Observations of radio emission from exoplanets and brown dwarfs suggest that such objects harbour atmospheric magnetized plasmas. However, the degree of thermal ionization for such substellar objects is insufficient to qualify the ionized component as a plasma, posing the question: what ionization processes can efficiently produce the required plasma? We propose Alfvén ionization as a mechanism for producing localized pockets of ionized gas in the atmosphere, having sufficient degrees of ionization (≥10−7) that they constitute plasmas. We outline the criteria required for Alfvén ionization to occur and justify its applicability in substellar atmospheres. For the model atmospheres considered here, our results show that degrees of ionization ranging from 10−6−1 can be obtained.