Thermospheric Parameters Research Articles

Wave-driven variability in the ionosphere-thermosphere-mesosphere system from TIMED observations: What contributes to the “wave 4”?

[1] Numerous observations and model studies made during the past 5 years have unequivocally revealed that the ionosphere and thermosphere owe a considerable amount of their longitudinal, local time, seasonal latitudinal and day-to-day variability to waves originating in the lower part of the atmosphere. The most prominent pattern is the four-peaked (“wave 4”) longitudinal structure frequently observed by (quasi-) Sun-synchronous satellites in a variety of ionospheric and thermospheric parameters. The “wave 4” has often been attributed to the diurnal, eastward, wave number 3 (DE3), nonmigrating tide alone. A more detailed analysis of TIMED observations, supported by physics-based empirical modeling and data from the CHAMP satellite, now indicates that this interpretation needs to be revised. Secondary wave generation due the nonlinear interaction between the migrating diurnal tide and the DE3 leads to a large stationary planetary wave 4 (SPW4) and a large semidiurnal, eastward wave number 2 (SE2) tide in the equatorial zonal wind at E-region heights. Combined amplitudes can equal those of the DE3. SE2 penetrates into the upper thermosphere with transequatorial wind speeds in excess of 10 m/s. This paper discusses the resulting implications for electric field generation in the E-region and tidal-ionosphere coupling in the F-region and provides observational constraints for future modeling efforts.

Modeling of multiple effects of atmospheric tides on the ionosphere: An examination of possible coupling mechanisms responsible for the longitudinal structure of the equatorial ionosphere

A number of recent studies have highlighted the observational evidence for a coupling between atmospheric tides in the thermosphere and the electron density structure of the ionosphere. The most commonly proposed mechanism to explain this is an electrodynamic coupling between tides at E region altitudes and ion drifts at F region altitudes. However, based on both the observational evidence from recent satellite missions such as those of the neutral winds associated with nonmigrating tides at F region altitudes, and considering the theoretical effects of atmospheric tides on the thermosphere and ionosphere, more than one coupling mechanism must be considered. We use Sami2 is Another Model of the Ionosphere to test a set of electrodynamic and chemical‐dynamical coupling mechanisms that could explain the link between tides in the thermosphere and the low‐latitude ionosphere. We investigate the possible role of the vertical drifts during the both the day and around sunset, perturbations to the thermospheric neutral density and thermospheric [O]/[N2], and tidal winds at F region altitudes. These simulations give an estimate of the sensitivity of the nighttime ionosphere to each of these coupling mechanisms. We then compare the results of these sensitivity tests with the effects of atmospheric tides on different thermospheric parameters as simulated by a self‐consistent model of the atmosphere‐ionosphere‐electrodynamic system (thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model). This comparison shows that in addition to the E region dynamo modulation, the potential coupling between tides and the ionosphere via changes in thermospheric [O]/[N2], meridional winds at F region altitudes, and modification of the vertical drifts around sunset could play an important role and all require further study, both with models and new observations.

Parameters of the ionospheric F 2 layer as a source of information on trends in thermospheric dynamics

The problem of determination of trends in thermospheric dynamics parameters (horizontal winds) on the basis of analysis of trends in various combinations of ionospheric F2-layer parameters is formulated. The previous attempts of the author in this direction are briefly described. It is shown that all studied parameters lead to the same result: after the “boundary date” (approximately 1980) a systematic change in these parameters (a long-term trend) is observed, this fact manifesting changes in the dynamical regime of the thermosphere because of cooling and contraction of the entire middle and upper atmosphere. The results of a search for trends in the hmF2 height for the moment (Т(ss) + 2h) are described. These trends are found higher than the hmF2 trends obtained earlier by various authors analyzing the hmF2 behavior at fixed moments of local time.

