Thermospheric Response Research Articles

Comparison of the observed and modeled low- to mid-latitude thermosphere response to magnetic activity: Effects of solar cycle and disturbance time delay

The performance of JB2008 and NRLMSISE-00 models, in describing the response of the thermosphere to magnetic activity are evaluated against total mass density retrieved from accelerometer measurements made onboard CHAMP satellite during 5 years. We show that the global low- to mid-latitude disturbance amplitude is correctly described by the JB2008 model for low solar activity conditions and by both the JB2008 and the NRLMSISE-00 models for high solar activity conditions. For low solar activity conditions, statistics based on almost 3 years of data confirm the large underestimation by the NRLMSISE-00 model quantified by Lathuillere et al. (2008) for the year 2004. We also found that the time delay between low- to mid-latitude global thermosphere disturbance and magnetic activity is statistically well estimated by the NRLMSISE-00 and JB2008 models for disturbed conditions. For moderately disturbed conditions however, the time delay estimated by the JB2008 model is too large by about 3 h. For very disturbed conditions, we found different time delays during day-time and night-time, using new geomagnetic proxies with a 30-min time resolution.

Dynamics of North American sector ionospheric and thermospheric response during the November 2004 superstorm

We present a study of ionospheric and thermospheric response during a November 9–10, 2004 major geomagnetic storm event (DsT ∼−300 nT). We utilize the North American sector longitude chain of incoherent scatter radars at Arecibo, Millstone Hill, and Sondrestrom, operating as part of a coordinated international mesosphere/lower thermosphere coupling study experiment. Total electron content (TEC) determinations from global positioning system (GPS) ground receivers, ground magnetometer traces from the Canadian CANOPUS array, Defense Meteorological Satellite Platform (DMSP) topside data, and global convection patterns from the SuperDARN radar network are analyzed to place the detailed radar data in proper mesoscale context. The plasmaspheric boundary layer (PBL) expanded greatly in the dusk sector during ring current intensification to span more than 25° of magnetic latitude, reaching as far south as 30° invariant latitude. Strong sub-auroral polarization stream velocities of more than 1 km/s were accompanied by large upwards thermal O+ fluxes to the overlying magnetosphere. The large PBL expansion subsequently exposed both Millstone Hill and Sondrestrom to the auroral convection pattern, which developed a complex multicell and reverse convection response under strongly northward IMF conditions during a period of global interplanetary electric field penetration. Large traveling atmospheric and ionospheric disturbances caused significant neutral wind and ion velocity surges in the mid-latitude and tropical ionosphere and thermosphere, with substorm activity launching equatorward neutral wind enhancements and subsequent mid-latitude dynamo responses at Millstone Hill. However, ionosphere and thermosphere observations at Arecibo point to significant disturbance propagation modification in the post-dusk sector PBL region.

Modeling the thermospheric response to solar flares

Measurements of the incoming solar extreme ultraviolet (EUV) irradiance now allow models to be driven at higher temporal resolution and with better accuracy than with proxy‐based models. Using solar irradiance measurements from the Solar EUV Experiment (SEE) instrument to drive the Global Ionosphere‐Thermosphere Model, the global thermospheric response to the 28 October 2003 and 6 November 2004 solar flares is presented. The model indicates that the thermospheric density at 400 km can increase by as much as 14.6% in under 2 hours because of the flare and takes 12 hours to settle to close to a nominal state. Intense dayside heating launches nightward propagating gravity waves that transport energy efficiently to the nightside at velocities near the local sound speed plus the bulk wind velocity. Measurements from the Champ satellite indicate that the simulated day to night propagation time is similar to the observed one. Substantial density enhancements occur near the midnight sector as the wave converges on itself. In some locations the nightside perturbations are as large as those on the dayside. The convergence of the wave also leads to significant adiabatic heating of the nightside thermosphere. Eventually, the wave is reflected back toward the dayside but is quickly damped.

