Fifth Solar Orbiter Workshop
September 10 - 14, 2012 - Brugge, Belgium

Session 1
Solar magnetism and the solar cycle

Type: Oral
Date: Monday, September 10, 2012
Time: 09:30 - 15:20
Remarks: Coffee & Posters: 10:30-11:00 & 15:20-16:00

Seq   Time   Title   Abs No
1   09:30   The solar dynamo
Brandenburg, Axel
Nordita, SWEDEN

Recent direct numerical simulations of convection in a rotating spherical wedge have now produced a cyclic magnetic field with equatorward migration around 20 degrees latitude and a polar branch at high latitudes (Kapyla, Mantere, Brandenburg 2012, ApJL, in press, arXiv:1205.4719). While they show us that magnetic field generation in the bulk of the convection zone is possible, and that cycle times can be long enough, the precise mechanism for migration near the surface remains unclear. It is not the near-surface shear layer, as has been speculated previously. When allowing for an exterior region above the convection zone to model the effects of a corona, one sees occasional events that resemble coronal mass ejections. While this modelling is still very simplistic, it does teach us an important lesson. We know that dynamos should shed negative magnetic helicity in the north and positive in the north, but both simulations and Ulysses observations now show that some distance away from the Sun the sign of magnetic helicity has reversed. Confirming this and determining the distance from the Sun where this happens (which is less than 1 AU) would be of great interest and may be possible with Solar Orbiter. An import aspect of any dynamo is the formation of active regions and eventually sunspots. It is generally believed that such structures arise from magnetic fields anchored at the bottom of the convection zone, but this may not be the case. Simulations now show that magnetic fields can spontaneously concentrate into structures on scales of several turbulent eddies (Brandenburg et al. 2011, ApJL 740 L50). While similar behavior is now also seen in numerical simulations (Stein & Nordlund 2012, ApJL 753, L13), it remains to be shown whether these mechanisms are related to each other. Both surface and helioseismological observations may help clarifying this question.

  Invited talk
2   10:00   Review on Helioseismology and Helioseismology with Solar Orbiter
Roth, Markus
Kiepenheuer-Institut für Sonnenphysik, GERMANY

In the past dedicated projects like the Solar and Heliospheric Observatory (SOHO) and the Global Oscillation Network Group (GONG) have pathed the way for the success of helioseismology. Recently the field took again a giant leap forward with the successful launch of the new space mission SDO. This talk will review the achievements and prospects of helioseismology, and will make a forward-look on the future developments of the field in regard of the upcoming Solar Orbiter space mission.

  Invited talk
3   11:00   solar magnetic field reversal and the role of dynamo families
BRUN, Sacha1; Derosa, M2; Hoeksema, T.3
1CEA-Saclay, FRANCE;
3Stanford solar group, UNITED STATES

We study how the solar magnetic field evolves from antisymmetric (dipolar) to symmetric (quadrupolar) state during the course of its 11-yr cycle. We show that based on equatorial symmetries of the induction equation, current Babcock-Leighton solar dynamo models excite mostly the antisymmetric (dipolar) family whereas a decomposition of the solar magnetic field data over the last 3/4 cycles (21 to 23/24) reveals that both families should be excited. We propose an alternative solar dynamo solution to reconcile models and observations.

4   11:20   Planning for Helioseismology with SO/PHI
Birch, A.1; Gizon, L.2; Hirzberger, J.2
1Max Planck Institute for Solar System Research, GERMANY;
2 Max Planck Institute for Solar System Research, GERMANY

Solar Orbiter presents an exciting opportunity for helioseismology. Some of the potential targets for helioseismology using PHI observations are large-scale flows (e.g., differential rotation, meridional flow), near-surface convection, sunspots, and the deep convection zone. Planning for data acquisition and processing for helioseismology requires knowledge of the science requirements as well as knowledge of the constraints from the instrument, the spacecraft, and the mission operations (e.g., telemetry, science windows, on-board processing capabilities). We will present a roadmap for obtaining reliable estimates for the detectability of helioseismology targets.

