Search for star-planet interaction

We analyse the chromospherical activity of stars with extrasolar planets and search for a possible correlation between the equivalent width of the core of Ca II K line and orbital parameters of the planet. We found a statistically significant evidence that the equivalent width of the Ca II K line reversal, which originates in the stellar chromosphere depends on the orbital period P_orb of the exoplanet. Planets orbiting stars with T_eff < 5500 K and with P_orb < 20 days generally have much stronger emission than planets at similar temperatures but at longer orbital periods. P_orb=20 days marks a sudden change in behaviour, which might be associated with a qualitative change in the star-planet interaction.Comment: 2 pages, 2 figures, to appear in the proceedings of IAU Symposium 282: "From Interacting Binaries to Exoplanets:Essential Modeling Tools

Dark Off-limb Gap:Manifestation of a Temperature Minimum and the Dynamic Nature of the Chromosphere

We study off-limb emission of the lower solar atmosphere using high-resolution imaging spectroscopy in the H$\beta$ and Ca II 8542 \r{A} lines obtained with the CHROMospheric Imaging Spectrometer (CHROMIS) and the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1-m Solar Telescope. The H$\beta$ line wing images show the dark intensity gap between the photospheric limb and chromosphere which is absent in the Ca II images. We calculate synthetic spectra of the off-limb emissions with the RH code in the one-dimension spherical geometry and find good agreement with the observations. The analysis of synthetic line profiles shows that the gap in the H$\beta$ line wing images maps the temperature minimum region between the photosphere and chromosphere due to the well known opacity and emissivity gap of Balmer lines in this layer. However, observed gap is detected farther from the line core in the outer line wing positions than in the synthetic profiles. We found that an increased microturbulence in the model chromosphere is needed to reproduce the dark gap in the outer line wing, suggesting that observed H$\beta$ gap is the manifestation of the temperature minimum and the dynamic nature of the solar chromosphere. The temperature minimum produces a small enhancement in synthetic Ca II line-wing intensities. Observed off-limb Ca II line-wing emissions show similar enhancement below temperature minimum layer near the edge of the photospheric limb.Comment: 14 pages, 8 figures, accepted in Ap

Flare-induced changes of the photospheric magnetic field in a δ\delta-spot deduced from ground-based observations

Aims: Changes of the magnetic field and the line-of-sight velocities in the photosphere are being reported for an M-class flare that originated at a $\delta$-spot belonging to active region NOAA 11865. Methods: High-resolution ground-based near-infrared spectropolarimetric observations were acquired simultaneously in two photospheric spectral lines, Fe I 10783 \AA\ and Si I 10786 \AA, with the Tenerife Infrared Polarimeter at the Vacuum Tower Telescope (VTT) in Tenerife on 2013 October 15. The observations covered several stages of the M-class flare. Inversions of the full-Stokes vector of both lines were carried out and the results were put into context using (extreme)-ultraviolet filtergrams from the Solar Dynamics Observatory (SDO). Results: The active region showed high flaring activity during the whole observing period. After the M-class flare, the longitudinal magnetic field did not show significant changes along the polarity inversion line (PIL). However, an enhancement of the transverse magnetic field of approximately 550 G was found that bridges the PIL and connects umbrae of opposite polarities in the $\delta$-spot. At the same time, a newly formed system of loops appeared co-spatially in the corona as seen in 171 \AA\ filtergrams of the Atmospheric Imaging Assembly (AIA) on board SDO. However, we cannot exclude that the magnetic connection between the umbrae already existed in the upper atmosphere before the M-class flare and became visible only later when it was filled with hot plasma. The photospheric Doppler velocities show a persistent upflow pattern along the PIL without significant changes due to the flare. Conclusions: The increase of the transverse component of the magnetic field after the flare together with the newly formed loop system in the corona support recent predictions of flare models and flare observations.Comment: 8 pages, 9 figures. Accepted for publication in Astronomy & Astrophysic

