Characteristics of plasma ring, surrounding the Earth at geocentric distances ∼7–10RE, and magnetospheric current systems

Journal of Atmospheric and Solar-Terrestrial Physics 99 (2013) 85–91

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Characteristics of plasma ring, surrounding the Earth at geocentric distances  7–10RE, and magnetospheric current systems E.E. Antonova a,b,n, I.P. Kirpichev b,a, V.V. Vovchenko b, M.V. Stepanova c, M.O. Riazantseva a,b, M.S. Pulinets a, I.L. Ovchinnikov a, S.S. Znatkova a a

Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, Russia Space Research Institute RAS, Moscow, Russia c Physics Department, Universidad de Santiago de Chile (USACH), Chile b

a r t i c l e i n f o

abstract

Article history: Received 16 March 2012 Received in revised form 18 August 2012 Accepted 30 August 2012 Available online 7 September 2012

There are strong experimental evidences of the existence of plasma domain forming a closed plasma ring around the Earth at geocentric distances  7–10RE. In this work, we analyze the main properties of this ring, using the data of the THEMIS satellite mission, acquired between April 2007 and September 2011. We also analyze the contribution of this ring to the storm dynamics. In particular, it is shown that the distribution of plasma pressure at  7–10RE is nearly azimuthally symmetric. However, the daytime compression of the magnetic field lines and the shift of the minimal value of the magnetic field to higher latitudes lead to the spreading of the transverse current along field lines and splitting of the daytime integral transverse current into two branches in Z direction. The CRC is the high latitude continuation of the ordinary ring current (RC), generated by plasma pressure gradients, directed to the Earth. We evaluated the contribution of the azimuthally symmetric part of the plasma ring to the Dst index for strong geomagnetic storms using the AMPTE/CCE radial profiles of plasma pressure published before, and showed that the contribution of the ring current including both RC and CRC is sufficient to obtain the observed Dst variation without the necessity to include the tail current system. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Plasma ring around the Earth High latitude transverse currents Magnetospheric storms and substorms Nature of Dst variation

1. Introduction Even very first results of particle observations in the magnetosphere of the Earth showed the existence of a region spaced from the geostationary orbit up to  10RE and named the region of quasitrapping (see, for example, Vernov et al., 1969). The trajectories of energetic particles in this region cross the magnetopause when the particle pitch angle is equal to 901. For particles with smaller pitch angles the drift shell splitting effect is observed, leading to the appearance of Shabansky-type orbits. Their drift trajectories are closed inside the magnetosphere (see Ukhorskiy et al., 2011, and references therein). Taking into consideration that effect of drift echo is observed up to  13RE near midnight (Hori et al., 2003), it is possible to locate the external boundary of the region of quasitrapping at such distances. Characteristics of plasma population in the region of quasitrapping are similar to that observed in the plasma sheet (see discussion in Antonova et al., 2011), as follows from both low altitude and near equatorial satellite observations. In particular,

n Corresponding author at: Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, Russia. Tel.: þ7 4959392810; fax: þ7 4959390896. E-mail address: [email protected] (E.E. Antonova).

1364-6826/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2012.08.013

Newell and Meng (1992), using DMSP satellite data, showed that particle precipitations near noon have the characteristics similar to the plasma sheet particles and come from the region located to the equator from the low latitude boundary layer (LLBL). Newell et al. (2009) developed a statistical model, which also contains a ring of precipitating particles having distribution functions similar to ones measured in the plasma sheet. The analysis of any crossing of the magnetopause using the THEMIS particle data shows the presence of plasma sheet-like plasma near noon. The detailed description of the THEMIS mission can be found in Angelopoulos (2008) and http://www.nasa.gov/mission_pages/ themis/. Analyzing statistical picture of GEOTAIL observations obtained by Nagata et al. (2008) at XGSW o0 (where GSW is the Geocentric Solar Wind coordinate system), it is also possible to select the ring-type distribution of plasma around the Earth. Despite of these results, the region of quasitrapping has not been selected as a magnetospheric domain, and the contribution of the corresponding transverse currents to the dynamics of the magnetosphere of the Earth has not been evaluated in details. It is well known that when the plasma is in a magnetosptatic equilibrium, the transverse currents inside the magnetosphere are mainly generated by the plasma pressure gradients. However, the correct estimation of plasma pressure requires measurements in situ of all plasma particles in a wide energy range. Since

