Citation: Zong, Q.-G., et al. (2009), Vortex-like plasma flow structures observed by Cluster at the boundary of the outer radiation belt and ring current: A link ...
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, A10211, doi:10.1029/2009JA014388, 2009
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Vortex-like plasma flow structures observed by Cluster at the boundary of the outer radiation belt and ring current: A link between the inner and outer magnetosphere Q.-G. Zong,1,2 Y. F. Wang,1 B. Yang,1 H. Zhang,3 A. M. Tian,1 M. Dunlop,4 T. A. Fritz,5 L. M. Kistler,6 A. Korth,7 P. W. Daly,7 and A. Pedersen8 Received 23 April 2009; revised 16 June 2009; accepted 20 July 2009; published 22 October 2009.
[1] Two vortex-like plasma flow structures have been observed at the outer radiation belt
and/or the ring current region on 11 April 2002, from 0415 to 0635 UT, when the Cluster fleet entered (in the Southern Hemisphere) and exited (in the Northern Hemisphere) the boundary layer of the inner magnetosphere near 2130 MLT. On 11 April 2002 during the period of interest, the solar wind speed was high, and the geomagnetic activity was moderate. These two vortices have opposite rotation directions and are characterized by bipolar signatures in the flow Vx components with peak-to-peak amplitudes of about 40 km/s. The inflection points of the plasma flow coincide precisely with the local maxima of the duskward core flow Vy (30 km/s) which exceed the surrounding flow by 3–4 times in magnitude for both vortices. A pair of bidirectional current sheets and bipolar electric fields (Ey) are found to be closely associated with these vortices. Whereas magnetic field disturbances are observed only in Bx and By components, the magnetic magnitude stays almost unchanged. Vortices observed both inbound and outbound at the boundary of the radiation belt at nearly the same location (L shell and latitude), suggesting they may last for more than 140 min. The scale sizes of the two vortices are about 810 km and 1138 km, respectively. Interestingly, it is found that Earth’s ionospheric singly charged oxygen are precipitating in the vortex dynamic process, having energies less than 1 keV and having a strong field-aligned pitch angle distribution. These plasma flow vortices are suggested to be formed at the interface between the enhanced ionospheric outflow stream from the polar ionosphere and a sudden braking and/or azimuthal deflection of bursty bulk flows generated by the tail reconnection. These observed flow vortices provide a link among the inner magnetosphere, the tail plasma sheet, and the Earth’s ionosphere by coupling magnetic shear stresses and plasma flow momentum. Citation: Zong, Q.-G., et al. (2009), Vortex-like plasma flow structures observed by Cluster at the boundary of the outer radiation belt and ring current: A link between the inner and outer magnetosphere, J. Geophys. Res., 114, A10211, doi:10.1029/2009JA014388.
1. Introduction [2] There are a number of boundaries in the inner magnetosphere between the hot tail plasma sheet, the cold 1 Institute of Space Physics and Applied Technology, Peking University, Beijing, China. 2 Center for Atmospheric Research, University of Massachusetts, Lowell, Massachusetts, USA. 3 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. 4 CCLRC Rutherford Appleton Laboratory, Didcot, UK. 5 Center for Space Physics, Boston University, Boston, Massachusetts, USA. 6 Space Science Center, University of New Hampshire, Durham, New Hampshire, USA. 7 Max-Planck-Institut fu¨r Sonnensystemforschung, Katlenburg-Lindau, Germany. 8 Department of Physics, University of Oslo, Oslo, Norway.
