SH51A-1589
Polar Magnetic Fields Observed During the Last Four Solar Minima
The Sun's polar fields during the current minimum are the weakest in at least four solar cycles. The field strengths are fairly symmetric, unlike at least the two previous minima. We compare data from the Wilcox Solar Observatory (WSO) and Michelson Doppler Imager (MDI) to follow the polar field changes since 1976. The polar field is never observed well from Earth because the ecliptic lies near the Sun's equator, and each year the view of the north (south) is completely hidden for several months around March 7 (September 7). Analysis of the most favorably oriented MDI synoptic maps each year allows us to derive the fairly slowly evolving large-scale polar magnetic field pattern from 1996 to the present. We account for differential rotation and other geometric effects. The analysis allows us to provide a useful interpolated or extrapolated correction that can be smoothly incorporated into the global synoptic or synchronic maps above about 70 degrees latitude. The polar field is important in modeling the large-scale coronal and heliospheric field, particularly at minimum. Even though there has been extremely little solar activity over the last several months, at the current solar minimum the structure of the corona is much less equatorial than usual, in part because the polar fields are relatively weak.
SH51A-1590
High-latitude activity and its relationship to the mid-latitude solar activity
The high-latitude activity at photosphere and corona, and their relation to the mid-latitude activity in cycle 23 have been explored using the MDI magnetic synoptic maps available on the SOHO web page with a new calibration and EIT synoptic maps. The EIT synoptic maps of EUV images in three lines Fe and in one line He II (171A, 195A, 284A and 304A) are obtained for period June 1996 - May 2006 (CR1911-CR2042) from the full disk EIT images. They are represented by values of the line intensity centered on the central meridian and can be directly compared with magnetic synoptic maps (MDI maps). It is found that the solar cycle dependence of the EUV polar corona occurs because of the large-scale topology of the solar corona and its relationship with the mid-latitude magnetic flux. It is seen more pronounced on the rising phase of the solar cycle due to the connectivity of the coronal structures extended from the mid-latitude to the high-latitude. But, after the solar cycle maximum the EUV polar corona shows a less dependence of the mid-latitude corona. In the polar regions the absent of the correlation of the unsigned magnetic flux and EUV corona occurs not only due to the effect of projection. But it tells us about the numerous emerging bi-polar and unipolar regions inside the polar region which does not contribute to the brightness of the EUV corona. Really, during the solar minimum when the polar magnetic field reaches its maximum and number of unipolar magnetic regions of strong magnetic field increases, we observe dimming and coronal holes instead of the bright EUV corona.
SH51A-1591
Tracking Vector Magnetograms with the Magnetic Induction Equation
The differential affine velocity estimator (DAVE) that we developed in 2006 for estimating velocities from line- of-sight magnetograms is modified to directly incorporate horizontal magnetic fields to produce a differential affine velocity estimator for vector magnetograms (DAVE4VM). The DAVE4VM's performance is demonstrated on the synthetic data from the anelastic pseudospectral ANMHD simulations that were used in the recent comparison of velocity inversion techniques by Welsch and coworkers. The DAVE4VM predicts roughly 95% of the helicity rate and 75% of the power transmitted through the simulation slice. Intercomparison between DAVE4VM and DAVE and further analysis of the DAVE method demonstrates that line-of-sight tracking methods capture the shearing motion of magnetic footpoints but are insensitive to flux emergence - the velocities determined from line-of-sight methods are more consistent with horizontal plasma velocities than with flux transport velocities. These results suggest that previous studies that rely on velocities determined from line-of-sight methods such as the DAVE or local correlation tracking may substantially misrepresent the total helicity rates and power through the photosphere.
SH51A-1592
Solar rotation rate at high latitude measured by MDI magnetograms
Using MDI 1-minute cadence magnetograms taken by a campaign program in March 2008, we study solar rotation rate at high latitude using an image cross correlation technique. The MDI magnetograms are first remapped onto the heliographic coordinates, and then averaged by 60 individual magnetograms in order to increase the ratio of signal-to-noise. The final data thus has a temporal resolution of 60 minutes. We compare our results with previous studies that usually measured the rotation rate in low latitude only, and also place a discussion here. SOHO is a project of international cooperation between ESA and NASA.
SH51A-1593
Predicting Solar Cycle 24
We review the Solar Polar Field Precursor Solar cycle Prediction Method. There are now four solar minimum direct measurements by WSO of the polar fields with a 3-arcminute aperture. The first two years of data were contaminated by scattered light from dirty optics, resulting in a reduction of the polar field values. Each percent of scattered light [measured just outside the limb] reduces the magnetic field by 3.5%. We correct for this contamination and present updated polar field values. Currently, the polar fields are the weakest since 1976. We present evidence from interplanetary magntic field measurements that the polar fields during the minimum ~1965 was lower than during the minimum ~1996. The current polar fields have not shown sign of weakening, and we estimate based on polar fields from the five most recent solar minima that solar cycle 24 will only reach a maximum of Rz ~70 ± 10, probably in 2013, corresponding to F10.7 values in the range of 125 ± 10.
