Time Of Substorm Onset Research Articles

Picturing Global Substorm Dynamics in the Magnetotail Using Low‐Altitude ELFIN Measurements and Data Mining‐Based Magnetic Field Reconstructions

AbstractA global reconfiguration of the magnetotail characterizes substorms. Current sheet thinning, intensification, and magnetic field stretching are defining features of the substorm growth phase and their spatial distributions control the timing and location of substorm onset. Presently, sparse in‐situ observations cannot resolve these distributions. A promising approach is to use new substorm magnetic field reconstruction methods based on data mining, termed SST19. Here we compare the SST19 reconstructions to low‐altitude electron losses and fields investigation (ELFIN) measurements of energetic particle precipitations to probe the radial profile of the equatorial magnetic field curvature during a 19 August 2022 substorm. ELFIN and SST19 yield a consistent dynamical picture of the magnetotail during the growth phase and capture its key features such as the formation of a thin current sheet and its earthward motion. Furthermore, they resolve a “checkmark” pattern of isotropic electron precipitation boundaries in the time‐energy plane, consistent with earlier observations but now over a broad energy range. It is shown that in the growth phase, the mismatch between SST19 and ELFIN latitudes is much less than one degree, the capability unattainable for any other empirical or first‐principles model.

Evolution of Energetic Proton Parallel Pressure Anisotropy at Geosynchronous Altitudes: Potential Role in Triggering Substorm Expansion Phase Onset

AbstractThe sequence of events associated with the triggering of energy release during substorm expansion phase onset is still not well‐understood. Oberhagemann and Mann (2020b, https://doi.org/10.1029/2019gl085271) proposed a new substorm onset mechanism, where the transition toward parallel proton pressure anisotropy during tail stretching in the late growth phase could trigger a pressure anisotropic ballooning instability. Here we examine the evolution of energetic proton parallel pressure anisotropy at geosynchronous altitudes, seeking evidence in support of the proposed substorm onset mechanism. We use the Geostationary Operational Environment Satellite (GOES) proton flux and magnetometer data combined with substorm onset indicators derived from ground‐based magnetometers. Superposed epoch analysis of substorm onset times for 2014 using the isolated substorm list (Ohtani & Gjerloev, 2020, https://doi.org/10.1029/2020ja027902) clearly shows signatures of energetic proton parallel pressure anisotropy immediately before substorm onset, potentially supportive of the Oberhagemann and Mann theory.

Verification of Substorm Onset From Intruding Flow Channels With High‐Resolution SuperDARN Radar Flow Maps

AbstractAuroral observations were first to identify the substorm, and later used to propose that substorm onset is triggered in the inner plasma sheet (equatorward portion of the auroral oval) by an intrusion of low entropy plasma comprising plasma sheet flow channels. Longitudinal localization makes the intruding flow channels difficult to observe with spacecraft. However, they are detectable in the ionosphere via the broader, two‐dimensional coverage by radars. Line‐of‐sight radar flow measurements have provided considerable support for the onset proposal. Here we use two‐dimensional, ionospheric flow maps for further testing. Since these maps are derived without the smoothing from global fits typically used for global convection maps, their spatial resolution is significantly improved, allowing representation of localized spatial structures. These maps show channels of enhanced ionospheric flow intruding to the time and location of substorm onset. We also see evidence that these intruding flows enter the plasma sheet from the polar cap, and that azimuthal spread of the reduced entropy plasma in the inner plasma sheet contributes to azimuthal onset spreading after initial onset. Identified events with appropriate radar data remain limited, but we have found no exceptions to consistency with flow channel triggering. Thus, these analyses strongly support the proposal that substorm onset is due to the intrusion of new plasma to the onset region. The lower entropy of the new plasma likely changes the entropy distribution of inner plasma sheet, a change possibly important for the substorm onset instability seen via the growing waves that demarcate substorm auroral onset.

