Comparison of ionospheric profile parameters with IRI-2012 model over Jicamarca

We used the hourly ionogram data obtained from Jicamarca station (12° S, 76.9° W, dip latitude: 1.0° N) an equatorial region to study the variation of the electron density profile parameters: maximum height of F2-layer (hmF2), bottomside thickness (B0) and shape (B1) parameter of F-layer. The period of study is for the year 2010 (solar minimum period).The diurnal monthly averages of these parameters are compared with the updated IRI-2012 model. The results show that hmF2 is highest during the daytime than nighttime. The variation in hmF2 was observed to modulate the thickness of the bottomside F2-layer. The observed hmF2 and B0 post-sunset peak is as result of the upward drift velocity of ionospheric plasma. We found a close agreement between IRI-CCIR hmF2 model and observed hmF2 during 0000-0700 LT while outside this period the model predictions deviate significantly with the observational values. Significant discrepancies are observed between the IRI model options for B0 and the observed B0 values. Specifically, the modeled values do not show B0 post-sunset peak. A fairly good agreement was observed between the observed B1 and IRI model options (ABT-2009 and Bill 2000) for B1.


Introduction
The bottomside ionospheric electron density profile below the F-layer are described by the maximum electron density (NmF2), the maximum height of F2 layer (hmF2), the thickness (B0) and the shape parameter (B1). These parameters are important for understanding the morphology of the ionosphere and for ionospheric modeling, e.g. the International Reference Ionosphere (IRI).The illustration of these profile parameters is given in figure 1. The altitude distributions of these profile parameters exhibit diurnal, seasonal and solar activity variations, which depend on the geographical location of the observational station. The B0 and B1 parameter were introduced by the IRI model to describe the ionospheric electron density thickness and shape below the F2 region. The B0 parameter is obtained from the ionospheric profile Ne (h) and calculated as the height difference between the peak height of F2-layer (hmF2) and the height where the electron density equals to 0.24*NmF2 in the absence of Fl- layer or the F1 peak height (hmF1) if the latter occur [1]. B1 determines the shape of the profile between the two heights from which B0 parameter is estimated [2]. In the previous works, the comparative study between the observational profile parameters and IRI-2001 model at Jicamarca station (12° S, 76.9° W, dip latitude: 1.0° N) has been earlier studied during solar maximum [3] and during solar minimum conditions [4]. A comparison between the observational B-parameter (B0 and B1) and the IRI-2012 model was recently carried out in the African sector [5], where they observe a fair agreement between the model predictions and B1 parameter. However, they observe that the model prediction agrees with the observational B0 values during the nighttime period and obtained some degree of differences during the pre-sunrise towards the midday period. A similar result was obtained at the Dibrugarh (27.5° N, 95° E, 43° dip), a low mid-latitude station where they have attributed the discrepancies between the IRI-2012 model option as due to the limitation of the data composition (location) used in building the ABT-2009 model option and the auto-scaling of F1 layer in the ionogram inversion software [6]. IRI model is a data-driven model which has been undergoing periodic updates in order to improve the model prediction. For this reason, the present study, re-examine the ionospheric parameters obtained from digisonde measurements at Jicamarca station, located in the Southern American sector and make a comparison with the updated version of the IRI-2012 model during the period (the year 2010) following the deep solar minimum.

Data Analysis
The DPS-4 digisonde measurement used in this study is located at Jicamarca (12° S, 76.9°W, dip latitude: 1.0° N), an equatorial location in the Southern American sector. We have used hourly ionograms during the year 2010, a year of low solar activity with 27-day averaged solar radio flux, F10.7 = 80 sfu. The ionogram traces were inverted into "true" height electron density profiles Ne(h) by the True Height Profile Inversion program (NHPC), which is run in the Digisonde Ionogram Data Visualization/ Editing Tool (SAO-X) software. The SAO explorer program allows us to estimate the profile parameters; hmF2, B0, and B1 from the raw ionogram data. The routine techniques for calculating B0 and B1 is by best fitting using the least-square fitting approach, the experimental profiles with the formula used in IRI model in equation (1 Monthly averages of these parameters are calculated from the selected ten (10) days quietest period. The quietest period is days with no record of geomagnetic disturbances. All the selected quiet period have ƩKp ≤ 24. Largely due to the paucity of data, only the months of January to July and November to December data are analyzed for this study. The months are then grouped into seasons: equinoctial months (March and April), summer months (November to February) and winter months (May and June), respectively. The observational results are compared with the IRI-2012 model. The IRI-2012 model is accessible via http://irimodel.org. In the IRI-2012 model, the IRI-CCIR option was used to compared with observational hmF2 because CCIR-maps of hmF2 was obtained through its close correlation with propagation factor M(3000)F2 [3]. For the B0 parameter, the IRI-2012 offers three choices: ABT-2009 model option [1], Bill-2000 model option [2] and Gul-1987 model option [7]. For the B1 parameter, Bil-2000 and Gul-1987 model options used the same B1 model and thus give same values of B1 prediction. The performance of the IRI model prediction relative to the observational results was estimated by finding the deviation of the model options from the experimental (observational) values.
Where the term ∆ is the deviation of the model prediction from the observational values ( ) and indicates the model option use for various profile parameters.