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these measurements are scarce, the number of works about the distribution of plasma pressure is rather limited. In particular, Tsyganenko and Mukai (2003) created a magnetotail plasma pressure model based on the GEOTAIL satellite data at geocentric distances larger than 10RE. Wang et al. (2009) obtained the global distribution of plasma pressure in the night sector also using GEOTAIL data. The global pressure distribution in the inner regions of the Earth’s magnetosphere at geocentric distances o8.8RE was obtained from the AMPTE/CCE equatorial satellite by Lui and Hamilton (1992), Lui (2003) and DeMichelis et al. (1997, 1999). The region of transition from dipole to tailward stretched field lines has been studied even less. Some pressure profiles in this region were obtained onboard the Interball/Tail probe by Antonova et al. (2001) and Kirpichev et al. (2005). The data provided by THEMIS satellite mission allow to obtain the global picture of pressure distribution inside the magnetosphere. Antonova et al. (2009a) obtained the daytime profile of plasma pressure distribution at the equatorial plane near noon up to the magnetopause using THEMIS-B satellite data for the period 2 June 2007–29 October 2007. Kirpichev and Antonova (2011) obtained the global picture of the diagonal components of the pressure tensor in a local magnetic field coordinate system averaged for the period of the solar minimum from August 2007 to September 2010. However, the dependence of the plasma pressure distribution on the solar wind dynamic pressure and interplanetary magnetic field orientation was not obtained. It is shown that plasma pressure is close to isotropic and plasma pressure gradients are directed mainly to the Earth. For plasmas in magnetostatic equilibrium with nearly isotropic plasma pressure, the transverse current (j?) is determined by the plasma pressure gradients as: j^ ¼ Brp=B2 ,

ð1Þ

where rp is the plasma pressure gradient and B is the magnetic field. From Eq. (1) it follows that when the plasma pressure gradient is directed to the Earth the corresponding transverse current is directed westward. This condition is fulfilled for the plasma ring, surrounding the Earth up to the dayside magnetopause. Nevertheless, to determine the density of the transverse current it is necessary to have the simultaneous measurements of pressure gradients and magnetic field. The orbits of the THEMIS satellites are located close to the equatorial plane. This allows to easily evaluate the integral transverse currents for the nighttime magnetosphere, for which the region with minimal value of the magnetic field is located close to the equatorial plane. In case of the dayside magnetosphere, the regions with the minimal values of the magnetic field are located away from the equatorial plane, leading to the spreading of the transverse current into two branches having the maxima in current density far from the equatorial plane (see Antonova et al., 2003, 2004). This difficulty can be overcome taking into consideration that the isotropic plasma pressure is constant along a magnetic field line. Therefore, it is possible to evaluate the current density at any point along the field line, using one of the magnetic field models, when the plasma pressure distribution in the equatorial plane is known, including the regions close to the minima of the magnetic field, far from the equatorial plane, where most of the transverse current is concentrated. The calculations made by Antonova et al. (2009a,b) using AMPTE/ CCE and THEMIS-B radial pressure profiles and Tsyganenko-2001 model showed that the integral transverse current at geocentric distances from 7.5 up to 10RE in the dayside is comparable to the nighttime current at the same geocentric distances. The centers of the averaged integral transverse current in these calculations were displaced on Zeff ffi 73RE from the equator. This finding suggests that surrounding the Earth plasma loop is a part of the ring current

Fig. 1. Scheme illustrating the structure of transverse currents closed inside the magnetosphere (ordinary ring current and cut-ring current). Arrows show current directions.

