Dayside Aurora Research Articles

Auroral Sequence Representation and Classification Using Hidden Markov Models

The naturally occurring aurora phenomenon is a dynamically evolving process. Taking temporal information into consideration, the auroral image sequence analysis is more reasonable and desirable than using static images only. However, the enormous richness of space structures and temporal variations make automatic auroral sequence analysis a particularly challenging task. In this paper, a hidden Markov model (HMM) based representation method including features of spatial texture and dynamic evolution is presented to characterize auroral image sequences captured by all-sky imagers (ASIs). The uniform local binary patterns are employed to describe the 2-D space structures of ASI images. HMM is feasible to characterize the doubly stochastic process involved in the auroral evolution-measurable polar light activities and hidden dynamic plasma processes. We present an affine log-likelihood normalization technique to manage the sequences with different lengths. The proposed method is used in the automatic recognition of four primary categories of ASI auroral observations between the years 2003 and 2009 at the Yellow River Station, Ny-Ålesund, Svalbard. The supervised classification results on manually labeled data in 2003 demonstrate the effectiveness of the proposed technique. Compared with frame-based classification, the higher accuracies and the lower rejection rates show the advantages of the sequence-based method. The occurrence distributions of the four aurora categories were obtained through automatic classification of data gathered from 2004 to 2009. Their agreement with the multiple-wavelength intensity distribution of the dayside aurora and the conclusions made from the frame-based method further illustrate the validity of our method on auroral representation and classification.

Photometry of dayside auroras during solar minimum 1This article is part of a Special issue that honours the work of Dr. Donald M. Hunten FRSC who passed away in December 2010 after a very illustrious career.

An examination is made of Antarctic dayside auroras to establish how they relate to solar wind strength under the quiet conditions of the recent extended solar minimum when the solar wind pressure was weak and the interplanetary magnetic field Bz is small. It is found that, during the many days of observation, the aurora is detected even with the most stable and quiet conditions. On such occasions the 630 nm OI emission can be as low as 50 R, but is unambiguously and continuously detectable through each noon. This is above an airglow intensity of about 30 R. For these quiet conditions there is no evident relation between the solar wind dynamic pressure or interplanetary magnetic field Bz and dayside auroral intensity. This suggests that there is no effective reconnection under these minimal conditions and the particle source for the dayside aurora could be within the magnetosphere.

Dayside corona aurora classification based on X-grey level aura matrices and feature selection

Dayside corona aurora is the main form of aurora at magnetic noon, which is generated by the dynamics process of the interaction of the sun and earth's magnetosphere. Hence the study of dayside corona aurora is of great importance to the analysis of ionosphere and its dynamic feature. This paper proposes a novel aurora texture extraction method based static image classification of dayside aurora, in which X-grey level aura matrices are designed to extract the features of the original aurora images. Besides, a dayside aurora classification algorithm for dayside aurora based on feature selection is proposed to handle the large quantities of aurora samples. To eliminate the effect of noise and solve the problem of high feature dimensionality, the ReliefF is adopted to select the effective feature vector. For classification, texture models are learned by using the support vector machine, then a given texture of aurora image can be classified into one of the pre-learned classes. All of the aurora images in the experiment are obtained from the all-sky aurora image system in the Chinese Arctic Yellow River Station. The experimental results illustrate the high effectiveness of the proposed dayside aurora classification algorithm.

On the relationship between flux transfer events, temperature enhancements, and ion upflow events in the cusp ionosphere

[1] A transit of the dayside aurora across the field-of-view of the EISCAT Svalbard Radar occurred on 20 December 1998. This offered an excellent opportunity to study the spatial structure of the cusp/cleft aurora using meridian scanning photometer and incoherent scatter radar. We were able to identify distinct regions of upflow driven by ion heating (type 1) and upflow driven by electron heating (type 2) around poleward moving auroral forms, a transient auroral feature tied to flux transfer events. A quiet period before the auroral transit allowed us to estimate a neutral temperature profile, which enabled calculation of the ion-neutral relative wind. We found evidence for purely ion heating-driven upflow equatorward of the cusp auroral boundary, and for electron heating-driven upflow near the equatorward auroral boundary. The greatest upflow occurred near the center of the cusp aurora when both ion and electron temperatures were enhanced. The observed upflows were greater than expected from ambipolar diffusion alone, suggesting that ion-neutral frictional heating did contribute to upflow events in most cases. The great variability observed in ion temperature indicates that the ion flow was greatly structured within the aurora. Type 1–2 upflows may be considered as spatial structures of active cusp. Upflows are observed at various times in their evolution, and one upflow event, estimated to be 5–10 minutes old, showed a lifting of the F region of some 100 km, indicating a hybrid of type 1 and type 2.

