Terminology

Terms in "Monthly overviews" (geomagnetic/geoelectric fields)

Geomagnetic storm

Large irregular disturbances in the geomagnetic field that occur almost simultaneously on a global scale. A geomagnetic storm with and without an abrupt onset is called sudden commencement storm and gradual commencement storm, respectively.
The geoelectric currents induced on the earth's surface by these disturbances also undergo similar changes.

Prominent phenomena

All short-period geomagnetic phenomena (lasting less than a day; si, bay, sfe, pc and pi, as defined below, classified either as A or B in Quality ranking) are designated as prominent.
However, this is not the case for rank A and B phenomena prior to December 2003 (limited designation).

si : An impulsive sudden change similar to an abrupt onset of the geomagnetic storm of sudden commencement type but not followed by major changes that would create an actual geomagnetic storm.
bay : A change with data resembling a bay, with gradual, steep or pulsating rises
sfe : A fishhook-shaped change with a short duration that occur during the day due to explosive events on the solar surface
pc : A continuous geomagnetic pulsation with periodic classification as pc1 - pc6

Continuous pulsation classification	Period
pc1	0.2 - 5 seconds
pc2	5 - 10 seconds
pc3	10 - 45 seconds
pc4	45 - 150 seconds
pc5	150 - 600 seconds
pc6	600 + seconds

pi : Irregular geomagnetic pulsation with periodic classification as pi1 to pi3, often preceding or accompanied by bay-shaped changes.

Irregular pulsation classification Period

pi1 1 - 40 seconds

pi2 40 - 150 seconds

pi3 150 + seconds

Irregular pulsation classification	Period
pi1	1 - 40 seconds
pi2	40 - 150 seconds
pi3	150 + seconds

K-index

The K-index expresses degrees of activity in geomagnetic variation. Days are divided into three-hour sections, with degrees of deviation from the quiet-day curve (daily variations in conditions of low geomagnetic activity) expressed in 10 grades from 0 to 9 on a quasi-logarithmic scale.
The total of the eight daily K-index values is called the daily K-index sum.
Kn and Ks index calculation is based on geomagnetic variations in the Northern Hemisphere (12 sites, including Memambetsu) and the Southern Hemisphere (9 sites).

Geomagnetic activity in the past 3 hours

Every hour the K-index is calculated for the pastthree hours and describe the geomagnetic activity with the following expressions depending on its value.

K index	Geomagnetic activity
2 or less	Calm
3/4	Moderately disturbed
5+	Disturbed

Daily geomagnetic activity

The following expressions are used according to the daily K-index sum.

Daily K index sum	Daily geomagnetic activity
3 or less	Very calm
4 - 10	Calm
11 - 16	Moderately Calm
17 - 22	Moderately disturbed
23+	Disturbed

Monthly geomagnetic activity

The following expressions are used according to the Monthly average of the daily K-index sum.

Monthly average of daily K index sum	Monthly geomagnetic activity
10.0 or less	Extremely low
10.0 - 11.5	Low
11.5 - 13.0	Slightly low
13.0 - 15.5	Normal
15.5 - 17.0	Slightly high
17.0 - 18.5	High
18.5+	Extremely high

Geomagnetic field components

F  : Total magnetic force (field intensity)
H  : Horizontal component (partial force in the horizontal plane)
Z  : Vertical component (downward along the plumb line)
D  : Declination (azimuth from true north, eastward positive)
I  : Inclination (dip angle from the horizontal plane, downward positive)
X  : Northern component (northward in geographic coordinates)
Y  : Eastern component (eastward in geographic coordinates)