SH44A-01 INVITED
The Buildup of Large-Scale Polar Magnetic Fields on the Sun: Small Things Can Make a Difference
Large-scale magnetic field patterns visible at high latitudes on the solar photosphere are thought to form primarily from the poleward transport of flux that has emerged at lower latitudes. It is only a small percentage of this lower-latitude (i.e., active region) flux, however, that makes it to the poles, as much active-region flux cancels during its emergence and subsequent dispersal. This dispersal is characterized by the shearing and advection caused by the surface differential rotation and meridional flows, as well as by constant buffeting by near-surface convection and interactions with nearby flux concentrations. Consequently, these processes can play an important role in the transport of flux to the poles and the buildup of the polar caps, and their nonlinear nature implies that interactions between patterns with differing spatial (and temporal) scales can affect the timing of the formation of the polar cap as well as its overall amplitude. Such possibilities are investigated using a surface flux-transport model, and implications regarding the heliosphere are discussed.
SH44A-02
Polar Magnetic Field Distribution as Observed by SOLIS and its relation with coronal structures
Chromospheric full-disk magnetograms obtained by the Vector Spectromagnetograph (VSM), from the Synoptic Optical Long-term Investigations of the Sun (SOLIS) project, are utilized to study the distribution of magnetic field flux elements within the polar caps. The density of magnetic flux concentrations is found to be uniform up to latitude of 75 degree and decreases significantly toward the solar pole. Our results should have implications for the processes carrying the magnetic flux from low to high latitudes (e.g., meridional circulation). This also has implications for numerous solar phenomena such as the formation and evolution of fine polar coronal structures (i.e., polar plumes).
SH44A-03 INVITED
Helioseismic Measurement of Subsurface Flows at Solar High Latitude
The solar polar magnetic field is of great interest as it is where solar magnetic field reversal starts. Surface and subsurface plasma flows are very important in understanding the field reversal because magnetic field flux is transported to solar high latitude from lower latitude according to flux transport theory. Local helioseismology has been able to derive subsurface flow fields, rotation rates, and meridional flows up to 30 Mm in depth. The results obtained by time-distance helioseismology during Solar Cycle 23 from SOHO/MDI have also revealed significant changes of the speed and the longitudinal structure of the flows. We used these measurements to compare with the magnetic flux transport determined from the magnetic field synoptic data. Furthermore, by use of MDI dynamic campaign observations and a recent high resolution observation of solar South Pole by Hinode, we explore the possibility to detect subsurface flow fields in solar high latitude.
SH44A-04 INVITED
The relation between the magnetic fields and the coronal activities in the solar polar region
The telescopes aboard Hinode satellite showed new views in solar physics. One of new discoveries obtained by Hinode is the activities in the polar region of the sun. Savcheva et al. (2007) investigated the coronal hole around the south pole using the high-cadence X-ray images, and they found that the appearance of X-ray jets in the polar coronal holes occurs at very high frequency -- about 60 jets/day on average. The other of new results obtained by Hinode is the magnetic field measurements of the solar polar region with high accuracy. Tsuneta et al. (2008) found that many vertically-oriented magnetic flux tube with field strength as strong as 1--1.2 kG that are scattered in latitude between 70 -- 90. If the trumpet-like magnetic structures extend to interplanetary space, there is possibility that they are the guide field of X-ray jets, coronal plumes and fast-solar wind. Thus, in order to understand the polar phenomena, it is very important to investigate the relation between the magnetic field distribution and the coronal structures. We examined the co-alignment between the Stokes-V maps of Na, the Stokes-Q maps of Fe, the X-ray images and the EUV images of the north polar region. At the result, we found that most of the trumpet-like magnetic structures around the pole do not associate with the coronal structures. It suggests that the energy injection for the fast-solar wind is not taking place in the corona. The other finding from the co-alignment is that X-ray jets are produced by the emerging/canceling fluxes. It is same as the jets in the active regions. The existence of the emerging flux near the pole suggests that the magnetic fields around the pole are provided from not only the active regions but also the emerging fluxes near the pole.
SH44A-05
3D Reconstruction of Polar Plumes From STEREO/SECCHI Images.
