The bottomside parameters B0, B1 obtained from incoherent scatter measurements during a solar maximum and their comparisons with the IRI-2001 model

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1 The bottomside parameters B, B obtained from incoherent scatter measurements during a solar maximum and their comparisons with the - model N. K. Sethi, K. K. Mahajan To cite this version: N. K. Sethi, K. K. Mahajan. The bottomside parameters B, B obtained from incoherent scatter measurements during a solar maximum and their comparisons with the - model. Annales Geophysicae, European Geosciences Union,, (6), pp <hal-75> HAL Id: hal-75 Submitted on Jan HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 Annales Geophysicae () : 87 8 c European Geophysical Society Annales Geophysicae The bottomside parameters B, B obtained from incoherent scatter measurements during a solar maximum and their comparisons with the - model N. K. Sethi and K. K. Mahajan Radio and Atmospheric Sciences Division, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi, India Received: 6 September Revised: January Accepted: February Abstract. High resolution electron density profiles (Ne) measured with the Arecibo (8. N, 66.7 W), Incoherent Scatter radar (I.S.) are used to obtain the bottomside shape parameters B, B for a solar maximum period (989 9). Median values of these parameters are compared with those obtained from the - model. It is observed that during summer, the values agree fairly well with the Arecibo values, though the numbers are somewhat larger during the daytime. Discrepancies occur during winter and equinox, when the underestimates B for the local times from about : LT to about : LT. Furthermore, the model tends to generally overestimate B at all local times. At Arecibo, B increases by about 5%, and B decreases by about % from solar minimum to solar maximum. Key words. Ionosphere (equational ionosphere; modeling and forecasting) Introduction The International Reference Ionosphere () is a global empirical model which specifies the monthly average of the electron density, electron temperature, ion temperature and ion composition. Over the years, testing and modification of has continued with extensive participation by a joint working group of the International Union of Radio Science (URSI) and the Committee on Space Research (COSPAR). These efforts have led to improvements through several versions (-78, -8, -86, -9, -95, -). Electron density distribution in the model below the F peak is described by the analytic function (Ramakrishnan and Rawer, 97): N(h) = NmF exp( x B )/ cos h(x), () with x = (hmf h)/b. () B is the bottomside thickness parameter and B determines the profile shape describing the bottomside electron Correspondence to: N. K. Sethi (nksethi@csnpl.ren.nic.in) density distribution below the F peak. In the widely used -9 (Bilitza, 99), there were two options for providing B. The old option was the standard model which made use of a table of values of B deduced from profile inversion of ionograms at mid-latitude stations. The second option (which was considered the better choice, especially at low-latitudes), used Gulyaeva s (987) model for B. This was based on half density height h.5 (the height below the F peak, where the density fell off to half the peak value: N(h.5) =.5 NmF). This B was given as B = (hmf h.5)/c, () where C was a function of B. Since the -9 assumed a constant value of for B most of the time, the parameter C correspondingly was assigned a value of (Bilitza, 99). In earlier studies, Mahajan et al. (995), using Arecibo measurements (a lowlatitude station) during solar minimum period (97 977) showed that B values based on Gulyaeva s 987 model were significantly larger, especially during daytime, for summer and equinox seasons, than those obtained from the measurements. Later, Pandey and Sethi (996), using I.S. measurements during daytime from Arecibo, for the solar minimum period (97 977), showed that the -9 values of electron densities below the F peak at all the heights were larger during summer and equinox months. A comparison of Ne profiles derived from ionogram at low-latitudes during solar minimum and maximum periods, with those obtained from -9, also showed similar results (De Gonzalez 996; Aggarwal et al., 996). Gulyaeva et al. (996) compared the ratio h.5/h mf using Arecibo Ne profiles during summer, with those obtained from the -9 model. Their analysis showed that the -9 underestimated the ratios at all local times in comparison to those obtained from the observations. In view of these and other discrepancies, the Task Force in 995 (Bilitza, 996) decided to establish B, B parameters directly from the measured profiles below the F peak down to. N mf. Following this, efforts were made by several workers to establish a set of B, B parameters, us-

