Northward Interplanetary Magnetic Field Research Articles

Comparative simulations of Kelvin–Helmholtz induced magnetic reconnection at the Earth's magnetospheric flanks

This study presents three-dimensional (3D) resistive Hall-magnetohydrodynamic simulations of the Kelvin–Helmholtz instability (KHI) dynamics at Earth's magnetospheric flanks during northward interplanetary magnetic field periods. By comparing two simulations with and without initial magnetic shear, we analyze the impact of distinct magnetic field orientations on plasma dynamics and magnetic reconnection events taking into account 3D mechanisms, such as KHI high latitude stabilization. The identical nature of the simulations, except for the presence/absence of an initial magnetic shear, enables, for the first time, a complete and coherent comparative analysis of the latitudinal distribution of KH vortices, current sheets, reconnection events, and the evolution of the mixing layer. In one configuration, a uniform magnetic field leads to double mid-latitude reconnection (MLR), while in the other, magnetic shear induces both type I vortex-induced reconnection (VIR) and MLR. Notably, the type I VIR observed in this second scenario results from the combined action of line advection and vortex-induced current sheet pinching (the classic mechanism driving two-dimensional type I VIR). Of particular importance is our quantification of newly closed field lines that experienced double reconnection, ultimately becoming embedded in solar wind plasma at low latitudes while remaining connected to magnetospheric plasma at high latitudes. The varying abundance of such lines in the two simulations holds implications for plasma transport at the magnetopause.

On the Importance of the Kelvin‐Helmholtz Instability on Magnetospheric and Solar Wind Dynamics During High Magnetic Shear

AbstractThe secondary processes driven by the velocity shear driven Kelvin‐Helmholtz Instability (KHI) in the magnetized plasmas have been shown to be important in producing plasma transport and heating from the shocked solar wind into the Earth's magnetosphere (MSP). The plasma transport into the MSP due to KHI has been shown to be strongest during northward interplanetary magnetic field (IMF) via KHI driven low‐ and mid‐latitude reconnection process. In a recent article, Li et al. (2023), https://doi.org/10.1029/2023gl105539 show Magnetosphere Multi‐Scale (MMS) spacecraft observations of multiple, Alfvénic reconnection jets during southward IMF at the dawn‐side MSP flank. The quasi‐periodic oscillations in plasma parameters and compressed, ion‐scale current sheets were strongly indicative of the MMS crossing regions of MSP‐like and magnetosheath‐like plasma within Kelvin‐Helmholtz waves. In this brief commentary, the importance of this new discovery for magnetospheric and solar wind dynamics is discussed.

Tracking the Subsolar Bow Shock and Magnetopause: Applying the Magnetosheath Velocity Gradient Method

AbstractBoth the bow shock and magnetopause move in response to varying solar wind and magnetospheric conditions. Tracking their locations can provide important clues to the state of the solar wind‐magnetosphere interaction, but is difficult with single spacecraft observations. This paper employs multipoint THEMIS observations of velocity gradients in the subsolar magnetosheath to remotely sense boundary locations on a continual basis for various solar wind conditions. We present three cases: (a) continuous northward IMF and no magnetopause motion; (b) southward IMF and no magnetopause motion with evidence of nightside activity; and (c) southward IMF and pressure increase with inward motion. When observing spacecraft are located near the Sun‐Earth line, inferred boundary locations agree well with the predictions of the BATS‐R‐US global magnetohydrodynamic model, confirming the utility of both the new method and the models. Results show that boundaries often lie nearly at rest with amplitudes less than 0.5 RE. They provide evidence indicating that nightside reconnection and a strong sunward convection in the outer magnetosphere can counteract the magnetopause erosion expected when a southward interplanetary magnetic field (IMF) initiates reconnection on the dayside magnetopause.

Direct Detection of Ongoing Magnetic Reconnection at Mercury's High‐Latitude Magnetopause

AbstractAn ongoing magnetic reconnection event was detected in the Mercury's high latitude magnetopause during a northward interplanetary magnetic field. The reconnection X‐line region was revealed in the Mercury's magnetopause based on the encountered flux ropes ejected away from this region both planetward and tailward. A series of magnetic flux ropes, known as flux transfer event shower were observed tailward of this X‐line region. These flux ropes were probably expanding and deflected as they were ejected away tailward from the X‐line region. Large‐amplitude variations in all three components of the magnetic field and a few small‐scale flux ropes were observed inside the X‐line region, which could be the seed of the flux rope shower at the magnetopause. The observations suggest that magnetic reconnection is highly dynamic and persistent in Mercury's magnetosphere.

