Picture of the Month

The energy from the solar interior is transported by convection to the solar surface. When observing the lowest layer of the solar atmosphere with a high-resolution solar telescope the granular structure of the hot up-flowing gas cells becomes recognizable. Recently, scientists from the Kiepenheuer Institut for Solar Physics have performed unprecedented spectroscopic observations with the Vacuum Tower Telescope (VTT) on Tenerife to systematically investigate the large-scale convective motion in the photosphere. The measurement of accurate absolute velocities was enabled by the scientific instrument LARS (Laser Absolute Reference Spectrograph) which employs a state-of-the-art Laser Frequency Comb as a calibration ruler for the solar spectrum.

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The solar 11-year cycle affects many of the observable values of the Sun. In the course of the solar cycle, the oscillation frequencies of the Sun and the amplitudes of these oscillations change marginally but measurably. Solar oscillations are standing sound waves inside the Sun. Similarly to sound waves here on Earth, the pressure within the medium is key to the way in these waves travel. This is why these oscillations are also referred to as p modes (p for pressure). Changes in the p-mode characteristics throughout the solar cycle are due to the varying strength of the magnetic field over time at different depths inside the Sun.

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The solar telescope GREGOR enables scientists to measure magnetic fields and flows on the Sun with unprecedented accuracy. The first results are now available.

At times of strong magnetic activity, the Sun offers spectacular sights of violent eruptions, evolving sunspots and strong magnetic fields. But there are also times at which it seems as though nothing is happening on the Sun surface with its almost regular pattern of grains referred to as granules. These ‘quiet’ regions also contain magnetic fields but they are very weak and therefore difficult to measure. Scientists at Kiepenheuer Institute for Solar Physics work with the GREGOR solar telescope, inaugurated in 2012, to analyse sunspots and their finely chased structure with an accuracy never seen before.

This and other first GREGOR results will be presented in nine articles of a special edition of Astronomy & Astrophysics.

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0.000 000 000 5 metre (= 0.5 nanometres = 0.5 nm), this tiny distance is how the two plates made of quartz glass fit together. Over an extension of 25 cm in diameter, the distance between the two plates varies by less than the distance of two neighbouring atoms in a silicon crystal. These two glass plates make up the Fabry-Perot interferometer of the VTF.

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The VTF (Visbile Tuneable Filter) is a two-dimensional high-resolution spectropolarimeter and is currently developed at the Kiepenheuer Institute. As one of the state-of-the-art scientific instruments at the future 4-m-class telescope DKIST on Hawaii it will play a major role in the next decade of solar observations.

The setup of the VTF consists of several Fabry-Pérot interferometers (FPI), a wavelength-dependend narrow pre-filter and a polarization modulator. Due to the requested accuracy of the physical measurements, an investigation of the instrumental impacts on the physical data acquisition was required in advance. Based on the inquired simulations, the requirements for manufacturing were inferred and strategies for the data calibration developed.

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LARS, the Laser Absolute Reference Spectrograph of the Kiepenheuer-Institute is a scientific instrument for the ultramodern observation of the Sun at the Vacuum Tower Telescope (VTT) at the Observatorio del Teide on Tenerife. LARS enables the measurement of the solar spectrum of a selected field-of-view with the high-resolution Echelle-spectrograph of the VTT. Additionally, the emission spectrum of the newly installed Laser Frequency Comb is superimposed with the solar spectrum. Since each emission peak of the comb spectrum represents one well-defined frequency, the solar spectrum can be calibrated on an absolute wavelength scale. Figuratively, the comb serves as a ruler for the spectral lines. The accuracy is of the order of m/s or below and is therefore a multiple better than former devices. The successful upgrade of the Laser Frequency Comb in May 2016 enables the continuous operation of this worldwide unique spectroscopic observation of the Sun.

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In a recently published work, KIS researchers and colleagues from MPS Göttingen present a new spherically geometric method for modelling the effect of flows on travel times of sound waves in the solar interior. This development is necessary for advancing our understanding of large-scale flows in the deep solar interior and thereby to gain insights on the origin of solar activity.

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The exact configuration of this magnetic field is important to understand the different features seen in sunspots. In the umbra, the dark center, the field is oriented perpendicular to the surface, while it is more horizontal in the penumbra, i.e. the surrounding region with narrow radially oriented filaments. Parts of the magnetic field continues into higher regions, while some of it reverses its polarity and returns back into the Sun. In the left panel of the figure we show observation with the Hinode satellite of the penumbra of a sunspot. Regions where the field returns below the surface are colored in red and blue. In the red regions, the polarity reversal is just above the surface, while it occurs in higher layers in the blue regions. The right panel show measurements with the new GREGOR Infrared Spectrograph (GRIS) at the Observatorio del Teide on Tenerife. The amount of red and blue regions in this observation is significantly less than the one found in Hinode data. The reason for this difference is subject of current studies at the KIS.

 

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On May 9th, Mercury will transit in front of the solar disk. To make this happen Earth has to be in a certain position in its orbit: The intersection points of the two planet’s orbits. They are called nodes. This happens twice a year: Around 8th of May and around 10th of November. In addition Mercury has to be in the corresponding position. This only happens every couple of years. In the 21st century there will be 14 Mercury transits: 5 in May and 9 in November. The last time it was in November 2006 and the next time it will happen in November 2019. Mercury’s angular diameter is too small to be observed by naked eye but the Kiepenheuer-Institute’s solar telescopes on Tenerife will observe the transit. It starts on 13:12 CEST and Mercury leaves the Sun at 20:40 CEST. During that time two kinds of observations will be performed: Mercury will be used as a target to determine optical properties of the telescope. Furthermore there will be measurements of the particle density of sodium close to Mercury’s surface: Various processes including the solar radiation releases a small amount of atoms (e.g. sodium) from the planet forming an extremely faint gas envelope (exosphere) and a tail....

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Today solar science is a diverse and rapidly evolving discipline. But the roots of solar science can be traced back to ancient times and beyond. Especially the observation of dark sunspots has a long tradition. While the oldest plausible record dates back to Chinese astronomers (800 BC), the first sunspot drawing was made in 1128 by the monk John of Worcester observing large sunspot formations visible to the naked eye.

At the beginning of the 17th century, several astronomical milestones were set to modernize the human understanding of the Sun. In 1609, Johannes Kepler expanded the heliocentric system to include elliptical orbits of the planets around the Sun. In the same year, the invention of the telescope became the starting shot in a new era of solar and stellar observations.

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