Quiet-time variations of F2-layer parameters at Jicamarca and comparison with IRI-2001 during solar minimum

We analyze Jicamarca ionograms to study the quiet-condition variations in the peak electron density (NmF2), its height (hmF2), and F2-layer thickness parameter (B0) of the equatorial F2 layer during solar minimum. The sunrise peak is found in hmF2 and B0 for all months. During daytime and nighttime, the variation in the hmF2 value is mainly responsible for that in NmF2 and B0. The sunset peaks of hmF2 and B0 exist in the equinoctial months, but not in the winter months. Moreover, the observed values of hmF2, NmF2, and B0 are generally similar to the modeled values of IRI-2001.     

Peak characteristics of F2 region over Tucumán: Predictions and measurements

Ionosonde measurements obtained at Tucumán are used to check the validity of the International Reference Ionosphere model to predict the maximum electron density of the F2 region (NmF2) and its height (hmF2) over this station. Data corresponding to different months and solar activity conditions are considered. CCIR and URSI options are used to model calculations. The results show that, generally, the predictions of hmF2 are better than those of NmF2. Disagreements between predicted and measured NmF2 values are observed and the consequences in the vertical total electron content modeling are stressed.     

Results of foF2 and Ne-h profiles at low latitude using recent digital ionosonde observations and their comparison with IRI-2000

Using digital ionosonde observations at low-latitude station, Delhi (28.6 N, 77.2 E, mag. dip 42.4 N), the diurnal and seasonal variations of the critical frequency of F2 layer (foF2) are analyzed from August 2000 to July 2001 during a high solar activity period. Also, noontime bottomside electron density (Ne-h) profiles, below the F2-peak, are derived from ionogram, using the POLAN (Report UAG-93, WDC-A, for Solar Terrestrial Physics, Boulder, Co.) program during the same period, and these profiles are then normalized to the peak height and density (hmF2, NmF2) of the F2-region. These observations are used to assess the predictability of the International Reference Ionosphere, IRI-2000 model (Radio Sc. 36(2) (2001) 261). Results show in general, a large variability, (1σ, σ is standard deviation), in foF2 during nighttime than daytime during winter and equinox, the variability of foF2 about the mean is about ±25% by night and ±15% by day. The IRI model shows a fairly good agreement with foF2 observations during daytime, however during nighttime, the discrepancies between the two exist. Comparative studies of the normalized observed profiles with those obtained with the IRI model (Bilitza, 2001) using both the options namely: Gulyaeva's (Adv. Space Res. 7 (1987) 39) model and B0-Table (Adv. Space Res. 25(1) (2000) 89), show that during all the seasons, in general, the B0-Tab option, reveals a better agreement with the observations, while the IRI model using Gulyaeva's option, overestimates the electron density distribution during summer and equinox, however, during winter, the model is close to the observations. The comparisons of average profile shape parameters (B0,B1) derived from noontime observed profiles, with those obtained, using B0-Tab option, in the IRI model, show a good agreement during all the seasons. However, B0, B1 obtained, using Gulyaeva's option in the IRI model, show a disagreement with the derived B0, B1 values during all the seasons, except during winter, for B0 parameter.     

Comparison between IRI predictions and digital ionosonde measurements of hmF2 at New Delhi during low and moderate solar activity

This paper deals with the diurnal and seasonal variations of height of the peak electron density of the F2-layer (hmF2) derived from digital ionosonde measurements at a low–middle-latitude station, New Delhi (28.6°N, 77.2°E, dip 42.4°N). Diurnal and seasonal variations of hmF2 are examined and comparisons of the observations are made with the predictions of the International Reference Ionosphere (IRI-2001) model. Our study shows that during both the moderate and low solar activity periods, the diurnal pattern of median hmF2 reveals a more or less similar trend during all the seasons with pre-sunrise and daytime peaks during winter and equinox except during summer, where the pre-sunrise peak is absent. Comparison of observed median hmF2 values with the IRI during moderate and low solar activity periods, in general, reveals an IRI overestimation in hmF2 during all the seasons for local times from about 06 LT till midnight hours except during summer for low solar activity, while outside this time period, the observed hmF2 values are close to the IRI predictions. The hmF2 representation in the IRI model does not reproduce pre-sunrise peaks occurring at about 05 LT during winter and equinox as seen in the observations during both the solar activity periods. The noontime observed median hmF2 values increase by about 10–25% from low (2004–2005) to high solar activity (2001–2002) during winter and equinox, while the IRI in the same time period and seasons shows an increase of about 10–20%. During summer, however, the observed noontime median hmF2 values show a little increase with the solar activity, as compared to the IRI with an increase of about 12%.     

