Auroral Breakup Research Articles

Auroral kilometric radiation at substorm onset

The temporal relationship of substorm signatures that occur in different regions of space is an important part of substorm research. In the present study we investigate the relative timing between sharp enhancement of auroral kilometric radiation (AKR) and auroral breakups. It is found, on the basis of 136 isolated substorm events identified with global auroral images from the Polar ultraviolet imager (UVI), that (1) 70% (83%) of the time AKR enhancements were detected within ±1 (±2) min of the auroral breakups; (2) AKR onset tends to occur, on an average, slightly later (0.36 min) than the corresponding auroral breakup; and (3) similar to previous study results, substorm‐associated AKR has a forbidden area at Polar altitude in the noon sector. These results suggest that when the satellite is suitably located, the enhancement of AKR is a good substorm onset indicator and it can be used to time substorm onsets adequately within the ∼1 min of uncertainties from the UVI observations, especially if confirmed by other substorm onset identifiers. The average frequency for the enhancement of AKR at substorm onset is found to be ∼300 kHz, corresponding to an upper limit of the AKR source altitude of ∼4700 km. The entire AKR frequency band expands to ∼64–650 kHz within a few minutes after onset, corresponding to a wider source altitude of ∼2100–12,000 km, a typical zone for auroral electron acceleration. We suggest that the fast expansion of the AKR source region along the local magnetic field lines, equivalent to the expansion of the auroral acceleration region, should be considered as one of the fundamental signatures in the substorm expansion phase.

Investigation of the conjugacy between auroral breakup and energetic electron injection

Auroral TV observations at Loparskaya, Kola Peninsula, and CRRES energetic electron and proton measurements during a moderate isolated substorm at 2000–2100 UT on February 13, 1991, are compared. The CRRES was at the apogee of orbit 494 in the outer radiation belt near the magnetic equator. CRRES footprint projection estimated by using a Tsyganenko magnetic field model was situated at the same latitudes several degrees eastward of active aurora during the substorm breakup or the intensification. The intensification consists of several activations of 1‐min duration, and this fine spatial and temporal structure is important for the conjugacy investigations. It is shown that injected energetic electrons have been accelerated at the same time as one of the auroral activations at the same latitude and approximately the same azimuthal extended region. The lack of enhanced electrons during other activations suggests that the acceleration region has a sharp equatorial boundary.

Thin current sheet evolution as seen in observations, empirical models and MHD simulations

The inner and midtail current sheet is examined during a substorm on Dec 10, 1996 using multispacecraft observations, empirical models, and global MHD simulations. Both the simulation and empirical magnetic field model show good agreement with observations, which substantially increases the confidence in both methods. Both locate the thin current sheet inner edge at ∼7 RE, and give a scale size of ∼15 RE in tailward and cross‐tail directions. The MHD simulation confirms the empirical model result that the thin current sheet remained stable after a local auroral breakup and indicates that the activity followed from flow channels generated at ∼40 RE independent of the thin current sheet evolution closer to the Earth.

Evaluation of low‐latitude Pi2 pulsations as indicators of substorm onset using Polar ultraviolet imagery

Impulsive Pi2 pulsations have long been recognized as one of the key signatures of magnetic activity during substorm periods due to their wide observable range both in latitudes and longitudes. It is well documented that there is usually more than one Pi2 wave burst associated with a substorm and only one of them corresponds to the onset of the substorm. This observational fact poses obstacles to determining substorm onsets with Pi2 signals. Although the Pi2 have become one of the most popular indicators for substorm onsets, the reliability of using the Pi2 in this fashion has not been seriously investigated. In this paper we address this question with a statistical approach by using ∼650 auroral substorm onsets identified with Polar ultraviolet images for a time interval from April 1996 to May 1997. A comparison of the low‐latitude Pi2 pulsation onsets from Kakioka (L = 1.07) with the auroral breakups indicates that identifying substorm onset with the Pi2 alone is often ambiguous. Of a total of 119 isolated (defined as ∼10 min of quiet time preceding the onset) Pi2 bursts seen within ∼10 min from a magnetic positive bay, there were 65 events (∼55%) taking place within 3 min from breakups and 34 events (29%) indicating no sign of an auroral breakup within 10 min of the Pi2 burst. This result suggests that Pi2 may not be as a good indicator of the substorm onset as it was thought to be. Interestingly, it is always possible to associate Pi2 pulsations with some forms of auroral intensification. When compared to auroral breakups, Pi2 onsets are subject to a small delay of 1 – 3 min, with a peak around l min. Delays of Pi2 onsets are revealed to be a function of location relative to auroral breakup. This dependence is found to be consistent with the time of flight for a fast‐mode wave, in a plasmapause cavity mode model, propagating in the magnetosphere.

