {\it Wind} reveal a very similar magnetic ejecta at both planets. We consider the CME expansion as measured by the ejecta duration and the decrease of the magnetic field strength between Mercury and Earth and the velocity profile measured {\it in situ} by {\it Wind}. The long-duration magnetic ejecta (20 and 42 hours at Mercury and Earth, respectively) is found to be associated with a relatively slowly expanding ejecta at 1 AU, revealing that the large size of the ejecta is due to the CME itself or its expansion in the corona or innermost heliosphere, and not due to a rapid expansion between Mercury at 0.45 AU and Earth at 1 AU. We also find evidence that the CME sheath is composed of compressed material accumulated before the shock formed, as well as more recently shocked material., Comment: Accepted to JGRA

Long periods of strong southward magnetic fields are known to be the primary cause of intense geomagnetic storms. The majority of such events are caused by the passage over Earth of a magnetic ejecta. Irrespective of the interplanetary cause, fast-forward shocks often precede such strong southward B$_{z}$ periods. Here, we first look at all long periods of strong southward magnetic fields as well as fast-forward shocks measured by the \textit{Wind} spacecraft in a 22.4-year span. We find that 76{\%} of strong southward B$_{z}$ periods are preceded within 48 hours by at least a fast-forward shock but only about 23{\%} of all shocks are followed within 48 hours by strong southward B$_{z}$ periods. Then, we devise a threshold-based probabilistic forecasting method based on the shock properties and the pre-shock near-Earth solar wind plasma and interplanetary magnetic field characteristics adopting a `superposed epoch analysis'-like approach. Our analysis shows that the solar wind conditions in the 30 minutes interval around the arrival of fast-forward shocks have a significant contribution to the prediction of long-duration southward B$_{z}$ periods. This probabilistic model may provide on average a 14-hour warning time for an intense and long-duration southward B$_{z}$ period. Evaluating the forecast capability of the model through a statistical and skill score-based approach reveals that it outperforms a coin-flipping forecast. By using the information provided by the arrival of a fast-forward shock at L1, this model represents a marked improvement over similar forecasting methods. We outline a number of future potential improvements., Comment: published in Space Weather, 22 Nov 2018

Measurements of coronal mass ejections (CMEs) by multiple spacecraft at small radial separations but larger longitudinal separations is one of the ways to learn about the three-dimensional structure of CMEs. Here, we take advantage of the orbit of the Wind spacecraft that ventured to distances of up to 0.012 astronomical units (au) from the Sun-Earth line during the years 2000 to 2002. Combined with measurements from ACE, which is in a tight halo orbit around L1, the multipoint measurements allow us to investigate how the magnetic field inside magnetic ejecta (MEs) changes on scales of 0.005 - 0.012 au. We identify 21 CMEs measured by these two spacecraft for longitudinal separations of 0.007 au or more. We find that the time-shifted correlation between 30-minute averages of the non-radial magnetic field components measured at the two spacecraft is systematically above 0.97 when the separation is 0.008 au or less, but is on average 0.89 for greater separations. Overall, these newly analyzed measurements, combined with 14 additional ones when the spacecraft separation is smaller, point towards a scale length of longitudinal magnetic coherence inside MEs of 0.25 - 0.35 au for the magnitude of the magnetic field but 0.06 - 0.12 au for the magnetic field components. This finding raises questions about the very nature of MEs. It also highlights the need for additional "mesoscale" multi-point measurements of CMEs with longitudinal separations of 0.01 - 0.2 au., Comment: Published in ApJL, 6 pages

