The structure of the magnetotail is investigated in a laboratory simulated magnetosphere. Particular emphasis is placed on the region of distant magnetotail where the closed field line region of the plasma sheet terminates and the process of reconnection takes place. Our study builds upon the previous investigation of the magnetotail where the main results were based on the magnetic field measurements in the tail region of the simulated magnetosphere. In this paper, more elaborate measurements of plasma flow and electric field are presented. Besides these measurements, this region of distant magnetotail is also explored by high resolution imaging with a gated optical imager (GOI) and by digital image analysis. These images clearly reveal a Y-type magnetic neutral line for the northward 'interplanetary' field (IMF) and a usual X-type for the southward IMF that confirms our previous results deduced from the magnetic field measurements. In the neighborhood of these neutral points a strong component of dawn to dusk electric field (E(sub y)) and a counterstreaming plasma flow is also observed. Plasma flow is measured by using a double sided Faraday cup which is also used to measure the y-component of tail current (J(sub y)) at different locations. These measurements reveal that the tail current is not carried by ions as previously thought, rather it is carried by electrons alone

Birn, J.

Minami, S.

Rahman, H. U.

Wessel, F. J.

Yur, G.

English

NASA Technical Reports Server

NASA-CR-IQ3OQ0
l_Iagnetotail Structures in a Simulated Earth's Magnetosphere
G. Yur and H. U. Rahman
Institute of Geophysics and Planetary Physics, University of California, Riverside
J. Birn
LANL, Los Alamos, New Mexico
F. J. Wessel
Department of Physics, University of California, Irvine
S. Minami
Department of Electrical Engineering, Osaka City University, Osaka, Japan
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Abstract
Structure of the magnetotail is investigated in a laboratory simulated magnetosphere.
Particular emphasis is placed on the region of distant magnetotail where the closed field
line region of the plasma sheet terminates and the process of reconnection takes place.
Our study builds upon the previous investigation of the magnetotail (Birn et al. 1992)
where the main results were based on the magnetic field measurements in the tail region
of the simulated magnctosphere. In this paper, more elaborate measurements of plasma
flow and electric field are presented. Besides these measurements, this region of distant
magnetotail is also explored by high resolution imaging with a gated optical imager (GOI)
and the digital image analysis. These images clearly reveal a Y-type magnetic neutral
line for the northward "interplanetary" field (IMF) and a usual X-type for the southward
IMF that confirms our previous results deduced from the magnetic field measurements.
In the neighborhood of these neutral points a strong component of dawn to dusk electric
field (Ey) and a counterstreaming plasma flow is also observed. Plasma flow is measured
by using a double sided Faraday cup which is also used to measure the y-component of
tail current (J_) at different locations. These measurements reveal that the tail current
is not carried by ions as previously thought, rather it is carried by electrons alone.
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https://ntrs.nasa.gov/search.jsp?R=19930017955 2020-03-17T05:57:44+00:00Z
1. Introduction
There has been a great deal of interest in the space physics community to understand
the structure of earth's magnetotail under varying conditions of the solar wind plasma
and its associated IMF. Both in situ observations and computer modelling has been used
in the past to understand its structure and its variations imposed by external changes.
The region of distant neutral line where the region of closed plasma sheet terminates
and probable reconnection of magnetic field lines occurs is one of the most interesting
and poorly understood region as far as the in situ observations are concerned. Until
recently, it was explored only by one satellite ISEE 3 that was basically launched to
monitor the solar wind parameters. In order to obtain the global features of this region
with respect to the varying conditions of the solar wind plasma, a coordinated multi-
satellite system is required. Therefore, we approach this problem by modelling the entire
system of magnetosphere in laboratory by interacting a magnetized plasma flow with a
dipole magnetic field. Our previous paper (Birn et al., 1992) presented the magnetic
field measurements and numerical modelling of the tail region with particular emphasis
on the distant neutral line. The main results of that study was the identification of a
Y-type magnetic neutral line for the northward IMF and an X-type neutral line for the
southward IMF. Those results strongly support the occurance of high-latitude night side
reconnection, as suggested by Dungey[1963] for northward IMF and more recently by
Gosling et al. [1991].
In the present paper we are again concentrating in the same region and present further
measurements of electric field and plasma flow for different polarities of the IMF. Also
the global images of the simulated magnetosphere together with the detail digitization of
the distant tail region are presented. In section 2 we will present these images and the
