Low Solar Activity Conditions Research Articles

Anomalous variations in the ionospheric F 2-layer structure at geomagnetic midlatitudes of the Southern and Northern hemispheres at the transition from summer to winter conditions under low solar activity

The structure and dynamics of the ionosphere and plasmasphere at low solar activity under quiet geomagnetic conditions on January 15–17, 1985, and July 10–13, 1986, over Millstone Hill station and Argentine Islands ionosonde, the locations of which are approximately magnetically conjugate, have been theoretically calculated. The detected correction of the model input parameters makes it possible to coordinate the measured and calculated anomalous variations in the electron density NmF2 at the height hmF2 of the ionospheric F2 layer over Argentine Islands ionosonde as well as the calculated and measured values of NmF2 and electron temperature at the hmF2 height over Millstone Hill station. It has been shown that vibrationally excited N2 and O2 molecules almost do not influence the formation of the winter anomaly under the conditions of low solar activity. A difference between the influence of electronically excited O+ on N e ions under winter and summer conditions forms not more than 11% of the N e winter anomaly event in the F 2 layer and topside ionosphere. The model without electronically excited O+ ions reduces the duration of the N e winter anomaly event. It has been shown that the seasonal variations in the composition of the neutral atmosphere form mainly the NmF2 winter anomaly event over the Millstone Hill radar at low solar activity.

Ionospheric variability over Grocka during low solar activity conditions

The variability of the critical frequency of F2 layer, foF2, over ionospheric station Grocka (44.48N, 20.31E) has been studied during the declining phase of solar cycle 23 from 2004 to 2006. The variability index was introduced to identify the daily and seasonal patterns characterizing the local mid-latitude ionosphere during quiet and disturbed geomagnetic conditions. In addition, the behaviour of the vertical total electron content values, vTEC, obtained from global positioning system (GPS) measurements in the surrounding area under these conditions is reported. The analysis shows a number of interesting features representative of the ionospheric variability relevant for ionospheric modelling as well as ionospheric propagation applications based on a single station approach.

Little or no solar wind enters Venus’ atmosphere at solar minimum

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

Electron temperature climatology at Millstone Hill and Arecibo

In this paper, ionospheric electron temperature (Te) data for more than two solar cycles measured by the incoherent scatter radars (ISR) at Millstone Hill (42.6°N, 71.5°W) and Arecibo (18.3°N, 66.7°W) are compared with the theoretical Te calculated from the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (NCAR‐TIEGCM) to investigate the temporal variations of Te. The comparisons are made for both low and high solar activity conditions and for three seasons: equinox, summer, and winter. The observations show that the diurnal variation of Te is characterized by morning and evening peaks at Arecibo and by a morning peak at Millstone Hill. The occurrence and strength of the peaks at Arecibo are significantly different from those at Millstone Hill. Daytime Te tends to increase with solar activity at both stations below ∼300 km. Te above 300 km generally decreases with solar activity; however, it increases with solar activity in equinox and summer at Arecibo, whereas it does so only in summer at Millstone Hill. The TIEGCM model can reproduce these variations. However, the modeled evening peak is weaker than that from observations at Arecibo. The simulations show that the daytime bulge of Te tends to occur at low latitudes and high solar activity, as seen in the observations, and the significant morning peak at low solar activity over Arecibo is associated with the equatorial anomaly. Moreover, an interesting feature predicted by the model is that the midday Te at the F2 peak height increases with solar activity when F10.7 values are less than about 100 × 10−22 W m−2 Hz−1 or larger than 190 × 10−22 W m−2 Hz−1 at Millstone Hill; so does at Arecibo when F10.7 values are larger than 100 × 10−22 W m−2 Hz−1. As a result, a positive correlation between daytime Te and Ne occurs under these conditions.

On the response of the exospheric temperature to the auroral heating impulse during geomagnetic disturbances