Seasonal variation of the total electron content, maximum electron density and equivalent slab thickness at a South-American station

Total electron content (TEC) and foF2 ionosonde data obtained at Tucumán (26.9°S; 65.4°W) from April 1982 to March 1983 (high solar activity period) are analyzed to show the seasonal variation of TEC, NmF2 (proportional to square of foF2) and the equivalent slab thickness EST. Bimonthly averages of the monthly median for January–February, April–May, July–August and October–November have been considered to represent summer, autumn, winter and spring seasons, respectively. The results show that the higher values of TEC and maximum electron density of F2-layer NmF2 are observed during the equinoxes (semiannual anomaly). During daytime, both in TEC and in NmF2 the seasonal or winter anomaly can be seen. At nighttime, this effect is not observed. Also, the observed NmF2 values are used to check the validity of International Reference Ionosphere (IRI) to predict the seasonal variability of this parameter. In general, it is found that averaged monthly medians (obtained with the IRI model) overestimate averaged monthly median data for some hours of the day and underestimate for the other hours.     

Comparison between IRI-2001 predictions and observed measurements of hmF2 over three high latitude stations during different solar activity periods

The monthly median values of the height of peak electron density of the F2-layer (hmF2) derived from ionosonde measurements at three high latitude stations, namely Narssarssuaq (NAR) (61.2 °N, 314.6 °E), Sondrestrom (SON) (67°N, 309.1°E) and College (COL) (69.9°N, 212.2°E) were analyzed and compared with the International Reference Ionosphere (IRI-2001) model, using Comité Consultatif International des Radio communications) (CCIR and Union Radio-Scientifique Internationale (URSI) options. The analysis covers hmF2 values for March Equinox (February, March, April), June Solstice (May, June, July), September Equinox (August, September, October), and December Solstice (November, December, January), during periods of high (2000–2001), medium (2004–2005) and low (2007–2008) solar activity. Generally, the IRI-2001 prediction follow fairly well the diurnal and seasonal variation patterns of the observed values of hmF2 at all the stations. However, IRI-2001 overestimates and underestimates hmF2 at different times of the day for all solar activity periods and in all the seasons considered. The percentage deviation never exceeded 20%, except during DEC SOLS at COL and SON and during MARCH EQUI at SON during low solar activity period. For all solar activity periods considered, both the URSI and CCIR options of the IRI-2001 model give hmF2 values close to the ones measured, but the URSI option performed better than the CCIR option.     

Comparisons of observed ionospheric F2 peak parameters with IRI-2001 predictions over South Africa

The monthly means of the ionospheric F2 peak parameters (foF2 and hmF2) over three stations in South Africa (Grahamstown, 33.3°S, 26.5°E, Madimbo, 22.4°S, 26.5°E, and Louisvale, 28.5°S, 21.2°E) were analyzed and compared with IRI-2001, using CCIR (Comité Consultatif International des Radio communications) and URSI (Union Radio-Scientifique Internationale coefficients) options. The analysis covers a few selected quiet and disturbed days during various seasons represented by the months of January, April, July and October 2003. IRI-2001 generally overestimates hmF2 for both quiet and disturbed days and it overestimates and underestimates foF2 at different times for all the stations. In general, foF2 is predicted more accurately by IRI-2001 than hmF2, and on average, the CCIR option performed better than the URSI option when predicting both foF2 and hmF2.In general, the model generates good results, although some improvements are still necessary to be implemented in order to obtain better predictions. There are no significant differences in the model predictions of hmF2 and foF2 for quiet and disturbed days.     