A Statistical Study of Auroral Medium Frequency Bursts and Anomalous Incoherent Scatter Radar Echoes

AbstractDifferent types of incoherent scatter radar (ISR) echoes are observed associated with aurora, including some which have been interpreted as signatures of cavitating Langmuir turbulence (CLT). Akbari et al. (2013) https://doi.org/10.1002/jgra.50314 discussed two instances of correlation between CLT and naturally occurring radio emissions called medium frequency burst (MFB) which occur at substorm onsets. Based on that observation, radio detections of MFB from Toolik Lake Observatory have been applied to investigate occurrence of CLT in ISR data from Poker Flat Incoherent Scatter Radar and their possible correlation with MFB. Of 131 MFB events, 25 occurred within 15 min of an ISR echo detection, compared to 6 of 116 intervals of a control set with similar local time and seasonal distribution. The difference is significant at the 10−4 level, suggesting that ISR echoes are more probable during substorm onset times identified using MFB as a proxy. However, only four observed ISR echoes coincident with one MFB event showed both specific characteristics consistent with CLT. Furthermore, investigation of the angle of arrival of MFB suggests that the electromagnetic emissions do not originate from the plasma volume where the ISR detects the echoes. The small number of coincident ISR echoes and MFB is expected due to the different volumes in which the emissions and the echoes are detected. 50% of the MFB events occurred within 20 min of a substorm onset independently identified versus 8% of the control set intervals, confirming the correlation of MFB with substorm onsets.

Ionospheric Electron Density and Conductance Changes in the Auroral Zone During Substorms

AbstractEnhanced precipitation of magnetospheric energetic electrons during substorms increases ionospheric electron density and conductance. Such enhancements, which have timescales of a few hours, are not reproduced by the existing ionospheric models. We use the linear prediction filter (LPF) method to reconstruct the substorm‐related response of electron densities and integral conductances from long‐term ionospheric observations made by the European Incoherent SCATer radar located at Tromsø. To characterize the intensity of substorm dipolarization at a 5 min time step, we use the midlatitude positive bay index. We build response functions (LP filters) as a function of substorm time between T0−1 h and T0 + 4 h (T0 is a substorm onset time) in different magnetic local time (MLT) sectors to estimate the magnitude and delays of the ionospheric density response at different altitudes. Systematic and large relative changes are mostly observed in the D‐ and E regions. The duration of the response is about 3 h. It starts and reaches maximum magnitude near midnight, propagating from there toward the east and decaying after passing into the noon‐evening sector. The reliability of LPF results is confirmed by the consistency of D‐region response with independently derived response of the auroral absorption. Whereas strong ionization increases are seen in both E‐ and D‐regions on the nightside, the D‐region response is stronger in the morning‐dayside sector. Such MLT variation corresponds to the drift motion and precipitation of the high‐energy electrons injected in the nightside magnetosphere during substorm dipolarization. The inferred ionization changes result in strong enhancements of integral Hall (and Pedersen) conductance in the nightside auroral zone, where intense auroral currents are known to occur during substorms

Using Multiple Signatures to Improve Accuracy of Substorm Identification

AbstractWe have developed a new procedure for combining lists of substorm onset times from multiple sources. We apply this procedure to observational data and to magnetohydrodynamic (MHD) model output from 1–31 January 2005. We show that this procedure is capable of rejecting false positive identifications and filling data gaps that appear in individual lists. The resulting combined onset lists produce a waiting time distribution that is comparable to previously published results, and superposed epoch analyses of the solar wind driving conditions and magnetospheric response during the resulting onset times are also comparable to previous results. Comparison of the substorm onset list from the MHD model to that obtained from observational data reveals that the MHD model reproduces many of the characteristic features of the observed substorms, in terms of solar wind driving, magnetospheric response, and waiting time distribution. Heidke skill scores show that the MHD model has statistically significant skill in predicting substorm onset times.