Hydrodynamic planetary thermosphere model: 2. Coupling of an electron transport/energy deposition model

An electron transport/energy deposition model is expanded to include atomic nitrogen and is coupled with a 1‐D hydrodynamic thermosphere model. The coupled model is used to investigate the response of the Earth's thermosphere under extreme solar EUV conditions and is compared with previous studies. It is found that (1) the parameterization of Swartz and Nisbet (1972) underestimates the ambient electron heating by photoelectrons significantly in the upper thermosphere of the Earth under conditions with greater than 3 times the present solar EUV irradiance; (2) the transition of the Earth's thermosphere from a hydrostatic equilibrium regime to a hydrodynamic regime occurs at a smaller solar EUV flux condition when enhanced, more realistic, and self‐consistent, ambient electron heating by photoelectrons is accounted for; (3) atomic nitrogen becomes the dominant neutral species in the upper thermosphere (competing against atomic oxygen) under extreme solar EUV conditions, and the electron impact processes of atomic nitrogen are important for both the chemistry and energetics in the corresponding thermosphere/ionosphere; (4) N+ remains a minor ion compared to O+, even when atomic nitrogen dominates the exobase; and (5) adiabatic cooling does not play an important role in electron gas energy budget. These findings highlight the importance of an electron transport/energy deposition model when investigating the thermosphere and ionosphere of terrestrial planets in their early evolutionary stages.

A statistical study of the observed and modeled global thermosphere response to magnetic activity at middle and low latitudes

From one year (2004) of thermosphere total density data inferred from CHAMP/STAR accelerometer measurements, we calculate the global thermosphere response to auroral magnetic activity forcing at middle and low latitudes using a method based on a singular value decomposition of the satellite data. This method allows separating the large‐scale spatial variations in the density, mostly related to altitude/latitude variations and captured by the first singular component, from the time variations, down to timescales on the order of the orbital period, which are captured by the associated projection coefficient. This projection coefficient is used to define a disturbance coefficient that characterizes the global thermospheric density response to auroral forcing. For quiet to moderate magnetic activity levels (Kp < 6), we show that the disturbance coefficient is better correlated with the magnetic am indices than with the magnetic ap indices. The latter index is used in all empirical thermosphere models to quantify the auroral forcing. It is found that the NRLMSISE‐00 model correctly estimates the main features of the thermosphere density response to geomagnetic activity, i.e., the morphology of Universal Time variations and the larger relative increase during nighttime than during daytime. However, it statistically underestimates the amplitude of the thermosphere density response by about 50%. This underestimation reaches 200% for specific disturbed periods. It is also found that the difference between daytime and nighttime responses to auroral forcing can statistically be explained by local differences in magnetic activity as described by the longitude sector magnetic indices.

Global observations of electromagnetic and particle energy flux for an event during northern winter with southward interplanetary magnetic field

Abstract. The response of the polar ionosphere–thermosphere (I-T) system to electromagnetic (EM) energy input is fundamentally different to that from particle precipitation. To understand the I-T response to polar energy input one must know the intensities and spatial distributions of both EM and precipitation energy deposition. Moreover, since individual events typically display behavior different from statistical models, it is important to observe the global system state for specific events. We present an analysis of an event in Northern Hemisphere winter for sustained southward interplanetary magnetic field (IMF), 10 January 2002, 10:00–12:00 UT, for which excellent observations are available from the constellation of Iridium satellites, the SuperDARN radar network, and the Far-Ultraviolet (FUV) instrument on the IMAGE satellite. Using data from these assets we determine the EM and particle precipitation energy fluxes to the Northern Hemisphere poleward of 60° MLAT and examine their spatial distributions and intensities. The accuracy of the global estimates are assessed quantitatively using comparisons with in-situ observations by DMSP along two orbit planes. While the location of EM power input evaluated from Iridium and SuperDARN data is in good agreement with DMSP, the magnitude estimated from DMSP observations is approximately four times larger. Corrected for this underestimate, the total EM power input to the Northern Hemisphere is 188 GW. Comparison of IMAGE FUV-derived distributions of the particle energy flux with DMSP plasma data indicates that the IMAGE FUV results similarly locate the precipitation accurately while underestimating the precipitation input somewhat. The total particle input is estimated to be 20 GW, nearly a factor of ten lower than the EM input. We therefore expect the thermosphere response to be determined primarily by the EM input even under winter conditions, and accurate assessment of the EM energy input is therefore key to achieving a comprehensive understanding of the I-T system, particularly during active times when the energy input increases markedly and expands well equatorward of nominal auroral latitudes.

