A comparative analysis of the role of interplanetary magnetic field (IMF) and sudden impulse (SI) in triggering a substorm

During a typical Akasofu-type of substorm, the southward component of IMF Bz is necessary prior to the onset. However, a sudden compression of solar wind, if intense enough, can also sometimes trigger a substorm, and is independent of the IMF orientation. The Akasofu-type substorm and the Impulse-induced substorm may differ in their occurrence mechanism and ground-based observations. This is shown using the initial four substorm events discussed in this paper having distinctly different IMF and sudden impulse conditions. A question then arises is how will these signatures vary when both sudden impulse and a southward component of IMF Bz are present prior to the onset. To account for the same, we analyze two substorm events of 05th April 2010 and 22nd June 2015. The substorm onsets on these days not just coincided with the sudden impulse but also a southward component of IMF Bz was present prior to the onsets. The present study accounts for the similarities and differences among isolated IMF induced substorms, isolated impulse-induced substorms and when both sudden impulse and a southward component of IMF Bz are present. We examined the relative dominance between the two factors in triggering a substorm using ground-based and satellite-based observations. If IMF Bz is near zero, a strong pressure pulse and/or large IMF By can lead to particle precipitation away from the usual midnight. To further ensure whether a pressure pulse or IMF By predominantly influences the substorm onset location, a statistical analysis of isolated substorms will be needed.     

On the association of quiet-time Pi2 pulsations with IMF variations

The association of quiet-time Pi2 pulsations with the variations of the interplanetary magnetic field (IMF) has been examined by using three reported events, occurring during extremely quiet intervals, of which the first was on 10 March 1997, the second 27 December 1997, and the third 11 May 1999. For the first event, the onset time of ground Pi2s maps to the IMF structure bearing a variation cycle of north-to-south and north again as seen by Wind in the upstream region and Geotail in the magnetosheath. Likewise, the second and the third events have respectively, four and three recurrent turnings propagating to the Earth sensed by multiple satellites. The comparison of geomagnetic perturbations, auroral brightenings, and energetic particle data in the magnetotail with the IMF observations shows successive substorm-like activations accompanied by ground Pi2 onsets. For a clear variation cycle, the first Pi2 burst appears 36 ± 8 min after southward turning of the IMF and the second one follows14 ± 4 min after a northward turning. Moreover, ground Pi2 onsets recur under low IMF clock angle conditions. These observational results can be interpreted with the prevailing models of externally triggered substorm. But the solar wind coupling to the magnetosphere under quiet conditions proceeds in a less efficient way than under substorm time conditions. Consequently, we suggest that recurrent quiet-time Pi2s can be associated with IMF variations and their cause can be the same as those for substorm times.     

The plasma sheet and boundary layers under northward IMF: A multi-point and multi-instrument perspective

During conditions of northward interplanetary magnetic field (IMF), the near-tail plasma sheet is known to become denser and cooler, and is described as the cold-dense plasma sheet (CDPS). While its source is likely the solar wind, the prominent penetration mechanisms are less clear. The two main candidates are solar wind direct capture via double high-latitude reconnection on the dayside and Kelvin–Helmholtz/diffusive processes at the flank magnetopause. This paper presents a case study on the formation of the CDPS utilizing a wide variety of space- and ground-based observations, but primarily from the Double Star and Polar spacecraft on December 5th, 2004. The pertinent observations can be summarized as follows: TC-1 observes quasi-periodic (∼2 min period) cold-dense boundary layer (compared to a hot-tenuous plasma sheet) signatures interspersed with magnetosheath plasma at the dusk flank magnetopause near the dawn-dusk terminator. Analysis of this region suggests the boundary to be Kelvin–Helmholtz unstable and that plasma transport is ongoing across the boundary. At the same time, IMAGE spacecraft and ground based SuperDARN measurements provide evidence of high-latitude reconnection in both hemispheres. The Polar spacecraft, located in the southern hemisphere afternoon sector, sunward of TC-1, observes a persistent boundary layer with no obvious signature of boundary waves. The plasma is of a similar appearance to that observed by TC-1 inside the boundary layer further down the dusk flank, and by TC-2 in the near-Earth magnetotail. We present comparisons of electron phase space distributions between the spacecraft. Although the dayside boundary layer at Polar is most likely formed via double high-altitude reconnection, and is somewhat comparable to the flank boundary layer at Double Star, some differences argue in favour of additional transport that augment solar wind plasma entry into the tail regions.     