On the basis of an assumed impulse-type energy input at auroral oval due to the particle precipitation, the response of the exospheric temperature T ∞ has been investigated in terms of changes in input energy, geomagnetic indices and latitude under simplified assumptions It is shown that the resulting T ∞ is proportional to the peak heating rate and inversely proportional to the gradient of the atmospheric density. Even during low solar activity and quiet geomagnetic conditions, the precipitated electron energy density in the thermosphere is found in the range of about 1–1.5×10 −6 erg/cm 3 s, reaching to about 3×10 −6 erg/cm 3 s at 2000 K and is easily supported by the solar wind–magnetospheric interaction. The auroral energy inputs lasting less than 2 h or so hardly generate geomagnetic storms. The inhomogeneous energy equation has been solved in horizontal two dimensions in presence of horizontal heat conduction, to study the T ∞ at other latitudes and its time delay with respect to the peak impulse heating time. This time delay is independent of the storm intensity, i.e., proxy geomagnetic indices and is about 6–7 h at low and mid latitudes and about 3 h or less at high latitudes in agreement with recent coupled magnetospheric–ionospheric–thermospheric models and with the available observed values. The diffusion coefficient of horizontal heat conduction deduced is about 7×10 4 m 2/s. The role of the horizontal heat conduction in the energy budget needs further investigation.

A Modeling Study of the Influence of Electric Fields on Ionosphere Semiannual Variations at Mid- and Low Latitudes

With a theoretic ionospheric model, we investigate the semiannual variations of the ionospheric F2 region peak electron density (NmF2) at mid- and low latitudes under the low solar activity and quiet geomagnetic conditions, and focus on how the electric fields modify the semiannual component of NmF2. The result indicates that when the input electric fields have no semiannual variation, the NmF2 at the geomagnetic equator also has some semiannual component, and the amplitudes of semiannual component decrease as the latitude increases. When the input electric fields have some semiannual component, the amplitudes of NmF2 semiannual component is strongly modified, and seems to be dependent upon magnetic latitudes. It is also found the modification of electric fields on the NmF2 may have a close relationship with the ‘fountain effect’ in the equatorial ionosphere.

An empirical model of electron temperature in the Indian topside ionosphere for solar minimum based on SROSS C2 RPA data

An empirical model of electron temperature ( T e) for Indian equatorial and low latitudes at the altitude of 500 km is presented. The model, applicable for low solar activity, is based on the observation that electron temperature and electron density ( N e) in the F region of the Indian equatorial and low latitude ionosphere are inversely correlated during the daylight hours (0600–1800 LT) at all latitudes and in all seasons. Nighttime (1900–0500 LT) T e was found to be independent of N e. Electron temperature and electron density measured by the Retarding Potential Analyzer (RPA) instrument on board the Indian SROSS C2 satellite during the solar minimum period January 1995 to December 1996 have been used to derive the relationship between T e and N e. The satellite observations correspond to an average altitude of 500 km. Regression coefficients were obtained from least square fits for all data during daylight hours from 10°S to 15°N magnetic latitudes at 5° intervals. Using the coefficients and corresponding N e data for summer, winter and equinox, T e was calculated for the daylight hours. Average nighttime T e was taken as the model value for these hours. Latitudinal profiles were obtained employing a system of associated Legendre polynomials. The modelled and observed electron temperature was compared and found to be in good agreement. The model is limited to fixed altitude and low solar activity conditions.

Analysis of the orbital decay of spherical satellites using different solar flux proxies and atmospheric density models

In order to highlight the performances of the JR-71 and MSISE-90 atmospheric density models and also to investigate whether the use of the E 10.7 index, instead of the F 10.7 cm solar radio flux, improves the trajectory modeling of low Earth objects, the orbital decay of 11 spherical satellites was analyzed. The data were distributed over a full solar activity cycle and covered altitudes in between 150 and 1500 km. The observed evolution of the mean semi-major axis was obtained by processing the historical two-line elements record. For each satellite, environmental condition, atmospheric model and solar flux proxy, a drag coefficient was computed by fitting the observed semi-major axis decay, comparing the results with the theoretical estimates. Below 400 km, both models overestimated the atmosphere density, independently on the solar flux proxy and environmental conditions. MSISE-90, with F 10.7, proved to be the best model to compute the air density, in low solar activity conditions. JR-71, with F 10.7, was generally more accurate over the maximum of the 23rd solar cycle, while MSISE-90 revealed to be the best model in the increasing phase of the solar flux, when using E 10.7. Between 750 and 850 km, the average accuracy of the models was generally better than 15% and the choice of the atmospheric density model was relatively inconsequential, both in conditions of low and high solar activity. A quite large uncertainty was instead observed in moderate conditions, when using E 10.7 in both models. At 1500 km, both models underestimated the air density by more than 35%, in low solar activity conditions. For high solar fluxes, the density was well represented by JR-71, both using F 10.7 and E 10.7, while it resulted to be underestimated, by about 25%, when using MSISE-90, both with E 10.7 and F 10.7. In general, more accurate orbital decay fits were obtained with E 10.7, which seems able to better describe the atmosphere density changes induced by a varying solar activity. In conclusion, none of the models analyzed was able to accurately describe the atmosphere density in all orbital regimes and environmental conditions, independently on the solar flux proxy.

