Stephan Buchert - Academia.edu (original) (raw)

Papers by Stephan Buchert

Research paper thumbnail of Coordinated ground-based, low altitude satellite and Cluster observations on global and local scales during a transient post-noon sector excursion of the magnetospheric cusp

Annales Geophysicae, 2001

On 14 January 2001, the four Cluster spacecraft passed through the northern magnetospheric mantle... more On 14 January 2001, the four Cluster spacecraft passed through the northern magnetospheric mantle in close conjunction to the EISCAT Svalbard Radar (ESR) and approached the post-noon dayside magnetopause over Greenland between 13:00 and 14:00 UT. During that interval, a

Research paper thumbnail of Lower-thermosphere–ionosphere (LTI) quantities: current status of measuring techniques and models

Annales Geophysicae, 2021

The lower-thermosphere-ionosphere (LTI) system consists of the upper atmosphere and the lower par... more The lower-thermosphere-ionosphere (LTI) system consists of the upper atmosphere and the lower part of the ionosphere and as such comprises a complex system coupled to both the atmosphere below and space above. The atmospheric part of the LTI is dominated by laws of continuum fluid dynamics and chemistry, while the ionosphere is a plasma system controlled by electromagnetic forces driven by the magnetosphere, the solar wind, as well as the wind dynamo. The LTI is hence a domain controlled by many different physical processes. However, systematic in situ measurements within this region are severely lacking, although the LTI is located only 80 to 200 km above the surface of our planet. This paper reviews the current state of the art in measuring the LTI, either in situ or by several different remote-sensing methods. We begin by outlining the open questions within the LTI requiring high-quality in situ measurements, before reviewing directly observable parameters and their most important derivatives. The motivation for this review has arisen from the recent retention of the Daedalus mission as one among three competing mission candidates within the European Space Agency (ESA) Earth Explorer 10 Programme. However, this paper intends to cover the LTI parameters such that it can be used as a background scientific reference for any mission targeting in situ observations of the LTI.

Research paper thumbnail of Daedalus: a low-flying spacecraft for in situ exploration of the lower thermosphere–ionosphere

Geoscientific Instrumentation, Methods and Data Systems, 2020

The Daedalus mission has been proposed to the European Space Agency (ESA) in response to the call... more The Daedalus mission has been proposed to the European Space Agency (ESA) in response to the call for ideas for the Earth Observation program's 10th Earth Explorer. It was selected in 2018 as one of three candidates for a phase-0 feasibility study. The goal of the mission is to quantify the key electrodynamic processes that determine the structure and composition of the upper atmosphere, the gateway between the Earth's atmosphere and space. An innovative preliminary mission design allows Daedalus to access electrodynamics processes down to altitudes of 150 km and below. Daedalus will perform in situ measurements of plasma density and temperature, ion drift, neutral density and wind, ion and neutral composition, electric and magnetic fields, and precipitating particles. These measurements will unambiguously quantify the amount of energy deposited in the upper atmosphere during active and quiet geomagnetic times via Joule heating and energetic particle precipitation, estimates of which currently vary by orders of magnitude Published by Copernicus Publications on behalf of the European Geosciences Union. 154 T. E. Sarris et al.: Daedalus: a low-flying spacecraft for in situ exploration between models and observation methods. An innovation of the Daedalus preliminary mission concept is that it includes the release of subsatellites at low altitudes: combined with the main spacecraft, these subsatellites will provide multipoint measurements throughout the lower thermosphereionosphere (LTI) region, down to altitudes below 120 km, in the heart of the most under-explored region in the Earth's atmosphere. This paper describes Daedalus as originally proposed to the ESA.

Research paper thumbnail of Investigation of energy transport and thermospheric upwelling during quiet magnetospheric and ionospheric conditions from the studies of low- and middle-altitude cusp

Annales Geophysicae, 2015

We investigate energy fluxes and small, kilometrescale Birkeland currents in the magnetospheric c... more We investigate energy fluxes and small, kilometrescale Birkeland currents in the magnetospheric cusp at a 1-3 Earth radii altitude and in the ionosphere using satellites when they were, according to the Tsyganenko model, in magnetic conjunction within 50-60 km and up to 15 min apart. We use Cluster and CHAMP satellites, and study three conjunction events that occurred in 2008 and 2009, when the Cluster spacecraft were crossing the cusps at only a few Earth radii altitude. Our goal is to understand better the influence of processes in the magnetospheric cusp on the upper thermosphere and its upwelling which was usually observed by the CHAMP satellite passing the cusp. Three studied events occurred under relatively quiet and steady magnetospheric and ionospheric conditions, which explains why observed thermospheric density enhancements were rather low. Our findings point out that for each studied event soft electron precipitation influences thermospheric density enhancements in a way that stronger electron precipitation produces stronger thermospheric upwelling. Therefore, in the case of these weak events, soft electron precipitation seems to be more important cause of the observed, thermospheric density enhancements than is the Joule heating.

Research paper thumbnail of Localized field‐aligned currents in the polar cap associated with airglow patches

Journal of Geophysical Research: Space Physics, 2016

Propagation and evolution of electric fields associated with solar wind pressure pulses based on ... more Propagation and evolution of electric fields associated with solar wind pressure pulses based on spacecraft and ground-based observations, J.

Research paper thumbnail of Swarm and ESR observations of the ionospheric response to a field‐aligned current system in the high‐latitude midnight sector

Geophysical Research Letters, 2015

We present a conjunction between the Swarm fleet and the European Incoherent Scatter Svalbard Rad... more We present a conjunction between the Swarm fleet and the European Incoherent Scatter Svalbard Radar (ESR) on 9 January 2014. The Swarm orbit in the early phase of the mission gives us the unique opportunity of sequencing the temporal evolution of the observed field-aligned current system in the nightside, near magnetic local midnight. These field-aligned currents are seen to move poleward through the radar field of view and to affect the observed ionosphere. The upward field-aligned current (FAC) is responsible, at least in part, for the heating of the ionospheric electrons. It is less clear whether the downward FAC cools the ionosphere. We use the TRANSCAR model of the ionosphere to quantify the thermoelectric effect that comes into play. Finally, we compare the plasma parameters measured by the Langmuir probe on board Swarm and the ESR and conclude on an agreement within the errors.