3 88 N. K. Sethi and K. K. Mahajan: The bottomside parameters B, B obtained from incoherent scatter measurements ARECIBO, June 989 ARECIBO, SUMMER, Height / Km Obs. Cal LT : hrs B / Km 5 Height / Km E+5 E+6 E+7 Nm / (cm-) ARECIBO, June 989 Obs. Cal. LT : hrs E+5 E+6 E+7 Nm / (cm-) Fig.. Sample profiles obtained using best B, B parameters derived from I.S. measurements at Arecibo for daytime, midnight. Calculated profiles are shown by solid lines. The model profile is shown as dash line. ing data from various stations (Huang and Reinisch, 996; Adeniyi and Radicella 998; Sethi et al., ; Mahajan and Sethi, ). Their results showed that B varied between and 9 with higher values by night and lower ones by day, while in the -9, B was kept at a constant value of for most of the cases. These workers also reported that B showed a strong diurnal variation with higher values around noon. In view of the discrepancies between the -9 model and the ionosonde, as well as the radar measurements, the model has since been revised and named - (Bilitza, ). The B, B values in the model are primarily based upon the ionosonde data base, generated through the ICTP efforts (Bilitza et al., ) and provide solar activity, as well as seasonal variations. We have recently acquired incoherent scatter Ne h data for the years (a period of high solar activity) from the Arecibo observatory (Qihou Zhou,, private communication). In this paper, we compute the B, B parameters for these data and compare these values with those obtained during the solar minimum by Sethi et al. (), Mahajan and Sethi (), and with the -. B / Km B / Km 5 ARECIBO, WINTER, ARECIBO, EQUINOX, Fig.. A scatter plot showing diurnal variation of the parameter B derived from the I.S. measurements at Arecibo for summer, winter and equinox. Experimental data The electron density profiles used in the present study, as mentioned earlier, have been taken from the Arecibo observatory, Puerto Rico, USA. The effective height resolution is.6 km, which was achieved by using s µs pulse length. The time resolution is about min. The power profile is converted into an electron density profile by using a simultaneously measured electron to ion temperature ratio profile. The profile is finally calibrated by reading NmF from an on-site ionosonde. We have employed the data for a period of solar maximum from March 989 to January 99, containing

4 N. K. Sethi and K. K. Mahajan: The bottomside parameters B, B obtained from incoherent scatter measurements 89 B / Km B / Km B / Km ARECIBO, SUMMER, 989 ARECIBO, WINTER, ARECIBO, EQUINOX, Fig.. Variation of median values of B observed at Arecibo and those obtained from the model for summer, winter and equinox. The median values are shown as a solid line and the model values are shown as a dashed line. around 7 N e profiles. Analysis and results The technique for obtaining B, B from the observed profiles is demonstrated in Figs. a and b. These figures show some sample Ne profiles truncated at. NmF below the F peak. The profile function, described by Eq. (), has been fitted to these profiles and the best B, B parameters have been obtained. The best fit profiles are shown as solid lines and the profiles are shown by dashed lines. Since the - has B, B models for the three seasons, summer, winter and equinox, we also have grouped the N(e) profiles into these three seasons. For summer, we have 675 profiles, for winter, 79 profiles and for equinox, 5787 profiles. These profiles, as stated earlier, were truncated at. NmF and were then fitted to the profile function, to find optimum B, B parameters. Median values for each of these parameters were calculated for -min bins of local time. The B, B values predicted by - (Bilitza, ) were obtained for RZ = 5, day 5 of June, March and December, as representative of summer, equinox and winter, respectively. Figures a c show mass plots of B against local time for summer, winter and equinox seasons. It is observed that for the same local time and in the same season, B shows a large scatter, and varies by as much as a factor of during all the seasons. In Figs. a c, we compare the plots of median B obtained from the incoherent scatter measurements and the model for summer, winter and equinox, respectively. It can be seen from Fig. a that during summer, the median value of B varies from around 9 km during night to about 5 km in the pre-noon hours, and thereafter, it falls gradually to about 8 km around midnight. However, it shows a good agreement with the predicted values. The percentage difference between the two is less than 5%. During the winter and equinox seasons, as can be seen from Figs. b and c, the median value of B increases from about 9 km in the morning (7: LT) to about 5 km in the afternoon (5: 6: LT) and thereafter, it falls to about km around midnight. B shows an increasing trend from : to : LT with values varying from 75 km to 75 km (during winter) and 8 to km (during equinox). In this time period, generally underestimates B. It may also be noted that the predicted values of B do not show any local time change during winter. The percentage difference between the predicted and the median values varies from about 5 to 65% during winter and from about 5 to 5% during equinox, for the local times between : to : LT. The difference between the two, outside this time period, remains at less than % for both the seasons. Diurnal variations of mass plots of the parameter B are shown in Figs. a c for summer, winter and equinox, respectively. It can be seen that B shows a large dispersion during the night hours, with values varying between and 6. The median values and the predicted values of B are shown in Figs. 5a c. The median values vary between.5 and, in general. The values exhibit more or less a similar diurnal trend, but with somewhat higher values during all the seasons and at all local times. The thickness and shape parameters are expected to show solar activity variations, as seen in - (Bilitza, ). We have, therefore, compared in Figs. 6a d the median values of these parameters for the solar maximum (989 99) with those obtained earlier by us (Mahajan and Sethi, ) for solar minimum (97 977), along with the predicted values. For low solar activity, the predicted B, B (Bilitza, ) values were obtained for RZ = 5, day 5

