Negative Bay Research Articles

Influence of geomagnetic disturbances on scintillations of GLONASS and GPS signals as observed on the Kola Peninsula

We have compared effects of geomagnetic disturbances during magnetic storms of various types (CME and CIR) and during an isolated substorm on scintillations of GLONASS and GPS signals, using a Septentrio PolaRx5 receiver installed in Apatity (Murmansk Region, Russia). We analyze observational data for 2021. The magnetic storms of November 3–4, 2021 and October 11–12, 2021 are examined in detail. The November 3–4, 2021 magnetic storm was one of the most powerful in recent years. The analysis shows that the scintillation phase index reaches its highest values during nighttime and evening substorms (σϕ≈1.5–1.8), accompanied by a negative bay in the magnetic field. During magnetic storms, positive bays in the magnetic field, associated with an increase in the eastward electrojet, lead, however, to quite comparable values of the phase scintillation index.
 An increase in phase scintillations during nighttime and evening disturbances correlates with an increase in the intensity of ULF waves (Pi3/Pc5 pulsations) and with the appearance of aurora arcs. This confirms the important role of ULF waves in forming the auroral arc and in developing ionospheric irregularities. The predominance of the green line in the spectrum of auroras indicates the contribution of disturbances in the ionospheric E layer to the scintillation increase. Pulsating auroras, associated with ionospheric disturbances in the D layer, do not lead to a noticeable increase in phase scintillations. Analysis of ionospheric critical frequencies according to ionosonde data from the Lovozero Hydrometeorological Station indicates the contribution of the sporadic Es layer of the ionosphere to jumps in phase scintillations.
 The difference between phase scintillation values on GLONASS and GPS satellites during individual disturbances can be as great as 1.5 times, which may be due to different orbits of the satellites. At the same time, the level of GLONASS/GPS scintillations at the L2 frequency is higher than at the L1 frequency. We did not find an increase in the amplitude index of scintillations during the events considered.

Влияние геомагнитных возмущений на сцинтилляции сигналов ГЛОНАСС и GPS спутников по данным наблюдений на Кольском полуострове

We have compared effects of geomagnetic disturbances during magnetic storms of various types (CME and CIR) and during an isolated substorm on scintillations of GLONASS and GPS signals, using a Septentrio PolaRx5 receiver installed in Apatity (Murmansk Region, Russia). We analyze observational data for 2021. The magnetic storms of November 3–4, 2021 and October 11–12, 2021 are examined in detail. The November 3–4, 2021 magnetic storm was one of the most powerful in recent years. The analysis shows that the scintillation phase index reaches its highest values during nighttime and evening substorms (σϕ≈1.5–1.8), accompanied by a negative bay in the magnetic field. During magnetic storms, positive bays in the magnetic field, associated with an increase in the eastward electrojet, lead, however, to quite comparable values of the phase scintillation index.
 An increase in phase scintillations during nighttime and evening disturbances correlates with an increase in the intensity of ULF waves (Pi3/Pc5 pulsations) and with the appearance of aurora arcs. This confirms the important role of ULF waves in forming the auroral arc and in developing ionospheric irregularities. The predominance of the green line in the spectrum of auroras indicates the contribution of disturbances in the ionospheric E layer to the scintillation increase. Pulsating auroras, associated with ionospheric disturbances in the D layer, do not lead to a noticeable increase in phase scintillations. Analysis of ionospheric critical frequencies according to ionosonde data from the Lovozero Hydrometeorological Station indicates the contribution of the sporadic Es layer of the ionosphere to jumps in phase scintillations.
 The difference between phase scintillation values on GLONASS and GPS satellites during individual disturbances can be as great as 1.5 times, which may be due to different orbits of the satellites. At the same time, the level of GLONASS/GPS scintillations at the L2 frequency is higher than at the L1 frequency. We did not find an increase in the amplitude index of scintillations during the events considered.

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.

