Thermospheric Composition Changes Research Articles

Ionospheric response to the initial phase of geomagnetic storms: Common features

Ionospheric responses to the initial phases of three geomagnetic storms: 2–5 April 2004, 7–9 November 2004, and 13–16 December 2006, were compared using both ground‐based GPS total electron content (TEC) data and coupled magnetosphere ionosphere thermosphere (CMIT) model simulations. The onset times for these storms all occurred at local daytime in the North American sector. This similarity of onset times and other factors resulted in some common features in their ionospheric response. These common features include (1) enhanced TEC (positive response) at low and middle latitudes in the daytime, (2) depleted TEC (negative response) around the geomagnetic equator in the daytime, (3) a north‐south asymmetry in the positive response as the northern hemispheric response appeared to be more pronounced, and (4) negative response at high latitudes as the storms progressed. The CMIT model captured most of these features. Analysis of model results showed that storm‐time enhancements in the daytime eastward electric field were the primary cause of the observed positive storm effects at low and middle latitudes as well as the negative response around the geomagnetic equator in the daytime. These eastward electric field enhancements were caused by the penetration of high latitude electric fields to low latitudes during southward interplanetary magnetic field (IMF) periods, when IMF Bz oscillated between southward and northward direction in the initial, shock phase of the storms. Consequently, the ionosphere was lifted up at low and middle latitudes to heights where recombination was weak allowing the plasma to exist for a long period resulting in higher densities. In addition, the CMIT model showed that high‐latitude negative storm responses were related to the enhancements of molecular nitrogen seen in TIMED/Global Ultraviolet Imager observations, whereas the negative storm effects around the geomagnetic equator were not associated with thermospheric composition changes; they were the result of plasma transport processes.

Ionospheric storms at geophysically-equivalent sites – Part 1: Storm-time patterns for sub-auroral ionospheres

Abstract. The systematic study of ionospheric storms has been conducted primarily with groundbased data from the Northern Hemisphere. Significant progress has been made in defining typical morphology patterns at all latitudes; mechanisms have been identified and tested via modeling. At higher mid-latitudes (sites that are typically sub-auroral during non-storm conditions), the processes that change significantly during storms can be of comparable magnitudes, but with different time constants. These include ionospheric plasma dynamics from the penetration of magnetospheric electric fields, enhancements to thermospheric winds due to auroral and Joule heating inputs, disturbance dynamo electrodynamics driven by such winds, and thermospheric composition changes due to the changed circulation patterns. The ~12° tilt of the geomagnetic field axis causes significant longitude effects in all of these processes in the Northern Hemisphere. A complementary series of longitude effects would be expected to occur in the Southern Hemisphere. In this paper we begin a series of studies to investigate the longitudinal-hemispheric similarities and differences in the response of the ionosphere's peak electron density to geomagnetic storms. The ionosonde stations at Wallops Island (VA) and Hobart (Tasmania) have comparable geographic and geomagnetic latitudes for sub-auroral locations, are situated at longitudes close to that of the dipole tilt, and thus serve as our candidate station-pair choice for studies of ionospheric storms at geophysically-comparable locations. They have an excellent record of observations of the ionospheric penetration frequency (foF2) spanning several solar cycles, and thus are suitable for long-term studies. During solar cycle #20 (1964–1976), 206 geomagnetic storms occurred that had Ap≥30 or Kp≥5 for at least one day of the storm. Our analysis of average storm-time perturbations (percent deviations from the monthly means) showed a remarkable agreement at both sites under a variety of conditions. Yet, small differences do appear, and in systematic ways. We attempt to relate these to stresses imposed over a few days of a storm that mimic longer term morphology patterns occurring over seasonal and solar cycle time spans. Storm effects versus season point to possible mechanisms having hemispheric differences (as opposed to simply seasonal differences) in how solar wind energy is transmitted through the magnetosphere into the thermosphere-ionosphere system. Storm effects versus the strength of a geomagnetic storm may, similarly, be related to patterns seen during years of maximum versus minimum solar activity.

