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On the prediction of F1 ledge occurrence and critical frequency
Adv. Space Rex Vol. 24 No. 9, pp. 1773-1775, 1997 01997 COSPAR. Published by Elsevicr Science Ltd. All rights reserved Printed in Great Britain 0273-l 177197 $17.00 + 0.00 PII: SO273-1177(97)00589-9
ON THE PREDICTION OF Fl LEDGE OCCURRENCE AND CRITICAL FREQUENCY C. Scotto*,
M. Mosert de Gonz&lez**, S. M. Radicella*** and B. Zolesi*
Qtituto Nazionale di Geofsica, Via di Vigna Murata 605. 00143 Roma, Italy **Grupo de Ionosfera de1 CASLEO, San Juan, Argentina ***International Center for Theoretical Physics, Trieste, Italy
ABSTRACT The probability of occurrence of Fl ledge is presented for different solar position and activity conditions by using the hourly ionogram scaling information given by the monthly bulletins of ionospheric data. The information considered includes cases of L condition. An updated and selected data base is used to test the Du Charme et al. (1973) formula adopted in the IRI model taking into account alternative solutions for the particular restrictions imposed by the IRI for high values of solar zenith angle. Special attention is given to the critical frequency prediction for twilight hours. 01997 COSPAR. Published by Elsevier Science Ltd. INTRODUCTION The International Reference Ionosphere (IRI) model (Bilitza and Rawer, 1990) of electron density profiles with height considers the Du Charme et al. (1973) formula [from now on: DC] to calculate the value of foF1 needed to estimate the amount of ionization in the intermediate layer (150 to 220 km) of the ionosphere. The DC formula has been calculated using a data base from 1954 through 1966 with 39 selected stations. The DC formulation assumes limits for the presence offoF as a function of the solar zenith angle and the solar activity given by the RIZindex. In addition to these limits the IRl model assumes that an Fl ionization is never present during winter and during nighttime. This paper utilizes a data base different from the one used in the original DC paper to evaluate the validity of the formula and the relevance of the limits imposed in such a paper and in the IRI model, using 22 years of data (from 1969 to 1990) extracted from 104 stations available on the Ionospheric Digital Database of the National Geophysical Data Center of Boulder, USA. This paper also takes into account the presence of the L condition. L condition describes cases where the ionogram trace shows a Fl ledge rather than a distinct Fl cusp. Numerical values of the critical frequency @FI) are diffkult to obtain in this cases. This condition is sometimes seen before ionospheric sunrise and after sunset. A probability function that could replace the limits mentioned above is introduced.
ANALYSIS OF DATA Table 1 shows the average differences between the calculated values of foFl and the median values obtained from the data base described above. These differences are given for ranges of geomagnetic latitudes and RIZ.These results confirm the validity of the DC formula to calculatefoF1. Table 2 shows the average differences when foFl is calculated without the limits imposed by DC and IRI including values observed before sunrise and after sunset. The values obtained indicate that the DC formula is able to predict foF1 also outside these limits including values observed before sunrise and after sunset.The probability function that was calculated from the data base assumes a dependence on cos(x) , Rn, and k. The expression used is: fYX,k,R,,)=[0.5+0.5.cos(~)]‘,
where y =a+ba3L+c.h2, 1773
(1)
C.
1774
Scott0et al.
and
a =2.9798+0.0853993a R,, b = 0.01069-0.0021967~ R,, c= -0.000256409+0.0000146678~ R,,
(2)
Figure 1 shows the smoothed values of gamma as a function of geomagnetic latitude for two different levels of RM when only cases with numerical values forfoFI are included (thin lines), and when also the cases with L condition are included (thick line). It can be seen that in this last case gamma can be considered a constant y=2.36, almost independent of geomagnetic latitude and RD.
Table 1 Average Differences / MHz between the Calculated Values offoF and Experimental Median Values in the Imposed Limits only . . 1c latitu& R&Q 50 < RIZ< 1OQ RIZ> 10Q all the RIZ O O
and Experimental all
0.216 0.264 0.218
Figure 2 shows the smoothed probability function calculated empirically as a function of solar zenith angle for two geomagnetic latitudes and for two levels of RIZwhen only numerical values of foFl are considered (thin lines) and when also L conditions are included. The results show that a unique probability function can be assumed independent of geomagnetic latitude and solar activity if numerical foFI and L condition cases are considered together.
-
Fl Fl or L condition
I0
E” E
m 0)
8
6 4 2
I
I 60
Geom3ignetic
latitude
/ ’
Fig. 1. The smoothed values of y as a function of geomagnetic latitude h for two different levels of RIZwhen only cases with numerical values for foFZ are observed (thin lines) and including L condition cases (thick line).
Prediction of F1 Ledge
> .=
0
u
0
6
Geomagnetic
latitude
= 0”
Geomagnetic
latitude
= 40”
1775
z
n
cu 9
0
0.4
ii
0 2
Fig.2. The smoothed probability function calculated empirically as a function of solar zenith angle for two geomagnetic latitudes and for two levels of R/t when only numerical values offoF are considered (thin lines) and when also L conditions are included (thick lines).
CONCLUSIONS 1) The present results confirm the validity of the DC formula to predict values offoFI, including zenith angles beyond the limits specified in the original paper. 2) These limits could be substituted by the function given above, which indicate the probability of occurrence of a Fl layer. 3) The presence of an L condition that is ignored by the DC original formulation would be taken into account if the limits were replaced by the probability function given in Eq. (1). REFERENCES Bilitza, D. and K. Rawer, “New options for IRI electron density in the middle ionosphere”, Adv. Space Res. , 10, #II, 117 (1990). Du Chat-me, E.D. , L.E. Petrie and R. Eyfrig, “A method for predicting the Fl layer critical frequency based on Zurich smoothed sunspot number”, Radio Science, 8, #lo, 837 (1973).