A statistical analysis of substorm associated tail activity

Substorm onset timing is a critical issue in magnetotail dynamics research. Solar wind energy is accumulated in the magnetosphere and the configuration of the magnetosphere evolves toward an unstable state during the growth phase. At some point, the expansion phase begins and the stored energy is released through a variety of processes that return the magnetosphere to a lower energy state. In recovery the various processes die away. Unfortunately, the ground and magnetospheric signatures of onset, i.e. energy release, can be seen both in the growth phase prior to onset and in the expansion phase after onset. Some investigators refer to each of these events as a substorm. Tail observations suggest that most substorms have one event that differentiates the behavior of the tail field and plasma. We refer to this time as the “main substorm onset”. Each substorm associated phenomenon is timed independently and then compared with main substorm onsets. ISEE-2 tail observations are used to examine the tail lobe magnetic conditions associated with substorms because ISEE-2 orbit has a high inclination and frequently observes lobe field. Approximately 70 ∼ 75% of tail lobe B_t and B_z change are associated with the main substorm onset. If the satellite is more than 3 Re above (below) the neutral sheet, 86% (57%) of plasma pressure dropouts are associated with substorms. We interpret our results as evidence that the effect of the growth phase is to drive the magnetosphere towards instability. As it approaches global instability local regions become temporarily unstable but are rapidly quenched. Eventually one of these events develops into the global instability that releases most of the stored energy and returns the magnetosphere to a more stable configuration.     

Multisatellite investigations of substorm onsets

Substorm onsets, identified by Pi2 pulsations observed on the AFGL Magnetometer Network, have been studied using ISEE 1 electric and magnetic field data and GOES 2 and GOES 3 magnetic field data. The relative positions of the spacecraft with respect to the substorm current system were determined from the Pi2 polarizations. One onset occurred when ISEE 1 and GOES 2 were on the same field line but in opposite hemispheres. During this onset ISEE 1 and GOES 2 observed magnetic signatures which appear to be due to conjugate field-aligned-currents flowing out of the western edge of the westward electrojets. A broadband burst of wave noise was seen in the ISEE 1 electric field at the same time as field-aligned-currents were observed. These may be current driven ion cyclotron waves. A three minute perturbation in the electric field data prior to the initial substorm onset indicates that there was an azimuthal westward flow of plasma starting ～ 1 min before the substorm onset.     

Case study of September 24–26, 1998 magnetic storm

We compute global magnetospheric parameters based upon solar wind data obtained from the WIND spacecraft upstream. Using the paraboloid magnetospheric model, calculations of the dynamic global magnetospheric current systems have been made. The solar wind dynamic pressure, the interplanetary magnetic field, the strength of the tail current, and the ring current control the polar cap and auroral oval size and location during the magnetic storm. The model calculations demonstrate that the polar cap and the auroral oval areas are mainly controlled by the tail current. The substorm onset at 0630 UT on September 25, 1998 happened near the minimum in the main phase field depression. The substorm expansion onset time is also marked by a sudden enhancement in the solar wind dynamic pressure and an enhancement in the tail current. The magnetic signatures of these two effects cancel each other, which explains why the D_st profile shows no strong time variation during the substorm. Evidence for the substorm expansion includes not only the signature in the AL index but also the strong asymmetry of the low latitude magnetic disturbances (substorm positive bay signature). Model calculations were checked by comparison with the GOES 8 and 10 magnetic field measurements.     