Hydrodynamic planetary thermosphere model: 1. Response of the Earth's thermosphere to extreme solar EUV conditions and the significance of adiabatic cooling

It has been suggested that the exobase temperature of early terrestrial planetary atmosphere could have reached over 10,000 K. Although such high exobase temperatures should have caused the major gases at the exobase to experience fast Jeans escape, and the entire thermosphere should have experienced hydrodynamic flow, hydrostatic equilibrium was assumed to be valid in this earlier model. In this paper we develop a multicomponent hydrodynamic thermosphere model to self‐consistently study the Earth's thermosphere under extreme solar EUV conditions. The model is validated against observations and other models for the present Earth's thermosphere. Simulations show that if forced in hydrostatic equilibrium and maintaining the current composition, the Earth's thermosphere could experience a fast transition to an atmospheric blowoff state when exposed to solar EUV radiation stronger than certain critical flux. When hydrodynamic flow and its associated adiabatic cooling are included, atmospheric blowoff is prevented and Earth's exobase temperature decreases with increasing solar EUV beyond the critical solar EUV flux. Simulations show that the transition of the thermosphere from the hydrostatic equilibrium regime to the hydrodynamic regime occurs when the exobase temperature reaches 7000 to 8000 K if atomic O and N dominate the upper thermosphere. The fast variations of the bulk motion velocities under different exobase temperatures suggest that the adiabatic cooling effect could have kept the exobase temperature lower than ∼1000 K if light gases such as atomic hydrogen were the dominant species in the Earth's thermosphere. We propose that hydrodynamic flow and associated adiabatic cooling should exist in the thermospheres of a broad range of early and/or close‐in terrestrial type planets and that the adiabatic cooling effect must be included in the energy balance in order to correctly estimate their thermospheric structures and their evolutionary paths.

Analysis of thermospheric response to magnetospheric inputs

The influence of the high‐latitude energy inputs and heating distributions on the global thermosphere are investigated by coupling a new empirical model of the Poynting flux with the NCAR‐TIEGCM. First, in order to show the contribution of the electric field variability to the energy input and thermospheric temperature, model results are compared for simulations where Joule heating is calculated with the average electric field (called “simple Joule heating”) and where Joule heating is adjusted according to the Poynting flux from the empirical model. In the northern (summer) hemisphere, the Poynting flux has a peak in the dayside cusp, which is missing in the altitude‐integrated simple Joule heating. The hemispheric integral of the Poynting flux is approximately 30% larger than the integral of simple Joule heating, and the polar average (poleward of 40°) temperature calculated with the Poynting flux increases by 85–95 K, which is more than 50% of the temperature increase caused by the polar energy inputs. Second, three different methods to distribute the Poynting flux in altitude are investigated. Different heating distributions cause a difference in the polar average of heating per unit mass as high as 40% at 160 km altitude. Consequently, the difference of the polar average temperature among these three cases is close to 30–50 K. These results suggest that not only the total amount of energy input, but the way that the energy is distributed in altitude is significant to the impact of the magnetosphere on the thermosphere and ionosphere.

Driving the TING model with GAIM electron densities: Ionospheric effects on the thermosphere

In order to study the effects of ionospheric plasma densities on the thermosphere, the electron and O+ densities in the Thermosphere Ionosphere Nested Grid (TING) model were replaced by electron densities from the Global Assimilation of Ionospheric Measurements (GAIM) model at pressure level z = 0 (about 210 ∼ 230 km altitude) and above. This GAIM‐derived TING model (G‐TING) was then run for the period 1–4 April 2004. The TING model was also run as a stand‐alone coupled model (S‐TING), without using the GAIM electron densities, for the same period. The resulting thermospheric responses from the two simulations were compared at around the F region peak altitude. There was an extended quiet period during this interval followed by a moderate geomagnetic storm (Kp ∼ 6.0). The ingestion of the GAIM electron densities had little effect on the neutral temperature and composition during the quiet period before the storm, but there were noticeable global effects on the neutral winds, mainly as a result of changes in the ion drag force. During disturbed periods, changed electron densities produced much more significant effects on the global thermosphere. The increased auroral electron densities enhanced the Joule heating rate, in addition to the ion drag, which resulted in significant global changes not only in the neutral winds, but also in the neutral temperature and composition. The results of our study confirm that having correct ionospheric plasma densities is critical to accurately predicting the thermosphere in the coupled thermosphere‐ionosphere model.

