A statistical study on the occurrence characteristics of low-to-midlatitude ionospheric density enhancements (plasma blobs)

A statistical study on the occurrence characteristics of global low-to-midlatitude topside ionospheric density enhancements (plasma blobs) has been carried out using the ROCSAT-1 (Republic of China Satellite-1) data during high solar activity period of 1999 to 2004. The latitudinal distribution of plasma blobs indicates that the occurrence rates increase from the dip latitude ±15° to midlatitudes that appear to complement the latitudinal distribution of equatorial plasma bubbles (EPBs). For the seasonal distribution, it is found that the maximum occurrence rate appears during the June solstice in both northern and southern hemispheres. The local-time distribution indicates a maximum occurrence rate around midnight, and maintains a high occurrence rate until about 04 h in local time in contrast to a rapid decrease after midnight for the EPB occurrences. Negative correlation with the solar activity is found for the occurrence of plasma blobs that is opposite to the positive correlation with the solar activity for the EPB occurrences. All these different and contrasting occurrence characteristics between the low-to-midlatitude plasma blob occurrence and the EPBs imply that the occurrence mechanism for the low-to-midlatitude plasma blobs should have little relationship to that of EPBs. From the fact that an upward/outward plasma drift and the increase of O+ species are observed inside the plasma blob, an eastward polarization electric field should directly appear at topside ionosphere for the occurrence of plasma blobs. This polarization electric field should be mapped from the nighttime midlatitude Es layer instability process where the polarization electric field is generated.