Model Lower Boundary Research Articles

Ionospheric Interhemispheric Asymmetry Induced by Planetary Wave Under Solar Minimum Condition

AbstractThe westward quasi‐6‐day planetary wave with zonal wave number 1 (Q6DW) is important in the Earth's atmospheric coupling. The International Global Navigation Satellite System Service total electron content (TEC) maps show that the Q6DW‐related ionospheric perturbations exhibit clear interhemispheric asymmetry, which do not always agree with the interhemispheric asymmetry feature of the background TEC. The National Center for Atmospheric Research Thermosphere‐Ionosphere‐Mesosphere Electrodynamic General Circulation Model is then used to investigate the reasons for planetary wave‐induced interhemispheric asymmetry by imposing Q6DW forcing at the model lower boundary at vernal equinox under solar minimum condition. The interhemispheric asymmetry features of the ionospheric responses to Q6DW are well captured by our model simulations. For example, the southern branch of the Q6DW perturbation in F region is ~26% larger than that in the Northern Hemisphere at 30°E, while the background Model TEC is~4.7% stronger in the Northern Hemisphere. We found that the vertical plasma transportation induced by Q6DW perturbations in the neutral wind fields in F region show the same asymmetric features with TEC, which leads to corresponding ionospheric perturbations by changing the local recombination effect. Our further simulations show that the vertical plasma drifts induced by Q6DW through E × B process can also cause interhemispheric asymmetry in Model TEC. It is also found that the zonal component of Q6DW perturbations contributes more to the asymmetry than the meridional component through both neutral wind direct transportation effects and E region wind dynamo effects. Moreover, the ionospheric asymmetry of the Q6DW‐related oscillations also exhibit clear longitudinal variabilities.

Numerical simulation of the 6 day wave effects on the ionosphere: Dynamo modulation

AbstractThe Thermosphere‐Ionosphere‐Mesosphere Electrodynamics General Circulation Model (TIME‐GCM) is used to theoretically study the 6 day wave effects on the ionosphere. By introducing a 6 day perturbation with zonal wave number 1 at the model lower boundary, the TIME‐GCM reasonably reproduces the 6 day wave in temperature and horizontal winds in the mesosphere and lower thermosphere region during the vernal equinox. The E region wind dynamo exhibits a prominent 6 day oscillation that is directly modulated by the 6 day wave. Meanwhile, significant local time variability (diurnal and semidiurnal) is also seen in wind dynamo as a result of altered tides due to the nonlinear interaction between the 6 day wave and migrating tides. More importantly, the perturbations in the E region neutral winds (both the 6 day oscillation and tidal‐induced short‐term variability) modulate the polarization electric fields, thus leading to the perturbations in vertical ion drifts and ionospheric F2 region peak electron density (NmF2). Our modeling work shows that the 6 day wave couples with the ionosphere via both the direct neutral wind modulation and the interaction with atmospheric tides.

Nonmigrating tidal modulation of the equatorial thermosphere and ionosphere anomaly

AbstractThe modulation of nonmigrating tides on both the ionospheric equatorial ionization anomaly (EIA) and the equatorial thermosphere anomaly (ETA) is investigated on the basis of simulations from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME‐GCM). Our simulations demonstrate the distinct features of the EIA and ETA seen in observations after the inclusion of field‐aligned ion drag in the model. Both the EIA and the ETA in the constant local time frame display an obvious zonal wave‐4 structure associated with the modulation of nonmigrating tides. However, the modeled EIA and ETA show a primary zonal wave‐1 structure when only the migrating tides are specified at the model lower boundary. Our simulations reveal that the zonal wave‐4 structure of the ETA under both low and high solar activity conditions is mainly caused by the direct response of the upper thermosphere to the diurnal eastward wave number 3 and semidiurnal eastward wave number 2 nonmigrating tides from the lower atmosphere. There is a minor contribution from the ion‐neutral coupling. The zonal wave‐4 structure of the EIA is also caused by these nonmigrating tides but through the modulation of the neutral wind dynamo.