Anomalous electron density events in the quiet summer ionosphere at solar minimum over Millstone Hill

This study compares the observed behavior of the F region ionosphere over Millstone Hill with calculations from the IZMIRAN model for solar minimum for the geomagnetically quiet period 23–25 June 1986, when anomalously low values of hmF2(<200 km) were observed. We found that these low values of hmF2 (seen as a G condition on ionograms) exist in the ionosphere due to a decrease of production rates of oxygen ions resulting from low values of atomic oxygen density. Results show that determination of a G condition using incoherent scatter radar data is sensitive both to the true concentration of O⁺ relative to the molecular ions, and to the ion composition model assumed in the data reduction process. The increase in the O⁺ + N₂ loss rate due to vibrationally excited N₂ produces a reduction in NmF2 of typically 5–10%, but as large as 15%, bringing the model and data into better agreement. The effect of vibrationally excited NO⁺ ions on electron densities is negligible.     

Comparison of the measured and modelled electron densities and temperatures in the ionosphere and plasmasphere during 20/30 January, 1993

We present a comparison of the electron density and temperature behaviour in the ionosphere and plasmasphere measured by the Millstone Hill incoherent-scatter radar and the instruments on board of the EXOS-D satellite with numerical model calculations from a time-dependent mathematical model of the Earths ionosphere and plasmasphere during the geomagnetically quiet and storm period on 20/30 January, 1993. We have evaluated the value of the additional heating rate that should be added to the normal photoelectron heating in the electron energy equation in the daytime plasmasphere region above 5000 km along the magnetic field line to explain the high electron temperature measured by the instruments on board of the EXOS-D satellite within the Millstone Hill magnetic field flux tube in the Northern Hemisphere. The additional heating brings the measured and modelled electron temperatures into agreement in the plasmasphere and into very large disagreement in the ionosphere if the classical electron heat flux along magnetic field line is used in the model. A new approach, based on a new effective electron thermal conductivity coefficient along the magnetic field line, is presented to model the electron temperature in the ionosphere and plasmasphere. This new approach leads to a heat flux which is less than that given by the classical Spitzer-Harm theory. The evaluated additional heating of electrons in the plasmasphere and the decrease of the thermal conductivity in the topside ionosphere and the greater part of the plasmasphere found for the first time here allow the model to accurately reproduce the electron temperatures observed by the instruments on board the EXOS-D satellite in the plasmasphere and the Millstone Hill incoherent-scatter radar in the ionosphere. The effects of the daytime additional plasmaspheric heating of electrons on the electron temperature and density are small at the F-region altitudes if the modified electron heat flux is used. The deviations from the Boltzmann distribution for the first five vibrational levels of N₂(v) and O₂(v) were calculated. The present study suggests that these deviations are not significant at the first vibrational levels of N₂ and O₂ and the second level of O₂, and the calculated distributions of N₂(v) and O₂(v) are highly non-Boltzmann at vibrational levels v > 2. The resulting effect of N₂(v > 0) and O₂(v > 0) on NmF2 is the decrease of the calculated daytime NmF2 up to a factor of 1.5. The modelled electron temperature is very sensitive to the electron density, and this decrease in electron density results in the increase of the calculated daytime electron temperature up to about 580 K at the F2 peak altitude giving closer agreement between the measured and modelled electron temperatures. Both the daytime and night-time densities are not reproduced by the model without N₂(v > 0) and O₂(v > 0), and inclusion of vibrationally excited N₂ and O₂ brings the model and data into better agreement.     

Bottomside profile shape parameters during low solar activity and comparisons with IRI-2007 model

Modern digital ionosonde measurements at low–middle latitude station, New Delhi, India, are used to assess the IRI-2007 model for the bottomside profile shape parameters B0 and B1 during solar minimum. Comparative analysis shows that in general, the IRI (B0 Table) option reveals better agreement with the B0 observations during daytime in all the seasons, while outside this time period, the IRI (Gulyaeva) predicted B0 values are closer to the observations. For B1 parameter, both the options in the IRI reproduce similar diurnal variations in all the seasons and are closer to observed values except during pre-sunrise and post-sunset hours.     

