Global UltraViolet Imager

The annual asymmetry in the <i>F</i> <sub>2</sub> layer during deep solar minimum (2008-2009): December anomaly

<p>Annual January/July midlatitude daytime asymmetry in monthly median <em>N<sub>m</sub>F</em><sub>2</sub> and model thermospheric parameters has been considered during deep solar minimum, (2008\textendash2009), when solar and geomagnetic activities were at the lowest level, to analyze the background effect due to the Sun-Earth minimum distance, perihelion, in the vicinity of the December solstice. Averaged over 10 midlatitude station pairs, the <em>N<sub>m</sub>F</em><sub>2</sub> asymmetry was found to be ≈1.23, while the average asymmetry for the annual component in <em>N<sub>m</sub>F</em><sub>2</sub> variations is ≈1.17. Annual asymmetry in monthly median neutral composition and temperature predicted by Mass Spectrometer Incoherent Scatter 86 (MSIS86) and MSISE00 thermospheric models along with the 7\% increase in solar EUV flux in the vicinity of the December solstice is sufficient to explain the observed annual asymmetry in <em>N<sub>m</sub>F</em><sub>2</sub>. A hierarchy of aeronomic parameters responsible for the observed asymmetry in <em>N<sub>m</sub>F</em><sub>2</sub> has been established: the main contributor is atomic oxygen\textemdashabout 50\% of the total effect, [N<sub>2</sub>] contributes around 35\% strongly compensating the [O] contribution, and solar EUV and <em>T<sub>n</sub></em> provide \&lt;10\% each. The zonal mean annual asymmetry in MSIS86 atomic oxygen column density was shown to be 1.18 at low and middle latitudes, and this is close to the estimated asymmetry for the annual component in <em>N<sub>m</sub>F</em><sub>2</sub> variations. The earlier proposed mechanism of the December anomaly is considered as a plausible one to explain the 1.18 January/July asymmetry in the atomic oxygen variations and consequently the <em>N<sub>m</sub>F</em><sub>2</sub> annual daytime asymmetry at middle latitudes under the deep solar minimum.</p>
Year of Publication
Journal of Geophysical Research: Space Physics
Number of Pages
Date Published