Sudden Fmin Enhancements and Sudden Cosmic Noise Absorptions

J. Geomag. Geoelectr., 27, 95-112, 1975 Sudden fminEnhancements and Sudden Cosmic Noise Absorptions Associated with Solar X-Ray Flares Teruo SATo Physics Department, Hyogo College of Medicine, Mukogawa-cho, Nishinomiya, Hyogo, Japan (Received November 28, 1974; Revised May 23, 1975) Sudden fmin enhancements (SFmE's) and sudden cosmic noise absorptions (SCNA's) associated with increments of X-ray fluxes during solar flares are studied on the basis of X-ray flux data measured by SOLRAD 9 and 10 satellites. Some statistical analyses on SFmE's observed at five observatories in Japan, corresponding to increased X-ray fluxes in the 1-8 A band are made for 50 solar flare events during the period January 1972 to December 1973, and value of fmin is expressed as functions of cos x(x; solar zenith angle) and 1-8 A band X-ray flux. Similar study is also made for SCNA's observed by 30 MHz riometer at Hiraiso for 15 great solar flare events during the same period, together with 27.6 MHz riometer data reported by SCHWENTEK(1973) and 18 MHz data published by DESHPANDEand MITRA (1972b). It is found that fm, value (MHz) and SCNA value (L, dB) of a radio wave with frequency f(MHz) are related to X-ray flux (F0, ergcm-2 sec-1) in the 1-8A band and to cos x, by following approximate expressions, fmin(MHz)=l0Fo4cos1/2x, L(dB)=4.37X103f-2Fo2cosx, respectively. Blackout seems to occur for Fo values causing fmin's greater than about 5 MHz. It is shown that these expressions can be derived from a brief theoretical calculation of radio wave absorption in the lower ionosphere. Also it is suggested that threshold X-ray fluxes in the 1-8 A band which may produce a minimum SFME(2MHz), blackout and minimum SCNA (0.27-0.36dB for 30 MHz noise) are 1.6x10-3, 6.2x10-2 and (3-8)x10-3erg cm-2 sec-1, respectively, for cos x=1. 1. Introduction It has been well established that enhanced X-ray emissions associated with solar flares produce excess ionizations at various heights of the ionosphere and cause sudden ionospheric disturbances (SID's). Especially, X-ray emissions with wavelengths less than 10 A produce intense ionizations at levels below A about 100km (KREPLINet al., 1962; ALLEN, 1965) and they are responsible for 95 96 T. SATO sudden fmin enhancement (abbreviated as SFmE), or blackout (BO) in the vertical incidence sounding, as well as sudden fieldstrength anomaly (SFA), sudden enhancement of atmospherics (SEA), sudden phase anomaly (SPA) and sudden cosmic noise absorption (SCNA). In recent years X-ray emissions mainly with wavelengths less than 20 A have been monitored continuously by satellites, and using these data a number of authors have studied various X-ray flare effects in the lower ionosphere and propagation of radio waves. For example, KREPLIN et al. (1970) reported a relationship between flare X-ray and radio wave emission. SENGUPTA (1971) treated X-ray effects on VLF and HF wave propagations and electron concentration profiles. In a series of papers DESHPANDE et al. (1972a, b), DESHPANDE and MITRA (1972a, b, c) and MITRA and ROWE (1972) made an extensive analysis on various kinds of the SID effects including variations of wave field intensity, phase height, ionization profile and cosmic noise absorption. Recently, OHLE et al. (1974) pointed out that flare X-ray effects in the lower ionosphere depend strongly on season and solar zenith angle, and TAUBENHEIM et al. (1974) analysed phase-height variations in SFA effects. In spite of the comprehensive studies mentioned above, SFmE's during solar flares have been studied very little up to date. Also quantitative relationship between X-ray fluxes and SCNA values seems to be not yet established. The aim of this paper is to examine statistically fmin,and SCNA values corresponding to X-ray fluxes during solar flares and to try to derive expressions showing dependence of fmin and SCNA on X-ray flux and solar zenith angle. A brief theory is presented to endorse the two derived expressions. Also threshold X-ray fluxes which will produce, respectively, minimum fmin blackout and minimum SCNA are estimated. 