A model of thermospheric temperature and composition

An empirical model of thermospheric temperature (T_∞T₁₂₀, and s) and composition (H, He, N, O, N₂, O₂, and Ar) was derived from measurements of 8 satellites (AE-C, AE-E, AEROS-A, AEROS-B, ARIEL-3, ESRO-4, OGO-6, and SAN MARCO-3) and 4 incoherent scatter stations (Arecibo, Jicamarca, Millstone Hill, and St Santin). The altitude covered extends from 120 km up to about 600 km over the time period 1967 to 1976. The analytical framework used in the model resembles closely the MSIS setup: time independent terms, solar flux terms, geomagnetic activity (K_p) effect, annual (semiannual) and diurnal (semidiurnal, terdiurnal) variations, longitudinal terms, the U.T. effect, and corrections compensating for deviations from diffusive equilibrium at altitudes below 200 km. The model describes quiet to medium disturbed geomagnetic conditions (K_p ? 4) at solar fluxes (10.7cm) ranging from 60 to 180 × 10^?22 Wm^?2Hz^?1. To get an impression of the accuracy presently obtained, the model is compared with MSIS, Jacchia (1977), and the models of Thuillier (T_∞ and Engebretson (N). The best agreement is found for the temperature and the constituents He, O, and N₂ with increasing deviations in the order of H, N, Ar, and O₂.     

Some properties of geomagnetic field pulsations in the 0.1–1.0-Hz frequency range: A quantitative description and comparison with the satellite and ground-based observations

By using an image-dipole magnetic field model for a variety of plasma density profiles we have studied the latitude effect of the 0.1–1.0-Hz hydromagnetic wave propagation in the Earth's magnetosphere. On comparing the results of signal group delay time calculations for dipole and model magnetic fields with ground and satellite observations we obtain some propagation characteristics of Pc1s and localize the regions of their generation. Our results show that most high-latitude Pc1 events are generated in the outer magnetosphere in accordance with ground and satellite observations and theoretical considerations. The non-dipole geometry of the geomagnetic field in the outer magnetosphere (at geomagnetic latitudes φ₀ > 66°, L > 6) has a significant effect on the hydromagnetic wave propagation.     

Ion temperature troughs in the equatorial topside ionosphere

The Retarding Potential Analyzer aboard OGO-6 sometimes recorded marked depressions of ion temperature as the satellite crossed the equatorial region. These “T_i troughs” occur at heights between about 700 km and the satellite apogee at 1100 km. At the centre of a trough, close to the dip equator, T_i is frequently 500–1000 K below its value at the northern and southern edges, which are usually 15°–20° in latitude from the centre of the trough. At a given season and local time, the occurrence, symmetry, depth and position of the troughs often vary markedly with longitude. The troughs have no particular association with equatorial troughs of ion concentration N_i.As suggested by Hanson, Nagy and Moffett, the T_i troughs appear to be caused by transequatorial winds that drive F region plasma along geomagnetic field lines. The plasma is adiabatically cooled as it is driven upwards on the “upwind” side of the dip equator, and heated as it descends on the “downwind” side. The available data on the occurrence of troughs at different longitudes, local times and seasons are reasonably consistent with wind directions deduced from Jacchia's model and the OGO-6 thermospheric model of Hedin et al., and with the north-south asymmetries of the tropical 630 nm airglow observed by OGO-4 and OGO-6. Factors determining the latitudinal extent of the troughs are discussed and some questions for further study are listed.     

Variations in the atmospheric neutral density at 145km

A least-squares multiple linear regression is performed on orbital decay density data obtained from precise orbital analysis of 22 low-perigee (130–160 km) Air Force satellites. Variations related to solar activity, the semi-annual effect, geomagnetic activity, and the zenith angle of the Sun are in agreement with the model of Jacchia (1971). Density variations in longitude and latitude are also deduced and compared with recent results from other investigations within this altitude regime.     

Global atomic oxygen density derived from OGO-6 1304 Å airglow measurements

The OGO-6 UV photometer experiment measured the atomic oxygen OI 1304 Å triplet in the Earth's dayglow between 400 and 1100 km. We have analyzed the data for the period 15 September–25 October 1969 by obtaining best-fit models in which the 1304 Å emission is excited by solar resonance scattering and photoelectron excitation. Provided the excitation processes are specified, we find a unique relationship between the vertical column density of atomic oxygen and the zenith 1304 Å intensity. This is essentially independent of the atmospheric temperature. Because of the large numerical uncertainties, the excitation sources are determined from the 1304 Å data and quiet-time in situ measurements of atomic oxygen density. They are found to be in good agreement with recent solar measurements of the 1304 Å lines and with calculations of the photoelectron excitation source. The deduced variations of atomic oxygen column densities over the daytime atmosphere are found to agree well with the Jacchia 1971 models. During the geomagnetic storm, the column density generally increased above a fixed altitude. However, the latitudinal dependence is complex. Following the strong geomagnetic activity between 15 September and 1 October, depletions in atomic oxygen are observed. At times, there is evidence of high-altitude transport of atomic oxygen from high latitude to low latitude.     

