Negative Magnetic Bays Research Articles

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.

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

“POLAR” SUBSTORMS AND THE HARANG DISCONTINUITY

The substorms that we term as “polar” substorms, are recorded in the evening sector of the Earth at the geomagnetic latitudes above 70° MLAT under the absence of negative magnetic bays at the lower latitudes. Like the “classical” substorms, the “polar” substorms are accompanied by aurora arcs brightening, poleward expansion, substorm current wedge formation. The onsets of “polar” substorms are typically located near 70° MLAT at ~19-23 MLT. Other important structure, namely, the Harang discontinuity (the evening narrow latitude-zone between the westward and eastward electrojets), is often observed in the same area and at the same MLT interval. Our aim is to study a possible relationship between the location of the “polar” substorms and the Harang discontinuity (HD). Using the IMAGE geomagnetic data, we found that the “polar” substorm onsets exhibit a tendency to occur near the HD latitude. The longitudinal relationship between the “polar” substorms and the HD was studied basing on the ionospheric AMPERE measurements by the 66 simultaneous satellites. We revealed the ground-based magnetic vortex associated with FACs enhancement near the eastward edge of the Harang discontinuity region separated the evening “polar” substorm development and the after-midnight westward electrojet location. Two typical events of the “polar” substorms are discussed in detail.

The Magnetic Storm of August 25–26, 2018: Dayside High Latitude Geomagnetic Variations and Pulsations

The features of daytime high latitude geomagnetic disturbances and geomagnetic pulsations during the recent strong magnetic storm on August 25–26, 2018, which occurred at the end of the decline phase of the 24th solar activity cycle with a very low level of solar flare activity, are considered. As a rule, during this phase of the solar activity cycle, magnetic storms are caused by high-speed solar wind flows from coronal holes; however, the magnetic storms during the decay of the 24th cycle were caused by coronal mass ejections (CMEs). It was shown that, despite very weak disturbances on the Sun and a low solar wind speed, a rather strong magnetic storm occurred in the Earth’s magnetosphere in August 2018 (Dst = –171 nT). The storm SC with a slight (~25 nT) jump in the Dst index caused quite intense daytime geomagnetic pulsations ipcl at the latitudes of the possible position of the daytime polar cusp. A feature of the recovery phase of this storm was the development of a magnetosphere substorm on a global scale, i.e., the appearance of a negative magnetic bay recorded simultaneously in the auroral night sector and in the polar latitudes of the day sector. A possible interpretation is considered.

Large Magnetic Storm on September 7–8, 2017: High-Latitude Geomagnetic Variations and Geomagnetic Pc5 Pulsations

The features of daytime high-latitude geomagnetic variations and geomagnetic pulsations in the Рс5 range during the recent, large, two-stage magnetic storm of September 7–8, 2017 are studied. The discussed disturbances were observed at the recovery phase of the first stage of the storm after the interplanetary magnetic field (IMF) turned northward. It is shown that the large sign-alternating variations in Ву and Bz components of the IMF caused intense geomagnetic disturbances up to 300–400 nT with a quasi-period of ~20 min in the daytime sector of polar latitudes, probably in the region of the daytime polar cusp. These disturbances may have reflected quasi-period motions of the daytime magnetopause and may have resulted from nonlinear transformation of the variations in the interplanaterary magnetic field in the magnetosheath or in the magnetospheric entry layers. The appearance of high-latitude long-period variations was accompanied by the excitation of bursts (wave packets) of geomagnetic Pc5 pulsations. The onset of Pc5 pulsation bursts often coincided with a sudden northward turn of the IMF. It was discovered for the first time that the development of a “daytime polar substorm,” i.e., a negative magnetic bay in the daytime sector of polar latitudes, led to a sudden termination of the generation of geomagnetic Pc5 pulsations over the entire latitude range in which these oscillations were recorded before the appearance of the daytime bay.

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.

