What is Space Weather and how does it affect us?
Space weather is the result of particles emitted by the Sun, termed the solar wind, interacting with the Earth's magnetic field. Large eruptions from the Sun, called Coronal Mass Ejections (CMEs), pose a hazard to the ground infrastructure, such as the high-voltage power network, in the form of Geomagnetically induced currents (GIC). GICs are excess currents in the power network that are created by the rapid variations of the magnetic field and in extreme cases have caused damage to transformers causing the power grid to temporarily shut down. These space weather events are the same natural phenomenon as those responsible for the Aurora becoming visible at mid-latitudes.
There are important questions about the hazard posed by space weather that are unanswered. These include:
How big can geomagnetic storms be?
How often do storms of this size occur?
What is the series of events that can lead to the largest geomagnetic storms?
What are the effects on modern technology?
What records do we have of historical geomagnetic storms?
The British Geological Survey (BGS) holds records for eight geomagnetic observatories operating in the UK going back to 1847. The digital era of geomagnetic observations began in the 1980s providing us with high-quality recordings of the Earth’s magnetic field but prior to this all data were recorded on photographic paper. The problem with the digital dataset is the dearth of very large geomagnetic storms. As shown in Figure 1, we have enjoyed an unusually quiescent era of solar activity since the 1960s.
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| Figure 1: Geomagnetic activity from 1880 to 2020, expressed by the daily Aa index. Figure produced as part of the British Geological Survey and the ESA Space Safety Programme (https://swe.bgs.ac.uk/bgs/indices.shtml?index=Aaindexdaily). |
What is the Carrington Event?
These historical records contain some of the largest known geomagnetic storms including the famous Carrington event of 1st-2nd September 1859 – one of the large storms on record. The event and its lesser-known precursor were recorded at two rival observatories both operating in London at the time, Kew and Greenwich. This provides a great opportunity to cross-compare the two observatories only 20km apart.
The paper magnetograms provide a unique example of near-continuous measurements for the Carrington event and pre-cursor storm. We manually extracted the digital time series of Kew and Greenwich records of three components of vertical, horizontal and declination of magnetic field from 25-Aug to 05-Sep-1859 by digitizing the historic records. To assist with scaling the magnetograms into accurate time and magnetic units, we use published journal papers from the period to benchmark our interpretation and spot values recorded at Greenwich. This isn’t without its own problems, including:
the poor quality of parts of the recorded traces
overlap or missing traces
Issues with instrumentation at the time of recording
Lack of metadata to scale recorded traces to modern SI units of magnetic field.
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| Figure 2: Paper magnetograms recorded on photographic paper for observatories in London, UK. (a, b) Declination angle at Kew from 10:20UT 02-Sep-1859 to 12:05UT 05-Sep-1859 (c, d) Declination angle and Horizontal Force at Greenwich from 12:00UT 01-Sep-1859 to 12:00UT 03-Sep |
Figure 3 shows the reconstructed time series of the magnetic field at both the Greenwich and Kew observatories. Reconstructing the days prior to the famous 1-2 September storms enables us to see the series of events that led to such an extreme geomagnetic storm. When storms occur in close succession, the first storm essentially clears the interplanetary space of solar wind and enables the subsequent storms to have greater impact on Earth. We can see evidence for a large storm during the 28th-30th storm for example. The period around August and September 1859 was unusually stormy compared to the modern era.
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| Figure 3: Digitized magnetograms of (a) horizontal magnetic field strength, (b) declination, (c) vertical magnetic field strength, at Kew (orange) and Greenwich (blue) observatories from 25- Aug to 05-Sep-1859. Highlighted green is the Carrington-observed flare at ~11:15-11:23 01 September 1859. Highlighted in grey are evidence for suspected earlier solar flares. |
The analogue record is a rich and yet untapped source of information about geomagnetic activity in the past. With a more focussed effort and new digitisation tools, the community may be able to find better answers to the great unknowns of space weather hazards in future.
E. Eaton1, C. Beggan1, E. Lawrence1, E. Clarke1, K. Matsumoto2, H. Hayakawa2
1British Geological Survey, 2Nagoya University
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