This work extends recent efforts on the force-free modeling of large flux rope-type structures (magnetic clouds, MCs) to much smaller spatial scales. We first select small flux ropes (SFRs) by eye whose duration is unambiguous and which were observed by the Solar Terrestrial Relations Observatory (STEREO) or Wind spacecraft during solar maximum years. We inquire into which analytical technique is physically most appropriate. We consider three models: (i) linear force-free field ($\bigtriangledown\times$ B = $\alpha (r) $ B) with a specific, prescribed constant $\alpha$ (Lundquist solution), and (ii) with $\alpha$ as a free constant parameter (Lundquist-alpha solution), (iii) uniform twist field (Gold-Hoyle solution). We retain only those cases where the impact parameter is less than one-half the FR radius, $R$, so the results should be robust (29 cases). The SFR radii lie in the range [$\sim$ 0.003, 0.059] AU. Comparing results, we find that the Lundquist-alpha and uniform twist solutions yielded comparable and small normalized $\chi^2$ values in most cases. We then use Grad-Shafranov (GS) reconstruction to analyze these events further. We then considered the twist per unit length, $\tau$, both its profile through the FR and its absolute value. We find $\tau$ to lie in the range [5.6, 34] turns/AU. The GH model-derived $\tau$ values are comparable to those obtained from GS reconstruction. We find that twist unit length ($L$) is inversely proportional to $R$, as $\tau \sim 0.17/R$. We combine MC and SFR results on $\tau (R)$ and give a relation which is approximately valid for both sets. The axial and azimuthal fluxes, $F_z$ and $F_\phi$, vary as $\approx 2.1 B_0 R^2 \times10^{21}$ Mx and $F_{\phi}/L \approx 0.36 B_0 R \times10^{21}$Mx/AU. The relative helicity per unit length, $H/L \approx 0.75 B_0^2 R^3$$\times 10^{42}$ Mx$^2$/AU., Comment: abstract shortened for arxiv, 31 pages, 15 Figures, in press at Solar Physics

We identify all fast-mode forward shocks, whose sheath regions resulted in a moderate (56 cases) or intense (38 cases) geomagnetic storm during 18.5 years from January 1997 to June 2015. We study their main properties, interplanetary causes and geo-effects. We find that half (49/94) such shocks are associated with interacting coronal mass ejections (CMEs), as they are either shocks propagating into a preceding CME (35 cases) or a shock propagating into the sheath region of a preceding shock (14 cases). About half (22/45) of the shocks driven by isolated transients and which have geo-effective sheaths compress pre-existing southward Bz. Most of the remaining sheaths appear to have planar structures with southward magnetic fields, including some with planar structures consistent with field line draping ahead of the magnetic ejecta. A typical (median) geo-effective shock-sheath structure drives a geomagnetic storm with peak Dst of -88 nT, pushes the subsolar magnetopause location to 6.3 Re, i.e. below geosynchronous orbit and is associated with substorms with a peak AL-index of -1350 nT. There are some important differences between sheaths associated with CME-CME interaction (stronger storms) and those associated with isolated CMEs (stronger compression of the magnetosphere). We detail six case studies of different types of geo-effective shock-sheaths, as well as two events for which there was no geomagnetic storm but other magnetospheric effects. Finally, we discuss our results in terms of space weather forecasting, and potential effects on Earth's radiation belts., Comment: 15 pages, 8 Figures, accepted by JGR

We report on 49 fast-mode forward shocks propagating inside coronal mass ejections (CMEs) as measured by Wind and ACE at 1 AU from 1997 to 2006. Compared to typical CME-driven shocks, these shocks propagate in different upstream conditions, where the median upstream Alfv{\'e}n speed is 85 km s$^{-1}$, the proton $\beta = 0.08$ and the magnetic field strength is 8 nT. These shocks are fast with a median speed of 590 km s$^{-1}$ but weak with a median Alfv{\'e}nic Mach number of 1.9. They typically compress the magnetic field and density by a factor of 2-3. The most extreme upstream conditions found were a fast magnetosonic speed of 230 km s$^{-1}$, a plasma $\beta$ of 0.02, upstream solar wind speed of 740 km s$^{-1}$ and density of 0.5 cm$^{-3}$. Nineteen of these complex events were associated with an intense geomagnetic storm (peak Dst under $-100$ nT) within 12 hours of the shock detection at Wind, and fifteen were associated with a drop of the storm-time Dst index of more than 50 nT between 3 and 9 hours after shock detection. We also compare them to a sample of 45 shocks propagating in more typical upstream conditions. We show the average property of these shocks through a superposed epoch analysis, and we present some analytical considerations regarding the compression ratios of shocks in low $\beta$ regimes. As most of these shocks are measured in the back half of a CME, we conclude that about half the shocks may not remain fast-mode shocks as they propagate through an entire CME due to the large upstream and magnetosonic speeds., Comment: Accepted to JGR, 17 pages, 2 tables, 10 figures