ON THE PREDICTION OF Fl LEDGE OCCURRENCE AND CRITICAL FREQUENCY C. Scotto*,
M. Mosert de Gonz&lez**, S. M. Radicella*** and B. Zolesi*
Qtituto Nazionale di Geofsica, Via di Vigna Murata 605. 00143 Roma, Italy **Grupo de Ionosfera de1 CASLEO, San Juan, Argentina ***International Center for Theoretical Physics, Trieste, Italy
ABSTRACT The probability of occurrence of Fl ledge is presented for different solar position and activity conditions by using the hourly ionogram scaling information given by the monthly bulletins of ionospheric data. The information considered includes cases of L condition. An updated and selected data base is used to test the Du Charme et al. (1973) formula adopted in the IRI model taking into account alternative solutions for the particular restrictions imposed by the IRI for high values of solar zenith angle. Special attention is given to the critical frequency prediction for twilight hours. 01997 COSPAR. Published by Elsevier Science Ltd. INTRODUCTION The International Reference Ionosphere (IRI) model (Bilitza and Rawer, 1990) of electron density profiles with height considers the Du Charme et al. (1973) formula [from now on: DC] to calculate the value of foF1 needed to estimate the amount of ionization in the intermediate layer (150 to 220 km) of the ionosphere. The DC formula has been calculated using a data base from 1954 through 1966 with 39 selected stations. The DC formulation assumes limits for the presence offoF as a function of the solar zenith angle and the solar activity given by the RIZindex. In addition to these limits the IRl model assumes that an Fl ionization is never present during winter and during nighttime. This paper utilizes a data base different from the one used in the original DC paper to evaluate the validity of the formula and the relevance of the limits imposed in such a paper and in the IRI model, using 22 years of data (from 1969 to 1990) extracted from 104 stations available on the Ionospheric Digital Database of the National Geophysical Data Center of Boulder, USA. This paper also takes into account the presence of the L condition. L condition describes cases where the ionogram trace shows a Fl ledge rather than a distinct Fl cusp. Numerical values of the critical frequency @FI) are diffkult to obtain in this cases. This condition is sometimes seen before ionospheric sunrise and after sunset. A probability function that could replace the limits mentioned above is introduced.
ANALYSIS OF DATA Table 1 shows the average differences between the calculated values of foFl and the median values obtained from the data base described above. These differences are given for ranges of geomagnetic latitudes and RIZ.These results confirm the validity of the DC formula to calculatefoF1. Table 2 shows the average differences when foFl is calculated without the limits imposed by DC and IRI including values observed before sunrise and after sunset. The values obtained indicate that the DC formula is able to predict foF1 also outside these limits including values observed before sunrise and after sunset.The probability function that was calculated from the data base assumes a dependence on cos(x) , Rn, and k. The expression used is: fYX,k,R,,)=[0.5+0.5.cos(~)]‘,
where y =a+ba3L+c.h2, 1773
(1)
C.
1774
Scott0et al.
and
a =2.9798+0.0853993a R,, b = 0.01069-0.0021967~ R,, c= -0.000256409+0.0000146678~ R,,
(2)
Figure 1 shows the smoothed values of gamma as a function of geomagnetic latitude for two different levels of RM when only cases with numerical values forfoFI are included (thin lines), and when also the cases with L condition are included (thick line). It can be seen that in this last case gamma can be considered a constant y=2.36, almost independent of geomagnetic latitude and RD.
Table 1 Average Differences / MHz between the Calculated Values offoF and Experimental Median Values in the Imposed Limits only . . 1c latitu& R&Q 50 < RIZ< 1OQ RIZ> 10Q all the RIZ O
and Experimental all
0.216 0.264 0.218
Figure 2 shows the smoothed probability function calculated empirically as a function of solar zenith angle for two geomagnetic latitudes and for two levels of RIZwhen only numerical values of foFl are considered (thin lines) and when also L conditions are included. The results show that a unique probability function can be assumed independent of geomagnetic latitude and solar activity if numerical foFI and L condition cases are considered together.
-
Fl Fl or L condition
I0
E” E
m 0)
8
6 4 2
I
I 60
Geom3ignetic
latitude
/ ’
Fig. 1. The smoothed values of y as a function of geomagnetic latitude h for two different levels of RIZwhen only cases with numerical values for foFZ are observed (thin lines) and including L condition cases (thick line).
Prediction of F1 Ledge
> .=
0
u
0
6
Geomagnetic
latitude
= 0”
Geomagnetic
latitude
= 40”
1775
z
n
cu 9
0
0.4
ii
0 2
Fig.2. The smoothed probability function calculated empirically as a function of solar zenith angle for two geomagnetic latitudes and for two levels of R/t when only numerical values offoF are considered (thin lines) and when also L conditions are included (thick lines).
CONCLUSIONS 1) The present results confirm the validity of the DC formula to predict values offoFI, including zenith angles beyond the limits specified in the original paper. 2) These limits could be substituted by the function given above, which indicate the probability of occurrence of a Fl layer. 3) The presence of an L condition that is ignored by the DC original formulation would be taken into account if the limits were replaced by the probability function given in Eq. (1). REFERENCES Bilitza, D. and K. Rawer, “New options for IRI electron density in the middle ionosphere”, Adv. Space Res. , 10, #II, 117 (1990). Du Chat-me, E.D. , L.E. Petrie and R. Eyfrig, “A method for predicting the Fl layer critical frequency based on Zurich smoothed sunspot number”, Radio Science, 8, #lo, 837 (1973).