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Register a new userExploring the radial evolution of Interplanetary Coronal Mass Ejections using EUHFORIA
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Coronal Mass Ejections (CMEs) are large-scale eruptions from the Sun into interplanetary space. Despite being major space weather drivers, our knowledge of the CME properties in the inner heliosphere remains constrained by the scarcity of observations at distances other than 1 au. Furthermore, most CMEs are observed in situ by single spacecraft, requiring numerical models to complement the sparse observations available. We aim to assess the ability of the linear force-free spheromak CME model in EUHFORIA to describe the radial evolution of interplanetary CMEs, yielding new context for observational studies. We model one well-studied CME, and investigate its radial evolution by placing virtual spacecraft along the Sun-Earth line in the simulation domain. To directly compare observational and modelling results, we characterise the interplanetary CME signatures between 0.2 and 1.9 au from modelled time series, exploiting techniques traditionally employed to analyse real in situ data. Results show that the modelled radial evolution of the mean solar wind and CME values is consistent with observational and theoretical expectations. The CME expands as a consequence of the decaying pressure in the surrounding wind: the expansion is rapid within 0.4 au, and moderate at larger distances. The early rapid expansion could not explain the overestimated CME radial size in our simulation, suggesting this is an intrinsic limitation of the spheromak geometry used. The magnetic field profile indicates a relaxation of the CME during propagation, while ageing is most probably not a substantial source of magnetic asymmetry beyond 0.4 au. We also report a CME wake that is significantly shorter than suggested by observations. Overall, EUHFORIA provides a consistent description of the radial evolution of solar wind and CMEs; nevertheless, improvements are required to better reproduce the CME radial extension.
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This study aims to provide a reference to different magnetic field models and reconstruction methods for interplanetary coronal mass ejections (ICMEs). In order to understand the differences in the outputs of those models and codes, we analyze 59 events from the Coordinated Data Analysis Workshop (CDAW) list, using four different magnetic field models and reconstruction techniques; force-free fitting (Goldstein,1983,Burlaga,1988,Lepping et al.,1990), magnetostatic reconstruction using a numerical solution to the Grad-Shafranov equation (Hu and Sonnerup, 2001), fitting to a self-similarly expanding cylindrical configuration (Marubashi and Lepping, 2007) and elliptical, non-force free fitting (Hidalgo,2003). The resulting parameters of the reconstructions for the 59 events are compared statistically, as well as in selected case studies. The ability of a method to fit or reconstruct an event is found to vary greatly: the Grad-Shafranov reconstruction is successful for most magnetic clouds (MCs) but for less than 10% of the non-MC ICMEs; the other three methods provide a successful fit for more than 65% of all events. The differences between the reconstruction and fitting methods are discussed, and suggestions are proposed as to how to reduce them. We find that the magnitude of the axial field is relatively consistent across models but not the orientation of the axis of the ejecta. We also find that there are a few cases for which different signs of the magnetic helicity are found for the same event when we do not fix the boundaries, illustrating that this simplest of parameters is not necessarily always well constrained by fitting and reconstruction models. Finally, we look at three unique cases in depth to provide a comprehensive idea of the different aspects of how the fitting and reconstruction codes work.
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. We study two Earth-directed CMEs using the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) model, testing the predictive capabilities of a linear force-free spheromak CME model initialised using parameters derived from remote-sensing observations. Using observation-based CME input parameters, we perform MHD simulations of the events with the cone and spheromak CME models. Results show that spheromak CMEs propagate faster than cone CMEs when initialised with the same kinematic parameters. We interpret these differences as due to different Lorentz forces acting within cone and spheromak CMEs, leading to different CME expansions. 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(Bz) up to 12-60(22-40) percentage points compared to a cone model. Considering virtual spacecraft located around Earth, B(Bz) predictions reach 45-70%(58-78%) of the observed peak values. The spheromak model predicts inaccurate magnetic field parameters at Earth for CMEs propagating away from the Sun-Earth line, while it successfully predicts the CME properties and arrival time in the case of strictly Earth-directed events. 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 around it.