Changes of thermospheric composition and ionospheric density caused by quasi 2 day wave dissipation

AbstractUsing the thermosphere‐ionosphere‐mesosphere electrodynamics–general circulation model, we investigate the effect of quasi 2 day wave (QTDW) dissipation on thermospheric composition (O/N2) and ionospheric electron density during solar minimum. The overall thermospheric and ionospheric changes induced by the QTDW are evaluated by running the model with and without QTDW forcing imposed at the model lower boundary. The dissipation of the westward propagating QTDW in the lower thermosphere causes westward mean wind acceleration and drives a poleward meridional circulation. The circulation induced by the QTDW, as determined by the difference between the mean wind patterns of a run with the QTDW and a base run without the QTDW, enhances the mixing of constituents in the lower thermosphere. Through molecular diffusion, the decrease of the O mixing ratio and the increase of the N2 and O2 mixing ratios propagate from the lower thermosphere into the upper thermosphere. As a result, the O/N2 ratio near the ionospheric F2 peak is reduced by about 16–20% at low and midlatitudes. This in turn produces an approximately 16–32% depletion in the F2 peak electron density at low and midlatitudes. The simulated decrease of electron density during a QTDW event is in quantitative agreement with published observations. This work suggests a new major pathway for the traveling planetary wave from the lower atmosphere to affect the thermosphere and ionosphere via dissipation and mean wind acceleration.

Dependence of the climatological axial latitudes of the tropospheric westerlies and storm tracks on the latitude of an extratropical oceanic front

Major “storm tracks”, where migratory cyclones and anticyclones recurrently develop, are observed around midlatitude oceanic frontal zones with strong meridional gradient of sea‐surface temperature (SST). A set of atmospheric general circulation model experiments is performed with zonally uniform SST prescribed at the model lower boundary. The latitudinal SST profile for each hemisphere is characterized by a single front. The frontal latitude is varied systematically from one experiment to another, while the intensity of the frontal gradient is kept unchanged. Though idealized, the experiments reveal a climatological tendency for a low‐level storm track to be organized along or slightly poleward of the SST front if located in the subtropics or midlatitudes. As a surface manifestation of an eddy‐driven polar‐front jet (PFJ), surface westerly axis tends to form on the poleward flank of the front. This anchoring effect of the SST front is also hinted at upper levels, but the climatological positions of the storm track and PFJ are less sensitive to the frontal latitude. For the SST front at subpolar latitude, the joint primary axes of the upper‐level storm track and PFJ form in midlatitudes away from the SST front. Their positions correspond to their counterpart simulated with a particular SST profile from which frontal gradient has been removed, suggesting that the anchoring effect of a subpolar SST front on the storm track and PFJ is overshadowed by atmospheric internal dynamics, namely, the self‐maintenance mechanism of a midlatitude storm track and PFJ through their interactions.

Influences of the Kuroshio/Oyashio Extensions on Air–Sea Heat Exchanges and Storm-Track Activity as Revealed in Regional Atmospheric Model Simulations for the 2003/04 Cold Season*

Abstract Influences of oceanic fronts in the Kuroshio and Oyashio Extension (KOE) region on the overlying atmosphere are investigated by comparing a pair of atmospheric regional model hindcast experiments for the 2003/04 cold season, one with the observed finescale frontal structures in sea surface temperature (SST) prescribed at the model lower boundary and the other with an artificially smoothed SST distribution. The comparison reveals the locally enhanced meridional gradient of turbulent fluxes of heat and moisture and surface air temperature (SAT) across the oceanic frontal zone, which favors the storm-track development both in winter and spring. Distinct seasonal dependency is found, however, in how dominantly the storm-track activity influences the time-mean distribution of the heat and moisture supply from the ocean. In spring the mean surface sensible heat flux (SHF) is upward (downward) on the warmer (cooler) side of the subarctic SST front. This sharp cross-frontal contrast is a manifestation of intermittent heat release (cooling) induced by cool northerlies (warm southerlies) on the warmer (cooler) side of the front in association with migratory cyclones and anticyclones. The oceanic frontal zone is thus marked as both the largest variability in SHF and the cross-frontal sign reversal of the SHF skewness. The cross-frontal SHF contrasts in air–sea heat exchanges counteract poleward heat transport by those atmospheric eddies, to restore the sharp meridional gradient of SAT effectively for the recurrent development of atmospheric disturbances. Lacking this oceanic baroclinic adjustment associated with the SST front, the experiment with the smoothed SST distribution underestimates storm-track activity in the KOE region. In winter the prevailing cold, dry continental airflow associated with the Asian winter monsoon induces a large amount of heat and moisture release even from the cooler ocean to the north of the frontal zone. The persistent advective effects of the monsoonal wind weaken the SAT gradient and smear out the sign reversal of the SHF skewness, leading to weaker influences of the oceanic fronts on the atmosphere in winter than in spring.

