Solar Epoch Research Articles

The validity of George's (1971) method for reducing Al absorption data

George's study of the global morphology of the quantity ∝ Nv · dh in the D- and E-regions of the ionosphere was criticised by Dyson and Bennett on the grounds that his data reduction technique was invalid at low latitudes. This paper refutes such a criticism by demonstrating that George's Φ function is invariant with respect to latitude. The Φ function is found to constitute a powerful transform for the reduction of Al absorption data obtained at different frequencies, locations, seasons and solar epochs.

Seasonal and solar cycle variations of equatorial spread- F in the American zone

The range type of spread- F near the magnetic equator seems to be associated with the post sunset uplifting of the F-layer which itself is caused by the strong eastward horizontal electric field in the evening before its reversal to the normal nighttime westward direction. The frequency type of spread- F seems to be the latter development of the F-region irregularities. During any solar epoch, the occurrence of range spread is maximum diuranally at 2100 LT and seasonally around November and February months, a secondary peak develops around 0500 during June solstices. The occurrence of frequency spread is maximum around 0000 LT diurnally and seasonally around June-July. The solar cycle effect of range spread is quite significant raising the rate and peak value of post sunset increase of its occurrence; however, the occurrence decreases very rapidly with time after 2100 LT during high sunspot years. Further, the range spread occurs fairly frequently during the post midnight hours of the low sunspot years especially for June solstices. Thus the relation between sunspots and range spread is positive for pre-midnight hours and strongly negative for post midnight hours giving an inverse relation between sunspots and whole night average range spread.

Collision frequency and transport properties of electrons in the ionosphere

A unified ionospheric electron collision frequency model profile 〈 ν g 〉 has been calculated in the height range 50–500 km. The computed profile accounts for the electron collisions with the neutral particles as well as the ions. Experimental values of momentum transfer cross-sections have been used for electron collisions with N 2, O 2 and Ar and theoretical values for N, O, He and H. It is observed that the electron-ion collisions 〈 ν ei 〉 dominate over the electron-neutral collisions 〈 ν en 〉 above 170 km. However, 〈 ν e− N 〉 is of the same order of magnitude as 〈 ν e− O 〉 in the height range 170–210 km. Above 360 km 〈 ν e− O 〉 becomes more important among the neutrals. The temporal, seasonal and solar epoch variations of 〈 ν ei 〉 are also shown. A typical electron collision frequency profile 〈 ν g 〉 corresponding to the exospheric temperature of 1100 K has been compared with the available experimental results for D, E and F-regions obtained from different experimental techniques. This profile has been used to determine the electrical, thermal, heat flow and current flow conductivities, the mobility and the diffusivity of electrons. The results so obtained are found to be in good agreement with the earlier results.

Range and frequency spreadF at Huancayo

The mean occurrence of frequency type of spreadF at Huancayo has been shown to have practically no solar cycle dependence. The occurrence of range type of spreadF is shown to be inversely related to sunspot number. The range spread occurs mostly around 2100 LT for any of the solar epoch, while frequency spread has maximum occurrence between 2300 and 0000 LT in high sunspot years and between 0000 and 0100 LT in low sunspot years. The seasonal variation in the occurrence of either type of spreadF shows minimum in northern solstices (June months) and maximum occurrence in southern solstices (December months). The post-sunset rise ofF layer is most predominant during high sunspot years. These results point out the inadequacy of the theory of spreadF based entirely on the post-sunset upward rise of theF region after sunset.

Ionospheric total electron content and slab-thickness at low latitudes in Indian zone

Observations of Faraday rotation of beacon signals from low orbiting satellite BE-B recorded at one station near the dip equator (Kodaikanal, dip 3·4° N) and at another station near the peak of the equatorial anomaly (Ahmedabad, dip 34° N) give a complete coverage of the equatorial anomaly belt in Indian zone. Contours of total elctron content (TEC) are obtained on a grid of latitudeversus local time for the different seasons of low (1964–66) and high (1967–69) solar activity epochs in the latitude belt 10° S to 26° N dip latitude. The development of the equatorial anomaly and its dependence on season and solar activity are discussed. Using similar contours of F2 layer critical frequency, f0F2 contours of equivalent slab-thickness, τ are also constructed. The dependence of τ on season and solar activity and its implications on temperature are discussed.

A study of the ionospheric electron content at Ibadan, 1965–1968

Results of the analysis of Faraday rotation of signals from the BE-B (S66) satellite are presented. These results indicate a ‘bite-out’ in the average diurnal cycle of the electron content which was not found in a previous study. Linear relationships for solar epoch control are established for the first time for Ibadan data and compared with results from other zones. Whilst the Ibadan data show a general agreement with other results in the correlation with solar flux index, S, they differ from other results in the correlation with the Zurich sunspot number index, R Z .

