E. Spanswick

Energetic particle precipitation into the middle atmosphere triggered by a coronal mass ejection

Precipitation of relativistic electrons into the atmosphere has been suggested as the primary loss mechanism for radiation belt electrons during large geomagnetic storms. Here we investigate the geographical spread of precipitation as a result of the arrival of a coronal mass ejection (CME) on 21 January 2005. In contrast to previous statistical studies we provide one of the first attempts to describe the geographic and temporal variability of energetic particle precipitation on a global scale using an array of instruments. We combine data from subionospheric VLF radio wave receivers, the high-altitude Miniature Spectrometer (MINIS) balloons, riometers, and pulsation magnetometers during the first hour of the event. There were three distinct types of energetic electron precipitation observed, one globally, one on the dayside, and one on the nightside. The most extensively observed form of precipitation was a large burst starting when the CME arrived at the Earth, where electrons from the outer radiation belt were lost to the atmosphere over a large region of the Earth. On the dayside of the Earth (10–15 MLT) the CME produced a further series of precipitation bursts, while on the nightside dusk sector (∼20 MLT) a continuous precipitation event lasting ∼50 min was observed at 2.5 < L < 3.7 along with Pc 1–2 pulsations observed with a ground-based magnetometer. These observations suggest that the generation of energetic electron precipitation at the inner edge of the outer radiation belt from electromagnetic ion cyclotron (EMIC) wave scattering into the loss cone is the most direct evidence to date connecting EMIC activity and energetic precipitation.

Low-energy ion precipitation structures associated with pulsating auroral patches

Pulsating auroras often appear in forms of geo-stable or slowly convecting “patches.” These patches can maintain their rough shape and size over many sequences of luminosity pulsations, yet they slowly drift with ionospheric E × B convection. Because of these characteristics, there has long been a speculation that the pulsating auroral patch (PAP) is connected to flux tubes filled with enhanced cold plasma. In this study, we perform a survey on pulsating auroral events when the footprints of low-Earth-orbit satellites traversed the PAPs, with a focus on the low-energy particle signatures associated with the PAPs. As a result, we identified, in a majority (~2/3) of events, the existence of a low-energy ion precipitation structure that is collocated with the PAP, with core energies ranging from several tens of eV up to a few hundred eV. This result supports the hypothesis that a PAP connects to flux tubes filled with enhanced cold plasma. We further propose that the plasma outflows from the ionosphere are the origin of such cold plasma flux tubes. We suggest that the PAP is formed by a combination of high-energy electrons of a magnetospheric origin, the low-energy plasma structure of an ionospheric origin, and certain ELF/VLF waves that are intensified and modulated in interactions with both the hot and cold plasma populations.

Global observations of substorm injection region evolution: 27 August 2001

We present riometer and in situ observations of a substorm electron injection on 27 August 2001. The event is seen at more than 20 separate locations (including ground stations and 6 satellites: Cluster, Polar, Chandra, and 3 Los Alamos National Laboratory (LANL) spacecraft). The injection is observed to be dispersionless at 12 of these locations. Combining these observations with information from the GOES-8 geosynchronous satellite we argue that the injection initiated near geosynchronous orbit and expanded poleward (tailward) and equatorward (earthward) afterward. Further, the injection began several minutes after the reconnection identified in the Cluster data, thus providing concrete evidence that, in at least some events, near-Earth reconnection has little if any ionospheric signature.

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.

Coordinated ionospheric observations indicating coupling between preonset flow bursts and waves that lead to substorm onset

A critical, long-standing problem in substorm research is identification of the sequence of events leading to substorm expansion phase onset. Recent Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager (ASI) array observations have shown a repeatable preonset sequence, which is initiated by a poleward boundary intensification (PBI) and is followed by auroral streamers moving equatorward (earthward flow in the plasma sheet) and then by substorm onset. On the other hand, substorm onset is also preceded by azimuthally propagating waves, indicating a possible importance of wave instability for triggering substorm onset. However, it has been difficult to identify the link between fast flows and waves. We have found an isolated substorm event that was well instrumented with the Poker Flat incoherent scatter radar (PFISR), THEMIS white-light ASI, and multispectral ASI, where the auroral onset occurred within the PFISR and ASI fields of view. This substorm onset was preceded by a PBI, and ionospheric flows propagated equatorward from the polar cap, crossed the PBI, and reached the growth phase arc. This sequence provides evidence that flows from open magnetic field lines propagate across the open-closed boundary and reach the near-Earth plasma sheet prior to the onset. Quasi-stable oscillations in auroral luminosity and ionospheric density are found along the growth phase arc. These preonset auroral waves amplified abruptly at the onset time, soon after the equatorward flows reached the onset region. This sequence suggests a coupling process where preexisting stable waves in the near-Earth plasma sheet interact with flows from farther downtail and then evolve to onset instability.

