Auroral Oval Research Articles

The record of the magnetic storm on 15 May 1921 in Stará Ďala (present-day Hurbanovo) and its compliance with the global picture of this extreme event

Abstract. This paper deals with the most intense magnetic storm of the 20th century, which took place on 13–15 May 1921. Part of this storm was observed in the magnetic declination and vertical intensity at Stará Ďala, currently known as Hurbanovo. However, the sensitivity of the magnetometer was not determined there in the years when the storm occurred. Here, we estimated the sensitivity scale values on the basis of data from before and after the studied event. The resulting digitized Stará Ďala’s data for 13–15 May 1921 are the main contribution of this work. The data were also put into the context of the records from other observatories. The overall picture of the geomagnetic field variations compiled from the observations by worldwide observatories, including Stará Ďala, suggests that the auroral oval got close to Stará Ďala and other European mid-latitude observatories in the morning hours on 15 May 1921.

Automatic Identification of Auroral Substorms Based on Ultraviolet Spectrographic Imager Aboard Defense Meteorological Satellite Program (DMSP) Satellite

An auroral substorm is an important physical process of energy accumulation and explosive release in the Earth’s magnetosphere, and is an important research object of space environment monitoring and space weather warnings. A westward traveling surge (WTS) is a typical auroral physical process of an auroral substorm. Its static characteristic is auroral folding at the polar boundary of an auroral oval and its dynamic characteristic is the westward motion of auroral folding. According to the static characteristic of a WTS, we defined a set of feature parameters based on the morphology and designed a set of automatic detection and discrimination methods; that is, the WTS was identified by using the extracted features and pattern recognition approaches. This approach was tested by using the aurora data of the ultraviolet auroral spectral imager of the Defense Meteorological Satellite Program (DMSP) satellite. The results showed that the accuracy rate of automatic recognition was 61.39%~63.61% and the precision rate was 55.52%~57.92%. The experimental results showed that the approach was effective at detecting the typical characteristics of an auroral substorm (WTS).

Spectral Characteristics of Phase Fluctuations at High Latitude

AbstractIt has long been established that ionospheric electron density irregularities cause random rapid amplitude and phase fluctuations in trans‐ionospheric radio signals. Various types of ground‐based measurements, including radar and Global Navigation Satellite System (GNSS) receivers, have been utilized to gain insights into the plasma mechanisms that could lead to the development of these ionospheric irregularities. The spectral characteristics of radio wave signals emitted by GNSSs and detected by the Canadian High Arctic Ionospheric Network Global Positioning System (GPS) receivers are analyzed to try and identify the spectral signatures of the ionospheric plasma irregularities. In the polar region, we have confirmed that phase fluctuations in the GPS signal are not always accompanied by amplitude fluctuations. For the first time, a systematic comparison is conducted on the spectral properties of phase‐fluctuation events in the absence and presence of amplitude scintillations, respectively. More specifically, a spectral characterization is performed on 41 events recorded by three receivers positioned within the auroral oval. When fluctuations above the background level are solely observed in the signal's phase, the spectrum of phase variations exhibits a consistent steepness. Conversely, the presence of amplitude scintillation is accompanied by an increase in power at higher frequencies, resulting in shallower spectra. This analysis provides yet another physical element suggesting that the rapid fluctuations observed in the signal's amplitude may be caused by Fresnel‐scale irregularities arising from a gradient‐drift‐like instability while the phase variations seem to be consistent with a refractive mechanism caused by large‐scale ionospheric structures.

Climatology of quiet time geomagnetic field variations at two locations in Antarctica

During geomagnetic quiet conditions, Indian Antarctic stations are considered to be located outside the auroral oval: Maitri (CGM coordinates: 63.3°S, 54.2°E) is equatorward and Bharati (CGM coordinates: 74.8°S, 98.4°E) is poleward of the auroral oval. Simultaneous observations of magnetic field variations at these two locations for 10-years (2013–2022) provide an opportunity to study quiet-time magnetic field patterns, if any. Geomagnetic quiet days with ΣKp ≤ 3 are selected, during which the lower values of solar wind and interplanetary parameters are also confirmed. Maitri station exhibits clear southern hemispheric solar quiet (Sq) type of magnetic field variation on geomagnetic quiet days in all seasons, indicating the influence of ionospheric dynamo due to thermospheric winds. Interestingly, Bharati station also displays regular and systematic magnetic field variations in all three components. The D-component at Bharati exhibits very strong variation at early morning hours (7–8 MLT), which is ∼2–4 times stronger than that of H-component, driving strong equatorward/northward currents during all seasons. Both stations show annual type of seasonal variation with peak amplitude during summer and least during winter. The schematic illustration of global and polar Sqs proposed here explains the results obtained through 10-years’ statistical study.

