SEARCH

SEARCH BY CITATION

Keywords:

  • nonmigrating tides;
  • nitric oxide;
  • thermosphere

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Analysis
  5. 3. Results
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[1] The large influence of nonmigrating tides on nitric oxide variability in the lower thermosphere is revealed for the first time using data from the TIMED (tides) and SNOE (nitric oxide) satellites. Between March and October, a wave-4 like longitude variation in nitric oxide density residuals is observed with peak-to-peak values of 40% between 30°S–30°N and 100–135 km. This variation becomes wave-3 like in December and January with wave-4/wave-3 transitions in November and February. It is quantitatively shown that the nitric oxide longitude variations are caused by nonmigrating tides in neutral gas density. The DE3 (diurnal eastward wavenumber 3) tidal component causes the observed wave-4 variation. The combination of DE2 (diurnal eastward wavenumber 2) and SE1 (semidiurnal eastward wavenumber 1) causes the observed wave-3 variation. These results suggest that similar effects will be found in other constituents and airglow emissions important to the aeronomy of the mesosphere and lower thermosphere.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Analysis
  5. 3. Results
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[2] Solar atmospheric tides are global-scale, harmonic waves of given frequency, propagation direction, and zonal wavenumber in temperature, wind, and neutral gas density. They are forced by insolation absorption in the troposphere/stratosphere, by non-linear wave-wave interaction in the stratosphere/mesosphere and by latent heat release in deep convective tropical clouds. Growing exponentially when propagating upward from their source regions, tides often dominate the thermal and dynamical structure of the mesosphere/lower thermosphere (MLT) region. Observations made by the SABER and TIDI instruments on the TIMED satellite have recently resulted in the first climatologies of the most important tidal wave components in temperature, and zonal and meridional winds, respectively [Forbes et al., 2006, 2008; Zhang et al., 2006; Oberheide et al., 2006, 2007]. An important result was that nonmigrating (non-Sun-synchronous) wave components are much larger than hitherto anticipated and often exceed their migrating (Sun-synchronous) counterparts.

[3] The nonmigrating tide that has so far attracted most attention is DE3 because it is the single largest component during most parts of the year and because it is the likely cause for the so-called “wave-4” signature observed by Sun-synchronous satellites in equatorial F-region parameters [e.g., Immel et al., 2006; Hagan et al., 2007]. Here, we denote a tidal component by a letter/number code. DWs or DEs is a westward or eastward propagating diurnal tide, respectively, with zonal wavenumber s. For semidiurnal tides, “D” is replaced by “S”, and D0, S0 are zonally symmetric oscillations. Hence, the migrating diurnal and semidiurnal tides are DW1 and SW2, respectively. All other components are nonmigrating tides, i.e., DE3 is the eastward propagating diurnal tide with zonal wavenumber 3.

[4] As the tidal temperature and wind perturbations are large in the MLT region, and even responsible for F-region variations, it is now reasonable to assume that they will also induce some composition variability in the E-region. To the best of our knowledge, this has so far only been studied for migrating tides [e.g., Marsh and Russell, 2000; Ward, 1999] but not for nonmigrating tides. The purpose of this work is therefore to identify such nonmigrating tidal signatures in thermospheric composition, analyze their seasonal variations and investigate their likely origin.

[5] The composition data used here come from more than three years of nitric oxide (NO) density observations made by the SNOE satellite [Barth et al., 2003; Barth and Bailey, 2004]. Hence, the appropriate tidal parameter to compare with is neutral gas density and not temperature or wind.

2. Data and Analysis

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Analysis
  5. 3. Results
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[6] The tidal neutral gas density variations are inferred from the above-mentioned 2002–2006 monthly mean climatologies of diurnal and semidiurnal temperature and horizonal wind tides as measured by the SABER and TIDI instruments on the TIMED satellite. They are referred to as tidal densities and are computed as exemplified by Oberheide and Forbes [2008] for the DE3 component but now extended to the full set of tidal components included in the TIMED climatologies.

[7] Figure 1 shows the resulting seasonal evolution at 0° latitude and 105 km altitude, given as percentage variations of the background density. The migrating tides DW1 and SW2 are only shown for comparison purposes and are not further discussed as they vanish in the tidal fields discussed below (see section 3 for reasoning). The single most important nonmigrating tide is DE3 that by far exceeds all other components between April and November with a pronounced late summer maximum but a deep winter minimum. Note that DE3 is then exceeded by DE2 and DW2. The leading semidiurnal components during winter are SW2 and SE1 with SE2 generally more important during summer.

image

Figure 1. Time evolution of (a) diurnal and (b) semidiurnal tidal density amplitudes at 0° latitude and 105 km altitude as inferred from TIMED observations. Diurnal and semidiurnal components with the same color/line style have the same zonal wavenumber (number in parentheses, 0–4) when observed from a Sun-synchronous satellite. Components not shown are small.