A revised method to extract thermospheric parameters from incoherent scatter observations

Height distribution of ionospheric plasma parameters in the F2-region is closely related to height distribution of the main thermospheric parameters. Therefore, they can be extracted from ionospheric observations solving an inverse problem of aeronomy. A self-consistent approach to the Ne(h) modeling at the F2-region heights has been applied to solve the problem. Using routine incoherent scatter radar observations (Ne(h), Te(h), Ti(h), Vi(h) profiles) the method yields a self-consistent set of main aeronomic parameters responsible for the F2-region formation. The list of derived parameters includes: neutral temperature profile Tn(h) depending on the exospheric temperature Tex, the temperature at 120 km T120 and the shape-parameter S, which determine the temperature profile, concentration of neutral species [O], [O2], [N2], vertical plasma drift W, which may be converted to the meridional thermospheric wind Vnx, total solar EUV flux and ion composition (O+, O2 +, NO+, N2 +, N+) as a result of Ne(h) fitting. Therefore, the method gives a complete description of the upper atmosphere condition in the vicinity of incoherent scatter facility for the periods of observation. Analysis of all available EISCAT (CP-1, CP-2) observations has shown wide deviations from MSIS-86 model predictions for geomagnetically disturbed conditions while the retrieved parameters are close to the model ones for quiet periods. The approach turns out to be very useful for physical analyses of the F2-layer disturbance mechanisms giving a complete picture of the phenomenon in question. Limitations and problems related to method application are discussed. Under existing conditions when thermospheric observations are not conducted currently the proposed method may be considered a real tool for thermosphere investigation and monitoring at least for the periods of ISR observations.

Ozone content of the atmosphere. 2. Variability of the total ozone content in the atmosphere and the horizontal wind in the lower thermosphere (Central Europe)

The correlation between total ozone content lower thermosphere horizontal wind parameters, and standard indices of solar activity and geomagnetic activity has been studied. The satellite measurements of TOC for five observatories in Central Europe and the lower thermosphere wind measurements for Collm observatory (Germany) were used for 1996–2003. The quasi-periodic structure of these variations and the correlation between the corresponding periodograms were also studied. The quantitative evaluation of the statistically significant correlations and common periodicities were revealed.

Comparison between some methods for estimating thermospheric parameters using ionospheric incoherent scatter measurements

Three methods for estimating the atomic oxygen density and exospheric temperature using the ionospheric incoherent scatter measurements have been compared. These methods are based on the usage of the relationship obtained from the energy conservation equation. They are applicable under the conditions of the atmosphere composed of oxygen atoms and ions. The first method is based on the regression analysis of finding minimal deviations of the calculated electron densities (N) from the values measured at altitudes of 300–450 km. The second method is based on solving the set of equations of the analytical expression relating N to ionospheric characteristics. These equations are given for several altitudes. The third method is related to the selection of such atomic oxygen densities at which the neutral gas temperature remains constant at an altitude above 350 km. All methods give similar results. The calculations based on the measurements performed at the Irkutsk radar on September 23, 1998, showed that the atomic oxygen densities at 400 km, estimated using the first and second methods, account for 0.48 of the value predicted by the MSIS-86 model, and such densities estimated using the third method account for 0.59 of the model value. In this case the estimated exospheric temperatures are larger than the values obtained using the thermospheric model by 11% for the first and second methods and by 14% for the third method.

Upper thermospheric ion-neutral coupling from combined optical and radar experiments over Svalbard

Abstract. The response of thermospheric neutral parameters such as winds and temperatures to rapid changes in geophysical conditions has usually been considered to be relatively slow, on the order of hours, and steady, representing an integration of more rapid ionospheric changes. Quantifying the relevant ion-neutral coupling has proved difficult due to a lack of relevant laboratory data for the most important collisions, namely between neutral atomic oxygen and its first ion. As a result the representation of ion-neutral coupling in numerical models of the upper atmosphere has often produced poor comparison to experimental data. Using a unique combination of spatially extended ion and neutral thermospheric parameters we show that the neutral response can be very rapid, within 15 min, to imposed forcing via ion-neutral coupling. The array of complementary instrumentation measuring the thermosphere above Svalbard in the Northern Hemisphere allows detailed study of the causes and effects from both the ion and neutral perspectives. The implications for development and testing of the thermospheric numerical models is discussed.

