Ian Schofield

Ground‐based ELF/VLF chorus observations at subauroral latitudes—VLF‐CHAIN Campaign

We report observations of very low frequency (VLF) and extremely low frequency (ELF) chorus waves taken during the ELF/VLF Campaign observation with High-resolution Aurora Imaging Network (VLF-CHAIN) of 17–25 February 2012 at subauroral latitudes at Athabasca (L=4.3), Canada. ELF/VLF waves were measured continuously with a sampling rate of 100 kHz to monitor daily variations in ELF/VLF emissions and derive their detailed structures. We found quasiperiodic (QP) emissions whose repetition period changes rapidly within a period of 1 h without corresponding magnetic pulsations. QP emissions showed positive correlation between amplitude and frequency sweep rate, similarly to rising-tone elements. We found an event of nearly simultaneous enhancements of QP emissions and Pc1/electromagnetic ion cyclotron wave intensities, suggesting that the temperature anisotropy of electrons and ions developed simultaneously at the equatorial plane of the magnetosphere. We also found QP emissions whose intensity suddenly increased in association with storm sudden commencement without changing their frequency. Falling-tone ELF/VLF emissions were observed with their rate of frequency change varying from 0.7 to 0.05 kHz/s over 10 min. Bursty-patch emissions in the lower and upper frequency bands are often observed during magnetically disturbed periods. Clear systematic correlation between these various ELF/VLF emissions and cosmic noise absorption was not obtained throughout the campaign period. These observations indicate several previously unknown features of ELF/VLF emissions in subauroral latitudes and demonstrate the importance of continuous measurements for monitoring temporal variations in these emissions.

Study of Pc1 pearl structures observed at multi-point ground stations in Russia, Japan, and Canada

We investigate possible generation mechanisms of Pc1 pearl structures using multi-point induction magnetometers in Athabasca in Canada, Magadan in Russia, and Moshiri in Japan. We selected two Pc1 pulsations that were simultaneously observed at the three stations and applied a polarization analysis. In case 1, on 8 April 2010, Pc1 pearl structures were slightly different in some time intervals at different stations, and their polarization angles varied depending on the frequencies at the three stations. Case 2, on 11 April 2010, showed Pc1 pearl structures that were similar at different stations, and their polarization angle was independent of frequency at all three stations. In order to understand these differences, we performed two simple model calculations of Pc1 pearl structures under different conditions. The first model assumes that Pc1 waves propagated from a latitudinally extended source with different frequencies at different latitudes to the observation points, representing beating of these waves in the ionosphere. The second model considers Pc1 waves for which different frequencies are mixed at a point source to cause the beating at the source point, indicating that the Pc1 pearl structures are generated in the magnetosphere. The first model shows slightly different waveforms at different stations. In contrast, the second model shows identical waveforms at different stations. From these results, we conclude that, in case 1, Pc1 pearl structures were caused by beating in the ionosphere. On the other hand, in case 2, they were the result of magnetospheric effects. We suggest that beating processes in the ionosphere could be one of the generation mechanisms of Pc1 pearl structures.

The AUTUMNX magnetometer meridian chain in Québec, Canada

The AUTUMNX magnetometer array consists of 10 THEMIS-class ground-based magnetometers deployed to form a meridian chain on the eastern coast of Hudson Bay in eastern Canada, a second partial chain one hour of magnetic local time further east, and one magnetometer at an intermediate midlatitude site. These instruments, augmented by those of other arrays, permit good latitudinal coverage through the auroral zone on two meridians, some midlatitude coverage, and detection of magnetic field changes near the sensitive infrastructure of the Hydro-Québec power grid. Further, they offer the possibility for conjugate studies with Antarctica and the GOES East geosynchronous satellite, and complement the Chinese International Space Weather Meridian Circle Program. We examine current world distribution of magnetometers to show the need for AUTUMNX, and describe the instrumentation which allows near-real-time monitoring. We present magnetic inversion results for the disturbed day February 17, 2015, which showed classic signatures of the substorm current wedge, and developed into steady magnetospheric convection (SMC). For a separate event later that day, we examine a large and rapid magnetic field change event associated with an unusual near-Earth transient. We show GOES East conjugacy for these events.