Plasma Observations in the Distant Magnetotail During Intervals of Northward IMF

AbstractWe examine a 6‐day traversal of the magnetotail by the ARTEMIS satellites during an interval of prolonged northward IMF. The electrostatic analyzer (ESA) onboard the ARTEMIS spacecraft measures high ion and electron fluxes at approximately 60 RE downtail in regions of the magnetotail which would normally be the magnetotail lobe, containing open flux evacuated of plasma. We interpret these observations as trapped plasma on closed magnetic flux indicating that the magnetotail is closed or partially closed but extends at least as far as ∼60 RE downtail. We find that the occurrence of plasma in the magnetotail and the closure of the magnetosphere results in distinct changes to the magnetotail structure including a reduction in the magnetic field strength and pressure as well as a narrowing of the tail by approximately 20 RE.

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.

Statistical Analysis of Electric Currents Within the Magnetosheath Using Dayside Magnetospheric Multiscale Mission Observations

AbstractEarth's magnetosheath is the region of shocked plasma that mediates coupling between the solar wind and magnetosphere. Magnetohydrodynamic (MHD) simulations predict electric current closure across the magnetosheath from the bow shock to the magnetopause. These currents provide a J × B force that diverts plasma flow along the flanks of the magnetosphere. Observations by the NASA Magnetospheric Multiscale (MMS) mission show that within the magnetosheath there are large amplitude, localized currents during periods of intense turbulence. We perform a statistical analysis of magnetic field data from the first 6 years (2015–2021) of the MMS mission during intervals when the satellites are on the dayside and generate statistical maps of electric current derived using the curlometer technique. We find that during the low magnetosonic Mach number regime (MMS < 5), the predicted current closure pattern becomes apparent for northward and southward IMF orientations, but not dawnward or duskward. For MMS > 5, results suggest that for all IMF orientations this large‐scale current closure pattern is not apparent, even after separating out quasi‐perpendicular (θbn ≥ 45°) and quasi‐parallel (θbn < 45°) bow shock conditions. Instead, the magnetosheath is dominated by small‐scale filamented current sheets that may be attributed to magnetosheath turbulence.

Location and Timing of Magnetic Reconnections in Earth's Magnetotail: Accomplishments of the 29‐Year Geotail Near‐Earth Magnetotail Survey

AbstractThe spacecraft Geotail surveyed the near‐Earth plasma sheet from XGSM = −10 to −31 RE and YGSM = −20 to +20 RE during the period from 1994 to 2022. It observed 243 magnetic reconnection events and 785 tailward flow events under various solar wind conditions during plasma sheet residence time of over 23,000 hr. Magnetic reconnections associated with the onset of magnetospheric substorms occur mostly in the range XGSM = −23 to −31 RE. When the solar wind is intense and high substorm activities continue, magnetic reconnection can occur closer to the Earth. The YGSM locations of magnetic reconnections depend on the solar wind conditions and on previous substorm activity. Under normal solar wind conditions, magnetic reconnection occurs preferentially in the pre‐midnight plasma sheet. Under conditions with intense (weak) solar wind energy input, however, magnetic reconnection can occur in the post‐midnight (duskside) plasma sheet. Continuous substorm activity tends to shift the magnetic reconnection site duskward. The plasma sheet thinning proceeds faster under intense solar wind conditions, and the loading process that provides the preconditions for magnetic reconnection becomes shorter. When magnetic flux piles up during a prolonged period with a strongly northward‐oriented interplanetary magnetic field (IMF) Bz, the time necessary to provide the preconditions for magnetic reconnection becomes longer. Although the solar wind conditions are the primary factors that control the location and timing of magnetic reconnections, the plasma sheet conditions created by preceding substorm activity or the strongly northward IMF Bz can modify the solar wind control.

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.