On relative timing in substorm onset signatures

We investigate the timing of popular substorm onset signatures to understand their temporal relationship. Proxies for substorm onsets include auroral breakups, high‐latitude magnetic bays, low‐latitude Pi2 bursts, dispersionless injections at geostationary orbits, and auroral kilometric radiation. We use the auroral breakup, identified with Polar UVI images, as a common reference time frame to calibrate the others. Results, illustrated by two well‐defined auroral substorms, unambiguously indicate that none of the four frequently used substorm onset proxies can provide a consistent timing of substorm onset. This inconsistency in substorm onset timing is attributed as a consequence of temporal and spatial limitations on each observational technique. A delay between the proxy identifiers and the auroral breakup is found to be typical. It is therefore strongly suggested from this study result that a common reference time frame for substorm onset is necessary, and we propose it should be auroral breakups. We argue that there is a need for an intercalibration of magnetospheric substorm phenomenology by using a unified definition of the substorm onset.

Characteristics of pseudobreakups and substorms observed in the ionosphere, at the geosynchronous orbit, and in the midtail

We present a comprehensive study of a sequence of two substorms and multiple pseudobreakups using optical, magnetic and incoherent scatter radar measurements, energetic particles from two geosynchronous satellites and particle and field data from the Geotail spacecraft located at Xgsm ∼ −86 RE. Following conventional nomenclature, we classified as pseudobreakups those auroral breakups which did not exhibit significant poleward expansion (< 2° magnetic latitude). Auroral intensifications following substorm breakups were also observed, and were classified separately. Pseudobreakups were found not to differ from substorm breakups in longitudinal extent (from 1.3 to 6.1 hours of magnetic local time), or in duration (from 5 to 16 minutes). In general, the ionospheric currents producing ground magnetic disturbances were more intense during substorms than pseudobreakups. We found that pseudobreakups are associated with the same magnetospheric processes as substorm breakups which involve current wedge formation, midlatitude magnetic Pi2 pulsations and energetic particle injections at the geosynchronous altitude. Moreover, pseudobreakups are associated with magnetic reconnection in the near‐Earth region, evidenced by the typical subsequent detection of a plasmoid at Geotail. This implies that the magnetotail volume influenced by a pseudobreakup is quite large in radial distance. We conclude that there is no definitive qualitative distinction between pseudobreakups and substorms but there is a continuum of states between the small pseudobreakups and large substorms.

On the dynamic cusp aurora and IMF By

Using a new form of presentation of optical data from meridian scanning photometers, we demonstrate two important aspects of cusp dynamics concerning transients called midday auroral breakups or poleward moving auroral forms (PMAFs). Many previous studies have associated PMAFs with pulsed (in time) reconnection at the magnetopause, where these transients are thought to represent the auroral footprint of reconnected magnetic flux tubes (flux transfer events) and their motion is thought to reflect the motion of the corresponding flux tubes. Some studies have indicated that there is a prenoon‐postnoon asymmetry in the occurrence and motion pattern of PMAFs related to the east‐west component of the interplanetary magnetic field (IMF) By. In two case studies we point out the strong modulation in intensity that exists between the PMAF and the surrounding cusp emission, evident in both the red and green lines. We then show cases where the local occurrence of PMAFs is dependent on IMF By in the sense that PMAF activity at ∼1100 magnetic local time ceases when IMF By changes from positive to negative values. This is explained by reconfigurations of the magnetopause reconnection geometry and ionospheric convection pattern causing PMAFs to disappear from the field of view of the optical instruments, an interpretation which is supported by local ground magnetic field observations.

