Bay Onset Research Articles

Association of Equatorward Extending Auroral Streamers With Ground Magnetic Perturbations and Geosynchronous Injections

AbstractAuroral streamers have been found to be a particularly important cause of magnetic perturbations within the auroral oval during substorms. We present a detailed investigation on the association of equatorward extending auroral streamers with ground magnetic perturbations at locations initially equatorward of the auroral oval using Time History of Events and Macroscale Interactions imagers and ground magnetometers. Based on the analysis of four events, we show that negative perturbations in the horizontal (H) component at initially subauroral latitudes were not seen prior to streamer onset, and the peak negative H‐bay at subauroral latitudes coincided with the equatorward extending auroral streamers. Although stations close to the poleward boundary of auroral oval showed modest H‐component perturbations in response to the substorm onset and poleward auroral intensifications, the abrupt and large responses within the oval were associated with the onset of streamers. Results confirm that ground magnetic perturbations that are traditionally perceived as signatures of magnetic substorm onset are not always an auroral substorm onset but can also be related to auroral streamers. These results are supported by the remarkable correlation of positive bay onset and sudden Pi2 enhancement with the onset of auroral streamers, irrespective of the classical substorm onset. In addition, the magnitude of positive bay and amplitude of Pi2 pulsations, particularly at subauroral latitudes, are enhanced during the events of equatorward extending streamers. In general, all streamer events were accompanied by dipolarization and electron flux enhancement at geosynchronous orbit that continue to strengthen during the emergence of bright streamers.

Thin Current Sheet Behind the Dipolarization Front

AbstractWe report a unique conjugate observation of fast flows and associated current sheet disturbances in the near‐Earth magnetotail by MMS (Magnetospheric Multiscale) and Cluster preceding a positive bay onset of a small substorm at ∼14:10 UT, September 8, 2018. MMS and Cluster were located both at X ∼ −14 RE. A dipolarization front (DF) of a localized fast flow was detected by Cluster and MMS, separated in the dawn‐dusk direction by ∼4 RE, almost simultaneously. Adiabatic electron acceleration signatures revealed from the comparison of the energy spectra confirm that both spacecraft encounter the same DF. We analyzed the change in the current sheet structure based on multi‐scale multi‐point data analysis. The current sheet thickened during the passage of DF, yet, temporally thinned subsequently associated with another flow enhancement centered more on the dawnward side of the initial flow. MMS and Cluster observed intense perpendicular and parallel current in the off‐equatorial region mainly during this interval of the current sheet thinning. Maximum field‐aligned currents both at MMS and Cluster are directed tailward. Detailed analysis of MMS data showed that the intense field‐aligned currents consisted of multiple small‐scale intense current layers accompanied by enhanced Hall‐currents in the dawn‐dusk flow‐shear region. We suggest that the current sheet thinning is related to the flow bouncing process and/or to the expansion/activation of reconnection. Based on these mesoscale and small‐scale multipoint observations, 3D evolution of the flow and current‐sheet disturbances was inferred preceding the development of a substorm current wedge.

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

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

Polar-equatorial ionospheric currents driven by the region 2 field-aligned currents at the onset of substorms

[1] The dawn-to-dusk convection electric field increases during the growth phase of substorms, driving DP2 currents composed of two-cell current vortices in high latitude and leading to an increase in the eastward electrojet at the dayside dip equator (EEJ). During the expansion phase of substorms, electric field and currents are often reversed in direction to the normal DP2 currents at subauroral to equatorial latitudes when the convection electric field reduces abruptly. The reversed current at the dayside dip equator appears as a counterelectrojet (CEJ) and causes an equatorial enhancement of the negative bay in the afternoon sector. Conversely, the convection electric field increases during substorms as deduced from sawtooth events, causing an increase in the EEJ. In this study, by analyzing isolated substorms with magnetometer array data and SuperDARN (Super Dual Auroral Radar Network) convection maps, we deduce that the reversed electric field and currents develop at the subauroral-to-equatorial latitudes at the onset of substorms, while the convection electric field increases at auroral latitudes. These observations imply that both the region 1 and region 2 field aligned currents (R1 and R2 FACs) develop on the dayside concurrently with the current wedge responsible for the midlatitude positive bay at midnight. The substorm-associated R2 FACs are strong enough to cause the reversed current at the subauroral latitude and the CEJ at the equator. We also deduce that the dayside equatorial CEJ begins simultaneously with or even earlier than the midlatitude positive bay at midnight while the midlatitude positive bay onset is delayed by several minutes as the station departs from the midnight meridian. These observations suggest that the substorm begins with the intensification of the R2 FACs responsible for the equatorial CEJ.

