Camilla Scolini

We investigate whether Alfvénic fluctuations (AFs) can affect the structure of magnetic ejecta (ME) within interplanetary coronal mass ejections (ICMEs). We study an ICME observed on 2001 December 29 at 1 au by the Advanced Composition Explorer (ACE) and Wind, at a total angular separation of ~0 . ° 8 (~ 0.014 au). We focus on the correlation and complexity of its magnetic structure measured between two spacecraft in association with large-amplitude AFs. The Alfvénicity of the ME is investigated in terms of the residual energy and cross helicity of fluctuations. We find that as for the event of interest, large-amplitude AFs occur in the rear region of the ME at both Wind and ACE with a duration of about 6 hr. We compare the correlation of the magnetic field strength and vector components measured between Wind and ACE, and investigate complexity in terms of the magnetic hodograms. The region showing AFs is found to be associated with a decreased correlation of the magnetic field components and an increased complexity of the ME magnetic configuration detected at ACE and Wind, which may be due to the fact that the two spacecraft crossing the same ME along different trajectories likely sampled AFs in different oscillation phases. Combining multipoint in situ measurements and remote-sensing observations of the ICME source region, we further discuss different potential sources of the AFs.

In this study, we utilize magnetic field data from the Juno mission’s cruise phase to visually identify and analyze 338 interplanetary small flux ropes (SFRs) across a heliocentric distance range of 1–5.5 au. The events are uniformly distributed across heliocentric distances, showing no clear trend. Through superposed epoch analysis, we find that the average SFR magnetic field profiles are symmetric and show little variation across the observed heliocentric distances. Additionally, we observe a slight increasing trend in the mean duration of SFRs, indicating minimal expansion during propagation. Furthermore, we determine that the SFR mean magnetic field dependence on distance is best fit by two separate power laws, exhibiting a steeper decay from 1 to ∼2.1 au and a shallower decay from ∼2.1 to 5.5 au. Near 1 au, the statistical decay rate of the mean magnetic field magnitude of SFRs is slightly higher than that of the interplanetary magnetic field (IMF), suggesting that SFRs may become indistinguishable from the IMF over time. This finding implies that SFRs detected at greater radial distances are possibly generated in situ as opposed to near the Sun. However, only ∼26% of the total population of SFRs in our catalog occurs within 1 day from the heliospheric current sheet (HCS), indicating a very limited association between the occurrence of the majority of SFRs and the presence of the HCS. These results raise questions about the origin of SFRs detected at larger distances, encouraging further exploration for alternatives to the conventional generation mechanisms.

Based on decades of single-spacecraft measurements near 1 au as well as data from heliospheric and planetary missions, multi-spacecraft simultaneous measurements in the inner heliosphere on separations of 0.05–0.2 au are required to close existing gaps in our knowledge of solar wind structures, transients, and energetic particles, especially coronal mass ejections (CMEs), stream interaction regions (SIRs), high speed solar wind streams (HSS), and energetic storm particle (ESP) events. The Mission to Investigate Interplanetary Structures and Transients (MIIST) is a concept for a small multi-spacecraft mission to explore the near-Earth heliosphere on these critical scales. It is designed to advance two goals: (a) to determine the spatiotemporal variations and the variability of solar wind structures, transients, and energetic particle fluxes in near-Earth interplanetary (IP) space, and (b) to advance our fundamental knowledge necessary to improve space weather forecasting from in situ data. We present the scientific rationale for this proposed mission, the science requirements, payload, implementation, and concept of mission operation that address a key gap in our knowledge of IP structures and transients within the cost, launch, and schedule limitations of the NASA Heliophysics Small Explorers program.

Aims.We present validation results for calculations of arrival times and geomagnetic impact of coronal mass ejections (CMEs) using the cone and spheromak CME models implemented in EUropean Heliospheric FORecasting Information Asset (EUHFORIA). Validating numerical models is crucial for ensuring their accuracy and performance with respect to real data.Methods.We compared CME plasma and magnetic field signatures measured in situ by satellites at the L1 point with the simulation output of EUHFORIA. The validation of this model was carried out using two datasets in order to ensure a comprehensive evaluation. The first dataset focuses on 16 CMEs that arrived at Earth, offering specific insights into the model’s accuracy in predicting arrival time and geomagnetic impact. Meanwhile, the second dataset encompasses all CMEs observed over eight months within Solar Cycle 24, regardless of whether or not they arrived at Earth, covering periods of both solar minimum and maximum activity. This second dataset enables a more comprehensive evaluation of the model’s predictive precision in term of CME arrivals and misses.Results.Our results show that EUHFORIA provides good estimates in terms of arrival times, with root mean square error (RMSE) values of 9 hours. Regarding the number of correctly predicted ICME arrivals and misses, we find a 75% probability of detection in a 12 hour time window and 100% probability of detection in a 24 hour time window. The geomagnetic impact forecasts measured by theK_pindex provide different degrees of accuracy ranging from 31% to 69%. These results validate the use of cone and spheromak CMEs for real-time space weather forecasting.

A plethora of coronal models, from empirical to more complex magnetohydrodynamic (MHD) ones, are being used for reconstructing the coronal magnetic field topology and estimating the open magnetic flux. However, no individual solution fully agrees with coronal hole observations and in situ measurements of open flux at 1 au, as there is a strong deficit between the model and observations contributing to the known problem of the missing open flux. In this paper, we investigate the possible origin of the discrepancy between modeled and observed magnetic field topology by assessing the effect on the simulation output by the choice of the input boundary conditions and the simulation setup, including the choice of numerical schemes and the parameter initialization. In the frame of this work, we considered four potential field source surface-based models and one fully MHD model, different types of global magnetic field maps, and model initiation parameters. After assessing the model outputs using a variety of metrics, we conclude that they are highly comparable regardless of the differences set at initiation. When comparing all models to coronal hole boundaries extracted by extreme-ultraviolet filtergrams, we find that they do not compare well. This mismatch between observed and modeled regions of the open field is a candidate contributing to the open flux problem.

