Evaluation of CME Arrival Prediction Using Ensemble Modeling Based on Heliospheric Imaging Observations.

In this study, we present a new method for forecasting arrival times and speeds of coronal mass ejections (CMEs) at any location in the inner heliosphere. This new approach enables the adoption of a highly flexible geometrical shape for the CME front with an adjustable CME angular width and an adjustable radius of curvature of its leading edge, i.e. the assumed geometry is elliptical. Using, as input, STEREO heliospheric imager (HI) observations, a new elliptic conversion (ElCon) method is introduced and combined with the use of drag-based model (DBM) fitting to quantify the deceleration or acceleration experienced by CMEs during propagation. The result is then used as input for the Ellipse Evolution Model (ElEvo). Together, ElCon, DBM fitting, and ElEvo form the novel ElEvoHI forecasting utility. To demonstrate the applicability of ElEvoHI, we forecast the arrival times and speeds of 21 CMEs remotely observed from STEREO/HI and compare them to in situ arrival times and speeds at 1 AU. Compared to the commonly used STEREO/HI fitting techniques (Fixed-$\Phi$, Harmonic Mean, and Self-similar Expansion fitting), ElEvoHI improves the arrival time forecast by about 2 hours to $\pm 6.5$ hours and the arrival speed forecast by $\approx 250$ km s$^{-1}$ to $\pm 53$ km s$^{-1}$, depending on the ellipse aspect ratio assumed. In particular, the remarkable improvement of the arrival speed prediction is potentially beneficial for predicting geomagnetic storm strength at Earth.

Timely and accurate prediction of coronal mass ejections (CMEs) is vital for mitigating the potential impact of severe space weather events on critical infrastructures. Currently, manual detection and tracking of CMEs as they traverse the heliosphere are the norm. This presentation introduces an innovative approach: the development and implementation of a machine learning algorithm for automatic detection and tracking of CMEs, leveraging data from various heliospheric imager (HI) instruments. The wealth of active spacecraft equipped with HI instruments provides a unique opportunity to train the algorithm using diverse datasets.This work gains significance in light of the upcoming launch of Vigil, a space weather monitor scheduled for deployment in the early 2030s at the L5 point. Vigil will continuously observe and provide real-time HI observations along the Sun-Earth line. Our presentation showcases preliminary outcomes from an automated CME detection and tracking algorithm, demonstrating its effectiveness with training on STEREO-HI data. We also discuss potential future steps and challenges in the development and testing of this algorithm, emphasizing its role in advancing operational space weather prediction capabilities, especially in anticipation of the Vigil mission.

<p>Understanding the evolution of the solar wind is fundamental to advancing our knowledge of energy and mass transport in the solar system, rendering it crucial to space weather and its prediction. The advent of truly wide-angle heliospheric imaging has revolutionised the study of Coronal Mass Ejections (CMEs) by enabling their direct and continuous observation out to 1 AU and beyond. A catalogue of CMEs has been compiled using data from the Heliospheric Imagers (HIs) on board the two STEREO spacecraft, which began as part of the FP7 HELCATS project. The mission was launched in 2006 and continues to provide data, therefore spanning 13 years, over which more than two-thousand CMEs have been observed using HI. To these CMEs, we apply geometric models that make use of both single-spacecraft and stereoscopic observations in order to determine their kinematic properties. These include CME speed, acceleration, propagation direction and launch time. The resulting kinematic properties and their statistics are discussed in the context of existing CME catalogues produced from coronagraph observations. This is done with emphasis on how the different models we apply influence our results and how these differences evolve over the solar cycle and as the angular separation of the STEREO spacecraft increases throughout the mission.</p>

Geometric modeling of Coronal Mass Ejections (CMEs) is a widely used tool for assessing their kinematic evolution. Furthermore, techniques based on geometric modeling, such as ELEvoHI, are being developed into forecast tools for space weather prediction. These models assume that solar wind structure does not affect the evolution of the CME, which is an unquantified source of uncertainty. We use a large number of Cone CME simulations with the HUXt solar wind model to quantify the scale of uncertainty introduced into geometric modeling and the ELEvoHI CME arrival times by solar wind structure. We produce a database of simulations, representing an average, a fast, and an extreme CME scenario, each independently propagating through 100 different ambient solar wind environments. Synthetic heliospheric imager observations of these simulations are then used with a range of geometric models to estimate the CME kinematics. The errors of geometric modeling depend on the location of the observer, but do not seem to depend on the CME scenario. In general, geometric models are biased towards predicting CME apex distances that are larger than the true value. For these CME scenarios, geometric modeling errors are minimised for an observer in the L5 region. Furthermore, geometric modeling errors increase with the level of solar wind structure in the path of the CME. The ELEvoHI arrival time errors are minimised for an observer in the L5 region, with mean absolute arrival time errors of 8.2 ± 1.2 h, 8.3 ± 1.0 h, and 5.8 ± 0.9 h for the average, fast, and extreme CME scenarios.

