The European Space Agency's three Swarm satellites were launched on 22 November 2013 into nearly polar, circular orbits, eventually reaching altitudes of 460 km (Swarm A and C) and 510 km (Swarm B). ...Swarm's multiyear mission is to make precision, multipoint measurements of low‐frequency magnetic and electric fields in Earth's ionosphere for the purpose of characterizing magnetic fields generated both inside and external to the Earth, along with the electric fields and other plasma parameters associated with electric current systems in the ionosphere and magnetosphere. Electric fields perpendicular to the magnetic field
B→ are determined through ion drift velocity
v→i and magnetic field measurements via the relation
E→⊥=−v→i×B→. Ion drift is derived from two‐dimensional images of low‐energy ion distribution functions provided by two Thermal Ion Imager (TII) sensors viewing in the horizontal and vertical planes;
v→i is corrected for spacecraft potential as determined by two Langmuir probes (LPs) which also measure plasma density ne and electron temperature Te. The TII sensors use a microchannel‐plate‐intensified phosphor screen imaged by a charge‐coupled device to generate high‐resolution distribution images (66 × 40 pixels) at a rate of 16 s−1. Images are partially processed on board and further on the ground to generate calibrated data products at a rate of 2 s−1; these include
v→i,
E→⊥, and ion temperature Ti in addition to electron temperature Te and plasma density ne from the LPs.
Key Points
Swarm TII sensors provide ion velocity and temperature from core ion distribution function images
CCD‐based imaging detectors in the TIIs support event rates that are much higher than possible with charge amplifier‐based systems
Swarm LPs use a harmonic‐mode technique to obtain electron density, temperature, and spacecraft potential at high rates
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Little is currently known about the optical phenomenon known as Steve. The first scientific publication on the subject suggests that Steve is associated with an intense subauroral ion drift (SAID). ...However, additional inquiry is warranted as this suggested relationship as it is based on a single case study. Here we present eight occurrences of Steve with coincident or near‐coincident measurements from the European Space Agency's Swarm satellites and show that Steve is consistently associated with SAID. When satellite observations coincident with Steve are compared to that of typical SAID, we find the SAID associated with Steve to have above average peak ion velocities and electron temperatures, as well as extremely low plasma densities.
Key Points
Eight Steve events were identified in all‐sky imager measurements with coincident or near‐coincident measurements from the Swarm satellites
In all cases, evidence of subauroral ion drifts are observed in Swarm measurements
All measurements of SAID overlapping with Steve have very fast ion flows, high electron temperatures, and extremely low plasma densities
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
High‐resolution in situ measurements from the three Swarm spacecraft, in a string‐of‐pearls configuration, provide new insights about the combined role of flow channel events and particle impact ...ionization in creating F region electron density structures in the northern Scandinavian dayside cusp. We present a case of polar cap patch formation where a reconnection‐driven low‐density relative westward flow channel is eroding the dayside solar‐ionized plasma but where particle impact ionization in the cusp dominates the initial plasma structuring. In the cusp, density features are observed which are twice as dense as the solar‐ionized background. These features then follow the polar cap convection and become less structured and lower in amplitude. These are the first in situ observations tracking polar cap patch evolution from creation by plasma transport and enhancement by cusp precipitation, through entrainment in the polar cap flow and relaxation into smooth patches as they approach the nightside auroral oval.
Key Points
Novel technique using string‐of‐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
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Magnetic reconnection leads to energy conversion in large volumes in space but is initiated in small diffusion regions. Because of the small sizes of the diffusion regions, their crossings by ...spacecraft are rare. We report four-spacecraft observations of a diffusion region encounter at the Earth's magnetopause that allow us to reliably distinguish spatial from temporal features. We find that the diffusion region is stable on ion time and length scales in agreement with numerical simulations. The electric field normal to the current sheet is balanced by the Hall term in the generalized Ohm's law, E(n) approximately jxB/ne.n, thus establishing that Hall physics is dominating inside the diffusion region. The reconnection rate is fast, approximately 0.1. We show that strong parallel currents flow along the separatrices; they are correlated with observations of high-frequency Langmuir/upper hybrid waves.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Theoretical models and observations have suggested that the increasing greenhouse gas concentration in the troposphere causes the upper atmosphere to cool and contract. However, our understanding of ...the long‐term trends in the upper atmosphere is still quite incomplete, due to a limited amount of available and well‐calibrated data. The European Incoherent Scatter radar has gathered data in the polar ionosphere above Tromsø for over 33 years. Using this long‐term data set, we have estimated the first significant trends of ion temperature at altitudes between 200 and 450 km. The estimated trends indicate a cooling of 10–15 K/decade near the F region peak (220–380 km altitude), whereas above 400 km the trend is nearly zero or even warming. The height profiles of the observed trends are close to those predicted by recent atmospheric general circulation models. Our results are the first quantitative confirmation of the simulations and of the qualitative expectations.