Substorm Triggering by the Solar Wind

AbstractThe coupling function monitors solar wind driving of geomagnetic activity. Magnetic indices, including the polar cap (PC) index, the auroral lower (AL) index, and the midlatitude positive bay index (MPB), provide measures of the magnetospheric response. Sharp changes in these are associated with the onset of a substorm and a maximum in coupling. Many reinterpret this association as evidence that northward turnings of the interplanetary magnetic field (IMF) trigger onset. We investigate this hypothesis using a list of over 100,000 negative bay onsets. When data are available, we calculate coupling and its time derivatives. From these, we construct ensembles of data segments centered on the onsets.`For all lists, a pulse occurs 20 min before onset and appears to trigger onset. By correlating the average derivative with every event in the ensemble, we conclude that a pulse of either sign precedes 40% of all onsets. More than half of the triggered onsets display a positive pulse, and slightly fewer show a negative pulse. Substorms following a positive pulse are stronger than those after a negative pulse. The pulse is usually hidden by noise, but ensemble averaging and correlation analysis can detect the pulse in the coupling data.

Radial Interplanetary Magnetic Field‐Induced North‐South Asymmetry in the Solar Wind‐Magnetosphere‐Ionosphere Coupling: A Case Study

AbstractIn this paper, we present a case study of the radial interplanetary magnetic field (IMF Bx)‐induced asymmetric solar wind‐magnetosphere‐ionosphere (SW‐M‐I) coupling between the northern and southern polar caps using ground‐based and satellite‐based data. Under prolonged conditions of strong earthward IMF on 5 March 2015, we find significant discrepancies between polar cap north (PCN) and polar cap south (PCS) magnetic indices with a negative bay‐like change in the PCN and a positive bay‐like change in the PCS. The difference between these indices (PCN‐PCS) reaches a minimum of −1.63 mV/m, which is approximately three times higher in absolute value than the values for most of the time on this day (within ±0.5 mV/m). The high‐latitude plasma convection also shows an asymmetric feature such that there exists an additional convection cell near the noon sector in the northern polar cap, but not in the southern polar cap. Meanwhile, negative bays in the north‐south component of ground magnetic field perturbations (less than 50 nT) observed in the nightside auroral region of the Northern Hemisphere are accompanied with the brightening and widening of the nightside auroral oval in the Southern Hemisphere, implying a weak, but clear energy transfer to the nightside ionosphere of both hemispheres. After the hemispheric asymmetries in the polar caps disappear, a substorm onset takes place. All these observations indicate that IMF Bx‐induced single lobe reconnection that occurred in the Northern Hemisphere plays an important role in hemispheric asymmetry in the energy transfer from the solar wind to the polar cap through the magnetosphere.

Magnetospheric response to the interaction with the sporadic solar wind diamagnetic structure

We report the results of a study on the movement of the solar wind diamagnetic structure (DS), which is a sequence of smaller-scale microDS being part of the May 18, 2013 coronal mass ejection, from a source on the Sun to Earth’s surface. DS determined from the high negative correlation coefficient (r=–0.9) between the IMF modulus (B) and the SW density (N) on the ACE and Wind satellites at the L1 point, on the THB and THC satellites (r=–0.9) in near-Earth orbit, and on the THA satellite inside the magnetosphere is carried by the solar wind from the Sun to Earth’s orbit, while maintaining its fine internal structure. Having a large size in the radial direction (≈763 Rᴇ, where Rᴇ is the Earth radius), DS flows around the magnetosphere. At the same time, microDS of size ≤13 Rᴇ passes through the bow shock and magnetopause as a magnetized plasmoid in which the ion concentration increases from 10 cm⁻³ to 90 cm⁻³, and the velocity decreases as it moves toward the magnetotail. When a microDS passes through the magnetopause, a pulsed electric field of ~400 mV/m is generated with subsequent oscillations with a period of T~200 s and an amplitude of ~50 mV/m. The electric field accelerates charged particles of the radiation belt and produces modulated fluxes of protons in an energy range 95–575 keV on the day side and electrons in 40–475 keV and protons in 95–575 keV on the night side. In the duskside magnetosphere (19–23 MLT), the substorm activation is observed in geomagnetic pulsations and auroras, but without a magnetic negative bay. In the post-midnight sector (01–05 MLT), a sawtooth substorm occurs without the growth phase and breakup with deep modulation of the ionospheric current and auroral absorption. The duration of all phenomena in the magnetosphere and on Earth is determined by the period of interaction between DS and the magnetosphere (~4 hrs). To interpret the regularities of the magnetospheric response to the interaction with DS, we consider alternative models of the impulsive passage of DS from SW to the magnetosphere and the classical model of reconnection of IMF and the geomagnetic field.