Satellite and ground‐based observations of auroral energy deposition and the effects on thermospheric composition during large geomagnetic storms: 1. Great geomagnetic storm of 20 November 2003

This report describes thermospheric composition and particle precipitation changes that occurred during the period of the great geomagnetic storm of 20–21 November 2003, an event that was associated with the passage of a magnetic cloud past the Earth. These changes are compared to those observed during geomagnetic activity on 17 November 2003 and during the intervening quieter period. The data used are obtained from (1) ground‐based magnetometers, an imaging riometer, a scanning Doppler imaging Fabry‐Perot, and photometers from stations in Alaska, (2) photometers from Canadian sites, (3) NOAA POES and DMSP particle sensors, and (4) the TIMED Global Ultraviolet Imager far UV sensor. The composition changes associated with the input of auroral particle and Joule energy showed larger depletions in atomic oxygen on 20 November than on the other nights and greater changes than are seen in the Naval Research Laboratory Mass Spectrometer and Incoherent Scatter (NRLMSIS) model atmosphere. NRLMSIS does better in reproducing the changes during the great magnetic storm with its long duration auroral energy input than during the shorter time duration geomagnetic activity that occurred on 17 November. During the nights with the largest changes in composition the input of Joule energy dominates over auroral particle energy. It is shown that the particle energy distributions associated with the 20–21 November storm in the period around and after the passage of the magnetic cloud had lower average energies and were enhanced at energies below 0.1 keV than those that caused auroral displays on the preceding days.

The global ionospheric asymmetry in total electron content

We use the NASA-JPL global ionospheric maps of total electron content (TEC), obtained from observations of GPS satellite signals, to construct average noon TEC maps for one-month periods centered on the June and December solstices of 2002. By combining data from equivalent points in the northern and southern hemispheres, we find that the combined north+south TEC in December substantially exceeds that in June, almost everywhere. The global average of an ‘Asymmetry Index’ (AI) used to characterize such patterns, AI=(Dec−June)/(Dec+June), results in AI=0.15 for 2002, far greater than the value of 0.035 that corresponds to the annual variation of the solar irradiance due to Sun-Earth distance. Since the F2-layer makes the major contribution to TEC, we regard this as a manifestation of the F2-layer annual asymmetry, discovered some decades ago but never explained. The neutral atomic oxygen/molecular nitrogen concentration ratio, derived from the GUVI experiment on the TIMED satellite, shows a similar but smaller asymmetry (AI=0.06), providing some evidence that the ionospheric asymmetry is related to changes in thermospheric composition, with a still smaller AI=0.03 in the NRL MSIS thermospheric model.

Quiet‐time seasonal behavior of the thermosphere seen in the far ultraviolet dayglow

The TIMED/GUVI instrument is a far ultraviolet spectrograph that obtains images in five spectrally resolved wavelength channels. These images yield information on the dayside composition, temperature, solar EUV flux, large‐scale wave structures, and auroral processes. In this paper we present an overview analysis of Earth‐disk images for four seasons (March, July, and September 2002 and January 2003). Days were selected during geomagnetically quiet periods when the Sun was nearly in the orbital plane (noon orbits). Two of GUVI's five channels (designated as 135.6 and LBHS and dominated by OI 135.6 nm and short wavelength N2 LBH band emission, respectively) are used when the instrument is in its imaging mode. These data are used to derive O/N2 (column density ratio referenced to an N2 column density of 1017 cm−2). The AURIC model is used to generate a lookup table that relates O/N2 to the ratio of 135.6 to LBHS for a given solar zenith angle. Global images of derived O/N2 (designated as GUVI O/N2) are presented for the 4 days. The initial validation of the retrieved composition ratio comes from comparison with the NRLMSIS model. Good overall qualitative agreement is obtained between GUVI and NRLMSIS. Both data and model exhibit similar latitudinal behaviors on the near‐solstice days, namely a distinct gradient with O/N2 decreasing from the winter to the summer hemisphere. Reductions in O/N2 in the vicinity of magnetic poles are seen in both GUVI and NRLMSIS images. Globally, O/N2 is smaller at the solstices and may be explained by the “thermospheric spoon” mechanism discussed by Fuller‐Rowell [1998]. Alternatively, the greater overall values at the equinoxes may arise in part from global response to greater Joule heating at these times of the year. The sensitivity of O/N2 to scalings of the N2 LBH cross section and solar EUV below 20 nm is also addressed in response to recent papers on these topics. This initial look at the GUVI data demonstrates great promise of FUV remote sensing as a way to observe thermospheric composition changes over broad geographic scales.

