Substorm Research Articles

Climatological Statistics of Extreme Geomagnetic Fluctuations With Periods From 1 s to 60 min

AbstractUsing a global database of 125 magnetometers covering several decades, we present occurrence statistics for fluctuations of the horizontal geomagnetic field (dBh/dt) exceeding the 99.97th percentile (P99.97) for both ramp changes (Rn) and the root‐mean‐square (Sn) of fluctuations over periods,τ, from 1 to 60 min and describe their variation with geomagnetic latitude and magnetic local time (MLT). Rates of exceedance are explained by reference to the magneto‐ionospheric processes dominant in different latitude and MLT sectors, including ULF waves, interplanetary shocks, auroral substorm currents, and traveling convection vortices. By fitting generalized Pareto tail distributions aboveP99.97, we predict return levels (RLs) forRnandSnover return periods of between 5 and 500 years. P99.97and RLs increase monotonically with frequency (1/τ) (with a few exceptions at auroral latitudes) and this is well modeled by quadratic functions whose coefficients vary smoothly with latitude. For UK magnetometers providing 1‐s cadence measurements, the analysis is extended to cover periods from 1 to 60 s and empirical Magnetotelluric Transfer functions are used to predict percentiles and return levels of the geoelectric field over a wide frequency range (2 × 10−4to 4 × 10−2 Hz) assuming a sinusoidal field fluctuation. These results help identify the principal causes of field fluctuations leading to extreme geomagnetically induced currents (GIC) in ground infrastructure over a range of timescales and they inform the choice of frequency dependence to use withdBh/dtas a GIC proxy.

A Magnetospheric Driver of Westward Traveling Surge: Plasma‐Sheet Bubble

AbstractThe westward traveling surge (WTS) is one of the most significant manifestations of auroral substorms. The formation and evolution of WTS are the key to understanding the dynamics of substorm. However, the generation mechanism of WTS remains unclear. This study investigates the magnetospheric driver of WTS using numerical simulations of the Inertialized Rice Convection Model. We model a typical WTS event by specifying a low‐entropy bubble injection from the magnetotail. The simulated characteristics of the WTS are consistent with typical observed features. Simulation confirms that the bubble injection creates an intense upward Region‐1 field‐aligned current to flow into the ionosphere, which is responsible for the prominent auroral structure. Moreover, the calculated speed of westward expansion of the bubble in the magnetosphere agrees very well with the speed of the westward surge in the ionosphere. Therefore, the plasma‐sheet bubble is believed to be a possible magnetospheric source of the WTS.

Prompt Penetration and Substorm Effects Over Jicamarca During the September 2017 Geomagnetic Storm

AbstractWe used reanalyzed Jicamarca radar measurements to study the response of equatorial ionospheric electrodynamics and spread F during the main phase of the large September 2017 geomagnetic storm. Our observations near dusk on 7 September show very large upward drifts followed by a large short‐lived downward drift perturbation that completely suppressed the lower F region plasma irregularities and severely decreased the backscattered power from the higher altitude spread F. We suggest that this large short‐lived westward electric field perturbation is most likely of magnetospheric origin and is due to a sudden and very strong magnetic field reconfiguration. Later in the early night period, data indicate large, mostly upward, drift perturbations generally consistent with standard undershielding and overshielding electric field effects, but with amplitudes significantly larger than expected. Our analysis suggests that occurrence of storm‐time substorms is one of the major factors causing the large nighttime westward and eastward electric field perturbations observed at Jicamarca near the storm main phase. Our analysis also suggests that magnetospheric substorms play far more important roles on the electrodynamics of the equatorial nighttime ionosphere than has generally been thought.

A Novel Magnetosphere–Ionosphere Coupling Model for the Onset of Substrom Expansion Phase

A novel magnetosphere–ionosphere (M-I) coupling model is proposed to simulate the brightening of the onset auroral arc of a magnetospheric substorm event. The new M-I coupling model is modified from the M-I coupling model proposed by the Alaska research team in 1988. We adjust the magnetospheric boundary conditions by including the Hall effects in the thin current sheet and allowing the spatial distributions of the reflection–transmission coefficient to vary with time. As a result, brightening and poleward drifting of multiple auroral arcs appear for the first time in an M-I coupling model. The new results indicate that the coupled Hall effects in the near-Earth plasma sheet and the E-region ionosphere play a vital role in triggering the onset of a magnetospheric substorm.

