Morning polar substorms and variations in the atmospheric electric field

The effects of morning magnetospheric substorms in the variations in near-Earth atmospheric electricity according to the observations of the electric field vertical component (E _z ), at Hornsund polar observatory (Spitsbergen). The E _z, data, obtained under the conditions of fair weather (i.e., in the absence of a strong wind, precipitation, and fog), are analyzed. An analysis of the observations indicated that the development of a magnetospheric substorm in the Earth’s morning sector is as a rule accompanied by positive deviations in E _z, independently of the Hornsund location: in the polar cap or at its boundary. In all considered events, Hornsund was located near the center of the morning convection vortex. In the evening sector, when Hornsund fell in the region of evening convection vortex, the development of a geomagnetic substorm was accompanied by negative deviations in E _z., It has been concluded that the variations in the atmospheric electric field E _z), at polar latitudes, observed during the development of magnetospheric substorms, result from the penetration of electric fields of polar ionospheric convection (which are intensified during a substorm) to the Earth’s surface.     

Isolated nighttime substorms and morning geomagnetic Pc5 pulsations from ground-based and satellite (THEMIS) observations

The analysis results of a complex of phenomena that were developing in the evening and morning magnetospheric and ionospheric sectors during two events (January 18 and February 19, 2008) are presented. The analysis is based on the observation data in the magnetotail from the THEMIS satellites and ground-based observations in the morning (MIRACLE network) and nighttime (THEMIS ground-based network) sectors. The events with moderate substorms in the nighttime sector were preceded by strong geomagnetic Pc5 pulsations in the morning sector, the regime of which changed during the development of auroral disturbances. The substorms were accompanied by dipolizations in the magnetotail at distances of ~10 Re and unexpected jump-like fluxes of ～200-keV electrons. The fluxes appeared within several minutes after a breakup at three central THEMIS satellites simultaneously spaced up to 1.7 Re. According with the ASC data at the NAL observatory (3 frames/min) and with the THEMIS network of ASC data, onset of auroral activations in the night and morning sectors occurred simultaneously. Probable reasons for the sudden suppression or intensification of Pc5 pulsations are discussed.     

Wave structure of magnetic substorms at high latitudes

A new type of high-latitude magnetic bays is revealed at geomagnetic latitudes higher than 71°, called ??polar substorms.?? It is shown that polar substorms differ from both classical substorms and high-latitude geomagnetic disturbances of the type of polar boundary intensifications (PBIs). While classical substorms start at latitudes below 67° and then expand poleward, polar substorms start almost simultaneously in the evening-night polar region of the oval. In contrast to PBIs, accompanied by auroral streamers expanding southward, polar substorms are accompanied by auroral arcs quickly traveling northward. It is shown that polar substorms are observed before midnight (20?C22 MLT) under weak geomagnetic activity (Kp ?? 2) during the late recovery phase of a magnetic storm. It is shown that a typical feature of polar substorms is the simultaneous excitation of highly intensive Pi2 and Pi3 geomagnetic pulsations at high latitudes, which exceed the typical amplitude of these pulsations at auroral latitudes by more than an order of magnitude. The duration of pulsations is determined by the substorm duration, and their amplitude decreases sharply at geomagnetic latitudes below ??71°. It is suggested that pulsations reflect fluctuations in ionospheric currents connected with polar substorms.     

Features of the planetary distribution of auroral precipitation characteristics during substorms

A planetary pattern of substorm development in auroral precipitation has been constructed on the basis of the F6 and F7 satellite observations. The behavior of the auroral injection boundaries and characteristics of precipitating electrons in various precipitation regions during all phases of a statistically mean magnetospheric substorm with an intensity of AL ～ ?400 nT at a maximum is considered in detail. It is shown that during a substorm, the zone of structured auroral oval precipitation AOP and the diffuse auroral zone DAZ are the widest in the nighttime and daytime sectors, respectively. In the daytime sector, all precipitation regions synchronously shift equatorward not only at the origination phase but during the substorm development phase. The strongest shift to low latitudes of the daytime AOP region is observed at a maximum of the development phase. As a result of this shift, the area of the polar cap increases during the phases of substorm origination and development. It is shown that the average position of the precipitation boundaries and the energy fluxes of precipitating electrons at each phase are linearly related to the intensity of a magnetic disturbance. This makes it possible to develop a model of auroral precipitation development during each phase of substorms of any intensity.     