We provide a newly developed approach to determine the 3D structure of high latitude open magnetic fields as visible in polar plumes. Our data are image pairs taken by the SECCHI-EUVI telescopes on board of the two STEREO spacecraft. From these image pairs we identify the plumes by two different methods. The first method identifies the plume axes by the local intensity maxima and the second method uses image processing tools such as the Hough Transform. The Hough Transform transfers plumes from images into points, called Hough coordinates, which can directly be used to calculate the 3D location of the plume. The automatic plume detection by the Hough transform method is well suited to study their temporal evolution. The capability of our code has been investigated with synthetic images taken from a 3D magnetohydrostatic corona model (Neukirch, 1995) and we apply our methods to study polar plumes observed from two vantage viewpoints. We identify the locations of the footpoints of the polar plumes in the photosphere as well as their inclination relative to the line-of-sight and to their local radial direction. The relationship between plume and bright point are investigated. With the help of SOHO/SUMER observations we derive the density scale height in the plumes. We found that plumes are not the main contributor to the fast solar wind.
SH44A-06
Recent studies of the behavior of the Sun's white-light corona over time
Predictions of upcoming solar cycles are often related to the nature and dynamics of the Sun's polar magnetic field and its influence on the corona. For the past 30 years we have a more-or-less continuous record of the Sun's white-light corona from groundbased and spacebased coronagraphs. Over that interval, the large scale features of the corona have varied in what we now consider a "predictable" fashion--complex, showing multiple streamers at all latitudes during solar activity maximum; and a simple dipolar shape aligned with the rotational pole during solar minimum. Over the past three decades the white-light corona appears to be a better indicator of "true" solar minimum than sunspot number since sunspots disappear for months (even years) at solar minimum. Since almost all predictions of the timing of the next solar maximum depend on the timing of solar minimum, the white-light corona is a potentially important observational discriminator for future predictors. In this contribution we describe recent work quantifying the large-scale appearance of the Sun's corona to correlate it with the sunspot record, especially around solar minimum. These three decades can be expanded with the HAO archive of eclipse photographs which, although sparse compared to the coronagraphic coverage, extends back to 1869. A more extensive understanding of this proxy would give researchers confidence in using the white-light corona as an indicator of solar minimum conditions.
SH44A-07
The Flux of Open and Torroidal Interplanetary Magnetic Field as a Function of Heliolatitude and Solar Cycle
Analyses performed during the previous 11-year phase of the solar cycle attempted to measure the flux of open and toroidal magnetic field lines [Bieber and Rust, ApJ, 453, 911, 1995] and associate the toroidal flux with coronal mass ejections (CMEs) [Smith and Phillips, JGR, 102, 249, 1997]. Since that time Ulysses has made three polar passes and spent at least eight years at polar latitudes, enabling us to examine the underlying assumption of the earlier studies that using near-ecliptic latitude measurements could serve as a proxy for polar-latitude observations. We find confirmation of the claims that the present solar minimum has experienced a strong decrease in open flux, but we also find evidence of past conditions at this same level. We find that torroidal flux is virtually negligable at higher latitudes as measured by the Ulysses spacecraft, even during times of solar maximum, and attribute this to the sub-photospheric winding of the Sun's magnetic field as illustrated by the familiar butterfly diagram. Our observations of the rate of toroidal flux ejection, 7 × 1022 Mx/year, sets a lower limit on the amount of magnetic flux that can be ejected by CMEs near solar maximum.
SH44A-08 INVITED
Future high-latitude observations anticipated from the Solar Orbiter mission
As one of the cornerstones of the HELEX programme, the Solar Orbiter mission is currently scheduled for launch in 2015. After an initial cruise phase, Solar Orbiter will reach its science orbit in 2018. This orbit comprises initially a nearly Sun-synchronous phase at a distance of only 0.22 AU from Sun center. In a later stage, the orbital inclination will be raised, thus allowing Solar Orbiter to reach solar latitudes of about 35 degrees, and making it the first mission after Ulysses to study the Sun from a high-latitude vantage point. In contrast to Ulysses, however, Solar Orbiter will carry a complementary suite of both, in-situ and remote- sensing instruments, which will allow the study of the solar atmosphere to be extended to the largely unexplored polar regions of the Sun. The polar magnetic fields are responsible for the polar coronal holes driving the fast solar wind, but are poorly known. From its vantage point outside the ecliptic, Solar Orbiter will uncover the surface and sub-surface flows at the poles, the polar magnetic field structure and its evolution. It will provide new insights into the formation of the polar coronal holes, the nature of their boundaries and the acceleration of the fast solar wind emanating from the holes. The potential of Solar Orbiter for investigating the acceleration mechanism of the fast wind, the plumes and X-ray jets at the poles, and the high-latitude meridional circulation will be discussed.