Direct Observation of Low‐Energy Electron Precipitation in the Subauroral Region

AbstractThis study used data collected by the Defense Meteorological Satellite Program (DMSP) and the Radiation Belt Storm Probes (RBSP) to identify a unique subauroral arc in the duskside, which is associated with precipitations of both ions and electrons. The auroral arc was found to be separated from the auroral oval, extending from 14 to 19 magnetic local time hours. Subauroral arcs are usually generated by energetic protons with energies above approximately 10 keV, while enhanced fluxes of low‐energy (<200 eV) electrons were revealed to occur and last for a few hours in this arc region from in situ observations by the DMSP satellite. The observations from RBSP‐B indicate that the low‐energy electron fluxes, with pitch angles near the loss cone, were accompanied by intensified electromagnetic ion cyclotron (EMIC) waves. It is suggested that the enhanced fluxes of cold electrons were heated by the Landau damping of the observed H+ band EMIC waves, which was in the frequency range below the local H+ gyrofrequency and above the local He+ gyrofrequency. Additionally, both electron density and temperature in the ionosphere increased significantly. This study, using conjugate observations of magnetospheric and ionospheric spacecraft, providing evidence of precipitation of low‐energy electrons within the subauroral arc region, thereby offering new insight into understanding the ionospheric effects of cold electron heating by EMIC waves.

First direct measurement and characterisation of plasma waves, originating from outer space, in the polar upper atmosphere, achieved in the Larsemann-Vestfold region by winter traverses onto the icecap

The geomagnetic field focuses onto the polar regions near the auroral oval, which the Larsemann Hills are in proximity to. Solar disturbances cause instabilities in the geomagnetic field and the associated electrically-charged particle (plasma) population, which translate into waves that propagate along geomagnetic field lines towards the Earths polar regions. As the waves enter the electrically charged upper atmosphere (ionosphere) they convert to a mode that propagates parallel to the Earth's surface in a waveguide. Hence, energy from outer space is distributed into the polar atmosphere, particularly near the auroral oval. A series of winter vehicle traverses in the Larsemann-Vestfold region deployed and operated a sensor array, with international co-operation, to measure and characterise the waveguide for the first time at polar latitudes. The directions of areas of origin for the plasma waves and propagation properties could be assessed using the array, not previously possible at permanent stations. Similar waves from sources equatorward of the auroral oval have been recently observed by new radar techniques, which could also be employed at high-latitudes, where the waves have recently been shown to affect the lower atmosphere.

Modeling TEC Irregularities in the Arctic Ionosphere Using Empirical Orthogonal Function Method

AbstractWe develop a climatological model for the Arctic ionosphere based on more than one solar cycle of data (2010–2021) of the rate of change of the total electron content index (ROTI) maps from the International GNSS Service (IGS). The IGS ROTI maps are daily averaged in magnetic latitude and local time coordinates. To develop a climatological model, the ROTI maps are decomposed into base functions and coefficients using the empirical orthogonal function (EOF) method. The EOF method converges very quickly, and the first four EOFs reflect the majority (96%) of the total data variability. Furthermore, different EOF components can reflect different drivers of ionospheric irregularities. The first EOF reflects the averaged ROTI activity and the impact of the solar radiation and geomagnetic activity; the second EOF reflects the impact of the interplanetary magnetic field (IMF) Bz and electric field; the third and fourth EOFs reflect the dawn‐dusk asymmetry around the auroral oval and polar cap, and they can be related to the IMF By. To build an empirical model, we fit the EOF coefficients using helio‐geophysical indices from four different categories (solar activity; geomagnetic indices; IMF; the solar wind coupling function). The final EOF model is dependent on seven selected indices (F10.7P, Kp, Dst, Bt, By, Bz, and EKL). The statistical data‐model comparisons show satisfactory results with a good correlation coefficient. However, the model cannot capture the significant expansion of the dayside ROTI activity during strong geomagnetic storms. Future effort is needed to provide corrections to the model during severe storms.

