Forecasting the Structure and Orientation of Earthbound Coronal Mass Ejections

Abstract

Coronal Mass Ejections (CMEs) are the key drivers of strong to extreme space weather storms at the Earth that can have drastic consequences for technological systems in space and on ground. The ability of a CME to drive geomagnetic disturbances depends crucially on the magnetic structure of the embedded flux rope, which is thus essential to predict. The current capabilities in forecasting in advance (at least half-a-day before) the geoeffectiveness of a given CME is however severely hampered by the lack of remote-sensing measurements of the magnetic field in the corona and adequate tools to predict how CMEs deform, rotate and deflect during their travel through the coronal and interplanetary space as they interact with the ambient solar wind and other CMEs. These problems can lead not only to overor underestimation of the severity of a storm, but also to forecasting “misses” and “false alarms” that are particularly difficult for the end-users. In this paper, we discuss the current status and future challenges and prospects related to forecasting of the magnetic structure and orientation of CMEs. We focus both on observational and modeling (first-principle and semi-empirical) based approaches, and discuss the spaceand ground-based observations that would be the most optimal for making accurate space weather predictions. We also cover the gaps in our current understanding related to the formation and eruption of the CME flux rope and physical processes that govern its evolution in the variable ambient solar wind background that complicate the forecasting. c ©2019 American Geophysical Union. All Rights Reserved. Plain-language summary: Coronal Mass Ejections (CMEs) are gigantic magnetized plasma clouds that are frequently expelled from the Sun. Practically all strong and extreme space weather disturbances in the near-Earth space environment are caused by CMEs that propagate in a few days from the Sun to the Earth. Space weather disturbances are related to various harmful effects to modern technology both in space and on ground which can lead to substantial economic losses. Forecasting the CME properties at least half a day before their impact on Earth is thus essential for our society. Our ability to provide accurate predictions of space weather consequences of CMEs is however currently quite modest. The key challenges are related to observational and modeling limitations, and complex evolution CMEs may experience as they propagate from Sun to Earth. This paper discusses the current status and future prospect in forecasting key CME properties using both observations and simulations. c ©2019 American Geophysical Union. All Rights Reserved. 1. Coronal Mass Ejections close to the Sun and in interplanetary space The largest space weather storms at Earth are caused by coronal mass ejections [CMEs; e.g., Webb and Howard , 2012], gigantic plasma clouds that are powered by the complex and ever-changing magnetic field of the Sun. Loop-like magnetic arcades extend from the surface of the Sun to the solar atmosphere and become sheared and twisted by the motion of their footpoints or newly emerging magnetic field. When enough twisting and energization has occurred, the structure may suddenly lose its balance hurling billions of tons of plasma at speeds up to several thousand kilometers per second away from the Sun [e.g., Forbes , 2000; Chen, 2017]. In remote-sensing observations, CMEs are best seen with white-light coronagraphs. A coronagraph creates an artificial solar eclipse; it blocks the bright solar disk and records sunlight that has scattered from coronal electrons [e.g., Billings , 1966]. After their discovery in early 1970s, CMEs were defined as transient and bright features propagating outward through the coronagraph field-of-view [Munro et al., 1979; Hundhausen et al., 1984]. Figure 1 shows a CME observed by the two COR2 coronagraphs [Howard et al., 2008a] onboard Solar TErrestrial RElations Observatory (STEREO) spacecraft. Signatures of CMEs are also observed using a wide range of other wavelengths [e.g., Howard and DeForest , 2012], such as Extreme UltraViolet (EUV) emission that comes from various ionization states of heavy ions in the corona and chromosphere, X-rays and radio waves. CMEs are also inherently related to other eruptive phenomena at the Sun, namely solar flares and prominence eruptions; they all often originate nearly simultaneously from the c ©2019 American Geophysical Union. All Rights Reserved. destabilization of the same large-scale magnetic field structure [e.g., Zhang et al., 2001; Temmer et al., 2008]. After their eruption from the Sun, CMEs propagate through the heliosphere. The fastest CMEs reach the orbit of the Earth (one astronomical unit, AU; 149 597 871 kilometers) in less than a day and slower ones typically in few days [e.g., Gopalswamy et al., 2001a; Owens and Cargill , 2004; Liu et al., 2014]. When observed in interplanetary space, CMEs