Effects of substorms during HF propagation in the auroral oval

The specific features of radio propagation from the viewpoint of physics of processes in the polar ionosphere have been studied in the present work based on the oblique-incidence sounding of the ionosphere (OISI) on the St. Petersburg-Belyi Nos (Amderma) polar radio path during substorm activity in the summer months of 1997. The OISI data were used to find the following parameters: maximum observable frequency during signal reflection from the E _s layer (EsMOF), maximum observable frequency during signal reflection from the F ₂ layer (F2MOF), and lowest observable frequencies for the E _s and F ₂ layers (EsLOF and F2LOF, respectively). Absolute MOF and LOF values were also found out. The total number of received rays was determined in addition to the above parameters. Isolated substorms against a quiet background were selected for the studies. These substorms resulted in substantial changes in the ionospheric radio channel and propagation conditions along the path. The results of the studies are as follows. (1) The following distinct regularities in the HF propagation along the path have been determined: (i) the range of operational frequencies Δ = MOF-LOF becomes substantially narrower during substorms; (ii) the radio propagation mechanism changes during a substorm; (iii) during substorms, the auroral absorption substantially and partially increases in the course of the expansion and recovery phases, respectively; (iv) multiray effect sharply increases at the beginning of the substorm active phase (T ₀). (2) The indications of changes in the radio propagation parameters, which can possibly be used to predict the beginning of substorm development, have been formulated. (3) All revealed regularities in the HF propagation in the auroral zone have been explained from the geophysical viewpoint. It is important to use these regularities to organize radio communication and to solve the problems within the scope of the Space Weather Program.     

A multipoint study of a substorm occurring on 7 December, 1992, and its theoretical implications

On 7 December 1992, a moderate substorm was observed by a variety of satellites and ground-based instruments. Ionospheric flows were monitored near dusk by the Goose Bay HF radar and near midnight by the EISCAT radar. The observed flows are compared here with magnetometer observations by the IMAGE array in Scandinavia and the two Greenland chains, the auroral distribution observed by Freja and the substorm cycle observations by the SABRE radar, the SAMNET magnetometer array and LANL geosynchronous satellites. Data from Galileo Earth-encounter II are used to estimate the IMF B_z component. The data presented show that the substorm onset electrojet at midnight was confined to closed field lines equatorward of the pre-existing convection reversal boundaries observed in the dusk and midnight regions. No evidence of substantial closure of open flux was detected following this substorm onset. Indeed the convection reversal boundary on the duskside continued to expand equatorward after onset due to the continued presence of strong southward IMF, such that growth and expansion phase features were simultaneously present. Clear indications of closure of open flux were not observed until a subsequent substorm intensification 25 min after the initial onset. After this time, the substorm auroral bulge in the nightside hours propagated well poleward of the pre-existing convection reversal boundary, and strong flow perturbations were observed by the Goose Bay radar, indicative of flows driven by reconnection in the tail.     

Spatial-temporal dynamics of auroras during the magnetic storm main phase

The structure and dynamics of auroras in the midnight sector during substorms, which develop during the magnetic storm main phase as compared to the characteristics of a typical auroral substorm, have been studied using the ground-based and satellite observations. It has been found out that a difference from the classical substorm is observed in auroras during the magnetic storm main phase. At the beginning of the storm main phase, the series of pseudobreakups with the most pronounced jump-like motion toward the equator shifts to lower latitudes. The substorm expansion phase can be observed not only as arc jumps to higher latitudes but also as an explosive expansion of a bright diffuse luminosity in all directions. During the magnetic storm main phase, auroras are mainly characterized by the presence of stable extensive rayed structures and by the simultaneous existence of different auroral forms, typical of different substorm phases, in the TV camera field of view.     

Propagation of HF radiowaves on high-latitude radio paths during the magnetically disturbed period of October 24–28, 2003

The behavior of the HF signal parameters during magnetic storms and substorms has bee experimentally studied simultaneously on the Kiruna-Kirkenes auroral path, Kiruna-Longyearbyen polar path, and Murmansk-St. Petersburg subauroral path. The first two paths are equipped with the instruments making it possible to measure the values of the signal-to-noise ratio, Doppler frequency shift, and elevation angle. The method of oblique sounding of the ionosphere (OSI) was used on the Murmansk-St. Petersburg path. Two substantial substorms, a moderate storm, and an intense storm occurred during the studied period. Some new regularities have been revealed. On the Kiruna-Kirkenes and Kiruna-Longyearbyen paths, the signalto-noise ratio increased (due to the transition from the F ₂ signal reflections to the E _s reflections), the elevation angle increased (due to an increase in the ionospheric F ₂ layer height and a decrease in the critical frequency), and the Doppler shift increased (due to the variations in ionization and the appearance of ionospheric irregularities during a substorm) when the signal was reflected from the F ₂ layer close to the moment of the substorm or storm beginning T ₀. It is possible to control the so-called “main effect” in the ionosphere on the Murmansk-St. Petersburg path.     

