Ionosphere-thermosphere System Research Articles

Dynamics of the Thermosphere

Major advances in our understanding of the dynamics of Earth's thermosphere (≈ 90-500 km) during the past 25 years are reviewed. Since the thermosphere is primarily an externally-forced system, a broad overview of the energy input, conversion and transport mechanisms in the ionosphere-thermosphere system is first provided. This serves as background and context for the non-specialist. Then, several broad areas of progress are in turn discussed in some detail: (i) the role of solar thermal tides in imposing significant longitudinal variability in the lower thermosphere (≈ 100-150 km), and affecting the zonal mean circulation at these altitudes; (ii) the zonal mean circulation of the thermosphere, the changes in O and N2 relative densities that accompany it, and the competing roles of solar radiative heating and Joule (ohmic) heating in determining the overall structure of this circulation; (iii) polar and auroral thermosphere dynamics, and connections to relevant magnetosphere and ionosphere processes; and (iv) the global response to geomagnetic disturbances, i.e., relatively sudden injections of energy and momentum from the magnetosphere. The paper concludes with a personal assessment of future research directions and scientific questions that remain to be addressed in forthcoming decades.

Local time dependant response of Indian equatorial ionosphere to the moderate geomagnetic storms

The electrodynamics and neutral dynamics of the equatorial ionosphere undergo significant changes during geo-magnetically disturbed periods as a result of the high and low latitude electro-dynamical coupling. These changes may sometimes trigger the plasma instabilities that lead to the onset of spread-F and scintillations even at the L-band frequencies. The response of the equatorial ionosphere–thermosphere system over the Indian sector to moderate geomagnetic storms of 11th Feb 2004 and 9th Mar 2004 has been studied using the ground based measurements from the Indian equatorial and low latitude region. A large vertical drift of the equatorial F-layer followed by nearly simultaneous onset of Spread-F on ionograms and scintillations at VHF (244 MHz) and L-band (1.5 GHz) frequencies, have been observed during the early evening periods that correspond to the time of rapid decrease (−25 to −30 nT/h) in Sym-H index due to prompt penetration of eastward electric fields into equatorial and low latitudes. The Spread-F and scintillations continued to exist for longer durations up to post-midnight to pre-dawn hours, possibly due to the combined action of prompt penetration and long-lived Disturbance Dynamo electric fields. Due to prolonged geomagnetic activity during the subsequent days (morning-to-noon hours), strong reversal in the Equatorial ElectroJet current is observed and the Equatorial Ionization Anomaly (EIA) is significantly suppressed under the influence of westward disturbance dynamo electric fields. The post-sunset enhancements in the vertical drift of the equatorial F-layer are also reduced significantly from their quiet day patterns and the subsequent occurrence of Spread-F and scintillations is inhibited.

Comparative investigations of equatorial electrodynamics and low-to-mid latitude coupling of the thermosphere-ionosphere system

Abstract. The thermospheric midnight temperature maximum (MTM) is a highly variable, but persistent, large scale neutral temperature enhancement which occurs at low latitudes. Its occurrence can impact many fundamental upper atmospheric parameters such as pressure, density, neutral winds, neutral density, and F-region plasma. Although the MTM has been the focus of several investigations employing various instrumentation including photometers, satellites, and Fabry-Perot interferometers, limited knowledge exists regarding the latitude extent of its influence on the upper atmosphere. This is largely due to observational limitations which confined the collective geographic range to latitudes within ±23°. This paper investigates the MTM's latitudinal extent through all-sky imaging observations of its 6300Å airglow signature referred to by Colerico et al. (1996) as the midnight brightness wave (MBW). The combined field of view of three Southern Hemisphere imaging systems located at Arequipa, Peru, and Tucuman and El Leoncito, Argentina, for the first time extends the contiguous latitudinal range of imager observations to 8° S-39° S in the American sector. Our results highlight the propagation of MBW events through the combined fields of view past 39° S latitude, providing the first evidence that the MTM's effect on the upper atmosphere extends into mid-latitudes. The observations presented here are compared with modeled 6300Å emissions calculated using the NCAR thermosphere-ionosphere-electrodynamic general circulation model (TIEGCM) in conjunction with an airglow code. We report that at this time TIEGCM is unable to simulate an MBW event due to the model's inability to reproduce an MTM of the same magnitude and occurrence time as those observed via FPI measurements made from Arequipa. This work also investigates the origins of an additional low latitude airglow feature referred to by Colerico et al. (1996) as the pre-midnight brightness wave (PMBW) and described as an enhancement in 6300Å emission which occurs typically between 20:00-22:00 LT and exhibits equatorward propagation. We present the first successful simulation of a PMBW event using the TIEGCM and the airglow code. We find that the PMBW's origin is electro-dynamical in nature, resulting from the expected evening decay of the inter-tropical arcs.

