Ionospheric disturbances, often associated with geomagnetic storms, may cause threats to radio systems used for communication and navigation. One example is the super storm on 20 November 2003, when plenty of strong and unusual perturbations were reported. This paper reveals additional information on the dynamics in the high‐latitude ionosphere over Europe during this storm. Here analyses of wavelike traveling ionospheric disturbances (TIDs) over Europe are presented, based on estimates of the total electron content (TEC) derived from ground‐based Global Navigation Satellite System (GNSS) measurements. These TIDs are ionospheric signatures of thermospheric surges initiated by space weather events. The source region of these TIDs is characterized by enhanced spatial gradients, TEC depression, strong uplift of the F2 layer, the vicinity of the eastward auroral electrojet, and strong aurora E layers. Joule heating is identified as the most probable driver for the TIDs observed over Europe during 20 November 2003. The sudden heating of the thermosphere leads to strong changes in the pressure and thermospheric wind circulation system, which in turn generates thermospheric wind surges observed as TID signatures in the TEC. Either the dissipation of the eastward auroral electrojet or particle precipitation are considered as the source mechanism for the Joule heating. In the course of the storm, the TEC observations show a southward shift of the source region of the TIDs. These meridional dislocation effects are obviously related to a strong compression of the plasmasphere. The presented results demonstrate the complex interaction processes in the thermosphere‐ionosphere‐magnetosphere system during this extreme storm.

The ultimate formation mechanism of the substorm current wedge (SCW) remains to date unclear. In this study, we investigate its relationship to plasma flows at substorm onset and throughout the following expansion phase. We revisit the case of 8 September 2002, which has been defined as one of the best textbook examples of a substorm because of its excellent coverage by both spacecraft in the magnetotail and ground-based observatories. We found that a dense sequence of arrival of nightside flux transfer events (NFTEs; which can be understood as the lobe magnetic signature due to a bursty bulk flow travelling earthward in the central plasma sheet) in the near-Earth tail leads to a modulation (and further step-like builtup) of the SCW intensity during the substorm expansion phase. In addition, we found that small SCWs are created also during the growth phase of the event in association with another less intense sequence of NFTEs. The differences between the sequence of NFTEs in the growth and expansion phase are discussed. We conclude that the envelope of the magnetic disturbances which we typically refer to as an intense magnetic substorm is the result of a group or sequence of more intense and more frequent NFTEs.

22 The ultimate formation mechanism of the substorm current wedge (SCW) remains to-date 23 unclear. In this study, we investigate its relationship to plasma flows at substorm onset and 24 throughout the following expansion phase. We revisit the case of September 8, 2002, which has 25 been defined as “one of the best textbook examples of a substorm” because of its excellent 26 coverage by both spacecraft in the magnetotail and ground-based observatories. We found that a 27 dense sequence of arrival of nightside flux transfer events (NFTEs, which can be understood as 28 the lobe magnetic signature due to a bursty bulk flow travelling earthward in the central 29 plasmasheet) in the near-Earth tail leads to a modulation (and further step-like built-up) of the 30 SCW intensity during the substorm expansion phase. In addition, we found that small SCWs are 31 created also during the growth phase of the event in association with another less intense 32 sequence of NFTEs. The differences between the sequence of NFTEs in the growth and 33 expansion phase are discussed. We conclude that the envelope of the magnetic disturbances 34 which we typically refer to as an intense magnetic substorm is the result of a group or sequence 35 of more intense and more frequent NFTEs. 36

Almost 40 years ago the concept of the substorm current wedge was developed to explain the magnetic signatures observed on the ground and in geosynchronous orbit during substorm expansion. In the ensuing decades new observations, including radar and low-altitude spacecraft, MHD simulations, and theoretical considerations have tremendously advanced our understanding of this system. The AMPTE/IRM, THEMIS and Cluster missions have added considerable observational knowledge, especially on the important role of fast flows in producing the stresses that generate the substorm current wedge. Recent detailed, multi-spacecraft, multi-instrument observations both in the magnetosphere and in the ionosphere have brought a wealth of new information about the details of the temporal evolution and structure of the current system. While the large-scale picture remains valid, the new details call for revision and an update of the original view. In this paper we briefly review the historical development of the substorm current wedge, review recent in situ and ground-based observations and theoretical work, and discuss the current active research areas. We conclude with a revised, time-dependent picture of the substorm current wedge that follows its evolution from the initial substorm flows through substorm expansion and recovery.

