Global Thermospheric Circulation Research Articles

Daytime mid-latitude F2-layer Q-disturbances: A formation mechanism

Negative and positive near noontime prolonged (≥3 hours) F2-layer Q-disturbances with deviations in NmF2 > 35% occurred at Rome have been analyzed using aeronomic parameters inferred from fp180 (plasma frequency at 180 km height) and foF2 observations. Both types of NmF2 perturbations occur under quiet (daily Ap < 15 nT) geomagnetic conditions. Day-to-day atomic oxygen [O] variations at F2-region heights specify the type (positive or negative) of Q-disturbance. The [O] concentration is larger on positive and is less on negative Q-disturbance days compared to reference days. This difference takes place not only on average but for all individual Q-disturbances in question. An additional contribution to Q-disturbances formation is provided by solar EUV day-to-day variations. Negative Q-disturbance days are characterized by lower hmF2 while positive – by larger hmF2 compared to reference days. This is due to larger average Tex and vertical plasma drift W on positive Q-disturbance days, the inverse situation takes place for negative Q-disturbance days. Day-to-day changes in global thermospheric circulation may be considered as a plausible mechanism. The analyzed type of F2-layer Q-disturbances can be explained in the framework of contemporary understanding of the thermosphere-ionosphere interaction based on solar and geomagnetic activity as the main drivers.

Atomic oxygen distributions in the Venus thermosphere: Comparisons between Venus Express observations and global model simulations

Nightglow emissions provide insight into the global thermospheric circulation, specifically in the transition region (∼70–120 km). The O 2 IR nightglow statistical map created from Venus Express (VEx) Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) observations has been used to deduce a three-dimensional atomic oxygen density map. In this study, the National Center of Atmospheric Research (NCAR) Venus Thermospheric General Circulation Model (VTGCM) is utilized to provide a self-consistent global view of the atomic oxygen density distribution. More specifically, the VTGCM reproduces a 2D nightside atomic oxygen density map and vertical profiles across the nightside, which are compared to the VEx atomic oxygen density map. Both the simulated map and vertical profiles are in close agreement with VEx observations within a ∼30° contour of the anti-solar point. The quality of agreement decreases past ∼30°. This discrepancy implies the employment of Rayleigh friction within the VTGCM may be an over-simplification for representing wave drag effects on the local time variation of global winds. Nevertheless, the simulated atomic oxygen vertical profiles are comparable with the VEx profiles above 90 km, which is consistent with similar O 2 ( 1Δ) IR nightglow intensities. The VTGCM simulations demonstrate the importance of low altitude trace species as a loss for atomic oxygen below 95 km. The agreement between simulations and observations provides confidence in the validity of the simulated mean global thermospheric circulation pattern in the lower thermosphere.

Processes of auroral thermal structure at Jupiter: Analysis of multispectral temperature observations with the Jupiter Thermosphere General Circulation Model

We use our three‐dimensional Jupiter Thermosphere General Circulation Model (JTGCM) to quantify thermal processes that take place in the auroral thermosphere. These processes tend to control the thermal budget in the Jovian ovals and polar caps and maintain thermospheric temperatures consistent with those derived from multispectral observations of Jupiter's aurora. The main heat source in the JTGCM that drives the thermospheric flow is high‐latitude joule heating resulting from frictional motion of the ions relative to the neutrals. A secondary source of heating that dominates the exospheric region of the Jovian ovals is the auroral process of particle precipitation. Both sources of high‐latitude heating in the JTGCM are strongly related to the current system in the outer magnetosphere that allows plasma to flow in and out of the Jovian ionosphere. The mapping of this flow to ionospheric altitudes gives rise to an ion drag process that dominates the neutral momentum forcing near the altitude of the ionospheric peak. We find that the ion drag and joule heating inputs in the JTGCM significantly intensify the underlying global thermospheric circulation, thereby affecting the distribution of the neutral temperature. Global simulations of the Jovian thermospheric dynamics indicate strong neutral outflows from the auroral ovals with velocities up to 1.9 km s−1 and subsequent convergence and downwelling at the Jovian equator. Such circulation is shown to be an important mechanism for transporting significant amounts of auroral energy to the rest of the planet and for regulating the global heat budget in a manner consistent with temperature observations of Jupiter's oval and polar cap regions. Adiabatic expansion of the neutral atmosphere resulting from outward flows is found to be an important source of cooling for the auroral exosphere. The distribution of neutral temperature from 1 μbar to 1 nbar is determined by the thermal balance between the total heating caused by joule and adiabatic heating processes and dynamical cooling mainly from the hydrodynamic advection process. The thermal balance of the Jovian thermosphere below the homopause (≥1 μbar) is shown to be dominated primarily by wind transport processes. The heating in this region both for the Jovian ovals and polar caps is due to hydrodynamic advection and adiabatic compression processes. This dynamical heating is efficiently dissipated by hydrocarbon cooling through CH4 and C2H2 infrared radiation at 7.8 and 12.6 μm, respectively.

