Electrostatic discharges in Saturn's B-ring

The Voyager observations of electrical discharges in Saturn's rings strongly support earlier speculations on the role played by electrostatics, magnetic fields, and lightning phenomena in the primitive solar system. They also suggest conditions then by direct analogy rather than by extrapolating backwards through time from conditions now. The observed discharges show a pronounced 10^h periodicity, which suggests a source in Keplerian orbit at 1.80 ± 0.01 Saturn radii (1 R_S = 60,330 km). In that region, the B ring is thicker than optical depth 1.8 for about 5,000 km. At 1.805 ± 0.001 Saturn radii, however, the ring is virtually transparent for a gap of width 200 m. We conclude that a small satellite orbits Saturn at that radius and clears the gap. The gap edges must prevent diffusive filling of the gap by fine material which is especially abundant at this position in the rings and would otherwise destroy the gap in minutes. The discharges represent the satellite's interaction with the outer edge of the gap. Spoke formation may involve the interaction of ring material in the vicinity of the gap.     

Structure of the equatorial lower ionosphere from the Thumba Langmuir probe experiments

A Langmuir probe designed and developed at the Physical Research Laboratory, Ahmedabad has been used on a variety of rockets since 1966 from the Thumba Equatorial Rocket Launching Station, TERLS (8°31'N, 76°52'E, dip.lat. 0°47'S) to study the structure of the equatorial lower ionosphere. Good quality data is available from a set of twenty five rocket flights conducted during the period 1966 to 1978. This data has been obtained using a single standardised instrument at a single location and using a uniform procedure for data reading and analysis, and adopting a calibration procedure to convert the measured probe currents into electron densities which involves a height dependent calibration factor. The data has been used to establish the gross features of the equatorial lower ionosphere under daytime, night time, morning twilight and evening twilight periods.     

Spatial asymmetry in topside ion density and vertical E × B plasma drift velocity within 75°E–95°E

The ion density measured by the Ionospheric Plasma and Electrodynamics Instrument (IPEI) on board the ROCSAT -1 over the 75°E and 95°E meridian at 600km altitude has been utilized to examine the latitudinal and longitudinal distribution within the Indian sector, in particular, the north-south and east-west asymmetries of the equatorial ionization anomaly (EIA). A longitudinal gradient in ion density at 600?km higher towards 95°E develops during the noontime and afternoon hours when the EIA is at its peak. The density gradient persists till evening hours when pre-reversal enhancements occur. The vertical E?×?B plasma drift velocity measured simultaneously by ROCSAT -1 for the same space-time configuration has also been studied. In addition to diurnal, seasonal and solar activity variations in E?×?B drift velocity, the longitudinal gradient is also observed. The EIA at the altitude of 600?km peaks at different latitudes and are mostly asymmetric about the magnetic equator. From midnight till 0800 LT, the ion density across the equator is nearly uniform in the equinoxes. But in the solstices, the density exhibits a north-south gradient. In the June solstice, density is higher in the northern hemisphere and decreases gradually towards south. The gradient in density reverses in December solstice. Normally, the EIA peaks within 1200 LT and 1600 LT while around 2000 LT, pre-reversal enhancement of ionization occurs affecting the EIA evening structure. The strength of the EIA also exhibits seasonal, year-to-year and hemispheric variations. The longitudinal asymmetry of drift velocity along 75°E and 95°E longitude sectors is the contributing factor behind the observed longitudinal asymmetry in ion density. Significant positive correlation between the strength of the EIA and E?×?B drift is observed in both longitudes.     

All-sky-imaging capabilities for ionospheric space weather research using geomagnetic conjugate point observing sites

Optical signatures of ionospheric disturbances exist at all latitudes on Earth—the most well known case being visible aurora at high latitudes. Sub-visual emissions occur equatorward of the auroral zones that also indicate periods and locations of severe Space Weather effects. These fall into three magnetic latitude domains in each hemisphere: (1) sub-auroral latitudes ～40–60°, (2) mid-latitudes (20–40°) and (3) equatorial-to-low latitudes (0–20°).Boston University has established a network of all-sky-imagers (ASIs) with sites at opposite ends of the same geomagnetic field lines in each hemisphere—called geomagnetic conjugate points. Our ASIs are autonomous instruments that operate in mini-observatories situated at four conjugate pairs in North and South America, plus one pair linking Europe and South Africa. In this paper, we describe instrument design, data-taking protocols, data transfer and archiving issues, image processing, science objectives and early results for each latitude domain. This unique capability addresses how a single source of disturbance is transformed into similar or different effects based on the unique “receptor” conditions (seasonal effects) found in each hemisphere. Applying optical conjugate point observations to Space Weather problems offers a new diagnostic approach for understanding the global system response functions operating in the Earth’s upper atmosphere.     

