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

[1] This paper is a companion to a paper by Liang et al. (2011) which reports a causal connection between the intensification of electrostatic ECH waves and the postmidnight diffuse auroral activity in the absence of whistler mode chorus waves at L = 11.5 on the basis of simultaneous observations from THEMIS spacecraft and NORSTAR optical instruments during 8-9 UT on February 5, 2009. In this paper, we use the THEMIS particle and wave measurements together with the magnetically conjugate auroral observations for this event to illustrate an example where electrostatic electron cyclotron harmonic (ECH) waves are the main contributor to the diffuse auroral precipitation. We use the wave and particle data to perform a comprehensive theoretical and numerical analysis of ECH wave driven resonant scattering rates. We find that the observed ECH wave activity can cause intense pitch angle scattering of plasma sheet electrons between 100 eV and 5 keV at a rate of >10 À4 s À1 for equatorial pitch angles a eq < 30°. The scattering approaches the strong diffusion limit in the realistic ambient magnetic field to produce efficient precipitation loss of <$5 keV electrons on a timescale of a few hours or less. Using the electron differential energy flux inside the loss cone estimated based upon the energy-dependent efficiency of ECH wave scattering for an 8-s interval with high resolution wave data available, the auroral electron transport model developed by Lummerzheim (1987) produced an intensity of $2.3 kR for the green-line diffuse aurora. Separately, Maxwellian fitting to the electron differential flux spectrum produced a green-line auroral intensity of $2.6 kR. This is in good agreement with the $2.4 kR green-line auroral intensity observed simultaneously at the magnetic foot point (as inferred using the event-adaptive model of Kubyshkina et al. (2009Kubyshkina et al. ( , 2011) of the location where the in situ observations were obtained. Our results support the scenario that enhanced ECH emissions in the central plasma sheet (CPS) can be an important or even dominant driver of diffuse auroral precipitation in the outer magnetosphere. This paper is an important compliment to recent work that has shown lower band and upper band chorus to be mainly responsible for the occurrences of diffuse aurora in the inner magnetosphere.Citation: Ni, B., J. Liang, R. M. Thorne, V. Angelopoulos, R. B. Horne, M. Kubyshkina, E. Spanswick, E. F. Donovan, and D. Lummerzheim (2012), Efficient diffuse auroral electron scattering by electrostatic electron cyclotron harmonic waves in the outer magnetosphere: A detailed case study,

We present a statistical study of relativistic electron counts in the electron radiation belt across a range of drift shells (L * > 4) combining data from nine combined X-ray dosimeters (CXD) on the global positioning system (GPS) constellation. The response of the electron counts as functions of time, energy and drift shell are examined statistically for 67 solar wind stream interfaces (SIs); two-dimensional superposed epoch analysis is performed with the CXD data. For these epochs we study the radiation belt dropouts and concurrent variations in key geophysical parameters.At higher L * we observe a tendency for a gradual drop in the electron counts over the day preceding the SI, consistent with outward diffusion and magnetopause shadowing. At all L * , dropouts occur with a median time scale of 7 h and median counts fall by 0.4-1.8 orders of magnitude. The central tendencies of radiation belt dropout and recovery depend on both L * and energy. For 70 per cent of epochs Sym-H more than −30 nT, yet only three of 67 SIs did not have an associated dropout in the electron data. Statistical maps of electron precipitation suggest that chorus-driven relativistic electron microbursts might be major contributors to radiation belt losses under high-speed stream driving.

We present the first observations of ionospheric phenomena using the newly deployed Transition Region Explorer (TREx) Spectrograph. On the night of 10 April 2018, STEVE (Strong Thermal Emission Velocity Enhancement) and the Picket Fence optical structures were observed by the spectrograph in Lucky Lake, Saskatchewan. STEVE contains an enhancement of the OI red‐line (630‐nm) emission and a continuum which spans the visible wavelengths. Based upon its spectrum, we assert that the characteristic mauve color of STEVE is a result of this continuum. The spectrum of the Picket Fence contains a strong OI green‐line (557.7‐nm) emission similar to that produced in typical auroral structures. From their spectra, we assert that the Picket Fence is caused by particle precipitation and thus that the Picket Fence is a form of aurora, while STEVE's spectrum confirms that it is not aurora.

