Thickness Of Low-latitude Boundary Layer Research Articles

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.

Determining the thickness of the low-latitude boundary layer in the Earth’s magnetosphere

We describe a method for determining the thickness of the low-latitude boundary layer (LLBL) of the Earth’s magnetosphere at the dayside near the equatorial plane based on the data gathered by a single satellite that traverses the layer and measures the plasma velocity. The method may be applied when the position of the magnetopause and the magnetosheath parameters fluctuate. The necessity of taking the presence of outer and inner LLBL regions into account is analyzed. The developed method is tested using the analysis results of two almost simultaneous close traverses of the magnetopause completed by the THEMIS mission satellites that provided relatively precise data on the LLBL thickness. It is shown that the developed method makes it possible to determine the LLBL thickness with an accuracy of ∼10%.

Saturn's low-latitude boundary layer: 1. Properties and variability

[1] Transport of solar wind plasma into a planetary magnetosphere produces an internal boundary layer adjacent to the magnetopause that contains a mixture of solar wind and magnetospheric plasma. The potential processes responsible for this transport are varied, and their relative importance remains unclear, as well as how their competition varies between the magnetized planets. In this paper we examine the properties and variability of Saturn's low-latitude boundary layer (LLBL) using observations made by the Cassini spacecraft; the electron structure of Saturn's LLBL is examined in a companion paper. The duration of spacecraft excursions into the LLBL was generally between ∼3 and ∼23 min, and the electron environment of Saturn's LLBL was intermediate between that of the magnetosheath and that of the magnetosphere, as expected. Estimates of the speed of the magnetopause current layer are of the order of 100 km s−1, and the estimated thickness of the LLBL is of the order of 1 Saturn radius. Our results do not reveal a strong influence of the orientation of the interplanetary magnetic field on the thickness of Saturn's LLBL, which is unlike its terrestrial counterpart. There is also no clear dawn-dusk asymmetry in the thickness of Saturn's LLBL, which might be expected on the basis of our understanding of the growth of the Kelvin-Helmholtz instability at Saturn's magnetopause. We discuss the implications of these findings for the physics of Saturn's magnetospheric boundary and what this could mean for our understanding of how boundary layers are formed in planetary magnetospheres.

Dayside auroras during unusually large positive values of the IMF B z component

The characteristics of dayside auroras during the large (16-24 nT) positive values of the IMF Bz component, observed on January 14, 1988, during the interaction between the Earth's magnetosphere and the body of the interplanetary magnetic cloud, have been studied based on the optical observations on Heiss Island. A wide band of diffuse red luminosity with an intensity of 1-2 kilorayleigh (kR) was observed during 6 h in the interval 1030-1630 MLT at latitudes higher than 75° CGL. Rayed auroral arcs, the brightness of which in the 557.7 nm emission sharply increased to 3-7 kR in the postnoon sector immediately after the polarity reversal of the IMF B y component from positive to negative, were continuously registered within the band. Bright auroral arcs were observed at the equatorward edge of red luminosity. It has been found out that the red auroral intensity increases and the band equatorward boundary shifts to lower latitudes with increasing solar wind dynamic pressure. However, a direct proportional dependence of the variations in the auroral fea� tures on the dynamic pressure variations has not been found. It has been concluded that the source of bright discrete auroras is located in the region of the lowlatitude boundary layer (LLBL) on closed geomagnetic field lines. The estimated LLBL thickness is ~3 Re. It has been concluded that the intensity of the dayside red band depends on the solar wind plasma density, whereas the position of the position equatorward boundary depends on the dynamic pressure value and its variations.

