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Magnetic Indices

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  • United States Geological Survey, Denver, United States
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Magnetic Indices

that the level of magnetic activity is proportional to the inverse Rossby
number of the stara result that is consistent with the emission
data. Clearly, as there is a small sample of stars for which we have a
detailed record of magnetic activity and this record is comparatively
short, there is a need to continue these observations as they give
us great understanding of the magnetic properties of our nearest
starthe Sun.
S. Tobias
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Cross-references
Dynamo, Solar
MAGNETIC INDICES
Magnetic indices are simple measures of magnetic activity that occurs,
typically, over periods of time of less than a few hours and which is
recorded by magnetometers at ground-based observatories (Mayaud,
1980; Rangarajan, 1989; McPherron, 1995). The variations that
indices measure have their origin in the Earths ionosphere and magne-
tosphere. Some indices having been designed specifically to quantify
idealized physical processes, while others function as more generic
measures of magnetic activity. Indices are routinely used across the
many subdisciplines in geomagnetism, including direct studies of the
physics of the upper atmosphere and space, for induction studies of
the Earths crust and mantle, and for removal of disturbed-time mag-
netic data in studies of the Earths deep interior and core. Here we
summarize the most commonly used magnetic indices, using data from
a worldwide distribution of observatories, those shown in Figure M31
and whose sponsoring agencies are given in Table M1.
Range indices Kand K
p
The 3-h Kinteger index was introduced by Bartels (1938) as a mea-
sure of the range of irregular and rapid, storm-time magnetic activity.
It is designed to be insensitive to the longer term components of mag-
netic variation, including those associated with the overall evolution of
a magnetic storm, the normal quiet-time diurnal variation, and the very
much longer term geomagnetic secular variation arising from core con-
vection. The Kindex is calculated separately for each observatory,
and, therefore, with an ensemble of Kindices from different observa-
tory sites, the geography of rapid, ground level magnetic activity can
be quantified.
When it was first implemented, the calculation of Krelied on the
direct measurement of an analog trace on a photographic record.
Today, in order to preserve continuity with historical records, computer
programs using digital data mimic the original procedure. First, the
diurnal and secular variations are removed by fitting a smooth curve
to 1-min horizontal component (H) observatory data. The range of
the remaining data occurring over a 3-h period is measured. This is
then converted to a quasilogarithmic Kinteger, 0, 1, 2, ..., 9, accord-
ing to a scale that is specific to each observatory and which is designed
to normalize the occurrence frequency of individual Kvalues among
the many observatories and over many years.
A qualitative understanding of the Kindex and its calculation can be
obtained from Figure M32. There we show a trace, Figure M32a,of
the horizontal intensity at the Fredericksburg observatory recording
magnetically quiet conditions during days 299301 of 2003, followed
by the sudden commencement in day 302 and the subsequent develop-
ment of the main and recovery phases of the so-called great Halloween
Storm. In Figure M32b we show, on a logarithmic scale, the range of
the Fredericksburg data over discrete 3-h intervals, and in Figure M32c
we show the Kindex values themselves. Note the close correspon-
dence between the magnetogram, the log of the range and the Kindex.
This storm is one of the 10 largest in the past 70 years since continu-
ous measurements of storm size have been routinely undertaken. For
more information on this particular storm, see the special issue of the
Journal of Geophysical Research, A9, 110, 2005.
Planetary-scale magnetic activity is measured by the K
p
index
(Menvielle and Berthelier, 1991). This is derived from the average of
fractional Kindices at 13 subauroral observatories (Table M1) in such
a way as to compensate for diurnal and seasonal differences between
the individual observatory Kvalues. The final K
p
index has values
0, 0:3, 0:7, 1:0, 1:3, ... etc. For illustration, in Figure M33 we show
magnetograms from the 13 observatories contributing to K
p
, recording
the Halloween Storm of 2003, along with the K
p
index itself. The dis-
tribution of observatories is far from uniform, with a predominant
representation from North America and Europe, and very little repre-
sentation from the southern hemisphere. In fact, in Figure M33,itis
easy to see differences during the storm in the magnetograms among
the different regional groupings of observatories. Although geographic
bias is an obvious concern for any index intended as a measure of pla-
netary-scale magnetic activity, K
p
has proven to be very useful for
scientific study (e.g., Thomsen, 2004). And, since it has been continu-
ously calculated since 1932, K
p
lends itself to studies of magnetic dis-
turbances occurring over many solar cycles.