Self-Similar Expansion (SSE) fitting models the leading edge of the Coronal Mass Ejection (CME) as a self-similar expansion sphere with a fixed propagation direction and a fixed angular width. Cone angle of the heliocentric cone which is tangent to the sphere usually refers to the angular width of the CME, ranging from 0 to 90 degrees. Angular widths of Fixed-Φ (FΦ) and Harmonic-mean (HM) fitting are defined, corresponding to 0 and 90 degrees of SSE model, respectively. Modified SSE (MSSE) fitting discussed in this paper models the leading edge of the CME as a self-similar expansion semi-sphere. It is able to extract CME parameters using MSSE fitting, including propagation direction, velocity and cone angle of the heliocentric cone, whose conic section is across the center of the semi-sphere, ranging from 0 to 90 degrees. Cone angles of FΦ and HM fitting are defined, corresponding to 0 and 45 degrees of MSSE model, respectively. Leading edge of the CME can be assumed as a heliocentric semi-sphere when cone angle is assumed to be 90 degrees. MSSE fitting expands the morphology of leading edge of the CME and considers cone model as a reference to determine whether and when the CME will reach a satellite. 23 CMEs were analyzed based on the observations by Heliospheric Imager (HI) from one spacecraft of STEREO, combined with in-situ observations of solar wind on STEREO and ACE. We found that the error of the CME propagation direction, arrival speed and theoretical arrival time analyzed by MSSE fitting decreased, compared to FΦ, HM and SSE fitting.

The field of heliospheric imaging has matured significantly over the last 10 years—corresponding, in particular, to the launch of NASA's STEREO mission and the successful operation of the heliospheric imager (HI) instruments thereon. In parallel, this decade has borne witness to a marked increase in concern over the potentially damaging effects of space weather on space and ground‐based technological assets, and the corresponding potential threat to human health, such that it is now under serious consideration at governmental level in many countries worldwide. Hence, in a political climate that recognizes the pressing need for enhanced operational space weather monitoring capabilities most appropriately stationed, it is widely accepted, at the Lagrangian L1 and L5 points, it is timely to assess the value of heliospheric imaging observations in the context of space weather operations. To this end, we review a cross section of the scientific analyses that have exploited heliospheric imagery—particularly from STEREO/HI—and discuss their relevance to operational predictions of, in particular, coronal mass ejection (CME) arrival at Earth and elsewhere. We believe that the potential benefit of heliospheric images to the provision of accurate CME arrival predictions on an operational basis, although as yet not fully realized, is significant and we assert that heliospheric imagery is central to any credible space weather mission, particularly one located at a vantage point off the Sun‐Earth line.

Using imaging data from the Solar TErrestrial RElations Observatory (STEREO) mission, we empirically reconstruct the time-dependent three-dimensional morphology of a coronal mass ejection (CME) from 2008 June 1, which exhibits significant variation in shape as it travels from the Sun to 1 AU, requiring us to abandon the assumption of self-similar expansion. We model the CME as a flux rope that is rather fat relative to its longitudinal extent close to the Sun, but which becomes thinner and flatter on top as the flux rope moves outward. We find best agreement with the STEREO images when the flux rope's west leg is assumed to be rotated 35° below the ecliptic plane. This orientation is consistent with previously published inferences about this CME's orientation from an analysis of in situ observations of the event from June 6 to June 7, when the CME hits STEREO-B. The agreement between these two very different kinds of analysis is encouraging. Close to 1 AU, the CME not only hits STEREO-B but also strikes a comet (Comet C/2007 W1 Boattini), which provides an additional constraint for our reconstruction efforts. Finally, we find that this CME is very instructive for assessing different methods of extracting kinematic information from measurements of elongation angles from the Sun, which is a complicated issue for measurements far from the Sun. The "fixed-ϕ" assumption that we have used successfully in the past does not work well here, and we discuss the implications for extracting reliable kinematic information from heliospheric imaging.

