New Observations Needed to Advance Our Understanding of Coronal Mass Ejections

Aims. We introduce a new model for coronal mass ejections (CMEs) that has been implemented in the magnetohydrodynamics (MHD) inner heliosphere model EUHFORIA. Utilising a linear force-free spheromak (LFFS) solution, the model provides an intrinsic magnetic field structure for the CME. As a result, the new model has the potential to predict the magnetic components of CMEs at Earth. In this paper, we present the implementation of the new model and show the capability of the new model. Methods. We present initial validation runs for the new magnetised CME model by considering the same set of events as used in the initial validation run of EUHFORIA that employed the Cone model. In particular, we have focused on modelling the CME that was responsible for creating the largest geomagnetic disturbance (Dst index). Two scenarios are discussed: one where a single magnetised CME is launched and another in which we launch all five Earth-directed CMEs that were observed during the considered time period. Four out of the five CMEs were modelled using the Cone model. Results. In the first run, where the propagation of a single magnetized CME is considered, we find that the magnetic field components at Earth are well reproduced as compared to in-situ spacecraft data. Considering a virtual spacecraft that is separated approximately seven heliographic degrees from the position of Earth, we note that the centre of the magnetic cloud is missing Earth and a considerably larger magnetic field strength can be found when shifting to that location. For the second run, launching four Cone CMEs and one LFFS CME, we notice that the simulated magnetised CME is arriving at the same time as in the corresponding full Cone model run. We find that to achieve this, the speed of the CME needs to be reduced in order to compensate for the expansion of the CME due to the addition of the magnetic field inside the CME. The reduced initial speed of the CME and the added magnetic field structure give rise to a very similar propagation of the CME with approximately the same arrival time at 1 au. In contrast to the Cone model, however, the magnetised CME is able to predict the magnetic field components at Earth. However, due to the interaction between the Cone model CMEs and the magnetised CME, the magnetic field amplitude is significantly lower than for the run using a single magnetised CME. Conclusions. We have presented the LFFS model that is able to simulate and predict the magnetic field components and the propagation of magnetised CMEs in the inner heliosphere and at Earth. We note that shifting towards a virtual spacecraft in the neighbourhood of Earth can give rise to much stronger magnetic field components. This gives the option of adding a grid of virtual spacecrafts to give a range of values for the magnetic field components.

<p>Previous studies of Coronal Mass Ejections (CMEs) have shown the importance of understanding their geometrical structure and internal magnetic field configuration for improving forecasting at Earth. The precise prediction of the CME shock and the magnetic cloud arrival time, their magnetic field strength and the orientation upon impact at Earth is still challenging and relies on solar wind and CME evolution models and precise input parameters. In order to understand the propagation of CMEs in the interplanetary medium, we need to understand their interaction with the complex features in the magnetized background solar wind which deforms, deflects and erodes the CMEs and determines their geo-effectiveness. Hence, it is important to model the internal magnetic flux-rope structure in the CMEs as they interact with CIRs/SIRs, other CMEs and solar transients in the heliosphere. The spheromak model (Verbeke et al. 2019) in the heliospheric wind and CME evolution simulation EUHFORIA (Pomoell and Poedts, 2018), fits well with the data near the CME nose close to its axis but fails to predict the magnetic field in CME legs when these impact Earth (Scolini et al. 2019). Therefore, we implemented the FRi3D stretched flux-rope CME model (Isavnin, 2016) in EUHFORIA to model a more realistic CME geometry. Fri3D captures the three-dimensional magnetic field structure with parameters like skewing, pancaking and flattening that quantify deformations experienced by an interplanetary CME. We perform test runs of real CME events and validate the ability of FRi3D coupled with EUHFORIA in predicting the CME geo-effectiveness. We have modeled two real events with FRi3D. First, a CME event on 12 July 2012 which was a head-on encounter at Earth. Second, the flank CME encounter of 14 June 2012 which did not leave any magnetic field signature at Earth when modeled with Spheromak. We compare our results with the results from non-magnetized cone simulations and magnetized simulations employing the spheromak flux-rope model. We further discuss how constraining observational parameters using the stretched flux rope CME geometry in FRi3D affects the prediction of the magnetic field strength in our simulations, highlighting improvements and discussing future perspective.</p><p><em>This research has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870405 (EUHFORIA 2.0)</em></p>

