Air density at heights near 200km from the orbit of 1970-65D

Cosmos 359 rocket, 1970-65D, entered orbit on 22 August 1970, with an initial perigee height of 209 km and inclination 51·2°, and decayed on 6 October 1971. Using the values of perigee height from RAE orbits and decay rates from USAF Spacetrack bulletins, 146 values of air density have been calculated between August 1970 and September 1971, mainly at heights between 180 and 230 km.On ten occasions in 1971 when there were substantial geomagnetic disturbances there were sudden increases in density, the largest being about 32 per cent.When the density was corrected to a fixed height and allowance was made for the day-tonight variation and the effects of solar activity, the dominant feature was a semi-annual variation, with maxima in density centred at 6 November 1970 and 7 April 1971, and minima centred at 5 January and 28 July 1971. The maxima in density are nearly equal and exceed the minima by about 50 per cent.     

Variations in air density from January 1972 to April 1975 at heights near 200 km

Variations in air density have been determined using the orbit of the satellite Cosmos 462, 1971-106A, which entered orbit on 3 December 1971 with an initial perigee near 230 km and inclination 65.75°, and decayed on 4 April 1975. Accurate orbits determined at 85 epochs give perigee height correct to about 200 m throughout the satellite's lifetime. Using these values of perigee height and orbital decay rates from NORAD elements, 604 values of air density at half a scale height above perigee have been evaluated. These densities have been compared with values from the COSPAR International Reference Atmosphere 1972, taking account of variations due to solar activity and geomagnetic disturbances, and day-to-night variations, to reveal the residual variations in density at a series of standard heights, 245, 240, 232 and 213 km.The main residual variation is semi-annual, with maxima usually in April and October, and minima usually in January and July; but it is irregular in phase and shape. The amplitude of the semi-annual variation is remarkably constant from year to year between 1972 and 1975, and considerably greater than that given by CIRA 1972: the April/July density ratio is 1.68, not 1.32 as in CIRA; the October–November maxima are all lower than the April maxima, whereas CIRA gives the opposite; the July minima are 18% lower than the January minima, as opposed to 10% in CIRA.A standardized semi-annual density variation for the early 1970s is presented, with January minimum of 0.94, April maximum of 1.28, July minimum of 0.77 and October–November maximum of 1.22. In addition, three other recurrent variations are recognizable: in each year the density has a subsidiary minimum in May and maximum in June; there are low values in mid November and high values in late December.     

The helium drag effect on the orbit of Explorer 19

The orbit of Explorer 19 (1963-53A) has been determined at 60 epochs between February 1976 and October 1976 from over 3000 observations. Using values of the orbital decay rate corrected for the effects of solar radiation pressure, 58 values of air density at a height of 900 km have been evaluated. After correcting for solar and geomagnetic activity and seasonal-latitudinal and diurnal variations in the exospheric temperature, the residual variation exhibited modulations associated with the ‘winter helium bulge’.An examination of three different models of the helium variation has indicated a procedure, which combines distinct features of the CIRA (1972) and Jacchia (1977) model atmospheres, for determining the atmospheric drag effect on Explorer 19. It is proposed that this technique may be equally applicable to any satellite in near-polar orbit at an equivalent height.     

Air density at heights near 150 km in 1970, from the orbit of Cosmos 316 (1969-108A)

Cosmos 316 (1969-108A) was launched on 23 December 1969 into an orbit with an initial perigee height of 154 km at an inclination of 49.5° to the equator. Being very massive, Cosmos 316 had a longer lifetime than any previous satellite with such a low initial perigee: it remained in orbit until 28 August 1970. Because of its interest for upper-atmosphere research, the satellite was intensively observed, and accurate orbits are being determined at RAE from all available observations. Using perigee heights from the RAE orbits so far computed, and decay rates from Spacetrack bulletins, 102 values of air density have been obtained, giving a detailed picture of the variations in density at heights near 150 km between 24 December 1969 and 28 August 1970. The three strongest geomagnetic storms, on 8 March, 21 April and 17 August 1970, are marked by sudden increases in density of at least 23, 15 and 24 per cent respectively. With values of density extending over eight months, it is possible for the first time to examine a complete cycle of the semi-annual variation at a height near 150 km: the values of density, when corrected to a fixed height, exhibit minima in mid January and early August; at the intervening maximum, in April, the density is 30 per cent higher than at the minima.     

