JANUARY 20^th 2005 GLE

The particle spectra of SPE (solar particle events) are strongly different from one event to another. As a consequence some events of same importance at medium energy can strongly differ when higher energies (which are of interest for aviation dosimetry) are considered. This is the case for recent events : in October and November 2003 three important flares were observed. The associated GLE enhancements were all lower than 10 % above the galactic cosmic ray level, as recorded by Kerguelen neutron monitor. The GLE that occured on 20 January 2005 had similar characteristics in X rays and for particles with energies of about 10 MeV, but an enhancement of 178 % was observed with Kerguelen neutron monitor at energy of about 1 GeV (1 GeV=10³ MeV=10⁶ KeV=10⁹ eV). The GLE of the 20 January 2005 is among the largest observed during the last fifty years. In fact it is the forth behind the GLE 5 of 1956 (2400 %), the GLE 31 of 1978 (214 %) and the GLE 42 of 1989 (270 %).

GLE profile
This figure shows the time profiles of the GLE observed with Kerguelen and with Terre Adélie French neutron monitors. Kerguelen, at middle latitude (49° S) is representative in fact of the enhancement observed around the Earth. Because the anisotropy of the particles of this specific GLE is extremely high, Terre Adélie, at high latitude (67° S), shows a much stronger enhancement of 2650 % (5 minute sampling). The enhancement is localised on a very specific area of Antarctica and as shown by the figure, the increase is much shorter than for the Kerguelen time profile. On the contrary, close to the North pole, the intensity is lower by about 20 % from what could be expected without anisotropy.

Radiation dose impact
Next figure (1.5M!) shows a world map of the dose rate received at subsonic altitude (39,000 feet) in the course of the GLE (increase at Kerguelen station). The regions in white and light green close to the equator are well protected all along the GLE, whereas a maximum of 138 μSv.h^-1 is estimated close to the poles (regions in red). If the anisotropy mentioned above is taken into account, people flying from Paris to San Francisco during the GLE potentially received about 140 μSv (74 μSv from the GLE and 66 μSv from galactic cosmic rays). An hypothetical long-haul flight at 39,000 feet above Terre Adélie (Antarctica) would have received a dose of 294 μSv from the GLE. Please note that the figure has not been updated with the most recent version of the model, it nevetheless illustrates the latitudinal effect of the geomagnetic field shielding.