The use of far ultraviolet remote sensing to monitor space weather

This paper discusses the connection between changes in Earth's thermosphere and ionosphere induced by changes in the Earth's local space environment (or “space weather”) and the phenomena observed in far ultraviolet images of the Earth. Two new experiments, the Global Ultraviolet Imager (GUVI) and the Special Sensor Ultraviolet Imager (SSUSI), will provide a new capability for monitoring changes in thermospheric composition and ionospheric density as they change in response to space weather. These sensors provide a ten-fold improvement in spatial and temporal resolution and a greater than ten-fold improvement in sensitivity over that provided by sensors on the POLAR and IMAGE satellites. These sensors are expected to provide new insights into the mesoscale coupling between the ionosphere and thermosphere, as well as allowing us to develop a better specification of the high latitude convection electric field pattern.

Annual and semiannual variations of the midlatitude ionosphere under low solar activity

The annual and semiannual variations of the midlatitude ionosphere under low solar activity are studied using middle and upper (MU) radar (135°E, 35°N) incoherent scatter observations and Sheffield University plasmasphere‐ionosphere model (SUPIM). The variations of the daytime electron density (Ne) and electron and ion temperatures (Te and Ti) at 200–600 km altitudes measured by the radar under low solar activity (F10.7 ≤ 120) are satisfactorily reproduced for the first time by incorporating the radar measured values of the magnetic meridional neutral wind velocity (Uθ) and northward perpendicular plasma drift velocity (V⟂) into SUPIM that uses mass spectrometer incoherent scatter 1986 (MSIS‐86) for neutral densities and neutral temperatures. The study shows that the annual and semiannual variations of the midlatitude ionosphere during daytime at altitudes near and above the ionospheric peak under low solar activity depend more on the direct effect of the neutral wind arising through the changes in ionospheric height than on the indirect effect arising through the changes in thermospheric composition (or atomic to molecular concentration ratio); the indirect effect, however, predominates on the variations at altitudes below the ionospheric peak. The electron temperature (Te) undergoes similar but almost opposite seasonal and semiannual variations as the electron density. The ion temperature (Ti) is closer to neutral temperature than to electron temperature at altitudes up to ∼400 km and shows comparatively weak annual and semiannual variations.

Simultaneous observations of lower thermospheric composition change during moderate auroral activity from Kangerlussuaq and Narsarsuaq, Greenland

To obtain information regarding the spatial scales involved in lower thermospheric composition changes in the auroral zone, simultaneous observations were carried out using photometers at Kangerlussuaq and Narsarsuaq, Greenland, two sites separated by ∼750 km in distance. At night, Kangerlussuaq, located at 67°N geographic is most often at the northern edge of the auroral oval, while Narsarsuaq, at the southern tip of Greenland, is more often in or at the southern edge of the auroral oval. On January 29–30, 1998, moderate auroral activity was recorded from both sites. Besides the photometer systems, observations were obtained during part or all of this period from Polar satellite instruments, from the Greenland magnetometer chain, from the CANOPUS chain of magnetometers and photometers, from DMSP, and from the Sondrestrom incoherent scatter radar. The analysis of these data show the necessity of considering both nonlocal and local sources in modeling aurorally induced changes in lower thermospheric composition.

Analysis of the positive ionospheric response to a moderate geomagnetic storm using a global numerical model

Abstract. Current theories of F-layer storms are discussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time dependent numerical model of the thermosphere-ionosphere-plasmasphere-magnetosphere system including electrodynamical coupling effects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena. The study focuses on positive ionospheric storm effects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical effects of both neutral meridional winds and electric fields caused by the disturbance dynamo effect. The penetration of short-time electric fields of magnetospheric origin during storm intensification phases is shown for the first time in this model study. Comparisons of the calculated thermospheric composition changes with satellite observations of AE-C and ESRO-4 during storm time show a good agreement. The empirical MSISE90 model, however, is less consistent with the simulations. It does not show the equatorward propagation of the disturbances and predicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of [O]/[N2] at high latitudes decreases significantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at fixed altitudes) above the quiet reference level. Meanwhile, the ionospheric storm is positive there. We conclude that the positive phase of the ionospheric storm is mainly due to uplifting of ionospheric F2-region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic field lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calculated zonal electric field disturbances also help to create the positive ionospheric disturbances both at middle and low latitudes. Minor contributions arise from the general density enhancement of all constituents during geomagnetic storms, which favours ion production processes above ion losses at fixed height under day-light conditions.Key words: Atmospheric composition and structure (thermosphere · composition and chemistry) · Ionosphere (ionosphere · atmosphere interactions; modelling and forecasting)

