Brian J. Jackel

We report an event study of STEVE on 17 July 2018, with focus on the optical spectra and emission altitudes of STEVE. We find that the STEVE comprises two traces, one at a higher elevation angle and the other at a lower elevation angle. The two traces merge into one when viewed near the zenith. Spectrograph measurements show that both STEVE traces are characterized by enhancements over broadband wavelengths, that is, an airglow continuum, but they differ in their red‐line (630 nm) component: The higher‐elevation STEVE contains substantial red‐line enhancement over background, while the lower‐elevation STEVE does not. Based upon triangulation analyses using multiple optical instruments, we evaluate that the two STEVE traces are likely emitted from distinctly different altitudes: The higher‐elevation STEVE comes from ~250‐km altitude, while the lower‐elevation one is from ≤150‐km altitude. Our results impose implications and constraints on the possible underlying mechanisms of STEVE.

The geoeffectiveness of solar wind shocks depends on angle with respect to the Sun‐Earth line, with highly angled solar wind shocks being less geoeffective than nearly frontal solar wind shocks. However, it is unclear whether this holds for the orientation of structures in nonshocked solar wind. In this paper, we perform a mutual information analysis of 18 years of in situ solar wind and ground magnetometer data in order to investigate the effects of solar wind phase front orientation on solar wind geoeffectiveness (indicated by SuperMAG SME, the SuperMAG enhanced version of the AE index). Since geomagnetic response is strongly influenced by interplanetary magnetic field (IMF) B_z, and IMF B_z affects phase front orientation, we use conditional mutual information to account for the effect of B_z on geomagnetic activity. In contrast to what has been found for solar wind shocks, we find that during times of IMF B_z > 0, phase fronts aligned with the average Parker spiral direction (45° azimuth, 0° inclination) tend to be associated with higher geomagnetic activity (SME > 500 nT) than would be expected if IMF B_z and phase front orientation quantities were unrelated. During times of IMF B_z < 0, there is no connection between solar wind phase front orientation and geomagnetic activity (SME). We believe that Parker spiral‐aligned phase fronts being associated with higher geomagnetic activity during times of IMF B_z > 0 is due to constant phase front orientation allowing for more efficient energy transfer either through viscous interaction or high‐latitude reconnection.

Citizen scientists, along with satellite and ground-based sensors, have revealed a new arc boundary at subauroral latitudes.

We present a comprehensive survey of 630 nm (red‐line) emission discrete auroral arcs using the newly deployed Redline Emission Geospace Observatory. In this study we discuss the need for observations of 630 nm aurora and issues with the large‐altitude range of the red‐line aurora. We compare field‐aligned currents (FACs) measured by the Swarm constellation of satellites with the location of 10 red‐line (630 nm) auroral arcs observed by all‐sky imagers (ASIs) and find that a characteristic emission height of 200 km applied to the ASI maps gives optimal agreement between the two observations. We also compare the new FAC method against the traditional triangulation method using pairs of all‐sky imagers (ASIs), and against electron density profiles obtained from the Resolute Bay Incoherent Scatter Radar‐Canadian radar, both of which are consistent with a characteristic emission height of 200 km.

Intense zonal ion velocity jets in the northern nightside auroral zone are measured during quiet geomagnetic conditions by the Swarm satellites around 500 km altitude. These velocity jets, exceeding 1 km/s in over 50% of orbits measured, range from 20 to 100 km in meridional thickness and reach a maximum at the boundary between upward and downward field‐aligned current. On average they represent a potential difference of approximately 3 kV between the R1/R2 currents. This boundary also separates different regions of electron temperature and meridional flow and is associated with ion upflows and anisotropic heating. Both dawnward and duskward velocity jets are observed, including some oppositely directed pairs bounding regions of upward field‐aligned current. Coincident ground‐based observations place ion velocity jets adjacent to auroral arcs, embedded in the auroral electrojets. Previous literature has focused on fast flows occurring in regions of relative low conductivity surrounding auroral arcs, typically during geomagnetically active conditions, and does not address the occurrence frequency of these events. We show ion velocity jets to be a persistent and ubiquitous property of the electrodynamics of quiet time R1/R2 current closure near midnight in the winter hemisphere.

Observations of optical aurora typically require the operation of sensitive instruments at remote field sites. Absolute radiometric calibration of these devices is essential for quantitative comparison over time and with other measurements. In this study we present absolute calibration of a proton auroral photometer using star transits observed during regular data collection. This requires absolute flux spectra with sufficient resolution to account for structure in stellar H_β absorption line profiles. Several flux spectral catalogs are combined and corrected for systematic differences. The resulting estimates of instrumental sensitivity are consistent with darkroom calibration to roughly 15%.

An instrument designed to measure the location and brightness of auroral emissions from energetic proton precipitation is described. This photometer scans from the north to south horizon with a stepper motor and mirror. The scans are configured in software for a 30 s cadence with equally spaced samples along a meridian at constant altitude. Broadband light is separated into two channels with a novel optical splitter. This splitter uses a filter that has high transmission for the signal channel and high reflection on both the long‐ and short‐wavelength sides to reflect the combined background passbands, directing each channel to its respective detector. The half‐cone angle and angle of incidence of this splitter filter allow for an overall compact optical design that also provides superior sensitivity in both signal and background channels. The signal channel is 3 nm wide full width at half maximum (FWHM) at 486.1 nm, and the background channel comprises two 3 nm wide FWHM passbands at 480 nm and 495 nm created by a single filter. Both of these channels are measured with photomultiplier tubes in photon‐counting mode. Calibrations indicate a response of around 1000 c/s per rayleigh. Data are currently acquired in 5 ms bins with a Nyquist frequency of 100 Hz. The first system (Forty‐Eight Sixty‐One (FESO)‐1) has been operating at Athabasca University since February 2014, and the second system (FESO‐2) was deployed at Lucky Lake, Saskatchewan, in October 2015. The improved sensitivity over legacy instruments and the simultaneous measurement of signal and background enable operation during intervals with dynamic electron aurora and scattered moonlight.

Observations of astronomical sources provide information that can significantly enhance the utility of auroral data for scientific studies. This report presents results obtained by using Jupiter for field cross calibration of four multispectral auroral meridian scanning photometers during the 2011–2015 Northern Hemisphere winters. Seasonal average optical field-of-view and local orientation estimates are obtained with uncertainties of 0.01 and 0.1°, respectively. Estimates of absolute sensitivity are repeatable to roughly 5 % from one month to the next, while the relative response between different wavelength channels is stable to better than 1 %. Astronomical field calibrations and darkroom calibration differences are on the order of 10 %. Atmospheric variability is the primary source of uncertainty; this may be reduced with complementary data from co-located instruments.

Observations of astronomical sources provides information that can significantly enhance the utility of auroral data for scientific studies. Jupiter is used for field cross-calibration of 4 multi-spectral auroral meridian scanning photometers during 2011–15 northern hemisphere winters. Seasonal average optical field-of-view and local orientation estimates are obtained with uncertainties of 0.01° and 0.1° respectively. Estimates of absolute photometric sensitivity are repeatable to roughly 5 % from one month to the next, while the relative response between different wavelength channels is stable to better than 1 %. Astronomical field calibrations and darkroom calibration differences are on the order of 10 %. Atmospheric variability is the primary source of uncertainty; this may be reduced with data from co-located instruments such as all-sky imagers.