A New Magnetic Storm Model
Robert B. Sheldon and Harlan E. Spence
Boston University Center for Space Physics
725 Commonwealth Av, Boston, MA 02215
18 July 1996
Recent observations of magnetic storms from the new perspective of the
POLAR orbit have elucidated the crucial role that parallel electric
fields and the ionosphere play in the development of a magnetic
storm. This model has implications for the empirical formulas linking
the solar wind with Dst, for composition changes in the ring current,
for precipitation and subauroral arcs seen at earth, for ionospheric
signatures seen by radar or riometers, and not least of all, energetic
neutral atoms. The model predicts that a large polar cap electric
field, which persists on the order of an hour or more, exceeds the
ability of the tail to shield out the polar cap potential, and
produces a strong cross-tail electric field in the plasmasheet. This
field convects plasmasheet ions into the inner magnetosphere, where
the opposing grad B and EXB drifts act as a selective
filter permitting a single energy the deepest penetration. This
monoenergetic band of ions generates a field-aligned potential drop
that attempts to confine the ions to the equator. Thus equatorial
electrons are accelerated through the potential and precipitated deep
into the ionosphere E-layer where they produce riometer and radar
signatures. Simultaneously, ionospheric ions are extracted and
energized into the ring current. Ion cyclotron waves, generated by
this unstable particle population, then perpendicularly heat these ion
beams and scatter them out of the loss cone so that they become a
permanent part of the ring current. This entire dynamic structure
would rapidly decay if it were not for the continuous power input of
the hot magnetospheric ions which in the frame of the ionosphere,
convect through the cold plasma population. When the cross-tail field
switches off, the convection power source is removed, the parallel
electric fields vanish, and the hot ions are trapped in the ring
current and subsequently decay through charge exchange.
The elements of this new magnetic storm model have all been presented
before, but they have lacked a coherent, causal chain, and above all,
a quantifiable model that had predictive ability. For example,
(Smith and Hoffman ) presented the time-dependent model
of ring current injection, but were unable to predict how deep the
injection would occur or subsequent Dst development of a
storm. Similarly, Hamilton et. al., ) measured the
enhanced ionospheric oxygen content of great storms, but were unable
to predict which storms or how much oxygen was to be expected. Our
model takes as its input the cross-tail potential at the inner edge of
the plasmasheet and the plasmasheet density at this location, to
predict all of the above quantities. With a suitable inner tail model,
we should be able to use only the solar wind density and electric
field , Ey=Bz * Vx, to make these predictions.
The POLAR spacecraft is in a polar, 9 x 2 Re 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 (Figure 1) 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 ). On this day the
Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD)
Blake et. al., ) experiment detected two monoenergetic bands
superposed on the night side ring current (figure 1): a monoenergetic
population of trapped ions at ~90 keV, and a monoenergetic beam
of field-aligned ions at ~40 keV.
FIGURE 1: POLAR/CEPPAD/IPS data on April 14-15, 1996, displaying
energy spectral counts of three successive passes in the
90o head in the energy bands from 24--138 keV.
FIGURE 2: POLAR/CEPPAD/IPS data on April 15, 1996, displaying roll
modulation of the counts in the 90o head in the energy
bands from 15--1500 keV. Dotted white and blue lines are 90o
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 and Hoffman ). 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. Support for this hypothesis came from the extensive GGS
Examination of WIND data (Ogilvie et al., ) 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 et al., ) 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 ) predicted in excess of 150 kV
across the polar cap for these solar wind conditions.
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., ) showed a large H
deflection with almost no Z deflection, indicative of strong overhead
The ionosphere responded strongly at this time (private communication,
J. Aarons, 1996) as ionospheric scintillations were enhanced.
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, (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 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.
Thus we conclude that these field-aligned ions are in situ
accelerated during the time of the measurement.
"Zipper" events, (Fennell et al.  identified first by
Kaye et al. ), have the same bimodal pitch-angle
distributions, though at somewhat lower energy. They found that the
"zippers" were rich in O+, and concluded that they were
observing beams coming from the ionosphere. Since beams are seen at
auroral latitudes, they concluded that they were on flux tubes
connected to auroral arcs. 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 20, it found an anomalously large
amount of O+ in the ring current. POLAR/TIMAS (Shelley
et al.,  private communication W.K. Peterson, 1996) did
detect E<20 keV O+\ beams for this day. 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 ~50 keV below the nose ions,
it appears there must be a causal connection.
Whipple  argues that one can produce a
field-aligned electric field if the electron and ion pitch angle
distributions (PADs) are not identical. In our case, the hot
monoenergetic nose ions are superposed on a cold plasmasphere electron
population producing a parallel electric field of several kTe. If
the energetic ion density is greater than the plasmaspheric cold
electron density, a second ion-dominated solution to the Whipple
equations is possible at approximately the ion beam parallel
FIGURE 3: Peak fits to 96s averages of the 90 head showing the
counts in successive 96s intervals throughout the nightside pass. The
three peaks are the beam, nose and ring current ions.