Magnetospheric Response to a Pressure Pulse in a Three‐Dimensional Hybrid‐Vlasov Simulation

AbstractVlasiator is a high‐performance ion‐kinetic code that is now conducting 3D hybrid‐Vlasov simulations of the global magnetosphere. We use Vlasiator to investigate the impact of a pressure pulse with southward‐oriented magnetic field on the Earth's magnetosphere. The simulation driving parameters are comparable to conditions that have led to geomagnetic storms. Our pressure pulse simulation reproduces many physical effects, namely the expansion of the auroral oval, the development of field‐aligned currents, enhanced particle precipitation near the open/closed field line boundary, and compression of Earth's magnetopause. This demonstrates the effectiveness of the hybrid‐Vlasov approach for moderate driving conditions. Our investigation of the time‐dependent magnetopause compression motivates a generalization of the existing theory. Specifically, we find that accounting for the finite transition time of the solar wind dynamic pressure improves the model's description of the magnetopause oscillations.

Structure of the High-Latitude Noon Ionosphere of the Southern Hemisphere

The structure of the winter noon ionosphere of the southern hemisphere was studied. This structure includes the dayside cusp, associated high-latitude ionospheric trough (HLT), main ionospheric trough (MIT), electron density (Ne) peak at latitudes about 70°, mid-latitude ring ionospheric trough (RIT), and low-latitude quasi-trough. Data from the CHAMP satellite in the southern hemisphere for quiet geomagnetic conditions under high solar activity were selected for analysis. The DMSP satellite data and a model of auroral diffuse precipitation were also used. This model represents two zones of auroral diffuse precipitation on the equatorward and poleward edges of the auroral oval. It is shown that the situation in the winter noon ionosphere of the southern hemisphere depends cardinally on longitude. At sunlit longitudes, only the HLT is observed, and MIT is formed in the shadow region. At intermediate longitudes, both troughs can be observed and, therefore, there is a problem of their separation. The positions of all structures of the ionosphere depend on the longitude; in particular, the positions of the daytime MIT are changed by 6°−7°. At latitudes of the dayside cusp, both the peak and the minimum of Ne can be observed. A high and narrow peak of Ne is regularly recorded in the CHAMP data at latitudes of the equatorward zone of diffuse precipitation (68°−72°). In the shadow region, this peak forms the MIT poleward wall, and at sunlit longitudes a quasi-trough equatorward of this peak is sometimes observed. The RIT is rarely formed during the day, only at the American and Atlantic longitudes.

Вклад ионосферных возмущений, регистрируемых радаром EISCAT, в сцинтилляции GPS-сигналов

It is performed an analysis of the influence of ionosphere disturbances on GPS scintillations at the auroral oval using receiver at Skibotn station (SKN, CGM — 66.28N, 103.41E). The ionosphere parameters were determined using EISCAT UHF radar data. We analyze the events for the years 2015-2022 when the GPS and EISCAT data were available. In this paper we present typical events. The optical aurora observations at Skibotn stations in 557.7 and 630.0nm spectrum lines were used as well. It is shown that the strongest GPS phase scintillations (σΦ > 1) were produced by the ionosphere disturbances in the E-region (100–120 km), these disturbances were accompanied by the appearance of the auroral arcs. Ionosphere disturbances in the F-region (> 200 km) do not produce any significant phase scintillations. The night-time and evening substorms accompanied by the negative magnetic bay produce strongest GPS phase scintillations. At the same time, positive magnetic bays, associated with the growth of the eastern electrojet, during magnetic storm produce comparable phase scintillation as night-time substorms. We did not find a clear growth of the amplitude scintillations during considered events.

