SEARCH

SEARCH BY CITATION

Keywords:

  • planetary waves;
  • ionosphere;
  • equatorial ionization anomaly

Abstract

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

[1] In situ electron density measurements from the CHAMP satellite and global positioning system total electron content (TEC) observations are used to illustrate how quasi-16-day planetary wave activity influences the structure of the low-latitude ionosphere. SABER temperature measurements reveal that quasi-16-day oscillations in the dynamo region (110 km) during the time interval of 1 December 2005 to 1 March 2006 are most dominant in the zonal mean. Enhancements in equatorial zonal mean temperatures in the E-region are connected with enhancements at ±10–20° magnetic latitude in zonal mean CHAMP electron densities and TEC. At a near-constant altitude of 350 km, the oscillations in daytime electron densities are greater than 40 percent of background levels. A local time effect is also observed in the planetary wave influence on the F-region electron densities. These results demonstrate that vertically propagating planetary waves induce significant variability in the low-latitude F-region ionosphere and that these effects are seen at all longitudes.

1. Introduction

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

[2] Oscillations at F-region altitudes at periods of 2, 5, 6–7, 9–10, and 12–18 days have been attributed to planetary wave activity in the middle atmosphere [Altadill et al., 2001, 2003; Forbes and Leveroni, 1992; Forbes et al., 1997; Lastovicka et al., 2003]. Although several mechanisms have been proposed (see review by Forbes [1996]), the manner in which planetary waves drive ionospheric variations still remains unknown. A key problem is that planetary waves are not capable of propagating above about 100–110 km [Pogoreltsev et al., 2007], and therefore more indirect scenarios, such as modulation of dynamo-generated electric fields that map into the F-region, appear to be required.

[3] Oscillations at planetary wave periods in the equatorial ionization anomaly (EIA) have been linked to changes in E-region winds which, through the dynamo mechanism, create oscillations in the strength of the equatorial electric field [Chen, 1992; Forbes and Leveroni, 1992]. The effect of planetary waves on the low-latitude F-region ionosphere has typically been studied using either a single ground-based location or a chain of stations at a fixed longitude [Chen, 1992; Forbes and Leveroni, 1992; Yi and Chen, 1993]. The latitudinal and longitudinal extent to which planetary waves influence the low-latitude ionosphere is therefore less well studied. At mid-latitudes, the quasi-16-day oscillation is typically a global scale phenomena, while shorter period planetary waves are more limited in longitude [Altadill and Apostolov, 2003; Lastovicka et al., 2006]. Using total electron content (TEC) observations at mid-latitudes, Borries et al. [2007] found planetary wave periodicities to be most dominant in the zonal mean. However, these periodicities were attributed to periodic geomagnetic disturbances as opposed to vertically-propagating waves. Vineeth et al. [2007] recently demonstrated that the occurrence of quasi-16-day oscillations in mesospheric winds during December 2005–March 2006 were linked to similar periodicities in the equatorial electrojet (EEJ). Due to the close relationship between EEJ electric fields and those driving the EIA, one might expect that quasi-16-day variations also occurred in the F-region ionosphere during this period. The question is, what are the magnitude and global extent of these F-region effects?

[4] The objective of the present study is thus to examine the extent to which the quasi-16-day planetary wave influences the low-latitude F-region. We use in situ electron density measurements from the CHAllenging Minisatellite Payload (CHAMP) satellite and global positioning system (GPS) TEC to demonstrate that the quasi-16-day oscillation during the time period of December 2005 to March 2006 influenced the zonal mean F-region ionosphere. (It was presumed by Vineeth et al. [2007] that the quasi-16-day oscillation penetrated into the 100–170 km dynamo region in order to modulate the EEJ electric field). We also utilize TIMED/SABER temperature data to verify that a quasi-16-day periodicity existed within the dynamo region during this period. Additionally, the local time dependence of the planetary wave effect is explored.

2. Data and Methods

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

[5] In order to observe the effect of planetary wave activity in the dynamo region we analyze SABER temperature data. The SABER temperature data consists of residuals from the tidal fits that are presented by Forbes et al. [2008]. These residuals are spectrally analyzed with respect to time and longitude, and average amplitudes calculated within the 12–18 day period range are used to provide a measure of the strength for the quasi-16-day wave for longitudinal wavenumbers s = 0, 1, 2 and 3.

[6] The Planar Langmuir Probe onboard CHAMP provides in situ electron density measurements at 15-second intervals (∼120 km. in-track distance). The electron density measurements are at a near constant height and thus are sensitive to changes in the F-layer peak height. During late 2005 and early 2006 the equatorial height of CHAMP was around 345 km. There is also an approximately 10 km. change in altitude between −30° and +30° latitude which may introduce a slight hemispheric asymmetry. Despite these challenges, CHAMP measurements of electron density are advantageous due to its near-polar, Sun-synchronous orbit that precesses in local time at the rate of ∼5.44 minutes per day. This means that CHAMP samples 15–16 different longitudes per day giving a global view of the ionosphere. The changing local time of the measurements introduces a trend in the data when an extended time period is considered. To eliminate this trend a cubic polynomial is fit to daily zonal mean electron densities and we analyze the residuals to determine the effect of planetary wave activity.

