Observation of 40keV Field-Aligned SubAuroral Ion Beams
Robert B. Sheldon1, Harlan E. Spence1
and Joseph F. Fennell2
1Boston University Center for Space Physics
725 Commonwealth Av, Boston, MA 02215
2The Aerospace Corporation
El Segundo, CA
18 June 1997
Abstract
Recent observations by CEPPAD/IPS from the new perspective of the
POLAR orbit reveal a close association of field-aligned ionospheric
beams with convecting plasmasheet "nose" ions. April 15, 1996 was
marked by a large southward Bz accompanying a fast solar wind
shock. This event triggered a strong cross-tail convection electric
field that pushed plasmasheet ions deep into the magnetosphere. When
the POLAR spacecraft passed through the inner magnetosphere, it
observed both an unusually energetic "nose" plasmasheet ion injection at
~90 keV, as well as a peculiar field-aligned beam at ~40
keV. These beams appear to be dominated by oxygen ions, which would
place their
origin in the ionosphere. Both populations existed from L=7--3, for
a duration of at least 2 hours. We speculate that the nose ions create
a parallel electric field that is responsible for the extended
ionospheric beam signature.
The POLAR spacecraft is in a polar, 9 x 2 Re orbit that in April 1996,
was in the noon-midnight meridian plane with perigee over the south
pole, making two diagonal passes through the midnight and noon
radiation belts. The Comprehensive Energetic Particle and Pitch Angle
Distribution (CEPPAD) (Blake et al. [1995]) IPS experiment,
consists of a multiple solid state detectors that detect ions between
15--1500 keV in 9 elevation angles and 16 or more azimuthal bins. A
typical pass on April 14, 1996, (see fig 1 panel a), shows
an energetic particle population whose average energy is proportional
to the |B|.
This is the normal distribution for ions diffusing in L-shell from a
source region in the plasmasheet (Sheldon [1993b]), showing the
adiabatic energization expected from Liouville's theorem. On April 15,
however, IPS detected two nearly monoenergetic bands superposed on the
night side ring current (panel b) at ~90 keV, and at ~40 keV. Unlike
the ring current, these two bands dropped to lower energies with lower
L-shell, displaying an apparent non-adiabatic behavior. The bands
became less distinct on the dayside and more sporadic, but apparently
exist throughout the ring current. To better understand these ions, we
examine only the lowest 8 energy channels and display the roll
modulation as the spacecraft rotates parallel and perpendicular to the
magnetic field (fig 2).
POLAR/CEPPAD/IPS data on (a) April 14, and (b) April 15,
and (c) April 16, 1996, displaying energy spectra summed over pitch
angles from 20--1500 keV.
We observed that the upper, ~90 keV band is strongly peaked around
90o and the lower band is peaked near 0o. Now a
monoenergetic trapped population is possible when a strong cross-tail
electric field drives ions against the grad B drift deep into
the magnetosphere (Smith [1974]). Such a "nose" event must be
nearly 90o trapped particles because of the large increase
in |B| while convecting from the plasmasheet, which is the
characteristic of the upper energy band in our data. Since nose events
are highly correlated with storms, and storms are defined by Dst, we
turn to the preliminary Dst provided by the Kyoto University web
site.
After subtracting the ionospheric Sq contribution using the "quietest
day of the month" (April 7/8) method, we find a moderate storm of at
least -63nT on the first hour of April 15. More
significantly, we find a very large asymmetric (ASY) component on that
day. Examination of the one-minute symmetric H components show that a
sudden increase preceded the storm decrease, thereby cancelling most
of the storm onset in Dst. This missing current shows up clearly in
the ASY component, leading us to believe that a stronger than normal
electric field had distorted the ring current, shifting the index from
symmetric to asymmetric, and disguising the magnitude of this storm in
Dst. Additional support for the nose event identification with a
storm injection came from the extensive GGS database.
POLAR/CEPPAD/IPS data on April 15, 1996, displaying roll
modulation of the counts in the 90o head in the energy bands
from 24--138 keV. Dotted white and blue lines are 90 and 60o
pitchangles.