Characterization of GPS-TEC over African equatorial ionization anomaly (EIA) region during 2009–2016

This study characterizes total electron content (TEC) measured by Global Positioning System (GPS) over African equatorial ionization anomaly (EIA) region for 2009–2016 period during both quiet geomagnetic conditions (Kp?≤?1) and normal conditions (1?>?Kp?≤?4). GPS-TEC data from four equatorial/low-latitude stations, namely, Addis Ababa (ADIS: 9.04°N, 38.77°E, mag. lat: 0.2°N) [Ethiopia]; Yamoussoukro (YKRO: 6.87°N, 5.24°W, mag. lat: 2.6°S) [Ivory Coast]; Malindi (MAL2; 3.00°S, 40.19°E, mag. lat: 12.4°S) [Kenya] and Libreville (NKLG; 0.35°N, 9.67°W, mag. lat: 13.5°S) [Gabon] were used for this study. Interesting features like noontime TEC bite-out, winter anomaly during the ascending and maximum phases of solar cycle 24, diurnal and seasonal variations with solar activity have been observed and investigated in this study. The day-to-day variations exhibited ionospheric TEC asymmetry on an annual scale. TEC observed at equatorial stations (EIA-trough) and EIA-crest reach maximum values between ～1300–1600 LT and ～1300–1600 LT, respectively. About 76% of the high TEC values were recorded in equinoctial months while the June solstice predominantly exhibited low TEC values. Yearly, the estimated TEC values increases or decreases with solar activity, with 2014 having the highest TEC value. Solar activity dependence of TEC within the EIA zone reveals that both F10.7?cm index and EUV flux (24–36?nm) gives a stronger correlation with TEC than Sunspot Number (SSN). A slightly higher degree of dependence is on EUV flux with the mean highest correlation coefficient (R) value of 0.70, 0.83, 0.82 and 0.88 for quiet geomagnetic conditions (Kp?≤?1) at stations ADIS, MAL2, NKLG, and YKRO, respectively. The correlation results for the entire period consequently reveals that SSN and solar flux F10.7?cm index might not be an ideal index as a proxy for EUV flux as well as to measure the variability of TEC strength within the EIA zone. The estimated TEC along the EIA crest (MAL2 and NKLG) exhibited double-hump maximum, as well as post-sunset peaks (night time enhancement of TEC) between ～2100 and 2300 LT. EIA formation was prominent during evening/post-noon hours.     

Proposal for a reference model of the middle atmosphere of the southern hemisphere

Available rocketsonde information has been used to compile tables of monthly mean temperature, pressure, density and zonal wind for the middle atmosphere of the southern hemisphere with the purpose of revising similar tables presented to COSPAR earlier. The altitude range is 25 to 80 km in steps of 5 km. The latitude range is 0° to 70°S with a 10° step. The compatability of different sets of temperature measurements is discussed. Mean values of temperature, pressure and zonal wind obtained for the southern hemisphere are compared with northern hemisphere model values. Large differences between the hemispheres (up to 20°C in temperature, 20–30% in pressure, 30–50 m/s in wind) imply that reference atmospheres such as CIRA should be complemented by southern hemisphere climatology.     

Seasonal,annual, and interannual variability in MLT quasi-two-day waves over the low-latitude region Kolhapur (16.8°N; 74.2°E)

