Temporal variations of solar EUV, UV, and 10,830‐Å radiations

Abstract

The temporal characteristics of the full-disk chromospheric EUV fluxes agree well with those of the ground-based'measurements of the chromospheric He I absorption line at 10,830 A and differ systematically from those of the coronal EUV and 10.7-cm flux. The ratio of the flux increase during the rise of solar cycle 21 to that during solar rotation variations is uniformly high for the chromospheric EUV and corroborating 10,830-/!/fluxes, highest for the transition region and'"cool" coronal EUV fluxes (T < 2 x 106 øK), and lowest for the "hot" coronal EUV and 10.7-em flux. The rise and decay rates of episodes of major activity progress from those for the hot coronal EUV lines and the 10.7-cm flux to slower values for the chromospheric H Lyman alpha line, 10,830-A line, and photospheric 2050-/!• UV flux. We suggest that active region remnants contribute significantly to the solar cycle increase and during the decay of episodes of major activity. The ratio of power in 13-day periodicity to that for 27 days is high (1/3) for the photospheric UV flux, medium (1/6) for the chromospheric EUV and 10,830-A fluxes, and small to negligible for the hot coronal EUV fluxes. These ratios are used to estimate the dependence of active region emission on the solar central meridian distance for chromospheric and coronal EUV fluxes. SOLAR UV AND EUV RADIATION The solar UV full-disk flux at 2050 A, measured by the NIMBUS 7 satellite, was shown by Donnelly et al. [1985] to vary in close agreement with ground-based measurements of the He I absorption line at 10,830/•. These variations included the following three time scales: (1) short-term variations with quasi-periodicity of about 13.5 or 27.5 days that are caused by solar rotation of active region plages, (2) intermediate-term enhancements of several months caused by episodes of major activity, and (3) long-term solar cycle variations. These UV variations were also shown to differ from those of the 10.7-cm solar radio flux (F10) during episodes of major activity and when the 13.5-day periodicity was strong in the UV flux [Donnelly et al., 1983]. This paper extends the research of UV and 10,830-• temporal characteristics by including comparisons with the solar extreme ultraviolet (EUV) flux measurements from the AE-E satellite. The remainder of this section introduces the data studied and our general knowledge of solar UV and EUV radiations, including their terrestrial importance, spectral differences, magnitudes of variations, and solar source regions. The next section describes the AE-E data studied. Then the temporal characteristics and comparisons of the EUV, UV, and 10,830-/• measurements for short-, intermediate-, and long-term variations are presented. Next, the implications of the 13-day periodicity on the dependence of active region radiation on the region's central meridian distance (CMD) are discussed for chromospheric and coronal EUV lines. Finally, the relation of our results to the temporal characteristics of ground-based measures of solar This paper is not subject to U.S...

Ionospheric effects of the solar flares as deduced from global GPS network data

“…Our results is consistent with the findings reported by (Donnelly, 1969), (Donnelly, 1971), (Donnelly, 1976), (Donnelly et al, 1986a), where a study of extreme UV (EUV) flashes of solar flares observed via SFD was made. In the cited references it was shown that the relative strength of impulsive EUV emission from flares decreases with increasing CMD and average peak frequency deviation is also significantly lower for SFD's associated with Hα flares at large CMD.…”

Evolutions of various solar indices around sunspot maximum and sunspot minimum years

“…Donnelly et al (1986) showed that the ratio of the amplitudes of the long-term variations to short-term variations was highest (about 2 to 3) for EUV (170-660Å), about 2 for chromospheric UV, EUV and 10 830Å He I, and less than 1.5 for F10, Rz and Ca-K Plage index. Several details about the 13-day and 27-day variations were presented by them.…”

“…The next major attempt to make a comparative study of the variations of different solar indices was by Donnelly et al (1986), using the AE-E data of 15 wavelength groups (170-1220Å, Hinteregger et al, 1981), the He I line 10 830Å, the F10 flux, and sunspots. Donnelly et al (1986) showed that the ratio of the amplitudes of the long-term variations to short-term variations was highest (about 2 to 3) for EUV (170-660Å), about 2 for chromospheric UV, EUV and 10 830Å He I, and less than 1.5 for F10, Rz and Ca-K Plage index.…”

Association between ionospheric maximum electron density and lower stratospheric temperature

“…Some authors using the sunspot number Rz or the 10.7-cm solar radio flux, F•o.7, as an indicator of solar activity variability, concluded that stratospheric temperature variations did not show a consistent relationship with the solar cycle variations [Mohanakumar, 1987]. Indeed, the solar spectral range mainly involved in stratospheric heating (200-300 nm), UV, has also temporal variations that differs from those of Rz or F•o.7 [Donnelly et al, 1986]. Labitzke and Van Loon [1994] [Van Loon and Labitzke, 1993] have analyzed the northern hemisphere stratospheric circulation.…”

“…The EUV flux is parameterized as a linear function of either Rz or F• o.v [Cook et al, 1980;Tobiska and Barth, 1990;Donnelly et al, 1986]. So that, the peak electron density of the ionospheric F 2 layer (at around 400 km height), N .... which is proportional to EUV flux [Hoegy and Mahajan, 1992 Table 1.…”