Positive and negative feedbacks in the magnetosphere-ionosphere coupling

We study dynamics of field-aligned currents (FACs) during the expansion phase (EP) of two selected summer and winter substorms, by applying the magnetogram inversion technique (MIT) to the data of the world network of ground-based stations. The empirical MIT data and the modified Iijima and Potemra (I-P) model are presented. It is supposed a sharp increase in the upward FAC in I-P Region 1 and associated upward inter-hemispheric current in the dusk sector of the summer hemisphere during EP – in correspondence with theoretical models. Simultaneously (relative to the substorm onset time), and with the same value range, the downward inter-hemispheric current should enhance in the conjugate sector of the winter hemisphere. This FACs restructuring scenario in two hemispheres may clarify some of the unresolved issues in substorm physics. E.g., we describe the strong seasonal variation in the magnetic and auroral activity that is practically absent or unknown in literature even for the Northern hemisphere. Trying to answer this and other questions debated in literature, we study and describe strong feedbacks between the intensity of night FACs flowing in/out of each hemisphere, on the one hand, and the ionosphere conductivity or electric field, on the other. These magnetosphere-ionosphere feedbacks cause, in particular, the above inter-hemispheric currents and consequences prior and during the EP of the addressed substorms.

Global Energy Dynamics During Substorms on 9 March 2008 and 26 February 2008 Using Satellite Observations and the WINDMI Model

AbstractWe analyze three substorms that occur on (1) 9 March 2008 05:14 UT, (2) 26 February 2008 04:05 UT, and (3) 26 February 2008 04:55 UT. Using ACE solar wind velocity and interplanetary magnetic field Bz values, we calculate the rectified (southward Bz) solar wind voltage propagated to the magnetosphere. The solar wind conditions for the two events were vastly different, 300 kV for 9 March 2008 substorm, compared to 50 kV for 26 February 2008. The voltage is input to a nonlinear physics‐based model of the magnetosphere called WINDMI. The output is the westward auroral electrojet current which is proportional to the auroral electrojet (AL) index from World Data Center for Geomagnetism Kyoto and the SuperMAG auroral electrojet index (SML). Substorm onset times are obtained from the superMAG substorm database, Pu et al. (2010, https://doi.org/10.1029/2009JA014217), Lui (2011, https://doi.org/10.1029/2010JA016078) and synchronized to Time History of Events and Macroscale Interactions during Substorms satellite data. The timing of onset, model parameters, and intermediate state space variables are analyzed. The model onsets occurred about 5 to 10 min earlier than the reported onsets. Onsets occurred when the geotail current in the WINDMI model reached a critical threshold of 6.2 MA for the 9 March 2008 event, while, in contrast, a critical threshold of 2.1 MA was obtained for the two 26 February 2008 events. The model estimates 1.99 PJ of total energy transfer during the 9 March 2008 event, with 0.95 PJ deposited in the ionosphere. The smaller events on 26 February 2008 resulted in a total energy transfer of 0.37 PJ according to the model, with 0.095 PJ deposited in the ionosphere.

Tailward Propagation of Magnetic Energy Density Variations With Respect to Substorm Onset Times

AbstractDuring geomagnetic substorms, around 1015 J of energy is extracted from the solar wind and processed by the Earth's magnetosphere. Prior to the onset of substorm expansion phases, this energy is thought to be largely stored as an increase in the magnetic field in the magnetotail lobes. However, how, when, and where this energy is stored and released within the magnetotail is unclear. Using data from the Cluster spacecraft and substorm onsets from Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE), we examine the variation in the lobe magnetic energy density with respect to substorm onset for 541 isolated onsets. Based on a cross‐correlation analysis and a simple model, we deduce the following: On average, the magnetic energy density increases approximately linearly in the hour preceding onset and decreases at a similar rate after onset. The timing and magnitude of these changes varies with downtail distance, with observations from the mid‐tail ( ) showing larger changes in the magnetic energy density that occur ∼20 min after changes in the near‐tail ( ). The decrease in energy density in the near‐tail region is observed before the ground onset identified by SOPHIE, implying that the substorm is driven from the magnetotail and propagates into the ionosphere. The implication of these results is that energy in the near‐tail region is released first during the substorm expansion phase, with energy conversion propagating away from the Earth with time.