Automatic Classification of Dayside Aurora in All-Sky Images Using a Multi-Level Texture Feature Representation

In this paper, we propose an aurora classification method using a multi-level feature representation aimed to capture both global and local texture information, and to reduce the feature space dimension substantially. First-order and second-order statistics are computed for an input image and its low-frequency scaled images at three lower levels obtained using wavelet decomposition. The features include gray level distribution, co-occurrence matrix features, and run-length matrix features. A support vector machine (SVM) classifier was trained and tested on a Chinese Arctic Yellow River Station dayside aurora image dataset. Classification performance was evaluated and compared with those of k-nearest neighbor (KNN) classifiers and back-propagation neural networks (BPNN). To explore the possibility of using a smaller feature space, we used a Minimum-Redundancy Max-Relevance feature selection strategy. The result shows that there is only indistinct performance decrease by reducing the feature vector from a total of 88 to the most discriminatory 38 features. This proves that our multi-level feature representation is very robust.

Different response of dayside auroras to increases in solar wind dynamic pressure

[1] Variations in the solar wind dynamic pressure can create changes in the auroral brightness, especially in the dayside region. In this study, mainly using Polar Ultraviolet Imager auroral images and NASA OMNI magnetic field and plasma data, we investigate the relationship between interplanetary parameters and dayside aurora intensification. We first identify the dayside aurora cases caused by the solar wind dynamic pressure increases from 1997 through 1999 and divide these cases into two types: one with intensification and the other without significant intensification following the increase in the solar wind dynamic pressure. Then, case by case, we examine the separate roles of the solar wind density and velocity on auroral brightness. Our result demonstrates that the dayside aurora intensification requires the contributions of both the density and solar wind velocity increases. The solar wind density increase alone (without velocity increase) could not create the dayside aurora intensification.

Decrease of auroral intensity associated with reversal of plasma convection in response to an interplanetary shock as observed over Zhongshan station in Antarctica

[1] We examined temporal variations of a dayside aurora and corresponding ionospheric plasma convection observed by an all-sky camera (ASC) and the Super Dual Auroral Radar Network (SuperDARN) over Zhongshan (ZHS), located at −74.5° in magnetic latitude (MLAT) in Antarctica, during a geomagnetic sudden commencement (SC) event that occurred on 27 May 2001. Simultaneous ASC observations at South Pole (SP, −74.3° MLAT) were also analyzed. During the SC time, ZHS and SP were located in the postnoon (1610 MLT) and prenoon (1100 MLT) sectors, respectively. Before the SC onset (1458UT), the ASC at ZHS observed an auroral arc with moderate intensity in the poleward direction of the field of view (FOV), and the SuperDARN radar detected sunward ionospheric plasma flow over ZHS. Just after the SC onset, the auroral intensity over ZHS decreased rapidly and the direction of the plasma flow was reversed to antisunward. Decrease of auroral intensity and reversal of the associated plasma convection in response to a sudden increase of the solar wind dynamic pressure at the early stage of a SC event has never been reported before. We suggest that these observational results were generated by a downward field-aligned current (FAC) and are consistent with a physical model of SC. The model predicts the appearance of a pair of FACs flowing downward (upward) in the postnoon (prenoon) sector at the very beginning of the SC, which is also supported by our observations. Consistence of the detailed observations with the model will be discussed in the paper, and we argue that here we present the first optical observational evidence supporting the validity of the model.

IMF dependence of Saturn's auroras: modelling study of HST and Cassini data from 12–15 February 2008

Abstract. To gain better understanding of auroral processes in Saturn's magnetosphere, we compare ultraviolet (UV) auroral images obtained by the Hubble Space Telescope (HST) with the position of the open-closed field line boundary in the ionosphere calculated using a magnetic field model that employs Cassini measurements of the interplanetary magnetic field (IMF) as input. Following earlier related studies of pre-orbit insertion data from January 2004 when Cassini was located ~ 1300 Saturn radii away from the planet, here we investigate the interval 12–15 February 2008, when UV images of Saturn's southern dayside aurora were obtained by the HST while the Cassini spacecraft measured the IMF in the solar wind just upstream of the dayside bow shock. This configuration thus provides an opportunity, unique to date, to determine the IMF impinging on Saturn's magnetosphere during imaging observations, without the need to take account of extended and uncertain interplanetary propagation delays. The paraboloid model of Saturn's magnetosphere is then employed to calculate the magnetospheric magnetic field structure and ionospheric open-closed field line boundary for averaged IMF vectors that correspond, with appropriate response delays, to four HST images. We show that the IMF-dependent open field region calculated from the model agrees reasonably well with the area lying poleward of the UV emissions, thus supporting the view that the poleward boundary of Saturn's auroral oval in the dayside ionosphere lies adjacent to the open-closed field line boundary.

