Lexicon: CME (“solar storm”)
☞ Coronal mass ejection (“solar storm”)
Coronal Mass Ejection (CME) – popularly and scientifically also called “solar storm”: Huge accumulations of plasma that are ejected by the sun and can cause auroras when they reach the earth as ICME (interplanetary CME). The term ICME is not often used, although it is useful to distinguish between CMEs close to the sun and interplanetary CMEs, as CMEs change greatly the further away from the sun they are.
Other term: Transient, especially for ejecta with an unknown origin and ejection time, not necessarily with a CME structure.
ICMEs are responsible for around 90% of all major geomagnetic storms. Calculating their path, their development and the specific effects of an impact is difficult and often flawed, especially when it comes to the time of arrival. The existence of CMEs as an independent phenomenon and trigger of auroras has only been scientifically recognised and accepted since the late 1990s.
☞ The solar flare myth – Gosling, 1993 (agupubs.onlinelibrary.wiley.com/doi/10.1029/93JA01896)
For many, coronal mass ejections are one of the most exciting phenomena in space weather. In English, the term “solar storm” is very popular as a colloquial term. The German translation “Sonnensturm” is rarely used and tends to cause confusion, as ‘Sturm’ and ‘storm’ are not congruent in meaning and can easily be confused with geomagnetic storms or radiation storms.
![Schematic of an ICME and upstream shock. Source: Zurbuchen and Richardson [2006].](https://raumwetter.de/wp-content/uploads/2024/10/Schematic-of-an-ICME-and-upstream-shock-Source-Zurbuchen-and-Richardson-2006.jpg)
Source: Zurbuchen and Richardson [2006].
A fast ICME is preceded by a shock front that produces energetic particles, followed by the turbulent “sheath”, usually an area with a high plasma density, and only then does the actual ICME bulk follow with another front. A CME consists of plasma (charged particles), energetic particles (“Solar Energetic Particles”/”Energetic Storm Particles”) and extremely long magnetic field lines, which can still be connected to the sun days later. These are usually twisted into each other, in which case they are referred to as a “flux rope” because they look like a rope, with flux standing for the magnetic flux, i.e. the magnitude of the magnetic field (analogous to electric current as the magnitude of the electric field). The entire structure is referred to as a magnetic cloud if it corresponds to a well-formed ICME (see figure above). Within flux ropes, the plasma density and also the temperature are low.
Geomagnetic effects such as auroras or induced currents in overhead power lines occur during the passage of the Earth mainly due to the interaction of the magnetic fields (coupling or repulsion depending on polarity) and the relatively high speed of the solar wind as a driver. The plasma density in the solar wind only plays a very minor role in the formation of a geomagnetic storm with a bright aurora – contrary to what many aurora observers assume (together with solar wind speed as dynamic pressure).
Source: Kataoka and Miyoshi (2006)
ICMEs take between one and five days to reach us. Slower ones are barely distinguishable from the background solar wind and become part of it. Extremely fast, high-energy ICMEs can also make it from the sun to Earth in less than 24 hours. This is very rare, but in such cases they can cause memorable events.