Simultaneous Development of Multiple Auroral Substorms: Double Auroral Bulge Formation

AbstractThe expansion phase of auroral substorms is characterized by the formation of an auroral bulge, and it is generally considered that a single bulge forms following each substorm onset. However, we find that occasionally two auroral intensifications takes place close in time but apart in space leading to the formation of double auroral bulges, which later merge into one large bulge. We report three such events. In those events the westward auroral electrojet intensified in each auroral bulge, and geosynchronous magnetic field dipolarized in the same sector. It appears that two substorms took place simultaneously, and each substorm was accompanied by the formation of its own substorm current wedge system. This finding strongly suggests that the initiation of auroral substorms is a local process, and there is no global reference frame for their development. For example, ideas such as (i) the auroralbreakup takes place in the vicinity of the Harang reversal and (ii) the westward traveling surge maps to the interface between the plasma sheet and low‐latitude boundary layer, do not necessarily hold for every substorm. Even if those ideas may be suggestive of causal magnetospheric processes, the reference structures themselves are probably not essential. It is also found that despite the formation of two distinct auroral bulges, the overall magnetosphere‐ionosphere current system is represented by one globally coherent system, and we suggest that its structure is determined by the relative intensities and locations of the two substorm current wedges that correspond to the individual auroral bulges.

Surface electrostatic discharge of charged typical space materials induced by strong electromagnetic interference

Surface charging of typical space materials such as polyimide and high density polyethylene may sometimes cause electrostatic discharge (ESD) under natural conditions. That is because polymer materials usually present lower surface discharge strength compared with their bulk breakdown strength, especially in vacuum. However, the space environment is very complicated, extremely hostile environments such as solar storms and magnetospheric substorms may happen and bring strong electromagnetic interference (EMI), which may induce ESD in spacecraft more easily and cause great damage to those spacecraft. In this paper, an innovative platform which can simulate the ESD of materials induced by strong EMI of microwave sources is set up. The influence of different factors such as power density, repetition frequency of microwave source, electromagnetic wave angle and distance between high voltage and ground electrode are researched. In addition, we perform particle-in-cell simulations to illustrate multipactor dynamics under the influence of strong electromagnetic waves. The induced discharging can be attributed to the electric field component of strong electromagnetic waves.

Data Mining Reconstruction of Magnetotail Reconnection and Implications for Its First-Principle Modeling

Magnetic reconnection is a fundamental process providing topological changes of the magnetic field, reconfiguration of space plasmas and release of energy in key space weather phenomena, solar flares, coronal mass ejections and magnetospheric substorms. Its multiscale nature is difficult to study in observations because of their sparsity. Here we show how the lazy learning method, known as K nearest neighbors, helps mine data in historical space magnetometer records to provide empirical reconstructions of reconnection in the Earth’s magnetotail where the energy of solar wind-magnetosphere interaction is stored and released during substorms. Data mining reveals two reconnection regions (X-lines) with different properties. In the mid tail (∼30RE from Earth, where RE is the Earth’s radius) reconnection is steady, whereas closer to Earth (∼20RE) it is transient. It is found that a similar combination of the steady and transient reconnection processes can be reproduced in kinetic particle-in-cell simulations of the magnetotail current sheet.