Atmospheric densities derived from CHAMP/STAR accelerometer observations

The satellite CHAMP carries the accelerometer STAR in its payload and thanks to the GPS and SLR tracking systems accurate orbit positions can be computed. Total atmospheric density values can be retrieved from the STAR measurements, with an absolute uncertainty of 10–15%, under the condition that an accurate radiative force model, satellite macro-model, and STAR instrumental calibration parameters are applied, and that the upper-atmosphere winds are less than 150 m/ s . The STAR calibration parameters (i.e. a bias and a scale factor) of the tangential acceleration were accurately determined using an iterative method, which required the estimation of the gravity field coefficients in several iterations, the first result of which was the EIGEN-1S (Geophys. Res. Lett. 29 (14) (2002) 10.1029) gravity field solution. The procedure to derive atmospheric density values is as follows: (1) a reduced-dynamic CHAMP orbit is computed, the positions of which are used as pseudo-observations, for reference purposes; (2) a dynamic CHAMP orbit is fitted to the pseudo-observations using calibrated STAR measurements, which are saved in a data file containing all necessary information to derive density values; (3) the data file is used to compute density values at each orbit integration step, for which accurate terrestrial coordinates are available. This procedure was applied to 415 days of data over a total period of 21 months, yielding 1.2 million useful observations. The model predictions of DTM-2000 (EGS XXV General Assembly, Nice, France), DTM-94 (J. Geod. 72 (1998) 161) and MSIS-86 (J. Geophys. Res. 92 (1987) 4649) were evaluated by analysing the density ratios (i.e. “observed” to “computed” ratio) globally, and as functions of solar activity, geographical position and season. The global mean of the density ratios showed that the models underestimate density by 10–20%, with an rms of 16–20%. The binning as a function of local time revealed that the diurnal and semi-diurnal components are too strong in the DTM models, while all three models model the latitudinal gradient inaccurately. Using DTM-2000 as a priori, certain model coefficients were re-estimated using the STAR-derived densities, yielding the DTM-STAR test model. The mean and rms of the global density ratios of this preliminary model are 1.00 and 15%, respectively, while the tidal and latitudinal modelling errors become small. This test model is only representative of high solar activity conditions, while the seasonal effect is probably not estimated accurately due to correlation with the solar activity effect. At least one more year of data is required to separate the seasonal effect from the solar activity effect, and data taken under low solar activity conditions must also be assimilated to construct a model representative under all circumstances.

Analysis of TEC data from the TOPEX/Poseidon mission

TOPEX/Poseidon mission has provided an extensive database of vertical TEC over the ocean since August 1992. Data from nearly 10 years of TOPEX TEC observations were analyzed to study the TEC climatology. First, TEC data were binned by season, geomagnetic activity, and solar activity to create longitudinally averaged TEC maps in magnetic latitude and local time. These maps show the annual and semiannual anomalies well known from climatological studies of NmF2 but lack the seasonal anomaly because of the longitudinally averaged binning. The equatorial anomaly is the most prominent feature in the maps, and they show strong TEC variations with solar activity but relatively weak variations with geomagnetic activity in our three Kp bins. Compared with the low solar activity conditions (F10.7 < 120), the TEC values for F10.7 ≥ 120 are much larger and the equatorial anomaly lasts longer into the night, up to midnight. During geomagnetically active periods, the TEC maps generally show a noticeable increase at low latitudes for F10.7 < 120, but this effect is barely detectable for F10.7 ≥ 120. Finally, three longitudinal bins (Indian, Pacific, and Atlantic) were added in order to see how the TEC morphology varies with longitude. The TEC measurements display strong longitudinal variations that closely follow the longitudinal variation of the magnetic declination. In the southern Pacific, where the declination is positive and large, the diurnal TEC variations significantly differ from those in the other longitude sectors, where the magnetic declination is negative in the Southern Hemisphere. Also, at noon, the phase of the longitudinal TEC variation is typically opposite to that at midnight.

