Relationship between F2 layer critical frequency and solar activity indices during different solar epochs

Transcription

1 Indian Journal of Radio & Space Physics Vol 42, April 2013, pp Relationship between F2 layer critical frequency and solar activity indices during different solar epochs S O Ikubanni 1,$,*, B O Adebesin 1, S J Adebiyi 1 & J O Adeniyi 2 1 Department of Physical Sciences (Physics), Landmark University, P M B 1001, Omu-Aran, Nigeria 2 Department of Physics, University of Ilorin, P M B 1551, Ilorin, Nigeria $ ikubanni.stephen@lmu.edu.ng Received 5 July 2012; revised 7 February 2013; accepted 26 February 2013 This paper aims at investigating the relationship between F2 layer critical frequency (fof2) and solar activity indices viz. smoothed twelve-month running mean of the sunspot number (R12) and the solar radio flux of 10.7cm wavelength (F10.7) during three different solar epochs. Data from Ouagadougou, an African low latitude station (latitude 12.4 N, longitude 1.5 W, dip 5.7), and the National Geophysical Data Centre were used for this study. Distinct characteristics were observed during different solar epochs and during each season. Sunspot number and solar radio flux agreed better during the year of moderate solar activity than during the year of low solar activity. The average correlation coefficients between fof2 and F10.7 obtained for high, moderate and low solar activity were 0.17, 0.67, and 0.44, respectively; while correlation coefficients between fof2 and R12 obtained were 0.05, 0.66, and 0.49, respectively. Irrespective of solar epochs and the trend of the indices, a trough in fof2 value was observed towards the end of June solstice. Relationship between fof2 and the solar activity indices agreed reasonably for the three solar epochs during December solstice. A strong solar activity dependence of fof2 around equinoxes than solstices was also observed. Factors that could cause the F2 layer variation were discussed. Keywords: F2 layer critical frequency, Solar activity, Solar radio flux, Sunspot number PACS Nos: dj; qd 1 Introduction Earth s Ionosphere is formed mainly through the photoionization by solar EUV and X-rays. Although the EUV flux data is limited 1, some solar activity indices have served as a very good proxy. Examples of these indices are the sunspot number and the solar radio flux at 10.7cm wavelength. Solar activity varies over different timescales and the 11-year solar cycle (SC) is its most prominent variation. SC variation of solar activity appears repeatedly but varies from cycle to cycle 2. Various researchers have observed the ionosphere to behave differently during different solar epochs 3,4. Rishbeth & Mendillo 5 have attributed the variation of the ionosphere to the solar, geomagnetic, and meteorological influence. They stated that geomagnetic and meteorological sources are more peculiar to the day-to-day variations while solar source contributes more to the month-to-month or year-to-year variations. The Sun is very quiet during the minimum and active during the maximum of solar cycle 6. Sunspot counts are very low at solar minimum and highest at solar maximum. The solar radio flux at 10.7cm wavelength, which is a proxy of the EUV radiation, is observed to follow the same variation pattern as of the sunspot number. There is an increase in EUV by as much as a factor of 3 over a SC 7. The increase causes a significant SC induced modulation in the ionosphere 1. Significant variations of solar EUV irradiance have been described using its long term continuous measurements 8. Different models have been developed from these measurements 6,9. Most significantly, Bilitza 1 had advocated the continual measurement of the EUV for use in the IRI model. In most ionospheric models, such as Xu et al. 10, solar activity indices, such as sunspot number and 10.7cm solar radio flux, were used rather than the EUV due to their long-term availability. However, variation of sunspot number and 10.7cm flux during different phases of solar cycle and their influence over ionosphere, in general, and across different seasons near equator and low latitude, in particular, has not been investigated in detail. In the present study, the variation of two common indices, the sunspot number and the solar radio flux 10.7cm wavelength, and their relation to the critical frequency of the F2 layer across different seasons of the year during different phases of the solar cycle has been investigated. The different

