Geology Reference
In-Depth Information
Fig. 4.14
Examples of
Solar eruptions.
Left
:
Coronal mass ejection on
April 16, 2012.
Right
:X
1.9 class solar flare on
November 3, 2011 (Credit:
NASA Solar Dynamics
Observatory)
At the magnetopause, the magnetic and
dynamic pressures balance out. Therefore, we
can use (
4.58
)and(
4.56
) to estimate the distance
of the magnetopause from the Earth's centre.
To this purpose, we approximate the Earth's
equatorial plane, where the value (
4.58
)ismore
appropriate, the magnitude
B
is given by:
core and crustal components), thereby, the pres-
ence of strong external field components impedes
a correct application of the method of calculation
of magnetic anomalies that will be described later
in this chapter. The strongest source of geomag-
netic field disturbance is represented by
geomag-
netic storms
. These events are associated with
large sudden variations of solar wind dynamic
pressure at the magnetopause, which follow the
impact of
coronal mass ejections
(CME) and
solar flare
particles (Fig.
4.14
).
A CME is a form of extensive and explosive
solar mass release that produces strong pertur-
bations of the solar wind, which reaches speeds
as high as 2,800 km s
1
during these events.
A solar flare is a more local event than CMEs,
which produces flashes of light for short time
intervals ranging from a few minutes to a few
hours (Schunk and Nagy
2009
). These explosions
can send bursts of energetic particles into the so-
lar wind, determining magnetic storms. A storm
results from compression of the magnetosphere
due to the arrival of the shockwave and can be
particularly strong when the increased solar wind
pressure is associated with a large southward IMF
component (Yokoyama and Kamide
1997
). The
typical time that a CME takes to reach the Earth
is 2
3 days. The resulting geomagnetic field
disturbance can have serious consequences for
the human electric infrastructures and networks,
and it is hardly a good idea to take magnetic
measurements during a storm. At mid-latitudes,
about one storm per year produces an external
field whose horizontal component
H
>250 nT,
and about ten storms per year have
H
>50 nT
(Campbell
2003
). Some events can produce ex-
tremely strong variations of geomagnetic field
4 r
3
Š
7:9
10
15
1
0
m
B.r/
D
(4.59)
r
3
wherewehaveassumedthat
m
D
7.9
10
22
Am
2
.
Let
R
p
be the distance of the magnetopause from
the Earth's centre in the dayside equatorial plane.
Equating (
4.58
) with (
4.56
) gives an estimate of
this quantity:
m
2
!
1=6
10
7
8 P
d
R
p
D
Š
8:5R
e
(4.60)
where
R
e
Š
6,371 km is the Earth's radius. This
is the upstream distance of the magnetopause in
normal conditions. However, strong solar wind
conditions can push the magnetopause well in-
side the geostationary orbit of satellites (
6.6
R
e
). In the nightside region, the magnetopause is
on average
30
R
e
from the ecliptic plane.
The importance for plate tectonics practition-
ers to have a basic understanding of the magne-
tosphere and related processes arises from the in-
fluence that short-period geomagnetic field time
variations have on the measurement of magnetic
anomalies produced by sea floor crustal magne-
tization. Marine geophysics campaigns require
precise determinations of the total geomagnetic
field strength of internal origin (which includes