A slow mode wave as a possible source of Pi 2 and associated particle precipitation: a case study

Abstract. An intensification of auroral luminosity referred to as an auroral break-up often accompanies the onset of geomagnetic pulsation (Pi 2) at the dip-equator. One such auroral break-up occurred at 2239 UT on 16 June, 1986, being accompanied by weak substorm activity (AE~50 nT) which was recorded in all-sky image of Syowa Station, Antarctica (66.2°S, 71.8°E in geomagnetic coordinates). The associated Pi 2 magnetic pulsation was detected by a fluxgate magnetometer in the afternoon sector at the dip-equator (Huancayo, Peru; 1.44°N, 355.9° in geomagnetic coordinates; 12.1°S, 75.2°W in geographic coordinates; L=1.00). In spite of the large separation of the two stations in longitude and latitude, the auroral break-up and subsequent luminosity modulation were seen to be correlated with the wave form of the ground Pi 2 pulsation. This occurred in such a way that the luminosity maximum was seen to occur at the phase of maximum amplitudes of Pi 2 wave form. We argue that the observed correlation could be interpreted as indicating a Pi 2-modulation of a field-aligned acceleration of the low energy electrons that may occur near the equator of the midnight magnetosphere.Key words. Magnetospheric physics (auroral phenomena; energetic particles · precipitating; MHD waves and instabilities)

A slow mode wave as a possible source of Pi 2 and associated particle precipitation: a case study

An intensification of auroral luminosity referred to as an auroral break-up often accompanies the onset of geomagnetic pulsation (Pi 2) at the dip-equator. One such auroral break-up occurred at 2239 UT on 16 June, 1986, being accompanied by weak substorm activity (AE~50 nT) which was recorded in all-sky image of Syowa Station, Antarctica (66.2°S, 71.8°E in geomagnetic coordinates). The associated Pi 2 magnetic pulsation was detected by a fluxgate magnetometer in the afternoon sector at the dip-equator (Huancayo, Peru; 1.44°N, 355.9° in geomagnetic coordinates; 12.1°S, 75.2°W in geographic coordinates; L=1.00). In spite of the large separation of the two stations in longitude and latitude, the auroral break-up and subsequent luminosity modulation were seen to be correlated with the wave form of the ground Pi 2 pulsation. This occurred in such a way that the luminosity maximum was seen to occur at the phase of maximum amplitudes of Pi 2 wave form. We argue that the observed correlation could be interpreted as indicating a Pi 2-modulation of a field-aligned acceleration of the low energy electrons that may occur near the equator of the midnight magnetosphere.Key words. Magnetospheric physics (auroral phenomena; energetic particles · precipitating; MHD waves and instabilities)

Plasma and magnetic flux transport associated with auroral breakups

Auroral breakups are the first visible sign of a substorm expansion onset. Keying the plasma sheet behavior to onset times of auroral breakups may help to identify the substorm onset process. With this goal in mind, we have identified a list of auroral breakups based on global imaging data from the Ultraviolet Imager of the POLAR spacecraft for two favorable viewing periods and have examined the simultaneous plasma measurements in the tail from GEOTAIL. Synoptic patterns of plasma transport and magnetic field changes in the tail surrounding the times of auroral breakups are constructed. The results indicate that the plasma sheet activities associated with auroral breakups are transient and spatially localized. These findings are consistent with the scenario in which expansion phase activities are dominated by localized, transient disturbances as portrayed by the substorm synthesis model.

Short‐duration convection bays and localized interplanetary magnetic field structures on November 28, 1995

