3.8 “Problem” and “stealth” CMEs

So-called “problem” geomagnetic storms lack historically obvious signatures of related solar activity, such as flares and large disappearing filaments (e.g., Dodson and Hedeman, 1964; McAllister et al., 1996, and references therein). An event on 6 – 10 January 1997 was the first such problem storm for which the antecedent CME was actually observed (Webb et al., 2000bJump To The Next Citation Point). The associated solar surface activity was so weak and unimpressive that, had the faint LASCO halo CME not been observed, the storm would not have been forecast. During the LASCO period, partial or full halo CMEs that had no obvious surface association were usually attributed to “backside” events directed away from the Sun-Earth line (e.g., Demastus et al., 1973; Munro et al., 1979; Webb and Hundhausen, 1987Jump To The Next Citation Point). Non-halo CMEs occurring near the limb were mostly attributed to unseen sources behind the limb.

The absence of solar surface activity with observed CME activity is not a new observation (Howard and Harrison, 2012Jump To The Next Citation Point).UpdateJump To The Next Update Information The launch of STEREO in 2006, however, afforded us the opportunity to study the origins of CMEs simultaneously from multiple lines of sight. Robbrecht et al. (2009a) presented a study of a streamer blowout CME without a clear source region. The STEREO spacecraft were sufficiently widely separated (53°) that the CME and its source region could be viewed edge-on in STEREO A and face-on in STEREO B. STEREO B saw the CME as a faint halo and it was detected in-situ as a magnetic cloud 5 days later. Robbrecht et al. suggested that the CME originated high enough up in the corona such that no surface signatures were evident. Subsequently, Ma et al. (2010) performed a statistical study of all CMEs observed during the first 8 months of 2009 when the STEREO lines of sight were nearly perpendicular to each other. They found that about a third of the CMEs were “stealth”, having no distinct surface association, and tending to be slow, i.e., < 300 km s–1. Faint coronal changes could be detected in about half of the stealth CMEs, again suggesting a higher launch site. It is noted that this period was during the recent unusual extended solar minimum, so the fraction of such CMEs may be different at other times. Howard and Harrison (2012) in a recent review paper discuss the use of the term “stealth CME”.Update


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