Examination of WIND data (fig 3.)Ogilvie et al., [1995]) for
April 14 showed that there were several Bz<0 periods lasting
for 1 hour or less. Corresponding CU and CL derived from the CANOPUS
array (Rostoker et al. [1995]) show that these periods led to
substorms with riometer absorption signatures at auroral
latitudes. However the storm trigger appears to be the strong
southward turning of Bz < -10 occurring at 2000 UT, accompanied by a
jump in the solar wind speed from 450 km/s to 600 km/s, which produced
an even larger Bz in the compressed magnetic field of the
magnetosheath. This period of strong southward Bz lasted more
than 3 hours, effectively saturating the ability of the tail to shield
out the polar cap potential. The IZMIRAN model (Papitashvili
[1994]) predicted in excess of 150 kV across the polar cap for these
solar wind conditions.
April 14-15, 1996 ISTP data. CANOPUS (a) CU (b) CL and riometers at
(c) L=12, (d) L=8, (e) L=6.6, and (f) L=4.4. (g) WIND/MFI Bz and
(h) WIND/SWE solar wind speed.
The CANOPUS array detected a magnetic bay, a nearly equal response of
AU and AL, suggesting that the current systems had moved equatorward,
overhead of the magnetometers. Indeed, the Halley Bay magnetometer at
L~4 (Dudeney et al. [1995]) showed a large H deflection with
almost no Z deflection, indicative of strong overhead currents. The
ionosphere responded strongly at this time (private communication,
J. Aarons, 1996) as ionospheric scintillations were enhanced. While
CANOPUS riometers at L > 6 recorded very little activity, the riometer
at L=4.4 as well as the Halley Bay riometer at L~4, (private
communication A. Rodger, 1996) showed an extremely intense and narrow
absorption feature at this time, indicating that precipitation had
penetrated to low latitudes, deep in the magnetosphere and down to
E-layer ionospheric depths.
From these observations, as well as complementary ground observations,
we surmise that after several intense substorms had pumped up the
plasmasheet, a strong convection field injected the plasma to at least
L=4.4, which POLAR/CEPPAD observed as a 90 keV band. The large
asymmetric component of Dst is evidence that this current was observed
by the low latitude stations.
The band of 40 keV field-aligned ions, however, are harder to
explain. They have the wrong pitchangles to have convected from the
plasmasheet, because the adiabatic decompression involved in
backtracing them to their origin would place them in the plasmasheet
loss cone. But ions of this energy would not have had access to the
plasmasheet simultaneously with higher energy ions, since the
magnetosphere is a "notch filter" for only one energy. This implies
that these ions are trapped on closed drift orbits. But if they
undergo the same processes as the adiabatically energized ring
current, which can be seen simultaneously with the banded
distribution, they would not be monoenergetic, nor would they track
the energy of the nose ions so precisely. That is, if they had resided
for any length of time in the magnetosphere, the same convection that
brought in the plasmasheet ions would disperse these ions as well.
If we examine the loss cones in high time resolution, (though our
spatial resolution remains 12 x 22 degrees) it is clear that one loss
cone is substantially more filled up than the other, which is
inconsistent with any mechanism that relied on convecting ions. Thus
we conclude that these field-aligned ions are in situ
accelerated during the time of the measurement.
The composition experiment, POLAR/CAMMICE, was turned off during this
period, so we cannot determine the E>30keV O+
content for April 15, however when it was switched back on, around
April 19, it found an anomalously large amount of O+ in
the ring current (RC) in two energy bands centered at 40 and 100
keV. Because of the short lifetime of O+ against charge
exchange, such a measurement is consistent with a storm injection
occurring only a few days previously, since between storms, CAMMICE
detects no O+ enhancements. A similar storm on March 21
showed that the IPS field-aligned beams are simultaneous with the
double peaked O+ spectrum observed by CAMMICE.
POLAR/TIMAS (Shelley et al. [1995]) (private communication
W.K. Peterson, 1996) did detect E<20keV O+ beams
for this day.
Therefore we conclude that on April 15 we are observing O+ ions
accelerated to ~30--40 keV in field-aligned beams, presumeably by
strong parallel electric fields in the ionosphere. Since these ions
track ~50 keV below the nose ions, it appears there must be a
causal connection, which becomes more apparent when we fit the
spectral shape of these populations.
