Steady Magnetospheric Convection Research Articles

Dependence of the global dayside reconnection rate on interplanetary magnetic field By and the earth’s dipole tilt

In the recent years, significant attention has been given to the combined effect of Interplanetary Magnetic Field (IMF) duskward component (By) and dipole tilt on the global magnetosphere-ionosphere system response. Numerous studies have pointed out that when the Earth’s magnetic dipole is tilted away from the Sun (negative dipole tilt during northern winter), and IMF has a positive By component, the effects on ionospheric currents, particle precipitation, ionospheric convection, and average size of the auroral oval, is significantly more enhanced, compared to when IMF By is negative. Furthermore, this IMF By polarity effect reverses when Earth’s dipole is tilted in the opposite direction. The underlying cause has remained unclear. Our analysis shows that substorms tend to be stronger during the same IMF By and dipole tilt polarity combination. Taken together with earlier results showing also more frequent substorms during the same conditions, our observations suggests that when IMF By and dipole tilt have opposite signs, there is a more efficient global dayside reconnection rate. We also show analysis of the occurrence frequency of periods of Steady Magnetospheric Convection, substorm onset latitude, and the isotropic boundary of proton precipitation, that are all consistent with our conclusion that the combination of IMF By and dipole tilt polarity affect the global dayside reconnection rate.

Сопряженность Pi3/Ps6 геомагнитных пульсаций по данным наблюдений в Антарктике и Гренландии

We consider interhemispheric properties of fine structure of substorm – quasi-periodic geomagnetic fluctuations, Pi3 pulsations, using data from conjugate magnetometers in Antarctica and Greenland. Pi3 pulsations are found to accompany both the substorm expansion/recovery phases and the steady magnetospheric convection (SMC) events. The epicenter of Pi3 power is at the same latitude as maximal amplitude of magnetic bays. The interhemispheric properties of Pi3 pulsations are not consistent: in some events, coherent in-phase magnetic oscillations are observed in both hemispheres, in others, periodic variations are observed in one hemisphere only. When Pi3 pulsations are observed in both conjugate sites, their H-components are in-phase, which corresponds to the fundamental mode of field line oscillations between high-conductive ionospheres. Conjugate observations have provided an additional information on an elusive mechanism of Pi3 pulsations.

Properties of Storm‐Time Magnetic Flux Transport

AbstractMagnetospheric convection, the transport of magnetic flux, along with mass, and energy, plays a vital role in magnetosphere‐ionosphere coupling and ring‐current energization. It is particularly intense during magnetic storms, a mode of circulation driven by prolonged and strong solar wind driving. A large body of work has quantified the evolution of flux circulation during nonstorm times, in particular during substorms and steady magnetospheric convection events (SMCs). However, transport during storms has not been duly characterized. Using magnetotail flux observations we explore storm‐time modes of convection and compare them to their nonstorm‐time counterparts. We find that storm‐time convection often encompasses loading‐unloading periods (akin to substorms) and SMCs (akin to those during nonstorm times). However, loading‐unloading events and SMCs are present during only 25% and 21% (respectively) of the (95) storms examined. We also investigate the solar wind conditions during these modes for storm versus nonstorm times. We find that these modes have characteristic profiles in AE, solar wind driving, and open flux content during storms that are similar to their nonstorm‐time counterparts, but larger in magnitude. Therefore, these convection modes occur during storm and nonstorm periods alike, and in particular they are not required for a storm to progress. Thus, what makes storms special, their intense ring current, is likely not attributable to the (un)steadiness of flux circulation, but to the prolonged and intense background flux circulation within which these modes are embedded, and hence to the ultimate strength of the solar wind driver.

