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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Terrestrial space weather involves the transfer of energy and momentum from the solar wind into geospace. Despite recently discovered seasonal asymmetries between auroral forms and the intensity of ...emissions between northern and southern hemispheres, seasonally averaged energy input into the ionosphere is still generally considered to be symmetric. Here we show, using Swarm satellite data, a preference for electromagnetic energy input at 450 km altitude into the northern hemisphere, on both the dayside and the nightside, when averaged over season. We propose that this is explained by the offset of the magnetic dipole away from Earth's center. This introduces a larger separation between the magnetic pole and rotation axis in the south, creating different relative solar illumination of northern and southern auroral zones, resulting in changes to the strength of reflection of incident Alfvén waves from the ionosphere. Our study reveals an important asymmetry in seasonally averaged electromagnetic energy input to the atmosphere. Based on observed lower Poynting flux on the nightside this asymmetry may also exist for auroral emissions. Similar offsets may drive asymmetric energy input, and potentially aurora, on other planets.
Large thermospheric neutral density enhancements in the cusp region have been examined for many years. The Challenging Minisatellite Payload (CHAMP) satellite for example has enabled many ...observations of the perturbation, showing that it is mesoscale in size and exists statistically over solar cycle timescales. Further studies examining the relationship with magnetospheric energy input have shown that fine‐scale Poynting fluxes are associated with the density perturbations on a case‐by‐case basis, whilst others have found that mesoscale downward fluxes also exist in the cusp region statistically. In this study, we use nearly 8 years of the overlapping Super Dual Auroral Radar Network and Active Magnetosphere and Planetary Electrodynamics Response Experiment datasets to generate global‐scale patterns of the high‐latitude and height‐integrated Poynting flux into the ionosphere, with a time resolution of 2 min. From these, average patterns are generated based on the interplanetary magnetic field orientation. We show the cusp is indeed an important feature in the Poynting flux maps, but the magnitude does not correlate well with statistical neutral mass density perturbations observed by the CHAMP satellite on similar spatial scales. Importantly, the lack of correlation between mesoscale height‐integrated Poynting fluxes and the cusp neutral mass density enhancement gives possible insight into other processes that may account for the discrepancy, such as energy deposition at finer scale sizes or at higher altitudes than captured.
Key Points
Statistical patterns of the total downward Poynting flux into the atmosphere have been derived using Super Dual Auroral Radar Network and Active Magnetosphere and Planetary Electrodynamics Response Experiment data
Statistical patterns of neutral mass density perturbations as a percentage of the background density have been derived using Challenging Minisatellite Payload data
Mesoscale downward Poynting flux in the cusp region do not correlate very well with neutral mass density enhancements at a similar scale
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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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Underestimation of the transfer of energy between the magnetosphere and ionosphere, the Poynting flux, is a persistent issue in space weather studies and the high‐latitude ionospheric models. Thought ...to be due to the inability to resolve small‐scale fluctuations of the ionospheric electric field, this underestimation could lead to significant further underestimations in parameters such as the thermospheric mass density and consequential satellite drag. Utilizing 16 Hz ion velocity and magnetic field measurements from the Swarm satellite mission, we examine the observed Poynting flux due to electric field fluctuations on very small spatial scales (∼1 km), and then artificially smooth the data to increase the observed scale. We quantify the decrease of integrated Poynting flux, poleward of 60/‐60° geomagnetic latitude, with increasing spatial scale. The decrease can be underestimated by as much as 15% by increasing scale from 1 km to only 8.6 km, or 16–2 Hz equivalent, with upward Poynting flux decreasing significantly faster. Our results thus point to a significant Alfvén wave driven component of the Poynting flux on kilometer scales. Additionally, we observe a northern hemisphere preference for increased Poynting flux, of which we examine its dependence on scale size and interplanetary magnetic field.
