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%.     

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.     

Comparison of satellite measurements of the low-latitude nighttime upper ionosphere with IRI

In this paper, we report the results of our comparison study between satellite measurements and the International Reference Ionosphere (IRI) model on the seasonal and longitudinal changes of the low-latitude nighttime topside ionosphere during the period of solar maximum from June 2000 to July 2001. Satellite measurements were made by KOMPSAT-1 and DMSP F15 at 685 km altitude and 840 km altitude, respectively. The results show that the IRI2001 model gives reasonable density estimations for the summer hemisphere and the March equinox at both altitudes. However, the observed wintertime densities are smaller than the predictions of the IRI2001 model, especially at a higher (840 km) altitude, manifesting strong hemispheric asymmetries. The observed electron temperatures generally reside between the two estimations of IRI2001, one based on the Aeros–ISIS data and the other based on Intercosmos, and the latter estimation better represents the observations. With more or less monotonic increase with latitude, the temperature profiles of the IRI2001 model do not predict the enhancement seen around 15° magnetic latitude of the winter hemisphere. Longitudinal variation, probably caused by the zonal winds, is seen in all seasons at both altitudes, while the IRI2001 model does not show a large variation. The observed density and temperature show significant changes according to the F10.7 values in the whole low-latitude region from 40°S to 40°N geomagnetic latitude. The effect is manifested as increases in the density and temperature, but not in the hemispheric asymmetry or in the longitudinal variation.     

A comparison of results derived from scaling VS chirp-ionosonde ionograms with the international reference ionosphere (IRI)

The results derived from processing vertical-incidence ionograms obtained with the chirp-ionosonde at Irkutsk for different winter time intervals (February) and at equinox are presented. The peak height hmF2 was determined by Dudeney's formula based on ionogram parameters, including the coefficient M(3000). The algorithm is suggested for determining the coefficient M(3000) in the automatic mode using the conventional form of the transfer curve method without invoking a standard transparency called the “transfer curve”. The parameters foF2 and hmF2 are compared with the international reference ionosphere (IRI-95) model. It is found that in most cases the values of the foF2 and hmF2 parameters, calculated in the IRI-95 model, are similar to the median ones. It is confirmed that for practical purposes where it is necessary to know the radio wave propagation conditions along the propagation path, the IRI model is convenient and attractive.     

太阳活动低年南极中山站电离层F层的平均特性

根据１９９５-１９９７年３年中山站数字式电离层测高仪的数据,分析了中山站不同季 节Ｆ层的临频变化特点．中山站夏季主要受太阳光光化电离的影响,Ｆ层临频随地方时的变 化与中纬台站相似;两分季,极隙区软电子沉降的作用显著,Ｆ层临频随磁地方时而变化,有 较明显的磁中午现象．冬季,太阳全天处于地平线以下,中山站Ｆ层临频的变化主要受极隙 区软电子沉降和极区等离子体漂移的影响,其峰值变化处于碰中午和地方时中午之间．中山 站夏季全天都能观测到Ｆ层的存在;两分季Ｆ层在地方时子夜附近的出现率较少;冬季月份 在磁地方时午后和子夜Ｆ层出现率明显减少,这可能与南半球冬季的高纬槽和极洞有关．对 Ｆ层不均匀区的分析认为,中山站在ｔ_(ＬＴ)为１６：００左右处于极光带赤道侧,２０：００左右进入极盖 区。     

高纬电离层气候学特征研究--EISCAT雷达观测及与IRI模式的比较

利用1988～1999年欧洲非相干散射EISCAT（European Incoherent Scatter）雷达观测数据,对不同太阳活动周相、不同季节的极光椭圆区电离层F区电子密度进行统计分析,研究其气候学特征,并与IRI 2001模式比较.EISCAT观测到的电子密度显示出显著的太阳活动高年“冬季异常”和太阳活动低年半年变化等现象.EISCAT实测电子密度随时间和高度的平均二维分布和500 km高度以下总电子含量TEC,从总体来看与IRI 2001模式预测结果符合较好.但高年在TEC达到最大值前后,IRI 2001模式预测的电子密度高度剖面与EISCAT观测结果有显著差别：F2峰以上IRI 2001模式预测的电子密度过大,造成TEC明显高于雷达观测值.另外,在太阳活动下降相,EISCAT观测显示出明显的半年周期季节变化特征,但IRI 2001模式未能预测出此下降相季节变化.     