Magnetic Storms Accompanied with Steady Magnetospheric Convection and Characteristics of the Ring Current during the Storms

In this paper,12 magnetic storms accompanied with Steady Magnetospheric Convection (SMC) were selected from 2001 to 2017 based on our new criterion. It was found that these magnetic storms have the same characteristics: a long main phase up to about 10 hours; a minimum platform period for each of their SYM-$H$, which lasts about 3 to 10 hours; when these magnetic storms occur, the partial ring current is located at the dusk-side constantly, and its duration is equal to the stable southward driving time of the $B_z$ component of IMF. In addition, the ion loss time during its platform period was calculated, though there were some differences in the lifetime of the ions in different events, the lifetime of the ions in our events is much longer than models. And there is no obvious relationship between the lifetime of the ions and the length of the platform period. Based on our study, the decay of ring current is slowed down as the the SMC event during the magnetic storm brings more energy.

Magnetospheric Flux Throughput in the Dungey Cycle: Identification of Convection State During 2010

AbstractWe quantify the contributions of different convection states to the magnetic flux throughput of the magnetosphere during 2010. To do this we provide a continuous classification of convection state for the duration of 2010 based upon observations of the solar wind and interplanetary magnetic field, geomagnetic indices, and field‐aligned currents measured by the Active Magnetosphere and Planetary Electrodynamics Response Experiment. Convection states are defined as (1) quiet, (2) weak activity, substorm (3) growth, (4) expansion and (5) recovery phases, (6) substorm driven phase (when relatively steady magnetospheric convection occurs), (7) recovery bays (when recovery phase is accompanied by a negative excursion of the AL electrojet index), and (8) periods of multiple intensifications (storm‐time periods when continuous short‐period AL activity occur). The magnetosphere is quiet for 46% of the time, when very little convection takes place. The majority of convection occurs during growth and driven phases (21% and 38%, respectively, of open magnetic flux accumulation by dayside reconnection). We discuss these results in the context of the expanding/contracting polar cap model of convection, and describe a framework within which isolated substorms and disturbances during periods of more continuous solar wind‐magnetosphere driving can be understood.

Magnetic Storms Accompanied with Steady Magnetospheric Convection and Characteristics of the Ring Current during the Storms

参考活跃的磁层稳态对流标准，选取了2001—2017年12个伴随磁层稳态对流的磁暴，研究发现这些磁暴存在以下共性：有长达约10h的漫长主相；其SYM-<i>H</i>存在一个最小值的平台期，约持续3～10h；这些磁暴发生时，部分环电流持续位于昏侧，其持续时间和行星际磁场分量<i>B</i>_<i>z</i>的稳定南向驱动时间相等.此外，这些磁暴发生时，其平台期的环电流离子的寿命为2.4～5.5h，比一般的大磁暴事件中离子寿命长，且其寿命与平台期长短没有明显关系.伴随稳态对流的大磁暴发生时，环电流离子寿命长，环电流衰减慢，推测是稳态对流期间能量持续而稳定注入磁层导致的.

FEATURES OF EXCITATION AND AZIMUTHAL AND MERIDIONAL PROPAGATION OF LONG-PERIOD Pi3 OSCILLATIONS OF THE GEOMAGNETIC FIELD ON DECEMBER 8, 2017

We study the Pi3 pulsations (with a period T=15–30 min) that were recorded on December 8, 2017 at ground stations in the midnight sector of the magnetosphere at the latitude range of DP2 current system convective electrojets. We have found that Pi3 are especially pronounced in the pre-midnight sector with amplitude of up to 300 nT and duration of up to 2.5 hrs. The pulsation amplitude rapidly decreased with decreasing latitude from F′=72° to F′=63°. The event was recorded during the steady magnetospheric convection. In the southward Bz component of the interplanetary magnetic field, irregular oscillations were detected in the Pi3 frequency range. They correspond to slow magnetosonic waves occurring without noticeable variations in the dynamic pressure Pd. Ground-based geomagnetic observations have shown azimuthal propagation of pulsations with a 0.6–10.6 km/s velocity east and west of the midnight meridian. An analysis of the dynamics of pulsations along the meridian has revealed their propagation to the equator at a velocity 0.75–7.87 km/s. In the projection onto the magnetosphere, the velocities are close in magnitude to the observed propagation velocities of substorm injected electrons. In the dawn-side magnetosphere during ground-observed Pi3 pulsations, compression mode oscillations were recorded.&#x0D; We conclude that propagation of geomagnetic field oscillations in this event depends on the dynamics of particle injections under the action of a large-scale electric field of magnetospheric convection, which causes the plasma to move to Earth due to reconnection in the magnetotail. Small-scale oscillations in the magnetosphere were secondary, excited by the solar wind oscillations penetrating into the magnetosphere.

