Ilpo Virtanen

Asymmetry of solar polar fields and the southward shift of HCS observed by Ulysses

[1] We study the hemispheric asymmetry of high-latitude unipolar fields and the latitudinal shift of the heliospheric current sheet (HCS) using Ulysses magnetic field observations during the perihelion passes in 1994-1995, 2001-2002, and 2007. Using the cumulative flux density and the best fit lines to its high-latitude observations in the two hemispheres, we find that the absolute value of the high-latitude radial field of the Southern Hemisphere is larger than in the north during both minimum time scans in 1994-1995 and 2007. The hemispheric difference is about 0.2 nT during both scans, suggesting that the northern field area is some 5%―10% (5%―15%) larger than the southern area and that the HCS is shifted southward by about -2° during both scans. The results resolve the discrepancy between earlier results in 1994-1995 and clarify similar observations in 2007 in the ecliptic. They also verify the southward shift of the HCS during the exceptional solar cycle 23. We also study the detailed structure of the equatorial region and find that generally the Ulysses observations compare favorably with simultaneous heliospheric magnetic field predictions given by Wilcox Solar Observatory synoptic maps. Using a simple HCS model, we find that even in case of southward shifted HCS, the highest peak of the cumulative flux density could be located above the equator. Thus, Ulysses observations around the heliographic equator cannot alone give an unambiguous information of the HCS shift, which emphasizes the importance of studying the high-latitude sections of Ulysses orbit.

The last dance of the bashful ballerina

Aims. The heliospheric magnetic field (HMF) has long been hemispherically asymmetric so that the field in the northern hemisphere is weaker and the area larger than in the south. This asymmetry, also called the bashful ballerina, has existed during roughly three-year intervals of the late declining to minimum phase of solar cycles 16‐22. We study the HMF and its hemispheric asymmetry during the exceptional solar cycle 23. Methods. We use NASA National Space Science Data Center OMNI database, which contains all solar wind and HMF observations at the Earth’s orbit, and coronal field predictions by Wilcox Solar Observatory. We present a new method to study the global hemispheric asymmetry by using the power n of the radial decrease of the radial field from the coronal source surface to 1 AU. Results. We find that the HMF is exceptional at low latitudes in solar cycle 23: while the typical latitudinal variation was attained in the north in 2008, it did not take place in the south until Spring 2009. Thus, the Rosenberg-Coleman rule is abnormally delayed or broken for the first time in 50 years. The n-values verify the clear northern dominance in cycles 21‐22. However, the low-latitude observations depict a considerably smaller asymmetry in cycle 23, although Ulysses observations at high latitudes show an equally large asymmetry in 2007 and in 1994‐1995. We argue that the weak low-latitude visibility of the asymmetry in cycle 23 is due to the exceptionally weak polar fields, leading to large tilt angle and a wide current sheet. Conclusions. We note that the exceptional properties of cycle 23 (weak dynamo, large tilt, small asymmetry) agree with the long-term evolution of hemispheric asymmetry viewed at the Earth. The active Sun is seen as more asymmetric at the Earth than the quiet Sun because the polar coronal holes with unipolar fields extend closer to the equator, allowing their asymmetry to be viewed even at low latitudes. We suggest that, after the period of weak activity and small asymmetry at 1 AU that started with cycle 23, the hemispheric asymmetry will again, withthe increasingly active cycles, become better visible at 1 AU but the asymmetry will be oppositely oriented, including a northward shifted current sheet, and larger areas but weaker intensities in the south. Thus, the ballerina should no longer be systematically bashful for some 100‐150 years.

Photospheric and coronal magnetic fields in six magnetographs - I. Consistent evolution of the bashful ballerina

Aims. We study the long-term evolution of photospheric and coronal magnetic fields and the heliospheric current sheet (HCS), especially its north-south asymmetry. Special attention is paid to the reliability of the six data sets used in this study and to the consistency of the results based on these data sets. Methods. We use synoptic maps constructed from Wilcox Solar Observatory (WSO), Mount Wilson Observatory (MWO), Kitt Peak (KP), SOLIS, SOHO/MDI, and SDO/HMI measurements of the photospheric field and the potential field source surface (PFSS) model. Results. The six data sets depict a fairly similar long-term evolution of magnetic fields and the heliospheric current sheet, including polarity reversals and hemispheric asymmetry. However, there are time intervals of several years long, when first KP measurements in the 1970s and 1980s, and later WSO measurements in the 1990s and early 2000s, significantly deviate from the other simultaneous data sets, reflecting likely errors at these times. All of the six magnetographs agree on the southward shift of the heliospheric current sheet (the so-called bashful ballerina phenomenon) in the declining to minimum phase of the solar cycle during a few years of the five included cycles. We show that during solar cycles 20–22, the southward shift of the HCS is mainly due to the axial quadrupole term, reflecting the stronger magnetic field intensity at the southern pole during these times. During cycle 23 the asymmetry is less persistent and mainly due to higher harmonics than the quadrupole term. Currently, in the early declining phase of cycle 24, the HCS is also shifted southward and is mainly due to the axial quadrupole as for most earlier cycles. This further emphasizes the special character of the global solar field during cycle 23.

