Substorm and pseudo-substorm Pi2 pulsations observed during the interval of quasi-periodic magnetotail flow bursts: A case study

Abstract

We studied the relationship between midtail flow bursts observed by the Geotail spacecraft and eight Pi2 pulsations near midnight observed at low-latitude Kakioka (KAK, L = 1.26) and high-latitude Tixie (TIX, L = 5.9) stations on 26 October (day 299) 1997, 1100–1600 UT. The Pi2 pulsations at KAK have a great similarity with those at TIX with an out of phase signature. Three of the Pi2 bursts were associated with substorm onsets/intensifications and other five events were associated with pseudo-substorm onsets. The pseudo-substorm Pi2 pulsations exhibited longitudinal phase variations similar to substorm-related Pi2 pulsations. From this observation we suggest that pseudo-substorm associated current system is morphologically the same as substorm current wedge. The substorm Pi2s are enhanced at higher frequency band (∼ 15–20 mHz) than the frequency band (∼6–15 mHz) of pseudo-substorm Pi2s. We do not attribute these frequency variations to the change of the plasmapause distance, which is favored in the plasmaspheric resonance model. During the five-hour interval, Geotail observed quasi-periodic high-speed flow bursts (perpendicular flow velocity V⊥x > 300 km/s) preceding the low-latitude Pi2 pulsations by ∼35–150 s. It is found that there is no obvious relationship between the speed of the earthward flow burst events and the power of the Pi2 events. This means that enhanced flow speed is not a main factor in controlling a Pi2 power. The waveform and period of the Pi2 pulsations are different from those of the flow bursts except for one event, which was previously reported as BBF-driven Pi2.