domain, in which region from  3.5RE up to 7RE is the region of ordinary ring current (RC) and the region at greater distances up to magnetopause near noon is the region of high latitude continuation of this current. Such current structure was named the cut-ring current (CRC) (see discussion in Antonova, 2003, 2004), considering that the transverse current splits into two branches in the dayside magnetosphere (see Fig. 1). The daytime part of CRC is located relatively close to the magnetopause and far from the equatorial plane. This could be the reason why it was not included into the traditional versions of Tsyganenko geomagnetic field models. It is interesting to mention that current lines corresponding to CRC have recently appeared in MHD modeling (see Liemohn et al., 2011). It is necessary to stress that the CRC has the same nature as ordinary westward ring current. Both of them are generated by plasma pressure gradients directed to the Earth. However, in contrast to ordinary ring current formed mainly by ions with energy 100 keV, CRC is formed by ions and electrons having the same characteristics as the plasma sheet particles. Although the drift shells of the energetic particles with the Shabansky-type orbits also split into two branches, the contribution of energetic particles to the CRC is not significant, because its density is too low. During the last 50 years, there have been numerous studies on the relative contributions of different magnetospheric currents to the Dst index. At the beginning, the ring current was considered as a main source of negative values of Dst variation. However, this point of view was later criticized as it could not explain some features of Dst dynamics including, for example, the decrease in 9Dst9 index after substorm onset or effect of Iyemori and Rao (1996). To overcome this difficulty, it was suggested that the tail current also significantly contributes to the Dst variation (see Discussion in Greenspan and Hamilton, 2000). The discussion about the main sources of the Dst variation is still open and there are also a significant number of works in favor of the ring current as a main source of Dst (see, for example, Roeder et al., 1996; Jordanova et al., 1998; Greenspan and Hamilton, 2000). In this context, the CRC could be important for the correct estimation of the Dst variation during geomagnetic storms. Although the dayside current is split into two branches, the azimuthally symmetric part of this current could be significant. This would shift the inner edge of the plasma sheet and tail current to the distances larger than traditionally established. Greenspan and Hamilton (2000) established L¼7 as an outer boundary for the ring current considering that during disturbed times the nightside inner edge of the plasma sheet is typically located near that L value (Parks, 1991). The existence of the

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plasma ring surrounding the Earth and CRC shifts the inner edge of the plasma sheet and tail current to larger L. Greenspan and Hamilton (2000) also showed that the plasma pressure continues to be nearly isotropic during geomagnetic storms. This means that the relation (1) can be used to estimate the contribution of the corresponding transverse current to the Dst variation. In this work, we obtain the averaged distribution of plasma pressure using the data of five THEMIS satellites for the period from August 2007 to September 2011, organizing the data according to the solar wind dynamic pressure and interplanetary magnetic field (IMF) orientation (Section 2). We also show that the symmetric part of the surrounding the Earth ring current produces the main contribution to the Dst variation during main phase of magnetic storm (Section 3). We also discuss the importance of CRC for the correct reproduction of the Dst variation during storms (Section 4). Section 5 contains the main conclusions.

2. Distribution of pressure in the plasma ring obtained from THEMIS satellite data We analyze the spatial distribution of the diagonal components of the pressure tensor, i.e., the pressure components aligned with the magnetic field, measured by the satellite, (p99) and perpendicular to it (p?). To calculate p99 and p?, we use the moments of ion and electron distribution functions obtained using two different THEMIS instruments. The electrostatic analyzer (ESA) measures ions in the energy range from 1.6 eV to 25 keV and electrons with energies from 2 eV to 32 keV. The solid state telescope (SST) measures ions with energies from 25 keV to 6 MeV and electrons with energies from 25 to 900 keV (Angelopoulos, 2008; McFadden et al., 2008, http://www.nasa.gov/mission_pages/themis/). In this study, we use particle spectra obtained by combining the ESA and SST measurements. When the satellites are located at the distances closer than 6RE, the ESA measurements are distorted by the penetrating radiation (McFadden et al., 2008) which can be the source of uncertainty of pressure at Ro6RE.. We excluded the time intervals for which the values obtained by ESA and SST instruments in the overlapped energy intervals exceeded 50%. The instruments did not allow determining the ion composition. Therefore, we assume that protons make the main contribution, which is a good approximation for magnetically quiet conditions (Daglis et al., 1999). Local magnetic field with the satellite spin time resolution (3 s) was obtained from the FGM magnetometer data (Auster et al., 2008).

