Spectroscopic study of “magnetic tornadoes"
Tornado-like prominences were first described by Pettit (1932) but have not been paid much attention thereafter. Recently, several groups (e.g., Su et al. 2012; Wedemeyer-Boehm et al. 2012) observed “magnetic tornadoes” with the AIA instrument onboard SDO, and pointed at possible connections among vortex motion on the surface, filament barbs, and solar tornadoes. They proposed a new idea of filament formation and eruption, in which vortices and the rotation of magnetic structures in the corona (“solar magnetic tornadoes”) play a central role. However, whether these magnetic structures are indeed rotating, is a key question in this respect. In order to observationally determine the rotational motion against alternative interpretations and to relate them to the overall filament dynamics, spectroscopic observations in combination with imaging data are necessary.
Alternative interpretations of the imaging observations include oscillating motions and plasma motions on helical magnetic fields. Recent observations by Orozco Suarez et al. (2012) show some evidence of rotating plasma motions in filament barbs, but the evidence is still not solid enough to confirm such general tornado scenario. Therefore, Doppler observations of filament barbs and tornado-like structures on the solar limb are essential for distinguishing rotating motion from oscillations and plasma motions along twisted structures. If the barbs are indeed rotating, the Doppler maps are expected to show: a) blue shifts on one side and red shifts on the other side of the “tornado” axis, and this pattern not periodically reversing (which would hint at oscillations); or b) blue shifts when the structure moves to one side and red shift when the structure moves to the other side (off axis rotation).
We have guaranteed observing time at CoMP-S and Kanzelhoehe for the period 2013 September 9 - 29, and ask for EIS support during one week of this campaign. According to the long term meteorological statistics of the atmospheric conditions at the Lomnicky Peak Observatory, September is the best observing period for the coronographic observation by CoMP-S at Lomnicky Stit. Preferred observation time is from 5:00 to 9:00 UT (valid for the period mentioned above; the best observing conditions are shifted to 7:00 UT - 10:00 UT during the winter time).
2) SDO/AIA: The Atmospheric Imaging Assembly (AIA; Lemen et al. 2012 ) on board the Solar Dynamics Observatory (SDO; Pesnell et al., 2012 ) is an array of 4 telescopes that together provide full-disk images of the solar corona in 10 UV and EUV wavelengths with a high temporal cadence (about 12 s) and spatial resolution (about 0.6 arcsec; 4096x4096-pixel images). AIA data available at Level 1.0 (corrected for bad pixels and spikes) are processed to Level 1.5 data ready for further scientific analysis by using SolarSoft routines.
The “tornado” funnels are observed best in the AIA Fe IX 171 A (0.63 MK) passband, where they appear as dark, cone-shaped column structures connecting the solar surface and the top of the prominence.
3) The CoMP-S is a 2D multi-channel spectro-polarimeter installed at the Lomnicky Peak Observatory (Slovakia). It is attached to a 200/3000 ZEISS coronagraph. The coronagraph is diffraction limited from 530 nm to 1083 nm by changing focus of the objective lens only within a range of 80 mm. This allows us to take the sequential scans in wavelength through the profiles of several spectral lines, namely: Fe XIV 530.3 nm, Ca XV 569.5 nm, Fe X 637.5 nm, Fe XI 789.2 nm, Fe XIII 1074.7 nm, 1079.8 nm, He I 587.6 nm, H I 656.3 nm, Ca II 854.2 nm and He I 1083.0 nm. The spatial resolutions of the coronagraph at the wavelengths 530 nm, 656 nm and 1083 nm are 0.67“, 0.82“ and 1.36“, respectively. Two 16-bit detectors with 2560 × 2160 of 6.5 micron square pixels are used to record the images.
For the observations of the solar tornadoes we plan to perform high cadence 2D spectroscopy in the Hα line (the cadence of the full spectral scans will be at the order of 20 seconds). We plan to scan the line profile in 11 points allowing us to detect Doppler shifts up to ± 35 km/s.
4) Kanzelhoehe Observatory (Austria; www.kso.ac.at) performs high-cadence imaging of the full solar disk with a refractor with d/f = 100/2000 and a Lyot band-pass filter centred at the H-alpha spectral line at 656.3 nm with a full-width-at-half-maximum of 0.07 nm. The images are recorded by a CCD camera with 2048 x 2048 Pixels, 12 bit dynamic range, and include frame selection. The spatial resolution is ~ 1 arcsec. In order to support of the magnetic tornado campaign, H-alpha imaging sequences with a time cadence of 6 seconds will be performed in order to get insight into the overall context and dynamics of the filaments/ prominences under study.
Wedemeyer-Boehm, S., Scullion, E., Steiner, O., Rouppe van der Voort, L., de La Cruz Rodriguez, J. Fedun, V., Erdelyi, R. 2012, Magnetic tornadoes as energy channels into the solar corona, Nature 486, 505-508.
Orozco Suarez, D., Asensio Ramos, A., Trujillo Bueno, J. 2012, Evidence for rotational motions in the feet of a quiescent solar prominence, ApJ Lett. 761, L25.
- Veronig, A.M. - Gomory, P. - Kienreich, I. W. - Muhr, N. - Vrsnak, B. - Temmer, M. - Warren, H.P.: Plasma Diagnostics of an EIT Wave Observed by Hinode/EIS and SDO/AIA. The Astrophysical Journal Letters, Volume 743, Issue 1, article id. L10, 7 pp. (2011).
- Harra, L.K. - Sterling, A.C. - Gomory, P - Veronig, A.M.: Spectroscopic Observations of a Coronal Moreton Wave. The Astrophysical Journal Letters, Volume 737, Issue 1, article id. L4, 6 pp.(2011).
The PI of the HOP-237, i.e. of the programme we are asking for support now, is Dr. Yang Su. The HOP-237 was run only once.
- Su, Y. - Gomory, P. - Veronig, A. M. - Temmer, M. - Wang, T. - Vanninathan, K. - Gan, W. - Li, Y.: Solar Magnetized Tornadoes: Rotational Motion in a Tornado-like Prominence. The Astrophysical Journal Letters, Volume 785, Issue 1, article id. L2,6 pp. (2014).
During its first run in 2013, we obtained a good dataset that allowed us to identify the rotational motion within, a key characteristic of tornado-like prominences. Two papers have been published from this dataset.
During the second run in 2014, we made a small modification to the list of spectral lines to better understand plasma density inside tornado. However, due to the presence of coronal loops along the light of sight, we didn't get a clear structure of the tornado itself. However for the first time, we observed one close to disk centre, which allowed us to study the magnetic field and plasma motions at the feet of tornadoes. The work is still in progress.
Recently, Terry Kucera and Peter Young informed us of an issue with EIS, that the PSF function may have caused the signature of rotational motion. However, it is difficult to identify/remove its effect. A simple way is to observe more tornadoes. All we need to do is to find a structure above the limb that is rotating oppositely to the PSF effect. We also hope to study plasma density within tornadoes.
Therefore, we request the third run, using the same selection of spectral lines as from the second run. We will also inform the ground observation team and IRIS team.