Магнитосферный отклик на взаимодействие с диамагнитной структурой спорадического солнечного ветра

We report the results of a study on the movement of the solar wind diamagnetic structure (DS), which is a sequence of smaller-scale microDS being part of the May 18, 2013 coronal mass ejection, from a source on the Sun to Earth’s surface. DS determined from the high negative correlation coefficient (r=–0.9) between the IMF modulus (B) and the SW density (N) on the ACE and Wind satellites at the L1 point, on the THB and THC satellites (r=–0.9) in near-Earth orbit, and on the THA satellite inside the magnetosphere is carried by the solar wind from the Sun to Earth’s orbit, while maintaining its fine internal structure. Having a large size in the radial direction (≈763 Rᴇ, where Rᴇ is the Earth radius), DS flows around the magnetosphere. At the same time, microDS of size ≤13 Rᴇ passes through the bow shock and magnetopause as a magnetized plasmoid in which the ion concentration increases from 10 cm⁻³ to 90 cm⁻³, and the velocity decreases as it moves toward the magnetotail. When a microDS passes through the magnetopause, a pulsed electric field of ~400 mV/m is generated with subsequent oscillations with a period of T~200 s and an amplitude of ~50 mV/m. The electric field accelerates charged particles of the radiation belt and produces modulated fluxes of protons in an energy range 95–575 keV on the day side and electrons in 40–475 keV and protons in 95–575 keV on the night side. In the duskside magnetosphere (19–23 MLT), the substorm activation is observed in geomagnetic pulsations and auroras, but without a magnetic negative bay. In the post-midnight sector (01–05 MLT), a sawtooth substorm occurs without the growth phase and breakup with deep modulation of the ionospheric current and auroral absorption. The duration of all phenomena in the magnetosphere and on Earth is determined by the period of interaction between DS and the magnetosphere (~4 hrs). To interpret the regularities of the magnetospheric response to the interaction with DS, we consider alternative models of the impulsive passage of DS from SW to the magnetosphere and the classical model of reconnection of IMF and the geomagnetic field.

Modulation of Magnetospheric Substorm Frequency: Dipole Tilt and IMF By Effects

AbstractSubstorm activity is heavily influenced by the Interplanetary Magnetic Field (IMF) Bz component and magnetospheric substorms occur most frequently when Bz is strongly negative. The substorm occurrence rate is also affected by the magnitude of the By component, but it is usually presumed that this contribution is independent of the sign of By. Using five independent substorm onset lists, we show that substorm activity does depend on the sign of By near the solstices. Specifically, we show that substorms occur more frequently when By and the dipole tilt angle Ψ have different signs as opposed to when they have the same sign. These results confirm that the magnetosphere exhibits an explicit dependence on the polarity of By for nonzero Ψ, as other recent studies have suggested, and imply variation in the dayside reconnection rate and/or the magnetotail response. On the other hand, we find no clear relationship between substorm intensity and By regardless of Ψ. Last, for the onset list based on identifying negative bays at auroral latitudes, we observe an overall trend of more frequent onsets for positive By, regardless of season. However, substorm frequency in the other four substorm lists does not exhibit an overall preference for positive By. We show that this phenomenon is very likely a consequence of the particular substorm identification method (i.e., identification of negative bays), which is affected by local ionospheric conditions that depend on By and Ψ.