A review of interplanetary discontinuities and their geomagnetic effects

Interplanetary discontinuities and their geomagnetic effects are reviewed for magnetospheric/space weather researchers. Discontinuities are particularly useful as diagnostics since they are clearly identifiable in interplanetary data and their geomagnetic effects are unambiguous most of the time. Directional discontinuities (DDs) are abrupt changes in the interplanetary magnetic field direction and plasma parameters. DDs may be rotational discontinuities (RDs), tangential discontinuities (TDs) contact discontinuities (CDs) or shocks (fast (FS), intermediate (IS) and slow (SS). Shocks can propagate in the direction of the driver (forward shocks or FSs) or opposite to the driver (reverse shocks of RSs). Discontinuities interacting with other discontinuities may create new discontinuities. Fast forward shocks (FFSs) are shown to energize trapped particles by compressive effects, cause dayside aurora, lead to the creation of new radiation belts and to trigger nightside sector magnetospheric substorms. Fast reverse shocks (FRSs) or reverse waves (RWs) lead to magnetospheric expansions and the cessation of geomagnetic activity. TD-bow shock interactions create hot flow anomalies (HFAs) which then lead to outward expansions of the local magnetopause and dayside auroral enhancements. Some DD crossings may cause sudden southward IMF turnings. These cause magnetic reconnection and energy input into the magnetosphere–ionosphere–magnetotail system. Substorms sometimes occur thereafter. DDs that entail northward IMF turnings may lead to the triggering of substorms.

The 4-emission-core structure of dayside aurora oval observed by all-sky imager at 557.7 nm in Ny-Ålesund, Svalbard

A survey of dayside aurora excitation at 557.7 nm, which was acquired from an all-sky imager at Yellow River Station in Ny-Ålesund, Svalbard, shows that there are 4 intense emission maxima in the dayside oval, centered near 0630/76, 0830/76, 1400/75, and 1600/75 (unit: MLT/MLAT), respectively. Tracing the magnetospheric sources of these cores along geomagnetic field lines, the 0830- and 1400-MLT cores correspond with the prenoon and postnoon magnetospheric boundary layers (MBLs), and the 0630- and 1600-MLT cores are located at the dusk and dawn MBLs, respectively. The potential solar wind-magnetosphere dynamic processes resulting in these auroral features are discussed.

Spatial texture based automatic classification of dayside aurora in all-sky images

A spatial texture based representation method including features of intensity, shape and texture, was utilized to characterize all-sky auroral images. The combination of the local binary pattern (LBP) operator and a delicately designed block partition scheme achieved both global shapes and local textures capabilities. The representation method was used in automatic recognition of four primary categories of discrete dayside aurora using observations between years 2003–2009 at the Yellow River Station, Ny-Ålesund, Svalbard. The supervised classification results on labeled data in 2003 were in accordance with the labeling by scientists considering both spectral and morphological information. The occurrence distributions of the four categories were obtained through automatic classification of data between 2004–2009, which confirm the multiple-wavelength intensity distribution of dayside aurora, and further provide morphological interpretation of auroral types.

Ion velocity filter effect observed in dayside hydrogen aurora

Observations of dayside auroral hydrogen emissions of Hα (λ656.3 nm) were carried out using spectrometers on Svalbard at Ny‐Ålesund (NYA: 76.3°N, 111.0°E CGM) and Longyearbyen (LYR: 75.3°N, 111.9°E CGM). Using a Monte Carlo model, simulated Doppler profiles were fitted to the spectra to estimate precipitating proton energy. A difference in energy was found between the sites for approximately 45 minutes. When combined with measurements of antisunward convection, the energies are consistent with the inverse variation of ion energy with latitude observed by satellites in the cusp region, known as the ion velocity filter. This is the first measurement of the ion velocity filter effect in the dayside aurora using ground‐based optical instrumentation. The increasing difference in energy observed is interpreted as a measure of the decreasing merging rate at the magnetopause.