Further investigations of the effects of neutral-air winds on the ionospheric F-layer

Ionospheric data obtained at middle latitudes during summer are compared with values of ƒ 0F2 calculated by solving simultaneously the F-layer continuity equation and the equation of motion of the neutral atmosphere. The ‘evening enhancements’ and ‘midnight maxima’ of ƒ 0F2 which occur over certain regions of the Earth in summer are shown to be caused almost entirely by neutral-air winds. These winds produce very different ionospheric effects at similar latitudes in the northern and southern hemispheres. Certain day-to-day changes in the diurnal behaviour of the ionosphere at sunspot minimum could be explained if the phase of the wind differs from day to day; such changes in the phase of the wind need not necessarily be accompanied by corresponding changes in the time at which the atmospheric ‘diurnal bulge’ occurs. Diurnal variations of ƒ 0F2 having different shapes are observed at different solar epochs and these can be reproduced theoretically provided that the effects of neutral-air winds are included in the calculations.

Lunar tidal oscillations in horizontal magnetic intensity at Kodaikanal during periods of low and high sunspots

The paper describes the lunar daily (L) variations at fixed lunar ages and the lunar monthly (M) variations at fixed solar hours in horizontal magnetic intensity (.H) at Kodaikanal for the low sunspot period, Jan, 1951 to Dec, 1955; and for the high sunspot period Jan. 1956 to Dec, 1960. The lunar daily variations at any of the seasons or solar activity epochs are found to follow Chapman's phase law: L=Cn sin [n~+(.n-2)v+an]. With the increase of solar activity the phase of Ls wave remains constant for each of the seasons, but the amplitude increases during D. and E. months and slightly decreases during the months, The lunar semi monthly (.M2) waves at fixed solar hours vary with the solar time in the same way as the electrojet current, i.e., the amplitude starts increasing with sunrise reaches a maximum near noon and decreases to a low value by sunset. The ratio of lunar semidiurnal (LB} wave to the solar semidiurnal (82) wave for any of the seasons decreases with solar activity. The amplitudes of LB or M2 wave at Kodaikanal are much smaller than the corresponding values at Huancayo indicating the longitudinal variation in the lunar daytime effects in H along the magnetic equator. The lunar semimonthly tides for the daytime hours are predominantly under the control of lunar time during the D. months and of lunar age during the J .months.

Decay rate of recovery phase of Geomagnetic Storms And dissipation of associated Ring Currents

Decay rates of the recovery phases of some 130 geomagnetic storms recorded at Alibag during the period 1924 to 1959, classified as great, are investigated with a view to ascertain (1) the precise relation between amplitude of depression of the main phase and the rate of decay of the recovery, phase of storm, (2) the possibility of more than one ring current being associated with each storm and (3) the solar-cycle variation in the decay rate of the recovery phase. Decay times are determined for the first 36 hours of the recovery phase by fitting an exponential curve and finding the time constant for the amplitude of the fitted curve to decay to l/e of the initial amplitude. As a second method, free-hand smooth curves are drawn best fitting the 36-hourly depressions and picking up the time for the initial amplitudes, given by the fitted curve, to decay to 1/2 its initial value. The recovery phase appears to have two distinct portions, an initial portion of less than 12 hours with a rapid decay rate and a later portion with a slower decay rate, suggesting the possibility of two ring currents being associated with each storm. Decay rates for the two portions of the recovery phase are separately investigated by the method of free-hand smooth curves. The generally known characteristic that decay rates are faster for the larger intensity storms is seen, but considerable scatter in decay rates is observed for any storm-intensity group. The scatter is less for decay rates of the initial portion 01 the recovery phase. There is a clear tendency, especially for storms of more than 200y (main-phase depression, for decay rates to be faster for the solar minimum epoch than for the maximum epoch. But the relative rates are far from the factor 3 expected by Dessler et al. This factor is, however, approached if extreme values of recovery rates of the initial portion of the recovery phase only are considered. For the later portions of the recovery phase no significant differences in decay rates are observed for the two solar epochs. Currents of the order of 101 amperes are associated with the inner ring current, which is considered large enough to produce shielding effects on the outer ring current field. The main phase depression appears, therefore, to be largely the effect of the inner ring current, especially for the large intensity storms.

