M. Förster

Ionospheric response to the 2015 St. Patrick's Day storm: A global multi‐instrumental overview

We present the first multi-instrumental results on the ionospheric response to the geomagnetic storm of 17–18 March 2015 (the St. Patricks Day storm) that was up to now the strongest in the 24th solar cycle (minimum SYM-H value of A233 nT). The storm caused complex effects around the globe. The most dramatic positive ionospheric storm occurred at low latitudes in the morning (~100–150% enhancement) and postsunset (~80–100% enhancement) sectors. These significant vertical total electron content increases were observed in different local time sectors and at different universal time, but around the same area of the Eastern Pacific region, which indicates a regional impact of storm drivers. Our analysis revealed that this particular region was most concerned by the increase in the thermospheric O/N 2 ratio. At midlatitudes, we observe inverse hemispheric asymmetries that occurred, despite the equinoctial period, in different longitudinal regions. In the European-African sector, positive storm signatures were observed in the Northern Hemisphere (NH), whereas in the American sector, a large positive storm occurred in the Southern Hemisphere, while the NH experienced a negative storm. The observed asymmetries can be partly explained by the thermospheric composition changes and partly by the hemispherically different nondipolar portions of the geomagnetic field as well as by the IMF By component variations. At high latitudes, negative ionospheric storm effects were recorded in all longitudinal regions, especially the NH of the Asian sector was concerned. The negative storm phase developed globally on 18 March at the beginning of the recovery phase.

Estimating the capture and loss of cold plasma from ionospheric outflow

An important source of magnetospheric plasma is cold plasma from the terrestrial ionosphere. Low energy ions travel along the magnetic field lines and enter the magnetospheric lobes where they are convected toward the tail plasma sheet. Recent observations indicate that the field aligned ion outflow velocity is sometimes much higher than the convection toward the central plasma sheet. A substantial amount of plasma therefore escapes downtail without ever reaching the central plasma sheet. In this work, we use Cluster measurements of cold plasma outflow and lobe convection velocities combined with models of the magnetic field in an attempt to determine the fate of the outflowing ions and to quantify the amount of plasma lost downtail. The results show that both the circulation of plasma and the direct tailward escape of ions varies significantly with magnetospheric conditions. For strong solar wind driving with a southward interplanetary magnetic field, also typically associated with high geomagnetic activity, most of the outflowing plasma is convected to the plasma sheet and recirculated. For periods with northward interplanetary magnetic field, the convection is nearly stagnant, whereas the outflow, although limited, still persists. The dominant part of the outflowing ions escape downtail and are directly lost into the solar wind under such conditions.

Calculated and observed ionospheric parameters for a Magion 2 passage and EISCAT data on July 31, 1990

Ionospheric parameters, calculated by using a global numerically self-consistent model of the thermosphere, ionosphere, and protonosphere (GSM TIP), are compared with experimental data. For July 31, 1990, there are ground-based Common Programme 1 observations from the European Incoherent Scatter (EISCAT) radar station below about 600-km height and simultaneous satellite data obtained onboard Magion 2 of the Active mission which passed near the EISCAT station around 1540 UT at an altitude of about 2500 km. The main goal of this paper is to estimate the ability of the first-principles mathematical GSM TIP model to reproduce the real ionospheric situation in a wide altitude/latitude range. Numerical calculations were performed with the complete theoretical thermosphere-ionosphere-protonosphere model. The results presented show good agreement between the incoherent scatter radar measurements (Ne, Te, Ti, and ) and model calculations under certain model assumptions. Model runs were performed to study the influence of additional local and nonlocal heating sources, the role of vibrationally excited molecular nitrogen, and different convection patterns for the conditions under study. There is relatively good agreement between model results and experimental electron concentration data along the Magion 2 satellite orbit. It was found that the best fit results are achieved with an additional plasmaspheric heat source, while the importance of vibrationally excited molecular nitrogen has not yet been firmly established.

Interhemispheric differences in ionospheric convection: Cluster EDI observations revisited

The interaction between the interplanetary magnetic field and the geomagnetic field sets up a large-scale circulation in the magnetosphere. This circulation is also reflected in the magnetically connected ionosphere. In this paper, we present a study of ionospheric convection based on Cluster Electron Drift Instrument (EDI) satellite measurements covering both hemispheres and obtained over a full solar cycle. The results from this study show that average flow patterns and polar cap potentials for a given orientation of the interplanetary magnetic field can be very different in the two hemispheres. In particular during southward directed interplanetary magnetic field conditions, and thus enhanced energy input from the solar wind, the measurements show that the southern polar cap has a higher cross polar cap potential. There are persistent north-south asymmetries, which cannot easily be explained by the influence of external drivers. These persistent asymmetries are primarily a result of the significant differences in the strength and configuration of the geomagnetic field between the Northern and Southern Hemispheres. Since the ionosphere is magnetically connected to the magnetosphere, this difference will also be reflected in the magnetosphere in the form of different feedback from the two hemispheres. Consequently, local ionospheric conditions and the geomagnetic field configuration are important for north-south asymmetries in large regions of geospace.