Dayside auroras during unusually large positive values of the IMF B z component

The characteristics of dayside auroras during the large (16-24 nT) positive values of the IMF Bz component, observed on January 14, 1988, during the interaction between the Earth's magnetosphere and the body of the interplanetary magnetic cloud, have been studied based on the optical observations on Heiss Island. A wide band of diffuse red luminosity with an intensity of 1-2 kilorayleigh (kR) was observed during 6 h in the interval 1030-1630 MLT at latitudes higher than 75° CGL. Rayed auroral arcs, the brightness of which in the 557.7 nm emission sharply increased to 3-7 kR in the postnoon sector immediately after the polarity reversal of the IMF B y component from positive to negative, were continuously registered within the band. Bright auroral arcs were observed at the equatorward edge of red luminosity. It has been found out that the red auroral intensity increases and the band equatorward boundary shifts to lower latitudes with increasing solar wind dynamic pressure. However, a direct proportional dependence of the variations in the auroral fea� tures on the dynamic pressure variations has not been found. It has been concluded that the source of bright discrete auroras is located in the region of the lowlatitude boundary layer (LLBL) on closed geomagnetic field lines. The estimated LLBL thickness is ~3 Re. It has been concluded that the intensity of the dayside red band depends on the solar wind plasma density, whereas the position of the position equatorward boundary depends on the dynamic pressure value and its variations.

Response of dayside auroras to abrupt increases in the solar wind dynamic pressure at positive and negative polarity of the IMF B z component

The optical observations on Heiss Island (Φ′ = 75.0°) have been used to study the characteristics of auroras in the near-noon MLT sector after abrupt increases in the solar wind dynamic pressure at negative and positive polarity of the IMF B z component. It has been found out that the 427.8 and 557.7 nm emission intensities considerably increased at B z 0, fluxes of harder (E > 1 keV) precipitating electrons were superimposed on the soft spectrum of precipitating particles in the equatorial part of the red luminosity band. This red band part was hypothetically caused by the low-latitude boundary layer (LLBL) on closed lines of the geomagnetic field, the estimated thickness of which is ∼3 R e . The 557.7 nm emission intensity increased during 3–5 min after SC/SI and was accompanied by the displacement of the red band equatorward boundary toward lower latitudes. The displacement value was ∼150–200 km when the dynamic pressure abruptly increased by a factor of 3–5. After SC/SI, the 630.0 nm emission intensity continued increasing during 16–18 min. It is assumed that the time of an increase in the red line intensity corresponds to the time of saturation of the magnetospheric boundary layers with magnetosheath particles after an abrupt increase in their density.

Dynamics of the dayside Aurora Australis

Current dayside optical studies of Aurora Australis from the Amundsen-Scott Research Station at the South Pole (74 degrees magnetic latitude) show some striking differences from optical results reported from Svalbard. A 6-channel meridian scanning photometer operating during the past three austral winters shows, in particular, the 630 nm emission is much lower, on average, than the Arctic dayside aurora and very weak on some days. The 558 nm intensity is higher relative to 630 nm suggesting the incoming electrons have a higher average energy. There are notable differences in auroral forms, giving further evidence of asymmetries in the two dayside ovals.

Synoptic distribution of dayside aurora: Multiple-wavelength all-sky observation at Yellow River Station in Ny-Ålesund, Svalbard

Observations acquired from three-wavelength (427.8, 557.7 and 630.0 nm) all-sky imagers (ASIs) at Yellow River Station (YRS) in Ny-Ålesund, Svalbard, are used to examine the synoptic distribution of dayside aurora. The results demonstrate that the maximum emission regions (MERs) at each wavelength are all located in the postnoon sector, but have rather different magnetic local time (MLT) distributions from each other. The so-called 15 MLT “hot spot” is the overlapping region of the MERs at three wavelengths, and the prenoon “warm spot” is characterized uniquely by an increase of emissions at the 557.7 nm wavelength. The detailed dayside auroral spectra and morphology as a function of MLT are discussed.