Seasonal features in the spread-F probability near midnight over Moscow

Using the data of Moscow station for 1975–1985, the seasonal features in the dependence of the spread-F probability P near midnight on the levels of solar and geomagnetic activity have been analyzed. It has been found that the P dependence on solar activity is most substantial in winter and fall, the P dependence on geomagnetic activity is maximal during equinoxes, and the P dependence on solar activity prevails in summer but is much weaker than in winter and fall. Based on the qualitative analysis of the known mechanisms of the midlatitude spread-F, the regression equation, which shows the P dependence on the solar activity level and thermospheric parameters (temperature and density) at a fixed average level of geomagnetic activity, has been obtained. In this equation the character of the seasonal changes in P is determined by the thermospheric parameters, the relative contribution of which depends on solar activity. The found dependence of the character of the P seasonal variations on the solar activity level has been interpreted based on this equation.

Causal link of the wave‐4 structures in plasma density and vertical plasma drift in the low‐latitude ionosphere

We investigate the annual and local time variations of the wave‐4 structures in the plasma density and vertical drift in the low‐latitude F region by analyzing the measurements from the first Republic of China satellite (ROCSAT‐1) and conducting simulations with the Global Ionosphere and Plasmasphere (GIP) model. The GIP model uses apex magnetic coordinates with International Geomagnetic Reference Field (IGRF) for magnetic field, neutral wind from HWM‐07, and thermospheric parameters from the NRLMSISE‐00 model. In order to understand how the vertical drifts relate to the longitudinal structure of the topside ionosphere, we apply the equatorial vertical drifts observed from ROCSAT‐1 to drive the GIP model. The model well reproduces the longitudinal structure in electron density, and the magnitudes of electron density are comparable with ROCSAT‐1 measurement at 600 km. The ROCSAT‐1 observations of the vertical drift and plasma density show maximum amplitudes of their wave‐4 components in July–September and minimum amplitudes in December–February. An eastward shift of the wave‐4 components with increasing local time is observed in both the density and the vertical drift. The GIP model density showed similar annual and local time variations of the wave‐4 component. Since the model uses the observed equatorial vertical E × B drift as an input, the results indicate the vertical drifts are essential in the formation and evolution of the longitudinal wave‐4 density structure. The amplitude of the eastward propagating diurnal tide (DE3) at 110 km shows similar annual and local time variations as the F region parameters, supporting the link between the DE3 tide, vertical E × B drift, and F region plasma density on a global scale.

Longitudinal differences in the equatorial spread F characteristics between Vietnam and Brazil

We present the results of a comparative study of the equatorial spread F (ESF) and the F layer critical parameter, the base height of the F layer bottomside ( h′ F) over the two equatorial sites, Ho Chi Minh City – HCM (dip latitude: 2.9°N) in Vietnam and Sao Luis – SL (dip latitude: ∼2°S) in Brazil. The study utilizes simultaneous data collected by a CADI at HCM and a digisonde at SL during the year 2002 with the monthly mean solar 10.7 cm flux (F10.7) varying from ∼120 to ∼185. This study focuses on the quiet time seasonal behavior of the F layer parameters in the two widely separated longitude sectors, and addresses the question as to what can we learn from such comparative studies with respect to the ambient ionospheric and thermospheric parameters that are believed to control the ESF generation and hence its longitudinal occurrence pattern. The observed differences/similarities in the diurnal and seasonal patterns of the F Layer height vis-à-vis the ESF occurrences are evaluated in terms of the known longitudinal differences in the F layer heights, thermospheric meridional winds and the geomagnetic peculiarities of the two sites.