Ionospheres of Venus and Mars: a comparative study

Magnetometer measurements on the Pioneer Venus Orbiter (PVO) have shown that Venus is a non-magnetic planet and similar measurements on the Mars Global Surveyor (MGS) have demonstrated that, except for some very localized magnetic fields of crustal origin, Mars intrinsic magnetic field is rather weak. As a consequence, solar wind interacts directly with the atmospheres and ionospheres of these planets. Much has been learnt about the ionosphere of Venus from the large database generated by the various aeronomy experiments on the PVO, especially during the solar maximum. For example, the major ion in the upper ionosphere (topside) of Venus is O +, and is controlled by diffusion, while O 2 + dominates in the bottomside (below 200 km) and is in photochemical equilibrium. Mars ionosphere is relatively less explored and most of the information has come from radio occultation experiments, which have provided a large data base on the electron density profiles for various solar activity conditions. Only two direct ion measurements exist due to Viking Landers, which correspond to solar minimum conditions and have shown that O 2 + dominates in most part of the Mars ionosphere. However, its height distribution is in variance with that expected theoretically from the diffusive equilibrium state. During high solar activity when Venus ionosphere is robust, solar wind interaction results in a sharp boundary of the ionosphere, called the ionopause. However, during solar minimum, when the ionosphere is weak, the solar wind compresses the Venus ionosphere to its limiting altitude in the photodynamical regime forming a thick ionopause. The diffusion region (or the topside ionosphere) is then nearly extinct. This thick ionopause is also seen at times of high solar wind dynamic pressure (Psw) during solar maximum with O + still as the dominant ion there. The diffusion region is nearly extinct again. Since Mars ionosphere is weak, particularly during low and moderate solar activity conditions, one expects the solar wind to compress its ionosphere in the photodynamical regime forming a thick ionopause. Consequently, the diffusion region (or the topside ionosphere) would be nearly extinct most of these times. The observed Viking O 2 + profiles and the radio occultation Ne profiles, which are indeed greatly compressed provide evidence in this regard. Thus the observed topside ionospheric profiles of Venus and Mars basically represent the ionopause regions, only exception being the solar maximum when an extended topside ionosphere exists at Venus.

Equatorial F2-peak parameters in the IRI model

We have used measurements of an ionosonde station near the magnetic equator in Ouagadougou, Burkina Faso to evaluate the ability of the International Reference Ionosphere (IRI) model to correctly represent ionospheric F2 peak parameters in this region. The data represent conditions of high and low solar activity. Comparing the URSI and CCIR option for the F2 plasma frequency, foF2, we find that for low solar activity both options agree quite well with the ionosonde foF2 values and overall the CCIR maps show a slightly better fit. During high solar activity discrepancies are found during nighttime and the URSI maps providing the overall better fit. The measured F2 peak height values, hmF2, are compared on one hand with IRI predictions that are obtained based on the CCIR model for the propagation factor M(3000)F2 and on the other hand with the ionosonde-measured M(3000)F2 values. As expected using the measured values results in more accurate predictions. It is important to note that with the measured M(3000)F2 values IRI predicts the characteristics post-sunset that is seen in the measurements but not in the IRI predictions with the CCIR-M(3000)F2 model.

First in situ observations of equatorial ionospheric bubbles by Indian satellite SROSS‐C2 and simultaneous multisatellite scintillations

The first observation of equatorial ionospheric irregularities by RPA probe of the Indian low Earth orbiting satellite SROSS‐C2 is presented in this paper. Amplitude scintillations of medium Earth orbiting Global Positioning System (GPS) satellites and geostationary FLEETSATCOM (244 MHz, 73°E) and INMARSAT (1.5 GHz, 65°E) signals recorded simultaneously at Calcutta (lat: 22.97° N, long: 88.50°E geographic; dip: 32°N) are used for a coordinated study of equatorial F region irregularities in the Indian zone. Cases of ionospheric irregularities identified from the SROSS‐C2 records obtained during the initial one‐and‐a‐half years since its launch in May 1994 have been analyzed. Some events of in situ ion density irregularities are compared with scintillations simultaneously observed on the transionospheric satellite links. Intense bite‐outs of ion density (maximum relative irregularity amplitude ΔN/N ∼ 65%) were detected on one occasion (October 29, 1994) coupled with deep fadings (S4 ∼ 1 at VHF, ∼0.52 at L‐band, and ∼0.69 at GPS L1 frequency) on ground‐based satellite links. An estimate of scintillation indices from the observed in situ density deviations compares well with the ground‐based measurements. The development of intense equatorial bubbles even on a day like October 29, 1994, under low solar activity conditions, may be attributed to a prompt penetration of magnetospheric electric field equatorwards during the main phase of a magnetic storm in progress [maximum negative excursion of Dst ∼ −127 nT at 1600UT (2100MLT) with a dDst/dt rate −37 nT/hr at 1300–1400UT (1800–1900MLT)]. The drift velocity and spatial extent of these irregularities have been estimated from ground‐based observations.

