GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 9, 1307, 10.1029/2001GL013950, 2002
2D plasma sheet ion density and temperature profiles for northward
and southward IMF
Simon Wing and Patrick T. Newell
The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
Received 21 August 2001; revised 5 December 2001; accepted 14 December 2001; published 8 May 2002.
[1] We previously developed a method for inferring plasma sheet
ion temperature and density from ionospheric observations. Using
this method and DMSP observations for 1992, we present 2D
profiles of the equatorial plasma sheet ion density and temperature
for southward and northward IMF for b2i > 64 or Kp < 3.
During periods of northward IMF, cold dense ions can be found
plentifully along the plasma sheet flanks. However, during periods
of southward IMF, the presence of these cold dense ions is
noticeably diminished, especially at the dusk flank where the
density peak is less discernable. These cold dense ions have been
previously interpreted in terms of magnetosheath ion entry into the
plasma sheet. Our result suggests that any mechanism proposed to
transport magnetosheath ions from dusk LLBL to the plasma sheet
along the dusk flank during periods of northward IMF would have
to be able to do so efficiently over a large spatial scale.
INDEX
TERMS: 2764 Magnetospheric Physics: Plasma sheet; 2748
Magnetospheric Physics: Magnetotail boundary layers; 2744
Magnetospheric Physics: Magnetotail; 2784 Magnetospheric
Physics: Solar wind/magnetosphere interactions
1. Introduction
[2] The Earth’s magnetotail plasma is highly variable spatially
as well as temporally. The 1D plasma sheet temperature (T ),
density (n), and pressure ( p) profiles along the midnight meridian
or along dawn-dusk are well known [e.g., Angelopoulos et al.,
1993; Spence et al., 1989; Huang and Frank, 1994; Baumjohann
et al., 1989]. These plasma sheet parameters have strong dependence on the geomagnetic activity [e.g., Huang and Frank, 1994;
Baumjohann et al., 1989, 1990] and on the solar wind and IMF
[e.g., Borovsky et al., 1998; Terasawa et al., 1997]. These
parameters are obtained from in situ measurements taken by
satellites that sampled only relatively small portions of the
magnetotail. As a result, these studies typically have to average
over large areas that are in tens to hundreds of cubic Earth radii
(RE3 ).
[3] We have previously developed a method to infer the
plasma sheet p, n, and T from ionospheric observations. Ionospheric polar orbiting satellites move rapidly, with a typical
orbital period near 100 min, allowing them to image the plasma
sheet 25 times per day. Plasma in the plasma sheet has been
observed to be nearly isotropic [e. g., Spence et al., 1989; Kistler
et al., 1992; Huang and Frank, 1994] and as a result p, n, and T
are conserved along the field line. Because of the abundance of
the ionospheric observations, inferred 2-D spatial profiles of
plasma sheet ion temperature, density, and pressure could be
constructed at unprecedented fine spatial resolution [Wing and
Newell, 1998].
[4] Magnetosheath ion entry into the plasma sheet has been
inferred from observations of cold dense ions in the plasma sheet
flanks in several Geotail passes [e.g., Fujimoto et al., 1998].
Copyright 2002 by the American Geophysical Union.
0094-8276/02/2001GL013950$05.00
Using observations from the same satellite, Terasawa et al.
[1997] show that during periods of northward IMF plasma sheet
ions are on average denser and colder and there are temperature
minima and density maxima near the dawn and dusk flanks.
However, because of limited spatial coverage, these and other
similar results from in situ observations were obtained by
averaging over a large region, blurring the spatial extent of these
cold dense ions. Recently Fairfield et al. [2000] and Otto and
Fairfield [2000] present a Geotail event near the dusk flank
during a period of northward IMF and an MHD simulation of the
same event. Based on their detail analysis, these authors argue for
Kelvin-Helmhotz (KH) instability as an important mechanism for
bringing magnetosheath ions from the dusk flank into the plasma
sheet during periods of strongly northward IMF.
