Correlations Between the Thermosphere's Semiannual Density Variations and Infrared Emissions Measured With the SABER Instrument

Abstract

This paper presents measurements of the amplitudes and timings of the combined, annual, and semiannual variations of thermospheric neutral density, and a comparison of these density variations with measurements of the infrared emissions from carbon dioxide and nitric oxide in the thermosphere. The density values were obtained from measurements of the atmospheric drag experienced by the Challenging Minisatellite Payload, Gravity Recovery and Climate Experiment A, Gravity field and Ocean Circulation Explorer, and three Swarm satellites, while the optical emissions were measured with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. These data span a time period of 16 years. A database containing global average densities that were derived from the orbits of about 5,000 objects (Emmert, 2009, https://doi.org/10.1029/2009JA014102, 2015b, https://doi.org/10.1002/2015JA021047) was employed for calibrating these density data. A comparison with the NRLMSISE-00 model was used to derive measurements of how much the density changes over time due to these seasonal variations. It is found that the seasonal density oscillations have significant variations in amplitude and timing. In order to test the practicality of using optical emissions as a monitoring tool, the SABER data were fit to the measured variations. Even the most simple fit that used only filtered carbon dioxide emissions had good correlations with the measured oscillations. However, the density oscillations were also well predicted by a simple Fourier series, contrary to original expectations. Nevertheless, measurements of the optical emissions from the thermosphere are expected to have a role in future understanding and prediction of the semiannual variations.

Keywords: satellite drag; semiannual variation; thermosphere composition; thermosphere emissions; thermospheric dynamics.

Figure 1

Global totals of nitric oxide (NO) and carbon dioxide (CO₂) emissions measured by the SABER instrument. The thin green and brown lines show the daily mean power, in gigawatts, for NO and CO₂, respectively. The thicker lines having similar colors show the same data after smoothing with a moving average of 31 days. The superposed, black lines show the result of similar smoothing with a yearly average. SABER = Sounding of the Atmosphere using Broadband Emission Radiometry.

Figure 2

From Emmert (2015b), global average thermospheric mass density derived from the orbits of about 5,000 objects, obtained from orbit data from 1967 to 2013. Results are graphed for an altitude of 400 km, with the green line showing daily values, monthly running averages in orange, and yearly running averages in blue.

Figure 3

(blue) Natural log of the ratio of CHAMP densities with respect to MSIS. (red) Corresponding TLE log‐ratios, interpolated to the altitude of CHAMP. (black) The difference between the TLE and CHAMP log‐ratios. CHAMP = Challenging Minisatellite Payload; TLE = two‐line element.

Figure 4

(blue) Natural log of the ratio of GRACE A densities with respect to MSIS. (red) Corresponding TLE log‐ratios, interpolated to the altitude of CHAMP. (black) The difference between the TLE and GRACE A log‐ratios. GRACE A = Gravity Recovery and Climate Experiment A; TLE = two‐line element.

Figure 5

(blue) Natural log of the ratio of GOCE densities with respect to MSIS. (red) Corresponding TLE log‐ratios, interpolated to the altitude of CHAMP. (black) The difference between the TLE and GOCE log‐ratios. GOCE = Gravity field and Ocean Circulation Explorer; TLE = two‐line element.

Figure 6

(blue) Natural log of the ratio of Swarm A densities with respect to MSIS. (red) TLE log‐ratios, interpolated to the initial altitude of Swarm A. (green) Result from fitting the red line to the SABER global NO and CO₂ emissions. (black) The difference between the green and blue lines. SABER = Sounding of the Atmosphere using Broadband Emission Radiometry; TLE = two‐line element.

Figure 7

Amplitude response of the digital band‐pass filter.

Figure 8

Percentage change in atomic oxygen needed in the MSIS model to match the measured densities, after band‐pass filtering. Each line color represents the results from a different satellite, indicated within the legend in the box. The brown line at the bottom shows the solar F _10.7 index. GRACE A = Gravity Recovery and Climate Experiment A; CHAMP = Challenging Minisatellite Payload; GOCE = Gravity field and Ocean Circulation Explorer.

Figure 9

Logarithm of the ratio of the measured and MSIS model densities, both mapped to 400 km, after band‐pass filtering. Each line color represents the results from a different satellite, indicated within the legend in the box. GRACE A = Gravity Recovery and Climate Experiment A; CHAMP = Challenging Minisatellite Payload; GOCE = Gravity field and Ocean Circulation Explorer; SAV = semiannual variation.

Figure 10

The measured CO₂ and NO emissions, after band‐pass filtering. SABER = Sounding of the Atmosphere using Broadband Emission Radiometry.

Figure 11

Result of fitting the semiannual variations with SABER measurements. The light‐blue, green, and dark‐blue lines show the measured semiannual variations from the CHAMP, GOCE, and Swarm A satellites. The graph shows the percentage change in atomic oxygen needed in the MSIS model to match the measured densities, after band‐pass filtering. The orange line shows a simple fit using only CO₂ measurements, and the red line shows the best fit that uses both NO and CO₂ measurements. The black line shows the result of fitting the measurements with a simple Fourier series, a function of time only. Only the results from CHAMP and GOCE, up to 2013, were used in the fits, while the Swarm A results are used for comparison. The 16‐year time period has been divided into four rows, as indicated with the dates on each horizontal axis, to improve the resolution. CHAMP = Challenging Minisatellite Payload; GOCE = Gravity field and Ocean Circulation Explorer.