Copyright 2009 by the American Geophysical Union. 0148-0227/09/2009JA014388$09.00
plasmaspheric plasma and the trapped radiation belt/ring current region, such as Alfven boundaries, plasmapause [Carpenter, 1963] and plasmapause boundary layer [Carpenter and Lemaire, 2004]. The population in the outer boundary of the radiation belt may depend on the particle’s energy and their equatorial pitch angle [Imhof et al., 1993]. The formation of the outer boundary layer of the radiation belt/ring current may be mostly caused by the population losses due to precipitation and drift losses. Recently, a sharp boundary layer, having a thickness which is comparable to the ion gyroradius between the inner magnetosphere and active outer plasma sheet, has been observed [Sergeev et al., 2003; Apatenkov et al., 2008]. [3] In the magnetotail, bursty bulk flows (BBFs) play very important roles in transporting magnetic flux, mass and energy from the distant tail to the near-Earth region [Baumjohann et al., 1990; Angelopoulos et al., 1992]. One of the remaining questions of BBFs regards their
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dissipation, i.e., the ‘‘fate’’ of these earthward moving bulk flows [Zong et al., 2008]. The Earthward moving BBFs should be stopped where the BBF is pressure balanced by the near-Earth magnetic pressure and the plasma thermal B2 pressure (nk0T + 2m ) [Shiokawa et al., 1997], where n 1/cm3 0 and T 1 keV. It has been suggested that oxygen BBFs may populate the inner magnetosphere directly if BBFs contain large amounts oxygen ions of ionospheric origin [Zong et al., 2008], although not all BBFs could reach the inner magnetosphere or even to geosynchronous orbit [Ohtani et al., 2006]. The Earth’s ionospheric outflows contribute a very important portion to the inner magnetosphere and to the near-Earth plasma sheet particle population, especially during geomagnetic active time [e.g., Kistler et al., 2006]. Outflowing ions (H+, He+, O+) from the ionosphere are typically characterized by a strong maximum in the flux parallel to the magnetic field [Shelley et al., 1976]. Oxygen ions originating from the polar ionosphere have been observed in the distant magnetotail (up to 200 RE) [Zong et al., 1997]. Ionospheric out flow depends strongly on geomagnetic activity, for example, it has been found that the median O+ density and pressure in the tail plasma sheet are 5 times higher during storm times than during nonstorm times [Kistler et al., 2006]. [4] Plasma flow vortices are common space physics phenomena [Lundin and Marklund, 1995]. They have been reported since the 1970 in the Earth’s magnetosphere [Hones, 1976, 1978, 1981, 1983], although most of them are found in the magnetospheric low-latitude boundary layer (LLBL) region [e.g., Fairfield et al., 2000; Otto and Fairfield, 2000; Hasegawa et al., 2004]. The plasma vortex in the tail plasma sheet is characterized by pronounced vertical motion in a plane approximately parallel to the ecliptic plane. These vortices found in the morningside rotate clockwise and those found in the eveningside rotate counterclockwise. The period for plasma vortex flow to rotate a full cycle is found to vary from 5 to 20 min and a street of vortices could last several hours [Hones, 1978, 1981, 1983]. [5] Plasma vortex flows in the plasma sheet are thought to be important in transportation of the kinetic energy from fast flow or BBFs in the magnetotail to the near Earth region [Hasegawa, 1979; Snekvik et al., 2007; Keiling et al., 2009]. A sudden breaking and/or azimuthal deflection of ~ BBF flows may generate a plasma flow vortices W = r V at the boundary between the tail plasma sheet and the inner magnetosphere as suggested by Hasegawa [1979] and Vasyliunas [1984]. The theoretical relation between the field-aligned current J// and plasma vorticity is [Hasegawa, 1979; Keiling et al., 2009] J== ¼ Bi
Z
ion eq
r d W dl== B dt B
where W is the vorticity, B is the magnetic field, r is the mass density, and the integration is from the magnetic equatorial plane to the ionosphere along a certain field line. Field-aligned currents, thus, are generated to transport transverse momentum along magnetic field lines [Pritchett and Coroniti, 2000]. These observed field-aligned currents provide a link between the inner magnetospheric boundary
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and the Earth’s ionosphere by coupling magnetic shear stresses and plasma flow momentum from the boundary toward the ionosphere. Further, it is suggested that magnetospheric plasma vortices could be related to the ionospheric vortex convection flows and the ionospheric aurora streamers through field-aligned currents [Moretto et al., 2002; Keiling et al., 2009]. [6] In light of the above developments this paper reports vortex-like plasma flow structures observed at the boundary of the outer radiation belt and ring current by the Cluster constellation with high temporal and spatial resolution data sets. In this paper, we suggest that plasma flow structures are formed at the interface between the enhanced ionospheric outflow stream dominated by singly charged oxygen ions and a sudden breaking and/or azimuthal deflection of BBF flows generated in the magnetotail.
