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Retrospective of the Solar Extreme Events of October-November, 2003: similarity and diversity
Unusual situations on the Sun are interesting from several points of view. For astrophysics, extremely high and low activity states of the nearest star may indicate on the level of fluctuations and regular trends, which are important, but not completely incorporated in current theoretical paradigms of the stellar evolution including also cyclic and chaotic components. For solar-terrestrial physics, the extreme states of solar activity are especially important because of many influences on the heliosphere, magnetosphere, ionosphere, and the upper atmosphere. Practical consequences of severe and extreme perturbations on the Sun are well known and their role increases with the growing sensitivity of possible biological and technical impacts. Extreme or extraordinary events and situations are not common by definition. They could be similar in some aspects when remaining quite different in many parameters. These events are rare. Hence, statistical methods are not applicable for their studies. Dynamical descriptions and forecasts are also restricted by severe predictability limits stemming from the not complete knowledge as well as simplifications in models. Moreover, principal difficulties arise on this way because of the unstable behavior, non-unique solutions and diverging phase space trajectories around turning and bifurcation points of the governing equations, both in macroscopic and microscopic descriptions, which often lead to inherent unpredictability or limited predictability in the better case. The only known way to overcome these difficulties with the intermittent turbulence is to construct physical descriptions and classifications of extreme events based on similarity and scaling laws. Dimensionless parameters and scaling approaches can be used for the quantitative evaluation of the similarity and diversity degree of different solar extreme situations of the type observed in October-November, 2003. Two examples demonstrate the usefulness of the scaling approach. First, the dimensionless 'velocity-emission numbers' which represent the ratios of the kinetic plasma energy (or power) to that of the radiation delimit flare-like and CME-like situations on the Sun and in the heliosphere. This approach leads to the conclusion that solar flares and CMEs are concurrent manifestations of the enhanced dissipation and radiation of the solar atmosphere. They represent two sides of one physical process when described in the framework of the MHD formulation with dissipation and radiation, but not the cause or the sequence of each other. Second, the dimensionless set of 'Trieste numbers' is appropriate for the quantitative demarcation between physically closed and open situations in all senses (against the energy, momentum, mass flows as well as geometry). Complicated turbulent situations and simple laminar regimes coexist. They are not absolutely defined, but can be relatively delimited by the number of essential scales or degrees of freedom involved,- many versus one or few respectively. In most cases, the processes look as intermittent between laminar and turbulent states. More common dimensionless parameters taken from dissipative MHD and plasma kinetic formulations are also very helpful. The conclusion in this part is that solar and heliospheric structures under consideration are essentially open against the energy, momentum and mass transports which needs non-local descriptions taking into account couplings between different scales in space and time. atmosphere as well as the involvement of more powerful thermal and luminosity drivers of the magnetic activity on the Sun, causal relation of CMEs and flares to subphotospheric processes. Future imaging observations of the Sun with a high space-time resolution in white light and in different spectral ranges from space could bring a better understanding of the origins of extreme (high and low) activity situations.