Polar and Equatorial Ionospheric Electrodynamical Coupling Under a Prolonged Northward Bz Interval

AbstractThe interplanetary magnetic field (IMF) significantly influences the global ionospheric electrodynamics, but it is still largely unknown the high‐ and low‐latitude ionospheric electrodynamical coupling under long‐duration northward IMF Bz (NBz) and By conditions. During the long‐duration NBz and duskward By conditions (Kp < 2, AE < 100 nT, SYM‐H > −25 nT) on 20 August 2014, six pairs of magnetometer data showed that daytime equatorial electrojet (EEJ) underwent strong decreases (reach up to 90%) at wide longitudes. Thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) captured well the decrease and the control simulations revealed that both effects of the NBz and By significantly change the plasma convection, Joule heating, and thermospheric winds at high latitudes; and penetration electric field (PEF) due to the NBz plays an important role in the decrease of the equatorial electric field. This study indicates the significant NBz effects on the equatorial ionospheric electrodynamics even during weak geomagnetic conditions, apart from the widely believed meteorological effects. Further study should be taken to disclose the relative contribution between geomagnetic and meteorological effects on the decrease of the equatorial electric field.

Data‐Based Modeling of the Magnetosheath Magnetic Field

AbstractA quantitative model of the magnetosheath (MS) magnetic structure is developed, using a multi‐year set of Geotail, Themis, Cluster, and MMS magnetometer and plasma instrument data. The MS database is created using an identification algorithm, based on observed magnetic field magnitudes and proton densities, normalized by their concurrent interplanetary values, followed by additional filtering with the help of standard bow shock (BS) and magnetopause (MP) models. The model architecture is based on the toroidal/poloidal formalism and a coordinate system that naturally accounts for the tailward flaring of both boundaries. The magnetic field expansions include 960 free coefficients, derived by fitting the model to a grand data set, split into independent training and validation subsets with 1,291,380 and 411,933 1‐min records, respectively. The model faithfully reproduces basic types of the interplanetary magnetic field (IMF) wrapping around the MP. Regular IMF sectors result in strongly dawn‐dusk asymmetric draping, with much larger magnitudes at the quasi‐perpendicular dusk side of BS, and weaker at the quasi‐parallel dawn side, where the MS field lines are bent and dragged tailward. Except in the case of the flow‐aligned IMF orientation, the subsolar field steadily grows toward the MP, and the effect is clearly IMF Bz‐dependent: the field and its gradient are larger (smaller) for northward (southward) IMF Bz, implying a pile‐up of the magnetic flux in the first case and stronger reconnection in the second. Model distributions of the MS field magnitude reveal local depressions, associated with polar cusps near the high‐latitude limits of data coverage.

Anomalous Response of Mercury’s Magnetosphere to Solar Wind Compression: Comparison to Earth

Magnetic field intensity increases when solar wind compresses a planet’s magnetosphere. The compression can be measured using the ratio of compressed magnetic fields to purely dipolar magnetic fields just inside the magnetopause. For Earth, the ratio is proportional to the subsolar standoff distance of the magnetopause. Data from in-orbit observations by the MESSENGER spacecraft indicate an opposite ratio for Mercury; the compression ratio is inversely proportional to the subsolar standoff distance. The additional magnetic fields induced by currents at the top of Mercury’s core enhance the total magnetic field strength. We also evaluated differences in the subsolar standoff of Mercury’s magnetopause according to the north–south polarity of the interplanetary magnetic field (IMF). Previous studies have not identified meaningful differences in subsolar standoff distance between those in northward versus southward IMF polarities for Mercury; however, we found that the difference is statistically significant at a large IMF B Z (15–20 nT). The magnetic reconnection that occurs behind the cusp for a large northward IMF transfers the magnetic flux to the dayside and increases the subsolar standoff distance. The eroded magnetic flux for a large southward IMF is compensated by the induced magnetic fields.