Особенности возбуждения и распространения по азимуту и меридиану длиннопериодных Pi3 колебаний геомагнитного поля 8 декабря 2017 г.

We study the Pi3 pulsations (with a period T=15–30 min) that were recorded on December 8, 2017 at ground stations in the midnight sector of the magnetosphere at the latitude range of DP2 current system convective electrojets. We have found that Pi3 are especially pronounced in the pre-midnight sector with amplitude of up to 300 nT and duration of up to 2.5 hrs. The pulsation amplitude rapidly decreased with decreasing latitude from F′=72° to F′=63°. The event was recorded during the steady magnetospheric convection. In the southward Bz component of the interplanetary magnetic field, irregular oscillations were detected in the Pi3 frequency range. They correspond to slow magnetosonic waves occurring without noticeable variations in the dynamic pressure Pd. Ground-based geomagnetic observations have shown azimuthal propagation of pulsations with a 0.6–10.6 km/s velocity east and west of the midnight meridian. An analysis of the dynamics of pulsations along the meridian has revealed their propagation to the equator at a velocity 0.75–7.87 km/s. In the projection onto the magnetosphere, the velocities are close in magnitude to the observed propagation velocities of substorm injected electrons. In the dawn-side magnetosphere during ground-observed Pi3 pulsations, compression mode oscillations were recorded.&#x0D; We conclude that propagation of geomagnetic field oscillations in this event depends on the dynamics of particle injections under the action of a large-scale electric field of magnetospheric convection, which causes the plasma to move to Earth due to reconnection in the magnetotail. Small-scale oscillations in the magnetosphere were secondary, excited by the solar wind oscillations penetrating into the magnetosphere.

Substorm Onset Latitude and the Steadiness of Magnetospheric Convection

AbstractWe study the role of substorms and steady magnetospheric convection (SMC) in magnetic flux transport in the magnetosphere, using observations of field‐aligned currents by the Active Magnetosphere and Planetary Electrodynamics Response Experiment. We identify two classes of substorm, with onsets above and below 65° magnetic latitude, which display different nightside field‐aligned current morphologies. We show that the low‐latitude onsets develop a poleward‐expanding auroral bulge, and identify these as substorms that manifest ionospheric convection‐braking in the auroral bulge region as suggested by Grocott et al. (2009, https://doi.org/10.5194/angeo-27-591-2009). We show that the high‐latitude substorms, which do not experience braking, can evolve into SMC events if the interplanetary magnetic field remains southward for a prolonged period following onset. We conclude that during periods of ongoing driving, the magnetosphere displays repeated substorm activity or SMC depending on the rate of driving and the open magnetic flux content of the magnetosphere prior to onset. We speculate that sawtooth events are an extreme case of repeated onsets and that substorms triggered by northward‐turnings of the interplanetary magnetic field mark the cessation of periods of SMC. Our results provide a new explanation for the differing modes of response of the terrestrial system to solar wind‐magnetosphere‐ionosphere coupling by invoking friction between the ionosphere and atmosphere.

Magnetotail Configuration During a Steady Convection Event as Observed by Low‐Altitude and Magnetospheric Spacecraft