Air density at heights near 200 km from the orbit of 1969-20B

The rocket of Cosmos 268, 1969-20B, entered orbit on 5 March 1969, with an initial perigee height of 230 km and inclination of 48.40°. Accurate orbits were computed at RAE from all available observations. Using the values of perigee height from the RAE orbit and decay rates from Spacetrack bulletins, 103 values of density have been calculated between July 1969 and February 1970. On three occasions when geomagnetic activity was strong there were sudden increases in density. When the density was corrected to a fixed height, the semi-annual variation was apparent. There was a strong minimum in July 1969, a maximum in October–November 1969 and a weak minimum in January 1970.     

Determination and geophysical interpretation of the orbit of China 2 rocket (1971-18B)

The orbit of China 2 rocket, 1971-18B, has been determined at 114 epochs throughout its 5-yr life, using the RAE orbit refinement program PROP 6, with more than 7000 radar and optical observations from 83 stations.The rocket passed slowly enough through the resonances 14:1, 29:2, 15:1 and 31:2 to allow lumped geopotential harmonic coefficients to be calculated for each resonance, by least-squares fittings of theoretical curves to the perturbation-free values of inclination and eccentricity. These lumped coefficients can be combined with values from satellites at other inclinations, to obtain individual harmonic coefficients.The rotation rate of the upper atmosphere, at heights near 300 km, was estimated from the decrease in orbital inclination, and values of 1.15, 1.05, 1.10 and 1.05 rev/day were obtained between April 1971 and January 1976. From the variation in perigee height, 25 values of density scale height were calculated, from April 1971 to decay. Comparison with values from the COSPAR International Reference Atmosphere 1972 shows good agreement between April 1971 and October 1975, but the observational values are 10% lower, on average, than CIRA thereafter.A further 1400 observations, made during the final 15 days before decay, were used to determine 15 daily orbits. Analysis of these orbits reveals a very strong West-to-East wind, of 240 ± 40 ms^?1, at a mean height of 195 km under winter evening conditions, and gives daily values of density scale height in the last 7 days before decay.     

Upper-atmosphere features revealed by the orbit of 1980-43A

The satellite NOAA-B (1980-43A) was launched in May 1980 into an orbit with perigee height near 260 km and apogee height 1440 km, at an inclination of 92.2°.The lifetime was 11 months. The orbit has been determined at 40 epochs between October 1980 and May 1981 from about 3000 radar and optical observations. The average orbital accuracy, radial and cross-track, was about 100 m, with rather better accuracy in the final 14 days.The variation of orbital inclination has been analysed to determine two good values of atmospheric rotation rate, namely 1.10 ± 0.10 rev day^?1 at 300 km (average local time) and 1.15 ± 0.06 rev day^?1 at 225 km (evening).The effect of atmospheric rotation on the precession of the orbital plane of an actual satellite has never previously been detected; it is clearly apparent for 1980-43A in its last days and conforms to the expected theoretical change.The variation of perigee height has been analysed to determine ten values of atmospheric density scale height, for heights of 280–370 km. These values, accurate to about 3%, exceed by 15% the values indicated by the COSPAR International Reference Atmosphere. Solar activity was higher in the years 1980–1981 than at any time since early 1958 and it appears that the CIRA model underestimates the density and density scale height at high levels of solar activity.     