On the relationship between shock‐induced polar magnetic bays and solar wind parameters

There has been an impression for more than half a century that reductions in the north‐south component of the Earth's magnetic field in the polar region (often called “negative magnetic bays” or “polar magnetic bays”) may occur immediately after compression of the magnetosphere by interplanetary (IP) shock impacts, in particular when the magnetosphere is “preconditioned” by southward interplanetary magnetic field (IMF). A literature search suggests that this view has not been rigorously verified nor disproved. This paper reports a study of 43 IP shock events to illustrate the effect of IP shock‐magnetosphere coupling on the high‐latitude auroral electrojets. Specifically, we correlate the strength of magnetic bays (inferred from minimum values of the AL index within a 30‐min window after shock impact) with solar wind parameters and their combinations within the same time window. It is surprisingly found that the strength of magnetic bays correlates much better with solar wind parameters downstream of a shock (correlation coefficient, r, up to 0.86) than solar wind parameters upstream of a shock (r up to 0.6). Therefore whether or not an interplanetary shock can “induce” a magnetic bay depends on the concurrent solar wind and IMF conditions. This strongly suggests that enhancements of the westward auroral electrojet during magnetosphere compression are primarily associated with a directly driven process. It is also found that the solar wind dynamic pressure, which is traditionally considered a secondary effect, contributes equally with IMF Bz to the westward auroral electrojet.

Investigation of external triggering of substorms with Polar ultraviolet imager observations

The triggering mechanism(s) for the substorm expansion phase onset is one of the outstanding issues in magnetospheric physics. Previous studies have shown that an impingement of the high solar wind dynamic pressure may lead to the expansion phase onset of a substorm. These previous studies typically used a negative magnetic bay as the main proxy for a substorm, but we now know that some magnetic bays are associated not with substorms but with the enhancement of convection. Therefore it is reasonable to cast doubts on the compression trigger mechanism. In this study we use classical substorm onsets, “auroral breakups,” which are the most reliable substorm onset indicator when identified with global auroral images, to reinvestigate this issue. We examine 43 interplanetary shock events that occurred between 1996 and 1999 with simultaneous global auroral images from the Polar ultraviolet imager images and the auroral electrojet indices. It is found, indeed, that ∼52% of the shocks produce magnetic bays (AL < −100 nT). While most of the shocks enhance auroral luminosities, only 4 events (∼9%) appear to have an auroral breakup preceded by an SSC/SI by 20 min (two events by 10 min). These results strongly indicate interplanetary shocks can produce negative magnetic bays but not auroral breakups (thus christened “compression bays”). An examination of precisely timed interplanetary magnetic field (IMF) during 11 substorms that occurred near the shock‐induced SSCs/SIs indicates that northward turnings of IMF occurred more than 50% of time. Given such a low probability, it is concluded that shock compression is not likely to trigger substorms but enhances magnetospheric currents and auroral particle precipitation. In line with many previous study results, on the other hand, the northward turnings of IMF can be a plausible substorm triggering mechanism.

Bursty bulk flow intrusion to the inner plasma sheet as inferred from auroral observations

The physics of the interaction of bursty bulk flows (BBFs) with the near‐Earth plasma is not yet known in detail. We address this issue with comprehensive observations made with the MIRACLE network during a well‐documented streamer event. In this case an equatorward intruding auroral streamer inclined from northwest (NW) to southeast (SE) reached the region of the proton oval of 486 nm (Hβ) luminosity. This Hβ band is assumed to map poleward of the ≥30 keV proton isotropic boundary, which is at geocentric distances ∼7–8 RE according to the appropriate magnetic field models. A significant enhancement (30%) in the Hβ luminosity was recorded just at the time and in the location where the equatorward end of the streamer reached the Hβ band. This enhancement implies a corresponding pressure enhancement in the equatorial magnetosphere. The simultaneous poleward shift of the Hβ emission suggests that also a magnetic field dipolarization took place in the same region. An NW‐aligned electrojet (300–400 km in width) accompanied the equatorward development of the streamer (with streamer locating at the dusk flank of this electrojet), and it produced a sharp negative magnetic bay (∼300 nT) resembling a substorm onset. Both auroral and equivalent current observations suggest that the plasma jet was not completely dissolved or stopped during this interaction, but rather diverted in the azimuthal direction. Equivalent currents reconstructed from ground magnetic field observations indicate that a R1‐type field‐aligned current system (upward current ∼0.5 MA) accompanied the streamer development. However, our observations do not show any signatures of a R2‐type current system expected to build up during the flow braking and diversion according to previously published simulation studies.

