Northward Interplanetary Magnetic Field Conditions Research Articles

Intermittent Lobe Reconnection Under Prolonged Northward Interplanetary Magnetic Field Condition: Insights From Cusp Spot Event Observations

AbstractPrevious studies have suggested that lobe reconnection under stable northward interplanetary magnetic field (IMF) conditions should be continuous. However, to what extent it can be considered as continuous is unknown. Auroral cusp spots have been considered a signature of lobe reconnection. Here, we show that during a cusp spot event continuously observed by the Defense Meteorological Satellite Program for approximately 4 hr under stable IMF conditions, the ground radars recorded multiple intermittent equatorward‐moving radar forms (EMRFs) near local noon and multiple intermittent poleward‐moving radar forms (PMRFs) at post‐noon, each lasting ∼20–30 min. These intermittent EMRFs and PMRFs are suggested to correspond to intermittent lobe reconnections occurring at the cusp's poleward boundary near local noon and the duskside boundary at post‐noon, respectively. These findings challenge the previously held notion of continuous lobe reconnection under stable IMF conditions.

Probabilistic modelling of substorm occurrences with an echo state network

Abstract. The relationship between solar-wind conditions and substorm activity is modelled with an approach based on an echo state network. Substorms are a fundamental physical phenomenon in the magnetosphere–ionosphere system, but the deterministic prediction of substorm onset is very difficult because the physical processes that underlie substorm occurrences are complex. To model the relationship between substorm activity and solar-wind conditions, we treat substorm onset as a stochastic phenomenon and represent the stochastic occurrences of substorms with a non-stationary Poisson process. The occurrence rate of substorms is then described with an echo state network model. We apply this approach to two kinds of substorm onset proxies. One is a sequence of substorm onsets identified from auroral electrojet intensity, and the other is onset events identified from activity of Pi2 pulsations, which are irregular geomagnetic oscillations often associated with substorm onsets. We then analyse the response of substorm activity to solar-wind conditions by feeding synthetic solar-wind data into the echo state network. The results indicate that the effect of the solar-wind speed is important, especially for Pi2 substorms. A Pi2 pulsation can often occur even if the interplanetary magnetic field (IMF) is northward, while the activity of auroral electrojets is depressed during northward IMF conditions. We also observe spiky enhancements in the occurrence rate of substorms when the solar-wind density abruptly increases, which might suggest an external triggering due to a sudden impulse of solar-wind dynamic pressure. It seems that northward turning of the IMF also contributes to substorm occurrences, though the effect is likely to be minor.

3D GUMICS Simulations of Northward IMF Magnetotail Structure

AbstractThis study presents a re‐evaluation of the Kullen and Janhunen (2004, https://doi.org/10.5194/angeo-22-951-2004) global northward interplanetary magnetic field (IMF) simulation, using the Grand Unified Magnetosphere–Ionosphere Coupling Simulation version 4 (GUMICS‐4), a global MHD model. We investigate the dynamic coupling between northward IMF conditions and the Earth’s magnetotail and compare the results to observation‐based mechanisms for the formation of transpolar arcs. The results of this study reveal that under northward IMF conditions (and northward IMF initialization), a large closed field line region forms in the magnetotail, with similarities to transpolar arc structures observed from spacecraft data. This interpretation is supported by the simultaneous increase of closed flux measured in the magnetotail. However, the reconnection configuration differs in several respects from previously theorized magnetotail structures that have been inferred from both observations and simulations results and associated with transpolar arcs. We observe that dawn–dusk lobe regions form as a result of high‐latitude reconnection during the initialization stages, which later come into contact as the change in the IMF By component causes the magnetotail to twist. We conclude that in the GUMICS simulation, transpolar arc‐like structures are formed as a result of reconnection in the magnetotail, rather than high‐latitude reconnection or due to the mapping of the plasma sheet through a twisted magnetotail as interpreted from previous analysis of GUMICS simulations.