The Growth of Ring Current/SYM‐H Under Northward IMF Bz Conditions Present During the 21–22 January 2005 Geomagnetic Storm

AbstractThe total energy transfer from the solar wind to the magnetosphere is governed by the reconnection rate at the magnetosphere edges as the Z‐component of interplanetary magnetic field (IMF Bz) turns southward. The geomagnetic storm on 21–22 January 2005 is considered to be anomalous as the SYM‐H index that signifies the strength of ring current, decreases and had a sustained trough value of −101 nT lasting more than 6 hr under northward IMF Bz conditions. In this work, the standard WINDMI model is utilized to estimate the growth and decay of magnetospheric currents by using several solar wind‐magnetosphere coupling functions. However, it is found that the WINDMI model driven by any of these coupling functions is not fully able to explain the decrease of SYM‐H under northward IMF Bz. A dense plasma sheet along with signatures of a highly stretched magnetosphere was observed during this storm. The SYM‐H variations during the entire duration of the storm were only reproduced after modifying the WINDMI model to account for the effects of the dense plasma sheet. The limitations of directly driven models relying purely on the solar wind parameters and not accounting for the state of the magnetosphere are highlighted by this work.

GNSS Scintillations in the Cusp, and the Role of Precipitating Particle Energy Fluxes

AbstractUsing a large data set of ground‐based GNSS scintillation observations coupled with in situ particle detector data, we perform a statistical analysis of both the input energy flux from precipitating particles, and the observed occurrence of density irregularities in the northern hemisphere cusp. By examining trends in the two data sets relating to geomagnetic activity, we conclude that observations of irregularities in the cusp grows increasingly likely during storm‐time, whereas the precipitating particle energy flux does not. We thus find a weak or nonexistent statistical link between geomagnetic activity and precipitating particle energy flux in the cusp. This is a result of a previously documented tendency for the cusp energy flux to maximize during northward IMF, when density irregularities tend not to be widespread, as we demonstrate. At any rate, even though ionization and subsequent density gradients directly caused by soft electron precipitation in the cusp are not to be ignored for the trigger of irregularities, our results point to the need to scrutinize additional physical processes for the creation of irregularities causing scintillations in and around the cusp. While numerous phenomena known to cause density irregularities have been identified and described, there is a need for a systematic evaluation of the conditions under which the various destabilizing mechanisms become important and how they sculpt the observed ionospheric “irregularity landscape.” As such, we call for a quantitative assessment of the role of particle precipitation in the cusp, given that other factors contribute to the production of irregularities in a major way.

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.

Rayleigh‐Taylor Instability Observed at the Dayside Magnetopause Under Northward Interplanetary Magnetic Field

AbstractUnder northward interplanetary magnetic field, periodical fluctuations with the period of 85 s have been observed by one of the Time History of Events and Macroscale Interactions during Substorms spacecraft at the dayside magnetopause when the solar wind dynamic pressure suddenly dropped. The observed magnetic field distortions, characterized by the compression of the main component as well as the flapping of the perpendicular components, are in line with the theoretical predictions of the Rayleigh‐Taylor (R‐T) instability excited at the magnetopause. With the convective electric field and the mean electric field removed, the perturbations in the electric field are identified as typical two sinusoidal signals with a 90° phase difference, consistent with previous prediction of the attendant electrostatic field arising from the R‐T instability. The transverse motions of the plasmas resulting from the electric drifting driven by the electrostatic field are observed both in pitch‐angle distributions and in distribution functions. The growth rate of the R‐T instability is about 43 s, implying that the instability has time to grow during the disturbances. Using the Gauss theorem, the calculated net charge density based on the electric field observation is one part of 27 compared to the totally observed plasma density, indicating that only a tiny fraction of the charge separates to set up the electrostatic field, which is due to a collective effect in the plasma.

Magnetopause Standoff Position Changes and Geosynchronous Orbit Crossings: Models and Observations

AbstractThis research examines the ability of current physics‐based models to predict the magnetopause location. We use the Run‐On‐Request capabilities at the Community Coordinated Modeling Center at NASA GSFC with 4 magnetohydrodynamic (MHD) models. The magnetopause position prediction and response time to the solar wind changes is then compared to extreme solar wind conditions where magnetopause crossing of geosynchronous orbit have been observed by the Geostationary Operational Environmental Satellite (GOES) satellites. Rigorous analysis/comparison of observations and models is critical in determining magnetosphere dynamics for model validation. This paper is a preliminary effort defining the metrics necessary to understand the current magnetosphere model capabilities and challenges. Results show that there are discrepancies between the MHD models' standoff positions of the dayside magnetopause for the same solar wind conditions on events that included (a) an increase in solar wind dynamic pressure and a step function in the Interplanetary Magnetic Field Bz component; (b) nominal solar wind conditions (values of approximated 400 km/s for solar wind speed and 5 nT for the magnetic field magnitude) with a northward IMF; and (c) compression caused by several coronal mass ejections impacting the near Earth environment. Overall, the models predicted different magnetopause subsolar locations sometimes in the order of 3 RE. Contingency tables were calculated to show model performance in comparison with the data observed with the GOES 13/15 geosynchronous orbit for extreme events and skill scores were calculated.