Observations and simulations of the ionospheric and thermospheric response to the December 2006 geomagnetic storm: Initial phase

We have investigated the thermospheric and ionospheric response to the 14–15 December 2006 geomagnetic storm using a Coupled Magnetosphere Ionosphere Thermosphere (CMIT) 2.0 model simulation. In this paper we focus on observations and simulations during the initial phase of the storm (about 8 h), when the shock was driving changes in geospace. The global ionospheric maps of total electron content (TEC), ionosonde data at four stations and Millstone Hill incoherent scatter radar (ISR) observations are compared with the corresponding simulation results from the CMIT model. The observations showed significant positive storm effects occurred in the Atlantic sector after the onset of this storm. The CMIT model is able to capture the temporal and spatial variations of the ionospheric storm effects seen in the GPS TEC observations, although the model slightly underestimates the daytime positive ionospheric storm in the South American sector. The simulations are also in agreement with the ionosonde and ISR ionospheric measurements. Term analysis of the ion continuity equation demonstrates that changes in the electric fields play a dominant role in generating the observed ionospheric positive storm effect in the American sector during the initial phase, although neutral winds and composition changes also contribute. The difference in the strength of the enhancements over North and South America can be explained by the slope of the topside electron density profiles in the two hemispheres. In the southern hemisphere electron densities decrease slowly with altitude, whereas the decrease is much more rapid in the northern (winter) hemisphere. The electric fields, therefore, cannot cause large increases in electron density by uplifting the plasma, so positive storm effects are small in the southern hemisphere compared with the northern hemisphere, even though the increase in hmF2 is greater in the southern hemisphere. Nighttime changes in electron density in other longitude sectors are small, because the topside electron densities also decrease slowly with altitude at night.

Role of neutral atmospheric dynamics in cusp density and ionospheric patch formation

Ionospheric patches are islands of plasma transiting the polar cap, with density at least double the surrounding background plasma. Since their discovery 25 years ago, many mechanisms for their production have been proposed and examined. However all of these mechanisms consider only electric fields and charged particles as candidate mechanisms for patch formation, particularly transient changes in these electrodynamic terms. Here for the first time, we call attention to the role of thermospheric dynamics in Patch formation. We show that the thermospheric response to transient heating events near the cusp must significantly increase exospheric densities over the cusp, and also that a plausible doubling of these densities near and above 400 km altitude leads to a new patch production mechanism. The underlying processes must drive a solar cycle variation of how many hours UT experience strong polar patches and scintillation. These processes must be added to present thinking and modeling.

Contrasting behavior of the thermosphere and ionosphere in response to the 28 October 2003 solar flare

We examined the thermospheric and ionospheric responses to the solar flare on 28 October 2003, utilizing simultaneous observations of the electron and neutral density from the CHAMP satellite. Rapid thermospheric response within a few minutes was observed. In addition, the neutral and plasma perturbations contrasted each other remarkably. First, their temporal development differed. Though started nearly simultaneously, the plasma perturbation developed much faster and to a larger amplitude than its neutral counterpart. Second, their latitudinal distributions differed. At the initial stage of the response, the neutral density was enhanced by 20% almost homogeneously at all latitudes below 50°N/S. In comparison, the plasma disturbance exhibited a distinctive latitudinal structure, with largest density enhancements of 68% at the dip equator, moderate increase of ∼20% at midlatitudes, and depression up to 35% around 15°N/S. This suggests a decoupling between the neutral and plasma disturbances during this stage. The plasma‐neutral coupling via ion drag was found to become important about 2–3 hours after the flare bursts. Another interesting feature is that the equatorial ionization anomaly was significantly weakened during the flare. The observations demonstrated that electrodynamics related to the equatorial fountain dominated the photochemistry in controlling the flare‐induced plasma density disturbances on 28 October 2003. This differs considerably from the nearly linear cos(SZA) dependence of flare‐induced total electron content enhancements.