There are several other indices related to the Kand K
p
.A
k
and A
p
are linear versions of Kand K
p
.K
n
,A
n
,K
s
, and A
s
are similar to K
p
and A
p
except that they use, respectively, northern and southern hemi-
sphere observatories; their global averages are K
m
and A
m
. The aa
index is like the K
p
except that it utilizes only two, roughly antipodal,
observatories, one in the northern hemisphere and one in the southern
hemisphere. aa has been continuously calculated since 1868, making it
one of the longest historical time series in geophysics.
Auroral electrojet indices AU, AL, AE, AO
During magnetic storms, particularly during substorms, magneto-
spheric electric currents are often diverted along field lines, with
MAGNETIC INDICES 509
current closure through the ionosphere. To measure the auroral zone
component of this circuit, Davis and Sugiura (1966) defined the aur-
oral electrojet index AE. Ideally, the index would be derived from data
collected from an equally spaced set of observatories forming a neck-
lace situated underneath the northern and southern auroral ovals.
Unfortunately, the southern hemispheric distribution of observatories
is far too sparse for reasonable utility in calculating AE, and the north-
ern hemispheric observatories only form a partial necklace, due to the
present shortage of reliable observatory operations in northern Russia.
Progress is continuing, of course, to remedy this shortcoming, but for
Table M1 Summary of index observatories used here
Agency Country Observatory Observatory Index
Geoscience Australia Australia Canberra CNB K
p
Geological Survey of Canada Canada Fort Churchill FCC AE
Geological Survey of Canada Canada Meanook MEA K
p
Geological Survey of Canada Canada Ottawa OTT K
p
Geological Survey of Canada Canada Poste-de-la-Baleine PBQ AE
Geological Survey of Canada Canada Yellowknife YKC AE
Danish Meteorological Institute Denmark Brorfelde BFE K
p
Danish Meteorological Institute Denmark Narsarsuaq NAQ AE
GeoForschungsZentrum Potsdam Germany Niemegk NGK K
p
GeoForschungsZentrum Potsdam Germany Wingst WNG K
p
University of Iceland Iceland Leirvogur LRV AE
Japan Meteorological Agency Japan Kakioka KAK D
st
Geological and Nuclear Science New Zealand Eyerewell EYR K
p
National Research Foundation South Africa Hermanus HER D
st
Swedish Geological Survey Sweden Abisko ABK AE
Swedish Geological Survey Sweden Lovoe LOV K
p
British Geological Survey United Kingdom Eskdalemuir ESK K
p
British Geological Survey United Kingdom Hartland HAD K
p
British Geological Survey United Kingdom Lerwick LER K
p
US Geological Survey United States Barrow BRW AE
US Geological Survey United States College CMO AE
US Geological Survey United States Fredericksburg FRD K
p
US Geological Survey United States Honolulu HON D
st
US Geological Survey United States San Juan SJG D
st
US Geological Survey United States Sitka SIT K
p
Figure M31 Map showing geographic distribution of magnetic index observatories.
510 MAGNETIC INDICES
now the partial necklace of northern hemisphere observatories is used
to calculate an approximate AE.
The calculation of AE is relatively straightforward. One-min reso-
lution data from auroral observatories are used, and the average hori-
zontal intensity during the five magnetically quietest days is
subtracted. The total range of the data from among the various AE
observatories for each minute is measured, with AU being the highest
value and AL being the lowest value. The difference is defined as
AE ¼AU AL, and for completeness the average is also defined
as AO ¼1=2ðAU ALÞ. For illustration, in Figure M34, we show
magnetograms from the eight auroral observatories contributing to
AE, during the Halloween Storm of 2003, along with the AE and its
attendant relatives.
