Auroral Observations Research Articles

Automatic recognition of poleward moving auroras from all-sky image sequences based on HMM and SVM

We present an automatic method to recognize the poleward moving auroras (PMAs) from all-sky image sequences. A simplified block matching algorithm combined with an orientation coding scheme and histogram statistics strategy was utilized to estimate the auroral motion between interlaced images. An all-sky image sequence was first modeled by hidden Markov models (HMMs) and then represented by HMM similarities. The imbalanced classification problem, i.e., non-PMA events far outnumbering PMA events, was addressed by the metric-driven biased support vector machine (SVM). The proposed method was evaluated using auroral observations in 2003 at the Chinese Yellow River Station. Five days observations were manually labeled as PMA or non-PMA events considering both the keogram and all-sky image information. The supervised classification experiments were carried out and achieved satisfactory results. We further detected PMAs from auroral observations in the remaining days and the resultant double-peak occurrence distribution was compared with that of the well-known poleward moving auroral forms (PMAFs).

Sophus Peter Tromholt: an outstanding pioneer in auroral research

Abstract. The Danish school teacher Sophus Peter Tromholt (1851–1896) was self-taught in physics, astronomy, and auroral sciences. Still, he was one of the brightest auroral researchers of the 19th century. He was the first scientist ever to organize and analyse correlated auroral observations over a wide area (entire Scandinavia) moving away from incomplete localized observations. Tromholt documented the relation between auroras and sunspots and demonstrated the daily, seasonal and solar cycle-related variations in high-latitude auroral occurrence frequencies. Thus, Tromholt was the first ever to deduce from auroral observations the variations associated with what is now known as the auroral oval termed so by Khorosheva (1962) and Feldstein (1963) more than 80 yr later. He made reliable and accurate estimates of the heights of auroras several decades before this important issue was finally settled through Størmer's brilliant photographic technique. In addition to his three major scientific works (Tromholt, 1880a, 1882a, and 1885a), he wrote numerous short science notes and made huge efforts to collect historical auroral observations (Tromholt, 1898). Furthermore, Tromholt wrote a large number of popular science articles in newspapers and journals and made lecture tours all over Scandinavia and Germany, contributing to enhance the public educational level and awareness. He devoted most of his life to auroral research but as a self-taught scientist, he received little acclaim within the contemporary academic scientific society. With his non-academic background, trained at a college of education – not a university – he was never offered a position at a university or a research institution. However, Sophus Tromholt was an outstanding pioneer in auroral research.

Multi-scale auroral observations in Apatity: winter 2010–2011

Abstract. Routine observations of the aurora are conducted in Apatity by a set of five cameras: (i) all-sky TV camera Watec WAT-902K (1/2"CCD) with Fujinon lens YV2.2 × 1.4A-SA2; (ii) two monochromatic cameras Guppy F-044B NIR (1/2"CCD) with Fujinon HF25HA-1B (1:1.4/25 mm) lens for 18° field of view and glass filter 558 nm; (iii) two color cameras Guppy F-044C NIR (1/2"CCD) with Fujinon DF6HA-1B (1:1.2/6 mm) lens for 67° field of view. The observational complex is aimed at investigating spatial structure of the aurora, its scaling properties, and vertical distribution in the rayed forms. The cameras were installed on the main building of the Apatity division of the Polar Geophysical Institute and at the Apatity stratospheric range. The distance between these sites is nearly 4 km, so the identical monochromatic cameras can be used as a stereoscopic system. All cameras are accessible and operated remotely via Internet. For 2010–2011 winter season the equipment was upgraded by special blocks of GPS-time triggering, temperature control and motorized pan-tilt rotation mounts. This paper presents the equipment, samples of observed events and the web-site with access to available data previews.

Efficient diffuse auroral electron scattering by electrostatic electron cyclotron harmonic waves in the outer magnetosphere: A detailed case study

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 >10−4 s−1 for equatorial pitch angles αeq < 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. (2009, 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.

