Some O+ ions ("conics") also seem accelerated in different ways by plasma processes associated with the aurora. These ions are accelerated by plasma waves in directions mainly perpendicular to the field lines. They, therefore, start at their "mirror points" and can travel only upward. As they do so, the "mirror effect" transforms their directions of motion, from perpendicular to the field line to a cone around it, which gradually narrows down, becoming increasingly parallel at large distances where the field is much weaker.