Analysis of the positive ionospheric response to a moderate geomagnetic storm using a global numerical model

Current theories of F-layer storms are discussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time dependent numerical model of the thermosphere-ionosphere-plasmasphere-magnetosphere system including electrodynamical coupling effects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena. The study focuses on positive ionospheric storm effects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical effects of both neutral meridional winds and electric fields caused by the disturbance dynamo effect. The penetration of short-time electric fields of magnetospheric origin during storm intensification phases is shown for the first time in this model study. Comparisons of the calculated thermospheric composition changes with satellite observations of AE-C and ESRO-4 during storm time show a good agreement. The empirical MSISE90 model, however, is less consistent with the simulations. It does not show the equatorward propagation of the disturbances and predicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of [O]/[N2] at high latitudes decreases significantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at fixed altitudes) above the quiet reference level. Meanwhile, the ionospheric storm is positive there. We conclude that the positive phase of the ionospheric storm is mainly due to uplifting of ionospheric F2-region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic field lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calculated zonal electric field disturbances also help to create the positive ionospheric disturbances both at middle and low latitudes. Minor contributions arise from the general density enhancement of all constituents during geomagnetic storms, which favours ion production processes above ion losses at fixed height under day-light conditions.Key words: Atmospheric composition and structure (thermosphere · composition and chemistry) · Ionosphere (ionosphere · atmosphere interactions; modelling and forecasting)

Thermospheric composition changes deduced from geomagnetic storm modeling

To test various hypotheses about positive ionospheric storm development, we numerically simulated the magnetic storm of 24–27 January 1974 to obtain the global pattern of its thermospheric and ionospheric effects. We use a global self‐consistent numerical model of the thermosphere‐ionosphere‐magnetosphere system which calculates all plasma parameters as well as electric fields both of magnetospheric and thermospheric dynamo origin. The predicted neutral composition changes are consistent with observations of the AE‐C and ESRO‐4 satellites. The storm‐to‐quiet density ratio of [O]/[N2] (at a fixed height) does not change significantly at middle and low latitudes while the absolute density of all neutral constituents is enhanced. The density ratio at a fixed pressure level can be slightly enhanced under certain conditions. Positive ionospheric storm effects during the main phase of the storm are shown to be mainly due to the uplifting of the ions along the field lines by an equatorward directed disturbance wind component at middle latitudes and their equatorward (convergent) movement at low latitudes. This is driven by the passage of travelling atmospheric disturbances and by the large‐scale disturbance wind circulation. Under daylight conditions minor contributions arise from the general thermal density enhancement of all constituents favoring positive ionospheric response due to a stronger ion production rather than loss. Height dependent deviations from diffusive equilibrium of minor thermospheric constituents are due to the relatively rapid (in comparison with their diffusion) vertical bulk motion.

Storm time dependence of equatorial disturbance dynamo zonal electric fields

We use Jicamarca radar observations of F region vertical plasma drifts and auroral electrojet indices during 1968–1988 to study the characteristics and temporal evolution of equatorial disturbance dynamo zonal electric fields. These electric fields result from the dynamo action of storm time winds and/or thermospheric composition changes driven by enhanced energy deposition into the high‐latitude ionosphere during geomagnetically active conditions. The equatorial vertical drift perturbations last for periods of up to 30 hours after large increases in the high‐latitude currents. On the average, this process can be described by two basic components with time delays of about 1–12 hours and 22–28 hours between the high‐latitude current enhancements and the equatorial velocity perturbations. Our data indicate strong coupling between dynamo processes with different timescales. The short‐term disturbance dynamo drives upward equatorial drifts (eastward electric fields) at night with largest amplitudes near sunrise and small downward drifts during the day. These perturbation drifts are in good agreement with results from the Blanc‐Richmond disturbance dynamo theory. The dynamo process with time delays of about a day drives upward drift velocities at night with largest values near midnight and downward drifts in the sunrise‐noon sector. In this case, the amplitudes of the disturbance drifts maximize during geomagnetically quiet times preceded by strongly disturbed conditions. We also present results of a new equatorial storm time dependent empirical model which illustrate the characteristics of the vertical disturbance dynamo drifts.

