Energy‐Dispersive Field‐Aligned Warm Ion Enhancement in the Plasma Sheet During a Substorm Growth Phase: A THEMIS Event

We present an event of field‐aligned warm (10–1,000 eV) ion enhancement with an energy‐dispersion signature with increasing energy that has never been reported before and propose that such dispersion was a result of outflow ions being gradually accelerated by intensifying upward field‐aligned potential. The energy‐dispersive enhancement with increasing energy from ∼20 eV up to several hundreds of eV in ∼10 min was observed in the plasma sheet around 01 hr magnetic local time by two spacecraft at r ∼ 7 and 9 RE, respectively, with the spacecraft at larger r observing the dispersion ∼20 min earlier than the other. This event occurred during the growth phase of a small non‐storm time substorm. Observations in the ionosphere and ground in the vicinity of the spacecraft's footprint indicate that the magnetospheric convection and field‐aligned currents (FACs) were enhanced and there was an indication of upward field‐aligned potential associated with FACs. We propose that enhanced Poynting flux associated with enhanced convection first drove ∼20 eV outflow ions to the plasma sheet to cause the initial enhancement, then the increasing intensities of FACs and the associated upward field‐aligned potential gradually increased the energy of the outflow ions to above 100 eV, thus resulting in the observed energy‐dispersive enhancement with increasing energy. In addition, the earthward penetration of FACs may explain why the enhancements were observed at larger r earlier. Key Points Enhancement of field‐aligned warm ions observed in the plasma sheet was energy‐dispersive with increasing energy from 20 eV to >100 eV The probe at larger r observed the energy‐dispersive enhancements 20 min earlier than did the probe at smaller r The enhancements were likely caused by enhanced convection and the dispersion was likely due to acceleration by field‐aligned potential