Download figure to PowerPoint

[8] The SNOE version 2, level 3 NO densities analyzed here have been measured between 11 March 1998 and 30 September 2000 [Barth et al., 2003; Barth and Bailey, 2004]. They can be downloaded at http://lasp.colorado.edu/snoedata/ on a 10° latitude grid between 90°S–90°N with 24° longitude resolution, and between 97–150 km altitude with 3.3 km vertical resolution. Since SNOE was in a Sun-synchronous orbit and its NO measurements were limited to daylight, all data have been collected at almost the same local solar time (LST), that is, close to 10.5 hour between 10.3 hour (March 1998) and 11.2 hour (September 2000).

[9] The constant LST of the NO data prevents a full tidal (i.e., wavenumber/frequency) analysis because tides are LST dependent. It nevertheless provides an excellent opportunity to study longitudinal variations introduced by tides because the observed phasing of a single tidal component will not change as a function of longitude.

[10] In this work, tides in the SNOE data are studied on a climatological basis, that is, all SNOE data are averaged into composite monthly means. This averaging preserves the tidal signatures because the LST does not change from one day to another. There will be some inherent smoothing of the tidal longitude structure because of short-term and inter-annual variability but this is just as for the TIMED tidal climatologies used for the tidal density computation.

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Analysis
  5. 3. Results
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[11] The top plots of Figure 2 show two examples of the longitude/altitude structure of the monthly-averaged NO densities. A pronounced longitudinal variation that closely resembles a zonal wavenumber 4 (wave-4) is clearly visible in the fall equinox data (September, left). In contrast, the winter solstice (December, right) variation is less pronounced and more zonal wavenumber 3 (wave-3) like. The middle plots show the same data but as percentage residuals, that is, after subtraction of and division by the zonal mean. Peak-to-peak variations can approach 40% within 40–60° longitude intervals. The wave-4/wave-3 structure in the September/December data residuals can be followed up to about 135 km. At altitudes above, the leading zonal wavenumber is more difficult to identify but it generally becomes wave-2 like.

image

Figure 2. Monthly-averaged (top) NO observations from SNOE, (middle) NO residuals from SNOE, and (bottom) tidal density residuals from TIMED at 0° latitude for (a) September and (b) December. Local solar time is 10.5 h. The wave-4 structure in the September NO residuals changes to wave-3 in December and can be followed up to 135 km. Longitudes of relative minima and maxima agree well with the TIMED tidal density residuals in the 100–135 km altitude range.

Download figure to PowerPoint

[12] The bottom plots of Figure 2 show the corresponding TIMED tidal density residuals for 10.5 hour LST. They are reconstructed from the full set of diurnal and semidiurnal density components by accounting for their relative phasing and by removing their zonal mean as for the NO residuals. The TIMED density residuals closely resemble the wave-4/wave-3 pattern observed in the 100–135 km SNOE data. Even the longitudes of relative minima/maxima, that is, the tidal phases, match each other. The quantitative agreement between the residual magnitudes around 110 km altitude is remarkable and still reasonable for altitudes above and below.

[13] Since this striking match strongly suggests that the longitudinal variations in the 100–135 km NO data are caused by tidal density variations, one has to remember how tides are observed from Sun-synchronous satellites. Migrating (Sun-synchronous) tides are always measured in the same phase, that is, as a zonally symmetric feature because these waves move westward with the relative motion of the Sun just as the satellite orbit does. The observed zonal wavenumber of a nonmigrating tide also differs from its true zonal wavenumber. A westward (eastward) propagating tide of zonal wavenumber s is observed as a zonal wavenumber s − 1 (s + 1) if diurnal and as s − 2 (s + 2) if semidiurnal.

[14] Hence, the removal of the zonal means in the SNOE and TIMED residuals also removes the migrating tides from both data sets and any tidal signatures are caused by nonmigrating tides alone. The time evolution of the tidal density components in Figure 1 now suggests that the September wave-4 signature is predominantly caused by DE3 and the December wave-3 signature by DE2 with some SE1 contributions. However, this tidal interpretation needs to be verified at other latitudes since tides are global phenomena.

[15] Figure 3 thus provides latitude/longitude cuts at 107 km (SNOE) and 105 km (TIMED) for the same two months. Nitric oxide residuals at latitudes poleward of 30° are affected by geomagnetic effects and cannot be analyzed for tides. However, the wave-4 signature in September (left) persists at all latitudes shown and is well reproduced by the tidal density residuals. The change to wave-3 in December (right) is less pronounced at latitudes poleward of 20°S where a wave-2 like signature is observed and where the good agreement with the tidal density residuals deteriorates. Whether this is a tidal effect or whether it is caused by non-tidal (i.e., geomagnetically driven) effects has not yet been resolved. Nevertheless, the horizontal structure of the NO residuals and their overall excellent agreement with the tidal density residuals gives further confidence in the tidal interpretation.

image

Figure 3. Monthly-averaged (top) NO residuals at 107 km from SNOE and (bottom) tidal density residuals at 105 km from TIMED for (a) September and (b) December. Local solar time is 10.5 h. The wave-4 structure in the September NO residuals persists at all latitudes shown. The change to wave-3 in December occurs in the whole latitude range although it is less pronounced poleward of 20°S. Longitudes of relative minima and maxima agree well with the TIMED tidal density residuals.

