Balfour Stewart reported that the magnetic storm from the Carrington Super Flare began at 05:00 GMT on the morning of September 2, 1859 as recorded by self-recording magnetograph at the Kew Observatory.
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Balfour Stewart reported that the magnetic storm from the Steward Super Flare began at 22:30 GMT on the evening of August 28, 1859 as recorded by self-recording magnetograph at the Kew Observatory.
Other types of magnetograph use narrowband filter imaging to produce a measurement of the first few moments of the spectral line, and operate much more quickly: the HMI instrument on board the Solar Dynamics Observatory will produce a vector magnetogram every few minutes.