The three spectral peaks all have differing shape so that we have used
three separate functional forms to fit them. Adiabatic energization
and diffusion explain the highest energy ring current peak, which we
are able to fit very well with a Gaussian in log(Energy) space. The
middle "nose" ion peak is represented in our instrument by a response
only one or two channels, which does not constrain the functional form
very much. We have chosen to fit this peak with a Gaussian in linear
space simultaneously with the lowest energy "beam" peak. The beam
reflects a highly asymmetric shape with a long tail toward lowest
energies but a sharp cutoff at high energies. After much
experimentation, we found that a Chapman layer function, described as
y=exp(1+x-exp(x)), gave the best fit with the fewest number of
free parameters. We have thus fit 16 energy channels with 9
parameters, carrying out this fit for 96s averages throughout the
nightside pass, fig 4.
Successive 96s spectra (symbols) and three peak fit (line)
for April 15 data set.
From the fits shown above, we plot the center energy and the width of
the peak in figure 5. to demonstrate how the three
populations, beam, nose and RC, evolve during this orbit. The RC is
adiabatically energized so that as the model equatorial magnetic field
increases the energy increases, as noted by the squares and dashed
line. A latitude effect is apparent in the RC due to the more rapid
loss of low energy ions at higher latitudes. This energization is not
seen by the nose ions because they are not an equilibrium trapped
population, rather we are seeing the subset of plasmasheet ions that
have convective access to this location, so that stronger B-field ions
also began with smaller magnetic moment (Sheldon [1994b]). The 90 keV
energy of the nose ions is consistent with a large polar cap
potential. Most importantly, we find that the ion beam energy, though
broader because of the asymmetric shape, tracks the beam energy very
precisely when we plot it as a ratio. That is, rather than a constant
energy difference, the beam is a constant fraction of the nose ion
energy, as shown by the X's in the above figure. This is a most
important clue, since the nose energy is completely determined by
details of the magnetic topology and convection electric field,
whereas the beam is a local, in situ phenomenon.
Peak fits to 96s averaged spectra of the 90o head.
Ring current ([]), nose ion (*), beam ion (triangle)
energies, and the ratio of nose/beam X10 (X) are
plotted. Error bars are FWHM from peak fits, since fitting errors are
negligible. Overplotted dashed line is the model equatorial magnetic
field intensity logarithmically scaled from 20--1000 nT; and the model
magnetic latitude (dotted line) linearly scaled from -40 to 40
degrees, with the equator marked with a vertical dotted
line.
Since these ion beams track the nose ions so closely, we assume that
the acceleration process must involve an in situ parallel
electric field that is somehow related to the nose ions. Alfven's
pioneering work on parallel electric fields comes to mind
(Alfven [1963]) (though we prefer the treatment of
Whipple [1977]) where he argues that one can produce a
field-aligned electric field if the electron and ion phase space
densities are not identical. Using this event as a guide, a 90 keV
monoenergetic nose ion population of ions with equatorial pitch angles
less than 90o (Sheldon [1994b]) was superposed on a
cold plasmasphere electron population of very different pitch angles.
According to (Whipple [1977]), this would produce a parallel
electric field of several kT_e pointing toward the
equator. That is, since a PA < 90o ion spends most of its
time away from the equator, the electrons at the equator will
experience a force pulling them toward higher latitudes so as to
shield the ion charge, and ions will be repelled toward the
equator. This then, is the correct sign of the electric field to
account for the ion beams observed by POLAR near the equator. Thus it
appears that a parallel electric field is a natural consequence of
storm injections, and may provide an important link in the storm
progression. The presence of oxygen in the beam suggests that this
parallel field penetrates down to nearly ionospheric depths and may
play an important part in the development of Dst during storms. A
more detailed theory will be described in a later paper.
This study was supported by NASA contract NAS5-30368 and NSF grant
ATM-9458424. The authors gratefully acknowledge K. Ogilvie,
E. Shelley, J. Dudeney, G. Rostoker, B. Blake and T. Fritz for the use
of their data.
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