Characteristics of Auroral Electron Precipitation at Geomagnetic Latitude 67° Over Tromsø

AbstractWe use the EISCAT incoherent scatter radar data measured in years 2001–2021 to study statistical characteristics of 1–100 keV electron precipitation at 66.7° MLAT over Tromsø. Peak energies, auroral powers and number fluxes of precipitating electrons are derived from electron density altitude profiles measured along the geomagnetic field line during periods of no photoionization. The method allows us to include energetic 30–100 keV electrons, which are poorly covered in earlier satellite‐based studies. Locations of the radar within the auroral oval are determined using a model with the 1‐hr Hpo geomagnetic index as input. The average peak energy of precipitating electrons increases almost monotonically from evening (18 MLT) to morning hours (09 MLT). The 30–50 keV electrons dominate the energetic electron precipitation before 06 MLT, after which the 50–100 keV precipitation becomes dominant. Large auroral powers (>60 mWm−2) are observed in the 18–02 MLT sector in the main auroral oval. We obtain occurrence rate of electron precipitation in Tromsø by calculating the fraction of data points with auroral power larger than 2 mWm−2. The occurrence rate peaks during the declining phases of solar cycles (sc), in 2002–2004 for sc23 and in 2015–2017 for sc24, caused by variations in geomagnetic activity. In addition, the occurrence rate has maxima during March and September, minimum in December to January, and it increases monotonically from evening to morning hours, reaching maximum at 05–06 MLT.

Локальная диагностика наличия полярных сияний на основе интеллектуального анализа геомагнитных данных

Despite the existing variety of approaches to monitoring space weather and geophysical parameters in the auroral oval region, the issue of effective prediction and diagnostics of auroras as a special state of the upper ionosphere at high latitudes remains virtually unresolved.
 In this paper, we explore the possibility of local diagnostics of auroras through mining of geomagnetic data from ground-based sources. We assess the significance of indicative variables and their statistical relationship.
 So, for example, the application of Bayesian inference to the data from the Lovozero geophysical station for 2012–2020 has shown that the dependence of a posteriori probability of observing auroras in the optical range on the state of geomagnetic parameters is logarithmic, and the degree of its significance is inversely proportional to the discrepancy between empirical data and approximating function.
 The accuracy of the approach to diagnostics of aurora presence based on the random forest method is at least 86 % when using several local predictors and ~80 % when using several global geomagnetic activity indices characterizing the geomagnetic field disturbance in the auroral zone.
 In conclusion, we discuss promising ways to improve the quality metrics of diagnostic models and their scope.

Local diagnostics of aurora presence based on intelligent analysis of geomagnetic data

Despite the existing variety of approaches to monitoring space weather and geophysical parameters in the auroral oval region, the issue of effective prediction and diagnostics of auroras as a special state of the upper ionosphere at high latitudes remains virtually unresolved.
 In this paper, we explore the possibility of local diagnostics of auroras through mining of geomagnetic data from ground-based sources. We assess the significance of indicative variables and their statistical relationship.
 So, for example, the application of Bayesian inference to the data from the Lovozero geophysical station for 2012–2020 has shown that the dependence of a posteriori probability of observing auroras in the optical range on the state of geomagnetic parameters is logarithmic, and the degree of its significance is inversely proportional to the discrepancy between empirical data and approximating function.
 The accuracy of the approach to diagnostics of aurora presence based on the random forest method is at least 86 % when using several local predictors and ~80 % when using several global geomagnetic activity indices characterizing the geomagnetic field disturbance in the auroral zone.
 In conclusion, we discuss promising ways to improve the quality metrics of diagnostic models and their scope.

Auroral Oval Morphology: Dawn‐Dusk Asymmetry Partially Induced by Earth's Rotation