磁层亚暴膨胀相的近地触发模型

本文指出现有亚暴的中性线模型其源区在赤疲乏面上离地球太远；以ＧＥＯＳ－２的观测资料为依据，提出了亚暴膨胀相的一个近地触发模型－气球模不稳定性模型，该模型认为，在增长相期间到达Ｒ≈（６－１０）ＲＥ的近地等离子体片内边缘区，出现指向地球方向的离子压强梯度，越尾电流强度增大，磁力线向磁尾拉伸。当等离子体片变薄，电子沉降增强，极光带电离层电导率骤增时，气球模不稳定性在近地等离子体片内边缘区被激发，场向电流     

Particle simulation study of substrom triggering with a southward IMF

This paper reports the spatial and temporal development of bursty bulk flows (BBFs) created by reconnection as well as current disruptions (CDs) in the near-Earth tail using our 3-D global electromagnetic (EM) particle simulation with a southward turning interplanetary magnetic field (IMF) in the context of the substorm onset. Recently, observations show that BBFs are often accompanied by current disruptions for triggering substorms. We have examined the dynamics of BBFs and CDs in order to understand the timing and triggering mechanism of substorms. As the solar wind with the southward IMF advances over the Earth, the near-Earth tail thins and the sheet current intensifies. Before the peak of the current density becomes maximum, reconnection takes place, which ejects particles from the reconnection region. Because of earthward flows the peak of the current density moves toward Earth. The characteristics of the earthward flows depend on the ions and electrons. Electrons flow back into the inflow region (the center of reconnection region), which provides current closure. Therefore the structure of electron flows near the reconnection region is rather complicated. In contrast, the ion earthward flows are generated far from the reconnection region. These earthward flows pile up near the Earth. The ions mainly drift toward the duskside. The electrons are diverted toward the dawnside. Due to the pile-up, dawnward current is generated near Earth. This dawnward current dissipates rapidly with the sheet current because of the opposite current direction, which coincides with the dipolarization in the near-Earth tail. At this time the wedge current may be created in our simulation model. This simulation study shows the sequence of the substorm dynamics in the near-Earth tail, which is similar to the features obtained by multisatellite observations. Identification of the timing and mechanism of triggering substorm onset requires further studies in conjunction with observations.     

Polar CEPPAD/IPS energetic neutral atom (ENA) images of a substorm injection

The CEPPAD Imaging Proton Spectrometer on the POLAR spacecraft has proven to perform very well as an Energetic Neutral (ENA) atom imager, despite the fact that it was designed primarily for measuring energetic ions in-situ. ENAs emitted from the ring current can be detected during storm- as well as quiet-time conditions and can be monitored continuously for many hours at a time when Polar is situated in the polar cap. In addition, we are able to routinely detect ‘bursts’ of ENA emissions in response to substorm-associated ion injections. In this paper, we present ENA images of a single such event together with global auroral imager data from the POLAR VIS instrument. LANL geosynchronous energetic particle data, and ground magnetic Pi2 data in order to establish that such bursts are indeed caused by substorm injections.     

Solar origin of near-relativistic impulsiveelectron events

There is increasing evidence suggesting that coronal acceleration supplies at least part of the particles observed during solar energetic particle events, yet coronal processes tend to be mostly disregarded in these studies. This is often due to the fact that the coronal restructuring in the early development of the associated flare and/or coronal mass ejection event is extremely fast (on the order of a few minutes) and can encompass most of the solar disk, thus requiring a full disk solar imager with very high time-cadence, and wide spectral coverage. An important subset of the energetic particle events are the near-relativistic impulsive electron events detected near Earth: their onsets can be traced back to a release time in the low corona with accuracies on the order of a couple of minutes. We investigate a series of impulsive electron events from 1998 to 2001 using energetic electron data measured in situ by the Electron, Proton, and Alpha Monitor (EPAM) experiment on the Advanced Composition Explorer (ACE) spacecraft, and radio coronal observations from the Nanqay Radioheliograph, the Decametric Array from Nanqay and the WAVES experiment on the WIND spacecraft. EPAM measures electrons in the energy range from 40 to 300 keV over a wide range of look directions and with better than 1 minute time resolution, while the Nançay radioheliograph provides images of the solar corona at 5 different frequencies with time cadence of 8 images per second and per frequency. This study focuses on the events which correspond to a delay, between the inferred injection times of the electrons at the Sun, and the electromagnetic emissions from flares, of at least 5 minutes. Radio signatures are found near the estimated time of the electron release for each of the events. The timing and spectral characteristics of the radio emissions, when compared with the properties of the particles seen at EPAM, strongly support an acceleration process in the corona but at highly variable heights from one event to the other.     