Research paper thumbnail of The science case for the EISCAT_3D radar

Progress in Earth and Planetary Science, 2015

The EISCAT (European Incoherent SCATer) Scientific Association has provided versatile incoherent ... more The EISCAT (European Incoherent SCATer) Scientific Association has provided versatile incoherent scatter (IS) radar facilities on the mainland of northern Scandinavia (the EISCAT UHF and VHF radar systems) and on Svalbard (the electronically scanning radar ESR (EISCAT Svalbard Radar) for studies of the high-latitude ionised upper atmosphere (the ionosphere). The mainland radars were constructed about 30 years ago, based on technological solutions of that time. The science drivers of today, however, require a more flexible instrument, which allows measurements to be made from the troposphere to the topside ionosphere and gives the measured parameters in three dimensions, not just along a single radar beam. The possibility for continuous operation is also an essential feature. To facilitatefuture science work with a world-leading IS radar facility, planning of a new radar system started first with an EU-funded Design Study (2005-2009) and has continued with a follow-up EU FP7 EISCAT_3D Preparatory Phase project (2010-2014). The radar facility will be realised by using phased arrays, and a key aspect is the use of advanced software and data processing techniques. This type of software radar will act as a pathfinder for other facilities worldwide. The new radar facility will enable the EISCAT_3D science community to address new, significant science questions as well as to serve society, which is increasingly dependent on space-based technology and issues related to space weather. The location of the radar within the auroral oval and at the edge of the stratospheric polar vortex is also ideal for studies of the long-term variability in the atmosphere and global change. This paper is a summary of the EISCAT_3D science case, which was prepared as part of the EU-funded Preparatory Phase project for the new facility. Three science working groups, drawn from the EISCAT user community, participated in preparing this document. In addition to these working group members, who are listed as authors, thanks are due to many others in the EISCAT scientific community for useful contributions, discussions, and support.

Research paper thumbnail of Evidence of small-scale field aligned current sheets from the low and middle altitude cusp continuing in the ionosphere

We investigate kilometer-scale field-aligned currents that were detected both in the magnetospher... more We investigate kilometer-scale field-aligned currents that were detected both in the magnetospheric cusp at a few Earth radii altitude and in the ionosphere by satellites that were, according to the Tsyganenko model, within a few tens of kilometers and minutes on the same magnetic field line. Also thermosphere up-welling that often accompanies the dayside field-aligned currents in the inner cusp was seen. We used Cluster and CHAMP satellites, and searched for conjunctions during the whole year of 2008, as then the Cluster spacecrafts were mostly at mid-altitudes when crossing the cusps. We focus on two case studies from this period. Evidence is presented that sheets of small scale field-aligned current continue through the low altitude cusp and ionosphere. The ionospheric current densities are not particularly strong, a few micro A/m2 at about 340 km, and several tens of nA/m2 at about 20000 km, implying that these currents might be relatively common events, but are hard to detect d...

Research paper thumbnail of The evolution of flux pileup regions in the plasma sheet: Cluster observations

Journal of Geophysical Research: Space Physics, 2013

Bursty bulk flows (BBFs) play an important role for the mass, energy, and magnetic flux transport... more Bursty bulk flows (BBFs) play an important role for the mass, energy, and magnetic flux transport in the plasma sheet, and the flow pattern in and around a BBF has important consequences for the localized energy conversion between the electromagnetic and plasma mechanical energy forms. The plasma flow signature in and around BBFs is often rather complicated. Return flows and plasma vortices are expected to exist at the flanks of the main flow channel, especially near the inner plasma sheet boundary, but also farther down-tail. A dipolarization front (DF) is often observed at the leading edge of a BBF, and a flux pileup region (FPR) behind the DF. Here we present Cluster data of three FPRs associated with vortex flows observed in the midtail plasma sheet on 15 August 2001. According to the principles of Fu et al. (2011, 2012c), two of the FPRs are considered to be in an early stage of evolution (growing FPRs). The third FPR is in a later stage of evolution (decaying FPR). For the first time, the detailed energy conversion properties during various stages of the FPR evolution have been measured. We show that the later stage FPR has a more complex vortex pattern than the two earlier stage FPRs. The two early stage FPR correspond to generators, E J < 0, while the later stage FPR only shows weak generator characteristics and is instead dominated by load signatures at the DF, E J > 0. Moreover, to our knowledge, this is one of the first times BBF-related plasma vortices have been observed to propagate over the spacecraft in the midtail plasma sheet at geocentric distances of about 18R E. Our observations are compared to recent simulation results and previous observations.

Research paper thumbnail of Swarm in situ observations of F region polar cap patches created by cusp precipitation

Geophysical Research Letters, 2015

• Novel technique using stringof-pearls spacecraft to resolve plasma dynamics • In situ Swarm obs... more • Novel technique using stringof-pearls spacecraft to resolve plasma dynamics • In situ Swarm observations of the creation and evolution of polar cap patches • Cusp ionization can dominate the creation and initial structuring of patches

Research paper thumbnail of Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere

Research paper thumbnail of Magnetospheric energy budget during huge geomagnetic activity using Cluster and ground-based data

Journal of Geophysical Research, 2006

The Cluster spacecraft crossed the magnetopause at the duskward flank of the tail while the Europ... more The Cluster spacecraft crossed the magnetopause at the duskward flank of the tail while the European Incoherent Scatter (EISCAT) radars and magnetometers observed the ionosphere during a sequence of intense substorm-like geomagnetic activity in October 2003. We attempt to estimate the local and global energy flow from the magnetosheath into the magnetotail and the ionosphere under these extreme conditions. We make for the first time direct observational estimates of the local solar wind power input using Cluster measurements. The global power input based on Cluster observations was found to be between 17 and 40 TW at the onset of the substorm intensification. However, spacecraft observations and global modelling of the magnetotail suggest that it is most probably closer to 17 TW. This is more than two times lower than the predicted parameter value (37 TW). Energy deposition in the ionosphere has been estimated locally with EISCAT and globally with the assimilated mapping of ionospheric electrodynamics (AMIE) technique. The amount of the global solar wind power input (17 TW) that is dissipated via Joule heating in the ionosphere is found to be 30%. The corresponding ratio based on empirical estimates is only 3%. However, empirical proxies seem to underestimate the magnitude of the Joule heating rate as compared to AMIE estimates ($ a factor 4) and the parameter is more than twice as large as the Cluster estimate. In summary, the observational estimates provide a good balance between the energy input to the magnetosphere and deposition in the ionosphere. Empirical proxies seem to suffer from overestimations (parameter) and underestimations (Joule heating proxies) when pushed to the extreme circumstances during the early main phase of this storm period.