This study presents the quasi-two-day wave (Q2DW) characteristics of the mesosphere and lower thermosphere (MLT) region obtained by taking hourly mean values of horizontal wind velocities for 4? years (August 2013–July 2017) through continuous measurements using a medium-frequency (MF) radar (operating frequency – 1.98?MHz) located at the low-latitude Indian station Kolhapur (16.8°N; 74.2°E). The MF radar located at Kolhapur was upgraded in 2013, and these results of Q2DW have been reported for the first time after upgrading. The present study investigated variability in seasonal, annual, interannual, and solar indices of Q2DWs traveling in zonal (EW) and meridional (NS) components in the MLT region. The Q2DW activity is observed to be stronger during austral summer (January–February) (EW?=?～5?m/s and NS?=?～8–10?m/s) than during boreal summer (June–July) (EW = ～5 m/s and NS = ～6–8?m/s). The Q2DW amplitudes are larger in the meridional component than in the zonal one. A strong semiannual oscillation (SAO) has been observed in Q2DWs, with peak during January–February and June–July. In addition, small enhancement is seen in meridional Q2DW in October (～5–6?m/s). It is observed that the entire spectrum (40–60?h) measured between 86 and 94?km contributes to the SAO amplitudes during January–February and June–July, whereas the waves measured between 42?h and 52?h contribute to enhancement in October similar to that reported elsewhere. In general, the Q2DW amplitude shows large interannual variability. The easterlies developed in the global circulation model in Northern hemisphere during May intensify up to around summer solstice. Q2DW activity peaks during westerly shear zone and intensifies with time at a lower thermospheric altitude (above 90?km). Small positive correlations (r?=?0.2 for sunspot number and r?=?0.1 for 10.7?cm solar flux) have been observed between Q2DW amplitudes and solar activity.     

The pioneer venus orbiter ultraviolet spectrometer experiment: Analysis of hydrogen lyman alpha data

Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUVS) HI 1216Å data from six (6) orbits are analyzed. Analysis of subsolar region periapsis data show that for an exobase temperature of 305K, the exobase density is $\text{5 ± 2(4)}{}^{\mathrm{@}}\text{cm}{}^{\mathrm{?3}}$ and the column abundance of atomic hydrogen between 110 and 200 km is 2.4 ± 0.8(13) cm^?2. The upward flux through the exobase is determined to be 7.5 ± 2.5(7)/cm²s. Apoapsis data were analyzed for both evening and morning geometries. We conclude: (1) the observed limb profiles show a diurnal variation consistent with Brinton et al.; (2) the model temperature field provides a good fit to the morning data, but the morning temperature field must be used to match the evening data; and (3) the theoretical Ly α limb intensity profiles are sensitive to small changes in the shape and magnitude of the variation of exobase hydrogen with solar zenith angle. The solar Ly α flux at line center required to fit the magnitude of the data is 8(11) photons/cm²s Å at Venus.     

Voyager photopolarimeter observations of Saturn and Titan

The Voyager 2 photopolarimeter experiment observed the intensity and polarization of scattered sunlight from the atmospheres of Saturn and Titan in the near-UV at 2640 Å and in the near-IR at 7500 Å. Measurements of Saturn's limb brightening and polarization at several phase angles up to 70° indicate that a significant optical depth of UV absorbers are present in the top 100 mbar of Saturn's atmosphere in the Equatorial Zone and north polar region, and possibly at other latitudes as well. UV absorbers are prominent in polar regions, suggesting that charged particle precipitation from the magnetosphere may be important in their formation.The whole-body polarization of Titan is strongly positive in both the UV and near IR. If spherical particles are responsible for the polarization, no single size distribution or refractive index can account for the polarization at both wavelengths. The model atmosphere proposed by Tomasko and Smith [1], characterized by a gradient in particle size with altitude, seems capable of explaining the Voyager observations. If non-spherical particles predominate, the Voyager observations place important constraints on their scattering properties.     

Longitudinal variations of the ionospheric trough position

For the first time a comprehensive pattern of the longitudinal effect of the ionospheric trough position was obtained. We present new results with longitudinal variations of the winter trough position as a function of geomagnetic latitude for both hemispheres and conditions of high and low solar activity and all local time hours. We used a large observational data set obtained onboard the Kosmos-900, Interkosmos-19 and CHAMP satellites for quiet geomagnetic conditions. We found that a magnitude of the trough position longitudinal effect averaged for a fixed local time is greater in the daytime (6–8°) than in the nighttime (3–5°). The longitudinal effect magnitude reaches its maximum (16°) in the morning (at 08 LT) in the Southern hemisphere at high solar activity. But on certain days at any solar activity the longitudinal effect magnitude can reach 9–10° even at night. The shape of the longitudinal effect was found to differ significantly in two hemispheres. In the Northern hemisphere the trough is usually closest to the pole in the eastern (American) longitudinal sector, and in the Southern hemisphere the trough is closest in the western (Eurasian) longitudinal sector. The magnitude and shape of the longitudinal effect is also different during low and high solar activity. The Global Self-consistent Model of the Thermosphere, Ionosphere, and Protonosphere (GSM TIP) simulations demonstrate that during low solar activity, the longitudinal variations of the daytime trough position is mainly determined by longitudinal variations of the ionization function, formed due to the longitudinal variations in the solar zenith angle and the atomic oxygen density distribution. The longitudinal variations of the nighttime trough position is formed by the longitudinal variations in ionization of precipitating auroral particles, neutral atmosphere composition, and electric field.     