A comparison of ionospheric vertical drift velocities measured by Digisonde and Incoherent Scatter Radar at the magnetic equator

Using data acquired with a Digisonde DPS-4 and an Incoherent Scatter Radar both from Jicamarca Radio Observatory, Peru, comparisons of ionospheric vertical drift velocities derived from these two instruments are carried out in this work. They show good agreement during sunset and evening hours (around sunset, up to approximately 22 LT), and also, during the post-midnight (between 02 and 03 LT) and sunrise hours (until 08 LT), when the F-layer bottom side height is around and above 300 km, as well as during events of magnetically disturbed periods when the ionospheric plasma drift may be influenced by the perturbation electric fields arising from (1) direct penetration from high to low magnetic latitudes, and (2) disturbance winds dynamo action driven by Joule heating due to increased energy and momentum deposition into the high-latitude thermosphere–ionosphere system. The results of this study lend confidence to using Digisonde drift data to study phenomena related to night-time magnetically disturbed periods.

Bimodal nature of solar wind–magnetosphere–ionosphere–thermosphere coupling

The solar wind–magnetosphere–ionosphere–thermosphere system has two ways to transfer the force that the solar wind exerts at the magnetopause to the Earth: (1) a dipole interaction mode and (2) a thermospheric drag mode. This paper uses MHD simulations to discuss both modes comparatively. The dipole interaction mode is relatively well known and so is used as a model against which to contrast the less well known thermospheric drag mode. The thermospheric drag mode itself consists of two distinct mechanisms, a direct mechanism in which the solar wind pulls on the thermosphere and (here discussed for the first time) an indirect mechanism involving an interaction between the region 1 current system and the geomagnetic dipole which increases the drag on the thermosphere nearly an order of magnitude over the direct drag mechanism. This additional force appears to be necessary to account for observed high-latitude thermospheric winds that correlate with IMF Bz.

Application of thermospheric general circulation models for space weather operations

Solar irradiance is the dominant source of heat, ionization, and dissociation of the thermosphere, and to a large extent drives the global dynamics, and controls the neutral composition and density structure. Neutral composition is important for space weather applications because of its impact on ionospheric loss rates, and neutral density is critical for satellite drag prediction. Recent observations from the Global Ultraviolet Imager (GUVI) instrument on the TIMED satellite depict the global distribution of neutral species; in particularly, the ratio of the height-integrated atomic oxygen to molecular nitrogen. The global circulation controls the neutral species composition structure, which in turn is very dependent on the balance between solar heating at the lower latitudes, and magnetospheric forcing at high latitudes. The observed latitude structure of O/N 2 therefore imposes constraints on external sources of energy to the thermosphere–ionosphere system, and is valuable information for use in physical models. One of the uses of thermospheric general circulation models for space weather operations lies in their utility as state propagators in data assimilation techniques. The physical models can match empirical models in accuracy provided accurate drivers are available, but their true value comes when combined with data in an optimal way. Recent results show that using a physical model within a data assimilation system is only beneficial if accurate drivers of the model are available. The use of O/N 2 data to tune the model forcing is therefore particularly important in the context of the recent development of a Kalman filter data assimilation model for use within the Global Assimilation of Ionospheric Measurement (GAIM) program. The need for adequate drivers is just as relevant in the application to neutral density, for satellite orbit prediction and re-entry estimation. Both applications rely on accurate estimates of the solar EUV and magnetospheric drivers.

