Alfven Boundaries: Noses and Zippers
Robert B. Sheldon and Harlan E. Spence
Boston University Center for Space Physics
725 Commonwealth Av, Boston, MA 02215
18 July 1996
On April 15, 1996, POLAR was in a noon-midnight meridional plane and
observed an atypical energetic particle distribution, characterized by
a band of nearly monochromatic 90o ions at 60 keV, and a second,
nearly monochromatic band of of field-aligned ions around 40
keV. These ions persisted from L=7-3.5. Below L~3.5 the bands
vanished, but reappeared as the spacecraft exited the magnetosphere.
The upper band is identified as a "nose" event, first described by
Smith [1974]. The bimodal distribution is reminiscent of "zipper"
events discussed by Fennell [1981], though this may be the
most energetic zipper event ever described. After examining the solar
wind monitor and ground stations, we develop a scenario to explain
this distribution, where the nose ions injected from the plasmasheet
produce a parallel electric field that extracts oxygen from the
ionosphere. Although we lack the Dst, we hypothesize that we were
observing a classical ~-100 Dst magnetic storm, albeit from a new
perspective. We generalize this observation by proposing that all
storm injections involve intense nose events that produce parallel
electric fields that populate the ring current with ionospheric
oxygen.
The POLAR spacecraft is in a polar orbit that on April 15 was in the
noon-midnight meridian with perigee over the south pole. Thus it made
two diagonal passes through the midnight and noon radiation belts. A
typical pass shows an energetic particle population whose average
energy is inversely proportional to L-shell. This is the normal
distribution for ions diffusing in L-shell from a source region in the
plasmasheet (Sheldon [1993a]). On this day the Comprehensive
Energetic Particle and Pitch Angle Distribution (CEPPAD)
(Blake et al. [1995]) instrument detected an unusual distribution
consisting of two monoenergetic bands of ions superposed on the night
side radiation belt, and partially observed on the dayside (Figure
1). Since the spacecraft was preparing to flip its axes, a maneuver
intended to keep the auroral imagers in shadow, we supposed
initially that we were seeing interference from hydrazine thrusters.
However the thrusters had not fired, and on closer examination, the
two bands showed all the characteristics of a monoenergetic beam of
trapped ions at 60 keV, and a monoenergetic beam of field-aligned ions
at 40 keV with a region of depleted plasma between the bands.
FIGURE 1: POLAR/CEPPAD/IPS ion "zipper" spectrogram taken on April 15, 1996.
A monoenergetic beam is possible when a strong cross-tail electric
field drives ions against the grad B drift deep into the
magnetosphere. That is, since the energy-dependent
grad B drift is clockwise and the energy-independent ExB
drift is counter-clockwise, the only ions that can deeply
penetrate are those whose energy is such that the two drifts nearly
cancel. Such so-called "nose events" (Smith [1974]) must be nearly
90o trapped particles because of the adiabatic compression
caused by the large increase in |B| while convecting from the
plasmasheet, which is the characteristic of the upper energy band in
the data. Do we find any evidence for strong convection on April 14--15
that would support this hypothesis?
Examination of WIND data (Ogilvie [1995]) for April 14 showed that
there were several Bz < 0 periods lasting for 1 hour or less.
Corresponding AU and AL derived from the CANOPUS array
(Rostoker [1995]) show that these periods led to intense substorms
with riometer absorption signatures at auroral latitudes. However the
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 effect
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 shed stress via substorms. 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 [1995]) showed a large H deflection with almost
no Z deflection, indicative of strong overhead currents.
Ionospheric scintillations became very disturbed at this time as
well. The IZMIRAN model (Papitashvili [1994]) predicted in excess of
150 kV across the polar cap for these solar wind conditions. While
CANOPUS riometers recorded very little activity at auroral latitudes,
the riometer at L=4.4, as well as the Halley Bay riometer at L~4,
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 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 60 keV band.
The monoenergetic field-aligned ions, however, are harder to
explain. They have the wrong pitch angles to have convected from the
plasmasheet, because the adiabatic decompression 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 in the magnetosphere, 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. Thus we conclude that these field-aligned ions are in situ
accelerated during the time of the measurement. What could this
acceleration be?
Again, we recall that "zipper" events, (Fennell [1981]) but identified
first by Kaye [1981]), had the same bimodal distributions. They
found that the field-aligned low energy component was rich in O+,
and concluded that they were observing beams coming from the
ionosphere. Since beams are seen at auroral latitudes, they supposed
that their satellite, SCATHA, was on flux tubes connected to auroral
arcs. Unfortunately the POLAR composition experiment, CAMMICE was
turned off during this period, so we cannot verify this assumption for
April 15. However when CAMMICE/MICS was switched back on, around April
20, it found an anomalously large amount of O+ in the ring
current. The only time for which CAMMICE/MICS observed more O+, was
March 21, 1996, which had a less well defined, but unmistakeable
"zipper" distribution observed by CEPPAD/IPS. So we conclude that
on April 15 we are observing O+ ions accelerated to ~30--40 keV
by strong parallel electric fields in the ionosphere. Since these ions
track ~20 keV below the nose ions, it appears there must be a
connection between them. What could be the connection between nose
events and ionospheric beams?