Spanish eyewitness accounts of the great space weather event of 1859

Exceptional geomagnetic storms were observed in late August and early September 1859. Auroras were observed around the world, and telegraph wires were damaged. Until now, there has been no evidence published of auroral observations in Spain during this famous space weather event. This paper presents Spanish observations that show the aspect of this great aurora from Spain, and the concurrent effects on European telegraph wires. We also computed the variation of the geomagnetic declination in Spain during recent centuries to put these records into context.

Johann Christian Heuson, a little-known auroral scholar of the early 18th century

Abstract. Heuson published two booklets of observations of the aurora of 17/18 February, 1/2 March 1721, and 16/17 November 1729 together with contemporary thoughts about the subject. His work characterizes him as a well-educated scholar familiar with contemporary auroral observations and theories. Heuson rejects views of the aurora as an omen or portent of divine wrath, but explains the aurora as a natural phenomenon and is thus in line with other well-known auroral scholars of that time.

Small satellite REIMEI for auroral observations

This paper describes the outline and the five years' on-orbit results of the small scientific satellite REIMEI for aurora observations and demonstrations of advanced small satellite technologies. REIMEI is a small satellite with 72 kg mass, and is provided with three-axis attitude control capabilities for aurora observations. REIMEI was launched into a nearly sun synchronous polar orbit on Aug. 23rd, 2005, from Baikonur, Kazakhstan, by Dnepr rocket. REIMEI satellite has been satisfactorily working on the orbit for five years at present as of January, 2011. Three-axis control is achieved with accuracy of 0.1° (3 σ). Multi-spectrum images of aurora are taken with 8 Hz rate and 2 km spatial resolution to investigate the aurora physics. REIMEI is performing the simultaneous observation of aurora images and particle measurements. REIMEI indicates that even a small satellite launched as a piggy-back can successfully perform unique scientific mission purposes.

Auroral electrojets during deep solar minimum at the end of solar cycle 23

[1] We investigate the auroral electrojet activity during the deep minimum at the end of solar cycle 23 (2008–2009) by comparing data from the IMAGE magnetometer chain, auroral observations in Fennoscandia and Svalbard, and solar wind and interplanetary magnetic field (IMF) observations from the OMNI database from that period with those recorded one solar cycle earlier. We examine the eastward and westward electrojets and the midnight sector separately. The electrojets during 2008–2009 were found to be weaker and at more poleward latitudes than during other times, but when similar driving solar wind and IMF conditions are compared, the behavior in the morning and evening sectors during 2008–2009 was similar to other periods. On the other hand, the midnight sector shows distinct behavior during 2008–2009: for similar driving conditions, the electrojets resided at further poleward latitudes and on average were weaker than during other periods. Furthermore, the substorm occurrence frequency seemed to saturate to a minimum level for very low levels of driving during 2009. This analysis suggests that the solar wind coupling to the ionosphere during 2008–2009 was similar to other periods but that the magnetosphere-ionosphere coupling has features that are unique to this period of very low solar activity.

On the simultaneity of substorm onset between two hemispheres

Simultaneous observations of auroral kilometric radiation from the Northern and Southern Hemispheres showed some cases in which the buildup of field-aligned acceleration occurred only in one hemisphere at the substorm onset. This indicates that a substorm does not always complete the current system by connecting the cross-tail current with both northern and southern ionospheric currents. Conjugate auroral observations showed that in one case, the auroral breakup in the Northern and Southern Hemispheres was not simultaneous; rather, they occurred a few minutes apart. This time difference in the breakup between two hemispheres suggests that the local auroral ionosphere controls auroral breakup in each hemisphere independently. The evidence in this study may indicate that the buildup of the field-aligned acceleration region at the auroral breakup does not result only from the magnetospheric process and that the auroral ionosphere finally controls and/or ignites the substorm onset, that is, the auroral breakup.

