DE3 Tide Research Articles

Influence of DE3 tide on the equinoctial asymmetry of the zonal mean ionospheric electron density

Through respectively adding September DE3 tide and March DE3 tide at the low boundary of Global Coupled Ionosphere-Thermosphere-Electrodynamics Model, Institute of Geology and Geophysics, Chinese Academy of Sciences (GCITEM-IGGCAS), we simulate the influence of DE3 tide on the equinoctial asymmetry of the zonal mean ionospheric electron density. The influence of DE3 tide on the equinoctial asymmetry of the zonal mean electron density varies with latitude, altitude, and solar activity level. Compared with the density driven by the September DE3 tide, the March DE3 tide mainly decreases the lower ionospheric zonal mean electron density and mainly increases the electron density at higher ionosphere. In the low-latitude ionosphere, DE3 tide drives an equatorial ionization anomaly (EIA) structure at higher ionosphere in the relative difference of zonal mean electron density, which suggests that DE3 tide affects the longitudinal mean equatorial vertical E × B plasma drifts. Although the lower ionospheric equinoctial asymmetry driven by DE3 tide mainly decreases with the increase of solar activity, the asymmetry at higher ionosphere mainly increases with solar activity. However, EIA in equinoctial asymmetry mainly decreases with the increase of solar activity.

New perspectives on thermosphere tides: 2. Penetration to the upper thermosphere

In this paper, we present new multi-year and 72-day mean seasonal-latitudinal tidal structures in exospheric temperature derived from joint analysis of CHAMP and GRACE accelerometer measurements. These results include diurnal tides DE3, DE2, D0, and DW2 and semidiurnal tides S0, SE1, SE2, SE3, SW4, and SW6. We also employ Hough mode extensions (HMEs) and the Climatological Model of Thermosphere Tides (CTMT) to ascertain whether the observed structures are consistent with those observed at 110 km and presented in part 1 of this study. The aggregate sum of all the tidal components is shown to impose considerable longitude and month-to-month variability on the exosphere temperature tidal spectrum. Please see related article: http://www.earth-planets-space.com/content/66/1/136 .

Global response of the ionosphere to atmospheric tides forced from below: Comparison between COSMIC measurements and simulations by atmosphere‐ionosphere coupled model GAIA

This paper for the first time presents a detailed comparison between simulated and observed global electron density responses to different atmospheric tides forced from below. The recently developed Earth's whole atmospheric model from the troposphere to the ionosphere, called GAIA, has been used for the simulation of the electron density tidal responses. They have been compared with the extracted from the COSMIC electron density data tidal responses for the period of time October 2007 to March 2009. Particular attention has been paid to the nonmigrating DE3/DE2 and migrating DW1, SW2 and TW3 electron density responses. The GAIA model reproduced quite well the COSMIC DE3/DE2 responses. Both simulations and observations revealed three altitude regions of enhanced electron density responses: (1) an upper level response, above 300 km height, apparently shaped mainly by the “fountain effect”; (2) a response located near altitudes of ∼200–270 km, and (3) a lower thermospheric response situated near 120–150 km height. A possible mechanism is suggested for explaining the two lower level responses. For the first time the GAIA model simulations supported the observational evidence found in the COSMIC measurements that the ionospheric WN4 (WN3) longitude structure is not generated only by the DE3 (DE2) tide as it has been often assumed. As regards the comparison of the migrating DW1, SW2 and TW3 responses the obtained results clearly demonstrate that the GAIA model reproduce very well of the SW2 and TW3 COSMIC electron density responses. The only main discrepancy is seen in the migrating DW1 response; the observation does not support the splitting of the simulated response at both sides of the equator. This is due mainly to the difference between the SABER and GAIA SW2 tide in the lower thermosphere as it turned out that the DW1 electron density response strongly depends on the mean features of the lower thermospheric SW2 tide.