Electron density profiles deduced from GPS TEC, O+-H+ transition height and ionosonde data

A new formula of the electron density profiles above the peak height is introduced. The formula is based on the Epstein layer and depends on the O and H scale heights and the OH transition level. Both scale heights have a ratio 1:16 reduced by a factor representing the change from magnetic field line direction to vertical direction. The bottom-side part of TEC (calculated by using foF2, M3000F2 and foE measurements) is subtracted from the GPSderived TEC at the same location. The topside TEC, together with the empirically obtained O-H transition level, are then used to deduce the unknown scale heights. The method is demonstrated on actual data covering low and high solar activity conditions.

Comparison and Accuracy Assessment of Semi-Empirical Atmosphere Models through the Orbital Decay of Spherical Satellites

A detailed comparison and assessment of the accuracy of semi-empirical atmospheric density models (JR-71, MSIS-86/90, TD-88) were carried out by analyzing the orbital decay of nine spherical satellites in the 150-1500 km altitude range. The orbital decay data used were distributed over a full solar activity cycle (1987-1999). The drag coefficients which were estimated by fitting the observed semimajor axis evolution with a high accuracy orbit propagator were compared with those computed by theoretical analysis. While MSIS-86/90 proved to be the best model to compute the air density below 400 km, in low solar activity conditions, JR-71 was more accurate at greater altitudes and/or solar fluxes. TD-88, defined below 700 km, was quite accurate (10%) only during periods of moderate solar activity. Between 350 and 800 km, the average accuracy of JR-71 and MSIS-86/90 was generally better than 15% and, sometimes, even better than 10%. However, at 1500 km and during low solar activity conditions, both models underestimated the air density by 60%, while below 350 km, still during low solar activity conditions, they overestimated the air density by 20-25% and 10-20%, respectively.

Atmospheric effects of precipitating energetic hydrogen atoms on the Martian atmosphere

The Martian atmosphere is under the influence of an intense flux of precipitating energetic (≲1 keV) hydrogen atoms. In the solar wind and in the magnetosheath, fast hydrogen atoms are produced by charge exchange between solar wind protons and the hydrogen corona. Atmospheric effects of the precipitating hydrogen atoms are thus manifestations of the direct interaction between solar wind protons and the planetary neutrals. A three‐dimensional Monte Carlo simulation model has been developed to study different atmospheric effects of the precipitating hydrogen atoms. The model is used to calculate the altitude profiles of the energy deposition rates, the ion production rates, and the photon emission rates at different solar zenith angles under low solar activity conditions. The peak loss and production rates under typical solar wind conditions caused by precipitating hydrogen atoms are estimated to be ∼1% of the corresponding peak values due to extreme ultraviolet radiation but comparable or larger than effects of H+ and O+ precipitation at low altitudes. The results indicate that a substantial part of the incoming particle and energy flux is scattered back from the Martian atmosphere.

Upper limits for the Martian exospheric number density during the Planet B/Nozomi mission

Dissociative recombination (DR) of ionospheric O 2 + ions is an important source of suprathermal atomic oxygen in the exosphere as previous studies about the Martian upper atmosphere have shown. Because of the weaker gravitational attraction a hot oxygen corona on Mars should be denser than that observed on Venus. Since the most important mechanism for the production of the hot oxygen atoms in the Martian exosphere is DR, we investigated the variability of this production mechanism depending of solar activity. The Japanese Nozomi spacecraft will have the possibility to detect with the neutral mass spectrometer (NMS) for the first time in-situ the theoretically predicted hot oxygen corona on Mars, if the corona number density above the cold background atmosphere is of the order of 10,000 cm −3 . Due to a problem in the propulsion system Nozomi failed its planned arrival rendevouzs with Mars in October 1999 and will, therefore, arrive at the red planet not before January 2004. Solar activity will reach its maximum in 2001, so the related production rate of hot oxygen atoms will be in the medium range during the new arrival date of Nozomi. We used the ionospheric profiles from the Viking mission for low solar activity conditions ( F 10.7≈70) and the Mariner 9 mission with a solar activity of about 120 for medium solar wind activity. The latter is comparable to the level we expect for the Mars arrival of Nozomi. The resulting influence of the hot oxygen corona number density distribution was calculated with a Monte Carlo technique. This technique is used to compute a hot particle density distribution function. We studied the atomic diffusion process in the Martian atmosphere by simulating the collision probability, particle direction and energy loss after collisions by generating random numbers. Compared to previous studies we have improved the Monte Carlo model by using more and smaller altitude steps and more detailed treatment of particles with a temporary downward motion. This has resulted in an increased amount of collisions and a shift to lower energies in the energy spectrum. Our results show that the hot oxygen component should begin to dominate above the cold background atmosphere at an altitude of about 500 km above the Martian surface. The NMS instrument on board of Nozomi should detect the hot oxygen component after its arrival at Mars in January 2004, at an altitude of about 600 km above the Martian surface. Since the solar activity will decrease during the mission the measurements during the first orbits will be the most significant ones. The first in-situ measurements of the hot oxygen number density would be very important for adjusting atmospheric escape models by separating ballistic, satellite and escape trajectories of the hot oxygen atoms, which are significant for studies of the evolution and solar wind interaction of the Martian atmosphere.

