When the solar wind impacts the magnetic field of the earth, it bends some of the magnetic field lines from the polar regions so that some of them are pushed back into a comet-like tail. Other nearby field lines, however, remain close to the earth and connect back up with the opposite magnetic polar region. The process is very much like combing a part in your own hair. The result is a 'cusp' in the field lines which can act line a channel down which energetic particles can flow to deeper layers in the magnetosphere and the atmosphere.
The cusp is located on the day-side hemisphere of the earth in the northern hemisphere between March - June - September, when the earth's northern hemisphere is tilted towards the sun. The southern hemisphere has its cusp facing the sun between September - December - March.
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