The role of vibrationally excited nitrogen and oxygen in the ionosphere over Millstone Hill during 16-23 March, 1990

We present a comparison of the observed behavior of the F region ionosphere over Millstone Hill during the geomagnetically quiet and storm period on 16/23 March, 1990, with numerical model calculations from the time-dependent mathematical model of the Earths ionosphere and plasmasphere. The effects of vibrationally excited N₂(v) and O₂(v) on the electron density and temperature are studied using the N₂(v) and O₂(v) Boltzmann and non-Boltzmann distribution assumptions. The deviations from the Boltzmann distribution for the first five vibrational levels of N₂(v) and O₂(v) were calculated. The present study suggests that these deviations are not significant at vibrational levels v = 1 and 2, and the calculated distributions of N₂(v) and O₂(v) are highly non-Boltzmann at vibrational levels v > 2. The N₂(v) and O₂(v) non-Boltzmann distribution assumption leads to the decrease of the calculated daytime NmF2 up to a factor of 1.44 (maximum value) in comparison with the N₂(v) and O₂(v) Boltzmann distribution assumption. The resulting effects of N₂(v > 0) and O₂(v) > 0) on the NmF2 is the decrease of the calculated daytime NmF2 up to a factor of 2.8 (maximum value) for Boltzmann populations of N₂(v) and O₂(v) and up to a factor of 3.5 (maximum value) for non-Boltzmann populations of N₂(v) and O₂(v). This decrease in electron density results in the increase of the calculated daytime electron temperature up to about 1040/1410 K (maximum value) at the F2 peak altitude giving closer agreement between the measured and modeled electron temperatures. Both the daytime and nighttime densities are not reproduced by the model without N₂(v > 0) and O₂(v > 0), and inclusion of vibrationally excited N2 and O2 brings the model and data into better agreement. The effects of vibrationally excited O2 and N2 on the electron density and temperature are most pronounced during daytime.     

The ionospheric F2-region at low geomagnetic latitudes during the geomagnetic storms of 22–26 April 1990: Comparison of observed and modeled response

A comparison between the modeled NmF2 and hmF2 and NmF2 and hmF2, which were observed by the Kokubunji, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders and by the middle and upper (MU) atmosphere radar, have been used to study the time-dependent response of the low-latitude ionosphere to geomagnetic forcing during a time series of geomagnetic storms from 22 to 26 April 1990. The reasonable agreement between the model results and data requires the modified equatorial meridional E×B plasma drift, the modified HWM90 wind, and the modified NRLMSISE-00 neutral densities. We found that changes in a flux of plasma into the nighttime equatorial F2-region from higher L-shells to lower L-shells caused by the meridional component of the E×B plasma drift lead to enhancements in NmF2 close to the geomagnetic equator. The equatorward wind-induced plasma drift along magnetic field lines, which cross the Earth equatorward of about 20° geomagnetic latitude in the northern hemisphere and about −19° geomagnetic latitude in the southern hemisphere, contributes to the maintenance of the F2-layer close to the geomagnetic equator. The nighttime weakening of the equatorial zonal electric field (in comparison with that produced by the empirical model of Fejer and Scherliess [Fejer, B.G., Scherliess, L., 1997. Empirical models of storm time equatorial zonal electric fields. J. Geophys. Res. 102, 24047–24056] or Scherliess and Fejer [Scherliess, L., Fejer, B.G., 1999. Radar and satellite global equatorial F region vertical drift model. J. Geophys. Res. 104, 6829–6842) in combination with corrected equatorward nighttime wind-induced plasma drift along magnetic field lines in the both geomagnetic hemispheres are found to be the physical mechanism of the nighttime NmF2 enhancement formation close to the geomagnetic equator over Manila during 22–26 April 1990. The model crest-to-trough ratios of the equatorial anomaly are used to study the relative role of the main mechanisms of the equatorial anomaly suppression for the 22–26 April 1990 geomagnetic storms. During the most part of the studied time period, a total contribution from geomagnetic storm disturbances in the neutral temperature and densities to the equatorial anomaly changes is less than that from meridional neutral winds and variations in the E×B plasma drift. It is shown that the latitudinal positions of the crests are determined by the E×B drift velocity and the neutral wind velocity.     

