K. A. McWilliams

Direct Observations of the Evolution of Polar Cap Ionization Patches

Patchy Polar Cap Patches of enhanced density plasma in the polar ionosphere (or polar cap patches) disturb radio communications and satellite positioning at high latitudes during magnetospheric storms. Using data from Global Positioning System satellites and a high-frequency radar network, Q.-H. Zhang et al. (p. 1597) analyzed a magnetospheric storm driven by a strong coronal mass ejection from the Sun and followed the evolution and motion of a patch of ionization throughout the polar cap. The localized dayside flow response to the solar disturbance allowed a patch to be stored and grow in the dayside polar cap, and the gaps between patches were controlled by the onset of magnetic reconnection in the magnetospheres tail. Observations of ionospheric perturbations after a solar burst hit Earth show how a patch of ionization formed and evolved. Patches of ionization are common in the polar ionosphere, where their motion and associated density gradients give variable disturbances to high-frequency (HF) radio communications, over-the-horizon radar location errors, and disruption and errors to satellite navigation and communication. Their formation and evolution are poorly understood, particularly under disturbed space weather conditions. We report direct observations of the full evolution of patches during a geomagnetic storm, including formation, polar cap entry, transpolar evolution, polar cap exit, and sunward return flow. Our observations show that modulation of nightside reconnection in the substorm cycle of the magnetosphere helps form the gaps between patches where steady convection would give a “tongue” of ionization (TOI).

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.

Day‐night coupling by a localized flow channel visualized by polar cap patch propagation

We present unique coordinated observations of the dayside auroral oval, polar cap, and nightside auroral oval by three all-sky imagers, two Super Dual Auroral Radar Network (SuperDARN) radars, and Defense Meteorological Satellite Program (DMSP)-17. This data set revealed that a dayside poleward moving auroral form (PMAF) evolved into a polar cap airglow patch that propagated across the polar cap and then nightside poleward boundary intensifications (PBIs). SuperDARN observations detected fast antisunward flows associated with the PMAF, and the DMSP satellite, whose conjunction occurred within a few minutes after the PMAF initiation, measured enhanced low-latitude boundary layer precipitation and enhanced plasma density with a strong antisunward flow burst. The polar cap patch was spatially and temporally coincident with a localized antisunward flow channel. The propagation across the polar cap and the subsequent PBIs suggests that the flow channel originated from dayside reconnection and then reached the nightside open-closed boundary, triggering localized nightside reconnection and flow bursts within the plasma sheet.

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 ...

Solar–wind–magnetosphere–ionosphere interactions in the Earth's plasma environment

The properties of the Earths coupled magnetosphere–ionosphere system are dominated by its interaction with the solar–wind plasma, mediated by magnetic reconnection at the magnetopause interface. As a consequence, Earths magnetospheric dynamics depend primarily on the concurrent orientation of the interplanetary magnetic field (IMF). We illustrate current understanding of the system through the results of a number of recent case studies and highlight the remaining issues. The discussion centres on flux–transfer events and substorms during intervals of southward IMF and magnetopause and tail processes during intervals of northward IMF. We emphasize the great diagnostic power of combined in situ and remote–sensing observations from space and on the ground.

Localized polar cap flow enhancement tracing using airglow patches: Statistical properties, IMF dependence, and contribution to polar cap convection

Recent radar observations have suggested that polar cap flows are highly structured and that localized flow enhancements can lead to nightside auroral disturbances. However, knowledge of these flows is limited to available echo regions. Utilizing wide spatial coverage by an all-sky imager at Resolute Bay and simultaneous Super Dual Auroral Radar Network measurements, we statistically determined properties of such flows and their interplanetary magnetic field (IMF) dependence. We found that narrow flow enhancements are well collocated with airglow patches with substantially larger velocities (≥200 m/s) than the weak large-scale background flows. The flow azimuthal widths are similar to the patch widths. During the evolution across the polar cap, the flow directions and speeds are consistent with the patch propagation directions and speeds. These correspondences indicate that patches can optically trace localized flow enhancements reflecting the flow width, speed, and direction. Such associations were found common (~67%) in statistics, and the typical flow speed, propagation time, and width within our observation areas are 600 m/s, tens of minutes, and 200–300 km, respectively. By examining IMF dependence of the occurrence and properties of these flows, we found that they tend to be observed under By-dominated IMF. Flow speeds are large under oscillating IMF clock angles. Localized flow enhancements are usually observed as a channel elongated in the noon-midnight meridian and directed toward premidnight (postmidnight) for +By (−By). The potential drops across localized flow enhancements account for ~10–40% of the cross polar cap potential, indicating that they significantly contribute to polar cap plasma transport.