Research paper thumbnail of The role of the inner tail to midtail plasma sheet in channeling solar wind power to the ionosphere

Journal of Geophysical Research: Space Physics, 2012

In this article we use Cluster power density (E Á J) data from 2001, 2002, and 2004 to investigat... more In this article we use Cluster power density (E Á J) data from 2001, 2002, and 2004 to investigate energy conversion and transfer in the plasma sheet. We show that a southward IMF B z is favorable for plasma sheet energy conversion, and that there is an increased particle and Poynting flux toward the Earth at times when Cluster observes an enhanced energy conversion in the plasma sheet. Conversion from electromagnetic to kinetic energy is increasingly dominant farther down-tail, while the generation of electromagnetic power from kinetic energy becomes important toward the Earth with a maximum at roughly 10 R E. By linking observations of the key quantity E Á J to observations of the solar wind input and earthward energy flux, our results demonstrate the role of the inner tail to midtail plasma sheet as a mediator between the solar wind energy input into the magnetosphere and the auroral dissipation in the ionosphere.

Research paper thumbnail of Coordinated EISCAT Svalbard radar and Reimei satellite observations of ion upflows and suprathermal ions

Journal of Geophysical Research: Space Physics, 2008

Seasonal breeding of tropical freshwater fish may be synchronized with periods of high food consu... more Seasonal breeding of tropical freshwater fish may be synchronized with periods of high food consumption. We explored this hypothesis by studying the relationship between diet and reproductive activity of Creagrutus guanes (Teleostei, Characidae). Our results showed that C. guanes had a generalist and omnivorous diet dominated by aquatic insects (mainly Diptera larvae) and seeds. Creagrutus guanes did not show intersexual or ontogenetic variation in diet. Peaks of feeding activity during rainy months were not synchronized with breeding in dry months. Our results do not support the hypothesis that the reproductive season has to be synchronized with high food consumption. We discussed the hypothesis fat reserves may be an important factor for the desynchronization of peaks of feeding and reproduction as explanation of seasonal breeding of this species. A reprodução sazonal de peixes tropicais de água doce pode estar sincronizada com períodos de alto consumo de alimento. Nós examinamos esta hipótese estudando a relação entre dieta e atividade reprodutiva de Creagrutus guanes (Teleostei, Characidae). Nossos resultados mostraram que C. guanes teve uma dieta generalista e onívora, com predominância de insetos aquáticos (principalmente larva de Diptera) e sementes. Creagrutus guanes não apresentou variação intersexual ou ontogenética na dieta. Picos de atividade alimentar durante os meses chuvosos não foram sincronizados com atividade reprodutiva nos meses secos. Nossos resultados não sustentam a hipótese de que a estação reprodutiva está sincronizada com o consumo elevado de alimento. A reserva de gordura pode ser um fator importante para a dessincronização de picos de alimentação e reprodução (a reserva de gordura pode influenciar a ausência de sincronia entre os picos de alimentação e reprodução). São apresentadas hipóteses alternativas para a explicação de desova sazonal da espécie.

Research paper thumbnail of Extreme solar-terrestrial events of October 2003: High-latitude and Cluster observations of the large geomagnetic disturbances on 30 October

Journal of Geophysical Research: Space Physics, 2005

The extremely large solar eruption on 28 October 2003 caused an intense geomagnetic storm at Eart... more The extremely large solar eruption on 28 October 2003 caused an intense geomagnetic storm at Earth. A second solar eruption on 29 October resulted in a reintensification of the storm about a day later. Similarities and differences between these two events in terms of solar eruption, solar wind driver, and their resulting effect on the near-Earth environment are investigated and put into context of previous works on storm geoeffectivness. Within the second storm some of the strongest substorms in the history of magnetic recordings occurred in northern Scandinavia. The aim of this study is to investigate the cause and resulting effects of these extreme geomagnetic disturbances on the ionosphere and upper atmosphere, focusing on the northern Scandinavian sector where these disturbances reached extremely high values. During this time period, well after the initial arrival of the Interplanetary Coronal Mass Ejection (ICME), the Cluster spacecraft were located at the flank of the magnetospheric tail. The satellites were passed several times by an inward and consecutively outward moving magnetopause in close relation to the substorm intensifications in northern Scandinavia. We propose that the evolution of these magnetospheric substorm intensifications are influenced by the changing dynamics of the solar wind in the form of increased pressure occurring after a prolonged period of southward Interplanetary Magnetic Field (IMF) and thus excessive energy loading into the magnetosphere prior to the onset of the intensifications. We present evidence of external pressure pulse triggering and possibly also quenching of these substorm onsets and recoveries. In addition, EISCAT data have been used to investigate the detailed local behavior of the ionospheric plasma, giving rise to such extreme disturbances. We found that in this case, extreme combinations of enhanced conductivity and intense electric field resulted in very high current intensities (westward electrojet $7.4 MA) and very fast onset of such currents. The dB dt associated geomagnetically induced currents caused power failures in southern Sweden.