A statistical study of space hurricanes in the Northern Hemisphere

The space hurricane is a newly discovered large-scale three-dimensional magnetic vortex structure that spans the polar ionosphere and magnetosphere. At the height of the ionosphere, it has a strong circular horizontal plasma flow with a nearly zero-flow center and a coincident cyclone-shaped aurora caused by strong electron precipitation associated with intense upward magnetic field-aligned currents. By analyzing the long-term optical observation onboard the Defense Meteorological Satellite Program (DMSP) F16 satellite from 2005 to 2016, we found that space hurricanes in the Northern Hemisphere occur in summer and have a maximum occurrence rate in the afternoon sector around solar maximum. In particular, space hurricanes are more likely to occur in the dayside polar cap at magnetic latitudes greater than 80°, and their MLT (magnetic local time) dependence shows a positive relationship with the IMF (interplanetary magnetic field) clock angle. We also found that space hurricanes occur mainly under dominant positive IMF By and Bz and negative Bx conditions. It is suggested that the stable high-latitude lobe reconnection, which occurs under the conditions of a large Earth’s dipole tilt angle and high ionosphere conductivity in summer, should be the formation mechanism of space hurricanes. The result will give a better understanding of the solar wind–magnetosphere–ionosphere coupling process under northward IMF conditions.

The Dawn‐Dusk Tail Lobe Magnetotail Configuration and the Formation of Aurora Transpolar Arc

AbstractThe magnetotail configuration includes a northern tail lobe, a southern tail lobe, and a plasma sheet when IMF directs southward, as a result of the Dungey cycle. However, it is little known about the magnetotail configuration for northward IMF (NIMF) conditions. Using in situ data and corresponding simulations, we show that the magnetotail forms into a dawnside tail lobe and a duskside tail lobe under NIMF with significant By conditions. The data shows signatures related to magnetotail reconnection under such conditions. Cusp reconnection and such magnetotail reconnection may form a dawn‐dusk tail lobe configuration and its version of Dungey cycle. Simulation results also show that aurora transpolar arc may be caused by a process of reconfiguration of the dawn‐dusk tail lobe structure as a result of an IMF By reversal.

Simulating the Responses of the Magnetosphere–Ionosphere System to the IMF By Reversal

The interplanetary magnetic field (IMF) By component can cause asymmetric features in the magnetosphere-ionosphere system and influence the electromagnetic energy input. In this study, we use the Space Weather Modeling Framework (SWMF) to study the transient dynamics after a sudden change in the IMF By component exerts on the magnetosphere system. Simulation results reveal that under northward IMF conditions, an abrupt change of the IMF By from duskward to dawnward induces a dawnward geomagnetic field firstly in the cusp region and then in both the near-Earth and magnetotail regions. The signal of the new y-component of the geomagnetic field in the near-Earth region extends tailward while that in the magnetotail moves Earthward, leading the middle-tail region to react last. We investigate in detail the transitional change of the y-component of the geomagnetic field and compare it to the scenario under southward IMF conditions, and find that the latter case reacts in a much simpler manner and the responses are faster globally.

Transport Path of Cold‐Dense Plasmas in the Dusk Magnetotail Plasma Sheet: MMS Observations

AbstractThe near‐Earth plasma sheet becomes cold and dense under northward interplanetary magnetic field (IMF) condition, which suggests efficient solar wind plasma entry into the magnetosphere across the magnetopause for northward IMF and a possible contribution of ionospheric oxygen ion outflow. The cold and dense characteristics of the plasma sheet are more evident in the magnetotail flank regions that are the interface between cold solar wind plasma and hot magnetospheric plasma. Several physical mechanisms have been proposed to explain the solar wind plasma entry across the magnetopause and resultant formation of the cold‐dense plasma sheet (CDPS) in the tail flank regions. However, the transport path of the cold‐dense plasma inside the magnetotail has not been understood yet. Here, we present a case study of the CDPS in the dusk magnetotail by magnetospheric multiscale (MMS) spacecraft under strongly northward IMF and high‐density solar wind conditions. The ion distribution function consists of high‐ and low‐energy components, and the low‐energy one intermittently shows energy dispersion in the directions parallel and antiparallel to the local magnetic field. The time‐of‐flight analysis of the energy‐dispersed low‐energy ions suggests that these ions originate in the region farther down the tail, move along the magnetic field toward the ionosphere and then come back to the magnetotail by the mirror reflection. The pitch‐angle dispersion analysis gives consistent results on the traveling time and path length of the energy‐dispersed ions. Based on these observations, we discuss possible generation mechanisms of the energy‐dispersed structure of the low‐energy ions during the northward IMF.