Interhemispheric Conjugacy of Multiple Transpolar Arcs

AbstractMultiple transpolar arcs appearing simultaneously in the polar cap have gained much interest in recent years. By analyzing Defense Meteorological Satellite Program Special Sensor Ultraviolet Spectrographic Imagers data, we report for the first time, that less than half of the multiple arc events occur simultaneously in both hemispheres. In 60% of the cases, multiple arcs appear in only one hemisphere. There is a clear difference in interplanetary magnetic field (IMF) conditions for those two groups. Conjugate multiple arcs appear on average during stronger northward IMF and smaller IMF clock angles than non‐conjugate multiple arcs. Only non‐conjugate multiple arcs show a dependence on IMF BX. They form in the northern (southern) hemisphere during negative (positive) BX. An IMF BX induced interhemispheric asymmetry in the magnetospheric field line topology might explain why multiple arcs appear sometimes in only one hemisphere.

Energy transfer across magnetopause under dawn–dusk IMFs

A parametric study on the energy transfer of the solar wind across the magnetopause entering the magnetosphere is conducted using a global magnetohydrodynamic numerical simulation. The characteristics of the mechanical and electromagnetic energy distribution under the dawn–dusk interplanetary magnetic fields (IMFs) are investigated by analyzing magnetic reconnection and viscous effect, and compared with the radial and north–south IMFs. It is shown that (1) the interactions at the magnetopause and the transfer of energy across this boundary move in relation to the IMF orientation. (2) For the duskward IMF, the mechanical energy flow clearly enters the equatorial and low-latitude regions on the dayside, and the electromagnetic energy flow has a small inflow on the equatorial and low latitudes of the dayside. A significant energy inflow appears on the dawn side in the northern hemisphere and the dusk side in the southern hemisphere near the polar cusp. (3) The energy distribution characteristics across the magnetopause under dawn–dusk IMFs are mirror symmetric about the Y=0 plane. (4) For a magnetic field of 5 nT, the electromagnetic energy input under the dawn–dusk IMFs is twice as large as the mechanical energy and the electromagnetic energy under the radial IMF, which is five times as large as the electromagnetic energy during the pure northward IMF, but only half as large as the electromagnetic energy under the pure southward IMF. The mechanical energy input under dawn–dusk IMFs has the same magnitude as that under radial and north–south IMFs. The magnitude of the energy transfer rate for the dawn IMF and dusk IMF (about 3.5%) is between 1.71% for the northward IMF and 4.95% for the southward IMF, but higher than 2.22% for the radial IMF. The Akasofu-type energy-coupling formula, varepsilon, underestimates the energy input from the solar wind under B_{y} dominated IMF.

Seasonal and diurnal variations of Kelvin-Helmholtz Instability at terrestrial magnetopause

Kelvin-Helmholtz Instability is ubiquitous at Earth’s magnetopause and plays an important role in plasma entry into the magnetosphere during northward interplanetary magnetic fields. Here, using one solar cycle of data from NASA THEMIS (Time History of Events and Macro scale Interactions during Substorms) and MMS (Magnetospheric Multiscale) missions, we found that KHI occurrence rates show seasonal and diurnal variations with the rate being high near the equinoxes and low near the solstices. The instability depends directly on the Earth’s dipole tilt angle. The tilt toward or away from the Sun explains most of the seasonal and diurnal variations, while the tilt in the plane perpendicular to the Earth‐Sun line explains the difference between the equinoxes. The results reveal the critical role of dipole tilt in modulating KHI across the magnetopause as a function of time, highlighting the importance of Sun-Earth geometry for solar wind-magnetosphere interaction and for space weather.