Geology Reference
In-Depth Information
of solar flares, CMEs, and other visible changes
(sunspots). To measure the solar activity and
the corresponding geomagnetic disturbance, sev-
eral indices have been designed. In marine geo-
physics, the most useful measure of magnetic
activity is the global 3-h-range
Kp index
.This
index is determined as the arithmetic mean of
K index
values from 13 different geomagnetic
observatories. The
K
index is a quasi-logarithmic
scale that quantifies the disturbance in the
H
component of the geomagnetic field in a single
observatory. It is calculated from the amplitude
range of
H
in a 4-h interval, after removal of
diurnal and secular variations by fitting a smooth
curve to the observed values. The
Kp
scale ranges
between 0 and 9, and is expressed in thirds of a
unit: 0, 1/3, 2/3, 1, 4/3, ::: These quantities are
usually indicated as: 0,0
C
,1
,1,1
C
, etc. Usually,
Kp
2
C
indicates quietness, whereas
Kp
5is
indicative of a geomagnetic storm.
A less intense but still significant contribution
to the external geomagnetic field comes from
currents driven by tidal forces and winds circu-
lating in the Earth's
ionosphere
, a conducting
region of the terrestrial atmosphere where a large
density of free low-energy electrons and ions
is present. This region extends upwards from
70 to 80 km elevation to considerable altitudes,
where it merges with the magnetosphere. The
free electrons and ions that form the ionosphere
plasma are produced by ionization of neutral
atoms, as a consequence of both extreme ultra-
violet radiation from the Sun and collisions with
high-energy particles that penetrate the atmo-
sphere. The ionosphere currents are revealed by
a recurrent pattern on observatory magnetograms
during quiet days, with characteristic 24/
k
hour
period spectral components, where
k
D
1,2,3,4
(Campbell
2003
). Therefore, the typical magne-
togram pattern associated with these currents is
referred to as
quiet daily variations
. When the
small but persistent component arising from tidal
forces is removed, the changes are called
Sq
variations
(i.e., solar quiet). A theoretical plot of
Sq
components is illustrated in Fig.
4.16
.The
Sq
field varies slowly through the months of the year
and its contribution to magnetic measurements
changes with the geographic location, but the
Fig. 4.15
Monthly X-class flares and International
Sunspot Number and 11-year solar cycle. The plot shows
the increased number of X-class flares during the peaks of
solar activity (higher number of sunspots). However, flares
can also occur when the sunspot cycle is at the minimum
(Original figure in Hathaway
2010
, updated to 2014 by
courtesy of the author)
strength. Tsurutani et al. (
2003
) report 12 large
magnetic storms from 1859 to 1989, with
H
range
(difference between the maximum and minimum
of
H
during the storm) varying from 450 to
1,720 nT. The duration of the main phase of
these events is of the order of a few hours.
In general, during the main phase the magnetic
field at the Earth's surface is significantly de-
pressed. This depression has a strong latitudinal
dependence and is determined by the injection
of magnetotail plasma into the nightside mag-
netosphere. Within the inner magnetosphere, the
Lorentz force drives the high-energy protons to
the west, whereas electrons move eastward, thus
feeding and strengthen a current loop around
the Earth that is known as the
ring current
.
This current determines the formation of a north-
ward directed magnetic moment that opposes the
Earth's dipole moment, thus decreasing the net
magnitude of the geomagnetic field measured at
near-equatorial magnetic stations.
Regarding the recurrence interval of geomag-
netic storms, it is strictly related to the
solar
cycle
, which consists into a periodic change of
solar activity with average duration of
11 years
(Fig.
4.15
). The solar cycle is a magnetic phe-
nomenon that originates by dynamo processes
within the Sun. During an 11-year cycle, it is
possible to observe a rise and fall in the number