[5] Motivated by these results, this study investigates the
effect of the IMF Bz in the 2D equatorial plasma sheet ion n
and T profiles with the method developed in Wing and Newell
[1998].
2. Data Set
[6] For this study, we used data obtained from the SSJ4 instrument onboard DMSP satellites F8, F9, F10, and F11 for the entire
year of 1992. In 1992, there were at least three DMSP satellites in
operation simultaneously (F8, F10, and either F9 or F11); for one
month, March, all four were in operation. Because of its upward
pointing and limited pitch angle resolution, DMSP SSJ4 measures
only highly field-aligned precipitating particles at an altitude of
roughly 835 km.
[7] NASA NSSDC OMNIWeb provides the IMP-8 hourly
average solar wind data.
3. Method for Inferring Plasma Sheet Ion T, n,
and p from Ionospheric Observations
[8] The method for inferring the plasma sheet ion T, n, and p
from the DMSP SSJ4 measurements has been described fully
elsewhere [Wing and Newell, 1998, 2000] and is briefly summarized here.
[9] It has been known for more than two decades that
energetic ions observed in the topside ionosphere are isotropic
above a certain latitude [e.g., Bernstein et al., 1974]. Likewise, an
overwhelming number of in situ observations in the plasma sheet
tailward of 8 – 10 RE indicate that plasma is nearly isotropic,
irrespective of activity levels [e.g., Kistler et al., 1992; Spence
et al., 1989; Huang and Frank, 1994]. As a result, n, T, and p are
approximately conserved along the magnetic field line. The ions
maintain their isotropy by pitch angle scattering in the tail current
sheet [Lyons and Speiser, 1982]. Nearer to Earth (typically
earthward of 8 – 10 RE, although this distance varies with
magnetic activity), the field line becomes more dipolar, and the
pitch angle scattering ceases. As a result, our method does not
work in this region. Newell et al. [1996, 1998] have developed a
computer algorithm for identifying the equatorward isotropy
boundary in the ionosphere (b2i) or equivalently the earthward
boundary of the central plasma sheet. Sergeev et al. [1993]
21 - 1
21 - 2
WING AND NEWELL: 2D PLASMA SHEET ION DENSITY AND TEMPERATURE PROFILES
Figure 1. Two-dimensional equatorial profiles of plasma sheet ion (a) pressure, (b) temperature, and (c) density during quiet time,
normalized b2i = 66 – 68, average Kp 1.5 (from plates 1, 3, and 4 of Wing and Newell [1998]). The ion density profile for combined
quiet and moderate time, normalized b2i > 64 or Kp < 3 is shown in (d). Each point is averaged over 1 1 RE2 regions. Some regions
are left blank either because of insufficient data points or because plasma is anisotropic (in the region closer to the Earth).
showed that the ion isotropy boundary (or b2i) highly correlates
with the tail inclination angle measured in the same local time
sector by GOES geosynchronous satellites (correlation coefficient = 0.9). Therefore b2i can be used to modify and improve
ionosphere-magnetosphere T89 magnetic field model [Tsyganenko, 1989] tail mapping [Sergeev et al., 1993] and has been
adapted into our method. Because b2i has a local time variation,
it is normalized to its midnight value.
[10] Electron acceleration events are excluded from the data
because they distort the plasma sheet ion spectra, resulting in
distributions that do not represent those in the plasma sheet.
Instead of computing moment, which is widely used, each ion
spectrum is fitted to distribution functions (one-component Maxwellian, two-component Maxwellian, and k) and the best fit is
selected. This takes into account ions outside the detectors’
energy range.
