D. J. Knudsen

Width and structure of mesoscale optical auroral arcs

Arc widths were calculated for 3126 stable auroral arcs observed by an all-sky camera located in Gillam, Manitoba. The camera is filtered to accept 5577-A emissions and has a single-pixel spatial resolution of 1.7 km at zenith. The measured mean width of stable mesoscale arcs located within ±5° of magnetic zenith is 18 km with a standard deviation of 9 km. The width distribution exhibits a steep cutoff below 8 km; when combined with studies of small-scale auroral structure this cutoff suggests a gap in the occurrence of arcs with widths of order 1 km. This feature of the arc width spectrum argues against the notion of a turbulent cascade of energy from larger to small scales. Residuals from the Gaussian fits are only about 3% of the arc amplitude on average, indicating little sub-structure within arcs at scales down to the measurement resolution.

Spatial modulation of electron energy and density by nonlinear stationary inertial Alfvén waves

A cold, nonlinear, quasi-neutral, two-fluid model of field-aligned current (FAC) sheets with a uniform tangential electric field describes stationary, spatially periodic electromagnetic perturbations (stationary inertial Alfven (SIA) waves) which carry enhanced current, which can accelerate electrons parallel to a strong background magnetic field B0, and which cause large variations in plasma density. The waves propagate along B0 at speeds less than the Alfven speed VA. Waves can be divided into two branches according to the sign of their field-aligned phase velocity with respect to the direction of field-aligned electron drift. Waves propagating parallel to electron drift can accelerate low-energy electrons up to VA within steady state parallel sheets with widths of the order of the electron inertial length λe = c/ωpe, where ωpe is the electron plasma frequency. Waves propagating antiparallel to electron drift can accelerate electrons to velocities much greater than VA in steady state sheets which are orders of magnitude wider than λe and which are depleted in density. SIA waves also form when only an energetic subpopulation of electrons carries FAC through an ambient population initially at rest and can accelerate the ambient electrons to approach the energy of the subpopulation. These results extend the phenomenon of steady state electron acceleration by inertial Alfven waves to include much higher energies and wider spatial scales than previously recognized. Although the relatively simple model described here omits certain effects which are known to be important in the Earths auroral regions, several properties of SIA wave solutions suggest they may play a role in auroral arc formation, including steady state field-aligned electron acceleration over a broad range of spatial scales and energies, density variation, and the formation of multiple parallel structures.

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

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.

A low-energy charged particle distribution imager with a compact sensor for space applications

Low-energy plasmas having temperatures of order 1 eV or less are found commonly in the ionospheres and space environments of Earth and other planets. Measuring the density, temperature, drift velocities, phase-space anisotropies, and other properties of these plasmas presents numerous challenges. Examples are distortions of particle trajectories due to spacecraft wakes, spacecraft charging, and particle gyromotion in magnetized plasmas. Furthermore, these plasmas are known to organize into structures as small as tens of meters across, traversed by spacecraft in tens of milliseconds or less. The Suprathermal Plasma Imager (SPI) was developed to address these challenges. The SPI is optimized for measurements of particles with ∼1 eV energies, and of the suprathermal extension of those populations up to several hundred eV. The SPI is sensitive to particle flux intensities of order 6×105 cm−2 s−1 sr−1 eV−1 and greater. It produces 3024-pixel images corresponding to two-dimensional (angle/energy) cuts through p...

Observation of polar cap patches and calculation of gradient drift instability growth times: A Swarm case study

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 ...