We present a novel ionospheric tomography reconstruction method. The method is based on Bayesian inference with the use of Gaussian Markov random field priors. We construct the priors as a system of stochastic partial differential equations. Numerical approximations of these equations can be represented with linear systems with sparse matrices, therefore providing computational efficiency. The method enables an interpretable scheme to build the prior distribution based on physical and empirical information on the structure of the ionosphere. We show through synthetic test cases in a two-dimensional setup of latitude-altitude slices how this method can be applied to satellite-based ionospheric tomography and how information about the structure of the ionosphere can be implemented in the prior. The technique is capable of being easily extended to multifrequency tomographic analysis or used for the inclusion of other data sets of ionospheric electron density, such as ground-based observations by radars or ionosondes.

The source of the auroral phenomenon known as omega bands is not yet known. We examine in detail five different intervals when omega bands were observed on March 9th, 2008 between 0400 UT and 1100 UT over central Canada using both ground and space-based instrumentation. The THEMIS all sky imagers show the development of some of the omega bands from north–south streamers. Spherical elementary currents derived from ground magnetometer data indicate that the omega bands lie near the interface between the region 1 and region 2 currents in the post-midnight sector. THEMIS spacecraft data from the pre-midnight sector display multiple high speed flows and dipolarization features associated with high levels of geomagnetic activity, whereas four GOES geosynchronous spacecraft show multiple injections and dipolarization features. Magnetic field line tracing suggests that the magnetospheric location of the omega bands is at or just beyond geosynchronous orbit. We discuss in detail two potential source mechanisms for the omega bands: plasma sheet velocity shears and high speed flows in the magnetotail and relate the available data to those mechanisms. Our data and a magnetohydrodynamic (MHD) simulation support high speed flows in the magnetotail as the most likely generation mechanism, although the distribution of the magnetotail spacecraft does not provide unambiguous support for our interpretation of the source mechanism.

The European Space Agency (ESA) Swarm spacecraft mission is the first multisatellite ionospheric mission with two low-orbiting spacecraft that are flying in parallel at a distance of 100-140km, thus allowing derivation of spatial gradients of ionospheric parameters not only along the orbits but also in the direction perpendicular to them. A third satellite with a higher orbit regularly crosses the paths of the lower spacecraft. Using the Swarm magnetic and electric field instruments, we present a novel technique that allows derivation of two-dimensional (2-D) maps of ionospheric conductances, currents, and electric field in the area between the trajectories of the two lower spacecraft, and even to some extent outside of it. This technique is based on Spherical Elementary Current Systems. We present test cases of modeled situations from which we calculate virtual Swarm data and show that the technique is able to reconstruct the model electric field, horizontal currents, and conductances with a very good accuracy. Larger errors arise for the reconstruction of the 2-D field-aligned currents (FAC), especially in the area outside of the spacecraft orbits. However, even in this case the general pattern of FAC is recovered, and the magnitudes are valid in an integrated sense. Finally, using an MHD model run, we show how our technique allows estimation of the ionosphere-magnetosphere coupling parameter K, if conjugate observations of the magnetospheric magnetic and electric field are available. In the case of a magnetospheric multisatellite mission (e.g., the ESA Cluster mission) several K estimates at nearby points can be generated.

At 10:00 UT on 25 February 2008, Cluster 1 spacecraft crossed the near‐midnight auroral zone, at about 2 RE altitude, while two of the Time History of Events and Macroscale Interactions During Substorms (THEMIS) spacecraft, THD and THE, observed multiple flow bursts on the near‐conjugate plasma sheet field lines. The flow shear pattern at THEMIS was consistent with the vortical motion at duskside of a localized flow channel. Coinciding in time with the flow bursts, Cluster 1 observed bursts of counterstreaming electrons with mostly low energies (≤441 eV), accompanied by short time scale (<5 s) magnetic field disturbances embedded in flow‐associated field‐aligned current systems. This conjugate event not only confirms the idea that the plasma sheet flows are the driver of the kinetic Alfvén waves accelerating the low‐energy electrons but is a unique observation of disturbances in the high‐altitude auroral region relevant to the multiple plasma sheet flows.

