Collection of Geomagnetic indices and like
Väinö K. Lehtoranta 16.11.1997
aa index. A daily and half daily index of geomagnetic activity
determined from the k indexes scaled at two nearly antipodal stations at invariant
magnetic latitude 50 degrees (Hartland, England, and Canberra, Australia). The aa values
are in units of 1 nT. The index is available back to 1868, and is provided by the Institut
de Physique du Globe de Paris, France.
ak index. A 3-hourly "equivalent amplitude" index of
geomagnetic activity for a specific station or network of stations (represented
generically here by k) expressing the range of disturbance in the horizontal magnetic
field. "ak" is scaled from the 3-hourly K index according to the following
table:
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K 0 1 2 3 4 5 6 7 8 9 ak 0 3 7 15 27 48 80 140 240 400
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At SESC these values are used directly for operational purposes. But to convert the ak
values to nanoteslas (nT), a local (station-dependent) conversion factor must be found by
dividing the station's lower limit for K=9 by 250. For example, at Boulder and
Fredericksburg the lower limit for K=9 is 500 nT so the factor is 2; therefore the ak
values for these stations are in units of 2 nT. (To obtain an equivalent amplitude in
nanoteslas for Boulder or Fredericksburg, the index value must be doubled).
Ak index. A daily index of geomagnetic activity for a specific station or
network of stations (represented generically here by k) derived as the average of the
eight 3-hourly ak indexes in a Universal Time day.
am index. A mean, 3-hourly "equivalent amplitude" of
geomagnetic activity based on standardized K index data from a global network of 23
Northern and Southern Hemisphere stations by the Institut de Physique du Globe de Paris,
France; am values are given in units of 1 nT.
Am index. The daily Ak index determined from the eight daily am indexes.
An index. The daily Ak index determined from only the Northern Hemisphere
stations of the am index network.
ap index. A mean, 3-hourly "equivalent amplitude" of magnetic
activity based on K index data from a planetary network of 11 Northern and 2 Southern
Hemisphere magnetic observatories between the geomagnetic latitudes of 46 degrees and 63
degrees by the Institut fur Geophysik at Gottingen, F.R. Germany; ap values
Ap index. Formally the daily Ak index, determined from the eight daily ap
indexes. However, for daily operational uses (since several weeks are required to collect
the data and calculate the index), Air Force Space Forecast Center estimates the value of
the Ap index by measuring the geomagnetic field in near-real time at several Western
Hemisphere magnetometer stations and statistically weighting the data to represent the
Gottingen Ap. The value of this estimated Ap index is reported in SESC daily and weekly
summaries of geophysical activity.
C index. A subjective daily character figure (index) of geomagnetic
activity for a single observatory; for each UTC day the figure is 0 for very quiet
magnetic conditions, 1 for moderately disturbed conditions, and 2 for severely disturbed
conditions.
Ci index. The daily international magnetic character figure formed by
taking the arithmetic mean of the C index values from all reporting observatories.
Cp index. A daily index of geomagnetic activity analogous to the Ci
index, obtained from the sum of the eight daily values of the ap index. The range of Cp is
0.0 to 2.5, 2.5 representing the most disturbed.
Dst index. A measure of variation in the geomagnetic field due to the
equatorial ring current. It is computed from the H-components at approximately four
near-equatorial stations at hourly intervals. At a given time, the Dst index is the
average of variation over all longitudes; the reference level is set so that Dst is
statistically zero on internationally designated quiet days. An index of -50 or deeper
indicates a storm-level disturbance, and an index of -200 or deeper is associated with
middle-latitude auroras. Dst is determined by the World Data Center C2 for Geomagnetism,
Kyoto University, Kyoto, Japan.
K index. A 3-hourly quasi-logarithmic local index of geomagnetic activity
relative to an assumed quiet-day curve for the recording site. Range is from 0 to 9. The K
index measures the deviation of the most disturbed horizontal component (see geomagnetic
elements ).
Km index. A 3-hourly planetary index of geomagnetic activity calculated
by the Institut de Physique du Globe de Paris, France, from the K indexes observed at a
large, symmetrically located network of stations. The Km indexes are used to determine the
am indexes.
Kp index. A 3-hourly planetary index of geomagnetic activity calculated
by the Institut fur Geophysik der Gottingen Universitat, F.R. Germany, from the K indexes
observed at 13 stations primarily in the Northern Hemisphere. The Kp indexes, which date
from 1932, are used to determine the ap indexes.