AbstractThe auroral oval morphology has been investigated in previous studies presenting maps of average auroral precipitation. However, such distributions tend to emphasize auroral intensity rather than the actual extent of the auroral oval. We develop a statistical method to characterize the auroral oval morphology by using 20 years of electron energy flux measurements from the Defense Meteorological Satellite Program/Special Sensor J (DMSP/SSJ); instead of relying on auroral oval boundaries, we derive the probability of observing aurora from a threshold of 2.109 eV/cm2/s/sr above which the total energy flux of electrons (in the energy range 1–30 keV) is defined as aurora. We then investigate the auroral occurrence probability (AOP) in the magnetic latitude‐magnetic local time (MLat‐MLT) sectors covered by DMSP for various conditions related to geomagnetic activity. Regardless of those conditions, the AOP distributions reveal a width asymmetry with a wider dawn‐to‐noon sector (06–12 MLT) compared to the dusk‐to‐midnight sector (18–24 MLT), the dawn preference getting even more pronounced as the geomagnetic activity decreases. In the context of an open magnetosphere, we investigate the relation between the observed extent asymmetry in the auroral oval and the magnetospheric plasma convection. Representing the plasma sheet magnetic flux as a one‐dimensional fluid subject to production on the nightside (closing of flux via reconnection) and loss on the dayside (opening of flux), we highlight similarities with the AOP in terms of MLT asymmetries. Finally, making use of this fluid model, we demonstrate that the corotation influence on the plasma convection pattern is consistent with the dawn‐dusk asymmetry observed in the AOP distributions.

Auroral Morphological Changes to the Formation of Auroral Spiral During the Late Substorm Recovery Phase: Polar UVI and Ground All‐Sky Camera Observations

AbstractThe ultraviolet imager (UVI) of the Polar spacecraft and an all‐sky camera at Longyearbyen contemporaneously detected an auroral vortex structure (so‐called “auroral spiral”) on 10 January 1997. From space, the auroral spiral was observed as a “small spot” (one of an azimuthally aligned chain of similar spots) in the poleward region of the main auroral oval from 18 to 24 hr magnetic local time. These auroral spots were formed while the substorm‐associated auroral bulge was subsiding and several poleward‐elongated auroral streak‐like structures appeared during the late substorm recovery phase. During the spiral interval, the geomagnetically north‐south and east‐west components of the geomagnetic field, which were observed at several ground magnetic stations around Svalbard island, showed significant negative and positive bays caused by the field‐aligned currents related with the aurora spiral appearance. The negative bays were reflected in the variations of local geomagnetic activity index (SML) which was provided from the SuperMAG magnetometer network at high latitudes. To pursue the spiral source region in the magnetotail, we trace each UVI image along field lines to the magnetic equatorial plane of the nightside magnetosphere using an empirical magnetic field model. Interestingly, the magnetotail region corresponding to the auroral spiral covered a broad region from Xgsm ∼ −40 to −70 RE at Ygsm ∼ 8 to 12 RE. The appearance of this auroral spiral suggests that extensive areas of the magnetotail (but local regions in the ionosphere) remain active even when the substorm almost ceases, and geomagnetic conditions are almost stable.

Space–Time Structures in the Auroral Oval: Approaches to Modeling

The interaction of the magnetospheric–ionospheric (MI) system surrounding the Earth with the environment (solar wind) occurs in the form of a series of transient processes at different scales. The largest of them, magnetic storms, are obviously triggered by disturbances in the solar wind (direct driving). The role of the internal dynamics of the MI system, which is caused to a large extent by the nonlinearity and temporal delays of the loading–unloading processes of energy and particle from the solar wind into the magnetosphere, becomes more significant at smaller scales (substorms, pseudobreakups, injections, and activations). A typical dynamic state of the MI system is characterized as self-organized criticality or turbulence, which are characterized by statistical scale invariance (scaling) in the fluctuation distributions of many characteristics. The dynamics of the MI system is projected into the region of the auroral oval, the very existence of which is due to this dynamics. The space–time structure of auroral disturbances largely reflects the structure of processes in the MI plasma. The description of this structure is important both for studying the fundamental study of plasma processes and for many topical applied problems related to the propagation of radio waves in the ionosphere and vital activity at high latitudes. The paper discusses approaches and developments for constructing a model of the space–time structure of the auroral oval, based on fractal and multifractal characteristics.

A Comparative Study on the Hot Dense Plasma and Cold Patch by Using Multi‐Instrument Observations

AbstractThe polar cap hot patch is an enhanced density structure which is associated with particle precipitation, ion upflow and flow shears. Based on combined observations from DMSP satellites, EISCAT radar, and all‐sky imager at the Chinese Yellow River Station, the plasma characteristics and evolution of hot dense plasma and classical polar cap patch are investigated. Both of them show enhanced F region density, but the hot dense plasma is associated with higher electron temperature and enhanced E region electron density caused by particle precipitation. Compared to the cold patch, the hot dense plasma is closer to the cusp region and is associated with strong auroral emissions. Based on the joint observations, the evolution of the hot/cold dense plasma is discussed. In the initial phase the dense plasma comes from the sunlit lower latitude region, or from particle precipitation in the duskside auroral oval, and is heated locally by poleward moving auroral structures in the dayside auroral oval. Next the patches move poleward into the polar cap where a decaying electron temperature (i.e., less precipitation) gradually transform them into a cold patch.

