The geomagnetic storm of 2012 July 14-16.
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Figure 1 shows the combined results from my magnetometer and VLF radio receivers for July 12. The morning was associated with disturbed magnetometer traces (red and blue) following solar activity from previous days. Activity decreased to quiet conditions from 09:00 onwards, as can be seen by comparing this chart with the equivalent period of July 14. The received signal levels from the Anthorn (Cumbria) VLF transmitter on 19.6 kHz (pink trace) and from Ramsloh (Germany) on 23.4 kHz (green trace) show typical rapid night-time variations and the strong peaks and troughs associated with the changing ionisation levels in the upper atmosphere at dawn and dusk. The impact of the solar flare can be seen in the Sudden Ionospheric Disturbance (SID) which I timed as starting at 15:58 and reaching a maximum deviation at 16:41.
Figure 2 shows the magnetometer results for July 14, recording a quiet day until 18:00. The normal diurnal variation can be clearly seen on the blue trace (representing the vertical component) plus the traffic-induced noise on the red trace that I suffer from, other than the two hour break following the end of the milkman's deliveries at 02:30 UTC, despite the magnetometer being located 25 metres from the road. At 18:09 there is the start of a sudden change in the magnetic field, reaching a maximum deviation at 18:12. This heralded the start of a period of active/storm conditions which lasted into the early hours of July 16 with a particularly active period during the evening of July 15 (see Figure 3).
The duration between the time of the maximum SID displacement and the start of the magnetic activity was 49 hours 28 minutes, which represents the time taken for the CME to reach the Earth. CME transit durations are very variable, typically between 1 and 3 days, being driven by the CME's initial velocity and the ambient solar wind conditions at the time.
All observations were made from my home in Basingstoke, UK using the UKRAA magnetometer and VLF receivers. Further information on this event and others can be found in the monthly reports compiled by John Cook and archived on the RAG website at http:// www.britastro.org/radio/
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Paul Hyde, Coordinator