  Unmanned aerial vehicles (UAVs) and Terahertz (THz) technology are envisioned
to play paramount roles in next-generation wireless communications. In this
paper, we present a novel secure UAV-assisted mobile relaying system operating
at THz bands for data acquisition from multiple ground user equipments (UEs)
towards a destination. We assume that the UAV-mounted relay may act, besides
providing relaying services, as a potential eavesdropper called the untrusted
UAV-relay (UUR). To safeguard end-to-end communications, we present a secure
two-phase transmission strategy with cooperative jamming. Then, we devise an
optimization framework in terms of a new measure $-$ secrecy energy efficiency
(SEE), defined as the ratio of achievable average secrecy rate to average
system power consumption, which enables us to obtain the best possible security
level while taking UUR's inherent flight power limitation into account. For the
sake of quality of service fairness amongst all the UEs, we aim to maximize the
minimum SEE (MSEE) performance via the joint design of key system parameters,
including UUR's trajectory and velocity, communication scheduling, and network
power allocation. Since the formulated problem is a mixed-integer nonconvex
optimization and computationally intractable, we decouple it into four
subproblems and propose alternative algorithms to solve it efficiently via
greedy/sequential block successive convex approximation and non-linear
fractional programming techniques. Numerical results demonstrate significant
MSEE performance improvement of our designs compared to other known benchmarks.