Simulated equinoctial asymmetry of the ionospheric vertical plasma drifts

This paper studies the influence of the lower thermospheric tidal winds below 105 km on the equinoctial asymmetry of the equatorial vertical E × B plasma drifts (V⊥) using Theoretical Ionospheric Dynamo Model II of the Institute of Geology and Geophysics, Chinese Academy of Sciences (TIDM‐IGGCAS‐II) and tidal winds below 105 km from thermosphere‐ionosphere‐mesosphere‐energetics‐dynamics satellite Doppler Interferometer (TIDI) observations. Although a series of other nonmigrating tides also affect the V⊥ asymmetry, the simulated equinoctial asymmetry in V⊥ are mainly driven by the migrating diurnal tide (DW1), migrating semidiurnal tide (SW2), DE3, and DW2 nonmigrating tides. The asymmetry in daytime V⊥ varies with local time and longitude and mainly shows three features. First, the simulated daytime V⊥ during the March equinox is larger than that during the September equinox in most of longitudinal sectors. This asymmetry is mainly driven by the semiannual oscillation (SAO) of the migrating diurnal tide in the tropical mesosphere‐lower thermosphere (MLT) region, and the equinoctial asymmetry of the migrating semidiurnal tide also plays an important role in the generation of this asymmetry. Second, the daytime V⊥ asymmetry in the Eastern Hemisphere is more significant than that in the Western Hemisphere. Our simulation suggests that the longitudinal variations of the geomagnetic fields and DW2 tides play important roles in the generation of this hemisphere difference. Third, there is an obvious wave number 4 longitudinal structure in the V⊥ asymmetry. Our simulation suggests that this wave number 4 structure is mainly driven by the equinoctial asymmetry of the DE3 tide.

Simulated longitudinal variations in the E-region plasma density induced by non-migrating tides

Based on the GCITEM-IGGCAS model and tides from TIMED/SABER observations, the longitudinal variations of the E-region plasma densities, which is induced by non-migrating tides, are investigated. We simulated the intra-annual variation of the E-region plasma density, and found that equinoctial E-region plasma density shows an obvious wavenumber-4 longitudinal structure both in Equinox and in June Solstice, and a wavenumber-3 longitudinal structure in December Solstice. Our simulations suggest that DE3 tide can drive the wavenumber-4 structure in E-region plasma density, and DE2 tide can drive the wavenumber-3 structure. O2+ controls the longitudinal variations of total ions density at lower E-region, and NO+ mainly controls that at higher E-region. We also noticed that the longitudinal variations of O2+ and of NO+ show obvious phase differences.

Global Response of the Ionosphere to Atmospheric Tides Forced from Below: Recent Progress Based on Satellite Measurements

This paper provides an overview on the recent progress in studying the ionospheric response to atmospheric tides forced from below. The global spatial structure and temporal variability of the atmospheric temperature tides and their ionospheric responses are considered on the basis of modern satellite-board data (COSMIC and TIMED). The tidal waves from the two data sets have been extracted by one and the same data analysis method. The similarity between the lower thermospheric temperature tides and their ionospheric responses provides evidence for confirming the new paradigm of atmosphere-ionosphere coupling. This paper provides also new experimental results which give an explanation why the WN4 and partly WN3 longitude structures are so prominent pattern in the ionosphere. These results present evidence indicating that the WN4 (WN3) structure is not generated only by the DE3 (DE2) tide as it has been often assumed. The DE3 (DE2) tide remains the leading contributor, but the SPW4 and SE2 (SPW3, DW4 and SE1) waves have their effects as well in a way that the ionospheric response becomes almost double (1.5 time stronger). The paper presents also the global distribution and temporal variability of the sun-synchronous 24-h (DW1), 12-h (SW2) and 8-h (TW3) electron density oscillations. It has been shown that while the latitude and altitude structure of the ionospheric SW2 response is predominantly shaped by the migrating SW2 tide forced from below the DW1 response is mainly due to daily variability of the photo-ionization. The peculiar vertical structure of the ionospheric TW3 response, that shows downward/upward phase progression, calls for further study of the physical processes shaping this ionospheric response.