Coronal Mass Ejections (CMEs) are large-scale eruptions of plasma and magnetic fields from the Sun. They are considered to be the main drivers of strong space weather events at Earth. Multiple models have been developed over the past decades to be able to predict the propagation of CMEs and their arrival time at Earth. Such models require input from observations, which can be used to fit the CME to an appropriate structure.When determining input parameters for CME propagation models, it is common procedure to derive kinematic parameters from remote-sensing data. The resulting parameters can be used as inputs for the CME propagation models to obtain an arrival prediction time of the CME f.e. at Earth. However, when fitting the CME structure to obtain the needed parameters for simulations, different geometric structures and also different parts of the CME structure can be fitted. These aspects, together with the fact that 3D reconstructions strongly depend on the subjectivity and judgement of the scientist performing them, may lead to uncertainties in the fitted parameters. Up to now, no large study has tried to map these uncertainties and to evaluate how they affect the modelling of CMEs. Furthermore, when using these determined parameters as inputs into CME propagation models, they spread throughout the modelling domain and influence the final results of the simulation and the predicted arrival time of the modelled CME.Fitting a large set of CMEs within a selected period of time, we aim to investigate the uncertainties in the CME fittings in detail. Each event is fitted multiple times by different scientists. We discuss statistics on uncertainties of the fittings. We also present some first results of the impact of these uncertainties on CME propagation modelling.Acknowledgements: This work has been partly supported by the International Space Science Institute (ISSI) in the framework of International Team 480 entitled: Understanding Our Capabilities In Observing And Modeling Coronal Mass Ejections'.

Context. We explore the impact of interactions between coronal mass ejections (CMEs) – known as CME–CME interactions – on Earth using remote-sensing and in situ observations and estimate the amplification of the geo-effectiveness of the individual CMEs by a factor of ∼2 due to CME–CME interactions. Aims. We present 3D reconstructions of interacting CMEs, which provide essential information on the orientation and interaction of the events. Additionally, we analysed coronal evolution of CMEs and their in situ characteristics at 1 AU to explore the impact of interactions between CMEs on their geo-effectiveness. Methods. We analysed CME interaction using white light data from LASCO and STEREO COR-A. The reported CMEs were reconstructed using the gradual cylindrical shell (GCS) model and simulated self-consistently with the physics-based 3D MHD model EUHFORIA (EUropean Heliosphere FORecasting Information Asset). By running different simulations, we estimated the geo-effectiveness of both individual and interacting CMEs using an empirical relationship method for the disturbance storm index. Results. The SOHO/LASCO spacecraft observed three CMEs erupting from the Sun within an interval of 10 h during a very active period in early November 2021. There were two partial halo CMEs that occurred on 1 Nov. 2021 at 19:00 UT and 22:00 UT, respectively, from the active region 12887 (S28W58), and a third halo CME occurred from AR 12891 (N17E03) on 2 Nov. 2021 at 02:48 UT. By combining remote observations close to the Sun, in situ data at 1 AU, and further numerical analyses of each individual CME, we are able to identify the initial and interplanetary evolution of the CMEs. Conclusions. (i) White light observations and a 3D reconstruction of the CMEs show cannibalism by CME-2 on CME-1 and a flank interaction of CME-3 with the merged CME-1 and CME-2 at 45–50 Rs. (ii) Interacting CMEs exhibit an increase in geo-effectiveness compared to an individual CME.

<p>Our knowledge of the properties of Coronal Mass Ejections (CMEs) in the inner heliosphere is constrained by the relative lack of plasma observations between the 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, and 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 a 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.</p>

