GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 1, PAGES 111-113, JANUARY 1, 2001
A linkage between polar patches and plasmaspheric
drainage plumes
Yi-Jiun Su,1 Michelle F. Thomsen, and Joseph E. Borovsky
Space and Atmospheric Sciences Group, Los Alamos National Laboratory,
Los Alamos, New Mexico
John C. Foster
Atmospheric Sciences Group, MIT Haystack Observatory, Westford, Massachusetts
Abstract. Two distinct features are shown here to follow the same convection pattern: polar ionization patches
and plasmaspheric drainage plumes. In this study, the enhanced ionospheric electron concentration, observed from
Millstone Hill incoherent scatter radar, is mapped to the
equatorial plane. The resulting trajectory there is found
to coincide with the location of the draining plasmasphere
as observed from a geosynchronous satellite. The result indicates that both the F -region electron concentration and
the plasmaspheric drainage plume follow the newest separatrix (the boundary between open and closed drift trajectories) convecting sunward. Observations have shown that
the polar patches are transported over the polar cap into the
night side. Hence, we conclude that the draining plasmasphere should follow a similar trajectory transporting plasmaspheric material over the polar cap into the tail plasma
sheet.
Introduction
The characteristics of polar ionization patches, determined mainly from observations in the northern hemisphere,
were summarized by Rodger et al. [1994]. Polar patches are
regions of scale size 200-1000 km, with enhanced F -region
electron concentration and 630 nm airglow emission, that are
observed to drift anti-sunward across the polar cap. Patches
have been observed in summer and winter at sunspot maximum and minimum during periods when the interplanetary
magnetic field (IMF) BZ is negative (southward). The formation mechanism for polar patches remains a controversial
topic. One mechanism suggests that the cross polar cap potential rises abruptly, causing an increase in the size of the
convection pattern. This, in turn, brings sunlit, high concentration F -region plasma from low latitudes to the vicinity
of the polar cusp, which thereafter convects into the polar cap [c.f., Foster and Doupnik, 1984; Anderson et al.,
1988; Foster, 1989, 1993]. Another suggests that the energetic particle precipitation in the cusp and boundary layer
plays an important role in the formation of regions of enhancement of ionospheric plasma concentration near noon.
Subsequently, these features are transported into the polar
1 Now
at Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado.
Copyright 2001 by the American Geophysical
Paper number 2000GL012042.
0094-8276/01/2000GL012042$05.00
cap where they continue to evolve [c.f., Rodger et al., 1994].
In this report, we present evidence that supports the mechanism described in Foster and Doupnik [1984] and Foster
[1989, 1993], in which enhanced convection brings high F region electron concentration from low latitudes sunward
and poleward into the polar cap.
A plasmaspheric drainage plume is another distinct feature related to the Earth’s variable convection pattern. The
plasmasphere is the near-Earth region of the magnetosphere
that is populated by dense, cold plasma of ionospheric origin.
Under quiet conditions, the convection in the near-Earth
region is dominated by the corotational electric field, and
magnetic flux tubes in the outer plasmasphere follow closed
drift trajectories around the Earth filling with ionospheric
plasma. During periods of increased magnetospheric convection, loaded flux tubes from the outer plasmasphere find
themselves in the region of newly open drift trajectories and
drain toward the dayside magnetopause, producing what are
known as detached plasma regions [Chappell, 1974], plasmaspheric tails [c.f., Chen and Wolf, 1972; Chen and Grebowsky,
1974], or plasmaspheric drainage plumes [c.f., Elphic et al.,
1996; Borovsky et al., 1998; Thomsen et al., 1998a]. It has
been suggested that draining plasmaspheric material is on
flux tubes which undergo reconnection at the dayside neutral line and is subsequently transported toward the magnetotail, contributing ultimately to the plasma sheet population [e.g., Borovsky et al., 1997; Elphic et al., 1997]. Recent
geosynchronous observations have confirmed the participation of plasmaspheric material in the dayside magnetopause
reconnection [Su et al., 2000a]. Additionally, plasmaspheric
material has been observed on reconnected field lines in the
cusp by polar-orbiting satellites [Su et al., 2000b], supporting this idea of plasmaspheric circulation.
