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The ISWAT clusters H1+H2 have a focus on interplanetary space and its characteristics, especially on the large-scale co-rotating and transient structures impacting Earth. SIRs, generated by the interaction between high-speed solar wind originating in large-scale open coronal magnetic fields and slower solar wind from closed magnetic fields, are regions of compressed plasma and magnetic field followed by high-speed streams that recur at the ca. 27 day solar rotation period. Short-term reconfigurations of the lower coronal magnetic field generate flare emissions and provide the energy to accelerate enormous amounts of magnetised plasma and particles in the form of CMEs into interplanetary space. The dynamic interplay between these phenomena changes the configuration of interplanetary space on various temporal and spatial scales which in turn influences the propagation of individual structures. While considerable efforts have been made to model the solar wind, we outline the limitations arising from the rather large uncertainties in parameters inferred from observations that make reliable predictions of the structures impacting Earth difficult. Moreover, the increased complexity of interplanetary space as solar activity rises in cycle 25 is likely to pose a challenge to these models. Combining observational and modeling expertise will extend our knowledge of the relationship between these different phenomena and the underlying physical processes, leading to improved models and scientific understanding and more-reliable space-weather forecasting. The current paper summarizes the efforts and progress achieved in recent years, identifies open questions, and gives an outlook for the next 5-10 years. It acts as basis for updating the existing COSPAR roadmap by Schrijver+ (2015), as well as providing a useful and practical guide for peer-users and the next generation of space weather scientists., Comment: Accepted for publication in Advances in Space Research

We report the result of the first search for multipoint in situ and imaging observations of interplanetary coronal mass ejections (ICMEs) starting with the first Solar Orbiter (SolO) data in 2020 April - 2021 April. A data exploration analysis is performed including visualizations of the magnetic field and plasma observations made by the five spacecraft SolO, BepiColombo, Parker Solar Probe (PSP), Wind and STEREO-A, in connection with coronagraph and heliospheric imaging observations from STEREO-A/SECCHI and SOHO/LASCO. We identify ICME events that could be unambiguously followed with the STEREO-A heliospheric imagers during their interplanetary propagation to their impact at the aforementioned spacecraft, and look for events where the same ICME is seen in situ by widely separated spacecraft. We highlight two events: (1) a small streamer blowout CME on 2020 June 23 observed with a triple lineup by PSP, BepiColombo and Wind, guided by imaging with STEREO-A, and (2) the first fast CME of solar cycle 25 ($ \approx 1600$ km s$^{-1}$) on 2020 November 29 observed in situ by PSP and STEREO-A. These results are useful for modeling the magnetic structure of ICMEs and the interplanetary evolution and global shape of their flux ropes and shocks, and for studying the propagation of solar energetic particles. The combined data from these missions are already turning out to be a treasure trove for space weather research and are expected to become even more valuable with an increasing number of ICME events expected during the rise and maximum of solar cycle 25., Comment: in press at ApJ Letters (submitted 2021 September 15, revised 2021 November 24, accepted 2021 December 9). 11 pages, 3 figures, 1 table

The evolution and propagation of coronal mass ejections (CMEs) in interplanetary space is still not well understood. As a consequence, accurate arrival time and arrival speed forecasts are an unsolved problem in space weather research. In this study, we present the ELlipse Evolution model based on HI observations (ELEvoHI) and introduce a deformable front to this model. ELEvoHI relies on heliospheric imagers (HI) observations to obtain the kinematics of a CME. With the newly developed deformable front, the model is able to react to the ambient solar wind conditions during the entire propagation and along the whole front of the CME. To get an estimate of the ambient solar wind conditions, we make use of three different models: Heliospheric Upwind eXtrapolation model (HUX), Heliospheric Upwind eXtrapolation with time dependence model (HUXt), and EUropean Heliospheric FORecasting Information Asset (EUHFORIA). We test the deformable front on a CME first observed in STEREO-A/HI on February 3, 2010 14:49 UT. For this case study, the deformable front provides better estimates of the arrival time and arrival speed than the original version of ELEvoHI using an elliptical front. The new implementation enables us to study the parameters influencing the propagation of the CME not only for the apex, but for the entire front. The evolution of the CME front, especially at the flanks, is highly dependent on the ambient solar wind model used. An additional advantage of the new implementation is given by the possibility to provide estimates of the CME mass., Comment: 25 pages, 7 figures, 1 table, revised for Space Weather