2 74 INDIAN J RADIO & SPACE PHYS, APRIL 2013 phases are: high solar activity phase, low solar activity phase and the decreasing phase. 2 Data analysis The 11-year solar cycle (SC) is the most prominent variation of solar activity, which has a significant effect on the equatorial ionosphere. The relationship between the critical frequency of the F2 layer (fof2) and some solar activity indices was studied over a span 11-year solar cycle ( ) with the maximum activity at the middle of the cycle. The solar activity indices include: monthlyhourly averages of the F2 layer critical frequency (fof2 in MHz) deduced from ionograms; and two solar activity proxies, F10.7 index and R12, obtained from the National Geophysical Data Centre - NOAA Satellite and Information Service. The ionograms were recorded by the Ionospheric Prediction Services (IPS) 42 ionosonde located at Ouagadougou, Burkina Faso (latitude 12.4 N, longitude 1.5 W, dip 5.7). The F10.7 index refers to the flux of radio emission from the Sun at a wavelength of 10.7 cm (2.8 GHz frequency). It trails the changing pattern of the solar UV radiation over the SC and is measured in solar flux unit (sfu) (1 sfu = Wm -2 Hz -1 ) (Ref. 11). The R12 is the twelve-month smoothed relative sunspot number. It is used because of two reasons: (i) according to the National Geophysical Data Centre, the daily sunspot number has little, if any relationship with ionospheric variability; and (ii) monthly averaged values of F2 layer parameter is better correlated with the twelve-month smoothed relative sunspot number than the monthly and daily average sunspot values 1. Using the data, the following have been investigated: (i) trend of fof2 with F10.7 and R12 over a solar cycle; (ii) behaviour of fof2 in response to the solar activity indices during different solar epochs; and (iii) seasonal variation in the dependence of fof2 on the solar activity indices across the solar cycle. The graphs of R12, F10.7 and fof2 were plotted over a solar cycle and for different solar epochs and presented in respectively. Three typical years of different solar activity level were chosen for the study: (i) a year of high solar activity 1989; (ii) a year of moderate solar activity 1992; and (iii) a year of low solar activity The years were picked after studying the plot in Fig. 1. It must be noted that the year of moderate solar activity is in the decreasing phase of solar cycle Results Figure 1(a) shows the plot of twelve-month smoothed relative sunspot number (R12) and solar radio flux at 10.7 cm wavelength, F10.7 for different years in solar cycle 22 ( ). It was observed that the R12 and the F10.7 values were extremely low between 1985 and 1987 and between 1994 and There was rapid increase in both values between 1987 and Likewise, there was rapid decrease between 1991 and There was comparative equilibrium between 1989 and 1991 representing a period of very high values of both R12 and F10.7. However, two distinct peaks were observed in 1989 and The trough between these two peaks was more pronounced with F10.7 than with R12 as observed. Figure 1(b) shows the plot of R12 and the monthly averaged value of the F2 layer critical frequency (fof2) over solar cycle 22 ( ). This plot followed the same trend as in Fig. 1(a) and there were two distinct peaks. However, the occurrence of the peak differs for both parameters. A unique feature observed was that one of the peaks was higher than the other. The magnitude of the peak is found to be higher in 1989 for R12 and in 1991 for fof2. Figure 1 (c) shows the plot of F10.7 and fof2 over solar cycle 22. Observations similar to those for R12 and fof2 [in Fig. 1(b)] were recorded. Fig. 1 Yearly variation of F2 layer critical frequency (fof2), sunspot number (R12) and solar radio flux (F10.7) over a solar cycle: (a) R12 and F10.7; (b) R12 and fof2; and (c) F10.7 and fof2