Air density at heights near 435 km from the orbit of skylab 1 (1973-27A)

On February 8, 1974, Skylab 1 was manoeuvred into a near circular orbit of inclination 50.04° and perigee near 420km. Orbital parameters have been computed at forty-six epochs thereafter using all available observations. Using these orbital elements, supplemented by orbital decay rates derived from NORAD bulletins, 193 values of air density were determined between 23 February, 1974 and 11, August, 1976. Corrected to a fixed height and normalised with respect to exospheric temperature these values reveal the semi-annual variation, exhibiting maxima in March–April and October–November, and minima in January–February and July–August. For 1974–1976 the July minima are more pronounced than the January minima whilst the April and October maxima appear equal. Overall the variation is greater than that indicated by CIRA 1972.     

Mass density of the upper atmosphere derived from Starlette’s Precise Orbit Determination with Satellite Laser Ranging

The atmospheric mass density of the upper atmosphere from the spherical Starlette satellite’s Precise Orbit Determination is first derived with Satellite Laser Ranging measurements at 815 to 1115 km during strong solar and geomagnetic activities. Starlette’s orbit is determined using the improved orbit determination techniques combining optimum parameters with a precise empirical drag application to a gravity field. MSIS-86 and NRLMSISE-00 atmospheric density models are compared with the Starlette drag-derived atmospheric density of the upper atmosphere. It is found that the variation in the Starlette’s drag coefficient above 800 km corresponds well with the level of geomagnetic activity. This represents that the satellite orbit is mainly perturbed by the Joule heating from geomagnetic activity at the upper atmosphere. This result concludes that MSIS empirical models strongly underestimate the mass density of the upper atmosphere as compared to the Starlette drag-derived atmospheric density during the geomagnetic storms. We suggest that the atmospheric density models should be analyzed with higher altitude acceleration data for a better understanding of long-term solar and geomagnetic effects.     

Analysis of thermospheric density variations neglected in modern atmospheric models using accelerometer data

The accuracy of the atmospheric density and its spatiotemporal variations given by the NRLMSISE-00 atmosphere model at the solar minimum are estimated using density measurements of the CACTUS microaccelerometer at heights of 270–600 km. The model errors are found to be noticeably (by a factor of 2–3) higher than the errors in atmospheric densities obtained from satellite drag data at solar minima. Microaccelerometer density data are used to study short-period (during one orbit) spatiotemporal density variations. The analysis of density variations over one orbit reveals orographic and continental effects. The amplitudes of the continental and orographic effects are estimated at 10–15% at a height of 270 km and 40% at a height of 600 km.     

The calculated and observed diurnal variation of the ionosphere over Millstone Hill on 23–24 March 1970

A numerical model of current F-region theory is use to calculate the diurnal variation of the mid-latitude ionospheric F-region over Millstone Hill on 23–24 March 1970, during quiet geomagnetic conditions. From the solar EUV flux, the model calculates at each altitude and time step primary photoelectron spectra and ionization rates of various ion species. The photoelectron transport equation is solved for the secondary ionization rates, photoelectron spectra, and various airglow excitation rates. Five ion continuity equations that include the effects of transport by diffusion, magnetospheric-ionospheric plasma transport, electric fields, and neutral winds are solved for the ion composition and electron density. The electron and ion temperatures are also calculated using the heating rates determined from chemical reactions, photoelectron collisions, and magnetospheric-ionospheric energy transport. The calculations are performed for a diurnal cycle considering a stationary field tube co-rotating with the Earth; only the vertical plasma drift caused by electric fields perpendicular to the geomagnetic field line is allowed but not the horizontal drift. The boundary conditions used in the model are determined from the incoherent scatter radar measurements of T_e, T_i and O⁺ flux at 800km over Millstone Hill (Evans, 1971a). The component of the neutral thermospheric winds along the geomagnetic field has an important influence on the overall ionospheric structure. It is determined from a separate dynamic model of the neutral thermosphere, using incoherent scatter radar measurements.The calculated diurnal variation of the ionospheric structure agrees well with the values measured by the incoherent scatter radar when certain restrictions are placed on the solar EUV flux and model neutral atmospheric compositions. Namely, the solar EUV fluxes of Hinteregger (1970) are doubled and an atomic oxygen concentration of at least $\text{10}{}^{11}\text{cm}{}^3$ at 120 km is required for the neutral model atmosphere. Calculations also show that the topside thermal structure of the ionosphere is primarily maintained by a flow of heat from the magnetosphere and the night-time F2-region is maintained in part by neutral winds, diffusion, electric fields, and plasma flow from the magnetosphere. The problem of maintaining the calculated night-time ionosphere at the observed values is also discussed.     

