Stretched Field Lines Research Articles

Unusual Magnetospheric Dynamics During Intense Substorm Initiated by Strong Magnetospheric Compression

AbstractWe investigate an unusual sequence and peculiar features of magnetotail changes during a storm‐range substorm initiated by the interplanetary shock. Auroral observations and measurements at several favorably distributed magnetospheric spacecraft allowed the construction of an adaptive time‐dependent magnetospheric model to quantitatively characterize the configurational changes and mapping variations. Several passages of low‐altitude spacecraft in polar orbits near midnight help reveal the magnetic configuration of the nightside tail‐dipole transition region. In this event, an intense auroral and convection activity (accompanied by an up to 1,500 nT increase in the SuperMag AL‐index index) emerged in the highly compressed magnetosphere after the passage of interplanetary shock followed by strongly southward interplanetary magnetic field (IMF). This directly driven phase of the activity continued for an hour and resulted in the formation of a hybrid magnetic configuration with dipolarized midtail and stretched field lines in the transition region. Observations of energetic particle isotropy boundary latitudes near midnight are consistent with the modeled magnetic configuration. In concert with a downward turn of the solar wind (SW) flow, and weakening of the IMF driver and convection, an unusual stretching signature of the inner magnetosphere magnetic field was observed as close as at r ∼ 5 to 7 Re; which resulted mostly from an increasing downward tilt of the thin azimuthal current. Classic substorm breakup signatures commenced at fairly low, ∼60° magnetic latitude, deep in the closed field line region, in association with the current sheet upward motion. It was followed by strong stepwise poleward auroral expansion. We discuss how these signatures deviate from standard substorm scenarios and may be potentially imparted by the aforementioned changes in SW flow direction and pressure.

Energetic Dynamics of the Inner Magnetosphere in Contact with Fast Solar Wind Currents: Case of the Period 1964-2009

The Earth’s magnetosphere is a magnetic shield that protects Earth from high-energy particles and is subject to a series of internal processes caused by jets of the solar wind (SW) that destabilize it. These disturbances affect health as well as technology and become more extreme when SW is more accelerated. Thus, to better understand the impact of high-speed solar wind (HSSW) invasion on the dynamics of the magnetospheric system, a statistical study of HSSW populations was conducted for even (20 and 22) and odd (21 and 23) solar cycles. The regression analysis using the solar-derived fields from all solar cycles, indicates three states of the inner magnetosphere: 1) the 00:00UT-15:00UT period marked by a magnetic reconnection on the day side of the Earth closest to the Sun with the interplanetary magnetic field (IMF) facing South; 2) the 15:00UT-21:00UT period where IMF changes from South to North and remains there until 21:00UT; and 3) the 21:00UT-24:00UT period where there is a reconnection on the night side with stretched field lines. Observations made at different phases of solar activity lead us to suggest that the magnetospheric electric field (EM) and the Bz component of IMF (IMF-Bz) are strongly correlated not only at a particular time scale, but at different time scales. We believe that the daily fluctuations of the electrical and magnetic effects of magnetospheric origin currents play a very important role in the dayside magnetic reconnection rate. Moreover, examination of the cycles with different parities shows important amplitudes of the solar causes for the even cycles compared to the odd solar cycles. Therefore, even solar cycles have a strong influence on our socio-economic system compared to odd cycles.

Increasingly Parallel Pressure Anisotropic Ballooning: Substorm Growth Phase Drift Orbits Resulting in a Locally Ballooning Unstable Geomagnetic Tail

AbstractRecent work by Oberhagemann and Mann (2020, https://doi.org/10.1029/2019GL085271) has shown that ballooning instabilities in the transition region between dipolar and taillike fields associated with substorm onset may be triggered by an anisotropic pressure distribution that moves from an initially mild perpendicular anisotropy toward more parallel anisotropy. Here we examine the mechanism which can result in this transition toward more parallel pressure during growth phase tail stretching. We trace particles through a two‐dimensional model magnetic field to examine the evolution of particle distributions as they periodically drift through a tail which changes from a less stretched, quiet time to a more stretched late growth phase configuration. Our findings show that these particle pitch angle distributions become more parallel anisotropic through a combination of drift shell splitting and preferential de‐energization in the perpendicular direction. Drift shell splitting causes low pitch angle particles from a high‐pressure region to mix with high pitch angle particles from a low‐pressure region in a radially localized area in the tail transition region where high earthward pressure gradients exist. Preferential de‐energization occurs due to a transfer of particles from less stretched to more stretched field lines that reverses the usual balance between Fermi and betatron acceleration and causes particle pitch angle distributions to become more field aligned. Our model predicts onset characteristics and location in good agreement with observations, with the most unstable region consistent with the location of the auroral beads, which accompany the onset of the expansion phase of many substorms.

