This study has investigated the speed at which information can propagate between CME plasma parcels (the Alfvén speed, V A ), relative to the speed at which CME plasma parcels separate owing to radial propagation in spherical geometry [V G ]. Where V G exceeds V A , plasma parcels can no longer be considered to constitute a single, coherent magnetohydrodynamic (MHD) structure. Figure 4 illustrates this idea. It shows a CME travelling through fast solar wind, but the upper flank encounters a slow wind stream. This results in distortion of the magnetic field structure within the CME. An Alfven wave is launched at a speed V A from point P B , which lies within the CME at the latitude of the solar wind speed shear, towards a point P A , located near the centre of the CME. Geometric expansions means that P B is moving away from P A at a speed V G . If V G > V A , as shown in this example, information cannot travel between the two points. Thus P A and P B are effectively isolated, and the response of the CME at points P A and P B to a structured solar wind is entirely independent; there can be no action as a single body, regardless of the magnitude of restoring forces such as magnetic pressure and curvature forces. A similar effect is expected within the deflected solar wind flow in the sheath region ahead of a fast moving CME39. Due to the large V G , the deflected solar wind flow within the sheath (labelled V SH in Fig. 4)24 cannot keep pace with a point on the leading edge and thus does not flow around the obstacle, but piles up ahead of it.

Figure 4 A schematic of one flank of a CME (white) propagating through a structured solar wind, in the reference frame of a point P A , located close to the centre of the CME. The shock (thick black line), and CME leading/trailing edges move away from P A at the CME expansion speed, V EX . Fast solar wind, in beige, flows into the CME shock at a speed V TR + V EX − V FSW (V TR and V FSW are the CME transit speed and the fast solar wind speed, respectively). Slow solar wind, in blue, flows into the shock at a speed of V TR + V EX − V SSW , (where V SSW is the slow solar wind speed). The point P B , located at the fast/slow solar wind interface, experiences a distortion of the CME magnetic field and launches an Alfven wave at speed V A towards P A . Point P B , however, is moving away from P A due to geometric expansion at a speed V G , thus the information can never arrive. Similarly, V SH , the speed of the deflected solar wind flow in the sheath behind the shock, is smaller than V G and thus the sheath flow cannot travel around the CME. Full size image

We estimate V A and V G using an analytic model, allowing parameter space to be fully and efficiently explored. Where simplifying assumptions are required, they have been chosen as far as possible to act in the favour of CME coherence (e.g., limiting the expansion of CMEs to the radial direction reduces V G ; coherence is defined to be lost when V G exceeds V A , rather than when the information travel time becomes large compared to the CME life time; helium is not included in the Alfvén speed estimation, etc). Thus we effectively examine the “best case scenario” for CME coherence. Nevertheless, we find that all observed CMEs lose coherence over their full angular extent by 0.1 to 0.2 AU. Even considering Alfvén wave propagation over half the typical CME angular extent, which would allow, e.g., the east flank of an ICME to know what’s happening to the west flank, no observed CMEs are expected to maintain coherence to 1 AU; indeed, less than 0.5% of all observed CMEs are expected to maintain flank-to-flank coherence past 0.3 AU.

One aspect that requires further investigation is the assumption that the fastest information path between two points is a straight line. While this is true for the analytical model employed here, as it has constant magnetic field intensity within a CME, in a real magnetic cloud this need not be the case. For an ideal force-free magnetic flux rope, the magnetic field intensity is highest at the flux rope axis (i.e., the centre of the CME). Thus shorter information travel times between two points on the CME leading edge could, in principle, be obtained using a non-linear ray path taking advantage of the increased Alfvén speed deep within the CME. An alternative preferential wave path could be through the high magnetic field intensities in the sheath region ahead of a fast CME, though the sheath is often high plasma density too, meaning the Alfvén speed may not be enhanced. These dynamic effects will be fully investigated using numerical magnetohydrodynamic modelling of an erupting magnetic flux rope and ray-tracing at each time step. In practice, however, these effects are unlikely to provide significantly different results to those presented here. Any increased Alfvén speed will be offset by an increased path length, and compression of the CME leading edge by interaction with the ambient solar wind means the highest magnetic field intensities are usually located near the CME leading edge, not near the centre of the CME35.

In light of these findings, new approaches are required for the interpretation of CME observations. We discuss a few examples here. The highly structured intensity patterns routinely seen within CMEs in Heliospheric Imager (HI) observations40 by the STEREO spacecraft may be a direct result of both the scale of coherence within a CME and the variability of the solar wind through which a CME is travelling. These relatively small-amplitude, small-scale structures are unlikely to be a significant issue for interpretation of the global properties of CMEs, either with the geometric models applied to HI observations to determine CME speed and direction13, or to flux-rope models applied to in situ observations18. Larger amplitude gradients in the solar wind, however, such as a sharp latitudinal or longitudinal transition between fast and slow wind (Fig. 4), are likely to invalidate both forms of reconstruction technique by generating both large distortion to the CME shape and radically altering the pile-up of the solar wind plasma in the CME sheath, which is the plasma that is imaged by Thompson-scattered photospheric light. The results presented here also suggest CME arrival-time forecasting is sensitive to ambient solar wind structure at the local scale, not just at a global scale41: application of a drag equation to a CME’s interaction with the solar wind42 is only really valid along an individual radial flow line, not to the CME as a whole. We suggest CME reconstruction techniques need to be modified to incorporate information about solar wind structure, either from global MHD models or from previous solar wind observations (e.g., assuming corotation of the solar wind). Ultimately, this may require solar wind data assimilation, to best interpolate and extrapolate between the available observations using physics-based models32.