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The probability of occurrence of ionospheric F1 layer and L condition


A model as a Fortran routine providing as output the probability of occurrence of the F1 layer or L condition (considered together) can be downloaded here.

 

1. Probability function for F1 layer (Scotto et al., 1997)

The expression proposed was:

P( χ , λ , R12 ) = [ 0.5 + 0.5 · cos( χ ) ]γ .

Considering only the cases in which the critical frequency is observable it was found:

γ = a + b · |λ| + c · |λ|2 , where

a = 2.9798 + 0.0853993 · R12 ,

b = 0.01069 + 0.0021967 · R12 ,

c = -0.000256409 + 0.0000146678 · R12 .

 

2. Probability function for F1 layer or L condition (Scotto et al., 1997)

When the cases with L condition were also taken into account it was found that the gamma function can be assumed to be a constant ( γ=2.36 ) almost independent of geomagnetic latitude and R12.

P( χ , λ , R12 ) = [ 0.5 + 0.5 · cos( χ ) ]2.36 .

 

3. Improved probability function for F1 layer or L condition (Scotto et al., 1998)

The assessment of occurrence of the F1 layer and L condition is particularly important for the shape of vertical electron density profiles. For this reason a new probability function was proposed for these cases (Scotto et al., 1998). The following expression was suggested:

p( λ , χ ) = 1 - ( 1 / 90k(λ) ) χk(λ) ,

if χ < 90°,

p( λ , χ ) = 0 ,

if χ > 90°,

where

k( λ ) = 6.42182 - 0.00252479 · λ2 + 4.02531 · 10-7 · λ4 .

 

4. F1 or L condition as best option for an electron density profile model 

In order to estimate the electron density profile the F1 cusp (well defined) and L condition should be considered together. If there is only an L condition on a recorded trace, it is acceptable to estimate the electron density profile as though the F1 cusp was present. On the contrary, if an L condition is present, it is not acceptable to estimate the electron density profile as though the F1 layer was completely absent.

Figure

If there is only an L condition on a recorded trace, it is acceptable to estimate the electron density profile as though the F1 cusp was present (top panel). If an L condition is present, it is not acceptable to estimate the electron density profile as though the F1 layer was completely absent (bottom panel).

 

5. Estimation of probability of occurrence using tables

The probability of occurrence of the F1 layer and L condition exhibits observable variations also depending on season and solar activity. The expression above fits the experimental data quite well but does not take into account these variations, which are difficult to describe in a mathematical function. An alternative approach in order to resolve this issue is to compile tables of occurrence of the F1 layer for different seasons and solar activity levels.
Probability tables were first computed by Scotto (2002).

A Fortran routine based on recomputed tables can be downloaded here.
This routine uses as input the season, the smoothed Zurich sunspot number (R12), the geomagnetic latitude, and the solar zenith angle. It provides as output the probability of occurrence of the F1 layer or L condition (considered together).

 

6. References

 


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