Modelling F2-layer seasonal trends and day-to-day variability driven by coupling with the lower atmosphere

This paper presents results from the TIME-GCM-CCM3 thermosphere–ionosphere–lower atmosphere flux-coupled model, and investigates how well the model simulates known F2-layer day/night and seasonal behaviour and patterns of day-to-day variability at seven ionosonde stations. Of the many possible contributors to F2-layer variability, the present work includes only the influence of ‘meteorological’ disturbances transmitted from lower levels in the atmosphere, solar and geomagnetic conditions being held at constant levels throughout a model year. In comparison to ionosonde data, TIME-GCM-CCM3 models the peak electron density ( NmF2) quite well, except for overemphasizing the daytime summer/winter anomaly in both hemispheres and seriously underestimating night NmF2 in summer. The peak height hmF2 is satisfactorily modelled by day, except that the model does not reproduce its observed semiannual variation. Nighttime values of hmF2 are much too low, thus causing low model values of night NmF2. Comparison of the variations of NmF2 and the neutral [O/N 2] ratio supports the idea that both annual and semiannual variations of F2-layer electron density are largely caused by changes of neutral composition, which in turn are driven by the global thermospheric circulation. Finally, the paper describes and discusses the characteristics of the F2-layer response to the imposed ‘meteorological’ disturbances. The ionospheric response is evaluated as the standard deviations of five ionospheric parameters for each station within 11-day blocks of data. At any one station, the patterns of variability show some coherence between different parameters, such as peak electron density and the neutral atomic/molecular ratio. Coherence between stations is found only between the closest pairs, some 2500 km apart, which is presumably related to the scale size of the ‘meteorological’ disturbances. The F2-layer day-to-day variability appears to be related more to variations in winds than to variations of thermospheric composition.

Equinoctial transitions in the ionosphere and thermosphere

Abstract. Equinoctial summer/winter transitions in the parameters of the F2-region are analyzed using ground-based ionosonde and incoherent scatter observations. Average transition from one type of diurnal NmF2 variation to another takes 20–25 days, but cases of very fast (6–10 days) transitions are observed as well. Strong day-time NmF2 deviations of both signs from the monthly median, not related to geomagnetic activity, are revealed for the transition periods. Both longitudinal and latitudinal variations take place for the amplitude of such quiet time NmF2 deviations. The summer-type diurnal NmF2 variation during the transition period is characterized by decreased atomic oxygen concentration [O] and a small equatorward thermospheric wind compared to winter-type days with strong poleward wind and increased [O]. Molecular N2 and O2 concentrations remain practically unchanged in such day-to-day transitions. The main cause of the F2-layer variations during the transition periods is the change of atomic oxygen abundance in the thermosphere related to changes of global thermospheric circulation. A possible relationship with an equinoctial transition of atomic oxygen at the E-region heights is discussed.Key words. Atmospheric composition and structure (thermosphere – composition and chemistry) – Ionosphere (ionosphere- atmosphere interactions; ionospheric disturbances)