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We analyzed the data from August 2007 to September 2011. It was geomagnetically quiet time interval with Dst mainly higher than –40 nT having the minimal value of about –110 nT. We excluded from the analysis the intervals with considerably negative values of Dst. The equatorial plane (XY) was divided into 0.5RE bins in the solar–magnetospheric (SM) coordinate system in XSM and YSM directions, ranging between –0.5RE and 0.5RE in ZSM direction. The measurements on all five satellites were accumulated and averaged inside each bin. After that, the data of each bin were divided into subsets depending on the solar wing dynamic pressure and magnetic field. The solar wind data were obtained from the WIND database (http://cdaweb.gsfc.nasa.gov/). It was found that the solar wind dynamic pressure is the main parameter controlling the plasma pressure distribution during the analyzed period. Consequently, we selected obtained values in bins in accordance with the position of the magnetopause obtained using Shue et al. (1998) model when the IMF was close to zero and the subsolar point was localized in the intervals 11.570.25 RE, 117 0.25 RE, 10.570.25 RE, 1070.25 RE, and 9.570.25RE. Fig. 2a and b shows two-dimensional distribution patterns of the perpendicular and parallel pressure components when the solar wind dynamic pressure was equal to 2.5 nPa and the subsolar point in accordance with Shue et al. (1998) model was at 10RE. The results are presented without smoothing. We also added the contours of the average value of the local magnetic field magnitude measured by the satellites and the position of the magnetopause according to the model of Shue et al. (1998) when the subsolar point was at 10RE. White bins correspond to region with low statistics. The pressure values outside the magnetopause correspond to the averaged plasma pressure values in the magnetosheath. Such a pressure near the subsolar point can almost completely balance the magnetic field pressure inside the magnetosphere (see Znatkova et al., 2011, and references therein). Fig. 3a and b shows the radial profiles of the parallel and perpendicular pressure components at the day–night and dawn– dusk meridians, which make it possible to analyze the pressure distribution asymmetry. It can be seen that nighttime pressure is larger than daytime pressure that agrees with the results of Lui and Hamilton (1992). However, the asymmetry is not large (  2) and is probably connected to the nighttime plasma heating during substorms. The pressure in the dusk sector is higher than that in the dawn sector. However, we should note that the dawn–dusk asymmetry is only observed for the distances lower than 7RE increasing toward the Earth. For larger distances, the pressure is

Fig. 2. (a) Distribution of the average value of the pressure perpendicular to the magnetic field (p?) and (b) parallel to the magnetic field (pjj) obtained using ESA and SST THEMIS measurements from April 2007 to September 2011 when the solar wind dynamic pressure was equal to 2.5 nPa and subsolar magnetopause position in accordance with Shue et al. (1998) model was at 10RE. The thick line shows the magnetopause position in accordance with Shue model, thin lines show the local magnetic field constant value (in nT) contours obtained in accordance with FGM THEMIS measurements.

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Fig. 3. Radial profiles of the perpendicular pressure component at (a) the noon– midnight and (b) dawn–dusk meridians for the same as in Fig. 2 solar wind dynamic pressure 2.5 nPa and IMF Bz 40 and Bz o0. Night pressure is shown by solid thin line for IMF Bz 40 and solid thick line for IMF Bz o0. Noon pressure is shown by dashed thin line for IMF Bz 40 and dashed thick line for IMF Bz o0. Dawn pressure is shown by solid thin line for IMF Bz 40 and solid thick line for IMF Bz o 0. Dusk pressure is shown by dashed thin line for IMF Bz 40 and dashed thick line for IMF Bz o 0.

nearly azimuthally symmetric. Fig. 4a and b shows the pressure anisotropy p?/p99 at the day–night and dawn–dusk meridians. It can be seen that the pressure anisotropy depends on the geocentric distance and is insignificant at geocentric distances larger than 7.5RE that agrees with recent results of Wang et al. (2011) confirming the validity of the approximation of azimuthally symmetric isotropic pressure. The statistical errors resulting from the determination of the pressure values are not shown in Figs. 3 and 4. However, produced analysis shows that the differences in the perpendicular and parallel pressure components and the pressure azimuthal asymmetry are inside the statistical errors.

3. Symmetric part of the ring current as the source of Dst variation One can test the impact of the cut-ring current by developing the magnetic field model in which CRC is included. Unfortunately, until now, it was not possible to obtain the analytical expression for CRC magnetic field to be able to modify the existing models or to develop the new ones. However, it is possible to obtain the simplified estimation of the contribution of the whole ring current to the Dst if the radial distribution of the magnetospheric plasma pressure is

Fig. 4. Pressure anisotropy at (a) the noon–midnight and (b) dawn–dusk meridians for the same as in Fig. 2 solar wind dynamic pressure 2.5 nPa and IMF Bz40 and Bz o 0. Night pressure anisotropy is shown by solid thin line for IMF Bz40 and solid thick line for IMF Bz o0. Noon pressure anisotropy is shown by dashed thin line for IMF Bz4 0 and dashed thick line for IMF Bz o 0. Dawn pressure anisotropy is shown by solid thin line for IMF Bz40 and solid thick line for IMF Bz o0. Dusk pressure anisotropy is shown by dashed thin line for IMF Bz4 0 and dashed thick line for IMF Bz o 0.

known. It is necessary to remind that the value of the magnetic pressure in the external parts of the magnetic trap is comparable to the plasma pressure. Therefore, instead of the Dessler–Parker– Sckopke (DPS) relation, which predicts a linear dependence of the perturbation of the magnetic field at the surface of the Earth (DB) on the total ring current kinetic energy (Up), it is better to use expression, obtained by Carovillano and Maguire (1968):

