B.W. Joshua

Ionospheric foF2 morphology and response of F2 layer height over Jicamarca during different solar epochs and comparison with IRI-2012 model

Diurnal, seasonal and annual foF2 variability and the response of the F2-layer height over Jicamarca (11.9 °S, 76.8 °W, 1 °N dip) during periods of low (LSA), moderate (MSA) and high (HSA) solar activities was investigated. The relative standard deviation (VR) was used for the analysis. The F2-layer critical frequency pre-noon peak increases by a factor of 2 more than the post-noon peak as the solar activity increases. The variability coefficient (VR) is lowest during the day (7–16%) for the three solar epochs; increases during nighttime (20–26%, 14–26%, and 10–20%, respectively for the LSA, MSA and HSA years); and attained highest magnitude during sunrise (21–27%, 24–27%, and 19–30%, respectively in similar order). Two major peaks were observed in VR – the pre-sunrise peak, which is higher, and the post-sunset peak. Generally, the variability increases as the solar activity decreases. Annually, VR peaks within 23–24%, 19–24% and 15–24% for the LSA, MSA, and HSA periods, respectively. The ionospheric F2-layer height rises to the higher level with increasing solar activity. The foF2 comparison results revealed that Jicamarca is well represented on the IRI-2012 model, with an improvement on the URSI option. The importance of vertical plasma drift and photochemistry in the F2-layer was emphasized.

Ionospheric response to magnetic activity at low and mid-latitude stations

The F2-layer response to the moderate storm of 5–7 April 2010 was investigated using data from two equatorial stations (Ilorin: lat. 8.5°N, 4.5°E; Kwajalein: lat. 9°N, long. 167.2°E) and mid-latitude (San Vito: lat. 40.6°N, long. 17.8°E; Pruhonice: lat. 50°N, long. 14.6°E). Before storm commencement, enhancement, and depletion of NmF2 values were observed in the equatorial and mid-latitude stations, respectively, indicating the latitudinal dependence of the pre-storm event. All the stations with the exception of Kwajalein show positive phase in NmF2 response at the storm onset stage. Positive phase in NmF2 continues over Ilorin and appears on the daytime ionosphere of Kwajalein on 6 April, whereas negative phase suppressed the positive feature in Pruhonice and San Vito until the recovery condition. The differences in the response of F2-layer to the storm for the two equatorial stations were attributed to their longitudinal differences. On the average, both the AE and Dst indices revealed poor correlation relationship. More studies are required to ascertain this finding.

Assessment of IRI and IRI-Plas models over the African equatorial and low latitude region.

A reliable ionospheric specification by empirical models is important to mitigate the effects of the ionosphere on the operations of satellite based positioning and navigation systems. This study evaluates the capability of the International Reference Ionosphere (IRI) and IRI extended to the plasmasphere (IRI-Plas) models in predicting the Total Electron Content (TEC) over stations located in the Southern hemisphere of the African equatorial and low latitude region. TEC derived from Global Positioning System (GPS) measurements were compared with TEC-predicted by both the IRI-Plas 2015 model and the three topside options of the IRI 2012 model [i.e. NeQuick (NeQ), IRI 2001 corrected factor (IRI-01 Corr) and the IRI 2001(IRI-01)]. Generally, the diurnal and the seasonal structures of modeled-TEC follow quite well with the observed-TEC in all the stations, although with some upward and downward offsets observed during the daytime and nighttime. The prediction errors of both models exhibit latitudinal variation and these showed seasonal trends. The values generally decrease with increase in latitude. The TEC data-model divergence of both models is most significant at stations in the equatorial region during the daytime and nighttime. Conversely, both models demonstrate most pronounced convergence during the nighttime at stations outside the equatorial region. The IRI-Plas model, in general, performed better in months and seasons when the three options of the IRI model underestimate TEC. Factors such as the height limitation of the IRI model, the inaccurate predictions of the bottomside and topside electron density profiles were used to explain the data-model discrepancies.

Performance evaluation of GIM-TEC assimilation of the IRI-Plas model at two equatorial stations in the American sector

Empirical models of the ionosphere, such as the International Reference Ionosphere (IRI) model, play a vital role in evaluating the environmental effect on the operation of space-based communication and navigation technologies. The IRI extended to Plasmasphere (IRI-Plas) model can be adjusted with external data to update its electron density profile while still maintaining the overall integrity of the model representations. In this paper, the performance of the total electron content (TEC) assimilation option of the IRI-Plas at two equatorial stations, Jicamarca, Peru (geographic: 12°S, 77°W, dip angle 0.8°) and Cachoeira Paulista, Brazil (Geographic: 22.7°S, 45°W, dip angle �26°), is examined during quiet and disturbed conditions. TEC, F2 layer critical frequency (foF2), and peak height (hmF2) predicted when the model is operated without external input were used as a baseline in our model evaluation. Results indicate that TEC predicted by the assimilation option generally produced smaller estimation errors compared to the “no extra input” option during quiet and disturbed conditions. Generally, the error is smaller at the equatorial trough than near the crest for both quiet and disturbed days. With assimilation option, there is a substantial improvement of storm time estimations when compared with quiet time predictions. The improvement is, however, independent on storm’s severity. Furthermore, the modeled foF2 and hmF2 are generally poor with TEC assimilation, particularly the hmF2 prediction, at the two locations during both quiet and disturbed conditions. Consequently, IRI-Plas model assimilated with TEC value only may not be sufficient where more realistic instantaneous values of peak parameters are required.