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

Abstract

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 eects. 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 eects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical eects of both neutral meridional winds and electric ®elds caused by the disturbance dynamo eect. 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.

ULF fluctuations of the geomagnetic field and ionospheric sounding measurements at low latitudes during the first CAWSES campaign

“…On the other hand, each storm event has its own individual manifestation because of the complex nonlinear interactions of the contributing processes and the large variety of the initial and boundary conditions. An appropriate interplay between global-scale modelling studies (such as those dealing with the composition changes (Namgaladze et al, 2000) or with the interaction of disturbance dynamo electric fields (Maruyama et al, 2005)) and coordinated measurements, such as this first CAWSES Space Weather campaign, is important for the understanding of the complex storm events.…”

Modification of conductivity due to acceleration of the ionospheric medium

“…A pressure gradient is taken into account in the frame of a local model (Maeda, 1977). A complicated model like this of Namgaladze et al (2000) is necessary to analyze all forces which define the motion in the ionosphere.…”

Model interpretation of the ionospheric F-region electron density structures observed by ground-based satellite tomography at sub-auroral and auroral latitudes in Russia in January–May 1999

“…It makes it possible to compare the results obtained by the theoretical and empirical models of the neutral atmosphere. For the quiet conditions they differ insignificantly at least after one day of numerical integration, whereas they can differ greatly during geomagnetically disturbed conditions (Namgaladze et al, 2000a). In the present study, referring to a low geomagnetic activity, we used the neutral gas temperature, density and composition in MSISE-90 to calculate the variations of the ionospheric parameters.…”