How the thermospheric circulation affects the ionospheric F2-layer

Rishbeth, H.

doi:10.1016/s1364-6826(98)00062-5

Cited by 456 publications

(553 citation statements)

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“…Rishbeth and Garriott, 1969), the midday F2-layer may be assumed to be in a nearly steady state. The height hmF2 of the F2-layer peak is determined by a balance between the electron loss (coe cient b) and plasma di usion (coe cient D); it depends on the solar cycle, the solar zenith angle, and thermospheric winds.…”

Section: Theorymentioning

confidence: 99%

Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion

et al. 2000

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Abstract. The companion paper by Zou et al. shows that the annual and semiannual variations in the peak F2-layer electron density (NmF2) at midlatitudes can be reproduced by a coupled thermosphere-ionosphere computational model (CTIP), without recourse to external in¯uences such as the solar wind, or waves and tides originating in the lower atmosphere. The present work discusses the physics in greater detail. It shows that noon NmF2 is closely related to the ambient atomic/molecular concentration ratio, and suggests that the variations of NmF2 with geographic and magnetic longitude are largely due to the geometry of the auroral ovals. It also concludes that electric ®elds play no important part in the dynamics of the midlatitude thermosphere. Our modelling leads to the following picture of the global three-dimensional thermospheric circulation which, as envisaged by Duncan, is the key to explaining the F2-layer variations. At solstice, the almost continuous solar input at high summer latitudes drives a prevailing summer-to-winter wind, with upwelling at low latitudes and throughout most of the summer hemisphere, and a zone of downwelling in the winter hemisphere, just equatorward of the auroral oval. These motions a ect thermospheric composition more than do the alternating day/night (up-and-down) motions at equinox. As a result, the thermosphere as a whole is more molecular at solstice than at equinox. Taken in conjunction with the well-known relation of F2-layer electron density to the atomic/molecular ratio in the neutral air, this explains the F2-layer semiannual e ect in NmF2 that prevails at low and middle latitudes. At higher midlatitudes, the seasonal behaviour depends on the geographic latitude of the winter downwelling zone, though the e ect of the composition changes is modi®ed by the large solar zenith angle at midwinter. The zenith angle e ect is especially important in longitudes far from the magnetic poles. Here, the downwelling occurs at high geographic latitudes, where the zenith angle e ect becomes overwhelming and causes a midwinter depression of electron density, despite the enhanced atomic/ molecular ratio. This leads to a semiannual variation of NmF2. A di erent situation exists in winter at longitudes near the magnetic poles, where the downwelling occurs at relatively low geographic latitudes so that solar radiation is strong enough to produce large values of NmF2. This circulation-driven mechanism provides a reasonably complete explanation of the observed pattern of F2 layer annual and semiannual quiet-day variations.

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Section: Theorymentioning

confidence: 99%

Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion

et al. 2000

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“…This planetary-scale phenomenon is mainly associated with electric fields generated by nonmigrating atmospheric tides in the E-region height that vary with longitude [3]. At midlatitudes, some longitudinal variations, such as those in annual and semi-annual ionospheric changes, have been explained as the result of the separation of geographic and geomagnetic poles and the associated distance from the aurora zone at different longitude sectors [4].…”

Section: Introductionmentioning

confidence: 99%

Ionospheric longitudinal variations at midlatitudes: Incoherent scatter radar observation at Millstone Hill

Zhang

Coster

Holt

et al. 2012

Sci. China Technol. Sci.

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Incoherent scatter radar (ISR) extra-wide coverage experiments during the period of 1978-2011 at Millstone Hill are used to investigate longitudinal differences in electron density. This work is motivated by a recent finding of the US east-west coast difference in TEC suggesting a combined effect of changing geomagnetic declination and zonal winds. The current study provides strong supporting evidence of the longitudinal change and the plausible mechanism by examining the climatology of electron density Ne on both east and west sides of the radar with a longitude separation of up to 40 o for different heights within 300-450 km. Main findings include: 1) The east-west difference can be up to 60% and varies over the course of the day, being positive (East side Ne > West side Ne) in the late evening, and negative (West side Ne > East side Ne) in the pre-noon. 2) The east-west difference exists throughout the year. The positive (relative) difference is most pronounced in winter; the negative (relative) difference is most pronounced in early spring and later summer.3) The east-west difference tends to enhance toward decreasing solar activity, however, with some seasonal dependence; the enhancements in the positive and negative differences do not take place simultaneously. 4) Both times of largest positive and largest negative east-west differences in Ne are earlier in summer and later in winter. The two times differ by 12-13 h, which remains constant throughout the year. 5) Variations at different heights from 300-450 km are similar. Zonal wind climatology above Millstone Hill is found to be perfectly consistent with what is expected based on the electron density difference between the east and west sides of the site. The magnetic declination-zonal wind mechanism is true for other longitude sectors as well, and may be used to understand longitudinal variations elsewhere. It may also be used to derive thermospheric zonal winds.