AbstractMotivated by ongoing discussion regarding the magnetic configuration in the near‐Earth and midtail regions and its role in populating the inner magnetosphere during long‐duration steady magnetospheric convection (SMC) events, we analyze a rich collection of observations during ~10 hr of strong, steady solar wind driving. Auroral boundaries and regions of stretched and dipolarized magnetic field in the plasma sheet were monitored using solar electron loss cone anisotropy observed by low‐altitude spacecraft. Following a southward turning of the interplanetary magnetic field and a subsequent 3‐ to 4‐hr period of large‐scale substorm‐related reconfigurations and plasma injections, the near‐Earth magnetic configuration evolved into a nonstandard type, which lasted until the end of this SMC event (5 hr). During that time a dipolarized region with complicated Bz landscape persisted in the midtail, while the configuration was very stretched in the near tail. This was manifested as a highly depressed magnetic Bz component at geostationary orbit and as persistent nonadiabatic electron scattering at the periphery of the outer radiation belt. In addition, in situ observations suggest that a thin current sheet extended longitudinally toward the dawn terminator. In the return convection region near the terminator, observations of this azimuthal current sheet were sporadically interrupted/modulated by earthward convecting plasma structures, either remnants of reconnection‐produced plasma bubbles or flapping waves. The hybrid magnetotail configuration (dipolar in the midtail and stretched in the near tail) observed during this long‐duration SMC event poses a challenge for empirical magnetospheric modeling.

Comparison of the Ionosphere During an SMC Initiating Substorm and an Isolated Substorm

AbstractIn order to assess the effects of ionospheric feedback on different modes of energy transport in the magnetosphere, we investigate an isolated substorm and a steady magnetospheric convection (SMC) event with very similar solar wind drivers. The primary focus is on a comparison between the isolated substorm and the substorm that initiates the SMC. Auroral data from Polar UVI LBHl and LBHs, along with assimilative mapping of the ionosphere electrojet potential patterns are used as inputs to the global ionosphere‐thermosphere model to calculate conductances and Joule heating rates. Results from this study show that the conductances both before and during the events play a large role the ability of the magnetosphere to remain in steady driven state. The substorm that initiates the SMC event shows very different signatures in the ionosphere than isolated substorm; these signatures indicate that there is very weak substorm current wedge, or possibly a pseudo‐breakup.

SAPS‐Associated Explosive Brightening on the Duskside: A New Type of Onset‐Like Disturbance

AbstractQuasiperiodic energetic particle injections have been observed at geosynchronous orbit on the duskside during a steady magnetospheric convection event. We examine high‐resolution auroral imager data and ground magnetometer data associated with the first of these injections and conclude that it was not associated with classical substorm signatures. It is proposed that these injections are caused by the explosive nonlinear growth of a shear flow‐ballooning instability in the region where subauroral polarization streams (SAPS) also occur. It is suggested that interchange will occur preferentially in the low‐conductivity SAPS region since the magnetic Richardson number is lowest there and the “line‐tying” effect will also be least stabilizing there. We propose that the observed particle injection signatures and auroral morphology constitute a new type of SAPS‐associated explosive “onset‐like” disturbance that can occur during intervals of strong convection.

Comparative study of large‐scale auroral signatures of substorms, steady magnetospheric convection events, and sawtooth events

AbstractThis paper investigates the auroral evolution during different magnetospheric modes: substorms, steady magnetospheric convection, and sawtooth events. We undertake a superposed epoch analysis using data from the Imager for Magnetopause‐to‐Aurora Global Exploration Far Ultraviolet spectrographic imager and wideband imaging camera for each of these event types. We find that the auroral oval narrows and shows an equatorward movement prior to substorm onset. At substorm onset, the auroral oval brightens explosively near 23 magnetic local time (MLT). After this the aurorae expand poleward and the brightening stretches duskward and dawnward, with the duskward expansion being faster. Approximately 20 min after substorm onset, the aurorae begin to dim. Steady magnetospheric convection events with preceding substorms initially show the same signatures as substorms, but instead of the recovery after 20 min postonset, the aurorae stay bright for an extended period of time (at least 4 h after onset). Despite continued dayside driving of the system during steady magnetospheric convection events, we see a reconfiguration in the nightside auroral activity, taking place between 120 to 150 min after onset. Sawtooth events show very similar signatures to substorms, except for the auroral emission being much brighter, covering a wider MLT extent, and taking significantly less time to recover. The proton aurorae during substorms take ∼2–4 h to dim, during sawtooth events this process takes less than 1 h, despite enhanced reconnection rates. A similar effect is seen in the electron aurorae, albeit not as extreme.

