Equinox Transition Research Articles

Seasonal Transitions in the Thermosphere Inferred from Ionospheric Observations

Ionospheric observations along with CHAMP/STAR neutral gas density measurements were used to retrieve thermospheric parameters and to check whether the equinox transition season exists separately from the December solstice and June solstice seasons. Juliusruh and Boulder ionosonde stations located in “far-from-pole” and “near-pole” longitudinal sectors were analyzed during deep solar minimum in 2008–2009. The results were compared to GOLD column O/N2 ratio observations. The retrieved thermospheric parameters have shown that equinoctial transition period exists separately from the winter one at Juliusruh, while column O/N2 ratios, exospheric temperatures Tex, and vertical plasma drifts related to thermospheric winds retrieved at Boulder for the winter season do not significantly differ from vernal values. This means that the December solstice season just does not exist as it merges with the vernal season in the “near-pole” longitudinal sector. The obtained results indicate that two longitudinal sectors manifest different seasonal variations both in thermospheric circulation and neutral composition.

Delineating the effect of upward propagating migrating solar tides with the TIEGCM-ICON

Introduction: The vertical coupling of the lower and upper atmosphere via atmospheric solar tides is very variable and affects the thermosphere and ionosphere system. In this study, we use Ionospheric Connection (ICON) explorer data from 220–270 Day Of Year (DOY), 2020 when large changes in the migrating semidiurnal tide (SW2) and the zonal and diurnal mean (ZM) zonal wind occur within 8 days.Method: We use the ICON Level4 product, the thermosphere-ionosphere-electrodynamics general circulation model (TIEGCM) driven by tides fitted to ICON observations via the Hough Mode Extension (HME) method. The effect of the upward propagating tides is isolated by examining the difference between two TIEGCM simulations with and without tidal HME forcing at the model lower boundary.Results: The simulations reveals that the solar SW2 changes its latitudinal structure at 250 after DOY 232 from two peaks at mid latitudes to one broad low latitude peak, while at 110 km the two-peak structure persists. The ZM zonal wind at 250 km undergoes a similar dramatic change. These SW2 changes are associated with the prevalence of antisymmetric HMEs after DOY 232. The migrating diurnal, terdiurnal and quaddiurnal tides at 250 km undergo similar variations as SW2. TW3 is strong in the thermosphere and most likely caused by non-linear tidal interaction between DW1 and SW2 above 130 km. Surprisingly, the solar in situ forcing of TW3 and SW2 in the upper thermosphere is not nearly as important as their upward propagating tidal component. Associated with the strong dynamical changes, the zonal and diurnal mean NmF2 decreases by approximately 15%–20%, which has a major contribution from the O/N2 decrease by roughly 10%. These changes are stronger than general seasonal behavior.Discussion: While studies have reported on the dynamical changes via SW2 in the mesosphere-lower thermosphere (MLT) region during the equinox transition period, this study is, to our knowledge, the first to examine the effects of rapid changes in SW2 on the upper thermosphere and ionosphere. The study highlights the potential of using ICON-TIEGCM for scientific studies.

Migrating Semidiurnal Tide During the September Equinox Transition in the Northern Hemisphere

AbstractSpecified Dynamics Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension simulations are used to investigate the solar migrating semidiurnal tide (SW2) around September equinox at middle to high latitudes in the Northern Hemisphere. A pronounced minimum in SW2 occurs around September equinox, and is characterized by a ∼50% reduction in tidal amplitudes for 20–30 days. Analysis of the simulation results indicates that the SW2 minimum occurs due to the seasonal transition of the zonal mean zonal winds, which alter the generation and propagation of different symmetric and antisymmetric modes of SW2. In particular, the antisymmetric modes notably decrease due to the more hemispherically symmetric zonal winds around equinox. It is further demonstrated that interannual variability in the timing of the SW2 minimum is related to the timing of the seasonal transition of the zonal mean zonal winds in the middle atmosphere. This leads to an earlier occurrence of the SW2 minimum during years when the seasonal transition occurs earlier, such as the recent 2019 September equinox which saw an earlier transition of the Southern Hemisphere zonal mean zonal winds following the occurrence of a sudden stratosphere warming. The connection between the timing of the SW2 minimum in the Northern Hemisphere and the timing of the seasonal transition in the middle atmosphere winds is confirmed by seasonal variability of 12‐h tides deduced from specular meteor radar observations at middle to high latitudes in the Northern Hemisphere.

