Breakthrough Toward Understanding The Solar Wind Origin

Transcription

1 Breakthrough Toward Understanding The Solar Wind Origin A White Paper submitted to the 2010 Solar and Space Physics Decadal Survey Yuan-Kuen Ko 1, David H. Brooks 2, Steven R. Cranmer 3, George A. Doschek 1, Antoinette B. Galvin 4, Steve W. Kahler 5, J. Martin Laming 1, Susan T. Lepri 6, Mark Popecki 4, John C. Raymond 3, Yi-Ming Wang 1, Harry P. Warren 1, Peter R. Young 2 1 Naval Research Laboratory, 2 George Mason University, 3 Harvard-Smithsonian Center for Astrophysics, 4 University of New Hampshire, 5 Air Force Research Laboratory, 6 University of Michigan 1. Summary To make a breakthrough in the next decade toward understanding the origin of the solar wind, it is necessary to utilize solar spectroscopic observations and in situ solar wind ion composition measurements, and the synergy between the two is essential. Solar spectroscopic observations have revealed many important physical properties in various structures at the Sun from the chromosphere to the corona, through their powerful diagnostic capabilities in electron/ion temperature, density, abundance and flows. The solar wind ion composition measured in situ is directly related to these physical properties at the Sun where the solar wind is formed and released into the heliosphere. It is therefore clear that combining the unique diagnostic power of the two provides the most stringent constraints on where and how the solar wind is produced and on models of the solar wind formation. To advance our knowledge of the solar wind origin, it is then important to recognize the need in the next decade to continue supporting the collection and analysis of solar spectroscopic and in situ solar wind ion composition data. And we recommend that future mission designs addressing the solar wind origins need to take into account the co-existence and coordination of these two types of instruments. 2. The Diagnostic Power and the Open Questions for Solar Wind Origin One of the long-unsolved problems in solar and space physics is the origin of the solar wind: where and how it is produced, and what governs its variability. We know this is rooted in the solar magnetic field whose dynamics and interactions with the solar plasma generate the dynamical structures on the Sun, the heating of the corona and the acceleration of the solar wind. These processes manifest themselves in many observables. In this White Paper, we emphasize that the combined diagnostics from solar spectroscopic observations and in situ solar wind ion composition measurements are essential in leading us to the next breakthroughs toward understanding the origin of the solar wind formation. 2.1 Solar Wind Ion Composition Measurements The solar wind measured in-situ bears signatures of its formation in the solar atmosphere. Unlike particle speed, density and ion/electron kinetic temperatures that may change during transport in the heliosphere, the solar wind ion charge states are frozen-in close to the Sun (generally within 5 solar radii, e.g. Owocki et al. 1983, Burgi & Geiss 1986, Ko et al. 1997), and the elemental abundance under normal coronal/solar wind conditions is unlikely to change after it is released into the heliosphere. Therefore the solar wind ion composition

2 closely reflects the conditions at where the solar wind is being formed at the Sun. It is well known that the solar wind O +7 /O +6 ratios are anti-correlated with the solar wind speed implying that the coronal electron temperature in the slow wind source region is higher than that for the fast wind (e.g. Zurbuchen et al. 1999, Wang et al. 2009). It is also well known that the First Ionization Potential (FIP) effect exists in the slow wind and less so in the fast wind (e.g. Meyer 1985, Geiss et al. 1995, von Steiger et al. 2000, Zurbuchen et al. 2002). The similar difference in the electron temperature and FIP effect in solar active region (AR)/quiet Sun (QS) and the coronal hole (CH) structures hints at their association with the slow and fast wind, respectively. However, the variations in the ion composition indicate a far more complicated situation. Here we show two examples. Figure 1 shows the O +7 /O +6 freezing-in temperature versus proton speed in the solar wind measured by ACE that can be traced back to the boundary of either a north polar CH (NPCH) or a south polar CH (SPCH). It shows a clear distinction (regardless of the solar wind speed!) between the two sources that the solar wind originating from the NPCH has larger O 7+ /O 6+ freezing-in temperature than that from the SPCH. Such distinction was also observed by Ulysses (Ko et al. 1999, Zhang et al. 2002) but with opposite trend that the SPCH has larger O 7+ /O 6+ freezing-in temperature during those periods of study (years ). This implies that there are intrinsic differences in the processes and/or environment for the solar wind formation at the two polar CHs, and it is likely related to the properties in the two CHs such as magnetic field and size (Zhang et al. 2002). Figure 2 shows a time series of solar wind parameters measured by ACE in November Figure 1 Solar wind freezing-in temperature from the O +7 /O +6 ratios (6-hr mean) versus proton speed with footpoints at the low-latitude extension of 5 NPCHs and 3 SPCHs. Data are from ACE SWICS and SWEPAM during years (Ko et al. 2010, in preparation) The solar wind from Periods 1&2 are traced back (e.g. by either a PFSS or 3-D MHD model) to the boundary of an equatorial CH with a trailing AR. Period 3 is traced back to the boundary of another equatorial CH with a leading AR. We can see clear distinctions, especially in the solar wind ion composition data, not only between the fast and slow solar wind from the same CH (1&2), but also between the two slow solar wind periods (2&3) from the two CHs. These examples show us that the solar wind ion composition data are powerful diagnostics for the solar wind formation regions, and tell us that: We have learned much from past and current space missions about the collective behavior of the solar wind under various solar wind conditions in situ. To take a leap forward, it is necessary to look into the variations among individual streams, and to search for and understand the underlying cause for such variability. To do this, it is essential to investigate what processes and properties in the associated solar wind source regions contribute to and correlate with these variations.