5   11:40   Recent observations of solar magnetic fields with Hinode, SDO and Sunrise
Borrero, J.
Kiepenheuer Institut fur Sonnenphysik, GERMANY

In the solar Photosphere, the interaction between the magnetic field and convective flows gives raise to a rich variety of phenomena: magnetic flux emergence, magnetic reconnection, shear flows, helicity build-up and so on. These phenomena range from extremely small spatial and temporal scales (few km and seconds) to large and long-lived (several Mm and weeks). In recent years, a number of missions have been launched to study these phenomena at all these possible scales: SDO, Hinode and Sunrise. While SDO is more suitable to study large-scale and long-lived events, Hinode and Sunrise are ideal to investigate those occurring at the smallest spatial and temporal scales. In this contribution I will present some of the most recent and important findings made with these new telescopes and their attached instrumentation. In particular, I will mostly focus on those results where the evolution of the Photospheric magnetic field leads to enhanced activity in the Chromosphere and Corona. If time permits I will also address some of the problems the aforementioned instruments had to deal with, and how they might affect the measurements of the Photospheric magnetic field vector with the PHI instrument on Solar Orbiter.

  Invited talk
6   12:10   Small Magnetic Elements, Bright Points and Solar Irradiance
Domingo, V.; Cabello, j.; Blanco, J.
Universidad de Valencia, SPAIN

The population of small magnetic elements as bright points in the quiet sun is studied as a function of latitude, with particular emphasis on high latitudes, anticipating the polar caps observation by Solar Orbiter, with its contribution to the determination of the solar luminosity. We make use of the fact that the Sun's rotation axis is inclined with respect to the ecliptic plane to measure the fraction of quiet sun solar surface covered by bright points at the highest possible latitude visible from Earths point of view. The contribution of the quiet sun to the total solar irradiance and variation is discussed as well. The research is based on data obtained by the Hinode spacecraft and at the Swedish Solar Telescope at Roque de los Muchachos/La Palma observatory.

7   14:00   Reaction of the outer solar atmosphere to the solar cycle
Schrijver, C.J.
Lockheed Martin Advanced Technology Center, UNITED STATES

As the outer solar atmospere responds to the varying magnetic field from the small-scale granular field up to the global field that couples into the heliosphere, we observe changes in appearance on all scales and in spectral irradiance at all wavelengths. In looking forward to the Solar Orbiter mission, this talk highlights selected problem areas in our knowledge of the Sun's magnetic field to which that mission's observations are anticipated to contribute, including the 3D reconstruction of active-region coronae, the long-range couplings between active and quiet regions, the dynamics of the high-latitude field, and the topology of the coronal domain as the interface between photosphere and heliosphere.

  Invited talk
8   14:30   The solar cycle as seen in the heliospheric magnetic field
Owens, M.J.
University of Reading, UNITED KINGDOM

Abstract to follow.

  Invited talk
9   15:00   The heliospheric magnetic flux density through several solar cycles
Erdos, G.1; Balogh, A.2
1Wigner Research Centre for Physics, HUNGARY;
2Imperial College, London, UNITED KINGDOM

The open magnetic flux of the Sun is an important measure that characterises both the solar activity cycle and the state of the heliosphere. The open flux can be calculated from modelling the solar corona from measurements of the photospheric magnetic field, using different techniques; these techniques have in common in assuming that the open flux is constituted by magnetic field lines that are open at a certain height in the corona. In the heliosphere, the magnetic flux density can be measured directly by using the radial component of the magnetic field vector as measured by space probes. It has been shown (Smith, JGR, 126, 12101, 2011) that the effect of fluctuations needs to be taken into account when interpreting the measured radial field component in terms of flux density. However, as the fluctuations are symmetric around the average Parker field direction, this effect can be taken into account when estimating the open magnetic flux from spacecraft measurements. The flux density, when correctly determined from the magnetic field measurements, is generally independent of heliographic longitude and latitude (when referenced to 1AU), but depends on the solar magnetic field which varies with the phase of the solar activity cycle (as shown by Erdos & Balogh, ApJ, 753, 130 2012). Similarly, the flux density, when referred to 1 AU, is independent of heliocentric distance. By analyzing Helios 1 and 2 data we show, that the effect of the fluctuations in the magnetic field is small at close distance to the Sun. However, the increasing dominance of the fluctuations and the tightening of the Parker spiral make precise measurements more difficult at distances more than a few AU. We have used OMNI magnetic field data covering several solar cycles, together with Ulysses data to assess any heliolatitude and heliocentric distance dependence. We find a very close agreement between, on the one hand, the total open flux estimated from solar measurements and model calculations and the flux from the integrated flux density measured directly from the radial component of the measured heliospheric magnetic field when suitably corrected for the effects of the fluctuations.