1. What influence the low-latitude boundary layer formation?
S.S. Znatkova1, E.E. Atonova1,2,
M.S. Pulinets1, I.P. Kirpichev2
1 - SINP MSU, Moscow, Russia
2 - SRI RAS, Moscow, Russia

2. Content What is LLBL? LLBL-formation theories
location, plasma and magnetic field parameters, n-T diagrams, structure
LLBL-formation theories
Reconnection, KH-instability, impulsive penetration, injection,
diffusion, gradient drift
What influence the LLBL formation?
Plasma and magnetic field fluctuations in solar wind and magnetosheath
Influence of fluctuations in MSH on LLBL formation
Dependence of LLBL thickness on IMF orientation
Plasma penetration inside the magnetosphere depending on homogeneity of solar wind.
Conclusions

3. What is LLBL?
Low-latitude boundary layer (LLBL) - the region of closed field lines near equator just earthward of the magnetopause
Mixing region of cold dense plasma of the solar wind and hot plasma of magnetosphere
Controls the rate of mass, momentum and energy transfer between the magnetosheath and the magnetosphere
Along LLBL from noon to the tail, the flow moves faster and the layer becomes thicker
Location of magnetosphere boundary layers
[Panov et al., 2008]

4. LLBL case-study
6 June 2009, THEMIS-C crossed the magnetopause and LLBL
THEMIS-B was in solar wind. The IMF was oriented northward.

5. LLBL spectrograms and magnetic field
The energies of ions and electrons and the level of magnetic field fluctuations are averaged between the values in MSH and PS

6. N-T diagram for electrons
The LLBL is structured. We can distinguish outer and inner LLBL. The outer LLBL possess properties similar to the MSH plasma and magnetic field parameters, outer – to the PS. The outer LLBL can disappear during intervals of strongly southward IMF [Nemececk et.al.,2015]
Inner LLBL
Outer LLBL

7. N-T diagram for ions

8. LLBL formation theories: reconnection
Left picture – southward orientation, right – northward. During the southward IMF the magnetic field lines reconnect at the low latitudes, during the northward – near cusp and plasma flows along the magnetopause [Němeček et al., 2003 Onsager et al., 2003]. High-latitude reconnection can appear at both hemispheres forming closed field-lines in magnetosphere, contributing to the LLBL formation.
On the bottom picture there are magnetic field lines shown with arrows, plasma flow direction – flat arrows [Fedorov et.al.2003]. The most probably reconnection considered to be if the angle between magnetic field lines in MSH and magnetosphere is >160°.
Němeček et al., 2003
The reconnection theories rely on laminar plasma flow. The existence of high level of magnetic field and plasma parameter fluctuations in magnetosheath are not taken into account.
Fedorov et al., 2003

9. LLBL formation theories: impulsive penetration, plasma injection, diffusion and gradient drift
Impulsive penetration theory is based on heterogeneous distribution of plasma pressure along the magnetopause, plasma element with redundant pressure overcome the magnetic barrier and penetrate the magnetosphere [Lemaire, 1977; Lemaire and Roth, 1991; Echim and Lemaire, 2000, 2002]. In [Lemaire, 1977; Echim and Lemaire, 2000, 2002] this heterogeneity was connected with transversal heterogeneity of solar wind. In many works the high level of fluctuations in magnetosheath is noted. Such a turbulence can lead to formation of heterogeneity of plasma flow inside the magnetosheath during homogeneous solar wind parameters.
In the paper [Newell and Meng, 2003] is discussed the injection of magnetosheath plasma into magnetosphere on the closed field lines during the local increase of solar wind dynamic pressure without magnetic reconnection. The injections takes place far from noon and more often at the down flanks under northward or radial IMF and small Bx IMF components. Observed injections are in good correlation with strong splashes of solar wind pressure.
Another plasma penetration mechanism, discussed in [Newell and Meng, 2003 and Treumann and Sckopke, 1999] was diffusion, that takes place owing to development of instabilities connected with magnetic field gradients on the tangential discontinuity, such as LLBL.
In [Treumann and Baumjohann, 1988] it was shown that 5% of particles can penetrate the magnetosphere owing to gradient drift (ions from the dusk side, electrons from the down).

10. LLBL formation: Kelvin-Helmholtz instability
The Kelvin-Helmholtz instabilities originate from velocity gradient across the magnetopause [Mishin, 1993; Miura,1995; Otto and Nykyri, 2003].
Vortex structure on flanks during KH instability [Hasegava et.al.2004]
A rolled-up KH vortex is observed at the inner edge of the LLBL by THEMIS-E and –D satellites. Although a developed KH instability is expected predominantly at the flank magnetopause and under northward IMF, the vortex was observed very close to the subsolar point (YGSM ≈ 4.5 RE) and the radial IMF was transformed into a southward pointing magnetic field at the magnetopause. The amplitude of the vortex was very small, comparable with the LLBL thickness estimated as 0.25 RE.
A sketch of the KH wave observation in the LLBL.
(top) A time evolution of velocity vectors of THE and THD
(bottom) The magnetic field vectors. [Grygorov et al., 2016]

