Context. Coronal Mass Ejections (CMEs) are the primary source of strong space weather disturbances at Earth. Their geoeffectiveness is largely determined by their dynamic pressure and internal magnetic fields, for which reliable predictions at Earth are not possible with traditional cone CME models. Aims. We study two well-observed Earth-directed CMEs using the EUropean Heliospheric FORecasting Information Asset (EUH-FORIA) model, testing for the first time the predictive capabilities of a linear force-free spheromak CME model initialised using parameters derived from remote-sensing observations. Methods. Using observation-based CME input parameters, we perform magnetohydrodynamic simulations of the events with EUH-FORIA, using the cone and spheromak CME models.Results. Simulations show that spheromak CMEs propagate faster than cone CMEs when initialised with the same kinematic parameters. We interpret these differences as result of different Lorentz forces acting within cone and spheromak CMEs, which lead to different CME expansions in the heliosphere. Such discrepancies can be mitigated by initialising spheromak CMEs with a reduced speed corresponding to the radial speed only. Results at Earth evidence that the spheromak model improves the predictions of B (B z ) up to 12-60 (22-40) percentage points compared to a cone model. Considering virtual spacecraft located within ±10 • around Earth, B (B z ) predictions reach 45-70% (58-78%) of the observed peak values. The spheromak model shows inaccurate predictions of the magnetic field parameters at Earth for CMEs propagating away from the Sun-Earth line.Conclusions. The spheromak model successfully predicts the CME properties and arrival time in the case of strictly Earth-directed events, while modelling CMEs propagating away from the Sun-Earth line requires extra care due to limitations related to the assumed spherical shape. The spatial variability of modelling results and the typical uncertainties in the reconstructed CME direction advocate the need to consider predictions at Earth and at virtual spacecraft located around it.

Coronal mass ejections (CMEs) are the primary sources of intense disturbances at Earth, where their geo-effectiveness is largely determined by their dynamic pressure and internal magnetic field, which can be significantly altered during interactions with other CMEs in interplanetary space. We analyse three successive CMEs that erupted from the Sun during September 4-6, 2017, investigating the role of CME-CME interactions as source of the associated intense geomagnetic storm (Dst min = −142 nT on September 7). To quantify the impact of interactions on the (geo-)effectiveness of individual CMEs, we perform global heliospheric simulations with the EUHFORIA model, using observation-based initial parameters with the additional purpose of validating the predictive capabilities of the model for complex CME events. The simulations show that around 0.45 AU, the shock driven by the September 6 CME started compressing a preceding magnetic ejecta formed by the merging of two CMEs launched on September 4, significantly amplifying its B z until a maximum factor of 2.8 around 0.9 AU. The following gradual conversion of magnetic energy into kinetic and thermal components reduced the B z amplification until its almost complete disappearance around 1.8 AU. We conclude that a key factor at the origin of the intense storm triggered by the September 4-6, 2017 CMEs was their arrival at Earth during the phase of maximum B z amplification. Our analysis highlights how the amplification of the magnetic field of individual CMEs in space-time due to interaction processes can be characterised by a growth, a maximum, and a decay phase, suggesting that the time interval between the CME eruptions and their relative speeds are critical factors in determining the resulting impact of complex CMEs at various heliocentric distances (helio-effectiveness).