are commonly referred to as interplanetary CMEs [ICMEs; e.g., Kilpua et al., 2017] based on characteristic plasma, magnetic field and compositional signatures measured by in-situ instruments. The connection between CMEs and ICMEs has now been unambiguously established with the observations from STEREO heliospheric imagers [Harrison et al., 2005, 2018] starting from the Sun up to 1 AU [Davis et al., 2009; Möstl et al., 2009, 2017]. At the Earth’s distance, ICMEs have their radial sizes about 0.2-0.3 AU [e.g., Gosling et al., 1987; Klein and Burlaga, 1982; Jian et al., 2006] and on average they propagate past our planet in about one to two days. The magnetic field in ICMEs is typically strong, the magnetic pressure dominates the plasma pressure and the field direction rotates smoothly over a large angle [e.g., Burlaga et al., 1981; Klein and Burlaga, 1982]. These are signatures of a magnetic flux rope, a configuration where magnetic field lines wind about the central axis. The flux rope structure is a key factor making ICMEs such powerful drivers of intense space weather storms [e.g., Gosling et al., 1991; Huttunen et al., 2005; Zhang et al., 2007; Richardson and Cane, 2012]. Most importantly, flux ropes can provide sustained periods of strongly southward interplanetary magnetic field allowing solar wind energy, plasma and c ©2019 American Geophysical Union. All Rights Reserved. momentum to enter efficiently the Earth’s magnetosphere [e.g., Dungey , 1961; Vasyliunas , 1975; Pulkkinen, 2007]. The interplanetary counterpart of a CME, as shown in Figure 1, is given in Figure 2. This ICME shows a clear field rotation and enhanced magnetic field, featured also by a low ratio of plasma to magnetic pressure (plasma beta). The magnetic field is southward within the flux rope structure and it causes a moderate-level magnetic storm as recorded here by the Dst index which measures the strength of the equatorial ring current [e.g., Mayaud , 1980]. The leading shock wave is identified as an abrupt and simultaneous jump of the magnetic field magnitude and plasma parameters, and the sheath by compressed and turbulent plasma and magnetic field [e.g., Kilpua et al., 2017]. Despite decades of research, the accuracy of predicting space weather effects of CMEs in advance (at least half a day) remains rather modest. Direct observations of Earthbound CMEs are typically not available until Lagrangian point L1, about 1.5 million kilometers from the Earth towards the Sun, where it only takes less than an hour for an ICME to reach our planet. The success of long-lead time forecasting thus depends on predicting accurately (1) intrinsic properties of a CME when it erupts from the Sun, and (2) how intrinsic properties change during the propagation from Sun to Earth. As observations in the heliosphere are very limited, step (2) is typically covered by modeling (Figure 3). One of the most critical current issues is that there is no practical method to forecast the magnetic field in CMEs. Space weather forecasts are also very sensitive to variations in the intrinsic CME parameters, including their orientation, direction and magnetic field structure [e.g., Kay and Gopalswamy , 2017; Möstl et al., 2018]. Another key challenge is related to the complex and often drastic evolution that CMEs may experience during c ©2019 American Geophysical Union. All Rights Reserved. their travel through the corona and interplanetary space [e.g., Manchester et al., 2017; Lugaz et al., 2017; Török et al., 2018]. We focus in this paper on the current status of predicting the magnetic structure and orientation of CMEs. We start by discussing societal aspects of CME impacts (Section 2), and then key critical physical aspects of CMEs that make them such challenging phenomena for forecasting space weather (Section 3). In Section 4, we cover the estimations of intrinsic parameters of CMEs and in Section 5, we discuss the modeling of CMEs. Finally, in Section 6, we give key future approaches and prospects for improving CME forecasts. We cover here only those models that are targeted in running (now or in the future) in near-real-time and that focus on modeling the structure and evolution of CMEs. We also do not discuss here solar energetic particles that are another highly important aspect of space weather [e.g.,see reviews by Cane and Lario, 2006; Desai and Giacalone, 2016], but whose forecasting requires largely different approaches than forecasting the consequences of the direct interaction of a CME with the Earth’s magnetosphere. 2. Societal Aspect of CMEs The term “space weather” refers to the variable conditions on the Sun and in the solar wind that can cause disturbances in the near-Earth space, and in the upper part of the Earth’s atmosphere and affect the functioning and reliability of technological systems on ground or in space, and endanger human life or health. Direct interaction of solar wind transients with the Earth’s magnetosphere, such as CMEs, can lead to significant disturbances in the ge