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am INDICES

am indices :

am, an and as indices are derived from K indices scaled at observatories located in the subauroral zones of the Northern and Southern hemispheres. The stations are arranged in groups (G1 to G9), each group representing a longitude sector in one of the hemisphere :

Northern Hemisphere

Southern Hemisphere

Observatory

Corr.Geom. Lat.

Observatory

Corr. Geom. Lat.

G1

MGD

Magadan

53.8°

G6

EYR

Eyrewell

50.2°

 

PET

Petropavlosk

46.4°

 

CAN

Canberra

45.2°

 

MMB

Memambetsu

37.4°

 

 

 

 

G2

POD

Podkammenkaya

57.2°

G7

GNA

Gnangara

44.1°

 

SVD

Sverdlovsk

52.2°

 

AMS

Amsterdam

48.3°

G3

HAD

Hartland

50.0°

G8

PAF

Kerguelen

58.8°

 

NGK

Niemegk

48.8°

 

CZT

Crozet

52.4°

 

 

 

 

 

HER

Hermanus

41.1°

G4

OTT

Ottawa

58.9°

G9

AIA

Argentine Isl.

49.7°

 

FRD

Frederiksburg

51.8°

 

TRW

Trelew

27.8°

G5

NEW

Newport

55.2°

 

 

 

 

 

VIC

Victoria

53.9°

 

 

 

 

 

TUC

Tucson

39.7°

 

 

 

 

The corrected geomagnetic latitude indicated in this Table has been calculated by Mayaud for taking into account the actual topography of the main field.

For a given time interval, the K values measured at the observatories of one group are averaged and converted back to amplitude. These amplitudes are weighted for balancing the differences in longitude width of the sectors, and the hemispheric averages of these weighted amplitudes give rise to the three-hour an and as indices respectively; am is equal to (an + as) / 2. Am, An and As are the daily mean values of am, an and as. They are all expressed in nanoteslas.

For the sake of tradition and convenience, Km, Kn, Ks equivalent values are also made available by means of a conversion table (see below); they are as usually expressed by values from 0o to 9o, corresponding to the given interval of am (or an, or as).

am

0.0

1.4

3.4

5.4

7.4

10.4

13.4

16.4

20.4

26.4

Km

0o

0+

1-

1o

1+

2-

2o

2+

3-

 

am

26.4

33.4

40.4

50.4

60.4

70.4

86.4

103.4

120.4

146.4

Km

3o

3+

4-

4o

4+

5-

5o

5+

6-

 

am

146.4

173.4

200.4

243.4

286.4

330.4

386.4

443.4

500.4

611.4

Km

6o

6+

7-

7o

7+

8-

8o

8+

9-

9o

Monthly tables of am indices are given, each line corresponding to a Greenwich UT day. Following values are displayed from left to right:

- Km values for the 8 three-hour intervals ;

- daily sum Sum Km ;

- am values for the 8 three-hour intervals ;

- Am, daily mean value ;

A musical diagram of am indices is drawn from 1959 according to Bartels solar rotations.