5 8 N. K. Sethi and K. K. Mahajan: The bottomside parameters B, B obtained from incoherent scatter measurements B B B ARECIBO, SUMMER, 989 ARECIBO, WINTER, ARECIBO, EQUINOX, B B B ARECIBO, SUMMER, 989 ARECIBO, WINTER, ARECIBO, EQUINOX, 989 Fig.. A scatter plot showing diurnal variation of the parameter B derived from the I.S. measurements at Arecibo for summer, winter and equinox. Fig. 5. Variation of median values of B observed at Arecibo and those obtained from the model for summer, winter and equinox. The median values are shown as a solid line and the model values are shown as a dashed line. of June, March and December, as representative of summer, equinox and winter, respectively. The B, B values thus obtained for low and high solar activity periods are averaged over three months to represent all seasons. It can be seen from Figs. 6a and b that the parameter B increases by more than 5% from low solar activity to high solar activity, while the predicted values show an increase by about % during daytime and by about % during the night. As seen from the Figs. 6c and d, the parameter B decreases by about % from low solar activity to high solar activity. However, the does not exhibit any solar activity dependence. Conclusions Comparison of the - predicted B parameter with those observed at Arecibo show a fair agreement during summer for most of the local times. A larger difference between the two occurs during winter and equinox seasons, particularly between : to : LT, when the underesti-

6 N. K. Sethi and K. K. Mahajan: The bottomside parameters B, B obtained from incoherent scatter measurements 8 5 LSA (97-77) 5 HSA (989-9) B / Km B / Km LSA (97-77) (d) HSA (989-9) B B Fig. 6. Plots showing the median values of B, B parameters obtained from the I.S. measurements, along with the model during low and high solar activity periods. The median values are shown as solid lines and the model values are shown as dashed lines. Low and high solar activity values of B are shown in and and of B in and (d), respectively. mates B. During winter, the does not show any local time change for the parameter B. For other local times, the discrepancies are the least. For B, the model and the observations show a similar diurnal trend, with showing somewhat higher values for all the seasons and for all local times. In contrast to our observations, does not exhibit any solar activity dependence for B. The parameters B, B obtained from measurements can be used as input to the model, in order to achieve a more accurate shape of the electron density profile in the F-region below the peak. Acknowledgement. The Arecibo observatory is one of the major facility of the National Astronomy and Ionosphere Center, operated by Cornell University under a cooperative agreement with the National Science Foundation. We are thankful to Qihou Zhou, Arecibo Observatory, HC Box 5995 Arecibo, PR 6, Puerto Rico, USA for supplying us data. K. K. Mahajan is thankful to Council of Scientific and Industrial Research (CSIR) for the award of Emeritus Scientist position during the progress of this work. Topical Editor M. Lester thanks two referees for their help in evaluating this paper.

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