3 IKUBANNI et al.: RELATIONSHIP BETWEEN F2 LAYER CRITICAL FREQUENCY AND SOLAR ACTIVITY INDICES 75 Figure 2 shows the monthly variation of the F2 layer critical frequency (fof2), sunspot number (R12) and solar radio flux (F10.7) during different solar epochs and comprises of nine panels. Panels (a-c) represent the plots of R12 and F10.7; R12 and fof2 (MHz); F10.7 and fof2 (MHz), respectively for different months for the year 1989 [a year of high solar activity (HSA)]. Panels (d-f) represent the plots of R12 and F10.7; R12 and fof2 (MHz); F10.7 and fof2 (MHz), respectively for different months for the year 1992 [a year of moderate solar activity (MSA)]. Panels (g-i) represent the plots of R12 and F10.7; R12 and fof2 (MHz); F10.7 and fof2 (MHz), respectively for different months for the year 1985 [a year of low solar activity (LSA)]. The four vertical broken lines drawn on each plot segment each plot into seasons: March equinox between the first and the second line; June solstice between the second and the third line; September equinox between the third and the fourth line; and December solstice between the fourth and the first line. 3.1 High solar activity (HSA) year 1989 For the year of HSA, both F10.7 and R12 followed the same trend. There was a distinct seasonal variation as shown in panel (a). During the March equinox, decrease in solar activity was observed. The activity then increased again from the start of June solstice reaching a peak before decreasing to its lowest towards the end of the season. It then increased to another peak around mid-september solstice, decreased a little then increased through that season to another peak in early December solstice (November). For both indices, F10.7 index and R12, the peak in June solstice was the highest while the peak in December solstice was higher than that of September equinox. From panels (b and c), during March equinox, R12 and F10.7 decreased rapidly while fof2 increased but rather gradually. From the start of June solstice, R12 and F10.7 increased rapidly to a peak around mid-season and then decreased rapidly to a minimum towards the end of the season while fof2 decreased rapidly from the start of the season to a minimum, as the solar indices, towards the end. In the September equinox, the solar activity indices (R12 and F10.7) increased from the minimum in June solstice until mid-season; it then decreased towards the end of the season (say October). However, fof2 increased from the same point as the solar activity until around October before decreasing through the December solstice. There were slight differences in the trend of solar activity indices as presented by R12 and F10.7 in panel (a), especially in March equinox and December solstice. In other seasons, the trend was nearly uniform. It was observed that for R12, there was a crest around November and a trough between December and January. For F10.7, there is a crest around November but the decrease is continuous cutting through March equinox as reported earlier. It is worthwhile to note that during the year, two crests and one trough of fof2 values were observed. The first crest is formed towards the end of the March equinox and the second towards the end of the September equinox while the trough was formed towards the end of the June solstice. However, the latter crest was observed to be more pronounced than the former. 3.2 Moderate solar activity (MSA) year 1992 Both the trend of the solar activity indices and the fof2 through the year is observed to be clearly different in the year of MSA 1992 [panels (d-f)] from observations in the year of HSA 1989 [panels (a-c)]. During March equinox, decrease in solar activity (R12 and F10.7) index is observed from around February to a minimum around mid June solstice season. R12 and F10.7 then increased slightly before decreasing to another minimum in the early part of the September equinox. From there, it increased slightly till the season end before decreasing through December solstice. There seems to be a better agreement between solar activity indices and fof2 compared to what was observed in a year of HSA. There were periods and seasons when higher values of fof2 were observed. However, it was unlike the year of HSA, two plateaus of fof2 value spreading over two months were observed. The first plateau formed during the March equinox and the second, which is lower than the first, between mid-september equinox and early December solstice. The fof2 was observed not to respond to the slight increase in the solar activity level during the June solstice. 3.3 Low solar activity (LSA) year 1985 Panels (g-i) represent the trend of R12, F10.7 and fof2 in a year of LSA. The trend pattern of R12 and F10.7 [Panel (g)] was quite different from those observed in a year of HSA [panel (a)] and MSA [panel (d)], especially towards the end of March equinox through the next two seasons (June solstice and September equinox). Solar activity was relatively stable from the first month until towards the end of March equinox and then increased into June solstice.