Magnetic cloud boundary layer of 9 November 2004 and its associated space weather effects

On 9 November 2004, the WIND spacecraft detected a magnetic cloud boundary layer (MCBL) during the interval from 19:07 UT to 20:30 UT. Within the MCBL, there is intense southward magnetic field and the dynamic pressure is rather high, which makes it much geoeffective. Twenty‐three minutes later, the MCBL arrived on the magnetopause. An intense geomagnetic storm main phase was driven by the sustaining strong southward magnetic field within the MCBL. During the passage of the MCBL, a typical magnetospheric substorm was triggered. The substorm onset was synthetically identified by the aurora breakup, magnetic dipolarization, dispersionless particle injection, Pi2 pulsation, and the polar bay onset. The substorm triggering is related to the special magnetic and plasma structure within the MCBL. The MCBL accompanying adjacent sheath region formed a dynamic pressure enhancement region, which strongly compressed the magnetosphere and even pushed the magnetopause into the geosynchronous orbit so that two dayside spacecraft GOES‐10 and GOES‐12 were directly exposed in the magnetosheath for a long interval during the passage of the MCBL. In terms of Shue et al. (1998) model, the closest subsolar standoff distance even reached 5.1 RE during the passage of the MCBL. It can be inferred that the strong dynamic pressure and the strong discontinuities within the MCBL determine the intense compression effect. In addition, a very intense geomagnetically induced current (GIC) event was directly caused by the MCBL. Similar to this case, majority of MCBLs are dynamic pressure enhancement regions, and there are strong southward magnetic field and several strong discontinuities inside these regions, which can potentially drive large‐scale magnetospheric activities. In this paper, we take a case study to discuss the magnetospheric activities and the space weather effects caused by MCBLs.

Flow shear near the boundary of the plasma sheet observed by Cluster and Geotail

We report on the transient strong flow shear (large northward/southward electric field) events accompanied by energetic ion beams and localized field‐aligned currents observed at the boundary of the premidnight plasma sheet by Cluster in the Northern Hemisphere and Geotail in the Southern Hemisphere. The events took place associated with plasma sheet expansion during a substorm interval, with the main positive bay onset at 1155 UT on 10 October 2001. Typical timescales of these events were 1–5 minutes. Cluster multipoint analysis showed that the field‐aligned currents consist of upward and downward current layers, the latter located at the outermost edge of the plasma sheet and concentrated in a region with a thickness of 1600 km. Low‐energy proton flow suggested that the electric field was southward at the outer part and northward at the inner part, with a magnitude exceeding 10 mV/m. The electric field reversal region also corresponds to the boundary between beam‐like electrons and more isotropic electron distributions. Geotail observed corresponding plasma and field disturbances simultaneously inside the plasma sheet. We suggest that the strong bipolar electric fields could be related to the Hall effect of the transient reconnection process tailward of Cluster and Geotail and to the leading edge of the plasma flow jetting Earthward from the reconnection region.