Global and local Joule heating during substorms in St. Patrick\u2019s Day 2015 geomagnetic storm

The first super storm of solar cycle 24 occurred on “St. Patrick’s Day” (17 March 2015), with a minimum Dst level of − 223 nT. Five major substorms in this super storm were selected, with minimum values of local electrojet index (IL) ranging from − 1662 to − 673 nT. The selected substorms are all in the 22:00 MLT–06:00 MLT sector of the auroral oval region showing associated Pi2s and negative bays in the H-component of magnetograms, derived from the IMAGE magnetometer longitudinal (Fennoscandia) chain. The solar wind energy input is estimated as time integral of Akasofu’s epsilon parameter, determined from the SuperMAG magnetometer. The local ionospheric Joule heating (local JH) rate, in the midnight or post-midnight sectors, is estimated using a modified form of Ahn’s empirical conversion. The Global ionospheric Joule heating rate in the northern hemisphere (global JH) is taken from OpenGGCM model. For the substorm in the main phase of the superstorm, the local JH consumes only 9% (8%, if the IL is replaced by AL index in the empirical conversion relation) of the global JH. However, 40–86% (39–48%, if the IL is replaced by AL index in the empirical conversion relation) of global JH is consumed as local JH for the remaining substorms.

The Midlatitude Positive Bay Index and the Statistics of Substorm Occurrence

AbstractThe auroral breakup and expansion are essential features of a magnetospheric substorm. Negative bays beneath the aurora and positive bays at midlatitudes accompany the expansion. These effects are caused by the substorm current wedge. The negative bay strength is provided by the auroral electrojet lower (AL) index. At midlatitudes the positive bay is caused by the field‐aligned currents connected to the electrojet. The midlatitude positive bay (MPB) index measures these effects. Both AL and MPB can be used to time auroral expansion onset. This work creates lists of MPB onsets and compares them with lists from other sources including onsets derived from IMAGE and Polar spacecraft and ground all‐sky camera data. The lists show that these measurements are nearly simultaneous with satellite expansion onsets. Substorms occur most frequently during the declining phase of the solar cycle, at the equinoxes, and after the passage of a stream interface in a corotating interaction region. Both MPB and SML indices reveal three substorm phases: growth, expansion, and recovery. In addition, the MPB onset list, the SML onset list, and the IMAGE auroral onset list reveal the presence of a 45‐min and a 2 3/4‐hr quasi‐periodicity in substorm occurrence. A comparison of the various lists reveals radical difference in their waiting time distributions and relatively low levels of association between events in the lists. The results suggest that none of the lists adequately capture the complexity of substorm activity and further refinements are needed in every algorithm used for onset detection.

Relation of Field‐Aligned Currents Measured by the Network of Iridium® Spacecraft to Solar Wind and Substorms

AbstractThe strength of field‐aligned currents coupling the magnetosphere to the ionosphere was obtained by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) using the network of Iridium® spacecraft. The distribution of current was integrated giving total current in and out of the ionosphere on the dayside and nightside of the Earth in both hemispheres. The onset of auroral zone negative bays and midlatitude positive bays corresponds to an increase in nightside upward current. The total outward current tends toward saturation with increasing solar wind driver strength. The optimum solar wind coupling function for AL index predicts ~73% of the variance in nightside upward current. The dayside and nightside predictors of upward current rise to a peak at 30–45 min and decay slowly over 2.5 hr. Nightside response is delayed relative to dayside.

A Case Study of Connection Between Ground Magnetic Field Perturbations and Tail Current Sheet Bursty Flows at X = -60 RE.