North‐south asymmetries in the polar thermosphere‐ionosphere system: Solar cycle and seasonal influences

Previous studies have revealed that ion drift and neutral wind speeds at ~400 km in the polar cap (>80° magnetic latitude) are on average larger in the Northern Hemisphere (NH) than in the Southern Hemisphere, which is at least partly due to asymmetry in the geomagnetic field. Here we investigate for the first time how these asymmetries depend on season and on solar/geomagnetic activity levels. Ion drift measurements from the Cluster mission show little seasonal dependence in their north-south asymmetry when all data (February 2001–December 2013) are used, but the asymmetry disappears around June solstice for high solar activity and around December solstice for low solar activity. Neutral wind speeds in the polar cap obtained from the Challenging Minisatellite Payload spacecraft (January 2002–December 2008) are always larger in the summer hemisphere, regardless of solar activity, but the high-latitude neutral wind vortices at dawn and dusk tend to be stronger in the NH, except around December solstice, in particular, when solar activity is low. Simulations with the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) more or less capture the behavior of the ion drift speeds, which can be explained as a superposition of seasonal and geomagnetic field effects, with the former being stronger for higher solar activity. The behavior of the neutral wind speed and vorticity is not accurately captured by the model. This is probably due to an incorrect seasonal cycle in plasma density around ~400 km in CMIT, which affects the ion drag force. This must be addressed in future work.

An estimate of the non-barometric effect in the [O] height distribution at low latitudes during magnetically disturbed periods

Abstract ESRO-4 observations of atomic oxygen occasionally show a non-barometric height distribution between 250 and 350km altitude at low latitudes during magnetically disturbed periods. A new data analysis method has been developed. The method indicates that the excess of atomic oxygen [O] abundance over what may be expected from mere thermal expansion (i.e. a non-barometric effect) can reach levels of up to 25–35%. The onset of this effect in the [O] distribution coincides with the expected arrival time of a Travelling Atmospheric Disturbance (TAD) at low latitudes and is considered to be due to atmospheric gas downwelling.

Using MFACE as input in the UAM to specify the MIT dynamics

The magnetosphere-ionosphere-thermosphere (MIT) dynamic system significantly depends on the highly variable solar wind conditions, in particular, on changes of the strength and orientation of the interplanetary magnetic field (IMF). The solar wind and IMF interactions with the magnetosphere drive the MIT system via the magnetospheric field-aligned currents (FACs). The global modeling helps us to understand the physical background of this complex system. With the present study, we test the recently developed high-resolution empirical model of field-aligned currents MFACE (a high-resolution Model of Field-Aligned Currents through Empirical orthogonal functions analysis). These FAC distributions were used as input of the time-dependent, fully self-consistent global Upper Atmosphere Model (UAM) for different seasons and various solar wind and IMF conditions. The modeling results for neutral mass density and thermospheric wind are directly compared with the CHAMP satellite measurements. In addition, we perform comparisons with the global empirical models: the thermospheric wind model (HWM07) and the atmosphere density model (Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Extended 2000). The theoretical model shows a good agreement with the satellite observations and an improved behavior compared with the empirical models at high latitudes. Using the MFACE model as input parameter of the UAM model, we obtain a realistic distribution of the upper atmosphere parameters for the Northern and Southern Hemispheres during stable IMF orientation as well as during dynamic situations. This variant of the UAM can therefore be used for modeling the MIT system and space weather predictions.

Some aspects of modelling the high-latitude ionospheric convection from Cluster/Edi data

Measurements onboard Cluster satellites are briefly described, which form the base for determining the intensity and direction of the electric field in the magnetosphere. The aim of this paper is to describe (1) the methodology of calculating the potential distribution at the ionospheric level and the results of constructing spatiotemporal convection patterns for different orientations of the IMF vector in the GSM YZ plane; (2) derivation of basic convection patterns (BCPs), which allow to deduce the statistical ionospheric convection pattern at high latitudes for any IMF Bz and By values (statistical convection model) using different sets of independent data; (3) the consequences of enlarging the amount of data used for analysis; (4) the results of potential calculations with various orders of the spherical harmonics describing them; (5) determination of the cross-polar cap potential with different IMF sector widths (α from 45° down to 10°); (6) the results of our trials to determine the contribution of the IMF Bx component to the convection pattern.

Effect of the global neutral hydrogen distribution on the spatial structure and thermal balance in the upper ionosphere

Abstract A global numerical self-consistent and time-dependent model of the thermosphere, ionosphere and protonosphere (GSM TIP) was used to study the influence of atomic neutral hydrogen distribution in the exosphere on the upper ionosphere parameters for summer solstice conditions during high solar activity (F10.7 = 200). Theoretical model calculations were compared with measured ion and electron densities obtained by in situ observations along early morning and afternoon orbits of the satellite pair Active and Magion-2 on 31 July (1990). The satellites passed over Europe around 03:25 and 15:40 UT at an altitude of about 2000–2500 km during orbits 3805 and 3812. The numerical results were compared with O+, H+ Ne densities in the dawn sector and with the Ne concentration in the afternoon sector. We found that the atomic neutral hydrogen density is an essential model input parameter for upper ionosphere and plasmasphere studies. The inaccurate model input of the exospheric neutral hydrogen distribution can cause dramatic variations of the H+ density up to a factor of 2–4 and a 40% variation of the Ne concentration under the conditions of our study. On the other hand, the variability of the thermal latitudinal structure (Ti and Te) in the upper ionosphere in the altitude range from 2000 to 2500 km reaches only about 10–15%.

Modeling of the Ionospheric Current System and Calculating Its Contribution to the Earth’s Magnetic Field

The additional magnetic field produced by the ionospheric current system is a part of the Earth’s magnetic field. This current system is a highly variable part of a global electric circuit. The solar wind and interplanetary magnetic field (IMF) interaction with the Earth’s magnetosphere is the external driver for the global electric circuit in the ionosphere. The energy is transferred via the field-aligned currents (FACs) to the Earth’s ionosphere. The interactions between the neutral and charged particles in the ionosphere lead to the so-called thermospheric neutral wind dynamo which represents the second important driver for the global current system. Both processes are components of the magnetosphere–ionosphere–thermosphere (MIT) system, which depends on solar and geomagnetic conditions, and have significant seasonal and UT variations.