Reconstructing solar magnetic fields from historical observations I. Renormalized Ca K spectroheliograms and pseudo-magnetograms

Aims. The present work is the first in a series of articles that develop a new proxy to represent the evolution of magnetic activity in past solar cycles by combining the information from historical Ca II K line spectroheliograms and sunspot magnetic field measurements. Methods. We use synoptic (Carrington) maps from 1915‐1985 that were derived from daily Ca K line observations at Mount Wilson Observatory to identify the chromospheric plages and to create synoptic pseudo-magnetograms. We use historical observations of sunspot magnetic fields from 1917 to the present to assign polarity to pixels situated within plages. The original Ca K spectroheliograms are nonuniform in their brightness, and we develop a novel approach to re-normalize their intensities. Results. We show that a homogeneous long-term series of pseudo-magnetograms can be successfully constructed by combining sunspot field measurements and plages with renormalized intensities. In our tests, about 80% of pixels situated within plages showed the same magnetic polarity as the synoptic magnetograms taken with the Kitt Peak Vacuum Telescope. Finally, we discuss possible approaches to further improve the agreement between observed and pseudo-magnetograms.

Reconstructing solar magnetic fields from historical observations - II. Testing the surface flux transport model

Aims. We aim to use the surface flux transport model to simulate the long-term evolution of the photospheric magnetic field from historical observations. In this work we study the accuracy of the model and its sensitivity to uncertainties in its main parameters and the input data. Methods. We tested the model by running simulations with different values of meridional circulation and supergranular diffusion parameters, and studied how the flux distribution inside active regions and the initial magnetic field affected the simulation. We compared the results to assess how sensitive the simulation is to uncertainties in meridional circulation speed, supergranular diffusion, and input data. We also compared the simulated magnetic field with observations. Results. We find that there is generally good agreement between simulations and observations. Although the model is not capable of replicating fine details of the magnetic field, the long-term evolution of the polar field is very similar in simulations and observations. Simulations typically yield a smoother evolution of polar fields than observations, which often include artificial variations due to observational limitations. We also find that the simulated field is fairly insensitive to uncertainties in model parameters or the input data. Due to the decay term included in the model the effects of the uncertainties are somewhat minor or temporary, lasting typically one solar cycle.

Comparing coronal and heliospheric magnetic fields over several solar cycles

Here we use the PFSS model and photospheric data from Wilcox Solar Observatory, SOHO/MDI, SDO/HMI, and SOLIS to compare the coronal field with heliospheric magnetic field measured at 1 au, compiled in the NASA/NSSDC OMNI 2 data set. We calculate their mutual polarity match and the power of the radial decay, p, of the radial field using different source surface distances and different number of harmonic multipoles. We find the average polarity match of 82% for the declining phase, 78%–79% for maxima, 76%–78% for the ascending phase, and 74%–76% for minima. On an average, the source surface of 3.25 R S gives the best polarity match. We also find strong evidence for solar cycle variation of the optimal source surface distance, with highest values (3.3 R S ) during solar minima and lowest values (2.6 R S –2.7 R S ) during the other three solar cycle phases. Raising the number of harmonic terms beyond 2 rarely improves the polarity match, showing that the structure of the HMF at 1 au is most of the time rather simple. All four data sets yield fairly similar polarity matches. Thus, polarity comparison is not affected by photospheric field scaling, unlike comparisons of the field intensity.

Photospheric and coronal magnetic fields in six magnetographs - II. Harmonic scaling of field intensities

Context. Photospheric magnetic fields have been observed since the 1970s by several ground-based and satellite instruments. While the different instruments show a fairly similar large-scale structure and temporal evolution of the photospheric magnetic field, the magnetic field intensity varies significantly between the observations. Aims. We introduce a new method for scaling the photospheric magnetic field in terms of the harmonic expansion. Contrary to earlier scaling methods, the harmonic scaling method can be straightforwardly used for data sets of different resolutions. Methods. We use synoptic maps constructed from Wilcox Solar Observatory, Mount Wilson Observatory (MWO), Kitt Peak (KP), SOLIS, SOHO/MDI and SDO/HMI measurements of the photospheric field. We calculate the harmonic expansions of the magnetic field for all these data sets (for most, up to n = 180) and investigate the scaling of the harmonic coefficients between all possible pairs of data sets. Results. The six data sets generally scale to one another relatively well, with the exception of even axial terms, especially the g2 quadrupole for a few pairs of data sets. Differences in polar field observations, pole-filling methods and possible zero-level mainly affect the scaling of even axial terms. Scaling factors typically slightly increase with harmonic order. The mutual scaling between SOLIS and HMI is very good, and one single overall coefficient of approximately 0.8 would be a reasonable choice for those data sets. Our results suggest that the KP synoptic maps are offset by a few degrees with respect to MWO and MDI. We note that the new method gives a correct scaling for the low harmonic terms that are sufficient and necessary for coronal modeling.

Bihelical Spectrum of Solar Magnetic Helicity and Its Evolution

Using a recently developed two-scale formalism to determine the magnetic helicity spectrum (Brandenburg et al. 2017), we analyze synoptic vector magnetograms built with data from the Vector Spectromagnetograph (VSM) instrument on the \emph{Synoptic Optical Long-term Investigations of the Sun} (SOLIS) telescope during January 2010-July 2016. In contrast to an earlier study using only three Carrington rotations, our analysis includes 74 synoptic Carrington rotation maps. We recover here bihelical spectra at different phases of solar cycle~24, where the net magnetic helicity in the majority of the data is consistent with a large-scale dynamo with helical turbulence operating in the Sun. More than

Tilt of Sunspot Bipoles in Solar Cycles 15 to 24

20\%

Southward shift of the coronal neutral line and the heliospheric current sheet: Evidence for radial evolution of hemispheric asymmetry

of the analyzed maps, however, show violations of the expected sign rule.