Annual and seasonal variations in the low-latitude topside ionosphere

Annual and seasonal variations in the low-latitude topside ionosphere are investigated using observations made by the Hinotori satellite and the Sheffield University Plasmasphere Ionosphere Model (SUPIM). The observed electron densities at 600 km altitude show a strong annual anomaly at all longitudes. The average electron densities of conjugate latitudes within the latitude range ±25° are higher at the December solstice than at the June solstice by about 100% during daytime and 30% during night-time. Model calculations show that the annual variations in the neutral gas densities play important roles. The model values obtained from calculations with inputs for the neutral densities obtained from MSIS86 reproduce the general behaviour of the observed annual anomaly. However, the differences in the modelled electron densities at the two solstices are only about 30% of that seen in the observed values. The model calculations suggest that while the differences between the solstice values of neutral wind, resulting from the coupling of the neutral gas and plasma, may also make a significant contribution to the daytime annual anomaly, the E × B drift velocity may slightly weaken the annual anomaly during daytime and strengthen the anomaly during the post-sunset period. It is suggested that energy sources, other than those arising from the 6% difference in the solar EUV fluxes at the two solstices due to the change in the Sun-Earth distance, may contribute to the annual anomaly. Observations show strong seasonal variations at the solstices, with the electron density at 600 km altitude being higher in the summer hemisphere than in the winter hemisphere, contrary to the behaviour in NmF2. Model calculations confirm that the seasonal behaviour results from effects caused by transequatorial component of the neutral wind in the direction summer hemisphere to winter hemisphere.     

Solar cycle and seasonal variations in F-region vertical drifts over Kodaikanal,India

Measurements of the changes in phase path of F-region reflections at normal incidence at Kodaikanal (77° 28′E, 10° 14′N, dip 3°N) from February 1991 to February 1993 are used to determine the variation of the equatorial evening F-region vertical drifts (V_z) with season, solar and magnetic activity. It is found that on average, at Kodaikanal, the post-sunset peak in V_z(V_zp) is higher in equinox and local winter months than in local summer. The day-to-day variability in V_zp is highest in summer and lowest in winter. This seasonal trend persists even on magnetically quiet days (A_p\leq14). V_zp is found to increase with 10.7 cm solar flux in all three seasons but tends to saturate for large flux values (>230 units) during local summer and winter months. Magnetic activity [represented by A_p as well as the time-weighted accumulations of a_p and a_p ()] does not seem to have any statistically significant effect on V_zp, except during equinoctial months of moderate solar activity, when V_zp decreases as magnetic activity increases.     

The role of vibrationally excited oxygen and nitrogen in the ionosphere during the undisturbed and geomagnetic storm period of 6–12 April 1990

We present a comparison of the observed behavior of the F-region ionosphere over Millstone Hill during the geomagnetically quiet and storm periods of 6–12 April 1990 with numerical model calculations from the IZMIRAN time-dependent mathematical model of the Earths ionosphere and plasmasphere. The major enhancement to the IZMIRAN model developed in this study is the use of a new loss rate of O⁺(⁴S) ions as a result of new high-temperature flowing afterglow measurements of the rate coefficients K₁ and K₂ for the reactions of O⁺(⁴S) with N₂ and O₂. The deviations from the Boltzmann distribution for the first five vibrational levels of O₂(v) were calculated, and the present study suggests that these deviations are not significant. It was found that the difference between the non-Boltzmann and Boltzmann distribution assumptions of O₂(v) and the difference between ion and neutral temperature can lead to an increase of up to about 3% or a decrease of up to about 4% of the calculated NmF2 as a result of a respective increase or a decrease in K₂. The IZMIRAN model reproduces major features of the data. We found that the inclusion of vibrationally excited N₂(v > 0) and O₂(v > 0) in the calculations improves the agreement between the calculated NmF2 and the data on 6, 9, and 10 April. However, both the daytime and nighttime densities are reproduced by the IZMIRAN model without the vibrationally excited nitrogen and oxygen on 8 and 11 April better than the IZMIRAN model with N₂(v > 0) and O₂(v > 0). This could be due to possible uncertainties in model neutral temperature and densities, EUV fluxes, rate coefficients, and the flow of ionization between the ionosphere and plasmasphere, and possible horizontal divergence of the flux of ionization above the station. Our calculations show that the increase in the O⁺ + N₂ rate factor due to N₂(v > 0) produces a 5–36% decrease in the calculated daytime peak density. The increase in the O⁺ + O₂ loss rate due to vibrationally excited O₂ produces 8–46% reductions in NmF2. The effects of vibrationally excited O₂ and N₂ on N_e and T_e are most pronounced during the daytime.     