The global context of the 14 November 2012 storm event

AbstractFrom 2 to 5 UT on 14 November 2012, the Van Allen Probes observed repeated particle flux dropouts during the main phase of a geomagnetic storm as the satellites traversed the post‐midnight to dawnside inner magnetosphere. Each flux dropout corresponded to an abrupt change in the magnetic topology, i.e., from a more dipolar configuration to a configuration with magnetic field lines stretched in the dawn‐dusk direction. Geosynchronous GOES spacecraft located in the dusk and near‐midnight sectors and the LANL constellation with wide local time coverage also observed repeated flux dropouts and stretched field lines with similar occurrence patterns to those of the Van Allen Probe events. THEMIS recorded multiple transient abrupt expansions of the evening‐side magnetopause ∼20–30 min prior to the sequential Van Allen Probes observations. Ground‐based magnetograms and all sky images demonstrate repeatable features in conjunction with the dropouts. We combine the various in situ and ground‐based measurements to define and understand the global spatiotemporal features associated with the dropouts observed by the Van Allen Probes. We discuss various proposed hypotheses for the mechanism that plausibly caused this storm‐time dropout event as well as formulate a new hypothesis that explains the combined in situ and ground‐based observations: the earthward motion of magnetic flux ropes containing lobe plasmas that form along an extended magnetotail reconnection line in the near‐Earth plasma sheet.

Link between premidnight second harmonic poloidal waves and auroral undulations: Conjugate observations with a Van Allen Probe spacecraft and a THEMIS all‐sky imager

AbstractWe report, for the first time, an auroral undulation event on 1 May 2013 observed by the Time History of Events and Macroscale Interactions During Substorms (THEMIS) all‐sky imager (ASI) at Athabasca (L = 4.6), Canada, for which in situ field and particle measurements in the conjugate magnetosphere were available from a Van Allen Probe spacecraft. The ASI observed a train of auroral undulation structures emerging spontaneously in the premidnight subauroral ionosphere during the growth phase of a substorm. The undulations had an azimuthal wavelength of ~180 km and propagated westward at a speed of 3–4 km s−1. The successive passage over an observing point yielded quasiperiodic oscillations in diffuse auroral emissions with a period of ~40 s. The azimuthal wave number m of the auroral luminosity oscillations was found to be m~−103. During the event, the spacecraft—being on tailward stretched field lines ~0.5 RE outside the plasmapause that mapped into the ionosphere conjugate to the auroral undulations—encountered intense poloidal ULF oscillations in the magnetic and electric fields. We identify the field oscillations to be the second harmonic mode along the magnetic field line through comparisons of the observed wave properties with theoretical predictions. The field oscillations were accompanied by oscillations in proton and electron fluxes. Most interestingly, both field and particle oscillations at the spacecraft had one‐to‐one association with the auroral luminosity oscillations around its footprint. Our findings strongly suggest that this auroral undulation event is closely linked to the generation of second harmonic poloidal waves.