Importance of the Northward IMF for the Quasistatic Mesoscale Field‐Aligned Currents Embedded in the Diminished Region 1/2 Current System in the Dusk Sector

AbstractTo understand the features of the quasistatic field‐aligned currents (FACs) consisting of multiple mesoscale (∼10–100 km) upward and downward FAC regions in the duskside auroral oval, we examined the magnetic field data obtained through the constellation measurements of Swarm satellites for a duration of more than three years. We introduced a method of automated event identification and took the events in 577 satellite orbits that pass through the high‐latitude part of the 14–18 magnetic local time auroral oval. The results of the analysis reveal that the occurrence ratio of the FAC events increases with the IMF BZ, and that the event can be detected at a ratio exceeding 20% for BZ greater than 1 nT. This indicates that the quasistatic mesoscale FACs in the duskside sector are pertinent to a northward IMF condition and that they are not a simple remnant of the diminished Region 1. The results also show that the FAC density tends to increase with the solar wind proton density, implying that the latter is directly associated with FAC generation. The FAC events can occur in a wide range of solar zenith angles, that is, 65–135°, indicating that whether the ionosphere is sunlit or dark is not crucial for the appearance of the event. These results strongly suggest that multiple mesoscale FACs are produced in the low‐latitude boundary layer through a mechanism related to solar wind plasma entry processes that can be more easily attained as the northward component of the IMF increases.

Transpolar Arcs During a Prolonged Radial Interplanetary Magnetic Field Interval

AbstractTranspolar arcs (TPAs) are believed to predominantly occur under northward interplanetary magnetic field (IMF) conditions with their hemispheric asymmetry controlled by the Sun‐Earth (radial) component of the IMF. In this study, we present observations of TPAs that appear in both the northern and southern hemispheres even during a prolonged interval of radially oriented IMF. The Defense Meteorological Satellite Program (DMSP) F16 and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellites observed TPAs on the dawnside polar cap in both hemispheres (one TPA structure in the southern hemisphere and two in the northern hemisphere) during an interval of nearly earthward‐oriented IMF on October 29, 2005. The southern hemisphere TPA and one of the northern hemisphere TPAs are associated with electron and ion precipitation and mostly sunward plasma flow (with shears) relative to their surroundings. Meanwhile, the other TPA in the northern hemisphere is associated with an electron‐only precipitation and antisunward flow relative to its surroundings. Our observations indicate the following: (a) the TPA formation is not limited to northward IMF conditions; (b) the TPAs can be located on both closed field lines rooted in the polar cap of both hemispheres and open field lines connected to the northward field lines draped over one hemisphere of the magnetopause. We believe that the TPAs presented here are the result of both indirect and direct processes of solar wind energy transfer to the high‐latitude ionosphere.

Magnetotail Configuration Under Northward IMF Conditions

AbstractDue to the sparsity of space probes, it is still not clear on how the magnetic structure of the magnetotail looks like and how it evolves when the interplanetary magnetic field (IMF) directs northward. This simulation study uses two different global magnetosphere magnetohydrodynamics (MHD) models to simulate two northward IMF events and study the evolution of the magnetotail. Both models show that the magnetotail may form a structure that is composed of a dawnside tail lobe and a duskside tail lobe, under northward IMF conditions with significant By, instead of a northern tail lobe and a southern tail lobe under southward IMF conditions. In this magnetic configuration, a tail lobe extends a domain from northern (southern) cusp to southern (northern) IMF. The larger the magnitude of IMF clock angle, the longer and wider the magnetotail. Such magnetic configuration suggests that magnetotail reconnection is possible to occur when the dawnside tail lobe contacts with the duskside tail lobe and thus a substorm is also possible to occur under northward IMF conditions with significant By.

