Session 6 - Magnetic structure of the corona and its transition to the heliosphere
Conveners: David Berghmans (ROB), Marc DeRosa (LMSAL)Wednesday 31/10, 16:30 - 18:00
Thursday 1/11, 11:00 - 12:00
This session will focus on the outer edge of the AIA FOV, and beyond, where the plasma pressure increasingly dominates the magnetic pressure and the topology of streamers and pseudo-streamers fades in the solar wind. Both solar max and solar min configurations, and the evolution between, can be addressed by the near solar cycle coverage of SDO. Contemporary and upcoming EUV imagers with extended FOV and/or coronagraphs with low occultor are welcome to provide contributions to expand the insights obtained from SDO results.
Plenary speaker: Dan Seaton (NOAA)
Talks
Click here to toggle abstract display in the scheduleWednesday October 31, 16:30 - 18:00
| 16:30 | Exploring the Middle Corona with a New Generation of Instruments | Seaton, D | Invited Oral |
| Dan Seaton | |||
| CIRES, University of Colorado & NOAA NCEI | |||
| The so-called “middle corona” — the region of the corona between 1.5 and a few solar radii — is the region where CMEs are accelerated, where magnetic energy is released by reconnection during solar flares, and where the nascent solar wind begins to be accelerated as it flows out into the heliosphere. Observations of this region therefore hold much potential to address key questions in solar physics, but because observing this region is technically challenging, it remains relatively poorly explored. SWAP on PROBA2 and SUVI on GOES-R, both of which can observe the EUV corona to heights larger than 2 solar radii, have produced observations demonstrating the value of observing this region, and a new generation of instruments currently in development will push the frontiers of understanding of this region considerably farther in the coming years. In this talk I will discuss some of the key questions in solar coronal physics that we can answer by exploring the middle corona and discuss some current and future opportunities to explore the middle corona using a variety of techniques and instruments. | |||
| 17:00 | Counter-streaming flows in a giant quiet-Sun filament | Diercke, A | Oral |
| Andrea Diercke[1,2], Christoph Kuckein[1], Meetu Verma[1], Carsten Denker[1] | |||
| [1]Leibniz-Institut für Astrophysik Potsdam (AIP), [2]Universität Potsdam | |||
| A giant solar filament was visible on the solar surface between 2011 November 8-23. The filament stretched over more than half a solar diameter. Multi-wavelength data from the SDO instrument AIA (171, 193, 304, and 211 A) were used to examine counter-streaming flows within the spine of the filament. H-alpha images from the Kanzelhöhe Solar Observatory provided context information. We apply local correlation tracking (LCT) to a two-hour time series on 2011 November 16 of the AIA images to derive horizontal flow velocities of the filament. To enhance the contrast of the AIA images, noise adaptive fuzzy equalization (NAFE) is employed, which allows us to identify and quantify counter-streaming flows in the filament. We detect counter-streaming flows in the filament, which are visible in the time-lapse movies in all examined AIA wavelength bands. In the time-lapse movies, we see that these persistent flows lasted for at least two hours. Furthermore, by applying LCT to the images we clearly determine counter-streaming flows in time series of 171 A and 193 A images. In the 304 A wavelength band, we only see minor indications for counter-streaming flows with LCT, while in the 211 A wavelength band the counter-streaming flows are not detectable. The average horizontal flows reach mean flow speeds of 0.5 km/s. The highest horizontal flow speeds are identified in the 171 A band with flow speeds of up to 2.5 km/s. The results are averaged over a time series of 90 min. Because the LCT sampling window has a finite width, a spatial degradation cannot be avoided leading to lower estimates of the flow velocities as compared to feature tracking or Doppler measurements. The counter-streaming flows cover about 15-20% of the whole area of the EUV filament channel and are located in the central part of the spine. In conclusion, we confirm the omnipresence of counter-streaming flows also in giant quiet-Sun filaments. | |||
| 17:15 | Characteristics of ephemeral coronal holes | Inglis, A | Oral |
| Andrew Inglis[1], Rachel O'Connor[2], Dean Pesnell[1], Michael Kirk[1], Nishu Karna[3] | |||