Global ionospheric response to nonmigrating DE3 and DE2 tides forced from below

[1] In this paper we present the global spatial structure and seasonal variability of the COSMIC electron density response to the forced from below DE3 and DE2 tides during the period January 2008 to March 2009. The COSMIC electron density at fixed altitudes and SABER temperatures were utilized in order to define the ionospheric DE3 and DE2 tidal response to the DE3 and DE2 temperature tides coming from below. The tidal waves from both data sets have been extracted by the same data analysis method. The latitude structure of the ionospheric response for heights up to 450–500 km is distributed on both sides of the dip equator at about ±30° modified dip latitude; above 500 km the response is approximately confined over the equator. Two altitude regions of enhanced ionospheric response are found: an upper level response (above 300 km) and a bottom level one (below 250 km); the two regions are separated by a narrow altitude zone with no tidal response. The different phase structure of the ionospheric response in both altitude regions suggests that (1) the upper level response is mainly due to the DE3/DE2 modulated vertical plasma drift and (2) the bottom level response is complex and, in all likelihood, is caused by more than one mechanism.

Temporal modulation of the four‐peaked longitudinal structure of the equatorial ionosphere by the 2 day planetary wave

Observations of electron densities by the Constellation Observing System for Meteorology, Ionosphere, and Climate in August to October 2008 have shown a prominent four‐peaked longitudinal structure in the height of the F2 layer (hmF2) in the equatorial ionosphere. The development of this ionospheric structure in daytime is found to be consistent with the forcing by the eastward‐propagating nonmigrating diurnal tide with zonal wave number 3 (DE3). It is believed that tidal winds can modify the E region electric fields and subsequently produce variations in the ionosphere through the dynamo effect. This study reveals that the amplitude of the hmF2 four‐peaked longitudinal structure is subject to a 2 day periodic modulation on certain intervals in the two‐month time period. Simultaneously, wind measurements from the SKiYMET meteor radar at Thumba (8.5°N, 77°E), India indicate corresponding 2 day planetary wave activity in the mesosphere and lower thermosphere (MLT). The 2 day planetary wave has both zonal and meridional wind components, and it is the variability in the zonal component that most closely corresponds to F2 layer changes. The zonal wind observations by the radar also show that the amplitude of the diurnal tide is modulated by the 2 day wave. This study suggests that the identified 2 day variation of the hmF2 four‐peaked longitudinal structure in the equatorial ionosphere is caused by the interaction between the DE3 tide and the 2 day planetary wave.

Correlation between the ionospheric WN4 signature and the upper atmospheric DE3 tide

The present work studies the correlation relationship between the longitudinal ionospheric structure of wave number 4 (WN4) and the upper atmospheric tide of nonmigrating tidal mode DE3 (diurnal eastward wave number 3). Global ionospheric maps produced by the Jet Propulsion Laboratory were used to deduce the latitudinal integration of total electron content in the low‐latitude ionosphere, and TIDI/TIMED observations were used to retrieve the atmospheric zonal and meridional winds. By applying Fourier filtering and fitting techniques, the WN4 wave and DE3 tidal components are derived from the ionospheric and upper atmospheric observations, respectively. We found that the observed WN4 wave and DE3 zonal wind components experience very similar annual and interannual variations, but the DE3 meridional wind component behaves in a quite different manner. Both WN4 and DE3 zonal winds are very intense during northern summer and autumn; they also appear in the later spring, but tend to vanish in winter. Their amplitudes increase as the solar activity decreases, and both are stronger in the quasi‐biennial oscillation (QBO) eastward wind phase than in the westward phase. At the same time, the DE3 meridional wind likes to occur only in winter and seems not change with solar activity and QBO phase. We further studied the correlation between the WN4 wave and the two wind components of the DE3 tide. We found that the cross‐correlation coefficient between the WN4 wave and the DE3 zonal wind is much larger, while that between the WN4 wave and the DE3 meridional wind is relatively smaller. Such different correlations are attributed to the different latitudinal symmetry of different DE3 wind components. The DE3 zonal wind is likely in latitudinally symmetric tidal mode; hence, it can efficiently affect the F region ion drifts. In contrast, the meridional wind is mainly in antisymmetric mode and thus seldom affects the ionospheric drifts. The present results support the suggestion that the longitudinal WN4 structure in the ionospheric F region originates from the symmetric modes, mainly the zonal wind component, of the upper atmospheric nonmigrating tidal mode DE3 in the ionospheric E region.