Latitudinal structure of thermospheric composition perturbations

Abstract The N 2 O density ratio measured by the ESRO 4 satellite is used to describe the latitudinal structure of thermospheric composition changes. To isolate the geomagnetic activity effect, all N 2 O data are divided by the quiet-time ( Ap = 0) ESRO 4 model. Subsequently, the latitudinal profiles of the residuals are superimposed in such a way that the equatorward boundary of the composition disturbance serves as a common reference location. Finally, the medians of these superimposed profiles are determined for different geophysical conditions. This averaging procedure preserves the typical latitudinal structure of the disturbances. The results are summarized in a simple analytical model. It describes the latitudinal variation of the N 2 O perturbation at 280 km altitude as a function of local time, season, and level of magnetic activity. The N 2 O density ratio is also used to describe the perturbations of the individual gas densities. To establish the respective correlations, 22 storms observed during the ESRO 4 mission are analyzed. The magnetic activity effect is isolated by normalizing the data to prestorm conditions. It is found that at 280 km altitude and irrespective of season and local time, the Ar and N 2 densities increase and the O and He densities decrease with increasing N 2 O perturbation. Simple analytical functions are used to describe this behavior. Combined with the above model of the N 2 O perturbations, these algorithms successfully predict the magnetic activity effect on the Ar, N 2 , O, and He densities at 280 km altitude.

Latitude and solar-cycle patterns in the response of the ionosphere F2-layer to geomagnetic activity

The F2-layer response to geomagnetic activity is studied using the daily mean and local-time variations of the ( disturbed quiet ) ratio of the peak electron density NmF2. Data for 53 stations are binned by local time, season, and geomagnetic activity. The mean level has a well-defined variation with latitude, attributed to neutral composition changes caused by a global-scale storm circulation. There are systematic differences in the local-time average of the ( disturbed quiet ) ratio of NmF2 between three solar cycles, which may indicate long-term changes in thermospheric composition that possibly arise from different levels of solar activity in the three cycles. Comparison with storm simulations using a coupled thermosphere-ionosphere-plasmasphere model reinforce the conclusion that the principal F2-layer storm effects are due to composition changes.

Thermospheric composition changes seen during a geomagnetic storm

The largest magnitude winds observed using the instruments on board the Dynamics Explorer 2 (DE-2) satellite were measured during the large geomagnetic storm that occured on the 24 th of November 1982. Neutral temperatures exceeded 2000 K during this strom, these high temperatures, combined with the very large observed winds and the very full instrumental coverage available in both hemispheres, make it a unique event to study. In this paper we present results obtained using these DE-2 data and a time dependent simulation of the event made using the National Center for Atmospheric Research Thermosphere/Ionosphere General Circulation Model (NCAR-TIGCM). In general, the agreement between model calcuations and the data is very good, implying that most of the important physical processes controlling the energetics and dynamics of the thermosphere are reasonably well represented in the model. The modelled summer hemisphere changes in the mass mixing ratio of N 2(Ψ N2) are in very good agreement with the DE-2 data, and the overall global pattern of Ψ N2 in the model is also in good agreement with the averaged data in both hemispheres. This agreement allows us to study the physical processes occurring in the model with confidence that they are the same as those occuring in the “real” thermosphere. This short paper describes model-experiment comparisons for the November 24, 1982 geomagnetic storm, but does not include the processes responsible for these changes. A full description of them is available in the set of papers/1,2,3,4/.

On the possibility of auroral remote sensing with the Viking ultraviolet imager

An investigation was carried out to assess the value of ultraviolet auroral images for remote sensing of electron precipitation. We compared auroral images, obtained by both cameras of the Viking Ultraviolet Imager during April and early May 1986, with simultaneous measurements of electron precipitation from the DMSP F7 and HiLat satellites at low altitudes above the auroral zone. The camera signals in the pixels that viewed the low‐altitude satellite footprint were corrected for various instrumental and nonauroral background contributions and were converted to the equivalent signals for vertical viewing from Viking. The electron data were averaged over the image pixels and were used to normalize the imager signals to unit electron energy flux. The resulting quantity, here termed the “effective sensitivity,” showed large scatter about the mean values for both cameras that tended to mask any energy dependence except a decrease at electron energies above 6 keV. The mean effective sensitivities were 11.3 ± 1.1 digitization numbers (DN)/(erg cm−2 s−1) for the Lyman‐Birge‐Hopfield (LBH) camera and 15.9 ± 2.5 DN/(erg cm−2 s−1) for the 1304 camera. These values correspond to 39 and 55 DN per kR of λ 4278 (N2+ first negative (1 N) (0,1) band), respectively, for the two cameras and are nearly independent of electron energy below about 6 keV. A published model calculation of the LBH camera response overestimates the effective sensitivity by about 40%. The signal ratio in simultaneous images from both cameras was insensitive to electron energy in the two cases examined, consistent with the weak energy dependences of the effective sensitivities. Unusually high LBH camera effective sensitivities of 45–60 DN/(erg cm−2 s−1) were obtained during several orbits on May 2 and 3. These observations could not be accounted for by thermospheric composition changes during the geomagnetic storm on May 1 and 2 or by changes in the intrinsic gain of the camera. Some of the scatter in the measured effective sensitivities resulted from approximations required in comparing the images and particle data, which differed significantly in spatial scale. We conclude that images from either Viking camera may be used to infer the instantaneous distribution of electron energy deposition, with roughly 50% accuracy.