| [1] NASA Goddard Space Flight Center, [2] Smith College, [3] Harvard-Smithsonian Center for Astrophysics | |||
| Ephemeral coronal holes are short-lived, low-density regions of the solar corona observed as dark features at EUV wavelengths, and are associated with open magnetic field lines due to a single dominant polarity. These structures are distinct from longer-lived equatorial and polar coronal hole regions, and remain relatively unexplored. Ephemeral coronal holes are primarily characterized by a lifetime substantially less than a single solar disk crossing, typically lasting only a few days. We conduct a systematic search for these events using Atmospheric Imaging Assembly (AIA) data provided by the Solar Dynamics Observatory (SDO), examine their characteristic properties and investigate the relationship with the underlying magnetic field structure. A preliminary examination of the SDO/AIA database between 2010-2016 identified 5 clear examples of ephemeral coronal holes, suggesting that they are rare phemonena. For each event we determine its key properties, including spatial extent over time, growth and decay rates, total lifetime, average flux within the coronal hole, and associated magnetic field properties. We also examine links between the coronal hole evolution and granulation processes occuring in the lower atmosphere. Further research will reveal the prevalence of these apparently rare phenomena, and shed light on the properties of the magnetic field that lead to their rapid formation and dissolution. | |||
| 17:30 | Three-dimensional structure of a coronal streamer observed by SOHO/LASCO and STEREO/COR2 | Decraemer, B | Oral |
| Bieke Decraemer [1,2], Andrei Zhukov [1], Tom Van Doorsselaere | |||
| [1] Royal Observatory of Belgium, [2] Centre for mathematical Plasma Astrophysics - KU Leuven | |||
| Helmet streamers are a prominent manifestation of magnetic structures with current sheets in the Solar corona. These large-scale structures are regions with high plasma density, overlying coronal active regions. We investigate the 3D structure of coronal streamers, observed by white-light coronagraphs (SOHO/LASCO and STEREO/COR2). 3D reconstruction of coronal structures is often ambiguous. Inverse reconstructions are difficult, so we design a forward model based on plausible assumptions about the 3D streamer structure taken from previous physical models (a plasma sheet centered around a current sheet). The streamer stalk is approximated by a plasma sheet, with electron density that is characterized by three functions describing the radial, transverse and face-on profiles respectively. We simultaneously fit the observational data from SOHO and STEREO vantage points using a multivariate minimization algorithm. We demonstrate that our model can reasonably describe the observations. | |||
| 17:45 | Spatiotemporal Analysis of Coronal Loops Using SDO/AIA | Pascoe, D | Oral |
| David Pascoe | |||
| KU Leuven | |||
| The high spatial and temporal resolution provided by the Atmospheric Imaging Assembly of the Solar Dynamics Observatory has inspired the development of advanced observational techniques to probe the solar atmosphere. Combining forward modelling of the EUV profile of coronal loops with the seismological analysis of kink oscillations provides a powerful plasma diagnostic to strongly constrain properties such as the transverse density profile and magnetic field strength. Our method uses Bayesian analysis and Markov chain Monte Carlo sampling to apply our spatial and temporal models simultaneously to the data. We compare our results with numerical simulations to quantify the uncertainties associated with the assumptions of the underlying theoretical models. | |||
Thursday November 1, 11:00 - 12:00
| 11:00 | The Coronal Monsoon: Thermal Nonequilibrium reveled by periodic coronal rain | Auchère, F | Oral |
| Frédéric Auchère[1], Clara Froment[1], Elie Soubrié[1, 3], Patrick Antolin[2], Ramon Oliver[3] | |||
| [1]IAS, CNRS / Univ. Paris Sud XI, Orsay, France, [2]University of St Andrews, St Andrews, United Kingdom, [3]University of the Balearic Islands, Palma De Mallorca, Spain, Gabriel Pelouze[1], Alfred Voyeux[1] | |||