An anomalous subauroral red arc on 4 August, 1972: comparison of ISIS-2 satellite data with numerical calculations

This study compares the Isis II satellite measurements of the electron density and temperature, the integral airglow intensity and volume emission rate at 630 nm in the SAR arc region, observed at dusk on 4 August, 1972, in the Southern Hemisphere, during the main phase of the geomagnetic storm. The model results were obtained using the time dependent one-dimensional mathematical model of the Earth’s ionosphere and plasmasphere (the IZMIRAN model). The major enhancement to the IZMIRAN model developed in this study to explain the two component 630 nm emission observed is the analytical yield spectrum approach to calculate the fluxes of precipitating electrons and the additional production rates of N⁺₂, O⁺₂, O⁺(⁴S), O⁺(²D), O⁻(²P), and O⁺(²P) ions, and O(¹D) in the SAR arc regions in the Northern and Southern Hemispheres. In order to bring the measured and modelled electron temperatures into agreement, the additional heating electron rate of 1.05 eV cm⁻³ s⁻¹ was added in the energy balance equation of electrons at altitudes above 5000 km during the main phase of the geomagnetic storm. This additional heating electron rate determines the thermally excited 630 nm emission observed. The IZMIRAN model calculates a 630 nm integral intensity above 350 km of 4.1 kR and a total 630 nm integral intensity of 8.1 kR, values which are slightly lower compared to the observed 4.7 kR and 10.6 kR. We conclude that the 630 nm emission observed can be explained considering both the soft energy electron excited component and the thermally excited component. It is found that the inclusion of N₂(v > 0) and O₂(v > 0) in the calculations of the O⁺(⁴S) loss rate improves the agreement between the calculated N_e and the data on 4 August, 1972. The N₂(v > 0) and O₂(v > 0) effects are enough to explain the electron density depression in the SAR arc F-region and above F2 peak altitude. Our calculations show that the increase in the O⁺ + N₂ rate factor due to the vibrationally excited nitrogen produces the 5–19% reductions in the calculated quiet daytime peak density and the 16–24% decrease in NmF2 in the SAR arc region. The increase in the O⁺ + N₂ loss rate due to vibrationally excited O₂ produces the 7–26% decrease in the calculated quiet daytime peak density and the 12–26% decrease in NmF2 in the SAR arc region. We evaluated the role of the electron cooling rates by low-lying electronic excitation of O₂(a¹δ_g) and O₂(b¹σ_g⁺), and rotational excitation of O₂, and found that the effect of these cooling rates on T_e can be considered negligible during the quiet and geomagnetic storm period 3–4 August, 1972. The energy exchange between electron and ion gases, the cooling rate in collisions of O(³P) with thermal electrons with excitation of O(¹D), and the electron cooling rates by vibrational excitation of O₂ and N₂ are the largest cooling rates above 200 km in the SAR arc region on 4 August, 1972. The enhanced IZMIRAN model calculates also number densities of N₂(B³π_g⁺), N₂(C³π_u), and N₂(A³σ_u⁺) at several vibrational levels, O(¹S), and the volume emission rate and integral intensity at 557.7 nm in the region between 120 and 1000 km. We found from the model that the integral integral intensity at 557.7 nm is much less than the integral intensity at 630 nm.     

地磁静日期内北半球电离层NmF2的结构分布

用北半球陆地上48个地面电离层垂测站资料，以及国际参考电离层IRI 90模式，考察1985年1月6～7日F2层电子密度最大值日变化的纬度剖面和经度效应. 结果表明，在亚洲地区的20°N～30°N内，F2层赤道异常“喷泉效应”产生的NmF2“北驼峰”最高，其最大值出现在中午，或稍迟时间. 30°N ～50°N区域内，NmF2的白天峰值幅度逐渐下降，峰值时间移至午前约10:00 LT. 更高纬度(50°N～62°N )台站上，中午前后NmF2出现双峰，傍晚有谷值，夜间又再次抬升. 欧、美地区的低纬台站很少，但借助IRI 90模式分析可发现，在270°E经圈上，“喷泉效应”造成的“北驼峰”幅度最小，而且随纬度增大时，NmF2白天幅度下降也不明显，即纬度剖面的经度效应非常显著. 对中国、日本地区台站资料的小范围经度差异分析表明，在驼峰区的90°E～140°E内，各站NmF2无明显差别；但在中纬地区30°N～50°N内，中国西部上空NmF2白天变化幅度较大，且较为陡直，而中国东部和日本台站上空则相对平缓.     