Formation and characteristics of low latitude boundary layer

Characteristics of low latitude boundary layer (LLBL) of the Earth’s magnetosphere are investigated using data of Interball/Tail probe observations. The role of different processes of LLBL formation is discussed. The high level of magnetosheath turbulence is taken into account. It is shown that the turbulent nature of magnetic field and plasma fluctuations in the magnetosheath is one of the main factors determining the structure of LLBL. The results of Interball/Tail probe observations of the event 9 March 1996 are analyzed. The thickness of LLBL is determined for the number of cases. The change of LLBL thickness under the influence of the changes of solar wind parameters is investigated. It is shown that variability of solar wind conditions can be the important factor controlling LLBL thickness. Results of observations are compared with the theory which explains the value of LLBL thickness as the result of plasma transport inside the magnetosphere. It is shown that the theory gives the qualitative explanation of the observed dependence of LLBL thickness on solar wind parameters.

Variations of the flank LLBL thickness as response to the solar wind dynamic pressure and IMF orientation

Several mechanisms have been discussed as candidates for a formation of the low‐latitude boundary layer (LLBL) magnetic reconnection between the magnetospheric and magnetosheath magnetic fields, impulsive penetration of magnetosheath plasma, and viscous/diffusive mixing of plasma populations at the magnetopause. The observed fluctuations of plasma parameters inside the LLBL are attributed either to transient nature of the phenomena forming the layer or to sweeping of deformations of the magnetopause or an inner edge of the LLBL surface along the spacecraft. We are using the Interball‐1/Magion‐4 satellite pair separated by several thousands of kilometers in order to distinguish between spatial and temporal changes of the LLBL plasma population. Observations of LLBL crossings invoked by sudden changes of upstream conditions show that even during a strongly northward interplanetary magnetic field (IMF), the LLBL is relatively thin and follows the compression of the magnetopause induced by changes of the solar wind dynamic pressure. The thickness of the LLBL increases significantly (from a small fraction of RE to more than 1.4 RE) with increase in upstream density and IMF BY component. Our results indicate that the dusk LLBL is supplied by high‐latitude reconnection in the Southern (Northern) Hemisphere, when IMF BY is negative (positive) and lies on open field lines. The change of IMF BY direction leads to brief presence of LLBL plasma outside the magnetopause on magnetosheath field lines. Fluctuations of plasma parameters and magnetic field implicate the presence of surface waves on the inner edge of the LLBL, but no waves were observed on the magnetopause.

Quasiturbulent transport and LLBL properties

The results of experimental observations select two regions in the magnetospheric turbulent wake: plasma sheet and low latitude boundary layer (LLBL). Large-scale properties of LLBL are analyzed taking into account regular and turbulent plasma transport in Y direction and the high level of the observed turbulence. The application of the developed approach to the problem of the boundary layer formation gives the possibility to suggest the qualitative explanation of LLBL thickness during southward and northward interplanetary magnetic field (IMF) orientations and the dawn–dusk LLBL asymmetry.

Formation of the flank LLBL: A case study

A formation of the low-latitude boundary layer (LLBL) on open field lines during intervals of southward oriented interplanetary magnetic field is now understood in terms of dayside reconnection. However, observations of counterstreaming particles suggest that the LLBL can exist on closed field lines over some portion of the magnetopause. The fluctuations of plasma parameters inside a layer are attributed either to a transient nature of the processes forming a layer or to sweeping of deformations of the magnetopause or inner edge of the LLBL surfaces along the spacecraft. The INTERBALL-1 and MAGION-4 crossed the LLBL near the terminator during intervals of disturbed solar wind conditions. Two-point observations allow us to distinguish the temporal and spatial changes, to estimate the LLBL thickness and its variations and to find the sources of observed plasma populations. Observations suggest that the flank LLBL is supplied by high-latitude reconnection near the cusp and that it lies on open field lines.

Rapid changes in LLBL thickness

We employ data from two spacecraft, at the dawn flank of the magnetopause, to investigate fluctuations in the thickness of the low‐latitude boundary layer (LLBL). We show the LLBL is considerably thinner shortly after the detection of a flux transfer event than it was during the event. These data are shown to be consistent with the theory of transient increases in the open LLBL thickness caused by a pulse of enhanced reconnection at the magnetopause.