Forecasting the in situ properties of coronal mass ejections (CMEs) from remote images is expected to strongly enhance predictions of space weather, and is of general interest for studying the interaction of CMEs with planetary environments. We study the feasibility of using a single heliospheric imager (HI) instrument, imaging the solar wind density from the Sun to 1 AU, for connecting remote images to in situ observations of CMEs. We compare the predictions of speed and arrival time for 22 CMEs (in 2008-2012) to the corresponding interplanetary coronal mass ejection (ICME) parameters at in situ observatories (STEREO PLASTIC/IMPACT, Wind SWE/MFI). The list consists of front- and backsided, slow and fast CMEs (up to $2700 \: km \: s^{-1}$). We track the CMEs to $34.9 \pm 7.1$ degrees elongation from the Sun with J-maps constructed using the SATPLOT tool, resulting in prediction lead times of $-26.4 \pm 15.3$ hours. The geometrical models we use assume different CME front shapes (Fixed-$\Phi$, Harmonic Mean, Self-Similar Expansion), and constant CME speed and direction. We find no significant superiority in the predictive capability of any of the three methods. The absolute difference between predicted and observed ICME arrival times is $8.1 \pm 6.3$ hours ($rms$ value of 10.9h). Speeds are consistent to within $284 \pm 288 \: km \: s^{-1}$. Empirical corrections to the predictions enhance their performance for the arrival times to $6.1 \pm 5.0$ hours ($rms$ value of 7.9h), and for the speeds to $53 \pm 50 \: km \: s^{-1}$. These results are important for Solar Orbiter and a space weather mission positioned away from the Sun-Earth line., Comment: 19 pages, 13 figures, accepted for publication in the Astrophysical Journal

A significant portion of transients measured by spacecraft at 1 AU does not show the well-defined properties of magnetic clouds (MCs). Here, we propose a new class of complex, non-MC ejecta resulting from the interaction of two CMEs with different orientation, which differ from the previously studied multiple-MC event. At 1 AU, they are associated with a smooth rotation of the magnetic field vector over an extended duration and do not show clear signs of interaction. We determine the characteristics of such events based on a numerical simulation and identify and analyze a potential case in the long-duration CME measured in situ in 2001 March 19-22. Such events may result in intense, long-duration geo-magnetic storms, with sawtooth events, and may sometimes be misidentified as isolated CMEs., Comment: Accepted to GRL; 5 pages, slightly different figures in print version

We present a 2.5D MHD simulation of a magnetic flux rope (FR) propagating in the heliosphere and investigate the cause of the observed sharp plasma beta transition. Specifically, we consider a strong internal magnetic field and an explosive fast start, such that the plasma beta is significantly lower in the FR than the sheath region that is formed ahead. This leads to an unusual FR morphology in the first stage of propagation, while the more traditional view (e.g. from space weather simulations like Enlil) of a `pancake' shaped FR is observed as it approaches 1 AU. We investigate how an equipartition line, defined by a magnetic Weber number, surrounding a core region of a propagating FR can demarcate a boundary layer where there is a sharp transition in the plasma beta. The substructure affects the distribution of toroidal flux, with the majority of the flux remaining in a small core region which maintains a quasi-cylindrical structure. Quantitatively, we investigate a locus of points where the kinetic energy density of the relative inflow field is equal to the energy density of the transverse magnetic field (i.e. effective tension force). The simulation provides compelling evidence that at all heliocentric distances the distribution of toroidal magnetic flux away from the FR axis is not linear; with 80% of the toroidal flux occurring within 40% of the distance from the FR axis. Thus our simulation displays evidence that the competing ideas of a pancaking structure observed remotely can coexist with a quasi-cylindrical magnetic structure seen in situ., Comment: 11 pages of text + 6 figures. Accepted to ApJ on 16 Oct 2013

