In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

The sun decreases this entity's linking number, called its helicity, in coronal mass ejections. In the presence of this entity, the Lyman alpha transition exhibits ten lines instead of two, and the wave propagation vector is parallel to this entity in Alfven waves. An armature rotates within a stator that generates this phenomenon in a dynamo, and this entity is expelled from a superconductor in the Meissner effect. This quantity is proportional to the integral around a loop of wire of the wire element cross the unit vector to the wire over the distance to the wire squared. For 10 points, give this entity, which exerts a force on a charged particle proportional to velocity's cross product with it.
Science