The representation of the semi-annual variation of the geomagnetic activity by an annual sine wave

The seasonal variation of the geomagnetic activity shows two sharp maxima (in March and September) and two broader minima (in June and December). It can only poorly be described by a double sine wave. The double phase wave of geomagnetic activity can be transformed - by vertical mirroring of the half year part between the maxima - into a single phase wave, which is represented well by a single sine function. This function is determined here for C _i (the daily international character figure of geomagnetic activity) and for A _p (the equivalent daily amplitude, based on K _p, the geomagnetic planetary three-hour-range indices), for both in their ratios to the mean value over the year and then averaged over many years. To remove part of the irregularities the daily values of C _i and A _p were corrected for solar activity and reduced to quiet Sun circumstances. Mirroring back to the double phase function the geomagnetic variation is then represented by $$Ci({\text{or }}Ap) = Cm({\text{or }}Ap,m) - |A{\text{ sin}}(\lambda - \varphi )|$$ , in which m means the mirror value, A is the amplitude of the single sine curve, λ runs parallel to the Sun's longitude, ? is the phase constant and the bars indicate the absolute value. The data of the first maximum of the seasonal variation was found to vary between March 18 and 28 for different groups of years. The sharpness of the maxima may point out a resonance in the interaction between the solar wind and the magnetosphere. In the appendix the relation \(Ci = aR^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}} + b\) (R being the relative sunspot number) is brought forward. The values of the parameter b through the eleven-year period reveal an increasing influence of sunspot-free regions towards the minimum.     

Density variations in the lower thermosphere from analysis of the AE-C accelerometer measurements

Density measurements at 140, 160, 180 and 200km from the AE-C accelerometer experiment during 1974 are analyzed by least-squares multiple linear regression. The resulting empirical model is compared to the widely-used Jacchia (1971) model which is primarily based on satellite orbital decay data above 200 km. The semiannual and geomagnetic (K_p) variations derived from the AE-C data set are in good agreement with Jacchia's predictions. However, individual magnetic storm responses often exceed such average representations of the geomagnetic effect by more than a factor of two. The density is more variable with respect to the daily 10.7cm solar flux than Jacchia's model predicts. The regression analysis reveals a positive correlation with the absolute value of the geographic latitude. The statistical weakness of this latter effect reflects the transient, aperiodic nature of high-latitude heating events related to magnetic activity, which can only be properly analyzed on an individual basis. Finally, the sub-solar bulge effect enters the regression equation with about half the amplitude predicted by the Jacchia model at these heights.     

Variations in air density at heights near 200 km during 1967–1969, from the orbit of 1967-31A

Air density at a height of 180–200 km from July 1967 to September 1969 has been determined from analysis of the high eccentricity orbit of satellite 1967-31A. The data show good correlation between sudden density increase and geomagnetic disturbance. The increases for disturbances of equal strength are approximately 40% greater during night-time than daytime hours. The day-night influence is also observed in the changes in density with changes in the solar flux index, F₁₀. The 27-day density variation is predominant mainly during night-time, although the atmospheric response to F₁₀ variations is quite variable regardless of local time. A semi-annual variation of approx. 40% is observed. Also found is a 25% diurnal variation for heights near 170–180 km, which is in good agreement with the CIRA 1972 atmosphere.     