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“…Among different explanations proposed during the decades of the problem analysis [see Rishbeth, 1998], the suggestion by Rishbeth and Setty [1961] that F 2 layer seasonal anomaly is due to neutral composition variation may be considered as generally accepted [Rishbeth et al, 2000]. At present, one can see only a detailed elaboration of this concept [e.g., Pavlov and Pavlova, 2005].…”

Section: Introductionmentioning

confidence: 99%

On the mechanism of seasonal and solar cycleN_mF₂variations: A quantitative estimate of the main parameters contribution using incoherent scatter radar observations

Mikhailov

Perrone

2011

J. Geophys. Res.

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[1] Seasonal (winter/summer) and solar cycle N m F 2 variations as well as summer saturation effect in N m F 2 have been analyzed using Millstone Hill incoherent scatter radar (ISR) daytime observations. A self-consistent approach to the Ne(h) modeling has been applied to extract from ISR observations a consistent set of main aeronomic parameters and to estimate their quantitative contribution to the observed N m F 2 variations. The retrieved aeronomic parameters are independent of uncertainties in thermosphere and solar EUV empirical models, and this is a distinguishing feature of the present consideration. Different temperatures in winter and in summer in the course of solar cycle overlapped on the O + + N 2 reaction rate coefficient temperature dependence result in different N m F 2 dependences on solar activity: a steep practically linear increase with a tendency to turn up in January (contrary to international reference ionosphere prediction) and a slow increase with a tendency to saturate at high solar activity in July despite increasing solar EUV irradiation. In winter the EUV flux and thermospheric parameters provide approximately equal contributions to the N m F 2 increase, while in summer the contribution of thermospheric parameters is small. Both in winter and in summer the variations of atomic oxygen [O] are small at the F 2 layer peak, and its contribution is small compared to linear loss coefficient, b. It is shown that the summer saturation effect in N m F 2 under high solar activity is not just reduced to O/N 2 or EUV flux solar cycle variations but is determined by b via the g 1 temperature dependence. A new mechanism (qualitative) to explain the December anomaly in N m F 2 is proposed. It is based on the idea that the areas of atomic oxygen production and its loss are spatially separated and that time is required to transfer [O] from one area to the other where [O] associates in a three-body collision. Therefore, under a 7% increase in the O 2 dissociation rate due to the Sun-Earth distance decrease in December-January compared to June-July, an accumulation of atomic oxygen should take place in the thermosphere in the vicinity of the December solstice resulting in a 21% N m F 2 increase, which is close to the observed global December effect.Citation: Mikhailov, A. V., and L. Perrone (2011), On the mechanism of seasonal and solar cycle N m F 2 variations: A quantitative estimate of the main parameters contribution using incoherent scatter radar observations,

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How the thermospheric circulation affects the ionospheric F2-layer

Cited by 456 publications

References 77 publications

Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion

Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion

Ionospheric longitudinal variations at midlatitudes: Incoherent scatter radar observation at Millstone Hill

On the mechanism of seasonal and solar cycleN_mF₂variations: A quantitative estimate of the main parameters contribution using incoherent scatter radar observations

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How the thermospheric circulation affects the ionospheric F2-layer

Cited by 456 publications

References 77 publications

Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion

Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion

Ionospheric longitudinal variations at midlatitudes: Incoherent scatter radar observation at Millstone Hill

On the mechanism of seasonal and solar cycleNmF2variations: A quantitative estimate of the main parameters contribution using incoherent scatter radar observations

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On the mechanism of seasonal and solar cycleN_mF₂variations: A quantitative estimate of the main parameters contribution using incoherent scatter radar observations