18. Gopalswamy, 2006.


19. According to a conservative estimate of its intensity, “the Carrington flare was a >X10 soft x-ray event, placing it among the top ~100 flares of the last ~150 years.” See Cliver and Svalgaard, 2004, p. 410.


20. Bartels, 1937.


21. See Gopalswamy, 2006, p. 251, Figure 6.


22. Jackman, C.H., et al., Satellite measurements of middle atmospheric impacts by solar proton events in solar cycle 23, Space Sci. Rev. 125, 381-391, 2006.


23. For example, the largest >10 MeV SEP event of solar cycle 23 lasted 5½ days, from 1705 UT on November 4, 2001, until 0715 UT on November 10, 2001. (See Report of Solar and Geophysical Activity for November 10, 2001, issued jointly by NOAA and the USAF.)


24. McCracken, K.G., et al., Solar cosmic ray events for the period 1561-1994. 1. Identification in polar ice, 1561-1950, J. Geophys. Res. 106, 21585-21598, 2001.


25. McCracken, 2001; Shea, M.A., et al., Solar proton events for 450 years: The Carrington event in perspective, Adv. Space Res. 38, 232-238, 2006. Shea et al. give a >30 MeV proton fluence of 5.0 × 109 cm−2 for the August 1972 SEP event (Table 1). They state that this was the “first major large solar proton fluence event that was recorded by a spacecraft” and “it is this event against which most comparisons are made” (p. 236). It should be noted that their Table 1 also includes the SEP event of November 12, 1960, for which a fluence twice that of the August event is given (9 × 109 cm2). However, as Shea and Smart note in an earlier paper, there is considerable uncertainty about the actual value of the >30 MeV proton fluence during this event (Shea, M.A., and D.F. Smart, A summary of major solar proton events, Solar Physics 127, 297-320, 1990). For example, Kim et al. note that values as small as 1.3 × 109 cm−2 have been estimated for the November 1960 event (Kim, M.-H., X. Hu, and F.A. Cucinotta, Effect of shielding materials from SPEs on the lunar and Mars surface, paper presented at the AIAA Space 2005 Conference, August 30−September 1, 2005, AIAA 2005-6653, 2005).


26. The magnetosphere is the region of space dominated by the geomagnetic field. It is populated by electrically charged particles of varying composition (but mostly protons) originating in the solar wind and the ionosphere. The interaction with the solar wind stretches the magnetosphere on the anti-sunward side into a long, comet-like tail that can extend millions of miles downstream in the solar wind flow.


27. Cf. the magnetometer data from the Kew Observatory outside London, reproduced in Cliver, 2006, p. 123, Figure 4.


28. Tsurutani, B.T., et al., The extreme magnetic storm of 1-2 September 1859, J. Geophys. Res. 108(A7), 2003, doi:10.1029/2002JA009504.


29. Yokoyama, N., Y. Kamide, and H. Miyaoka, The size of the aurora belt during magnetic storms, Ann. Geophys. 16, 566-583, 1998.


30. Siscoe, G., N.U. Crooker, and C.R. Clauer, Dst of the Carrington storm of 1859, Adv. Space Res. 38, 173-179, 2006. The hourly Dst (disturbed storm time) index is the standard measure of magnetic storm intensity. It is derived from measurements made at four low-latitude magnetic observatories of the depression in the magnitude of the horizontal component of the geomagnetic field. The depression in the field is caused by an increase in the energy density of the ring current, a current system encircling Earth at low latitudes. It is the formation of a ring current that constitutes a magnetic storm. Use of the Colaba data for a Dst proxy assumes that the contribution of low-latitude auroral electrojects to the depression in H was insignificant. (For the opposite view, see Green and Boardsen, 2006, p. 134). It should be noted that Dst estimates for the September storm calculated on the basis of assumed solar wind parameters can yield higher values. Tsurutani et al., 2003, predict a Dst of −1760 nT. See also Li, X., et al., Modeling of the September 1-2, 1859, super magnetic storm, Adv. Space Res. 38, 273-279, 2006. In contrast, the upper limit Dst that Siscoe et al. derive from solar wind conditions is consistent with the proxy Dst of −850 nT.


31. Cliver and Svalgaard, 2004, p. 416, Table VI; Tsurutani et al., 2003.


32. According to Tsurutani et al., 2003, the storm had a single, brief (1-1.5 hrs) main phase and was caused by a magnetic cloud-type CME with an intense southward magnetic field and no contribution from a draped field in the sheath of shocked solar wind between the CME and the shock. Siscoe et al., 2006, on the other hand, hypothesize that the storm consisted of two main phases separated by a brief recovery. The first main phase was caused by a strongly southward sheath field; the second, by a northward-to-southward rotation of the field within the CME.


33. See note 30.


34. Stewart, B., On the great magnetic disturbance which extended from August 28 to September 7, 1859, as recorded by photography at the Kew Observatory, Phil. Trans. Royal Soc. 151, 423-430, 1861.


35. Nevanlinna, H., On geomagnetic variations during the August-September storms of 1859, Adv. Space Res. 42, 171-180, 2008.

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