Statistical study of the night-time F-layer dynamics at the magnetic equator in West Africa during the solar minimum period 1995–1997

Abstract. In this paper, we report on the night-time equatorial F-layer height behaviour at Korhogo (9.2° N, 5° W; 2.4° S dip lat), Ivory Coast, in the West African sector during the solar minimum period 1995–1997. The data were collected from quarter-hourly ionograms of an Ionospheric Prediction Service (IPS) 42-type vertical sounder. The main focus of this work was to study the seasonal changes in the F-layer height and to clarify the equinox transition process recently evidenced at Korhogo during 1995, the year of declining solar flux activity. The F-layer height was found to vary strongly with time, with up to three main phases. The night-to-night variability of these morphological phases was then analysed. The early post-sunset slow rise, commonly associated with rapid chemical recombination processes in the bottom part of the F layer, remained featureless and was observed regardless of the date. By contrast, the following event, either presented like the post-sunset height peak associated with the evening E × B drift, or was delayed to the midnight sector, thus involving another mechanism. The statistical analysis of the occurrence of these events throughout the solar minimum period 1995–1997 revealed two main F-layer height patterns, each characteristic of a specific season. The one with the post-sunset height peak was associated with the northern winter period, whereas the other, with the midnight height peak, characterized the northern summer period. The transition process from one pattern to the other took place during the equinox periods and was found to last only a few weeks. We discuss these results in the light of earlier works.

Equinox transition at the magnetic equator in Africa: analysis of ESF ionograms

Abstract. We study equatorial night-time F layer behaviour from quarter-hourly ionograms at Korhogo/Ivory Coast (9.2° N, 5° W, dip lat. −2.4°) during local Spring March–April 1995, declining solar flux period, according to the magnetic activity. The height and thickness of the F-layer are found to vary intensely with time and from one day to the next. At time of the equinox transition, by the end of March, a net change of the nightly height-time variation is observed. The regime of a single height peak phase before 22 March changes to up to three main F-layer height phases after 30 March, each associated to a dominant mechanism. The first phase is identified to the post-sunset E×B pulse, the second phase associated to a change in the wind circulation phenomenon and the third one attributed to pre-sunrise phenomena. The influence of the magnetic activity is identified by the increase in the second peak amplitude. After the 21 April magnetic-equinox period, the height-time morphology becomes more irregular suggesting meridional wind abatement. The initiation, the growth and the maintenance of ESF are explored in relation to these nightly variations. The Rayleigh-Taylor instability is clearly identified as main precursor phenomenon. This is followed by the P-type (F-layer peak spread) structures, the whole with no specific dependence on the magnetic activity and on the F-layer phases, in contrast to further I and F-type (Inside and Frequency spread) ESFs. We discuss our results in the light of recent advanced experiments in Peru and the pacific.

Longitudinal variability of mesospheric temperatures during equinox at middle and high latitudes

Airglow emission and temperature observations by the wind imaging interferometer (WINDII) on the upper atmosphere research satellite (UARS) and ground-based stations revealed a rapid 2-day rise in the nighttime emission rate in springtime followed by a subsequent decrease in the magnitude, which was termed ‘springtime transition’. A rapid temperature enhancement was also revealed in the average annual temperature at 87 km height, employing observations from 1991 to 1997 and more recent temperature data from 1998 to 1999. Large amplitude perturbations in mesospheric Na-lidar and OH rotational temperatures at 87 km around the autumnal equinox at mid-latitudes were also reported. Recently it has been suggested that the observed fall perturbation is a signature of mesospheric planetary waves of zonal wavenumber 1 (hereafter a wave 1) in the Northern hemisphere. The current study extends earlier work on mesospheric temperatures from the WINDII/UARS experiment and complementary ground-based observations to examine global spatial and temporal characteristics of the equinox transition at 87 km height. A 7-year climatology at middle and high latitudes, 40–60°N is presented revealing a V-shaped signature of temperature decrease with amplitude of ∼25 K below the annual mean and a duration of 3–4 weeks depending on the year, following fall equinox (Day 257–284). Satellite and ground-based temperature observations were successfully combined to infer tidal information, a key to the interpretation of the results obtained. After accounting for the diurnal tidal perturbation a distinct planetary wave structure was revealed at latitudes from 45°N to 65°N. Longitudinal temperature variations show a drop at 140–180°E on Day 264 at 50°N which extends to at least 60°N accompanied by wave signatures. Spectral analysis indicated planetary waves with waves 1 and 2 planetary perturbations maximizing on Day 263–265 (September 20–22). Various waveforms were also identified (waves 3–6) from which a waveform with a wavenumber 4 appeared to be the third most persistent wave signature during the period with a period of about 4 days. The spectral analysis also revealed a depressed planetary wave activity following fall equinox. It is concluded that the observed temperature perturbation is the likely result of in situ generated gravity waves at 87 km superposed on the planetary wave 1 propagating upwards from below enhanced by the reversal of zonal mean wind flow at equinox.