2. Data Data on solar X-ray flux analysed in this work are those in the 1-8 A band for 50 solar X-ray flare events during the period January 1972 to December 1973 measured by SOLRAD 9 and 10 satellites which were reported in SOLAR- GEOPHYSICAL DATA (1972-1974). The corresponding fmin data are taken from f-plots at Wakkanai (45.4N, 141.7E), Akita (39.7N, 140.1E), Kokubunji (35.7N, 139.5E), Yamagawa (31.2N, 130.6E) and Okinawa (26.3N, 127.8E), published in IONOSPHERICDATA IN JAPAN (1972, 1973). Values of f minmeasured at observatories in other countries are not used because capa- bilities of instruments might be different from those in Japan. Performances of instruments used for ionospheric vertical sounding at Japanese observatories are approximately the same, therefore, in this study they are assumed to be all the same. Some details of them are shown in Table 1. Analysed data for SCNA fmin Enhancements and SCNA's during Solar X-ray Flares 97 Table 1. Typical performance specifications of instruments for ionospheric vertical sounding in Japanese observatories. events are 30 MHz riometer data for 15 large flares observed at Hiraiso (36.4N, 140.6E), Japan, which are available from the data in 1972 and 1973, and in addition to this, 27.6 MHz riometer data observed at Lindau (51.7N, 10.1 E) during the period July 28 to August 12 1972, reported by SCHWENTEK (1973), and 18 MHz data published by DESHPANDEand MITRA (1972b), which were recorded at Delhi (28.6N, 77.2E) and other stations during 1967-1969, are used for comparison with Japanese data. 3. Results 3.1 Sudden fmin enhancement (SFmE) 3.1.1 Statistical results Figure 1 shows some examples of fmin enhancements associated with increased solar X-ray fluxes (i.e. SFmE's) observed at Yamagawa in Japan and Rostov in USSR. The figure indicates a clear one-to-one correspondence between X-ray flux and fmin enhancement. Figure 2 demonstrates seasonal and local time distributions of occurrence number of distinct SFmE's (including blackout) such as are shown in Fig. 1, who have peak fmin values larger than 3 MHz for the 1-8 A band fluxes exceeding 1 X 10-2ergcm-2 sec-1, observed at five observatories in Japan during the period January 1972 to October 1974. (The X-ray data measured from SOLRAD satellites were presented until April 1974. After May fmin enhancements which are regarded obviously as flare- associated enhancements are counted as SFmEevents.) Each SFmEis numbered when fmin enhancements are observed simultaneously at least at three observatories. The peak fmin, 3 MHz, is tentatively determined to be the threshold value, 98 T. SATO Fig. 1. An example of correspondence between solar X-ray flux enhancements in the 0-3 and 1-8 A bands and sudden fmin enhancements (SFmE's) which were observed during the period April 30 to May 3 Values of fmin are taken from f -plots at Yamagawa (31.2N, 130.6E) and Rostov (47.2N, 39.7E). 1972 1973 1974 Fig. 2. Occurrence number of sudden fmin enhancements (SFmE's, above 3 MHz) for 1-8 A fluxes greater than 1 x 10-2erg cm-2 sec-1, observed at five observatories in Japan during the period January 1972 to October 1974. The enhancement is numbered when the event occurred simultaneously at least at three observatories. fmin Enhancements and SCNA's during Solar X-ray Flares 99 because normal f minexceeds usually 2 MHz in the daytime in summer. Thin lines in Fig. 2 represent that fmin is between 3 and 5 MHz, whereas thick lines represent that fmin.is greater than 5 MHz or blackout occurs. It is recognized from this figure that there is a tendency that SFmE's occur mainly in equinoxes and summer in a wide range of local time, whereas in winter they take place in a narrow time range centered on local noon. These results, which seem to in- dicate an seasonal and local time dependence of SFmE, is presumed from a viewpoint that SFmE's are caused due to excess absorption of radio waves pass- ing through flare-associated ionization layers, whose magnitudes and locations depend on both X-ray flux and cos x (x; solar zenith angle) (CHAPMAN,1931). Since solar flares occur irregularly in time, we cannot exclude a possibility that SFmE's in any other year occur more often in winter than in summer or equinoxes. But this possibility seems to be low because X-ray flares responsible for SFmE's in winter are comparatively great flares. In fact, SFmE's in winter i Fig. 2 occur for the 1-8 A fluxes exceeding about 1 X 10-1erg cm-2 sec-1, whereas other SFmE's occur for the fluxes larger than about 1 x 10-2erg cm-2 sec-1. The relation of SFmEoccurrence to cos x is also expected from results demonstrated by OBAYASHI (1970), showing a dependence of fmin ion cos x. In order to examine the relationship among fmin, X-ray flux (F0)and cos x, fmin values (fmin'S)for 30 great X-ray events during the period January 1972 to December 1973 are plotted in Fig. 3 against X-ray flux in the 1-8 A band and cos X value.

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