COSMO. The COronal Solar Magnetism Observatory

Transcription

1 COSMO The COronal Solar Magnetism Observatory A Response to the Request for Information from the NRC Decadal Survey in Solar and Space Physics Steven Tomczyk 1*, Joan Burkepile 1, Roberto Casini 1, Giuliana de Toma 1, Yuhong Fan 1, Philip G. Judge 1, Haosheng Lin 2, Boon Chye Low 1, Scott W. McIntosh 1, Peter G. Nelson 1, Stanley C. Solomon 1 1 The National Center for Atmospheric Research, High Altitude Observatory 2 The University of Hawaii, Institute for Astronomy * Corresponding author, tomczyk@ucar.edu, Project website: November 12, 2010

2 Introduction Measuring magnetic fields in the solar corona is crucial to understanding and predicting the Sun s generation of space weather that affects communications, GPS systems, space flight, and power transmission (Hanslmeier, 2003; Lambour, et al., 2003; Iucci et al., 2006). Most forms of solar activity, including high-energy electromagnetic radiation, solar energetic particles, flares, and coronal mass ejections (CMEs), derive their energy from coronal magnetic fields. The corona is also the source of the solar wind with its embedded magnetic field that engulfs the Earth. The ability to measure coronal magnetic fields will significantly advance the understanding of the underlying physical processes, and will lead to improved predictions of hazardous space weather effects on Earth. The scientific community recognizes the critical need for accurate measurements of the coronal magnetic field. The last National Research Council decadal review of solar and space physics recognized that an important next step in solar physics is...to develop the science of coronal mapping for measurement of the structure and strength of the coronal magnetic field. (Lanzerotti, et al., 2003). Precise coronal magnetic field observations are essential to address some of the important questions that remain in solar physics. Yet until recently, magnetic fields in the corona have been extremely difficult to measure for three important reasons: 1) the magnetic fields in the corona are intrinsically weak; 2) the coronal spectroscopic lines are dimmer and broader than their photospheric counterparts; and 3) the optically thin corona requires interpretation of magnetic signatures integrated along extended path lengths. These challenges have been addressed recently using greatly improved infrared (IR) detectors, forward models, advanced diagnostic tools, and inversion codes. As a result, we now have the capability to directly measure the coronal magnetic field. Unfortunately, today s solar coronagraphs are too small to obtain the measurements at the required spatial and temporal resolution. Figure 1. The solar corona during the total eclipse of 11 July The COronal Solar Magnetism Observatory (COSMO) is a proposed suite of complementary ground-based instruments designed to study magnetic fields and plasma conditions in the corona. The central instrument is a 1.5m-aperture coronagraph that will obtain daily measurements of the strength and direction of coronal magnetic fields over a large field-of-view at the spatial and temporal resolutions required to address the outstanding problems in coronal physics. Supporting instruments focus on chromospheric and prominence magnetometry, and observing the electron scattered K-corona. This suite of comprehensive observations will not be provided by any current or proposed observatory; the COSMO facility will enable breakthrough science and enhance the value of data collected by other observatories on the ground (e.g. ATST, FASR, SOLIS) and in space (e.g. Hinode, STEREO, SDO, TRACE, GOES, and SOHO). COSMO will replace the current Mauna Loa Solar Observatory and be operated as an NCAR facility in collaboration with The University of Hawaii and The University of Michigan. COronal Solar Magnetism Observatory 1