亚暴期间极光电集流带的变化

极光活动加剧和极光电集流增强是磁层-电离层能量耦合的两种重要表现形式,它们同为磁层带电粒子向电离层沉降的结果,但是变化规律却非常不同.本文用地基磁场资料,反演极区等效电流体系,研究地磁平静期和扰动期极光电集流带的运动特点.研究表明,Harang间断把极光电集流带分为两段：下午—黄昏段的东向电集流带较弱,而晨侧和子夜—凌晨段的西向电集流带较强.在亚暴膨胀相,随着AE指数增大,整个极光卵向赤道扩展,而极光电集流带却表现出分段差异的特点：下午—黄昏东向电集流带向低纬移动,晨侧西向电集流带也向赤道移动,而子夜—凌晨西向电集流带则向极移动.电动力学分析表明,在不同地方时段,控制电流的主要因素不同,因而,电流及其磁扰有不同的特点：下午—黄昏东向电集流和晨侧西向电集流组成了DP2电流体系,主要受控于磁层对流电场,反映了“驱动过程”的行为;而子夜—凌晨西向电集流是DP1电流体系的基本部分,主要受控于电导率,反映了“卸载过程”的特点.     

Relation of auroral substorm onset to local AL index and dispersionless particle injections

In this study, we investigate the relation of auroral substorm onset to the sharp decrease in the local AL index (IL) during substorms. With a database of over 4200 onsets determined from auroral images, we have statistically examined the timing between the auroral substorm onset and the sharp decrease in the IL index, as determined with data from the IMAGE magnetometer network. From the database of onsets, 54 substorms were determined to be within 6° of the central meridian of the IMAGE ground array. Our superposed epoch median curve shows that the IL index begins to sharply decrease 3 min before the auroral onset, which is twice the 2 min resolution of the auroral imager. However, the mean difference determined by measuring the time between the start of the IL decrease and the auroral substorm onset is about 1.1±0.6 min. An analysis of the superposed epoch median curves of the SOPA particle data for the LANL spacecraft closest to the auroral onset meridian indicates that both the electron and proton injections begin about 3 min before the auroral onset. However, the mean time of the difference between the minimum of the particle dispersionless injection and the auroral onset is simultaneous within the uncertainty of the auroral onset and the error of the mean for the injection. The location of the electron injection relative to the IMAGE ground array seems to be 01–04 MLT, while the proton injection appears to be in the 22–01 MLT sector. These statistical results support the idea that the field aligned and ionospheric currents of the substorm current wedge begin to flow before the auroral onset.     

Global X-ray emission during an isolated substorm — a case study

The polar ionospheric X-ray imaging experiment (PIXIE) and the UV imager (UVI) onboard the Polar satellite have provided the first simultaneous global scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung and the auroral UV emissions. While the UV images in the Lyman–Birge–Hopfield-long band used in this study respond to the total electron energy flux which is usually dominated by low-energy electrons (<10 keV), the PIXIE images of X-ray bremsstrahlung above ∼2.7 keV respond to electrons of energy above ∼3 keV. Comparison of precipitation features seen by UVI and PIXIE provides information on essentially complementary energy ranges of the precipitating electrons. In this study an isolated substorm is examined using data from PIXIE, UVI, ground-based measurements, and in situ measurements from high- and low-altitude satellites to obtain information about the global characteristics during the event. Results from a statistical study of isolated substorms, which has reported a significant difference in the patterns of energetic electron precipitation compared to the less energetic precipitation are confirmed. A localized maximum of electron precipitation in the morning sector delayed with respect to substorm onset is clearly seen in the X-ray aurora, and the time delay of this morning precipitation relative to substorm onset strongly indicates that this intensification is caused by electrons injected in the midnight sector drifting into a region in the dawnside magnetosphere where some mechanism effectively scatter the electrons into the loss cone. In this study, we also present the results from two low-altitude satellite passes through the region of the localized maximum of X-ray emission in the morning sector. Measured X rays are compared with X-ray fluxes calculated from the electron spectral measurements. By fitting the electron spectra by a sum of two exponentials we obtain fairly good agreement between calculated and directly measured X-ray flux profiles.     