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.     

Quiet-condition hmF2, NmF2, and B0 variations at Jicamarca and comparison with IRI-2001during solar maximum

We use the measurements of the Jicamarca digisonde to examine the variations in F2 layer peak electron density (NmF2), its height (hmF2), and the F2 layer thickness parameter (B₀) near the dip equator. The hourly ionograms during geomagnetic quiet-conditions for a 12-month period close to the maximum solar activity, April 1999–March 2000, are used to calculate the monthly averages of these parameters, for each month. The averages are compared with the International Reference Ionosphere (IRI)-2001 model values. The results show that the higher hmF2 values during daytime, associated with the upward velocity, are mainly responsible for the greater values of NmF2 and B₀; while the nighttime lower hmF2, related to the downward velocity, are responsible for the smaller NmF2 and B₀. For daytime, hmF2 and NmF2 are correlated with the solar activity in the equinoctial and summer months. The hmF2 and B₀ peaks at sunset with an associated sharp decrease in NmF2 are presented in the equinoctial and summer months, but not in the winter months. Comparison of the measured hmF2 values with the International Radio Consultative Committee (CCIR) maps used in IRI-2001 (IRI-CCIR) reveals an IRI overestimate in hmF2 during daytime. The most significant discrepancy is that the IRI-CCIR does not model the post-sunset peak in hmF2. For the NmF2 comparison, the values obtained from both the CCIR and URSI maps are generally close to the observed values. For the B₀ comparison, the highest discrepancy between the observation and the Gulyaeva option (IRI-Gulyaeva) is the location of the annual maximum for the daytime values, also the winter daytime predictions are too low. Additionally, the significant negative difference between the observation and the B₀-table option (IRI-B₀-table) provides a slightly better prediction, except for 0400–1000 LT when the model significantly overestimates. The post-sunset peak in B₀ at some months is predicted by neither the IRI-Gulyaeva nor the IRI-B₀-table options.     

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.     

Seasonal variation of the auroral E-region neutral wind for different solar activities

Seasonal variations in the auroral E-region neutral wind for different solar activity periods are studied. This work is based on neutral wind data obtained over 56 days between 95–119 km altitude under geomagnetic quiet conditions (A_p<16) during one solar cycle by the European Incoherent Scatter radar located in northern Scandinavia. In general, the meridional mean wind shifts northward, and the zonal mean wind increases in eastward amplitude from winter to summer. The zonal mean wind blows eastward in the middle and lower E-region for each season and for each solar condition except for the equinox, where the zonal mean wind blows westward at and below 104 km. Solar activity dependence of the mean wind exists during the winter and equinox seasons, while in summer it is less prominent. Under high solar activity conditions, the altitude profiles of the horizontal mean winds in winter and the equinoxes tend to resemble those in summer. The horizontal diurnal tide is less sensitive to solar activity except during summer when the meridional amplitude increases by ∼10 m s⁻¹ and the corresponding phase shifts to a later time period (1–2 h) during high solar activity. Seasonal dependence of the semidiurnal tide is complex, but is found to vary with solar activity. Under low solar activity conditions the horizontal semidiurnal amplitude shows seasonal dependence except at upper E-region heights, while under high solar activity conditions it becomes less sensitive to seasonal effects (except for the meridional component above 107 km). Comparisons of mean winds with LF and UARS observations are made, and the driving forces for the horizontal mean winds are discussed for various conditions.     

Validation of IRI-2007 against TEC observations during low solar activity over Indian sector

Published values of Total Electron Content (TEC) measured by ATS-6 are used to assess the latest available IRI-2007 model during solar minimum over Indian sector covering equatorial to low-mid-latitudes stations. The study reveals that during all seasons and at all locations, in general, the TEC predicted by NeQuick and IRI01-corr options provided in the IRI-2007 model shows much better agreement with the TEC observations as compared to those generated by IRI-2001.option. TEC predicted using NeQuick option found to be little more closer to the observation except at equatorial station during daytime, while IRI-2001 option highly overestimates the TEC in all seasons and times.     

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.     

