Low-latitude Boundary Layer Research Articles

Stability of plasma cylinder with current in a helical plasma flow

Stability of a plasma cylinder with a current wrapped by a helical plasma flow is studied. Unstable surface modes of magnetohydrodynamic (MHD) oscillations develop at the boundary of the cylinder enwrapped by the plasma flow. Unstable eigenmodes can also develop for which the plasma cylinder is a waveguide. The growth rate of the surface modes is much higher than that for the eigenmodes. It is shown that the asymmetric MHD modes in the plasma cylinder are stable if the velocity of the plasma flow is below a certain threshold. Such a plasma flow velocity threshold is absent for the symmetric modes. They are unstable in any arbitrarily slow plasma flows. For all surface modes there is an upper threshold for the flow velocity above which they are stable. The helicity index of the flow around the plasma cylinder significantly affects both the Mach number dependence of the surface wave growth rate and the velocity threshold values. The higher the index, the lower the upper threshold of the velocity jump above which the surface waves become stable. Calculations have been carried out for the growth rates of unstable oscillations in an equilibrium plasma cylinder with current serving as a model of the low-latitude boundary layer (LLBL) of the Earth’s magnetic tail. A tangential discontinuity model is used to simulate the geomagnetic tail boundary. It is shown that the magnetopause in the geotail LLBL is unstable to a surface wave (having the highest growth rate) in low- and medium-speed solar wind flows, but becomes stable to this wave in high-speed flows. However, it can remain weakly unstable to the radiative modes of MHD oscillations.

Magnetospheric Ion Evolution Across the Low‐Latitude Boundary Layer Separatrix

AbstractOn 20 September 2015, the Magnetospheric Multiscale (MMS) spacecraft crossed the dusk magnetopause after a compression of the magnetosphere. Enhanced densities and fluxes of both colder (≤10 eV) and hotter (>1 keV) magnetospheric and magnetosheath heavy ion species were observed reaching the magnetopause. The evolution of the velocity distributions for H+, He+, and O+ measured by the Hot Plasma Composition Analyzer on MMS during this magnetopause crossing is presented. In particular, this study focuses on the changes in the species' distribution functions as MMS passes from the magnetosphere through the electron edge of the low‐latitude boundary layer (LLBL) separatrix and then into the LLBL. Two types of processes are suggested to play a role in the heating of colder magnetospheric ions across the LLBL separatrix in the region between the separatrix and the electron and ion edges of the LLBL. One mechanism leads to the formation and enhancement of ring distributions in this layer of the LLBL as the magnetospheric ions propagate across the separatrix. A second mechanism leading first to perpendicular heating and then to parallel heating of colder protons may arise from a possible two‐stream instability as the magnetospheric ions first encounter the warmer magnetosheath electrons in the electron layer and then the warmer magnetosheath ions between the electron and ion edges of the LLBL separatrix. Perpendicular heating of He+ and O+ is seen more so in the main reconnection exhaust, due to nonadiabatic behavior of these ions as they are accelerated up to the bulk flow speed.

The nonlinear behavior of whistler waves at the reconnecting dayside magnetopause as observed by the Magnetospheric Multiscale mission: A case study

AbstractWe show observations of whistler mode waves in both the low‐latitude boundary layer (LLBL) and on closed magnetospheric field lines during a crossing of the dayside reconnecting magnetopause by the Magnetospheric Multiscale (MMS) mission on 11 October 2015. The whistlers in the LLBL were on the electron edge of the magnetospheric separatrix and exhibited high propagation angles with respect to the background field, approaching 40°, with bursty and nonlinear parallel electric field signatures. The whistlers in the closed magnetosphere had Poynting flux that was more field aligned. Comparing the reduced electron distributions for each event, the magnetospheric whistlers appear to be consistent with anisotropy‐driven waves, while the distribution in the LLBL case includes anisotropic backward resonant electrons and a forward resonant beam at near half the electron‐Alfvén speed. Results are compared with the previously published observations by MMS on 19 September 2015 of LLBL whistler waves. The observations suggest that whistlers in the LLBL can be both beam and anisotropy driven, and the relative contribution of each might depend on the distance from the X line.

