Recently, it was reported (V. G. Eselevich, Planet. Space Sci., 42(7), 575–582, 1994) that preferential relationships exist between the transit velocity V T of earthward-directed interplanetary shocks and solar active processes, in particular, eruptive filaments outside active regions (the size of the erupting filament L f and solar flares (the value of the X-ray characteristic J). Unfortunately, statistical testing of the proposed associations was not accomplished, nor was the “geoeffectiveness” of the events adequately described. Reported here are the results of a re-examination of the 21 eruptive filaments (SSC-EF events) and 26 X-ray flares (SSC-F events) that have been associated with storm sudden commencements (SSCs) at Earth. Simple statistical testing refutes the claim that a preferential relationship exists between V T and L f, while it supports the claim that one exists between V T and J. More importantly, the inferred relationship between V T and J is found to be more complicated than previously thought. In particular, it now appears that SSC-F events may be separable into two groups, based on the value of J: a low- J group ( J ≤56), in which V T varies directly with J, and a high- J group ( J > 56), in which V T varies inversely with J. As a whole, high- J events are associated with shocks of higher average transit velocity than those of low- J events, and SSC-F events with shocks of higher average transit velocity than those of SSC-EF events. Further, high- J events tend to be of greater X-ray class (> M3), longer duration (> 80 min), and are more likely to be associated with type II IV radio emission (9 of 12) than low- J events. They also tend to occur in magnetically complex (gamma/delta configuration) active regions (10 of 12) that are large in areal extent (area >445 millionths of a solar hemisphere) on the day of flaring (9 of 12). Of the 9 solar proton events that affected the Earth's environment that were found to be associated with SSC-F events, six were high- J events. Concerning “geoeffectiveness”, there appears to be no preferential relationship between the value of the J-parameter and the most negative value of the Dst geomagnetic index Dst(min) following the SSC, which is found to usually occur at 6–14 h after SSC onset (18 of 26) and which ranged in value from −1 to −249 (having a median value of about −75). Of the 26 SSC-F events, only 14 can be associated with a Dst(min) ≤ −75, and of these only 7 were high- J events. Of the 14 storm-related events (i.e. Dst(min) ≤ −75), three have previously been identified as being either “magnetic clouds” or “bidirectional flows”, both manifestations of earthward-directed coronal mass ejections (CMEs). Superposed epoch analyses of selected solar wind parameters and Dst during the interval of storm-related SSC-F events demonstrate that geoeffective SSC-F events tend to be associated with solar wind flows that are faster, greater in magnetic field strength, and have a rotating field which has a strong southward component shortly after SSC onset, in comparison to SSC-F events that do not have Dst(min) ≤ −75. Therefore, it is inferred that geoeffective SSC-F events are probably fast earthward-directed CMEs. Although no single parameter is found that can serve as a predictor of high-skill level for determining the geoeffectiveness of an SSC-F event prior to its occurrence at Earth, one finds that knowledge of the flare's hemispheric location and appearance or lack of appearance of a two-ribbon structure is sufficient to correctly postdict the geoefectiveness of 20 out of 25 of the SSC-F events (80%). Surprisingly, the association or lack of association of metric type II IV radio emission as a characteristc for determining the geoeffectiveness of the SSC-F events proved unfruitful, as did, to a lesser extent, the duration of the X-ray emission.
Search from 250 M+ research papersSearch from 250 M+ research papers
Jun 1, 1998
Kazuto Ashihara + 3
Open Access