We report the first observations that connect a burst of fast flows and Pi2 fluctuations in the midtail current sheet at X ~-60 RE with ground magnetic negative bays and Pi2 pulsations at high latitudes near 70°. Both the flow burst and negative bays occurred around midnight and were isolated and lasted for ~10 min. The flow burst had the characteristics of a low-density plasma bubble. It was moving earthward at ~340 km/s and was accompanied by field-aligned currents. The timing and equatorward motion of magnetic bays can be accounted for by this earthward moving bubble. Enhanced Pi2 fluctuations were observed within both the bubble and the magnetic bays. Pressure anisotropy with higher parallel pressure was observed associated with the flow burst and was firehose unstable. We discuss firehose instability as one of the possible mechanisms for the Pi2 fluctuations.

Statistical study of polar negative magnetic bays driven by interplanetary fast‐mode shocks

AbstractThe question of whether an interplanetary (IP) fast‐mode shock can trigger the substorm expansion is still not resolved. Some scientists believe that the substorm expansion can be triggered by a sudden compression of the magnetosphere if a substorm growth phase is in progress (e.g., the magnetosphere precondition hypothesis). Such a hypothesis has not been rigorously tested. Here we study 258 positive sudden impulse (SI+) events associated with interplanetary fast‐mode shocks observed by the ACE spacecraft from 1998 to 2009. The westward auroral electrojet SML index, derived from the SuperMAG network geomagnetic field data, is used to check for negative magnetic bays. It is found that the probability of having a magnetic bay onset that follows a shock impact peaks (~27%) sharply and immediately (within 3 min) after the shock‐induced SI+. This indicates that the majority of the shock events do not lead to magnetic bays. A superposed epoch analysis of the shock events associated with (Type‐Y) and without (Type‐N) negative bays indicates that the Type‐Y events had stronger (~40%) solar wind driving than the Type‐N ones. This result generally supports the magnetosphere precondition hypothesis. We also found that (1) Type‐Y events are associated with even stronger (~56%) solar wind driving downstream of the shock than Type‐N events and (2) there is a good correlation between the SML and the polar cap magnetic PC indices both upstream and downstream of the shock. It is suggested that sharp decreases in SML immediately after a shock impact are not associated with the substorm‐driven DP‐1 current system but with the convection‐driven DP‐2 current system.

Pulsating aurora and quasiperiodic VLF hiss in the auroral zone morning sector: The event of December 30, 2011

The spatial–temporal variations in aurora and VLF emissions during an weak intensification in the auroral zone morning sector on December 30, 2011, have been analyzed. The event was accompanied by a negative bay (~70 nT) in the X component of the magnetic field at ground stations in northern Scandinavia. At the recovery phase of this bay, the precipitation zone moved and VLF emission frequency simultaneously increased over ten minutes, which may indicate that waves and precipitating electrons had a common source. VLF noise bursts in the 600–1000 Hz band with a characteristic modulation scale of ~10 s and the corresponding aurora intensifications localized in the ~100 km region were observed during the following ten minutes, which also confirms that recorded waves are related to electron precipitation. This correspondence of the pulsating aurora periods and VLF noise modulation has been revealed for the first time. The role of VLF wave generation processes during the cyclotron interaction with electrons in the magnetosphere and the propagation of these waves from the magnetosphere to the observation point are discussed.

Geomagnetic disturbances and pulsations as a high-latitude response to considerable alternating IMF Variations during the magnetic storm recovery phase (Case study: May 30, 2003)