Research paper thumbnail of Energy conversion regions as observed by Cluster in the plasma sheet

Journal of Geophysical Research: Space Physics, 2011

In this article we present a review of recent studies of observations of localized energy convers... more In this article we present a review of recent studies of observations of localized energy conversion regions (ECRs) observed by Cluster in the plasma sheet at altitudes of 15-20R E. By examining variations in the power density, E • J, where E is the electric field and J is the current density, we show that the plasma sheet exhibits a high level of fine structure. Approximately three times as many concentrated load regions (CLRs) (E • J > 0) as concentrated generator regions (CGRs) (E • J < 0) are identified, confirming the average load character of the plasma sheet. Some ECRs are found to relate to auroral activity. While ECRs are relevant for the energy conversion between the electromagnetic field and the particles, bursty bulk flows (BBFs) play a central role for the energy transfer in the plasma sheet. We show that ECRs and BBFs are likely to be related, although details of this relationship are yet to be explored. The plasma sheet energy conversion increases rather simultaneously with increasing geomagnetic activity in both CLRs and CGRs. Consistent with large-scale magnetotail simulations, most of the observed ECRs appear to be rather stationary in space but varying in time. We estimate that the ECR lifetime and scale size are a few minutes and a few R E , respectively. It is conceivable that ECRs rise and vanish locally in significant regions of the plasma sheet, possibly oscillating between load and generator character, while some energy is transmitted as Poynting flux to the ionosphere.

Research paper thumbnail of Transient reconnection in the cusp during strongly negative IMF B y

Journal of Geophysical Research, 2004

We investigate transient magnetic reconnection (flux transfer events, FTEs) in the cusp and its i... more We investigate transient magnetic reconnection (flux transfer events, FTEs) in the cusp and its ionospheric signatures. We present in situ observations by the Polar spacecraft of multiple impulsive plasma injections and related magnetopause crossings in the outer stagnant cusp of the northern hemisphere. At this time the interplanetary magnetic field (IMF) was northward, dominated by strong B y (À11 nT). Plasma injections are observed in the direction of the magnetic tension force due to the IMF B y and resemble the ''time of flight'' energy-latitude dispersion. Associated magnetopause crossings exhibit large magnetic field rotation (>150 degrees) and strong depression of the field magnitude (down to several percent of the value in the magnetosheath). We use conjugate observations from the Greenland magnetometer network and Sondrestrom incoherent scatter radar together with the Polar UV imager to show that the transient reconnection causes a direct response of cusp auroral brightness and ionospheric convection in the direction consistent with the IMF B y polarity. The observations, in space and on the ground, are consistent with a sequence of bulges launched by transient reconnection. We present a qualitative model of the bulge structure that is consistent with most of the Polar observations during the studied interval. Conjugate ground-based observations provide additional evidence for transient reconnection and are consistent with the ground signature of FTEs.

Research paper thumbnail of What high altitude observations tell us about the auroral acceleration: A Cluster/DMSP conjunction

Geophysical Research Letters, 2003

Magnetic conjugate observations by Cluster and DMSP F14 satellites are used to study the field li... more Magnetic conjugate observations by Cluster and DMSP F14 satellites are used to study the field lines of auroral arc. Cluster is well above the acceleration region and observes upward keV ion beams and bipolar electric structures. The integrated potential at Cluster altitudes shows a dip that is consistent with the keV electron acceleration energy at low altitude. The earthward Poynting flux at Cluster altitudes is comparable to the electron energy flux at low altitudes. Thus, for this event the auroral acceleration can be described as a quasi-stationary potential structure with equipotential lines reaching the Cluster altitudes. The arc forms at the outer edge of the plasma sheet at a density gradient. Multiple Cluster satellite measurements allow us to study the density increase associated with the development of the arc, and to estimate the velocity of the structure. The quasi-potential structure itself may be part of an Alfvén wave.

Research paper thumbnail of Temporal evolution of two auroral arcs as measured by the Cluster satellite and coordinated ground-based instruments

Annales Geophysicae, 2004

When mapped along geomagnetic field lines, the separation of the spacecraft in the ionosphere was... more When mapped along geomagnetic field lines, the separation of the spacecraft in the ionosphere was confined to within 110 km in latitude and 50 km in longitude. This constellation allowed us to study the temporal evolution of plasma with a time scale of a few minutes. Ground-based instrumentation used involved two all-sky cameras, magnetometers and the EISCAT radar. The main findings were as follows. Two auroral arcs were located close to the equatorward and poleward edge of a large-scale density cavity, respectively. These arcs showed a different kind of a temporal evolution. (1) As a response to a pseudo-breakup onset, both the up-and downward field-aligned current (FAC) sheets associated with the equatorward arc widened and the total amount of FAC doubled in a time scale of 1-2 min. (2) In the poleward arc, a density cavity formed in the ionosphere in the return (downward) current region. As a result of ionospheric feedback, a strongly enhanced ionospheric southward electric field developed in the region of decreased Pedersen conductance. Furthermore, the acceleration potential of ionospheric electrons, carrying the return current, increased from 200 to 1000 eV in 70 s, and the return current region widened in order to supply a constant amount of return current to the arc current circuit. Evidence of local acceleration of the electron population by dispersive Alfvén waves was obtained in the upward FAC region of the poleward arc. However, the downward accelerated suprathermal electrons must be further energised below Cluster in order to be able to produce the observed visible aurora.

Research paper thumbnail of Field-aligned currents and ionospheric parameters deduced from EISCAT radar measurements in the post-midnight sector

Annales Geophysicae, 2002

Attempting to derive the field-aligned current (FAC) density using the EISCAT radar and to unders... more Attempting to derive the field-aligned current (FAC) density using the EISCAT radar and to understand the role of the ionosphere on closing FACs, we conducted special radar experiments with the EISCAT radar on 9 October 1999. In order to derive the gradient of the ionospheric conductivity (grad) and the divergence of the electric field (div E) nearly simultaneously, a special experiment employed an EISCAT radar mode which let the transmitting antenna sequentially point to four directions within 10 min; two pairs of the four directions formed two orthogonal diagonals of a square. ∼ 100 km from the arc, with no distinct depletion of electron density. We also detected a region of the high arcassociated electric field, accompanied by the depletion of electron density above 110 km. Using auroral images, this region was identified as a dark spot with a spatial scale of over 150 × 150 km. The dark spot and the electron depletion were likely in existence for a limited time of a few minutes.

