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Abstract 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. 

Abstract The aim of this study is to use multispacecraft measurements of interplanetary magnetic clouds (MCs) to better constrain and understand the effect of expansion on their magnetic field properties. We develop a parameter (γ) for comparing magnetic field components measured at multiple spacecraft. We use the minimum variance technique on the magnetic field data to obtain the axial and azimuthal components. The parameterγacts at the front boundary as a measure of the global difference in the evolution with heliospheric distance of the axial and azimuthal magnetic field components of MCs. Our goal is to determine whether the studied MCs exhibit self-similar expansion and, if so, whether this expansion is predominantly isotropic or radial, based on the estimatedγ. Through our analysis of data from multiple spacecraft, we observe a notable consistency in theγvalues across the examples examined. We find that the overall expansion of these MCs tends to be isotropic, while the local expansion of MCs, derived from theγvalues measured at the rear boundary of MCs, usually shows anisotropic behavior, particularly when the distances between the observations from the two spacecraft are relatively short. This discovery offers insights for refining flux rope models and advancing our comprehension of the expansion processes associated with coronal mass ejections. 

Abstract The anisotropy of energetic particles provides essential information to help resolve the underlying fundamental physics of their spatial distributions, injection, acceleration, and transport processes. In this work, we report an energetic ion enhancement that is characterized by very large and long-lasting anisotropies observed by STEREO A and Solar Orbiter, which are nearly aligned along the same nominal Parker spiral. This ion enhancement appears at the rising phase of a widespread solar energetic particle event that was associated with the farside coronal mass ejection on 2022 February 15. According to our analysis, the long-lasting anisotropy resulted from the continuous injection of energetic ions from a well-connected particle source located beyond the STEREO A’s orbit. Solar Orbiter also observed an interval of very large anisotropy dominated exclusively by sunward streaming ions but with the additional implication that it detected the very early phase of ion injections onto magnetic field lines that newly connected to the particle source, which is likely the first reported event of this kind. These results further illustrate how energetic particle anisotropy information, in particular from multiple observer locations, can be used to disentangle the sources and transport processes of energetic ions, even when their heliospheric context is not simple. 

Abstract 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. The 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 aging, as well as spatial variations within a CME. This study uses multispacecraft 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 aging effect (the effect of the time evolution on in situ properties). We compile a database of 19 events from the past 4 decades measured by two spacecraft with a radial separation of <0.2 au and an angular separation of <10°. We find that the average magnetic field strength measured by the two spacecraft differs by 18% of the typical average value, which highlights nonnegligible spatial or temporal variations. For one particular event, measurements taken by the two spacecraft allow us to quantify and significantly reduce the aging 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 aging and that correcting for self-similar expansion does not capture the whole aging effect. 

Abstract A fundamental property of coronal mass ejections (CMEs) is their radial expansion, which determines the increase in the CME radial size and the decrease in the CME magnetic field strength as the CME propagates. CME radial expansion can be investigated either by using remote observations or by in situ measurements based on multiple spacecraft in radial conjunction. However, there have been only few case studies combining both remote and in situ observations. It is therefore unknown if the radial expansion in the corona estimated remotely is consistent with that estimated locally in the heliosphere. To address this question, we first select 22 CME events between the years 2010 and 2013, which were well observed by coronagraphs and by two or three spacecraft in radial conjunction. We use the graduated cylindrical shell model to estimate the radial size, radial expansion speed, and a measure of the dimensionless expansion parameter of CMEs in the corona. The same parameters and two additional measures of the radial-size increase and magnetic-field-strength decrease with heliocentric distance of CMEs based on in situ measurements are also calculated. For most of the events, the CME radial size estimated by remote observations is inconsistent with the in situ estimates. We further statistically analyze the correlations of these expansion parameters estimated using remote and in situ observations, and discuss the potential reasons for the inconsistencies and their implications for the CME space weather forecasting. 

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 degree (~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. 

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.