Telescopic and Microscopic Views of the Magnetosphere: Multispacecraft 
Observations 

D.N. Baker


The magnetospheric research community has long sought the capability to view the
Sun-Earth system in a global way and to probe concurrently the microphysical
details of key physical regions. This objective has now been substantially
realized with the combination of the CLUSTER and IMAGE missions. With the
additional use of SOHO, ACE, FAST, SAMPEX, POLAR, and geostationary orbit
spacecraft, there is a remarkable ability to apply both telescopic and
microscopic principles. As an example, a bright active region on the Sun gave
rise on 29 March 2001 to a fast halo coronal mass ejection (CME) event observed
by SOHO instruments. Subsequently on 31 March, a strong interplanetary shock
wave ahead of a magnetic cloud (probably arising from the earlier CME) passed
the ACE spacecraft and hit the EarthÕs magnetosphere. This driver compressed the
subsolar magnetopause to *4 RE geocentric distance and initiated a powerful
geomagnetic storm (minimum Dst ~ -360 nT). The CLUSTER set of four spacecraft
were located in the midnight sector of the magnetosphere near perigee (r~4 RE)
at ~0635 UT and observed a dispersionless injection of energetic (E *20 keV)
electrons in association with a large magnetospheric substorm expansion phase
onset (AE ~1200 nT). Concurrent to these in situ observations, the IMAGE
spacecraft was returning a sequence of global Energetic Neutral Atom (ENA)
images from the medium-energy (MENA) and high-energy (HENA) sensor systems.
These data showed a very prominent injection of substorm ions in the premidnight
(and postdusk) sector of the inner magnetosphere. This event is consistent with
a substorm onset that pushed the substorm Òinjection boundaryÓ far inside of
geostationary orbit and far toward the dusk sector. In another event on August
27, 2001, the IMAGE-CLUSTER combination provided evidence that magnetic
reconnection began in the mid-tail plasma sheet some 7 min prior to auroral
onset and brightening. Alternative interpreta! tions may also be possible even
with the multispacecraft data available. Notwithstanding, these data gave an
unprecedented view both telescopically and microscopically of a magnetospheric
substorm onset and help establish key process timing in the magnetosphere. Such
available events reveal the power of multispacecraft observations.

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Space Science Reviews, 109 (1-4), 133-153, 2003