Research paper thumbnail of Coordinated ground-based, low altitude satellite and Cluster observations on global and local scales during a transient post-noon sector excursion of the magnetospheric cusp

Annales Geophysicae, 2001

On 14 January 2001, the four Cluster spacecraft passed through the northern magnetospheric mantle... more On 14 January 2001, the four Cluster spacecraft passed through the northern magnetospheric mantle in close conjunction to the EISCAT Svalbard Radar (ESR) and approached the post-noon dayside magnetopause over Greenland between 13:00 and 14:00 UT. During that interval, a

Research paper thumbnail of Lower-thermosphere–ionosphere (LTI) quantities: current status of measuring techniques and models

Annales Geophysicae, 2021

The lower-thermosphere-ionosphere (LTI) system consists of the upper atmosphere and the lower par... more The lower-thermosphere-ionosphere (LTI) system consists of the upper atmosphere and the lower part of the ionosphere and as such comprises a complex system coupled to both the atmosphere below and space above. The atmospheric part of the LTI is dominated by laws of continuum fluid dynamics and chemistry, while the ionosphere is a plasma system controlled by electromagnetic forces driven by the magnetosphere, the solar wind, as well as the wind dynamo. The LTI is hence a domain controlled by many different physical processes. However, systematic in situ measurements within this region are severely lacking, although the LTI is located only 80 to 200 km above the surface of our planet. This paper reviews the current state of the art in measuring the LTI, either in situ or by several different remote-sensing methods. We begin by outlining the open questions within the LTI requiring high-quality in situ measurements, before reviewing directly observable parameters and their most important derivatives. The motivation for this review has arisen from the recent retention of the Daedalus mission as one among three competing mission candidates within the European Space Agency (ESA) Earth Explorer 10 Programme. However, this paper intends to cover the LTI parameters such that it can be used as a background scientific reference for any mission targeting in situ observations of the LTI.

Research paper thumbnail of Daedalus: a low-flying spacecraft for in situ exploration of the lower thermosphere–ionosphere

Geoscientific Instrumentation, Methods and Data Systems, 2020

The Daedalus mission has been proposed to the European Space Agency (ESA) in response to the call... more The Daedalus mission has been proposed to the European Space Agency (ESA) in response to the call for ideas for the Earth Observation program's 10th Earth Explorer. It was selected in 2018 as one of three candidates for a phase-0 feasibility study. The goal of the mission is to quantify the key electrodynamic processes that determine the structure and composition of the upper atmosphere, the gateway between the Earth's atmosphere and space. An innovative preliminary mission design allows Daedalus to access electrodynamics processes down to altitudes of 150 km and below. Daedalus will perform in situ measurements of plasma density and temperature, ion drift, neutral density and wind, ion and neutral composition, electric and magnetic fields, and precipitating particles. These measurements will unambiguously quantify the amount of energy deposited in the upper atmosphere during active and quiet geomagnetic times via Joule heating and energetic particle precipitation, estimates of which currently vary by orders of magnitude Published by Copernicus Publications on behalf of the European Geosciences Union. 154 T. E. Sarris et al.: Daedalus: a low-flying spacecraft for in situ exploration between models and observation methods. An innovation of the Daedalus preliminary mission concept is that it includes the release of subsatellites at low altitudes: combined with the main spacecraft, these subsatellites will provide multipoint measurements throughout the lower thermosphereionosphere (LTI) region, down to altitudes below 120 km, in the heart of the most under-explored region in the Earth's atmosphere. This paper describes Daedalus as originally proposed to the ESA.

Research paper thumbnail of Investigation of energy transport and thermospheric upwelling during quiet magnetospheric and ionospheric conditions from the studies of low- and middle-altitude cusp

Annales Geophysicae, 2015

We investigate energy fluxes and small, kilometrescale Birkeland currents in the magnetospheric c... more We investigate energy fluxes and small, kilometrescale Birkeland currents in the magnetospheric cusp at a 1-3 Earth radii altitude and in the ionosphere using satellites when they were, according to the Tsyganenko model, in magnetic conjunction within 50-60 km and up to 15 min apart. We use Cluster and CHAMP satellites, and study three conjunction events that occurred in 2008 and 2009, when the Cluster spacecraft were crossing the cusps at only a few Earth radii altitude. Our goal is to understand better the influence of processes in the magnetospheric cusp on the upper thermosphere and its upwelling which was usually observed by the CHAMP satellite passing the cusp. Three studied events occurred under relatively quiet and steady magnetospheric and ionospheric conditions, which explains why observed thermospheric density enhancements were rather low. Our findings point out that for each studied event soft electron precipitation influences thermospheric density enhancements in a way that stronger electron precipitation produces stronger thermospheric upwelling. Therefore, in the case of these weak events, soft electron precipitation seems to be more important cause of the observed, thermospheric density enhancements than is the Joule heating.

Research paper thumbnail of Localized field‐aligned currents in the polar cap associated with airglow patches

Journal of Geophysical Research: Space Physics, 2016

Propagation and evolution of electric fields associated with solar wind pressure pulses based on ... more Propagation and evolution of electric fields associated with solar wind pressure pulses based on spacecraft and ground-based observations, J.

Research paper thumbnail of Swarm and ESR observations of the ionospheric response to a field‐aligned current system in the high‐latitude midnight sector

Geophysical Research Letters, 2015

We present a conjunction between the Swarm fleet and the European Incoherent Scatter Svalbard Rad... more We present a conjunction between the Swarm fleet and the European Incoherent Scatter Svalbard Radar (ESR) on 9 January 2014. The Swarm orbit in the early phase of the mission gives us the unique opportunity of sequencing the temporal evolution of the observed field-aligned current system in the nightside, near magnetic local midnight. These field-aligned currents are seen to move poleward through the radar field of view and to affect the observed ionosphere. The upward field-aligned current (FAC) is responsible, at least in part, for the heating of the ionospheric electrons. It is less clear whether the downward FAC cools the ionosphere. We use the TRANSCAR model of the ionosphere to quantify the thermoelectric effect that comes into play. Finally, we compare the plasma parameters measured by the Langmuir probe on board Swarm and the ESR and conclude on an agreement within the errors.