Coronal mass ejections (CMEs) erupting from the host star are expected to affect the atmospheric erosion processes of planets. For planets with a magnetosphere, the embedded magnetic field in the CMEs is thought to be the most important parameter to affect planetary mass-loss. In this work, we investigate the effect of different magnetic field structures of stellar CMEs on the atmosphere of a hot Jupiter with a dipolar magnetosphere. We use a time-dependent 3D radiative magnetohydrodynamic (MHD) atmospheric escape model that self-consistently models the outflow from hot Jupiter’s magnetosphere and its interaction with stellar CMEs. For our study, we consider three configurations of magnetic field embedded in CMEs – (a) northward $B_z$ component, (b) southward $B_z$ component, and (c) radial component. We find that both the CMEs with northward $B_z$ and southward $B_z$ increase the planetary mass-loss rate when the CME arrives from the stellar side, with the mass-loss rate remaining higher for the CME with northward $B_z$ until it arrives on the opposite side. The largest magnetopause is found for the CME with a southward $B_z$ component. During the passage of a CME, the planetary magnetosphere goes through three distinct changes – (1) compressed magnetosphere, (2) enlarged magnetosphere, and (3) relaxed magnetosphere for all three CME configurations. The computed synthetic Ly $\alpha$ transit absorption generally increases when the CME is in interaction with the planet for all magnetic configurations but the maximum Ly $\alpha$ absorption is found for the case of radial CME with the most compressed magnetosphere.

Abstract. The apparent contradiction between small-scale source regions of, and large-scale coronal response to, coronal mass ejections (CMEs) has been a long-standing puzzle. For some, CMEs are considered to be inherently large-scale events – eruptions in which a number of flux systems participate in an unspecified manner, while others consider magnetic reconnection in special global topologies to be responsible for the large-scale response of the lower corona to CME events. Some of these ideas may indeed be correct in specific cases. However, what is the key element which makes CMEs large-scale? Observations show that the extent of the coronal disturbance matches the angular width of the CME – an important clue, which does not feature strongly in any of the above suggestions. We review observational evidence for the large-scale nature of CME source regions and find them lacking. Then we compare different ideas regarding how CMEs evolve to become large-scale. The large-scale magnetic topology plays an important role in this process. There is amounting evidence, however, that the key process is magnetic reconnection between the CME and other magnetic structures. We outline a CME evolution model, which is able to account for all the key observational signatures of large-scale CMEs and presents a clear picture how large portions of the Sun become constituents of the CME. In this model reconnection is driven by the expansion of the CME core resulting from an over-pressure relative to the pressure in the CME's surroundings. This implies that the extent of the lower coronal signatures match the final angular width of the CME.

High-resolution EUV spectroscopy provides an informative diagnostic tool for the early evolution of coronal mass ejections (CMEs) since it can directly measure many physical properties of CME plasma close to the Sun that cannot be determined from white-light coronagraphs or narrowband imagers. The Hinode/EUV Imaging Spectrometer (EIS) captured a series of high-resolution EUV spectra of the 2008 April 9 event, also known as the “Cartwheel CME,” during its initial acceleration period. CMEs are inherently dynamic events, driving the plasma from ionization equilibrium, which makes the nonequilibrium ionization (NEI) effects important when analyzing spectral observations. In this work, we performed simulations of the Cartwheel CME using the Alfvén Wave Solar atmosphere Model (AWSoM) to provide insight into the plasma structure and dynamics during the early evolution of this CME. For the first time, we combined self-consistent nonequilibrium charge state calculations in the EUV spectral line synthesis for a CME simulation to account for the plasma departures from ionization equilibrium everywhere in the CME. In this first paper of the series, we compare the simulation results to the EIS observations of this event to assess the quality of the AWSoM predictions. We find that the CME is out of equilibrium, indicating that NEI effects should be considered in the EUV spectral synthesis. Overall, the model reproduces the thermodynamic evolution of the CME in the low solar corona well, as compared to the EIS observations.

The shocks and magnetic clouds caused by Coronal Mass Ejections (CMEs) in the solar corona and interplanetary (IP) space play an important role in the study of space weather. In the present paper, numerical simulations of some simple CME models were performed by means of a finite volume, explicit solver to advance the equations of ideal magnetohydrodynamics. The aim is to quantify here both the effect of the background wind model and of the initial polarity on the evolution of the IP CMEs and the corresponding shocks. To simulate the CMEs, a high density-pressure plasma blob is superposed on different steady state solar wind models. The evolution of an initially non-magnetized plasma blob is compared with that of two magnetized ones (with both normal and inverse polarity) and the differences are analysed and quantified. Depending on the launch angle of the CME and the polarity of the initial flux rope, the velocity of the shock front and magnetic cloud is decreased or increased. Also the spread angle of the CME and the evolution path of the CME in the background solar wind is substantially different for the different CME models and the different wind models. A quantitative comparison of these simulations shows that these effects can be quite substantial and can clearly affect the geo-effectiveness and the arrival time of the events.

