October 2003 Solar Proton Event

On 28-30 October 2003 a very large SPE occurred. The solar proton >100 MeV flux reached values >100 cm-2s-1sr-1 and the time integrated flux was estimated to 1.2 x 107 cm-2sr-1 (Figure 1). This is the fourth largest event since 1962. The Thule neutron monitor indicates a 20% forbush decrease (not shown).

During the period from 16 October to 09 November 2003 GOMOS performed a series of occultation of star p Cet (visual magnitude 2.04, brightness temperature 4500 K) at a high and almost constant latitude (69°N-71°N). This star was chosen because it was the only one occulted in the auroral zone on dark limb around the SPE period. On 28th October GOMOS instrument was off due to an orbit correction of ENVISAT platform.

The evolution of O3 concentration at 48 km during the few days before and after the SPE shows a decrease of about 50% (Figure 2). During the same time, a spectacular increase of NO2 is observed from values near the noise level to about 5 x 108 mol cm-3 (~20 ppb). There is no indication of a return to previous values during the following 6 days (29 October to 3 November). The comparison of the mean profile for the period before and after the 28th October (Figures 3 and 4) shows that O3 is depleted down to at least 35 km. On the same time, the NO2 increase is observed at all altitudes above 37 km. The vertical integration of the difference between the O3 profile before and after the SPE gives a decrease of 7 Dobson Units (DU), more than 2% of the total column estimated to be about 300 DU at 70°N

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