CORRELATION BETWEEN EMISSION INTENSITIES IN DAYSIDE AURORAL ARCS AND PRECIPITATING ELECTRON SPECTRA

AbstractMore than 20000 dayside auroral arcs of the 557.7 and 630.0 nm emission intensities have been statistically studied, and the dependences of the I557.7/I630.0 ratio on the I557.7 emission intensity have been determined. The 557.7 nm emission intensity has two maximum values in the hot spot and warm spot regions, with average values of 2.2 and 2.9 kR, respectively. But there is a maximum near magnetic noon for 630.0 nm emission intensity, with an average value of 1.5 kR. In the I557.7 emission range 0.1∼10 kR, the I557.7/I630.0 ratio tends to increase from 0.2 to 9. The correlation between emission intensity and precipitating electron spectra has been investigated using 17 cases of DMSP passing through 40 auroral arcs above Chinese Arctic Yellow River Station (YRS). We obtain the equations that the average energy of the electrons is proportional to the I557.7/I630.0 ratio. There is a positive correlation between the total energy flux of the electrons and the I557.7 emission intensity. The typical region of electron precipitation, where the auroral arcs were observed, was the boundary plasma sheet (BPS) in the prenoon and postnoon sectors. We also found some low‐energy precipitating electrons from the region of mantle, where the arcs are located poleward of dayside auroral oval. The magnetic source region of the precipitating electrons with low energy was identified as the low latitude boundary layer (LLBL) adjacent to magnetic noon. Arcs are located in the lower latitude in this region.

Kelvin‐Helmholtz wave at the subsolar magnetopause boundary layer under radial IMF

AbstractWe present the first observation of the Kelvin‐Helmholtz (KH) rolled‐up vortex at the dayside magnetopause layers under a radial interplanetary magnetic field (IMF). The study uses measurements of four Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes aligned along the YGSE axis about 10 RE upstream of the Earth and located in different regions of the near‐Earth environment. THEMIS C and A serve as monitors of the quiet solar wind and fluctuating magnetosheath conditions, respectively, and THEMIS D and E observe the magnetopause and low‐latitude boundary layer (LLBL) crossings. The analysis shows the following: (1) a radial IMF changes to the southward pointing magnetosheath magnetic field; (2) dayside reconnection forms the thin but dense LLBL; (3) a large velocity shear at the LLBL inner edge excites a train of KH waves; and (4) in spite of a short path from the subsolar point (≈5 RE), one of the KH waves exhibits all features of a fully developed rolled‐up vortex.

Observations of whistler mode waves with nonlinear parallel electric fields near the dayside magnetic reconnection separatrix by the Magnetospheric Multiscale mission

AbstractWe show observations from the Magnetospheric Multiscale (MMS) mission of whistler mode waves in the Earth's low‐latitude boundary layer (LLBL) during a magnetic reconnection event. The waves propagated obliquely to the magnetic field toward the X line and were confined to the edge of a southward jet in the LLBL. Bipolar parallel electric fields interpreted as electrostatic solitary waves (ESW) are observed intermittently and appear to be in phase with the parallel component of the whistler oscillations. The polarity of the ESWs suggests that if they propagate with the waves, they are electron enhancements as opposed to electron holes. The reduced electron distribution shows a shoulder in the distribution for parallel velocities between 17,000 and 22,000 km/s, which persisted during the interval when ESWs were observed, and is near the phase velocity of the whistlers. This shoulder can drive Langmuir waves, which were observed in the high‐frequency parallel electric field data.