Polar observations of transverse magnetic pulsations initiated at substorm onset in the high‐latitude plasma sheet

This paper presents simultaneous observations of 6‐mHz magnetic pulsations in the nightside high‐latitude plasma sheet, inner plasma sheet at the geosynchronous distance, and auroral region on the ground in association with substorm onset. We study an isolated substorm (AE ∼ 300 nT) onset on 19 October 1999 at ∼0145 UT. The Polar spacecraft was located in the plasma sheet near the plasma sheet boundary and was magnetically conjugate with the Greenland west coast magnetometer chain. Polar measured large‐amplitude transverse magnetic (10 nT; toroidal) and electric (20 mV m−1) field oscillations (∼6 mHz) during the early development of the negative bay (300 nT) in Greenland. On the ground, pulsations (50 nT) with the same frequency were superimposed on the negative bay. The geostationary GOES 8 spacecraft was located 2 hours west of Greenland. It observed compressional magnetic field oscillations (1 nT and ∼6 mHz). At Polar the pulsations were initiated ∼3 min before the first outward propagating substorm signature, an abrupt enhancement of the plasma sheet electron fluxes. Such a rich data set allows us to study in detail the spatial origin of the pulsations, their timing with respect to the negative bay onset, and their role in initiation of the substorm current wedge. It is concluded that the pulsations were initiated between the plasma sheet boundary and the tailward expanding region of the enhanced plasma sheet electron fluxes. The pulsations were then later observed on the ground and at GOES 8. It can also be argued that the pulsations were an integral part of the formation of the substorm current wedge. Finally, we suggest that the pulsations were generated by periodic variations in the rate of the current diversion from the braking region of the earthward flows generated by reconnection at the near‐Earth neutral line.

Reconciliation of the substorm onset determined on the ground and at the Polar spacecraft

An isolated substorm on Oct. 17, 1997 during a close conjunction of the Polar spacecraft and the ground‐based MIRACLE network is studied in detail. We identify signatures of substorm onset in the plasma sheet midway between the ionosphere and the equatorial plasma sheet, determine their timing relative to the ground signatures, and discuss their counterparts on the ground and in the equatorial plasma sheet. The substorm onset is determined as the negative bay onset at 2040:42(±5 sec) UT coinciding with the onset of auroral precipitation, energization of plasma sheet electrons at Polar, and strong magnetic field variations perpendicular to the ambient field. Such accurate timing coincidence is consistent with the Alfvén transit time between Polar and the ionosphere. Furthermore, the timing of other field and particle signatures at Polar showed clear deviations from the onset time (±2 min). This suggests that the sequence of these signatures around the onset time can be used to validate the signatures predicted by various substorm onset models.

Substorm onset timing: The December 31, 1995, event

The objective of the present study is to examine the timing of various onset‐associated signatures and address the cause‐and‐effect relationship between the formation of a near‐Earth neutral line (NENL) and the trigger of tail current disruption. An event selected for this study took place on December 31, 1995. In this event the Geotail satellite was located at X = −30.3 RE in the midnight sector at a local time between the GOES 8 and 9 geosynchronous satellites. The timing of the Geotail observation of a fast (950‐km/s) tailward convection flow accompanied with southward Bz (< −10 nT) indicates that the near‐Earth reconnection process started at least 4 min before the ground substorm onset, which was identified by various signatures such as an auroral expansion, a Pi2 onset, a positive bay onset, and a negative bay onset. Both GOES satellites observed dipolarization. GOES 9 was located closer to the onset meridian and observed a sudden recovery (dipolarization) of the local magnetic field but with a noticeable (≈1 min) delay from the ground onset. This delay can be interpreted in terms of the earthward expansion of tail current disruption initiated outside of geosynchronous orbit. The timing of all these features is consistent with the idea that dipolarization is a pileup of magnetic flux conveyed from the NENL. However, a sharp decrease in the H component at GOES 9 prior to the local dipolarization onset and the sudden start of a substorm are difficult to explain in terms of this idea. It is asserted that tail current disruption is a unique process rather than a direct consequence of the NENL formation, although it is possible that the reconnection process sets up a favorable condition for triggering tail current disruption. The fast plasma flow in the plasma sheet ceased soon after the substorm onset, suggesting that during the expansion phase, the tail current disruption took over the near‐Earth reconnection process as a major role in the substorm dynamics.