Predictions of coronal mass ejection (CME) propagation and impact in the heliosphere, in either research or operational settings, are usually performed by employing magnetohydrodynamic (MHD) models. Within such simulations, the CME ejecta is often described as a hydrodynamic pulse that lacks an internal magnetic field and is characterized by a spherical geometry – leading to the so-called cone CME model. White-light observations of CMEs in the corona, however, reveal that the morphology of these structures resembles more closely that of a croissant, i.e., exhibiting an elongated cross-section of their front. It follows that, in space weather forecasts, the assumption of a spherical geometry may result in erroneous predictions of CME impacts in the heliosphere in terms of hit/miss and arrival time/speed, especially in the case of flank encounters. A spheroid CME model is expected to provide a more accurate description of the elongated morphology that is often observed in CMEs. In this paper, we describe the implementation and initial validation of the spheroid CME model within the MHD EUropean Heliospheric FORecasting Information Asset (EUHFORIA) code. We perform EUHFORIA simulations of an idealized CME as well as a “real” event to compare the spheroidal model with the traditional cone one. We show how the initial ejecta geometry can lead to substantially different estimates in terms of CME impact, arrival time/speed, and geoeffectiveness, especially with increasing distance to the CME nose.

Coronal holes are recognized as the primary sources of heliospheric open magnetic flux (OMF). However, a noticeable gap exists between in situ measured OMF and that derived from remote-sensing observations of the Sun. In this study, we investigate the OMF evolution and its connection to solar structures throughout 2014, with special emphasis on the period from September to October, where a sudden and significant OMF increase was reported. By deriving the OMF evolution at 1 au, modeling it at the source surface, and analyzing solar photospheric data, we provide a comprehensive analysis of the observed phenomenon. First, we establish a strong correlation between the OMF increase and the solar magnetic field derived from a potential-field source-surface model (cc _Pearson = 0.94). Moreover, we find a good correlation between the OMF and the open flux derived from solar coronal holes (cc _Pearson = 0.88), although the coronal holes only contain 14%–32% of the Sun’s total open flux. However, we note that while the OMF evolution correlates with coronal hole open flux, there is no correlation with the coronal hole area evolution (cc _Pearson = 0.0). The temporal increase in OMF correlates with the vanishing remnant magnetic field at the southern pole, caused by poleward flux circulations from the decay of numerous active regions months earlier. Additionally, our analysis suggests a potential link between the OMF enhancement and the concurrent emergence of the largest active region in solar cycle 24. In conclusion, our study provides insights into the strong increase in OMF observed during 2014 September–October.

Context. Coronal mass ejections (CMEs) are large-scale structures of magnetized plasma that erupt from the corona into interplanetary space. The launch of Solar Orbiter (SolO) in 2020 enables in situ measurements of CMEs in the innermost heliosphere, at such distances where CMEs can be observed remotely within the inner field of view of heliospheric imagers (HIs). It thus provides the opportunity for investigations into the correspondence of the CME substructures measured in situ and observed remotely. We studied a CME that started on 2022 March 10 and was measured in situ by SolO at ∼0.44 au. Aims. Combining remote observations of CMEs from wide-angle imagers and in situ measurements in the innermost heliosphere allows us to compare CME properties derived through both techniques, validate the estimates, and better understand CME evolution, specifically the size and radial expansion, within 0.5 au. Methods. We compared the evolution of different CME substructures observed in images from the HIs on board the Ahead Solar Terrestrial Relations Observatory (STEREO-A) and the CME signatures measured in situ by SolO. The CME is found to possess a density enhancement at its rear edge in both remote and in situ observations, which validates the use of the signature of density enhancement following the CMEs to accurately identify the CME rear edge. We also estimated and compared the radial size and radial expansion speed of different substructures in both observations. Results. The evolution of the CME front and rear edges in remote images is consistent with the in situ CME measurements. The radial expansion (i.e., radial size and radial expansion speed) of the whole CME structure consisting of the magnetic ejecta and the sheath is consistent with the in situ estimates obtained at the same time from SolO. However, we do not find such consistencies for the magnetic ejecta region inside the CME because it is difficult to identify the magnetic ejecta edges in the remote images.

Coronal mass ejections (CMEs) are large-scale eruptions with a typical radial size at 1 au of 0.21 au but their angular width in interplanetary space is still mostly unknown, especially for the magnetic ejecta (ME) part of the CME. We take advantage of STEREO-A angular separation of 20°–60° from the Sun–Earth line from 2020 October to 2022 August, and perform a two-part study to constrain the angular width of MEs in the ecliptic plane: (a) we study all CMEs that are observed remotely to propagate between the Sun–STEREO-A and the Sun–Earth lines and determine how many impact one or both spacecraft in situ, and (b) we investigate all in situ measurements at STEREO-A or at L1 of CMEs during the same time period to quantify how many are measured by the two spacecraft. A key finding is that out of 21 CMEs propagating within 30° of either spacecraft only four impacted both spacecraft and none provided clean magnetic cloud-like signatures at both spacecraft. Combining the two approaches, we conclude that the typical angular width of an ME at 1 au is ∼20°–30°, or 2–3 times less than often assumed and consistent with a 2:1 elliptical cross section of an ellipsoidal ME. We discuss the consequences of this finding for future multi-spacecraft mission designs and for the coherence of CMEs.