Key elements of auroral substorm development and their relationship to recent observations of detached sub-auroral phenomena including STEVE-like emissions

Since Akasofu introduced the concept of the substorm in 1964, numerous refinements have been added to the phenomenological model of how auroral substorms develop. Here, we review several of the most important of these including; polar cap patches, intensifications of the Polar Activated Band (PAB) at the poleward edge of the bulge, the development of new arcs poleward, the generation of auroral streamers, the evolution of streamers into auroral torches and omega bands, the association of streamers with particle injection and BBF activity in the tail, and wedgelet-like magnetic perturbations on the ground. We also review the concept of “contact breakups” which are breakups that appear to be triggered by the arrival of streamer-like forms in the equatorward regions of the oval. Finally, we remind the community that the addition of detached sub-auroral emissions was made to the phenomenological picture of substorm develop in the late 1960s along with the concept of auroral streamers. Newer observations of detached sub-auroral (STEVE-like emissions) from the Viking/UVI and POLAR/VIS imagers are presented which confirm that they are east-west aligned bands of emission that separate away from the equatorward edge of the auroral oval in response to intensifications of the poleward edge of the bulge and subsequent streamer production. We propose a new model for the formation of detached sub-auroral (STEVE-like) emissions in which a non-linear growth of a SAPS-driven instability (e.g. like the shear-flow/ballooning instability) at the plasmapause results in the disruption of the boundary separating hot plasma sheet particle from the cold plasmaspheric plasma. It is proposed that the resulting intermixing of plasma populations leads to both the observed STEVE-like emissions and also provides a source of cold plasma on open drift paths that can feed the long-lived drainage plumes that have recently been discovered.

Causality and Information Transfer Between the Solar Wind and the Magnetosphere–Ionosphere System

An information-theoretic approach for detecting causality and information transfer is used to identify interactions of solar activity and interplanetary medium conditions with the Earth’s magnetosphere–ionosphere systems. A causal information transfer from the solar wind parameters to geomagnetic indices is detected. The vertical component of the interplanetary magnetic field () influences the auroral electrojet (AE) index with an information transfer delay of 10 min and the geomagnetic disturbances at mid-latitudes measured by the symmetric field in the H component (SYM-H) index with a delay of about 30 min. Using a properly conditioned causality measure, no causal link between AE and SYM-H, or between magnetospheric substorms and magnetic storms can be detected. The observed causal relations can be described as linear time-delayed information transfer.

Magnetohydrodynamic Turbulence in the Earth’s Magnetotail From Observations and Global MHD Simulations

Magnetohydrodynamic (MHD) turbulent flows are found in the solar wind, the magnetosheath and the magnetotail plasma sheet. In this paper, we review both observational and theoretical evidence for turbulent flow in the magnetotail. MHD simulations of the global magnetosphere for southward interplanetary magnetic field (IMF) exhibit nested vortices in the earthward outflow from magnetic reconnection that are consistent with turbulence. Similar simulations for northward IMF also exhibit enhanced vorticity consistent with turbulence. These result from Kelvin-Helmholtz (KH) instabilities. However, the turbulent flows association with reconnection fill much of the magnetotail while the turbulent flows associated with the KH instability are limited to a smaller region near the magnetopause. Analyzing turbulent flows in the magnetotail is difficult because of the limited extent of the tail and because the flows there are usually sub-magnetosonic. Observational analysis of turbulent flows in the magnetotail usually assume that the Taylor frozen-in-flow hypothesis is valid and compare power spectral density vs. frequency with spectral indices derived for fluid turbulence by Kolmogorov in 1941. Global simulations carried out for actual magnetospheric substorms in the tail enable the results of the simulations to be compared directly with observed power spectra. The agreement between the two techniques provides confidence that the plasma sheet plasma is actually turbulent. The MHD results also allow us to calculate the power vs. wave number; results that also support the idea that the tail is turbulent.

The impact of cold electrons and cold ions in magnetospheric physics

A review of the impact of the cold-ion and cold-electron populations in the Earth’s magnetosphere is presented. The cold populations are defined by total energy less than approximately 100 eV, i.e. in the energy range which is strongly affected by spacecraft charging and that often dominates the total plasma density. We also include the warm plasma cloak in the review, since it overlaps partially with the cold energy range and is a population that is still not well understood.The known impacts of cold ions and cold electrons that are discussed are: the source of hot magnetospheric plasma, solar-wind/magnetosphere coupling, magnetotail reconnection and substorms, Kelvin–Helmholtz instabilities on the magnetopause, chorus, hiss, electromagnetic-ion-cyclotron and ultra-low-frequency wave–particle interactions, aurora structuring and spacecraft charging. Other possible impacts are associated with refilling on open-drift trajectories, the remnant layer and plasmapause disruption. A discussion of the difficulty of cold-plasma measurements and the need for new measurement techniques that measure the full cold-ion and cold-electron distribution functions is also presented.There remain a lot of unknowns about the cold-ion and cold-electron populations, associated with their origin, properties, drivers and impacts. These populations will need to be fully understood before the magnetosphere–ionosphere system can be fully understood.