SH51A-1594
Determination of Temperatures and Densities of Polar Coronal X-ray Jets Observed with Hinode XRT and EIS
In this work we present observations of polar coronal X-ray jets made with Hinode XRT and EIS during Septebmer 2008 joint observing program. We present the methods for determining temperature and density of jets as well as some statistics. The temperature sytucture of jets is considered and is related to the density measurements.
SH51A-1595
ULYSSES at High Solar Latitudes: The Signature of Evolving Turbulence in Quiet Fast Solar Wind
The Ulysses spacecraft spent many months at high heliospheric latitudes above the Sun's polar coronal holes, in the quiet fast solar wind with a highly ordered magnetic field. The power spectra of fluctuations in solar wind bulk parameters, such as the magnetic field, show inverse power law behaviour that contains both an inertial range of intermittent turbulence and, at lower frequencies, a scaling range with approximate "1/f" dependence. Here we use statistical analysis methods, such as the generalized structure function (GSF) and extended self-similarity (ESS), to quantify the scaling of the moments of the probability density function of fluctuations in the magnetic field. We present corroborating results from both the South 1994 and the North 1995 polar passes by Ulysses. GSF is sufficient to reveal approximate power law scaling for the "1/f" range, and we test the resultant scaling exponents for secular trends with heliospheric latitude and with radial distance from the sun. At higher frequencies, where we would anticipate an inertial range of turbulence, the scaling properties of the structure functions, as well as being characteristic of intermittent turbulence, also show evidence of an 'envelope' function which we identify. We find that within these regions of quiet fast polar flows, this envelope function does not vary with heliospheric radius or latitude, and in particular is the same for both polar passes. This suggests that while the higher frequency, or inertial range, fluctuations found in the solar wind at high heliospheric latitudes are consistent with locally evolving turbulence, their envelope also reflects some aspects of the evolution of coronal structure and flows into the quiet fast solar wind. RN acknowledges the STFC and UKAEA Culham for financial support and A. Balogh and the ULYSSES team for data provision.
SH51A-1596
Magnetic Sectors of the Heliospheric Field as Observed by Ulysses Through Two Solar Cycles
The nearly two solar cycles that Ulysses has spent in its polar orbit around the Sun has provided a data base of in situ measurements as a function of heliographic latitude. The polarity of the magnetic field is a well defined quantity which has characteristic variations vs heliographic latitude and time according to the phase of the solar sunspot cycle. We have determined the sector crossings for the whole 18 years long data base, and the location of the crossings were mapped back to the Sun using the solar wind velocity measured onboard. From the location of the sector boundaries the tilt angle of the heliospheric current sheet was estimated through the whole mission. Two shorter time intervals were analyzed in detail and compared, when Ulysses moved poleward of the magnetic sectors in 1993 and 2005, corresponding to the declining phases of cycles 22 and 23, respectively. The Ulysses observations were compared with the source surface potential field models, obtained from ground based photospheric magnetic field measurements by the Wilcox Solar Observatory. Systematic deviations have been found, which were interpreted by super radial expansion of the solar wind above coronal holes.
SH51A-1597
Solar Wind Measurements on Solar Orbiter: Discovering the Links Between the Solar Wind and the Atmosphere of our Sun
The Solar Orbiter spacecraft will be launched in 2015 and will include the Solar Wind Analyzer instrumentation suite. This space mission will allow for unprecedented data collection of particle characteristics near the Sun at various heliolatitudes during both the quiet and active phases of the solar cycle. The close proximity will allow for determination of the source regions on the sun for the observed events. Of particular interest will be the study of the origins of and processes related to solar energetic particles. This mission will lead to a better understanding of the Sun and the interstellar medium in our solar system.
SH51A-1598
Reconnection-Driven Changes of the Open-Closed Coronal Magnetic Field Boundary
We present recent 3D MHD simulation results with the ARMS code that show the dynamic evolution of the coronal helmet streamer belt boundary via magnetic reconnection processes. We start with an initial multipolar PFSS configuration with a coronal null point and associated topological features located under the streamer belt, well within the closed-field region. As this configuration is slowly energized via rotational shearing flows, volumetric currents form along the separatrix boundary and during the subsequent field evolution are compressed to a thin current sheet until the numerical resistivity mimics the onset of magnetic reconnection. The reconnection scenario is analogous to the coronal breakout-reconnection in CME modeling, but here, once the closed streamer belt flux has been transferred out of the way there is interchange reconnection that opens the outer-spine fieldline and creates a narrow channel of open field surrounding the AR sepratrix boundary. This topological evolution is best understood in the context of the Antiochos (2007) conjecture about the existence of "coronal hole canals". In addition, we will discuss the implication of the MHD simulation results for creating slow, unstructured, streamer blob-like transients and as one of the potential mechanisms operating at the open-closed magnetic field boundary that could lead to the formation of the slow solar wind. This work is supported, in part, by NSF ATM-0621725 and NASA NNX08AJ04G.