Thin filament simulations for Earth's plasma sheet: Tests of validity of the quasi‐static convection approximation

The main goal of this paper is to estimate the errors involved in applying a quasi‐static convection model such as the Rice Convection Model (RCM) or its equilibrium version (RCM‐E), which neglect inertial currents, to treat the injection of fresh particles into the inner magnetosphere in a substorm expansion phase. The approach is based on the idea that the dipolarization process involves earthward motion of a bubble that consists of flux tubes that have lower values of the entropy parameter than the surrounding medium. Our tests center on comparing MHD simulations with RCM‐ and RCM‐E‐like quasi‐static approximations, for cases where the bubble is considered to be a thin ideal‐MHD filament. Those quasi‐static solutions miss the interchange oscillations that are often a feature of the MHD results. RCM and, to a lesser extent, RCM‐E calculations tend to overestimate the westward electric field at the ionospheric footprint of the bubble and underestimate its duration. However, both get the time integral of the E × B drift velocity right as well as the net energization of the particles in the filament. The quasi‐static approximation is most accurate if its computed value of the braking time of the bubble's earthward motion is long compared to the period of the relevant interchange oscillation. Comparison of MHD filament simulations of interchange instability with corresponding RCM calculations suggests a similar validity criterion. For plasma sheet conditions, the quasi‐static approximation is typically best if the background medium has lowβ, worst if it consists of highly stretched field lines.

Storm-time ring current: model-dependent results

Abstract. The main point of the paper is to investigate how much the modeled ring current depends on the representations of magnetic and electric fields and boundary conditions used in simulations. Two storm events, one moderate (SymH minimum of −120 nT) on 6–7 November 1997 and one intense (SymH minimum of −230 nT) on 21–22 October 1999, are modeled. A rather simple ring current model is employed, namely, the Inner Magnetosphere Particle Transport and Acceleration model (IMPTAM), in order to make the results most evident. Four different magnetic field and two electric field representations and four boundary conditions are used. We find that different combinations of the magnetic and electric field configurations and boundary conditions result in very different modeled ring current, and, therefore, the physical conclusions based on simulation results can differ significantly. A time-dependent boundary outside of 6.6 RE gives a possibility to take into account the particles in the transition region (between dipole and stretched field lines) forming partial ring current and near-Earth tail current in that region. Calculating the model SymH* by Biot-Savart's law instead of the widely used Dessler-Parker-Sckopke (DPS) relation gives larger and more realistic values, since the currents are calculated in the regions with nondipolar magnetic field. Therefore, the boundary location and the method of SymH* calculation are of key importance for ring current data-model comparisons to be correctly interpreted.

Modeling the relationship between substorm dipolarization and dispersionless injection

[1] Significant populations of electrons with energies of tens of keV appear in the Earth's inner central plasma sheet during the substorm expansion phase. Increasing observational evidence indicates that these injections begin at a radially narrow but azimuthally extended transition between very stretched and less stretched field lines around 8 RE. In this work we suggest that the tailward retreat of this transition region can be responsible for adiabatic acceleration of electrons which is sufficient to cause ionospheric signatures of the dispersionless injections observed by riometers. To support this mechanism we develop a novel conceptual magnetotail magnetic field model with a few adjustable parameters which can be easily constrained by observations. Our calculations show that a tailward motion of the transition region at the speed of 0.8 RE/min is required to achieve good agreement with riometer observations.

Observations and simulations of a highly structured plasma sheet during northward IMF

We investigated a highly structured plasma sheet populated by counterstreaming beams observed by the Cluster and Double Star spacecraft on October 10, 2005, during an extended interval of northward interplanetary magnetic field (IMF). We used Wind spacecraft observations as input into our global magnetohydrodynamic (MHD) simulation of the event, and launched ions from the low‐latitude boundary layer, determined to be the source of the observed ions, in our large‐scale kinetic (LSK) particle tracing calculation that used the time‐dependent global fields from the MHD simulation. We found that a highly structured plasma sheet was formed in which the dawn and dusk halves were temporarily decoupled because of the presence of flux ropes in the center of the tail, possibly the result of localized near‐Earth reconnection. Our LSK simulation results showed good agreement with observed spectrograms and velocity distribution functions. On the dawn side, the ions launched in our simulation first formed a plasma sheet boundary layer (PSBL). Subsequent interactions of these ions with the current sheet scattered them and formed the central plasma sheet (CPS). On the dusk side, however, the topology of the magnetotail, including the formation of highly stretched field lines in the dusk flank and of a flux rope on open field lines in the center of the tail resulted in a structured plasma sheet, with counterstreaming beams present throughout the plasma sheet, including its center portion. This is very different from the classical picture of the PSBL in which layered counterstreaming beams exist only at the outer edges of the plasma sheet and do not penetrate the CPS.

