Jisheng Xu

Longitudinal development of low-latitude ionospheric irregularities during the geomagnetic storms of July 2004

[1]xa0During the period 22–28 July 2004, three geomagnetic storms occurred due to a sequence of coronal mass ejections. In this paper we present and discuss the ionospheric observations from a set of in situ satellites and ground-based GPS total electron content and scintillation receivers, a VHF radar, and two chains of ionosondes (∼300°E and ∼120°E, respectively) that provide the evolutionary characteristics of equatorial and low-latitude ionospheric irregularities versus longitude during these storm periods. It is found that the irregularities occurred over a wide longitudinal range, extending from around 300°E to 120°E on storm days 25 and 27 July 2004. On 25 July plasma bubbles (PBs) began premidnight in America and postmidnight in Southeast Asia. On 27 July the occurrence of irregularities followed the sunset terminator and was observed sequentially after sunset from American to Southeast Asian longitudes. Past studies have reported that storm-time low-latitude ionospheric irregularities are mostly confined to a narrower longitude range, <90°, after sunset hours and are associated with the prompt penetration of eastward electric fields (PPEFs) into low latitudes. In June solstice months the occurrence of range-type spread F or PBs is very low in Southeast Asian and South American sectors. In contrast, the present results indicate that geomagnetic storms triggered the wide longitudinal development of PBs. In the American sector this was probably due to the effects of PPEFs on both storm days. However, in the Southeast Asian sector the PBs on the 2 days probably arose from disturbance dynamo electric field (DDEF), PPEF, and gravity wave seeding effects. This study further shows that under complex storm conditions, besides the long duration or multiple penetrations, the combined effects of PPEFs and DDEFs could result in a wide longitude extent of ionospheric irregularities at times.

Ionospheric Correction Based on Ingestion of Global Ionospheric Maps into the NeQuick 2 Model

The global ionospheric maps (GIMs), generated by Jet Propulsion Laboratory (JPL) and Center for Orbit Determination in Europe (CODE) during a period over 13 years, have been adopted as the primary source of data to provide global ionospheric correction for possible single frequency positioning applications. The investigation aims to assess the performance of new NeQuick model, NeQuick 2, in predicting global total electron content (TEC) through ingesting the GIMs data from the previous day(s). The results show good performance of the GIMs-driven-NeQuick model with average 86% of vertical TEC error less than 10 TECU, when the global daily effective ionization indices (Az) versus modified dip latitude (MODIP) are constructed as a second order polynomial. The performance of GIMs-driven-NeQuick model presents variability with solar activity and behaves better during low solar activity years. The accuracy of TEC prediction can be improved further through performing a four-coefficient function expression of Az versus MODIP. As more measurements from earlier days are involved in the Az optimization procedure, the accuracy may decrease. The results also reveal that more efforts are needed to improve the NeQuick 2 model capabilities to represent the ionosphere in the equatorial and high-latitude regions.

On the occurrence of F region irregularities over Haikou retrieved from COSMIC GPS radio occultation and ground‐based ionospheric scintillation monitor observations

In this paper, the amplitude scintillation index (s4) derived from COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) Radio Occultation (RO) technique and ground-based Ionospheric Scintillation Monitor (ISM) at Haikou station (Geo. Lat.: 20.0°N, Geo. Lon.: 110.3°E, Mag. Lat.: 10.02°N) is used to investigate the morphology of F-region irregularities in the low latitudes of China. The RO events of tangent point within the range of 10-30°N latitude, 70-160°E longitude, and 150-500u2009km altitude are adopted to analyze the ionospheric scintillation characteristics. The percentage of ionospheric scintillation occurrence is computed to obtain its diurnal variations, seasonal trends and the dependence on solar and geomagnetic activities. Based on a statistical analysis of a long-term period dataset (year 2007 to 2013), we found that the ionospheric scintillation occurrence from both techniques show similar variations. After sunset (18 LT), the scintillation occurrence increases rapidly and reaches the maximum 3u2009hours later. Then it decreases rapidly till 04 LT and remains low level during the daytime. The ionospheric scintillation tends to occur more frequently during vernal and autumnal equinoxes, especially in March-April and September-October. The equinoctial asymmetry could be seen clearly from the ground-based ISM observations. The peak ionospheric scintillation occurrence time varies with seasons. It is reached latest in summer while in spring it is very close to that in autumn. The nighttime ionospheric scintillation occurrence tends to increase with increasing solar activities. The increasing tendency is more prominent in vernal and autumnal equinox than that in summer and winter. In general, the control of geomagnetic activities is apt to inhibit ionospheric scintillation at equinox nighttime. In summer and winter, the geomagnetic activities could either trigger or inhibit the generation of ionospheric irregularities in a much more complicated way. Thus it can be concluded that the tangent point location does accurately represent the scattering region, at least in an average sense. The RO technique is demonstrated to be a useful tool for remotely sensing the terrestrial ionosphere on a global scale down to the regional scale in terms of scintillation occurrence.