Abstract
AbstractSevere geomagnetic storms are driven by the coronal mass ejections (CMEs). Consequently, there has been a great deal of focus on predicting if and when a CME will arrive in near‐Earth space. However, it is useful to step back and ask, “How valuable is this information, in isolation, for making decisions to mitigate against the adverse effects of space weather?” While all severe geomagnetic storms are triggered by CMEs, not all CMEs trigger severe storms. Thus, even perfect knowledge of CME arrival time will provide “actionable” forecast information only in operational situations where false alarms can be tolerated. Of course, any CME transit model used to predict CME arrival time must also produce an estimate of CME speed at Earth. This can help discriminate between geoeffective and nongeoeffective CMEs, reducing false alarms and expanding the range of operational scenarios under which a forecast provides value. Thus, from an end‐user perspective, CME arrival speed should form part of the standard metric by which CME transit models are evaluated. Looking to the future, even coarse information about the CME magnetic properties would likely provide even greater forecast value. These points are illustrated by a simple analysis of solar wind data.
Highlights
30 years ago, the “The Solar Flare Myth” (Gosling, 1993) conclusively demonstrated that severe geomagnetic activity results from the passage of coronal mass ejections (CMEs; large episodic eruptions of coronal plasma and magnetic field) through near‐Earth space
As the passage of a CME through near‐Earth space is associated with increased probability of high geoeffectiveness, knowledge of CME arrival time is expected to valuable for space weather mitigation, even without further information about the CME properties
CME arrival time only constitutes “actionable” information in operational situations where false alarms can be tolerated
Summary
30 years ago, the “The Solar Flare Myth” (Gosling, 1993) conclusively demonstrated that severe geomagnetic activity results from the passage of coronal mass ejections (CMEs; large episodic eruptions of coronal plasma and magnetic field) through near‐Earth space. Over the intervening decades, there has been a great deal of effort directed toward forecasting if and when CMEs will arrive at Earth. Such forecasts are routinely initiated using observed properties of CMEs close to the Sun, the near‐Sun speeds derived from coronagraphs and, more recently, heliospheric imagers. Any method of forecasting CME arrival time must produce some predictions of CME arrival speed in near‐Earth space. Such speed estimates can differ substantially across the CME transit models, even when the same transit time is predicted.
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