We present ground‐based, plasma sheet, and magnetosheath observations of two subsequent short‐duration (10–20 min) increases of the postmidnight westward electrojet on November 28, 1995. Appearing as though small (150–200 nT) substorms, they were not accompanied by any substorm expansion onset signatures. Auroral breakup, worldwide Pi2 pulsations, and the corresponding plasma sheet activity, such as fast flows, current disruption, and plasmoid generation, were all observed only at the recovery of the second electrojet increase. These convection bays were associated with the equatorward expansion of the auroras and simultaneous magnetic variations in the polar cap and middle latitudes. Growth phase signatures of the lobe field increase and tailward stretching of magnetic field were also observed in the plasma sheet. Bursty bulk flows in the plasma sheet seem to be quenched at the onset of first convection bay and did not resume until the auroral breakup which concluded the second convection bay. A point of interest of this event was the “incomplete” convection/current system with a well‐developed dawn vortex in the absence of well‐defined dusk vortex; instead, a complicated transient activity dominated over the afternoon‐dusk local time sector. We interpret this asymmetry either in terms of the magnetopause encounter with the edge of the solar wind driver, i.e., strong southward IMF, which hits only the dawn part of the magnetosphere, or with an extremely slant interplanetary discontinuity. This unique configuration was inferred from observations of uncorrelated strong southward Bz events by the Wind and IMP 8 spacecraft in the dusk and dawn magnetosheath, respectively, as well as from the directional analysis of the interplanetary discontinuities which form the edges of these structures. We suggest that interaction of the magnetosphere with very slant solar wind discontinuities may bring various specific features to magnetospheric and ionospheric dynamics that have not been reported.

Conjugacy of isolated auroral arcs and nonconjugate auroral breakups

Fine examples of both conjugate and nonconjugate isolated auroral arcs were observed at two geomagnetically conjugate stations near L = 6, Syowa Station in Antarctica and Husafell in Iceland on September 12, 1988. These events exhibited some interesting characteristics. An auroral loop structure that appeared in both hemispheres was ∼2.0 times larger in the north‐south direction at Syowa than at Husafell. This scale difference is greater than expected from the difference in geographic and geomagnetic (IGRF) coordinates between the two points of observation. However, temporal and spatial variations in the loop structures were almost identical in both hemispheres. After the disappearance of the loop structure, closely conjugate auroras were formed. Nonconjugate auroral features appeared again at Syowa on the poleward side, while the equatorward auroras maintained conjugacy. The nonconjugate aurora at Syowa then began to break up, showing fast moving vortex‐like structures (auroral spirals). At this time, all auroral features at Husafell seemed to have their conjugate counterparts in equatorward auroras at Syowa and none exhibited rapid motions. These conjugate auroras at Husafell were gradually extending poleward, while the corresponding features at Syowa were compressed toward the equator and shrinking in size. The onset of auroral breakup was about one minute earlier at Syowa than at Husafell. The nonconjugate auroral features were reflected in corresponding magnetic field variations on the ground. The events summarized above give interesting clues to the development and decay of auroral conjugacy and the question why the beginning of auroral breakup is not simultaneous at conjugate stations. The time lag and nonconjugacy of auroral breakup in conjugate areas suggests that the triggering source of auroral breakup was not located near the equatorial plane in the magnetosphere but most likely in a localized region near the ionosphere in the southern hemisphere. The nonconjugate auroral spirals also suggest the existence of asymmetrical field‐aligned currents.

A substorm model with onset location tied to an auroral arc

Two important observational constraints on substorm onset theories are that (1) substorm onset occurs on field lines of a pre‐existing auroral arc, and (2) reduction of the cross‐tail current occurs in the near‐Earth magnetotail. Recent progress in linking generation of at least some auroral arcs to field line resonances (FLRs) and current disruption to possibly the cross‐field current instability (CCI) suggests a synergistic combination of these two theories. We suggest that as the cross‐tail current sheet thins to the extent that ions become unmagnetized at the end of substorm growth phase, the equatorial electric field associated with FLRs can amplify significantly the cross‐tail current to exceed the instability threshold. This combined theory naturally accounts for the observational constraints of substorm onset starting on a pre‐existing auroral arc and linking closely with current reduction in the near‐Earth magnetotail. It is compatible with the observation of the most equatorward auroral arc being the most likely one for initial brightening because the cross‐tail current is most intense in its near‐Earth end and is therefore most likely to exceed the unstable threshold for CCI. It is also consistent with auroral precipitation fading to be a common feature before an auroral breakup. Other predictions yet to be tested are that (1) the onset is more favorable for the phase of the resonance in which the resonance‐associated field‐aligned current is directed from the ionosphere to the magnetosphere, and (2) the onset is more favorable poleward of the resonance shell or auroral arc.