We report on a numerical investigation of two coronal mass ejections (CMEs) which interact as they propagate in the inner heliosphere. We focus on the effect of the orientation of the CMEs relative to each other by performing four different simulations with the axis of the second CME rotated by 90 degrees from one simulation to the next. Each magneto-hydrodynamic (MHD) simulation is performed in three dimensions (3-D) with the Space Weather Modeling Framework (SWMF) in an idealized setting reminiscent of solar minimum conditions. We extract synthetic satellite measurements during and after the interaction and compare the different cases. We also analyze the kinematics of the two CMEs, including the evolution of their widths and aspect ratios. We find that the first CME contracts radially as a result of the interaction in all cases, but the amount of subsequent radial expansion depends on the relative orientation of the two CMEs. Reconnection between the two ejecta and between the ejecta and the interplanetary magnetic field (IMF) determines the type of structure resulting from the interaction. When a CME with a high inclination with respect to the ecliptic overtakes one with a low inclination, it is possible to create a compound event with a smooth rotation in the magnetic field vector over more than 180 degrees. Due to reconnection, the second CME only appears as an extended "tail", and the event may be mistaken for a glancing encounter with an isolated CME. This configuration differs significantly from the one usually studied of a multiple-magnetic cloud event, which we found to be associated with the interaction of two CMEs with the same orientation., Comment: 15 pages, 10 figures, 1 table; revised version to ApJ

In 2010 May 23-24, SDO observed the launch of two successive coronal mass ejections (CMEs), which were subsequently tracked by the SECCHI suite onboard STEREO. Using the COR2 coronagraphs and the heliospheric imagers (HIs), the initial direction of both CMEs is determined to be slightly west of the Sun-Earth line. We derive the CME kinematics, including the evolution of the CME expansion until 0.4 AU. We find that, during the interaction, the second CME decelerates from a speed above 500 km/s to 380 km/s the speed of the leading edge of the first CME. STEREO observes a complex structure composed of two different bright tracks in HI2-A but only one bright track in HI2-B. In situ measurements from Wind show an "isolated" ICME, with the geometry of a flux rope preceded by a shock. Measurements in the sheath are consistent with draping around the transient. By combining remote-sensing and in situ measurements, we determine that this event shows a clear instance of deflection of two CMEs after their collision, and we estimate the deflection of the first CME to be about 10 degrees towards the Sun-Earth line. The arrival time, arrival speed and radius at Earth of the first CME are best predicted from remote-sensing observations taken before the collision of the CMEs. Due to the over-expansion of the CME after the collision, there are few, if any, signs of interaction in in situ measurements. This study illustrates that complex interactions during the Sun-to-Earth propagation may not be revealed by in situ measurements alone., Comment: 14 pages, 8 figures, 1 table, accepted to the Astrophysical Journal