Longitudinal variations of nighttime electron auroral precipitation in both the Northern and Southern hemispheres from the TIMED global ultraviolet imager

Using 6 years of Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) global ultraviolet imager auroral observations in both hemispheres, we have studied the longitudinal variations of auroral precipitation during the magnetic nighttime period of 2100-0300 magnetic local time. There was a strong seasonal dependence of the longitudinal variations of the aurora: (1) During solstices and for both hemispheres, auroral precipitation peaked between magnetic longitude (MLON) 210 degrees E and 360 degrees E in June and between MLON 120 degrees E and 300 degrees E in December. (2) In the equinoxes, the auroral longitudinal pattern was generally similar to that in local summer in each hemisphere, except that in the Northern Hemisphere the maximum precipitation was usually located in more westward longitudes in equinox than in summer. (3) The ratios between the maximum and the minimum of the precipitation energy flux along longitudes varied between 1.3 and 1.9, which were similar to those in previous studies. These features of the auroral longitudinal patterns did not change much from Kp = 1 to Kp = 4 conditions. Since the longitudinal distribution of auroral precipitation changed greatly with season in each hemisphere, the longitudinal variations of the magnetic field strength, which do not change with season, might not be the only process that caused the observed longitudinal variations of the aurora. Further data analysis shows that there was a significant negative correlation (coefficient vertical bar r vertical bar = similar to 0.4-0.8) between the peak auroral precipitation intensity and the solar-EUV-produced ionospheric conductivity of the same hemisphere (in summer and equinox) or of the conjugate hemisphere (in winter). These results indicate the important effects of solar-EUV-produced ionospheric conductivity, which has significant longitudinal variations, on the longitudinal patterns of the aurora at magnetic nighttime. Our results also suggest that the interhemispheric coupling during solstices might be an important factor that contributes to the longitudinal variations of the nighttime aurora. Our correlation analysis indicates that the hemispheric differences in the conjugate magnetic field strengths also contribute to the longitudinal variations of the aurora, although they appear not to be a major factor.

Periodic black auroral patches at the dawnside dipolarization front during a substorm

[1] Periodic black auroral patches were observed in the Canadian postmidnight sector by ground-based all-sky cameras during an auroral substorm. The aurora consisted of several black patches of ∼50 km (∼1° in longitude) scales that appeared along the equatorward edges of the proton auroras over a 3 h local time segment from Gillam to Sanikiluaq ground stations 15 min after the auroral intensification occurred in Alaska. To the best of our knowledge, this type of black auroras has never been reported before, although the surrounding luminous auroras evolve into the well-known torch structures later. In situ observations by the THEMIS satellites, three of which were close to being magnetically conjugate in the plasma sheet, as well as ground-based proton auroral observations, show that a dipolarization front and associated dawnward ion flows propagated eastward at poleward of the black patches coinciding with their appearance. The flow motions of auroras observed poleward of the black patches are possibly generated by decelerations of dawnward ion flow because of the consistency of both durations and positions. We speculate, based on manifestations of high-pressure regions in dipolarized flux tubes, the pressure-driven interchange instability formed a beading shape (periodic black patches) of field-aligned currents. The black auroral patches developed into torch structures after the flow disappearance, indicating the dawnward flows might stabilize and limit the instability within a narrow L shell where the pressure gradient is directed Earthward and flows cannot enter. We conclude that the periodic black patches arise as a consequence of a relaxation process of enhanced plasma pressure near the inner boundary of the plasma sheet during a substorm.

Danish auroral science history

Abstract. Danish auroral science history begins with the early auroral observations made by the Danish astronomer Tycho Brahe during the years from 1582 to 1601 preceding the Maunder minimum in solar activity. Included are also the brilliant observations made by another astronomer, Ole Rømer, from Copenhagen in 1707, as well as the early auroral observations made from Greenland by missionaries during the 18th and 19th centuries. The relations between auroras and geomagnetic variations were analysed by H. C. Ørsted, who also played a vital role in the development of Danish meteorology that came to include comprehensive auroral observations from Denmark, Iceland and Greenland as well as auroral and geomagnetic research. The very important auroral investigations made by Sophus Tromholt are outlined. His analysis from 1880 of auroral observations from Greenland prepared for the significant contributions from the Danish Meteorological Institute, DMI, (founded in 1872) to the first International Polar Year 1882/83, where an expedition headed by Adam Paulsen was sent to Greenland to conduct auroral and geomagnetic observations. Paulsen's analyses of the collected data gave many important results but also raised many new questions that gave rise to auroral expeditions to Iceland in 1899 to 1900 and to Finland in 1900 to 1901. Among the results from these expeditions were 26 unique paintings of the auroras made by the artist painter, Harald Moltke. The expedition to Finland was headed by Dan la Cour, who later as director of the DMI came to be in charge of the comprehensive international geomagnetic and auroral observations made during the Second International Polar Year in 1932/33. Finally, the article describes the important investigations made by Knud Lassen during, among others, the International Geophysical Year 1957/58 and during the International Quiet Sun Year (IQSY) in 1964/65. With his leadership the auroral and geomagnetic research at DMI reached a high international level that came to be the background for the first Danish satellite, Ørsted, successfully launched in 1999 and still in operation.

