Using the aa Index Over the Last 14 Solar Cycles to Characterize Extreme Geomagnetic Activity
Abstract Geomagnetic indices are routinely used to characterize space weather event intensity. The DST index is well resolved but is only available over five solar cycles. The aa index extends over 14 cycles but is highly discretized with poorly resolved extremes. We parameterize extreme aa activity...
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| Format: | Article |
| Language: | English |
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Wiley
2020-02-01
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| Series: | Geophysical Research Letters |
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| Online Access: | https://doi.org/10.1029/2019GL086524 |
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| author | S. C. Chapman R. B. Horne N. W. Watkins |
| author_facet | S. C. Chapman R. B. Horne N. W. Watkins |
| author_sort | S. C. Chapman |
| collection | DOAJ |
| description | Abstract Geomagnetic indices are routinely used to characterize space weather event intensity. The DST index is well resolved but is only available over five solar cycles. The aa index extends over 14 cycles but is highly discretized with poorly resolved extremes. We parameterize extreme aa activity by the annual‐averaged top few percent of observed values, show that these are exponentially distributed, and they track annual DST index minima. This gives a 14‐cycle average of ∼4% chance of at least one great ( DST<−500 nT) storm and ∼28% chance of at least one severe ( DST<−250 nT) storm per year. At least one DST=−809 [−663,−955] nT event in a given year would be a 1:151 year event. Carrington event estimate DST∼−850 nT is within the same distribution as other extreme activity seen in aa since 1868 so that its likelihood can be deduced from that of more moderate events. Events with DST≲−1,000 nT are in a distinct class, requiring special conditions. |
| format | Article |
| id | doaj-art-ef236766754a4c79b739e6fb4e75e861 |
| institution | DOAJ |
| issn | 0094-8276 1944-8007 |
| language | English |
| publishDate | 2020-02-01 |
| publisher | Wiley |
| record_format | Article |
| series | Geophysical Research Letters |
| spelling | doaj-art-ef236766754a4c79b739e6fb4e75e8612025-08-20T03:10:10ZengWileyGeophysical Research Letters0094-82761944-80072020-02-01473n/an/a10.1029/2019GL086524Using the aa Index Over the Last 14 Solar Cycles to Characterize Extreme Geomagnetic ActivityS. C. Chapman0R. B. Horne1N. W. Watkins2Centre for Fusion, Space and Astrophysics, Physics Department University of Warwick Coventry UKBritish Antarctic Survey Cambridge UKCentre for Fusion, Space and Astrophysics, Physics Department University of Warwick Coventry UKAbstract Geomagnetic indices are routinely used to characterize space weather event intensity. The DST index is well resolved but is only available over five solar cycles. The aa index extends over 14 cycles but is highly discretized with poorly resolved extremes. We parameterize extreme aa activity by the annual‐averaged top few percent of observed values, show that these are exponentially distributed, and they track annual DST index minima. This gives a 14‐cycle average of ∼4% chance of at least one great ( DST<−500 nT) storm and ∼28% chance of at least one severe ( DST<−250 nT) storm per year. At least one DST=−809 [−663,−955] nT event in a given year would be a 1:151 year event. Carrington event estimate DST∼−850 nT is within the same distribution as other extreme activity seen in aa since 1868 so that its likelihood can be deduced from that of more moderate events. Events with DST≲−1,000 nT are in a distinct class, requiring special conditions.https://doi.org/10.1029/2019GL086524space weatheraa indexCarrington eventgeomagnetic storm |
| spellingShingle | S. C. Chapman R. B. Horne N. W. Watkins Using the aa Index Over the Last 14 Solar Cycles to Characterize Extreme Geomagnetic Activity Geophysical Research Letters space weather aa index Carrington event geomagnetic storm |
| title | Using the aa Index Over the Last 14 Solar Cycles to Characterize Extreme Geomagnetic Activity |
| title_full | Using the aa Index Over the Last 14 Solar Cycles to Characterize Extreme Geomagnetic Activity |
| title_fullStr | Using the aa Index Over the Last 14 Solar Cycles to Characterize Extreme Geomagnetic Activity |
| title_full_unstemmed | Using the aa Index Over the Last 14 Solar Cycles to Characterize Extreme Geomagnetic Activity |
| title_short | Using the aa Index Over the Last 14 Solar Cycles to Characterize Extreme Geomagnetic Activity |
| title_sort | using the aa index over the last 14 solar cycles to characterize extreme geomagnetic activity |
| topic | space weather aa index Carrington event geomagnetic storm |
| url | https://doi.org/10.1029/2019GL086524 |
| work_keys_str_mv | AT scchapman usingtheaaindexoverthelast14solarcyclestocharacterizeextremegeomagneticactivity AT rbhorne usingtheaaindexoverthelast14solarcyclestocharacterizeextremegeomagneticactivity AT nwwatkins usingtheaaindexoverthelast14solarcyclestocharacterizeextremegeomagneticactivity |