Abstract: The characteristics of field-aligned currents (FACs) and the related physical quantities under radial interplanetary magnetic field (RIMF) from 2001 to 2009 were investigated. Statistical analysis and comparison with FACs under normal conditions were performed using CHAllenging Minisatellite Payload (CHAMP) data. The results showed that the intensity of noon FACs has clear dependence on the dipole tilt angle under normal conditions and that the ratio between summer and winter is about 3; however, the dependence disappears under RIMFs and the ratio decreases to 2. The magnitude of the interplanetary magnetic field (IMF), the merging electric field, solar wind dynamic pressure, and the auroral electrojet (AE) index decrease, while the disturbance storm time(Dst) index remains almost stable under RIMFs. The merging electric field is a controlling factor of FACs under normal conditions, but the control effect disappears during RIMFs. In the Northern Hemisphere, the intensity of FACs under IMF < 0 conditions is larger than that under IMF > 0 conditions (the situation is the converse in the Southern Hemisphere). Although the intensity of FACs under normal conditions is greater than that under RIMFs, the probability that the density of FACs under RIMFs is greater than the yearly average is about 0.2, and some geomagnetic disturbances, such as substorms, still occur under RIMFs.

Key words: field-aligned currents, radial interplanetary magnetic fields, merging electric field