Publication Date:
2021-02-10
Description:
Annual January/July midlatitude daytime asymmetry in monthly median NmF2 and model
thermospheric parameters has been considered during deep solar minimum, (2008–2009), 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 NmF2 asymmetry was found to be ≈1.23, while the average asymmetry for the annual
component in NmF2 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 NmF2. A hierarchy of aeronomic parameters responsible for the
observed asymmetry in NmF2 has been established: the main contributor is atomic oxygen—about 50% of the
total effect, [N2] contributes around 35% strongly compensating the [O] contribution, and solar EUV and Tn
provide 〈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 NmF2 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 NmF2 annual daytime asymmetry at middle latitudes under the deep solar minimum.
Description:
Published
Description:
1341-1354
Description:
2A. Fisica dell'alta atmosfera
Description:
JCR Journal
Description:
restricted
Keywords:
december anomaly
;
01. Atmosphere::01.02. Ionosphere::01.02.01. Ion chemistry and composition
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