Shock Induced Strong Substorms and Super Substorms: Preconditions and Associated Oxygen Ion Dynamics

It is well known that the interaction between interplanetary (IP) shocks and the Earth’s magnetosphere would generate/excite various types of geomagnetic phenomena. Progresses have been made on the Earth’s magnetospheric response to solar wind forcing in recent years in the aspects associated with magnetospheric substorms. Strong substorms and super substorms could be triggered externally by sudden changes of solar wind dynamic pressures. When a strong substorms (AE > 1000 nT) or super substorms (AE > 2000 nT) occurs, singly charged oxygen ions escaped from the Earth’s ionosphere are found to be a dominated ion population in the magnetotail and in the inner magnetosphere—ring current region. The products of a strong substorms or super substorms- plasmoid, burst bulk flows are also found to contain significant oxygen ions, even substorm injections can be dominated by oxygen ions. Thus, the magnetospheric dynamic must consider the contributions from the heavy oxygen ions. Also, the IP shock induced super substorms associated electromagnetic pulses (dB/dt) would shift the energetic particle (injections) inward and accelerate existing population significantly. Extensive attempts have also been made to understand how the solar wind energy couples with the magnetosphere to excite magnetospheric substorms. The statistical analysis shows that strong substorms (AE > 1000 nT) and super substorms (AE > 2000 nT) triggered by interplanetary shocks are most likely to occur under the southward interplanetary magnetic field (IMF) and fast solar wind pre-conditions. In addition, strong substorms after the IP shock arrival are more likely to occur when IMF points toward (away from) the Sun around spring (autumn) equinox, which can be ascribed to the Russell-McPherron effect. Thus, the southward IMF precondition of an interplanetary shock and the Russell-McPherron effect can be considered as precursors of a strong substorm and/or super substorm triggered by IP shocks. Moreover, the average duration of CME sheath region which is just behind the interplanetary shock are found to be about 7 hours. This indicates that southward IMF compressed by shock could last at least 7 hours long in the downstream of the interplanetary shock (sheath region) if a southward IMF pre-condition is present, which explains why the largest substorm often occur in the CME sheath.

Are Dawn Storms Jupiter's Auroral Substorms?

AbstractDawn storms are among the brightest events in the Jovian aurorae. Up to now, they had only been observed from Earth‐based observatories, only showing the Sun‐facing side of the planet. Here, we show for the first time global views of the phenomenon, from its initiation to its end and from the nightside of the aurora onto the dayside. Based on Juno's first 20 orbits, some patterns now emerge. Small short‐lived spots are often seen a couple of hours before the main emission starts to brighten and evolve from a straight arc to a more irregular one in the midnight sector. As the whole feature rotates dawn‐ward, the arc then separates into two arcs with a central initially void region that is progressively filled with emissions. A gap in longitude then often forms before the whole feature dims. Finally, it transforms into an equatorward‐moving patch of auroral emissions associated with plasma injection signatures. Some dawn storms remain weak and never fully develop. We also found cases of successive dawn storms within a few hours. Dawn storms thus share many fundamental features with the auroral signatures of the substorms at Earth, despite the substantial differences between the dynamics of the magnetosphere at the two planets.