Characteristics of GPS TEC variations in the polar cap ionosphere

AbstractThis paper presents statistical characteristics (occurrence rate, amplitude, and frequency) of low‐frequency (<100 mHz) variations in total electron content (TEC) observed in the polar cap ionosphere. TEC variations were primarily associated with mesoscale (tens to hundreds of kilometers) ionization structures and were observed by five Global Positioning System (GPS) receivers over a 6 year period (2009–2014). The altitude of ionization structures was estimated by using colocated ionosonde radars. High data rate receivers combined with broad spatial coverage of multisatellite TEC measurements provided high‐resolution magnetic local time/latitude maps of TEC variation characteristics, which were examined as a function of solar cycle and season. These high‐resolution maps improve upon the current observational picture of mesoscale structuring in the polar cap and provide accurate links to particular magnetospheric source regions. Occurrence of TEC variations was consistently highest in dayside regions mapping to low latitude and plasma mantle boundary layers, while largest‐amplitude TEC variations were observed in dayside regions close to the polar cusp, and lower latitudes around midnight. Occurrence and amplitude of TEC variations increased significantly during the ascending phase of the solar cycle, independent of solar wind conditions, while seasonal statistics showed highest dayside occurrence and amplitude in winter months, lowest in summer, and highest nightside occurrence and amplitude around equinox. A surprising result in the frequency distributions of TEC variations was discrete frequencies of about 2 and 4 mHz, which appeared to originate from regions corresponding to the plasma mantle, immediately poleward of the polar cusp.

The dependence of the LLBL thickness on IMF [formula omitted] and [formula omitted] components

The thickness of the low latitude boundary layer (LLBL) is studied as a function of interplanetary magnetic field (IMF) using the data of THEMIS mission. The data from intersections of LLBL by Themis-A and -C satellites are analyzed. Solar wind parameters are provided by Themis-B satellite located before the bow shock. We use earlier developed method of LLBL thickness determination based on the analysis of the variation of plasma velocity in the layer perpendicular to the magnetopause. The database for the present analysis consists of 109 single satellite LLBL crossings where the values of LLBL thickness are obtained. The time shift of solar wind propagation from the spacecraft performing measurements outside the bow shock to the LLBL is taken into account. We analyze the dependence of LLBL thickness on IMF Bz and By using data of IMF measurements with 3s resolution and produce the 180s averaging of these data. Large scattering of the values of LLBL thickness and the weak dependence on IMF is demonstrated. Dawn–dusk asymmetry of LLBL thickness is not observed. The dependence of LLBL thickness on IMF clock angle is discussed.

Statistical study of the ULF Pc4–Pc5 range fluctuations in the vicinity of Earth’s magnetopause and correlation with the Low Latitude Boundary Layer thickness

The main generation mechanisms for the Earth’s Low Latitude Boundary Layer (LLBL) are considered to be magnetic reconnection, viscous interactions such as Kelvin–Helmholtz instability and associated plasma mixing and diffusion. We have performed a statistical study of the Ultra Low Frequency (ULF) fluctuation power at the Pc4–Pc5 range using ≈6years of THEMIS measurements of the plasma velocity and magnetic field. The results reveal a clear dawn–dusk asymmetry showing that the fluctuation power is typically more enhanced in the vicinity of the magnetopause downstream of the quasi-parallel shock. The statistical study of the Vx-component of the plasma velocity indicates that the LLBL is also thicker on the dawn-sector. These results may suggest that the physical mechanisms that provide power in the Pc4–Pc5 range are more effective on the dawn-sector and provide a means for a more effective LLBL generation.

Geomagnetic disturbances and pulsations as a high-latitude response to considerable alternating IMF Variations during the magnetic storm recovery phase (Case study: May 30, 2003)

Features of high-latitude geomagnetic disturbances during the magnetic storm (Dst min =–144 nT) recovery phase were studied based on the observations on the Scandinavian profile of magnetometers (IMAGE). Certain non-typical effects that occur under the conditions of large positive IMF Bz values (about +20–25 nT) and large negative IMF By values (to–20 nT) were revealed. Thus, an intense (about 400 nT) negative bay in the X component of the magnetic field (the polar electrojet, PE) was observed in the dayside sector at geomagnetic latitudes higher than 70°. As the IMF B y reverses its sign from negative to positive, the bay in the X component was replaced by the bay in the Y component. The possible distribution of the fieldaligned currents of the NBZ system was analyzed based on the CHAMP satellite data. The results were compared with the position of the auroral oval (the OVATION model) and the ion and electron flux observations on the DMSP satellite. Analysis of the particle spectra indicated that these spectra correspond to the auroral oval dayside sector crossings by the satellite, i.e., to the dayside projection of the plasma ring surrounding the Earth. Arguments are presented for the assumption that the discussed dayside electrojet (PE) is localized near the polar edge of the dayside auroral oval in a the closed magnetosphere. The features of the spectral and spatial dynamics of intense Pc5 geomagnetic pulsations were studied in this time interval. It was established that the spectrum of high-latitude (higher than ~70°) pulsations does not coincide with the spectrum of fluctuations in the solar wind and IMF. It was shown that Pc5 geomagnetic pulsations can be considered as resonance oscillations at latitudes lower than 70° and apparently reflect fluctuations in turbulent sheets adjacent to the magnetopause (the low-latitude boundary layer, a cusp throat) or in a turbulent magnetosheath at higher latitudes.