CRRES/Ground-based multi-instrument observations of an interval of substorm activity

Observations are presented of data taken during a 3-h interval in which five clear substorm onsets/intensifications took place. During this interval ground-based data from the EISCAT incoherent scatter radar, a digital CCD all sky camera, and an extensive array of magnetometers were recorded. In addition data from the CRRES and DMSP spacecraft, whose footprints passed over Scandinavia very close to most of the ground-based instrumentation, are available. The locations and movements of the substorm current system in latitude and longitude, determined from ground and spacecraft magnetic field data, have been correlated with the locations and propagation of increased particle precipitation in the E-region at EISCAT, increased particle fluxes measured by CRRES and DMSP, with auroral luminosity and with ionospheric convection velocities. The onsets and propagation of the injection of magnetospheric particle populations and auroral luminosity have been compared. CRRES was within or very close to the substorm expansion phase onset sector during the interval. The onset region was observed at low latitudes on the ground, and has been confirmed to map back to within L=7 in the magnetotail. The active region was then observed to propagate tailward and poleward. Delays between the magnetic signature of the substorm field aligned currents and field dipolarisation have been measured. The observations support a near-Earth plasma instability mechanism for substorm expansion phase onset.     

Particle-precipitation fine structure during the development of substorms at high latitudes

The energy of precipitating particles that cause auroras can be characterized by the ratio of different atom and molecule emissions in the upper atmospheric layers. It is known that the spectrum of precipitating electrons becomes harder when substorms develop. The ratio of the I ₆₃₀₀ red line to the I ₅₅₇₇ green line was used to determine the precipitating-electron spectrum hardness. The I ₆₃₀₀/I ₅₅₇₇ parameter was used to roughly estimate the electron energy in auroral arcs observed in different zones of the auroral bulge at the bulge poleward edge and within this bulge. The variations in the emission red and green lines in auroral arcs during substorms that occurred in the winter season 2007–2008 and in January 2006 were analyzed based on the zenith photometer and all-sky camera data at the Barentsburg and Longyearbyen (LYR) high-latitude observatories. It has been indicated that the average value of the I ₆₃₀₀/I ₅₅₇₇ emission ratio for arcs within the auroral bulge is larger than this value at the bulge poleward edge. This means that the highest-energy electron precipitation is observed in arcs at the poleward edge of the substorm auroral bulge.     

Effect of magnetic storms in variations in the atmospheric electric field at midlatitudes

The observations of the variations in the vertical component of the atmospheric electric field (E _z ) at Swider midlatitude Poland observatory (geomagnetic latitude 47.8°) under the conditions of fair weather during 14 magnetic storms have been analyzed. The effect of the magnetic storm main phase in the daytime midlatitude variations in E _z in the absence of local geomagnetic disturbances has been detected for the first time. Considerable (～100–300 V m^?1) decreases in the electric field strength (E _z ) at Swider observatory were observed in daytime simultaneously with the substorm onset in the nighttime sector of auroral latitudes (College observatory). The detected effects indicate that an intensification of the interplanetary electric field during the magnetic storm main phase, the development of magnetospheric substorms, and precipitation of energetic electrons into the nighttime auroral ionosphere can result in considerable disturbances in the midlatitude atmospheric electric field.     

Magnetotail stability and dynamics: Progress 1991–1993

The progress in investigations of magnetotail stability and dynamics is reviewed for the years 1991–1993. Major advances have been made in understanding magnetotail convection, near-tail properties relevant for substorm onset, the change of thermal plasma properties during substorms, and ion acceleration. Also, attempts have been made to integrate different aspects of substorms and different substorm models. Major outstanding problems concern the onset mechanism, electron energization, and substorm recovery.     