Magnetic reconnection during steady magnetospheric convection and other magnetospheric modes

Abstract. We use remote sensing of the proton aurora with the IMAGE-FUV SI12 (Imager for Magnetopause to Aurora Global Exploration–Far Ultraviolet–Spectrographic Imaging at 121.8 nm) instrument and radar measurements of the ionospheric convection from the SuperDARN (Super Dual Aurora Radar Network) facility to estimate the open magnetic flux in the Earth's magnetosphere and the reconnection rates at the dayside magnetopause and in the magnetotail during intervals of steady magnetospheric convection (SMC). We find that SMC intervals occur with relatively high open magnetic flux (average ∼ 0.745 GWb, standard deviation ∼ 0.16 GWb), which is often found to be nearly steady, when the magnetic flux opening and closure rates approximately balance around 55 kV on average, with a standard deviation of 21 kV. We find that the residence timescale of open magnetic flux, defined as the ratio between the open magnetospheric flux and the flux closure rate, is roughly 4 h during SMCs. Interestingly, this number is approximately what can be deduced from the discussion of the length of the tail published by Dungey (1965), assuming a solar wind speed of ∼ 450 km s−1. We also infer an enhanced convection velocity in the tail, driving open magnetic flux to the nightside reconnection site. We compare our results with previously published studies in order to identify different magnetospheric modes. These are ordered by increasing open magnetic flux and reconnection rate as quiet conditions, SMCs, substorms (with an important overlap between these last two) and sawtooth intervals.

Effects of auroral potential drops on plasma sheet dynamics

AbstractThe reaction of the magnetosphere‐ionosphere system to dynamic auroral potential drops is investigated using the Lyon‐Fedder‐Mobarry global model including, for the first time in a global simulation, the dissipative load of field‐aligned potential drops in the low‐altitude boundary condition. This extra load reduces the field‐aligned current (j||) supplied by nightside reconnection dynamos. The system adapts by forcing the nightside X line closer to Earth, with a corresponding reduction in current lensing (j||/B = constant) at the ionosphere and additional contraction of the plasma sheet during substorm recovery and steady magnetospheric convection. For steady and moderate solar wind driving and with constant ionospheric conductance, the cross polar cap potential and hemispheric field‐aligned current are lower by approximately the ratio of the peak field‐aligned potential drop to the cross polar cap potential (10–15%) when potential drops are included. Hemispheric ionospheric Joule dissipation is less by 8%, while the area‐integrated, average work done on the fluid by the reconnecting magnetotail field increases by 50% within |y| &lt; 8 RE. Effects on the nightside plasma sheet include (1) an average X line 4 RE closer to Earth; (2) a 12% higher mean reconnection rate; and (3) dawn‐dusk asymmetry in reconnection with a 17% higher rate in the premidnight sector.

Influence of ion outflow in coupled geospace simulations: 2. Sawtooth oscillations driven by physics‐based ion outflow

AbstractWe present the first simulations of magnetospheric sawtooth oscillations under steady solar wind conditions that are driven internally by heavy ion outflow from a physics‐based model. The simulations presented use the multifluid Lyon‐Fedder‐Mobarry magnetohydrodynamics model two‐way coupled to the ionosphere/polar wind model (IPWM). Depending on the type of wave‐particle interactions utilized within IPWM, the coupled simulations exhibit either sawtooth oscillations or steady magnetospheric convection. Contrasting the simulations that do and do not develop sawtooth oscillations yields insights into the relationship between outflow and sawtooth oscillations. The total outflow rate is not an adequate predictor of the convection mode that will emerge. The simulations that develop sawtooth oscillations are characterized by intense outflow concentrated in the midnight auroral region. This outflow distribution mass loads the tail reconnection region without excessively mass loading the dayside reconnection region and leads to an imbalance between the dayside and nightside reconnection rates.