3 Key Science Drivers The proposed COSMO design emerged from a broad base within the solar and heliospheric communities through the COSMO Science Advisory Panel which identified important questions that this facility can uniquely address. COSMO s unprecedented observations will contribute significantly to answering the following questions about fundamental coronal processes. 1. What determines the magnetic structure of the corona? COSMO will investigate large-scale coronal structures in relation to surface field advection, differential rotation, and photospheric flux emergence. COSMO will provide new information on the evolution and interactions between magnetically closed and open regions that determine the changing structure of the heliospheric magnetic field. 2. What is the physics of the storage and explosive release of magnetic energy in the solar atmosphere that produce CMEs? COSMO will detect the polarization signatures of non-potential fields to gain insight into the roles of energy storage, magnetic reconnection, helicity creation and transport, and flux emergence in CME formation. COSMO will also provide white light images of CMEs in the low corona that are needed to determine the basic properties of CMEs in the early stages of formation. 3. What is the nature of the changes in coronal magnetic structure that accompany the 11-year solar cycle? COSMO will provide daily observations of the coronal magnetic field down to 1 Gauss over a 1 field-of-view and will obtain observations approximately 300 days per year. Long-term synoptic observations of the coronal magnetic field will directly address how the coronal field responds to the sunspot cycle and how the switch in polarity of the global field manifests itself in the heliosphere. 4. How are prominences formed, how do they evolve, and how are they related to CMEs? COSMO will provide routine observations of prominence, filament, and chromospheric magnetic fields, and prominence flows. These observations will place constraints on prominence densities and determine how prominence and coronal magnetic fields interact, how and where magnetic energy is stored (e.g. flux and helicity transport) and how it is released (e.g. instabilities, reconnection, dissipative heating). 5. Where do CME-associated shocks form, and what is their role in accelerating solar energetic particles? Particles accelerated by CME-driven shocks have the highest particle energies and pose the greatest space weather hazards. COSMO can detect compressions and distortions in the field due to the formation and passage of a CME and associated waves. Density compressions resulting from shock formation can be large enough to produce intensity enhancements in white light coronal images (Vourlidas et al. 2003). COSMO has the potential to simultaneously detect both the shock and CME locations. 6. What is the role of magnetic reconnection and flares in CME formation? COSMO will provide the first routine, high time cadence measurements of coronal magnetic fields in and around flares and associated CMEs as seen over the solar limb. These high time cadence observations can help determine when, where and how magnetic energy is released by the CME and by the dissipative processes that result in flares. COronal Solar Magnetism Observatory 2

4 Observational Requirements These science questions have been translated into observation and telescope requirements by the COSMO Science Advisory Panel and the Co-Principal Investigators team. The COSMO science questions will be addressed through synoptic observation of coronal magnetic fields over a 1 field-of-view, with a spatial resolution of 5 arcseconds at a temporal cadence of 10 minutes. COSMO will be sensitive to magnetic field strengths of a few Gauss for the faintest coronal structures and less than a Gauss for the brightest. In order to do this, COSMO will need to observe features down to a few millionths of the brightness of the solar disk, which requires a large aperture telescope, 1.5m in diameter. The large field-of-view is needed to observe the global properties of the corona and is required to address all of the science questions, especially to track the outward motion and other basic properties of CMEs and prominences as they are ejected from the corona. It is also necessary to observe as low in the corona as possible to understand how the corona continually reacts to changes occurring in the photosphere and chromosphere. High temporal cadence is needed to capture the relentless dynamic evolution of the coronal plasma structure and explosive disturbances (e.g., MHD waves, shocks, prominence eruptions, CMEs, and flares) that are the essence of science questions 2, 4, 5, and 6. The physical properties of CMEs and eruptive prominences are best determined from the polarization signal of broadband filtered white-light observations because the scattered light from the corona is partially polarized. The Science Advisory Panel and Co-PIs have determined that a spatial resolution of 5 arc seconds is sufficient to address many of the scientific questions pertaining to the global magnetic structure of the corona. The COSMO Science Advisory Panel has also identified the specific need to directly measure the magnetic field in the chromosphere and in prominences (science question 4). Recent observations suggest that MHD waves in the upper chromosphere have sufficient energy to accelerate the solar wind outside of active regions (De Pontieu et al. 2007; Aschwanden, et al. 2007); COSMO will provide routine magnetic field measurements in this key region. These measurements will provide critical information on plasma conditions in the very low atmosphere that are needed to couple the coronal magnetic fields with those measured at photospheric heights. Technical Overview The science goals and observational requirements outlined in the previous section will be met with a facility consisting of two major elements: a large-aperture coronagraph with its own pointing system and dome, and a solar-pointed spar for supporting instrumentation in a smaller adjacent dome. A rendering of the large-aperture coronagraph and dome is shown in Figure 2. Figure 2. Concept for the large aperture coronagraph and dome. A protective snout extending through the telescope slot when the dome is open allows the lens to be continually washed in HEPA-cleaned air. The sealed He-filled tube protects the back surface of the lens from contamination and improves internal seeing. The large-aperture coronagraph will have instrumentation to measure coronal magnetic fields. In order to exploit these measurements COronal Solar Magnetism Observatory 3