[7] To complement the CHAMP in situ electron density observations, TEC measurements at a fixed local time are also analyzed. The TEC measurements come from the International GNSS Service (IGS) global ionosphere maps (GIMs) [Dow et al., 2005]. The GIMs are obtained from a global network of GPS receivers and have a temporal resolution of 2 hours and spatial resolution of 5° in longitude and 2.5° in latitude. The TEC observations at a fixed local time are analyzed in an identical manner to the CHAMP electron densities.

3. Results and Discussion

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

[8] Average SABER temperature amplitudes at 110 km for 12–18 day periods and zonal wavenumbers s = 0 and s = 1, from 1 December 2005 to 31 March 2006, are shown in Figure 1. The SABER temperature amplitudes exhibit a general intensification of activity during January and February 2006, similar to the interval of time identified by Vineeth et al. [2007]. The enhancements are much more prominent in the s = 0 component as opposed to s = 1, and are not seen in s = 2 and s = 3 (not shown). This demonstrates that in the dynamo region all longitudes appear to be equally influenced by the planetary wave oscillation.

image

Figure 1. (a) s = 0 and (b) s = 1 SABER temperature average amplitudes with 12–18 day periods at 110 km.

Download figure to PowerPoint

[9] Connections between temperature perturbations in the E-region and electron densities in the F-region have previously been hypothesized through changes in the strength of the dynamo-generated electric fields thought to be generated by the accompanying wind fields [England et al., 2006; Immel et al., 2006]. Given the quasi-16-day oscillations in SABER zonal mean temperatures at 110 km, we expect similar periodicities to be present in the low-latitude F-region ionosphere. Lomb-Scargle periodogram [Lomb, 1976; Scargle, 1982] analyses presented in Figure 2 indeed reveals quasi-16-day periodicities in CHAMP electron densities and TEC. Significant differences are apparent in the strength of the quasi-16-day periodicity between the magnetic equator and 15° magnetic latitude and between daytime and nighttime which will be discussed later. Figures 2a and 2b also reveal elevated power within the 12–18 day period band in Kp and SOHO 0.1–50 nm EUV flux, respectively. A larger peak in Kp is observed at 9-days which is attributed to periodic forcing due to solar wind high-speed streams during late 2005 and early 2006 [Thayer et al., 2008]. The 9-day period in Kp is stronger than the peak near 16-days, while the opposite is true in electron densities and TEC. Energy near quasi-16-day periods is also observed in SOHO EUV measurements. We have compared the 27-day oscillations in SOHO EUV and daytime CHAMP electron densities and established a linear relationship between the two. Applying this relationship to the quasi-16-day oscillation, the EUV flux variability is not able to account for the corresponding variability in electron density. We therefore conclude that the quasi-16-day oscillations observed in the ionosphere are due to coupling with the lower atmosphere and not recurrent geomagnetic activity or solar flux variability.

image

Figure 2. Lomb-Scargle periodograms during the time period of 1 Dec. 2005–1 Mar. 2006. (a) Kp (b) SOHO EUV flux in 0.1–50 nm (c) CHAMP in situ electron density for ascending portion of the orbit at the magnetic equator (solid) and 15° magnetic latitude (dashed). (d) Same as Figure 2c except for the descending portion of the orbit. (e) TEC at 10 LT at the magnetic equator(solid) and 15° magnetic latitude (dashed). (f) Same as Figure 2e except for 22 LT.

Download figure to PowerPoint

[10] We now turn our attention to the temporal and local time response of the low-latitude ionosphere. Raw and residual CHAMP electron densities for the descending portion of the orbit are given in Figures 3a and 3b, respectively. To emphasize the quasi-16-day periodicity, a bandpass filter centered at 15 days with half-power points at 12 and 18 days is applied. The bandpass filtered residuals are shown in Figures 3c and 3d for the descending (daytime) and ascending (nighttime) portions of the orbit. Enhancements in the EIA crests that are greater than 45% of the cubic trend removed from the data are observed in the daytime around days 31–33, 46–48, and 60–62. These enhancements are associated with 15–20% decreases in the equatorial electron density. During these same time periods, 15–20% decreases are also observed in the nighttime electron densities between −15° and +15° magnetic latitude. The timing of these enhancements is similar to the enhancements observed in the SABER planetary-wave-period temperature amplitudes with the exception of the peak in electron density at days 31–33 which may be due to differences in local time between the measurements. This does, however, demonstrate that the enhancements in temperature observed in the dynamo region are likely connected with changes in the strength of the electric field which creates the changes in the strength of the EIA observed by the CHAMP satellite.

image

Figure 3. (a) Zonal mean CHAMP in situ electron densities for the descending portion of the orbit prior to the removal of a cubic polynomial. (b) Electron density residuals for the descending portion of the orbit expressed as a percentage of the trend. (c) Bandpass filtered electron density residuals for the descending portion of the orbit expressed as a percentage of the trend. The LT of equatorial crossing is overlayed. (d) Same as Figure 3c except for the ascending portion of the orbit. (e) Bandpass filtered TEC and CTR at 10 LT. Filtered TEC is expressed as a percentage of the cubic trend that was removed. (f) Same as Figure 3e except for at 22 LT.