Diurnal variations of hmF2, B0, and B1
For the equinoctial months, the diurnal monthly mean values of hmF2, B0, and B1 are given in the left panel in figure 2(a, d and g). In figure 2(a), hmF2 generally increases during the sunrise hour to ~1000 LT. The observed increase in hmF2 can be attributed to the plasma drift during the daytime and this occurrence can be explained as due to the cross product of electric field (E) and magnetic field (B) at the equatorial latitude. Consequently, the ExB drift raises electron density to a higher altitude where the ratio of gyrofrequency to the ion collision frequency is large. This process is generally known as Equatorial Anomaly (EIA). At midday, the increase in hmF2 ceased and the magnitude remains fairly constant. The restriction of the growth of plasma height is known as ionospheric ceiling [8]. The ionospheric ceiling is due to the isoelectron density delineations which are aligned with the magnetic field lines near the dip equator [8]. These mechanisms affect the morphology of B0 parameter as well (see figure 2d). Observe that B0 increases during sunrise towards the midday. At post sunset, around 1800-1900 LT, hmF2 increased gradually and reached a peak value at about 2000 LT for the months of March and April. The observed hmF2 post sunset peak is as a result of the upward velocity of the pre-reversal enhancement (PRE). The PRE resulting from eastward polarized electric field causing the rise of plasma to drift upward (ExB drift velocity) because of F-region dynamo mechanism when the conductivity of E-layer already decayed at post-sunset. The post-sunset peak is observable for B0 as well. The increase in the magnitude of B0 around this time indicates that PRE upward velocity not only modulates the height of F2-layer but also increases the bottomside thickness (B0) below the peak height of F2-layer. During the nighttime period towards the pre-noon around 2100 to 0700 LT, the magnitude of hmF2 decreases and this also affect the values of B0 parameter. In general, the B0 values during the daytime period are higher than the nighttime period. We observe that B1 do not respond to any of the mechanism (earlier mentioned) that causes redistribution of electron density at the bottomside F2 region. The B1 values are centered on ~2 during the periods from 0800-1800 LT and The diurnal monthly average values of the ionospheric profile parameters of hmF2 (figure 2b), B0 (figure 2e) and B1 (figure 2h) for the summer months are given in figure 2 (middle panel). Generally, the morphologies of these parameters are similar to that in the equinoctial months. We observe that post sunset peak values of hmF2 and B0 are smaller as compared to the equinoctial months. The late reversal time of PRE values during the summer month at about 2100-2200 LT is attributed to the longer persistent of the upward velocity of the lifted ionospheric plasma to higher altitude [3]. The diurnal monthly averages of the ionospheric profile parameters hmF2 (figure 2c), B0 (figure 2f) and B1 (figure 2i) for the winter months are given in the right panel of figure 2. We observe that the amplitude of hmF2 is lowest in the winter than in the other seasons. The post-sunset peak of B0 parameter diminishes and the midday values are lower when compared with other seasons. Since B0 is measured from the peak height of F2 layer to the peak height of F1 layer or to the h0.24 (height of 0.24*NmF2) when F1 does not exist, this explains why the variations in the magnitude of either hmF2 or NmF2 affect the observed values of B0. A close observation in the variation of B1parameter revealed a similar trend across the three seasons. Observe in figure 2, that the midday minimum values of B1 coincide with the period in which the thickness parameter B0 reaches its maximum value. This observation has been reported in similar findings at Cyprus (35° N, 33° E) by Panda and Haralambous [9]. can be observed during 0000-7000 LT for all the months. In term of the season, the greatest discrepancies are observed during the summer months (January, February, November, and December) and equinoctial months (March and April). For Bill-2000 option, a small deviation between the model prediction and the observational results is found between the period 0000-3000 LT for all the months. During 0400-1000LT a negative deviation was observed. Our result is consistent with a study carried out by Lee et al. [4] and with similar findings by Kalita and Bhuyan [6]. For the ABT-2009 model option, a small deviation between the modeled values and the observational results is observed during 1400 LT and 2200-2400 LT for all the months. A negative difference between the observational results and ABT-2009 model prediction is observed during the daytime between 1000-1300 LT in all the months.

Conclusion
The electron density profile parameters obtained from the digisonde sounder installed at Jicamarca station during the year 2010 were used to study the variation of hmF2, B0, and B1 during solar minimum. We observed that the magnitude of both hmF2 and B0 are highest during the equinoctial months (April and March) than the rest of the months. We observe the effect of upward PRE velocity to be responsible for the observable hmF2 and B0 post-sunset peak. The prediction made by the  IRI-CCIR hmF2 model during the pre-noon period is close to the measured hmF2 values. However, significant disagreements between the model and the observational values of hmF2 were found during 0700-1500 LT at all months. For the observed B0 compared with IRI model, all the model options give a positive and negative deviation from the observational values. The B1 parameter does not exhibit a significant seasonal variability and has small value during the daytime than during the morning and nighttime periods. The IRI model options gave a closer value with the observed B1result.