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How Does Nitric Oxide Exhibit Strong Solar-geospace Coupling in Thermosphere?
Date: 2018/04/04 Author: TANG Chaoli

Recently, researchers at Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institute of Physical Science have investigated the global variations of nitric oxide infrared radiative cooling rate (NO-IRCR) and its response to solar and geomagnetic activity in the thermosphere during 2002-2017. The response ranges of the NO-IRCR to solar and geomagnetic activity at different altitudes and latitudes are first reported.

Nitric oxide (NO) is one of the most significant constituents in cooling the thermosphere via infrared radiation (IR) above 120 km, especially during solar storm time. NO emission at 5.3 μm is the major infrared radiative cooling mechanism in the thermosphere. The NO infrared radiative cooling plays a critical role on the atmospheric thermal structure.

The energy balance and thermal structure of upper atmosphere are dominated by dynamics and solar activity cycle. Incoming solar radiation provides the external forcing for the thermosphere atmosphere system. The NO-IRCR in the thermosphere atmosphere is greatly affected by solar variability. The response of NO-IRCR to solar activity affects the energy budget balance.

On the basis of a large volume of data recorded in the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) dataset, researchers statistically found that the latitudinal distribution characteristics of NO-IRCR are nearly symmetric between the two hemispheres shown in fig.1a.

In addition, they found that the observed interannual variations in the NO-IRCR clearly follow the 11 solar cycle in the thermosphere shown in fig.1b. And the close relationship between NO-IRCR and solar activity in thermosphere regions has been found. For example, at 175km, the correlation coefficient between NO-IRCR and F10.7 (which is an index of solar activity) is as high as 0.97 (see Fig.1c).

The 95% confidence limits of NO-IRCR to solar activity and geomagnetic activity were also investigated by a multiple linear regression fit using the F10.7 and Kp indices shown in fig.1d.

The results show that the variation of NO-IRCR is dominated by solar activity between ~190 km and ~225 km, and there is a strong geomagnetic component to the NO-IRCR variability between 100 km and ~190 km. The influence of solar activity on the variations of NO-IRCR gradually decreases with increasing latitude shown in fig.1e, and the influence of geomagnetic activity on NO-IRCR is inverse, i.e, increases with the latitude increasing shown in fig.1f.

Quantifying the spatial and temporal variability of NO-IRCR in the thermosphere will increase the understanding of how upper atmospheric NO cooling behaves, and could be used to increase the accuracy of future space weather and climate models.

Link to the paper: Global distribution and variations of NO infrared radiative flux and its responses to solar activity and geomagnetic activity in the thermosphere.

The global distribution and variations of NO-IRCR and its responses to solar activity and geomagnetic activity in the thermosphere:  (a) The normalized latitudinal variations of NO-IRCR in different altitude layer with contour interval of 10%; (b) The normalized interannual variations of NO-IRCR in different altitude layer during 2002–2016 with contour interval of 15%; (c) Annual-mean of NO-IRCR and F10.7 at 175 km altitude layer; (d) The values of F(2,12) for 90 altitude layers, and the top right corner of the graph is a partial magnification of the abscissa; (e) Contour of the solar response of NO-IRCR for the spatial bins with contour interval of 2%/10 SFU using the multiple linear regression; (f) Contour of the geomagnetic response of NO-IRCR for the spatial bins with contour interval of 1%/10NT using the multiple linear regression (Image by Tang Chaoli).





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