Comparative study of short‐term diurnal tidal variability

The wind and temperature measurements from an unusually long period operation of the sodium lidar at Colorado State University (41°N, 105°W) around September equinox 2003 showed significant short‐term tidal variability. Coincident with the large tidal changes, a strong temperature inversion layer was also observed above 90 km. Examination of the simultaneous temperature measurement from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument, on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite, not only confirms the existence of the inversion layer but also reveals the global nature of the inversion, suggesting the presence of a transient planetary wave in the mesosphere. The large tidal variability, therefore, is probably a consequence of the interaction between the transient planetary wave and tides. This possibility is investigated by using the NCAR thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) and by comparing model results with the lidar, SABER, and TIMED Doppler Interferometer (TIDI) measurements. With a large transient planetary wave specified at the model lower boundary, the model is able to produce strong diurnal tidal variability comparable to that from the lidar observation, and the modeled temperature inversion is similar to that from the SABER measurement. The model results suggest that the planetary/tidal wave interaction excites nonmigrating tides and modulates the gravity modes and/or the rotational modes of the diurnal migrating tide. Among the nonmigrating tides, the diurnal zonally symmetric (S = 0) component is the strongest, and its interaction with the planetary wave leads to a strong diurnal eastward wave number 1 component.

Development of a simple groundwater model for use in climate models and evaluation with Gravity Recovery and Climate Experiment data

Groundwater interacts with soil moisture through the exchanges of water between the unsaturated soil and its underlying aquifer under gravity and capillary forces. Despite its importance, groundwater is not explicitly represented in climate models. This paper developed a simple groundwater model (SIMGM) by representing recharge and discharge processes of the water storage in an unconfined aquifer, which is added as a single integration element below the soil of a land surface model. We evaluated the model against the Gravity Recovery and Climate Experiment (GRACE) terrestrial water storage change (ΔS) data. The modeled total water storage (including unsaturated soil water and groundwater) change agrees fairly well with GRACE estimates. The anomaly of the modeled groundwater storage explains most of the GRACE ΔS anomaly in most river basins where the water storage is not affected by snow water or frozen soil. For this reason, the anomaly of the modeled water table depth agrees well with that converted from the GRACE ΔS in most of the river basins. We also investigated the impacts of groundwater dynamics on soil moisture and evapotranspiration through the comparison of SIMGM to an additional model run using gravitational free drainage (FD) as the model's lower boundary condition. SIMGM produced much wetter soil profiles globally and up to 16% more annual evapotranspiration than FD, most obviously in arid‐to‐wet transition regions.

Using redundancy analysis to improve dynamical seasonal mean 500 hPa geopotential forecasts

AbstractIn this study, we evaluate and compare 500 hPa geopotential height hindcast skill in two large dynamical hindcast experiments performed with the Canadian Climate Centre second generation general circulation model (GCM). In one hindcast experiment, seasonal hindcasts are made from lagged initial conditions observed at the beginning of each season. The sea‐surface temperatures (SSTs) required by the model during each forecast period are forecast by persisting the SST anomalies observed during the month just prior to the forecast period. The second hindcast experiment consists of an ensemble of simulations in which continuously evolving observed SSTs are specified at the model's lower boundary. These hindcasts do not benefit from re‐specification of the initial state at the beginning of each season, but they do enjoy the benefit of ‘perfect’ SST forecasts. We also demonstrate the use of a regression technique, called redundancy analysis (RA), for statistically improving the skill of both types of dynamical hindcast. The results indicate that specification of the initial state at the beginning of each season adds skill to the seasonal hindcasts, even though SSTs at the lower boundary are imperfectly specified. We also find that the model can predict the mean state of the North Atlantic Oscillation (NAO) with some skill in boreal winter and spring when the initial state is specified at the beginning of each season. The results also indicate that statistical post‐processing with the RA technique improves the (cross‐validated) skill of both types of dynamical hindcast. Copyright © 2001 Royal Meteorological Society