Relationship of the diffuse aurora and SAR arc dynamics to substorms and storms

Investigation results of a diffuse aurora (DA) and stable auroral red (SAR) arc dynamics based on spectrophotometric observations at the Yakutsk meridian (199°E geomagnetic longitude) are presented. The relationship of an equatorward extension of DA in the 557.7 nm emission to a substorm growth phase during the magnetospheric convection intensification after the turn of IMF B_Z to the south is shown. The formation of SAR arc during the substorm expansion phase is investigated. The association of SAR arc dynamics with the development of asymmetric ring current (substorm injection) during the main phase of a storm is analyzed. It is shown how the pulsating precipitations of energetic ring current particles develop in the outer plasmasphere based on photometric observations.     

Dynamics of solar cosmic ray events: Processes at large heliocentric distances (⪢1 AU)

Observations of solar cosmic ray events far from the sun (?1 AU) became possible after the launch of Pioneer 10 in 1972. Four spacecraft have now travelled beyond the orbit of Jupiter - Pioneer 10/11 and Voyager 1/2 — and are producing a growing body of distant observations of solar cosmic ray events. Initial studies using Pioneer 10/11 data out to ～6 AU interpreted flare particle observations in terms of a diffusion model, including the effects of convection and adiabatic energy loss. This model enjoyed general success in explaining the time-intensity profiles in cases where the spacecraft connection longitude at the sun did not change significantly with time. The results implied that the radial diffusion coefficient (K_r) increased slowly with distance over that radial range. More recent results at larger distances imply that K_r may begin to decrease beyond ～5 AU. It is not yet clear whether the standard diffusion model will be adequate to explain solar events well beyond 5 AU. The fact that large events at very large distances can last up to two solar rotations implies that solar wind stream structure will also play a role in the event dynamics. In general, however, observations at large distances offer perhaps the best hope of separating interplanetary propagation effects from coronal storage and propagation effects which frequently dominate observed event profiles at 1 AU.     

Evaluation of substorm models with Cluster observations of plasma flow reversal in the magnetotail

We evaluate two prevailing substorm models with an event of plasma flow reversal from tailward to Earthward detected by Cluster at the downstream distance of ∼19 R_E in the magnetotail during a substorm on August 22, 2001. We use the unique capability of Cluster measurements in determining gradients to examine the associated current density, Lorentz force, and current dissipation/dynamo term. In association with plasma flow reversal, it is found that (1) there was no clear quadrupole magnetic perturbation signature, (2) the x-component of the Lorentz force did not change sign, (3) the y-component of the product of the current density and the electric field was occasionally negative indicative of a dynamo effect, and (4) the timing sequence of flow reversal from the Cluster configuration did not match tailward motion of a single plasma flow source. These observations are consistent with the near-Earth initiation model for substorms with multiple current disruption sites moving progressively tailward near the late stage of substorm expansion.     