This talk reviews the interplanetary scintillation (IPS) technique, which allows for the determination of speed and density turbulence of the solar wind in the three-dimensional (3D) inner heliosphere. When sufficient number of scintillating sources are employed to suitably probe a given part of interplanetary space, IPS observations are powerful to study the ambient solar wind as well as to track coronal mass ejections (CMEs) in the distance range of Sun to Earth's orbit. A number of CMEs have been tracked over the entire Sun-Earth distance, by combining the white-light images from the LASCO/SOHO space mission and IPS measurements obtained from the Ooty Radio Telescope (ORT) and ISEE Multi-antenna system. Since the IPS observations at Ooty are on a grid of large number of radio sources, they provide image of disturbance associated with the CME at different distances from the Sun before its arrival at the near-Earth space. The results on the CME “speeddistance” evolution indicate a two-level deceleration: a low decline in speed at distances within or about 100 solar radii and a rapid decrease at larger distances from the Sun. However, the radial decline rates of speed, respectively within and above 100 solar radii, differ between different CMEs and suggest the involvement of input energy associated with the CME eruption in the propagation as well as the dynamics of the ambient solar wind. The typical linear size evolution of the CME with heliocentric distance suggests a pressure balance maintained between the CME driver gas and the ambient solar wind. Further, IPS estimates of density turbulence and speed have been used as inputs to the University of California, San Diego (UCSD) timedependent tomography to reconstruct the full 3D distributions of both solar wind speed and density throughout the inner heliosphere. The usefulness of IPS measurements is discussed on the possibility of forming a basic model to forecast the arrival of CMEs at 1 AU.

Using in situ measurements and remote-sensing observations, we study two coronal mass ejections (CMEs) that left the Sun on 2012 June 13–14 and impacted both Venus and Earth while the planets were in close radial alignment. The two CMEs generate multiple fronts in Solar Terrestrial Relations Observatory (STEREO)/Heliospheric Imager (HI) images, which can also be observed in the “J-map” as bifurcated features. We present the “ghost front” model to combine remote observations from STEREO/SECCHI and in situ observations from the Wind and Venus Express (VEX) spacecraft, and to derive the kinematics and propagation directions of the CMEs. By fitting the observations of multiple fronts to a kinematically evolving flux rope model and assuming the CMEs undergo deceleration through frictional drag with a steady-state solar wind, we confirm that the outer and inner fronts of the CMEs as detected in HI images are consistent with peaks in Thomson scattered light returned from the flank and nose of a single front for each CME. An interaction takes place between CME-1 and CME-2 that can be observed in the HI-1 field of view (FOV) before CME-1 encounters Venus. The multipoint in situ observations of the shock–CME interaction event serve as further evidence of the interaction between CMEs. The arrival times calculated from the ghost front model are within 2.5 hr of those observed at VEX and Wind. Our analysis indicates that ghost fronts could provide information about the longitudinally extended shape of the CME in the FOV of HI-1, which can be used to improve the forecast of interplanetary CME arrival time at Earth.

Coronal mass ejections (CMEs) are large-scale eruptions of magnetized plasma from the Sun's lower atmosphere, significantly influencing space weather and planetary environments. To improve predictions of CME arrival and their impacts on Earth and its surroundings, a deeper understanding of their origins, initiation, and complex early evolution is crucial. While coronagraphic observations have been essential for studying the dynamics of CMEs, they cannot capture the initial, critical phase of CME development. Consequently, investigating indirect phenomena in the lower solar atmosphere has become essential. One of the most prominent indirect indicators associated with CMEs is coronal dimming. These are localized, sudden decreases in coronal emission observed at extreme ultraviolet and soft X-ray wavelengths, which evolve rapidly during the lift-off and early expansion phases of CMEs. Coronal dimmings have been interpreted both as “footprints” of the erupting magnetic structure and as indicators of coronal mass loss in the lower corona.I will review recent advancements in using coronal dimmings to diagnose CMEs. Topics covered will include statistical studies linking dimming characteristics to CME mass and speed, the use of dimmings as early indicators of CME propagation direction, and insights into the magnetic topology and reconfiguration of the early CME stages based on dimming locations and fine structure. Additionally, the potential role of dimmings in the pre-event phase preceding CME onset will be discussed. Finally, I will highlight future research directions and underexplored areas in CME science, emphasizing the untapped potential of coronal dimmings in advancing our understanding of these dynamic solar events.