Simultaneous observations of earthward flow bursts and plasmoid ejection during magnetospheric substorms

Examination of observations taken by radially aligned International Solar Terrestrial Physics spacecraft in the nightside magnetosphere on 9 July 1997 has revealed close temporal correlations between earthward flow bursts in the plasma sheet and the ejection of plasmoids. A one‐dimensional model of plasma sheet flow is applied to these observations to determine the time and location for the initiation of lobe flux tube reconnection. For the single clear flow burst‐plasmoid pair observed during the first substorm and the three pairs produced by the second substorm, lobe flux reconnection was inferred to have started at X ∼ −15 to −18 RE, respectively, about 2–5 min prior to the observations of substorm expansion phase onset. These time delays and propagation speeds are shown to be consistent with the measured plasma sheet bulk flow speeds. Substorm expansion phase onset was essentially coincident with the arrival of the flow bursts at Geotail, which was located near the inner edge of the plasma sheet at X ∼ −9 RE. The dipolarization of the magnetic field at geosynchronous orbit, auroral kilometric radiation (AKR) emissions, Pi2 pulsations, high‐latitude negative magnetic bays, and auroral breakup marking substorm expansion onset are all coincident within the ±1 min resolution of the measurements. Accordingly, it appears that earthward of the inner edge of the plasma sheet, where Geotail was located, substorm effects propagated at speeds comparable to the Alfven speed characteristic of the high‐latitude inner magnetosphere, ∼103 km s−1. In summary, the results of our investigation strongly support the modern near‐Earth neutral line (NENL) model of substorms in which the onset of lobe flux tube reconnection in the near tail is followed ∼2–5 min later by the braking of earthward flow bursts as they encounter the inner magnetosphere and within ∼1 min, by Pi2s generations, current wedge development, and AKR and auroral expansion, and finally, ∼10–20 min later, by the tailward retreat of the neutral line and either the development of a new NENL or the initiation of the recovery phase.

Penetration of auroral electric fields to the equator during a substorm

We have studied the negative magnetic bay associated with the substorm that occurred on April 20, 1993, and have found that it is markedly enhanced at the daytime dip equator, coherent with that at afternoon subauroral latitudes. The amplitude of the negative bay decreases monotonously with the latitude, but it is amplified at the dip equator by a factor of 2.5 compared to the low‐latitude negative bay. This latitudinal profile implies that in addition to the three‐dimensional current system in the magnetosphere, DP ionospheric currents originating in the polar ionosphere contribute greatly to negative bays. Penetration of the convection electric field and the effect of a shielding electric field due to Region 2 (R2) field‐aligned currents (FACs) are examined on the basis of European Incoherent Scatter (EISCAT) and International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations made in the afternoon sector. The northward electric field at EISCAT (66° corrected geomagnetic latitude (CGMLAT)) is well correlated with the magnetic field X component at Nurmijärvi (56° CGMLAT) during the presubstorm period, but the coherency breaks down during the substorm cycle. By assuming that the R2 FACs intensify the northward electric field at EISCAT but reduce it at Nurmijärvi, we demonstrate that the R2 FACs grow concurrently, although delay by some 17 min, with the convection electric field. Our analytical results indicate that the convection electric field decreases abruptly during the substorm and that the shielding electric field overcomes the convection electric field at around the peak of the negative bay, owing to its delayed reaction. The equatorial negative bay is thus due to an overshielding effect caused by the electric field associated with the R2 FACs.

Geomagnetic storm and substorm aurora observed from Spitsbergen

ABSTRACTThe present study confirms that the auroras of the nightside oval population consist of two main types, namely storm and substorm aurora. Storm-type aurora, which is relatively infrequent, results from bombardment of the upper ionosphere by fast particle streams generated in the solar wind by cataclysmic solar events related to coronal mass ejections. The associated turbulent plasma that is injected into the magnetosphere produces great magnetic storms of world-wide dimensions that may last for days. In contrast, substorm aurora is a frequent (almost daily) occurrence that lasts for an hour or two around geomagnetic midnight. It is generated by the impulsive release of stored magnetospheric energy from the substorm onset region in the Earth's magnetotail and is associated with localised negative magnetic bays in the H (horizontal)-component of the Earth's magnetic field in the vicinity of the auroral oval.