| We report on the discovery of periodic coronal rain in an off-limb sequence of SDO/AIA images. The showers are co-spatial and in phase with periodic (6.6 hr) intensity pulsations of coronal loops of the sort described by Auchère et al. (2014) and Froment et al. (2015, 2017. These new observations make possible a unified description of both phenomena. Coronal rain and periodic intensity pulsations of loops are two manifestations of the same physical process: evaporation / condensation cycles resulting from a state of thermal nonequilibrium (TNE). The fluctuations around coronal temperatures produce the intensity pulsations of loops, and rain falls along their legs if thermal runaway cools the periodic condensations down and below transition-region (TR)temperatures. This scenario is in line with the predictions of numerical models of quasi-steadily and footpoint heated loops. This event of periodic coronal rain is compared with a similar event showing only pulsations at coronal temperatures but no significant cool rain fall. For both events we have stereoscopic observations from the SDO and STEREO spacecraft which allows reconstruction of the 3D loop geometries. Comparison with numerical simulations suggest that these two events correspond to two regimes of TNE: one with "full condensations" (coronal rain) and another in which "incomplete condensations" start to develop but are pushed down one loop leg before they can reach chromospheric temperatures. These new observations impose severe constrains on the spatio-temporal distribution of coronal heating. | |||
| 11:15 | A new method for measuring relative abundances in the solar corona | Zambrana prado, N | Oral |
| Zambrana Prado Natalia[1], Buchlin Eric[1] | |||
| [1]Institut d’Astrophysique Spatiale, CNRS, Univ. Paris-Sud, Université Paris Saclay | |||
| Linking the Solar Wind to its origin in the solar atmosphere is a difficult task. One way forward is to use composition data measured in situ and remotely. Indeed, different structures on the Sun have different abundances, that become frozen at a certain height, and therefore we can determine where certain wind plasma detected in situ comes from. However, systematically determining these abundances from remote-sensing observations is difficult because it usually first requires an accurate determination of the Differential Emission Measure (DEM). We present a new method to measure relative abundances using UV spectroscopy, which aims at being independent from the DEM. This method relies on optimizing linear combinations of spectral lines. We test this method using DEMs obtained from AIA observations and creating synthetic intensities with them. This allows us to test the method accurately and to find the best linear combinations. This method could be used semi-automatically for optimal abundance determinations from existing observations as well as for designing new observations such as those from the SPICE spectrometer from the future Solar Orbiter mission. | |||
| 11:30 | Using SDO/AIA to Understand the Thermal Evolution of Solar Prominence Formation | Viall, N | Oral |
| Nicholeen Viall [1], Therese Kucera [1], and Judith Karpen [1] | |||
| [1] NASA/GSFC | |||
| We investigate prominence formation using time series analysis of Solar Dynamics Observatory’s Atmospheric Imaging Assembly (SDO/AIA) data. We examine the thermal properties of forming prominences by analyzing observed light curves using the same technique that we have already successfully applied to active regions to diagnose heating and cooling cycles. This technique tracks the thermal evolution using emission formed at different temperatures, made possible by AIA's different wavebands and high time resolution. We also compute the predicted light curves in the same SDO/AIA channels of a hydrodynamic model of thermal nonequilibrium formation of prominence material, an evaporation-condensation model. In these models of prominence formation, heating at the foot-points of sheared coronal flux-tubes results in evaporation of material of a few MK into the corona followed by catastrophic cooling of the hot material to form cool (~10,000 K) prominence material. We investigate prominences from different viewing angles to evaluate possible line of sight effects. We demonstrate that the SDO/AIA light curves for flux tubes undergoing thermal nonequilibrium vary at different locations along the flux tube, especially in the region where the condensate forms, and we compare the predicted light curves with those observed. | |||
| 11:45 | Modelling the Multi-Stranded Nature of Coronal Loops | Williams, T | Oral |
| Thomas Williams | |||
| UCLAN | |||
