Poleward Boundary Intensifications Research Articles

Preonset time sequence of auroral substorms: Coordinated observations by all‐sky imagers, satellites, and radars

Using two conjunction events of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) imagers and spacecraft as well as the Super Dual Auroral Radar Network (SuperDARN) and Poker Flat Incoherent Scatter Radars, we show that longitudinally narrow flow bursts in the nightside polar cap can precede poleward boundary intensifications (PBIs) that are followed by equatorward moving north‐south (N‐S) arcs, including those leading to substorm onset instability within the near‐Earth plasma sheet. The association between the ionospheric flows and PBIs indicates that enhanced flows on open field lines may contribute to parallel potential drop increase, triggering of magnetotail reconnection, and to the earthward flows leading to N‐S arcs and to substorm onset. We also investigated differences between N‐S arc sequences that do and do not lead to substorm expansion onset. We found that the two types of N‐S arcs have similar characteristics, indicating that their corresponding plasma sheet flow properties could also be similar. There is, however, one difference between the sequences of N‐S arc evolution. Each N‐S arc leads to small intensification of the growth phase arc, and when the onset‐related N‐S arc reaches the equatorward portion of the auroral oval, the preexisting growth phase arc is much brighter than at the times of non‐onset‐related N‐S arcs. Assuming that the growth phase arc is related to pressure gradients at the inner edge of the plasma sheet, this difference indicates that the near‐Earth plasma pressure distribution at the time of plasma sheet fast flows is crucial in substorm triggering. These observations suggest that substorm onset instability is possible only when the preexisting inner plasma sheet pressure is sufficiently large.

Reconnection electric field estimates and dynamics of high-latitude boundaries during a substorm

Abstract. The dynamics of the polar cap and the auroral oval are examined in the evening sector during a substorm period on 25 November 2000 by using measurements of the EISCAT incoherent scatter radars, the north-south chain of the MIRACLE magnetometer network, and the Polar UV Imager. The location of the polar cap boundary (PCB) is estimated from electron temperature measurements by the mainland low-elevation EISCAT VHF radar and the 42 m antenna of the EISCAT Svalbard radar. A comparison to the poleward auroral emission (PAE) boundary by the Polar UV Imager shows that in this event the PAE boundary is typically located 0.7° of magnetic latitude poleward of the PCB by EISCAT. The convection reversal boundary (CRB) is determined from the 2-D plasma drift velocity extracted from the dual-beam VHF data. The CRB is located 0.5–1° equatorward of the PCB indicating the existence of viscous-driven antisunward convection on closed field lines. East-west equivalent electrojets are calculated from the MIRACLE magnetometer data by the 1-D upward continuation method. In the substorm growth phase, electrojets together with the polar cap boundary move gradually equatorwards. During the substorm expansion phase, the Harang discontinuity (HD) region expands to the MLT sector of EISCAT. In the recovery phase the PCB follows the poleward edge of the westward electrojet. The local ionospheric reconnection electric field is calculated by using the measured plasma velocities in the vicinity of the polar cap boundary. During the substorm growth phase, values between 0 and 10 mV/m are found. During the late expansion and recovery phase, the reconnection electric field has temporal variations with periods of 7–27 min and values from 0 to 40 mV/m. It is shown quantitatively, for the first time to our knowledge, that intensifications in the local reconnection electric field correlate with appearance of auroral poleward boundary intensifications (PBIs) in the same MLT sector. The results suggest that PBIs (typically 1.5 h MLT wide) are a consequence of temporarily enhanced longitudinally localized magnetic flux closure in the magnetotail.

Observation of an inner magnetosphere electric field associated with a BBF-like flow and PBIs

Abstract. Themis E observed a perpendicular (to the magnetic field) electric field associated with an Earthward plasma flow at XGSM=−9.6 RE on 11 January 2008. The electric field observation resembles Cluster observations closer to Earth in the auroral region. The fast plasma flow shared some characteristics with bursty bulk flows (BBFs) but did not meet the usual criteria in maximum velocity and duration to qualify as one. Themis C observed the same flow further downtail but Themis D, separated by only 1 RE in azimuthal direction from Themis E, did not. At the time of the electric field and ion flow event, the all-sky imager and ground-based magnetometer at Rankin Inlet observed Poleward Boundary Intensifications (PBIs) and a negative bay signature. None of the other Themis ground-based observatories recorded any significant auroral or magnetic field activity, indicating that this was a localized activity. The joint Themis in situ and ground-based observations suggest that the two observations are related. This indicates that auroral electric fields can extend to regions much farther out than previously seen in Cluster observations.

Periodic traveling compression regions during quiet geomagnetic conditions and their association with ground Pi2

Abstract. Recently, Keiling et al. (2006) showed that periodic (~90 s) traveling compression regions (TCRs) during a substorm had properties of Pi2 pulsations, prompting them to call this type of periodic TCRs "lobe Pi2". It was further shown that time-delayed ground Pi2 had the same period as the lobe Pi2 located at 16 RE, and it was concluded that both were remotely driven by periodic, pulsed reconnection in the magnetotail. In the study reported here, we give further evidence for this association by reporting additional periodic TCR events (lobe Pi2s) at 18 RE all of which occurred in succession during a geomagnetically very quiet, non-substorm period. Each quiet-time periodic TCR event occurred during an interval of small H-bay-like ground disturbance (<40 nT). Such disturbances have previously been identified as poleward boundary intensifications (PBIs). The small H bays were superposed by Pi2s. These ground Pi2s are compared to the TCRs in the tail lobe (Cluster) and both magnetic pulsations and flow variations at 9 RE inside the plasma sheet (Geotail). The main results of this study are: (1) Further evidence is given that periodic TCRs in the tail lobe at distances of 18 RE and ground Pi2 are related phenomena. In particular, it is shown that both had the same periodicity and occurred simultaneously (allowing for propagation time delays) strongly suggesting that both had the same periodic source. Since the TCRs were propagating Earthward, this source was located in the outer magnetosphere beyond 18 RE. (2) The connection of periodic TCRs and ground Pi2 also exists during very quiet geomagnetic conditions with PBIs present in addition to the previous result (Keiling et al., 2006) which showed this connection during substorms. (3) Combining (1) and (2), we conclude that the frequency of PBI-associated Pi2 is controlled in the outer magnetosphere as opposed to the inner magnetosphere. We propose that this mechanism is pulsed reconnection based on previous results which combined modeled results and observations of substorm-related periodic TCRs and ground Pi2. (4) We show that TCRs with small compression ratios (ΔB/B<1%) can be useful in the study of magnetotail dynamics and we argue that other compressional fluctuations with ΔB/B<1% (without having all of the characteristic signatures of TCRs) seen in the tail lobe could possibly be related to the same mechanism that generates TCR with ΔB/B>1% (which are more commonly studied). (5) Finally, it is noted that both quiet time and substorm-related periodic TCRs had remarkably similar periods in spite of the drastically different geomagnetic conditions prevailing during the events which poses the important question of what causes this periodicity under these different conditions.

PFISR observations of strong azimuthal flow bursts in the ionosphere and their relation to nightside aurora

Flow bursts within the ionosphere are the ionospheric signatures of flow bursts in the plasma sheet and have been associated with poleward boundary intensifications (PBIs). Some PBIs extend equatorward from the polar cap boundary, where they can be roughly divided into north–south-aligned and east–west-aligned structures. In this paper, we present two flow burst events observed by the new Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) in the pre-midnight auroral zone on 28 April 2007, one towards the west and the other towards the east. In both cases, enhanced flows lasted for about 8–10 min with peak velocities exceeding 1500 m/s. The concurrently measured electron density showed that the flow bursts occurred in low conductivity regions. However, near the poleward (equatorward) edge of the westward (eastward) flow burst, strong electron density enhancements were observed in the E region, indicating the presence of discrete auroral arcs. Auroral images from the Polar spacecraft were available at the time of the eastward flow burst and they indicate that this burst was associated with an east–west-aligned auroral structure that connected at later MLT to a north–south structure. In addition, simultaneous precipitating particle energy spectrum measured by the the Defense Meteorological Satellites Program (DMSP) F13 satellite reveals that this auroral structure resulted from mono-energetic electron precipitation associated with a significant field-aligned potential drop. These observations show direct evidence of the relationship between flow bursts, field-aligned currents and auroral intensifications, and suggest that eastward/westward flow bursts are associated with east–west-oriented PBI structures that have extended well within the plasma sheet. This is in contrast to the equatorward-directed flow that has been previously inferred for PBIs near the polar cap boundary and for north–south auroral structures. This paper illustrates the use of the PFISR radar for studying the magnetosphere–ionosphere coupling of flow bursts.

Relation of substorm onset to Harang discontinuity

In this study we investigate the relation of auroral substorm onset to the Harang discontinuity (HD). Various studies have reported that the substorm onset occurs poleward, equatorward, or within the HD. The motivation for the present study is to further investigate this relation with a much larger database. Using a database of over 3700 onsets determined from auroral images taken on the IMAGE spacecraft, we statistically examine the magnetic latitude location of the onset with respect to the location of the HD. The location of the discontinuity is determined with data from the IMAGE ground magnetometer network. We define the location of the HD as the transition from relatively strong eastward to relatively strong westward equivalent ionospheric currents at the latitude of the main nightside auroral electrojet flow. Our results show that the location of 75 onsets that occurred above the IMAGE ground array about 31% occurred within 6° of latitude of the HD observed during the growth phase of the substorm and 37% occur within 5° of latitude of the HD observed during the expansion phase. We also find that 23% of the onsets occur in association with vortices of ionospheric currents observed during the growth phase. The remaining 31% of the onsets do not appear to be associated with a HD or a vortex during the growth phase. Ten of the initial 75 onsets were identified as likely pseudo breakups and not fully developed substorms. Seven of the initial 75 onsets were probably poleward boundary intensifications (PBI). This study demonstrates that approximately 2/3 of the auroral substorm onsets do not occur within or near the HD identified in the growth phase. These results do not support the prediction made by Lyons et al. (2003) that substorm auroral onset should occur within the upward field aligned current in the Harang discontinuity.

Two classes of earthward fast flows in the plasma sheet

In this study we first identify earthward plasma sheet fast flows from Geotail plasma and magnetic fields with a criterion of V⊥x > 300 km/s, where V⊥x is the X component of the plasma flow perpendicular to the ambient magnetic field. We then estimate rates of change of the nightside auroral power over the courses of the fast flows using Polar Ultraviolet Imager auroral images. It is found that 68 earthward fast flows observed at ∣Y∣ < 6 RE during 1997–1998 can be classified into two classes. One class of the earthward fast flows (Class I) was often observed near X = ∼−10 RE and the other class (Class II) was found at X < −15 RE. The auroral power rates of change of the fast flows in Class I in terms of time are found to be high, indicating that the total auroral power on the nightside was significantly increasing during these fast flows. The corresponding auroral images show an apparent substorm bulge developed on the nightside, i.e., a significant global auroral development. The auroral power rates for most of the earthward fast flows in Class II are low. The auroral features, such as poleward boundary intensifications and pseudobreakups, are found to be associated with these fast flows. Some of the earthward fast flows in Class II can propagate earthward and provide a favorable condition for a substorm onset, leading to an auroral bulge developed on the nightside. We have also tested another criterion of VxBz > 2 mV/m for an identification of fast flows, where Vx is the X component of the plasma flow and Bz is the Z component of the geomagnetic field. It is found that V⊥x is a better parameter than VxBz to differentiate the two classes of earthward fast flows in the plasma sheet.

Response of the Earth's magnetosphere to changes in the solar wind

The solar wind couples to the magnetosphere via dynamic pressure and electric field. Pressure establishes the size and shape of the system, while the electric field transfers energy, mass, and momentum to the magnetosphere. When the interplanetary magnetic field (IMF) is antiparallel to the dayside magnetic field, magnetic reconnection connects the IMF to the dipole field. Solar wind transport of the newly opened field lines to the nightside creates an internal convection system. These open field lines must ultimately be closed by reconnection on the nightside. For many decades, it was thought that a magnetospheric substorm was the process for accomplishing this and that all magnetic activity was a consequence of substorms. It is now recognized that there are a variety of modes of response of the magnetosphere to the solar wind. In this paper, we briefly describe these modes and the conditions under which they occur. They include substorms, pseudo-breakups, poleward boundary intensifications (PBI), steady magnetospheric convection (SMC), sawtooth injection events, magnetic storms, high-intensity long-duration continuous AE activities (HILDCAAs), and storm-time activations. There are numerous explanations for these different phenomena, some of which do not involve magnetic reconnection. However, we speculate that it is possible to interpret each mode in terms of differences in the way magnetic reconnection occurs on the nightside.

Auroral poleward boundary intensifications (PBIs): Their two‐dimensional structure and associated dynamics in the plasma sheet

Auroral poleward boundary intensifications (PBIs) are typically seen both in ground meridian scanning photometers (MSP) and in ground and spacecraft auroral images. They appear as a localized increase in intensity at or near the magnetic separatrix. This increase is often seen to extend equatorward through the ionospheric mapping of the plasma sheet. PBIs are associated with plasma sheet flow bursts and are thus important for the remote monitoring of plasma sheet dynamics. From the study of simultaneous ground MSP observations, IMAGE FUV auroral images, and Geotail plasma sheet data, we find that PBIs correlate well with plasma sheet fast flows observed within the local time sector of the PBIs and that there can be several PBIs over the longitudinal range of fast flows in the tail. We infer that every north‐south PBI is the ionospheric signature of a fast flow channel in the plasma sheet and that many fast flow channels exist simultaneously over a width of the plasma sheet that can comprise the whole width of the plasma sheet or only a part of it. Also, we find that there is a local time dependence on the type of PBI structure. Most PBIs are narrow auroral structures that are not strictly north‐south oriented. Instead, PBIs are tilted counterclockwise away from the north‐south direction, leading to a preferred orientation that is approximately aligned with a line going from 0300 magnetic local time (MLT) to 1700 MLT. This results in PBIs that are closer to north‐south (NS) structures in the postmidnight sector and closer to east‐west (EW) near the dusk sector. In the premidnight sector (2200–0000 MLT), PBIs start as EW arcs and then tilt and become primarily NS structures. We further found for one event that the PBI fast flows have a large Vy component resulting in tail convection that is both earthward and dawnward in the region of Geotail. We suggest that the continuous and strongly positive interplanetary magnetic field (IMF) By may order the two‐dimensional convection as observed, thus offering a possible explanation for the alignment direction of PBIs in the ionosphere under the assumption that fast flow channels align themselves with the background convection. However, the projection of the PBI structures into the tail using the T96 model suggests that all PBIs, both EW and NS, map to radially stretched channels in the tail that do not have a significant dawnward component. Future work is needed to clarify this apparent contradiction. Finally, frequency analysis indicates that the PBI/bursty bulk flow (BBF) period is characterized by oscillations in the velocity and magnetic field with frequencies of ∼0.6 mHz and ∼1.3–1.5 mHz. This oscillation in velocity is superposed on the background strong convection.

Pi2 pulsations associated with poleward boundary intensifications during the absence of substorms

Pi2 pulsations during the intervals of extremely quiet geomagnetic conditions (Kp = 0) have been reported by Sutcliffe and Lyons [2002]. These authors observed that several Pi2 bursts occurred simultaneously at high (magnetic latitude = 71°) and low (42°) latitudes during the absence of magnetospheric substorms and found that the bursts are strongly correlated with poleward boundary intensifications (PBIs). The authors discussed the correlation between the PBI‐associated Pi2 (PBI‐Pi2) bursts and enhancements of energetic particle fluxes in the plasma sheet, but they did not focus on the wave properties of the PBI‐Pi2 pulsations. In this study we examine whether the PBI‐Pi2 pulsations at middle/low latitudes exhibit spatial variations similar to substorm‐associated Pi2 pulsations. Using ground‐based data from latitudinally and longitudinally extended magnetometer network and spacecraft data in the duskside, we investigate the spatial variation of the frequency, amplitude, phase, and interstation coherence of the PBI‐Pi2 events. We show that the PBI‐Pi2 pulsations in this study have different features at different local times and suggest that their period and duration are determined at a source region, where fast earthward flows brake.

Observations of the phases of the substorm

Following the database of large‐scale vortices during pseudo‐breakup and breakup registered by the Gillam All‐Sky Imager, we selected one event (19 February 1996) for a detailed consideration. This event is a sequence of pseudo‐breakup and local substorm, and breakup followed by the large substorm, which is isolated from the previous pseudo‐breakup by the second growth phase. Commencement of these elements of auroral activity was clearly seen above the Churchill line of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS; pseudo‐breakup was completely covered by the field of view of the Gillam All‐Sky Imager). Geotail was located at ∼19 RE in the equatorial plane of midnight sector, which, along with supporting observations from two geostationary satellites (GOES 8 and 9), allowed for a comparison of ground‐based, geostationary orbit and midtail signatures. The pseudo‐breakup consisted of two distinct stages: a near‐exponential arc intensity growth and a poleward vortex expansion that started simultaneously with dipolarization in the inner magnetosphere. The latter corresponded to explosive onset of short‐period (tens of millihertz) pulsations observed at geostationary orbit and on the ground in the vicinity of the arc. No significant disturbances poleward of the vortex were observed. Pseudo‐breakup was followed by the second growth phase, which involved a significant thinning of the plasma sheet. Breakup was of a similar two‐stage character as the pseudo‐breakup. Full onset of the expansive phase that followed breakup was seen simultaneously by all instruments including Geotail, which detected strong perturbations in the midtail. The expansive phase onset launched the second postbreakup package of Pi2 pulsations that were of larger amplitude. Finally, during the substorm recovery phase, the poleward boundary intensifications (PBIs) were observed as long‐period, on the order of 10 min, pulses of electron precipitation. PBI commencement coincided with bursty flows and pulses of plasma energization in the midtail. Observed features support recent ideas claiming that we are dealing with processes (breakup, full onset of the expansive phase, and PBIs) of a distinct physical nature that require different commencement thresholds, namely, the inner plasma sheet instability (pseudo‐breakup and breakup), midtail reconnection (expansive phase onset), and further magnetotail dynamics during the recovery phase (PBIs).

Two‐dimensional structure of auroral poleward boundary intensifications

We investigate the two‐dimensional structure of auroral poleward boundary intensifications (PBIs). PBIs are a nightside auroral intensification that has been studied primarily with ground‐based meridian scanning photometers (MSPs). They have a signature that in the MSP data, appears as an increase in intensity at or near the magnetic separatrix and is often seen to extend equatorward. They are also associated with fast flows in the tail and are thus important to the dynamics of the plasma sheet. MSP data provide information about the temporal evolution of the aurora in one spatial dimension, in this case roughly along a magnetic meridian. This paper is motivated by a desire to determine the physics of PBIs that is revealed by their two‐dimensional structure. To do this, we have identified a number of PBI events in the CANOPUS Rankin Inlet and Gillam MSPs that occurred at times when high‐resolution, two‐dimensional images of the aurora over the same region were also available. The two‐dimensional images used in this study were obtained by the Freja UV imager, from October 1992 to January 1993, and by the CANOPUS Gillam all‐sky imager during the winter viewing season of 1996–1997. We find that PBIs, as observed by the MSPs, are either equatorward extending or non‐equatorward extending. Equatorward extending PBIs are either north‐south aligned structures or east‐west arcs propagating mostly equatorward, but we were not able to determine without doubt which type is the most prevalent. We suggest that equatorward extending PBIs may be the auroral footprint of two major modes of energy transfer in the plasma sheet: multiple, narrow, earthward fast‐flow channels in the plasma sheet and sequences of azimuthally broad and primarily earthward propagating phase fronts initiating near the separatrix. Nonequatorward extending PBIs are found to mostly be a series of multiple bead‐like intensifications along the poleward boundary of the aurora zone. Such PBIs may be evidence for shear instabilities at the separatrix boundary on the flanks of the magnetotail.

Multistage substorm expansion: Auroral dynamics in relation to plasma sheet particle injection, precipitation, and plasma convection

We present observations of the auroral expansions during two substorms, focusing on multistage intensifications and the morphology of the poleward boundary, and relate these auroral observations to the local plasma convection and plasma sheet dynamics. The observations are made by meridian scanning photometers and an all‐sky camera (ASC) at Ny Ålesund, Svalbard (76° magnetic latitude (MLAT)), an ASC in Lovozero, Russia (64° MLAT), the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer chain in Svalbard and Scandinavia, the HYDRA instrument on Polar located at the inner edge of the plasma sheet, particle detectors on DMSP F13 and DMSP F14 traversing the ionospheric projection of the plasma sheet, and the CUTLASS Finland HF radar. In each substorm the aurora between 70° and 80° MLAT consisted of two branches separated by ∼5° in MLAT. The higher‐latitude branch (at ∼75°–78°MLAT) was subject to a sequence of short‐lived (∼1–2 min) intensifications, so‐called “poleward boundary intensifications” (PBIs), recurring at ∼3‐min intervals. Subsequent to each brightening, auroral forms traveled equatorward at a speed of ∼1.0–1.5 km s−1. On Polar the PBIs are related on a one‐to‐one basis with injections of electrons in the 5‐ to 20‐keV energy range at the inner edge of the equatorial plasma sheet with predominantly a trapped distribution, delayed by ∼5 min. Electron precipitation within 60°–77° MLAT, corresponding to a large radial extent of the plasma sheet, is documented by DMSP flights in the 1800–2000 magnetic local time (MLT) sector. In discussing the branches of the high‐latitude aurora within the context of current understanding of the relation of bursty bulk flows to substorm expansion phase dynamics, we note the following: (1) the initial auroral breakup located at 63°–64° MLAT near the equatorward edge of plasma sheet precipitation, which was followed by (2) two successive brightenings/auroral expansions appearing within 72°–74° MLAT/∼2100 MLT, separated by 14 min, (3) a 20‐min‐long brightening sequence in the poleward auroral branch (75°–78° MLAT), consisting of six discrete events (PBIs) within the boundary plasma sheet precipitation, and (4) the presence of auroral vortex motion/strong field‐aligned current sheets in some of these PBIs, which were accompanied by (5) electron injections at the inner edge of the plasma sheet, (6) brightenings of the lower‐latitude auroral branch when equatorward moving auroral forms (EMAFs/streamers) arrive there, and (7) localized bursts of equatorward ionospheric convection at speeds of 0.5–1 km s−1 in the latitude range of the EMAFs/streamers. The documented associations between PBIs/EMAFs, plasma sheet injections, and the local convection events are explained in terms of a substorm scenario involving bursty bulk flows in the late expansion phase.

Auroral poleward boundary intensifications and tail bursty flows: A manifestation of a large‐scale ULF oscillation?

Auroral zone observations often show significant ULF power. We have analyzed auroral and plasma sheet observations during two prolonged periods of strongly southward and relatively steady interplanetary magnetic field (IMF). We find evidence that auroral poleward boundary intensifications (PBIs), which have large intensity and occur repetitively throughout such periods, may be a manifestation of a large‐scale ULF oscillation mode that strongly perturbs the plasma sheet and the auroral ionosphere. If this is correct, then ULF modes would be a major component of tail dynamics, of magnetosphere coupling to the ionosphere, and of auroral zone disturbances during periods of enhanced convection. They would simultaneously affect a large region of the nightside, extending along auroral zone field lines from the ionosphere to the equatorial plasma sheet and extending from field lines that lie near the magnetic separatrix to, at times, as close to the Earth as synchronous orbit. They would also occasionally have amplitudes as large as the changes that occur in association with other auroral zone disturbances such as substorms. Here we have found peak‐to‐peak amplitudes as high as several hundred nanoteslas in ground X, an order of magnitude in synchronous energetic proton fluxes, ∼20–40 nT in synchronous magnetic field components, ∼20 nT in tail magnetic field components, ∼1000 km/s in tail flow speeds, and ∼400 m/s in ionospheric flow speed. We find evidence for significant power at 0.5–0.7 mHz (∼25–30 min period), significant power at a possible second harmonic (∼1.1–1.3 mHz), and power at frequencies that could be higher harmonics simultaneously within the auroral ionosphere and within the nightside plasma sheet.

Association between quiet‐time Pi2 pulsations, poleward boundary intensifications, and plasma sheet particle fluxes

Pi2 pulsations have been observed to occur at low latitudes during extremely quiet conditions. We have found an excellent correlation between an occurrence of such a Pi2 train and high‐latitude Pi2 pulsations and bay‐like disturbances in ground magnetic field of the type associated with poleward boundary intensifications (PBIs). The Pi2s and the PBIs that are inferred to occur simultaneously, show a ∼30 min repetition rate. They also exhibit an excellent correlation with energetic particle enhancements in the near Earth tail plasma sheet. These associations demonstrate the large‐scale nature of the magnetosphere‐ionosphere disturbance that gives rise to PBIs and shows that the disturbance may have important effects on energetic plasma sheet particles. They show for the first time that PBIs are associated with clearly observable Pi2 pulsations at low latitudes. Our observations are consistent with the possibility that PBIs are part of a large‐scale magnetospheric oscillation with ∼30 min period.

Auroral disturbances during the January 10, 1997 magnetic storm

It is well known that intense and frequent auroral‐zone disturbances, often attributed to substorms, occur during magnetic storms. We examine observations during the January 10, 1997 main phase and find that observed auroral‐zone activity was dominated by a combination of global auroral and current enhancements, which are a direct response to solar wind dynamic pressure enhancements, and poleward boundary intensifications, which are localized in longitude and have an auroral signature that moves equatorward from the magnetic separatrix. Poleward and azimuthally expanding regions of auroral activity which accompany substorms are found to contribute significantly less to the observed activity. This suggests that poleward boundary intensifications and dynamic pressure responses may be an important cause of disturbances during periods of enhanced convection such as magnetic storms and convection bays.

The auroral signature of earthward flow bursts observed in the magnetotail

We present direct evidence that transient Earthward flow bursts in the magnetotail can produce an observable signature in the optical aurora. This signature is north‐south aligned auroral structures that are extensions of transient intensifications near the poleward boundary of the auroral oval. Our study focuses on the period from 0500 to 0700 UT on January 7, 1997, during which five distinct flow bursts are observed in the Geotail data. At that time, the spacecraft was located approximately 30 RE downtail on field lines that project down to the CANOPUS array of ground based instruments. We find that each of the flow bursts seen in the Geotail data is associated with an auroral poleward boundary intensification (PBI) observed in the CANOPUS meridian scanning photometer (MSP) data, which appears as a north‐south aligned auroral structure in the CANOPUS all‐sky imager (ASI) data. Based on these observations we estimate that the fast flows originated between 50 and 100 RE downtail.

Geomagnetic disturbances: characteristics of, distinction between types, and relations to interplanetary conditions

This tutorial emphasis disturbances in auroral emissions and ionospheric currents and their relation to interplanetary conditions and the overall level of geomagnetic activity. Auroral zone disturbances are divided into three fundamentally different types: poleward boundary intensifications (PBIs), substorms, and effects of solar wind dynamic pressure enhancements. The most common type of auroral-zone disturbance is the PBI, which occurs during all levels of geomagnetic activity. PBIs have an auroral signature that often can be seen to move equatorward from the magnetic separatrix. They are typically associated with ground magnetic perturbations of few tens of nT, but perturbations can be as high as ∼500 nT . Individual PBIs are longitudinally localized, associated with the longitudinally localized flow bursts in the tail plasma sheet, and occasionally traverse essentially the entire latitudinal extent of the plasma sheet. PBIs appear to generally be the dominant type of auroral-zone disturbance during periods of enhanced magnetospheric convection, including the growth phase of substorms, convection bays, and the main phase of magnetic storms. Substorms are a far more dramatic and large scale, but far less common, disturbance than PBIs. They occur after a ≳30 min growth-phase period of enhanced convection. It is now known that at least ∼50% of substorms are associated with IMF changes that lead to a reduction in the strength of convection. However, it has not yet been shown whether or not all are most substorm onsets are caused by these types of IMF changes. Auroral activity during substorms typically initiates within a ∼1–2 h MLT sector near the equatorward boundary of the auroral oval and then expands both poleward and azimuthally. Substorms are associated with ground magnetic disturbances that range from ∼50 to ∼2000 nT , a reduction in strength of the cross-tail current, a poleward displacement of the inner edge of the plasma sheet, and a large release of plasma and magnetic field energy from the region earthward of the new inner edge of the plasma sheet. The reduction of cross-tail current is also believed to often be associated with a severance, and loss from the magnetotail, of the outer portion of the plasma sheet ( r≳25 R E). Recent studies have shown that solar wind dynamic pressure increases caused large auroral-zone disturbances during a stormtime period of strongly enhanced convection, affecting the poleward boundary, latitudinal width, and intensity of the auroral oval. Dynamic pressure increases also appear to also enhance the entire magnetospheric current system, including the magnetopause, cross-tail, region 1 field-aligned, and global ionospheric currents. Thus, in addition to PBIs, significant variations in solar wind dynamic pressure should be considered as a possibly important source of geomagnetic disturbances during periods of enhanced magnetospheric convection.