Thermospheric composition variations due to nonmigrating tides and their effect on ionosphere

The TIMED/GUVI thermospheric composition (O/N2 ratio) over 2009 reveals clear wave‐3 (summer season) and wave‐4 (Fall season) longitudinal dependence. The wave peak to valley varies from 7 to 11% of the background O/N2. The low Kp (yearly mean of 0.9) and low solar EUV flux indicates that the wave features were not caused by geomagnetic activity or solar EUV variation. A more likely explanation is nonmigrating diurnal eastward propagating DE2 and DE3 tides. Model calculations indicate that a 10% variation in O/N2 makes 1/3 contribution to the nightside equatorial 135.6 nm intensities compared to the variation due to the E‐region dynamo electric field. This study confirms that nonmigrating tides can penetrate toward the upper thermosphere, both O/N2 and the E‐region dynamo should be considered in analyzing ionospheric density variations due to nonmigrating tides. Comparison with MSIS results indicates that the nonmigrating tides have stronger effect on O/N2 than the migrating tides.

Strong evidence for the tidal control on the longitudinal structure of the ionospheric F‐region

This paper presents for the first time the global latitude structure and seasonal variability of the ionospheric response to the forced from below DE3 and DE2 tides during the period of time January 2008–March 2009. The COSMIC hmF2 and SABER temperature data have been utilized in order to define the ionospheric DE3 and DE2 tidal response to the DE3 and DE2 temperature tides propagating from below. The COSMIC DE3 and DE2 hmF2 tidal oscillations are derived by the same method as the tides seen the SABER temperatures. It has been shown that the longitude wave‐4 and wave‐3 hmF2 structures observed respectively in September and May 2008 are forced mainly by DE3 (wave‐4) and DE2 (wave‐3) temperature tides coming from below. The longitude wave‐3 hmF2 structure observed in January 2008 however is forced by the combined action of the DE2 temperature tide coming from below and the SPW3 probably generated in‐situ.

Simulated wave number 4 structure in equatorial F‐region vertical plasma drifts

This article investigates the wave number 4 longitudinal structure in equatorial vertical E × B plasma drifts (V⊥), using the Theoretical Ionospheric Dynamo Model, Institute of Geology and Geophysics, Chinese Academy of Sciences, Version II model and the DE3 tide wind from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED)/TIMED Doppler interferometer (TIDI) observations. We simulate this longitudinal structure and examine its intra‐annual variations and the effect of different DE3 tide components and of different current components on it. We find that this longitudinal structure shows obvious intra‐annual variation, being strongest in northern summer and autumn and weakest in northern winter. This result is consistent with the intra‐annual variation observed by Ren et al. (2009b). Our simulations also suggest that the eastward propagation of the DE3 tide can cause the eastward shift of the wave number 4 structure in V⊥. The wave number 4 structures in V⊥ are mainly driven by a zonal wind component of DE3 tide, which contributes much more than the meridional wind component of DE3 tide to the wave number 4 structure in V⊥. With the influence of the coupling of ionospheric electric fields along the highly conducting magnetic field lines, the symmetric wind component of DE3 tide plays a more important role in the wave number 4 structure in V⊥ than the antisymmetric component does. The intra‐annual variations of the wave number 4 structure in V⊥ are mainly controlled by the symmetric wind component of DE3 tide. Our simulations also suggest that the wave number 4 structures in V⊥ are mainly driven by the Hall current.