Research paper thumbnail of The science case for the EISCAT_3D radar

Progress in Earth and Planetary Science, 2015

The EISCAT (European Incoherent SCATer) Scientific Association has provided versatile incoherent ... more The EISCAT (European Incoherent SCATer) Scientific Association has provided versatile incoherent scatter (IS) radar facilities on the mainland of northern Scandinavia (the EISCAT UHF and VHF radar systems) and on Svalbard (the electronically scanning radar ESR (EISCAT Svalbard Radar) for studies of the high-latitude ionised upper atmosphere (the ionosphere). The mainland radars were constructed about 30 years ago, based on technological solutions of that time. The science drivers of today, however, require a more flexible instrument, which allows measurements to be made from the troposphere to the topside ionosphere and gives the measured parameters in three dimensions, not just along a single radar beam. The possibility for continuous operation is also an essential feature. To facilitatefuture science work with a world-leading IS radar facility, planning of a new radar system started first with an EU-funded Design Study (2005-2009) and has continued with a follow-up EU FP7 EISCAT_3D Preparatory Phase project (2010-2014). The radar facility will be realised by using phased arrays, and a key aspect is the use of advanced software and data processing techniques. This type of software radar will act as a pathfinder for other facilities worldwide. The new radar facility will enable the EISCAT_3D science community to address new, significant science questions as well as to serve society, which is increasingly dependent on space-based technology and issues related to space weather. The location of the radar within the auroral oval and at the edge of the stratospheric polar vortex is also ideal for studies of the long-term variability in the atmosphere and global change. This paper is a summary of the EISCAT_3D science case, which was prepared as part of the EU-funded Preparatory Phase project for the new facility. Three science working groups, drawn from the EISCAT user community, participated in preparing this document. In addition to these working group members, who are listed as authors, thanks are due to many others in the EISCAT scientific community for useful contributions, discussions, and support.

Research paper thumbnail of Evidence of small-scale field aligned current sheets from the low and middle altitude cusp continuing in the ionosphere

We investigate kilometer-scale field-aligned currents that were detected both in the magnetospher... more We investigate kilometer-scale field-aligned currents that were detected both in the magnetospheric cusp at a few Earth radii altitude and in the ionosphere by satellites that were, according to the Tsyganenko model, within a few tens of kilometers and minutes on the same magnetic field line. Also thermosphere up-welling that often accompanies the dayside field-aligned currents in the inner cusp was seen. We used Cluster and CHAMP satellites, and searched for conjunctions during the whole year of 2008, as then the Cluster spacecrafts were mostly at mid-altitudes when crossing the cusps. We focus on two case studies from this period. Evidence is presented that sheets of small scale field-aligned current continue through the low altitude cusp and ionosphere. The ionospheric current densities are not particularly strong, a few micro A/m2 at about 340 km, and several tens of nA/m2 at about 20000 km, implying that these currents might be relatively common events, but are hard to detect d...

Research paper thumbnail of The evolution of flux pileup regions in the plasma sheet: Cluster observations

Journal of Geophysical Research: Space Physics, 2013

Bursty bulk flows (BBFs) play an important role for the mass, energy, and magnetic flux transport... more Bursty bulk flows (BBFs) play an important role for the mass, energy, and magnetic flux transport in the plasma sheet, and the flow pattern in and around a BBF has important consequences for the localized energy conversion between the electromagnetic and plasma mechanical energy forms. The plasma flow signature in and around BBFs is often rather complicated. Return flows and plasma vortices are expected to exist at the flanks of the main flow channel, especially near the inner plasma sheet boundary, but also farther down-tail. A dipolarization front (DF) is often observed at the leading edge of a BBF, and a flux pileup region (FPR) behind the DF. Here we present Cluster data of three FPRs associated with vortex flows observed in the midtail plasma sheet on 15 August 2001. According to the principles of Fu et al. (2011, 2012c), two of the FPRs are considered to be in an early stage of evolution (growing FPRs). The third FPR is in a later stage of evolution (decaying FPR). For the first time, the detailed energy conversion properties during various stages of the FPR evolution have been measured. We show that the later stage FPR has a more complex vortex pattern than the two earlier stage FPRs. The two early stage FPR correspond to generators, E J < 0, while the later stage FPR only shows weak generator characteristics and is instead dominated by load signatures at the DF, E J > 0. Moreover, to our knowledge, this is one of the first times BBF-related plasma vortices have been observed to propagate over the spacecraft in the midtail plasma sheet at geocentric distances of about 18R E. Our observations are compared to recent simulation results and previous observations.

Research paper thumbnail of Swarm in situ observations of F region polar cap patches created by cusp precipitation

Geophysical Research Letters, 2015

• Novel technique using stringof-pearls spacecraft to resolve plasma dynamics • In situ Swarm obs... more • Novel technique using stringof-pearls spacecraft to resolve plasma dynamics • In situ Swarm observations of the creation and evolution of polar cap patches • Cusp ionization can dominate the creation and initial structuring of patches

Research paper thumbnail of Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere

Research paper thumbnail of Magnetospheric energy budget during huge geomagnetic activity using Cluster and ground-based data

Journal of Geophysical Research, 2006

The Cluster spacecraft crossed the magnetopause at the duskward flank of the tail while the Europ... more The Cluster spacecraft crossed the magnetopause at the duskward flank of the tail while the European Incoherent Scatter (EISCAT) radars and magnetometers observed the ionosphere during a sequence of intense substorm-like geomagnetic activity in October 2003. We attempt to estimate the local and global energy flow from the magnetosheath into the magnetotail and the ionosphere under these extreme conditions. We make for the first time direct observational estimates of the local solar wind power input using Cluster measurements. The global power input based on Cluster observations was found to be between 17 and 40 TW at the onset of the substorm intensification. However, spacecraft observations and global modelling of the magnetotail suggest that it is most probably closer to 17 TW. This is more than two times lower than the predicted parameter value (37 TW). Energy deposition in the ionosphere has been estimated locally with EISCAT and globally with the assimilated mapping of ionospheric electrodynamics (AMIE) technique. The amount of the global solar wind power input (17 TW) that is dissipated via Joule heating in the ionosphere is found to be 30%. The corresponding ratio based on empirical estimates is only 3%. However, empirical proxies seem to underestimate the magnitude of the Joule heating rate as compared to AMIE estimates ($ a factor 4) and the parameter is more than twice as large as the Cluster estimate. In summary, the observational estimates provide a good balance between the energy input to the magnetosphere and deposition in the ionosphere. Empirical proxies seem to suffer from overestimations (parameter) and underestimations (Joule heating proxies) when pushed to the extreme circumstances during the early main phase of this storm period.

