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Seasonal & Diurnal Sunlit differences

Intrinsic Source of Asymmetry

Relative Importance – Primary


Diurnal: Earth rotates around its own axis at a period about 24 hours, leading to the diurnal variations.

Seasonal: Due to the changes of the relative angle between the Earth’s self-rotation angle and its orbital plane as the Earth rotates around the Sun, seasons occur. The seasons are different in the Northern Hemisphere and Southern Hemisphere, indicating there could be significant asymmetry of the sunlit around summer and winter solstices.

(credit: National Geographic [Illustration by Mary Crooks])

Earth’s self-rotation and Earth’s orbital motion around the Sun.

Solar EUV Conductance: Due to the asymmetric sunlit in different hemispheres, the solar EUV conductance are different in hemispheres. This is obvious around the winter and summer solstices. However, even on equinoxes, due to the offset between Earth’s rotation axis and the magnetic dipole axis, the geomagnetically high-latitude regions in the Northern and Southern hemispheres receive different amounts of the sunlit. 

Precipitation: The precipitation of the energetic electrons can be affected by the background solar EUV conductance at high latitudes. Especially, those electron precipitations accelerated by the field-aligned currents (FACs) are more intense during the winter than during the summer (Newell et al., 1996). 

Thermospheric neutral density and temperature: The thermospheric neutral density and temperature are generally higher in the summer hemisphere than the winter hemisphere. 

Joule heating: Joule heating in the summer hemisphere is typically higher that in the winter hemisphere. As a result, the ratio of the atomic oxygen (O) to the molecular Nitrogen (N2) in the summer hemisphere is generally lower than that in the winter hemisphere due to the transport process. Therefore, the electron density in the ionosphere F region in the summer hemisphere is lower in the summer hemisphere than the winter hemisphere. This phenomenon is called “winter anomaly” (Rishbeth and Garriot, 1969). 

Modeling Capability:

GITM captures both diurnal and seasonal asymmetries in the solar EUV radiations but does not have any feedback from the magnetosphere since it is a numerical model for the Earth’s ionosphere and thermosphere.


Includes both diurnal and seasonal asymmetries in the solar EUV radiation

Includes both diurnal and seasonal asymmetries in the solar EUV radiation


Newell, P., Meng, CI. & Wing, S. Relation to solar activity of intense aurorae in sunlight and darkness. Nature 393, 342–344 (1998). https://doi.org/10.1038/30682

Rishbeth, H., & Garriott, O. K. (1969). Introduction to ionospheric physics.

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