Atmospheric density at 169 km from accelerometer measurements and orbital decay of a low altitude satellite

Atmospheric densities at 169 km have been obtained for the period 19 August–3 September 1970 from the measurements of an accelerometer on a low altitude satellite and from the orbital decay of the same satellite. Three different sets of local time and latitude conditions were provided by the data; two from the accelerometer measurements, before and after perigee, and one at perigee, from the orbital decay data. Under the generally quiet magnetic activity conditions that prevailed during the data-taking period, the short term density fluctuations were found to be poorly correlated with the small K_p variations. However, on the greater time scale of a day, a definite relationship was found between the daily average density and the daily geomagnetic index A_p. Further, the increase in the density corresponding to A_p was largest at the highest latitude. The high latitude accelerometer data exhibited a quasi-daily periodicity, with maximum densities occurring when the satellite was within the dayside cusp. This effect also appeared to depend on the degree of auroral electrojet activity as defined by the AE index. Comparisons of the data with the Jacchia?70 and ?71 models indicated that these models may give density values which are too small for the conditions and time period corresponding to the data.     

Relations between turbulent regions of interplanetary magnetic field and Jovian decametric radio wave emissions from the main source

Jovian decametric radio wave emissions that were observed at Goddard Space Flight Center, U.S.A. for a period from 1 October to 31 December, 1974 and data obtained at Mt Zao observatory, Tohoku University, Japan, for a period from 14 July to 6 December, 1975 have been used to investigate the relationship of the occurrence of the Jovian decametric radio waves (JDW), from the main source, to the geomagnetic disturbance index, ΣK_p. The dynamic cross-correlation between JDW and ΣK_p indicates an enhanced correlation for certain values of delay time. The delay time is consistent with predicted values based on a model of rotating turbulent regions in interplanetary space associated with two sector boundaries of the interplanetary magnetic field, i.e. the rotating sector boundaries of the interplanetary magnetic field first encounter the Earth's magnetosphere producing the geomagnetic field disturbances, and after a certain period, they encounter the Jovian magnetosphere. There are also cases where the order of the encounter is opposite, i.e. the sector boundaries encounter first Jovian magnetosphere and encounter the Earth's magnetosphere after a certain period.     

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.     

A study of geomagnetic storms at Beijing in terms of solar wind

With the aid of the Akasofu's energy coupling function between the solar wind and the magnetosphere, we have made in this paper an analysis of about 20 geomagnetic storms recorded at Beijing during the period of years 1966 to 1972. There is a close correlation between the energy coupling function ? and the geomagnetic indices a_p and K_p. All in all an empirical formula as ? ～ 1?2 × 10¹⁷a_p has been found for the geomagnetic storms occurred in a low latitude station, i.e. Beijing of China. Comparisons of the horizontal component H_max (in γ) and ?(10¹⁸ erg s^?1) in Table 1 indicate that the development of storm main phase at Beijing depends very much on the ? values thus involved. Also, these are well illustrated for several individual storms as mentioned in the second section of the paper. In concluding this paper some brief discussions are made and included. It is hoped that geomagnetic observations in the middle and low latitudes from our vast country should make further contributions to the study of solar wind-magnetosphere coupling, including the Akasofu's energy coupling function.     

The effect of a normal magnetic field component on current sheet stability

A $\text{2}\text{2}\text{5}\text{-}\text{dimensional}$ particle-mesh computer model for the simulation of the current-sheet region of the geomagnetic tail is described. Important features are (a) the use of Fast Fourier techniques for the efficient solution of Ampere's equation, (b) the incorporation of sources and sinks of particles, (c) facilities for simulating finite width effects and (d) the option of including a normal magnetic field component linking through the sheet.Simulations carried out using this model indicate that current sheets with a non-zero normal magnetic field component and an infinite width are stable. The particles trace out Speiser-like orbits in such a case. Sheets with B_normal = 0 and a finite width are unstable with respect to the ion tearing-mode instability. However the presence of a normal magnetic field stabilises the system provided ρ₀<2L_y where ρ₀ is the characteristic length associated with the normal field and where L_y is the width of the sheet.On the basis of these results it is suggested that a geomagnetic substorm occurs when the normal magnetic field drops below the critical value needed for stability.     

