1. Research Center for Astronomy and Applied Mathematics, Academy of Athens, 115 27 Athens, Greece
2. Department of Physics, University of Cincinnati, 45269 Ohio, USA
3. Department of Mechanical Engineering, Technological Education Institute of Central Macedonia, 621 24 Serres, Greece
4. Department of Physics, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece
5. Department of Physics, University of Florida, 32611-8440 Florida, USA