Observations of Mirror Mode Structures in the Dawn‐Side Magnetosphere

AbstractWe present observations of mirror mode structures in the dawn‐side magnetosphere from the Magnetospheric Multiscale Mission. The observations were made during a period of relatively stable northward interplanetary magnetic field conditions. We observed magnetic troughs only with reductions of up to 90% in the magnitude of the local magnetic field. The structures were closely aligned with the local magnetic field and had sizes perpendicular to the magnetic field of the order of 1Re and ion densities up to twice that of the surrounding regions. We used ion, magnetic field, and electric field data to examine the plasma conditions inside and outside of these structures to investigate the stability and evolution of the structures and estimate the magnitude of changes in the ion distributions. Comparing the ion bulk properties to theoretical predictions, we concluded that variations in the parameters with trough depth were due to the spacecraft traversing them at different locations along their length. Our calculations of heating and cooling of the ions at different pitch angles is consistent with theory. We conclude that these troughs were likely long‐lived, bistable structures that originated in the magnetosheath near the nose of the magnetopause and were captured into the magnetosphere by dual‐lobe reconnection.

Global Dynamics of the Earth's Magnetosphere During Northward IMF Conditions in the Era of the Heliophysics System Observatory

Global Dynamics of the Earth's Magnetosphere During Northward IMF Conditions in the Era of the Heliophysics System Observatory

Preservation and variation of ion-to-electron temperature ratio in the plasma sheet in geo-magnetotail

The ion-to-electron temperature ratio is a good indicator of the processes involved in solar wind plasma entering and being transported inside Earth’s plasma sheet. In this study, we have demonstrated that patchy magnetic reconnection has the potential to preserve the ion-to-electron temperature ratio under certain conditions. If the charged particles are non-adiabatically accelerated no more than once in a single reconnection, the temperature ratio would be preserved; on the other hand, this ratio would not be preserved if they are accelerated multiple times. Consequently, under a northward interplanetary magnetic field (IMF) condition, the reconnection in the nonlinear phase of the Kelvin–Helmholtz instability is the dominant process for solar-originated plasma entering the Earth’s magnetosphere, and the ion-to-electron temperature ratio is preserved inside the plasma sheet. When the direction of the IMF is southward, the reflection of electrons from the magnetic mirror point, and subsequent multiple non-adiabatic accelerations at the reconnection site, are the primary reasons for the observed low ion-to-electron temperature ratio close to the Earth at midnight. While reconnections that occur in the night-side far tail might preserve the ratio, turbulence on the boundaries of the bursty bulk flows (BBFs) could change the ratio in the far tail through the violation of the <i>frozen-in</i> condition of the ions. The plateau in the contour of the calculated ion-to-electron temperature ratio in the down tail distance between 40 and 60 Earth radii may explain the strong correlation between the ion and electron temperatures in the outer central plasma sheet, which has not been clearly understood till date.

The Quasipersistent Feature of Highly Structured Field‐Aligned Currents in the Duskside Auroral Oval: Conjugate Observation Via Swarm Satellites and a Ground All‐Sky Imager

AbstractDuring northward interplanetary magnetic field (IMF), irregular magnetic perturbations were observed in the duskside aurora oval via Swarm satellites instead of large‐scale Region 1/2 magnetic perturbations. By taking advantage of Swarm constellation measurements, and their conjugate observations with an all‐sky imager on the ground, the features of the irregular magnetic perturbations were examined. Detailed analysis of the data from Swarm A and Swarm C for two events demonstrated that the irregular magnetic perturbations are a result of highly structured quasistatic field‐aligned currents (FACs), not dynamic Alfvén waves. The typical latitudinal size of the upward FACs is 20–30 km. In each region of the upward FACs, 630‐nm aurora emissions are relatively strong, indicating that the energy flux of precipitating electrons having energies of a few hundred electron volts is high in each of the upward FAC regions. The enhanced mesoscale auroras continued to exist for at least approximately 30 min. These indicate that the mesoscale FAC structures also have quasipersistent features. The precipitating particle data from the Defense Meteorological Satellite Program satellite, which passed through the field of view of the all‐sky imager, indicate that the source of the precipitating particles is the duskside low‐latitude boundary layer (LLBL). We suggest that the highly structured quasipersistent FACs flow along the magnetic field lines connected to the duskside LLBL where cold dense ions exist. The highly structured FACs in the duskside aurora oval are the phenomena that are pertinent to the magnetosphere for a northward IMF condition, not a simple remnant of the typical Region 1.