We present a study of coronal mass ejections (CMEs) which impacted one of the STEREO spacecraft between January 2008 and early 2010. We focus our study on 20 CMEs which were observed remotely by the Heliospheric Imagers (HIs) onboard the other STEREO spacecraft up to large heliocentric distances. We compare the predictions of the Fixed-Phi and Harmonic Mean (HM) fitting methods, which only differ by the assumed geometry of the CME. It is possible to use these techniques to determine from remote-sensing observations the CME direction of propagation, arrival time and final speed which are compared to in situ measurements. We find evidence that for large viewing angles, the HM fitting method predicts the CME direction better. However, this may be due to the fact that only wide CMEs can be successfully observed when the CME propagates more than 100 deg from the observing spacecraft. Overall eight CMEs, originating from behind the limb as seen by one of the STEREO spacecraft can be tracked and their arrival time at the other STEREO spacecraft can be successfully predicted. This includes CMEs, such as the events on 4 December 2009 and 9 April 2010, which were viewed 130 deg away from their direction of propagation. Therefore, we predict that some Earth-directed CMEs will be observed by the HIs until early 2013, when the separation between Earth and one of the STEREO spacecraft will be similar to the separation of the two STEREO spacecraft in 2009--2010., Comment: 21 pages, accepted to Solar Physics

The Fixed-\Phi (F\Phi) and Harmonic Mean (HM) fitting methods are two methods to determine the average direction and velocity of coronal mass ejections (CMEs) from time-elongation tracks produced by Heliospheric Imagers (HIs), such as the HIs onboard the STEREO spacecraft. Both methods assume a constant velocity in their descriptions of the time-elongation profiles of CMEs, which are used to fit the observed time-elongation data. Here, we analyze the effect of aerodynamic drag on CMEs propagating through interplanetary space, and how this drag affects the result of the F\Phi and HM fitting methods. A simple drag model is used to analytically construct time-elongation profiles which are then fitted with the two methods. It is found that higher angles and velocities give rise to greater error in both methods, reaching errors in the direction of propagation of up to 15 deg and 30 deg for the F\Phi and HM fitting methods, respectively. This is due to the physical accelerations of the CMEs being interpreted as geometrical accelerations by the fitting methods. Because of the geometrical definition of the HM fitting method, it is affected by the acceleration more greatly than the F\Phi fitting method. Overall, we find that both techniques overestimate the initial (and final) velocity and direction for fast CMEs propagating beyond 90 deg from the Sun-spacecraft line, meaning that arrival times at 1 AU would be predicted early (by up to 12 hours). We also find that the direction and arrival time of a wide and decelerating CME can be better reproduced by the F\Phi due to the cancellation of two errors: neglecting the CME width and neglecting the CME deceleration. Overall, the inaccuracies of the two fitting methods are expected to play an important role in the prediction of CME hit and arrival times as we head towards solar maximum and the STEREO spacecraft further move behind the Sun., Comment: Solar Physics, Online First, 17 pages

A goal of the NASA STEREO mission is to study the feasibility of forecasting the direction, arrival time and internal structure of solar coronal mass ejections (CMEs) from a vantage point outside the Sun-Earth line. Through a case study, we discuss the arrival time calculation of interplanetary CMEs (ICMEs) in the ecliptic plane using data from STEREO/SECCHI at large elongations from the Sun in combination with different geometric assumptions about the ICME front shape (Fixed-\Phi (FP): a point and harmonic Mean (HM): a circle). These forecasting techniques use single-spacecraft imaging data and are based on the assumptions of constant velocity and direction. We show that for the slow (350 km/s) ICME on 2009 February 13-18, observed at quadrature by the two STEREO spacecraft, the results for the arrival time given by the HM approximation are more accurate by 12 hours than those for FP in comparison to in situ observations of solar wind plasma and magnetic field parameters by STEREO/IMPACT/PLASTIC, and by 6 hours for the arrival time at Venus Express (MAG). We propose that the improvement is directly related to the ICME front shape being more accurately described by HM for an ICME with a low inclination of its symmetry axis to the ecliptic. In this case the ICME has to be tracked to > 30{\deg} elongation to get arrival time errors < 5 hours. A newly derived formula for calculating arrival times with the HM method is also useful for a triangulation technique assuming the same geometry., Comment: ApJ, accepted