4. The Quiet Time 2D Plasma Sheet T, n, and
p Profiles
[11] The inferred 2D magnetically quiet time ion p, T, and n
profiles in the neutral sheet averaged in 1 1 RE2 bins are shown in
Figures 1a – 1c respectively. The data in these figures have an
average Kp 1.5 and b2i = 66 – 68. As shown in Figures 1a and
1b, pressure and temperature are higher in the dusk-premidnight
than in the postmidnight-dawn sector in the near Earth region,
between 8 – 10 RE from the Earth. This result is consistent with
WING AND NEWELL: 2D PLASMA SHEET ION DENSITY AND TEMPERATURE PROFILES
21 - 3
Figure 2. The IMF Bz dependence of the plasma sheet ion density and temperature constructed from the same data set used in Figure 1d,
b2i > 64 or Kp < 3. The ion density 2D profiles for the northward and southward IMF are shown in (a) and (b) respectively and their
corresponding temperature profiles are shown in (c) and (d) respectively.
the E B and the gradient/curvature motions of ions originating
from the deep tail and low-latitude boundary layer (LLBL). The
E B convection moves ions of all energies Earthward, but the
curvature and gradient duskward (or westward) drift separates
the ions by energy. Colder ions convect Earthward with little
westward/duskward displacement, but hotter ions drift further
duskward, resulting in a higher temperature and pressure in the
dusk-premidnight sector at approximately 8 – 10 RE away from the
Earth. Figures 1b and 1c show that the temperature is colder and
density higher near both flanks, where most likely there is mixing
of colder magnetosheath like ions from LLBL and hotter ions from
the plasma sheet [e.g., Fujimoto et al., 1998; Eastman et al., 1985;
Spence and Kivelson, 1993]. The Finite-width magnetotail convection model calculation, which takes into account deep tail and
dawn LLBL ions undergoing E B and gradient/curvature drifts,
shows that (a) pressure and temperature have a maximum in the
near-Earth dusk sector and (b) along the dawn flank, the temperature has a minimum and the density has a maximum [Spence and
Kivelson, 1993]. The model does not include the dusk LLBL ion
source, which explains the absence of the temperature minimum
and the density maximum in the dusk flank. Figures 1b and 1c
suggest that LLBL ions may enter the plasma sheet along the dusk
flank, which is the main topic in the next two sections of this paper.
The plasma pressure also has a maximum near midnight that may
have resulted partly from weaker magnetic field and pressure
balance. In addition, bursty bulk flows (BBFs) which occur more
frequently near the midnight meridian may also contribute to the
higher pressure and temperature near midnight meridian [e.g.,
21 - 4
WING AND NEWELL: 2D PLASMA SHEET ION DENSITY AND TEMPERATURE PROFILES
Angelopoulos et al., 1993]. Comparisons of Figures 1c with in
situ measurements [e.g., Huang and Frank, 1994; Baumjohann
et al., 1989, 1990; Lennarsson, 1992; Lennartsson and Shelley,
1986] and model calculations (Spence and Kivelson [1993] and
Wang et al. [2001]) are in general very good [Wing and Newell,
1998; Wang et al., 2001].
[14] Acknowledgments. The DMSP SSJ4 instrument was designed
and built by Dave Hardy and colleagues at the AFRL. Fred Rich has been
helpful in our acquiring this data, as has the World Data Center in Boulder,
Colorado. NASA NSSDC OMNIWeb provided the IMP-8 solar wind data.
The work of SW was supported by NSF Space Weather Grant ATM9819705 and NASA Grant NAG5-10971; the work of PTN was supported
by AFOSR Grant F49620-00-1-0172.
References
5. IMF Bz Effect on Plasma Sheet Ion n and T
[12] We have previously separated the plasma sheet ion n, T,
and p by b2i or magnetic activity [Wing and Newell, 1998].
Motivated by recent results [e.g., Fairfield et al., 2000; Otto and
Fairfield, 2000], we searched for the IMF Bz dependence on the
plasma sheet ion n and T. In order to increase the number of
points, we included data for quiet and moderate time, normalized
b2i > 64 or Kp < 3. Figure 1d shows the 2D equatorial density
profile when data for all IMF Bz. Figure 1d shows that the density is
generally higher than the quiet time density, in agreement with
previous in situ measurements [e.g., Baumjohann et al., 1989].