Interplanetary coronal mass ejections (ICMEs) often consist of a shock wave, sheath region, and ejecta region. The ejecta regions are divided into two broad classes: magnetic clouds (MC) that exhibit the characteristics of magnetic flux ropes and non-magnetic clouds (NMC) that do not. As CMEs result from eruption of magnetic flux ropes, it is important to answer why NMCs do not have the flux rope features. One claims that NMCs lose their original flux rope features due to the interactions between ICMEs or ICMEs and other large scale structures during their transit in the heliosphere. The other attributes this phenomenon to the geometric selection effect, i.e., when an ICME has its nose (flank, including leg and non-leg flanks) pass through the observing spacecraft, the MC (NMC) features will be detected along the spacecraft trajectory within the ejecta. In this Letter, we examine which explanation is more reasonable through the geometric properties of ICMEs. If the selection effect leads to different ejecta types, MCs should have narrower sheath region compared to NMCs from the statistical point of view, which is confirmed by our statistics. Besides, we find that NMCs have the similar size in solar cycles 23 and 24, and NMCs are smaller than MCs in cycle 23 but larger than MCs in cycle 24. This suggests that most NMCs have their leg flank pass through the spacecraft. Our geometric analyses support that all ICMEs should have a magnetic flux rope structure near 1 AU.
A sample of isolated Earth-impacting ICMEs that occurred in the period January 2008 to August 2014 is analysed in order to study in detail the ICME in situ signatures with respect to the type of filament eruption related to the corresponding CME. For Earth-directed CMEs, a kinematical study was performed using the STEREO-A, B COR1 and COR2 coronagraphs and the Heliospheric Imagers HI1. Based on the extrapolated CME kinematics, we identified interacting CMEs, which were excluded from further analysis. Applying this approach, a set of 31 isolated Earth-impacting CMEs was unambiguously identified and related to the in situ measurements recorded by the Wind spacecraft. We classified the events into subsets with respect to the CME source location as well as with respect to the type of the associated filament eruption. Hence, the events are divided into three subsamples: active region (AR) CMEs, disappearing filament (DSF) CMEs, and stealthy CMEs. The related three groups of ICMEs were further divided into two subsets: magnetic obstacle (MO) events (out of which four were stealthy), covering ICMEs that at least partly expose characteristics of flux ropes, and ejecta (EJ) events, not showing such characteristics. In the next step, MO-events were analysed in more detail, considering the magnetic field strengths and the plasma characteristics in three different segments of the ICMEs, defined as the turbulent sheath (TS), the frontal region (FR), and the MO itself. The analysis revealed various well-defined correlations for AR, DSF, and stealthy ICMEs, which we interpreted considering basic physical concepts. Our results support the hypothesis that ICMEs show different signatures depending on the in situ spacecraft trajectory, in terms of apex versus flank hits.
Fast interplanetary coronal mass ejections (interplanetary CMEs, or ICMEs) are the drivers of strongest space weather storms such as solar energetic particle events and geomagnetic storms. The connection between space weather impacting solar wind disturbances associated with fast ICMEs at Earth and the characteristics of causative energetic CMEs observed near the Sun is a key question in the study of space weather storms as well as in the development of practical space weather prediction. Such shock-driving fast ICMEs usually expand at supersonic speed during the propagation, resulting in the continuous accumulation of shocked sheath plasma ahead. In this paper, we propose the sheath-accumulating propagation (SAP) model that describe the coevolution of the interplanetary sheath and decelerating ICME ejecta by taking into account the process of upstream solar wind plasma accumulation within the sheath region. Based on the SAP model, we discussed (1) ICME deceleration characteristics, (2) the fundamental condition for fast ICME at Earth, (3) thickness of interplanetary sheath, (4) arrival time prediction and (5) the super-intense geomagnetic storms associated with huge solar flares. We quantitatively show that not only speed but also mass of the CME are crucial in discussing the above five points. The similarities and differences among the SAP model, the drag-based model and the`snow-plough model proposed by citet{tappin2006} are also discussed.
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