| It is thought that many coronal loops which appear as monolithic structures may be comprised of smaller, sub-resolution strands. One case for this school of thought is the simultaneous presence of red- and blue-shifted Doppler velocities of various spectral lines in the same structure. In this study we investigate this phenomenon using our Multi-Stranded Hydrodynamics (MSHD) code, which has been updated to better handle the numerical complexities of the transition region. Matching the synthetic Doppler velocities from MSHD with observations from the EUV Imaging Spectrometer (EIS) and Marshall Grazing Incidence X-ray Spectrometer (MaGIXS), it may be possible to infer some of the underlying properties of the loop which cannot be observed. This may include the number of strands the loop is comprised of, the number and magnitude of nanoflare bursts occurring, the loop temperature, density, etc. The improved MSHD code could prove to be a useful tool in helping to understand the underlying physics of coronal loops and the nature by which they are heated. | |||
Posters
Click here to toggle abstract display in the liste-Posters
| On the Vertical Magnetic Field at the Umbral Boundary of a long-lived Sunspot | Schmassmann, M |
|---|---|
| Markus Schmassmann, Rolf Schlichenmaier, Nazaret Bello {Gonz\'alez} | |
| Kiepenheuer-Institut fuer Sonnenphysik (KIS), Freiburg i.Br., Germany | |
| In a statistical study of sunspots in 79 active regions observed with SP/Hinode, {Jur{\v c}{\'a}k}~et~al.\ (2018) found no dependence on sunspot size of the vertical magnetic field component $B_\text{ver}$ averaged along the umbral boundary. They concluded that the absolute value of $B_\text{ver}$ at the umbral boundary is the same for all spots. We present here our investigation of the temporal evolution of $B_\text{ver}$ averaged along the umbral boundary of one long-lived sunspot during its stable phase. We use HMI/SDO data of the full disc passage of NOAA AR 11591. Contours of continuum intensity at $I_\text{c}=0.5I_\text{qs}$, whereby $I_\text{qs}$ refers to the average over the quiet sun areas, are used to extract the magnetic field along the umbral boundary. Projection effects due to different formation heights of the Fe\,\textsc{i}~617.3\,nm line and continuum are taken into account. We find that during the first disc passage, $B_\text{ver}$ remains constant at 1693\,G with a root-mean-square deviation of 15\,G, whereas the magnetic field strength varies substantially (mean 2171\,G, rms of 48\,G) and shows a long term variation. Compensating for formation height has little influence on the mean value along each contour, but reduces the variations along the contour when away from disc centre, yielding a better match between the contours of $B_\text{ver}=1693\,$G and $I_\text{c}=0.5I_\text{qs}$. We will show that during the disc passage of a stable sunspot, its umbral boundary can equivalently be defined by using the continuum intensity $I_\text{c}$ or the vertical magnetic field component $B_\text{ver}$, while contours of fixed magnetic field strength fail to outline the umbral boundary. | |
p-Posters
| Exceptional Extended Field of View Observations by SWAP on 1 and 3 April 2017 | O'hara, J |
|---|---|
| Jennifer O'Hara[1], Marilena Mierla[1,2], Elena Podladchikova[1], Elke D'Huys[1], Matthew J West[1] | |
| [1]Solar–Terrestrial Center of Excellence – SIDC, Royal Observatory of Belgium, Brussels, Belgium, [2]Institute of Geodynamics of the Romanian Academy, Bucharest, Romania | |
| On the 1st and 3rd April 2017 two large solar eruptions, which were associated with an M4.4 and M5.8 class flare, respectively, were observed on the solar western limb with the PROBA2/SWAP telescope. The large field of view of SWAP combined with the exceptional circumstances of the satellite being off-pointed in a favorable position to view the events, provide us with the rare opportunity to study these eruptions up to approximately 2 solar radii, where space-based coronagraph observations begin. SWAP observations reveal off-limb erupting features as well as on disk EUV waves initiated by these eruptions. Using this unique set of observations, the evolution of these two events is tracked and the propagating speeds of both the eruptions and the on-disk EUV waves are calculated. | |
| Solar Coronal Jets Extending beyond the AIA Field of View Observed during the 2017 August 21 | Hanaoka, Y |