Q index. A 15-minute index of geomagnetic activity intended for
high-latitude (auroral) stations. After quiet diurnal variations
are removed, Q is the largest deviation scaled from the undisturbed level for the two
horizontal components. (This differs from the K index, which is scaled from the largest
relative deviation.) The 15-minute periods are centered on the hour and at 15, 30, and 45
minutes past each hour. The range of Q is from 0 to 11; the upper limit, in nanoteslas,
for each index value is given below.
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Q = 0 1 2 3 4 5 6 7 8 9 10 (T) 11 (E)
10 20 40 80 140 240 400 660 1000 1500 2200 Unlimited
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Note: Not to be confused with Qe (effective Q index) from the USAF 55th Space
Weather Squadron (55SWS) (= auroral activity index), presented on the pages of NWRA
(Northwest Research Associates, Inc.) Qe is being used as an input to space-weather
models.
Miscellaneous geomagnetic terms
geomagnetic activity. Natural variations in the geomagnetic field
classified quantitatively into quiet, unsettled, active, and geomagnetic storm levels
according to the observed a index:
Category Range of index
quiet 0 - 7
unsettled 8 - 15
active 16 - 29
minor storm 30 - 49
major storm 50 - 99
severe storm 100 - 400
geomagnetic elements. The components of the geomagnetic field at the
surface of the earth. These elements are usually denoted thus in the literature:
X - the geographic northward component
Y - the geographic eastward component
Z - the vertical component, reckoned positive downward
H - the horizontal intensity, of magnitude sq rt((X)squared + (Y)squared)
F - the total intensity sq rt((H)squared + (Z)squared)
I - the inclination (or dip) angle, arctan (Z/H)
D - the declination angle, measured from the geographic north direction to the H component
direction, positive in an eastward direction.
D = arctan (Y/X)
However, in SESC use, the geomagnetic northward and geomagnetic eastward components are
called the H and D components. The H axis direction is defined by the mean direction of
the horizontal component of the field; the D component is expressed in nanoteslas and is
related to the direction of the horizontal component relative to geomagnetic north by
using the small-angle approximation. Thus the D component = H (the horizontal intensity)
multiplied by delta D (the declination angle relative to geomagnetic north, expressed in
radians).
geomagnetic field. The magnetic field in and around the earth. The
intensity of the magnetic field at the earth's surface is approximately 32,.000 nT at the
equator and 62,000 nT at the north pole (the place where a compass needle points
vertically downward). The geomagnetic field is dynamic and undergoes continual slow
secular changes as well as short-term disturbances (see geomagnetic activity ). The
geomagnetic field can be approximated by a centered dipole field, with the axis of the
dipole inclined to the earth's rotational axis by about 11.5 degrees. Geomagnetic dipole
north is near geographic coordinate 78.3 N 69 W (Thule, Greenland), and dipole south is
near 79 S 110 E (near Vostok, Antarctica). The observed or dip poles, where the magnetic
field is vertical to the earth's surface, are near 76 N 101 W, and 66 S 141 E. The adopted
origin of geomagnetic longitude is the meridian passing through the geomagnetic poles
(dipole model) and the geographic south pole. (See also corrected geomagnetic
coordinates.)
geomagnetic storm. A worldwide disturbance of the earth's magnetic field,
distinct from regular diurnal variations. A storm is precisely defined as occurring when
the daily Ap index exceeds 29. (See geomagnetic activity ).
Initial Phase: Of a geomagnetic storm, that period when there may be an increase
of the middle-latitude horizontal intensity (H) (see geomagnetic elements ) at the surface
of the earth. The initial phase can last for hours (up to a day), but some storms proceed
directly into the main phase without showing an initial phase.
Main Phase: Of a geomagnetic storm, that period when the horizontal magnetic
field at middle latitudes is generally decreasing, owing to the effects of an increasing
westward-flowing magnetospheric ring current. The northward component can be depressed as
much as several hundred nanoteslas in intense storms. The main phase can last for hours,
but typically lasts less than 1 day.