Coordinated observation of the dayside magnetospheric entry and exit of the THEMIS satellites with ground‐based auroral imaging in Antarctica

Data from the five‐satellite Time History of Events and Macroscale Interactions during Substorms (THEMIS) constellation suitably located to study solar wind magnetospheric coupling during the austral winter of 2007 were compared to data from ground‐based all‐sky imagers (ASIs) at South Pole (74° magnetic latitude) and at AGO‐1 (80° magnetic latitude). The THEMIS constellation entered and exited the magnetosphere near magnetic midday on 10 and 12 August 2007, respectively. On 12 August interplanetary magnetic field (IMF) (Bz > 0) the dayside aurora was located more poleward between AGO‐1 and South Pole. The THEMIS satellites traversing the magnetopause saw it move in and out several times during the satellite crossings. The inward motion of the magnetopause was sometimes correlated with equatorward expansions of the aurora and sometimes with solar wind pressure pulses as seen by Geotail. The Bz > 0 auroral latitude was consistent with dayside cusp “spot” seen by the IMAGE spacecraft and had been associated with footprints of “steady state” lobe reconnection. The aurora consisted of a continuous stream of poleward moving auroral forms (PMAF) even during a period of slightly Bz > 0 and By = 0. On 10 August 2007 IMF Bz < 0 the dayside aurora was located more equatorward, over South Pole, and the THEMIS satellites crossed the bow shock from the magnetosheath into the solar wind. Negative Bz pulses observed at the satellite were correlated with large poleward expansions of the dayside aurora consistent with reconnection in the subsolar region accompanied by simultaneous plasma density increases at THEMIS B satellite consistent with outward radial motion of the bow shock.

The role of external triggers in flow shear arcs in the dayside aurora

Abstract. In case studies we relate dayside auroral transients to IMF By-distorted plasma convection cells based on high-resolution observations from the ground. We selected three days representing positive and negative IMF By conditions when SuperDARN returned reliable dayside convection patterns in the sector of our optical observations from Ny Ålesund, Svalbard (76° MLAT). We combine two perspectives on the dayside aurora, the local and the global. In the first we derive the fine-structure of dayside precipitation/convection as a function of magnetic latitude (MLAT) and magnetic local time (MLT), which is necessary to understand the local M-I coupling processes (Birkeland current structure). The larger perspective (quasi-global dayside aurora) may be used to shed light on the solar wind-magnetosphere interconnection topology. The auroral morphology consists of brightening events and poleward moving auroral forms (PMAFs) in the pre- and postnoon sectors longitudinally separated by a band of strongly attenuated aurora near noon. We find that the MLT-dependent spatial structure in the dayside aurora (PMAFs/prenoon – "midday gap aurora" – PMAFs/postnoon) which is present during stable IMF conditions is altered by temporal structure during intervals of IMF/solar wind plasma transients. The focus is on the PMAF substructure (so-called "rebrightening forms") which we identify as dynamical plasma flow shear arcs (FSAs) in By-distorted dawn- and dusk-centered convection cells in the close vicinity of the cusp.

Changes of dayside auroral distribution caused by a solar wind pressure pulse and associated interplanetary magnetic field disturbances

Abstract. Global auroral images from the IMAGE satellite were used to study statistically changes of the dayside aurora spatial distribution after an abrupt solar wind pressure increase, or so-called "Sudden Impulse" (SI). Contributions from IMF changes associated with a SI were also investigated. The effects of the IMF and pressure variations were separated using a multi-factor correlation analysis. The first prominent effect due to pressure increase is the auroral intensification equatorward of the middle dayside oval within 6 min after a SI occurred. This is consistent with the midday sub-auroral patches. The second effect due to pressure increase is the auroral intensification at high latitudes in the vicinity of the polar cap boundary. For the first 6 min the auroral intensification is most prominent in the postnoon sector. Later on (6–20 min) the intensification occurs in the prenoon sector. The most obvious effect of IMF changes is the "IMF By" effect, an intensification (fading) of the most poleward auroral forms when IMF By becomes negative (positive). This effect occurs 6–20 min after changes in the interplanetary medium. Such an effect is consistent with the IMF By-related system of field-aligned currents. No significant motion of the dayside auroral oval was observed associated with IMF Bz variations. This can be explained by a response time to IMF Bz changes larger than 20 min.

Magnetospheric application of high-altitude long-duration balloon technology: Daylight auroral observations

Daylight auroral imaging is a proposed application of the NASA high-altitude long-duration balloon technology. This paper discusses the theoretical background of this application and test observations, for proof of the feasibility. It is demonstrated that nitrogen auroral emissions in the near-infrared band are detectable at altitudes of 35–40 km and above using a near-infrared InGaAs camera. The purpose of such observations is to identify auroral small-scale structures that are manifestations of auroral particle accelerations and the solar wind – magnetosphere – ionosphere interaction. Use of this new approach will enable studies of the dayside aurora, low-latitude aurora, and storm-time and substorm-time auroral conjugacy.