Comment on “Substorm triggering by new plasma intrusion: THEMIS all‐sky imager observations” by Y. Nishimura et al.

[1] In their recent paper “Substorm triggering by new plasma intrusion: THEMIS all‐sky imager observations,” Nishimura et al. [2010, hereafter N2010] describe a sequence of events leading to substorm auroral onset. They describe a distinct and repeatable sequence of substorm onset precursor events and discuss their finding in the light of different substorm onset models. They present three cases of ground‐based auroral observations in detail showing the development of a poleward boundary intensification (PBI) followed by a north‐south aligned auroral arc moving equatorward toward the latitude of the subsequent substorm onset. This development of a PBI and north‐south aurora is linked to fast plasma flows in the magnetotail resulting from enhanced reconnection near the open‐closed field line boundary. The conclusion is drawn that the earthward transport of new plasma leads to the development of a near‐ earth instability and auroral breakup about 5.5 min after the PBI formation. [2] The event studies are accompanied by a statistical analysis of more than 200 substorm onsets that were observed by All‐Sky Imagers (ASI) of the large THEMIS Ground‐Based Observatories (GBO) array [Harris et al., 2008; Mende et al., 2008]. Using the results of their statistical study the authors determine the occurrence probability of precursor auroral forms before substorm onset. They report a very impressive 84.1% occurrence frequency for the development of N‐S or E‐W auroral forms prior to substorm onset. However, here we will show that several aspects of their event selection and statistical analysis are flawed, leading to grossly exaggerated results.

Substorm triggering by new plasma intrusion: THEMIS all‐sky imager observations

A critical, long‐standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset. Based on event and statistical analysis of THEMIS all‐sky imager data, we show that there is a distinct and repeatable sequence of events leading to onset, the sequence having similarities to and important differences from previous ideas. The sequence is initiated by a poleward boundary intensification (PBI) and followed by a north‐south (N‐S) arc moving equatorward toward the onset latitude. Because of the linkage of fast magnetotail flows to PBIs and to N‐S auroras, the results indicate that onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre‐existing open‐closed field line boundary. The flows carry new plasma from the open field line region to the plasma sheet. The auroral observations indicate that Earthward‐transport of the new plasma leads to a near‐Earth instability and auroral breakup ∼5.5 min after PBI formation. Our observations also indicate the importance of region 2 magnetosphere‐ionosphere electrodynamic coupling, which may play an important role in the motion of pre‐onset auroral forms and determining the local times of onsets. Furthermore, we find motion of the pre‐onset auroral forms around the Harang reversal and along the growth phase arc, reflecting a well‐developed region 2 current system within the duskside convection cell, and also a high probability of diffuse‐appearing aurora occurrence near the onset latitude, indicating high plasma pressure along these inner plasma sheet field lines, which would drive large region 2 currents.

Ground-based observations of diffuse auroral structures in conjunction with Reimei measurements

Abstract. We present results from ground-based auroral observations coordinated with the Japanese satellite, Reimei, that took place during the winters of 2006, 2007 and 2008 at Poker Flat, Alaska. Comparable temporal and spatial resolution for the optical and in situ particle data, allowed for investigation of small scale and/or rapidly time-varying auroral structures. Four satellite passes through diffuse auroral structures were identified. The structures within the aurora, whether stationary or time-varying (pulsating aurora), were most closely correlated with the highest energy precipitating electrons measured by these detectors (8 to 12 keV). This relation is found to be consistent across all four examples, revealing that the electron precipitation responsible for these diffuse auroral structures is primarily that of the ≥8 keV electrons.