[1] We present ground-based and in situ observations from March 13, 2007. The THEMIS satellites were in the evening sector conjugate to THEMIS ground-based imagers. At $0507 UT there was an optical onset on inner CPS field lines. This involved near-simultaneous brightening of 1 MLT hour longitudinal segment of the onset arc. The part of the arc that brightened was that closest to the equatorward boundary of the diffuse (proton) aurora. Within one minute, a dipolarization front moved across four THEMIS satellites. Based on their locations, the order in which they detected the dipolarization front, and the auroral evolution, we assert that the expansion phase began earthward of the four satellites and evolved radially outwards. We conclude that this onset occurred in an azimuthally localized region of highly stretched field lines.

[1] We present observations of a substorm that occurred on February 25, 2008. Auroral onset was observed with a multi-spectral (l = 427.8, 557.7 and 630.0 nm) and white light all sky imager at Gillam, Canada. An equatorward moving diffuse auroral patch was observed in the l = 630.0 nm images at least six minutes prior to auroral onset. This form emerged from the background noise poleward of the eventual onset arc and intensified as it moved equatorward. Auroral expansion onset occurred when this form reached the onset arc location. Flows were detected by THEMIS probes P3 (TH-D) and P4 (TH-E) near X $ À11 R E nearly 90 seconds prior to auroral expansion. A small discrete arc was observed in the l = 557.7 nm images at the westward and equatorward edge of the diffuse 630.0 nm patch nearly 2 minutes prior to expansion onset, suggesting a field-aligned current of a substorm current wedge geometry. We conclude that the equatorward moving l = 630.0 nm diffuse auroral patch was generated by processes associated with an earthward moving flow burst that formed prior to auroral substorm onset. Citation: Kepko, L., E.

There has been an exciting recent development in auroral research associated with the discovery of a new subauroral phenomenon called STEVE (Strong Thermal Emission Velocity Enhancement). Although STEVE has been documented by amateur night sky watchers for decades, it is as yet an unidentified upper atmosphere phenomenon. Observed first by amateur auroral photographers, STEVE appears as a narrow luminous structure across the night sky over thousands of kilometers in the east-west direction. In this paper, we present the first statistical analysis of the properties of 28 STEVE events identified using Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager and the Redline Emission Geospace Observatory (REGO) database. We find that STEVE occurs about 1 hr after substorm onset at the end of a prolonged expansion phase. On average, the AL index magnitude is larger and the expansion phase has a longer duration for STEVE events compared to subauroral ion drifts or substorms. The average duration for STEVE is about 1 hr, and its latitudinal width is~20 km, which corresponds to~¼ of the width of narrow auroral structures like streamers. STEVE typically has an equatorward displacement from its initial location of about 50 km and a longitudinal extent of 2,145 km. Plain Language Summary Strong Thermal Emission Velocity Enhancement (STEVE) is anatmospheric phenomenon that manifests across the night sky as an extremely thin yet long ribbon of vibrant purple and white hues. Although STEVE has been well documented by amateur auroral photographers for several decades, the scientific community only recently stumbled upon this phenomenon. In this paper, we report on the first statistical analysis of STEVE's optical characteristics using ground-based all-sky imagery and examined satellite data to determine the geomagnetic conditions favorable for the formation of STEVE. Our results verify that STEVE is narrow in the north-south direction, but it extends over a wider east-west region. We have also determined that STEVE displaces southward over its lifetime in most observations. More interestingly, all 28 STEVE events identified in this study were observed at the end of a prolonged substorm expansion phase. More recently, Gallardo-Lacourt et al. (2018) analyzed data from the Polar-Orbiting Environmental Satellite (POES)-17 satellite for one STEVE event identified by Time History of Events and Macroscale Interactions GALLARDO-LACOURT ET AL. 9893

Abstract. High-latitude irregularities can impair the operation of GPS-based devices by causing fluctuations of GPS signal amplitude and phase, also known as scintillation. Severe scintillation events lead to losses of phase lock, which result in cycle slips. We have used data from the Canadian High Arctic Ionospheric Network (CHAIN) to measure amplitude and phase scintillation from L1 GPS signals and total electron content (TEC) from L1 and L2 GPS signals to study the relative role that various high-latitude irregularity generation mechanisms have in producing scintillation. In the first year of operation during the current solar minimum the amplitude scintillation has remained very low but events of strong phase scintillation have been observed. We have found, as expected, that auroral arc and substorm intensifications as well as cusp region dynamics are strong sources of phase scintillation and potential cycle slips. In addition, we have found clear seasonal and universal time dependencies of TEC and phase scintillation over the polar cap region. A comparison with radio instruments from the Canadian GeoSpace Monitoring (CGSM) network strongly suggests that the polar cap scintillation and TEC variations are associated with polar cap patches which we therefore infer to be main contributors to scintillation-causing irregularities in the polar cap.