The solar zenith angle (SZA) dependence of the conductance is studied and a simple theoretical form for the Hall‐to‐Pedersen conductance ratio is developed, using the peak plasma production height. The European Incoherent Scatter (EISCAT) radar observations at Tromsø (67 MLAT) on 30 March 2012 were used to calculate the conductance. The daytime electric conductance is associated with plasma created by solar extreme ultraviolet radiation incident on the neutral atmosphere of the Earth. However, it has been uncertain whether previous conductance models are consistent with the ideal Chapman theory for such plasma productions. We found that the SZA dependence of the conductance is consistent with the Chapman theory after simple modifications. The Pedersen conductance can be understood by approximating the plasma density height profile as being flat in the topside E region and by taking into account the upward gradient of atmospheric temperature. An additional consideration is necessary for the Hall conductance, which decreases with increasing SZA more rapidly than the Pedersen conductance. This rapid decrease is presumably caused by a thinning of the Hall conductivity layer from noon toward nighttime. We expressed this thinning in terms of the peak production height in the Chapman theory.

The complex image method (CIM) is an efficient tool to calculate the electromagnetic field at the earth’s surface produced by 3D ionospheric current systems when the earth has a layered conductivity structure. The calculations are applicable to the estimation of source effects on magnetotelluric data. In this paper CIM is used in connection with some typical high-latitude ionospheric events: a westward travelling surge, a Harang discontinuity, an omega band, and a giant pulsation. The complicated ionospheric current systems are constructed of short horizontal current filaments with vertical currents at both ends. The currents are given numerically on a 50 km × 50 km grid covering a region of even 1000 km × 2000 km. The investigations indicate that the source distortion very much depends on the event, and may be significant in a wide period range, especially for a resistive earth structure. The source effect seems quite unpredictable. Sometimes the apparent resistivity is larger and sometimes smaller than the plane wave value. At times the source effect is very small even if the ionospheric current is strongly inhomogeneous.

We present a novel algorithm for ionospheric tomography and the latest results obtained with it. Ionospheric tomography is mathematically a sparse limited-angle tomography problem and therefore severely ill-posed. This means that to obtain reasonable reconstructions the problem needs a strong regularisation. In ionospheric tomography the regularisation is often implemented with a limited set of base functions, Tikhonov regularisation, initial profiles for iterative methods, or with a different combinations of these schemes. These methods have been shown to produce satisfactory reconstructions, but the role of the regularisation and how much the chosen method actually constraints the possible outcomes is not always clear. Our tomography scheme is implemented in the Bayesian statistical framework (Markkanen et al. Ann. Geophys., vol. 13, pp. 1277-1287, 1995). In Bayesian inference the regularisation is given as an a priori distribution. The a priori distribution contains the information about the unknown parameters before the measurements. The second step is to build a likelihood function for the unknown parameters, given the observed measurements. By combining the likelihood with the prior density function, we obtain the a posteriori distribution, from where we can obtain the estimate with the highest probability, based on the information in our disposal. Here we give the a priori distribution with the novel correlation priors (Roininen et al. Inverse Probl. Imag., vol. 5, issue 2, pp. 611-647, 2011). Essentially the correlation priors are Gaussian Markov random fields, wherein we can state the physical assumptions of the ionosphere in an interpretable manner. With this implementation we get a very thorough understanding of what is the role of the regularising a priori distribution. On addition to that, the statistical framework also gives a very natural way to combine different ionospheric measurements. Therefore we can use measurements from Low Earth Orbit (LEO) a- d Global Positioning System (GPS) satellites as well as ground based measurements in the same model. Starting from 2011, as a part of TomoScand project, Finnish Meteorological Institute has been installing a new receiver network for LEO beacon satellites. At the moment, together with the receiver network of Sodankyla Geophysical Observatory, we collect data from 11 receivers stations. The northern most receiver located in Svalbard and southern most in Estonia. We also collect data from 86 GPS receiver stations provided by Finnish company Geotrim. We present the latest results with the new network and algorithm in a 2-dimensional case, however, the goal in the near future is to extend the model to a 3-dimensional case to cover over Scandinavia.

