1. Monitoring program of selected Fraunhover lines over the 11-year cycle of solar activity
S. Osipov, R. Kostik, N. Shchukina
Main Astronomical Observatory (MAO), NAS, 03143, Kyiv, Ukraine
FIG.1. Coelostat and additional mirrows of the ACU-5 telescope.
MAO planet
AIMS: To date, there are several programs of long-term monitoring of Fraunhofer lines in the solar spectrum. Doyle et al. (2001) studied the changes of Mn I nm and Сa II H nm lines in the solar flux spectrum (i.e.; “the Sun as a star”) recorded during 1978−1993. Over 35 years Livingston et al. (2010 and references therein) observed chromospheric (Ca II H & K, CN 388.3, Ca I 854.2, He I 1083 nm) and photospheric (Fe I , C I , and Ti II nm) Fraunhofer lines, integrated over the full disk and (some of them) for a small circular region near the center of the solar disk. Since 2006 SOLIS (Synoptic Optical Long-term Investigations of the Sun) provides autonomous full-disk spectral, magnetic, and imaging measurements to help understanding solar activity and its effect on the Earth's climate and atmosphere ( Nevertheless, the Sun as a star monitoring of spectral lines reflects variations of solar activity and makes it impossible to study separately the cycle variations of the quiet Sun. For this reason, it is still unknown how the physical parameters of the quiet solar atmosphere change over the 11-year cycle of the solar activity. Our long-term monitoring program of selected Fraunhofer lines aims to clarify this question.
OBSERVATIONS: Our research is based on high (R = 330,000) spectral resolution observations of the quiet Sun using the horizontal solar telescope ACU-5 of the Main Astronomical Observatory of the National Academy of Sciences of Ukraine (FIG.1, 2). The diagnostics of the solar cycle variation of the quiet solar atmosphere includes observations of the H alpha, Ca II H & K and more than 40 spectral lines of neutral and ionized chemical elements (C I, Ca I, Cr I, Mn I, Fe I, Fe II, Ti II) in nine spectral regions from 393 nm to 657 nm (393.0–393.7, 524.4–525.2, 532.0–532.8, 537.6–538.5, 539.0–539.8, 558.4–559.2, 624.9–625.7, 632.6–633.3, 655.3–657.3 nm). The formation heights of these lines cover the large portion of the photosphere, the temperature minimum and the lower chromosphere. Since we perform observations at three positions on the solar disk (center, North and South poles) with the entrance slit height of 2 arc min. The exposure time is in the range of 1−2.5 s. The observations are performed daily, when the weather conditions allow.
The daily observing program consists in using the spectrograph in one pass mode. Compared to the double pass mode, such a scheme degrades the instrumental profile, but makes it possible to measure line bisectors with higher precision (FIG 3). In addition, it enables to make faster observations reducing seeing effects. We developed the methods to account for the scattered light, instrumental profile, turbulence inside the telescope, etc. This allows to record the Fraunhofer lines on long time scales with a high precision metrological stability.
Observational season lasts from March to October. In total we have 284 days of observations between 2012 and 2016.
ACU-5
ACU-5 HORIZONTAL SOLAR TELESCOPE was set into operation in 1966 (Kostik & Shchukina, 1997). In 2011 we performed its reconstruction. The main telescope characteristics are: a 440 mm coelostat and additional mirrors, a 440-mm main mirror with a focal length of 17.5 m. A focal length of the spectrograph collimator and camera mirrors are 7 m. Currently, the telescope is equipped with a CCD camera SBIG ST-8300M. Its array (17.96 х mm) has 3326 х 2504 pixels at 5.4 microns. The grating has a ruled area of 140x150 mm with 600 lines per mm. The instrumental profile at nm obtained with a helium-neon laser in the fourth order has a half width of 18−20 mÅ (Osipov, 2015).
FIG. 3. Bisector noise (red lines) and bisector (green or black lines) of the Fe I nm line. Top: observations at ACU-5. Middle: Liège atlas. Bottom: FTS atlas.
FIG. 2. Main mirrow (left) and spectrograph (right) of ACU-5. D
FWHM EW
FIG. 4. Variations of the line core depth (D, left), full width at half maximum (FWHM, middle) and equivalent width (EW, right) of the three Fe I lines obtained at ACU-5 telescope over observing seasons of
FIG. 5. Same as in FIG.4 but for the bisector deflection of the two Fe I lines.
RESULTS: FIGURES 4 and 5 illustrate the first results of our observations. FIGURE 4 shows variations with time of the line core depth (D), full width at half maximum (FWHM) and the equivalent width (EW) of a weak (λ nm), moderate (λ nm), and strong (λ nm) Fe I lines obtained for the quiet regions of the solar disk center during observations at the ACU-5 telescope. Time variations of the bisector deflection of the last two lines are shown in FIG.5. According to Gurtovenko & Kostik (1989) the formation region of the weak line extends from the bottom of the photosphere (the wings) to a height HD =178 km (the line core). The moderate line is formed in the layer between the lower and middle photosphere (HD =315 km). The formation region of the strong Fe I λ nm line covers the whole photosphere. Its core intensity comes from the temperature minimum (Shchukina & Trujillo Bueno, 2001). Blue circles shown in the FIG. 4 and 5 represent values averaged over the observing seasons of , while the vertical lines are 95% confidence intervals for random errors. These figures demonstrate that the line parameter variations are within a few tenths of a percent. The amplitudes of the FWHM and equivalent width variations are below 1 mÅ. Changes of the bisector deflection are below 10 m/s.
We compared these variations with the total unsigned magnetic field of the Sun measured by Wilcox Solar Observatory over the years 2012−2016 (see FIG.6). The first two red points shown in FIG. 6 are on the ascending stage of the 24th cycle of solar activity. Next two red points present two peaks of the activity observed in 2014 and The last point is on the descending part of the cycle. Our measurements reveal that the line core depths (FIG.4, left panel), FWHM (Fig.4, middle panel) and the bisector deflection (i.e. efficiency of convection, FIG.5) of all considered Fe I lines track the variations of the total unsigned magnetic flux. Interestingly, their line depths increase with the total unsigned magnetic flux, while their full widths FWHM become narrower as we approach the peak of the solar activity. FIGURE 5 (right panel) shows that the equivalent widths of these lines are not correlated with the solar activity cycle. The behavior of these parameters can be explained by assuming that during the maximum of the solar activity deep layers of the quiet photosphere become slightly hotter. Consequently, the line wing and local continuum intensities grow and the Fe I lines become deeper and narrower.
FIG.6. The total unsigned magnetic field of the Sun in Gauss as measured by Wilcox Solar Observatory over Blue circles indicate the value of the magnetic field for observing days at the ACU-5. Red circles are averaged over each period.
REFERENCES
Doyle J. G. et al. 2001, A&A, 369, L13
Livingston W. et al. 2010, MmSAI,
Gurtovenko, E. A. & Kostik, R. I. 1989, Fraunhofer Spectrum and the System of Solar Oscillator Strengths (Russian language ed.; Kiev: Naukova Dumka)
Kostik, R.I. & Shchukina, N.G. 1997, in ASP Conference Series, Advances in Physics of Sunspots, 118, ed. B. Schmieder, J.C. del Toro Iniesta, M. Vazquéz, 372
Osipov, S. N. 2015, Kinemat. Phys. Celest. Bodies, 31, 261
Shchukina, N. & Trujillo Bueno, J. 2001, ApJ, 550, 970