Prompt thermospheric response to the 6 November 2001 magnetic storm

Thermospheric densities observed by the STAR accelerometer on CHAMP, near 400 km altitude, are used to examine the response to the major geomagnetic storm on 6 November 2001. This is the first large storm studied with CHAMP data at these near dawn‐dusk local times. New features of the neutral density response at both global and local temporal/spatial scales have been revealed. Prompt response of the neutral atmosphere observed at the beginning of the storm was enriched by short‐ and meso‐scale perturbations. Simultaneous observations of electromagnetic and ionospheric plasma perturbations and particle precipitations from the DMSP satellites, electron energy fluxes from NOAA‐14 and Polar/UVI specified the energy source and the time delay. The suggested mechanism for this prompt thermospheric response appears to be due mainly to enhanced perpendicular electric fields. Large‐scale, global changes caused by the geomagnetic storm show generally greater enhancements in the winter (northern) hemisphere at both 0700 and 1900 local time and greater at 0700 LT than 1900 LT.

Thermospheric response to ion heating in the dayside cusp

Recent observations from the CHAMP satellite indicate that neutral density enhancements are common in the northern dayside cusp. The neutral density in this region can be nearly a factor of 2 larger than in adjacent regions of the thermosphere on the poleward and equatorward sides of the cusp. The presence of density enhancements implies that the neutral atmosphere is being heated in the cusp region causing upwelling. A high-resolution model of the global thermosphere is used to study the thermospheric response to heating in the northern dayside cusp. It is found that heating in the cusp results in the creation of a neutral fountain. Specifically, upward drift of the thermosphere within the cusp region is followed at higher altitudes by poleward and equatorward movement out of the cusp region and the gradual subsidence of the neutral gas. Density enhancements of the magnitude observed by the CHAMP satellite occur in the model results for sufficiently strong heating in the cusp. Neutral temperature enhancements also occur and are strongest near the poleward and equatorward boundaries of the cusp region.

Solar Variability and Climate Impact on Terrestrial Planets

Some possible factors of climate changes and of long term climate evolution are discussed with regard of the three terrestrial planets, Earth, Venus and Mars. Two positive feedback mechanisms involving liquid water, i.e., the albedo mechanism and the greenhouse effect of water vapour, are described. These feedback mechanisms respond to small external forcings, such as resulting from solar or astronomical constants variability, which might thus result in large influences on climatic changes on Earth. On Venus, reactions of the atmosphere with surface minerals play an important role in the climate system, but the involved time scales are much larger. On Mars, climate is changing through variations of the polar axis inclination over time scales of ∼105–106 years. Growing evidence also exists that a major climatic change happened on Mars some 3.5 to 3.8 Gigayears ago, leading to the disappearance of liquid water on the planet surface by eliminating most of the CO2 atmosphere greenhouse power. This change might be due to a large surge of the solar wind, or to atmospheric erosion by large bodies impacts. Indeed, except for their thermospheric temperature response, there is currently little evidence for an effect of long-term solar variability on the climate of Venus and Mars. This fact is possibly due to the absence of liquid water on these terrestrial planets.

The ionospheric and thermospheric response to CMEs: Challenges and successes

Studies of the ionospheric response to the impact of CMEs on geospace began long before the space age. Early successes included the discovery that the F 2 region ionosphere reacted to the resulting geomagnetic storms in different ways depending on the latitude of the observations. At high latitudes this layer is depleted (negative storm effects), whereas at low latitudes it is enhanced (positive storm effects). Much of the subsequent work on the effects of geomagnetic storms has involved refining this simple picture and understanding the reasons why this behavior occurs. The basis of this understanding begins with magnetosphere–ionosphere–thermosphere (MIT) coupling. Energy that is transferred from the magnetosphere into the thermosphere/ionosphere system manifests itself in a number of ways, the most important of which is Joule heating. Other important processes include momentum transfer from the ions to the neutrals and the various manifestations of the heat transfer associated with precipitating particles. While these processes are known in principle, their details and evolution through the time of the passage of a CME across geospace are less well understood. Also, the impact of a geomagnetic storm on the ionosphere and thermosphere is seen not just locally in the region where MIT coupling primarily occurs, but over the whole globe. There is still much debate about the processes that act to transfer energy from the high-latitude regions to other parts of the globe. The aim of this paper is to highlight these challenges and to suggest the main methods by which they should be attacked.