Disturbed vertical E×B plasma drifts in the equatorial F2 region at solar minimum deduced from observed NmF2 and hmF2 variations

Ground-based ionosonde and magnetic-field observations on the equatorial station Huancayo, ESRO4 neutral-composition measurements, and theoretical model calculations were used to analyze disturbed E×B vertical plasma drift during the phase of solar minimum in 1973. Vertical drifts calculated for disturbed days do not show the systematic decrease often mentioned in publications, and demonstrate strong dependence on IMF-B_z changes. It is confirmed with the help of our drift calculations that B_z turnings to a northward direction result in a decrease (up to reversal) of normal Sq (eastward during daytime and westward at nighttime) in the zonal component of electric field. Southward B_z excursions enhance normal E_y both in daytime and nighttime hours. Model predictions of E_y’s reaction to IMF-B_z changes are discussed.     

The effects of nitric oxide cooling and the photodissociation of molecular oxygen on the thermosphere/ionosphere system over the Argentine Islands

In the past the global, fully coupled, time-dependent mathematical model of the Earths thermo-sphere/ionosphere/plasmasphere (CTIP) has been unable to reproduce accurately observed values of the maximum plasma frequency, foF2, at extreme geophysical locations such as the Argentine Islands during the summer solstice where the ionosphere remains in sunlight throughout the day. This is probably because the seasonal dependence of thermospheric cooling by 5.3 m nitric oxide has been neglected and the photodissociation of O₂ and heating rate calculations have been over-simplified. Now we have included an up-to-date calculation of the solar EUV and UV thermospheric heating rate, coupled with a new calculation of a diurnally varying O₂ photodissociation rate, in the model. Seasonally dependent 5.3 m nitric oxide cooling is also included. With these important improvements, it is found that model values of foF2 are in substantially better agreement with observation. The height of the F2-peak is reduced throughout the day, but remains within acceptable limits of values derived from observation, except at around 0600 h LT. We also carry out two studies of the sensitivity of the upper atmosphere to changes in the magnitude of nitric oxide cooling and photodissociation rates. We find that hmF2 increases with increased heating, whilst foF2 falls. The converse is true for an increase in the cooling rate. Similarly increasing the photodissociation rate increases both hmF2 and foF2. These changes are explained in terms of changes in the neutral temperature, composition and neutral wind.     

Diurnal and seasonal variation of B0,B1 parameters during high solar activity period at low mid-latitude and their comparisons with IRI-2001 model

Diurnal and seasonal variations of bottom side electron density profile shape parameters B0, B1, representing the bottom side F2-layer thickness and shape, are examined using modern digital ionosonde observations at a low-middle latitude station, New Delhi (28.6^°N, 77.2^°E, dip 42.4^°N) for high solar activity (HSA) (2001–2002). Median values of these parameters are obtained at each hour during different seasons and compared with the predictions of the latest version of the international reference ionosphere (IRI), IRI-2001 model using both the options namely: IRI (Gulyaeva) and IRI (B0 Tab.). Results show in general, a large variability in B0, and B1 parameters during all the seasons, the variability is larger during nighttime than by daytime. The diurnal variation of median B0, in general, show more or less similar trends with diurnal maximum occurring around noontime, except during summer, when it occurs between 09 and 10 LT. Variation pattern of B1 in general, is identical in all the seasons with lower values of B1 by daytime than by night. Comparative studies of B0 with those obtained with the IRI model show that in general, IRI (B0 Tab.) option reveals better agreement with the observations during all the seasons for local times from about 10 LT to about 16 LT, while outside this time period IRI (Gulyaeva) matches well with the observations. The predicted B1 parameter, using IRI (B0 Tab.) is close to observations in terms of diurnal variation, while B1 using IRI (Gulyaeva) option, assumes a fixed value of 3 at all local times irrespective of season.