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Derivation, Meaning, and Use of Geomagnetic Indices

Derivation, Meaning, and Use of Geomagnetic Indices

Pierre-Noél Mayaud (Editor)

ISBN: 978-1-118-66383-7

Mar 2013, American Geophysical Union

154 pages

Select type: O-Book


Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 22.

Indices have evolved in fields where there are massive data ensembles to be digested and interrelated. Usually, an index will be formed from one specific variable that tends to characterize the whole. Thus we have indices such as the Gross National Product, the Consumer Price Index, sunspot numbers, and birth and death rates. Geomagnetic indices have enjoyed a long history that dates back to the days when observers would watch through a magnifying glass the oscillatory motions of the end of a long compass needle. Geomagnetic indices have been particularly successful because they can characterize the state of affairs of the entire magnetosphere as well as that of the surrounding interplanetary medium. Furthermore, of all the relevant parameters that might be used to form an index, geomagnetic variations have one of the longest histories, and they are supported by one of the oldest and best-established worldwide networks of observatories.

Acknowledgments ii

Foreword vi

1. Introduction 1

2. What Is a Geomagnetic Index? 2

3. A Historical Review of Past Indices 4

3.1. Daily Range at a Given Observatory 4

3.2. Indices C and Ci (and C9) 4

3.3. Classification of Days 7

3.4. Declination Ranges at Greenwich 9

3.5. The Numerical Magnetic Character 10

3.6. The Norwegian 'Storminess' Index 10

3.7. The u Measure of Geomagnetic Activity 11

3.8. The W Index 14

3.9. From Past Indices to the Present IAGA Indices 15

4. Range Indices (K, R, Q) 16

4.1. A Preliminary Description 16

4.2. The Range or Other Alternatives 18

4.3. The Length of the Constant Time Interval 23

4.4. The Zero Level 24

4.5. Use of the Largest Range in the Horizontal Components 27

4.6. Classes of Ranges and K Indices 30

4.7. Subjective Character of the K Scalings? 38

4.8. Conclusion 40

5. Planetary Indices Derived from K Indices (Kg, am, and aa) 40

5.1. The Kp Index 40

5.2. The am Range Index 52

5.3. The aa Index 76

5.4. A Quantitative Comparison of the Average am, aa, and ap Indices 81

6. Classification of Days or of Any Interval of Time 85

6.1. The International Classification 86

6.2. Classification of Time Intervals From an Estimation of the Activity Level 93

7. The AU, AL, and AE Indices 96

7.1. The Ideal Scheme of the AU, AL, and AE Indices 96

7.2. The Actual Derivation 97

7.3. Tabulation of the Indices 105

7.4. Artificial and True Features in AU and AL Indices 106

8. The Dst Index 115

8.1. The Previous Attempts 117

8.2. The Ideal Scheme of the Dst Index 119

8.3. The Actual Derivation 121

8.4. Tabulation of the Dst Index 123

8.5. Artificial and True Features of the Dst Index 123

8.6. The DR Indices 129

9. Conclusion 129

9.1. Are Other Geomagnetic Indices Needed? 129

9.2. Meaning and Use of the Present Indices 131

Appendix A. A Possible Classification of the Irregular Variations 134

Worldwide Irregular Variations 134

Truly worldwide irregular variations (with a zonal component) 134

Potentially worldwide irregular variations (without zonal component) 135

Nonworldwide Irregular Variations 135

Conclusion 136

Appendix B. List of Observatories 137

References 139

List of Geomagnetic Index Symbols 144

Thematic Index 145