A Modeling Study on the Responses of the Mesosphere and Lower Thermosphere (MLT) Temperature to the Initial and Main Phases of Geomagnetic Storms at High Latitudes

AbstractJoule heating and radiative cooling usually play key roles in high‐latitude thermospheric temperature changes during geomagnetic storms. In the mesosphere and lower thermosphere (MLT), however, the causes of storm‐time temperature changes at high latitudes are still elusive. Here, we elucidate the nature and mechanisms of MLT temperature variations at high latitudes during the 10 September 2005 storm by diagnostically analyzing the MLT thermodynamics in the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM) simulations. In the storm's initial and main phases, the MLT temperature decreases at 0:00 local time (LT)−12:00 LT, but increases in the 12:00 LT–24:00 LT sector at high latitudes. Afterward, the temperature decrease disappears and temperature increase occurs at all local times in the high latitudes. Adiabatic heating/cooling and vertical advection associated with vertical winds are the main drivers of high‐latitude temperature changes in the entire altitude range of the MLT region. However, around the auroral oval and above ∼100 km, the Joule heating rate is comparable to the heating caused by vertical advection and adiabatic heating/cooling associated with vertical winds and becomes one of the major contributors to total heating in the high‐latitude MLT region. The effects of Joule heating can penetrate down to ∼95 km. Horizontal advection also plays a key role in storm‐time MLT temperature changes inside the polar cap and becomes larger than the adiabatic heating/cooling above ∼105 km.

Space–Time Structures in the Auroral Oval: Approaches to Modeling

The interaction of the magnetospheric–ionospheric (MI) system surrounding the Earth with the environment (solar wind) occurs in the form of a series of transient processes at different scales. The largest of them, magnetic storms, are obviously triggered by disturbances in the solar wind (direct driving). The role of the internal dynamics of the MI system, which is caused to a large extent by the nonlinearity and temporal delays of the loading–unloading processes of energy and particle from the solar wind into the magnetosphere, becomes more significant at smaller scales (substorms, pseudobreakups, injections, and activations). A typical dynamic state of the MI system is characterized as self-organized criticality or turbulence, which are characterized by statistical scale invariance (scaling) in the fluctuation distributions of many characteristics. The dynamics of the MI system is projected into the region of the auroral oval, the very existence of which is due to this dynamics. The space–time structure of auroral disturbances largely reflects the structure of processes in the MI plasma. The description of this structure is important both for studying the fundamental study of plasma processes and for many topical applied problems related to the propagation of radio waves in the ionosphere and vital activity at high latitudes. The paper discusses approaches and developments for constructing a model of the space–time structure of the auroral oval, based on fractal and multifractal characteristics.

Features of Polar Substorms: An Analysis of Individual Events

Polar substorms include substorms observed at geomagnetic latitudes above 70° MLAT in theabsence of simultaneous negative magnetic bays at lower latitudes, that is, substorms on the compressed contractedauroral oval. The general morphological features of polar substorms are considered based on theexample of individual events registered on Svalbard arch. It is shown that polar substorms, like “classical”substorms, are characterized by the formation of a substorm current wedge and a steplike movement to thepole after the onset of a substorm, generation of Pi2 geomagnetic pulsations, and an increase of the PC-indexof the polar cap before the onset of the substorm. At the same time, there are certain differences between polarsubstorms and “classical” substorms; namely, they start on more distant L-shells, develop in the region of acontracted auroral oval, occur at earlier pre-midnight hours, and generate only at low solar wind speeds andweakly disturbed geomagnetic conditions. It has been suggested that polar substorms may be a specific typeof “classical” substorms that develop in the evening sector under magnetically quiet or weakly disturbed conditionswhen the auroral oval is concracted. The source of polar substorms may also be a local intensificationof previously existing substorms in the post-midnight sector