Plain Language Summary
At Earth's high‐latitudes, energy from space weather enters the upper atmosphere (>100 km) and is deposited mostly as heat. This heat can have numerous knock‐on effects on the atmosphere, such as causing the air density to increase, which poses a risk to satellites orbiting at the same altitude. It is thus very important to accurately quantify the energy that enters the atmosphere from space, but it has been found that a large proportion of the energy is released on very small spatial scales (on the order of kilometers). These scales are often difficult to measure due to resolution limitations of most instruments, however, the Swarm constellation of satellites are equipped with electric field instruments that can retrieve measurements 16 times per second (16 Hz), equivalent to observing spatial scales of around 1 km. In this study, we use nearly 7 years of high‐resolution Swarm data to calculate the average space weather energy input, known as the Poynting flux, and investigate the effect of artificially smoothing out the data to simulate larger spatial scales. We find significant underestimations of Poynting flux at larger scales compared to the smaller ones, which stresses the importance of small‐scale measurements in estimating the space weather—atmosphere energy budget.
Key Points
High‐resolution Swarm satellite data is used to examine Poynting flux across various spatial scales statistically
Poynting flux decreases significantly with increasing spatial scale, dropping faster at scales under 10 km
Area Integrated Poynting flux is 9–28% larger in the northern hemisphere than in the south
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Storm-time topside ionosphere plasma composition, especially the light ion fraction, is an important parameter which controls magnetosphere–ionosphere coupling, plays a part in the growth of local ...instabilities, and provides information about the ring current, ion upflow, movement of ionization and other important physical processes and parameters. Ion composition is difficult to estimate on fine scales as empirical models tend to be parametrized by fixed inputs, ignoring the role of memory in plasma, and to preferentially capture large scales, while ground radars have limited coverage. In particular, ionospheric composition measurements at mid-latitude are lacking. Here we show, using the new Swarm SLIDEM effective ion mass measurement, a superposed epoch analysis of storm-time dayside and nightside effective ion mass changes, demonstrating the extent and timescales of motion of the O+/H+ transition height with the main phase of geomagnetic storms, as well as directly observing evidence for the latitude dependence of these dynamics.
Graphical Abstract
Current methods for estimating ion density on Swarm rely on the assumption of 100% O + and no along-track ion velocity flows. These assumptions are routinely violated, particularly on the nightside ...and during high-latitude and polar cap traversals, compromising the accuracy of the measurements. The use of faceplate current data along with the Langmuir probe ion admittance measurements, and orbital-motion limited (OML) theory, make it possible to relax some of the assumptions inherent in current ESA Swarm density estimates. This further yields along-track ion drift and effective ion mass estimates. This paper describes the theoretical basis for estimating revised ion density, providing a new estimate for effective ion mass, as well as an alternative way of estimating along-track ion drift. The complete Swarm historical data set has been generated and validated using empirical models (International Reference Ionosphere, and an empirical electric field model), as well as ground-spacecraft conjunctions. Case studies and statistical results reveal clear geophysical signatures in the new product of light ions at low- and mid-latitudes and along-track ion drift at high latitudes, and their response to space weather.
Graphical Abstract
Key points
A new data product for Swarm along-track ion drift velocity, density and effective mass has been derived
The addition of faceplate current to ion admittance enables a refinement to Swarm ion density
The estimations have been validated against a variety of independent measurements and empirical models
The electrodynamics associated with dual discrete arc aurora with antiparallel flow along the arcs were observed nearly simultaneously by the enhanced Polar Outflow Probe (e‐POP) and the Swarm A and ...C spacecraft. Auroral imaging from e‐POP reveals 1–10 km structuring of the arcs, which move and evolve on second timescales and confound the traditional single‐spacecraft field‐aligned current algorithms. High‐cadence magnetic data from e‐POP show 1–10 Hz, inferred Alfvénic, perturbations coincident with and at the same scale size as the observed dynamic auroral fine structures. High‐cadence electric and magnetic field data from Swarm A reveal nonstationary electrodynamics involving reflected and interfering Alfvén waves and modulation consistent with trapping in the ionospheric Alfvén resonator (IAR). These observations suggest a role for Alfvén waves, perhaps also the IAR, in discrete arc dynamics on 0.2–10 s timescales and ~1–10 km spatial scales and reinforce the importance of considering Alfvén waves in magnetosphere‐ionosphere coupling.