The classic substorm current wedge model represents ground and space magnetic perturbations measured during substorms. We have developed an inversion technique to calculate parameters determining the intensity and geometry of the current system using magnetic field data at midlatitudes. The current wedge consists of four segments: a sheet-like field-aligned current downward to the ionosphere postmidnight, a westward current across the auroral bulge, an upward sheet-like current from the westward surge premidnight, and an eastward current in the equatorial plane. The model has five parameters including the current strength, the locations, and breadths of the two field-aligned current sheets. Simultaneous changes in the ring current are represented by the superposition of a symmetric ring current and a partial ring current characterized by three additional parameters. Parameters of the model are determined as a function of time based on midlatitude ground magnetometers, using realistic field lines and accounting for Earth's induction. The model is validated by a variety of techniques. First, the model predicts more than 80% of the variance in the observations. Second, the intensity of the current wedge and the ring current follows the same trends of the westward electrojet and the ring current indices. Third, the intensity of the westward electrojet agrees extremely well with the intensity of the current wedge. Finally, spacecraft observations of the aurora correspond with the evolution deduced from the model. This model of the substorm current wedge provides a valuable tool for the study of substorm development and its relation to phenomena in space.

We introduce a new coherent dual-channel beacon satellite receiver intended for ionospheric tomography. The measurement equation includes neutral atmosphere and ionosphere propagation effects, relative errors in satellite and receiver clocks, and residual Doppler shifts caused by errors in the satellite ephemeris. We also investigate the distribution of errors for phase curve measurements and the use of phase curve measurements for limited angle tomography using the framework of statistical linear inverse problems. We describe the design of our beacon satellite receiver software and present one possible hardware configuration. Finally, we present results obtained using a network of four newly developed receivers and compare the results with those of an existing ionospheric tomography network at Sodankyl Geophysical Observatory.

Global Pi2 pulsations have mainly been associated with either low/middle latitudes or middle/high latitudes and, as a result, have been treated as two different types of Pi2 pulsations, either the plasmaspheric cavity resonance or the transient response of the substorm current wedge, respectively. However, in some reports, global Pi2 pulsations have a single period spanning low/middle/high latitudes. This “super” global type has not yet been satisfactorily explained. In particular, it has been a major challenge to identify the coupling between the source region and the ground. Here we report two consecutive super global Pi2 events which were observed over a wide latitudinal and longitudinal range. Using four spacecraft that were azimuthally spread out in the nightside and one spacecraft in the tail lobe, it was possible to follow the Pi2 signal along various paths with time delays from the magnetotail to the ground. Furthermore, it was found that the global pulsations were a combination of various modes including the transient Alfvén and fast modes, field line resonance, and possibly a forced cavity‐type resonance. As for the source of the Pi2 periodicity, oscillatory plasma flow inside the plasma sheet during flow braking (e.g., interchange oscillations) is a likely candidate. Such flow modulations, resembling the ground Pi2 pulsations, were recorded for both events.

Magnetic reconnection in Earth's magnetotail produces fast earthward flows in the plasma sheet. Tailward flows are often observed associated with the earthward flows. Both return flow vortices at the flanks of an earthward flow channel and rebound of the earthward flow from the intense dipolar magnetic field of the inner magnetosphere have been shown to explain tailward flows observed near Earth. We combine plasma sheet measurements from Cluster with conjugate ground‐based magnetic and auroral data to examine the development of earthward and tailward flow signatures during a substorm onset. We show for the first time observations of ionospheric signatures that appear to be associated with rebound flows. Because of the highly dynamic magnetotail configuration, special care is taken with the satellite footprint mapping. The ionospheric footprints produced by the event oriented AM02 model drift equatorward and poleward in response to tail magnetic field stretching and dipolarization, respectively. The footprint motion matches that of the ambient ionospheric structures, and the plasma flow measured by Cluster agrees with that inferred from the conjugate ionospheric observations, confirming the validity of the AM02 mapping. The ionospheric signatures of fast earthward flows during a substorm onset are shown to resemble the known signatures of quiet‐time flows, including equatorward propagating auroral streamers inside a channel of enhanced poleward equivalent current. However, the large‐scale dipolarization results in additional poleward expansion of the signatures, as has been predicted by simulations.