We determine the three‐dimensional geometry and deprojected mass of 29 well‐observed coronal mass ejections (CMEs) and their interplanetary counterparts (ICMEs) using combined Solar Terrestrial Relations Observatory ‐ Solar and Heliospheric Observatory white‐light data. From the geometry parameters, we calculate the volume of the CME for the magnetic ejecta (flux‐rope type geometry) and sheath structure (shell‐like geometry resembling the (I)CME frontal rim). Working under the assumption that the CME mass is roughly equally distributed within a specific volume, we expand the CME self‐similarly and calculate the CME density for distances close to the Sun (15–30 Rs) and at 1 AU. Specific trends are derived comparing calculated and in‐situ measured proton densities at 1 AU, though large uncertainties are revealed due to the unknown mass and geometry evolution: (1) a moderate correlation for the magnetic structure having a mass that stays rather constant (cc ≈ 0.56 − 0.59), and (2) a weak correlation for the sheath density (cc ≈ 0.26) by assuming the sheath region is an extra mass—as expected for a mass pile‐up process—that is in its amount comparable to the initial CME deprojected mass. High correlations are derived between in‐situ measured sheath density and the solar wind density (cc ≈ −0.73) and solar wind speed (cc ≈ 0.56) as measured 24 h ahead of the arrival of the disturbance. This gives additional confirmation that the sheath‐plasma indeed stems from piled‐up solar wind material. While the CME interplanetary propagation speed is not related to the sheath density, the size of the CME may play some role in how much material could be piled up.

Coronal mass ejections (CMEs) are the major space weather drivers, and an accurate modeling of their onset and propagation up to 1 AU represents a key issue for more reliable space weather forecasts. In this paper we use the newly developed EUropean Heliospheric FORecasting Information Asset (EUHFORIA) heliospheric model to test the effect of different CME shapes on simulation outputs. In particular, we investigate the notion of “spherical” CME shape, with the aim of bringing to the attention of the space weather community the great implications of the CME shape implementation details for simulation results and geoeffectiveness predictions. We take as case study an artificial Earth‐directed CME launched on 6 June 2008, corresponding to a period of quiet solar wind conditions near Earth. We discuss the implementation of the cone model used to inject the CME into the modeled ambient solar wind, running several simulations of the event and investigating the outputs in interplanetary space and at different spacecraft and planetary locations. We apply empirical relations to simulation outputs at L1 to estimate the expected CME geoeffectiveness in terms of the magnetopause stand‐off distance and the induced Kp index. Our analysis shows that talking about spherical CMEs is ambiguous unless one has detailed information on the implementation of the CME shape in the model. All the parameters specifying the CME shape in the model significantly affect simulation results at 1 AU as well as the predicted CME geoeffectiveness, confirming the pivotal role played by the shape implementation details in space weather forecasts.

Context. Eruptive events such as coronal mass ejections (CMEs) and flares accelerate particles and generate shock waves which can arrive at Earth and can disturb the magnetosphere. Understanding the association between CMEs and CME-driven shocks is therefore highly important for space weather studies. Aims. We present a study of the CME/flare event associated with two type II bursts observed on September 27, 2012. The aim of the study is to understand the relationship between the observed CME and the two distinct shock wave signatures. Methods. The multiwavelength study of the eruptive event (CME/flare) was complemented with radio triangulation of the associated radio emission and modelling of the CME and the shock wave employing MHD simulations. Results. We found that, although temporal association between the type II bursts and the CME is good, the low-frequency type II (LF-type II) burst occurs significantly higher in the corona than the CME and its relationship to the CME is not straightforward. The analysis of the EIT wave (coronal bright front) shows the fastest wave component to be in the southeast quadrant of the Sun. This is also the quadrant in which the source positions of the LF-type II were found to be located, probably resulting from the interaction between the shock wave and a streamer. Conclusions. The relationship between the CME/flare event and the shock wave signatures is discussed using the temporal association, as well as the spatial information of the radio emission. Further, we discuss the importance and possible effects of the frequently non-radial propagation of the shock wave.