Electron and ion edges and the associated magnetic topology of the reconnecting magnetopause

AbstractUsing high‐resolution burst mode THEMIS data, we have examined in detail the electron and ion edges of the reconnecting magnetopause and the associated magnetic topologies of 23 high shear reconnecting magnetopause crossings. The electron edge is identified as the most earthward detection of entering magnetosheath electrons and the accompanying first loss of magnetospheric electrons. The electron edge thus marks the most earthward measurable open (reconnected) field line. The ion edge, identified as the most earthward detection of entering magnetosheath ions, was always detected either sunward of the electron edge or simultaneous with it, indicating that the entire low‐latitude boundary layer (LLBL) was on open field lines. The radial separation of the electron and ion edges is due to a time‐of‐flight effect associated with the fact that the entering magnetosheath electrons have considerably higher speeds than the entering magnetosheath ions. Importantly, our study reveals that an examination of three‐dimensional particle distributions covering the entire range of energies of the various magnetosheath and magnetospheric populations present is essential for a correct determination of the magnetic topology of the LLBL. Deducing the topology from electron pitch angle distributions covering a limited energy range can lead to incorrect deduction of the topology.

MESSENGER observations of the dayside low‐latitude boundary layer in Mercury's magnetosphere

AbstractObservations from MErcury Surface Space ENvironment GEochemistry, and Ranging (MESSENGER)'s Magnetometer and Fast Imaging Plasma Spectrometer instruments during the first orbital year have resulted in the identification of 25 magnetopause crossings in Mercury's magnetosphere with significant low‐latitude boundary layers (LLBLs). Of these crossings 72% are observed dawnside and 65% for northward interplanetary magnetic field. The estimated LLBL thickness is 450 ± 56 km and increases with distance to noon. The Na+ group ion is sporadically present in 14 of the boundary layers, with an observed average number density of 22 ± 11% of the proton density. Furthermore, the average Na+ group gyroradii in the layers is 220 ± 34 km, the same order of magnitude as the LLBL thickness. Magnetic shear, plasma β and reconnection rates have been estimated for the LLBL crossings and compared to those of a control group (non‐LLBL) of 61 distinct magnetopause crossings which show signs of nearly no plasma inside the magnetopause. The results indicate that reconnection is significantly slower, or even suppressed, for the LLBL crossings compared to the non‐LLBL cases. Possible processes that form or impact the LLBL are discussed. Protons injected through the cusp or flank may be important for the formation of the LLBL. Furthermore, the opposite asymmetry in the Kelvin‐Helmholtz instability (KHI) as compared to the LLBL rules out the KHI as a dominant formation mechanism. However, the KHI and LLBL could be related to each other, either by the impact of sodium ions gyrating across the magnetopause or by the LLBL preventing the growth of KH waves on the dawnside.

Thickness of the low-latitude boundary layer at different levels of magnetic field fluctuations in the magnetosheath

The role of a high fluctuations level in the Earth’s magnetosheath in plasma penetration into the magnetosphere and in the formation of the low-latitude boundary layer (LLBL) has been considered based on the events that occurred on November 1 and 5, 2007, using the THEMIS-A satellite observations. During the selected LLBL crossings the satellite was measuring behind the quasi-parallel and quasi-perpendicular bow shocks. The angle between the magnetic field direction in the solar wind and the normal to the bow shock (ΘBn) has been taken as a parameter reflecting the level of magnetic field and plasma paremeters fluctuations in the magnetosheath. It has been indicated that a thick LLBL is observed when angle ΘBn is small and the turbulence level in the magnetosheath is high. When angle ΘBn is large, the layer thickness decreases. The possible mechanisms by which a thick LLBL is formed are discussed.

