Mapping the Heliosphere: 3-D Trajectory of Solar Electrons
In following solar corpuscular emissions away from the sun, simultaneous
radio triangulation between the WIND and ULYSSES spacecraft was used for
the first time to determine remotely the 3-D trajectory in interplanetary
space of fast moving solar electrons. These energetic electrons ejected
from the sun follow the interplanetary magnetic field lines and generate
radio emissions as they travel. The radio trajectory indirectly measures
both the interplanetary magnetic field topology and the speed of the
electron beam. Measurements of the exact wavelengths of the radio
emissions also provide the interplanetary plasma densities along the path
of the electron beam. The dual spacecraft observations of the exciter
electron speeds, radio beaming geometry and emission brightness
temperature, permit tight constraints on possible emission mechanisms
responsible for the solar radio bursts.
Lunar Wake Physics
WIND investigators have vigorously exploited the spacecraft’s
traversals of the lunar wake with the result that the prevailing MHD
picture of the wake has been greatly modified. Instead of a trailing shock
wave 2 or 3 RL behind a fill-in region, the wake appears far more
complicated with its great extension, surprisingly stable ion beams driven
by flanking ambipolar electric fields, and trailing electrostatic wave
activity. Motivated by these new observations, it has become clear that a
kinetic approach to modeling the region is needed. Further studies will
only enhance our understanding of this interesting plasma laboratory and,
in addition, clarify the wake physics of small bodies, for example,
Deimos, Phobos, asteroids etc., and their interaction with the solar wind.
Such studies will increase the scientific yield of planned missions to
WIND Unique Accomplishments:
- Tracing out the spiral structure of the IMF remotely from type III
- 3-D measurements of the solar wind distribution functions and
energetic particles, including composition.
- Radical improvement in our understanding of the lunar wake, with its
extended electrostatic tail, plasma wings, surprisingly stable ion beams.
- Multipoint observations of the heliospheric current sheet structure.
- Isotopic composition of solar wind and energetic particles.
- Measurements of ionosphere modification by radio waves, transmitted
from the HAARP facility in Alaska.
- High resolution observations of gamma ray transients.
- Observations of galactic center radiation - potential signature of a
massive black hole.
Geoeffectiveness of Solar Events
With the importance of magnetic clouds established, the question of the
"geoeffectiveness" of different magnetic clouds has become important. Data
from the fleet of ISTP/GGS spacecraft were used in a comparison of the
effects of three clouds. They show that geoeffectiveness depends on the
amplitude, duration and abruptness of change of interplanetary parameters.
However, the total energy input into the magnetosphere in the form of
Poynting flux integrated over time seems to show a larger range than the
various parameters used to measure the effect. During this period of low
speed CMEs, the plasma density plays the major role in compressing the
magnetopause. Studies of the effects of a fast stream overtaking a cloud
can better be studied during solar minimum, since clouds during solar
maximum tend to be isolated.
Initial analyses of data from WIND, POLAR, SAMPEX and several other
spacecraft have been used to determine the geoeffectiveness of magnetic
clouds in driving relativistic electron acceleration within the
magnetosphere. A two-step process has been suggested, whereby strong
substorm activity caused by a large negative interplanetary magnetic field
in the leading portion of the cloud produces a seed population of
magnetospheric electrons. A high level and long duration of
large-amplitude, low-frequency wave activity, which can occur
concurrently, is an indication of a global wave field in which time
varying magnetic fields are large. The induced electric fields could be
very effective in accelerating electrons throughout the outer radiation
zone. When these phenomena have been observed, the relativistic electrons
display a global, coherent and intense acceleration.
Bursty Bulk Flows
An example of the utility of special spacecraft configurations is
illustrated by the event on March 27, 1996, when WIND was in the dusk
magnetotail near perigee and very close to GEOTAIL (4 RE apart), while
IMP-8 monitored the solar wind. The IMP 8 measurements document the
gradual input of magnetic flux to the magnetosphere (see figure) that is
in contrast to the bursty conversion of stored energy into heated flowing
plasmas seen by WIND and GEOTAIL. Furthermore, the flux transport
associated with these bursts can be very different at the two spacecraft
(especially at 1430 UT) demonstrating that the spatial scale of the bursts
is a small fraction of the magnetotail dimension. Preliminary indications
are that the brightenings seen in the global auroral images are well
associated with the flow bursts.
GEOTAIL Unique Accomplishments:
- First fully 3-D mapping of the global magnetotail structure.
- Typical location of the nightside reconnection line established at 25
- Established the spatial distributions of low energy ions and wideband
plasma waves in the distant tail.
- Discovered new kinetic effects of nightside reconnection - two
electron and two ion populations.
