A geomagnetic storm is a complex process: its various features act at dierent heights. In the F2 layer the midlatitude eect is basically an ionospheric response to storm-induced changes in the neutral atmosphere, which are primarily a consequence of a strong Joule heating in the auroral thermosphere. At lower heights the role of ionization and photochemical processes increases due to shorter electron lifetimes. At the base of the F1 layer (160-170 km) the storm eect is almost absent. At

http://elpub.wdcb.ru/journals/ijga/v02/gai99312/gai99312.pdf

Eects of geomagnetic storms on the ionosphere and atmosphere

The coupled ionosphere–thermosphere–plasmasphere system is very complex. The study of its interrelationships during geomagnetically disturbed conditions is an especially challenging task.Significant progress has been achieved during the last few years in developing comprehensive theoretical models to describe its global behaviour.Moreover, more simple, specialized numerical modelling of some specialaspects of storm behaviour and/or regional models have contributedto the progress in this field.This paper summarizes recent developments in upper ionosphereand plasmasphere storm studies and modelling.From an observational point of view the upper ionosphere/plasmasphereregion is well reflected in radio beacon measurements providing the totalelectron content (TEC). The development of space-based radio navigation systems such as GPS offersnew opportunities to derive TEC on both regional and global scale.Combining TEC with ionosonde data enables the variability of the shape of the electron density distribution during storms to be studied.We present some examples of co-ordinated investigation,made during the CEDAR storm study intervals.

Geomagnetic storm effects on the topside ionosphere and plasmasphere : A compact tutorial and new results

The response of the thermosphere and ionosphere to the famous January 10, 1997, geomagnetic storm is simulated using the thermosphere-ionosphere-electrodynamics general circulation model with realistic, time-dependent distributions of ionospheric convection and auroral precipitation as inputs. The simulation results show a dominant positive storm phase of increased F layer electron density over much of the northern winter hemisphere, but a negative storm phase with reduced electron density at middle and low latitudes is also evident in the simulation. The coexistence of both positive and negative storm phases is a result of the complex dynamical and chemical interactions between charged particles and neutral gases. The impulsive magnetospheric energy inputs via auroral precipitation and Joule heating generate traveling atmospheric and ionospheric disturbances (TADs and TIDs) which propagate from the northern auroral zone to lower latitudes and penetrate well into the Southern Hemisphere. The simulation results demonstrate that positive storm phases are caused primarily by enhanced auroral precipitation over high latitudes and by TIDs at middle and low latitudes. Globally speaking, composition changes in terms of enhancements in the N2/O ratio are mainly responsible for negative storm effects. However, although there is some correlation between increases in N2/O and decreases in the F layer critical frequency ƒoF2 in the winter hemisphere during the storm main phase and early recovery phase, the overall changes in ƒoF2 are also determined by other processes, such as the ionization production associated with enhanced auroral precipitation and the variations associated with TIDs. In the low to middle-latitude region changes in ƒoF2 approximately anticorrelate with changes at the height of the F layer electron density peak (e.g., hmF2) at 70°W during the storm main phase as well as its early recovery phase. This is attributed in part to the relation that exists between meridional wind velocity and vertical shear of that velocity for aurorally produced TADs.

Coexistence of ionospheric positive and negative storm phases under northern winter conditions: A case study

. 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 (1965–1991) 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 frame-work 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 (1965–1991). Key words. Ionosphere (ionosphere-atmosphere interactions; ionospheric disturbances)

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An interpretation of the ƒo F2 and hm F2 long-term trends in the framework of the geomagnetic control concept

Measurements of the Earth’s magnetic field collected by low-Earth-orbit satellites such as Swarm and CHAMP, as well as at ground observatories, are dominated by sources in the Earth’s interior. However these measurements also contain significant contributions from more rapidly-varying current systems in the ionosphere and magnetosphere. In order to fully exploit magnetic data to probe the physical properties and dynamics of the Earth’s interior, field models with suitable treatments of external sources, and their associated induced signals, are essential. Here we review the methods presently used to construct models of the internal field, focusing on techniques to handle magnetospheric and ionospheric signals. Shortcomings of these techniques often limit the quality, as well as spatial and temporal resolution, of internal field models. We document difficulties in using track-by-track analysis to characterize magnetospheric field fluctuations, differences in internal field models that result from alternative treatments of the quiet-time ionospheric field, and challenges associated with rapidly changing, but spatially correlated, magnetic signatures of polar cap current systems. Possible strategies for improving internal field models are discussed, many of which are described in more detail elsewhere in this volume.

Challenges handling magnetospheric and ionospheric signals in internal geomagnetic field modelling

Current theories of F-layer storms are dis- cussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time de- pendent numerical model of the thermosphere-iono- sphere-plasmasphere-magnetosphere system including electrodynamical coupling eAects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena. The study focuses on positive iono- spheric storm eAects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical eAects of both neutral meridional winds and electric fields caused by the disturbance dynamo eAect. The penetration of short-time electric fields of magneto- spheric origin during storm intensification phases is shown for the first time in this model study. Compar- isons 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 consis- tent with the simulations. It does not show the equatorward propagation of the disturbances and pre- dicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of (O)/(N2) at high latitudes decreases signif- icantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at fixed 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 F2-region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic field lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calcu- lated zonal electric field 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 fixed height under day- light conditions.

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

To test various hypotheses about positive ionospheric storm development, we numerically simulated the magnetic storm of 24–27 January 1974 to obtain the global pattern of its thermospheric and ionospheric effects. We use a global self-consistent numerical model of the thermosphere-ionosphere-magnetosphere system which calculates all plasma parameters as well as electric fields both of magnetospheric and thermospheric dynamo origin. The predicted neutral composition changes are consistent with observations of the AE-C and ESRO-4 satellites. The storm-to-quiet density ratio of [O]/[N2] (at a fixed height) does not change significantly at middle and low latitudes while the absolute density of all neutral constituents is enhanced. The density ratio at a fixed pressure level can be slightly enhanced under certain conditions. Positive ionospheric storm effects during the main phase of the storm are shown to be mainly due to the uplifting of the ions along the field lines by an equatorward directed disturbance wind component at middle latitudes and their equatorward (convergent) movement at low latitudes. This is driven by the passage of travelling atmospheric disturbances and by the large-scale disturbance wind circulation. Under daylight conditions minor contributions arise from the general thermal density enhancement of all constituents favoring positive ionospheric response due to a stronger ion production rather than loss. Height dependent deviations from diffusive equilibrium of minor thermospheric constituents are due to the relatively rapid (in comparison with their diffusion) vertical bulk motion.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999GL900514

A. A. Namgaladze

Papers

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

Thermospheric composition changes deduced from geomagnetic storm modeling