4 76 INDIAN J RADIO & SPACE PHYS, APRIL 2013 Fig. 2 Monthly variation of the F2 layer critical frequency (fof2), sunspot number (R12) and solar radio flux (F10.7) during different solar epochs; (1st line) in a year of high solar activity: (a) R12 and F10.7; (b) R12 and fof2; (c) F10.7 and fof2; (2nd line) in 1992-a year of moderate solar activity: (d) R12 and F10.7; (e) R12 and fof2; (f) F10.7 and fof2; (3rd line) in a year of low solar activity: (g) R12 and F10.7; (h) R12 and fof2; and (i) F10.7 and fof2

5 IKUBANNI et al.: RELATIONSHIP BETWEEN F2 LAYER CRITICAL FREQUENCY AND SOLAR ACTIVITY INDICES 77 Two peaks were observed during the June solstice. The first peak was formed in the early season and the second, which was higher than the first, was formed towards the end of the season. After the second peak, there was a rapid decrease in solar activity level, which reached a minimum around mid-september equinox. It then increased again towards the end of the season and became relatively stable from then and through the December solstice. In panels (h and i), it was observed that although R12 and F10.7 was relatively stable, fof2 increased slightly from the first month to early part of the March equinox. It then decreased gradually from the peak at the early part of the season until towards the end of June solstice reaching a minimum towards the end of June solstice, which was similar to what was observed in both the year of HSA and MSA. However, the period of crest in fof2 was the same for the second peak in R12 and F10.7 during the June solstice. The second peak was higher among the two peaks during this season. As the solar activity continued, the indices (R12 and F10.7) decreased during September equinox to a minimum in mid-season and then increased towards the end of the season (say October). However, fof2 increased from the end of June solstice till around October before decreasing through the December solstice It is worthwhile to note that during the year, two crests and one trough of fof2 values were observed (similar to observation during the year of HSA). However, unlike HSA, the first crest is formed at the start of March equinox. Although it was generally observed that the fof2 trend look similar in different solar epochs, the solar activity trend was not similar for different solar epochs. The plots in Fig. 3 and the statistical presentation in Table 1 show that the dependence of fof2 on either indices is strongest in moderate solar activity (MSA) and weakest in high solar activity (HSA). This dependence is inferred from the magnitude of the correlation between the parameters under consideration. Unlike during other solar epochs, the dependence of fof2 on each of the solar activity indices during HSA period is observed to differ significantly. Likewise, Fig. 4 illustrates the trend and increase in the fof2 with respect to increase in the solar activity indices. Magnitude of the increase in fof2 is observed to correspond well to that of the solar activity indices for a period of about five equinoctial months during high solar activity. The Table 1 Correlation coefficient (r) between fof2 and the solar activity indices during different solar activity epochs Correlation coefficient (r) Solar epoch F10.7 vs R12 fof2 vs R12 fof2 vs F10.7 LSA MSA HSA Fig. 3 Correlation of the solar activity indices (R12 and F10.7) with ionospheric F2 critical frequency (fof2) for: (1st column): LSA; (2nd column): MSA; and (3rd column): HSA [r is the correlation coefficient of the linear fit; R 2 is the determination coefficient of the quadratic fit]

6 78 INDIAN J RADIO & SPACE PHYS, APRIL 2013 Fig. 4 Deviations of the monthly values of the three parameters [fof2 (MHz), F10.7 (sfu) and R12] in (a) HSA; and (b) MSA from observations in minimum solar activity (LSA) increase in solar activity corresponds well for a period of 9 to 10 months during MSA. It is of interest that the maximum deviation in fof2 is found during March equinox in MSA while the minimum deviation is found during June solstice in HSA. 4 Discussion The relationship between solar activity indices (R12 and F10.7) and the F2 layer critical frequency have been investigated during different solar epochs. The significantly low linear correlation coefficient between fof2 and either of the parameters may be due to the saturation phenomenon reported in earlier studies. However, compared to other solar epochs, the significant difference between the linear correlation coefficient (r) of: fof2 versus F10.7, and fof2 versus R12 in HSA is suggested to be due to the non-linear (quadratic) relationship between F10.7 and R12. It is known that in a quadratic function, there is a nonlinear value of the dependent variable as the independent variable reaches a threshold and surpasses it. The fof2 saturates at high solar activity due to high electron density in the ionosphere 12. Additionally, significant variations with season and solar activity level have been identified. This is similar to the observation of Brum et al. 13 where fof2 has strong solar activity dependence, i.e. increase with increasing sunspot number but