Apart from the higher density in Figure 1d, Figures 1d and 1c are
similar in that both show the presence of density peaks at both
the dusk and dawn flanks. However, starkly contrasting pictures
of the plasma sheet flanks emerge when the data in Figure 1d are
separated by the sign of the 2-component of IMFs. This was done
using IMP-8 hourly average IMF data. The results are shown in
Figures 2a and b for northward and southward IMF, respectively.
As previously reported, the plasma sheet ion density is higher
during periods of northward IMF than southward IMF [e.g.,
Terasawa et al., 1997], but the profile is presented here in more
detail in 2D. During periods of northward IMF, density peaks
appear along the plasma sheet dawn and dusk flanks. However,
during periods of southward IMF, in the dawn flank the density
peak is smaller and narrower (in the y direction) and in the dusk
flank it is even less discernable. During periods of northward
IMF, ion temperature has minima along the dawn and dusk flanks
as shown in Figure 2c. However, during periods of southward
IMF, the temperature minima are smaller and narrower, as shown
in Figure 2d.
6. Discussion and Summary
[13] For the first time, the IMF Bz dependence of the plasma
sheet ion n and T profiles is exhibited in fine spatial detail in 2D.
The plasma sheet ion n and T profiles generally differ for the
northward and southward IMF cases, but the most remarkable
contrast occurs in the density profiles along the plasma sheet dusk
flank. During periods of northward IMF, ions are colder and denser
along both flanks of the plasma sheet, i.e. adjacent to the magnetopause. However, during periods of southward IMF, this density
peak is smaller along the dawn flank, and it is barely discernible
along the dusk flank. Likewise, the temperature minima are smaller
and narrower at the flanks, but the dawn-dusk asymmetry appears
not to be as strong as in the density profile. This result is consistent
with the previous Geotail observations [Terasawa et al., 1997], but
here the spatial extent of these cold dense ions is shown in detail in
2D. The presence of the cold dense ions along the plasma sheet
flanks has been interpreted as a strong indication of magnetosheath
ion entry [e.g., Fujimoto et al., 1998]. Consequently, the result here
indicates that far more magnetosheath ions enter from the dusk
LLBL during periods of northward IMF than during periods of
southward IMF. Otto and Fairfield [2000] and Fairfield et al.
[2000] demonstrate that KH instability can transport massive
magnetosheath ions effectively across the magnetopause dusk
flank. Other mechanisms may also be possible and need to be
investigated. However, the result here indicates that any such
mechanism would have to be able to transport ions efficiently
over a very large spatial scale.
Angelopoulos, V., C. F. Kennel, F. V. Coroniti, R. Pellat, H. E. Spence,
M. G. Kivelson, R. J. Walker, W. Baumjohann, W. C. Feldman, J. T.
Gosling, and C. T. Russell, Characteristics of ion flow in the quiet state of
the inner plasma sheet, Geophys. Res. Lett., 20, 1711 – 1714, 1993.
Bernstein, W., B. Hultqvist, and H. Borg, Some implications of low altitude
observations of isotropic precipitation of ring current protons beyond the
plasmapause, Planet. Space Sci., 22, 767, 1974.
Baumjohann, W., G. Paschmann, and C. A. Cattell, Average plasma properties in the central plasma sheet, J. Geophys. Res., 94, 6597 – 6606, 1989.
Baumjohann, W., G. Paschmann, and H. Luhr, Pressure balance between
lobe and plasma sheet, Geophys. Res. Lett., 17, 45 – 48, 1990.
Borovsky, J. E., M. F. Thomnsen, and R. C. Elphic, The driving of the plasma
sheet by the solar wind, J. Geophys. Res., 103, 17,617 – 17,639, 1998.
Eastman, T. E., L. A. Frank, and C. Y. Huang, The boundary layers as the
primary transport regions of the Earth’s magnetotail, J. Geophys. Res.,
90, 9541– 9560, 1985.
Fairfield, D. H., A. Otto, T. Mukai, S. Kokubun, R. P. Lepping, J. T.