Download figure to PowerPoint

[11] The CHAMP observations are at a near constant height and the oscillations in the EIA could be the result of only changes in the F-layer peak height. We have analyzed GPS TEC measurements in order to demonstrate how the planetary wave activity influences the ionosphere in an integrated sense and to examine the response at fixed local times. Bandpass filtered TEC at 10 LT and 22 LT are shown in Figures 3e and 3f, respectively. We have also overlayed the bandpass filtered crest-to-trough ratio (CTR) which provides a means of assessing the strength of the EIA [Mendillo et al., 2000]. The bandpass filtered CTR shows that changes in the strength of the EIA at quasi-16-day periods occurs during the time period when similar periodicities were observed in SABER zonal mean temperatures and CHAMP electron densities. The enhancements observed in TEC during this time period are only 15% of the background level, which is significantly smaller than in electron density at a near-constant height. In the anomaly crest regions at ±10–20° magnetic latitude, a significant local time effect is observed. At these latitudes, daytime TEC increases are nearly inphase with nighttime decreases in TEC. We have looked at other local time pairs and observed similar anti-correlation between daytime and nighttime TEC oscillations.

[12] One feature that is particularly interesting is the larger oscillations in nighttime TEC compared to daytime TEC. This is opposite to what is observed in CHAMP electron densities, where the daytime enhancements were significantly greater than those at night. Vineeth et al. [2007] observed quasi-16-day enhancements in the strength of the CEJ during this time period and this may be related to the difference between TEC and CHAMP observations. The days with enhanced CEJ will produce greater downward E × B drift during the evening hours. Downward drifts will increase the loss rate, generating the periodic decreases that are observed in TEC. The difference between the strength of the daytime and nighttime TEC oscillations may be due to the relative strength of the EEJ/CEJ as well as the smaller background TEC levels at night. The increased downward drifts associated with the CEJ will also suppress the height of the F-layer. This will alter the height of the electron density observations with respect to the F-region peak and is thought to be responsible for producing the smaller nighttime modulations in CHAMP electron densities compared to the daytime.

4. Conclusions

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

[13] The results presented provide insight into the extent to which planetary waves may influence the low-latitude ionosphere. Whereas previous analyses have focused on a single longitude [e.g., Chen, 1992; Forbes and Leveroni, 1992; Vineeth et al., 2007], we have demonstrated that at low-latitudes the quasi-16-day planetary wave appears to be a global phenomena that influences the ionosphere at all longitudes in a similar manner. This is consistent with the longitudinal extent of the quasi-16-day planetary wave influence on F-region electron densities at mid-latitudes [Altadill and Apostolov, 2003; Lastovicka et al., 2006]. Quasi-16-day enhancements in zonal mean temperature in the dynamo region are connected with similar oscillations in the strength of the EIA, further demonstrating the strong coupling between the lower-atmosphere and the ionosphere. At a constant height, these oscillations are significant and reach 45% of the background electron density. There is also a significant local time effect with the planetary wave activity producing daytime enhancements and nighttime decreases in the EIA crest region.

[14] In the present analysis we have explored the effect of quasi-16-day oscillations during the time period of 1 December 2005 to 1 March 2006. A sudden stratospheric warming (SSW) also occurred during this time period and Chau et al. [2009] recently demonstrated a connection between SSWs and the low-latitude ionosphere. The quasi-16-day oscillations in the low-latitude F-region electron densities and the SSW may be related, however, this is beyond the scope of the present study. As we have only studied a single event, it is unclear whether the features we have observed are a consistent feature of how planetary waves influence the low-latitude ionosphere. Moreover, we have only examined the effect of the quasi-16-day wave and whether or not planetary waves of other periodicities exhibit similar characteristics is unclear. Shorter period planetary waves may influence the low-latitude ionosphere on a less global scale [Altadill and Apostolov, 2003; Lastovicka et al., 2006]. A detailed analysis of the longitudinal extent that planetary waves of different periods influence the low-latitude F-region ionosphere is necessary in order to improve our understanding of how vertically propagating waves influence the low-latitude ionosphere.

Acknowledgments

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

[15] This work is supported by NASA grant award NNX08AF22G under the TIMED Guest Investigator Program. The authors thank the various providers of data used in the present study.

References

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