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1.
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%.  相似文献   

2.
Arecibo (18.4 N, 66.7 W) incoherent scatter (IS) observations of electron density N(h) are compared with the International Reference Ionosphere (IRI-95) during midday (10/14 h), for summer, winter and equinox, at solar maximum (1981). The N(h) profiles below the F2 peak, are normalized to the peak density NmF2 of the F region and are then compared with the IRI-95 model using both the standard B0 (old option) and the Gulyaeva-B0 thickness (new option). The thickness parameter B0 is obtained from the observed electron density profiles and compared with those obtained from the IRI-95 using both the options. Our studies indicate that during summer and equinox, in general, the values of electron densities at all the heights given by the IRI model (new option), are generally larger than those obtained from IS measurements. However, during winter, the agreement between the IRI and the observed values is reasonably good in the bottom part of the F2 layer but IRI underestimates electron density at F1 layer heights. The IRI profiles obtained with the old option gives much better results than those generated with the new option. Compared to the observations, the IRI profiles are found to be much thicker using Gulyaeva-B0 option than using standard B0.  相似文献   

3.
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.  相似文献   

4.
Electron and ion temperature (Te and Ti) 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 Te (∼1000 K) is independent of the season and the solar flux whereas Ti exhibits positive correlation with the solar activity during all three seasons. In the early morning hours during summer, Te is higher by ∼500 K than other seasons in all three levels of solar activity. During winter and equinox in the early morning hours, Te and Ti are higher during low solar activity, showing a negative correlation with solar flux. During daytime, the Ti increases with the solar flux in winter and summer solstice, but is independent in equinox. IRI underestimates Te and Ti during the morning period by 50–75% in the equatorial and near-equatorial stations during all levels of solar activities.  相似文献   

5.
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.  相似文献   

6.
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.  相似文献   

7.
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.  相似文献   

8.
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.  相似文献   

9.
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.  相似文献   

10.
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.  相似文献   

11.
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12.
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.  相似文献   

13.
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 (Ap<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.  相似文献   

14.
In this study, foF2 data obtained from an equatorial station in West Africa were subjected to an occurrence probability distribution test. This was done on an hourly basis, for all the 24 h of the day. The results show that the probability (Np) of predicting foF2 within the range±of a standard deviation (σ) centered on the mean (μ) is ⩾0.68 is at least about 70% of the hourly set of data considered in this study irrespective of time of the day, season or solar cycle period. The distribution is not, however, perfectly symmetrically distributed around the mean. The seasonal hourly averages of foF2 were compared with those of IRI predictions. The IRI representation was found to be very good at low and moderate solar activity for both day and nighttime when the ITU-R coefficients are used. This is also true of the daytime at high solar activity. The night time prediction is only fairly good when the URSI coefficients are used for the prediction.  相似文献   

15.
Using ionosonde made observations at Concepción (36.8°S; 73.0°W) for the 1958–1994 interval, long-term trends of critical frequency (foF2) and peak height (hmF2) of the ionospheric F2-layer are analysed. The trends found for different times-of-day and all seasons are consistent with an increasing diurnal-variation amplitude of both foF2 and hmF2. An increasing hmF2 trend of up to 1.5 km/year found between midnight and dawn during winter has no precedent. It is suggested that these long-term amplitude changes may be associated with changes in the prevailing thermospheric meridional neutral winds.  相似文献   

16.
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.  相似文献   

17.
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.  相似文献   

18.
The variability of foF2 in different phases of solar cycle 23   总被引：1，自引：0，他引：1  
In this paper we examined the variations of the foF2 with solar activity for different local time and different seasons. Beside this we evaluated International Reference Ionosphere (IRI) models at different phases of solar cycle 23, different latitudes and different local time. We studied F2 layer critical frequency (foF2) of the ionosphere by using the flare index calculated by the Kandilli Observatory. For this purpose, we identified the months similar with high flare activity during the solar cycle 23. We chose 6 months which represented the different phases (ascending branch, maximum and descending branch) of the solar cycle. We also took into account the fact that these months were in different seasons. The hourly monthly means of observed foF2 data from four ionosonde stations for 6 months were calculated. On the other hand, the identical foF2 values of the same months were calculated for the year 1996, which is the minimum year of the previous solar activity cycle. We subtracted the foF2 values of 1996 from the values of the selected months of the last solar cycle to obtain the residuals, Δ(foF2). Then the magnitude of the residuals is compared through the cycle. We used IRI-2007 as well as IRI-2001 models to see the degree of deviation of the observed results from the predicted ones. We found that the predicted values of the ΔfoF2, which are calculated by the IRI-2007, fitted well with the observed Δ(foF2) and showed that the Δ(foF2) are dependent on the solar cycle variations in general.  相似文献   

19.
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.  相似文献   

20.
《Journal of Atmospheric and Solar》2002,64(12-14):1413-1423
In this paper, the ionospheric electron content (IEC) profile features, have been utilized for extraction of a few parameters and the parameters are used for assertion of a certain day as quiet (Q) or disturbed (D). The IEC values taken for the study cover two solar activity periods (high and low) and are based on VHF RB data collected over Guwahati (26.2°N, 91.75°E). The paper describes methods for extraction of parameters like profile factor, P and anomaly factor, PEA from IEC profiles separately for three seasons (summer, winter and equinox). The definitions of Q and D days are made through profile features and the threshold values of ΣKp for each season are evaluated. The relations between ΣKp and P factor, ΣKp and PEA are established after corrections for solar activity condition. The prediction and assertion of Q/D days are then made by examining IEC profiles for the cases where IEC data were not used for the parameter extraction.  相似文献   

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