Interannual variations in the stratospheric circulation with a perfectly steady troposphere

The seasonal and interannual variations of the stratospheric circulation are investigated using a quasi‐geostrophic, β‐plane channel model of the middle atmosphere. In the horizontal the model truncation retains at most two zonal and meridional wavenumbers. There are 27 layers in the vertical. All conditions at the model's lower boundary in the lower stratosphere are constant in time. No interannual variations are found if the flow is forced to a radiative equilibrium condition that is annually periodic with a radiative damping time of ∼20 days in the lower stratosphere. The seasonal evolution of the stratospheric circulation differs for different amplitudes of zonal wavenumbers 1 and 2 prescribed at the model's lower boundary (hB1 and hB2, respectively). When hB2 is small (50 m), variations are weak during winter and strong during spring, thus resembling the southern stratosphere. When hB2 is larger (150 m), variations have significant intensity during the entire winter, thus resembling the northern stratosphere. It is demonstrated that interannual variations can be obtained during winter and spring if two requirements are met. First, the radiative damping time is doubled in the lower stratosphere to a value in closer agreement with observational estimates. Second, if the model's truncation includes the antisymmetric mode about the channel center. The two requirements complement each other in the following way: Larger radiative damping times imply that the system can “remember” different conditions at the beginning of the previous summer, when otherwise the system memory is lost. Nonsymmetric conditions about the channel center imply that the critical level of the mean zonal wind in early winter does not extend across the entire domain. It is concluded that intraseasonal and interannual variations in the stratospheric circulation could exist even in the presence of planetary waves with constant amplitude in the troposphere and radiative forcing in the stratosphere, which is annually periodic.

Seasonal skill and predictability of ECMWF PROVOST ensembles

AbstractVariations in seasonal‐forecasting skill and predictability during the 15 years (1979–93) of the European Centre for Medium‐Range Weather Forecasts (ECMWF) re‐analysis (ERA), have been studied using 120‐day ensemble integrations of the ECMWF model. These integrations form part of the European Union PROVOST (PRediction Of climate Variations On Seasonal to interannual Time‐scales) project. Observed sea surface temperatures (SSTs) were updated daily at the model's lower boundary. Two major and three moderate El Niño Southern Oscillation (ENSO) events occurred during the ERA period. Results are interpreted as giving an upper bound on the predictive skill of a coupled ocean‐atmosphere system as a function of season, location, and state of ENSO.The model systematic error was found to be comparable with a typical amplitude of interannual variation. When standardized by the corresponding ERA anomaly variance, systematic error appears to be largest in boreal spring in the northern extratropics, and in boreal summer in the tropics.Ensemble‐mean skill scores were found to be positive overall. Apart from the northern winter season, the ensemble‐mean skill for months 2–4 drops significantly when compared with months 1–3. The interannual variation of skill scores is much larger for the European region than for the hemispheric domain. Over the northern hemisphere, skill is much higher when only ENSO years are considered; for Europe, the enhancement in skill for ENSO years is much weaker.Estimates of intrinsic predictability were made for each year of the dataset. These estimates, defined both by a t‐test and variance ratio, indicate generally high predictability in years when ENSO was strong. Apart from northern winter, the predictability estimates also showed a systematic drop between months 1–3 and months 2–4. It is therefore concluded that the fall in skill scores between months 1–3 and 2–4 indicates more a weakening of the impact of initial conditions (ICs) than, say, an increase in the effects of model error. In order to study this further, the relative impacts of SSTs and ICs, including land surface ICs, on interannual variation of precipitation have been examined in an additional set of experiments. Overall, SSTs have a dominant role, though the impact of ICs is not negligible.The predictability of tropical and extratropical precipitation is also discussed. The level of skill for precipitation in the extratropics is generally lower than in the tropics. However, within the tropics there are regions where the precipitation exhibits chaotic behaviour and is correspondingly less predictable.