Timing and location of substorm onsets from THEMIS satellite and ground based observations

Abstract. The unprecedented coverage of the THEMIS GBO station network coupled with high temporal and spatial resolution allowed us to determine the various stages of the global scale developments of the optical aurora at substorm onsets. We identified several steps of the substorm onset auroral phenomena and we suggest that the most rapid development is the starting of the Substorm Poleward Expansion (SPE) and it is most useful for accurate timing of the substorm onset. The physical significance of this step is the start of the large scale substorm energy dissipation in the atmosphere due to particle precipitation and auroral electrojet currents. We also recognized several pre-cursor features. We also measured the time of arrival of magnetic impulses associated with the same substorms at the THEMIS satellites. We used these times and a simple model with assumed iono-acoustic speeds in the range of 300–800 km/s to calculate the location and time of the origin of the magnetic impulses propagating from substorm onset. The assumption was made that the substorm occurred between two THEMIS satellites and the impulses propagated away from a singular starting point in and out along the magneto tail GSM-x axis. This technique is only useful in cases where the ground based signature of the substorm is very close in local time (or longitude) to the foot of the field lines of the THEMIS satellites. The x distance of the calculated origins were naturally highly dependent on the assumed propagation velocity model and the associated magneto-sonic speed. The resulting x distances of the starting point for the three events ranged between 11 and 17.6 RE. denoting a starting region that requires highly stretched field lines to map to the auroral onset latitude but which is generally considered to be too close for neutral line formation. The corresponding start times were in the range of 0 to 170 s prior SPE depending strongly on the assumed propagation speed.

Magnetic Effects on the Coalescence of Kelvin-Helmholtz Vortices

We simulate the coalescence process of MHD-scale Kelvin-Helmholtz vortices with the electron inertial effects taken into account. Reconnection of highly stretched magnetic field lines within a rolled-up vortex destroys the vortex itself and the coalescence process, which is well known in ordinary fluid dynamics, is seen to be inhibited. When the magnetic field is initially antiparallel across the shear layer, on the other hand, multiple vortices are seen to coalesce continuously because another type of magnetic reconnection prevents the vortex decay. This type of reconnection at the hyperbolic point also changes the field line connectivity and thus leads to large-scale plasma mixing across the shear layer.

Structure of an MHD‐scale Kelvin‐Helmholtz vortex: Two‐dimensional two‐fluid simulations including finite electron inertial effects

Our two‐dimensional two‐fluid simulations including finite electron inertial effects of an MHD‐scale Kelvin‐Helmholtz (KH) vortex show that the plasma mixing and transport across the Earth's tail‐magnetopause during northward interplanetary magnetic field (IMF) conditions can be caused by the coupling between the KH vortex and magnetic reconnection. First, it is found by systematical two‐dimensional simulations under various fundamental conditions that two types of magnetic reconnection are driven in a KH vortex that are crucial factors for determining the structure of the KH vortex itself. One, named “type I reconnection,” occurs in the case where the magnetic field components along the k vector of Kelvin‐Helmholtz instability (KHI) are antiparallel across the velocity shear layer (antiparallel case). Another, named “type II reconnection,” is driven in the case where the velocity shear is strong enough to produce highly rolled‐up KH vortices and highly stretched field lines therein (strong KHI case). It is also found that type I reconnection leads directly to plasma mixing across the shear layer and that type II reconnection forms magnetic islands and plasma contained in these magnetic islands can be transferred from one side to the other across the shear layer. Next, linear analyses of the Earth's magnetopause‐like cases under northward IMF show that the antiparallel case is obtained rather commonly and also that there is a significant possibility that the strong KHI case may be important. Furthermore, two‐fluid simulations of the magnetopause‐like cases actually show the occurrence of type I and type II reconnection in two dimensions. These results indicate that type I and type II reconnection may play a substantial role in the plasma mixing and transport across the magnetopause.