varying with seasons. These variations have been observed to be due to factors, such as thermospheric wind 14, neutral winds, dynamo electric field 15, which varies seasonally 16, its combined effect 17 and the varying Sun-Earth distance due to the Earth s elliptic orbit round the Sun. Solar activity dependence of fof2 is strongest in the year of MSA (which is the decreasing phase of the solar cycle) than in HSA and LSA (the LSA year is in the cycle 21/22 minimum). Second, the relationship between either R12 or F10.7 and fof2 in the low latitude region varies significantly with season. The variation during HSA and LSA is significant in the first half of the year (from early March equinox to the end of June solstice). However, it is more pronounced in the solstice than the equinox as seen in Fig. 2 and according to Yang & Chen 16, neutral winds are stronger in solstices than in equinoxes. The increasing strength of the winds tends to cause depletion in the low latitude electron density (and consequently a drop in the fof2) during solstices. The above result is consistent with the result of Liu et al. 8, who studied the solar activity dependence of the dayside NmF2. They found stronger solar activity dependence around equinoxes than solstices. Furthermore, they noticed an equinox asymmetry, with stronger dependence near the March equinox than near the September equinox. According to Liu et al. 18, this equinox asymmetry should be caused by other factors besides the solar activity level. Whalen 19 also observed a roughly linear growth rate of NmF2 with solar activity at 2100 hrs LT. He observed clear higher growth rates near equinoxes than near the December solstice and higher growth rates near the December solstice than near the June solstice. The observed relationships in the present study plausibly agree for the three solar epochs during the December solstice which may be due to an insignificant effect of E B ion drift during this period. The vertical E B force - which is induced by the atmospheric tidal forcing, is due to the conjunction of an eastward-westward electric field, E and the Earth s magnetic field, B that is horizontal around the equatorial region have major influence on the electron movement in the equatorial region 20. It is

7 IKUBANNI et al.: RELATIONSHIP BETWEEN F2 LAYER CRITICAL FREQUENCY AND SOLAR ACTIVITY INDICES 79 known that fof2 is related to peak electron density, N max by the equation fof2 ( 80.8 ) Nmax =, where, N max, is the peak plasma or electron density. This E B force causes the drifting of the electron either upward (when E is westward) or downward (when E is eastward), thereby depleting or enhancing the electron density especially of the low latitude. Equatorial ionosphere s electrodynamics and chemical processes as well as its seasonal, longitudinal, and diurnal variations are much due to the E B vertical drift 21. The thermospheric wind can also indirectly cause seasonal variation by changing the variation produced by the E B drift 14. Moreover, seasonal variation of fof2 may be due to the changes in neutral composition. Stubbe (Ref. 22 and the references therein) had earlier stated that seasonal F-region variation is caused by changes in the composition of the neutral gas. The densities of these gases vary significantly across seasons due to the meridional winds irrespective of the solar activity. The horizontal component, which is parallel to the Earth s magnetic field, B, is able to cause the drifting of ion, although weakly, in a similar role to zonal electric field 23. The vertical component, which is perpendicular in direction to B, would have a nearly negligible effect. Hence, the ion drift away from a geographical longitude and electron density is enhanced at such latitude. The meridional wind is a source of reduction in the seasonal anomaly at an equatorial region and for low solar activity. Hence, it can be suggested that meridional wind effect decreases with decreasing latitude and solar activity. The equinoctial asymmetry in fof2 has been reported in earlier studies. 24,25 The present work revealed asymmetry in the peak of fof2 around two equinoxes. It was observed in the present study that the asymmetry is not pronounced at low solar levels but well pronounced at MSA and HSA. According to previous studies, the asymmetry may be attributed to the meridional neutral winds that modulate the height of the F2 layer. Meridional winds have no effect on the ionospheric F2 at the equator since it is almost parallel to the Earth s magnetic field. The effect becomes noticeable away from the equator and it increases with increasing latitude. The wind is known to move from the summer hemisphere to the winter hemisphere during solstices while it moves away from the equator to the poles during equinoxes. The summer-winter movement results in enhanced electron density in the December solstice and depletion during June solstice. In addition, the effect increases with rising solar activity. The poleward movement causes electron density enhancement in both equinoctial periods, but the September equinox experiences larger winds than the