Modulation of Magnetospheric Substorm Frequency: Dipole Tilt and IMF By Effects

AbstractSubstorm activity is heavily influenced by the Interplanetary Magnetic Field (IMF) Bz component and magnetospheric substorms occur most frequently when Bz is strongly negative. The substorm occurrence rate is also affected by the magnitude of the By component, but it is usually presumed that this contribution is independent of the sign of By. Using five independent substorm onset lists, we show that substorm activity does depend on the sign of By near the solstices. Specifically, we show that substorms occur more frequently when By and the dipole tilt angle Ψ have different signs as opposed to when they have the same sign. These results confirm that the magnetosphere exhibits an explicit dependence on the polarity of By for nonzero Ψ, as other recent studies have suggested, and imply variation in the dayside reconnection rate and/or the magnetotail response. On the other hand, we find no clear relationship between substorm intensity and By regardless of Ψ. Last, for the onset list based on identifying negative bays at auroral latitudes, we observe an overall trend of more frequent onsets for positive By, regardless of season. However, substorm frequency in the other four substorm lists does not exhibit an overall preference for positive By. We show that this phenomenon is very likely a consequence of the particular substorm identification method (i.e., identification of negative bays), which is affected by local ionospheric conditions that depend on By and Ψ.

Comparing Three Approaches to the Inducing Source Setting for the Ground Electromagnetic Field Modeling due to Space Weather Events

AbstractGround‐based technological systems, such as power grids, can be affected by geomagnetically induced currents (GIC) during geomagnetic storms and magnetospheric substorms. This motivates the necessity to numerically simulate and, ultimately, forecast GIC. The prerequisite for the GIC modeling in the region of interest is the simulation of the ground geoelectric field (GEF) in the same region. The modeling of the GEF in its turn requires spatiotemporal specification of the source which generates the GEF, as well as an adequate regional model of the Earth’s electrical conductivity. In this paper, we compare results of the GEF (and ground magnetic field) simulations using three different source models. Two models represent the source as a laterally varying sheet current flowing above the Earth. The first model is constructed using the results of a physics‐based 3‐D magnetohydrodynamic (MHD) simulation of near‐Earth space, the second one uses ground‐based magnetometers’ data and the Spherical Elementary Current Systems (SECS) method. The third model is based on a “plane wave” approximation which assumes that the source is locally laterally uniform. Fennoscandia is chosen as a study region and the simulations are performed for the September 7–8, 2017 geomagnetic storm. We conclude that ground magnetic field perturbations are reproduced more accurately using the source constructed via the SECS method compared to the source obtained on the basis of MHD simulation outputs. We also show that the difference between the GEF modeled using laterally nonuniform source and plane wave approximation is substantial in Fennoscandia.

A Review of Studies of Geomagnetic Storms and Auroral/Magnetospheric Substorms Based on the Electric Current Approach

The progress of space physics is reviewed from my personal point of view, particularly how I have reached my present understanding of auroral substorms and geomagnetic storms from the time of the earliest days of space physics. This review is somewhat unique in two ways. First of all, instead of taking the magnetic field line approach (including magnetic reconnection), I have taken the electric current approach; it consists of power supply (dynamo), transmission (currents/circuits), and dissipation (auroral/magnetospheric substorms). This is the basic way to study electromagnetic phenomena and it is much more instructive in understanding the physics involved in the chain processes. Secondly, this is not a textbook-like review, but it is hoped that my humble experience may be useful to see how a new science of space physics has evolved with a number of controversies. On the other hand, it can be seen that the electric current approach is still in a very rudiment stage. Thus, new generations of researchers are most welcome in taking this new way of studying auroral/magnetospheric substorms and geomagnetic storms.