Comparison of CHAMP and TIME‐GCM nonmigrating tidal signals in the thermospheric zonal wind

Four years (2002–2005) of continuous accelerometer measurements taken onboard the CHAMP satellite (orbit altitude ∼400 km) offer a unique opportunity to investigate the thermospheric zonal wind on a global scale. Recently, we were able to relate the longitudinal wave‐4 structure in the zonal wind at equatorial latitudes to the influence of nonmigrating tides and in particular to the eastward propagating diurnal tide with zonal wave number 3 (DE3). The DE3 tide is primarily excited by latent heat release in the tropical troposphere in deep convective clouds. In order to investigate the mechanisms that couple the tidal signals to the upper thermosphere, we undertook a comparison with the thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) developed at the National Center for Atmospheric Research (NCAR). We ran the model for a day in March, June, September, and December and applied the same processing steps to the model output as was done for the CHAMP tidal analysis. The main results of the comparison can be summarized as follows: (1) TIME‐GCM simulations do not correctly reproduce the observed intra‐annual variations of DE3 and the eastward propagating diurnal tide with zonal wave number 2 (DE2). (2) Simulations of DE3 for June are more successful. Both TIME‐GCM and CHAMP show an increase in DE3 amplitudes with decreasing solar flux level. (3) The amplitudes of the simulated westward propagating diurnal tide with zonal wave number 2 (DW2) and the standing diurnal tide (D0) increase with increasing solar flux in June. The predicted dependence of DW2 and DO on solar flux is also observed by CHAMP.

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

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

Intra‐annual variation of wave number 4 structure of vertical E × B drifts in the equatorial ionosphere seen from ROCSAT‐1

This paper studies the intra‐annual variation of the wave number 4 structure in the equatorial vertical E × B drifts at high solar flux level based on of the ROCSAT‐1 observations during the interval from 1999 to 2004. It is found that the daytime longitudinal structure is significant in northern summer and early autumn but weak in northern winter. This is consistent with the intra‐annual variations of the tidal mode DE3. At night, the intra‐annual variation of the wave number 4 structure shows some differences from that in daytime, which may relate to different dynamo mechanisms operating in different local time. We also found that the wave number 4 structure mainly shifts eastward during daytime in most of months, which is coincident with the eastward propagation of the DE3 tide. However, it is largely disturbed near sunrise and sunset. We attribute this disturbance as the jumping of the main dynamo region between E and F layers, as well as the different dynamo mechanisms in different dynamo regions.

Global structures of the DE3 tide

The Hough mode decomposition (HMD) is used to investigate the global structures of the eastward propagating diurnal tide of zonal wavenumber 3 (DE3). The tide is delineated by using the SABER/TIMED temperatures collected during 2002–2006. The HMD analysis results show that the DE3 tide is primarily dominated by two leading propagating Hough modes, i.e., (−3, 3) and (−3, 4) modes; the influences.of the other Hough modes including trapped modes can be neglected. Based upon the HMD analysis results, this paper first reported the maximum of the tidal activity in the MLT region. The results show that the DE3 tide exhibits annual unimodal distribution with the maximal amplitude occurring at 110 km in late summer (around July each year). Moreover, characteristic 2-year period variation is observed in the (−3, 3) Hough mode. And this type of inter-annual variation is further reflected in the tidal amplitude at 110 km height. For example, corresponding to the 2-year variation of the (−3, 3) mode, the DE3 tidal amplitude exhibits two substantially enhanced activities with maximal amplitude exceeding 12 K in 2002 and 2004, respectively. Moreover, current investigation results indicate that the influence of the second propagating Hough mode, (−3, 4) mode, is important, in particular at the height under 100 km, where the DE3 amplitudes exhibit antisymmetric distribution with respect to the equator. The (−3, 4) mode exhibits bimodal distribution over a yearly course, which dominates the DE3 tide in the lower mesosphere. For example, two maximal DE3 activities were observed in late-winter-to-earlyspring and late-autumn-to-early-winter, respectively. The first maximum is seen in the south of the equator, and the second maximum is in the north of it.