March equinox 25. In agreement with Bailey et al. 24, the peak in March equinox is higher than that of the September equinox during MSA. In HSA, the peak in September equinox is higher. This can be attributed to the prevailing wind, which becomes larger during September equinox as solar activity increases. The plateau observed in MSA, as oppose the almost sharp peak observed in HSA, could also be attributed to the meridional winds. It is known that as solar activity increases, the effect of the wind also increases. Hence, at HSA, the electron enhancement around December solstice causes the fof2 to rise to a higher value while the subsequent depletion around June solstice causes a sharp peak followed by a sharp drop in the fof2 values. In June solstice, fof2 value reached to its minimum irrespective of the solar activity level; the differences in the trend of the indices during the three solar epochs did not affect fof2 in this season. This implies that minimum fof2 during June solstice is not due to solar activity trend but due to the maximum Sun- Earth distance (shorter daytime solar radiation influx) in the Northern hemisphere in this season. Maximum Sun-Earth distance implies a shorter time for ion production and longer time for the recombination process compared to other seasons. However, this is not the sole cause of the minimum. From the above, it is assumed that fof2 for the season with minimum Sun-Earth distance (December solstice) should strongly follow the solar activity proxy trend. However, this is not the case as the better agreement between the fof2 and the solar activity indices are observed during the equinoxes than during December solstice. Hence, it is suggested that observations during December solstice might be typically due to a reduced daytime eastward electric field, which results into a slower diffusion (upward movement) of the electron from the equatorial region, and an enhanced nighttime westward electric field which results in increased electron density due to faster downward movement of electrons at the equatorial region. Furthermore, the dependence of fof2 on either of the indices is flawed by the phenomenon of hysteresis. This hysteresis is observed in the form of

8 80 INDIAN J RADIO & SPACE PHYS, APRIL 2013 time lag in the plots. This phenomenon is pronounced in March and September equinoxes during a year of HSA [panels (b and c)], also in June solstice in the three solar epochs [panels (b, c, e, f, h and i)] Recently Özgüç et al. 3 reported the presence of this phenomenon. The result presented in Fig. 3 confirms the existence of some form of saturation in the fof2. Saturation effect has been reported to be a feature of the low latitude fof2 at high solar flux levels. 5 Conclusion Data set spanning a solar cycle 22 from an ionosonde located in an African equatorial region of Ouagadougou, Burkina Faso (latitude 12.4 N, longitude 1.5 W, dip 5.7) and two solar activity proxies (F10.7 index and R12) have been used. The trend of fof2 over the cycle and during different phases of the cycle has been observed. Trend of solar activity, as represented by the indices, varies across each season in different solar epochs. Likewise, the annual variation is distinct for the three years of study. The indices are maximum during HSA (mid-june solstice) and LSA (throughout the June solstice with two peaks) but minimum during MSA (mid-june solstice).the peak of the indices during MSA is observed in the early part of the March equinox. There is a better agreement between R12 and F10.7 during MSA and LSA than during HSA. A peak was seen in each equinoxes as observed in the three different solar activity phases, the peak in the September equinox is higher than that of March equinox during HSA and LSA. However, the difference in the height is more pronounced during HSA. During MSA, the peak (plateau) in March equinox is higher than that of the September equinox. In addition, the trend of fof2 and solar activity indices is better in September equinox and December solstice, partly good in March equinox but seems to be out of phase in June solstice with the appearance of the hysteresis effect. From the present observations, it can be inferred that predicting fof2 based on the solar activity indices R12 and F10.7 is best in a year of MSA and during the September equinox. Other factors such as E B vertical drift, neutral composition, thermospheric wind and the Sun-Earth distance contribute more to the behaviour of the ionospheric parameter in other seasons. One can conclude that the ionospheric variations may be partly due to the influence of the solar and geomagnetic activities as well as other factors and not mainly due to geomagnetic activities and is in agreement with Forbes et al. 30 and Zhang et al. 31. However, the varying influence of these other factors and their quantitative analysis across each seasons could not be ascertained in the present study. Acknowledgement The authors are thankful to the Ionospheric station at Ouagadougou, Burkina Faso for providing the ionospheric data for this work. 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