Download figure to PowerPoint

[16] This is further supported when examining the data during the reminder of the year (Figure 4). A clear wave-4 signature dominates the NO and tidal density residuals between March and October as it is expected since DE3 is the single most important nonmigrating tide then (Figure 1). December and January are wave-3 like because the combined DE2 and SE1 response (both are observed as a wave-3) is ∼5% and therefore larger than all other components. Figure 4 also provides the diurnal and semidiurnal contributions to the tidal densities. While semidiurnal tides are comparatively less important between March and April, their contribution equals or even exceeds those of the diurnal tides during winter. The NO residuals in February and November are more irregular than during other months as they are observed in the wave-3/wave-4 transition periods. Figure 1 shows that a number of diurnal and semidiurnal components are about equally large then. Hence, their superposition will be very sensitive to even small changes in their relative contributions. Considering that, NO and tidal density residuals in February and November still compare remarkably well although with larger differences than during the reminder of the year.

image

Figure 4. Equatorial NO residuals at 107 km (thick red line, SNOE), tidal density residuals at 105 km (thick blue line, TIMED). Diurnal (thin dotted blue line) and semidiurnal (thin dashed blue line) contributions are also shown. Agreement is obtained throughout the year, with somewhat larger differences in February and November when the transition between wave-4 and wave-3 takes place.

Download figure to PowerPoint

4. Discussion and Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Analysis
  5. 3. Results
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[17] To investigate potential geomagnetic and solar effects on the results, the whole analysis was repeated for low Ap and F10.7 levels (not shown). About 1/3 of the SNOE data are for Ap ≤ 6 and F10.7 ≤ 132 [Barth et al., 2003]. Using SNOE data within these limits changed some details in the vertical and horizontal NO residual structures but the overall effect was small and the level of agreement with the TIMED tidal densities remains unaffected.

[18] Removing the zonal mean from the NO densities as function of geomagnetic instead of geographic latitude (not shown) partly removed the odd December signal in the 20°S–30°S latitude band (Figure 3b) which may be a hint for its geomagnetic origin. But more important, it often disturbed the wave-4 and wave-3 signatures in the equatorial NO residuals. This comes not unexpected as the natural tidal coordinates are geographical.

[19] Above 135 km, the tidal densities do not longer match the NO residuals. As this occurs as a sudden effect in a thin layer about 5–10 km thick, this may be indicative of non-tidal effects becoming important. A closer investigation of their nature and origin is beyond the scope of this work although altitude-dependent NO production processes may play a role [e.g., Siskind and Rusch, 1992].

[20] Between 100–135 km, however, the longitude structure in the low-latitude NO residuals is clearly a result of nonmigrating tidal density variations. To the best of our knowledge, this is the first report and interpretation of these striking variations that can be as large as 40% (peak-to-peak) in the vicinity of the NO maximum between 105 and 110 km. DE3 introduces the wave-4 like signature between March and October. DE2 and SE1 introduce the wave-3 like signature in December and January. This demonstrates the tropospheric influence on thermospheric NO since DE3, DE2, SE1 are all forced by latent heat release in deep convective tropical clouds [Oberheide et al., 2006, 2007]. November and February are transition months between wave-4 and wave-3 since a number of nonmigrating tidal components are about equally important then. The nonmigrating tide signatures are similar to the equatorial NO density variations as function of solar activity reported by Barth et al. [2003, Figure 6]. Hence, the nonmigrating tides are a first order effect in understanding NO variability on time scales of a few days or less. The implications need to be further investigated, i.e., how the tidally induced NO variations may affect Earth's upper atmospheric energy budget since NO acts as the atmosphere's natural thermostat via its 5.3 μm emissions [Mlynczak et al., 2007].

[21] Our results have significant wider implications. First, the results presented here confirm, in a sense even validate, the tidal densities inferred from the TIMED tidal climatologies. More importantly, large longitude variations due to nonmigrating tides may exist in many other quantities of aeronomic interest and importance in the lower thermosphere. Based on previous studies devoted to the effects of migrating tides [e.g., Forbes and Hagan, 1980; Forbes, 1981; Angelats i Coll and Forbes, 1998] significant longitudinal variations may be present in atomic oxygen densities, O/N2 ratios, 5577 Å green-line and OH emissions, D- and E-region ion densities, to name a few. Finally, since the peak tidal density variations occur in a height region important for re-entry vehicles, they need to be included into forthcoming improvements of corresponding drag models.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Analysis
  5. 3. Results
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[22] We gratefully acknowledge the SNOE team for providing the NO data. JO is supported under DFG CAWSES grant OB299/2-2. JMF is supported under AFOSR MURI grant FA9550-07-1-0565 and grant NNX07AB74G from the NASA TIMED Program.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Analysis
  5. 3. Results
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References