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

Multi-wavelength observations of the 2014 June 11 M3.9 flare:Temporal and spatial characteristics

We present multi-wavelength observations of an M-class flare (M3.9) that occurred on 2014 June 11. Our observations were conducted with the Dunn Solar Telescope (DST), adaptive optics, the multi-camera system ROSA (Rapid Oscillations in Solar Atmosphere) and new HARDcam (Hydrogen-Alpha Rapid Dynamics) camera in various wavelengths, such as Ca~II~K, Mg~I~b$_2$ (at 5172.7 Ang), and H$\alpha$ narrow-band, and G-band continuum filters. Images were re-constructed using the Kiepencheuer-Institut Speckle Interferometry Package (KISIP) code, to improve our image resolution. We observed intensity increases of $\approx$120-150% in the Mg, Ca~K and H$\alpha$ narrow band filters during the flare. Intensity increases for the flare observed in the SDO EUV channels were several times larger, and the GOES X-rays increased over a factor of 30 for the harder band. Only a modest delay is found between the onset of flare ribbons of a nearby sympathetic flare and the main flare ribbons observed in these narrow-band filters. The peak flare emission occurs within a few seconds for the Ca~K, Mg, and H$\alpha$ bands. Time-distance techniques find propagation velocities of $\approx$60 km/s for the main flare ribbon and as high as 300 km/s for smaller regions we attribute to filament eruptions. This result and delays and velocities observed with SDO ($\approx$100 km/s) for different coronal heights agree well with the simple model of energy propagation versus height, although a more detailed model for the flaring solar atmosphere is needed. And finally, we detected marginal quasi-periodic pulsations (QPPs) in the 40--60 second range for the Ca~K, Mg and H$\alpha$ bands, and such measurements are important for disentangling the detailed flare-physics.Comment: 16 Pages, 7 Figures, 1 Table (1 video in on-line journal); Accepted in Research in Astronomy and Astrophysic

Multi-wavelength imaging and spectral analysis of jet-like phenomena in a solar active region using IRIS and AIA

High-resolution observations of dynamic phenomena give insights into the properties and processes that govern the low solar atmosphere. We present an analysis of jet-like phenomena emanating from a penumbral footpoint in active region (AR) 12192 using imaging and spectral observations from the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. These jets are associated with line-of-sight Doppler speeds of ±10–22 km s−1 and bright fronts that seem to move across the plane-of-sky at speeds of 23–130 km s−1. Such speeds are considerably higher than the expected sound speed in the chromosphere. The jets have signatures that are visible both in the cool and hot channels of IRIS and AIA. Each jet lasts on average 15 minutes and occurs 5–7 times over a period of 2 hr. Possible mechanisms to explain this phenomenon are suggested, the most likely of which involve p-mode or Alfvén wave shock trains impinging on the transition region and corona as a result of steepening photospheric wavefronts or gravity waves

Association between Tornadoes and Instability of Hosting Prominences

We studied the dynamics of all prominence tornadoes detected by the Solar Dynamics Observatory/Atmospheric Imaging Assembly from 2011 January 01 to December 31. In total, 361 events were identified during the whole year, but only 166 tornadoes were traced until the end of their lifetime. Out of 166 tornadoes, 80 (48%) triggered CMEs in hosting prominences, 83 (50%) caused failed coronal mass ejections (CMEs) or strong internal motion in the prominences, and only 3 (2%) finished their lifetimes without any observed activity. Therefore, almost all prominence tornadoes lead to the destabilization of their hosting prominences and half of them trigger CMEs. Consequently, prominence tornadoes may be used as precursors for CMEs and hence for space weather predictions.Comment: 16 pages, 5 figures, Accepted in Ap

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