3 Figure 2 Left panels: ACE measurements of solar wind parameters (1 hour data from SWEPAM, MAG and SWICS. Black curve is from 6-hour smoothing.). Three solar wind periods are marked in different color blocks, and the numbers are written on the top center panel. Period 1 (light-blue) is mostly in the fast solar wind (>500 km/s). Period 2 (dark-blue) and 3 (beige) are in the slow solar wind. The spiral angle indicates that Periods 1&2 and Period 3 are in different interplanetary magnetic field sectors divided by a heliospheric current sheet. Right panels: (Top): Coronal magnetic field configuration on the ecliptic cut plane for this period (Carrington Rotation (CR) 2036) overlaid on the coronal density contours, calculated from the MAS model run at the Community Coordinated Modeling Center ( Black (red) lines represent open (closed) field lines and the associated open field regions corresponding to the 3 periods are marked. Time increases (CR longitude decreases) clockwise starting from right at CR longitude of 360 degrees. (Middle and bottom): SOHO EIT λ195 and MDI magnetogram synoptic maps for CR2036 (EIT map: courtesy of Nathan Rich; MDI map from: with the approximate footpoint locations for the 3 periods indicated (coordinates are obtained from the PFSS model). We can see that Periods 2 and 3 have similar proton speed and density, but are very different in the proton temperature and ion composition signatures (O+7/O+6 ratio, average charge of Fe ions, Fe/O (the FIP bias) and He/p density ratios). This indicates that there are intrinsic differences in the formation of the solar wind between the two CH source regions. 2.2 Solar Spectroscopic Observations The solar spectroscopic observations, both on disk and off limb, provide rich information about the electron/ion temperature, density, abundance and flows in various structures at the

