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Recent values of the three principal modulation parameters

The three potentials considered here are:
  • The heliocentric potential (O'Brien, 1976) which is used for example as an input for galactic cosmic ray dose calculation with the model CARI.
  • The modulation potential (Nymmik et al., 1996). The energy spectrum of the different primary galactic particles and nuclei can be deduced from the modulation potential.
  • The deceleration potential (Badhwar and O'Neill, 1996) which is for example an input of models such as EPCARD to calculate the radiation dose induced by galactic cosmic ray aboard aircrafts (see SIEVERT).
The following table gives the recent monthly modulation parameters : heliocentric and deceleration potentials are monthly average based on neutron monitor observations, and modulation potential, based on RI12 smoothed sunspot number, is averaged over 12 months and corresponds to a rigidity of 7 GV. Also given are the measured counts of the Kerguelen neutron monitor (used to calculate the heliocentric potential) and the estimated counts of the Climax monitor (used to calculate the deceleration parameter). This estimate is deduced with a linear regression method from Kerguelen observations (correlation coefficient = 0.987 and standard error of estimate = 43 for counts of the order of 4000).

Recent Potentials
  month/year heliocentric* modulation* deceleration* measured** estimated***  
    potential potential potential Kerguelen counts Climax counts  
    * in MV     ** in counts/hour/400     *** in counts/hour/100
References :
O'Brien K.: 1971, Nuovo Cimento, A3, 52-78O
Nymmik R.A., Panasyuk M.I. et Suslov A.A.: 1996, Adv. Space Research, 17, (2)19 (2)30.
Badhwar G.D.and O'Neill P.M.: 1996, Adv. Space Research, 17, (2)7-(2)17.


On the calculation of the heliocentric potential given here

The monthly values of the heliocentric potential are calculated using the observed monthly averages of the cosmic ray intensity measured with the French neutron monitor located at Port-aux-Français, in the Kerguelen Islands (South Indian Ocean). The original heliocentric potential is given by the US FAA site. The heliocentric potential data given by this site for the period 1964 to 1997 have been fitted with the Kerguelen neutron monitor output using a 2nd degree polynom. This function is then used to calculate the heliocentric potential given here, deduced from the Kerguelen observations. The same is done with the Dumont d'Urville neutron monitor (in Terre Adélie, Antarctic) as a back-up.

The following figure shows the monthly relative difference (in %) between the dose calculated with CARI-6 model, for a flight from Paris to San Francisco, with the original heliocentric potential on the one hand and with the potential deduced from Kerguelen observations on the other hand. The difference is only of a few percents. The period shown is the period from 1964 to 1997, used to fit the data. After 1997 the difference is larger, possibly due to changes of monitors, but it remains smaller than 6 % pic to pic, which is still a good precision for a biological dose.