Determination of ionospheric electron content from the Faraday rotation of geostationary satellite signals

Calculations using a wide range of model ionospheres (with a peak at 300 km) show that the integrated electron content up to the height of the satellite could be up to four times the value deduced from Faraday rotation measurements. However, using a fixed mean field height of 400 km, the observed Faraday rotation gives the electron content up to a height h_F of 2000 km with an accuracy of ±3 per cent. For observations at different magnetic and geographic latitudes, and geostationary satellites at different longitudes, the optimum value of h_F varies by only ±200 km. Night-time increases in the height of the ionosphere have little effect on h_F, but increase the mean field height to about 470 km. Using a fixed value of 420 km, with h_F = 2000 km, gives an accuracy of ±5 per cent under most conditions.     

A new theoretical approach to the modelling of toroidal Pc5 pulsations

A model is developed to represent a toroidal mode of Pc5 geomagnetic pulsations. It is shown that this model is consistent in its predictions, such as the latitude profiles of amplitude and phase and their dependence on the height integrated Pedersen conductivity, Σ_p, with those of Walker's (1980) theory. It is also shown that this theory is relatively easily capable of accommodating (i) a variety of field line plasma mass density distributions, (ii) a variety of external excitation schemes, (iii) unequal Σ_p's at each end of the field lines and (iv) non-dipolar geomagnetic fields. The theory yields the transient as well as the steady state response, an important feature permitting application to short-lived events or to those for which the generator is amplitude modulated. It is shown, for instance, that the amplitude-latitude profile varies during the transient. It is also shown that the steady state latitude profiles of amplitude and phase are the dual of those observed as a function of frequency when the excitation frequency is scanned through a resonance. A more realistic steady state energy flow from a generator along the field lines to the ionosphere is inherent in this theory compared with that from the mode to the ionosphere which is inherent in Walker's theory.     

A pictorial comparison of interplanetary magnetic field polarity,solar wind speed,and geomagnetic disturbance index during the sunspot cycle

Observations of interplanetary magnetic field polarity, solar wind speed, and geomagnetic disturbance index (C9) during the years 1962–1975 are compared in a 27-day pictorial format that emphasizes their associated variations during the sunspot cycle. This display accentuates graphically several recently reported features of solar wind streams including the fact that the streams were faster, wider, and longer-lived during 1962–1964 and 1973–1975 in the declining phase of the sunspot cycle than during intervening years (Bame et al., 1976; Gosling et al., 1976). The display reveals strikingly that these high-speed streams were associated with the major, recurrent patterns of geomagnetic activity that are characteristic of the declining phase of the sunspot cycle. Finally, the display shows that during 1962–1975 the association between long-lived solar wind streams and recurrent geomagnetic disturbances was modulated by the annual variation (Burch, 1973) of the response of the geomagnetic field to solar wind conditions. The phase of this annual variation depends on the polarity of the interplanetary magnetic field in the sense that negative sectors of the interplanetary field have their greatest geomagnetic effect in northern hemisphere spring, and positive sectors have their greatest effect in the fall. During 1965–1972 when the solar wind streams were relatively slow (500 km s^-1), the annual variation strongly influenced the visibility of the corresponding geomagnetic disturbance patterns.Visiting Scientist, Kitt Peak National Observatory, Tucson, Arizona.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Long-period pulsations of the Earth's magnetic field with periods more than 20 minutes before proton flares on the sun

Long-period (more than 20 min) quasi-periodic pulsations (QPP) occurring in the Earth's magnetic field (EMF) before the proton flare are studied by the method of spectral correlation analysis of geomagnetic field H-component. The corresponding data have been obtained at six stations located from 12°41'E up to 180° 52'E and from 52°04'N up to 68°52'N.QPP space-time distribution is shown to be correlated with that of the Earth's ionosphere current systems. The results obtained indicate that QPP of the EMF are influenced by QPP of the solar X-ray and ultraviolet radiation modulated by oscillation processes in the active solar region.     

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.     

A model of the terrestrial ionosphere in the altitude interval 50–4000 km II. Molecular ions (N2 +, NO+, O2 +) and electron density

Empirical models of molecular ion densities (N₂ ⁺, NO⁺, O₂ ⁺) and the electron density (N _e ) are presented in the altitude interval 50–4000 km as functions of time (diurnal, annual), space (position, altitude) and solar flux (F _10.7). Using observations of 6 satellites (AE-C, AE-D, AE-E, ALOUETTE-2, ISIS-1, ISIS-2), 4 incoherent scatter stations (Arecibo, Jicamarca, Millstone Hill, St Santin) and more than 700 D-region profiles, this model describes the global gross features of the ionosphere for quiet geophysical conditions (K _p 3).The molecular ion densities and the electron density increase with increasing altitude up to a maximum (or several maxima) - and decrease from thereon with increasing height. Between ~80 and 200 km, the main ionic constituents are NO⁺ and O₂ ⁺; below ~80 km cluster ions are predominating. During local summer conditions the molecular ions and N _e increase around polar latitudes and decrease correspondingly during local winter. The diurnal variations are intrinsically coupled to the individual plasma layers; in general, the molecular ion and electron densities are enhanced during daytime and depleted during nighttime (for details and exceptions, see text).     