Meridional ionospheric electric field caused by curvature current in the magnetosphere

The bending of geomagnetic field lines towards the geotail produces a curvature drift of charged particles parallel to the geomagnetic axis. The divergence of the current so produced forms Birkeland current to the ionosphere where a meridional electric field is created. This field would drive ionospheric currents to form a negative magnetic bay in the dawn sector of the auroral zone and a positive one in the dusk sector. Also it would cause a dawn-dusk field across the polar cap.

Auroral zone E-region motions deduced from spaced receiver observations

E-region drift velocities in the Auroral Zone have been observed by the spaced receiver technique at Tromsø, Norway (69.66°N, 18.94°E). Drift velocities have been obtained for disturbed as well as quiet magnetic conditions. The drift velocities derived are mainly eastward in strong negative magnetic bays and westward in strong positive bays. At midday a northward velocity is observed during quiet conditions. Several observations reveal a rather erratic drift pattern, particularly during medium disturbed conditions. These results are attributed to a lack of a dominating driving force in the ionosphere during such periods.

On the morphology of energetic (≥30keV) electron precipitation at the onset of negative magnetic bays

Multiple balloon recordings of bremsstrahlung X-rays supported by recordings of cosmic noise absorption have been used to study in detail energetic (≥30 keV) electron precipitation events occurring near local midnight at the onset of the expansion phase of magnetospheric substorms. This type of precipitation occurs during the first 5–10 min after bay onset and can usually be distinguished from the subsequent bay-associated precipitation by its characteristic time structure, variation in energy spectrum, and higher intensities. During this same interval, the poleward border of the precipitation region moves rapidly towards higher latitudes with speeds of typically 1–2 km/s, whereas the equatorward border seems to move slowly towards lower latitudes. The northward expansion starts just poleward of the lowest latitude reached during the slow equatorward motion of the preceding growth-phase precipitation. The previous narrow precipitation region may thus expand to as much as 10° of invariant latitude within a few minutes. Within the expanding region there are additional intrinsic temporal variations. As the flux of precipitating electrons tends to be most intense and most energetic near the poleward border, recordings made northward of the latitude where the poleward motion started tend to give the appearance of an impulsive precipitation event. The bay-onset precipitation starts abruptly at the onset of Pi 2 magnetic pulsations. Associated with these pulsations there are modulations of the flux of precipitating electrons. An intensified westward electrojet appears to have its center in the equatorward part of the precipitation region. It is suggested that the poleward expansion is associated with the expansion of the plasma sheet earthward of a newly formed X-type neutral line, and is caused by a sudden enhancement of field-line re-connection across the neutral sheet. The intense, more energetic electron precipitation at the poleward border of the precipitation region then takes place along the outer border of the expanding plasma sheet.

On the morphology of energetic (≥30keV) electron precipitation during the growth phase of magnetospheric substorms

The morphology of energetic (≥30 keV) electron precipitation during the growth phase of magnetospheric substorms has been investigated using measurements of auroral-zone bremsstrahlung X-rays obtained from multiple balloon flights and supplementing riometer recordings. Growth-phase precipitation typically starts about one hour before the onset of a negative magnetic bay and occurs in a limited region parallel to the auroral oval around local midnight. The precipitation is first observed in the northern part of the auroral zone and moves southwards with a speed of 5–10 km/min. To the north this precipitation therefore ceases well before bay onset whereas a continuous transition from ‘prebay’ precipitation to bay-associated precipitation takes place in the south. A decrease in the intensity or at least a levelling off may occur some minutes before bay onset. The southward movement of the precipitation region is associated with a similar movement of a weak ionospheric current system. The events studied were all associated with a southward-pointing interplanetary magnetic field and with growth-phase conditions in the magnetotail. It is suggested that growth-phase precipitation originates from the ‘horns’ of the plasma sheet. The equatorward motion of the precipitation is then a consequence of an expansion of the polar cap, a thinning of the plasma sheet, and an equatorward motion of its inner edge. It is also suggested that this precipitation provides a stabilization of the outer boundaries of the plasma sheet by restricting the ionospheric mobility of the bordering field lines through enhanced conductivity.