We present a numerical investigation of the coronal evolution of a coronal mass ejection (CME) on 2005 August 22 using a 3-D thermodynamics magnetohydrodynamic model, the SWMF. The source region of the eruption was anemone active region (AR) 10798, which emerged inside a coronal hole. We validate our modeled corona by producing synthetic extreme ultraviolet (EUV) images, which we compare to EIT images. We initiate the CME with an out-of-equilibrium flux rope with an orientation and chirality chosen in agreement with observations of a H-alpha filament. During the eruption, one footpoint of the flux rope reconnects with streamer magnetic field lines and with open field lines from the adjacent coronal hole. It yields an eruption which has a mix of closed and open twisted field lines due to interchange reconnection and only one footpoint line-tied to the source region. Even with the large-scale reconnection, we find no evidence of strong rotation of the CME as it propagates. We study the CME deflection and find that the effect of the Lorentz force is a deflection of the CME by about 3 deg/Rsun towards the East during the first 30 minutes of the propagation. We also produce coronagraphic and EUV images of the CME, which we compare with real images, identifying a dimming region associated with the reconnection process. We discuss the implication of our results for the arrival at Earth of CMEs originating from the limb and for models to explain the presence of open field lines in magnetic clouds., Comment: 14 pages, 8 Figures, accepted to Astrophysical Journal

Using data from the Heliospheric Imagers (HIs) onboard STEREO, it is possible to derive the direction of propagation of coronal mass ejections (CMEs) in addition to their speed with a variety of methods. For CMEs observed by both STEREO spacecraft, it is possible to derive their direction using simultaneous observations from the twin spacecraft and also, using observations from only one spacecraft with fitting methods. This makes it possible to test and compare different analyses techniques. In this article, we propose a new fitting method based on observations from one spacecraft, which we compare to the commonly used fitting method of Sheeley et al. (1999). We also compare the results from these two fitting methods with those from two stereoscopic methods, focusing on 12 CMEs observed simultaneously by the two STEREO spacecraft in 2008 and 2009. We find evidence that the fitting method of Sheeley et al. (1999) can result in significant errors in the determination of the CME direction when the CME propagates outside of 60deg \pm 20 deg from the Sun-spacecraft line. We expect our new fitting method to be better adapted to the analysis of halo or limb CMEs with respect to the observing spacecraft. We also find some evidence that direct triangulation in the HI fields-of-view should only be applied to CMEs propagating approximatively towards Earth (\pm 20deg from the Sun-Earth line). Last, we address one of the possible sources of errors of fitting methods: the assumption of radial propagation. Using stereoscopic methods, we find that at least seven of the 12 studied CMEs had an heliospheric deflection of less than 20deg as they propagated in the HI fields-of-view, which, we believe, validates this approximation., Comment: 17 pages, 6 figures, 2 tables, accepted to Solar Physics

Interplanetary coronal mass ejections (ICMEs) have complex magnetic and density structures, which is the result of their interaction with the structured solar wind and with previous eruptions. ICMEs are revealed by in situ measurements and in the past five years, through remote-sensing observations by heliospheric imagers. However, to understand and analyze these observations often requires the use of numerical modeling. It is because no instruments can yet provide a simple view of ICMEs in two or three dimensions. Numerical simulations can be used to determine the origin of a complex ejecta observed near Earth, or to analyze the origin, speed and extent of density structures observed remotely. Here, we review and discuss recent efforts to use numerical simulations of ICMEs to investigate the magnetic topology, density structure, energetics and kinematics of ICMEs in the interplanetary space. After reviewing existing numerical models of ICMEs, we first focus on numerical modeling in support of the SMEI and STEREO observations. 3-D simulations can help determining the origins of the fronts observed by SECCHI and SMEI, especially for complex events such as the January 24-25, 2007 CMEs. They can also be used to test different methods to derive ICME properties from remote observations, to predict and explain observational effects, and to understand the deceleration and deformation of ICMEs. In the last part, we focus on the numerical investigation of non-magnetic cloud ejecta. We discuss how simulations are crucial to understand the formation of non-twisted ejecta and the formation of complex ejecta due to the interaction of multiple ICMEs., Comment: 18 pages, 9 figures, accepted to JASTP