5 to their full potential for advancing science, crucial information on prominence and chromospheric magnetic fields and the dynamical conditions of the embedded plasma will be collected by two additional instruments: a chromosphere and prominence magnetometer and a white-light K-coronagraph. These supporting instruments are designed to provide temporal and spatial resolutions that meet or exceed the temporal and spatial resolutions of the coronal magnetic field observations from the large-aperture coronagraph. Figure 3. Photon noise limited LOS magnetic field strength sensitivity using the FeXIII nm line vs. telescope aperture for an integration time of 10 minutes assuming a background sky brightness of 5 µb sun. The lines plotted correspond to various intensities of the corona in units of millionths of the disk intensity. It is possible to achieve magnetic field sensitivity of 1 Gauss in 10 minutes for coronal structures with brightness of 2 µb sun in the Fe XIII nm line with a 1.5-m aperture telescope. Structures as faint as 0.5 µbsun can be observed with a sensitivity of 3.5 Gauss in 10 minutes, or 2 Gauss in 30 minutes. Large-aperture coronagraph The most important instrument in the COSMO suite is a large-aperture coronagraph with post-focus instrumentation to measure coronal magnetic fields. This solar telescope performs coronal magnetometry using the emission lines of Fe XIII at and nm and the He I chromospheric emission line at nm. An overview (Judge, et al. 2001) of methods for measuring coronal magnetic fields identified the most promising techniques. COSMO will use the Zeeman effect and Hanle effect in scattering polarization to study the widest possible range of magnetic fields in the corona and chromosphere. The line-of-sight (LOS) strength of the coronal magnetic field can be measured directly through the Zeeman effect observed in the circular polarization, and the plane-of-sky (POS) direction of the magnetic field is determined from the linear polarization of the scattered radiation. These magnetic field measurements will have a dynamic range from a fraction of a Gauss to several thousand Gauss. This is critical to provide information on both large-scale quiet coronal fields and active region fields. To meet the science goals discussed above requires observation of the coronal magnetic field: with 1 Gauss sensitivity, and 5 arcseconds spatial resolution, for corona with 2 µb sun brightness, in 10 minutes integration time, for a sky background level of 5 µb sun. A calculation of the available coronal flux (Figure 3) indicates that this demanding requirement can only be met with a coronagraph having an aperture of at least 1.5 m with an extremely low level of instrumentally scattered light and a 1º field-of-view. The aperture requirement is driven by the need to collect sufficient photons to achieve a photon noise limited magnetic field measurement at the required level. Since the solar corona is a million times fainter than the nearby solar photosphere, observing it requires a coronagraph with extremely low levels of scattered light. A detailed engineering study of scattering from lenses and mirrors was conducted (Nelson 2006a; 2008) and showed that a lens objective scatters a factor of 10 times less light than a mirror with the same quality of figure, and a factor of four times less than a mirror with the same level of dust contamination. The huge scattering advantage of a lens over a mirror led us to choose a refracting design for the COSMO large-aperture coro- COronal Solar Magnetism Observatory 4