Birkeland currents during substorms: Statistical evidence for intensification of Regions 1 and 2 currents after onset and a localized signature of auroral dimming

AbstractWe conduct a superposed epoch analysis of Birkeland current densities from AMPERE (Active Magnetosphere and Planetary Electrodynamics Response Experiment) using isolated substorm expansion phase onsets identified by an independently derived data set. In order to evaluate whether R1 and R2 currents contribute to the substorm current wedge, we rotate global maps of Birkeland currents into a common coordinate system centered on the magnetic local time of substorm onset. When the latitude of substorm is taken into account, it is clear that both R1 and R2 current systems play a role in substorm onset, contrary to previous studies which found that R2 current did not contribute. The latitude of substorm onset is colocated with the interface between R1 and R2 currents, allowing us to infer that R1 current closes just tailward and R2 current closes just earthward of the associated current disruption in the tail. AMPERE is the first data set to give near‐instantaneous measurements of Birkeland current across the whole polar cap, and this study addresses apparent discrepancies in previous studies which have used AMPERE to examine the morphology of the substorm current wedge. Finally, we present evidence for an extremely localized reduction in current density immediately prior to substorm onset, and we interpret this as the first statistical signature of auroral dimming in Birkeland current.

SAPS onset timing during substorms and the westward traveling surge

AbstractWe present multispacecraft observations in the magnetosphere and conjugate ionosphere of the onset time of subauroral polarization streams (SAPS) and tens of keV ring current injections on the duskside in three individual substorms. This is probably the first unequivocal determination of the substorm SAPS onset timing. The time lag between the SAPS and substorm onsets is much shorter than the gradient‐curvature drift time of ∼10 keV ions in the plasmasphere. It seemingly depends on the propagation time of substorm‐injected plasma from the dipolarization onset region to the plasmasphere, as well as on the SAPS position. These observations suggest that fast onset SAPS and ring current injections are causally related to the two‐loop system of the westward traveling surge.

Relationship between wave-like auroral arcs and Pi2 disturbances in plasma sheet prior to substorm onset.

Wave-like substorm arc features in the aurora and Pi2 magnetic disturbances observed in the near-Earth plasma sheet are frequently, and sometimes simultaneously, observed around the substorm onset time. We perform statistical analyses of the THEMIS ASI auroral observations that show wave-like bright spot structure along the arc prior to substorm onset. The azimuthal mode number values of the wave-like substorm arcs are found to be in the range of ~100–240 and decrease with increasing geomagnetic latitude of the substorm auroral arc location. We suggest that the azimuthal mode number is likely related to the ion gyroradius and azimuthal wave number. We also perform correlation study of the pre-onset wave-like substorm arc features and Pi2 magnetic disturbances for substorm dipolarization events observed by THEMIS satellites during 2008–2009. The wave-like arc brightness structures on the substorm auroral arcs tend to move azimuthally westward, but with a few exceptions of eastward movement, during tens of seconds prior to the substorm onset. The movement of the wave-like arc brightness structure is linearly correlated with the phase velocity of the Pi2 δBy disturbances in the near-Earth plasma sheet region. The result suggests that the Pi2 transverse δBy disturbances are related to the intensifying wave-like substorm onset arcs. One plausible explanation of the observations is the kinetic ballooning instability, which has high azimuthal mode number due to the ion gyroradius effect and finite parallel electric field that accelerates electrons into the ionosphere to produce the wave-like arc structure.

Statistical visualization of the Earth's magnetotail and the implied mechanism of substorm triggering based on superposed-epoch analysis of THEMIS data