global modelling of the magnetosphere. The plasma flow and electric field measurements
will be presented in section 3. Section 5 will provide the conclusion of this paper.
2. Simulation Procedure and Imaging
The details of the experimental set up is presented in some of our previous papers
(Rahman, el.al,1991; 1989; Bivn et al, I992). The simulation is performed by interacting
an intense magnetized plasma beam produced by a large plasma gun with a strong dipole
field to form a magnetospherethat scalesto the Earth's magnetosphere.The plasmaand
other parametersarepresentedin Table 1. The dimensionlessparametersclearly showa
better agreementswith the MHD scalingrequirements(Rahman et. al, I991) except that
the ratio of ion-Larmour radius with the size of the magnetosphere (riL/Xo) satisfies only
marginally.
In the past, images were taken by using an open shutter camera. Light emitted
from the hot plasma or from the background low density neutral gas molecules provides a
source for these images. Variation in the light intensity develops due to the diverse plasma
condition that exist in the magnetosphere. For example the bow shock, magnetopause
boundary, neutral sheet, reconnection regions, plasma entry regions in the polar cusp and
auroral formation regions at the poles are relatively brighter than the usual background.
Since the density of the simulated solar wind plasma is ,,_ 1013 cm -3, the strong light
emitted by the brightest regions overexpose the film and a high resolution image cannot
be obtained by simple open shutter technique. Therefore, the images taken by an open
shutter camera show only limited features like magnetopause boundary, near earth neutral
sheet and the auroral regions. The structures like bow shock, reconnection regions and
the distant magnetotall could not be identified from those images.
The gated optical imager (GOI) provides a unique capability to obtain high resolution
and also the dynamic behavior of the magnetosphere. Figure 1 shows the schematic dia-
gram of GOI which is used to obtain these images. Light emitted from the magnetosphere
is focused on a biased cathode that emits electrons. These electrons are accelerated, by
applying a square pulse of few micro-second duration, which is much shorter than the
entire plasma pulse of 100 #sec, and then passed through a microchannel plate where the
electron flux is amplified because of the secondary emission by almost a million times.
This secondary electron flux is energized by applying a high voltage of 6-7 kV which can
reconstruct the magnetospheric image on a phosphorus screen that is finally photographed
on an ordinary film. Using this technique we were able to image for the first time, the
detailed structures of the bow shock and the distant magnetotail. These images can be
further enhanced by using a CCD camera and computer aided digitizing techniques. In
this way not only the qualitative informations but also some quantitative information can
be extracted from this imaging technique.
Fig. 2 representsa sample imageof a laboratory magnetospherefor the southward
polarity of IMF with an appliedstrengthof 200G. The magneticfield that penetratesinto
the plasmais always lessthan 100 G with an insignificant reduction in the plasmaflow
speed( Wessel et al. 1990). Therefore the Alfvgn Mach number is almost 2 for all these
images. The exposure time of this image is 5_sec. A series of images taken during the
entire interaction period have shown that this type of global structure persists 80% of the
interaction time. Plasma entry from the polar cusp, formation of bow shock, elongated
magnetotail and auroral formation can be clearly identified in this image. An extended X-
type neutral line can be clearly seen in the current sheet of the magnetotail that persists
up 40#secs. The lower panel of Fig. 2 is digitized version of the distant neutral line
region that shows more clearly the formation of the X-type reconnection in the tail region
that corresponds exactly the same position where the neutral line was predicted from the
magnetic field measurements. This image reveals the presence of a hot trapped plasma
between the earth and the neutral line region and a hot flaring plasma away from the
neutral line. Figures 4 presents the image for the northward IMF with 200 G applied
magnetic field. In this image, a clear separation of bow shock and magnetospause is not
very obvious, however, this image clearly reveals an open magnetosphere. An extended
neutral sheet in the tail region is also very obvious that have a strong resemblance with
the predicted Y-type magnetic neutral line. The region is further clarified in the digitized
version presented in the lower panel which seems to be again consistent with our previous
measurements of the magnetic field (Birn et al., 1992). The complete dynamic evolution
of the laboratory magnet is presented in a separate paper (Yur et al., I993).
3. Plasma Flow and Electric Field Measurements
To measure the plasma flow and identify different boundaries of the simulated
magnetosphere, a new type of double sided Faraday cup is developed. The schematic of
this Faraday cup is shown in Fig. 4 which has a dimensions of 1 cm 3. This combination
of two Faraday cups can measure simultaneously the counter streaming fluxes of either
ions or electrons. For this paper we used this probe to measure the ion flux (en,vi) at