High-latitude irregularities during the magnetic storm of October 31 to November 1, 1972

By using the scintillation of satellite radio beacons and radio stars as the means of determining the intensity of F layer irregularities, observations of the high-latitude irregularity region were made during the magnetic storm of October 31 to November 1, 1972. At ssc of this winter storm the irregularity region from 60° to 63° invariant latitude was affected almost immediately. However, scintillation intensities were high only for a period of the order of many minutes and subsided again before the expansion phase of the storm. During the expansion phase the motion of the storm irregularities could be tracked equatorward particularly during the occurrence of one intense negative magnetic bay in the H component. The velocity was high at invariant latitudes of 53°–60° (of the order of 300–600 m/s) but slowed to 100–200 m/s from 49° to 53° and then to even lower velocities at 45°, the lowest latitude of the observations. A comparison with auroral photographs indicates that even 3° equatorward of the optical aurora the irregularity intensities at F layer heights were considerable. High levels of scintillation activity were also noted poleward of the aurora. Heat conduction due to auroral current energy dissipation or from isotropic electric fields is postulated as the source for the production of intense F layer irregularities during these severe magnetic storms. The precipitation of low-energy electrons is another (but less attractive) hypothesis.

Morphology and fine time structure of an early-morning electron precipitation event

Simultaneous balloon recordings of auroral-zone X-rays from precipitating electrons, covering a range of L-values from ≈5 to ≈7.5, are presented. The precipitation event was observed in the early morning sector (from about 0200 to 0500 local magnetic time), and was associated with a negative magnetic bay. Before the bay, precipitation associated with the growth phase of the substorm was observed at high L-values. After bay onset, precipitation was observed over the whole range of L-values covered, but with a delayed onset in the southern part of the precipitation region as compared with the onset of cosmic noise absorption in the local midnight sector. At high L-values the X-ray flux was completely unstructured and drizzle-like, both before and after bay onset. At low L-values, where precipitation occurred only after bay onset, the event was splash-like with X-ray bursts of typically 4–6 sec duration apparently rising out of the cosmic-ray background. The precipitation bursts had spatial extensions of 300–400 km. They were accompanied by weak magnetic impulses which were, both temporally and spatially, closely related to the X-ray bursts. The unstructured precipitation at high L-values was apparently associated with and extending along the auroral electrojet, presumably representing freshly accelerated particles. The highly structured and burst-like precipitation to the south seems to have come from a cloud of electrons drifting out from the acceleration region, from which wave-particle instabilities or some other mechanism caused electrons to be precipitated.

Onset of magnetospheric substorms

An examination of the onset of magnetospheric substorms is made by using ATS 5 energetic particles, conjugate balloon X rays and electric fields, all-sky camera photographs, and auroral-zone magnetograms. It is shown that plasma injection to ATS distances, conjugate 1- to 10-keV auroral particle precipitation, energetic electron precipitation, and enhancements of westward magnetospheric electric-field component all occur with the star of slowly developing negative magnetic bays. No trapped or precipitating energetic-particle features are seen at ATS 5 when later sharp negative magnetic-bay onsets occur at Churchill or Great Whale River.

Interplanetary magnetic-field variations and substorm activity

A fine time-scale study of interplanetary magnetic field (IMF) variations and auroral-zone magnetograms during active and moderately active days show that the time delay between the southward turning of the IMF and the first sign of a negative magnetic bay is typically less than 15 min. During the moderately active period, 88% of all substorms were associated with southward interplanetary magnetic fields. Conversely, 80% of all large southward IMF events were associated with auroral-zone negative bays; during some events, however, magnetic bays could not be found, even by using high-latitude stations. It is concluded that the main mechanism for the triggering of magnetospheric substorms is the southward turning of the IMF.