6 nagraph. To evaluate the feasibility of constructing a 1.5-m lens, we conducted a finite element analysis (Nelson, 2006b; 2008) which showed that gravitational flexure of this lens would have a negligible impact on the image quality. The FEA analysis also considered stress-induced birefringence, and this was found to be well below acceptable levels. We note that blanks of fused silica of sufficient size and quality for the COSMO large aperture coronagraph are now readily available, due in large part to developments in support of optical fibers for telecommunications. A fused silica lens with a diameter of 1.6 m will be used as a camera lens in the Large Synoptic Survey Telescope (LSST; Oliver et al. 2008). Post-focus instrumentation for the coronagraph includes a narrow band tunable filter to observe the entire field-of-view, and a fiber-fed spectropolarimeter capable of high spectral resolution line profile observations. Both of these instruments are based on prototypes developed over the past decade (see below). Chromosphere and prominence magnetometer (ChroMag) The second instrument in the suite is the ChroMag, an imaging polarimeter for magnetic and plasma diagnostics of chromosphere and prominences, using the spectral lines of He I (587.6 and 1083 nm) and Hα (656.3 nm), both on the solar disk and above the limb. This instrument will have the ability to observe magnetic fields and LOS Doppler shifts (Casini 2007). Measuring prominence magnetic fields embedded within the corona provides a more complete picture of magnetic field configurations that are needed to understand the role of prominences in CME formation and initiation. The ChroMag design (Elmore 2007b) will utilize a 20-cm aperture telescope and a tunable filter to measure the polarization across the line profile with a bandwidth of nm in the visible region and nm in the IR. It will have a full FOV of 2.5 solar radii and a spatial resolution of 2.3 arcseconds. White-light K-coronagraph The third element of the suite is a 20-cm aperture K-coronagraph that will measure white light coronal polarization brightness. This will provide a direct measurement of the column density of coronal electrons. It will reside on the solar-pointed spar in a small adjacent dome and provide observations of CME formation and early acceleration in the very low corona (down to 1.05 R sun ). It replaces the 1970s-vintage Mauna Loa Mk4 system, the world s only ground-based whitelight coronameter. Modern detector technology will be employed to significantly improve spatial resolution, temporal resolution, and signal-to-noise out to 2.5 R sun (Elmore 2007a). Given the absence of a white-light coronagraph on the SDO spacecraft, the K- coronagraph will be an essential complement to space- and ground-based coronal instruments. Technical Readiness Prototype instruments are producing results that, although limited by the small aperture of their telescopes, illustrate the potential and feasibility of the COSMO post-focus instruments. One prototype resides on Solar-C, the University of Hawaii 46-cm aperture coronagraph at Haleakala (Kuhn, et al. 2003). It employs IR detector technology in an Optical Fiberbundle Imaging Spectropolarimeter (OFIS) to record the complete polarization (Stokes I,Q,U,V) of the forbidden line of Fe XIII at nm. Figure 4 shows a measurement of the LOS strength of the magnetic field (Lin, et al. 2004). Each fiber in the 16 x 8 fiber bundle subtended 20 arc seconds, and the observation required 70 minutes of integration time. Notably, the errors on the LOS component of the magnetic field are less than COronal Solar Magnetism Observatory 5

7 1 Gauss. The precision on the magnetic field achieved with this instrument demonstrates the feasibility of the technique; however, the limited FOV and the coarse spatial and temporal samplings are not sufficient to address the science questions posed above. Another prototype application of coronal polarimetry with new IR detector technology is illustrated in Figure 5. This shows a map of the coronal field direction and strength measured using the NCAR/HAO Coronal Multichannel Polarimeter (CoMP) on the 20-cm aperture OneShot coronagraph at NSO s Sacramento Peak Observatory in New Mexico (Tomczyk et al. 2008). The CoMP instrument can observe the coronal magnetic field direction and strength, and plasma motion using the near infrared FeXIII emission lines with a full view of the low corona (~1.03 to 1.4 R sun ). The CoMP observations are photon noise limited, required an integration time of 2.42 hours, and were made at a spatial resolution of 9 arc seconds. Unique high cadence observations made with CoMP in October 2005 have produced the detection of ubiquitously propagating Alfvén waves in the solar corona (Tomczyk et al. 2007). These waves are a possible source of mechanical energy needed to heat the quiet corona and act as a driver for the fast solar wind (DePontieu et al. 2007). Measurement of the speed of wave propagation can also be used to infer the strength and orientation of the magnetic field through the recently developed technique of coronal seismology (e.g. Tomczyk and McIntosh 2009). This provides an important additional constraint on the magnetic field complementary to the polarization measurements. Figure 4. Coronal magnetic field observations from Solar-C and OFIS shown as a contour plot of the LOS magnetic field strength determined from circular polarization, superposed on EIT observations for context. Figure 5. Coronal magnetic field observations from CoMP. Left: the direction of the magnetic field shown as vectors on top of the FeXIII intensity. Right: the LOS component of the coronal magnetic field. The field strengths measured by OFIS and CoMP are in the range of 1-50 Gauss. These instrument developments demonstrate that technological advances, primarily in IR detector technology, now make it possible to measure the weak fields in the solar corona. However, these prototype results are severely limited by the modest apertures of the available coronagraphs. The science goals of COSMO require a coronagraph with an aperture of 1.5 m. COSMO as a Complement to ATST The Advanced Technology Solar Telescope (ATST) is a Major Research Equipment project to build a 4-m aperture solar telescope which has the ability to measure coronal magnetic fields above the limb using the same Zeeman and Hanle diagnostics as COSMO. Due to its large aperture, the ATST COronal Solar Magnetism Observatory 6