OEDIPUS-C topside sounding of a structured auroral E region

The Observations of Electric-field Distributions in the Ionospheric Plasma—A Unique Strategy C (OEDIPUS-C) rocket payload was launched from Poker Flat, Alaska, into an evening aurora at 0638 UT, on November 7, 1995. The payload included a tethered HF transmitter-receiver pair which acted as a topside sounder. The bistatic (two-point) configuration allowed an in situ calibration of the radiated power. The conditions in the magnetosphere and ionosphere during the experiment were monitored by a ground-based network of instruments and by instruments on the GOES 7 satellite in a geosynchronous orbit. In this paper we present results of the data analysis of topside ionograms that were obtained during the down-leg flight of OEDIPUS-C (OC). The relatively low altitudes through which OC carried out topside sounding make the resulting ionograms a novel data set. Ionospheric reflections of the 10-W transmissions were detected at payload heights between 780 and 160 km on the down leg. Near apogee at 824 km, extremely low electron densities (∼100 cm−3) were observed. The monotonic rise in electron density at the payload from apogee to reentry clearly showed that there was no ionospheric F layer peak. The topside-sounding echoes came from all heights between the payload and the E layer peak around 100 km altitude. Strong X-mode ionospheric reflections plus strong O-mode ground reflections were observed. OC thus has provided a close-hand view of a thick, highly structured, auroral E layer sounded at small ranges. The RF signal was efficiently guided along the magnetic field aligned density depletions that were located at the equatorward edges of auroral arcs. Large pulse-to-pulse variations in the amplitude of the ionospheric reflection are not explained by ducting in the geometric-optics sense.

Anisotropic core ion temperatures associated with strong zonal flows and upflows

The Swarm satellites observe strongly anisotropic ion temperatures at 500 km altitude. The ion temperature anisotropy ratios going up to 5 for the strongest electric fields presented in this paper. The largest observed anisotropy ratios exceed the values predicted by theories of collisional heating in strong flows by a factor of 2, indicating that collisional cross sections should perhaps be revised for O+ ion colliding with O. Temperature anisotropy is also found not to be a simple function of electric field strength. This could be understood in terms a time delay needed to advect hot anisotropic ion velocity distributions from strongly collisional regions below 400 km to weakly collisional regions at 500 km and above. The mirror force associated with these events is insufficient to account for the observed upward flows (>500 m/s). For gyroresonant heating to be consistent with our observations, an additional mechanism for field-aligned acceleration is required.

Thermal ion imagers and Langmuir probes in the Swarm electric field instruments

The European Space Agencys three Swarm satellites were launched on November 22, 2013 into nearly-polar, circular orbits, eventually reaching altitudes of 460 km (Swarm A and C) and 510 km (Swarm B). Swarms multi-year mission is to make precision, multi-point measurements of low-frequency magnetic and electric fields in Earths 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 ( 66x40 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.

Tethered two-point measurements of solitary auroral density cavities

The OEDIPUS-C sounding rocket observed localized plasma density depletions in the topside auroral ionosphere using two Langmuir probes situated on separate subpayloads several hundred meters apart. One probe was biased in electron saturation (+5 V) and the second in ion saturation (-5 V). Both of these measurements are consistent with density depletions of tens of percent. The depletions have cross-field dimensions of tens of meters and occur in regions of VLF hiss, suggesting they are likely signatures of lower hybrid solitary structures. Time delays between event detections on the two subpayloads are consistent with spatially localized structures traversed in succession. The two-point measurements show directly that the cavities extend at least 800 m along the geomagnetic field line, and the apparent electric potential within them is depressed by roughly 0.5 V.

Space Weather opportunities from the Swarm mission including near real time applications

Sophisticated space weather monitoring aims at nowcasting and predicting solar-terrestrial interactions because their effects on the ionosphere and upper atmosphere may seriously impact advanced technology. Operating alert infrastructures rely heavily on ground-based measurements and satellite observations of the solar and interplanetary conditions. New opportunities lie in the implementation of in-situ observations of the ionosphere and upper atmosphere onboard low Earth orbiting (LEO) satellites. The multi-satellite mission Swarm is equipped with several instruments which will observe electromagnetic and atmospheric parameters of the near Earth space environment. Taking advantage of the multi-disciplinary measurements and the mission constellation different Swarm products have been defined or demonstrate great potential for further development of novel space weather products. Examples are satellite based magnetic indices monitoring effects of the magnetospheric ring current or the polar electrojet, polar maps of ionospheric conductance and plasma convection, indicators of energy deposition like Poynting flux, or the prediction of post sunset equatorial plasma irregularities. Providing these products in timely manner will add significant value in monitoring present space weather and helping to predict the evolution of several magnetic and ionospheric events. Swarm will be a demonstrator mission for the valuable application of LEO satellite observations for space weather monitoring tools.