The increase in velocity over time.
Alpha particles are ions.
ACRs are pick-up ions that are tossed around in and out of the solar wind termination shock until some gain energy and are thrown back inside the heliosphere. The ones that are thrown in are then called ACRs. They are a type of cosmic ray. Antimatter Matter composed of the antiparticles of normal matter. A particle and its antiparticle have a number of opposite properties.