Local time dependent Pi2 frequencies observed by THEMIS spacecraft near dawn and dusk in the inner magnetosphere

We report an example of Pi2 pulsation exhibiting different frequency between dawn and dusk. This Pi2 pulsation occurred around 1932 UT on February 14, 2008, and was observed at low-latitude Bohyun (BOH, L = 1.35, MLT = 3.8) and Hermanus (HER, L = 1.83, MLT = 20.1) stations. The pulsation starts simultaneously at both stations but the frequency is higher at BOH than at HER. At the time of the Pi2, THEMIS-A (THA) and THEMIS-D (THD) were on dawn (L = 2.9, MLT = 5.3) and dusk (L = 2.8, MLT = 18), respectively, in the inner magnetosphere. We find a nearly identical waveform and period between THA ${\delta}Bz$ and BOH ${\delta}H$ and between THD ${\delta}Bz$ and HER ${\delta}H$ with a near $0^{\circ}$ phase delay. This observation implies that Pi2-associated fastmode wave in the inner magnetosphere is not excited globally as a single frequency at all longitudes. We suggest that the different frequency between dawn and dusk is due to dawn-dusk asymmetry of the plasmasphere.