A theoretical study of thermospheric composition perturbations during an impulsive geomagnetic storm

The compositional response of the neutral thermosphere to an impulsive geomagnetic storm has been investigated using a numerical simulation made with the National Center for Atmospheric Research thermospheric general circulation model (NCAR‐TGCM). Calculated time‐dependent changes in neutral thermospheric composition have been studied, together with detailed neutral parcel trajectories and other diagnostic information from the model, to gain a greater understanding of the physical mechanisms responsible for composition variability during geomagnetic storms and, in particular, to investigate the causes of the positive and negative ionospheric storm effects. The following principal results were obtained from this study. (1) Calculated perturbations in thermospheric composition following the onset of an impulsive geomagnetic storm were found to be in good qualitative agreement with the previous experimental statistical study of storm time thermospheric morphology by Prölss (1981). (2) During the initial (onset) phase of the simulated storm, upward vertical winds occurred in the auroral zone and downward winds occurred in the central magnetic polar cap. (3) The largest perturbations in mass mixing ratio of nitrogen (ΨN2) at F region altitudes were found to be associated with parcels of neutral gas that travelled through the cusp region and with parcels that were trapped within the auroral zone for a long time. (4) Storm time enhancements in ΨN2 were found to occur in the midnight and early morning sectors both within and equatorward of the auroral zone, and these were determined to be associated with the advective effects of the large antisunward polar cap neutral winds. The physical size of the region of enhanced ΨN2 values was determined to be related to the time taken for individual parcels of air to corotate out of the large day‐to‐night thermospheric wind jet, and thus to the magnitudes of the antisunward polar cap winds. (5) Decreases in ΨN2 were found to occur in regions of downward vertical winds and in regions into which relatively nitrogen‐poor air was advected by the horizontal wind. (6) Calculated compositional perturbations following the onset of the storm were found to propagate to mid‐latitudes due to the advective effects of equatorward winds, leading to the mid‐latitude positive ionospheric storm effects. This last result is in disagreement with the earlier study of Rishbeth et al. (1985), who found the compositional disturbance to be largely restricted to the auroral zone on a constant‐pressure surface. However, it is consistent with the more recent study undertaken by Fuller‐Rowell et al. (1991), who demonstrated that composition changes were seen well beyond the auroral oval.

Lower thermospheric composition changes derived from optical and radar data taken at Sondre Stromfjord during the Great Magnetic Storm of February 1986

Several nights of optical and radar data were obtained from February 3 to 10, 1986 at Sondre Stromfjord, Greenland during a great geomagnetic storm when Ap reached 202. These data, excluding O I (630.0 nm), were used to monitor changes in O and O2 densities in the region from 100 to 200 km and are consistent with results of Hecht et al. (1989) wherein the O I (630.0 nm) emission was used. That is, there were large depletions of O in the E region during the peak of the storm. Using 24‐hour Ap values both MSIS‐83 and MSIS‐86 underestimated the depletion of O during the storm. MSIS‐86 did noticeably worse than did MSIS‐83, probably a result of the low exospheric temperature predicted by MSIS‐86. However, when 3‐hour ap values were used both models did better in predicting the measured O depletions, although MSIS‐86 again was less successful than was MSIS‐83. The data results also show that the enhancements in O2 were less than predicted by MSIS models in either mode. From 0200–0230 UT on February 9, 1986, a time just after the peak of geomagnetic activity as measured by Kp, the O mixing ratio near 135 km was found to increase by nearly a factor of 2. This increase is consistent with the passage of an atmospheric gravity wave through this altitude region providing that the scale height of O above 140 km is greater than predicted by the model.