2. Observations [7] On 11 April 2002, from 0400 to 0700 UT, the fleet of Cluster spacecraft [Escoubet et al., 1997] were traveling across the inner magnetosphere (start at GSE: X = 3.5, Y = 1.0, Z = 3.6 RE, 2100 MLT) with separations of less than 0.1 L shells between the spacecraft; see Figure 1. [8] Figure 1a shows the Cluster trajectory during 0400– 0700 UT, 11 April 2002 in GSM X– Z plane together with Tsyganenko T89 [Tsyganenko , 1989] magnetic field configuration. Figures 1b and 1c show Cluster configuration as referred to C3 at 0420 (the center time for the time interval marked ‘‘vortex 1’’; see Figure 2 for details) and 0627 UT (the center time for the time interval marked ‘‘vortex 2’’; see Figure 2 for details), respectively. The black, red, green and blue triangles represent C1, C2, C3 and C4, respectively. The Cluster constellation were traveling across the equator from south to north in the nightside (2130 MLT). Figure 2 summarizes the Cluster observations of plasma flow vortices from 0415 to 0430 UT and 0620 to 0635 UT on 11 April 2002 which occur when the Cluster fleet entered (in the Southern Hemisphere) and exited (in the Northern Hemisphere) the boundary layer of the inner magnetosphere (the outer radiation belt and/or the ring current region). The two vortex regions in Figure 2 are highlighted in yellow. This event was first brought to our attention by Vallat et al. [2007], who focused on multiple nose-like events in the inner magnetosphere rather than the boundary layers. It should be noted that both ‘‘vortices’’ (observed at 0420 UT and 0627 UT) were observed at rather high latitudes (invariant latitude 650 700); the magnetic field was mainly in XY plane where Bz is relatively small (see Figure 2). [9] Figures 2a and 2b reveal the variation of the plasma electron number density obtained from the spacecraft potential measurements, taken by the EFW instrument [Pedersen et al., 2008] in the time period from 0415 UT to 0430 UT and from 0620 UT to 0635 UT, respectively. The plasma density inside the vortices is comparable with that in the adjacent outer radiation belt; however, the plasma density enhanced greatly at the inner edge (possibly the plasmapause boundary layer) of the plasma flow vortices. The energetic electron fluxes (>30 keV) from the RAPID instrument [Wilken et al., 2001] are shown in Figures 2c and 2d.
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Figure 1. (a) Cluster trajectory during 0400 – 0700 UT, 11 April 2002, in the noon-midnight meridian plane of the geocentric solar magnetic (GSM) system. The Cluster constellation is traveling across the equator from south to north on the morningside (2130 MLT). The magnetic field configuration is obtained by the Tsyganenko T89 Model. Cluster configuration as referred to C3 at (b) 0420 (the center time for vortex 1; see Figure 2 for details) and (c) 0627 UT (the center time for vortex 2; see Figure 2 for details), respectively. The black, red, green, and blue triangles represent C1, C2, C3, and C4, respectively. [10] Figures 2e – 2h show the plasma flow obtained by the CIS/HIA instrument [Re`me et al., 2001]. For the first vortex (Figures 2e and 2g), a clear sudden change of the Vx component of the plasma flow from negative to positive (bidirectional signature) is observed, whereas in the second vortex, Vx shows an obvious positive to negative bidirectional signature. Both vortices display a peak-to-peak magnitude of about 40 km/s in the Vx component coincident with a peak value of about 30 km/s in the Vy component. [11] The inflection points of the Vx bidirectional signatures for both Southern and Northern hemispheres are at 0419:35 UT and 0627:30 UT coinciding with the maximum values in the Vy components. Strong axial plasma flows at the structure’s center have a duration of about 195s and 180s which significantly exceed the adjacent background flows. Figures 2i and 2j show the Ey components in GSE coordinates from the EFW instrument. The bipolar-like signatures are also observed, as marked by the dashed lines, although the bipolar signature is not significant in the first vortex. The magnetic fields Bx, By, Bz and the magnitude Bt are given in Figures 2k – 2p. The Bz components are relatively small with respect to the Bx and By component. [12] The disturbances are seen basically in the Bx and By components with comparison to the magnetic field magnitude Bt. In addition, such disturbances were not present in the Bz component. Data from different spacecraft are marked in different colors. In Figures 2f, 2h, and 2j, the time series of the plasma flow and observed electric field (Vx, Vy and Edusk) for spacecraft C3 has been shifted according to C1. It can be seen that the main features (the plasma flow, the electric fields, and energetic electrons)