Steinberg, A. J. Lazarus, and T. Yamamoto, Geotail observations of the
Kelvin-Helmholtz instability at the equatorial magnetotail boundary for
parallel northward fields, J. Geophys. Res., 105, 21,159 – 21,173, 2000.
Fujimoto, M., T. Terasawa, T. Mukai, Y. Saito, T. Yamamoto, and S. Kokubun, Plasma entry from the flanks of the near-Earth magnetotail: Geotail observations, J. Geophys. Res., 103, 4391– 4408, 1998.
Huang, C. Y., and L. A. Frank, A statistical survey of the central plasma
sheet, J. Geophys. Res., 99, 83 – 95, 1994.
Kistler, L. M., E. Mobius, W. Baumjohann, G. Paschmann, and D. C.
Hamilton, Pressure changes in the plasma sheet during substorm injections, J. Geophys. Res., 97, 2973 – 2983, 1992.
Lennartsson, W., and E. G. Shelley, Survey of 0.1- to 16-keV/e plasma
sheet ion composition, J. Geophys. Res., 91, 3061 – 3076, 1986.
Lennarsson, W., A scenario for solar wind penetration of Earth’s magnetic
tail based on ion composition data from ISEE 1 spacecraft, J. Geophys.
Res., 97, 19,221 – 19,238, 1992.
Lyons, L. R., and T. W. Speiser, Evidence for current sheet acceleration in
the geomagnetic tail, J. Geophys. Res., 87, 2276 – 2286, 1982.
Newell, P. T., Y. I. Feldstein, Y. I. Galperin, and C.-I. Meng, Morphology of
nightside precipitation, J. Geophys. Res., 101, 10,737 – 10,748, 1996.
Newell, P. T., V. A. Sergeev, G. R. Bikkuzina, and S. Wing, Characterizing
the state of the magnetosphere: Testing the ion precipitation maxima
latitude (b2i) and the ion isotropy boundary, J. Geophys. Res., 103,
4739 – 4745, 1998.
Otto, A., and D. H. Fairfield, Kelvin-Helmholtz instability at the magnetotail boundary: MHD simulation and comparison with Geotail observations, J. Geophys. Res., 105, 21,175 – 21,190, 2000.
Sergeev, V. A., M. Malkov, and K. Mursula, Testing the isotropic boundary
algorithm method to evaluate the magnetic field configuration in the tail,
J. Geophys. Res., 98, 7609 – 7620, 1993.
Spence, H. E., M. G. Kivelson, R. J. Walker, and D. J. McComas, Magnetospheric plasma pressures in the midnight meridian: Observations from 2.5
to 35 RE, J. Geophys. Res., 94, 5264 – 5272, 1989.
Spence, H. E., and M. G. Kivelson, Contributions of the low-latitude
boundary layer to the finite width magnetotail convection model, J. Geophys. Res., 98, 15,487 – 15,496, 1993.
Terasawa, T., et al., Solar wind control of density and temperature in the
near-Earth plasma sheet: WIND/GEOTAIL collaboration, Geophys. Res.
Lett., 24, 935 – 938, 1997.
Tsyganenko, N. A., A magnetospheric magnetic field model with a warped
tail current sheet, Planet Space Sci., 37, 5 – 20, 1989.
Wang, C.-P., L. R. Lyons, M. W. Chen, and R. A. Wolf, Modeling the quiet
time inner plasma sheet protons, J. Geophys. Res., 106, 6161 – 6178,
2001.
Wing, S., and P. T. Newell, Central plasma sheet ion properties as inferred
from ionospheric observations, J. Geophys. Res., 103, 6785 – 6800, 1998.
Wing, S., and P. T. Newell, Quiet time plasma sheet ion pressure contribution to Birkeland currents, J. Geophys. Res., 105, 7793 – 7802, 2000.
S. Wing and P. T. Newell, The Johns Hopkins University Applied Physics
Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland, USA. (simon.
[email protected]; [email protected])