GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L02101, doi:10.1029/2005GL024574, 2006
Polar survey of magnetic field in near tail: Reconnection rare inside
9 RE
Y. S. Ge and C. T. Russell
Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA
Received 6 September 2005; revised 31 October 2005; accepted 2 December 2005; published 17 January 2006.
[1] We have examined the magnetic field at the apogee of
the Polar spacecraft for three seasons during which time
apogee was close to the equatorial plane and within 2 hours
of midnight. The magnetic field magnitude in this region is
rarely below 5 nT. However, when it is low, the magnetic
field is very dynamic, with current sheet thinnings, plasma
sheet expansions, and magnetic field dipolarizations. At
times of low magnetic field strength we have examined the
component normal to the magnetospheric equator for the
presence of southward magnetic fields. However, in the
period studied only northward fields were present in this
region with one brief exception due to a bent current layer
and not due to local reconnection. Thus it is very rare that
the neutral point is as close as 9 RE. Citation: Ge, Y. S., and
reconnection. Much earlier McPherron et al. [1973] inferred
beneath the location of OGO-5 a neutral point as close as 9
RE. Polar with its apogee at 9 RE spends much of its orbit in
this radial range. The period when Polar’s apogee precessed
through the equatorial plane gives us a chance to determine
how often near-Earth reconnection is seen at this close
distance.
[3] In this paper, we survey the passes of Polar spacecraft
that traverse the near-Earth current sheet. We examine both
the strength of the magnetic field and its southward component near current sheet crossings to investigate the
possibility of magnetic reconnection in this region.
C. T. Russell (2006), Polar survey of magnetic field in near tail:
Reconnection rare inside 9 RE, Geophys. Res. Lett., 33, L02101,
doi:10.1029/2005GL024574.
2. Polar Observations
1. Introduction
[2] The near-tail region at 9 RE is a very important part of
the magnetosphere, standing at the gateway between the
dipolar region of the magnetosphere, and the tail and plasma
sheet. Here the magnetic field crossing the equatorial plane
is much weaker than near synchronous orbit, and is quite
dynamic [e.g., Takahashi et al., 1987]. Reconnection, that is
responsible for much of this dynamic behavior, is believed
to occur at greater geocentric distances. Geotail observations have been interpreted as showing that magnetic
reconnection most frequently takes place at radial distances
of 20– 30 RE [e.g., Nagai et al., 1998] in association with
substorm onsets. However, the reconnection point can only
be detected where there are measurements and some spacecraft have detected reconnection much closer to the Earth.
Cluster has observed the tailward passages of X-line over
the spacecraft at 18 RE [e.g., Baker et al., 2002] and at
16.4 RE [e.g., Runov et al., 2003]. Paschmann et al. [1985]
examined fast tailward flows near 14 Re just after the onset
of the CDAW-6 substorm on March 22, 1979 with ISEE 1
and 2. The closest observation of a southward component of
the tail field was at 8 RE by AMPTE/CCE [e.g., Takahashi
et al., 1987]. However, Lui et al. [1988] has argued that
these observations were not consistent with an X-type
neutral-line geometry because of inconsistencies of particle
and field signatures. Recently during an extremely intense
substorm on October 30, 2003 [Miyashita et al., 2005],
Geotail observed a large southward magnetic field at
8.5 RE, which very possibly results from the magnetic
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2005GL024574$05.00
[4] The Polar spacecraft was launched in 1996 into a 9
RE apogee polar orbit with its line of apsides oriented so
that in one year it precessed to where apogee was directly
over the north polar cap. During Polar’s continued operation
its orbit has remained polar and the line of apsides has
continued to precess so that in 2002 it was in the equatorial
plane. This configuration allows Polar to probe the nearEarth tail in the neighborhood of 9 RE. One limitation of
Polar for this study is that Polar is not operated in eclipse.
Thus we excluded the region within about 1 RE of EarthSun line.