Storm time global thermosphere: A driven‐dissipative thermodynamic system

Orbit‐averaged mass densities and exospheric temperatures ∞ inferred from measurements by accelerometers on the Gravity Recovery and Climate Experiment (GRACE) satellites are used to investigate global energy Eth and power Πth inputs to the thermosphere during two complex magnetic storms. Measurements show , ∞, and Eth rising from and returning to prevailing baselines as the magnetospheric electric field ɛVS and the Dst index wax and wane. Observed responses of Eth and ∞ to ɛVS driving suggest that the storm time thermosphere evolves as a driven‐but‐dissipative thermodynamic system, described by a first‐order differential equation that is identical in form to that governing the behavior of Dst. Coupling and relaxation coefficients of the Eth, ∞, and Dst equations are established empirically. Numerical solutions of the equations for ∞ and Eth are shown to agree with GRACE data during large magnetic storms. Since ∞ and Dst have the same ɛVS driver, it is possible to combine their governing equations to obtain estimates of storm time thermospheric parameters, even when lacking information about interplanetary conditions. This approach has the potential for significantly improving the performance of operational models used to calculate trajectories of satellites and space debris and is also useful for developing forensic reconstructions of past magnetic storms. The essential correctness of the approach is supported by agreement between thermospheric power inputs calculated from both GRACE‐based estimates of Eth and the Weimer Poynting flux model originally derived from electric and magnetic field measurements acquired by the Dynamics Explorer 2 satellite.

Observations of the low‐latitude solar eclipse on 8 April 2005 by CHAMP

Measurements of ionospheric and thermospheric parameters obtained by the Challenging Minisatellite Payload (CHAMP) satellite were analyzed for the time period of the solar eclipse on 8 April 2005. We have analyzed the electron density and temperature, as well as the thermospheric mass density and the neutral wind velocity. Because of the location of the eclipse close to the equator, a study of the equatorial electrojet is also included. During closest approach, CHAMP passed the region of 30% obscuration. Significant modifications of the ionosphere at F2 layer altitudes could be observed. The plasma fountain at the equator is strongly enhanced, resembling post‐sunset conditions. The thermospheric mass density and zonal wind show no eclipse‐related effect. The equatorial electrojet strength is reduced by almost 40%. This is about twice the amount expected from E region electron depletion at the CHAMP footprint.

Influence of geomagnetic activity on the O I 630.0 and 557.7 nm dayglow

The one‐dimensional fluid/kinetic code TRANSCAR is used to simulate the effects of geomagnetic activity on the atomic oxygen (3P‐1D) red and (1D‐1S) green thermospheric dayglows at 630.0 and 557.7 nm, associated with the modification of the density structure of the neutral atmosphere. It is found that when magnetic activity increases from quiet to strong, the altitude of the peak of both emissions increases by <10%, the peak intensity of the 557.7 nm thermospheric layer decreases by about 40%, and the peak intensity of the 630.0 nm layer remains almost constant. The prevailing production and quenching processes are reviewed and their variations are explained in terms of changes in the ionospheric and thermospheric parameters which are induced by the rise of the geomagnetic activity. Together with this model study a 4‐year set of WINDII data is analyzed. The measurements confirm the trends previously revealed by the TRANSCAR model. No statistical variation of the 630.0 nm peak emission is seen, while the anticorrelation between the 557.7 nm thermospheric peak intensity and the magnetic activity is clearly found.

Geomagnetic storm effects at F1-layer heights from incoherent scatter observations

Abstract. Storm effects at F1-layer heights (160–200 km) were analyzed for the first time using Millstone Hill (mid-latitudes) and EISCAT (auroral zone) incoherent scatter (IS) observations. The morphological study has shown both increases (positive effect) and decreases (negative effect) in electron concentration. Negative storm effects prevail for all seasons and show a larger magnitude than positive ones, the magnitude of the effect normally increasing with height. At Millstone Hill the summer storm effects are small compared to other seasons, but they are well detectable. At EISCAT this summer decrease takes place only with respect to the autumnal period and the autumn/spring asymmetry in the storm effects is well pronounced. Direct and significant correlation exists between deviations in electron concentration at the F1-layer heights and in the F2-layer maximum. Unlike the F2-layer the F1-region demonstrates a relatively small reaction to geomagnetic disturbances despite large perturbations in thermospheric parameters. Aeronomic parameters extracted from IS observations are used to explain the revealed morphology. A competition between atomic and molecular ion contributions to Ne variations was found to be the main physical mechanism controlling the F1-layer storm effect. The revealed morphology is shown to be related with neutral composition (O, O2, N2) seasonal and storm-time variations. The present day understanding of the F1-region formation mechanisms is sufficient to explain the observed storm effects.Key words. Atmospheric composition and structure (thermosphere-composition and chemistry); ionosphere (ion chemistry and composition; ionospheric disturbances)