Changes in the ionospheric profile and the faraday factor M with Kp

Using incoherent scatter data from Millstone Hill, we investigated the variations in the shape of the daytime, mid-latitude ionospheric electron density profile associated with changes in geomagnetic activity. The analysis performed was to deduce the dependence upon the 3-hr geomagnetic index Kp of h(N_m), h(0·7 N_m) above and below N_m, the plasma scale height H_T in the range 500–1000 km, and the ratio $\text{N(1000)}\text{N(h}{}_{\mathrm{m}}\text{)}$. The electron density data used spanned the solar maximum years 1968–1971. Daytime data from the period 1000 to 1600 LT were averaged separately for summer, winter and spring-fall. It is shown that the mean value $\text{M}$ of the factor M = B cos θ sec χ used by Titheridge (1972) to relate the Faraday rotation $\text{Ω}$ from a geostationary satellite to the total electron content N_N up to 2000 km is practically the same (to within 1–2 per cent) as the $\text{M}$ value used to relate the N_T and $\text{Ω}$ values both computed up to 1000 km. Taking advantage of this identity, we have used the linear relationship obtained between the ionospheric parameters and Kp to deduce the height at which $\text{M}$ should be evaluated as a function of Kp.     

Daytime ionosphere of Venus as studied with Veneras 9 and 10 dual-frequency radio occultation experiments

We present results of the dual-frequency radio sounding of the Venusian ionosphere carried out by the Venera 9 and 10 satellites in 1975. Thirteen height profiles of electron density for different solar zenith angles varying from 10 to 87° have been obtained by analyzing the refraction bending of radiorays in the sounded ionssphere. The main maximum of electron density at a height of 140–150 km depends on the solar zenith angle and is 1.4 to 5 × 10⁵ cm^?3. The lower maximum is determined definitely to be at ～130 km high. In the main and lower maxima the electron density variations with solar zenith angle are in good agreement with the Chapman layer theory. For the first time it is found that the height of the upper boundary for the daytime ionosphere (h_i) depends regularly on the solar zenith angle. At Z_⊙ < 60°, h_i does not exceed 300 km while at Z_⊙ > 60°, it increases with Z_⊙ and comes up to ～ 600 km at Z_⊙ ～ 80°.     

Propagation of the geomagnetic density and temperature effect into the exosphere

The propagation of the geomagnetic effect into the exosphere is investigated based on a free-flight particle kinetic model of exospheric densities and temperatures. Exobasic neutral gas conditions and their variations during a geomagnetic storm occurrence are adopted as given by the OGO-6 model. The contributions of particles originating at different exobasic locations to the density and temperature at exospheric regions are taken into account according to the time needed to reach these regions. A short-time geomagnetic variation of exobasic conditions is simulated by a Gaussianshaped A_p -index variation with an FWHM of 20 min. It is then shown that the relative amplitude and the half width of the geomagnetic density variation increase strongly with exospheric heights. The density peak and the main temperature peak are shown to be delayed by more than one and two hours, respectively, at heights above 10,000 km. The temperature variation changes from a singlepeaked to a double-peaked structure at greater exospheric heights. It is shown that the exospheric density response to geomagnetic disturbances is detectable in observations of the geocoronal He-1-584 Å resonance radiation.     

The semi-annual variation in air density for 1974–1978 from the orbit of 1972-05B

Values of air density at 712 epochs between August 1973 and September 1978 have been determined using orbital elements of 1972-05B with orbital decay rates from NORAD bulletins. Normalised to a series of fixed heights and cleared of the effects of solar activity, geomagnetic activity and the diurnal variation, the residual air density was analysed for the semi-annual variation. This variation exhibited maxima usually in April and October and minima usually in January and July.

For 1974–1978, this study revealed near-identical values of the April and October maxima and a July minimum 12% stronger than the January minimum. Further, the shape and phase of the variation exhibited an irregular pattern from year-to-year. Overall the amplitude of the variation was considerably greater than that given in the atmospheric models (CIRA, 1972; Jacchia, 1977). Other observations included the presence of subsidiary minima and maxima in late May and June respectively during 1977 and 1978 and a general increase in air density from mid 1977 onwards, relative to the atmospheric models.