一九八○年二月十六日云南地区日全食的电离层效应

本文借助于Budden的频高图分析方法,分析了云南省瑞丽县弄岛街1980年2月16日日全食期间的电离层资料,计算了日食日及控制目的剖面,给出了剖面参数,得到了E层临频及最大电子浓度所在高度在日食过程中的变化,也研究了F层的日食效应,指出了在各选定频率上的反射虚高和真高在日食过程中的变化,发现在300公里以下的各给定高度上,尤其是在270公里附近的高度上,其电子浓度明显地下降。     

Predicted and measured electron density at 600 km altitude in the South American peak of the equatorial anomaly

Measurements of the electron density at 600 km altitude (N₆₀₀) were obtained with the Hinotori satellite launched by the Institute of Space and Astronautical Science of Japan. These measurements were used to check the validity of the International Reference Ionosphere (IRI) model in predicting the electron density at that altitude in the South American peak of the equatorial anomaly. The measurements correspond to the longitude zone from 285 to 369° and −15° geomagnetic latitude. To model the electron density at 600 km altitude, two cases were considered, namely (i) N₆₀₀ was calculated with the IRI model at 10° intervals within the corresponding longitudinal zone and mean values were obtained, and (ii) N₆₀₀ was calculated with the IRI using ionosonde data as input coefficients in the model. The data used for this study were measured almost simultaneously with the total electron content data used in a previous work. The results show good predictions at hours of minimum ionisation for the equinox and the December solstice. For the June solstice, the best agreement was obtained around noon. However, strong disagreements were observed in some cases such as the equinox at 15:00 LT, suggesting that there is a need to improve the modeled topside profile.     

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.     

Climatology of ionospheric upper transition height derived from COSMIC satellites during the solar minimum of 2008

The electron density profiles retrieved from the Constellation Observing System for Meteorology, Ionosphere & Climate (COSMIC) satellite Radio Occultation (RO) observations during 2008 are used to derive ionospheric upper transition height, where the density of O⁺ is equal to that of light ions (mainly H⁺ and He⁺). It is found that the ionosphere upper transition height is very low, with significant local time, latitude and seasonal variations, during the extremely low solar minimum of 2008. The transition height is higher in the daytime than at night, except over middle latitude region of winter hemisphere, where the transition height has minimum in the morning. There is a pronounced peak over equator for all seasons. The transition height is higher in summer than in winter hemisphere. Our results have comparability with C/NOFS satellite observations around the equatorial region during June–August of 2008. However, the IRI model gives much higher transition height than those from COSMIC and cannot reproduce its latitude and season variations well during 2008.     

Solar activity dependence of electron and ion temperatures using SROSS C2 RPA data and comparison with IRI model

Electron and ion temperature (T_e and T_i) data observed using RPA on board SROSS C2 satellite are investigated for the variation with local time, season, latitude (0–30°N geographic) over a half of a solar cycle (1995–2000). The nighttime T_e (∼1000 K) is independent of the season and the solar flux whereas T_i exhibits positive correlation with the solar activity during all three seasons. In the early morning hours during summer, T_e is higher by ∼500 K than other seasons in all three levels of solar activity. During winter and equinox in the early morning hours, T_e and T_i are higher during low solar activity, showing a negative correlation with solar flux. During daytime, the T_i increases with the solar flux in winter and summer solstice, but is independent in equinox. IRI underestimates T_e and T_i during the morning period by 50–75% in the equatorial and near-equatorial stations during all levels of solar activities.     

Features of the Behavior of Ionospheric Parameters В0 and В1 in the High-Latitude Zone

Geomagnetism and Aeronomy - The climatological features and behavior of the parameters B0_Neh_I, B1_Neh_I found from N(h) profiles calculated by the curves from the virtual heights during...     

Local enhancements in the horizontal distribution of the electron density at heights of the topside ionosphere near the dayside polar cap boundary

The structure of the electron density horizontal distribution at heights of the topside ionosphere in the Northern Hemisphere under quiet magnetic conditions in the polar peak region has been analyzed based on the measurements conducted on board the STSAT-1, DMSP F13, and DMSP F15 satellites during the period of moderate solar activity. The sector near 1000 MLT (magnetic local time) for the spring equinox season has been studied most thoroughly. It has been obtained that the polar peak consists of localized irregularities in the electron density of a specific form, inside which the electron density exceeds its background values by several tens of percent. The characteristic dimensions of the irregularities vary from a few degrees to several tens of degrees in the meridional and zonal directions, respectively. The irregularities are centered along the dayside boundary of the polar cap and coincide with the region of “soft” electron precipitation (with the average energy lower than 500 eV and with the flux density above 10⁷ electrons cm^?2 s^?1 sr^?1).