Whipple [1977] and Chiu [1978] argue that one can produce a
field-aligned electric field if the electron and ion pitch angle
distributions (PADs) are not identical. In this case, the ions mirror
at a different location on the field line than the electrons,
producing a double layer and strong parallel electric field. A nose
event, because of its monoenergetic character at near 90o
pitch angles, would produce just such a peculiar PAD. If the energetic
particle density is comparable or greater than the plasmasphere, it
seems likely that the inner magnetospheric plasma cannot neutralize
the electric field by itself, but must extract ionospheric
particles. Naturally the electric field created by this mechanism must
be less than the energy content of the generating ions, so that this
second band of ions remains at a distinct energy lower than the nose
ions, in agreement with the observations. Such a parallel electric
field also accelerates electrons down into the ionosphere to produce
riometer absorption events. If the electric field is colocated with
the nose plasma, the absorption must be a narrow strip in latitude, as
seen in the CANOPUS data.
If this be the mechanism that generates field aligned electric fields
and oxygen beams, then we have elucidated a new paradigm for magnetic
storms. It is generally thought that magnetic storms are
characterized by intense convection fields that bring plasmasheet
plasma deep into the ring current. If this be true of all magnetic
storms, then all storms should also create parallel fields and extract
oxygen from the ionosphere. Thus models that attempt to predict Dst
from solar wind parameters need to include this additional source of
ring current plasma, which we expect to be a non-linear
contribution. That is, big storms extract more oxygen than small
storms, with a larger Dst effect.
Support for this theory, suprisingly came during the same COSPAR
session where our paper was presented. M. Grande (this issue) calculated the
O+/He++ ratio for the entire CRRES data set, about 14 months. He
plotted this ratio versus Dst, and found a distinct linear trend; the
larger the Dst, the larger the ratio, as we would predict. A second
study by Mursula [1996] showed that the PC1 waves associated with
ion-cyclotron waves (ICW) usually accompany storms. But during the
most intense part of the main phase of a storm, the PC1 waves occurred
primarily at dusk and the frequency dropped down to below the oxygen
gyrofrequency. These observations are suggestive of an oxygen rich
plasma occuring at dusk, the location of the deepest penetration of
the nose ions and also in agreement with our theory. T. Lui (this issue)
presented
neutral atom composition measurements during a magnetic storm with
GEOTAIL/EPIC data. The H and O spectra match nicely with the H+
and O+ spectra observed with AMPTE/CCE in a 1986 storm. But the
time decay of the GEOTAIL species were identical, suggesting that the
H+/O+ ratio was constant during the recovery phase of a
storm. This is not consistent with their different loss cross
sections, unless a production mechanism for O+ is found. Thus this
observation weakly supports our theory above as well. Finally,
Horne [1996] addressed one weakness with this model, that ionospheric
beams should mirror in the opposite ionosphere and be lost unless they
scatter out of the loss cone. R. Horne presented wave growth studies
showing that oxygen should be very unstable to waves that transversely
heat the oxygen and trap it in the magnetosphere. These are the waves
probably observed by Mursula, although Horne had modelled only one of
two branches of the dispersion relation.
FIGURE 2: Schematic storm development.
(1) IMF Ey exceeds some threshold for t>1 hr.
(2) Cross-tail E-field develops when near-earth plasmasheet
current systems saturate.
(3) Convection drives a band of plasmasheet
"nose" ions deep into the magnetosphere.
(4) Parallel E-fields develop as
nose ion densities dominate over plasmasheet electrons.
(5) O+ ions are drawn out of the ionosphere.
(6) Ion-cyclotron waves scatter O+ out of
the loss cone, and they become trapped in the ring current.
We have attempted to build a new paradigm of magnetic storms that
incorporates field-aligned electric fields as an intrinsic part of the
storm development. A storm proceeds then with the following steps
(FIGURE 2):
- It may be necessary to have a critical density plasmasheet
for a magnetic storm, in which case, precursor substorm activity would
be a prerequisite.
- Large solar wind Ey for an extended period
(>1hr) saturates the tail electric field shielding.
- This produces a strong cross-tail convection electric field.
- A monoenergetic
"nose" event penetrates deep into the magnetosphere from the (pumped up?)
plasmasheet.
- Simultaneously a parallel electric field develops as
the energetic nose event plasma dominate over the plasmaspheric cold
plasma.
- This parallel electric field extracts and energizes
ionospheric plasma including H+ and O+.
- Either the nose or
the field-aligned plasma produce intense ICW which perpendicularly
heat the beams and trap them in the magnetosphere.
- When the
convection switches off, the ring current is trapped and begins to
decay through charge exchange.
Blake, J. B. et al.
CEPPAD: Comprehensive energetic particle and pitch angle
distribution experiment on POLAR.
In C. T. Russell, editor, The Global Geospace Mission, pages
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