Sunspots, solar radio noise, solar EUV and ionosphericfoF2

R. P. KANE

lnstituto National de Pesquisas Espaciais, INPE C.P. 515, Sao Jose dos Campos. SP. Brazil

(Recriwd infinal,fimn 27 May 199 1)

Abstract-Plots of monthly median noon time fiiF2 values at Manila, Kodaikanal and Vanimo w solar EUV tlux (170 190 A), smoothed sunspot number R,, and smoothed solar radio noise (10.7 cm) flux indicate saturation effects in the.fbF2 vs sunspot plots. The saturation diminishes in the,fijF2 vs solar radio noise plots and disappears for foF2 vs solar EUV plots. Thus, in the absence of EUV data, solar radio noise could be a reasonably good substitute, better than sunspots, for predictingJbF2 values on time scales of one month, or more.

I. INTRODUCTION

Ionospheric electron densities are produced mainly by

solar EUV and X-rays, which are known to have large solar cycle variations. In the absence of direct measurements of EUV, the long continuous data of sunspot numbers arc often used. In recent years, the solar radio noise flux at 10.7 cm (2800 MHz) has also

been used. For low values of the sunspot numbers, foF2 shows a good linear relationship but, at high

sunspot numbers, .fi,F2 seems to show suturation

effects. Also, for the same sunspot number&OF2 may show different values during the rise and fall of the solar cycle (GOPALA RAO and SAMBASIVA RAO, 1969). This ‘hysteresis’ effect is small at low and high lati- tudes but substantial at middle latitudes, and is attri- buted to possible geomagnetic storm effects.

In most of these studies, sunspots and the solar radio noise F(10.7) are bracketed in the same

category, so much so that JOACHIM (1966) established a formula relating F(10.7) to sunspots. From his study of correlations between thermospheric density and

temperature and solar EUV and solar radio noise, HEDIN (1984) reported that correlations were better

with EUV than with 10.7cm flux, when short period (days) variations were considered. Recently, LAKSHMI et ul. (1988) studied the relationship between the smoothed sunspot numbers (R,?), solar EUV (170- 190 A) and ionospheric JbF2 and showed that the

saturation effects found when ,foFZ was correlated with sunspots almost disappeared whenfoF2 was cor- related with solar EUV. The F(10.7) was not used in this study, but it was mentioned as equivalent to sunspots rather than to solar EUV. It is true that, in a gross way, sunspots and solar radio noise show similar solar cycle variations. However, on a finer scale. there are considerable differences. For example.

in solar cycle 21, the smoothed Zurich sunspot num- ber peaked earlier than F(10.7). A part of this differ- ence could be because of the fact that the Zurich sunspot number places IO times more weight on the number of groups of sunspots than on the number of spots (WILSON rt al., 1987). The solar radio noise

F( 10.7) seems to have variations somewhere between the variations of sunspots and variations of solar UV and EUV. Also solar emissions in different wavc- lengths reach their maximum at different times (DON-

NELLY et al., 1986). In this note we examine the data used by LAKSHMI et al. (1988), extending their study to F(10.7).

2. RESUI,TS

Figure 1 shows a plot of equinox monthly median

noonfbF2 values vs EUV, vs smoothed sunspot R,?

and vs smoothed F(10.7). The first two columns are the same as shown in Fig. I of LAKSHMI et cd. (I 988) for solar EUV and sunspots. The third column is our addition for F(10.7) solar radio noise. In the middle column. the saturation effect in ,fbF2, when plotted against sunspots, is seen for Manila and Vanimo. though not for Kodaikanal. In the first column for plots of.foF2 against solar EUV. the saturation effect is not seen. In the third column for plots of ,fbF2

against F(l0.7), the saturation effects are still them but much less as compared with the middle column. Thus, F(10.7) seems to have an effect in between the effect of sunspots (saturation) and the effect of solar EUV (no saturation).

Figure 2 shows a similar plot but for the summer

months (May, June, July, August). For Manila and Vanimo, the saturation effect is absent for EUV, large for sunspots and in between for solar radio noise.

463

464 R. P. KANE

EQUINOXES (1977 - 1980)

VANIMO (GEOG. LA; 27OS)

6~~~’ _i;J;::_~j

. .