[5] Near 9 RE the magnetic field is usually quiet and
somewhat weakened around the current sheet where BX
reverses. Shown in Figure 1 are 6-second Polar magnetic
field data [Russell et al., 1995] transformed into geocentric
solar magnetic (GSM) coordinates. The Polar spacecraft
passed the current sheet at around 1700 UT which is
indicated by the Bx reversal. The minimum magnetic
strength (and the component of the field normal to the
current sheet) is about 30 nT.
[6] However, at times the magnetic field is highly disturbed and a strong tail current reduces the magnitude to a
low value. Figure 2 shows plots of a Polar crossing through
the current sheet on October 19, 2003. Eight hours of
6-second magnetometer data are shown. When Polar
crossed the current sheet at 2337 UT, the magnetic field
magnitude was less than 7 nT. Such a low field is rarely
seen at this distance. However, the component normal to the
magnetic equator did not become negative, minimizing
close to 7 nT also. The x and y components are approaching
zero and the magnetic field is mainly in the normal
component at the crossing of the current sheet.
[7] The disturbances during the crossing period are very
interesting. Before Polar passed the current sheet, there were
two increases in Bz component which are the signatures of
dipolarizations when close to the current sheet. One was
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Figure 3. Minimum magnetic field each orbit (2001–
2003) when Polar was at apogee and within 2 hours LT of
midnight.
[8] To determine the typical behavior of the field in this
region we have examined all the passes near midnight in
2001, 2002 and 2003. A wide range of field strengths are
seen at Polar’s apogee on the nightside. Figure 3 shows the
minimum field strength observed on each orbit in the three
years when Polar’s apogee was within 2 hours local time of
midnight. The survey was performed using 6-second data.
Orbits with eclipse during the crossing of the current sheet
are not included.
[9] Typically the field strength is close to 20 nT. However, the magnetic field can be much weaker and much
stronger than that. Occasionally it is less than 5 nT, in 18 of
271 counted crossings. Some crossings with high field
strengths of 30– 65 nT were observed. The contribution of
the dipole field of the Earth at 9 RE on the magnetic equator
is about 40 nT. So we can see that the field in this region is
generally weakened by the cross-tail current. Events with
field strength lower than 5 nT while rare are very relevant to
the question of the innermost occurrence of magnetic
reconnection.
[10] However, when we examine the north-south component of the magnetic field for these low field periods, we see
no evidence for a southward IMF inside of Polar’s apogee.
In particular, we examined the highest resolution (0.120 sec)
data when tail field was lower than 5 nT and no negative z
component of magnetic field was found except on one orbit
shown on Figure 4. During this orbit on September 19,
Figure 2. Time series of magnetic field on a disturbed day,
October 19, 2003 (6s resolution). Dashed lines show the
dipolarizations seen by Polar.
Figure 4. Time series of magnetic field on a disturbed day,
September 19, 2003 (0.12s resolution).
Figure 1. Time series of magnetic field on a quiet day
when Polar near center of near tail region (6s resolution).
close to 2100 UT and another occurred less than 1 hour
later. At this time, the Polar spacecraft was in the south tail
lobe field. Two more clear dipolarizations were observed
after crossing the current sheet. At 0020 UT on Oct 20,
2003, Bz component increased from 7 nT in current sheet to
above 30 nT. The immediate decrease in Bx component and
in the magnetic field strength indicates that the plasma sheet
expanded and Polar reentered hot plasma region. Then the
plasma sheet thinned and magnetic field recovered to lobe
field level. At 0100 UT, the Bz component rose again
accompanied with a sudden drop of Bx component, signaling another dipolariztion.
3. Statistics
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[12] The solar wind dynamic pressure controls the field
strength in the near tail where the magnetopause flares
[Petrinec and Russell, 1996]. Thus we would expect that
there would be an anti-correlation of the field at Polar’s
apogee near midnight with ru2 the dynamic pressure of the
solar wind. Most of the variance would be due to N rather
than u because of the much greater variability of the solar
wind density. We note that VBz controls the flaring angle by
moving more flux from the dayside to the tail. This increases
the strength of the field in the tail without changing ru2.