Magnetopause poleward of the cusp: Comparison of plasma and magnetic signature of the boundary for southward and northward directed interplanetary magnetic field

A coherent data set of high-latitude dayside magnetopause encounters by old (Heos 2, Hawkeye, Prognoz 7, 8) and new (Polar, Interball Tail, Cluster) spacecraft is needed to build a realistic model of the magnetopause (MP) including an indentation in the cusp. In building such a coherent data set a caution is necessary as the dayside magnetopause at high-latitudes may be less clearly defined than in the case of observations at low latitudes. It is due to expected presence of bundles of newly-reconnected magnetic field lines forming an extended boundary layer on the magnetosheath (MS) side of the magnetopause in the cusp region. Moreover, numerical magnetohydrodynamic (MHD) models of the solar wind-magnetosphere interaction predict that under northward interplanetary magnetic field (IMF) an additional thin current sheet should form inside the magnetopause at high latitudes on the dayside (e.g., Wu, 1983; Palmroth et al., 2001). Such a thin currect sheet is absent in empirical magnetosphere models. This internal current sheet, if a real one, may be mistaken for the magnetopause if magnetic field data are only taken into account and/or plasma data are unavailable. The Interball-Tail orbit allows for a full transition of magnetopause boundary layers at high-latitudes. We compare plasma and magnetic field signatures of the magnetopause poleward of the cusp for southward and northward IMF. The distance between the magnetic signature of the magnetopause (the current layer) and a cold and laminarly antisunward flowing MS plasma (so called free-flow MS) was found to be 0.5 to 1 R_E, at least. These observations were made under nominal solar wind of v350 km/s and p_dyn=1 to 4 nPa. We also observed several transient magnetic field reversals in the cusp related to pulses of solar wind dynamic pressure and/or the IMF discontinuity arrival. These transient reversals occurred at the same distance to the model MP as well defined full MP crossing, so most probably they represent just short encounters with the magnetopause current layer. Our analysis suggests that an indentation of the magnetopause with a subtle structure dependent on the local magnetic shear would explain and allow to predict the magnetic configuration in the high-altitude cusp.     

ISEE-1 and 2 observations of field-aligned currents in the distant midnight magnetosphere

Magnetic field measurements obtained in the nightside magnetosphere by the co-orbiting ISEE-1 and 2 spacecraft have been examined for signatures of field-aligned currents (FAC). Such currents are found on the boundary of the plasma sheet both when the plasma sheet is expanding and when it is thinning. Plasma sheet boundary layer current structure and substorm associated dynamics can be determined using the two spacecraft, although for slow traversals of the FAC sheet the spatial/temporal ambiguity is still an issue. We often find evidence for the existence of waves on the plasma sheet boundary, leading to multiple crossings of the FAC sheet. At times the boundary layer FAC sheet orientation is nearly parallel to the X-Z GSM plane, suggesting ‘protrusions’ of plasma sheet into the lobes. The boundary layer current polarity is, as expected, into the ionosphere in the midnight to dawn local time sector, and outward near dusk. Current sheet thicknesses and velocities are essentially independent of plasma sheet expansion or thinning, having typical values of 1500 km and 20–40 km/s respectively. Characteristic boundary layer current densities are about 10 nanoamps per square meter.     

近地尾向流和磁场——TC-1卫星观测结果

2004年10月12日,在01:30—04:30 UT期间,位于向阳侧磁层顶附近的Geotail卫星探测到行星际磁场为持续南向.此太阳风条件驱动了一个小磁暴,Sym-H指数在04:12 UT达到最小值-33 nT.在磁暴主相期间,AE指数维持在较高的水平,其最大值达400 nT.02:00—03:00 UT期间,TC-1卫星在近地磁尾(-10.6,3.2,-0.1)R_e处观测到明显的亚暴膨胀相特征和磁场偶极化过程.在偶极化前1 min,有较强的(v_x<-100 km/s)持续时间超过3 min的尾向流发生.分析发现该尾向流具有低温、高密度和沿磁场流动的特点,这说明尾向流具有来源于电离层风的特征.尾向流期间,TC-1观测的磁场分量B_x和总的磁场强度增加,磁倾角减小,磁场结构变成非偶极型,说明尾向流对磁场结构有一定的影响,文中尝试给出了相应的物理解释.观测表明,该事例中的近地磁尾尾向流可能对磁场偶极化过程的发生有重要意义.     