Analysis of temperature versus density plots and their relation to the LLBL formation under southward and northward IMF orientations

AbstractThe paper reports a clear dependence of a basic structure of two sublayers of the low‐latitude boundary layer (LLBL) on northward and southward interplanetary magnetic field (IMF) orientations. Regardless of different processes responsible for a formation and evolution of the entire LLBL, the outer part of the LLBL is significantly influenced by the sign of the IMF BZ component. Under northward IMF conditions this layer is present, whereas it is missing during a southward pointing IMF. This behavior can be understood in terms of a motion of reconnection spots due to the changes of the orientation of the magnetosheath magnetic field in the vicinity of the magnetopause. Our case and statistical studies demonstrate that the changes of the LLBL structure can be observed in the subsolar region as well as on flanks near the dawn‐dusk meridian. Moreover, the study emphasizes a role of magnetosheath magnetic field variations on the boundary layer formation.

The dependence of the strength and thickness of field-aligned currents on solar wind and ionospheric parameters.

Sheared plasma flows at the low-latitude boundary layer (LLBL) correlate well with early afternoon auroral arcs and upward field-aligned currents. We present a simple analytic model that relates solar wind and ionospheric parameters to the strength and thickness of field-aligned currents (Λ) in a region of sheared velocity, such as the LLBL. We compare the predictions of the model with DMSP observations and find remarkably good scaling of the upward region 1 currents with solar wind and ionospheric parameters in region located at the boundary layer or open field lines at 1100-1700 magnetic local time. We demonstrate that [Formula: see text] and Λ ~ L when Λ/L < 5 where L is the auroral electrostatic scale length. The sheared boundary layer thickness (Δ m ) is inferred to be around 3000 km, which appears to have weak dependence on Vsw. J‖ has dependencies on Δ m , Σ p , nsw, and Vsw. The analytic model provides a simple way to organize data and to infer boundary layer structures from ionospheric data.

Magnetospheric boundary perturbations on MHD and kinetic scales

AbstractTo study the magnetopause on both MHD and kinetic scales, we have analyzed two Time History of Events and Macroscale Interactions during Substorms/Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun magnetopause crossings under steady slow‐solar wind and minimum magnetic shear conditions. These events approximate a ground state of the magnetospheric boundary with minimum influences from large‐solar wind disturbances and magnetic reconnection. Our observations reveal evidence for the Kelvin‐Helmholtz instability, the quasi‐periodicity of magnetopause surface waves accompanied by highly asymmetrical plasma signatures between the inbound (from magnetosheath to low‐latitude boundary layer (LLBL)) and the outbound (from LLBL to magnetosheath) magnetopause crossings. Stronger plasma and magnetic gradients were observed during the outbound crossings but more gradual and volatile variations at higher frequencies during the inbounds. The scale lengths of the magnetic and plasma gradients were comparable or less than the proton gyroradius. Enhancements of lower hybrid waves occurred at the locations of strong gradients or variations. We interpreted the collocations of the lower hybrid waves and plasma gradients and their variations in terms of (1) lower hybrid instabilities that directly convert solar wind flow energy into lower hybrid waves and other wave modes in the LLBL, or (2) Kelvin‐Helmholtz instability and magnetic reconnection which produce the conditions for the lower hybrid instabilities to grow. The rate of ion diffusion across the magnetopause caused by the lower hybrid instability is marginally sufficient to populate the LLBL. The diffusion coefficient of O+ is ~30 times larger than that of H+. The lower hybrid waves could contribute to the energy dissipation at plasma gradients in magnetopause surface wave structures and limit Kelvin‐Helmholtz instability growth further downstream.

Turbulent plasma transport across the Earth's low‐latitude boundary layer

AbstractThe Kelvin‐Helmholtz instability is a key process for the transport of solar wind plasma into the Earth's magnetosphere when the interplanetary magnetic field (IMF) is northward. Previous kinetic simulations for symmetric layers have demonstrated that the flow vortices compress the magnetopause current layer and induce magnetic reconnection, leading to the rapid streaming of solar wind plasma into the vortices along newly reconnected field lines. Using fully kinetic 3‐D simulations, we demonstrate that the inherent density asymmetry across the boundary layer leads to a spectrum of oblique interchange instabilities along the magnetospheric side of the vortices. These secondary instabilities give rise to turbulence, which transports the solar wind plasma originally stored within the flow vortices deep into the magnetosphere. Simple estimates suggest that this turbulent transport may contribute significantly to the formation of the Earth's low‐latitude boundary layer and the cold‐dense plasma sheet during prolonged periods of northward IMF.

