[2] Strongest perturbations on the Sun and in the heliosphere are of great practical interest from the point of view of the space weather evaluation and forecast (R. Schwenn, Space weather: Solar perspective, available at http://solarphysics.livingreviews.org/Articles/lrsp-2006-2/; T. Pulkkinen, Space weather: Terrestrial perspective, available at http://solarphysics.livingreviews.org/Articles/lrsp-2007-1/). Several difficulties exist: (1) the statistics of such events is rare by definition of extreme events; (2) observational information about physical parameters is limited and far from being complete; and (3) the reliable theoretical models are still not available.
[3] The aim of this paper is to discuss some differences and common characteristics of the recent extreme solar events, their heliospheric manifestations and related geomagnetic storms. We consider mainly the declining phase of the 23rd solar cycle and some other cases. We remark on some common and specific properties of these extreme events, which were also partially investigated before [Panasyuk et al., 2004; Veselovsky et al., 2004; Yermolaev et al., 2005]: (1) the broad diversity of their parameters, which makes all such events unique and not similar in some sense; (2) the absence of universality even under similar manifestations of biggest solar flares and coronal mass ejections; (3) the global and multiple character of the most powerful perturbations on the Sun and in the heliosphere; and (4) the relation of such events to the longitudinal asymmetry of the Sun and to the longer-duration variations in the solar interiors including cyclic and sporadic ones. Scaling approaches are outlined for the physical and statistical classifications.
[4] In November 2004, the solar activity was high and generated interplanetary and terrestrial storms, which were unexpected by the warning centers. Indeed, the low and moderate activity was predicted for the solar rotation period from 3 November up to 29 November with only possible solar flares of M class. Warnings were issued only when the halo coronal mass ejections (CMEs) appeared in the field of view of the SOHO/LASCO coronographs. A similar situation happened in September 2005. The activity in October-November 2003 and in September 2005 according to several parameters belongs to the strongest one not only during the 23rd solar cycle but also during the whole period of observations. In October-November 2003, the series of coronal mass ejections and solar flares occurred which leaded to big geomagnetic storms.
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