Neutral density response to the solar flares of October and November, 2003

In this paper we report the first measurements of the thermosphere neutral density response to a solar EUV flare. Two X17 flares are considered: The first on 28 October, 2003, and the second on 4 November, 2003. The density measurements are provided by accelerometers on the GRACE and CHAMP satellites near 490 km and 400 km, respectively. X‐ray fluxes are provided by GOES‐12, and EUV fluxes by the SEE instrument on TIMED. The thermosphere density increases associated with the first flare are about 50–60% at low to mid‐latitudes. The time to peak response is 72 ± 47 minutes with respect to the peak EUV flux emitted by the flare, while the recovery to prior quiescent levels takes nearly 12 hours with an approximate decay time constant of 8 hours. The response to the second flare is about 35–45% with similar response and recovery times. The above density enhancements correspond to exosphere temperature increases of about 125–175 K and 100–125 K, respectively. These results provide new opportunities for the testing and validation of models over time scales similar to those of basic mechanisms governing the thermosphere response, such as thermal conduction, radiative cooling, and restoration of diffusive equilibrium.

Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA)—Japanese sounding rocket campaign—

Abstract Japanese sounding rocket “S-310-35” was launched from And/0ya Rocket Range in Norway on December 13, 2004 during Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign, in which the rocket-borne in-situ measurements and ground-based measurements were coordinated to carry out a comprehensive observation of the thermospheric response against the auroral energy input. The instruments on board the rocket successfully performed their measurements during the flight, and thereby the temperature and density of molecular nitrogen, auroral emission rate, and the ambient plasma parameters were derived. Simultaneous measurements by the ground-based instruments provided neutral wind, neutral temperature, the auroral images and the ionospheric parameters near the rocket trajectory. This paper introduces science objectives, experimental outline, and preliminary scientific results of the DELTA campaign and explains geophysical condition at the time of the rocket launch, while the companion papers in this special issue describe more detailed results from each instrument.

Response of Mesosphere and Lower Thermosphere to a Localized and Time-Variable Thermal Source

采用二维交替方向隐格式(ADI),数值模拟了中高层大气对一个位于对流层高度范围的局地时变热源的响应.分析了这一过程中大气响应的能量密度,水平波数,垂直波数以及频率.模拟结果表明:响应波动携带能量向上传播,集中在较为狭窄的水平区域,该区域关于源的水平位置对称;相对于动量源驱动,热源的水平尺度对响应的水平波长决定性作用更加明显;对于高频振荡的热源,其响应的频率和源一致;响应的垂直波长(41 km)比源的垂直尺度(6 km)大得多.

A global circulation model of Saturn's thermosphere

We present the first 3-dimensional self-consistent calculations of the response of Saturn's global thermosphere to different sources of external heating, giving local time and latitudinal changes of temperatures, winds and composition at equinox and solstice. Our calculations confirm the well-known finding that solar EUV heating alone is insufficient to produce Saturn's observed low latitude thermospheric temperatures of 420 K. We therefore carry out a sensitivity study to investigate the thermosphere's response to two additional external sources of energy, (1) auroral Joule heating and (2) empirical wave heating in the lower thermosphere. Solar EUV heating alone produces horizontal temperature variations of below 20 K, which drive horizontal winds of less than 20 m/s and negligible horizontal changes in composition. In contrast, Joule heating produces a strong dynamical response with westward winds comparable to the sound speed on Saturn. Joule heating alone, at a total rate of 9.8 TW, raises polar temperatures to around 1200 K, but values equatorward of 30° latitude, where observations were made, remain below 200 K due to inefficient meridional energy transport in a fast rotating atmosphere. The primarily zonal wind flow driven by strong Coriolis forces implies that energy from high latitudes is transported equatorward mainly by vertical winds through adiabatic processes, and an additional 0.29–0.44 mW/m 2 thermal energy are needed at low latitudes to obtain the observed temperature values. Strong upwelling increases the H 2 abundances at high latitudes, which in turn affects the H + 3 densities. Downwelling at low latitudes helps increase atomic hydrogen abundances there.