- Detailed study of bursty bulk flows, first sampled by ISEE, as a key
mechanism for redistribution of flux within the magnetosphere.
- Realization that ionospheric ions are a component of the
tailward-flowing plasma, even in the distant tail.
- Realization that the 2-D plasmoid picture from ISEE-3 must be replaced
by one of fully 3-D flux rope structures which have now been mapped by
Geotail and modeled theoretically.
- Detection of abrupt jumps in electric field waveform measurements
associated with electrostatic "noise".
- Extended and multiple crossings of bow shock and magnetosheath
The process of magnetic reconnection is of fundamental importance to the
magnetosphere and to astrophysical plasmas because it converts magnetic
energy into kinetic energy. The ISTP/GGS spacecraft have observed
reconnection or its effects at the equatorial magnetopause, over the polar
cap and in the geomagnetic tail. On May 29, 1996, under northward IMF
conditions, WIND detected an unusually large density enhancement which
compressed the magnetopause sufficiently that POLAR near its 8 RE apogee
at high latitude encountered the heart of the reconnection process between
the strong northward magnetic field of the solar wind and the magnetic
field from the Earth. For the first time a spacecraft passed from one
ejection region through the region of interconnection (the "diffusion"
region) and out the other ejection region. With POLAR the microphysics of
this poorly understood mechanism which determines so much of the global
rearrangement of magnetic flux in the magnetosphere has been probed.
Figure 2-9. Magnetic flux input (IMP-8) and
transport (WIND and GEOTAIL) on March 27, 1996.
Figure 2-10. POLAR UVI observations on
March 27, 1996.
Sources of Magnetospheric Plasmas
The answer to the important question of the sources of magnetospheric
plasmas is emerging from GGS measurements. First, GEOTAIL detected the
frequent occurrence of cool, tailward flowing H+, O+ (and sometimes even
He+) ions along field lines of the tail lobes, with surprisingly high
densities (up to 1/cc) that even exceed those of the plasma sheet. The
ionosphere is the only possible source of O+ and He+ ions, where such ions
have previously been observed moving upward during geomagnetically active
times. POLAR has now detected these ions streaming along magnetic field
lines at high latitudes over the polar cusp, thus connecting the
ionospheric and magnetotail measurements. These ions form an important
source of particles for the nightside plasma sheet as they drift slowly
across field lines and toward the equator in the expected dawn to dusk
tail electric field. Many of these first time measurements have been made
possible by the incorporation aboard POLAR of a spacecraft potential
control drive (PSI) which neutralizes the charge acquired by the
GEOTAIL sampling of the He++ ions near the magnetotail boundary has also
confirmed that solar wind/magnetosheath plasma is a contributing source to
the plasma sheet population.
POLAR Unique Accomplishments:
- First tri-spectral global imaging of aurora simultaneously in visible,
ultraviolet and x-ray wavelengths.
- Confirmed role of parallel electric fields in the acceleration of
electrons into the ionosphere.
- Detection of thermal ion plasmas at high altitudes made possible by
spacecraft charge neutralization.
- Found spatially extensive density cavities due to parallel electric
fields over the auroral oval.
- Found new region of large parallel electric fields at distances >6 Re.
- Discovered major acceleration region in dayside cusp containing ions
as energetic as 2 MeV.
- Found surprising appearances of energetic ions over the polar cap.
- Derived time-dependent ionospheric conductivity maps from auroral
imaging at two wavelengths.
- Co-discovered a third tail of sodium atoms in Comet Hale-Bopp.
- Confirmed existence of atmospheric holes.
- Discovered large objects containing water streaking towards the Earth
and dissipating in the atmosphere.
The ring current development and decay are very important steps in the
flow of energy through geospace. From its vantage point at high latitudes,
POLAR has a unique panoramic view of the entire ring current that has
allowed the first, time-dependent, global observations of the asymmetric
properties of the ring current. In regions devoid of ion fluxes the
energetic neutral atoms (ENAs) from charge exchange between energetic ring
current ions and the ambient geocorona have been detected. When integrated
globally these measurements show a remarkable measure of the strength of
the ring current.