Over the past decades, missions at the L1 point have been providing solar wind and interplanetary magnetic field measurements that are necessary for forecasting space weather at Earth with high accuracy and a lead time of a few tens of minutes. Improving the lead time, while maintaining a relatively high level of accuracy, can be achieved with missions sunward of L1, so‐called sub‐L1 monitors. However, too much is unknown to plan for sub‐L1 monitors as operational missions: both the orbital requirements of such missions, and the achievable accuracy of forecasts based on their measurements have not been quantitatively defined. We review here some proposed mission concepts and explain the knowledge gaps related to coronal mass ejections (CMEs) that require a space weather research or science mission. We first show how STEREO‐A measurements in 2023 can be used as a proof of concept of the use of sub‐L1 monitor slightly off the Sun‐Earth line to forecast the Dst index. We then highlight that separations of are needed to ensure that CMEs measured by a sub‐L1 monitor impact Earth. Next, we show that measurements with angular separations of have negligible errors but separations of a few degrees can result in significant errors in lead time and in the forecasted magnetic field strength of CMEs. We also discuss how CME evolution over the last 0.05–0.2 au before impacting Earth is strongly under‐constrained and needs to be better understood before using measurements of sub‐L1 monitors for real‐time space weather forecasting.

Parker Solar Probe (PSP; launched in 2018) observes the Sun from unprecedented close-in and out-of-ecliptic orbits. The unique and high-resolution data from the Wide-Field Imager for Solar Probe (WISPR) aboard PSP give us new insights about the initiation and early evolution of Coronal Mass Ejections (CMEs) in the inner Heliosphere. We investigate the morphology and propagation behavior of distinct small-scale structures associated with a CME caused by a filament eruption, together with blobs related to the post-CME current sheet. Within this work we want to answer the following questions: how do the small scale magnetic field structures develop, how do they change in shape over time and what is their relation with the erupting filament and flux rope, respectively. The fast PSP motion at perihelion allows one to have views from different angles of the same event, hence we apply a single-spacecraft triangulation technique to derive coordinates and kinematics of each tracked feature. We find distinct groups of small-scale features which appear to be the building blocks of the global CME. We categorised the small scale magnetic structures based on their morphology and extent in longitude and latitude. We obtained a large range of longitudes among the different blobs related to the CME-aftermath. Thread-bundles are identified in the inner Heliosphere, which might be related to the vertical threads that are seen evolving during the filament eruption. Finally, we discuss on the different global appearances of the CME as observed from 1 AU compared to 0.18 AU (PSP).

Our knowledge of the physical processes carrying information across Coronal Mass Ejections (CMEs), thereby controlling the way CME structures respond to external disturbances  in interplanetary space, is still incomplete. One prominent question is whether CMEs are “coherent” structures, i.e. potentially capable of responding in a uniform manner to external forces, and across what (macroscopic) spatial scales does such coherence exist. A necessary condition for different regions within a given CME to exhibit a coherent behavior is that they must be causally connected to the perturbation source. Past studies suggested interplanetary CMEs may behave as coherent structures only locally, and indicated that the spatial scale of coherence may be hampered by interactions with large-scale structures in the solar wind. Additionally, observational studies often assumed the correlation in the magnetic field profiles at different spacecraft locations as a proxy for coherence, but the physical link between correlation and coherence is still to be established. Characterizing the physical mechanisms mediating CME structural changes in different solar wind conditions at a fundamental level is therefore imperative to better understand their evolution and impact on space-borne and ground-based anthropic activities. In this study, we investigate the role of Alfvénic fluctuations (AFs) as mediators of coherence within interplanetary CMEs, and the physical relationship between correlation and coherence using multi-point observations near 1 au. In order to determine if and to what extent AFs alter CMEs at different spatial scales, we compare CME signatures at multiple spacecraft in terms of presence/absence of AFs, AF properties (if present), and correlation of magnetic signatures.  We contextualize the results in terms of the CME interaction history and the causal connection of different spacecraft observations. This study reveals how AFs affect the correlation of CME magnetic signatures across different spatial scales, and helps reconcile correlation scales within CMEs with their coherent behavior. 