The recent launch of Solar Orbiter and BepiColombo opened a brief window in which these two spacecraft were positioned in a constellation that allows for the detailed sampling of any Earth-directed CMEs. Fortunately, two such events occurred with in situ detections of an ICME by Solar Orbiter on the 19th of April and the 28th of May 2020. These two events were subsequently also observed in situ by BepiColombo and Wind around a day later. We attempt to reconstruct the observed in situ magnetic field measurements for all three spacecraft simultaneously using an empirical magnetic flux rope model. This allows us to test the validity of our flux rope model on a larger and more global scale and allows for cross-validation of the analysis with different spacecraft combinations. Finally, we can also compare the results from the in situ modeling to remote observations obtained from the STEREO-A heliospheric imagers. We make use of the 3D coronal rope ejection model in order to simulate the ICME evolution. We adapt a previously developed ABC-SMC fitting algorithm for the application to multi point scenarios. We show that we are able to generally reconstruct the flux ropes signatures at three different spacecraft positions simultaneously using our model in combination with the flux rope fitting algorithm. For the well-behaved 19th of April ICME our approach works very well. The 28th of May ICME, on the other hand, shows the limitations of our approach. Unfortunately, the usage of multi-point observations for these events does not appear to solve inherent issues, such as the estimation of the magnetic field twist or flux rope aspect-ratios due to the specific constellation of the spacecraft positions. As our general approach can be used for any fast forward simulation-based model we give a blueprint for future studies using more advanced ICME models., Comment: 11 pages, 7 figures

On 2020 April 19 a coronal mass ejection (CME) was detected in situ by Solar Orbiter at a heliocentric distance of about 0.8 AU. The CME was later observed in situ on April 20th by the Wind and BepiColombo spacecraft whilst BepiColombo was located very close to Earth. This CME presents a good opportunity for a triple radial alignment study, as the spacecraft were separated by less than 5$^\circ$ in longitude. The source of the CME, which was launched on April 15th, was an almost entirely isolated streamer blowout. STEREO-A observed the event remotely from -75.1$^\circ$ longitude, which is an exceptionally well suited viewpoint for heliospheric imaging of an Earth directed CME. The configuration of the four spacecraft has provided an exceptionally clean link between remote imaging and in situ observations of the CME. We have used the in situ observations of the CME at Solar Orbiter, Wind, and BepiColombo, and the remote observations of the CME at STEREO-A in combination with flux rope models to determine the global shape of the CME and its evolution as it propagated through the inner heliosphere. A clear flattening of the CME cross-section has been observed by STEREO-A, and further confirmed by comparing profiles of the flux rope models to the in situ data, where the distorted flux rope cross-section qualitatively agrees most with in situ observations of the magnetic field at Solar Orbiter. Comparing in situ observations of the magnetic field between spacecraft, we find that the dependence of the maximum (mean) magnetic field strength decreases with heliocentric distance as $r^{-1.24 \pm 0.50}$ ($r^{-1.12 \pm 0.14}$), in disagreement with previous studies. Further assessment of the axial and poloidal magnetic field strength dependencies suggests that the expansion of the CME is likely neither self-similar nor cylindrically symmetric.

Studying the ambient solar wind, a continuous pressure-driven plasma flow emanating from our Sun, is an important component of space weather research. The ambient solar wind flows in interplanetary space determine how solar storms evolve through the heliosphere before reaching Earth, and especially during solar minimum are themselves a driver of activity in the Earth's magnetic field. Accurately forecasting the ambient solar wind flow is therefore imperative to space weather awareness. Here we present a machine learning approach in which solutions from magnetic models of the solar corona are used to output the solar wind conditions near the Earth. The results are compared to observations and existing models in a comprehensive validation analysis, and the new model outperforms existing models in almost all measures. In addition, this approach offers a new perspective to discuss the role of different input data to ambient solar wind modeling, and what this tells us about the underlying physical processes. The final model discussed here represents an extremely fast, well-validated and open-source approach to the forecasting of ambient solar wind at Earth., Comment: 15 pages, 6 figures, 3 tables

Geomagnetic storms resulting from high-speed streams can have significant negative impacts on modern infrastructure due to complex interactions between the solar wind and geomagnetic field. One measure of the extent of this effect is the Kyoto $Dst$ index. We present a method to predict $Dst$ from data measured at the Lagrange 5 (L5) point, which allows for forecasts of solar wind development 4.5 days in advance of the stream reaching the Earth. Using the STEREO-B satellite as a proxy, we map data measured near L5 to the near-Earth environment and make a prediction of the $Dst$ from this point using the Temerin-Li $Dst$ model enhanced from the original using a machine learning approach. We evaluate the method accuracy with both traditional point-to-point error measures and an event-based validation approach. The results show that predictions using L5 data outperform a 27-day solar wind persistence model in all validation measures but do not achieve a level similar to an L1 monitor. Offsets in timing and the rapidly-changing development of $B_z$ in comparison to $B_x$ and $B_y$ reduce the accuracy. Predictions of $Dst$ from L5 have an RMSE of $9$ nT, which is double the error of $4$ nT using measurements conducted near the Earth. The most useful application of L5 measurements is shown to be in predicting the minimum $Dst$ for the next four days. This method is being implemented in a real-time forecast setting using STEREO-A as an L5 proxy, and has implications for the usefulness of future L5 missions., Comment: 20 pages, 6 figures, in press at AGU Space Weather