Cluster observations of the substructure of a flux transfer event: analysis of high-time-resolution particle data

Abstract. Flux transfer events (FTEs) are signatures of transient reconnection at the dayside magnetopause, transporting flux from the dayside of the magnetosphere into the magnetotail lobes. They have previously been observed to contain a combination of magnetosheath and magnetospheric plasma. On 12 February 2007, the four Cluster spacecraft were widely separated across the magnetopause and observed a crater-like FTE as they crossed the Earth's dayside magnetopause through its low-latitude boundary layer. The particle instruments on the Cluster spacecraft were in burst mode and returning data providing 3-D velocity distribution functions (VDFs) at 4 s resolution during the observation of this FTE. Moreover, the magnetic field observed during the event remained closely aligned with the spacecraft spin axis and thus we have been able to use these 3-D data to reconstruct nearly full pitch angle distributions of electrons and ions at high time resolution (up to 32 times faster than available from the normal mode data stream). These observations within the boundary layer and inside the core of the FTE show that both the interior and the surrounding structure of the FTE consist of multiple individual layers of plasma, in greater number than previously identified. Our observations show a cold plasma inside the core, a thin layer of antiparallel-moving electrons at the edge of FTE itself, and field-aligned ions with Alfvénic speeds at the trailing edge of the FTE. We discuss the plasma characteristics in these FTE layers, their possible relevance to the magnetopause reconnection processes and attempt to distinguish which of the various different FTE models may be relevant in this case. These data are particularly relevant given the impending launch of NASA's MMS mission, for which similar observations are expected to be more routine.

Properties of Kelvin‐Helmholtz waves at the magnetopause under northward interplanetary magnetic field: Statistical study

AbstractWe search the plasma and magnetic field data of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes B and C during 2008 and 2009 for observation evidences of the Kelvin‐Helmholtz instability (KHI). Fourteen KHI events with rolled‐up vortices are identified under the northward interplanetary magnetic field (IMF) at the low‐latitude boundary layer (LLBL). We collect another 42 events reported from the observations of the Geotail, Double Star TC‐1, and Cluster for a statistical study of the KH wave properties. All the 56 rolled‐up KH wave events are quantitatively characterized by the dominant period, phase velocity, and the wavelength. We further explore the relationship between the KH wave period and the solar wind velocity (VSW) and the IMF clock angle. It is found that the KH period tends to be shorter under a higher VSW, and longer with a larger IMF clock angle. The spatial distribution of the KH wavelength shows a longitudinal growth with increasing distance from the subsolar point along the flank magnetopause. The statistical results provide new insights for the development of KH waves and their connection with the interplanetary conditions and deepen our understanding of the KHI at the magnetopause.

Day‐night coupling by a localized flow channel visualized by polar cap patch propagation

We present unique coordinated observations of the dayside auroral oval, polar cap, and nightside auroral oval by three all-sky imagers, two Super Dual Auroral Radar Network (SuperDARN) radars, and Defense Meteorological Satellite Program (DMSP)-17. This data set revealed that a dayside poleward moving auroral form (PMAF) evolved into a polar cap airglow patch that propagated across the polar cap and then nightside poleward boundary intensifications (PBIs). SuperDARN observations detected fast antisunward flows associated with the PMAF, and the DMSP satellite, whose conjunction occurred within a few minutes after the PMAF initiation, measured enhanced low-latitude boundary layer precipitation and enhanced plasma density with a strong antisunward flow burst. The polar cap patch was spatially and temporally coincident with a localized antisunward flow channel. The propagation across the polar cap and the subsequent PBIs suggests that the flow channel originated from dayside reconnection and then reached the nightside open-closed boundary, triggering localized nightside reconnection and flow bursts within the plasma sheet.