This study is focused on mapping the enhanced F -region
electron density, observed by the Millstone Hill radar, to
the equatorial plane and comparing its trajectory with the
draining plasmasphere observed from the geosynchronous
satellite 1989-046 for the storm event of March 1990.
Results
The three-hourly averaged KP values on March 17-21,
1990, are shown in Figure 1. The F -region electron densities
observed at 500 km altitude by the Millstone Hill incoherent
scatter radar were presented in panels (a) and (d) of their
Figure 1 by Foster [1993] for the time interval from 1200 UT
on March 18 to 1200 UT on March 19 and from 1200 UT
on March 20 to 1200 UT on March 21, respectively; these
111
112
SU ET AL.: POLAR PATCHES AND PLASMASPHERIC DRAINAGE PLUMES
Figure 1. Three-hourly averaged Kp values on March 17-21,
1990. Each minor tick mark on the horizontal axis represents a
3-hour interval.
intervals are shown here by horizontal bars on the KP plot
in Figure 1. Panels (a) and (d) of Foster’s Figure 1 are
re-plotted on polar dial plots shown in our Figures 2a and
2b, respectively. The radial and azimuthal axes are geodetic latitude and magnetic local time (MLT), respectively.
There are two branches of high F -region density: One is the
branch corotating with the Earth at low latitudes; the other
is a smaller piece extending sunward and poleward. Foster
[1993] termed the Kp -dependent observations of this latter
feature storm-enhanced density (SED), and identified this
feature as sunward-advecting solar-produced plasma which
had corotated past noon at lower latitudes, before being entrained in the convection pattern in the afternoon sector
and brought back toward noon, eventually convecting into
the polar cap and becoming polar patches. We should note
here that only regions with electron densities greater than
105.8 cm−3 are plotted in our Figure 2. The electron density
is typically below 105.8 cm−3 during a quiet-time interval.
An example of a quiet-time interval can be found in Figure 1c of Foster [1993]. The purpose of setting this density
threshold is to focus on the evolution of SED (the smaller
branch extended sunward and poleward in our Figure 2a
and 2b).
In Figures 2c and 2d, the ionospheric distributions of Figures 2a and 2b are mapped to the equatorial plane of the
magnetosphere along magnetic field lines specified by the
Tsyganenko (T89) [Tsyganenko, 1989] external field model
combined with the IGRF internal field. The positive XGEO
and YGEO in Figures 2c and 2d are in the sunward and
duskward directions, respectively. Superimposed on these
mappings as bold lines are the locations where the Los
Alamos magnetospheric plasma analyzer (MPA) on satellite 1989-046 observed cold plasma with ion densities ≥ 10
cm−3 at geosynchronous orbit. These cold, dense ions are
of plasmaspheric origin, and their location indicates that
they are part of the drainage plume produced by the enhanced convection associated with the elevated KP . Details on the MPA instrument are provided by Bame et al.
[1993]. A typical example of observations at geosynchronous
orbit is presented by McComas et al. [1993]. The typical
ion density in the trough region [Thomsen et al., 1998b] is
less than 1 cm−3 , and the plasmaspheric density is greater
than 10 cm−3 with a saturation density ∼ 100-200 cm−3
at geosynchronous orbit (6.6 RE ). In the cases presented
here, the cold ion density jumps from less than a few cm−3
to more than 10 cm−3 when the satellite passes from the
trough region to the plasmasphere; hence we set the lowest
plasmaspheric density threshold at 10 cm−3 . In Figure 2c,
the plasmaspheric interval started at 2218 UT (1155 LT) on
March 18 and ended at 0230UT (1607 LT) on March 19. The
gap discontinuity between the bold line segments is due to
a data gap. In Figure 2d, the plasmaspheric interval started
at 0000 UT (1337 LT) and ended at 0228 UT (1605 LT)
on March 21. Magnetosheath ions were present at geosynchronous orbit from 2320-2400 UT on March 20 (see dotted
curve in Figure 2d) at which time the magnetopause had
been driven within geosynchronous orbit due to the strong
solar wind pressure.