Research paper thumbnail of The role of the inner tail to midtail plasma sheet in channeling solar wind power to the ionosphere

Journal of Geophysical Research: Space Physics, 2012

In this article we use Cluster power density (E Á J) data from 2001, 2002, and 2004 to investigat... more In this article we use Cluster power density (E Á J) data from 2001, 2002, and 2004 to investigate energy conversion and transfer in the plasma sheet. We show that a southward IMF B z is favorable for plasma sheet energy conversion, and that there is an increased particle and Poynting flux toward the Earth at times when Cluster observes an enhanced energy conversion in the plasma sheet. Conversion from electromagnetic to kinetic energy is increasingly dominant farther down-tail, while the generation of electromagnetic power from kinetic energy becomes important toward the Earth with a maximum at roughly 10 R E. By linking observations of the key quantity E Á J to observations of the solar wind input and earthward energy flux, our results demonstrate the role of the inner tail to midtail plasma sheet as a mediator between the solar wind energy input into the magnetosphere and the auroral dissipation in the ionosphere.

Research paper thumbnail of Coordinated EISCAT Svalbard radar and Reimei satellite observations of ion upflows and suprathermal ions

Journal of Geophysical Research: Space Physics, 2008

Seasonal breeding of tropical freshwater fish may be synchronized with periods of high food consu... more Seasonal breeding of tropical freshwater fish may be synchronized with periods of high food consumption. We explored this hypothesis by studying the relationship between diet and reproductive activity of Creagrutus guanes (Teleostei, Characidae). Our results showed that C. guanes had a generalist and omnivorous diet dominated by aquatic insects (mainly Diptera larvae) and seeds. Creagrutus guanes did not show intersexual or ontogenetic variation in diet. Peaks of feeding activity during rainy months were not synchronized with breeding in dry months. Our results do not support the hypothesis that the reproductive season has to be synchronized with high food consumption. We discussed the hypothesis fat reserves may be an important factor for the desynchronization of peaks of feeding and reproduction as explanation of seasonal breeding of this species. A reprodução sazonal de peixes tropicais de água doce pode estar sincronizada com períodos de alto consumo de alimento. Nós examinamos esta hipótese estudando a relação entre dieta e atividade reprodutiva de Creagrutus guanes (Teleostei, Characidae). Nossos resultados mostraram que C. guanes teve uma dieta generalista e onívora, com predominância de insetos aquáticos (principalmente larva de Diptera) e sementes. Creagrutus guanes não apresentou variação intersexual ou ontogenética na dieta. Picos de atividade alimentar durante os meses chuvosos não foram sincronizados com atividade reprodutiva nos meses secos. Nossos resultados não sustentam a hipótese de que a estação reprodutiva está sincronizada com o consumo elevado de alimento. A reserva de gordura pode ser um fator importante para a dessincronização de picos de alimentação e reprodução (a reserva de gordura pode influenciar a ausência de sincronia entre os picos de alimentação e reprodução). São apresentadas hipóteses alternativas para a explicação de desova sazonal da espécie.

Research paper thumbnail of Extreme solar-terrestrial events of October 2003: High-latitude and Cluster observations of the large geomagnetic disturbances on 30 October

Journal of Geophysical Research: Space Physics, 2005

The extremely large solar eruption on 28 October 2003 caused an intense geomagnetic storm at Eart... more The extremely large solar eruption on 28 October 2003 caused an intense geomagnetic storm at Earth. A second solar eruption on 29 October resulted in a reintensification of the storm about a day later. Similarities and differences between these two events in terms of solar eruption, solar wind driver, and their resulting effect on the near-Earth environment are investigated and put into context of previous works on storm geoeffectivness. Within the second storm some of the strongest substorms in the history of magnetic recordings occurred in northern Scandinavia. The aim of this study is to investigate the cause and resulting effects of these extreme geomagnetic disturbances on the ionosphere and upper atmosphere, focusing on the northern Scandinavian sector where these disturbances reached extremely high values. During this time period, well after the initial arrival of the Interplanetary Coronal Mass Ejection (ICME), the Cluster spacecraft were located at the flank of the magnetospheric tail. The satellites were passed several times by an inward and consecutively outward moving magnetopause in close relation to the substorm intensifications in northern Scandinavia. We propose that the evolution of these magnetospheric substorm intensifications are influenced by the changing dynamics of the solar wind in the form of increased pressure occurring after a prolonged period of southward Interplanetary Magnetic Field (IMF) and thus excessive energy loading into the magnetosphere prior to the onset of the intensifications. We present evidence of external pressure pulse triggering and possibly also quenching of these substorm onsets and recoveries. In addition, EISCAT data have been used to investigate the detailed local behavior of the ionospheric plasma, giving rise to such extreme disturbances. We found that in this case, extreme combinations of enhanced conductivity and intense electric field resulted in very high current intensities (westward electrojet $7.4 MA) and very fast onset of such currents. The dB dt associated geomagnetically induced currents caused power failures in southern Sweden.