Abstract. To investigate the physical mechanism responsible for substorm triggering, we performed a superposed-epoch analysis using plasma and magnetic-field data from THEMIS probes. Substorm onset timing was determined based on auroral breakups detected by all-sky imagers at the THEMIS ground-based observatories. We found earthward flows associated with north–south auroral streamers during the substorm growth phase. At around X = −12 Earth radii (RE), the northward magnetic field and its elevation angle decreased markedly approximately 4 min before substorm onset. Moreover, a northward magnetic-field increase associated with pre-onset earthward flows was found at around X = −17 RE. This variation indicates that local dipolarization occurs. Interestingly, in the region earthwards of X = −18 RE, earthward flows in the central plasma sheet (CPS) reduced significantly approximately 3 min before substorm onset, which was followed by a weakening of dawn-/duskward plasma-sheet boundary-layer flows (subject to a 1 min time lag). Subsequently, approximately 1 min before substorm onset, earthward flows in the CPS were enhanced again and at the onset, tailward flows started at around X = −20 RE. Following substorm onset, an increase in the northward magnetic field caused by dipolarization was found in the near-Earth region. Synthesizing these results, we confirm our previous results based on GEOTAIL data, which implied that significant variations start earlier than both current disruption and magnetic reconnection, at approximately 4 min before substorm onset roughly halfway between the two regions of interest; i.e. in the catapult current sheet.

Automated determination of auroral breakup during the substorm expansion phase using all‐sky imager data

AbstractThis paper describes a novel method for quantitatively and routinely identifying auroral breakup following substorm onset using the Time History of Events and Macroscale Interactions during Substorms all‐sky imagers. Substorm onset is characterized by a brightening of the aurora that is followed by auroral poleward expansion and auroral breakup. This breakup can be identified by a sharp increase in the auroral intensity i(t) and the time derivative of auroral intensity i(t). Utilizing both i(t) and i'(t), we have developed an algorithm for identifying the time interval and spatial location of auroral breakup during the substorm expansion phase based solely on quantifiable characteristics of the optical auroral emissions. We compare the time interval determined by the algorithm to independently identified auroral onset times from three previously published studies. In each case the time interval determined by the algorithm is within error of the onset independently identified by the prior studies. We further show the utility of the algorithm by comparing the breakup intervals determined using the automated algorithm to an independent list of substorm onset times. We demonstrate that 50% of the breakup intervals characterized by the algorithm are within the uncertainty of the times identified in the list. The quantitative description and routine identification of an interval of auroral brightening during the substorm expansion phase provides a foundation for unbiased statistical analysis of the aurora and to probe the physics of the auroral substorm and identify the processes leading to auroral substorm onset.

OVATION‐SM: A model of auroral precipitation based on SuperMAG generalized auroral electrojet and substorm onset times

OVATION‐SM, an empirical model for different types of auroral energy flux, has been developed. OVATION‐SM is a linear combination of the SME index (or square root of SME index for monoenergetic auroras), time since the last substorm onset, and time until the next substorm onset. Because OVATION‐SM is based on ground magnetometer data and products of that data, it is possible to calculate continuous auroral power at a 1 min cadence for 30+ years. OVATION‐SM captures the gross auroral morphology, including onsets and other brightening and dimming events. The detailed spatial auroral morphology is beyond the scope of the current version of OVATION‐SM. OVATION‐SM explains more than 70% of the variance in Polar UV Imager nightside auroral power, which makes it a better predictor of nightside auroral power than any other model currently available.

Energetic particle injections to geostationary orbit: Relationship to flow bursts and magnetospheric state

To address the mechanism and factors controlling the injection of energetic particles to the geostationary orbit (GEO), we analyzed the appearance of injections at the GEO drift shell as observed by LANL spacecraft in the cases where the flow bursts and associated transient dipolarization were detected at the entry to the inner magnetosphere, in the high beta plasma sheet region on the nightside between 8 and 13 Re. We analyzed two different data sets, one including Geotail observations in 1995–2005 and another including a set of Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations in 2008–2009. We found that only a small portion of all flow bursts at 8–13 Re were associated with particle injection at GEO but that those injection‐associated flows had smaller values of plasma tube entropy parameter (PV5/3) as well as larger change of magnetic field north‐south component (dBz). This confirms a scenario that the bursty flows at the entry of the inner magnetosphere (8–13 Re) penetrate into GEO and produce there the energetic particles flux increase. According to the bubble theory of magnetotail plasma flows, the probability of the deep plasma penetration critically depends on how stretched the magnetospheric configuration is, and this dependence is statistically confirmed in a large database to be the major factor controlling the occurrence of GEO injections. We suggest using the background plasma tube entropy value in the nightside part of the GEO drift shell as a suitable parameter to predict the probability of particle injection to GEO. One more outcome of this study is that the energetic particle injections cannot reliably serve as a tool to identify the substorm onset times, as has been done in many past studies.