Growth and evolution of a plasmoid associated with a small, isolated substorm: IMP 8 and GEOTAIL measurements in the magnetotail

A tailward‐moving plasmoid was observed at the GEOTAIL satellite at a GSM position (−73.3, 18.1, −1.1) RE on September 16, 1993, at 0417 UT, when the IMP 8 satellite was at (−37.5, −2.5, 1.7) RE at the midnight plasma sheet/lobe interface. The first indication of the plasmoid formation a few minutes after the negative bay onset of a small, localized auroral substorm was the onset of tailward beams of energetic ions and electrons at GEOTAIL well within the plasma sheet. Earthward‐streaming energetic ions observed at IMP 8 a few minutes later suggest that the plasmoid evolved from within the plasma sheet to encompass the flux of nearly the entire thickness of the plasma sheet. The opposite direction of the anisotropies at IMP 8 and GEOTAIL suggest that the particle acceleration region was between X=−37.5 and −73 RE at that time. The isolated substorm associated with this plasmoid started equatorward of 67° latitude at a location which we map to near‐Earth nightside plasma sheet (|X| < 15 RE) based on ground observations of a field line resonance. The active electrojet did not expand poleward until at least 10 min after the detection of the acceleration region tailward of IMP 8 and at least several minutes after the core of the plasmoid had moved tailward of GEOTAIL. These observations reinforce by means of in situ, concurrent, multipoint measurements the attitude expressed recently by several researchers that the locations of lobe reconnection and equatorial projection of electrojet intensification during substorm expansion are distinctly different from each other.

The ground signatures of the expansion phase during multiple onset substorms

The ground signatures of multiple onset substorms have been investigated in night-side magnetograms from low to high latitudes and in observations of auroral-zone electron precipitation. Pi 2 onsets at three widely spaced stations are used for accurate timing of each onset. It is found that an evening auroral arc brightens at the onset of each Pi 2 train, also in the case of weak pulsations before the first low-latitude positive bay onset. The latter onset is, on the other hand, associated with the initiation of a westward travelling surge, and field-aligned currents moving with the surge cause a similar westward movement of the magnetic signatures in subauroral and low-latitude magnetograms. At the arrival of a surge at an evening side observatory, the westward electrojet is displaced rapidly poleward, with a sharp increase in local bay activity and high-energy electron precipitation. The westward expansion of new activity appears as a continuous motion along the oval and is associated with a local poleward displacement of the westward electrojet. Consecutive surge initiation and low-latitude onsets do not, however, always occur progressively farther west. Thus, the development of the expansion phase consists of a series of intensifications and auroral surge formations at 10–20 min intervals. Near the time of maximum auroral-zone bay activity and apparently also when maximum westward extent is reached, the whole nighttime oval seems to be shifted poleward. Our findings are thus not consistent with the Wiens and Rostoker (1975) northward-westward stepping model. An alternative model is therefore presented based on the fundamental role of the westward travelling surge in carrying substorm activity westward along the oval. The associated field-aligned current system will perturb the pre-existing magnetospheric current wedge and cause positive bay increases at low latitudes and westward moving magnetic signatures at subauroral stations.

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

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

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

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

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

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

Auroral-zone electron precipitation events observed before and at the onset of negative magnetic bays

Abstract Balloon recordings of bremsstrahlung X-raya from precipitating electrons are presented, showing impulsive X-ray events just at the onset of bay activity as well as weaker and smoother electron precipitation before the bay, the latter apparently being connected with a growth phase of the same substorm. The pre-bay events are generally very smoothly varying events, but show modulation effects associated with the occurrence of irregular magnetic pulsations. The impulsive precipitation events start very abruptly together with Pi 2 magnetic pulsations at bay onset, and also show modulation effects, so that maxima in the X-ray flux tend to accompany maxima in the H -component at auroral-zone stations. These X-ray pulsations have a shorter period than the simultaneous mid-latitude Pi 2, which we take to indicate that the impulsive precipitation events occur on field lines close to the inner edge of the neutral sheet.