A survey of quiet auroral arc orientation and the effects of the interplanetary magnetic field

Using data from the THEMIS All-Sky Imager array, we have carried out an extensive study of the orientation of quiet auroral arcs relative to the magnetic east-west direction. We used over 7500 images of quiet auroral arcs that were collected during extended solar minimum and at various geomagnetic latitudes and longitudes. For each arc, we determined its “tilt” (the angle the arc makes with the local magnetic east-west direction) and its “multiplicity” (whether or not the arc was part of a multiple-arc system). We have found that at more equatorward latitudes, arc tilts are within σSD = ±7.7∘. We determined that both single- and multiple-arc systems tend to tilt a few degrees to the south-east prior to 23 magnetic local time (MLT) and to the north-east afterward. This tilt appears to be more prominent at higher latitudes. We compared the auroral arc orientations to the mapping of equatorial contours of constant magnetic field strength into the ionosphere, where we used the T87 and T89 magnetic field models for quiet (Kp = 1,3) conditions for the mappings and to determine the constant equatorial magnetic field strength contours. We found that the MLT trends of the tilts are such that arc alignment appears to follow the constant magnetic field strength contours as projected into the ionosphere. We assert that the systematic dependencies of the orientation of auroral arcs indicate that arc morphology is governed by the large-scale structure of the magnetosphere as opposed to localized processes within the ionosphere. In addition, we studied the effects of the interplanetary magnetic field (IMF) on the location in MLT of the reversal of the arc tilts. We found that negative IMF Bxand Byconditions cause the reversal location to shift duskward of 23 MLT. Alternately, a positive IMF Bx, coupled with a negative By, results in a shift in reversal location toward magnetic midnight. This behavior is consistent with that found in studies of the MLT distribution of substorm onsets.

A statistical approach to determining energetic outer radiation belt electron precipitation fluxes

Subionospheric radio wave data from an Antarctic-Arctic Radiation-Belt (Dynamic) Deposition VLF Atmospheric Research Konsortia (AARDDVARK) receiver located in Churchill, Canada, is analyzed to determine the characteristics of electron precipitation into the atmosphere over the range 3  30 keV precipitation flux determined by the AARDDVARK technique was found to be ±10%. Peak >30 keV precipitation fluxes of AARDDVARK-derived precipitation flux during the main and recovery phase of the largest geomagnetic storm, which started on 4 August 2010, were >105 el cm−2 s−1 sr−1. The largest fluxes observed by AARDDVARK occurred on the dayside and were delayed by several days from the start of the geomagnetic disturbance. During the main phase of the disturbances, nightside fluxes were dominant. Significant differences in flux estimates between POES, AARDDVARK, and the riometer were found after the main phase of the largest disturbance, with evidence provided to suggest that >700 keV electron precipitation was occurring. Currently the presence of such relativistic electron precipitation introduces some uncertainty in the analysis of AARDDVARK data, given the assumption of a power law electron precipitation spectrum.

Energetic electron precipitation characteristics observed from Antarctica during a flux dropout event

Data from two autonomous VLF radio receiver systems installed in a remote region of the Antarctic in 2012 is used to take advantage of the juxtaposition of the L=4.6 contour, and the Hawaii-Halley, Antarctica, great circle path as it passes over thick Antarctic ice shelf. The ice sheet conductivity leads to high sensitivity to changing D-region conditions, and the quasi-constant L-shell highlights outer radiation belt processes. The ground-based instruments observed several energetic electron precipitation events over a moderately active 24-hour period, during which the outer radiation belt electron flux declined at most energies and subsequently recovered. Combining the ground-based data with low- and geosynchronous-orbiting satellite observations on 27 February 2012, different driving mechanisms were observed for three precipitation events with clear signatures in phase space density and electron anisotropy. Comparison between flux measurements made by Polar-orbiting Operational Environmental Satellites (POES) in low Earth orbit and by the Antarctic instrumentation provides evidence of different cases of weak and strong diffusion into the bounce-loss-cone, helping to understand the physical mechanisms controlling the precipitation of energetic electrons into the atmosphere. Strong diffusion events occurred as the 30 keV flux than was reported by POES, more consistent with strong diffusion conditions.

Using patchy pulsating aurora to remote sense magnetospheric convection

Five patchy pulsating aurora (PPA) patches have been identified in data obtained from the Gillam Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager (ASI). We found that azimuthal velocities of five patches derived from THEMIS ASI data were close to the same value as the local convection velocities as obtained from analysis of Super Dual Auroral Radar Network data, consistent with the idea that the patch motion is primarily due to E × B convection. We argue that this means that we can infer 2-D maps of the time-evolving convection from time sequences of PPA. Further, given that the E × B convection is understood to be a projection of magnetospheric convection, this means that, provided auroral and viewing conditions cooperate, patch motion can be used for remote sensing magnetospheric convection over across-extended regions with fairly high time resolution. This is the first detailed demonstration of the equivalence of patch velocities and E × B convection.

First observations from the RISR‐C incoherent scatter radar

First-light measurements from the Canadian face of the Resolute Bay Incoherent Scatter Radar (RISR-C) were taken in August of 2015. Data were taken for roughly 25 hours on both RISR-C and the North face of the Resolute Bay radar (RISR-N) in an 11-beam World Day mode. Overall, the measurements from the RISR-C radar are of high quality and consistent with results from the RISR-N radar. During the 25-hour period analyzed in this study, the ionosphere responded to changes in orientation of the Interplanetary Magnetic Field (IMF). During one particular event, a change from Bz negative to positive and By positive to negative caused the anti-sunward flow to stall, and a strong dawn-to-dusk flow, with decreased electron density and increased ion temperature, replaced it in the RISR-C field-of-view. Overall, it is clear that measurements from the RISR-C radar will complement and greatly expand the scope of ionospheric polar cap measurements.