| Y. Hanaoka[1,2], R. Hasuo[1,2], T. Hirose[2], A. C. Ikeda[2], T. Ishibashi[2], N. Manago[2], Y. Masuda[2], S. Morita[2,3], J. Nakazawa[2], O. Ohgoe[1,2], Y. Sakai[2,4], K. Sasaki[2], K. Takahashi[3], and T. Toi[2] | |
| [1]National Astronomical Observatory of Japan, [2]Solar Eclipse Digital Imaging and Processing Network, [3]NPO Kwasan Astro Network, [4]Chiba Prefectural Tsurumaisakuragaoka High School | |
| We observed six polar coronal jets extending beyond 2 Rsun during the solar eclipse on 2017 August 21 in white-light images. All of them were found in polar plumes as narrow structures above EUV jets observed in the low corona observed with the AIA of the SDO. The EUV observation shows much more jets, but the jets whose brightnesses are comparable to ordinary soft X-ray jets and that occurred in the polar regions near the eclipse period were observed as eclipse jets without exception. These results mean that the ordinary polar jets actually extend beyond the field-of-view of the AIA and probably escape from the Sun as part of the solar wind. Solar eclipse observations enable us to observe the corona between about 1.2-2.0 Rsun, which is difficult to observe with the current spaceborne instruments. Observing beyond the AIA field-of-view is promising to know the larger scale aspects of the coronal dynamic phenomena seen in the low height. | |
| Long-term evolution of the solar corona using SWAP data | Mierla, M |
| Marilena Mierla[1,2], Elke D’Huys[1], Daniel B. Seaton[3], David Berghmans[1], Matt West[1], Elena Podladchikova[1], Laurence Wauters[1], Jan Janssens[1] | |
| [1] Royal Observatory of Belgium, Brussels, Belgium, [2] Institute of Geodynamics of the Romanian Academy, Bucharest, Romanian, [3] NOAA, Boulder, USA | |
| In this work, we use the PROBA2/SWAP images to study the evolution of the large-scale structures of the solar corona observed in the EUV during the solar cycle 24 (from 2010 to 2018). We will discuss the evolution of the corona at different heights above the solar surface and the evolution of the corona over the poles. We compare it with the sunspot number evolution. | |
| Reflection of Acoustic Modes on Sunspots | Waidele, M |
| Matthias Waidele, Kolja Glogowski, Markus Roth | |
| Kiepenheuer-Institut für Sonnenphysik | |
| Sunspots are known to strongly influence solar acoustic modes. There is a variety of possible interactions of the magnetic field and the waves, one of them being reflection. Assuming that part of a wave got reflected at the subsurface magnetic fluxtube it should be detectable at the surface again. In our studies we use the helioseismic Fourier-Hankel analysis method to decompose sunspot data of 6 days recorded by SDO/HMI into in and outgoing waves. The power spectrum of outgoing waves shows a signal that could theoretically be contributed to wave reflection at the sunspot. | |
| Power spectrum power-law indices as a diagnostic of coronal heating | Ireland, J |
| Jack Ireland[1], Nicholeen Viall[2], Stephen Bradshaw[3], Michael Kirk[4] | |
| [1] ADNET Systems, Inc., MD, USA / NASA GSFC, MD, USA, [2] NASA GSFC, [3] Rice University, TX, USA, [4] Catholic University, DC, USA. | |
| We investigate the coronal heating of active regions by bringing together novel data analysis techniques with hydrodynamic modeling in a new and unique way. Viall & Klimchuk 2011, 2012, 2014, 2017 have shown that the timing of active region coronal emission brightenings in multiple channels of Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO/AIA) follows that expected from simulations of a nanoflare-heated corona. Using Numerical HYDrodynamic RADiative Emission Model for the Solar Atmosphere (HYDRAD)-based simulations of AIA emission for an AR, Bradshaw & Viall 2016 have shown that the timing of coronal emission brightenings is dependent on the properties of the nanoflare energy distribution and occurrence rate. Relatedly, Ireland et al. 2015 show that average power spectra $P(f)$ (where $f$ is frequency) of time series of AIA 171Å and 193Å AR images are dominated by power laws, $P(f)~f^{-z}, z>0$. Ireland et al. 2015 show that a distribution of exponentially decaying events of emission $E$ along the line-of-sight, where $N(E)~E^{-m}$ and the size of the emission depends on its duration $T$ such that $E~T^{k}$ creates a power law power spectrum $P(f)~f^{-k(2-m)}$. We present analyses that test the hypothesis that a distribution of nanoflare events causes both the emission power-law power spectrum in AIA time-series as well as the observed brightening time-lags. Firstly, we show that the power-law indices of Fourier power spectra of the same simulated data described in Bradshaw & Viall 2016 depends on the frequency of nanoflares used. Secondly, using the same observational AIA time-series data analyzed by Viall & Klimchuk (2012), we obtain correlations of the cross-channel time-lags with the power-law indices of Fourier power spectra in each AIA channel. Finally, the ability of power-law indices and time-lags together to constrain the underlying nanoflare frequency distribution is discussed. | |