Unusual spatial-temporal dynamics of geomagnetic disturbances during the main phase of the extremely strong magnetic storm of November 7–8, 2004

The spatial dynamics of geomagnetic variations and pulsations, auroras, and riometer absorption during the development of the main phase of the extremely strong magnetic storm of November 7–8, 2004, has been studied. It has been indicated that intense disturbances were observed in the early morning sector of auroral latitudes rather than in the nighttime sector, as usually takes place during magnetic storms. The unusual spatial dynamics was revealed at the beginning of the storm main phase. A rapid poleward expansion of disturbances from geomagnetic latitudes of 65°–66° to 74°–75° and the development of the so-called polar cap substorm with a negative bay amplitude of up to 2500 nT, accompanied by precipitation of energetic electrons (riometer absorption) and generation of Pi2–Pi3 pulsations, were observed when IMF B _z was about ?45 nT. The geomagnetic activity maximum subsequently sharply shifted equatorward to 60°–61°. The spatial dynamics of the westward electrojet, Pi2–Pi3 geomagnetic pulsations, and riometer absorption was similar, which can indicate that the source of these phenomena is common.     

Variations in the ULF index of geomagnetic pulsations during strong magnetic storms

The level of wave geomagnetic activity in the morning, afternoon, and nighttime sectors during strong magnetic storms with Dst varying from ?100 to ?150 nT has been statistically studied based on a new ULF wave index. It has been found out that the intensity of geomagnetic pulsations at frequencies of 2–7 mHz during the magnetic storm initial phase is maximal in the morning and nighttime sectors at polar and auroral latitudes, respectively. During the magnetic storm main phase, wave activity is maximal in the morning sector of the auroral zone, and the pulsation intensity in the nighttime sector is twice as low as in the morning sector. It has been indicated that geomagnetic pulsations excited after substorms mainly contribute to a morning wave disturbance during the magnetic storm main phase. During the storm recovery phase, wave activity develops in the morning and nighttime sectors of the auroral zone; in this case nighttime activity is also observed in the subauroral zone.     

Variations in the geomagnetic field and auroras during the main phase of a large magnetic storm of November 20, 2003

The substorm characteristics during the main phase of a large magnetic storm of November 20, 2003, are studied based on the data of TV observations of auroras and auroral absorption at Tixie Bay station and at the global network of magnetic stations. The contribution of auroral particles, responsible for the emission of discrete auroras, has been estimated based on an analysis of the spatial-time variations in the auroral luminosity intensity. This contribution accounted for ～40% of the total luminous flux, which is approximately twice as large as was previously observed in substorm disturbances. Responses of the solar wind and IMF parameters in substorms and variations in the magnetic indices, characterizing geomagnetic activity in the northern polar cap and ring current (PCN, ASY-H and SYM-H), have been detected. The spatial-time distribution of the equivalent ionospheric currents has been constructed, and the total value of these currents along the meridian has been determined based on the [Popov et al., 2001] method and using the IMAGE magnetic data. It has been obtained that the maximal total equivalent ionospheric current in the premidnight sector (～2000 MLT) leads the minimal value of the SYM-H index by ～1.5 h.     

Equatorial ionospheric response to isolated auroral substorms over a solar cycle (1980-85): evidence of longitudinal anomaly

The equatorial ionospheric response to 228 isolated, rapid-onset auroral substorms (as defined from the auroral electrojet index AE) was found from enhancements of the virtual (minimum) height of the F-region (h′F) in the declining phase of a solar cycle (1980-85). The responses, found for three longitudinal sectors at the equator: Africa (Ouagadougou and Dakar), Asia (Manila) and America (Huancayo), were compared with the response close to the auroral source region at Yakutsk (northern Siberia). The auroral substorm onsets were centered at 17 and 15 UT at sunspot maximum (1980-82) and minimum (1983-85), preceding by 3–5 h the period of post-sunset height rise in the African sector whereas other sectors were in the early afternoon (Huancayo) and morning (Manila). The African response, particularly at Ouagadougou, was distinctly different from other sectors. In the sunspot maximum years (1980-81) the auroral surges were followed after about 3 h by a sharp depression (h′F<0) in the post-sunset height rise, with a period of little or no response (h′F=0) in 1982. A response polarity reversal (h′F>0) was noted in this sector for sunspot minimum (1983-85) when large h′F enhancements were observed at the sunset region. The responses in the Asian and American sector were positive except for a case in Huancayo when response was negative, following an auroral surge before the sunset at this station. It appears that the aurorally generated large-scale travelling ionospheric disturbances (LSTIDs), which first cause positive height enhancements in a sub-auroral location (Yakutsk), subsequently affect the unstable post-sunset ionosphere in the equatorial Africa.