The global distribution and variability of stratospheric constituents measured by LIMS

The purpose of the Nimbus 7 LIMS experiment was to sound the composition and structure of the upper atmosphere and provide data for study of photochemistry, radiation, and dynamics processes. Vertical profiles were measured of temperature and ozone (O₃) over the 10-km to 65-km range and water vapor (H₂O), nitrogen dioxide (NO₂), and nitric acid (HNO₃) over the 10-km to ～50-km range. Latitude coverage extended from 64°S to 84°N. Several general features of the atmosphere have emerged from data analyses thus far. Nitrogen dioxide exhibits rapid latitudinal variations in winter and shows hemispheric asymmetry with generally higher vertical column amount in the summer hemisphere. HNO₃ data show that this gas is highly variable with altitude, latitude, and season. Smallest mixing ratios occur in the tropics, and the largest values occur in the high latitude winter hemisphere. The results show that O₃, NO₂, and HNO₃ are strongly affected during a stratospheric warming. There is a persistently low water vapor mixing ratio in the tropical lower stratosphere (～2–3 ppmv), a poleward gradient at all times in the mission, and evidence of increasing mixing ratio with altitude at tropical and middle latitudes.     

AIRS观测资料研究全球平流层重力波特性

利用2012—2014年1月和7月AIRS（Atmospheric Infrared Sounder）第79通道的观测数据，分析了平流层重力波活动强弱的全球分布以及重力波发生频率的全球分布；分析了重力波活动随纬度和经度的变化特征，给出了重力波活动在全球范围内的热点区域及其活动强度；对比了白天与夜间的重力波活动强度及发生频率.研究表明重力波活动强度呈现出随纬度变化的特征，在低纬度地区（0°—30°），冬季半球重力波活动强度低，夏季半球重力波活动高；在中高纬度地区，冬季半球重力波活动强度高而夏季半球重力波活动强度低.在1月，全球重力波活动有4个突出的热点区域，分别为50°N附近欧洲大陆与大西洋交接地带、北美洲与大西洋交接地，20°S附近南美洲与大西洋交接地区、非洲与印度洋交接地区.在7月，重力波活动突出的地方为巴塔哥尼亚至南极半岛地区，以及50°S和75°E附近的印度洋区域.重力波活动强度在夜间大于白天，但是夜间的强重力波活动区域小于白天.      

Scientific results from the pioneer Saturn infrared radiometer

The Pioneer 11 Infrared Radiometer instrument made observations of Saturn and its rings in broadband channels centered at 20 and 45 μm and obtained whole-disk information on Titan. A planetary average effective temperature of 96.5±2.5 K implies a total emission 2.8 times the absorbed sunlight. Correlation with radio science results implies that the molar fraction of H₂ is 90±3% (assuming the rest is He). Temperatures at the 1 bar level are 137 to 140 K; regions appearing cooler may be overlain by a cloud acting as a 124 K blackbody surface. A minimum temperature averaging 87 K is reached near 0.06 bars. Ring boundaries and optical depths are consistent with those at optical wavelengths. Ring temperatures are 64–86 K on the south (illuminated) side, ～54 K on the north (unilluminated) side, and at least 67 K in Saturn's shadow. There is evidence for a south to north drop in ring temperatures. Titan's 45 μm brightness temperature is 75±5 K.     

Mean winds of the mesosphere and lower thermosphere (60–110 km): A global distribution from radar systems (MF,Meteor, VHF)

During the last decade a large number of radars (～12) have been developed, which have produced substantial quantities of tidally-corrected mean winds data. The distribution of the radars is not global, but many areas are well covered: the Americas with Poker Flat (65°N), Saskatoon (52°N), Durham (43°N), Atlanta (34°N), Puerto Rico (18°N); Europe with Kiruna (68°), Garchy (47°N) and Monpazier (44°N); and Oceania with Christchurch (44°S), Adelaide (35°S), Townsville (20°S), and Kyoto (35°N). Zonal and meridional wind height-time cross-sections from $\text{60}\text{80}$ km (MF/Meteor Radar) to ～110 km have been prepared for the last 5–6 years. They are compared with cross-sections from CIRA-72 for zonal winds, and Groves (1969) for meridional winds.It is shown that while CIRA-72 is still a useful model for many purposes, significant differences exist between it and the new radar data. The latter demonstrate important seasonal, latitudinal, longitudinal and hemispheric variations. The new meridional cross-sections are of great value. The common features with Groves (1969) are the equatorward cells in summer near 85 km; however their strength (～10 ms^?1) and size are less. Systematic and somewhat different variations emerge at higher (?52°N) and middle (35–44°) latitudes.     