8 is optimized for high spatial resolution observations over a maximum 5 arcminute field-ofview (Figure 6). ATST will be used to pursue a range of user defined observations on the solar disk and above the limb, but with a field-of-view that is small compared to large scale coronal structures. The capabilities of ATST are complementary to the large fieldof-view, dedicated measurements provided by COSMO. Sun ATST FOV COSMO FOV Figure 6. Comparison of the FOV of the COSMO large coronagraph (orange, 1.0 ) with the ATST FOV (blue, 0.08 ). The size of the Sun (red, 0.5 ) is shown for comparison. The light gathering ability of a telescope is given by the product of the area of the collecting aperture and the solid angle of the field-of-view. While ATST has a factor of 7 greater aperture area, that advantage is more than compensated by the factor of 144 larger solid angle of COSMO. The COSMO coronagraph will have a light gathering power that exceeds that of the ATST by a factor of 20. This, coupled with its dedicated nature, makes COSMO ideally suited for observation of the corona over a range of spatial and temporal scales that will not be available to the ATST. Such observations are crucial to advance understanding of the structure and dynamics of the solar corona. COSMO COST ESTIMATE We have completed a conceptual design of the COSMO facility and suite of instruments. From this, we estimate that the COS- MO facility can be designed and constructed in five years at an approximate cost of $20M. This includes telescopes, instruments and buildings. Note that the design and construction of the COSMO K-coronagraph has been fully funded by NSF through NCAR at a cost of $2.2M and the design is currently underway. We expect to deploy the K-coronagraph to MLSO early in Following the completion of the COSMO facility, we will transfer the K-coronagraph to the COSMO spar. The recurring operational cost of the COSMO facility will be borne mostly by the existing HAO operational effort. This currently consists of 3.0 FTE observing personnel at MLSO and 3.0 FTE data management personnel at HAO in Boulder. Once COSMO is complete, the existing MLSO facility will be closed and the HAO effort transferred to the new observatory, thus minimizing recurring operations costs. There will be an increase in data processing and reduction costs due to the greater volume and complexity of the COS- MO data. Therefore, the estimated annual incremental operational cost will be: 1.0 FTE data management personnel, or approximately $200K/year. This level of data management support will allow COSMO to provide all key observations to the user community as a variety of data products: quick-look real-time images, movies, and activity alerts; level 1 data products, activity catalogues, and databases; and level 2 fully processed data products designed for easy use with other datasets and community models for data mining and reanalysis. COSMO will have an open data policy. The Mauna Loa Solar Observatory has been serving the solar and heliospheric community for over 40 years. Based on this, we expect the COSMO facility will provide unique and valuable observations to the community over its expected 40 year lifetime. COronal Solar Magnetism Observatory 7