Seasonal variation of the longitudinal structure of the equatorial ionosphere: Does it reflect tidal influences from below?

We have examined the longitudinal structure of the equatorial ionosphere at 400‐km altitude in the noon and postsunset local time sectors in different seasons using 6 years of F‐region plasma density observations from the CHAMP satellite. A four‐peak wave structure is observed in both local time sectors. In the noon sector at a fixed solar flux level, this structure is observed to be most prominent around September equinox and weakest around December solstice. This seasonal dependence agrees well with that of the nonmigrating diurnal tides DE3, hence supporting a close coupling between the ionosphere and the mesosphere‐lower thermosphere possibly via the DE3 tidal modulation of the E‐layer dynamo. In the postsunset sector, however, such agreement cannot be claimed. In this sector, although the four‐peak structure can be observed in all seasons at moderate and high solar flux levels, its seasonal dependence does not follow that of the DE3 tides. This structure becomes indiscernible near solstices at low solar flux levels. Furthermore, the postsunset four‐peak wave structure exhibits larger amplitude than that during daytime, hence indicating that it is more likely an amplified feature rather than a remnant of the daytime structure. The prereversal enhancement is speculated to be a possible candidate to cause this amplification.

Annual Variation and Global Structures of The DE3 Tide

The SABER/TIMED temperatures taken in 2002–2006 are used to delineate the tidal signals in the middle and upper atmosphere. Then the Hough mode decomposition is applied with the DE3 tide, and the overall features of the seasonal variations and the complete global structures of the tide are observed. Investigation results show that the tide is most prominent at 110 km with the maximal amplitude of > 9K, and exhibits significant seasonal variation with its maximum amplitude always occurring in July every year. Results from the Hough mode decomposition reveal that the tide is composed primarily of two leading propagating Hough modes, i.e., the (−3,3) and the (−3,4) modes, thus is equatorially trapped. Estimation of the mean amplitude of the Hough modes show that the (−3,3) mode and (−3,4) mode exhibit maxima at 110km and 90 km, respectively. The (−3,3) mode plays a predominant role in shaping the global latitude-height structure of the tide, e.g., the vertical scale of > 50km at the equator, and the annual course. Significant influence of the (−3,4) mode is found below 90km, where the tide exhibits anti-symmetric structure about the equator; meanwhile the tide at northern tropical latitudes exhibits smaller vertical wavelength of about 30 km.

Tidal propagation of deep tropical cloud signatures into the thermosphere from TIMED observations

The diurnal eastward zonal wavenumber‐3 (DE3) tide is excited in the tropical troposphere by latent heat release in deep convective clouds. Its direct propagation into the thermosphere is explored using Hough Mode Extension (HME) analysis of temperature T, and zonal and meridional wind (u, v) measurements from SABER and TIDI on board the TIMED satellite. HMEs provide observation‐based tidal information about parameters not measured (vertical wind w, density ρ) and at latitudes and altitudes not covered by the two instruments. DE3 in (u, v, T, ρ) maximizes around 100 km. The thermospheric (180 km) signal is negligible for (v, ρ) but still considerable for (u, w, T). Maximum amplitudes are 6 m/s (u), 0.12 m/s (w), and 8 K (T). The HME analysis also shows the quantitative consistency of the DE3 tides from SABER and TIDI in the mesosphere/lower thermosphere region. This allows one to interpret the seasonal DE3 variation in terms of symmetric vs. antisymmetric modes.