Propagation characteristics of Pi 2 pulsations observed at high‐ and low‐latitude MAGDAS/CPMN stations: A statistical study

Abstract

[1] The objective of this study is to understand better the propagation of Pi 2 waves in the nighttime region. We examined Pi 2 oscillations that showed high correlation between high-and low-latitude Magnetic Data Acquisition System/Circum Pan-Pacific Magnetometer Network stations (correlation coefficient: jgj ! 0.75). For each horizontal component (H and D) we examined the magnetic local time (MLT) dependence of the delay time of high-latitude Pi 2 oscillations that corresponds to the highest correlation with the low-latitude Pi 2 oscillation. We found the delay time of the high-latitude H showed remarkable MLT dependence, especially in the premidnight sector: we found that in the premidnight sector the high-latitude H oscillation tends to delay from the low-latitude oscillation (<100 s). On the other hand, the delay time of the high-latitude D oscillation was not significant ($±10 s) in the entire nighttime sector. We propose a Pi 2 propagation model to explain the observed delay time of high-correlation highlatitude H. The model quantitatively explains the trend of the event distribution. We also examined the spatial distribution of high-correlation Pi 2 events relative to the center of auroral breakups. It was found that the high-correlation Pi 2 events tend to occur away from the center of auroral breakups by more than 1.5 MLT. The present result suggests that the high-correlation H component Pi 2 oscillations at high latitude are a manifestation of forced Alfvén waves excited by fast magnetosonic waves.

AKR modulation and global Pi2 oscillation

“…The shift values were the delay time for the maximum correlation coefficient between GAM H and each station's H. Correlation coefficients were calculated for the interval from 10:18:00 to 10:30:00 UT. The correlation coefficients for each station were TIK: 0.90, CHD: 0.81, ZYK: 0.92, MGD: 0.93, respectively (i.e., high-correlation Pi2 [Uozumi et al, 2009]). Figure 5 clearly shows that after the phase shift completed, the relative phase differences kept among stations until the end of the Pi2 event.…”

“…On the other hand, they found that in the premidnight sector the highlatitude H oscillation tended to delay from the low-latitude H oscillation (DT < 100 s) despite they were highly correlated each other. To explain the delay time, Uozumi et al [2009] developed a Pi2 propagation model in the magnetosphere, and confirmed that observed delay times were basically explained by the model estimation.…”

“…Olson [1999] and Yumoto et al [2001] stated that a compressional pulse (fast mode wave) causes field line resonances and surface waves at the plasmapause as well as cavity waves in the inner magnetosphere. Low-latitude Pi2 are recognized as a cavity oscillation [e.g., Yumoto, 1990;Takahashi et al, 2003] and are directly driven process by earthward propagating compressional pulses from the magnetotail [e.g., Kepko et al, 2001;Uozumi et al, 2009]. On the other hand, oscillations of substorm currents are detected across the nightside of the Earth at onset as the midlatitude and highlatitude portion of Pi2.…”

“…In order to clarify which Pi2 mode(s) corresponded to AKR modulation, nightside Pi2 signatures of this event, which were observed away from the WTS, have been interpreted in detail by adopting the aspect of Lester et al [1983Lester et al [ , 1984 and Uozumi et al [2009]. Then we compared the Pi2 signatures with AKR modulation.…”

“…Thus many of past studies investigated each Pi2 component. Uozumi et al [2009] made a comparative study between high-and low-latitude Pi2. They demonstrated that some Pi2 oscillations, which were observed simultaneously in high-and low-latitude regions, were highly correlated each other (high-correlation Pi2, correlation coefficient: |g| > 0.75).…”