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The combination of nonorthogonal multiple access (NOMA) technique and unmanned aerial vehicle (UAV) provides an effective solution for achieving massive connections and improving spectrum efficiency. However, the related security risk becomes serious due to the line-of-sight (LoS) channels involved and high transmit power for weaker users in NOMA-UAV networks. In this article, a UAV-assisted NOMA transmission scheme is proposed to achieve secure downlink transmission via artificial jamming, where a UAV flies straightly to serve multiple ground users in the presence of a passive eavesdropper. During the flight, only the closest NOMA users are chosen to connect with the UAV in each time slot to achieve high LoS probability. To balance the security and transmission performance, the tradeoff between the jamming power and the sum rate is investigated by jointly optimizing the power allocation, the user …