Earth Planets Space, Vol. 61 (No. 5), pp. 595-597, 2009LETTER
NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.
(Received June 21, 2008; Revised October 27, 2008; Accepted October 27, 2008; Online published May 29, 2009)
In this letter, I show that the discrepancies in the geoeffectiveness of halo coronal mass ejections (CMEs) reported in the literature arise due to the varied definitions of halo CMEs used by different authors. In particular, I show that the low geoeffectiveness rate is a direct consequence of including partial halo CMEs. The geoeffectiveness of partial halo CMEs is lower because they are of low speed and likely to make a glancing impact on Earth.
Key words: Coronal mass ejections, geomagnetic storms, geoeffectiveness, halo CMEs.