Equatorial storm indices D
st
and A
sym
One of the most systematic effects seen in ground-based magnetometer
data is a general depression of the horizontal magnetic field as
recorded at near-equatorial observatories (Moos, 1910). This is often
interpreted as an enhancement of a westward magnetospheric equator-
ial ring current, whose magnetic field at the Earths surface partially
cancels the predominantly northerly component of the main field.
The storm-time disturbance index D
st
(Sugiura, 1964) is designed to
measure this phenomenon. D
st
is one of the most widely used indices
in academic research on the magnetosphere, in part because it is well
motivated by a specific physical theory.
The calculation of D
st
is generally similar to that of AE, but it is
more refined, since the magnetic signal of interest is quite a bit smaller.
One-min resolution horizontal intensity data from low-latitude obser-
vatories are used, and diurnal and secular variation baselines are sub-
tracted. A geometric adjustment is made to the resulting data from
each observatory so that they are all normalized to the magnetic equa-
tor. The average, then, is the D
st
index. It is worth noting that, unlike
the other indices summarized here, D
st
is not a range index. Its relative
A
sym
is a range index, however, determined by the difference between
the largest and smallest disturbance field among the four contributing
observatories.
In Figure M35 we show magnetograms from the four observatories
contributing to D
st
and A
sym
, for the Halloween storm of 2003, along
with the indices themselves. The commencement of the storm is easily
identified, and although the magnetic field is very disturbed during the
first hour or so of the storm, the disturbance shows pronounced long-
itudinal difference, and hence a dramatically enhanced A
sym
. With the
subsequent worldwide depression of Hthrough to the beginning of
day 303 the storm is at its main phase of development. During this
time D
st
becomes increasingly negative. It is of interest to note that
it is during this main phase that AE is also rapidly variable, signally
the occurrence of substorms with the closure of magnetospheric elec-
tric currents through the ionosphere. AE diminishes during the recov-
ery period of the storm as D
st
also pulls back for its most negative
values and A
sym
is diminished. Toward the end of day 303 the second
act of this complicated storm begins, with a repeat of the observed
relationships of the various indices.
Figure M32 Example of (a) magnetometer data, horizontal
intensity (H) from the Fredericksburg observatory recording the
Halloween Storm of 2003, (b) the maximum range of Hduring
discrete 3 h intervals, and (c) the Kindex for Fredericksburg.
Figure M33 Example of (a) magnetometer data, horizontal
intensity (H), from the observatories used in the calculation of the
K
p
index, together with (b) the corresponding K
p
index. The
observatories have been grouped into North American, European,
and southern hemisphere regions in order to highlight similarities
of the data within each region and differences in the data across
the globe.
MAGNETIC INDICES 511
Ava ilability
Magnetic indices are routinely calculated by a number of different
agencies. Intermagnet agencies routinely calculate K indices for their
observatories (www.intermagnet.org). The GeoForschungsZentrum in
Potsdam calculates K
p
(www.gfz-potsdam.de). The Kyoto World Data
Center calculates AE and D
st
(swdcwww.kugi.kyoto-u.a c.jp). Other
agencies supporting the archiving and distribution of the indices
include the World Data Centers in Copenhagen (web.dmi.dk/fsweb/
projects/wdcc 1) and Boulder (www.ngdc.noaa.gov), as well as the
International Service of Geomagnetic Indices in Paris (www.cetp.
ipsl.fr).
Jeffrey J. Love and K.J. Remick
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Mayaud, P.N., 1980. Derivation, Meaning, and Use of Geomagnetic
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McPherron, R.L., 1995. Standard indices of geomagnetic activity. In
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Sugiura, M., 1964. Hourly values of equatorial D
st
for the IGY. Annals
of the International Geophysical Year,35: 945948.
Thomsen, M.F., 2004. Why K
p
is such a good measure of magne-
tospheric convection. Space Weather,2: S11004, doi:10.1029/
2004SW000089.