observed by different spacecraft for the two vortices are essentially identical, indicating that these two vortices are spatial in nature rather than being a temporal effect. The differences (less than 2%) in Bx and By observed by different S/C, visible in Figures 2k– 2p, are most likely due to different locations of spacecraft. [13] Figures 2q and 2r present the evolution of the estimated total current density Jt that is calculated by the linear interpolation approach for the time period of interest [Robert et al., 1998]. The maximum current reaches 150 and 100 nA/m2 in the two vortices, respectively. These currents are significantly larger than those normally found in the magnetotail current sheet (5 nA/m2) [Thompson et al., 2005]. The angles between the current and the magnetic field vector are shown in Figures 2s and 2t. It can be seen that before in vortex 1 centered at 0419:30 UT and vortex 2 centered at 0627:30 UT, these currents are antiparallel to the field line with angles between 150° and 180°. However, at the centers of both vortices these currents are field-aligned dominated since the angles are mostly between 0 and 30°. [14] As one can see from Figure 2, there is an existing sharp transition of the electron flux with more than 3 orders from 102 to 105 (cm2 s1 sr1) which are closely related to these observed vortices. Such a stable and sharp energetic particle boundary was first reported by Sergeev et al. [2003]. By timing the sharp boundaries in plasma electron density observed by all four spacecraft, the orientation and the moving speed of the boundary can be determined based on the triangulation method [Russell et al., 1983]. This normal is determined as (0.39, 0.78, 0.50 in GSE) for vortex 1 in the Southern Hemisphere and (0.23, 0.027,
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Figure 2. (a and b) Plasma density obtained from spacecraft potential; (c and d) energetic electron flux; (e – h) plasma flow Vy and Vx; (i – p) duskward electric field and magnetic field components Bx and By and the total magnetic field Bt; and (q – t) the current obtained with curlometer technique and the angle between the current and magnetic field lines. Plasma flow vortices observed by the Cluster spacecraft at the boundary layers of the outer radiation belt region are highlighted in yellow. The exact times from every s/c passing the boundary are shown in Figures 2a and 2b. Figures 2f, 2h, and 2j, the time series of the plasma flow, and observed electric field (Vx, Vy, and Edusk) for spacecraft C3 are shifted according to C1.
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Figure 3. Cluster spacecraft tetrahedron parameters. Elongation and planarity for both (a) vortex 1 (0415 –0430) and (b) vortex 2 (0620– 0635) on 11 April 2002 perigee pass, during which the spacecraft separation was around 100 300 km. (c) The influence of the tetrahedron shape (characterized by the tetrahedron parameters) on the estimate of jJj [after Robert et al., 1998]. Blue and red lines demarcate the extreme values (blue lines for vortex 1 and red lines for vortex 2) taken by the two parameters for spacecraft separation distances during 11 April 2002; the maximum separation is 300 km. It can be seen that the maximum uncertainty on jJj is never more than 15% for both cases. 5 of 11
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Figure 4. Hodograms showing plasma flow vortices 1 and 2 obtained at the boundary layers of the outer radiation belt in the minimum variance coordinate system (rVl, rVm, and rVn) with the axes corresponding to the maximum, intermediate, and minimum variance, respectively. Here ‘‘s’’ and ‘‘e’’ denote the start and end of the traces. 0.97 in GSE) for vortex 2 in the Northern Hemisphere. These thicknesses and speeds are determined as 35 km and 0.72 km/s for the boundaries related to vortex 1 in the Southern Hemisphere and 20 km and 0.42 km/s for the electron wall related to vortex 2 in the Northern Hemisphere, respectively. On the basis of the observed time intervals of the two vortices, the thickness of both vortices can further be estimated to 810 and 1138 km. The thicknesses of those electron ‘‘walls’’ found here are about 35 and 20.5 km (one ion gyroradius scale), agreeing with results obtained by Sergeev et al. [2003] and Apatenkov et al. [2008], respectively. [15] In order to analyze the error range of curlometer technique, two parameters were introduced to characterize the tetrahedron geometry: the elongation, or prolateness and the planarity, or oblateness besides the size scale of the tetrahedron [Robert et al., 1998; Vallat et al., 2005]. As we can see from Figure 1, near the perigee, the Cluster tetrahedron shape is along the inbound or outbound leg (mainly in GSM Z direction) of the Cluster orbit. The Cluster tetrahedron has large elongation and cannot be considered as a regular pattern [Robert et al., 1998]. [16] Figure 3 shows Cluster spacecraft tetrahedron parameters: elongation and planarity for both vortex 1 (0415 – 0430) and vortex 2 (0620 – 0635) on 11 April 2002 perigee pass (Figures 3a and 3b), during which the spacecraft separation was around 100 300 km. Figure 3c [after Robert et al., 1998] shows the influence of the tetrahedron shape (characterized by the tetrahedron parameters) on the estimate of jJj. Blue or red lines demarcate the extreme values (blues lines for vortex 1 and red lines for vortex 2) of the two parameters. It can be seen that the maximum uncertainty of jJj is never more than 15% for both cases.