Dynamical processes related to the atomic oxygen equinox transition

Both ground based and satellite observations have shown a “springtime transition” in the night-time OI 557.7 nm airglow emission and O 2 atmospheric band emission characterized by a relatively persistent period of very low emission rates around spring equinox at mid and high latitudes, indicating depletion of the peak atomic oxygen between 95 and 100 km . The salient feature of this depletion is reproduced by the NCAR thermosphere–ionosphere–mesosphere-electrodynamics general circulation model simulation. Analysis of the simulation results shows that the mean circulation in the upper mesosphere/mesopause and lower thermosphere is altered significantly by changes in gravity wave filtering and gravity wave forcing around equinox. The change first starts in form of a distinctive circulation cell in the summer lower thermosphere when the westward jet in the summer stratosphere and mesosphere begins to reverse. The circulation cell then becomes deeper and expands to lower altitudes and to the other hemisphere. This circulation can span a large vertical depth and drives an exchange of mass and chemical species between the upper mesosphere and lower thermosphere. The variation of the peak atomic oxygen concentration around equinox is caused by this exchange, and the vertical transport of atomic oxygen dominates the diffusive transport in this process. Additional variability of the atomic oxygen and corresponding airglow emissions may result from the variability of gravity wave sources, planetary waves and transience of their amplitudes and phases, and circulation change due to late and final stratospheric sudden warming.

Equinox transition in wind and airglow observations

The equinox transition marks two significant intervals in the annual variability of the mesosphere and lower thermosphere. A scenario of this phenomenon is emerging in which for atomic oxygen, the winter period is characterized by downward flow, leading to enhanced oxygen concentrations and high levels of oxygen airglow. These high winter airglow levels are punctuated by large fluctuations associated with the highly variable winter dynamics, including stratospheric warmings, superposed on a monotonic downward trend. The summer is a time of less variability and weaker airglow emission associated with atmospheric upwelling, ending with a buildup of atomic oxygen in the fall. Within this general picture the observations depend significantly on latitude. At high latitudes the effects of upwelling and downwelling are clear and these persist to latitudes as low as 40°, but at still lower mid-latitudes a semi-annual variation with equinoctial maxima seems to dominate, as observed by earlier workers. This scenario is supported by the ground-based and satellite observations presented here. The dynamical coupling with the airglow is explored by comparing airglow observations at Stockholm (59.5°N, 18.2°E) with the zonal wind observations at nearby Juliusruh (54.6°N, 13.4°E), while WINDII data are used to connect the two as well as providing mid-latitude data. In fitting these equinox transition observations into the annual variation these observations are compared with predictions from the TIME-GCM model, with good overall agreement.

Correction to “The role of ring current nose events in producing stable auroral red arc intensifications during main phase: Observations during September 19‐24, 1984, Equinox Transition Study”

Correction to “The role of ring current nose events in producing stable auroral red arc intensifications during main phase: Observations during September 19‐24, 1984, Equinox Transition Study”

The role of ring current nose events in producing stable auroral red arc intensifications during the main phase: Observations during the September 19‐24, 1984, Equinox Transition Study

