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The Rocket Experiment for Neutral Upwelling 2 (RENU2) rocket was launched on 13 December 2015 at 07:34 UT. The payload transited the cusp region during a neutral upwelling event, supported by a comprehensive set of onboard and ground-based instrumentation. RENU2 data highlight two important processes. One is that a proper understanding of neutral upwelling by Poleward Moving Auroral Forms (PMAFs) requires a treatment that mimics the quasiperiodic passage of a sequence of PMAFs. As a PMAF reaches a flux tube, its physical consequences must be determined including the residual history of effects from previous passages, implying that understanding such a process requires an accounting of the system hysteresis. Second, RENU2 observations suggest that neutral density enhancements driven by precipitation and/or Joule heating can be highly structured in altitude and latitude. In addition, timescales involving neutral dynamics suggest that the structuring must be slowly changing, for example, over the course of 10 to tens of minutes. publishedVersion

Rapid fading of auroral activity a few minutes before substorm breakup has earlier been analyzed in case-studies. Here we report on a study in which all-sky camera (ASC) and magnetic data over 3 years were examined to find breakups that were accompanied by a preceding fading. To illustrate typical features of the fading effect we analyze three events in detail and discuss seven other events to find the spatial and temporal behavior of the fading and the global conditions favoring this phenomenon, which is not associated with every breakup. In these ten events the precipitation diminished typically for about 2 min and a local breakup followed after 2–3 min. Usually the arc which broke up had faded earlier. Comparison with geostationary electron flux recordings shows that in many cases the global onset had already taken place when the fading was recorded at a different longitude. Thus fading is not just a growth-phase phenomenon as often thought, but can also appear as a precursor of the approaching auroral bulge. The AE index and solar-wind data reveal that the fading has a tendency to take place during magnetically disturbed conditions caused by continuous energy input from the solar wind. Furthermore, while a widely recognized phenomenon, we have found that the fading prior to breakup is not a very common feature in the spatio-temporal scale of auroral ASC recordings. In many cases the deepness of the fading had a longitudinal dependence, which leads to the suggestion that this phenomenon is related to azimuthal gradients in the tail magnetic field and/or plasma pressure. Possible scenarios causing fading both before and after the onset are discussed based on a few previously presented theoretical auroral-arc models.

This work presents a novel image accumulation filter technique that reveals small scale features and details from intense luminosity or high dynamic range video recordings. It was discovered and developed from the analyses of the Norwegian Broadcasting Corporation (NRK) film of the total solar eclipse that occurred Friday 20th of March 2015 in Longyearbyen (78° N, 15° E) on Svalbard, Norway. The result of the filter is fused with a High Dynamic Range (HDR) image of the Corona and the Solar Dynamic Observatory (SDO) image of the solar disk.

Every winter, the all-sky cameras (ASCs) in the MIRACLE network take images of the night sky at regular intervals of 10–20 s. This amounts to millions of images that not only need to be pruned, but there is also a need for efficient auroral activity detection techniques. In this paper, we describe a method for performing automated classification of ASC images into three mutually exclusive classes: aurora , no aurora , and cloudy . This not only reduces the amount of data to be processed, but also facilitates in building statistical models linking the magnetic fluctuations and auroral activity helping us to get a step closer to forecasting auroral activity. We experimented with different feature extraction techniques coupled with Support Vector Machines classification. Color variants of Scale Invariant Feature Transform (SIFT) features, specifically Opponent SIFT features, were found to perform better than other feature extraction techniques. With Opponent SIFT features, we were able to build a classification model with a cross-validation accuracy of 91%, which was further improved using temporal information and elimination of outliers which makes it accurate enough for operational data pruning purposes. Since the problem is essentially similar to scene detection, local point description features perform better than global- and texture-based feature descriptors.