Features of high-latitude geomagnetic disturbances during the magnetic storm (Dst min =–144 nT) recovery phase were studied based on the observations on the Scandinavian profile of magnetometers (IMAGE). Certain non-typical effects that occur under the conditions of large positive IMF Bz values (about +20–25 nT) and large negative IMF By values (to–20 nT) were revealed. Thus, an intense (about 400 nT) negative bay in the X component of the magnetic field (the polar electrojet, PE) was observed in the dayside sector at geomagnetic latitudes higher than 70°. As the IMF B y reverses its sign from negative to positive, the bay in the X component was replaced by the bay in the Y component. The possible distribution of the fieldaligned currents of the NBZ system was analyzed based on the CHAMP satellite data. The results were compared with the position of the auroral oval (the OVATION model) and the ion and electron flux observations on the DMSP satellite. Analysis of the particle spectra indicated that these spectra correspond to the auroral oval dayside sector crossings by the satellite, i.e., to the dayside projection of the plasma ring surrounding the Earth. Arguments are presented for the assumption that the discussed dayside electrojet (PE) is localized near the polar edge of the dayside auroral oval in a the closed magnetosphere. The features of the spectral and spatial dynamics of intense Pc5 geomagnetic pulsations were studied in this time interval. It was established that the spectrum of high-latitude (higher than ~70°) pulsations does not coincide with the spectrum of fluctuations in the solar wind and IMF. It was shown that Pc5 geomagnetic pulsations can be considered as resonance oscillations at latitudes lower than 70° and apparently reflect fluctuations in turbulent sheets adjacent to the magnetopause (the low-latitude boundary layer, a cusp throat) or in a turbulent magnetosheath at higher latitudes.

Substorm evolution of auroral structures

AbstractAuroral arcs are often associated with magnetically quiet time and substorm growth phases. We have studied the evolution of auroral structures during global and local magnetic activity to investigate the occurrence rate of auroral arcs during different levels of magnetic activity. The ground‐magnetic and auroral conditions are described by the magnetometer and auroral camera data from five Magnetometers — Ionospheric radars — All‐sky cameras Large Experiment stations in Finnish and Swedish Lapland. We identified substorm growth, expansion, and recovery phases from the local electrojet index (IL) in 1996–2007 and analyzed the auroral structures during the different phases. Auroral structures were also analyzed during different global magnetic activity levels, as described by the planetary Kp index. The distribution of auroral structures for all substorm phases and Kp levels is of similar shape. About one third of all detected structures are auroral arcs. This suggests that auroral arcs occur in all conditions as the main element of the aurora. The most arc‐dominated substorm phases occur in the premidnight sector, while the least arc‐dominated substorm phases take place in the dawn sector. Arc event lifetimes and expectation times calculated for different substorm phases show that the longest arc‐dominated periods are found during growth phases, while the longest arc waiting times occur during expansion phases. Most of the arc events end when arcs evolve to more complex structures. This is true for all substorm phases. Based on the number of images of auroral arcs and the durations of substorm phases, we conclude that a randomly selected auroral arc most likely belongs to a substorm expansion phase. A small time delay, of the order of a minute, is observed between the magnetic signature of the substorm onset (i.e., the beginning of the negative bay) and the auroral breakup (i.e., the growth phase arc changing into a dynamic display). The magnetic onset was observed to precede the structural change in the auroral display. A longer delay of a few minutes was found between the beginning of the growth phase and the first detected auroral structure.

Microclimate impacts survival and prevalence of Phytophthora ramorum in Umbellularia californica, a key reservoir host of sudden oak death in Northern California forests.

Phytophthora ramorum, an invasive pathogen and the causal agent of Sudden Oak Death, has become established in mixed-evergreen and redwood forests in coastal northern California. While oak and tanoak mortality is the most visible indication of P. ramorum’s presence, epidemics are largely driven by the presence of bay laurel (Umbellularia californica), a reservoir host that supports both prolific sporulation in the winter wet season and survival during the summer dry season. In order to better understand how over-summer survival of the pathogen contributes to variability in the severity of annual epidemics, we monitored the viability of P. ramorum leaf infections over three years along with coincident microclimate. The proportion of symptomatic bay laurel leaves that contained viable infections decreased during the first summer dry season and remained low for the following two years, likely due to the absence of conducive wet season weather during the study period. Over-summer survival of P. ramorum was positively correlated with high percent canopy cover, less negative bay leaf water potential and few days exceeding 30°C but was not significantly different between mixed-evergreen and redwood forest ecosystems. Decreased summer survival of P. ramorum in exposed locations and during unusually hot summers likely contributes to the observed spatiotemporal heterogeneity of P. ramorum epidemics.