An examination of stable auroral red (SAR) arc emissions over the last solar cycle (Slater and Kleckner, 1989) indicates that the strongest emissions, during the lifetime of a particular SAR arc, often occur in association with the main phase of the magnetic storm. Previous observations of thermal and energetic particle populations at high and low altitudes on SAR arc field lines by Kozyra et al. (1987a) indicate that the energy source for these emissions is the O+ in the ring current. The O+ content of the ring current increases with increasing magnetic activity reaching its maximum percentage contribution near minimum Dst for a particular storm. This variation in the O+ content of the ring current is inconsistent with an early main phase enhancement of SAR arc emissions. To investigate the source of main phase enhancements in SAR arc emissions, a study of the September 19‐24, 1984, magnetic storm period during which SAR arc emissions were observed by the ground‐based mobile automatic scanning photometer network in both the main and recovery phases is presented. The emissions associated with the main phase (∼ 400 R) were an order of magnitude greater than those associated with the recovery phase (tens of R). Energetic particle measurements from the DE 1 and AMPTH spacecraft, on field lines that map to the SAR arc position at low altitude, were examined to determine if differences in the energy sources during these time periods were evident. In agreement with previous work, ring current O+ supplied the bulk of the electron heating during storm recovery phase as a result of Coulomb collisions of O+ with the plasmaspheric electrons; contributions by ring current H+ were negligible. A new result of the present work is that an enhancement of the 15‐25 keV H+ component of the ring current during the main phase of the September 19 magnetic storm was responsible for an approximately one order of magnitude increase in the electron heating rate and SAR arc emissions during the main phase compared to the recovery phase. The increase in the H+ flux occurred in association with a ring current “nose event”, a front of ions injected into the inner magnetosphere in response to a discontinuous change in the cross‐tail electric field. The association between nose events and intensifications of SAR arc emissions in the main phase has not previously been explored but is a natural consequence of the injection of significant fluxes of relatively low‐energy ring current ions earthward of the plasmapause during early storm time ring current formation.

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.

Thermospheric tides at equinox: Simulations with coupled composition and auroral forcings: 1. Diurnal component

Thermospheric tidal components calculated by the National Center for Atmospheric Research thermospheric general circulation model have been revised by including the effects of self‐consistent composition couplings and auroral processes. Calculations are presented for equinox conditions at solar cycle minimum and maximum. The diurnal neutral winds and perturbation temperatures predicted by the model are compared with radar observations and the MSIS‐86 model at low, mid, and high latitudes. Sensitivity tests indicate that the feedback between composition and dynamics affects the diurnal tidal temperature amplitudes up to 50% and the diurnal winds by about 15%. The calculated phases are within an hour of uncoupled composition calculations. Auroral processes dominate the diurnal waves at high latitudes, where the wind amplitudes may be double or triple those calculated for solar forcing only. The high‐latitude energy and momentum sources must be included in the model formulations in order to reproduce observations of the September 1984 Equinox Transition Study exospheric temperatures.

Neutral and ion composition changes in the F region over Millstone Hill during the Equinox Transition Study

We use overhead ionospheric incoherent scatter radar data from Millstone Hill to deduce the stormtime variations of the thermospheric temperature, O and N2 densities, and F1 region ion composition during the Equinox Transition Study of September 17–24, 1984. Using the measured neutral temperature profile and O density at 400 km altitude, we extrapolate to determine O density changes in the lower thermosphere. Using these O densities and the measured electron densities, we deduce the variation of the N2 density. Our initial attempt leads to the deduction of large depletions in both O and N2 in the lower thermosphere. On using the MSIS model neutral temperatures for the extrapolation, however, we compute O depletions having magnitudes more comparable with theoretical predictions and N2 variations consistent with expected thermal expansion effects. We speculate that the radar‐measured neutral temperatures were contaminated by ion‐neutral frictional heating effects during the disturbed periods of the ETS and that this served to invalidate neutral density extrapolations made with these temperatures. Further, we compute the height of 50% O+ ion composition in the F1 region and the height of the peak electron density in the ionosphere, and, upon comparing these, find that the height of the peak on the disturbed days was located in the lower F1 region molecular‐ion layer rather than in the F2 region O+ layer. This occurrence has serious consequences for methods of deducing neutral winds from the height of the F layer, for these methods rely on the peak containing primarily O+ ions.

A study of the equinox transition in the ionosphere above Buenos Aires (34.5°S, 58.5°W) and Canberra (35.3°S, 211°W), as observed in the height of the F2 layer

The equinox transition (ET) in the thermosphere is analyzed on the basis of the hmF2 parameter for two southern hemisphere mid‐latitude ionosonde stations with 152° longitudinal separation (Buenos Aires and Canberra). The period under study covers August 1 to October 31, 1984, thus including the Equinox Transition Study core period. Such a sample is necessary for the correct interpretation of long‐lasting trends. ET features are observed at both locations, but important longitudinal differences are evident. Harmonic analysis of the data, performed to separate 24‐, 12‐, and 8‐hour oscillations shows reasonable agreement with meridional tidal wind models, particularly with respect to phases. Results indicate that the ET is a gradual phenomenon not restricted to the core period but starting before it. With respect to the longitudinal differences, the most prominent one is the difference in the amplitude of the 12‐hour oscillation, which is of greater magnitude over Buenos Aires than Canberra by at least a factor of 3. The 8‐hour oscillation at both locations shows a progressive increase in amplitude fluctuations after the core period with a poor correlation between sites, thus suggesting an important contribution due to local origin.