Simultaneous THEMIS in situ and auroral observations of a small substorm

We present ground‐based and in situ observations from March 13, 2007. The THEMIS satellites were in the evening sector conjugate to THEMIS ground‐based imagers. At ∼0507 UT there was an optical onset on inner CPS field lines. This involved near‐simultaneous brightening of 1 MLT hour longitudinal segment of the onset arc. The part of the arc that brightened was that closest to the equatorward boundary of the diffuse (proton) aurora. Within one minute, a dipolarization front moved across four THEMIS satellites. Based on their locations, the order in which they detected the dipolarization front, and the auroral evolution, we assert that the expansion phase began earthward of the four satellites and evolved radially outwards. We conclude that this onset occurred in an azimuthally localized region of highly stretched field lines.

Coordinated observation of field line resonance in the mid-tail

Abstract. Standing Alfvén waves of 1.1 mHz (~15 min in period) were observed by the Cluster satellites in the mid-tail during 06:00-07:00 UT on 8 August 2003. Pulsations with the same frequency were also observed at several ground stations near Cluster's footpoint. The standing wave properties were determined from the electric and magnetic field measurements of Cluster. Data from the ground magnetometers indicated a latitudinal amplitude and phase structure consistent with the driven field line resonance (FLR) at 1.1 mHz. Simultaneously, quasi-periodic oscillations at different frequencies were observed in the post-midnight/early morning sector by GOES 12 (l0≈8.7), Polar (l0≈11-14) and Geotail (l0≈9.8). The 8 August 2003 event yields rare and interesting datasets. It provides, for the first time, coordinated in situ and ground-based observations of a very low frequency FLR in the mid-tail on stretched field lines.

The main features of the ion spectra variations in the transition region from dipole to tailward streched field lines

The variations of the ion spectra in the transition region from dipole to tailward stretched field lines are analyzed on the base of INTERBALL/Tail satellite observations. The simultaneous measurements of the CORALL, DOK-2 and SKA-2 instruments were used for the obtaining the ion spectra in a wide energy range. The events of small spectra variations along the satellite trajectory are selected. Some possibilities to explain the observed feature are analysed including the satellite travelling along single field line, the compensation of the loses of particles due to drift effects by the adiabatic acceleration due to the regular earthward drift in the dawn-dusk electric field, and intense plasma mixing in the conditions of the nonconservation of the magnetic moments of particles. It is shown that the action of turbulent auroral electric fields is the most probable explanation of the observed effect.

Field line resonances in a stretched magnetotail: CANOPUS optical and magnetometer observations

Ground‐based observations support the existence of 1–4 mHz field line resonances (FLRs) at auroral latitudes. The low frequencies suggest that many nightside FLRs are excited on stretched magnetic field lines in the inner plasma sheet. Data from the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometers and photometers on 21 February 1993 indicate a FLR with a peak at 2.4 mHz, and that of 31 January 1997 indicate a peak at 1.3 mHz. In this paper, models of FLRs and models of stretched magnetotail topologies deduced from optical data are compared with the observations. The FLR model is independently constrained by a parametric near‐Earth magnetotail model that allows us to make direct comparisons with the observed FLR frequency. For the two case studies mentioned above, we are able to show that the theoretical FLR spectrum, computed on stretched field lines, is comparable to the low‐frequency experimental observations.

Pivotal role of magnetic reconnection in the Earth's magnetosphere

Magnetic reconnection plays a pivotal role in the physics of the Earth's magnetosphere. It determines the framework of the magnetosphere by defining the field-line topology where the field lines from the polar cap are open to the solar wind. The open field lines are produced by the reconnection on the dayside magnetopause, and the energy flow along these field lines is the predominant mode of the energy supply from the solar wind to the magnetosphere. The magnetotail is formed on the nightside of the Earth by the open field lines which are stretched by the solar wind. Then the reconnection occurs in the magnetotail and liberates the magnetic energy stored in the stretched field lines. The energy released by this reconnection accelerates the plasma in the magnetotail and produces the global disturbance called magnetospheric substorm. The above picture applies to a wide range of directions of the interplanetary magnetic field (IMF), but another mode of the reconnection occurs when the IMF direction is close to being northward.