4 Sun from the chromosphere to the corona. It is well known that CHs have lower electron density and temperature than the AR/QS structures, and the FIP bias is also lower (Raymond et al. 2001). It is also well known that there are considerable variations within individual types of structures in terms of these properties. In the past decade, there have been increasing efforts to address the problem of solar wind origins from solar spectroscopic observations. Examples are: ion temperature anisotropy and outflows above CHs and in coronal streamers (e.g. Cranmer et al. 1999, Miralles et al. 2001, Strachan et al. 2002, Frazin et al. 2003, Teriaca et al. 2003), outflows at CHs and ARs (e.g. Hassler et al. 1999, Tu et al. 2005, Doschek et al. 2008, Harra et al. 2008, Habbal et al. 2008), abundance variations in solar structures (e.g. Raymond et al. 1997, Ko et al. 2006, Brooks & Warren 2010), relating coronal and solar wind properties from quadrature observations (e.g. Suess et al. 2000, Poletto et al. 2002, Bemporad et al. 2003), and reconnection signatures at CH boundaries (e.g. Madjarska et al. 2004). Figure 3 Hinode/EIS observations of AR10978 in the Fe XIII λ line. From left to right: 1) EIT λ195 image on the day of the EIS observation. The square marks the field-of-view of the EIS raster on AR10978, 2) Intensity image of Fe XIII λ built from the EIS raster, 3) corresponding Doppler velocity map (blue/red shift indicating up/down flows), and 4) non-thermal velocity map (red/yellow represents higher values than green/blue). Small squares marked on the Doppler velocity map are the upflow regions at both the west and east sides of the AR extracted for detailed study. The average electron density, FIP bias (from the Si/S abundance ratio), upflow velocity, and non-thermal velocity for both sides of the selected upflow regions are written on the panels. Upflows of ~20 km/s and non-thermal broadening of ~40 km/s were seen at both sides of the AR where the 1-2 million degree emission was weak. Figure 3 shows one recent such attempt from the Hinode/EIS observation of AR10978 on Dec.12, 2007 when the AR was crossing the central meridian (Brooks & Warren 2010). This AR is bracketed by an equatorial CH at its west and the low-latitude extension of the south polar CH at its east. The blue-shift (upflow) regions at the two sides of the AR hint that they can be where the solar wind flows out of the Sun. This is consistent with the idea that these two coronal holes, as indicated from coronal field models (e.g. the PFSS model), are all open field regions that connect to the Earth. The Si/S abundance ratio at the west side of the AR upflow region seems to be also consistent with that in the solar wind measured by ACE three days later. If such one-to-one connection can be established in general by the analysis of many regions, much insight can be gained about how source region properties are reflected in the solar wind properties through the formation process of the solar wind. We have learned much about the collective properties of the solar structures and have obtained many clues about how and where the solar wind is formed at the Sun. Solar spectroscopic observations such as the above, when combined with solar wind ion

5 composition measurements, can take us a step further in relating the variations in the solar wind with those in its source regions. It is thus essential to identify and analyze properties in regions at the Sun that are directly associated with the solar wind measured in situ. 2.3 Open Questions for Solar Wind Origin As we are equipped with much knowledge gained from the wealth of in situ solar wind measurements at one end and solar observations at the other, the time is ripe to connect the dots between the two. And the synergy between the two is essential to help answer some of the most important open questions for solar wind origin that are still facing us. Where is the solar wind formed, and what governs its variability? What signatures in the coronal hole and/or its boundary with the AR/QS reveal the formation of the, e.g., 700 km/s solar wind versus the formation of the, e.g., 400 km/s solar wind? How do properties, such as temperature and abundance, observed at the Sun lead and relate to properties measured in the solar wind? What in the solar data can reveal/differentiate the solar wind formation processes that result in the solar wind variation in time and space? Can a given theory/model of the solar wind formation make correct predictions for the variations at both the source regions at the Sun and the associated solar wind stream? 1 How is the solar wind heated and accelerated from various processes such as waves and reconnection? What heating and acceleration mechanisms at the Sun can produce the temperature, density, bulk flows, non-thermal velocity profiles, and abundances that are consistent with observations both at the solar source and in the associated solar wind? How does the solar magnetic field manifest itself in the variations of these observables? 3. Next Steps Toward Understanding The Solar Wind Origin In order to make breakthroughs in the next decade toward understanding the origin of the solar wind formation, we should recognize that solar spectroscopic and in situ solar wind ion composition data are one essential component of key measurements in solar and heliophysics missions. It is important to continue supporting the collection and analysis of solar spectroscopic and in situ solar wind ion composition data from existing and future space missions. Continuing efforts should be encouraged toward the development of high sensitivity and high spectral/time/spatial resolution solar spectroscopic instruments, and high time resolution solar wind heavy ion measurements, that meet the requirements for resolving the fine-scale and dynamic structures relevant for solar wind formation. It is important that future solar missions have the ability to measure the composition of both low- and high-fip elements across a wide range of temperatures and densities. Specific efforts should be taken to identify emission lines of ions that can match the insitu measurements and provide optimal coordinated diagnostics between the spectroscopic and in situ ion composition data. 1 We also recognize the importance of coronal field and solar wind modeling in order to get the best estimates of the magnetic connection between the source regions and the in situ measurements.

