Statistical survey of earthbound interplanetary shocks, associated coronal mass ejections and their space weather consequences
School of Physics and Astronomy, University of Birmingham, Edgbaston B15 2TT, UK
Accepted: 19 May 2005
A comprehensive statistical analysis of events relevant to space weather over the 80 month period from January 1998 to August 2004 is presented. A database has been constructed using data from instruments from the SOHO, ACE, WIND and GOES spacecraft, as well as ground magnetometer data. Parameters investigated include times and epochs of halo and partial halo coronal mass ejections (HCMEs) along with details of the interplanetary shock at L1 (0.99 AU), namely the changes in the interplanetary magnetic field and solar wind density, and shock speed. Transit time to the Earth and average transient speed have also been determined, along with the projected speed and angular width of the HCME at the Sun. An estimate is made of the acceleration of the transients on their passage from the Sun to the Earth, and associated solar flare data are considered. Finally, the geoeffectiveness of the events are analysed using Ap, Dst and sudden commencement data. We found that just over a quarter of the 938 HCMEs observed by LASCO were associated with a forward shock near L1, suggesting that around half of the Earthbound HCMEs are either deflected away from the Sun-Earth line or do not form a shock. Around half of the shocks went on to cause a geomagnetic storm, consistent with a southward BIMF occurring 50% of the time. There was a general tendency for HCME and shock speeds to be more varied (with more events at higher speeds) around solar maximum, and most events decelerated in transit to the Earth, implying a speed “equalisation” between the HCME shock and surrounding solar wind, although an assumption of a constant acceleration appears to be invalid. Only around 40% of the shock/storms were associated with an X or M class flare, and there appears to be no relationship between flare intensity and any physical parameter close to the Earth, except in extreme cases. There was a tendency for HCME speed near the Sun to increase with flare intensity. This casts doubt on the validity of using flare data alone as an effective space weather forecaster.
Key words: Sun: coronal mass ejections (CMEs) / Sun: solar-terrestrial relations / Sun: flares / Sun: solar wind / interplanetary medium / shock waves
© ESO, 2005