Plain Language Summary
An ongoing question in space physics is whether the energy that powers the vibrant and dynamic aurora is the result of static electric fields or magnetic waves. We address this question using data from three observational satellites, e‐POP, Swarm A, and Swarm C. We compare electric and magnetic field measurements at the locations of the spacecraft to high‐speed images of the aurora below as the satellites traveled over northern Canada. We show that as the satellites traveled over the aurora they detected magnetic waves known as Alfvén waves. We argue that these waves play an important and underappreciated role in transporting energy from near‐Earth space to the atmosphere in order to power the aurora.
Key Points
High‐resolution e‐POP magnetic and auroral data reveal coherent, inferred Alfvénic, dynamics and structuring of discrete auroral arcs
Multipoint high‐cadence Swarm E‐ and B‐ field data reveal nonstationary electrodynamics involving reflected and interfering Alfvén waves
Suggests a role for Alfvén waves and possibly ionospheric Alfvén resonator (IAR) in discrete arc dynamics on 0.2–10 s and 1–10 km scales
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Abstract
By comparing data from 530 nightside auroral zone crossings of the three Swarm satellites with images from THEMIS ground‐based all‐sky imagers (ASIs), we find that 154 arc systems, ...containing two or more arcs, host regions of fluctuating magnetic and electric fields (Alfvén waves) up to 8 Hz. Many cases exhibit a standing wave pattern as seen by comparing the amplitude ratio and phase difference of electric and magnetic field Fourier components. The largest peak of Alfvénic cross‐power spectral density (CPSD, which is similar to Poynting flux) within a large‐scale current system tends to be narrow in latitude (about 50–100 km) and its magnitude increases with an increasing number of current sheets in the field‐aligned current (FAC) system. The peak of CPSD tends to be located on the poleward side of the current system but can extend as equatorward as the boundary between the Region 1 and Region 2 FACs. CPSD tends to peak either within downward current sheets (
of cases) or on the boundary between downward and upward FACs (
). Only
of peaks occur within upward current sheets, indicating that there is only a weak association with discrete auroral arcs, a finding which is confirmed by comparison with ASI images. These findings demonstrate that Alfvénic fluctuations occur over a much wider range of locations and conditions than previously reported.
Key Points
Roughly one third of nightside auroral zone crossings reveal regions of strong Alfvén wave activity up to 8 Hz
Alfvénic regions can occur in upward and downward FACs and at the boundary between them
Alfvénic regions occur over a much wider range of locations and conditions than previously reported
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Auroral arcs occur in regions of upward field‐aligned currents (FACs); however, the relation is not one to one, since kinetic energy of the current‐carrying electrons is also important in the ...production of auroral luminosity. Multiple auroral arc systems provide an opportunity to study the relation between FACs and auroral brightness in detail. In this study, we have identified two types of FAC configurations in multiple parallel arc systems using ground‐based optical data from the Time History of Events and Macroscale Interactions during Substorms all‐sky imagers, magnetometers and electric field instruments on board the Swarm satellites. In “unipolar FAC” events, each arc is an intensification within a broad, unipolar current sheet and downward return currents occur outside of this broad sheet. In “multipolar FAC” events, multiple arc systems represent a collection of multiple up/down current pairs. By collecting 17 events with unipolar FAC and 12 events with multipolar FACs, we find that (1) unipolar FAC events occur most frequently between 20 and 21 magnetic local time and multipolar FAC events tend to occur around local midnight and within 1 h after substorm onset. (2) Arcs in unipolar FAC systems have a typical width of 10–20 km and a spacing of 25–50 km. Arcs in multipolar FAC systems are wider and more separated. (3) Upward currents with more arcs embedded have larger intensities and widths. (4) Electric fields are strong and highly structured on the edges of multiple arc system with unipolar FAC. The fact that arcs with unipolar FAC are much more highly structured than the associated currents suggests that arc multiplicity is indicative not of a structured generator deep in the magnetosphere, but rather of the magnetosphere‐ionosphere coupling process.
Key Points
Multiple arcs with “unipolar” FAC are located within a broad, upward current sheet and downward currents occur outside of the arc system
Multiple arc systems with “multipolar FAC” represent a collection of multiple up/down current pairs
We study multiple arc system with “unipolar” and “multipolar” FAC and compare the MLT distribution, relation to substorm and arc morphology
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