Recent developments in ionosphere–thermosphere modeling with an emphasis on solar-variability

The Utah State University modeling developments have led to new results in several areas of ionospheric research. This review will present the most recent of these results and their synergisms with both ground-based and satellite-based observations. Particular attention is given to new results associated with solar variability, a central theme to the TIGER program. The effect on the ionosphere of differences in solar X-EUV spectra will be demonstrated by studies using the time-dependent ionospheric model, while their effect on the coupled thermosphere–ionosphere system will be shown using a new model, the global average ionosphere thermosphere model. How the dayside solar-produced plasma contributes to sub auroral polarization streams, storm enhanced densities, tongues of ionization, plasma patches, polar wind jets, and polar cap scintillations, will be described via recent modeling results from a variety of coupled or driven thermosphere, ionosphere, and magnetosphere models. The review will indicate the need for improved solar output specification, whether via observations or models. These improvements need to be over the flare through solar cycle time scale, with particular attention to the short wavelength end of the spectrum.

Equatorial ionosphere–thermosphere system: Electrodynamics and irregularities

The equatorial ionosphere and thermosphere constitute a coupled system, with its electro dynamical and plasma physical processes being responsible for a variety of ionospheric phenomena peculiar to the equatorial region. The most important of these phenomena are: the equatorial electrojet (EEJ) current system and its instabilities, the equatorial ionization anomaly (EIA), and the plasma instabilities/irregularities of the night ionosphere (associated with the plasma bubble events – ESF). They constitute the major topics of investigations having both scientific and practical objectives. The tidal wind interaction with the geomagnetic field is responsible for the atmospheric dynamo electric fields, that together with the wind system, drives the major phenomena, under quiet conditions. Drastic modifications of these phenomena can occur due to magnetospheric forcing under solar-, interplanetary- and magnetospheric disturbances. They can also undergo significant modifications due to forcing by atmospheric waves (such as planetary- and atmospheric gravity waves) propagating upward or from extra tropics. This article will focus on the ambient conditions of the ionosphere–thermosphere system and the electro dynamics and plasma instability processes that govern the plasma irregularity generation. Major emphasis is given to problems related to the structuring of the equatorial night ionosphere through plasma bubble/ESF irregularity processes. Specific topics to be covered will include: equatorial electric fields, thermospheric winds, sunset electrodynamic processes, plasma drifts, EEJ plasma instability/irregularity generation, nighttime/post sunset plasma bubble irregularity generation, and very briefly, disturbance electric fields and winds and their effect on the ionization anomaly, the TEC and ESF/plasma bubble irregularities.

An ensemble‐type Kalman filter for neutral thermospheric composition during geomagnetic storms

Global circulation models (GCMs) for the thermosphere ionosphere system have been in use for more than 20 years. In the beginning the GCMs were run on supercomputers, were expensive to run, and were used mainly to provide insight into the physics of the region and to interpret measurements. Advances in computer technology have made it possible to run GCMs on desktops and to compare their results with real‐time or near‐real‐time measurements. Today's models are capable of reproducing generic geomagnetic storm effects, but modeling specific storms is still a challenge because accurate descriptions of the energy input during storms are not easy to obtain. One way to compensate for the uncertainty in model inputs for a given period is to assimilate measurements into the model results. In this way, meteorologists have been improving their ability to model tropospheric weather for the last few decades. Data assimilation algorithms have seen an explosive growth in the last few years, and the time has come to apply such techniques to the thermospheric storm effects problem. We present results from an ensemble Kalman filter scheme that determines the best estimate of the global height‐integrated O/N2 ratio by combining GCM results and uncertainties with measurements and their errors. We describe the differences that result from the application of an ensemble Kalman filter to an externally forced system (neutral chemical composition) versus a system dominated by the initial condition and internal dynamics (tropospheric weather and ocean models). The results demonstrate that an ensemble of 10 members is able to characterize the state covariance matrix with sufficient fidelity to enable the Kalman filter to operate in a stable mode. Some information about the external forcing was extracted from the estimate of the state. The general trend of the forcing was followed by the filter, but departures were present over some periods.