"Thermospheric dynamics during September 18–19, 1984: 2. Validation of the NCAR Thermospheric General Circulation Model""

The validation of complex nonlinear numerical models such as the National Center for Atmospheric Research thermospheric general circulation model (NCAR TGCM) requires a detailed comparison of model predictions with data. The Equinox Transition Study (ETS) of September 17–24, 1984, provided a unique opportunity to address the verification of the NCAR TGCM, since unusually large quantities of high‐quality thermospheric and ionospheric data were obtained during an intensive observation interval. In a companion paper (paper 1) by Crowley et al. (this issue) a simulation of the September 18–19 ETS interval was described. Using a novel approach to modeling, the TGCM inputs were tuned where possible with guidance from data describing the appropriate input fields, and the arbitrary adjustment of input variables in order to obtain thermospheric predictions which match measurements was avoided. In the present paper the winds, temperatures, and densities predicted by the TGCM are compared with measurements from the ETS interval. In many respects, agreement between the predictions and observations is good. The quiet day observations contain a strong semidiurnal wind variation which is mainly due to upward propagating tides. The storm day wind behavior is significantly different and includes a surge of equatorward winds due to a global propagating disturbance associated with the storm onset. The density data confirm the existence of the newly discovered four‐cell high‐latitude density anomaly described in paper 1. A quantitative statistical comparison of the predicted and measured winds indicates that the equatorward winds in the model are weaker than the observed winds, particularly during storm times. This is consistent with predicted latitudinal temperature gradients and storm time density increases which are much smaller than the observed values. Soft particle precipitation or high‐altitude plasma heating is invoked as a possible source of the additional high‐latitude heating required by the model. A quiet day phase anomaly in the measured F region winds which is not reproduced by the model suggests the occurrence of an important unmodeled interaction between upward propagating semidiurnal tides and high‐latitude effects. Wind data from altitudes below 100 km indicate shortcomings in the generic equinox solar minimum tidal specification used in the TGCM. The lack of appropriate data to specify input fields seriously impairs our ability to generate realistic global thermospheric simulations. The problem is particularly acute in the southern hemisphere. Furthermore, disturbances generated in the southern hemisphere simulation propagate northward and degrade the northern hemisphere predictions. Thus, if the thermosphere in the northern hemisphere is ever to be fully understood, the southern hemisphere needs to be observed and simulated more accurately. Several improvements are suggested for future realistic time‐dependent simulations of specific intervals.

"Evidence for the equatorward penetration of electric fields, winds, and compositional effects in the Asian/Pacific sector during the September 17–24, 1984, ETS interval""

Data from a chain of eight ionosondes between 15° and 51° magnetic latitude near the −165° east magnetic meridian (Asian/Pacific sector) are analyzed to illustrate the equatorward penetration of ionospheric storm effects during the September 17–24, 1984, Equinox Transition Study (ETS) interval. Hourly values of measured f0F2, M(3000)F2, and f0E are used to infer the storm time deviations of f0F2, hmax, and an effective meridional wind speed from their average magnetically quiet values at the beginning of the interval. The latter parameters are subjected to a 5‐hour running average and are fit with Legendre polynomials in latitude to clarify the large‐scale dynamical features in a color‐coded display. The method illustrates the equatorward penetration of several phenomena in response to changing magnetic conditions, interpreted as follows: (1) Elevations in hmax of the order of 40 km occur down to 35°–45° magnetic latitude during the first, less intense, magnetic storm occurring on September 19; and elevations of the order of 50–70 km penetrate to at least 15° magnetic latitude during the more intense storm at the end of the interval. These effects are interpreted as primarily due to meridional winds driven by high‐latitude heating. (2) Daytime depressions in f0F2 occur on the following day (“storm negative phase”) of the order of 1.0–1.5 MHz (∼30% change) between 30° and 40° magnetic latitude after the first storm, and of the order of 2.0–2.5 MHz (∼45% change) down to 15°–20° magnetic latitude after the second storm, due to decreased [O]/[N2] ratios connected with the global heating and dynamical transport. (3) Enhancements in f0F2 of the order of 2.0–3.0 MHz (∼30–40% change) near 15°–20° magnetic latitude are interpreted as originating from a “pumping up” of the fountain effect by eastward electric fields penetrating to low latitudes during the early phases of the storms, and a secondary enhancement several hours later is consistent with the existence of an ionospheric disturbance dynamo. (4) Diurnal‐average meridional winds, inferred from the changes in hmax, illustrate disturbance mean meridional circulations of the order of 20–25 m s−1 and 30–50 m s−1 for the two disturbances, whose equatorward penetration is sensitive to the energetic input at high latitudes. This information forms one data set which numerical simulations of the ETS interval should strive to reproduce.