Research paper thumbnail of Energy conversion regions as observed by Cluster in the plasma sheet

Journal of Geophysical Research: Space Physics, 2011

In this article we present a review of recent studies of observations of localized energy convers... more In this article we present a review of recent studies of observations of localized energy conversion regions (ECRs) observed by Cluster in the plasma sheet at altitudes of 15-20R E. By examining variations in the power density, E • J, where E is the electric field and J is the current density, we show that the plasma sheet exhibits a high level of fine structure. Approximately three times as many concentrated load regions (CLRs) (E • J > 0) as concentrated generator regions (CGRs) (E • J < 0) are identified, confirming the average load character of the plasma sheet. Some ECRs are found to relate to auroral activity. While ECRs are relevant for the energy conversion between the electromagnetic field and the particles, bursty bulk flows (BBFs) play a central role for the energy transfer in the plasma sheet. We show that ECRs and BBFs are likely to be related, although details of this relationship are yet to be explored. The plasma sheet energy conversion increases rather simultaneously with increasing geomagnetic activity in both CLRs and CGRs. Consistent with large-scale magnetotail simulations, most of the observed ECRs appear to be rather stationary in space but varying in time. We estimate that the ECR lifetime and scale size are a few minutes and a few R E , respectively. It is conceivable that ECRs rise and vanish locally in significant regions of the plasma sheet, possibly oscillating between load and generator character, while some energy is transmitted as Poynting flux to the ionosphere.

Research paper thumbnail of Transient reconnection in the cusp during strongly negative IMF B y

Journal of Geophysical Research, 2004

We investigate transient magnetic reconnection (flux transfer events, FTEs) in the cusp and its i... more We investigate transient magnetic reconnection (flux transfer events, FTEs) in the cusp and its ionospheric signatures. We present in situ observations by the Polar spacecraft of multiple impulsive plasma injections and related magnetopause crossings in the outer stagnant cusp of the northern hemisphere. At this time the interplanetary magnetic field (IMF) was northward, dominated by strong B y (À11 nT). Plasma injections are observed in the direction of the magnetic tension force due to the IMF B y and resemble the ''time of flight'' energy-latitude dispersion. Associated magnetopause crossings exhibit large magnetic field rotation (>150 degrees) and strong depression of the field magnitude (down to several percent of the value in the magnetosheath). We use conjugate observations from the Greenland magnetometer network and Sondrestrom incoherent scatter radar together with the Polar UV imager to show that the transient reconnection causes a direct response of cusp auroral brightness and ionospheric convection in the direction consistent with the IMF B y polarity. The observations, in space and on the ground, are consistent with a sequence of bulges launched by transient reconnection. We present a qualitative model of the bulge structure that is consistent with most of the Polar observations during the studied interval. Conjugate ground-based observations provide additional evidence for transient reconnection and are consistent with the ground signature of FTEs.

Research paper thumbnail of What high altitude observations tell us about the auroral acceleration: A Cluster/DMSP conjunction

Geophysical Research Letters, 2003

Magnetic conjugate observations by Cluster and DMSP F14 satellites are used to study the field li... more Magnetic conjugate observations by Cluster and DMSP F14 satellites are used to study the field lines of auroral arc. Cluster is well above the acceleration region and observes upward keV ion beams and bipolar electric structures. The integrated potential at Cluster altitudes shows a dip that is consistent with the keV electron acceleration energy at low altitude. The earthward Poynting flux at Cluster altitudes is comparable to the electron energy flux at low altitudes. Thus, for this event the auroral acceleration can be described as a quasi-stationary potential structure with equipotential lines reaching the Cluster altitudes. The arc forms at the outer edge of the plasma sheet at a density gradient. Multiple Cluster satellite measurements allow us to study the density increase associated with the development of the arc, and to estimate the velocity of the structure. The quasi-potential structure itself may be part of an Alfvén wave.

Research paper thumbnail of Temporal evolution of two auroral arcs as measured by the Cluster satellite and coordinated ground-based instruments

Annales Geophysicae, 2004

When mapped along geomagnetic field lines, the separation of the spacecraft in the ionosphere was... more When mapped along geomagnetic field lines, the separation of the spacecraft in the ionosphere was confined to within 110 km in latitude and 50 km in longitude. This constellation allowed us to study the temporal evolution of plasma with a time scale of a few minutes. Ground-based instrumentation used involved two all-sky cameras, magnetometers and the EISCAT radar. The main findings were as follows. Two auroral arcs were located close to the equatorward and poleward edge of a large-scale density cavity, respectively. These arcs showed a different kind of a temporal evolution. (1) As a response to a pseudo-breakup onset, both the up-and downward field-aligned current (FAC) sheets associated with the equatorward arc widened and the total amount of FAC doubled in a time scale of 1-2 min. (2) In the poleward arc, a density cavity formed in the ionosphere in the return (downward) current region. As a result of ionospheric feedback, a strongly enhanced ionospheric southward electric field developed in the region of decreased Pedersen conductance. Furthermore, the acceleration potential of ionospheric electrons, carrying the return current, increased from 200 to 1000 eV in 70 s, and the return current region widened in order to supply a constant amount of return current to the arc current circuit. Evidence of local acceleration of the electron population by dispersive Alfvén waves was obtained in the upward FAC region of the poleward arc. However, the downward accelerated suprathermal electrons must be further energised below Cluster in order to be able to produce the observed visible aurora.

Research paper thumbnail of Field-aligned currents and ionospheric parameters deduced from EISCAT radar measurements in the post-midnight sector

Annales Geophysicae, 2002

Attempting to derive the field-aligned current (FAC) density using the EISCAT radar and to unders... more Attempting to derive the field-aligned current (FAC) density using the EISCAT radar and to understand the role of the ionosphere on closing FACs, we conducted special radar experiments with the EISCAT radar on 9 October 1999. In order to derive the gradient of the ionospheric conductivity (grad) and the divergence of the electric field (div E) nearly simultaneously, a special experiment employed an EISCAT radar mode which let the transmitting antenna sequentially point to four directions within 10 min; two pairs of the four directions formed two orthogonal diagonals of a square. ∼ 100 km from the arc, with no distinct depletion of electron density. We also detected a region of the high arcassociated electric field, accompanied by the depletion of electron density above 110 km. Using auroral images, this region was identified as a dark spot with a spatial scale of over 150 × 150 km. The dark spot and the electron depletion were likely in existence for a limited time of a few minutes.