Yearly variations in the low-latitude topside ionosphere

Abstract. Observations made by the Hinotori satellite have been analysed to determine the yearly variations of the electron density and electron temperature in the low-latitude topside ionosphere. The observations reveal the existence of an equinoctial asymmetry in the topside electron density at low latitudes, i.e. the density is higher at one equinox than at the other. The asymmetry is hemisphere-dependent with the higher electron density occurring at the March equinox in the Northern Hemisphere and at the September equinox in the Southern Hemisphere. The asymmetry becomes stronger with increasing latitude in both hemispheres. The behaviour of the asymmetry has no significant longitudinal and magnetic activity variations. A mechanism for the equinoctial asymmetry has been investigated using CTIP (coupled thermosphere ionosphere plasmasphere model). The model results reproduce the observed equinoctial asymmetry and suggest that the asymmetry is caused by the north-south imbalance of the thermosphere and ionosphere at the equinoxes due to the slow response of the thermosphere arising from the effects of the global thermospheric circulation. The observations also show that the relationship between the electron density and electron temperature is different for daytime and nighttime. During daytime the yearly variation of the electron temperature has negative correlation with the electron density, except at magnetic latitudes lower than 10°. At night, the correlation is positive.Key words: Ionosphere (equatorial ionosphere; ionosphere-atmosphere interactions; plasma temperature and density)

The equatorial F-layer: progress and puzzles

Abstract. This work reviews some aspects of the ionospheric F-layer in the vicinity of the geomagnetic equator. Starting with a historical introduction, brief summaries are given of the physics that makes the equatorial ionosphere so interesting, concentrating on the large-scale structure rather than the smaller-scale instability phenomena. Several individual topics are then discussed, including eclipse effects, the asymmetries of the `equatorial trough', variations with longitude, the semiannual variation, the effects of the global thermospheric circulation, and finally the equatorial neutral thermosphere, including `superrotation' and possible topographic influences.Keyword: Ionosphere (equatorial ionosphere)

Yearly variations in the low-latitude topside ionosphere

Observations made by the Hinotori satellite have been analysed to determine the yearly variations of the electron density and electron temperature in the low-latitude topside ionosphere. The observations reveal the existence of an equinoctial asymmetry in the topside electron density at low latitudes, i.e. the density is higher at one equinox than at the other. The asymmetry is hemisphere-dependent with the higher electron density occurring at the March equinox in the Northern Hemisphere and at the September equinox in the Southern Hemisphere. The asymmetry becomes stronger with increasing latitude in both hemispheres. The behaviour of the asymmetry has no significant longitudinal and magnetic activity variations. A mechanism for the equinoctial asymmetry has been investigated using CTIP (coupled thermosphere ionosphere plasmasphere model). The model results reproduce the observed equinoctial asymmetry and suggest that the asymmetry is caused by the north-south imbalance of the thermosphere and ionosphere at the equinoxes due to the slow response of the thermosphere arising from the effects of the global thermospheric circulation. The observations also show that the relationship between the electron density and electron temperature is different for daytime and nighttime. During daytime the yearly variation of the electron temperature has negative correlation with the electron density, except at magnetic latitudes lower than 10°. At night, the correlation is positive.Key words: Ionosphere (equatorial ionosphere; ionosphere-atmosphere interactions; plasma temperature and density)

Major phenomena of the equatorial ionosphere-thermosphere system under disturbed conditions

We present here a brief review of recent results related to the response of the major phenomena of the equatorial ionosphere-thermosphere system (EITS) to magnetospheric disturbances. Modifications to the equatorial ionization anomaly (EIA), equatorial spread F (ESF) and equatorial electrojet (EEJ) are produced by (a) disturbance electric fields resulting from prompt equatorward penetration of magnetospheric/high latitude electric fields, (b) disturbance dynamo driven by enhanced global thermospheric circulation resulting from energy input at high latitude, and (c) disturbance winds (zonal and meridional) modifying the equatorial thermospheric dynamics. While the disturbance electric fields could cause enhanced development or inhibition of these phenomena the disturbance winds seem to always inhibit them, especially the EIA and ESF. New results discussed concern the role of meridional winds in the disturbance electric field induced F layer height response over low latitudes, evidence on ESF/plasma bubble inhibition by disturbance meridional/trans-equatorial wind, EIA expansion in latitude reaching spectacular magnitude during intense storms, longitudinal/local time dependence of EIA enhancements; and common sources of ESF and EIA response features and related aspects, among others. We advance, further, suggestions on specific studies to be pursued for better understanding of the problems presented.

