Daytime F2-layer positive storm effect at middle and lower latitudes

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Abstract. The method earlier used for the foF2 long-term trends analysis is applied to reveal hmF2 long-term trends at 27 ionosonde stations in the European and Asian longitudinal sectors. Observed M(3000)F2 data for the last 3 solar cycles are used to derive hmF2 trends. The majority of the studied stations show significant hmF2 linear trends with a confidence level of at least 95% for the period after 1965, with most of these trends being positive. No systematic variation of the trend magnitude with latitude is revealed, but some longitudinal effect does take place. The proposed geomagnetic storm concept to explain hmF2 long-term trends proceeds from a natural origin of the trends rather than an artificial one related to the thermosphere cooling due to the greenhouse effect.

Section: Discussionmentioning

confidence: 82%

Long-term hmF2 trends in the Eurasian longitudinal sector from the ground-based ionosonde observations

Marín¹,

Михайлов

Morena³

et al. 2001

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“…An analysis of the F2-layer storm mechanisms for the lower latitude station Havana, with the same inv = 35 • (L = 1.5) as Alma-Ata, was made by Mikhailov et al (1995). According to AE-C and ESRO-4 satellite observations, geomagnetic disturbances result in an increase in the atomic oxygen absolute concentration, presumably due to the disturbed thermospheric circulation and downwelling at low latitudes, while the R = (O/N 2 ) storm /(O/N 2 ) quiet ratio remains practically unchanged at the heights of the F2-region (Prölss and von Zahn, 1977;Skoblin and Mikhailov, 1996;.…”

Section: Lower Latitudesmentioning

confidence: 99%

An interpretation of the ƒoF2 and hmF2 long-term trends in the framework of the geomagnetic control concept

Mikhailov

Marín

2001

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Abstract. Earlier revealed morphological features of the foF2 and hmF2 long-term trends are interpreted in the scope of the geomagnetic control concept based on the contemporary F2-layer storm mechanisms. The F2-layer parameter trends strongly depend on the long-term varying geomagnetic activity whose effects cannot be removed from the trends using conventional indices of geomagnetic activity. Therefore, any interpretation of the foF2 and hmF2 trends should consider the geomagnetic effects as an inalienable part of the trend analysis. Periods with negative and positive foF2 and hmF2 trends correspond to the periods of increasing or decreasing geomagnetic activity with the turning points around 1955, and the end of 1960s and 1980s, where foF2 and hmF2 trends change their signs. Such variations can be explained by neutral composition, as well as temperature and thermospheric wind changes related to geomagnetic activity variations. In particular, for the period of increasing geomagnetic activity positive at lower latitudes, but negative at middle and high latitudes, foF2 trends may be explained by neutral composition and temperature changes, while soft electron precipitation determines nighttime trends at sub-auroral and auroral latitudes. A pronounced dependence of the foF2 trends on geomagnetic (invariant) latitude and the absence of any latitudinal dependence for the hmF2 trends are due to different dependencies of NmF2 and hmF2 on main aeronomic parameters. All of the revealed latitudinal and diurnal foF2 and hmF2 trend variations may be explained in the framework of contemporary F2-region storm mechanisms. The newly proposed geomagnetic storm concept used to explain F2-layer parameter long-term trends proceeds from a natural origin of the trends rather than an artificial one, related to the thermosphere cooling due to the greenhouse effect. Within this concept, instead of cooling, one should expect the thermosphere heating for the period of increasing geomagnetic activity .Correspondence to: A. V. Mikhailov (avm71@orc.ru)

“…For day light conditions enhanced [O] density favours ion production over the loss processes which are enhanced to a lesser degree (see Appendix B of Mikhailov et al, 1995). According to Mikhailov et al (1995) this is ®rst true for the longitudinal sector where the ®rst storm onset took place at local night time. Di erent longitudinal sectors, which are marked by the storm onset time, therefore exhibit distinct F-layer positive storm mechanisms.…”

Section: ] ([O]/[n 2 ])mentioning

confidence: 99%

Analysis of the positive ionospheric response to a moderate geomagnetic storm using a global numerical model

Namgaladze

Förster²,

Yurik³

2000

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Abstract. Current theories of F-layer storms are discussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time dependent numerical model of the thermosphere±iono-sphere±plasmasphere±magnetosphere system including electrodynamical coupling e ects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exempli®es the complex storm phenomena. The study focuses on positive ionospheric storm e ects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical e ects of both neutral meridional winds and electric ®elds caused by the disturbance dynamo e ect. The penetration of short-time electric ®elds of magnetospheric origin during storm intensi®cation phases is shown for the ®rst time in this model study. Comparisons of the calculated thermospheric composition changes with satellite observations of AE-C and ESRO-4 during storm time show a good agreement. The empirical MSISE90 model, however, is less consistent with the simulations. It does not show the equatorward propagation of the disturbances and predicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of [O]/[N 2 ] at high latitudes decreases significantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at ®xed altitudes) above the quiet reference level. Meanwhile, the ionospheric storm is positive there. We conclude that the positive phase of the ionospheric storm is mainly due to uplifting of ionospheric F 2 -region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic ®eld lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calculated zonal electric ®eld disturbances also help to create the positive ionospheric disturbances both at middle and low latitudes. Minor contributions arise from the general density enhancement of all constituents during geomagnetic storms, which favours ion production processes above ion losses at ®xed height under daylight conditions.