SMLTM simulations of the diurnal tide: comparison with UARS observations

Abstract. Wind and temperature observations in the mesosphere and lower thermosphere (MLT) from the Upper Atmosphere Research Satellite (UARS) reveal strong seasonal variations of tides, a dominant component of the MLT dynamics. Simulations with the Spectral mesosphere/lower thermosphere model (SMLTM) for equinox and solstice conditions are presented and compared with the observations. The diurnal tide is generated by forcing specified at the model lower boundary and by in situ absorption of solar radiation. The model incorporates realistic parameterizations of physical processes including various dissipation processes important for propagation of tidal waves in the MLT. A discrete multi-component gravity-wave parameterization has been modified to account for seasonal variations of the background temperature. Eddy diffusion is calculated depending on the gravity-wave energy deposition rate and stability of the background flow. It is shown that seasonal variations of the diurnal-tide amplitudes are consistent with observed variations of gravity-wave sources in the lower atmosphere.

SMLTM simulations of the diurnal tide: comparison with UARS observations

Wind and temperature observations in the mesosphere and lower thermosphere (MLT) from the Upper Atmosphere Research Satellite (UARS) reveal strong seasonal variations of tides, a dominant component of the MLT dynamics. Simulations with the Spectral mesosphere/lower thermosphere model (SMLTM) for equinox and solstice conditions are presented and compared with the observations. The diurnal tide is generated by forcing specified at the model lower boundary and by in situ absorption of solar radiation. The model incorporates realistic parameterizations of physical processes including various dissipation processes important for propagation of tidal waves in the MLT. A discrete multi-component gravity-wave parameterization has been modified to account for seasonal variations of the background temperature. Eddy diffusion is calculated depending on the gravity-wave energy deposition rate and stability of the background flow. It is shown that seasonal variations of the diurnal-tide amplitudes are consistent with observed variations of gravity-wave sources in the lower atmosphere.

The impact of gravity waves on the Venus thermosphere and O2 IR nightglow

The terrestrial Fritts and Lu [1993] gravity wave parameterization, based on the theory of wave saturation, has been adopted within the Venus thermospheric general circulation model (VTGCM) to examine the impact of upward propagating gravity waves on the Venus thermospheric subsolar‐to‐antisolar (SS‐AS) circulation and the resultant O2 IR nightglow. It is shown that the drag force generated by the breakdown of gravity waves can effectively slow down the winds, which results in reproducing the observed day/night contrasts in temperatures and densities. With stronger drag force on the dawn terminator than on the dusk terminator, the apparently superrotating zonal wind can be forced by the dissipation of gravity waves. The equivalent superrotating zonal wind varies both with local time and altitude. The simulated distribution of the O2 IR nightglow is strongly sensitive to small changes of the characteristic intrinsic phase speeds at the model lower boundary. This sensitivity implies that minor variations in gravity wave momentum fluxes to the thermosphere can give rise to large O2 IR emission variations with time and space, in agreement with ground‐based observations.

Simulation of tides with a spectral mesosphere/lower thermosphere model

The spectral mesosphere/lower thermosphere model (SMLTM) is an extension into the thermosphere (up to ∼220 km) of the three‐dimensional middle atmospheric (∼15–120 km) model developed by Akmaev et al. [1992]. The model incorporates realistic parameterizations of radiative transfer and dissipation processes including eddy and molecular viscosity and thermal conductivity and ion drag. A discrete multi‐component gravity wave parameterization is optimized with respect to the zonal mean temperature distribution as represented in the CIRA‐1986 empirical model. The diurnal tide is generated by forcing specified at the model lower boundary and by in situ absorption of solar radiation. The semi‐diurnal tide is primarily generated by ozone absorption of solar UV radiation in the stratosphere. First results of tidal simulations indicate that even without any additional tuning the simulations compare well with recent observations from the Upper Atmosphere Research Satellite (UARS). This means that the background temperature and wind distributions, gravity wave momentum deposition, turbulent diffusion, and tidal forcing all interact self‐consistently and are in agreement with observational data. It is noted that simultaneous UARS wind and temperature measurements, if they become available, will provide key information needed to distinguish the primary mechanism for damping the diurnal tide.