Auroral fragmentation into patches

Auroral patches in diffuse auroras are very common features in the postmidnight local time. However, the processes that produce auroral patches are not yet well understood. In this paper we present two examples of auroral fragmentation which is the process by which uniform aurora is broken into several fragments to form auroral patches. These examples were observed at Athabasca, Canada (geomagnetic latitude: 61.7°N), and Tromso, Norway (67.1°N). Captured in sequences of images, the auroral fragmentation occurs as finger-like structures developing latitudinally with horizontal-scale sizes of 40–100 km at ionospheric altitudes. The structures tend to develop in a north-south direction with speeds of 150–420 m/s without any shearing motion, suggesting that pressure-driven instability in the balance between the earthward magnetic-tension force and the tailward pressure gradient force in the magnetosphere is the main driving force of the auroral fragmentation. Therefore, these observations indicate that auroral fragmentation associated with pressure-driven instability is a process that creates auroral patches. The observed slow eastward drift of aurora during the auroral fragmentation suggests that fragmentation occurs in low-energy ambient plasma.

Polar cap precursor of nightside auroral oval intensifications using polar cap arcs

Recent radar and optical observations suggested that localized fast flows in the polar cap precede disturbances within the nightside auroral oval. However, how commonly this connection occurs has been difficult to examine due to limited coverage of radar flow measurements and diffuse and dim nature of airglow patches. Polar cap arcs are also associated with fast flows in the polar cap and appear much brighter than patches, allowing evaluation of the interaction between polar cap structures and nightside aurora more definitively. We have surveyed data during six winter seasons and selected quasi-steady polar cap arcs lasting >1 h. Thirty-four arcs are found, and for the majority (~85%) of them, as they extend equatorward from high latitude, their contact with the nightside auroral poleward boundary is associated with new and substantial intensifications within the oval. These intensifications are localized (< ~1 h magnetic local time (MLT)) and statistically occur within 10 min and ±1 h MLT from the contact. They appear as poleward boundary intensifications in a thick auroral oval or an intensification of the only resolvable arc within a thin oval, and the latter can also exhibit substantial poleward expansion. When radar echoes are available, they corroborate the association of polar cap arcs with localized enhanced antisunward flows. That the observed oval intensifications are major disturbances that only occur after the impingement of polar cap arcs and near the contact longitude suggest that they are triggered by localized fast flows coming from deep in the polar cap.

Interplanetary magnetic field and solar cycle dependence of Northern Hemisphere F region joule heating

Joule heating in the ionosphere takes place through collisions between ions and neutrals. Statistical maps of F region Joule heating in the Northern Hemisphere polar ionosphere are derived from satellite measurements of thermospheric wind and radar measurements of ionospheric ion convection. Persistent mesoscale heating is observed near postnoon and postmidnight magnetic local time and centered around 70° magnetic latitude in regions of strong relative ion and neutral drift. The magnitude of the Joule heating is found to be largest during solar maximum and for a southeast oriented interplanetary magnetic field. These conditions are consistent with stronger ion convection producing a larger relative flow between ions and neutrals. The global-scale Joule heating maps quantify persistent (in location) regions of heating that may be used to provide a broader context compared to small-scale studies of the coupling between the thermosphere and ionosphere.

SuperDARN observations of quasi‐stationary mesoscale convection vortices in the dayside high‐latitude ionosphere

During the past few years of SuperDARN observations, mesoscale (500–1000 km diameter) clockwise-sense convection vortices which do not show significant movement in any direction have been observed in the postnoon sector. These convection vortices or small convection cells are herein named quasi-stationary convection vortices (QSCVs). In this paper, a statistical study of 28 QSCVs is presented, using Saskatoon-Kapuskasing radar data from the period October 1993 to March 1997. These QSCVs occur in the magnetic latitude interval 72°–81° between 1430 and 1630 magnetic local time (MLT) and are centered near the western end of the afternoon convection cell. The QSCVs occur mainly during southward interplanetary magnetic field (IMF) but sometimes for northward IMF. They appear at lower latitudes as the IMF Bz becomes more negative; they occur at higher latitudes and earlier local times for positive IMF By and at lower latitudes and later local times for negative IMF By. All the QSCVs appear to be associated with large-amplitude fluctuations either in the IMF or in the solar wind pressure, or in both. It is found that the time of the ionospheric convection change in response to the fluctuations in the IMF and solar wind pressure at the magnetopause is about 6–9 min. The observations have shown some evidence that the termination of the vortex is caused also by large changes in the IMF and/or solar wind pressure. The lifetime of the QSCVs is typically 10–20 min, but it can be as long as 50 min. The QSCVs have clockwise rotational sense and are closely associated with the region 1 field-aligned currents. Unlike traveling convection vortices, the QSCVs do not appear to be related to any instability at the magnetopause or low-latitude boundary layer. The large fluctuations in the solar wind pressure and IMF seem to be likely initiating mechanisms for the QSCVs.