In order to facilitate usage of optical data in space climate studies, we have developed an automated algorithm to quantify the complexity of auroral structures as they appear in ground-based all-sky images. The image analysis is based on a computationally determined “arciness” value, which describes how arc like the auroral structures in the image are. With this new automatic method we have analyzed the type of aurora in about 1 million images of green aurora (λ = 557.7nm) captured at five camera stations in Finnish and Swedish Lapland in 1996–2007. We found that highly arc like structures can be observed in any time sector and their portion of the auroral structures varies much less than the fraction of more complex forms. The diurnal distribution of arciness is in agreement with an earlier study with high arc occurrence rate in the evening hours and steadily decreasing toward the late morning hours. The evolution of less arc-like auroral structures is more dependent on the level of geomagnetic activity and solar cycle than the occurrence of arcs. The median arciness is higher during the years close to the solar minimum than during the rest of the solar cycle. Unlike earlier proposed, the occurrence rate of both arcs and more complex auroral structures increases toward the solar maximum and decreases toward the solar minimum. The cyclic behavior of auroral structures seen in our data is much more systematic and clear than previously reported visual studies suggest. The continuous arciness index describing the complexity of auroral structures can improve our understanding on auroral morphology beyond the few commonly accepted structure classes, such as arcs, patches, and omega bands. Arciness can further be used to study the relationship of auroral structures at different complexity levels and magnetospheric dynamics.

The Magnetometers Ionospheric Radars All-sky Cameras Large Experiment (MIRACLE) network monitors auroral activity in the Fennoscandian sector. Network stations cover the range of 55° to 57° magnetic latitude north and span two hours in magnetic local time. Seven of the MIRACLE network stations include digital all-sky cameras (ASCs). Up to recent years, the type of ASC used in the network consisted of an optical system, a charged coupled device (CCD) and an image intensifier enabling short exposure times. This system is referred to as an intensified CCD (ICCD) camera. As image intensifiers degrade over time, it has become necessary to replace the MIRACLE network ASCs with newer technology. Since 2007 the Sodankyla (SOD) and Kilpisjarvi (KIL) stations have been equipped with electron multiplying CCD (EMCCD) cameras. Both ICCD and EMCCD cameras in the MIRACLE network operate at three different wavelengths: 427.8 nm, 557.7 nm and 630.0 nm. The signal-to-noise ratio and dynamic range of the EMCCD camera is greater than that of the ICCD camera by about a factor of 2. The goal of this study is to investigate the differences between the two types of camera placed under the same dome at the Sodankyla station. We also compare ASC images with in situ particle data (total electron energy flux) from low-altitude satellite NOAA18. Our analysis includes two days with different conditions (faint emission and substorm activity). This study shows that the EMCCD camera has greater sensitivity for blue and red wavelengths allowing for better measurement of auroral emission. The dynamic imaging range of the EMCCD camera is about 1.7 times that of the ICCD camera. Moreover, the EMCCD camera data are well correlated with the NOAA18 satellite total electron energy flux (cross-correlation coefficient ∼0.8) for the 557.7 nm emission.

This study describes rapid prototype construction of small and lightweight push broom Hyper Spectral Imagers (HSI). The dispersive element housings are printed by a thermoplastic 3D printer combined with S-mount optical components and commercial off-the-shelf camera heads. Four models with a mass less than 200 g are presented with a spectral range in the visible to the near-infrared part of the electromagnetic spectrum. The bandpass is in the range from 1.4 - 5 nm. Three test experiments with motorized gimbals to stabilize attitude show that the instruments are capable of push broom spectral imaging from various platforms, including airborne drone to handheld operations. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

This study examines auroral colour camera data from the Canadian Dense Array Imaging SYstem (DAISY). The Dense Array consists of three imagers with different narrow (compared to all-sky view) field-of-view optics. The main scientific motivation arises from an earlier study by Knudsen et al. (2001) who used All-Sky Imager (ASI) combined with even earlier TV camera observations (Maggs and Davis, 1968) to suggest that there is a gap in the distribution of auroral arc widths at around 1 km. With DAISY observations we are able to show that the gap is an instrument artifact and due to limited spatial resolution and coverage of commonly used instrumentation, namely ASIs and TV cameras. If the auroral scale size spectrum is indeed continuous, the mechanisms forming these structures should be able to produce all of the different scale sizes. So far, such a single process has not been proposed in the literature and very few models are designed to interact with each other even though the range of their favourable conditions do overlap. All scale-sizes should be considered in the future studies of auroral forms and electron acceleration regions, both in observational and theoretical approaches.