Polarization analysis of VLF/ELF waves observed at subauroral latitudes during the VLF-CHAIN campaign

Chorus wave emissions are one of the most intense naturally occurring phenomena in the very low (VLF) and extremely low frequency (ELF) ranges. They are believed to be one of the major contributors to acceleration and loss of electrons in the radiation belts. During the VLF Campaign observation with High-resolution Aurora Imaging Network (VLF-CHAIN) from 17 to 25 February 2012, several types of VLF/ELF emissions, including chorus, were observed at subauroral latitudes in Athabasca, Canada. To our knowledge, there has not been any comprehensive study of the physical properties of such emissions at these latitudes. In this study, we calculate spectral and polarization parameters of VLF/ELF waves with high temporal resolution. We found that the polarization angle of several emissions depended on both frequency and time. We suggest that the frequency-dependent events, which usually last several tens of minutes, might be the consequence of the broadening of the ray path that the waves follow from their generation region to the ground. Furthermore, time-dependent events, also lasting tens of minutes, have a polarization angle that changes from negative to positive values (or vice versa) every few minutes. We suggest that this could be due to variations of the wave duct, either near the generation region or along the wave propagation path. Using another ground station in Fort Vermillion, Canada, about 450 km northwest of Athabasca, we tracked the movements of the ionospheric exit point of three chorus emissions observed simultaneously at both stations. Although we found that movement of the ionospheric exit point does not follow a general direction, it is subject to hovering motion, suggesting that the exit point can be affected by small-scale plasma processes.

New science in plain sight: Citizen scientists lead to the discovery of optical structure in the upper atmosphere

Citizen scientists, along with satellite and ground-based sensors, have revealed a new arc boundary at subauroral latitudes. A glowing ribbon of purple light running east-west in the night sky has recently been observed by citizen scientists. This narrow, subauroral, visible structure, distinct from the traditional auroral oval, was largely undocumented in the scientific literature and little was known about its formation. Amateur photo sequences showed colors distinctly different from common types of aurora and occasionally indicated magnetic field–aligned substructures. Observations from the Swarm satellite as it crossed the arc have revealed an unusual level of electron temperature enhancement and density depletion, along with a strong westward ion flow, indicating that a pronounced subauroral ion drift (SAID) is associated with this structure. These early results suggest the arc is an optical manifestation of SAID, presenting new opportunities for investigation of the dynamic SAID signatures from the ground. On the basis of the measured ion properties and original citizen science name, we propose to identify this arc as a Strong Thermal Emission Velocity Enhancement (STEVE).

Fast modulations of pulsating proton aurora related to subpacket structures of Pc1 geomagnetic pulsations at subauroral latitudes

To understand the role of electromagnetic ion cyclotron (EMIC) waves in determining the temporal features of pulsating proton aurora (PPA) via wave-particle interactions at subauroral latitudes, high-time-resolution (1/8 s) images of proton-induced N2>+ emissions were recorded using a new electron multiplying charge-coupled device camera, along with related Pc1 pulsations on the ground. The observed Pc1 pulsations consisted of successive rising-tone elements with a spacing for each element of 100 s and subpacket structures, which manifest as amplitude modulations with a period of a few tens of seconds. In accordance with the temporal features of the Pc1 pulsations, the auroral intensity showed a similar repetition period of 100 s and an unpredicted fast modulation of a few tens of seconds. Furthermore, these results indicate that PPA is generated by pitch angle scattering, nonlinearly interacting with Pc1/EMIC waves at the magnetic equator.

NetPICOmag: A low-cost networked magnetometer and its applications

NetPICOmag (NPM) is the culmination of a design effort to build a compact, low-cost, laboratory-grade, networked magnetometer designed for remote autonomous operation, suited for research and education. NPM allows wide placement of magnetometers sensitive enough to detect auroral activity and the daily variation, and is suitable for education projects and a range of geophysical applications. The use of networked microcontrollers and GPS timing is applicable to other small instruments for field or local deployment, and an onboard data logging capability has also been demonstrated. We illustrate the value of the placement of low-cost magnetometers to increase coverage in an area through the study of a Pc 5 pulsation event which took place on September 4, 2010. By combining results with those from auroral zone magnetometers supporting the THEMIS project, we find that the phase velocity of these morning sector pulsations was northward on the ground. The event took place under very quiet solar wind conditions, and credible mapping associates it with the inner magnetosphere. Another aspect beyond increasing areal coverage is increasing density of coverage, which becomes feasible with instruments of very low cost. We examine aspects of the April 5, 2010 space weather event which are possible to deduce from closely spaced magnetometers.