Initial results from the coupled magnetosphere ionosphere thermosphere model: magnetospheric and ionospheric responses

The Center for Integrated Space Weather Modeling (CISM) is developing a simulation package of the coupled Sun–Earth system by connecting existing models within each region. The Coupled Magnetosphere Ionosphere Thermosphere (CMIT) model combines the Lyon–Fedder–Mobarry global magnetohydrodynamic (MHD) magnetospheric (LFM) model with the Thermosphere Ionosphere Nested Grid (TING) model. The LFM uses the ideal MHD equations to model the interaction between the magnetospheric plasma and the solar wind. It includes the magnetosphere–ionosphere interaction by requiring the conservation of current flowing between the magnetosphere and a two dimensional ionosphere. TING is a three dimensional code designed to simulate the thermosphere–ionosphere system by solving the mass, momentum, and thermodynamic energy equations for the global thermosphere and ionosphere. Normally, TING uses parameterized models to describe the magnetospheric input into the ionosphere. We begin this paper with a description of how the TING model is used to replace the simple two dimensional ionosphere within the LFM to produce the CISM CMIT Model. The results from the coupled model for a series of steady IMF conditions are compared with the results from the LFM to show that these models have been successfully combined. The problem of high cross polar cap potentials remains, but the structure of the magnetosphere is not dramatically altered by the coupling and the ionospheric conductances show a more realistic distribution driven by EUV radiation as well as a more clearly defined auroral oval.

Initial results from the coupled magnetosphere–ionosphere–thermosphere model: thermosphere–ionosphere responses

The thermosphere–ionosphere nested grid (TING) model has been successfully coupled with the Lyon–Fedder–Mobarry (LFM) global magnetosphere MHD code. The coupling between these models is two-way: the LFM provides the TING model with global electric fields and precipitating electron energy fluxes, and the TING model feeds ionospheric conductances back to the LFM. This code coupling enables studies of the global energy budget of the magnetosphere–ionosphere–thermosphere system. In this paper, we present simulation results from the coupled magnetosphere–ionosphere–thermosphere (CMIT) model under solar minimum, northern hemisphere summer conditions. The IMF input to the CMIT model changed its direction every 4 h. Comparisons are made between the simulated results of the CMIT model and those of the stand-alone TING model. It is found that the CMIT model predicted higher cross polar cap potential drops than the empirical model used by the stand-alone TING model. The energy input to the upper atmosphere by precipitating electrons, however, was much lower in the CMIT model during the southward IMF interval. The simulated responses of the thermosphere and ionosphere were also significantly different. As a result of the greater Joule heating calculated in the CMIT model, neutral temperatures and winds were significantly enhanced in the CMIT model in comparison with the stand-alone TING model.

Interpre: a Windows software for semiautomatic scaling of ionospheric parameters from ionograms

By analysing an ionogram several important parameters like foF2 and MUF(3000)F2 can be found, that have a significant role in the studies concerning ionospheric physics and related phenomena. Furthermore, the observation of ionospheric plasma and the forecasting of physical phenomena connected to the Sun-Magnetosphere–Ionosphere–Thermosphere system holds a remarkable scientific interest with respect to space weather, because of the influence that these phenomena have on satellite and terrestrial communications. From this point of view, the development of models whose input are real-time ionospheric parameters has become fundamental. This paper will introduce a Visual Basic program for semi-automatic scaling of ionospheric parameters from ionograms. This software allows to increase the speed of the scaling phase performed by the operator trying also to improve the output in terms of reliability.