Electrodynamic patterns for September 19, 1984

Variations of the northern high‐latitude ionospheric convection pattern are estimated from combined ground‐based and satellite‐based measurements for the disturbed day of September 19, 1984, during the Equinox Transition Study (ETS) campaign period. The electric potential distribution is determined from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure of Richmond and Kamide (1988) on the basis of available electric field, conductance, and magnetic perturbation observations. As an improvement over previous data inversion studies of high‐latitude electrodynamics, we have incorporated satellite measurements of precipitating particles to help determine variations of the conductances in the auroral oval. These measurements are found to cause substantial modifications to the initial empirical conductance model. A large substorm commenced about 0920 UT and peaked about 1050 UT, followed by smaller substorms starting about 1430 UT and 1800 UT. The polar‐cap potential drop varied between about 30 kV and 160 kV during the day, and the integrated Joule heating over the northern polar region varied between 3×109 W and 8.6×1011 W. During the substorms the derived convection potential was considerably enhanced and distorted with respect to the empirical model of Foster et al. (1986). Strong eastward convection in the midnight sector of the auroral oval during the first large substorm was associated with a marked westward rotation of the two‐cell convection pattern at the peak of the substorm. These variations can be expected to have influenced significantly the dynamics of the high‐latitude ionosphere and thermosphere during the ETS period.

"Modeled solar EUV flux during the Equinox Transition Study: September 17–24, 1984""

A new empirical model of solar extreme ultraviolet (EUV) irradiance variability has been developed based on satellite and rocket EUV observations for all levels of solar activity. Its results for the Equinox Transition Study (ETS) period of September 17–24, 1984, have been calculated, and the tabulated results of photon and energy fluxes for each day of this period are presented. The model average total EUV full‐disk energy flux for the spectral range between 1.9 and 105.0 nm for the ETS period is 2.9 ergs cm−2 s−1, with a general secular increase in the total flux level between September 17 and September 24. This modeled rise corresponds to the increased EUV irradiance variability due to active region evolution and passage across the disk. The modeled EUV values in September 1984 near solar cycle 21 minimum conditions are higher than rocket‐ and satellite‐measured EUV values near solar minimum conditions of cycle 20. However, the results are consistent with recent trends in the literature to revise the solar cycle 20 minimum flux values upward.

Variations in the proton concentration and flux of the topside ionosphere at Arecibo during the September 1984 Equinox Transition Study

Analysis of incoherent scatter radar measurements at Arecibo routinely take into account the presence of light ions above an altitude of 400 km. Combined with field‐aligned ion velocity observations it is possible to determine the diurnal variations of the proton flux. We find that the near‐zero daytime proton outflow in the topside ionosphere during equinox is not generally balanced by a larger nighttime inflow, as appears to be the case for other ions such as O+. The proton flux and concentration does respond, however, to external influences such as disturbed magnetic conditions. One such isolated magnetic storm occurred during the day during the Equinox Transition Study, a World Day investigation in September 1984. We show the response to the storm of several measured topside parameters, in particular the proton concentration and its flux, where significant departures from their nominally zero conditions occurred.

The behavior of the electron density and temperature at Millstone Hill during the Equinox Transition Study September 1984

We present first results from a comparison of an extensive set of ionospheric storm measurements with a comprehensive model that includes interhemispheric coupling, photoelectrons, vibrationally excited N2, and the MSIS‐86 neutral atmosphere model. While the modeled electron density and temperature agree well with the measured density and temperature for the quiet day, the model does not reproduce the observed factor of 1.7 decrease in the electron density on the storm day. The model does not support a recent suggestion that vibrationally excited N2 might account for the negative phase of ionospheric storms. The most likely cause of the model failure is an inadequate decrease in the ratio of atomic oxygen to molecular neutral densities in the MSIS neutral atmosphere model for this particular storm. A factor of 3 to 5 decrease in the atomic oxygen density to molecular density ratio at 300 km is needed to explain the observed decrease in electron density. This result is important because (1) it indicates that the new MSIS model may not be reliable during particular magnetic storms and (2) it affects the accuracy of neutral winds calculated from radar measurements of the ion drift and also winds derived from the height of the F2 layer peak density.