The Solar Wind 109
TABLE 2 Characteristics of Interplanetary Coronal Mass Ejections at 1 AUCommon signatures:
Counterstreaming (along the field) suprathermal (energy>70 eV) electrons
Counterstreaming (along the field) energetic (energy>20 keV) protons
Helium abundance enhancement
Anomalously low proton and electron temperatures
Strong magnetic field
Lowplasma beta
Low magnetic field strength variance
Anomalous field rotation (flux rope)
Anomalous ionic composition (e.g., Fe^16 +,He+)
Cosmic ray depression
Average radial thickness: 0.2 AU
Range of speeds: 300–2000 km/s
Single point occurrence frequency:
∼72 events/year at solar activity maximum
∼8 events/year at solar activity minimum
Magnetic field topology: Predominantly closed magnetic loops rooted in Sun
Fraction of events driving shocks:∼1/3
Fraction of earthward-directed events producing large geomagnetic storms:∼1/6when ICMEs are confined largely to the low-latitude band
of solar wind variability.
7.5 The Magnetic Field Topology of ICMEs and the
Problem of Magnetic Flux Balance
The coronal expansion carries the solar magnetic field out-
ward to form the heliospheric magnetic field. In the qui-
escent wind, these field lines are usually “open” in the
sense that they connect to field lines of the opposite po-
larity only in the very distant heliosphere. CMEs, on the
other hand, originate in the corona in closed field regions
that have not previously participated directly in the solar
wind expansion and carry new magnetic flux into the he-
liosphere. The magnetic flux that each CME apparently
adds to the heliosphere must be balanced by removal of
magnetic flux elsewhere since the overall heliospheric mag-
netic field strength is roughly (within a factor of 2) con-
stant in time. Magnetic reconnection within the magnetic
“legs” of a CME close to the Sun appears to be the prime
way that this balance is achieved. Figure 10 illustrates that
such reconnection is inherently three-dimensional in na-
ture and initially produces helical magnetic field lines that
are partially disconnected from the Sun (see, also, Fig. 7b).
Sustained three-dimensional magnetic reconnection even-
tually produces open and disconnected field lines thread-
ing an ICME, both of which are sometimes observed. All
of the types of reconnection illustrated in Fig. 10 reduce
the amount of magnetic flux permanently added to the
heliosphere by an ICME. However, it is not presently clear
what mix of reconnections within the magnetic legs of
ICMEs and elsewhere in the solar atmosphere (e.g., at the
heliospheric current sheet) is actually responsible for main-
taining a rough long-term balance of magnetic flux in the
heliosphere.7.6 Field Line Draping About Fast Interplanetary
Coronal Mass Ejections
Because the closed field nature of ICMEs effectively pre-
vents any substantial interpenetration between the plasma
within an ICME and that in the surrounding wind, the am-
bient plasma and magnetic field ahead must be deflected
away from the path of a fast ICME. Figure 7b illustrates
that such deflections cause the ambient magnetic field to
drape about the ICME. The degree of draping and the re-
sulting orientation of the field ahead of an ICME depend
upon the relative speed between the ICME and the ambient
plasma, the shape of the ICME, and the original orienta-
tion of the magnetic field in the ambient plasma. Draping
plays an important role in reorienting the magnetic field
ahead of a fast ICME. On the other hand, conditions and
processes back at the Sun largely determine the field ori-
entation within ICMEs. As a final point of interest, Figure
7b also illustrates that, just as the bow wave in front of a
boat moving through water is considerably broader in ex-
tent than is the boat that produces it, so too is the shock in
front of a fast ICME somewhat broader in extent than is the