[17] One of the most interesting features observed at the boundaries in Figure 2 is that the enhancement in the plasma flow Vy appears at the center of the bidirectional flow Vx and wavelike electric field Ey. In order to inspect the vortex-like structures in detail, a minimum variance analysis on the mass flux (MVArv) is performed to investigate the morphology of these vortex-like structures [Sonnerup and Scheible, 1998]. The results are plotted in Figures 4 (left) and 4 (right). [18] In the minimum variance analysis on mass flux (MVArv) the coordinate system denotes the rVl axis is denoted to be along the direction of maximum variance for the plasma flow during the time interval of interest. The other two axes, rVm and rVn, are oriented along the intermediate and minimum flow variance directions, respectively. Figures 4 (left) and 4 (right) display the variation of plasma flow of these two vortices in MVA coordinates from 0418 to 0421 UT (3 min) and from 0626 to 0630 UT (4 min), respectively. [19] Figures 4 (left) and 4 (right) demonstrate the large amplitude rotations which exist for both vortices in the rVl and rVm plane. The directions of the principal axes in GSE coordinates are given as follows: the minimum variation direction for both two vortices, rVn, were (0.014, 0.387, 0.922) and (0.082, 0.120, 0.989); the intermediate variation direction, rVm, were (0.813, 0.532, 0.236) and (0.62, 0.769, 0.145); the maximum variation direction, rVl, were (0.582, 0.753, 0.307) and (0.778, 0.628, 0.011), respectively. A total of 45 and 60 plasma flow vectors were used in these two analysis. The eigenvalues are derived as [20.3, 547.7, 1087.3] and [14.3, 166.5, 326.9] and the ratio of intermediate to minimum eigenvalue are 27.0 and 11.6, respectively. The plasma flow illustrated in the hodograms
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Figure 5. Cluster CIS observations for vortex events 1 and 2. Shown are (a) the hydrogen energy spectrograms and (b and c) their pitch angle distributions for two energy ranges. Also shown are (d) the oxygen energy spectrograms and (e and f) their pitch angle distributions for two energy ranges. The small color boxes on the top represent the plasma flow vortex region. The two vortices are highlighted in orange and are labeled. in Figure 4 are the intermediate flow variance rVm rotated over the maximum flow variance rVl. The ratio (27.0 and 11.6) of the intermediate to minimum eigenvalues [Sonnerup and Cahill, 1967] indicates that the determination of the structures which are in these two cases fairly good. It
is interesting to point out that these two vortices rotation directions of the vortices are opposed, the one in the Southern Hemisphere rotates clockwise and the other one in the Northern Hemisphere rotates counterclockwise in the rVl and rVm plane.
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Figure 6. Stacked line plots of interplanetary and planetary parameters for the time interval 10 – 12 April 2002. (a) The proton temperature followed by (b) a high solar wind speed and (c) the solar wind azimuthal flow angle measured with the ACE spacecraft. Planetary (d) Kp, (e) AE, (f) equatorial geomagnetic Dst, and (g) corrected Dst* index. The stream interface is indicated by the dashed line, and the time period of the two observed vortices are shaded in pink. [20] To inspect the ion composition of these two vortices in detail, Cluster CIS-CODIF spectrograms and pitch angle distribution are given Figure 5, which shows the hydrogen energy spectrogram in the energy range from 30 eV to 36 keV (Figure 5a) associated with their pitch angle distributions for energies above 1 keV (Figure 5b) and energies below 1 keV (Figure 5c). For comparison, the oxygen energy spectrogram is given in Figure 5d, and their pitch angle distributions for high (>1 keV) and low energies (1 keV) and lower (1 keV) are isotropically distributed in agreement with hydrogen ions. However, lower-energy oxygen ions (