Cross-references
IAGA, International Association of Geomagnetism and Aeronomy
Ionosphere
Magnetosphere of the Earth
MAGNETIC MINERALOGY, CHANGES
DUE TO HEATING
Mineralogical alterations occur very often in rocks subjected to ther-
mal treatment. Laboratory heating may cause, in many cases, not only
magnetic phase transformations, but also changes in the effective mag-
netic grain sizes, the internal stress, and the oxidation state. The pre-
sence or absence of such alterations is crucial to the validity and
success of numerous magnetic studies.
The basic assumption in paleointensity determinations in the mea-
surement of anisotropy of thermoremanent magnetization is that the
rock is not modified during the different successively applied heating
treatments. For simple thermal demagnetization, the occurrence of
mineralogical alteration can introduce errors in the determination of the
magnetic carrier if the latter had undergone transformation a at lower
Figure M34 Example of (a) magnetometer data, horizontal
intensity (H), from the observatories used in the calculation of the
AE indices, together with (b) the corresponding AU and AL indices
and the (c) AE and AO indices.
Figure M35 Example of (a) magnetometer data, horizontal
intensity (H), from the observatories used in the calculation of the
D
st
indices, together with (b) the corresponding D
st
plotted as the
center trace and the maximum and minimum disturbance values,
the difference of which is A
sym
.
512 MAGNETIC MINERALOGY, CHANGES DUE TO HEATING
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An index, denoted by AE, is derived as a measure of global electrojet activity. The basic data used are 2.5-minute readings of the H trace in the standard magnetograms from seven auroral-zone observatories. The readings are referenced to a level determined for each observatory from quiet intervals. All the data from the seven observatories are then plotted against UT, and two envelopes are drawn to embrace all the points. The index AE at any epoch is defined by the distance (or separation) between the upper and lower envelopes at that epoch. When viewed as functions of UT the upper and lower envelopes themselves show development and decay of positive and negative variations. It is found that a positive excursion usually accompanies a negative (larger) variation, confirming the well known feature of polar disturbance. The AE index for a 6-day period, February 10–15, 1958, shows that polar disturbances statistically repeat with a time interval of about 4 hours, and that the average duration of their most active phase is a little more than 1 hour. It is pointed out that the average repetition time of 4 hours is comparable with that of the electron flux enhancements observed by Anderson et al. in the magnetosphere tail. It is suggested that polar disturbances are directly related to Anderson's ‘electron islands’ in the magnetosphere tail.
Article
The 3-hour K index was the first to provide an objective and quantitative monitoring of the irregular variations of the transient geomagnetic field observed in a given place. The use of K indices from a network of observatories to derive a planetary index of geomagnetic activity was suggested by Bartels when defining these indices. Then, Kp, am, Km, an, as, and aa were successively designed and accepted as International Association of Geomagnetism and Aeronomy indices. At present these K-derived planetary indices are routinely computed and circulated. They make long homogeneous data sets and are widely used for long-term and statistical studies in geomagnetism and solar-terrestrial physics. After a short description of the main features of transient geomagnetic activity a definition of the K index as a measure of the irregular activity is given with a summary of its basic characteristics. The derivation of K-derived planetary indices is described and discussed, and updated indications concerning their availability are presented. This short review provides users with the minimum required knowledge about these indices and may serve as an introduction to the Mayaud (1980) monograph.
Article
Contents: 1. Introduction. 2. Common indices of geomagnetic activity: derivation and significance. 3. Indices of magnetic pulsations. 4. Other miscellaneous indices. 5. Geomagnetic-activity indices in the physics of the magnetosphere. 6. Concluding remarks.
Colaba Magnetic Data, 1846 to 1905. 2. The Phenomenon and its Discussion
  • N A F Moos
Moos, N.A.F., 1910. Colaba Magnetic Data, 1846 to 1905. 2. The Phenomenon and its Discussion. Bombay, India: Central Government Press.
Derivation, Meaning, and Use of Geomagnetic Indices
  • P N Mayaud
Mayaud, P.N., 1980. Derivation, Meaning, and Use of Geomagnetic Indices, Geophysical Monograph 22. Washington, DC: American Geophysical Union.