6 Co-existence, and coordination in location and timing, between solar spectroscopic observations and in situ solar wind heavy ion measurements will achieve the greatest scientific benefit. Examples are: 1) on-disk observations 2 of structures near the Sunspacecraft line with the solar wind measured in situ 3, and 2) quadrature observation of the solar corona with the in-situ solar wind off the solar limb 4. References Bemporad A. et al. 2003, ApJ, 593, 1146 Brooks, D. H., & Warren, H. P. 2010, submitted to ApJ Burgi, A., & Geiss, J. 1986, Sol. Phys., 103, 347 Cranmer, S.P. et al. 1999, ApJ, 511, 481 Doschek, G. A. et al. 2008, ApJ, 686, 1362 Frazin, R. A., Cranmer, S. P., & Kohl, J. L. 2003, ApJ, 597, 1145 Geiss, J. et al 1995, Science, 268, 1033 Habbal, S. R., Scholl, I. F., & McIntosh, S. W. 2008, ApJ, 683, L75 Harra, L., K. et al. 2008, ApJ, 676, L147 Hassler, D. M. et al. 1999, Science, 283, 810 Ko, Y.-K., Fisk, L. A., Geiss, J., Gloeckler, G., & Guhathakurta, M. 1997, Sol. Phys., 171, 345 Ko, Y.-K., Gloeckler, G., Cohen, C. M. S., & Galvin, A. B. 1999, JGR, 104, Ko, Y.-K., Raymond, J. C., Zurbuchen, T. H., Riley, P., Raines, J. M. & Strachan, L. 2006, ApJ, 646, 1275 Madjarska, M. S., Doyle, J. G., & van Driel-Gesztelyi, L. 2004, ApJ, 603, L57 Meyer, J.-P. 1985, ApJS, 57, 173 Miralles, M. P., Cranmer, S. R., Panasyuk, A. V., Romoli, M., & Kohl, J. L. 2001, ApJ, 549, L257 Owocki, S. P., Holzer, T. E., & Hundhausen, A J., 1983, ApJ, 275, 354 Poletto, G. et al. 2002, JGR, 107, 1300 Raymond, J. C. et al. 1997, Sol. Phys., 175, 645 Raymond, J. C. et al. 2001, in Joint SOHO/ACE workshop "Solar and Galactic Composition". Edited by Robert F. Wimmer-Schweingruber. Publisher: American Institute of Physics Conference proceedings vol. 598 location: Bern, Switzerland, March 6-9, 2001, p49 Strachan, L., Suleiman, R., Panasyuk, A. V., Biesecker, D. A., & Kohl, J. L 2002, ApJ, 571, 1008 Suess, S. T., Poletto, G., Romoli, M., Neugebauer, M., Goldstein, B. E., & Simnett, G. 2000, JGR, 105, Teriaca, L., Poletto, G., Romoli, M., & Biesecker, D. A. 2003, ApJ, 588, 566 Tu, C.-Y. et al. 2005, Science, 308, 519 von Steiger, R. et al. 2000, JGR, 105, Wang, Y.-M., Ko, Y.-K., & Grappin, R. 2009, ApJ, 691, such as Hinode, the proposed Solar-C mission (Doschek et al.), the Solar Spectroscopy Explorer Mission (SSE) (Bookbinder et al.), and Fine scale Advanced Coronal and Transition region Spectrograph (FACTS) Mission (Korendyke et al.). The latter three are white papers submitted to the Decadal Survey. 3 such as ACE, STEREO, Solar Orbitor, and the proposed Solar Sentinels Mission (Szabo et al., a white paper submitted to the Decadal Survey). Note that there is no future solar wind heavy ion sensors planned at the Sun-Earth line (e.g. L1) post ACE. 4 such as the proposed Mission to the Sun-Earth L5 Lagrangian Point: An Optimal Platform for Heliophysics & Space Weather Research (Vourlidas et al.), and Ultraviolet Coronagraph Spectroscopy: A Key Capability for Understanding the Physics of Solar Wind Acceleration (Cranmer et al.). Both are white papers submitted to the Decadal Survey.

7 Zhang, J., Woch, J., Solanki, S. K., & von Steiger, R. 2002, GRL, 29, 1236 Zurbuchen, T. H., Hefti, S., Fisk, L. A., Gloeckler, G., & von Steiger, R. 1999, Space Sci. Rev., 87, 353 Zurbuchen, T. H., Fisk, L. A., Gloeckler, G., & von Steiger, R. 2002, GRL, 29, 1352