EFFECTS ON THE IONOSPHERE DUE TO PHENOMENA OCCURRING BELOW IT

The terrestrial thermosphere and ionosphere form the most variable part of theEarth's atmosphere. Because our society depends on technological systems thatcan be affected by thermospheric and ionospheric phenomena, understanding,monitoring and ultimately forecasting the changes of the thermosphere–ionosphere system are of crucial importance to communications, navigation and the exploration of near-Earth space. The reason for the extreme variability of the thermosphere–ionosphere system isits rapid response to external forcing from various sources, i.e., thesolar ionizing flux, energetic charged particles and electric fields imposed via the interaction between the solar wind, magnetosphere and ionosphere, as well as coupling from below (“meteorological influences”) by the upward propagating, broad spectrum,internal atmospheric waves (planetary waves, tides, gravity waves) generated in thestratosphere and troposphere. Thunderstorms, typhoons, hurricanes, tornadoes andeven seismological events may also have observable consequences in the ionosphere.The release of trace gases due to human activity have the potential to cause changes inthe lower and the upper atmosphere.A brief overview is presented concerning the discoveries and experimentalresults that have confirmed that the ionosphere is subject to meteorologicalcontrol (especially for geomagnetic quiet conditions and for middle latitudes).D-region aeronomy, the winter anomaly of radiowave absorption, wave-liketravelling ionospheric disturbances, the non-zonality and regional peculiaritiesof lower thermospheric winds, sporadic-E occurrence and structure, spread-Fevents, the variability of ionospheric electron density profiles and Total ElectronContent, the variability of foF2, etc., should all be considered in connection withtropospheric and stratospheric processes. “Ionospheric weather”, as a part of spaceweather, (i.e., hour-to-hour and day-to-day variability of the ionospheric parameters)awaits explanation and prediction within the framework of the climatological, seasonal,and solar-cycle variations.

The variability of Joule heating, and its effects on the ionosphere and thermosphere

Abstract. A considerable fraction of the solar wind energy that crosses the magnetopause ends up in the high-latitude thermosphere-ionosphere system as a result of Joule heating, the consequences of which are very significant and global in nature. Often Joule heating calculations use hourly averages of the electric field, rather than the time-varying electric field. This leads to an underestimation of the heating. In this paper, we determine the magnitude of the underestimation of Joule heating by analysing electric field data from the EISCAT Incoherent Scatter Radar, situated at the 67° E magnetic latitude. We find that the underestimation, using hourly-averaged electric field values, is normally ~20%, with an upper value of about 65%. We find that these values are insensitive to changes in solar flux, magnetic activity and magnetic local time, implying that the electric field fluctuations are linear related to the amplitude of the electric field. Assuming that these changes are representative of the entire auroral oval, we then use a coupled ionosphere-thermosphere model to calculate the local changes these underestimations in the heating rate cause to the neutral temperature, mean molecular mass and meridional wind. The changes in each parameter are of the order of a few percent but they result in a reduction in the peak F-region concentration of ~20% in the summer hemisphere at high latitudes, and about half of this level in the winter hemisphere. We suggest that these calculations could be used to add corrections to modelled values of Joule heating.Key words. Ionosphere (eletric fields and currents; ionospheric disturbances; polar ionosphere)

Outstanding problems in the equatorial ionosphere–thermosphere electrodynamics relevant to spread F

Dynamic coupling of the atmospheric regions involving upward energy transport coupled with the locally active thermal tidal modes establish the wind system of the thermosphere whose interaction with the magnetized conducting ionospheric layers produces the dynamo electric fields and currents that control the quiet time electrodynamic processes of the equatorial ionosphere–thermosphere system. The plasma fountain responsible for the ionization anomaly, the vertical and zonal plasma drifts of the post-sunset hours leading to plasma bubble/spread F irregularity generation, and the Hall electric field that drives the electrojet current system are among the most notable manifestations of the these processes. The electrodynamic processes to be discussed in this paper will concern mainly the equatorial spread F (ESF) irregularity generation and their variabilities. The key factors that control the ESF generation by generalized Rayleigh–Taylor instability process, such as the the prereversal enhancement electric field (vertical drift) that is controlled by post-sunset zonal wind and longitudinal conductivity gradients, meridional/transequatorial winds, flux tube integrated conducitvities, and the seed perturbations are discussed in some detail, focussing attention on the aspects of their seasonal, logitudinal and day-to-day variabilities. The role of the evening F layer vertical drift in the instability process leading to ESF has been the most extensively investigated experimentally so far. Recent theoretical and computational results have advanced greatly our understanding of the importance of the key factors in the development process of spread F. However, observational identification of the relative importance of some of the key control factors in a given spread F event or in its day-to-day variablity is still lacking, the main examples being that of the seed perturbation, meridional/transequatorial winds and integrated conductivities. However, some progress on understanding the seasonal pattern has been achieved. Enhanced conductivity below F region and in E layer in the longitude sector of the South Atlantic Magnetic Anomaly seems to influence the equatorial spread F electrodynamics.