DB=Bs ¼ ð2U p þ U b Þ=3U s ,

ð2Þ

where Bs is the magnetic field at the Earth’s equator, Us is the energy of the Earth’s dipole magnetic field outside the surface of the Earth, and Ub is the energy of the ring current magnetic field. Expressions (1) and (2) give the possibility to calculate the effect related to the distortion of the dipole magnetic field by moving particles. The ring current kinetic energy Up in DPS in accordance with (2) should be replaced by Up þUb/2. It is possible to calculate DB, if the radial distribution of isotropic plasma pressure p(r) is known, using the iterative solution method, which was frequently used in the first works on the Dst formation (see Vovchenko and Antonova, 2010, 2012, and references therein). The initial magnetic field not distorted by currents flowing in the plasma is considered as the dipole

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field. It is taken into account that plasma pressure is nearly isotropic during geomagnetic storms (Greenspan and Hamilton, 2000). It is well known that azimuthal distribution of plasma pressure during the main phase of the magnetic storm is very asymmetric with maximum near midnight and dusk. Therefore, to consider the symmetric part of the ring current it is necessary to use radial dependence of pressure p(r) measured in the day or dawn sectors. However, the high-orbiting satellites spend a few hours to cross a region sufficient for obtaining a radial pressure profile. For example, AMPTE/CCE crosses the ordinary ring current during 3 h (see Greenspan and Hamilton, 2000). This means that during geomagnetic storms, the configuration of the geomagnetic field and the distribution of plasma pressure in the inner magnetosphere may change significantly during the satellite crossing. Therefore, only long duration storms can be analyzed. Unfortunately, analyzed period of THEMIS operation does not contain such storms. This forced us to analyze only the AMPTE/CCE pressure profiles published in previous works. In particular, we analyze 16 pressure profiles (energy density) published by Hamilton et al. (1988) for the great magnetic storm of February 1986, and pressure profiles for single passes during storm main phases published by Krimigis et al. (1985) for the magnetic storm of 4–7 September 1984, and Greenspan and Hamilton (2000) for the magnetic storm 30 November 1988. Fig. 5a–c shows the values of the measured Dst variation and the values of Dst obtained using the iterative solution method. Calculated DB was multiplied by the coefficient of 3/2 to consider the induction currents in the Earth. Fig. 5 shows that our simplified calculations reproduce the value of Dst variation in spite of the rather simple model used and the absence of information about solar wind dynamic pressure and corresponding positive contribution of magnetopause currents to the Dst variation. Our simplified calculations reproduce the value of Dst variation during storm main phases. This suggests that azimuthally symmetric part of the ring current including CRC is the main source of the Dst variation during all phases of storm.

4. Discussion Our analysis has shown that it is possible to restore the traditional point of view that the decrease in the Dst value is

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mainly due to the development of the azimuthally symmetric part of the ring current. Introduction of CRC system allows obtaining the Dst very similar to that observed without the necessity to consider the contribution of the tail current in the region near the Earth. It is necessary to mentioned that the discussion of the contribution of tail current to the Dst variation started after the analysis of February 1986 storm by Hamilton et al. (1988), for which the Dessler–Parker–Sckopke relation was not fulfilled. Nevertheless, in this work the contribution of Earth’s currents to the Dst was not considered (see Discussion in Greenspan and Hamilton, 2000). Later, Greenspan and Hamilton (2000) tested the DPS relation during the maximum phase of 80 magnetic storms and found the strong linear correlation between ring current energy estimated from nightside ion measurements and the Dst index. On the contrary, dayside measurements of the ring current energy do not yield a robust correlation with Dst, showing large scattering of points (see Figure 5b of Greenspan and Hamilton, 2000). It seems to us that this result is mainly related to large distortion of daytime flux tubes and limiting the calculations of ring current energy to geocentric distances smaller than 7RE. This result can be significantly improved considering the existence of the high latitude portion of the plasma pressure ring at geocentric distances 47RE. The high latitude part of the plasma pressure profile is possible to restore using data of low orbiting satellites as it was done, for example, by Stepanova et al. (2008). It is also important to mention that the introduction of CRC removes the difficulties related to the Iyemori and Rao (1996) effect. It can be done, considering that the current, which circulates in the nightside inner magnetosphere, does not belong to the tail current and is a part of the whole ring current. Current disruption and dipolarization after substorm onset leads to the decrease in CRC and corresponding decrease in 9Dst9. The introduction of the CRC system also removes the difficulties, which arise when the significant contribution of the tail current to the Dst variation is considered, like apparent pressure disbalance, which should appear at the flanks of the magnetopause during the large tail current crossings (see Antonova, 2001). Contrary to the tail current, the CRC is closed inside the magnetosphere and its development does not produce the disbalance of plasma pressure at the magnetopause.