11. Magnetosheath turbulence
The examples of Low- and High- frequency variations of the ion flux and magnetic field magnitude in the magnetosheath of the Earth.
Red curves are simultaneous measurements in undisturbed solar wind
N.N. Shevyrev, G.N. Zastenker, P.E. Eiges, 2004
Different conditions of pressure balance take place at different places of the magnetopause. Sporadic plasma jets are formed due to the destruction of pressure balance.
Scheme, illustrating the turbulent character
of magnetic fluctuations in the magnetosheath

12. Case study: fluctuations in magnetosheath
Satellites location in Magnetosheath – Interplanetary medium reference frame [Verigin M. et al.2006], F – fractional distance, F = 0 at the magnetopause, F = 1 at the bow shock. A case study from [Rossolenko et al., 2008]
Geotail and Interball/Tail were in the dusk flank of the magnetopause on at UT
Fluctuations of solar wind velocity components <10km/s, fluctuations of IMF components were about ±2 nТ, fluctuations of nsw~1.5 cm-3
Nearly stable solar wind conditions

13. Case study: fluctuations in magnetosheath
Fluctuations of plasma velocity components in magnetosheath were about km/s (pannels 1-3)
Fluctuations of magnetic field components in magnetosheath ~ 20nТ (pannels 5-7)
Fluctuations of plasma density in magnetosheath ~ 30 cm-3 (pannel 4)
Rather turbulent magnetosheath is observed (in comparison with solar wind).

14. The influence of magnetosheath plasma parameter fluctuations on the LLBL formation
Sporadic plasma jets are formed due to the destruction of pressure balance. Observed reconnection of field lines is a consequence of such process. [Rossolenko et al., 2008]
Configuration of magnetic field lines in the XZ- plane corresponding the model of Tsyganenko-96 (solar wind parameters: By=-1 nT, Bz=-1 nT, nsw=10 cm-3, vsw=335 km/s)
The value of magnetic field under the magnetopause is often lower then the amplitude of magnetic field fluctuations in turbulent magnetosheath
Different conditions of pressure balance take place at different places of the magnetopause.

15. The dependence of LLBL thickness from 𝜽 𝑩𝒏 angle and the level of fluctuations in MSH
High level of fluctuations in magnetosheath
Pressure balance violation in different points of magnetopause
Plasma penetration inside magnetosphere, forming LLBL
Hypothesis: High level of fluctuations attracts more often plasma penetration into the magnetosphere and we should observe thick LLBL
Investigate the dependence of LLBL thickness from 𝜽 𝑩𝒏 angle, as it is an index of the level of fluctuations in MSH.

16. The dependence of LLBL thickness from 𝜽 𝑩𝒏 angle and the level of fluctuations in MSH
There are the dependences for the day side (X>0) and tail (X<0) of the magnetosphere.
The thickest LLBL is observed at θBn from 15° to 55° at the dayside magnetosphere (behind the quasiparallel bowshock). The tendency of LLBL thickness increase with the increase of level of magnetosheath fluctuations is studied in the dayside.
In the tail region (X<0) only a few points were observed with large LLBL thickness.
207 пересечение Темис-В
LLBL Thickness, km
LLBL Thickness, km
𝜽 𝑩𝒏
𝜽 𝑩𝒏

17. LLBL thickness dependence on BZ IMF
In [Haerendel, et al., 1978] it was marked that LLBL thickness has a low correlation with IMF BZ in the dayside magnetopause. But if X<0 the dependence is trailed as was shown in [Haerendel, et al., 1978], [Eastman and Hones, 1979], [Mitchell et al.,1987].
LLBL thickness was determined in [Mitchell et al.,1987] on the base of ISEE 1 data. The dependence of LLBL thickness (crossing time) from the value and orientation of magnetic field was received.
The results of the investigation were:
- LLBL thickness increases from dayside to tail.
- thick and thin LLBL is observed under southward IMF conditions in the tail and only thick under northward IMF conditions.
- LLBL is thicker under northward IMF conditions.
Mitchell et al.,1987

18. The correlation of Bz и By components in solar wind and near the subsolar magnetopause
The value of Bx-component near the magnetopause is fluctuating near zero independently from the averaging time.
By-component near the magnetopause, as it was shown in [Fairfield, 1967], is rather good correlated with By IMF component. The correlation coefficient of By IMF with By in magnetosheath increases with increasing of averaging time.
The correlations are almost absent for Bz-component.
The cases (~30% for averaged values in 26 investigated cases) when the sign of Bz IMF component doesn’t coinside with the sign of Bz near magnetopause were observed.
Pulinets et.al., 2012

19. The data set for investigation of LLBL thickness dependences on IMF components
LLBL crossings are picked out from the data of THEMIS mission ( The LLBL plasma parameters and magnetic field data are provided by Themis-A and -C satellites. The solar wind and IMF parameters are obtained from Themis-B satellite.
In this work [Znatkova et al., 2016] 109 LLBL intersections are carefully analyzed.