We present the first statistical analysis of complexity changes affecting the magnetic structure of interplanetary coronal mass ejections (ICMEs), with the aim of answering the questions: How frequently do ICMEs undergo magnetic complexity changes during propagation? What are the causes of such changes? Do the in situ properties of ICMEs differ depending on whether they exhibit complexity changes? We consider multispacecraft observations of 31 ICMEs by MESSENGER, Venus Express, ACE, and STEREO between 2008 and 2014 while radially aligned. By analyzing their magnetic properties at the inner and outer spacecraft, we identify complexity changes that manifest as fundamental alterations or significant reorientations of the ICME. Plasma and suprathermal electron data at 1 au, and simulations of the solar wind enable us to reconstruct the propagation scenario for each event, and to identify critical factors controlling their evolution. Results show that ∼65% of ICMEs change their complexity between Mercury and 1 au and that interaction with multiple large-scale solar wind structures is the driver of these changes. Furthermore, 71% of ICMEs observed at large radial (>0.4 au) but small longitudinal (<15°) separations exhibit complexity changes, indicating that propagation over large distances strongly affects ICMEs. Results also suggest that ICMEs may be magnetically coherent over angular scales of at least 15°, supporting earlier theoretical and observational estimates. This work presents statistical evidence that magnetic complexity changes are consequences of ICME interactions with large-scale solar wind structures, rather than intrinsic to ICME evolution, and that such changes are only partly identifiable from in situ measurements at 1 au.

We analyse in this work the propagation and geoeffectiveness of four successive coronal mass ejections (CMEs) that erupted from the Sun during 21-23 May 2013 and that were detected in interplanetary space by the Wind and/or STEREO-A spacecraft. All these CMEs featured critical aspects for understanding so-called "problem space weather storms" at Earth. In the first three events a limb CMEs resulted in moderately geoeffective in-situ structures at their target location in terms of the disturbance storm time (Dst) index (either measured or estimated). The fourth CME, which also caused a moderate geomagnetic response, erupted from close to the disc centre as seen from Earth, but it was not visible in coronagraph images from the spacecraft along the Sun-Earth line and appeared narrow and faint from off-angle viewpoints. Making the correct connection between CMEs at the Sun and their in-situ counterparts is often difficult for problem storms. We investigate these four CMEs using multiwavelength and multipoint remote-sensing observations (extreme ultraviolet, white light, and radio), aided by 3D heliospheric modelling, in order to follow their propagation in the corona and in interplanetary space and to assess their impact at 1 AU. Finally, we emphasise the difficulties in forecasting moderate space weather effects provoked by problematic and ambiguous events and the importance of multispacecraft data for observing and modelling problem storms.

Context. Coronal mass ejections (CMEs) are large-scale eruptions coming from the Sun and transiting into interplanetary space. While it is widely known that they are major drivers of space weather, further knowledge of CME properties in the inner heliosphere is limited by the scarcity of observations at heliocentric distances other than 1 au. In addition, most CMEs are observed in situ by a single spacecraft and in-depth studies require numerical models to complement the few available observations. Aims. We aim to assess the ability of the linear force-free spheromak CME model of the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) to describe the radial evolution of interplanetary CMEs in order to yield new contexts for observational studies. Methods. We modelled one well-studied CME with EUHFORIA, investigating its radial evolution by placing virtual spacecraft along the Sun–Earth line in the simulation domain. To directly compare observational and modelling results, we characterised the interplanetary CME signatures between 0.2 and 1.9 au from modelled time series, exploiting techniques that are traditionally employed to analyse real in situ data. Results. Our results show that the modelled radial evolution of the mean solar wind and CME values is consistent with the observational and theoretical expectations. The CME expands as a consequence of the decaying pressure in the surrounding solar wind: the expansion is rapid within 0.4 au and moderate at larger distances. The early rapid expansion was not sufficient to explain the overestimated CME radial size in our simulation, suggesting this is an intrinsic limitation of the spheromak geometry applied in this case. The magnetic field profile indicates a relaxation on the part of the CME structure during propagation, while CME ageing is most probably not a substantial source of magnetic asymmetry beyond 0.4 au. Finally, we report a CME wake that is significantly shorter than what has been suggested by observations. Conclusions. Overall, EUHFORIA provides a consistent description of the radial evolution of solar wind and CMEs, at least close to their centres. Nevertheless, improvements are required to better reproduce the CME radial extension.