基于COSMIC掩星精密定轨数据的等离子体层电子含量研究

等离子体层是日地环境重要的组成部分.本文利用COSMIC掩星精密定轨数据经处理后得到的podTec文件获取等离子体层电子含量（PEC）对等离子体层进行研究.将podTec数据进行处理后获得的PEC（pod-PEC）和IRI-Plas经验模型提供的PEC （IRI-PEC）进行对比，发现pod-PEC与IRI-PEC符合得较好.在低（0°—20°）、中（20°—50°）、高（50°—90°）修正地磁纬度带下，分析了COSMIC在太阳活动极大年（2014年）3，6，9和12月的pod-PEC，得到如下结论:PEC随着纬度升高而逐渐减少，且3，9月PEC在中低纬关于磁赤道的南北对称性较好，6月北半球各纬度带的PEC明显高于南半球同一纬度带的值，而12月情况则完全相反，南半球中纬的PEC甚至会等于北半球低纬的PEC值；PEC在白天高而晚上低，高纬地区的PEC昼夜变化不明显；PEC具有明显的季节性.对于北半球，一年中PEC最大值出现在春季，冬秋季次之，夏季最低，具有明显的年度异常现象.      

Latitudinal asymmetry in electron and ion density distribution in southern and northern hemispheres

With data of satellite INTERCOSMOS-BULGARIA-1300 northern and corresponding southern hemisphere plasma densities have been compared. Southern densities are greater in the ?90 to ?180 and +30 to +120° ranges. The opposite is true for ?60 to +30°. No asymmetry has been observed during daytime. These results are explained by the variations of the magnetic declination.     

Spatio-temporal structure of the wave packets generated by the solar terminator

Using long-term (1998--2009) total electron content (TEC) measurements from the GPS global network including dense network of GPS sites in USA and Japan, we have obtained the first data regarding the spatio-temporal structure and the statistics of medium-scale traveling wave packets (MS TWPs) excited by the solar terminator (ST). Total amount of the detected TWPs exceeds 565,000. There is no correlation between TWPs occurrence and geomagnetic and solar activity. We found that the diurnal, seasonal and spectral MS TWPs characteristics are specified by the solar terminator (ST) dynamics. MS TWPs are the chains of narrow-band TEC oscillations with single packet’s duration of about 1–2 h and oscillation periods of 10–20 min. The total duration of chain is about 4–6 h. The MS TWPs spatial structure is characterized by a high degree of anisotropy and coherence at the distance of more than 10 wavelengths. Occurrence rate of daytime MS TWPs is high in winter and during equinoxes. Occurrence rate of nighttime MS TWPs has its peak in summer. These features are consistent with previous MS travelling ionosphere disturbance (TID) statistics obtained from 630-nm airglow imaging observations in Japan. In winter, MS TWPs in the northern hemisphere are observed 3–4 h after the morning ST passage. In summer, MS TWPs are detected 1.5–2 h before the evening ST appearance at the point of observations, but at the moment of the evening ST passage in the magneto-conjugate point. The obtained results are the first experimental evidence for the hypothesis of the ST-generated ion sound waves.     

Observation of aphelion cloud belt over Martian tropics,its evolution,and associated dust distribution from MCS data

