Ap Index

A special scale is used to convert the Kp back to an equivalent gamma level (ap for planetary activity) that would be a typical field size in the middle latitudes. The daily average of the eight ap values becomes Ap, the equivalent daily amplitude index (Figure 4.19). Because a large number of the index

NOVEMBER 1998

Kp Three-Hourly Indices Day 1234 5 6 7 8 Ap

1

2-

1+

0+

1-

0

1-

2-

1-

4

2

2+

1 +

0

0+

1

0+

2+

1

4

3

1 +

2-

1-

2-

1-

2

2+

2+

6

4

3-

1-

1 +

2-

1

2-

1 +

1

6

5

2

1-

1 +

2-

3-

3

2

2+

8

6

3-

3 +

4 +

3

3+

4-

3+

4-

20

7

3-

3-

3

4

5

5

4

3+

26

8

6

8-

7 +

5-

4

3+

3-

3+

66

9

5-

6-

7-

6+

6

5+

7-

5-

75

10

4-

2+

1 +

2-

1-

1-

0+

0

6

11

0

0

0+

1 +

1-

0+

1-

2-

3

12

1-

1-

1 +

0+

1-

0+

1

2-

4

13

6

6-

6-

5

4 +

6-

6

6-

64

14

5+

6-

4 +

6-

5+

4-

4-

2+

41

15

2-

1

1

2+

3

2+

3+

3

10

16

2+

2+

2-

2-

1+

2-

1 +

3-

7

17

3

2+

2

1-

1-

1

0+

0+

6

18

2

2+

2+

2

2+

1 +

2

2-

7

19

3-

2

2-

2

2

2+

1+

1

7

20

1-

2

2+

3-

2

1

3+

3+

10

21

2

2

1 +

1 +

2-

2+

1

1 +

6

22

2+

2+

2+

1 +

1

2-

1

0+

6

23

3-

2+

2-

2

3

3-

2+

3+

11

24

2+

3

2

3-

3+

4

4

3-

16

25

2

3

2+

2

2-

4-

4-

3

13

26

2+

2

3

3

2

3 +

2

1 +

10

27

3

3

2

2

2+

3-

1-

1-

9

28

2+

3

1-

0+

1-

2+

2-

1+

6

29

2+

2+

1+

2-

1 +

0+

4-

2

8

30

0+

4

4-

3

2-

3 +

4 +

4 +

Mean 16

FIGURE 4.19 ► Table of planetary magnetic activity values of 3-hr K, daily Ap, and monthly mean Ap for November 1998. Table provided by H. Coffee at NGDC/NOAA.

stations are at high-latitude locations, the ap and Ap values are well correlated with the AE index already described. Average daily and monthly values of Ap are used for long-term comparisons to other solar-terrestrial processes. An 11-year solar activity related maximum (coming 1 or 2 years after the sunspot number maximum) is clearly evident in the monthly mean Ap values (Figure 4.20).

There is a favorable alignment between the arriving solar wind disturbance and the Earth's magnetosphere at the March and September equinoctial periods. At such times, large magnetic storms are more likely to occur. A study of

1930 1940 1950 1960 1970 1980 1990 2000

Number ot Days Ap>40 □ Sunspot Number

1930 1940 1950 1960 1970 1980 1990 2000

Number ot Days Ap>40 □ Sunspot Number

FIGURE 4.20 ► Comparison of sunspot number and number of days in each year that the Ap index exceeded 40 for the period of 1930 to 1997. Figure from E. Erwin of NGDC/NOAA.

60 years of magnetic storms clearly illustrates this seasonal preference (Figure 4.21).

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