This relationship, shown in Figure 2-11 for fifty days at the beginning
of 1997, clearly identifies the magnetic storm resulting from the January
6 CME and a recurrent storm in February. In addition, energetic neutrals
have been observed from substorm injection events on the nightside of the
A puzzling auroral form called the theta aurora, a double bar across the
polar cap, has been interpreted to indicate a highly distorted magnetic
tail, a bifurcation of the two open lobe cells. Now data from WIND, POLAR
and SuperDARN have been used to develop a model that successfully explains
this configuration. It predicts the form of the theta auroras after a
southward turning of the interplanetary magnetic field or a large jump in
the By component after a prolonged period of northward field. The
transpolar arc lies on closed magnetic field lines and maps to the plasma
sheet boundary layer. The analysis of in-situ plasma data placed with
respect to the auroral configuration by auroral images indicates common
electron distributions and ion composition in the theta aurora and the
poleward zone of the nightside auroral oval. Plasma convection derived
from the radar data exhibits a two-cell pattern that evolves into a
turbulent state having antisunward flow in the transpolar arc.
Global Convection Patterns
Another new technique developed within ISTP/GGS combines data from WIND
and the ground-based SuperDARN radar network with model convection
patterns to provide time-dependent high-latitude electrical potential
patterns. These global "images" provide a dynamic view of the manner in
which electric fields in the magnetosphere and high-latitude ionosphere
respond to changes in the solar wind. Recently, this capability has played
an integral role in many international campaigns supported by extended
real time coverage of the WIND spacecraft.
Hemispherical energy flux
The integrated hemispherical energy flux carried by auroral electron
precipitation into the atmosphere is a measurable parameter of the rate of
energy dissipation from the magnetosphere associated with auroral
activity. This hemispherical power has been used for several years in
time-dependent global models of the thermosphere and ionosphere. It has
been derived from electron precipitation measurements made by polar
orbiting satellites with extrapolations through models to the entire
auroral oval. Now auroral images of the entire oval at ultraviolet
wavelengths have been shown to provide substantial improvement in the
temporal resolution of this parameter which is now consistent with the
time scale of global auroral variability. Ultraviolet auroral images from
POLAR are also being used as quantitative remote diagnostics of the
auroral regions, yielding estimates of incident energy characteristics.
Theory and Modeling
Modelers are now able to run global simulations, using measured solar
wind parameters as input, that can be compared with GGS observations. A
major step toward quantitative capability was realized in the MHD codes
when it became computationally possible to move the tailward boundary to
great distances (X=-300 RE), well beyond the point where the plasma flow
becomes supersonic. This extension of the modeling domain allowed modelers
to eliminate unwanted feedback from reflections at the back boundary. The
simulation codes were checked for numerical accuracy and physics content
against textbook cases, generic IMF orientations, and with code
intercomparison. On a parallel basis, mission specific diagnostics and
visualization tools were developed to cast the simulation results and
observations in compatible terms. Furthermore, a complete end-to-end case
study (the May 19-20, 1996 ISTP event), led by the theory group resulted
in quantitative comparison between the ionospheric responses predicted by
the MHD and MAMI simulations, and observations. This study has led to a
better characterization of magnetosphere-ionosphere coupling that will be
used to improve the models.
Simulations and GEOTAIL data together have confirmed the great degree to
which the IMF, particularly its north-south component, controls
magnetotail structure. Long periods of steady northward IMF lead to a
balloon-like tail that is virtually closed within 200 Re; under
alternative IMF orientations, the magnetotail is usually very long and
flaps in response to changes in the solar wind direction in the so called
"wind-sock effect." The By component of the IMF twists the tail through
angles that in the distant tail can probably approach 90 degrees. Coupling
of fluid and kinetic models has allowed tracking of magnetotail ions, and
shed some light on what fractions originate from the ionosphere, mantle
and low latitude boundary layer.
Another exciting result emerging from the theory effort is the
suggestion that Poyting flux transferred from the solar wind into the
magnetosphere becomes focused in the region around X=-10 RE resulting in
strong adiabatic heating during the substorm growth phase. Observations
from the March 9, 1995 ISTP event are consistent with this notion. Another
hybrid model, one combining an MHD-radiation belt simulation, was used to
study the Sudden Storm Commencement of March 24, 1991; the resulting
dynamics and timing of the fluxes in the simulation were in excellent
agreement with those measured by the CRRES satellite.
Theory and Modeling Unique Accomplishments
- Global movies of the response of the magnetosphere to solar wind
variations of duration of days.
- Detailed maps of the origin of plasmas in the magnetosphere.
- Detailed coupling of magnetospheric input into a global atmospheric
- Movies of global atmospheric response to substorms.
- Simulation of the birth of a radiation belt owing to strong changes in
the solar wind.
- Detailed prediction of activity indices for the magnetosphere.
Ground-Based Unique Accomplishments
- Ground-truth for interpretation of space based auroral imaging and
resulting estimates of energy flux of electron auroral precipitation.
- Measurement of time-dependent electrical convection pattern
- Detailed tracking of ionospheric signatures of the dynamic boundaries
of electron precipitation regions.