Processes responsible for the compositional structure of the thermosphere

A diagnostic postprocessor analysis package used with runs of the National Center for Atmospheric Research thermospheric general circulation model (NCAR‐TGCM) has been extended to include the terms of the neutral composition equation that is solved by this model. The purpose of the development of this capability is to quantify the relative importance of the various physical and chemical mechanisms that force changes in neutral thermospheric composition for given geophysical situations. Such mechanisms include photodissociation of molecular oxygen and three‐body recombination of atomic oxygen, molecular and eddy diffusion and vertical and horizontal wind advection. Compositional term analysis calculations are presented using a diurnally‐reproducible TGCM run for a day of moderate geomagnetic activity (Kp = 3) near December solstice. Principal results for F region altitudes are in general agreement with previous work and include the following: (1) upper thermospheric composition is controlled by three major processes of vertical advection, horizontal advection and molecular diffusion; (2) the time scale for changes of 1/e in the mass mixing ratio for N2 and O resulting from these processes is typically ∼25–30 hours; (3) large gradients exist in the mass mixing ratios for N2 and O in the summer hemisphere high‐latitude region that, when combined with horizontal winds blowing across the magnetic pole, led to significant composition changes due to horizontal mass advection; (4) vertical advection is the major cause of the changes in mass mixing ratio of N2 and O at low and middle latitudes, with cooling and subsidence leading to decreases (increases) in the mixing ratios of N2 (O) at night, and heating and expansion leading to increase (decreases) during the daytime; (5) upward winds caused by Joule heating in the morning and evening sectors of the auroral oval cause large increases in the mixing ratio of N2 in these regions due to vertical mass advection. The principal results at ∼120 km altitude include the following: (1) photodissociation is an important term for O and O2 in this altitude region, becoming roughly commensurate with molecular diffusion, vertical and horizontal advection; (2) significant increases in the mass mixing ratio of O in the summer hemisphere occur due to photodissociation, especially in the daytime at middle latitudes; (3) downward molecular diffusion of O is important in controlling the mass mixing ratio for O, particularly in the high‐latitude winter hemisphere; (4) a complicated morphological pattern of horizontal and vertical advection maintains the mass mixing ratios at these altitudes; (5) vertical advection resulting from Joule heating in the morning and evening sectors of the auroral zone is of importance for the composition of the high‐latitude lower thermosphere; (6) typical time scales for maximum changes of 1/e in the mass mixing ratio of N2 are of the order of 60–90 hours.

Modelling of thermospheric composition changes caused by a severe magnetic storm

The UCL 3-dimensional time-dependent thermospheric model, with atomic and molecular components, is used to study composition changes in the neutral gas at F-layer heights produced by a severe magnetic storm. The computations give the mean molecular weight ( MW), temperature and winds as functions of latitude, longitude, height and time for a period of 30 h. Starting from quiet-day conditions, the simulation starts with a 6-h “substorm” period in which strong electric fields are imposed in the auroral ovals, accompanied by particle input. Weaker electric fields are imposed for the remaining 24 h of the simulation. The energy input causes upwelling of air in the northern and southern auroral ovals, accompanied by localized composition changes (increases of MW), which spread no more than a few hundred kilometres from the energy sources. There is a corresponding downward settling of air at winter midlatitudes and low latitudes, producing widespread decreases of MW at a fixed pressure-level. These storm effects are superimposed on the quiet-day summer-to-winter circulation, in which upwelling occurs in the summer hemisphere and down welling in the winter hemisphere. The composition changes seen at a fixed height differ somewhat from those at a fixed pressure-level, because of the expansion resulting from the storm heating. The results can be related to the well-known prevalence of “negative” F-layer storms (with decreases of F2-layer electron density) in summer, and “positive” F-layer storms in winter and at low latitudes. However, the modelled composition changes are not propagated far enough to account for the observed occurrence of negative storms at some distance from the auroral ovals. This difficulty might be overcome if particle heating occurs well equatorward of the auroral ovals during magnetic storms, producing composition changes and negative storm effects at midlatitudes. Winds do not seem a likely cause of negative storm effects, but other factors (such as increases of vibrationally-excited N 2) are possibly important.