Figure 2 shows that the plasmaspheric drainage plume
observed at geosynchronous orbit is colocated with the
mapped high-latitude ionospheric density enhancement described by Foster [1993]. It appears that, as a consequence of
increased geomagnetic activity, corotating low-latitude flux
tubes with enhanced ionospheric densities and high-density
plasmasphere now find themselves on an open trajectory.
Thus, both the enhanced F -region electron concentration
and outer plasmasphere on those flux tubes convect sunward
along the new open/closed drift separatrix. Furthermore,
the enhanced F -region plasma density had an observed velocity of 800 m s−1 , giving a 2-hour transit time from its
source at low latitude to the polar cap at noon [Foster, 1993].
This transit time is comparable to the transit time of draining plasmasphere with a typical observed velocity of ∼ 10
km s−1 from the dusk to the dayside magnetopause.
Figure 2. Polar dial plots of enhanced F -region electron densities as a function of geodetic latitude and magnetic local time
(MLT) observed from Millstone Hill radar on (a) March 18-19 and
(b) March 20-21, respectively. The regions of enhanced F -region
electron densities mapped to the equatorial plane are shown in
panels (c) and (d). Bold lines represent plasmaspheric observations (density ≥ 10 cm−3 ) obtained by MPA1989-046 at geosynchronous orbit. The segment of dotted curve in panel (d) represents an interval of magnetosheath observed by MPA.
SU ET AL.: POLAR PATCHES AND PLASMASPHERIC DRAINAGE PLUMES
Conclusion
Our results indicate that both the enhanced F -region
electron concentration and the draining plasmasphere follow the most recent open trajectory convecting sunward.
This result supports the polar patch transport mechanism
described by Foster and Doupnik [1984], in which the enhanced electron concentration convects from low latitudes
to the vicinity of the polar cusp due to an increasing electric
potential. Such plasma, seen at very high latitudes within
the polar cap, serves as a tracer of the convection pattern
away from the cleft.
Recently, the transport of polar patches has been reported by Crowley et al. [2000] during the March 21,
1990, storm period, using data obtained from the Oaanaaq
Digisonde, the Millstone Hill radar, and the DMSP F8 satellite. The Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique was adopted to track the motion
of an observed patch following a convection pattern. Their
study showed that a patch in the center of the polar cap was
convected into the nightside, and broken into blobs. Plasmaspheric material has been observed to participate in the
dayside magnetopause reconnection [Su et al., 2000a]. Plasmaspheric material has also been observed on reconnected
field lines in the cusp region after being dragged poleward
due to the tension force [Su et al., 2000b]. The results
presented here support the logical next step, namely that
plasmaspheric drainage material would follow a trajectory
similar to that of the polar patches, thus transporting plasmaspheric material over the polar cap and into the magnetotail. Magnetic flux tubes carrying plasmaspheric material
in the magnetotail can participate in nightside reconnection
and the injection of ionospheric/plasmaspheric ions into the
magnetotail acceleration region. The stripping off of the
plasmasphere/ionosphere by the enhanced dayside electric
field provides a source of ionospheric ions to the magnetosphere.
Acknowledgments. This work was supported by the
NASA ISTP Program and by the U.S. Department of Energy.
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J. E. Borovsky, Y.-J. Su, and M. F. Thomsen, Space and Atmospheric Sciences Group(NIS-1), MS D466, Los Alamos National
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J. C. Foster, Atmospheric Sciences Group, MIT Haystack Observatory, Westford, MA. (e-mail: [email protected])
(Received July 13, 2000; revised September 18, 2000;
accepted October 12, 2000.)