The Solar TErrestrial RElations Observatory (STEREO) and its heliospheric imagers (HI) have provided us the possibility to enhance our understanding of the interplanetary propagation of coronal mass ejections (CMEs). HI-based methods are able to forecast arrival times and speeds at any target and use the advantage of tracing a CME's path of propagation up to 1 AU. In our study we use the ELEvoHI model for CME arrival prediction together with an ensemble approach to derive uncertainties in the modeled arrival time and impact speed. The CME from 3 November 2010 is analyzed by performing 339 model runs that are compared to in situ measurements from lined-up spacecraft MESSENGER and STEREO-B. Remote data from STEREO-B showed the CME as halo event, which is comparable to an HI observer situated at L1 and observing an Earth-directed CME. A promising and easy approach is found by using the frequency distributions of four ELEvoHI output parameters, drag parameter, background solar wind speed, initial distance and speed. In this case study, the most frequent values of these outputs lead to the predictions with the smallest errors. Restricting the ensemble to those runs, we are able to reduce the mean absolute arrival time error from $3.5 \pm 2.6$ h to $1.6 \pm 1.1$ h at 1 AU. Our study suggests that L1 may provide a sufficient vantage point for an Earth-directed CME, when observed by HI, and that ensemble modeling could be a feasible approach to use ELEvoHI operationally., Comment: submitted to Space Weather

We present an advance towards accurately predicting the arrivals of coronal mass ejections (CMEs) at the terrestrial planets, including Earth. For the first time, we are able to assess a CME prediction model using data over 2/3 of a solar cycle of observations with the Heliophysics System Observatory. We validate modeling results of 1337 CMEs observed with the Solar Terrestrial Relations Observatory (STEREO) heliospheric imagers (HI) (science data) from 8 years of observations by 5 in situ observing spacecraft. We use the self-similar expansion model for CME fronts assuming 60 degree longitudinal width, constant speed and constant propagation direction. With these assumptions we find that 23%-35% of all CMEs that were predicted to hit a certain spacecraft lead to clear in situ signatures, so that for 1 correct prediction, 2 to 3 false alarms would have been issued. In addition, we find that the prediction accuracy does not degrade with the HI longitudinal separation from Earth. Predicted arrival times are on average within 2.6 +/- 16.6 hours difference of the in situ arrival time, similar to analytical and numerical modeling, and a true skill statistic of 0.21. We also discuss various factors that may improve the accuracy of space weather forecasting using wide-angle heliospheric imager observations. These results form a first order approximated baseline of the prediction accuracy that is possible with HI and other methods used for data by an operational space weather mission at the Sun-Earth L5 point., Comment: 27 pages, 7 figures, 3 tables, accepted for publication in the AGU journal "Space Weather" on 2017 July 1

Prediction of the effects of coronal mass ejections (CMEs) on Earth strongly depends on knowledge of the interplanetary magnetic field southward component, Bz. Predicting the strength and duration of Bz inside a CME with sufficient accuracy is currently impossible, which forms the so-called Bz problem. Here, we provide a proof-of-concept of a new method for predicting the CME arrival time, speed, Bz and the resulting Dst index at Earth based only on magnetic field data, measured in situ in the inner heliosphere (< 1AU). On 2012 June 12-16, three approximately Earthward-directed and interacting CMEs were observed the by the STEREO imagers, and by Venus Express (VEX) in situ at 0.72 AU, 6 degree away from the Sun Earth line. The CME kinematics are calculated using the drag-based and WSA-Enlil models, constrained by the arrival time at VEX, resulting in the CME arrival time and speed at Earth. The CME magnetic field strength is scaled with a power law from VEX to Wind. Our investigation shows promising results for the Dst forecast (predicted: -96 and -114 nT (from 2 Dst models), observed: -71 nT), for the arrival speed (predicted: 531 +- 23 km s-1, observed: 488 +- 30 km s-1) and timing (6 +- 1 hours after actual arrival time). The prediction lead time is 21 hours. The method may be applied to vector magnetic field data from a spacecraft at an artificial Lagrange point between the Sun and Earth, or to data taken by any spacecraft temporarily crossing the Sun--Earth line.