Multiple transpolar auroral arcs reveal insight about coupling processes in the Earth’s magnetotail

A distinct class of aurora, called transpolar auroral arc (TPA) (in some cases called "theta" aurora), appears in the extremely high-latitude ionosphere of the Earth when interplanetary magnetic field (IMF) is northward. The formation and evolution of TPA offers clues about processes transferring energy and momentum from the solar wind to the magnetosphere and ionosphere during a northward IMF. However, their formation mechanisms remain poorly understood and controversial. We report a mechanism identified from multiple-instrument observations of unusually bright, multiple TPAs and simulations from a high-resolution three-dimensional (3D) global MagnetoHydroDynamics (MHD) model. The observations and simulations show an excellent agreement and reveal that these multiple TPAs are generated by precipitating energetic magnetospheric electrons within field-aligned current (FAC) sheets. These FAC sheets are generated by multiple-flow shear sheets in both the magnetospheric boundary produced by Kelvin-Helmholtz instability between supersonic solar wind flow and magnetosphere plasma, and the plasma sheet generated by the interactions between the enhanced earthward plasma flows from the distant tail (less than -100 RE) and the enhanced tailward flows from the near tail (about -20 RE). The study offers insight into the complex solar wind-magnetosphere-ionosphere coupling processes under a northward IMF condition, and it challenges existing paradigms of the dynamics of the Earth's magnetosphere.

Decay of Kelvin-Helmholtz Vortices at the Earth's Magnetopause Under Pure Southward IMF Conditions.

At the Earth's low‐latitude magnetopause, clear signatures of the Kelvin‐Helmholtz (KH) waves have been frequently observed during periods of the northward interplanetary magnetic field (IMF), whereas these signatures have been much less frequently observed during the southward IMF. Here, we performed the first 3‐D fully kinetic simulation of the magnetopause KH instability under the southward IMF condition. The simulation demonstrates that fast magnetic reconnection is induced at multiple locations along the vortex edge in an early nonlinear growth phase of the instability. The reconnection outflow jets significantly disrupt the flow of the nonlinear KH vortex, while the disrupted turbulent flow strongly bends and twists the reconnected field lines. The resulting coupling of the complex field and flow patterns within the magnetopause boundary layer leads to a quick decay of the vortex structure, which may explain the difference in the observation probability of KH waves between northward and southward IMF conditions.

Simultaneous Observations of Poleward‐Moving Auroral Forms at the Equatorward and Poleward Boundaries of the Auroral Oval in Antarctica

AbstractPoleward‐moving auroral form (PMAF) is a type of dayside auroral pattern. The distinguishing characteristic of a PMAF is its separation from the auroral oval and subsequent poleward motion. It has been reported and widely accepted that PMAFs occur more frequently during periods with dominant southward interplanetary magnetic field (IMF). However, most ground‐based optical observations were made near magnetic latitudes of 75°, close to the equatorward boundary of the dayside auroral oval; therefore, PMAFs occurring at the poleward boundary were likely missed easily. Using all‐sky images from two stations located on the similar magnetic meridian in Antarctica at 80° and 75° magnetic latitude, and taking solar wind data close to the magnetosphere by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, we demonstrate that unlike the result from prior low‐latitude single‐station observations, the combined observations from both stations show an insignificant difference in the PMAF occurrence rate in the northward and southward IMF conditions and no clear sign suggesting that the southward IMF or southward turning of the IMF triggers PMAFs. With good agreements between PMAF occurrence rate and the intensity of the poleward drift, we suggest that PMAFs are more likely to be plasma patches torn away from the dayside auroral oval convecting antisunward and rather than direct foot points of reconnecting flux tubes.