[13] Figure 5 shows the relations between the median
field strength at the current sheet crossing and solar wind
density, dynamic pressure, and convected southward IMF
flux. The solar wind data used here is one minute
resolution ACE data and has been propagated to the
subsolar bow shock at (17, 0, 0) RE using the advection
technique of Weimer et al. [2003]. As expected, an anticorrelation is observed between the solar wind density and
the dynamic pressure with the near-tail magnetic field.
Clearly as we expect, larger solar pressures (mainly caused
by larger densities) compress the tail field more, enhancing
the tail current and producing lower fields near 9 RE.
Similarly the correlation with the convected southward
IMF in Figure 5c shows that the more southward IMF
flux is moved to the magnetotail, the lower is the nearEarth tail magnetic field. The correlation coefficients
between the median magnetic field strength and solar wind
density, dynamic pressure, and convected electric field are
0.75, 0.85, and 0.85 respectively.
5. Discussion and Conclusions
Figure 5. Relation between median tail magnetic field at
the current sheet crossing (minimum jBj) and solar wind
parameters: (a). Solar wind number density; (b). Solar wind
dynamic pressure; (c). Converted southward IMF flux.
Values shown are medians in overlapping (by half) bins. R is
the correlation coefficient of medians.
2003, a very brief (<1 second) southward component of
magnetic field was present at 0813 UT. This short period of
negative component is apparently caused by a bend in the
current layer as the magnetic field direction does not
approach the vertical (z) direction. This may be the same
reason that GOES 9 saw a transient southward field at
0600 UT on December 31, 1995 [see Ohtani et al., 1999,
Figure 9] Thus at least while Polar has been near the current
sheet and near midnight, there has been no clear crossing of
the X-line detected at Polar.
4. Control of the Field Strength in the
Near-Earth Tail
[11] The weakness of the field in the region near 9 RE is
due to the closure of the magnetic field associated with the
twin current cylindrical current systems encircling the tail
lobes that opposes the dipole magnetic field. Thus we would
expect that any mechanism that increased the magnetic field
strength in the tail would weaken the magnetic field near 9
RE especially if the inner edge of the tail current simultaneously moved toward the Earth.
[14] The evolution of the Polar orbit has enabled us to
probe the night magnetosphere in the equatorial plane near 9
RE. The typical field strength at 9 RE midnight is found to
be about 20 nT. On fewer than 7% of the orbits a magnetic
field strength under 5 nT was reached in the vicinity of the
current sheet. At these times the field was quite dynamic
with dipolarizations of the magnetic field. However, we find
no evidence that the neutral point moved inside Polar’s orbit
during our observations. Thus we conclude that the nearEarth reconnection is very rare seen at distance closer than 9
RE. This is consistent with the bursty bulk flow and
traveling convection region study of Slavin et al. [2005]
which shows that X-lines form earthward of Cluster apogee
at 19 RE only 20% of time. If we accept the interpretation of Lui et al. [1988] of the CCE southward fields as
inconsistent with reconnection, then there has only been one
demonstrated reconnection event in this region, that seen by
Geotail during the October 30, 2003 super storm [Miyashita
et al., 2005].We would expect that the weaker fields are
associated with stronger solar wind dynamic pressures and
also with increased southward IMF convected to the dayside
magnetopause that would increase the magnetic flux in the
tail and the flaring of the tail. Both the expected correlations
are observed. These correlations are consistent with the
rarity of southward magnetic fields inside of 9 RE near
midnight.
[15] Acknowledgments. The authors wish to thanks Dan Weimer and
James Weygand for providing time-propagated solar wind data. This work
was supported by the national Aeronautics and Space Administration under
grant NAG5-11324.
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Y. S. Ge and C. T. Russell, Institute of Geophysics and Planetary Physics,
University of California, Los Angeles, CA 90095-1567, USA. (yasong@
igpp.ucla.edu)
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