Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsm21a1661i&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SM21A-1661
Physics
2407 Auroral Ionosphere (2704), 2411 Electric Fields (2712), 2712 Electric Fields (2411), 2790 Substorms, 6929 Ionospheric Physics (1240, 2400)
Scientific paper
At the onset of magnetospheric substorms, impulsive hydromagnetic oscillations occur globally in the magnetosphere with a period range from 40 to 150 seconds [e.g. Saito, 1968]. They are called Pi 2 magnetic pulsations. Pi 2 has been studied with arrays of magnetometers on the ground and with in-situ observation by satellites [e.g. Yumoto, 2001]. However characteristics of Pi 2 electric pulsations in the low-latitude ionosphere have not been clearly identified yet. We have focused on measuring the Pi 2 electric pulsations by an FM-CW (Frequency Modulated Continuous Wave) radar and clarify their characteristics. In order to detect the ionospheric electric fields, we built a FM-CW (HF) radar at Sasaguri (Magnetic Latitude: 23.2 degree, Magnetic Longitude: 199.6 degree), Fukuoka, Japan in 2002. The radar provides the Doppler shift of launched wave frequencies, which corresponds to the height variation of the ionosphere, with a high- time resolution of 3 sec. When the eastward (westward) electric field penetrates into the low-latitude ionosphere, it drifts upward (downward) through the ExB drift. Thus, using the FM-CW radar we can measure east-west electric fields (Ey) in the ionosphere [see Ikeda et al., 2008]. In this study, we also used geomagnetic field data BH at Kujyu (KUJ; M. Lat. 23.6 degree, M. Lon. 203.2 degree), a part of Circum-pan Pacific Magnetic Network (CPMN) stations [cf., Yumoto et al., 2001]. The first Pi 2 event was observed at KUJ and Sasaguri simultaneously at 1332 UT (2232 LT) on 6 November, 2003. Also positive bay was detected in the H-component (BH) at KUJ. The dominant frequencies of the electric field Ey and magnetic field BH were identical. From a cross-correlation analysis between the BH at KUJ and Ey at Sasaguri, we found that the correlation coefficient is 0.80 and phase delay is about -100 degree. Takahashi et al. (JGR, 2001) showed the expected phase relation between magnetic field of cavity- mode Pi 2 and associated electric field. Based on their result, we suggest the phase delay of -100 degree indicates that the first Pi 2 event shows a radial mode structure of cavity-mode Pi 2. The second Pi 2 event was observed at 1415 UT (2315 LT) on the same day 6 November, 2003. The dominant frequency of Ey and BH were the same and phase delay is -148 or 39 degree. Since the phase delay is almost -180 or 0 degree, this Pi 2 event can be described by as a propagating mode. Further we analyzed 26 Pi 2 events that were recorded by a CPMN magnetometer at KUJ or Kagoshima (KAG; Magnetic Latitude: 21.9 degree, Magnetic Longitude: 203.2 degree) and the FM-CW radar simultaneously within a period from Nov. 2003 to Apr. 2004. The criteria of the analyzed Pi 2 events are that Pi 2 magnetic amplitude range of the BH is more than 1 nT at KUJ or KAG. In 12 of all 26 Pi 2 events, the dominant frequency of Ey and BH are identical. Moreover we examined the phase delay of 12 Pi 2 events, and found that the only 3 events in midnight sector show the radial mode structure of cavity-mode Pi 2. Other 9 events show a propagating mode of Pi 2. The observed low-latitude Pi 2s are concluded not to be described as a simple standing or propagating mode.
Ikeda Atsushi
Nozaki Kazuhiro
Shinohara Manabu
Uozumi Teiji
Yoshikawa Akira
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