On the relationship between transit velocity of interplanetary shocks and solar active processes

Recently, it was reported (V. G. Eselevich, Planet. Space Sci., 42(7), 575–582, 1994) that preferential relationships exist between the transit velocity VT of earthward-directed interplanetary shocks and solar active processes, in particular, eruptive filaments outside active regions (the size of the erupting filament Lf 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 VT and Lf, while it supports the claim that one exists between VT and J. More importantly, the inferred relationship between VT 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 VT varies directly with J, and a high-J group (J > 56), in which VT 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 IIIV 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 Earthʹs 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 flareʹs 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 IIIV 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.