The Earth's Dynamic Magnetotail

Geomagnetic field lines that are stretched on the nightside of the Earth due to reconnection with the interplanetary magnetic field constitute the Earth's magnetotail. The magnetotail is a dynamic entity where energy imparted from the solar wind is stored and then released to generate disturbance phenomena such as substorms. This paper gives an updated overview on the physics of the magnetotail by drawing heavily from recent research conducted with the GEOTAIL satellite. It summarizes firstly the basic properties of the magnetotail such as shape, size and magnetic flux content, internal motion and plasma regimes. Then it describes characteristics of tail plasmas of the solar-wind and the ionosphere origins. Thirdly it addresses acceleration and heating of plasmas in the magnetotail, where reconnection between the stretched field lines is the main driver but the site of the acceleration is not limited to the immediate vicinity of the neutral line. In the collisionless regime of the plasma sheet kinetic behaviors of ions and electrons control the acceleration process. The paper closes by enumerating the problems posed for future studies.

Relationship of the ionospheric convection reversal to the hard auroral precipitation boundary

Plasma drift and particle measurements from the DMSP spacecraft for the three Geospace Environmental Modeling (GEM) campaign periods (January 27–29, March 28–29, and July 20–21, 1992) have been used to study the relationship between the plasma convection reversal and the poleward boundary of the diffuse auroral zone characterized by hard electron precipitation with energies greater than 0.45 keV. This boundary, named the hard auroral precipitation boundary (or the HAP boundary) in this paper, is often regarded as the ionospheric footprint of the boundary between the central plasma sheet (CPS) and the boundary plasma sheet (BPS). By examining simultaneous ion drift and particle measurements from about 500 satellite passes we find that the large‐scale plasma flow in the morning and evening sectors changes its direction within the auroral oval at the HAP boundary. However, exceptions are found in the early morning sector between 0300 and 0600 MLT, where the convection reversal is sometimes (in 30% of the DMSP crossings) displaced poleward relative to the HAP boundary. It is shown that the shape of the region bordered by the HAP boundary can be roughly represented by a circle, whose size is influenced by the IMF Bz component. There is roughly a linear correlation between the diameter of this circle and the cross‐polar‐cap potential drop, with the best correlation coefficient of 0.65 for winter season. Our study suggests that the HAP boundary corresponds to the magnetospheric boundary between the quasi‐dipolar region and the region with more stretched field lines, and the source of the region 1 field‐aligned current is located near the HAP boundary. A By‐dependent shift of the HAP boundary with respect to the noon‐midnight meridian is also found. In the northern hemisphere, the shift is dawnward for positive By and duskward for negative By in the southern hemisphere, the shift is opposite to that in the northern hemisphere.

Low Altitude Image of Particle Acceleration and Magnetospheric Reconfiguration at Substorm Onset

We compare meridional distributions of auroral (0.3-20 keV) and energetic (30-300 keV) particle fluxes measured by two low-altitude polar-orbiting spacecraft which crossed the auroral oval before and shortly after the sudden substorm onset. 3-4 min after the substorm onset a very strong particle energization was observed with the particle flux increased by a factor 100-1000 at energies up to >300 keV. It was found in the middle of auroral oval, poleward of the isotropic boundary of energetic electrons detected prior to the substorm onset. Interpreting the variations of Energetic Particle (EP) flux and anisotropy (ratio of precipitated to trapped particle flux) in terms of particle scattering in the equatorial current sheet, we were able to characterize the configuration of equatorial magnetotail. The major acceleration/precipitation region in the center of auroral zone was inferred to be confined in the region of dipolarized magnetic field in the magnetotail. Further poleward (tailward in the magnetotail) the isotropic electron distributions indicated the presence of current sheet with very stretched magnetic field lines. Based on magnetic flux conservation in the flux tube, we estimate the width of transition region between dipolarized region and current sheet to be ∼0.5RE (about the gyroradius of plasma sheet proton in the equatorial magnetic field ∼5 nT). Such sharp transition may be formed by fast Earthward flow in the current sheet colliding with the dipolarized magnetic shells. An evidence supporting this interpretation is that the explosive westward electrojet was found at latitudes occupied by the current sheet but poleward of the dipolarized region. We also show another event in which the dipolarized region expanded up to the poleward boundary of auroral oval in 7 minutes after the substorm onset.