F2-region response to geomagnetic disturbances

The F2-region response to a geomagnetic storm usually called a ionospheric storm is a rather complicated event. It consists of the so-called positive an negative phases, which have very complicated spatial and temporal behavior. During the recent decade there was significant progress in understanding this behavior. The principal features of the positive and negative phase distribution and variations have been explained on the basis of the principal concept: during a geomagnetic disturbance there is an input of energy into the polar ionosphere, which changes thermospheric parameters, such as composition, temperature and circulation. Composition changes directly influence the electron concentration in the F2 region. The circulation spreads the heated gas to lower latitudes. The conflict between the storm-induced circulation and the regular one determines the spatial distribution of the negative and positive phases in various seasons. There are still problems unsolved. The most acute ones are: the appearance of positive phases before the beginning of a geomagnetic disturbance, the occurence of strong negative phases at the equator, the role of vibrationally excited nitrogen in forming the negative phase, and the relation of positive phases to the dayside cusp. There are indications that the f oF2 long-term trends revealed during the recent years may be explained by long-term trends of the number of negative ionospheric disturbances due to secular variations of the geomagnetic activity.

Plausible explanation for the equatorial temperature and wind anomaly (ETWA) based on chemical and dynamical processes

It has been proposed earlier through independent studies that either ‘chemical heating’ or ‘ion drag’ might be responsible for the generation of a large-scale low-latitude process, namely the Equatorial Temperature and Wind Anomaly (ETWA). The present study based on simultaneous measurements of ionospheric and thermospheric parameters by DE-2 satellite alongwith some of the standard models shows that both the processes are important and necessary for the generation of ETWA. A proper quantification has been attempted with reasonable success by taking into account both the chemical heating and ion drag. The chemical heating which operates irrespective of the time of the day, has been shown to account for 25–30 K (∼30%) of the neutral temperature enhancements in ETWA. On the other hand, the ion-drag associated heating, which is operative only during daytime conditions could raise the temperature by 60–80 K. Thus the relative importance of ion drag and chemical heating have been evaluated in arriving as a plausible explanation of ETWA.

Plasmaspheric storm time erosion

Unusually low whistler mode group delay times are observed by VLF Doppler receivers at both Faraday, Antarctica, and Dunedin, New Zealand, following magnetic storms. These are typically caused by plasmaspheric electron concentration depletions near L=2.4 and not by changes in the VLF wave propagation path. Using a data set that is almost continuous since 1986, we find that depletions during storms in the solar minimum of 1995 are significantly deeper than in the minimum of 1986. Event studies at Faraday show that the electron concentration depletions caused by storms were about a factor of 2 in 1986 and a factor of 3–4 in 1995, independent of the time of year. However, the depletions observed by both sites are significantly deeper than those observed in 1958 and 1961 using natural whistlers (i.e., factors of 2–4 compared to 1.3). The Sheffield University Plasmasphere Ionosphere Model (SUPIM) has been used to investigate possible causes of the plasmaspheric electron concentration depletions observed in the whistler mode data. Thermospheric parameters, including a reduction in the concentration of neutral hydrogen and oxygen at all altitudes, were perturbed by a factor of 10 from their normal levels. However, the plasmaspheric depletions produced were only of the order of 10% after 27 hours. It is unlikely therefore that thermospheric modifications alone are responsible for the depletions observed in the data. Additionally, a tube of plasma was moved to higher L shell under the influence of an equatorial meridional E × B drift velocity of 1000 m s−1 and showed levels of depletion of about a factor of 2. Although it is possible to generate plasmaspheric concentration depletions using the drifting tube model, the depletions are smaller than those observed and the outward E × B drift velocity needed is a factor of 2 greater than those reported previously at L=2.4. It is therefore unlikely that the tube drifting mechanism is the principal cause of the observed plasmaspheric electron concentration depletions at L=2.4. Although no mechanism is clearly identified in this study, the ground‐based results presented in this paper indicate erosion levels of similar structure and magnitude to electron concentration profiles from the ISEE 1 satellite in the aftermath of magnetic disturbances during 1983, thus providing a long‐term record of plasmaspheric erosion.