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 Bt and Bz 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.

Recent advances in understanding substorm dynamics

Magnetospheric substorms are elemental processes of solar wind energy storage and explosive release in Earth's magnetosphere. They encompass fundamental plasma physics questions, are ubiquitous during all types of geomagnetic conditions, contribute significantly to magnetic storms, and are a key element of Space Weather applications. This paper reviews recent major advances enabled by modern multi‐point space‐based and ground‐based platforms. These datasets have also empowered a system‐wide perspective and advanced modeling. We particularly highlight progress in two areas: (1) substorm onset timing and evidence for current sheet preconditioning and destabilization and (2) fast flows and dipolarizations, including the role of entropy in magnetotail plasma propagation.

Multiple magnetic dipolarizations observed by THEMIS during a substorm

Abstract. The magnetic field dipolarization in the vicinity of substorm onset and during substorm expansion phase during the period of 06:00–06:40 UT on 15 February 2008 is investigated with observations from multiple probes of THEMIS. It is found that the magnetic dipolarization at the substorm onset (the onset time was about 06:14 UT) was not accompanied by obvious magnetic disturbance and ion bulk speed variation. The magnetic dipolarizations taking place during the substorm expansion phase observed by P4~(−10.97, 2.04, −3.03) RE and P3~(−11.32, 1.15, −3.10) RE were mostly accompanied by high speed earthward ion bulk flow, but the magnetic dipolarizations occurring during the substorm expansion phase observed by P5~(−9.45, 1.07, −2.85) RE were not accompanied by high speed earthward ion bulk flow. Before substorm onset THEMIS P3, P4, P5 all observed the Bx component fluctuation with a period of about 300 s. After substorm onset earthward high speed ion bulk flow and significant magnetic disturbances both occurred at P3 and P4 locations. These results indicate that there is no one-to-one relationship between the near-Earth magnetic dipolarization and the earthward ion bulk flow. In particular, the magnetic dipolarization occurring on the earthward side of the inner near-Earth plasma sheet is not accompanied by high speed earthward ion bulk flow. The dipolarization at substorm onset is a local and small scale phenomenon. There are multiple magnetic dipolarizations occurring during the substorm expansion phase. The dipolarization process is very complex and is not simply an MHD process. It is accompanied by some kinds of plasma instabilities, the plasma sheet azimuthal expansion not only by earthward ion bulk flow during substorm. A sharp increase of the AE index does not always give an accurate substorm onset time for substorm analysis.

A superposed epoch investigation of the relation between magnetospheric solar wind driving and substorm dynamics with geosynchronous particle injection signatures

[1] We report a superposed epoch analysis of the hemispheric open magnetic flux, maximum nightside auroral intensity, geomagnetic activity, and solar wind and interplanetary magnetic field conditions around the time of substorm onset for three distinct categories of substorms defined by their energetic particle injection signatures. Substorms identified from global auroral imagery are classified into one of three categories based on their energetic particle injection signatures as seen at geosynchronous orbit by the Los Alamos National Laboratory spacecraft. Category 1 events are associated with a “classic” substorm injection, category 2 events show varied activity (i.e., energetic enhancements not following the evolution expected for classic substorms), and category 3 events show no apparent injection activity. The superposed epoch analysis reveals that the three distinct particle injection categories exhibit distinct differences in the level and continuity of magnetospheric driving by the solar wind, such that category 1 events can be described as classic substorm events, category 2 as continuously driven events, and category 3 as weak events. The results of this study suggest that the level and continuity of the dayside solar wind driving of the magnetosphere during substorms have a direct impact on the injection of energetic particles to geosynchronous orbit at substorm onset. These results could have considerable value in empirical predictions of the space weather environment.