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Ion Charge States and Potential Geoeffectiveness: The Role of Coronal Spectroscopy for Space‐Weather Forecasting

Severe space weather is driven by interplanetary coronal mass ejections (ICMEs), episodic eruptions of solar plasma, and magnetic flux that travel out through the heliosphere and can perturb the Earth's magnetosphere and ionosphere. In order for space‐weather forecasts to allow effective mitigating action, forecasts must be made as early as possible, necessitating identification of potentially “geoeffective” ICMEs close to the Sun. This presents two challenges. First, geoeffectiveness is primarily determined by the magnetic field intensity and orientation, both of which are difficult to measure close to the Sun. Second, the magnetic field evolves in transit between the Sun and the Earth, sometimes in a highly nonlinear way. Conversely, solar wind ion charge states, such as the ratio of O7+ to O6+, are fixed by the electron temperature at the coronal height where ion‐electron collisions are last possible as the ICME erupts. After this point, they are said to be “frozen in” as they do not evolve further as the ICME propagates through the solar wind. In this study we show that ion charge states, while not geoeffective in and of themselves, act as strong markers for the geoeffectiveness of the ICME. The probability of severe space weather is around 7 times higher in “hot” ICMEs than “cold” ICMEs, as defined by O7+/O6+. We suggest that coronal spectroscopy of ICMEs could complement current forecasting techniques, providing valuable additional information about potential geoeffectiveness.