Effects of Substorms on High‐Latitude Upper Thermospheric Winds

AbstractDuring magnetospheric substorms, high‐latitude ionospheric plasma convection is known to change dramatically. How upper thermospheric winds change, however, has not been well understood, and conflicting conclusions have been reported. Here, we study the effect of substorms on high‐latitude upper thermospheric winds by taking advantage of a chain of scanning Doppler imagers (SDIs), THEMIS all‐sky imagers (ASIs), and the Poker Flat incoherent scatter radar (PFISR). SDIs provide mosaics of wind dynamics in response to substorms in two dimensions in space and as a function of time, while ASIs and PFISR concurrently monitor auroral emissions and ionospheric parameters. During the substorm growth phase, the classical two‐cell global circulation of neutral winds intensifies. After substorm onset, the zonal component of these winds is strongly suppressed in the midnight sector, whereas away from the midnight sector two‐cell circulation of winds is enhanced. Both pre and postonset enhancements are ≥100 m/s above the quiet‐time value, and postonset enhancement occurs over a broader latitude and local‐time area than preonset enhancement. The meridional wind component in the midnight and postmidnight sectors is accelerated southward to subauroral latitudes. Our findings suggest that substorms significantly modify the upper‐thermospheric wind circulation by changing the wind direction and speed and therefore are important for the entire magnetosphere‐ionosphere‐thermosphere system.

How Do Auroral Substorms Depend on Earth's Dipole Magnetic Moment?

AbstractEarth's dipole magnetic moment M is known to decrease by ∼9% over the past 150 years. It has been argued that the decrease in M makes the near‐Earth space environment different. We investigated how the change in M affects the development of an auroral substorm by increasing and decreasing M by a factor of 1.5 in global magnetohydrodynamics simulation. The ionospheric conductivity decreases with increasing M, in accordance with the aid of empirical relations. When we imposed the southward interplanetary magnetic field, an auroral substorm took place regardless of M, but its development depends largely on M. When M is lower, (1) the expansion onset takes place later, (2) the auroral electrojet develops slowly, and (3) the maximum auroral electrojet increases. The first two consequences are probably associated with the slow magnetospheric convection as manifested by the polar cap potential drop. The third consequence is associated with the nonlinear dependence of substorm‐associated field‐aligned currents (FACs) on the ionospheric conductivity. The maximum values of the westward auroral electrojet and the net FACs increase with decreasing M, whereas the incident magnetic energy into the magnetosphere decreases with decreasing M. This implies that the efficiency of the generation of the substorm‐associated FACs increases with decreasing M. It is also found that, for the lower M‐value, the auroral oval shifts equatorward during the growth phase and expands more equatorward and poleward during the expansion phase. Evolution of substorms depends largely on the value of Earth's dipole moment and the ionospheric conductivity.

Relationship Between Geomagnetic Storms and Auroral/Magnetospheric Substorms: Early Studies

This paper describes a short story of how I learned in early days in space physics (1960–1970) that there are the direct and feed-back relationships between geomagnetic storms and auroral/magnetospheric substorms. In those days, both geomagnetic storms and auroral substorms were almost independent subjects. It is now understood that auroral substorms are directly related to the development of the ring current and thus of the main phase of geomagnetic storms. Further, we have begun to recognize that the growth of the ring current (caused by auroral/magnetospheric substorms) will change the internal structure of the magnetosphere, which in turn will change and could modify at least the intensity of auroral substorms. Thus, there are interesting feed-back processes between them. It is expected that this feed-back relationship between geomagnetic storms and auroral/magnetospheric substorms will become one of the major issues in magnetospheric physics in the future. In fact, an effort to understand this relationship will deepen our understanding of both geomagnetic storms and auroral/magnetospheric substorms. The progress in understanding the relationship between auroral substorms and geomagnetic storms is an example, in which it takes a long time to advance even one step. It is hoped that this paper will serve to learn the background in the development of space physics in the early days.

On the Impact of a Streaming Oxygen Population on Collisionless Magnetic Reconnection

AbstractUsing 2.5‐D Particle‐In‐Cell (PIC) simulations, we investigate how magnetotail reconnection is affected by a cold, streaming, oxygen plasma population, attributed to an ionospheric source, in the inflow region. As the tailward streaming oxygen reaches the current layer, a tailward motion of the reconnection site is induced. Due to the much longer cyclotron period of the oxygen ions, oxygen cannot couple as directly into the reconnection dynamics as protons. We find that the oxygen ions couple indirectly by means of impacting the electron dynamics. Therefore, a demagnetized species can, in fact, alter the dynamics of the reconnection site. We see further that the reconnection rate remains unchanged relative to a nonstreaming run. Our results may prove useful for understanding the development and dynamics of magnetospheric substorms and storms.