Two spacecraft observations of a reconnection pulse during an auroral breakup

At 1130 UT on November 28, 1995, two spacecraft, Interball‐Tail and Geotail, were in a favorable position to study the plasma sheet activity and an auroral breakup observed on the ground near the spacecraft ionospheric footpoints. Both spacecraft were near the neutral sheet, and they were nearly aligned along the magnetic meridian. During the auroral breakup observed at the equatorward half of the auroral oval (also registered as an AKR burst at Interball) both spacecraft simultaneously detected signatures of a reconnection pulse: The earthward plasma streaming and magnetic field dipolarization were observed at 12 RE at Interball, while the tailward energetic ion beam, then the tailward flow and the passage of a plasmoid were observed at 28 RE at Geotail. This pulse seem to proceed inside of the plasma sheet closed field lines, in the region of small (∼1 nT) background magnetic field at the neutral sheet. At Interball position the onset of fast earthward ion flow, likely initiated by the reconnection pulse, was followed by other manifestations (dipolarization, enhancements of the magnetic turbulence and the energetic particle flux, the intensification of field‐aligned currents). Auroral observations showed initial brightening delayed an approximately l min after the commencement of the reconnection pulse. The auroral intensification was not accompanied by a significant magnetic disturbance on the ground, and therefore the event can be classified as the pseudobreakup. We estimate magnetic flux transport characteristics and possible location of the onset region in the plasma sheet. We conclude that observations during this event are consistent with the initiation of an auroral breakup by some disturbance (e.g., Alfven wave) generated by the reconnection pulse that commenced in the neutral sheet at ∼15 RE distance.

Luminosity variations in several parallel auroral arcs before auroral breakup

Abstract. Variation of the luminosity in two parallel auroral arcs before auroral breakup has been studied by using digitised TV-data with high temporal and spatial resolution. The intervals when a new arc appears near already existing one were chosen for analysis. It is shown, for all cases, that the appearance of a new arc is accompanied by fading or disappearance of another arc. We have named these events out-of-phase events, OP. Another type of luminosity variation is characterised by almost simultaneous enhancement of intensity in the both arcs (in-phase event, IP). The characteristic time of IP events is 10–20 s, whereas OP events last about one minute. Sometimes out-of-phase events begin as IP events. The possible mechanisms for OP and IP events are discussed.

Luminosity variations in several parallel auroral arcs before auroral breakup

Variation of the luminosity in two parallel auroral arcs before auroral breakup has been studied by using digitised TV-data with high temporal and spatial resolution. The intervals when a new arc appears near already existing one were chosen for analysis. It is shown, for all cases, that the appearance of a new arc is accompanied by fading or disappearance of another arc. We have named these events out-of-phase events, OP. Another type of luminosity variation is characterised by almost simultaneous enhancement of intensity in the both arcs (in-phase event, IP). The characteristic time of IP events is 10–20 s, whereas OP events last about one minute. Sometimes out-of-phase events begin as IP events. The possible mechanisms for OP and IP events are discussed.

On the current-voltage relationship in auroral breakups and westwards-travelling surges

Abstract. Auroral precipitating electrons pass through an acceleration region before entering the atmosphere. Regardless of what produces it, a parallel electric field is assumed to cause the acceleration. It is well known that from kinetic theory an expression for the corresponding upward field-aligned current can be calculated, which under certain assumptions can be linearized to j∥=KV. The K constant, referred to as the Lyons-Evans-Lundin constant, depends on the source density and thermal energy of the magnetospheric electrons; it is an important parameter in magnetosphere-ionosphere coupling models. However, the K parameter is still rather unknown, and values are found in a wide range of 10–8–10–10 S m–2. In this study, we investigated how the type of auroral structure affects the K values. We look at onset and westwards-travelling surge (WTS) events and make comparisons with earlier results from observations of more stable auroral arcs. A new analysis technique for studying those magnetospheric parameters using ground-based measurements is introduced. Electron density measurements are taken with the EISCAT radar, and through an inversion technique the flux-energy spectra are calculated. Source densities, thermal energies and potential drops are estimated from fittings of accelerated Maxwellian distributions. With this radar technique we have the possibility to study the changes of the mentioned parameters during the development of onsets and the passage of surges over EISCAT. The study indicates that the linearization of the full Knight formulation holds even for the very high potential drops and thermal temperatures found in the dynamic onset and WTS events. The values of K are found to be very low, around 10–11 S m–2 in onset cases as well as WTS events. The results may establish a new technique where ionospheric measurements are used for studying the ionosphere-magnetosphere coupling processes.

