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take ve ltergrams across the Fe I line at 6173 A, separated by 69 m A (Borrero54 Gizon, Birch & Spruit |
Figure 24: The HMI instrument to be |
own in 2010 onboard NASA's Solar |
Dynamics Observatory. Courtesy of Philip Scherrer. |
et al. 2007). This line has a Lande factor g= 2:5 and therefore is better suited |
for the measurement of the vector magnetic eld (than e.g. the Ni 6768 line). A |
picture of the HMI |
ight model is shown in Figure 24 . The HMI instrument was |
delivered in November 2007 and has been integrated onto the SDO spacecraft. |
At the time of writing, the launch of SDO is scheduled for February 2010 from |
Cape Canaveral. |
11.2 Solar Orbiter |
Solar Orbiter is the next solar physics mission of the European Space Agency |
(ESA) and a logical step after SOHO. The target launch date is 2015. Solar |
Orbiter will use multiple gravity assist manoeuvres at Venus and the Earth such |
that the inclination of the orbit to the ecliptic will incrementally increase during |
the course of the mission (about 10 years) to reach heliographic latitudes of at |
least 30. The elliptical orbit will have a minimum perihelion distance of 0 :22 AU. |
The scientic payload will include a remote sensing package that will deliver |
0:5 arcsec pixel images of the solar photosphere (intensity, Doppler velocity, and |
magnetic eld). |
While the exact details of the orbit (and observation windows) are still be- |
ing discussed, it is clear that Solar Orbiter will oer unique opportunities forLocal Helioseismology 55 |
helioseismology (Woch & Gizon 2007). First, it will be possible to study the sub- |
surface |
ows and structure in the polar regions, which is not possible today and |
is important to understand the solar cycle. Second, Solar Orbiter will enable us |
to test and apply the concept of stereoscopic helioseismology. Stereoscopic helio- |
seismology combines observations from dierent vantage points. Solar Orbiter's |
orbit is particularly interesting as it will oer a large range of spacecraft-Sun- |
Earth angles. With observations from two widely dierent viewing angles (Solar |
Orbiter and another Earth or near-Earth experiment), it becomes possible to |
consider acoustic ray paths with very large separation distances (see Figure 6b). |
This is useful in local helioseismology to probe structures deep into the Sun, and, |
in particular, at the bottom of the convection zone. |
12 SUMMARY AND OUTLOOK |
Local helioseismology exploits the information contained in the local dispersion |
relation of the acoustic and surface-gravity waves (ring-diagram analysis) and |
in the correlations of the random wave eld (time-distance helioseismology and |
related methods) in order to study the subsurface structure and dynamics of the |
Sun in three dimensions. The high-quality observations from the GONG network |
and the SOHO satellite have made possible the study of the properties of the |
upper layers of the convection zone and their variations with the solar cycle. |
Local helioseismology has not reached maturity and there are many open ques- |
tions about data analysis methods and interpretation. The observational results |
which, in our view, are the most robust and physically sensible are sketched in |
Figure 25 and listed in the Summary Points below. Local helioseismology |
measures eects that are subtle, such as velocities of only a few m s 1. In addi- |
tion to approximations in the data interpretation, it is important to keep in mind |
that several sources instrumental errors can aect the measurements, e.g., plate |
scale errors and optical distortion (Korzennik, Rabello-Soares & Schou 2004) or |
uncertainties in the orientation of the image (Giles 2000). |
An important challenge for future work in local helioseismology is to detect |
signatures of magnetic elds at the base of the convection zone, where the eld |
is expected to be amplied by dierential rotation and stored until erupting to |
the surface as active regions. Direct detection through their eect on wave prop- |
agation properties is unlikely. Because of the high pressure at the base of the |
convection zone, the contrast in propagation speed is very much lower than in |
surface structures like sunspots, even at the inferred eld strengths of 105G. |
More promising is the prospect of detecting systematic |
ows that might be as- |
sociated with magnetic structures at the base of the convection zone. Easiest |
to detect would be azimuthal |
ows (variations in rotation rate), such as have |
already been reported on the time scale of the solar cycle. Even if the sensitivity |
of helioseismic methods turns out insucient to detect such deeply seated struc-56 Gizon, Birch & Spruit |
Figure 25: |
tures, it may well be sucient to rule out certain less-preferred classes of models |
for the solar cycle, such as convective dynamo models acting throughout the con- |
vection zone or in a shallow surface layer. An important class of |
ows would be |
the geostrophic |
ows caused by thermal eects of magnetic elds (see Section 7). |
Such disturbances are much easier to detect through their thermal winds than |
directly by their temperature contrast. They might be turned into a diagnostic |
of magnetic elds in deeper layers that can be probed with helioseismology. |
The availability of powerful computers provides exciting opportunities to de- |
vise, validate, and optimize improved methods of local helioseismology. Exploring |
these possibilities will be key to taking full advantage of the observations of solar |
oscillations. In Sections 4.2 and 6, we have shown examples of the usefulness of |
numerical simulations of wave propagation through prescribed reference sunspot |
models. Simulations of wave propagation in spherical geometry (e.g. Hanasoge |
et al. 2006) have been used in time-distance studies of the deep convection zone |
(e.g. Zhao et al. 2009) and to validate far-side imaging (Hartlep et al. 2008).Local Helioseismology 57 |
Figure 26: Radiative MHD simulation of a sunspot by Rempel et al. (2009). ( a) |
Bolometric intensity (black and white) and subsurface magnetic eld strength |
on a vertical cut through the center of the sunspot (in the range 0 { 8 kG). See |
Supplemental Movie 11 . (b) Power spectrum of the surface oscillations in the |
simulation. The blue line is the phase speed at the bottom of the box, above |
which the model is not realistic. |
It is now becoming possible to simulate the near surface layers of the Sun, in- |
cluding pores and sunspots, by numerically solving the radiative MHD equations |
(Rempel et al. 2009). Figure 26ashows a snapshot of the intensity and the |
magnetic eld for a sunspot simulation. In this simulation, the solar oscillations |
are naturally excited by the convection (see Figure 26b). This type of simulation |
provides a means for computing realistic time series of Dopplergrams, which can |
be used as input to all the methods of local helioseismology. With this type of |
data set, it will be possible to resolve some of the outstanding issues, for example |
regarding sunspot subsurface structure (Section 6.4). The recently achieved con- |
vergence of observations and realistic 3D radiative MHD simulations of sunspots |