The present study uses five Martian years of observations from Mars Climate Sounder onboard Mars Reconnaissance Orbiter for investigating the Aphelion Cloud Belt (ACB) over the tropics. Analysis of zonal mean water ice column opacity suggests that the spatial extension of the ACB is mainly confined over the tropics and mid-latitudes (-20 – 40°N) during L_S ~ 45 – 135° (L_S = 0° signifies northern spring equinox). The ACB is seen primarily in the nighttime only due to the truncation of the daytime profile observations at significantly higher altitudes (at ~30 km). Zonal mean ice extinction profiles show ACB’s altitudinal range within ~10 – 40 km, and the existence of a thin cloud band in the absence of a thick ACB during aphelion season. Three phases of the ACB could be identified as the formation phase during L_S = 45 – 75° (phase 1), the peak phase during L_S = 76 – 105° (phase 2), and the decaying phase during L_S = 106 – 135° (phase 3). Observation of the cloud latitude belt shows a northward movement starting from phase 2, prominent over regions nearby Lunae Planum and Xanthe Terra. During this phase, the top level of thick clouds within the ACB decreases to ~20 km in the southern hemisphere, while it increases a little over the northern hemisphere (NH). The decreasing tendency continues in phase 3 over the entire region ?10 – 10°N, and the thick cloud base moves higher over the NH, though the vertical depth of it becomes narrower than phase 2. Temperature profiles do not show any noticeable influence on the northward evolution of the ACB. However, the study at a regional level indicates a possible association of upper tropospheric dustiness with the ACB’s evolution. The mechanism is evident in the correlation analysis mostly at an altitude range of ~18 – 35 km. The migrating semidiurnal tide (SMD) as a proxy of dust or water ice forcing, and the calculated upper tropospheric dust radiative heating, shows an apparent northward movement of their peak amplitude within the three phases of the ACB. This match between the spatiotemporal variations of the SMD and the water ice was not observed previously. However, the correlating behavior seems to be prominent in the areas nearby Lunae Planum and Xanthe Terra and the upper-tropospheric region of the atmosphere.     

On the performance of IRI-2016 to predict the North-South asymmetry of the equatorial ionization anomaly around 73°E longitude

The ionospheric total electron content (TEC) in both northern and southern Equatorial anomaly regions are examined by using the Global Positioning System (GPS) based TEC measurements around 73°E Longitude in the Asian sector. The TEC contour charts obtained at SURAT (21.16°N; 72.78°E; 12.9°N Geomagnetic Lat.) and DGAR (7.27°S; 72.37°E; 15.3°S Geomagnetic Lat.) over 73°E longitude during a very low solar activity phase (2009) and a moderate solar activity (2012) phase are used in this study. The results show the existence of hemispheric asymmetry and the effects of solar activity on the EIA crest in occurrence time, location and strength. The results are also compared with the TEC derived by IRI-2016 Model and it is found that the North-South asymmetry at the EIA region is clearly depicted by IRI-2016 with some discrepancies (up to 20% in the northern hemisphere at SURAT and up to 40% in the southern hemisphere at DGAR station for June Solstice and up to 10% both for SURAT and DGAR for December Solstice). This discrepancy in the IRI-2016 model is found larger during the year 2012 than that during the solar minimum year 2009 at both the hemispheres. Further, an asymmetry index, (A_i) is determined to illustrate the North-South asymmetry observed in TEC at EIA crest. The seasonal, annual and solar flux dependence of this index are investigated during both solstices and compared with the TEC derived by IRI.     

Equatorial anomaly according to the Interkosmos-19 data and IRI model: A comparison

The comparison of the IRI model with the foF2 distribution in the equatorial anomaly region obtained by topside sounding onboard the Interkosmos-19 satellite has been carried out. The global distribution of foF2 in terms of LT-maps was constructed by averaging Intercosmos-19 data for summer, winter, and equinox. These maps, in fact, represent an empirical model of the equatorial anomaly for high solar activity F10.7 ~ 200. The comparison is carried out for the latitudinal foF2 profiles in the characteristic longitudinal sectors of 30, 90, 210, 270, and 330°, as well as for the longitudinal variations in foF2 over the equator. The largest difference between the models (up to 60%) for any season was found in the Pacific longitudinal sector of 210°, where there are a few ground-based sounding stations. Considerable discrepancies, however, are sometimes observed in the longitudinal sectors, where there are many ground-based stations, for example, in the European or Indian sector. The discrepancies reach their maximum at 00 LT, since a decay of the equatorial anomaly begins before midnight in the IRI model and after midnight according to the Interkosmos-19 data. The discrepancies are also large in the morning at 06 LT, since in the IRI model, the foF2 growth begins long before sunrise. In the longitudinal variations in foF2 over the equator at noon, according to the satellite data, four harmonics are distinguished in the June solstice and at the equinox, and three harmonics in the December solstice, while in the IRI model only two and one harmonics respectively are revealed. In diurnal variations in foF2 and, accordingly, in the equatorial anomaly intensity, the IRI model does not adequately reproduce even the main, evening extremum.