We discuss how simultaneous observations by multiple heliospheric imagers can provide some important information about the azimuthal properties of Coronal Mass Ejections (CMEs) in the heliosphere. We propose two simple models of CME geometry that can be used to derive information about the azimuthal deflection and the azimuthal expansion of CMEs from SECCHI/HI observations. We apply these two models to four CMEs well-observed by both STEREO spacecraft during the year 2008. We find that in three cases, the joint STEREO-A and B observations are consistent with CMEs moving radially outward. In some cases, we are able to derive the azimuthal cross-section of the CME fronts, and we are able to measure the deviation from self-similar evolution. The results from this analysis show the importance of having multiple satellites dedicated to space weather forecasting, for example in orbits at the Lagrangian L4 and L5 points., Comment: 7 pages, 4 figures, 1 table, accepted to ApJ

Most simulations of coronal mass ejections (CMEs) to date either focus on the interplanetary propagation of a giant plasma "blob" without paying too much attention to its origin and to the formation process or they focus on the complex evolution of the coronal magnetic field due to (sub-)photospheric motions which result in an eruption. Here, we present global simulations of CMEs where coronal motions are used to produce a realistic evolution of the coronal magnetic field and cause an eruption. We focus on active region 10069, which produced a number of eruptions in late August 2002, including the August 24, 2002 CME - a fast (~2000 km/s) eruption originating from W81-, as well as a slower eruption on August 22, 2002 (originating from W62). Using a three-dimensional magneto-hydrodynamic (MHD) simulation of these ejections with the Space Weather Modeling Framework (SWMF), we show how a realistic initiation mechanism enables us to study the deflection of the CME in the corona and in the heliosphere. Reconnection of the erupting magnetic field with that of neighboring streamers and active regions modify the solar connectivity of the field lines connecting to Earth and change the expected solar energetic particle fluxes. Comparing the results at 1 AU of our simulations with in situ observations by the ACE spacecraft, we propose an alternate solar origin for the shock wave observed at L1 on August 26., Comment: 4 pages, 2 figures, refereed proceedings for Solar Wind 12

We present general considerations regarding the derivation of the radial distances of coronal mass ejections (CMEs) from elongation angle measurements such as those provided by SECCHI and SMEI, focusing on measurements in the Heliospheric Imager 2 (HI-2) field of view (i.e. past 0.3 AU). This study is based on a three-dimensional (3-D) magneto-hydrodynamics (MHD) simulation of two CMEs observed by SECCHI on January 24-27, 2007. Having a 3-D simulation with synthetic HI images, we are able to compare the two basic methods used to derive CME positions from elongation angles, the so-called "Point-P" and "Fixed-Phi" approximations. We confirm, following similar works, that both methods, while valid in the most inner heliosphere, yield increasingly large errors in HI-2 field of view for fast and wide CMEs. Using a simple model of a CME as an expanding self-similar sphere, we derive an analytical relationship between elongation angles and radial distances for wide CMEs. This relationship is simply the harmonic mean of the "Point-P" and "Fixed-Phi'' approximations and it is aimed at complementing 3-D fitting of CMEs by cone models or flux rope shapes. It proves better at getting the kinematics of the simulated CME right when we compare the results of our line-of-sights to the MHD simulation. Based on this approximation, we re-analyze the J-maps (time-elongation maps) in January 26-27, 2007 and present the first observational evidence that the merging of CMEs is associated with a momentum exchange from the faster ejection to the slower one due to the propagation of the shock wave associated with the fast eruption through the slow eruption., Comment: 10 pages, 4 figures, accepted in Annales Geophysicae (Special Issue: Three eyes on the Sun - multi-spacecraft studies of the corona and impacts on the heliosphere)