The AUTUMNX magnetometer meridian chain in Québec, Canada

The AUTUMNX magnetometer array consists of 10 THEMIS-class ground-based magnetometers deployed to form a meridian chain on the eastern coast of Hudson Bay in eastern Canada, a second partial chain one hour of magnetic local time further east, and one magnetometer at an intermediate midlatitude site. These instruments, augmented by those of other arrays, permit good latitudinal coverage through the auroral zone on two meridians, some midlatitude coverage, and detection of magnetic field changes near the sensitive infrastructure of the Hydro-Quebec power grid. Further, they offer the possibility for conjugate studies with Antarctica and the GOES East geosynchronous satellite, and complement the Chinese International Space Weather Meridian Circle Program. We examine current world distribution of magnetometers to show the need for AUTUMNX, and describe the instrumentation which allows near-real-time monitoring. We present magnetic inversion results for the disturbed day February 17, 2015, which showed classic signatures of the substorm current wedge, and developed into steady magnetospheric convection (SMC). For a separate event later that day, we examine a large and rapid magnetic field change event associated with an unusual near-Earth transient. We show GOES East conjugacy for these events.

Can ring current stabilize magnetotail during steady magnetospheric convection?

AbstractThe present study investigates the role of the ring current in stabilizing the magnetotail during steady magnetospheric convection (SMC) events. We develop a method for estimation of the symmetric ring current intensity from the single spacecraft magnetic field observations. The method is applied to a large number of SMC events identified using three different automatic procedures adopted from the literature. It is found that the symmetric ring current can be weak or strong depending on a particular event. We find a significant fraction of events that have a rather weak symmetric ring current in spite of the strong solar wind driving during the event. These findings imply that the symmetric ring current plays no role in the magnetotail stabilization.

Are steady magnetospheric convection events prolonged substorms?

AbstractMagnetospheric modes, including substorms, sawtooth events, and steady magnetospheric convection events, have in the past been described as different responses of the magnetosphere to coupling with the solar wind. Using previously determined event lists for sawtooth events, steady magnetospheric convection events, and substorms, we produce a statistical study of these event types to examine their similarities and behavior in terms of solar wind parameters, auroral brightness, open magnetospheric flux, and geomagnetic indices. A superposed epoch analysis shows that individual sawteeth show the same signatures as substorms but occur during more extreme cases of solar wind driving as well as geomagnetic activity. We also explore the limitations of current methods of identifying steady magnetospheric convection events and explain why some of those events are flagged inappropriately. We show that 58% of the steady magnetospheric convection events considered, as identified by criteria defined in previous studies are part of a prolonged version of substorms due to continued dayside driving during expansion phase. The remaining 42% are episodes of enhanced magnetospheric convection, occurring after extended periods of dayside driving.

The importance of storm time steady magnetospheric convection in determining the final relativistic electron flux level

AbstractRelativistic electrons pose a space weather hazard to satellites in the radiation belts. Although about half of all geomagnetic storms result in relativistic electron flux enhancements, other storms decrease relativistic electron flux, even under similar solar wind drivers. Radiation belt fluxes depend on a complex balance between transport, loss, and acceleration. A critically important aspect of radiation belt enhancements is the role of the “seed” population—plasma sheet particles heated and transported earthward by magnetotail processes—which can become accelerated by wave‐particle interactions with chorus waves. While the effect of substorms on seed electron injections has received considerable focus, in this study we present a previously unexplored connection between quasi‐steady convection during steady magnetospheric convection (SMC) events and the transport and energization of electrons. SMC events are long‐duration intervals of enhanced convection without any substorm expansions and are an important mechanism in coupling magnetotail plasma populations to the inner magnetosphere. We find that storms with SMCs in the recovery phase are more likely to increase relativistic electron flux levels, while storms without SMCs are more likely to result in a decrease. Using particle measurements from the Time History of Events and Macroscale Interactions During Substorms mission, we show that phase space density of seed electron populations increases 1 h before SMC start and stays elevated through the duration of SMCs. Chorus activity is also elevated during SMC events. These results suggest that rather than hindering electron acceleration by diverting plasma away from the inner magnetosphere, SMC events appear to act to enhance and maintain seed electron populations.