Summary
A Ground Level Enhancement occurred on 14 July 2000 and was observed by at least 18 stations in the worldwide neutron monitor network. Earliest onset was observed between 10.30 and 10.35 UT at several stations. The event is associated with an X5.8 solar flare (importance 3B) from active region 9077 that commenced at 10.03 UT, reached its peak at 10.24 UT and ended at 10.43 UT. The largest neutron monitor response was observed at South Pole with a maximum 58.3% above the pre-increase level in 1-minute data. Comparison of Mt. Wellington observations with those of Hobart and the new monitor at Kingston led to a derived attenuation length for the GLE of 110 g cm-2. Analysis of the enhancement using the established technique described by Cramp et al. (1997) has been undertaken for every 5-minute interval from 10.30 UT to 20.00 UT using data from 25 stations (including some with no significant enhancement). The spectrum was fitted with a simple power law and initially had a slope of around -6, softening during the event to around -9. At onset the particle arrival was quite anisotropic but tended toward increasing isotropy for an hour or more. Later in the event, when only low energy protons remained, the pitch angle distribution became highly anisotropic again. The apparent “arrival direction” of the particles also changed markedly between 11.00 and 11.10 UT but varied more slowly before and after that transition. Cramp, J.L., Duldig, M.L., Flückiger, E.O., Humble, J.E., Shea, M.A., and Smart, D.F. (1997) JGR 102, 24237–24248. 1 Introduction. A very small increase was observed at Lomnicky Stit which has a vertical cutoff of approximately 4 GV. The technique for modelling the GLE response by neutron monitors has been developed over many years (Shea & Smart, 1982; Humble et al., 1991) and is described in detail in Cramp et al. (1997).