Nonlinear Response of the Cross Polar Cap Potential to Solar Wind Density Under Northward Interplanetary Magnetic Field

AbstractIt is commonly believed that the magnitude and orientation of interplanetary magnetic field (IMF) together with the solar wind (SW) velocity have the most important impact on the cross polar cap potential (φpc), so that little attention has been given to the effect of SW density, especially under northward IMF conditions. Previous studies have shown that φpc increases with SW density as a response to the changes in magnetosheath force balance, while our study shows that φpc has more complicated responses to the SW density depending on the magnitude of IMF rather than a simple linear response as reported previously. The φpc may be insensitive to SW density increasing at moderate IMF Bz (cf. 8 nT) and at intense Bz (20 nT) under large‐density conditions. The different behavior of SW density in regulating φpc is mainly due to the competing effects originated from viscous interaction and magnetic reconnection. Further, the physical mechanisms are explored, including the driving sources of the viscous potential and affecting factors of reconnection potential. These results pave the way for better understanding of the SW density effects on solar wind‐magnetosphere‐ionosphere (SW‐M‐I) interactions.

Unusual Location of the Geotail Magnetopause Near Lunar Orbit: A Case Study

AbstractThe Earth's magnetopause is highly variable in location and shape and is modulated by solar wind conditions. On 8 March 2012, the ARTEMIS probes were located near the tail current sheet when an interplanetary shock arrived under northward interplanetary magnetic field conditions and recorded an abrupt tail compression at ∼(‐60, 0, ‐5) RE in Geocentric Solar Ecliptic coordinate in the deep magnetotail. Approximately 10 minutes later, the probes crossed the magnetopause many times within an hour after the oblique interplanetary shock passed by. The solar wind velocity vector downstream from the shock was not directed along the Sun‐Earth line but had a significant Y component. We propose that the compressed tail was pushed aside by the appreciable solar wind flow in the Y direction. Using a virtual spacecraft in a global magnetohydrodynamic (MHD) simulation, we reproduce the sequence of magnetopause crossings in the X‐Y plane observed by ARTEMIS under oblique shock conditions, demonstrating that the compressed magnetopause is sharply deflected at lunar distances in response to the shock and solar wind VY effects. The results from two different global MHD simulations show that the shocked magnetotail at lunar distances is mainly controlled by the solar wind direction with a timescale of about a quarter hour, which appears to be consistent with the windsock effect. The results also provide some references for investigating interactions between the solar wind/magnetosheath and lunar nearside surface during full moon time intervals, which should not happen in general.

Generation of Turbulence in Kelvin‐Helmholtz Vortices at the Earth's Magnetopause: Magnetospheric Multiscale Observations

AbstractThe Kelvin‐Helmholtz instability (KHI) at Earth's magnetopause and associated turbulence are suggested to play a role in the transport of mass and momentum from the solar wind into Earth's magnetosphere. We investigate electromagnetic turbulence observed in Kelvin‐Helmholtz vortices encountered at the dusk flank magnetopause by the Magnetospheric Multiscale (MMS) spacecraft under northward interplanetary magnetic field (IMF) conditions in order to reveal its generation process, mode properties, and role. A comparison with another MMS event at the dayside magnetopause with reconnection but no KHI signatures under a similar IMF condition indicates that while high‐latitude magnetopause reconnection excites a modest level of turbulence in the dayside low‐latitude boundary layer, the KHI further amplifies the turbulence, leading to magnetic energy spectra with a power law index −5/3 at magnetohydrodynamic scales even in its early nonlinear phase. The mode of the electromagnetic turbulence is analyzed with a single‐spacecraft method based on Ampère's law, developed by Bellan (2016, https://doi.org/10.1002/2016JA022827), for estimating wave vectors as a function of spacecraft frame frequency. The results suggest that the turbulence does not consist of propagating normal‐mode waves but is due to interlaced magnetic flux tubes advected by plasma flows in the vortices. The turbulence at sub‐ion scales in the early nonlinear phase of the KHI may not be the cause of the plasma transport across the magnetopause but rather a consequence of three‐dimensional vortex‐induced reconnection, the process that can cause an efficient transport by producing tangled reconnected field lines.