An isolated substorm on Oct. 17, 1997 during a close conjunction of the Polar spacecraft and the ground-based MIRACLE network is studied in detail. We identify signatures of substorm onset in the plasma sheet midway between the ionosphere and the equatorial plasma sheet, determine their timing relative to the ground signatures, and discuss their counterparts on the ground and in the equatorial plasma sheet. The substorm onset is determined as the negative bay onset at 2040:42(±5 sec) UT coinciding with the onset of auroral precipitation, energization of plasma sheet electrons at Polar, and strong magnetic field variations perpendicular to the ambient field. Such accurate timing coincidence is consistent with the Alfven transit time between Polar and the ionosphere. Furthermore, the timing of other field and particle signatures at Polar showed clear deviations from the onset time (±2 min). This suggests that the sequence of these signatures around the onset time can be used to validate the signatures predicted by various substorm onset models.

Multiple and sporadic time-of-flight velocity dispersed ion structures (TDIS) are systematically observed above the ionosphere at ∼3 Re altitude by Interball/Auroral spacecraft near the poleward edge of the auroral bulge. These events represent direct snapshots of the impulsive ion acceleration process in the equatorial plasma sheet which allow us to study the details of the connection between ionospheric and plasma sheet manifestations of the magnetospheric substorm. Two events are analyzed during which the spacecraft footpoints passed over the Scandinavian ground network. We found that the TDIS correlate with the intensifications of westward current and auroral activations at the poleward edge of the bulge, which confirms the association of these dispersed ion beams with the temporal evolution of impulsive reconnection in the tail. Furthermore, we present direct evidence of an active neutral line in the magnetotail during one of the events using plasma sheet measurements made concurrently by the Interball/Tail and Geotail spacecraft. The 2–3 min repetition period of these ∼1 min long activations indicates a fundamental time constant of the substorm instability. On the other hand, the estimated injection distances of the energy-dispersed ions were inferred to be smaller than the estimated position of the reconnection region in the tail. We also found that the TDIS ion beams are released within the closed plasma flux tubes deep inside the plasma sheet, and yet they are synchronized with auroral activations at the poleward boundary. These facts imply that the ion beams are formed in a spatially extended region of the plasma sheet rather than in the close vicinity of the neutral line. We argue that braking of the reconnection-induced fast flow bursts when they interact with the closed plasma flux tubes and the earthward propagating fast wave electric field generated in the braking region may be important in forming the observed multiple, sporadic, energy-dispersed ion beams.

In this paper we report coordinated multispacecraft and ground-based observations of a double substorm onset close to Scandinavia on November 17, 1996. The Wind and the Geotail spacecraft, which were located in the solar wind and the subsolar magnetosheath, respectively, recorded two periods of southward directed interplanetary magnetic field (IMF). These periods were separated by a short northward IMF excursion associated with a solar wind pressure pulse, which compressed the magnetosphere to such a degree that Geotail for a short period was located outside the bow shock. The first period of southward IMF initiated a substorm growth. phase, which was clearly detected by an array of ground-based instrumentation and by Interball in the northern tail lobe. A first substorm onset occurred in close relation to the solar wind pressure pulse impinging on the magnetopause and almost simultaneously with the northward turning of the IMF. However, this substorm did not fully develop. In clear association with the expansion of the magnetosphere at the end of the pressure pulse, the auroral expansion was stopped, and the northern sky cleared. We will present evidence that the change in the solar wind dynamic pressure actively quenched the energy available for any further substorm expansion. Directly after this period, the magnetometer network detected signatures of a renewed substorm growth phase, which was initiated by the second southward turning of the IMF and which finally lead to a second, and this time complete, substorm intensification. We have used our multipoint observations in order to understand the solar wind control of the substorm onset and substorm quenching. The relative timings between the observations on the various satellites and on the ground were used to infer a possible causal relationship between the solar wind pressure variations and consequent substorm development. Furthermore, using a relatively simple algorithm to model the tail lobe field and the total tail flux, we show that there indeed exists a close relationship between the relaxation of a solar wind pressure pulse, the reduction of the tail lobe field, and the quenching of the initial substorm.

[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] With the deployment of the all-sky imager array of the THEMIS mission, we were able to construct a preliminary database of auroral substorm expansion phase onsets, from which we have established a number of common features characterizing the first tens of seconds of the substorm auroral intensification. We find that the intensification occurs within ∼10 sec over an arc segment extending approximately 1 h MLT and featuring wave-like formations distributed in longitude. The longitudinal wave number ranges between 100 and 300 such that the wavelength is comparable to the ion gyroradius in the central plasma sheet. The scale the intensification is about 10–30 sec. This study casts important observational constraints on substorm onset theories.