Present state of knowledge:
Following earlier observations by space-based white-light coronagraphs, considerable progress in understanding CMEs has been achieved using data from the SOHO mission, which provides continuous coverage of the Sun and combines coronagraphs with an EUV imager and off-limb spectrometer. Other spacecraft, such as ACE, WIND, Ulysses and STEREO, which carried comprehensive in-situ instrumentation, have contributed significantly to our understanding of the interplanetary manifestation of these events. With a full solar cycle of CME observations, the basic features of CMEs are now understood. CMEs appear to originate from highly-sheared magnetic field regions on the Sun known as lament channels, which support colder plasma condensations known as prominences. Eruptions develop in the low corona within 10-15 minutes, while the associated shocks cross the solar disk within 1 hour. CMEs reach speeds of up to 3000 km/s and carry energies (kinetic, thermal and magnetic) of ~1025 J (= 1032 ergs). They can also accelerate rapidly during the very early stages of their formation, with the CME velocity being closely tied, in time, to the associated flare’s soft X-ray light profile (Zhang and Dere 2006). High-resolution SOHO and STEREO coronagraph images have provided evidence for a magnetic flux rope structure in some CMEs as well as for post-CME current sheets. Both features are predicted by CME initiation models (e.g., Lin & Forbes 2000; Lynch et al. 2004).
STEREO observations have made it possible to chart in three dimensions the trajectories of CMEs in the corona and heliosphere, thereby improving our understanding of CME evolution and propagation. STEREO data have supported detailed comparison both of in-situ measurements with remote-sensing observations and of MHD heliospheric simulations with observations. The combination of high-cadence coronagraphic and EUV imaging simplifies the separation of the CME proper from its effects in the surrounding corona (Patsourakos and Vourlidas 2009) and allows a more accurate determination of its dynamics.
Despite the advances in our understanding enabled by SOHO and STEREO, very basic questions remain unanswered. These concern the source and initiation of eruptions, their early evolution, and the heliospheric propagation of CMEs. All current CME models predict that the topology of ICMEs is that of a twisted flux rope as a result of the flare reconnection that occurs behind the ejection. Observations at 1 AU, however, find that less than half of all ICMEs, even those associated with strong flares, have a flux rope structure (Richardson and Cane 2004). Many ICMEs at 1 AU appear to have a complex magnetic structure with no clearly-defined topology. Moreover, for ICMEs that do contain flux ropes, the orientation is often significantly different from that expected on the basis of the orientation of the magnetic fields in the prospective source region. CMEs are believed to originate from prominence eruptions, yet in ICMEs observed at 1 AU prominence plasma is very rarely detected. These major disconnects between theoretical models (of prominence eruption and CME propagation) and observations (remote and in situ) need to be resolved if any understanding of the CME process is to be achieved.
How Solar Orbiter will address this question:
To advance our understanding of the structure of ICMEs and its relation to CMEs at the Sun beyond what has been achieved with SOHO and STEREO requires a combination of remote-sensing and in-situ measurements made at close perihelion and in near-corotation with the Sun. Through combined observations with its magnetograph, imaging spectrograph, and soft X-ray imager, Solar Orbiter will provide the data required to establish the properties of CMEs at the Sun and to determine how coronal magnetic energy is released into CME kinetic energy, flare-associated thermal/non-thermal particle acceleration, and heating. Observations with the imaging spectrograph will be used to determine the composition of CMEs in the low corona and to establish how they expand and rotate and will also provide vital clues to the energy partition within a CME once it is released. Solar Orbiter will make comprehensive in-situ measurements of the fields and plasmas (particularly composition) of ICMEs following their release and, critically, prior to their processing during propagation in the heliosphere. These measurements will allow the properties of an ICME to be related to those of the CME at the Sun and to the conditions in the CME source region as observed by Solar Orbiter’s remote-sensing instruments and will make it possible to examine the evolution of CMEs in the inner heliosphere. Solar Orbiter’s combination of remote-sensing and in-situ observations will also establish unambiguously the magnetic connectivity of the ICME and reveal how the magnetic energy within flux ropes is dissipated to heat and accelerate the associated particles. Solar Orbiter data will also reveal how the structure of the magnetic field at the front of a CME evolves in the inner heliosphere – a critical link in understanding, and eventually predicting, how transient events on the Sun may determine the geoeffective potential of the event.
To fully understand the physical system surrounding CME ejection, the temporal evolution of active regions and CME-related shocks and current sheets must be tracked from their formation in the corona to their expulsion in the solar wind. During the mission phases when the spacecraft is in near-corotation with the Sun, Solar Orbiter will continuously observe individual active regions, free from projection complications, over longer periods than are possible from Earth orbit. Solar Orbiter will thus be able to monitor the development of sheared magnetic fields and neutral lines and to trace the flux of magnetic energy into the corona. Observations from the vantage point of near-corotation will make it possible to follow the evolution of the current sheet behind a CME with unprecedented detail and to clarify the varying distribution of energy in different forms (heating, particle acceleration, kinetic).
Detailed sub-objectives: