Convection Bays Research Articles

Superposed epoch study of PV5/3 during substorms, pseudobreakups and convection bays

We present a superposed epoch study to examine the entropy parameter PV5/3, for isolated substorms, pseudobreakups and convection bays observed by Geotail in the near‐Earth plasma sheet, by using a flux‐tube‐volume formula with coefficients adjusted to fit magnetic field models for a variety of conditions. It is found that the changes of PV5/3 for these three types of events are distinct: (1) for substorms, PV5/3 increases continuously during the growth phase, decreases after dipolarization, and maintains a low value of ∼0.08 nPa(RE/nT)5/3; (2) for pseudobreakups, PV5/3 changes in a way similar to substorms, but decreases more moderately and then gradually increases; (3) for convection bays, PV5/3 takes a nearly constant value of 0.06 nPa(RE/nT)5/3. These results suggest that the characteristics of the processes that violate conservation of PV5/3 play a central role in the temporal and spatial reduction of PV5/3 and thus determine the earthward plasma transport during active times.

High‐time resolution dayside convection monitoring by incoherent scatter radar and a sample application

Convection and its changes, driven by the solar wind, are related to geomagnetic disturbances such as substorms, dynamic pressure disturbances and convection bays. Furthermore, convection is theoretically related to the evolution of the Harang electric field reversal (or “discontinuity”), which has been related observationally and theoretically to these disturbances. Interplanetary measurements generally used for studying and making associations of convection with disturbances are acceptable for statistical studies, but spatial structure and propagation timing errors due to the highly variable orientation of interplanetary structures create significant ambiguity for case studies and have prevented definitive understanding. Moreover, small but geoeffective structure in the solar wind may be missed by even multispacecraft observations. Direct observation of dayside convection with high time resolution offers a potential solution to this problem. In this paper, we demonstrate that Sondrestrom incoherent scatter radar (ISR) observations have the ability to accurately monitor the dayside convection with ∼2.5 min temporal resolution. The observations clearly show the dayside convection responses to IMF Bz, By and solar wind dynamic pressure variations and thus shed new light on how the magnetosphere and ionosphere respond to different external driving. Then, we illustrate an application of this technique by showing how it can be applied to study substorm triggering. We plan to use this technique in the future to study the relationships between convection, other geomagnetic disturbances, and the Harang reversal, as well as to evaluate the preceding convection strength and duration and the convection changes that lead to substorm onset.

Corotating solar wind streams and recurrent geomagnetic activity: A review

Solar wind fast streams emanating from solar coronal holes cause recurrent, moderate intensity geomagnetic activity at Earth. Intense magnetic field regions called Corotating Interaction Regions or CIRs are created by the interaction of fast streams with upstream slow streams. Because of the highly oscillatory nature of the GSM magnetic field z component within CIRs, the resultant magnetic storms are typically only weak to moderate in intensity. CIR‐generated magnetic storm main phases of intensity Dst < −100 nT (major storms) are rare. The elongated storm “recovery” phases which are characterized by continuous AE activity that can last for up to 27 days (a solar rotation) are caused by nonlinear Alfven waves within the high streams proper. Magnetic reconnection associated with the southward (GSM) components of the Alfvén waves is the solar wind energy transfer mechanism. The acceleration of relativistic electrons occurs during these magnetic storm “recovery” phases. The magnetic reconnection associated with the Alfvén waves cause continuous, shallow injections of plasma sheet plasma into the magnetosphere. The asymmetric plasma is unstable to wave (chorus and other modes) growth, a feature central to many theories of electron acceleration. It is noted that the continuous AE activity is not a series of substorm expansion phases. Arguments are also presented why these AE activity intervals are not convection bays. The auroras during these continuous AE activity intervals are less intense than substorm auroras and are global (both dayside and nightside) in nature. Owing to the continuous nature of this activity, it is possible that there is greater average energy input into the magnetosphere/ionosphere system during far declining phases of the solar cycle compared with those during solar maximum. The discontinuities and magnetic decreases (MDs) associated with interplanetary Alfven waves may be important for geomagnetic activity. In conclusion, it will be shown that geomagnetic storms associated with high‐speed streams/CIRs will have the same initial, main, and “recovery” phases as those associated with ICME‐related magnetic storms but that the interplanetary causes are considerably different.

Substorm and convection bay compared: Auroral and magnetotail dynamics during convection bay

Using observations from eight spacecraft and a ground network, we study two subsequent bay‐like disturbances on December 10, 1996, initiated by southward interplanetary magnetic field intervals, one being a classic substorm and another one a convection bay. Both events showed enhanced convection and Dst decreases as well as Pi2 pulsations in the auroral zone. Contrasting to the well‐defined substorm signatures of the first event (poleward auroral expansion, substorm current wedge, strong particle injection to 6.6 RE) resulting from energy loading/unloading and near‐Earth reconnection in the tail, these signatures were virtually absent during the convection bay (CB). Distinctive features of the CB event were the same as those during the Steady Magnetospheric Convection intervals: (1) wide double oval at the nightside; (2) thick plasma sheet, relaxed lobe field, and enhanced magnetic flux closure (large Bz) and multiple bursty earthward flows (BBFs) in the midtail; (3) sporadic narrow soft injections to 6.6 RE; (4) auroral streamers associated with both BBFs and narrow injections. We emphasize the development of multiple and sporadic auroral streamers which start at the poleward oval boundary, propagate equatorward (in 3–8 min) and end with a long‐duration bright spot in the equatorward oval. We conclude that the plasma sheet and auroral dynamics during the convection bay was formed by sporadic narrow (a few RE wide) plasma streams (plasma bubbles) which transported the plasma sheet material from the distant magnetotail reconnection regions to the inner magnetosphere and may significantly contribute to the magnetospheric circulation on the nightside. We modeled the nightside tail configuration using magnetotail magnetic observations and low‐altitude particle boundaries to show that at the beginning of the convection bay the increase of magnetic flux tube volume with distance was small in the midtail. Therefore the “pressure crisis” in the tail was significantly reduced during the convection bay, and the efficient earthward transport by sporadic narrow plasma streams was probably able to balance the magnetospheric circulation to avoid the large‐scale instability of the magnetotail.

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.

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.

A scenario covering all phases of nightside convection with the roles of ion drifts and reconnection determined from the observed field‐aligned currents

The rate of transfer of magnetic flux into the nightside dipole‐like region is a function of the field‐aligned current (FAC) intensity and Nt–Nd, where Nt and Nd are the total number of plasma sheet ions on flux tubes in the near‐Earth tail and just inside the dipole‐like region. In addition, for slowly varying convection, the FAC intensity is a function of the magnetic flux in the tail. The above two relationships provide the basis for a scenario for convection. Convection bays and steady state occur if there is an ample supply of flux tubes with a small value of Nt in the near‐Earth tail. The growth phase occurs when a major increase in dayside reconnection leads to an increase in the amount of tail flux, which in turn causes increased convection into the nightside dipolar region. The plasma sheet thins until the predipolarization phase starts (reconnection or some similar process). The reconnection provides magnetic flux with reduced Nt, and also produces a change in the field‐aligned currents, both of which increase the dipolarization rate after electrical coupling with the ionosphere is established. At that time, either a pseudobreakup or a substorm onset will occur. The recovery phase is a convection toward a steady state, in which the plasma undergoes redistribution in latitude, local time, radial distance, and energy. It can be either fairly localized or magnetosphere‐wide, depending on the changes that have occurred in the dayside reconnection rate.

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.

Current sheet measurements within a flapping plasma sheet

Slow plasma flows perpendicular to the magnetotail neutral sheet and associated with flapping motions of the plasma sheet can occasionally exceed the sensitivity threshold of plasma instruments. This is the case in several events selected from 3 months of magnetotail observations by the three‐dimensional plasma instrument on the AMPTE/IRM satellite. By comparing the ion flow in the ZGSM direction and the correlated changes of the XGSM‐directed magnetic field, we estimate the current sheet flapping velocity to range from a few tens to a few hundreds of kilometers per second. Our measurements of the current sheet thickness (0.1 – 1 RE) and current density (10–30 nA/m2) were significantly different from estimates of these parameters based on magnetospheric models, suggesting a stretched field configuration. The observed events took place during geomagnetically disturbed periods. One of those periods was a 2‐hour‐long convection bay interval. The largest flapping flows were observed during the few events which also exhibited high‐speed Earthward flows, but the flapping velocity was smaller during the more common, slow convection intervals. This observation, as well as the unexpectedly large flapping velocities seen as close as 12 RE (i.e., close to the hinge point), suggests that flapping motions originate near the current sheet. Plasma sheet flapping tends to occur in conjunction with ground Pi2 bursts, but the exact onset times and the dominant periods of the two phenomena are generally different. Thus, although a common energy source of both phenomena may be intermittent current disruption/reconnection, the two phenomena are likely to couple differently to that free‐energy source. We discuss the vertical structure of the plasma sheet parameters during flapping plasma sheet episodes.

Spontaneous substorm onset during a prolonged period of steady, southward interplanetary magnetic field

We document a clear case of spontaneous substorm onset under conditions of steady southward interplanetary magnetic field on January 22, 1993. The substorm occurred at 0415 UT with an intensification at 0455 UT during an interval of convection bay activity but had most of the features associated with substorms. These features include high‐latitude and midlatitude magnetic bays at ground magnetometers, and magnetic field compression and energetic particle injection at geosynchronous altitude. In addition, a plasmoid release was observed in the distant tail plasma sheet by the Geotail spacecraft at a GSM position (−96, 1, −6) RE. We conclude that spontaneous‐onset substorms occur occasionally even during steady convection bay events. Such substorms and “classical” substorms progress in a very similar manner, irrespective of possible differences in their onset mechanism.

Large-scale fields and flows in the magnetosphere-ionosphere system

Advances in our understanding of the large-scale electric and magnetic fields in the coupled magnetosphere-ionosphere system are reviewed. The literature appearing in the period January 1991–June 1993 is sorted into 8 general areas of study. The phenomenon of substorms receives the most attention in this literature, with the location of onset being the single most discussed issue. However, if the magnetic topology in substorm phases was widely debated, less attention was paid to the relationship of convection to the substorm cycle. A significantly new consensus view of substorm expansion and recovery phases emerged, which was termed the ‘Kiruna Conjecture’ after the conference at which it gained widespread acceptance. The second largest area of interest was dayside transient events, both near the magnetopause and the ionosphere. It became apparent that these phenomena include at least two classes of events, probably due to transient reconnection bursts and sudden solar wind dynamic pressure changes. The contribution of both types of event to convection is controversial. The realisation that induction effects decouple electric fields in the magnetosphere and ionosphere, on time scales shorter than several substorm cycles, calls for broadening of the range of measurement techniques in both the ionosphere and at the magnetopause. Several new techniques were introduced including ionospheric observations which yield reconnection rate as a function of time. The magnetospheric and ionospheric behaviour due to various quasi-steady interplanetary conditions was studied using magnetic cloud events. For northward IMF conditions, reverse convection in the polar cap was found to be predominantly a summer hemisphere phenomenon and even for extremely rare prolonged southward IMF conditions, the magnetosphere was observed to oscillate through various substorm cycles rather than forming a steady-state convection bay.

Substorm signatures at synchronous altitude

The signatures of magnetospheric substorms near 6.6 RE are statistically examined by using data obtained on board ATS 6 by magnetic field and energetic particle measurements. It was confirmed that the configuration change toward a taillike field in the dusk‐to‐midnight sector typically begins about one hour before the onset of the expansion phase of a substorm. Configuration changes before the onsets of moderate substorms are characterized by a directional change of magnetic field at magnetic latitude of ∼10°, rather than a change in the field magnitude. During storm periods it was found that the magnetic field orientation occasionally becomes almost parallel to the magnetic equator before expansion phase onsets. In such cases, the field magnitude usually increases above background levels. These facts strongly suggest that a taillilke configuration is caused by an intensification and earthward motion of the cross‐tail current system, often close to 6.6 RE, than the development of diamagnetic ring current. The degree of taillike field distortion is statistically correlated with a subsequent field recovery during the expansion phase. Namely, a large field recovery at ATS 6 and associated large positive bays on the ground tend to follow a pronounced deviation from a quiet field level at ATS 6. The examination of ground magnetograms indicates that the development of Sqp or convection bay type current system is associated with the taillike field configuration. These results are discussed in term of a phenomenological model of magnetospheric substorms including the precursory or growth phase.

Multiple‐satellite studies of magnetospheric substorms: Distinction between polar magnetic substorms and convection‐driven negative bays

It is generally believed that long intervals of intense (500–1000 γ) negative auroral zone bays are caused by magnetospheric substorm occurrences at a high repetition rate. Since individual substorm phases may then overlap in time and occur within limited local time sectors, it may be difficult to identify their characteristic signatures. In this paper we have examined ground and multiple‐satellite recordings during an ∼5‐hour interval of persistent auroral zone activity in a systematic search for substorm expansion signatures. It appears that this enduring activity started as a rather weak substorm which thereafter developed into strong magnetospheric activity driven by a continuously southward interplanetary magnetic field (IMF). During this period there were no clear mid‐latitude positive bays, and pulsation recordings from different local time sectors showed only one instance of weak Pi 2‐like variations. The associated high‐latitude equivalent ionospheric current pattern is consistent with a convection system, and we refer to this activity as the convection bay. Following the plasma sheet and ground signatures of substorm recovery early in the convection bay, there was no definite indication of substorm expansions other than this weak magnetic pulsation in any recordings during the next ∼3½ hours until a clear substorm emerged. This substorm onset, which was associated with a not particularly strong auroral zone bay, was accompanied by the full complement of substorm expansion signatures. At this time a minimum plasma sheet thickness of only a few tenths of an earth radius was observed in the region where a neutral line appears to form during substorms. Prior to this onset, the plasma sheet was relatively thick, and it seems that the neutral line was located well tailward of the Vela orbit (r ∼ 18 RE). This event and other similar observations suggest that continuous auroral zone activity during southward IMF, such as a geomagnetic storm period, may be driven by a steady state magnetospheric convection. The long absence of clear substorm onset signatures may indicate that the nonsteady enhancements of convection associated with localized substorm expansions then make only small contributions to the total convection pattern. Similarly, the ground magnetic disturbances may then be caused mainly by Sqp currents and enhanced currents due to strong particle precipitation rather than by substorm‐associated current wedges. During this quasi‐steady state the tail reconnection rate appears nearly to balance the dayside merging rate, allowing most energy extracted from the solar wind to be released continuously rather than to be first stored in the tail. It seems that a large‐scale substorm expansion may be triggered if this convection mode is perturbed, for instance, by a significant reduction of the imposed convection electric field.