various regions of the tail. Fig. 5 presents the ion flux Ji versus z at z = 9.75cm behind
the model earth for the southward IMF of 200 and 300 G. This location is in the closed
field region and in the vicinity of the termination point. Significantly enhanced plasma
fluxes, both earthward and tailward, near the equatorial plane are quite obvious from
thesemeasurements.The tailward ion flux is almost twice as large asthe earthward ion
flux. The flux profile has a symmetrical behavior in both upper and lower lobes. Fig. 6
is the samedata at z - 6.75cm, much closer to the model earth, but only in the upper
lobe. In this case the flux in both direction has almost the same magnitude. This data
implies the presence of a trapped hot plasma with temperature of 50-100 eV in the near
earth tail region. Fig. 7 and 8 are the similar flux profiles for the northward IMF again at
6.75 cm and 9.75 cm respectively. Again the pattern is similar to the southward IMF case
except that now the net flux is somewhat smaller in magnitude. This implies that the
net plasma density in the case of northward IMF is lower than the southward IMF case
which may be due to the reduced level of entry from solar wind to the magnetosphere for
the northward IMF. The trapped plasma seems to have entered from the neutral point
in the distant tail region that can also be seen from the images presented in Fig. 2 and
3. We have also used this diagnostic to measure the fluxes along the y-axis that show
almost the same level of flux in both directions, which may be an indication that the tail
current is mainly carried by electrons. Using a double Langmuir probe we have also seen
a significant enhancement in plasma density and temperature in the regions of enhanced
ion fluxes. These measurements represent the first identification of plasma sheet and lobe
region s through plasma data in the simulated magnetotail.
The presence of the energized plasma in the photographic pictures possibly indicates
the effects of a localized electric field, which favors a dynamic picture. For this purpose
we performed some measurements of E U as function of z along the tail axis. For this
purpose a floating E-probe is used, the circuit diagram of which is given in Fig. 9. For
southward IMF of 200 and 300 gauss, the E_ as a function of z is presented in Fig. 10.
This figure shows that Ey is concentrated in the vicinity of the neutral point, having a
double peak structure. Fig. 11 shows Ey as a function of z for northward IMF. In this
case only a single flat peak appears with Ey gradually approaching zero as we move the
probe away from the model earth. The most interesting feature is that this E-field does
not correspond to the v × B.
4. Conclusions
In conclusion we have studied the structure and dynamics of the magnetotail in a
laboratory magnetosphere. The images taken by the GOP show that a quasi-stationary
magnetosphere develops in less than 2 /_secs and persists for almost 50 #secs without
any substantial variation in the global structures that is comparable to the tota! duration
of the plasma flow which is 70-100 _usecs. Global features like bow shock, magnetopause
boundary, plasma sheet in magnototail, reconnection region, auroral regions, plasma entry
regions etc. are first time clearly identified. For the southward IMF a usual X-type neutral
line region in the distant magnetotail can be easily identified from these images. On the
other hand, for the northward IMF a long plasma sheet originating from the model earth
is observed which is an indication of a Y-type magnetic neutral line. These results are
consistent with our previous magnetic field measurements in the tail region of the model
magnetosphere (Birn et a1.1992). Detailed measurements of plasma flux, electric and
magnetic field, and other plasma parameters support these observations.
Acknowledgments. This work was supported by National Science Foundation. One of the
author F. J. Wessel was supported by a grant from NASA.
References
Birn, J., G. yur, H. U. Rahman, S. Minami, On the termination of the closed field line
region of magnetotail, J. Geophys. Res., 97, 14833, 1992.
Dungey, J. W., Structure of the exosphere or adventures in velocity space, in Geophysics,
The Earth's Environment, edited by C. DeWitt, J. Hieblot, and A. Lebeau, p. 503,
Gordon and Breach, New York, N.Y., 1963.
Gosling, J. T., M. F. Thomsen, S. J. Bame, R. C. Elphic, and C. T. Russell, Observation
of reconnection of interplanetary and lobe magnetic field lines at the high-latitude
magnetopause, J. Geophys. Res., 96, 14097, 1991.
Rahman, H. U., G. Yur, R. S. White, and J. Birn, On the influence of the magnetization
of a model solar wind on a laboratory magnetosphere, J. Geophys. Res., 96, 7823, 1991.
Wessel, F. J., J. H. Song, N. Rostoker, G. Yur, H. U. Rahman, Fast magnetization of a
low-to-high beta plasma beam, Proceedings SPIE-The International Society for Optical
Engineering, vol. 1226, pp. 457, 1990.
Yur, G., H. U. Rahman, S. Minami, and J. Birn, Dynamic evolution of the laboratory
simulatedmagnetosphere,J. Geophysic. Res., submitted.
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northward polarity of MF. The electric field is seen to have a
strong componant around the neutral point.


Magnetotail structures in a simulated Earth's magnetosphere

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