On the relationship between the quasi‐biennial oscillation, total chlorine and the severity of the antarctic ozone hole

AbstractA 3‐dimensional model of the dynamics and radiation of the stratosphere and mesosphere is used to simulate austral spring conditions. An idealized representation of the southern hemisphere planetary waves is prescribed at the 316 mb model lower boundary, and the tropical zonal winds are relaxed towards an idealized time‐dependent representation of the quasi‐biennial oscillation (QBO). As the initial phase of the QBO is changed, the October mean lower‐stratospheric temperatures polewards of 75°S respond systematically, with the two extreme responses found for phases only 1/4 cycle apart rather than for opposite phases. The integrations are repeated for these two phases but with fully interactive photochemistry and different levels of chlorine, corresponding to conditions in about 1980, the mid to late 1980s, and those anticipated in about the year 2010. Increasing the chlorine amount produced systematically deeper ozone holes with areas significantly larger for conditions in the year 2010 than for those in the late 1980s, broadly in agreement with the secular trend observed by the Total Ozone Mapping Spectrometer. For the QBO phase giving lowest temperatures the simulated ozone holes are consistently deeper but with reduced sensitivity for high chlorine amounts. Differences in polar ozone amounts between the two phases are comparable with the observed interannual variability. In the model the results show that the QBO modulates the severity of the antarctic ozone hole directly through the wave‐induced transporting circulation rather than indirectly through the temperature‐dependent heterogeneous chemistry. Further, it is argued that the phases of the QBO producing the two extreme responses in the springtime ozone amounts over Antarctica are likely to depend, in reality, on details of the tropospheric planetary‐wave flux. The model results are also shown to be consistent with the observed downward trend in October antarctic temperatures since 1979.

Seasonal Development of the Extratropical QBO in a Numerical Model of the Middle Atmosphere

Abstract The seasonal (wintertime) development of middle atmosphere circulation in opposite phases of the equatorial quasi-biennial oscillation (QBO) was simulated with a three-dimensional nonlinear numerical model. In the stratosphere, the effect of equatorial QBO was generally consistent with the extratropical QBO observed by Holton and Tan, namely, a stronger midwinter polar vortex in the westerly phase, and vice versa. However, the extratropical response to the QBO was sensitive to other factors such as mesospheric gravity wave drag and the amplitude of Rossby waves specified at the model's lower boundary. The extratropical QBO was realistic only when a drag parameterization was included and Rossby wave amplitudes lay in an intermediate range close to the observed. At somewhat stronger forcing, the model's response was largest in the mesosphere where (in this case) westerlies were stronger in the easterly phase of equatorial QBO. This was apparently due to a shielding effect. The theory of planetary w...

Thermospheric tides at equinox: Simulations with coupled composition and auroral forcings: 2. Semidiurnal component

New simulations of semidiurnal thermospheric winds and perturbation temperatures calculated by the National Center for Atmospheric Research thermospheric general circulation model (TGCM) are presented for equinox conditions. The model now incorporates a self‐consistent calculation of neutral composition and includes high‐latitude energy and momentum sources associated with the aurora. The tidal waves propagating upward from the lower atmosphere are specified by wavelike distributions of the geopotential at the model lower boundary; the boundary geopotential is related to the other model fields through Hough mode tidal functions. The specification of the wave amplitudes and phases is based on lower atmosphere model predictions and thus represents the coupling between the upper and lower atmosphere in a more physically meaningful manner than previous calculations. In these simulations, asymmetric semidiurnal modes are also included. The revised TGCM calculations at geographic longitude 75°W are compared with observational data at low, middle, and high latitudes. In general, the agreement is reasonable, given the limited data available. The predicted solar cycle variation of the semidiurnal tides is consistent with previous works: the temperatures respond more strongly than the winds, and the solar cycle effects are most pronounced in the upper thermosphere. The lower thermosphere is dominated by the upward propagating waves from the lower atmosphere, with little contribution from solar forcing. These upward propagating waves are damped at lower altitudes as solar forcing increases. The effects of coupled composition and auroral forcings on the semidiurnal tidal components are investigated. Both processes alter the simulated solar‐driven tidal fields compared to calculations which neglected these processes: amplitudes may double or triple in restricted altitude and latitude regions, while the times of maxima may shift by a few hours. The TGCM simulations are compared with the Fourier‐decomposed exospheric temperature measured during the September 1984 Equinox Transition Study period. The semidiurnal temperature phase in particular is found to be strongly dependent on the level of geomagnetic activity. Thus the simulation of individual day observations will require careful specification of the auroral forcings which are input into the theoretical models. Simultaneous data obtained at different latitudes are vital to test and confirm the model predictions.