Electric field measurements at a southern mid-latitude station obtained using an HF digital ionosonde

Abstract It is important to understand the convection of the inner magnetosphere to fully describe the response of the low- to mid-latitude thermosphere-ionosphere system to geomagnetic storms. Realistic numerical simulations of mid-latitude electric fields suffer from limited knowledge of lower thermospheric winds and ionospheric conductivity on a global scale. Even empirical models of mid-latitude electric fields suffer from the paucity of measurements made by the handful of incoherent scatter radars concentrated in the American-European sector, and the intermittent satellite measurements made in other regions. Thus it would be very useful to show the extent to which Doppler velocity measurements made with the numerous digital ionosondes deployed around the globe can be used to infer F -region electric fields. The monthly average diurnal variation of Doppler velocity measured by a recently commissioned Digisonde at Bundoora (145.1°E, 37.7°S, geographic; 49°S magnetic) is seen to resemble the average diurnal variation of ion drift measured by the incoherent scatter radar at Millstone Hill (71.5°W 42.6°N; 57°N). Moreover, the Bundoora measurements exhibit the nighttime westward perturbation drifts found in Dynamics Explorer-2 ion drift measurements.

High‐latitude currents and their energy exchange with the ionosphere‐thermosphere system

The transfer of electrical energy at high latitudes between the magnetosphere and ionosphere‐thermosphere (IT) system is contained within the resulting currents and electric fields between the two systems. As such, the electrodynamic state of the IT system can influence how much of the electrical energy is transferred at any one time by modulating the currents that flow in the region. For this study, 1557 hours of detailed E region measurements by the Sondrestrom incoherent‐scatter radar from 1993 through 1998 were collected, of which 95 hours were used to determine the local electromagnetic energy transfer rates within the IT system and evaluate the role of the IT system in modifying this energy transfer. The measurements have been organized into 59 periods of enhanced electrical energy transfer and cover all universal times but are local to the region above Sondrestrom at 74.2° magnetic latitude. In many of the events studied it was found that the IT system acted to reduce the net electromagnetic energy transferred from the magnetosphere owing to the presence of neutral winds. The dawn sector proved to have on average greater passive energy deposition rates than in the dusk sector. The reduction of this rate by neutral winds also proved stronger in the dawn sector. This enhanced modulation caused the dawn‐dusk asymmetry in energy transfer to be less pronounced but still apparent in the electromagnetic energy transfer rate.

Modeling of the solar wind originated energy input for the study of effects on the terrestrial thermosphere and ionosphere-introduction

The development of a particle climatology for the purpose of studying solar particle fluxes and their effect on the terrestrial atmospheric system has been adopted as an activity of the International Solar Cycle Study, Working Group 1, Panel 2. The activity has been incorporated as a component of the Thermospheric-Ionospheric Geospheric Research (TIGER) Program. This climatology is an empirical model of particle input providing average spectral characteristics, precipitating particle fluxes, and ionization rate profiles as functions of latitude, local time, and activity level. It is being built upon a model already being developed with particle data from the Upper Atmosphere Research Satellite (UARS). This paper provides an introduction to our development of the particle climatology, which is described in more detail in the following paper. We review particle input models currently used to study the effects of particle energy input on the thermosphere-ionosphere system. We describe the objective of the climatology development, the optimal low-altitude database to be used, and the selection of indices to be incorporated into the model.

Ground measurements of thermospheric and ionospheric parameters: An introductive description of the existing resources

During the last decade a lot of progress has been made in the comprehension of the thermosphere-ionosphere system. Physical models are now able to describe this coupled system and comparisons with ground or space measurements are now done on different time and spatial scales. Starting with a review of the input parameters used in such modeling and of their outputs, we outlined the parameters that can be measured from ground. Among the different techniques of measurements, two are particularly important: the incoherent scatter technique that is reviewed by the paper of Kofman (this issue) and the optical interferometric technique that we briefly describe here.

Modeling of the thermosphere-ionosphere system

Empirical, semi-empirical, and theoretical models of the thermosphere and ionosphere and of photochemical processes occurring therein are briefly described. An overview is provided of this section of the issue, containing papers on these topics and on the measurement of thermospheric and ionospheric properties from space. The importance to these endeavors of accurate measurement and specification of the solar ultraviolet, extreme-ultraviolet, and soft x-ray irradiance is discussed.