N.K. Sethi

Performance of IRI model predictions of F-region for Indian latitudes

Observed f0F2 data from ionosonde measurements for three low latitude Indian stations namely Delhi (28.6°N, 77.2°E), Ahmedabad (23.0°N, 72.6°E) and Kodaikanal (10.2°N, 77.5°E) are analysed and compared to the IRI-90. The analysis covers all types of solar activity namely periods of high (R12 = 160), low (R12=15) and medium (R12 = 50) solar-activity for summer and winter. Discrepancies could be attributed to a longitude dependent shift in the latitudinal position of the equatorial anomaly crest towars higher latitudes that occurs with increaseing solar-activity.. This feature is not satisfactorily represented in any of the global options of the IRI.

Comparative study of TEC with IRI model for solar minimum period at low latitude

Measurements of electron density profiles carried out at the low latitude station Arecibo, with the incoherent scatter radar, are used to derive total electron content (TEC). Comparisons are made with the IRI-95 model. Some discrepancies are found between the observed and the IRI predicted TEC and these discrepancies are maximum in equinox and summer months and minimum in winter months. However, for winter, a fair agreement is found between the IRI and the Arecibo data. We attribute these discrepancies to the profile shapes in the IRI model.

Dependence of F2- peak height on solar activity: A study with incoherent scatter measurements

Abstract We have used about 16000 high resolution electron density profiles from Arecibo (18.4 N, 66.7 W, dip 50°) incoherent scatter radar for the years 1974–1977 and 1989–1990 to study the control of solar activity on the F2 peak height (hmF2). Although the general trend of diurnal variations in hmF2 are nearly identical (i.e. maximum around midnight and minimum around sunrise) during low as well as high solar activity, the median hmF2 increases by about 100 km from solar minimum to solar maximum. Further, there is a considerable day-to-day variability in hmF2. Comparison of measured hmF2 with the IRI (derived from numerical map of M(3000)F2) reveals that IRI generally overestimates this parameter during low solar activity and underestimates during high solar activity.

Comparison of hmF2 and midday bottomside electron density profile obtained from IRI and incoherent scatter measurements

Abstract About 2000 electron density height profiles observed with the Arecibo (magn.dip 50°) incoherent scatter radar are used to compare the diurnal and seasonal variations of hmF2 with the IRI-90. A good agreement is seen for the median values. The midday bottomside profiles normalized to the peak density are compared with IRI-90 for each season and it is found that IRI overestimates the electron density distribution in the region 100 km below the F2 peak for equinox and summer but matches well during winter.

Information about the E-region valley from incoherent scatter measurements

Abstract The valley between the E/F-regions was studied with electron density — height profiles from the Arecibo Incoherent Scatter Radar. Our analyses indicate that while the valley is rather small during the day it is very deep and wide during the night. No significant changes in the valley parameters were seen during the day but large changes occurred during the night. Intermediate layers also appear between the E and F regions during the night.

Variability of the F-region parameter h0.5

Abstract High resolution electron density profiles, measured with the Arecibo incoherent scatter radar have been used to derive h0.5, the height of the half density point. More than 2400 profiles, for the period August 1974 – May 1977, have been used to study the behaviour of this parameter. Our analysis shows that during the night, h0.5 varies mostly between 200 and 350 km, but during the day the excursion is larger and values lower than 150 km are also seen. These lower values are often coincident with the presence of a layer (F1) between h0.5 and hmF2. We have therefore, divided the daytime data into two classes, class ‘A’ where h0.5 is more than hmF1 and class ‘B’ where h0.5 is less than hmF1. We find that in both the classes, most of the variability in h0.5 is due to the variability in hmF2 and a linear relationship between h0.5 and hmF2 is seen, although the dispersion in class ‘B’ is larger. Within the same class, no difference is seen in the relationship between day and night. We also compare the parameter Y0.5 hmF2 obtained from the Arecibo measurements with that calculated from the IRI-90 model, based upon Gulyaevas (1987) formula. In class ‘A’ the median values of this parameter vary between 0.15 and 2.0 with no major seasonal differences. The IRI values, however, are significantly larger. On the other hand, class ‘B’ shows larger seasonal differences, particularly between summer and winter, but the discrepancy with the IRI is somewhat smaller.

Incoherent-scatter measurements of E-F valley and comparisons with theoretical and empirical models

High resolution electron density measurements from Arecibo incoherent scatter (i.s.) radar are used to make a detailed study of the E-F valley. Features of the important valley parameters like height, width and depth are examined. These features are then compared with the available theoretical and empirical models. The depth of the valley obtained from the empirical models agrees with i.s. measurements for near-noon periods, but disagrees with these measurements for pre-noon and post-noon periods. Further, the i.s. measurements indicate that E-F valley is rather small during daytime as compared to models which give larger width. During the night, the valley is quite wide and deep but the presence of sporadic-E (Es) contaminates the Ne-h profiles observed with the i.s. radar. As a result the valley parameters cannot be determined unambiguously during the night.

Equivalent slab thickness and its variability: a study with incoherent scatter measurements

Abstract In order to check its usefulness for the IRI, incoherent scatter (I.S.) radar measurements at Arecibo (18.4° N, 66.7° W), for the solar minimum period of 1974–1977 are used to study equivalent slab thickness (EST) and its variability. While systematic diurnal changes are apparent, day-to-day and hour-to-hour fluctuations can also not be overlooked. This variability is examined in relation to the simultaneously measured F-layer peak height and electron temperature. While during nighttime, EST shows dependence upon the F-layer peak height (hmF2), it shows by day a dependence on electron temperature.

Empirical models of parameters B0, B1 from Arecibo radar measurements

Abstract High resolution electron density profiles (Ne), measured with the Arecibo incoherent scatter radar (I.S.), are used to obtain the “best fit” bottomside parameters B0 and B1 by comparing the observed profiles with the IRI bottomside profile function. For each profile, the comparison is done for all heights between hmF2 and h0.24 - the altitude where density falls off to quarter of the peak value. However, when a F1 ledge is present, the comparison is limited to altitudes between hmF2 and hmF1. A large ‘day-to-day’ and hour-to-hour variability is seen in both B0 and B1. Median values of these parameters are used to generate empirical models. Comparisons are also made with the IRI model.

Bottomside parameters B0,B1 from Arecibo incoherent scatter radar measurements

Abstract About 1500 electron density (Ne) profiles observed with the Arecibo incoherent scatter (I.S.) radar have been used to obtain the “best” B0 and B1 parameters by fitting the observed profiles to the IRI bottomside profile function. Seasonal and diurnal variations of these parameters are obtained. The relative difference between the “best fit” profiles and the observed I.S profiles is also studied. This difference is rather small during the night for all the seasons, but is very large during the day, especially during summer and equinox seasons, when an F1 layer is present. About 70 % of the profiles show rather good agreement.