I would suggest you get the information about the energy for those electrons and positrons originated from cosmic rays, intensity of cosmic ray to estimate how many generated electrons.
You can estimate the current in the power line (from power and voltage) and calculate the magnetic field strength.

Please check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=36.0]Charged particle motion in E/B Field[/url],too!