Francisco Salvá’s auroral observations from Barcelona during 1780–1825

We present a record of auroral observations in Barcelona in the late 18th century and the early 19th century by Francisco Salvá. We have consulted the original manuscripts containing the meteorological readings made by Salvá. Dates of 19 aurorae and one brief auroral description have been identified. The annual number of aurorae observed by Salvá is compared with other auroral series at higher latitudes. Moreover, we have calculated the variation of geomagnetic latitude of Barcelona during the period under study. The main characteristic of this data set is an abrupt decrease of the annual number of auroral observations recorded in Barcelona around 1793.

Early observation of the aurora australis: AD 1640

David M Willis, Jose M Vaquero and F Richard Stephenson cast a critical eye over evidence of auroral observations in 17th-century Chile.

Characteristics of variations in the ground magnetic field during substorms at mid latitudes

Abstract. Substorms are known to cause geomagnetically induced currents (GIC) in power transmission lines through variations in the ground magnetic field. An improved knowledge and understanding of how the different phases of substorms affect the ground magnetic field will ultimately help to better understand how GIC arise. Although usually associated with high latitude power transmission networks, GIC potentially pose a risk to mid latitude networks such as the UK's National Grid. Using a list of substorm expansion phase onsets derived from auroral observations by the IMAGE-FUV satellite, this study examines 553 individual onsets. In order to cover mid latitudes, ground magnetometer data from the UK Sub-Auroral Magnetometer Network (SAMNET) are exploited. These high time resolution (5 s) data are used to study the ground magnetic field for an hour after onset, in particular the time derivative of the horizontal magnetic field, H. The data covers the period from 2000 to 2003 (just after solar maximum). Results are compared with a previous study of magnetic field variations at higher latitudes, using data with a much lower (1 min) cadence during substorms identified from geomagnetic indices during a period just after solar minimum.

Relations between proton auroras, intense electric field, and ionospheric electron density depletion

A case study with simultaneous European Incoherent Scatter and optical auroral observations was conducted in order to determine characteristics of the magnetosphere‐ionosphere coupling from the viewpoint of the electrodynamics in the ionosphere. Particularly focused on were the relationships between ionospheric electron density depletion, perpendicular electric fields, and proton auroras. Intense electron density depletion was observed in the E and F regions poleward and in the vicinity of a thin equatorward moving arc. This depletion was associated with an intense, equatorward perpendicular electric field close to ∼80 mV/m and most likely with a downward field‐aligned current (FAC), but it did not accompany detectable proton aurora. Hence the downward FA electric field in the lower magnetosphere associated with this depletion was weak or absent. The motion of the FAC system with the depletion and the arc presumably enabled the downward FAC to obtain enough current carriers as ionospheric electrons were lost by the evacuation process. The evacuation process associated with the downward FAC was, however, efficient enough to establish the depletion. On the other hand, a widely distributed proton aurora observed immediately after the depletion was associated with an intense, equatorward perpendicular electric field close to 90 mV/m, enhanced electron density, and most likely a downward FAC. No electron precipitation was associated with this proton aurora. Thus the electron density enhancement, providing the downward current carriers, had to be caused by the ionization of precipitating protons presumably accelerated by downward field‐aligned electric fields in the lower magnetosphere.

Oblique Double Layers: A Comparison between Terrestrial and Auroral Measurements

The S3-3, POLAR, and FAST satellite auroral observations of parallel and perpendicular electric field structures have been identified as belonging to a large "U"-shaped potential structure that supports oblique electric double layers. This interpretation is verified by terrestrial laboratory measurements of a self-consistently supported three-dimensional oblique current-free double layer. Its width is a few tens of Debye lengths, its oblicity (with respect to the magnetic field) varies from 0 up to 30 degrees, and its strength is a few times the electron temperature.