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Provisional local K-indices for last week.
In this figure we present last week´s local K-index at four stations. The stations are Ny-Ålesund (NAL) and Tromsø (TRO) in the auroral zone, Dombås (DOB) at subauroral latitude and Brorfelde (BFE) at magnetic mid-latitude. The co-ordinates are listed at The stack info page
Geomagnetic indices are used to compress the rich and complicated information of magnetogrammes into a manageable series of numbers. The method of compression depends on which characteristic of the magnetomgrammes we like to preserve. The K-index - also denoted K-number - has gained wide popularity as a simple mean to characterize the level of irregular geomagnetic disturbance. Regular phenomena, like the diurnal variation, are not included in the K-index.
The International Association for Geomagnetism and Aeronomy (IAGA) has adopted procedures for computing K-numbers. They are not suitable for near real time operations. Our procedure is therefore slightly different and the resulting numbers should be regarded as provisional in the IAGA sense. The deviations are, however, minor and insignificant in space weather monitoring. Our K-numbers are calculated from one minute resolution records of the field´s horizontal component. The regular diurnal variation is removed by an empirical model. Every three hour the range (maximum minus minimum) during the preceding three hours is calculated and a number from 0 to 9 - the K-number - is assigned by a look-up table. The 3-hour periods are 00-03 UT, 03-06 UT etc. until 21-24 UT, giving eight K-numbers per UT-day.
The table for conversion from range to K-number varies with the magnetic longitude of the station; the frequency of K=9 should be independent of the latitude, Thus the K-index describes the disturbance level relative to what is normal at the location; to get a 9 in the auroral zone requires a field variation of 2000 nT or more while 600 nT is sufficient at mid-latitude. The table below gives the numbers for the four stations used here.

 

Upper limit of 3-hour range  (nT)

K - number

NAL & TRO

DOB

BFE

0

20

8

6

1

40

15

12

2

80

30

24

3

160

60

48

4

280

105

84

5

480

180

144

6

800

300

240

7

1320

495

396

8

2000

750

600

9

>2000

>750

>600

16 August 2011, Truls Lynne Hansen