Spectral Characteristics and Formation Height of Off-limb Flare Ribbons

Flare ribbons are bright manifestations of flare energy dissipation in the lower solar atmosphere. For the first time, we report on high-resolution imaging spectroscopy observations of flare ribbons situated off-limb in the H$\beta$ and Ca II 8542 {\AA} lines and make a detailed comparison with radiative hydrodynamic simulations. Observations of the X8.2-class solar flare SOL2017-09-10T16:06 UT obtained with the Swedish Solar Telescope reveal bright horizontal emission layers in H$\beta$ line wing images located near the footpoints of the flare loops. The apparent separation between the ribbon observed in the H$\beta$ wing and the nominal photospheric limb is about 300 - 500 km. The Ca II 8542 {\AA} line wing images show much fainter ribbon emissions located right on the edge of the limb, without clear separation from the limb. RADYN models are used to investigate synthetic spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. The simulations show that, towards the limb, where the line of sight is substantially oblique with respect to the vertical direction, the flaring atmosphere model reproduces the high contrast of the off-limb H$\beta$ ribbons and their significant elevation above the photosphere. The ribbons in the Ca II 8542 {\AA} line wing images are located deeper in the lower solar atmosphere with a lower contrast. A comparison of the height deposition of electron beam energy and the intensity contribution function shows that the H$\beta$ line wing intensities can be an useful tracer of flare energy deposition in the lower solar atmosphereComment: 18 pages, 10 figures, accepted in Ap

Signatures of Helium Continuum in Cool Flare Loops Observed by SDO/AIA

We present an analysis of off-limb cool flare loops observed by SDO/AIA during the gradual phase of SOL2017-09-10T16:06 X8.2-class flare. In the EUV channels starting from the 335 {\AA} one, cool loops appear as dark structures against the bright loop arcade. These dark structures were precisely coaligned (spatially and temporally) with loops observed by SST in emission lines of hydrogen and ionized calcium. Recently published semi-empirical model of cool loops based on SST observations serves us to predict the level of hydrogen and helium recombination continua. The continua were synthesized using an approximate non-LTE approach and theoretical spectra were then transformed to AIA signals. Comparison with signals detected inside the dark loops shows that only in AIA 211 {\AA} channel the computed level of recombination continua is consistent with observations for some models, while in all other channels which are more distant from the continua edges the synthetic continuum is far too low. In analogy with on-disk observations of flares we interpret the surplus emission as due to numerous EUV lines emitted from hot but faint loops in front of the cool ones. Finally we briefly comment on failure of the standard absorption model when used for analysis of the dark-loop brightness.Comment: 9 pages, 2 figure

Magnetic Flux Emergence in a Coronal Hole

A joint campaign of various space-borne and ground-based observatories, comprising the Japanese Hinode mission (Hinode Observing Plan 338, 20 - 30 September 2017), the GREGOR solar telescope, and the Vacuum Tower Telescope (VTT), investigated numerous targets such as pores, sunspots, and coronal holes. In this study, we focus on the coronal hole region target. On 24 September 2017, a very extended non-polar coronal hole developed patches of flux emergence, which contributed to the decrease of the overall area of the coronal hole. These flux emergence patches erode the coronal hole and transform the area into a more quiet-Sun-like area, whereby bipolar magnetic structures play an important role. Conversely, flux cancellation leads to the reduction of opposite-polarity magnetic fields and to an increase in the area of the coronal hole.© 2020 Springer Nature Switzerland AG. Part of Springer Nature.J.P. acknowledges support from Leibniz-Institut für Sonnenphysik (KIS) on funding, computational resources, and material for the creation and preparation of this manuscript. D. U., J. I. C. R., and K. K received funding for this research project from the FWF under project grant P27800. P.G., S.J.G.M., and J.K. acknowledge project VEGA 2/0048/20. This work is part of a collaboration between AISAS and AIP supported by the German Academic Exchange Service (DAAD) under project No. 57449420. C.D., C.K., I.K., and M.V. acknowledge support by grant DE 787/5-1 of the Deutsche Forschungsgemeinschaft (DFG). M.T. acknowledges funding by the Austrian Space Applications Programme of the Austrian Research Promotion Agency FFG (859729, SWAMI). The support by the European Commission Horizon 2020 Programs under grant agreements 824135 (SOLARNET) and 824064 (ESCAPE) are highly appreciated. Thanks to Y. Hanaoka for providing data from the infrared Stokes polarization full-disc images from the Solar Flare Telescope. Moreover, the authors want to acknowledge SDO/AIA and SDO/HMI Data Science Centers and Teams. Data were obtained during a joint GREGOR campaign with support by Hinode, IRIS, VTT, plus Chrotel. The 1.5-meter GREGOR solar telescope was built by a German consortium under the leadership of KIS with AIP, and MPS as partners, and with contributions by the IAC and ASU. Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners, which is operated by these agencies in cooperation with ESA and NSC (Norway). We acknowledge data use from ACE and Wind spacecraft instruments, and to STEREO as well. We acknowledge data use from WDC of Geomagnetism, Kyoto, and LMSAL SolarSoft. NASA Astrophysics Data System (ADS) has been used as bibliographic engine.Peer reviewe