赤道電離圏電場イブニング
エンハンスメントの経度変化

*丸山 隆[1]

通信総合研究所[1]

Longitudinal variation of evening enhancement of equatorial ionospheric electric field

*Takashi Maruyama[1]
Communications Research Laboratory[1]

Topside electron density distribution based on the satellite sounding data is analyzed in connection with the longitudinal changes in the evening enhancement of the equatorial zonal electric field. Clear connection between the magnetic declination angle and the strength of the evening enhancement is found.

There is a great concern in the evening enhancement of the zonal electric field at the F region height of equatorial ionosphere that causes equatorial plasma bubbles. The basic mechanism of the evening enhancement is known to be the combined effect of the F-region dynamo action and abrupt decrease in the E-region electron density, and hence conductivities, near sunset terminator. However, detailed mechanism of the polarization and current system remain unresolved. Further more, the large day-to-day variability of the enhancement strength, seasonal and longitudinal changes are not well explained. As inferred from the basic mechanism, the angle between the magnetic meridian, in which the electric fields are mapped along the field line, and the sunset terminator, which determines the E-region condition, is expected to be connected with the strength of the electric field.
Recently, old topside sounding data are reprocessed and global distribution map of electron density at the satellite height (about 1100 km) are depicted. One of the distinct ionospheric effects of the zonal electric field is the formation of the equatorial anomaly consisting of equatorial bite out of foF2 and two crests at the off-equatorial latitudes in the both hemispheres. A counter part to this in the topside ionosphere is the density enhancement above the magnetic equator. Thus the electron density at the height of 1100 km gives a measure of eastward electric field.
During the northern winter season, electron density enhancement at the satellite height and bite out of foF2 corresponding to the evening enhancement of electric field are revealed at the longitudes with a large magnetic declination angle over the Atlantic Ocean, where the sunset line and magnetic meridian tend to be parallel during northern winter season. A qualitative agreement is found between the satellite observations and ground-based ionosonde observations at Fortaleza, Brazil and Huancayo, Peru. Further signature of topside electron density morphology during evening hours will be discussed.