The magnetosphere shields the earth from charged particles in solar winds.
A geomagetic reversal means that the magnetic field will be destabilized for some amount of time.
the aurora borealis and aurora australis are caused by the patterns of the magnetosphere causing the particles to collide with atoms in the ionosphere, releasing light energy.
However if the mangetospere were to destabilize, or dissapear completely, these lights would be intense everywhere around the planet, and the amount of radiation reaching the planet would be extreme, killing most and leaving others with cancers and other such problems due to radiation.