Key Points
Upper atmosphere cooling over the past 33 years is observed with EISCAT data
Above about 400 km height ionospheric warming is seen
The height profiles of the trends are close to those produced by the recent GCM
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The Swarm mission represents a strong new tool to survey polar cap patches and plasma structuring inside the polar cap. In the early commissioning phase, the three Swarm satellites were operated in a ...pearls‐on‐a‐string configuration making noon‐midnight transpolar passes. This provides an unparalleled opportunity to examine the potential role of the gradient drift instability (GDI) process on polar cap patches by systematically calculating GDI growth times during their transit across the pole from day to night. Steep kilometer‐scale gradients appeared in this study as initial structures that persisted during the approximate 90 min it took a patch to cross the polar cap. The GDI growth times were calculated for a selection of the steep density gradients on both the dayside and the nightside. The values ranged from 23 s to 147 s, which is consistent with recent rocket measurements in the cusp auroral region and provides a template for future studies. Growth times of the order of 1 min found both on the dayside and on the nightside support the existing view that the GDI may play a dominant role in the generation of radio wave scintillation irregularities as the patches transit the polar cap from day to night.
Key Points
New technique to assess GDI/polar cap plasma structuring using Swarm
Internal kilometer‐scale structures persist as patches convect across the polar cap
The GDI can act quickly on several kilometer‐scale gradients within polar cap patches
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We have investigated how geomagnetic activity, the solar wind (SW), and the interplanetary magnetic field (IMF) influence the occurrence of the F‐region/topside ionospheric ion upflow and downflow. ...Occurrence of dayside ion upflow observed with the European Incoherent Scatter Svalbard radar (ESR) at 75.2° magnetic latitude is highly correlated with the SW density, as well as with the strength of the IMF By component. We suggest that this correlation exists because the region where ion upflow occurs is enlarged owing to SW density and IMF By magnitude, but it does not move significantly in geomagnetic latitude. The occurrence frequency of dayside ion upflow displays peaks versus the geomagnetic activity index (Kp), SW velocity, and negative IMF Bz component; that is, ion upflow is less frequently seen at the highest values of these parameters. Dayside ion downflow in the F‐region/topside ionosphere occurs only when the Kp index and/or SW velocity are high or when IMF Bz is largely negative. The ion downflow is likely due to ballistic return of the ion upflow. We suggest that the region of ion upflow not only becomes larger but also moves equatorward with increasing Kp, SW velocity, and negative IMF Bz. The ESR can so be poleward of the upflow region and observe ions convecting poleward and returning ballistically downward.
We use Cluster spacecraft observations to study in detail the structure of a magnetic reconnection separatrix region on the magnetospheric side of the magnetopause about 50 ion inertial lengths away ...from the X‐line. The separatrix region is the region between the magnetic separatrix and the reconnection jet. It is several ion inertial lengths wide and it contains a few subregions showing different features in particle and wave data. One subregion, a density cavity adjacent to the separatrix, has strong electric fields, electron beams and intense wave turbulence. The separatrix region shows structures even at smaller scales, for example, solitary waves at Debye length scale. We describe in detail electron distribution functions and electric field spectra in the separatrix region and we compare them to a numerical simulation. Our observations show that while reconnection is ongoing the separatrix region is highly structured and dynamic in the electric field even if the X‐line is up to 50 ion inertial lengths away.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The Earth's neutral atmosphere is the driver of the well-known solar quiet (Sq) and other magnetic variations observed for more than 100 years. Yet the understanding of how the neutral wind can ...accomplish a dynamo effect has been incomplete. A new viable model is presented where a dynamo effect is obtained only in the case of winds perpendicular to the magnetic field B that do not map along B. Winds where u×B is constant have no effect. We identify Sq as being driven by wind differences at magnetically conjugate points and not by a neutral wind per se. The view of two different but entangled dynamos is favoured, with some conceptual analogy to quantum mechanical states. Because of the large preponderance of the neutral gas mass over the ionized component in the Earth's ionosphere, the dominant effect of the plasma adjusting to the winds is Joule heating. The amount of global Joule heating power from Sq is estimated, with uncertainties, to be much lower than Joule heating from ionosphere–magnetosphere coupling at high latitudes in periods of strong geomagnetic activity. However, on average both contributions could be relatively comparable. The global contribution of heating by ionizing solar radiation in the same height range should be 2–3 orders of magnitude larger.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The influence of solar activity upon ion upflow in the polar ionosphere was investigated using data obtained by the European Incoherent Scatter (EISCAT) Tromsø UHF radar between 1984 and 2008. In ...agreement with other work we find that the upward ion flux is generally higher when solar activity is high than when it is low. Ion upflow events and also the upward velocity behave the opposite: they are more frequently seen and higher, respectively, at times of low solar activity. In any year about 30–40% ion upflow is accompanied by ∼500 K higher electron temperature than the background temperature at 400 km altitude. Electron and ion heating in connection with upflow is nearly twice as prevalent during high solar activity as it is at low activity. The acceleration of ions by pressure gradients and ambipolar electric field becomes larger when solar activity is low than when it is high. This variation of the average acceleration is caused by the different shapes of electron density profiles for low and high solar activities. Ions start to flow up at above 450 km altitude when solar activity was high, and lower, at 300–500 km altitude, at low solar activity. It is suggested that the solar activity influences long‐term variations of the ion upflow occurrence because it modulates the density of neutral particles, the formation of the F2 density peak, and ion‐neutral collision frequencies in the thermosphere and ionosphere.
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