Imaging and EISCAT radar measurements of an auroral prebreakup event

Abstract. The results of coordinated EISCAT and TV-camera observations of a prebreakup event on 15 November 1993 have been considered. The variations of the luminosity of two parallel auroral arcs, plasma depletion on the poleward edge of one of these arcs as well as electron and ion temperatures in front of a westward travelling surge were studied. It was found that a short-lived brightening of a weak zenith arc before an auroral breakup was accompanied by fading of an equatorial arc and, vice versa. A plasma depletion in the E region was detected by the EISCAT radar on the poleward edge of the zenith arc just before the auroral breakup. The plasma depletion was associated with an enhancement of ion (at the altitudes of 150–200 km) and electron (in E region) temperatures. During its occurrence, the electric field in the E-region was extremely large (~150 mV/m). A significant increase in ion temperature was also observed 1 min before the arrival of a westward travelling surge (WTS) at the radar zenith. This was interpreted as the existence of an extended area of enhanced electric field ahead of the WTS.

Particle precipitation in auroral breakups and westward traveling surges

We have used data from the European Incoherent Scatter (EISCAT) facility to determine the characteristics of particle precipitation before and within auroral breakups and westward traveling surges. The EISCAT data of electron density versus altitude were transformed into energy spectra of the precipitating electrons [Kirkwood and Osepian, 1995]. We have used other ground‐based data such as auroral images and magnetic recordings to put the EISCAT observations into the context of the substorm development. In particular, we tried to determine the exact location of EISCAT with respect to the initial auroral forms of an onset. Characteristic differences are found in the deduced energy spectra of the precipitating electrons for the onset and the WTS events. Both the WTS and onset spectra showed a tendency of superposed spectral peaks at 10 keV and 20–30 keV respectively, which could be related to a near Earth acceleration mechanism. The conductance ratio in the onset events was twice as high as in the WTS events and also the measured particle fluxes were higher. We will discuss the implementations of our results for our understanding of particle acceleration mechanisms during the very onset of the substorm.

Current disruption in the Earth's magnetosphere: Observations and models

Observations and models of current disruption in the Earth's magnetosphere are briefly reviewed. At the approach of current disruption onset, the cross‐tail current sheet shows a rapid growth in the current density, a large upsurge in the duskward ion bulk speed to nearly the ion thermal speed, an increase in the plasma pressure and its isotropy, a rise in the plasma beta, and a decrease in the current sheet thickness to a length scale comparable to the thermal ion gyroradius. During current disruption, there are (1) large changes in the local magnetic and electric fields, (2) significant magnetic and electric fluctuations over a broad frequency range, (3) magnetic field‐aligned counterstreaming electron beams, (4) ion energization perpendicular to the magnetic field, and (5) reduction in the cross‐tail current by an amount similar to that built up during the growth phase. Observations further indicate that regions of local reversal of the north‐south magnetic field component are not necessarily sites of intense particle energization. Remote sensing of disruption activities shows that at least some current disruptions are not caused by a disturbance propagating earthward from the tail beyond 10 RE downstream. The timescale involved is comparable to or shorter than the ion gyroperiod. Current disruption thus has spatial and temporal scales outside the MHD regime. Several models for current disruption are briefly discussed. Two roles are considered for the cross‐field current instability proposed for current disruption. It can provide anomalous resistivity for magnetic reconnection as advocated by the traditional viewpoint or act singly to instigate global changes of the magnetosphere during the initial substorrn expansion phase. The latter role is elaborated by showing that the instability may modify significantly the local current density and any such process will alter the force equilibrium in the current sheet and give rise to an efficient plasma and energy transport on a global scale. Furthermore, such a process can generate field‐aligned current with intensity comparable to those associated with an auroral breakup arc at substorrn expansion onset. This scenario leads to a new emphasis that in addition to magnetic reconnection, rapid conversion of magnetic energy into particle energy in magnetotail systems may take place without a magnetic X line or separatrix playing the key role in energy conversion.