Coronal mass ejections (CMEs) are a major transient input of mass and energy into the solar wind. We review some of the past and present concepts that influence the development of models of coronal mass ejections, both for CME initiation and CME evolution and propagation in the solar wind. We use the flux cancellation model to illustrate present research on CMEs. Primarily for convenience, modeling of CME propagation has usually been treated separately from the initiation problem. We suggest that future computational modeling of interplanetary CMEs is likely to emphasize the need to study coronal initiation and solar wind propagation together.

We describe the world's largest synthesis radio telescope, the Very Large Array (VLA), and how it can be used to complement observations with the Solar and Heliospheric Observatory (SOHO) and the Yohkoh solar spacecraft. The VLA provides images with high spatial and temporal resolution, often across the visible solar disk. The VLA also detects nonthermal radiation that is not observed with SOHO and Yohkoh, and provides estimates for the coronal magnetic field strengths that are not directly measured by these spacecraft. The VLA data can be combined with SOHO CDS, SOHO EIT, or Yohkoh SXT observations to provide new insights to the compact, variable sources, called blinkers and bright points, in the solar transition region or low corona. A new 400 cm VLA system provides images of nonthermal burst activity associated with Coronal Mass Ejections (CMEs), and may detect thermal emission from CMEs, that can be compared with SOHO's LASCO and EIT instruments to obtain new information about the origin and evolution of CMEs.

Interplanetary coronal mass ejections (ICMEs) are the heliospheric counterparts of solar coronal mass ejections, and their comprehensive cataloging near Earth has enabled significant advances across space science disciplines. As a part of the Project Space Life by Eco Astronomy Inc, this review presents a synthesis of the Near-Earth ICME since 1996” dataset compiled by I. G. Richardson and H. V. Cane, highlighting its historical development, methodology, key findings, and broad applications. The dataset encompasses approximately 600 ICMEs observed at 1 AU from 1996 through early 2024, spanning solar cycles 23, 24, and the rise of 25. We discuss how ICMEs are identified using in-situ plasma, magnetic field, composition, and energetic particle signatures, and how this approach has evolved with improved data (e.g. ACE spacecraft measurements). The ICME catalogs reveals clear solar-cycle trends, ICME occurrence rises by an order of magnitude from solar minimum to maximum with the yearly counts peaking in 2000 (51 events) and dipping to only a few events at recent minima. It also captures intriguing temporal patterns such as a quasi-periodicity of ~150 days in ICME occurrence (Cane & Richardson, January 1, 2003). We synthesize the typical ICME properties (duration, speed, magnetic field strength) and how they vary between cycles: e.g. cycle 23 produced more frequent and faster ICMEs than the weaker cycle 24. The dataset’s extensive annotations (footnotes) link each ICME to associated phenomena like geomagnetic sudden commencements, storm indices, and cosmic ray depressions, allowing analysis of ICME impacts. We review how ~90% of major geomagnetic storms (Dst ≤ –100 nT) in 1996–2005 were caused by ICMEs or their driven shocks (Richardson & Cane, 2007), underlining their geo effectiveness. We summarize statistical correlations between ICME parameters (speed, magnetic field, size) and geomagnetic storm intensity and discuss notable events. Beyond geomagnetism, ICMEs are shown to modulate galactic cosmic rays, causing short-term intensity drops (Forbush decreases) in ~80% of cases (I & H, 2011) and contributing to long-term cosmic-ray modulation over the solar cycle. The rich Richardson-Cane ICME database has fostered multidisciplinary benefits: improving space weather forecasts (by providing empirical event data for CME arrival and storm predictions), informing heliospheric physics (e.g. ICME propagation and solar-cycle variability), elucidating cosmic ray transport (through ICME-caused particle decreases), and aiding studies of impacts on technological systems (satellites, power grids, GPS) and Earth’s atmosphere. We conclude by emphasizing the value of this continually updated ICME catalogs for future research and cross-disciplinary applications, and we include a comprehensive appendix of ICME analysis figures derived from the dataset.