近地尾向流和磁场偶极化——TC-1卫星观测结果

2004年10月12日, 在01:30---04:30 UT期间, 位于向阳侧磁层顶附近的Geotail卫星探测到行星际磁场为持续南向. 此太阳风条件驱动了一个小磁暴, Sym-H指数在04:12 UT达到最小值-33nT. 在磁暴主相期间, AE指数维持在较高的水平, 其最大值达400nT. 02:00---03:00 UT期间, TC-1卫星在近地磁尾(-10.6, 3.2, -0.1)Re处观测到明显的亚暴膨胀相特征和磁场偶极化过程. 在偶极化前1min, 有较强的(v_x<-100 km/s)持续时间超过3min的尾向流发生. 分析发现该尾向流具有低温、高密度和沿磁场流动的特点, 这说明尾向流具有来源于电离层风的特征. 尾向流期间, TC-1观测的磁场分量Bx和总的磁场强度增加, 磁倾角减小, 磁场结构变成非偶极型, 说明尾向流对磁场结构有一定的影响, 文中尝试给出了相应的物理解释. 观测表明, 该事例中的近地磁尾尾向流可能对磁场偶极化过程的发生有重要意义.      

2004年12月13日IMF北向期间极区亚暴电离层电动力学特征

利用KRM地磁反演方法, 结合北半球中高纬度地磁台站数据, 研究了2004年12月13日行星际磁场北向期间发生的亚暴事件, 极区电离层电动力学参量(电流矢量、等效电流函数以及电势)的分布特征. 结果表明, 在该亚暴膨胀相起始后, 午夜之前西向电集流急剧增强, 且等效电流体系表现为夜侧双涡, 同时伴随夜侧增强的南向电场. 由于极弱的直接驱动过程, 卸载过程引起的电离层效应得到清楚显示. 卸载过程在膨胀相期间起绝对主导性作用. 同时, 夜侧电导率的增强是电集流区域电流急剧增强的主要原因.      

亚暴起始地磁指数判断的统计研究

利用2004年地磁西向电急流 AL指数, 亚暴电急流AE指数和场向电流AF指数来确定亚暴起始, 并与2004年亚暴极光起始进行对比. 研究发现, 如果以极光亚暴起始为时间零点, 亚暴的西向电急流AL起始和电急流AE起始主要分布于-5~+6 min的时间范围内, 但在-9~+9 min的时间范围内也有个别事例. 场向电流 AF 起始分布较宽, 可以分布于-8~+7 min的时间范围内. 平均西向电急流AL起始, 电急流AE起始和场向电流AF起始分别为0.5, 0.5, -0.1min. 通常西向电急流AL起始与极光起始同时的概率最高, 而多数情况下电急流AE起始和场向电流AF起始提前极光起始1min. 这些地面磁场指数确定的亚暴起始分布, 随着亚暴强度的增大(即最小AL指数减少, 最大AE指数增大, 最大AF指数增大)而向极光亚暴起始靠近. 对于5个超级亚暴来说, 其西向电急流AL起始和电急流AE起始都发生在极光起始之前. 这些结果说明对于大亚暴, 电急流的增加要早于极光爆发.      

On the source region of flux transfer events

We have examined the region of occurrence of flux transfer events for three distinct orientations of the interplanetary magnetic field: nearly horizontal in the solar magnetospheric equator, diagonally southward at 45° to the magnetospheric equator and nearly due south. For horizontal IMF conditions the FTE's occur in a horizontal band about ± 6 R_E wide. For diagonally southward IMF conditions, the FTE's occur in a diagonal swath about ± 6 R_E wide passing through the subsolar point. For duskward but nearly due southward IMF conditions, our observations reveal FTE's throughout the northern morning quadrant. These observations are consistent with a near equatorial source for flux transfer events and hence with component merging and not anti-parallel merging. These observations also help understand the energetic ion anisotropies seen in these events.