Report on the Activity of Commission G

(15 November 2004)

 

1. Studies on topside ionosphere

In the evaluation of radio propagation delays through the ionosphere when the satellite elevation angle is low, so-called thin layer model of the ionosphere leads a large error depending on the azimuth angle. Thus we need information on the vertical distribution of the ionosphere. Topside electron density modeling is going on by comparing GPS derived total electron content (TEC) and ionospheric critical frequency (foF2) by National Institute of Information and Communications Technology (NICT) and Institute of Space and Aeronautical Science (ISAS)/JAXA groups. Topside electron density distribution shape is estimated by using non-diffusive equilibrium distribution model, i.e., the combination of the diffusive equilibrium and flux solution terms.

To verify an additional ionization layer predicted in the equatorial ionosphere, topside ionograms obtained by the Planetary Plasma Sounder (PPS) system on-board the Ohzora (EXOS-C) satellite were analyzed by the Tohoku University group. Based on the analysis of the PPS data of 8 passes in March and 11 passes in May, 1987, the ionization ledge observed in the local noon time period shows nature theoretically predicted for the F3 layer by Balan and Bailey (1995); namely, the ionization ledge has a tendency to rise with the upward velocity of about 32 m/s in this local time period. It was noted that some peaks of the ledge structure were located on the field line of higher latitude region than the field line of the equatorial anomaly crests. Also in their study, the occurrence probability of the ionization ledge shows a tendency contrary to that of the F3 layer reported by Balan et al. (2000). This difference may be due to the lack of data number. An extended statistical study is needed to clarify the seasonal dependence of the occurrence probability in the future studies.

 

2. Observation Campaigns

Geomagnetic-conjugate experiment of equatorial ionosphere over Indonesia-Thailand region

There are two research projects for studies of equatorial ionosphere in Southeast Asia. One is “Southeast Asia Equatorial Ionospheric Network (SEALION)” leaded by NICT. SEALION consists of three FM/CW ionosondes located along the longitude of 100 degrees East; two of them are at the magnetic conjugate points and one is near the magnetic equator. Another research project is “Coupling Processes in the Equatorial Atmosphere (CPEA)” that studies dynamical coupling processes in the equatorial atmosphere from the troposphere to the ionosphere. Research Institute of Sustainable Humanosphere (RISH), Kyoto University and Solar-Terrestrial Environmental Laboratory (STEL), Nagoya University jointly participate ionospheric studies under the CPEA. In October 2004, SEALION and CPEA conducted collaborative observation campaign with all of their instruments. Echoes from ionospheric irregularities associated with plasma bubbles were observed by the Equatorial Atmosphere Radar (EAR), while meridional structure of the ionosphere was observed by SEALION. In the EAR site CPEA team operated an airglow imager, GPS scintillation receivers, a magnetometer, etc. This is a very unique observation network for the study of the equatorial atmosphere. We anticipate revealing dynamical processes of the equatorial ionosphere and generation mechanisms of the plasma bubbles.

 

Rocket Experiment of momentum transfer between neutral atmosphere and ionosphere

A rocket experiment to observe F-region ionosphere is planned and proposed to ISAS/JAXA to be conducted in summer 2006 with an S-520 sounding rocket. The PI of the experiment is Prof. Ono, Tohoku University, and is joined by researchers from Hokkaido University, Tohoku University, Toyama Prefectural University, Nagoya University, Kyoto University, and NICT. Indian and Canadian researchers join the project, too. The S-520 rocket will be equipped with in-situ instruments to measure plasma parameters, i.e., plasma density, electric field, electron temperature, etc. A unique part of the rocket experiment is the Lithium (Li) release experiment in the ionosphere, which will measure neutral atmospheric winds. This experiment will observe both ionospheric plasma and neutral atmosphere, which will reveal momentum transfer between the neutral atmosphere and ionosphere. Ground-based observations are also planned in association with the rocket experiment. Dr. Yamamoto, Kyoto University is planning to operate a portable radar for observation of E-region irregularities and meteors. The radar experiment adds simultaneous data of electric field and neutral winds in the lower ionosphere, and would help study of ionospheric E- and F-region coupling processes.

 

Geomagnetic Conjugate Observations of Medium-Scale Traveling Ionospheric Disturbance (MSTID)

Prof. Ogawa and his research group in STEL, Nagoya University conducted geomagnetic conjugate observations of 630-nm airglow at Sata, Japan, and Darwin, Australia, with two all-sky CCD imagers. The observations were conducted in association with the CPEA project that studies coupling processes in the equatorial atmosphere. In their experiment, airglow perturbations caused by MSTIDs were simultaneously observed at both sites near midnight of August 9, 2002. The MSTID structures were mirrored in the northern and southern hemispheres connected by the geomagnetic field lines. This result suggests that polarization electric field (E_p) plays an important role in MSTIDs set up and grow in the ionosphere. E_p maps along B and drives the F-region plasma upward and downward in the direction of EXB, causing plasma density perturbations with mirrored structures in both the hemispheres. This electromagnetic coupling process between both the hemispheres is associated with the equatorial atmosphere at very high altitude. The research group now proposes a future plan to expand the inter-hemisphere comparisons of the mid-latitude ionosphere by including neutral-wind experiment with Fabry-Perot interferometries and ionospheric irregularity experiment with VHF/HF radars at the conjugate points in Japan and Australia. The experiment would be very helpful to reveal generation processes of the polarization electric field and source of the MSTIDs.

 

Reference

Otsuka, Y., K. Shiokawa, T. Ogawa, and P. Wilkinson (2004), Geomagnetic conjugate observations of medium-scale traveling ionospheric disturbances at midlatitude using all-sky airglow imagers, Geophys. Res. Lett., 31, L15803, doi:10.1029/2004GL020262.

 

Figure 1:

(a) Map showing the locations of observational sites and coverage of the airglow imagers (radius of 500 km). One site is located at Sata (31.0N, 130.7E), Japan and the other site at Darwin (12.4S, 131.0E), Australia. The geomagnetic field line connecting Darwin and its conjugate point is shown by a black curve. Red circle shows conjugate area connected by the geomagnetic field lines to the observational coverage of the Darwin imager. (b and c) Two-dimensional maps of 630-nm airglow intensity at (b) Sata and (c) Darwin at 1502 UT (2402 LT) on August 9, 2002. The all-sky images are converted to geographical coordinates assuming the emission layer at an altitude of 250~km. Color levels in each image show percentage of the airglow intensity deviations from 1-hour average to the background.

 

3. Coming Workshops

Domesitic

(1) Space Weather/Climate Symposium (24-26 November at Nagoya University)

(2) Mesosphere Thermosphere Ionosphere Workshop (25-26 November at Nagoya University)

(3) GPS/GNSS Symposium 2004 (17-19 November at Tokyo University of Marine Science and Technology.

International

(1) International Workshop on Seismo Electromagnetics (15-17 March 2005, Chofu, Tokyo, Japan)

(2) The 11th International Symposium on Equatorial Aeronomy (ISEA-11) (9-14 May2005 in Taipei, Taiwan) http://csrsddc.csrsr.ncu.edu.tw/isea-11.html

 (Prepared by T. Maruyama and M. Yamamoto)