Magnetic field inclination angle at geosynchronous orbit

Simultaneous magnetic field data from the geostationary satellites GOES 2 and GOES 3 have shown significant differences between the inclination angles measured at these two satellites. The relationship between the two inclination angles is examined for three magnetic activity groups and eight local time sectors by using data from GOES 2 and GOES 3 for the 12-month period from March 1979 to February 1980. During this time GOES 2 was 1.09 R_E off the geomagnetic equator in the northern geomagnetic hemisphere, while GOES 3 was separated from GOES 2 by 2 h L.T. and 0.47 R_E off the geomagnetic equator, also in the northern geomagnetic hemisphere. The average inclination angle is found to differ substantially from the predictions based on the previously published magnetic field models. The relationship between the two inclination angles is fitted by a simple model of magnetic field perturbation that varies as a power of distance from the Equator. During magnetic disturbances, the spatial variation of magnetic field perturbation on the nightside suggests that the ring current density increases with distance from the geomagnetic equator.     

Multiwavelength Study on Solar and Interplanetary Origins of the Strongest Geomagnetic Storm of Solar Cycle 23

We study the solar sources of an intense geomagnetic storm of solar cycle 23 that occurred on 20 November 2003, based on ground- and space-based multiwavelength observations. The coronal mass ejections (CMEs) responsible for the above geomagnetic storm originated from the super-active region NOAA 10501. We investigate the H?? observations of the flare events made with a 15 cm solar tower telescope at ARIES, Nainital, India. The propagation characteristics of the CMEs have been derived from the three-dimensional images of the solar wind (i.e., density and speed) obtained from the interplanetary scintillation data, supplemented with other ground- and space-based measurements. The TRACE, SXI and H?? observations revealed two successive ejections (of speeds ???350 and ???100 km?s^?1), originating from the same filament channel, which were associated with two high speed CMEs (???1223 and ???1660 km?s^?1, respectively). These two ejections generated propagating fast shock waves (i.e., fast-drifting type II radio bursts) in the corona. The interaction of these CMEs along the Sun?CEarth line has led to the severity of the storm. According to our investigation, the interplanetary medium consisted of two merging magnetic clouds (MCs) that preserved their identity during their propagation. These magnetic clouds made the interplanetary magnetic field (IMF) southward for a long time, which reconnected with the geomagnetic field, resulting the super-storm (Dst _peak=?472 nT) on the Earth.     

The westward thermospheric jet-stream of the evening auroral oval

One of the most consistent and often dramatic interactions between the high latitude ionosphere and the thermosphere occurs in the vicinity of the auroral oval in the afternoon and evening period. Ionospheric ions, convected sunward by the influence of the magnetospheric electric field, create a sunward jet-stream in the thermosphere, where wind speeds of up to 1 km s^?1 can occur. This jet-stream is nearly always present in the middle and upper thermosphere (above 200 km altitude), even during periods of very low geomagnetic activity. However, the magnitude of the winds in the jet-stream, as well as its location and range in latitude, each depend on geomagnetic activity. On two occasions, jet-streams of extreme magnitude have been studied using simultaneous ground-based and satellite observations, probing both the latitudinal structure and the local time dependence. The observations have then been evaluated with the aid of simulations using a global, three-dimensional, time-dependent model of thermospheric dynamics including the effects of magnetospheric convection and particle precipitation. The extreme events, where sunward winds of above 800 ms^?1 are generated at relatively low geomagnetic latitudes (60–70°) require a greatly expanded auroral oval and large cross-polar cap electric field ( ~ 150 kV). These in turn are generated by a persistent strong Interplanetary Magnetic Field, with a large southward component. Global indices such as K_p are a relatively poor indicator of the magnitude and extent of the jet-stream winds.     

Stellar occultation measurements of molecular oxygen in the lower thermosphere

Stellar ultraviolet light near 1500 Å is attenuated in the Earth's upper atmosphere due to strong absorption in the Schumann-Runge continuum of molecular oxygen. The intensity of stars in the Schumann-Runge continuum region has been monitored by the University of Wisconsin stellar photometers aboard the OAO-2 satellite during occultation of the star by the Earth's atmosphere. These data have been used to determine the molecular oxygen number density profile at the occultation tangent point. The results of 14 stellar occultations obtained in low and middle latitudes are presented giving the night-time vertical number density profile of molecular oxygen in the 140–200 km region. In general, the measured molecular oxygen number density is about a factor of 2 lower than the number densities predicted by the CIRA 1965 model. Also, the number density at a given height appears to decrease with decreasing solar activity. Measurements taken at low latitudes during the August 1970 geomagnetic storm showed a decrease in the molecular oxygen number density at a given height several days after the peak of the storm followed by a slow recovery to pre-storm densities.