| Multi-wavelength Observations of Flare-Induced Acoustic Waves Around Active Regions with SDO AIA | Pesnell, D |
| Teresa Monsue[1], W. Dean Pesnell[2], Frank Hil[3]l, Michael Kirk[2] | |
| [1] Vanderbilt University, Nashville, Tennessee, USA [2] NASA GSFC, Greenbelt, Maryland, USA [3] NSO, Boulder, Colorado, USA | |
| Active regions on the Sun are abundant with a variety of waves that are both acoustically helioseismic and magnetohydrodynamic in nature. The occurrence of a solar flare can disrupt these waves, through MHD mode-mixing or scattering by the excitation of these waves. We take a multi-wavelength observational approach to understand the source of these waves by studying active regions where flaring activity occurs. Utilizing a Fast Fourier Transform (FFT) algorithm, our approach is to search for signals within a time series of images by producing multi-frequency power map movies and spatially sampling the time series by radial sectors with constant area that minimizes the spatial variation of the acoustic power. With this application we are able to study the active region both spatially and temporally and correlate data over multiple wavelengths, allowing us to observe the behavior of the waves at different heights within the Solar atmosphere. We apply multi-wavelength measurements utilizing NASA’s SDO AIA 1700 (lower photosphere), 1600 (upper photosphere) and 304 (chromosphere) passbands. When we run power map movies of the chromosphere we are able to see a subtle propagating feature moving outward from the center of the flare; this could be an MHD-wave propagating outward by the flaring event. With our sector sampling method we observe power variation around the flaring active region. This power variation corresponds to the flare induced enhancement of the oscillations around the active region. Furthermore, there seems to be absorptive properties observed within the chromospheric line of the AIA 304 \AA\ passband. | |
| The spectral content of SDO/AIA 1600 and 1700 A filters from flare and plage observations | Milligan, R |
| Paulo Simoes[1,2], Hamish Reid[1], Ryan Milligan[1,3,4], Lyndsay Fletcher[1] | |
| [1]University of Glasgow, [2]Centro de Radio Astronomia e Astrofisica Mackenzie, [3]NASA Goddard Space Flight Center, [4]Department of Physics Catholic University of America | |
| The strong enhancement of the ultraviolet emission during solar flares is usually taken as an indication of plasma heating in the low solar atmosphere caused by the deposition of the energy released during these events. Images taken with broadband ultraviolet filters by the Transition Region and Coronal Explorer (TRACE) and Atmospheric Imaging Assembly (AIA 1600 and 1700 \AA) have revealed the morphology and evolution of flare ribbons in great detail. However, the spectral content of these images is still largely unknown. Without the knowledge of the spectral contribution to these UV filters, the use of these rich imaging datasets is severely limited. Aiming to solve this issue, we estimate the spectral contributions of the AIA UV flare and plage images using high-resolution spectra in the range 1300 to 1900 \AA\ from the Skylab NRL SO82B spectrograph. We find that the flare excess emission in AIA 1600 \AA\ is composed of the C IV 1550 \AA\ doublet (26\%), Si I continua (20\%), with smaller contributions from many other chromospheric lines such as C I 1561 and 1656 \AA\ multiplets, He II 1640 \AA, Si ii 1526 and 1533 \AA. For the AIA 1700 \AA\ band, C I 1656 \AA\ multiplet is the main contributor (38\%), followed by He II 1640 (17\%), and accompanied by a multitude of other chromospheric lines, with minimal contribution from the continuum. Our results can be generalized to state that the AIA UV flare excess emission is of chromospheric origin, while plage emission is dominated by photospheric continuum emission in both channels. | |