We describe an empirical model to predict the 1‐AU arrival of coronal mass ejections (CMEs). This model is based on an effective interplanetary (IP) acceleration described by Gopalswamy et al. [2000b] that the CMEs are subject to, as they propagate from the Sun to 1 AU. We have improved this model (1) by minimizing the projection effects (using data from spacecraft in quadrature) in determining the initial speed of CMEs, and (2) by allowing for the cessation of the interplanetary acceleration before 1 AU. The resulting effective IP acceleration was higher in magnitude than what was obtained from CME measurements from spacecraft along the Sun‐Earth line. We evaluated the predictive capability of the CME arrival model using recent two‐point measurements from the Solar and Heliospheric Observatory (SOHO), Wind, and ACE spacecraft. We found that an acceleration cessation distance of 0.76 AU is in reasonable agreement with the observations. The new prediction model reduces the average prediction error from 15.4 to 10.7 hours. The model is in good agreement with the observations for high‐speed CMEs. For slow CMEs the model as well as observations show a flat arrival time of ∼4.3 days. Use of quadrature observations minimized the projection effects naturally without the need to assume the width of the CMEs. However, there is no simple way of estimating the projection effects based on the surface location of the Earth‐directed CMEs observed by a spacecraft (such as SOHO) located along the Sun‐Earth line because it is impossible to measure the width of these CMEs. The standard assumption that the CME is a rigid cone may not be correct. In fact, the predicted arrival times have a better agreement with the observed arrival times when no projection correction is applied to the SOHO CME measurements. The results presented in this work suggest that CMEs expand and accelerate near the Sun (inside 0.7 AU) more than our model supposes; these aspects will have to be included in future models.

The images taken by the Heliospheric Imager (HI) instruments, part of the SECCHI imaging package on board the pair of STEREO spacecraft, provide information on the radial and latitudinal evolution of the plasma transported by coronal mass ejections (CMEs). In this case study, a CME, appearing near 15 UT on 2007 November 15 in SECCHI coronagraph images, leads to the formation of two out-flowing density structures (DSs) in the heliosphere. The analysis of time-elongation maps constructed from images obtained by the HI instruments shows that these DSs were propagating along the Sun–Earth line. A direct comparison of HI images and in situ measurements taken near Earth could therefore be performed. These two DSs are separated by a cavity associated with little brightness variation or equivalently little electron density variation. In situ measurements made in the solar wind near Earth on 2007 November 20 show that this cavity corresponds to a magnetic cloud (MC). While the leading DS is related to the sheath in front of the MC, the second DS is located on the sunward side of the MC where high-speed solar wind from a coronal hole catches up and interacts with the MC. We conclude that HI observes the sub-structures of a merged interaction region (MIR), a region of the interplanetary medium where the total solar wind pressure is greatly enhanced by the interaction of an MC with the ambient solar wind. This MIR caused the largest geomagnetic storm in 2007.

Numerical modeling of interplanetary coronal mass ejections and comparison with heliospheric images

The Radiation Assessment Detector (RAD) instrument onboard the Mars Science Laboratory (MSL) mission's Curiosity rover has been measuring galactic cosmic rays (GCR) as well as solar energetic particles (SEP) on the surface of Mars for more than 6 years since its landing in August 2012. The observations include a large number of Forbush decreases (FD) caused by interplanetary coronal mass ejections (ICMEs) and/or their associated shocks shielding away part of the GCR particles with their turbulent and enhanced magnetic fields while passing Mars. This study combines MSL/RAD FD measurements and remote tracking of ICMEs using the Solar TErrestrial RElations Observatory (STEREO) Heliospheric Imager (HI) telescopes in a statistical study for the first time. The large data set collected by HI makes it possible to analyze 149 ICMEs propagating toward MSL both during its 8‐month cruise phase and after its landing on Mars. We link 45 of the events observed at STEREO‐HI to their corresponding FDs at MSL/RAD and study the accuracy of the ICME arrival time at Mars predicted from HI data using different methods. The mean differences between the predicted arrival times and those observed using FDs range from −11–5 hr for the different methods, with standard deviations between 17 and 20 hr. These values for predictions at Mars are very similar compared to other locations closer to the Sun and also comparable to the precision of some other modeling approaches.

Between December 2010 and March 2013, volunteers for the Solar Stormwatch (SSW) Citizen Science project have identified and analyzed coronal mass ejections (CMEs) in the near real‐time Solar Terrestrial Relations Observatory Heliospheric Imager observations, in order to make “Fearless Forecasts” of CME arrival times and speeds at Earth. Of the 60 predictions of Earth‐directed CMEs, 20 resulted in an identifiable Interplanetary CME (ICME) at Earth within 1.5–6 days, with an average error in predicted transit time of 22 h, and average transit time of 82.3 h. The average error in predicting arrival speed is 151 km s−1, with an average arrival speed of 425km s−1. In the same time period, there were 44 CMEs for which there are no corresponding SSW predictions, and there were 600 days on which there was neither a CME predicted nor observed. A number of metrics show that the SSW predictions do have useful forecast skill; however, there is still much room for improvement. We investigate potential improvements by using SSW inputs in three models of ICME propagation: two of constant acceleration and one of aerodynamic drag. We find that taking account of interplanetary acceleration can improve the average errors of transit time to 19 h and arrival speed to 77 km s−1.