Are Stealth CMEs a New Space Weather Forecasting Extreme?

One of the underlying problems in the investigation of CME genesis and evolution is relating remote-sensing observations of coronal mass ejections (CMEs) to in-situ observations of interplanetary CMEs (ICMEs). Typically, remote-sensing observations of an eruption are first observed in the low corona, followed by coronagraph observations of the global structure of the CME projected onto the plane of the sky, and then finally local, highly-quantitative measurements of an ICME are made in situ along a spacecraft trajectory. However, the dramatic change in solar activity in recent years has raised awareness of "stealth" CMEs, which are CMEs that are often observed in coronagraph data but not in coronal images. These events often cause problem geomagnetic storms because they go unnoticed until they hit Earth. Largely identified during the deep minimum of cycle 23/24, stealth CMEs appear to be on the rise. Since solar cycle 25 brings with it the possibility of yet another low activity cycle, it is very likely that the number of stealth CMEs will remain a significant fraction of ejecta. We investigate the properties of stealth CMEs, paying special attention to their proximity to coronal holes. We note the existence of mismatched polarity reversals in the magnetic field and electron strahl measured in situ within ICMEs associated with stealth CMEs and discuss the plausibility of interaction with solar wind emanating from coronal holes as a key element of stealth CME eruption. That stealth ICME arrival at Earth is often accompanied by fast solar wind streams, which enhances their geoeffectiveness, means that these events often cause strong geomagnetic storms at Earth with little to no warning. Thus these near-invisible events represent a large gap in our ability to provide accurate storm predictions and have defined a new form of “extreme” when it comes to space weather forecasting.