20. The dependence of LLBL thickness on IMF <Bz>
The trends indicate the increase of the LLBL thickness under northward IMF.
However only a small number of points support this conclusion. We are not able to distinguish any strong dependence using 180s IMF <Bz> averaging.

21. The dependence of LLBL thickness on IMF <By>
We see slight tendency of the increase the LLBL thickness at negative <BY> at the dayside.
In the tail the tendency is inversed. The tendencies are not strong and we also cannot make the exact conclusions.

22. The dependence of LLBL thickness on IMF <By> for the dusk and dawn flanks of the magnetosphere
To define the dusk-dawn asymmetry we separated the cases on flanks. The dusk-dawn asymmetry is hard to distinguish.
The decreasing LLBL thickness with increasing By at the dusk flank in left Fig. relies on three outlying points, whereas the line showing this trend for the dawn flank (right Fig.) is nearly horizontal.

23. The dependence of LLBL thickness on the clock angle for dayside and tail magnetosphere
θ=tan-1(By/Bz), describes the
IMF orientation relative to the
Earth’s dipole.
Two peaks are seen: the thickest LLBL is observed when the clock angle is about 45° and 135° when the values of IMF components By and Bz were nearly equal.
15 cases that make the peak had the thickness value lager than 5000 km and was located at 1,5-3 Re in Zgsm. km
In these 15 cases the values of Bx IMF component are 2-3 times larger than the module of IMF By and Bz components averaged for 3 and 180s as well. We can say that there was radial IMF with dominant IMF Bx component.

24. The LLBL thickness dependence on the solar wind velocity
The probability of existing of sharp magnetosphere boundary (thin or absent boundary layer) increases with increasing of the solar wind velocity.
N – the number of magnetopause crossings. N\Np – the relative number of sharp boundary cases.
The authors of [Lemair, 1978] considered the «impulsive penetration» of plasma elements with increased concentration inside the magnetosphere as the mechanism of boundary layer formation. This mechanism describes the large probability of LLBL existing under lower solar wind velocity, because the high-speed flows of solar wind are more homogeneous than the slower.
[Bame S.J. et al., 1977]
Kotova G.A., 1984

25. The dependence of LLBL thickness on the solar wind velocity
We observe the thin LLBL if the solar wind velocity value is high at the dayside magnetopause.

26. Conclusions 1 Hypotheses that were investigated:
Influence of magnetosheath plasma and magnetic field fluctuations on magnetosheath plasma penetration into the magnetosphere. Magnetosheath parameter fluctuations are considered as the source of plasma jets in LLBL. As the fluctuation index 𝜃 𝐵𝑛 angle was used. We observed the tendency of increase of LLBL thickness at the dayside magnetosphere if the level of fluctuations is high (quasiparallel bowshock). Under such conditions the violation of pressure balance can take place at different points of magnetopause and plasma of magnetosheath can penetrate the magnetosphere forming the LLBL. Owing to the pressure balance violation in different points of magnetopause, the thin LLBL can be also observed. Another formation processes not depending on kinds of bowshock can also take place.
The LLBL thickness depends on IMF orientation. The dependence of LLBL thickness on IMF Bz differed in the dayside magnetosphere and in the tail. There is weak or absent dependences in the dayside magnetosphere. The thickest LLBL is observed at small |Bz|. The absence of the dependence can be connected with the high level of turbulence in magnetosheath and as it was shown in [Pulinets at.al.,2012] with the absence of correlation of Bz component in magnetosheath and Bz IMF. The thickest LLBL is observed under northward IMF in the tail of the magnetosphere.
The dependence of LLBL thickness on IMF By (averaged for 180s) was determined. The thickest LLBL is observed in dusk flank of the magnetosphere under IMF By<0 and in the dawn flank under IMF By>0.

27. Conclusions 2 Hypotheses that were investigated:
The LLBL thickness depends on orientation of IMF. During the investigation of the dependence of LLBL thickness on the clock angle for dayside and tail magnetosphere we investigate two peaks (15 cases with the LLBL thickness larger than 5 Re). They are observed when the clock angle is about 45° (from 30° to 70°) and 135° (from 120° to 160°). In these 15 cases the values of Bx IMF component were 2-3 times larger than the module of IMF By and Bz components averaged for 3 and 180s as well. We can say that there was radial IMF for these cases with thickest LLBL ( km). In [Grygorov et al., 2016] the possibility of development of KH vortices at inner edge of LLBL during radial IMF was described. Probably the satellite intersection of such a vortices can be interpreted as thick LLBL or they are the source of LLBL plasma.
Magnetosheath plasma penetration into the magnetosphere and LLBL formation can depend on the level of homogeneity of solar wind. The high-speed flows of solar wind are more homogeneous than the slower [Bame S.J. et al., 1977]. There is a big probability of thick LLBL existing under lower solar wind velocity. It is confirmed by the theory of «impulsive penetration» of plasma elements with increased concentration inside the magnetosphere as the mechanism of boundary layer formation [Lemair, 1978]. We observed the increase of LLBL thickness at lower solar wind velocities.

28. Thank You for the attention!