FIGURE 4: Peak fits to 96s averages of the 90 head. Ring
current (square), nose ion (+), and beam ion (triangle) energies
plotted versus time. The ratio of nose/beam X 10 is displayed
as crosses. Overplotted with different scales are: the model
equatorial magnetic field intensity plotted at the same scale from
20--1000 nT; and the model magnetic latitude (dotted line) from -40 to
40 degrees, with the equator marked with a vertical dotted line.
Figure 2 illustrates how the three populations (ring current,
"nose", and beam) evolve during this interval. First, the ring
current is adiabatically energized so that as the model equatorial
magnetic field increases the energy increases, as noted by the squares
and dashed line. This adiabatic energization is not seen by the nose
ions because we are not following a single ion trajectory, rather we
are seeing the nose ions that have access to this location, so that
stronger B-field ions also began with smaller magnetic moment. A more
detailed analysis of the pitch angles is needed to clarify the coupled
energy-L dependence of these ions. If the first solution to
Whipple  holds, then both the nose and the beam energies should
move in concert as we move along the field line. That is, as we
traverse a region of strong potential gradient, we should observe a
constant difference between these two energies.
We find instead that the beam ion energy tracks the nose ion energy
with a nearly constant ratio of 1/2, and not a constant
difference. This suggests that the field-aligned potential gradient is
closer to the ionosphere than our orbit, and that the beam energy is
not determined by the electron thermal temperatures or anisotropies,
but by the energy of the nose ions themselves, consistent with the
second, ion-dominated solution to the Whipple equilibrium.
Such a parallel electric field should also accelerate magnetospheric
electrons to 30 keV into the ionosphere, producing a riometer
absorption event. Since the electric field is colocated with the nose
plasma, the absorption must be a narrow strip in latitude but
distributed in longitude, as seen in the CANOPUS data. This type of
signature is typical of sub-auroral ion drifts (SAID) which have been
identified with substorm injections. Our mechanism would identify a
SAID with a storm injection, and also accounts for the E-layer keV
electron signature seen in subauroral riometer data.
We carefully distinguish between substorm and storm injections because
we feel the characteristic signatures of each are completely
different. A substorm involves a reconfiguration of the magnetic
field which produces a dB/dt electric field in the region between 6-10
Re near midnight. This inductive electric field in situ
accelerates the entire plasma population to ~30 keV, which is
observed in our instrument as an isotropic, dispersionless E<50 keV
enhancement over a restricted MLT and L-shell range. A storm
injection, on the other hand, is observed as a nose event, a
monoenergetic band of ions penetrating to subauroral L-shells and
existing over a broad range of MLT determined by the duration of the
cross-tail field. Energization of low energy plasma is not seen, but
adiabatic energization of the nose ions occurs as the ions convect
toward stronger B. From an ionospheric viewpoint, substorms are in the
auroral zone, whereas storms penetrate down to sub-auroral latitudes
albeit in a restricted latitude band.
If our mechanism generates field aligned electric fields and oxygen
beams for every storm injection, then we have elucidated a new model
for magnetic storms. Since it is generally thought that magnetic
storms are characterized by intense convection fields that bring
plasmasheet plasma deep into the ring current, then all storms should
create parallel fields and extract oxygen from the ionosphere as
well. Thus large storms should extract more oxygen than small storms,
with a larger Dst effect.
Several recent observations lend support for this theory. Analysis of
the CRRES data set shows that there is a positive correlation between
the magnitude of Dst and the O+ content of the ring current (
M. Grande et al., ). A Dst prediction filter Gleisner )
found that a neural network with one hidden layer, representing an
unknown quadratic dependence on solar wind Vx,Bz, and density, could
explain up to 84\% of the variance in Dst. If significant O+ is
extracted during the main phase as we predict, one would expect such a
non-linear dependence of Dst with Ey.
This extraction of ions and precipitation of electrons near midnight
will generate an outward flowing current which then drifts westward
with the bulk of the ring current. We expect that the disappearance of
the parallel field, occurring near the dusk terminator, will result in
a downward current thus completing the loop of the partial ring
current as measured by Suzuki  using Magsat data. They
concluded that 1/3--1/4 of ring current amperage was observed in the
partial ring current. If we assume that half of the partial ring
current is carried by upward flowing ions, then we estimate that 1/6 -
1/8 of the total ring current is composed of ions of ionospheric
A study of storm-time PC1 pulsations by Mursula et al. 
showed that the PC1 waves associated with ion-cyclotron waves during
the most intense part of the main phase of a storm occurred primarily
at dusk and the PC1 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.
Finally, GEOTAIL/EPIC observed energetic neutral atoms (ENA) during
the recovery phase of a magnetic storm (Lui et al., )
which showed the same H and O spectra as AMPTE/CCE H+ and
O+ spectra observed in a 1986 storm (Hamilton et al.
). 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 as we predict above. Recent ENA observations of a
storm made with POLAR/CEPPAD also show a strong asymmetry consistent
with the above picture.
We have attempted to build a new model 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:
- It may be necessary to have a critical density plasmasheet for a
magnetic storm, in which case, precursor substorm activity or a solar
wind density enhancement 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
- When the convection switches off, the ring current is trapped and
begins to decay through charge exchange.
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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(Due to spamming, delete asterisk)