The SECCHI instruments aboard the recently launched STEREO spacecraft enable for the first time the continuous tracking of coronal mass ejections (CMEs) from the Sun to 1 AU. We analyze line-of-sight observations of the January 24-25, 2007 CMEs and fill the 20-hour gap in SECCHI coverage in January 25 by performing a numerical simulation using a three-dimensional magneto-hydrodynamic (MHD) code, the Space Weather Modeling Framework (SWMF). We show how the observations reflect the interaction of the two successive CMEs with each other and with the structured solar wind. We make detailed comparison between the observations and synthetic images from our model, including time-elongation maps for several position angles. Having numerical simulations to disentangle observational from physical effects, we are able to study the three-dimensional nature of the ejections and their evolution in the inner heliosphere. This study reflects the start of a new era where, on one hand, models of CME propagation and interaction can be fully tested by using heliospheric observations and, on the other hand, observations can be better interpreted by using global numerical models., Comment: 18 pages, 6 figures, accepted to the topical issue of Solar Physics on STEREO results at solar minimum

We discuss how some coronal mass ejections (CMEs) originating from the western limb of the Sun are associated with space weather effects such as solar energetic particles (SEPs), shock or geo-effective ejecta at Earth. We focus on the August 24, 2002 coronal mass ejection, a fast (~ 2000 km/s) eruption originating from W81. Using a three-dimensional magneto-hydrodynamic simulation of this ejection with the Space Weather Modeling Framework (SWMF), we show how a realistic initiation mechanism enables us to study the deflection of the CME in the corona and the heliosphere. Reconnection of the erupting magnetic field with that of neighboring streamers and active regions modify the solar connectivity of the field lines connecting to Earth and can also partly explain the deflection of the eruption during the first tens of minutes. Comparing the results at 1 AU of our simulation with observations by the ACE spacecraft, we find that the simulated shock does not reach Earth, but has a maximum angular span of about 120$^\circ$, and reaches 35$^\circ$ West of Earth in 58 hours. We find no significant deflection of the CME and its associated shock wave in the heliosphere, and we discuss the consequences for the shock angular span., Comment: 7 pages, 4 figures, IAU 257 Symposium Proceedings

The interaction of multiple Coronal Mass Ejections (CMEs) has been observed by LASCO coronagraphs and by near-Earth spacecraft, and it is thought to be an important cause of geo-effective storms, large Solar Energetic Particles events and intense Type II radio bursts. New and future missions such as STEREO, the LWS Sentinels, and the Solar Orbiter will provide additional observations of the interaction of multiple CMEs between the Sun and the Earth. We present the results of simulations of two and more CMEs interacting in the inner heliosphere performed with the Space Weather Modeling Framework (SWMF). Based on those simulations, we discuss the observational evidence of the interaction of multiple CMEs, both in situ and from coronagraphs. The clearest evidence of the interaction of the CMEs are the large temperature in the sheath, due to the shocks merging, and the brightness increase in coronagraphic images, associated with the interaction of the leading edges. The importance of having multiple satellites at different distances and angular positions from the Sun is also discussed., Comment: 6 pages, 3 figures, ISROSES conference

We discuss features of coronal mass ejections (CMEs) that are specific to heliospheric observations at large elongation angles. Our analysis is focused on a series of two eruptions that occurred on 2007 January 24-25, which were tracked by the Heliospheric Imagers (HIs) onboard STEREO. Using a three-dimensional (3-D) magneto-hydrodynamic simulation of these ejections with the Space Weather Modeling Framework (SWMF), we illustrate how the combination of the 3-D nature of CMEs, solar rotation, and geometry associated with the Thomson sphere results in complex effects in the brightness observed by the HIs. Our results demonstrate that these effects make any in-depth analysis of CME observations without 3-D simulations challenging. In particular, the association of bright features seen by the HIs with fronts of CME-driven shocks is far from trivial. In this Letter, we argue that, on 2007 January 26, the HIs observed not only two CMEs, but also a dense corotating stream compressed by the CME-driven shocks., Comment: 5 pages, 2 figures, accepted for ApJ Letter