The relative importance of solar activity and anthropogenic influences on the ion composition, temperature, and associated neutrals of the middle atmosphere

The changes arising from variations in solar activity and human activities on atmospheric composition and thermal structure of the middle atmosphere, especially in the ion composition, are assessed using interactive one‐ and two‐dimensional chemical models of the stratosphere, mesosphere, and lower thermosphere. The changes from anthropogenic activities are less in magnitude but permanent in nature, and changes due to variations in the solar activity are comparatively higher but periodic. Our results may have bearing and could explain some of the results reported in recent literature on observed lower ionospheric data and strengthen the global change concept in this region. Model response to available observations is quite reasonable, and a good agreement on temperature trend and ionization parameters is found. The impact of increasing concentrations of several greenhouse gases (anthropogenic origin) and variations in solar activity on the ion composition, temperature, and neutral species (of interest to ionization) is examined in two different modeling studies. In each case, the effect of the other forcing is filtered out. A cooling of the order of −10°K in the mesosphere and a maximum of −14°K in the stratosphere is predicted for the double‐CO2 scenario. A decreasing trend in nitric oxide number density from −40% to −95% with altitude is noticed for the double‐CO2 case contrary to an increasing trend (10% to 140%) for high solar activity (HSA) as compared to low solar activity (LSA) conditions in the mesosphere and the lower thermosphere. As a result of changes in temperature and in the concentrations of NO, O2, H2O, O, CH3CN, a decreasing trend of −50% for the double‐CO2 case and an increasing trend of 60% for HSA as compared to LSA are predicted in NO+ ion number density near the mesopause for the tropics. The variations in water cluster ions are negligible in the stratosphere but reach to −50% in the lower thermosphere for the double‐CO2 scenario. The total ion density varies from −30% at 40 km to +30% at 90 km for HSA as compared to LSA conditions and from −5% to 12% from the stratosphere to the mesosphere for the double‐CO2 scenario.

A global measurement of lower thermosphere atomic oxygen densities

The first global measurements of the atomic oxygen fine structure emission line O(³P1) → O(³P2) at a wavelength of 63µm were performed by the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiments in November 1994 and in August 1997. The measured limb radiances were converted to atomic oxygen densities in the altitude regime from 130 km to 175 km using a radiative transfer code and MSIS thermospheric temperatures. The resultant densities are on the average about 40% below the predictions of the MSIS model for the low solar activity conditions of the CRISTA experiments and at all altitudes covered by the measurement. The densities exhibit considerable horizontal structures which are only in partial agreement with the MSIS predicted relative variations. The latitudinal distribution shows steeper decreases towards the poles than given in MSIS.

Dependencies of the amplitude of the temperature enhancement maximum and atomic oxygen concentrations in the mesopause region on seasons and solar activity level

Long term temperature measurements of the mesopause and lower thermosphere give a possibility to obtain altitude profiles of the mean annual temperature and to reveal a regular occurrence of the temperature maximum for heights of 85–95 km, the amplitude of which correlates with the solar activity level. Distributions with height of the atomic oxygen concentration for low and high solar activity conditions have been calculated on the basis of an empirical model of 557.7 nm emission variations and its photochemical theory. It is shown that there is the distinct correlation between an increment of the temperature and the density of the atomic oxygen. Apparently, a reaction of CO2 with O2 causes this phenomenon.