Results and Discussion

where n_e,n and n_e,s are the electron densities of the north and south peaks and n_e,t is the elec-tron density of the equatorial trough. The elecelec-tron density profile at each longitude can then be characterized by a single number. Note that the CTR is set to 1 when the EIA has not been well es-tablished at satellite altitudes. Alternatively, the latitude separation of the EIA peaks may be used to characterize the strength of the EIA. We have obtained similar results using both approaches, and have choosen to use the CTR for the present analysis.

3.1.2.2 Zonal Wind Amplitudes from TIMED

The nonmigrating tidal amplitudes of the equatorial zonal wind at 100 km are determined using a Hough Mode Extension (HME) analysis of temperature and zonal and meridional wind measurements from the SABER and TIDI instruments onboard the TIMED satellite [Oberheide and Forbes, 2008b]. An altitude of 100 km is used due to superior data quality at this altitude.

Since the nonmigrating tides are vertically propagating, observations at 100 km can be viewed as representative of the nonmigrating tides entering the dynamo region. A five-year mean (2002-2006) is used in order to determine the month-to-month variability in the leading nonmigrating tidal components. The equatorial zonal wind results are representative of the broader latitude region between ±30^◦ latitude.

3.1.3 Results and Discussion

The month-to-month variation of the equatorial zonal winds at 100 km for the leading non-migrating tidal components is shown in Figure 3.1. The leading tidal components are DE2, DE3, and SW 4 which when viewed at a fixed local time appear as wave-3, wave-4, and wave-2, respec-tively. The amplitude of each nonmigrating tide exhibits significant month-to-month variability.

In particular, it should be noted that during January the leading tidal component is SW 4 followed by DE2 and DE3. During March-November DE3 is greatest followed by DE2 and SW 4. In

December DE2 is the primary component followed by SW 4 and DE3. Given the premise that the E-region tides are expected to induce a longitudinal dependence in the dynamo electric fields, one may expect to observe a similar variability in the longitudinal structure of the EIA. Therefore, the dominant longitudinal structure of the EIA is expected to be wave-2 in January, wave-4 in July, and wave-3 in December. Other tidal components, many of which appear as wave-2, -3, and -4, are also present and we have focused here on the three leading tidal components. A detailed discussion of the various nonmigrating tides that appear as wave-2, -3, and -4 and the relative strengths of each is given in Forbes et al. [2008].

Representative global electron density maps for the four time periods considered are displayed in the left panel of Figure 3.2. A Fourier fit is performed using the CTR values at each longitude and the resulting wavenumber spectra are shown in the right panel. Similar to the variations seen in the zonal wind amplitudes, the primary component of the CTR amplitude exhibits intra-annual variability. In January 2003, the dominant component is wave-2 followed by wave-3 and wave-4 is the weakest component. The primary component changes to wave-4 in July 2004 and a similar amplitude is seen for waves 1-3 during this time period. In December 2004 and 2005 the dominant wavenumber is 3 followed by wave-2, -1 and -4. There are also significant changes in the amplitudes from December 2004 to December 2005 which is thought to be due to changes in the orbital altitude of the CHAMP satellite and/or changes in the level of solar activity (see Table 3.1).

Although the primary focus is on SW 4, DE2, and DE3 and their corresponding signatures in the ionosphere, a significant wave-1 component is also present. The offset of the geomagnetic field with respect to the geographic equator and longitudinal variation in the neutral winds create hemispheric asymmetries in the longitudinal structure of the ionosphere [McDonald et al., 2008].

The resulting hemispheric asymmetries will impact the CTR value and may be partly responsible for the observed wave-1 component. A wave-1 component can also result from the nonmigrating tides D0, DW 2, SW 1, and SW 3. These tides arise from nonlinear interaction of SP W 1 with DW 1 and SP W 1 with SW 2 [Angelats i Coll and Forbes, 2002; Hagan and Roble, 2001]. The months in question overlap with SP W 1 activity in the respective winter hemispheres indicating that the

Figure 3.1: Month-to-month variability in the equatorial zonal wind nonmigrating tidal amplitudes at 100 km derived from TIMED. Data represents a five-year mean from 2002-2006. The bracketed value indicates the longitudinal structure that is observed when the nonmigrating tide is viewed at a fixed local time.

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Figure 3.2: 10-day mean CHAMP in-situ electron densities. Left panel is data that has been binned in latitude and longitude and averaged. The right panel is the CTR wavenumber spectrum. Note that a different color scale is used for each figure.

observed wave-1 may partly be due to these nonmigrating tides.

We now turn our attention to discuss the results shown in Figure 3.2 in the context of Figure 3.1. Assuming that SW 4, DE2, and DE3 are comparable in terms of their efficiency in generating electric fields, we may expect similar seasonal variations in the CTR wavenumber spectrum and the zonal wind wavenumber spectrum at 100 km. This is a reasonable assumption considering the vertical structure of these tides presented by Forbes et al. [2008] indicates similar efficiency of electric field generation for these nonmigrating tides. In January the leading component in the zonal wind amplitude is SW 4 (wave-2) which is also the dominant wavenumber of the CTR. This demonstrates that SW 4 may be cabable of modulating the dynamo electric fields in order to create a wave-2 longitudinal structure in the EIA during January. During this same time period, the second leading component is observed to be wave-3 in both the zonal wind amplitude as well as the CTR. A wave-4 (DE3) structure dominates the zonal wind amplitude in July and this is also the dominant wavenumber of the CTR. In July 2004 the CTR amplitude for wave-2 and wave-3 are equivalent whereas from Figure 3.1 it is expected that wave-3 be greater than wave-2. The results shown in Figure 3.1 represent a five year mean and this discrepancy may be the result of year-to-year variability in the zonal wind. Lastly, a wave-3 structure is observed to be the primary component of both the zonal wind amplitude and the CTR during December. In both December 2004 and December 2005, wave-2 is the second major component of the CTR which is in agreement with the zonal wind amplitudes. Similar to SW 4 in January, this demonstrates that DE2 is likely able to influence the dynamo electric fields in order to create a wave-3 structure in the EIA during December.