Fig. 5. The results of the comparison of the value of measured (thick line) and calculated Dst (crosses) for three magnetic storms: (a) February 1986, (b) September 4–7, 1984, (c) November 30, 1988. Parts of the figures show satellite orbit positions. Calculations correspond to measured by AMPTE/CCE plasma pressure profiles published by Hamilton et al. (1988), Krimigis et al. (1985) and Greenspan and Hamilton (2000).

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It is interesting to mention that the analysis of the CRC is also important for the understanding of the processes leading to the development of an isolated substorm (see Lui, 2011). As it was shown by Akasofu (1964), the onset of an isolated substorm is located at the equatorial boundary of the auroral oval, generally identified as the inner plasma sheet boundary, located at  7RE, which is significantly shifted towards the Earth during magnetic storms. During the last decade, new experimental evidence pointed out that the onset of an isolated substorm is located at geocentric distances smaller than 10RE (see Lui, 2011 and references therein; Yahnin et al., 2002; Dubyagin et al., 2003). This means that the onset is located inside the CRC region, and the cutring current can affect the current disruption and the field line dipolarization during substorms. The location of the energetic ion injection boundary during substorms at geocentric distances o10RE (Lopez et al., 1990), not very far from the geostationary orbit, also leads to the same conclusion. The process of heating part of plasma population during substorm also is very important for the solution of the problem of the origin of the Earth’s external electron radiation belt (see Borovsky and Cayton, 2011, and references therein). Therefore, the fact that substorm onset is located inside the CRC should be considered during the analysis of the appearance of ‘‘electrons-killers’’, making the knowledge about plasma pressure distribution very relevant for the space weather forecast (see Antonova et al., 2003 and references therein). We also hope that the including of the additional current system, which extends the ordinary ring current region up to the daytime magnetopause, can be useful for the improving existing models of ring current formation such as models of Jordanova et al. (2009, 2010), Zaharia et al. (2006, 2010), Liemohn and Jazowski (2008), Wolf et al. (2007) and others.

5. Conclusions We summarize the data showing the existence of plasma ring at geocentric distances from  7RE up to  10RE having the same characteristics as plasma in the plasma sheet. At the same time, the discussed region is the region of quasitrapping for energetic particles. The nighttime part of the ring is ordinarily selected as the near Earth plasma sheet. The existence of the plasma ring surrounding the Earth with quasitrapped energetic particle population indicates that this ring is a well defined magnetospheric domain. The distribution of plasma pressure in the ring is obtained using data of THEMIS mission for the period August 2007– September 2011, which is one year larger than the period analyzed by Kirpichev and Antonova (2011) for obtaining the averaged picture of pressure distribution. It is shown that quite time averaged plasma pressure profile is near to azimuthally symmetric and plasma pressure is nearly isotropic. Radial gradient of plasma pressure has the earthward direction generating the transverse westward current, if we assume that the plasma is in a magnetostatic equilibrium. The value of dayside part of this current was previously underestimated, as it was not taken into account the compression of daytime field lines leading to the displacement of the location of the regions with minimal values of the geomagnetic field to higher latitudes. Estimations of daytime transverse currents, taking into account that these currents are not concentrated at the equatorial plane, allow obtaining comparatively high values of daytime transverse currents comparable with nighttime transverse currents at the same geocentric distances. Such feature gives the possibility to analyze the region of plasma ring surrounding the Earth as the high latitude continuation of the ordinary ring current.

Acknowledgments We are grateful to the National Aeronautics and Space Administration (NASA, contract no. NAS5_02099) and V. Angelopoulos for the THEMIS mission data; D. Larson and R.P. Lin for the SST data; C.W. Carlson and J.P. McFadden for the ESA data; and K.H. Glassmeier, U. Auster, and W. Baumjohann for the FGM data distributed under the guidance of the Technical University of Braunschweig and supported by the German Ministry for Economy and Technology and the German Center for Aviation and Space (DLR) under contract no. 50 OC 0302. This work was partially supported by the grants of Russian Foundation for Basic Research 10-05-00247, 12-05-01030, 12-02-00217, and FONDECYT grant 1110729.

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