9 REFERENCES Aschwanden, M. J., Winebarger, A., Tsiklauri, D., Peter, H. 2007, The Coronal Heating Paradox, ApJ, 659, Casini, R. 2007, Prominence and Filament Magnetometry Simulations, COSMO Tech Note 12, Boulder, Colorado, High Altitude Observatory. DePontieu, B., McIntosh, S.W., Carlsson, M., Hansteen, V.H., Tarbell, T.D., Schrijver, C.J., Title, A.M., Shine, R.A., Tsuneta, S., Katsukawa, Y., Ichimoto, K., Suematsu, Y., Shimizu, T., Nagata, S. 2007, Chromospheric Alfvenic Waves Strong Enough to Power the Solar Wind, Science, 318, Elmore, D. 2007a, Baseline Design of a Coronagraph to Measure K-corona Polarization Brightness, COSMO Tech Note 8, Boulder, Colorado, High Altitude Observatory. Elmore, D. 2007b, Baseline Design for a Prominence and Filament Magnetometer, COSMO Tech Note 11, Boulder, Colorado, High Altitude Observatory. Hanslmeier, A. 2003, Space Weather - Effects on radiation on manned space missions. Hvar Observatory Bulletin, 27, 159. Iucci, N., Dorman, L. I., Levitin, A. E., Belov, A. V., Eroshenko, E. A., Ptitsyna, N.G., Villoresi, G., Chizhenkov, G. V., Gromova, L. I., Parisi, M., Tyasto, M. I. and Yanke, V. G. 2006, Spacecraft operational anomalies and space weather impact hazards, Advances in Space Research, 37, 184. Judge, P. G., Casini, R., Tomczyk, S., Edwards, D. P. and Francis, E. 2001, Coronal Magnetometry: A Feasibility Study, NCAR Technical Report NCAR/TN-446-STR. Kuhn, J. R., Coulter, R., Lin, H. and Mickey, D. L. 2003, The SOLAR-C off-axis coronagraph, in Innovative Telescopes and Instrumentation for Solar Astrophysics, Proc. SPIE, 4853, 318. Lambour, R. L., Coster, A. J., Clouser, R., Thornton, L. E., Sharma, J., Cott, T. A. 2003, Operational impacts of space weather, Geophysical Research Letters, 30, 36. Lanzerotti, L. J., et al. 2003, The Sun to the Earth - and Beyond, A Decadal Research Strategy in Solar and Space Physics, N. R. C. Space Studies Board, The National Academies Press, Washington, D.C., p 54. Lin, H., Kuhn, J. R. and Coulter, R. 2004, Coronal Magnetic Field Measurements, ApJ, 613, L177. Nelson, P. G. 2006a, An Analysis of Scattered Light in Reflecting and Refracting Primary Objectives for Coronagraphs, COSMO Tech Note 4. Boulder, CO, High Altitude Observatory. Nelson, P. G. 2006b, A FEA of Meter-Class Refracting Primary Objectives for Coronal Polarimetry, COSMO Tech Note 2, Boulder, Colorado, High Altitude Observatory. Nelson, P. G. 2008, The Feasibility of Large Refracting Telescopes for Solar Coronal Research, in Ground-based and Airborne Telescopes II, Proc SPIE, 7012, COronal Solar Magnetism Observatory 8

10 Olivier, S. S., Seppala, L. and Gilmore, K. 2008, Optical Design of the LSST Camera in Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, Proc. SPIE, 7018, 70182G. Tomczyk, S., Card, G. L., Darnell, T., Elmore, D. F., Lull, R., Nelson, P. G., Streander, K. V., Burkepile, J., Casini, R., and Judge, P. 2008, An Instrument to Measure Coronal Emission Line Polarization, Solar Physics, 247, 411. Tomczyk, S. and McIntosh, S.W. 2009, Time-distance Seismology of the Solar Corona with CoMP, ApJ, in press. Tomczyk, S., McIntosh, S.W., Keil, S.L., Judge, P.G., Schad, T., Seeley, D.H. and Edmonson, J Alfven Waves in the Solar Corona, Science, 317, Vourlidas, A., Wu, S. T., Wang, A. H.;,Subramanian, P., Howard, R. A. 2003, Direct Detection of a Coronal Mass Ejection-Associated Shock in Large Angle and Spectrometric Coronagraph Experiment White-Light Images, ApJ, 598, COronal Solar Magnetism Observatory 9