Annual Report 1996-1997

of Division II,

Ionospheric and Magnetospheric Environment
Last modified: Tue Jun 10 14:52:50 1997

Chapter I
Waves in the Magnetosphere and Ionosphere

(1) ULF Waves in the Magnetosphere

We have been analyzing magnetometer data from the AMPTE CCE satellite (L=2-7) and the Kakioka ground station (L=1.2) to determine the mode structure of ULF waves in the inner magnetosphere. In this analysis the ground data are used as a reference for determining the radial phase structure of ULF waves observed by the satellite. A previous study which used the ETS-VI satellite found few cases of high ground-satellite coherence at L > 6. The present study, in contrast, found many high-coherence wave events in L < 6. High coherence is usually found between the compressional component in the magnetosphere and the H component on the ground, which leads us to argue that fast mode waves are the cause of ULF waves observed on the ground. Currently, we are investigating the radial mode structure of the compressional oscillations to test the theoretical prediction that cavity mode resonances exist in the plasmasphere.

We are investigating properties of magnetospheric ULF waves (Pc3 and Pi2 pulsations) by combining observations from satellites and ground-based observing sites including those along the 210 degree geomagnetic meridian. We have found that Pc3 pulsations exhibit an amplitude peak at L=1 and L=2. The former peak is accompanied by a rapid spatial phase variation, which is suspected to arise from an ionospheric conductivity effect. The latter, by contrast, does not accompany a phase variation, being inconsistent with the conventional field resonance theory. As for Pi2, Akebono provided an interesting case in which the Poynting flux associated with the pulsation was directed along the ambient magnetic field toward the ionosphere closer to the satellite. Energy flow is very large so that the Pi2 pulsation can be quickly damped if such Poynting flux exists over a wide range of L. This result poses a new question: What drives Pi2 pulsation for several oscillation periods?


Fig. 1.  Simultaneous observation of dayside Pc 3 waves from the GEOTAIL satellite and a ground magnetometer array. The L values of the satellite (GTL) and the ground stations are shown on the right-hand side. The field magnitude at the satellite and the H component on the ground are used to represent Pc3 activity. Similar wave packets are observed at different locations, but the phase and amplitude of Pc3 vary with L.

(2) Coordinated Ground-Based Network Observations

As one of the Solar Terrestrial Energy Program (STEP: 1990-1997), ground-based network observations of magnetic field and aurora along the meridians around 190, 210, and 250 degree geomagnetic longitudes are being conducted in collaboration with, and with support from, twenty-eight institutes and organizations in Australia, Indonesia, Japan, Palau, Papua New Guinea, Philippines, Russia, Taiwan, and U.S.A. Identical 26 fluxgate magnetometers and all-sky TV cameras are in operation. The magnetic field data of one-min average are available to the scientific community through data-network linked to the STE Laboratory database consisting of a UNIX workstation. One-second data are also available upon request. Summary plots for each day from May 1990 can be seen on WWW. We have received 30-40 data requests per year via e-mail since 1992.

(3) Low-Latitude ELF-VLF Emissions

Sudden impulse (SI) that occurred at 0606 UT on December 24, 1995 was observed at stations along the 210 degree magnetic meridian. In association with this SI, energetic particle precipitation and VLF waves were observed at high-latitude station, and strong low-latitude VLF emissions were detected at Moshiri Observatory (L=1.6), an evidence indicating that the SI energy was transferred into the low-latitude magnetosphere.


Fig. 2.  VLF emissions observed at Moshiri simultaneously with SI geomagnetic variation.

HTML created by Stephan Buchert