When a radio active element due to its instability of nucleus, it emits alpha particles in the form of rays. An alpha particle consists of 2 protons and 2 neutrons and no electrons. It can be written as α or
or He with 2+ superscript and 2 as subscript.
Ernest Rutherford discovered them. An alpha particle is produced when a helium atom loses its electrons , ie., alpha particle and helium nucleus are same.
It gets attracted towards negative electric charges - anodes. Alpha rays get deflected in an electric field due to coulomb's attraction force between charges.
Alpha rays also get deflected in a magnetic field because magnetism exerts force on moving charges. Alpha particle has 2 positive charges. They do not get attracted toward magnets. They get deflected in a direction perpendicular to the magnetic field. Moving charges produce a magnetic field and that interacts with external magnetic field.
If alpha is moving along a road, magnetic field is acting perpendicular to it across the road, then the force on it is in vertical direction - that is in to the sky. The alpha raises into the sky.
This is specified by Flemming's left hand rule.