Some peculiarities of altitudinal distribution of atomic oxygen at low latitudes during magnetic storms

Several storms were examined during the period of the ESRO-4 mission to study the altitudinal behaviour of atomic oxygen and molecular nitrogen at low latitudes. A special method of data reduction was applied to distinguish composition changes due to thermal expansion and those which resulted from some non-thermal process. It was shown that in the majority of cases studied there exists some latitudinal interval where changes of the N 2/O ratio from quiet to storm days are small compared with that observed in the purely thermal case. This latitudinal interval was found to be displaced to the winter hemisphere (not always across the equator) during solstice cases and the corresponding altitudinal interval was sometimes equal to the whole height range covered by the satellite. The effective temperature of atomic oxygen turned out to be greater than that deduced from the altitudinal distribution of N 2. This implies that there exists a non-thermal component of the [O] increase on a storm day at low latitudes and thus [O] is distributed in a non-barometric way. A possible mechanism responsible for this behaviour is downwelling due to the disturbed global thermospheric circulation.

Ionospheric F2 layer seasonal and semiannual variations

An extensive series of computations, using the Coupled Thermosphere‐Ionosphere‐Plasmasphere model (CTIP), has been undertaken to investigate the semiannual variation in peak noontime electron density, a common feature of the F2‐layer, particularly at low latitudes and in the southern hemisphere at mid‐latitudes. Results from the model reveal such a variation, most prominently, at mid‐latitudes, in the South American sector. An analysis of this phenomenon shows that it is intimately related to the large offset of the geomagnetic axis from Earths spin axis in the southern hemisphere. Because of this offset, a given geographic latitude in the South American sector corresponds to a lower magnetic latitude than in other sectors and is thus farther from the energy inputs associated with the auroral regions. As a result, the composition changes are much smaller during the winter months than at other longitudes, the mean molecular mass being essentially constant for a 4‐month period centered on the winter solstice. This result is understood in terms of the global thermospheric circulation. In the absence of any composition changes, noon ionospheric density is influenced primarily by the solar zenith angle. This angle reaches a maximum at the winter solstice, leading to diminished ion production, a minimum in NmF2, and therefore a semiannual variation overall. On the basis of the model results, the semiannual variation is seen as a feature of the midlatitude ionosphere at geographic longitudes opposite to the location of the geomagnetic pole. This phenomenon is seen in both northern and southern hemispheres, though the effect is much larger in the southern hemisphere as a result of the greater magnetic offset.

Effect of polar cap patches on the polar thermosphere

Polar cap patches are regions of enhanced ionization that appear when the interplanetary magnetic field is southward. They are created either in the dayside cusp or equatorward of the cusp in the sunlit hemisphere. Once formed, they convect in an antisunward direction across the dark polar cap at speeds of 100 m/s to about 2 km/s. The size of a plasma patch varies from about 100 to 1000 km, and its density relative to the background density varies from a few tens of percent to a factor of 100. Because propagating plasma patches might have a significant effect on the neutral atmosphere, a global thermospheric circulation model was used to calculate the response of the polar thermosphere to a “representative” plasma patch. The model predicts that a localized thermospheric disturbance is induced by and moves along with the propagating plasma patch. The moving disturbance is characterized by a neutral density depletion, an enhanced wind speed, an elevated neutral temperature, neutral gas up welling, and O/N2 composition changes. The thermospheric disturbance persists for a time that is much longer than the patch lifetime, and it spreads out over a region that is much larger than the patch dimensions as it dissipates.