Recovery Phase: Of a geomagnetic storm, that period when the depressed northward
field component returns to normal levels. Recovery is typically complete in one to two
days, but can take longer.
pulsation. A rapid fluctuation of the geomagnetic field having periods
from a fraction of a second to tens of minutes and lasting from minutes to hours. There
are two main patterns: Pc (a continuous, almost sinusoidal pattern), and Pi (an irregular
pattern). Pulsations occur at magnetically quiet as well as disturbed times. Pc's are
grouped, according to their physical and morphological properties, into five categories:
Pc1 - periods 0.2-5 s. May occur in bursts ("pearls"), or in consecutive groups
of pulsations with sharply decreasing frequency.
Pc2 - periods 5-10 s. Do not seem to be physically related to Pc1 or Pc3.
Pc3 - periods 10-45 s. Are observed over a wide range of latitudes.
Pc4 - periods 45-150 s. Are also known as Pc II or Pc.
Pc5 - periods 150-600 s. Are sometimes called giant micropulsations.
quiet day curve (QDC). Especially in connection with the components of
the geomagnetic field (see geomagnetic elements ), the trace expected in the absence of
activity. The K index and Q index are measured from deviations relative to a QDC. Riometer
and neutron monitor deviations are also measured relative to a QDC.
Sq. The diurnal variation of the geomagnetic field.The Sq variation is
explained in terms of solar tidal motions of the ionosphere and thermally driven
ionospheric winds.
substorm. A geomagnetic perturbation lasting 1 to 2 hours, which tends to
occur during local post-midnight nighttime. The magnitude of the substorm is largest in
the auroral zone, potentially reaching several thousand nanoteslas. A substorm corresponds
to an injection of charged particles from the magnetotail into the auroral oval.
sudden commencement ( SC, or SSC for Storm Sudden Commencement). An
abrupt increase or decrease in the northward component (see geomagnetic elements) of the
geomagnetic field, which marks the beginning of a geomagnetic storm. SCs occur almost
simultaneously worldwide but with locally varying magnitudes.
sudden impulse (SI + or SI - ). A sudden perturbation, positive or
negative, of several nanoteslas in the northward component (see geomagnetic elements ) of
the low-latitude geomagnetic field, not associated with a following geomagnetic storm. (An
SI becomes an SC if a storm follows.)
Space Weather Indices (Northwest Research Associates, Inc.)
This page provides access to plots of several indices of solar and geomagnetic activity
that are frequently used as inputs to space-weather models.
Brief definitions of each of the indices plotted are as follows:
F10.7 - Solar radio noise flux at a wavelength of 10.7 cm;
GPS-derived F10.7 - F10.7 value derived from GPS IONO coefficients;
SSNe - Effective sunspot number derived from ionospheric measurements;
IPS T index - Ionospheric index derived from ionospheric measurements;
Ap - Daily global index of geomagnetic activity.
Qe - Effective Q index (auroral activity index).
Kpe - Effective Kp index (auroral activity index).
Additional information defining the various indices are included with the expanded plots
that are obtained by clicking on the "button" versions of the plots on this
page. References for the various indices are listed at the end of each page of expanded
plots.
The data shown in these plots came from the following sources:
F10.7, SSN, and Ap from the NOAA Space Environment Center(SEC)
GPS IONO coefficients from NAWCWPNS, China Lake, CA
SSNe from the USAF 55th Space Weather Squadron (55SWS) via the NOAA SEC
SSNe from NWRA calculated using foF2 data from the NOAA SEC
IPS T index from the Austrailian Ionospheric Prediction Service (IPS) Radio and Space
Services
Qe from the USAF 55th Space Weather Squadron (55SWS) via the NOAA SEC
Kpe from NWRA calculated using DMSP SSJ/4 data from the NOAA SEC
The parameter Qe is the effective Q index derived from observations of the precipitating
electrons which are the source of high-latitude aurora. The measurements are from an
electron spectrometer on the USAF Defense Meteorological Satellite Program (DMSP)
satellites (part of the SSJ/4 sensor package). An algorithm developed by the Geophysics
Directorate of the USAF Phillips Laboratory is used by the USAF 55th Space Weather
Squadron (located at Falcon AFB, CO) to determine the equatorward boundary of auroral
electron precipitation from which the effective Q index is derived.
OTHER ARTICLES BY MR: VÄINÖ K. LEHTORANTA:
GLOSSARY OF SOLAR TERRESTRIAL TERMS
SAR GLOSSARY OF SOLAR TERRESTRIAL TERMS