Electric field effects on ionospheric and thermospheric parameters above the EISCAT station for summer conditions

Abstract. Numerical calculations of the thermospheric and ionospheric parameters above EISCAT are presented for quiet geomagnetic conditions in summer. The Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) was used. The numerical results were obtained both with a self-consistent calculation of the electric fields of magnetospheric and dynamo-action origin and with the magnetospheric electric fields only. It was found that the dynamo-electric field has some effect on the ionospheric convection pattern during quiet geomagnetic conditions. It has a marked effect mainly on the zonal neutral wind component above EISCAT (±20 m/s at 140 km altitude). We have studied the effects of various field-aligned current (FAC) distributions on thermosphere/ionosphere parameters and we show that a qualitative agreement can be obtained with region-I and -II FAC zones at 75° and 65° geomagnetic latitude, respectively. The maximum FAC intensities have been assumed at 03–21 MLT for both regions with peak values of 2.5×10–7 A m–2 (region I) and 1.25×10–7 A m–2 (region II). These results are in agreement with statistical potential distribution and FAC models constructed by use of EISCAT data. The lack of decreased electron density in the night-time sector as observed by the EISCAT radar was found to be due to the spatial distribution of ionospheric convection resulting from electric fields of magnetospheric origin.Key words. Electric fields and currents · Ionosphere- atmosphere interactions · Modelling and forecasting

Plasma drifts inferred from thermospheric neutral parameters during geomagnetic storms at 23°S

Abstract Nighttime thermospheric neutral winds and temperatures are derived from measurements of Doppler shifts and widths of the OI 630.0 nm airglow emission line, respectively, using a Fabry-Perot interferometer at Cachoeira Paulista (23°S, 45°W, 16°S dip latitude), Brazil. The observed nighttime East–West and North–South temperature gradients and neutral wind velocity variations are presented for two geomagnetic storm periods 7–10 July 1991 and 4–9 August 1991. Zonal plasma drifts inferred during an intense geomagnetic storm, from the nighttime neutral wind velocities and temporal temperature gradient variations, suggested that a strong zonal plasma drift (−240 m⧹s) on 9–10 July 1991 changed the direction of the zonal winds to westward at 01:30. Also it was noticed that, during the geomagnetic storm, the zonal temperature gradients were not well defined like those observed during quiet time. The meridional wind was towards the pole for the whole night during the geomagnetic storm of 9–10 July, a behaviour which is unusual during winter time (southern hemisphere) at Cachoeira Paulista. However, the meridional plasma drifts inferred did not show significant differences during the quiet and disturbed periods. The method presented to infer plasma drifts using the observed neutral parameters showed results comparable with those observed by incoherent scatter radar measurements at Arecibo.

Physical mechanism of strong negative storm effects in the daytime ionospheric F2 region observed with EISCAT

Abstract. A self-consistent method for daytime F-region modelling was applied to EISCAT observations during two periods comprising the very disturbed days 3 April 1992 and 10 April 1990. The observed strong Ne decrease at F2-layer heights originated from different physical mechanisms in the two cases. The negative F2-layer storm effect with an NmF2 decrease by a factor of 6.4 on 3 April 1992 was produced by enhanced electric fields (E≈85 mV/m) and strong downward plasma drifts, but without any noticeable changes in thermospheric parameters. The increase of the O+ + N2 reaction rate resulted in a strong enrichment of the ionosphere with molecular ions even at F2-layer heights. The enhanced electric field produced a wide mid-latitude daytime trough on 03 April 1992 not usually observed during similar polarization jet events. The other strong negative storm effect on 10 April 1990 with a complete disappearance of the F2-layer maximum at the usual heights was attributed mainly to changes in neutral composition and temperature. A small value for the shape parameter S in the neutral temperature profile and a low neutral temperature at 120 km indicate strong cooling of the lower thermosphere. We propose that this cooling is due to increased nitric oxide concentration usually observed at these heights during geomagnetic storms.Key words. Atmospheric composition and structure · Thermosphere · Ionosphere · Ion chemistry and composition · Atmosphere interactions