Physics and chemistry of upper atmospheres of planets from infrared observations

An important goal of the study of atmospheres of planets is to provide an understanding of the relationship and interaction among phenomena occurring at different altitude and pressure regions and the effects of external influences on these phenomena. To probe multiple pressure regimes in general requires measurements in different spectral regions and with a wide range of spectral resolutions. This presents formidable technical challenges. To directly probe phenomena in the upper neutral atmosphere (pressures ⩽30 mbar) is particularly difficult. Much of the information on local chemistry and physics is contained in the near Doppler profiles of infrared molecular lines formed in these regions and spectral resolving powers λ Δλ > 10 6 are required to adequately measure the lineshapes. The very high spectral resolution technique of infrared heterodyne spectroscopy capable of making such line measurements is briefly described. Several investigations are described to illustrate the range of phenomena that can be studied and the unique physical and chemical parameters that can be retrieved. These investigations include direct measurement of global thermospheric circulation on Venus using Doppler shifts of nonthermal CO 2 emission lines near 10 μm; the study of Jupiter's deep interior utilizing spectroscopy of H 2 quadrupole lines (17 and 28.2 μm>) formed in the stratosphere; the evaluation of photochemical processes which determine the composition of Mars and the outer planets by retrieving constituent abundances (O 3 near 9.5 μm on Mars and hydrocarbons on Jupiter and Neptune); and the determination of the compositional and thermal structure in Jupiter's auroral region using line profiles of auroral ethylene (C 2H 4) and ethane (C 2H 6) emission near 10.5 and 12 μm. The value of multispectral and temporal observations for more comprehensive studies of atmospheric phenomena and the internal and external processes driving them are also discussed.

Auroral zone thermospheric dynamics: 2. Individual nights

Ground‐based Fabry‐Perot spectrometer measurements of the O I 15867 K (630.0 nm; 1 K = 1 cm−1) emission line were obtained from College, Alaska, during the winters of 1981 to 1983. Averages of the temperatures and winds, derived from these measurements, show a general flow driven by ion drag due to magnetospheric convection. This hypothesis was supported by results from a global thermospheric circulation model. Individual nights, however, show differences from the average flow pattern. During the recovery phase of an auroral substorm, when the westward electrojet is over the observing site, the equatorward meridional wind decreases in magnitude. The zonal wind is generally westward in the evening and eastward in the morning. However, the crossover from westward to eastward flow is a function of the location of the substorm onset and the arrival of the westward electrojet over College. Zenith winds can be very large (200 m/s) and show no systematic pattern, perhaps due to a superposition of wave modes from the auroral zone source region. An example of a night with little auroral activity highlights the effects of magnetospheric convection and the auroral electrojet on the normal auroral zone wind pattern. Temperature measurements occasionally show large enhancements, usually associated with type‐A red aurora.

The effects of geomagnetic activity on the F-region of the ionosphere

A study is made of the effects of geomagnetic activity on F-region ionization by correlating the F-region parameters and the Kp and AE indices, using delays up to 33 h. The total electron content from a single station for the years 1971–74 as well as ƒ 0F2 from a longitudinal chain of stations around 150°E for the years 1968–74 were used. Initially the analyses were carried out using all the geomagnetic data. They were then divided according to high or low activity in order to separate magnetic storm from quiet magnetic effects. Data sets for each station consisted of 2-monthly segments for each hour of the day for each of the years. This enabled latitudinal, seasonal and sunspot cycle effects on the correlations to be detected. Significant positive correlations with delays greater than a few hours dominated the winter results at lower latitudes; the correlations being predominantly negative at higher latitudes. The summer results at all latitudes were dominated by negative correlations. Significant negative correlations with Kp at delays between 0–3 h were obtained for summer and autumn from higher latitude stations probably indicative of electric fields. The time lags greater than 3 h are consistent with changes in atmospheric neutral composition brought about by modification of the global thermospheric circulation following heating in the auroral zones. Analysis of hmF2 data for a mid latitude station showed a negative correlation of the peak height of the F-region with magnetic activity.

Depressions in midlatitude F-region under relatively quiet geomagnetic conditions

A study is made of the depressions in day time midlatitude F-region electron density under relatively quiet geomagnetic conditions, through a detailed analysis of five specific events, using total electron content data at Auckland (37.0°S, 175.0°E) and Invercargill (46.5°S, 168.4°E) and ƒ 0F 2 data at Auckland, Christchurch (43.5°S, 172.8°E), Campbell Island (52.5°S, 169.2°E) and several Australasian stations. In peak density, the depressions are noticed to have a latitudinal extent from about 27° to 53° with prominent effects at latitudes around 40° and with a longitudinal extent of about 3000 km. Based on the complementary experimental results from ionogram analysis and total electron content measurements at Auckland, it is suggested that the depressions studied here, all of which have been noticed to occur in association with auroral disturbances ( AE >300 γ), are due to changes in atmospheric neutral composition brought about by modifications in the global thermospheric circulation pattern following heating in the auroral zone.