Eastward electrojet enhancements during substorm activity

In this study, we use a semi-automatic routine to identify negative and positive bays in the IMAGE magnetometer data during seven months in 2003. The IMAGE stations have been divided into three latitude regions to monitor the time evolution and temporal relationship between the regions during substorms. In particular, we focus on the events where both positive and negative ground magnetic deflections are observed in different latitude regions. We found 101 events in total. We examine separately a subset of 32 events, for which the local electrojet index values are larger than the global ones, suggesting that the strongest activity at that time takes place within or very close to the local time sector of IMAGE. We systematically analyze the temporal difference and the intensity of the positive and negative bays. Our results show that the magnitude of the positive bay is on average about half of that of the negative bay. Two thirds of the positive bays within the IMAGE network peak earlier than the negative bays. Because the positive and negative bays occur meridionally very close together, we suggest that the enhancements of the westward current at the poleward part of the auroral oval and the eastward current within the return flow are very tightly coupled through field-aligned currents and closing horizontal currents. The substorm current system appears as a superposition on the large-scale current pattern in the vicinity of the evening sector shear flow region.

Interrelationship between preonset auroral and magnetic signatures at a geomagnetically conjugate Iceland‐Syowa pair

AbstractWe present the initiation and development of Pi1‐Pi2 band pulsations in concert with spatiotemporal evolution of preonset auroral arc during an isolated auroral substorm reported by Motoba et al. (2012), and their interhemispheric similarities/dissimilarities, based on the colocated optical and magnetic field measurements at a geomagnetically conjugate Iceland‐Syowa pair. At least ~7 min prior to the auroral expansion onset, the first identifiable signature of preonset arc began with a small‐scale (~30–50 km) azimuthal beading. Interestingly, the early development of the visible arc beading for ~3–4 min was not accompanied by any ground magnetic perturbation. Then the bead‐like forms in the arc evolved into brighter, larger undulations, eventually developing into the poleward expansion. Such a preonset optical sequence was almost identical at both stations. In the transition from the beads into undulations, Pi2 pulsations were first identified clearly, followed by Pi1 pulsations a few minutes later. Whereas the Pi2 onset was coincident with the initiation of progressive increase of the preonset arc luminosity, the Pi1 onset was associated with a subsequent steep increase. The Pi2 wave cycles superimposed on the magnetic negative bay were well correlated with the preonset arc luminosity time series. These results imply the potential linkage between the ground Pi1‐Pi2 signatures in the onset region and the preonset auroral arc processes. Although the Pi1‐Pi2 features were generally similar between the two hemispheres, there was also some dissimilarity in their temporal behaviors, which could reflect a small but noteworthy interhemispheric asymmetry in the overhead conjugate preonset aurora.

Regularities in the manifestation of earthquake precursors in the ionosphere and near-surface atmospheric electric fields in Kamchatka

The data of hourly measurements of ionospheric parameters in Petropavlovsk-Kamchatsk are analyzed for the period 1998–2002. In the vertical component of near-surface atmospheric quasistatic electric field Ez, earthquake precursors in the form of anomalous negative bays have been found earlier. In some cases, anomalously high sporadic layer Es, interpreted as an ionospheric precursor of an earthquake, was observed simultaneously with anomalous negative bays in Ez. All these cases were correlated with earthquakes of different magnitudes which occurred with a significant time delay (more than five days) after the precursor appearance. Based on the whole data set (including those for simultaneously measured Es and Ez), empirical dependences linking the prediction time of a precursor, earthquake magnitudes, and the distance from the observation point to the epicenter, are presented. It is shown that these dependences are close to those obtained earlier for long-term earthquake precursors in near-surface geophysical fields of the same seismoactive region. Estimates of the prediction time for earthquake precursors on the boundaries of preparation zones are presented.