Global storm time auroral X‐ray morphology and timing and comparison with UV measurements

The Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on NASA's Polar spacecraft provides the first global images of the auroral oval in X‐rays and allows very accurate measurements of the timing of geomagnetic disturbances to a degree of temporal resolution not available from previous imagers due to its photon counting characteristics. On October 19, 1998, a magnetic cloud associated with a CME encountered the Earth's magnetopause near 0500 UT, generating a magnetic storm that reached a minimum value in Dst of −139 nT. The z component of the interplanetary magnetic field (IMF) (Bz) remained remarkably steady for the first 10 hours of the storm as did the solar wind particle pressure. The PIXIE and UVI instruments on the Polar spacecraft were both imaging the auroral oval from 0800 to 1800 UT; six distinct impulsive auroral enhancements were observed by the imagers during this time period. Global imaging combined with geosynchronous particle observations allowed classification of the geomagnetic disturbances associated with the events. Only two of the events were classified as substorms; one was classified as a poleward boundary intensification, one was a convection bay, and one was a pseudobreakup. A sixth event occurred after a dramatic northward turning of the IMF at the end of the 10‐hour Bz south period but was very weak and transient. The effects of the northward turning were counteracted by a simultaneous increase in the By component of the IMF. The first sign of significant substorm activity occurred over 8 hours after the cloud encountered the Earth and was not associated with any change in the solar wind magnetic field or particle pressure. The cross polar cap potential remained large (> 100 kV), and most of the X‐ray emissions observed were associated with enhanced earthward convection caused by large cross‐tail electric fields; 50% were collected from the 0000 – 0600 magnetic local time (MLT) sector.

Association between Geotail plasma flows and auroral poleward boundary intensifications observed by CANOPUS photometers

Poleward boundary intensifications are nightside geomagnetic disturbances that have an auroral signature that moves equatorward from the poleward boundary of the auroral zone. They occur repetitively, so that many individual disturbances can occur during time intervals of ∼1 hour, and they appear to be the most intense auroral disturbance at times other than the expansion phase of substorms. We have used data from three nightside conjunctions of the Geotail spacecraft in the magnetotail with the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) ground‐based array in central Canada to investigate the relation between the poleward boundary intensifications and bursty plasma sheet flows and to characterize the bursty flows associated with the disturbances. We have found a distinct difference in plasma sheet dynamics between periods with, and periods without, poleward boundary intensifications. During periods with identifiable poleward boundary intensifications, the plasma sheet has considerable structure and bursty flow activity. During periods without such poleward boundary intensifications, the plasma sheet was found to be far more stable with fewer and weaker bursty flows. This is consistent with the intensifications being the result of the mapping to the ionosphere of the electric fields that give rise to bursty flows within the plasma sheet. Two different types of plasma sheet disturbance have been found to be associated with the poleward boundary intensifications. The first consists of plasma sheet flows that appear to be the result of Speiser motion of particles in a localized region of thin current sheet. The second, seen primarily in our nearest‐to‐the‐Earth example, consists of energy‐dispersed ion structures that culminate in bursts of low‐energy ions and isotropic low‐energy electrons and are associated with minima in magnetic field and temperature and maxima in ion density and pressure. Both types of plasma sheet disturbance are associated with localized regions of enhanced dawn‐to‐dusk electric fields and appear to be associated with localized enhanced reconnection. Our analysis has shown that poleward boundary intensifications are an important aspect of geomagnetic activity that is distinct from substorms. In addition to their very distinct auroral signature, we have found them to be associated with a prolonged series of ground magnetic Pi 2 pulsations and ground X component perturbations, which peak at latitudes near the ionospheric mapping of the magnetic separatrix, and with a series of magnetic Bz oscillations near synchronous orbit. Like substorms, the tail dynamics associated with the poleward boundary intensifications can apparently extend throughout the entire radial extent of the plasma sheet. Color versions of figures are available at http://www.atmos.ucla.edu/∼larry/geotail.html.