Earth's magnetic field acts as a shield against the bombardment of particles continuously streaming from the sun. Because the solar particles (ions and electrons) are electrically charged, they feel magnetic forces and most are deflected by our planet's magnetic field. Earth's magnetic field, which shields our planet from severe space weather, often develops two holes when the solar wind's magnetic field points North that allow the largest leaks of solar particles. Read more at: http://www.nasa.gov/mission_pages/themis/news/themis_leaky_shield.html
Credit: NASA CI Lab/Walt Feimer