CRITERIA FOR THE ASSOCIATION OF SOLAR FLARES WITH GEOMAGNETIC DISTURBANCES

The statistical relation between solar flares of importance [Formula: see text] and the onset of geomagnetic disturbances was studied for 1949–61. Disturbances were defined as an increase in ap[Formula: see text], occurring within 12 hours or less, and these disturbances include both gradual and sudden commencement events. Associated radio noise bursts at frequencies below 300 Mc were used to select 240 major-burst flares, and the effects of flare importance, time of solar epoch, and associated polar-cap-absorption (PCA) events were considered in turn. A detailed examination was made of the effect of flare heliographic location. There is a strong positive correlation between the occurrence of major-burst flares and geomagnetic disturbances 1 to 3 days afterwards. The degree of correlation is approximately doubled when the major-burst flare is associated with a PCA event. The most probable time delay between flares and disturbances is 1.5 to 2.0 days, and this timing is relatively constant over a wide range of conditions.The dominance of northern storm-flares (Bell 1961) was confirmed for 1949–61. Detailed analysis shows that the effect is strongest for heliographic latitudes above 15 °N. In addition to this effect, there was a consistent western bias of the storm-flares in both the northern and southern hemispheres, for flares associated with either gradual or sudden commencement (SC) events, with maximum concentration occurring at 10 °W. to 15 °W. Major-burst flares associated with PCA events also show this western bias; the data suggest that there is a systematic increase in east–west asymmetry in going from major-burst storm-flares to major-burst PCA flares to PCA flares accompanied by ground-level cosmic-ray increases. The corresponding longitudes of maximum concentration of the associated flares are about 10 °– 15 °W., 45 °W., and 75 °W.

A Note on the Changes of the Maximum of Ionization of the F1 Layer Associated with the Sunspot Activity

AbstractThe ionospheric F1 layer, whose behaviour conforms to the standard theory during low and moderate solar activity, shows considerable departure from it when the solar activity is very high. The simple relationship which has been shown to exist between foF1 and the mean Zurich sunspot number (Ratcliffe et al., 1956) no longer holds at very high sunspot numbers that were observed during the current solar epoch. Also, foF1 shows anomalous behaviour with respect to season and geographic latitude, similar to that of the F2 layer, at very high solar activity.

The absorption of short radio waves in the ionosphere

It is shown that the reflection coefficients (ρ) measured for radio waves on 2 and 4 Mc sec −1 reflected vertically from the ionosphere do not fit the expression −log ρ = A(ƒ + ƒ L) −2 (1) appropriate to “nondeviative” absorption. Since the two frequencies mentioned did not approach near to critical frequencies at the time of the measurements, this failure cannot be ascribed to excessive “deviative” absorption. If the measured values are inserted into the expression −log ρ = C + B(ƒ + ƒ L) −2 (2) it is found that 1. (a) C varies with the sun's zenith angle but not with solar epoch. 2. (b) B is a very variable quantity, it depends on the solar epoch and is greatest in winter. With values of B and C deduced from measurements on 2 and 4 Mc sec −1, equation (2) gives a reasonable approximation to the measured values of ρ on 2.4, 3.2, and 4.8 Mc sec −1. It is suggested that waves in the frequency range 2 to 4.8 Mc sec −1 are appreciably absorbed outside the nondeviating region, where equation (1) is appropriate.

Atmospheric tides in the ionosphere. II. Lunar tidal variations in the F region near the magnetic equator

Three years’ data for the heights and critical frequencies of the F 1 and F 2 regions at Huancayo, Peru, are examined for lunar tides. Semi-diurnal lunar tides are found in all quantities save f 0 F 1 . It is found that the lunar variation in the F 2 region depends on solar time in both phase and amplitude. At certain solar epochs it attains very large amplitudes, up to 60 km. in h max. F 2 and 28% in f 0 F 2 . The theory of these variations, originated in a previous paper, is further developed.

On the variability of the quiet-day diurnal magnetic variation at Eskdalemuir and Greenwich

§ 1. The solar diurnal magnetic variation on quiet days (conveniently denoted by S q ) undergoes more or less regular changes of two kinds: one, affecting both type and amplitude, in the course of each year—an annual variation; the other, affecting chiefly the amplitude, in the course of the sunspot cycle—this will be termed the solar-cyclic change. In addition, it undergoes irregular or fortuitous changes from day to day. These have scarcely been studied as yet, and there is no precise systematic information available concerning them. They are of considerable interest owing to their probable connection with day-to-day changes in the sun’s ultra-violet radiation. We therefore propose to examine them systematically, especially with reference to their similarity at different stations. In this paper we confine ourselves to the records for Eskdalemuir and Greenwich, the respective geographical co-ordinates being:— Latitudr. Longitude. ˚ ' ˚ ' Eskdalemuir . . . . . . . . . . . . 55 19 N 3 12W Greenwich . . . . . . . . . . . . . 51 28 N 0 0 The material used consists of the daily ranges, in all three magnetic elements, on 819 very quiet days. It is shown that the ranges suffice for the comparison of different days, since (§ 15) the irregular changes in S q , at any particular season and solar epoch, affect almost solely the amplitude of the daily variation, while the type, that is, the form of the curves representing the diurnal inequalities, remains constant.