43 + t i

d~_~h,_..j 6

40 80 IM 160 200 80 120 160 200

EUV FLUX (170-1908) SUNSPOT NO. (R,,) SOLAR FLUX (IO.7 CM )

Fig. 1. Plots of monthly median noontime.~~~2 vs EUV ilux (17&190 A), smoothed sunspot number Rt2

and smoothed solar radio noise (10.7cm) flux, for three low latitude stations, namely Manila (14.7 N. 121.1 ), Kodaikanal (10,2’N, 77.5”E) and Vanimo (2.7”S, 141.3-E), for equinoxes during 1977 1980.

The scatter is rather large. Part of the scatter could be due to the fact that the fnF2 is a function of solar zenith angle and, even in the same season, the zenith

angles change from month to month. Also, for noon data, transport phenomena are very important for low latitudes and these have seasonal variations. For prediction purposes, sunspots are generally used as the key parameter (REIIDY et GE., 1979). Since the non- linear term is smatier for (10.7)cm flux, it may be interesting to check whether predictions are improved by using the (10.7) cm flux instead of sunspots.

3. CONCLUSIONS AND DISCUSSION

From this study, it seems that, whereas on the time scale of a month (monthly median noon values), the fi7F2 shows saturation effects when plotted against sunspots, the saturation effects are diminished when plots are made against solar radio noise flux F(10.7) and disappear when plots are made against solar EUV. Thus, solar radio noise may be a reasonably good approxjmation for solar EUV for periods of one

month or more. For shorter periods. the F( 10.7) flux may not be an adequate substitute for solar EUV (DONNELLY er ul., 1986).

Solar EUV radiation in the range IO@- 1026 A is the primary cause of ionisation of the ionospheric E- and F-regions and of thermospheric heating. The H- Lyman alpha (1216,4) is a major source of ion pro- duction in the D-region. Radiation in the range 1026 1800 A produces atomic oxygen in the thermosphere and upper mesosphere, while solar UV in the range 1800-2900 8, produces ozone and heats up the strato- sphere. Solar UV is produced in the upper photo- sphere, in the region of photosphere-chromosphere temperature minimum and in the chromosphere. The

quiet-Sun soiar UV flux is quite large. On the other hand, solar EUV originates in the chromosphere, the chromosphere-corona transition region and the solar corona and its solar cycle changes are quite large and the amplitudes are different for the chromospheric and coronal emissions. The F( 10.7) has contributions from all the three solar regions. The large values (6s 70 units) even at. low sunspot activity come from the

Sunspots and ionospheric foF2

SUMMER ( 1977- 19801

6 ““““’ IIIIIII, ,,,I,/,, / 6 IO 14 18 22 0 40 80 I20 160 200 80 120 I60 200

6

EUV FLUX (170-190%) SUNSPOT NO. CR,*) SOLAR FLUX ( 10.7 CM)

Fig. 2. Plots of monthly median noontime,fbF2 vs EUV flux (170-190A). smoothed sunspot number K,, and smoothed solar radio noise (10.7cm) flux, for summer months of 1977 19X0.

465

chromospherc while the time-varying part comes from

the corona. The best measurements of the solar EUV made so far were the AE-E measurements of HIN- TI-~REGGER et N/. (1973), which ended shortly after 1980. However, measurements of solar X-rays and solar UV are being made with a series of satellites (WAGNER. 1988 ; HEATH and SCHLESINGER, 1986). For a limited period from November 1978 to December 1980. simultaneous data were available for EUV and UV, and DONNELLY et ul. (1986) have reported results of a comparative study. From these, as also from

further studies, DONNELLY (1989, 1990) concludes that F( 10.7) behaves like chromospheric emissions for

the long-term solar cycle and like the hot coronal

emissions for the short-term variations. For the USC of F(10.7) as a solar input, ionospheric and ther-

mospheric modellers (e.g. JACCHIA, 198 1) have found that using both the daily values and the RI-day run- ning averages simultaneously is more beneficial. All in all, in the absence of solar EUV and UV data. F(10.7) may be better than sunspots when making ionospheric predictions.

Acknowledgements-This work was partially supported by FNDCT Brazil under contrxt FINEP 537:CT.

DONNELLY R. F.

DONYELLY R. F.

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