JOP134 High-Resolution Study of Supergranulation: 
                Plasma Flows and Magnetic Field Structures

Received: 18 October 2000

    Participants: Nadege Meunier (THEMIS), Alexander Kosovichev, Tom Duvall,
    Laurent Gizon, Junwei Zhao and John Beck (MDI)

SCIENTIFIC OBJECTIVES

Although solar supergranulation has been observed for a long time,
its nature is not yet understood. A link between granulation and explosive
granules has been studied recently by Roudier et al. (1999) using very high
spatial resolution images. On the other side it has been shown that magnetic
flux tubes concentrate between the supergranulation cells (hence the name
of network) and diffusion of magnetic elements has been observed many
times by several teams.

However, some observational results have not been explained. For instance,
there is evidence that supergranules are cooler in the middle than
near the edges. This is not consistent with the conventional picture
of convective cells. Also, surprising strong downflows in the middle of
supergranules have been found from the MDI data by the time-distance technique.
In addition, supergranular cells observed in Doppler images
and by the time-distance technique rotate faster (~4%) than the
plasma (observed in the same Doppler images). In fact, supergranules
probably rotate faster than anything else in the Sun as recent helioseismology
results have indicated. Furthermore, magnetic flux tubes observed on
magnetograms
or spectroheliograms have an intermediate rotation rate although their
position is very close to the Doppler cells!

Our objective is to understand better the relations between
supergranulation flows and the magnetic network (as well
as the nature of supergranulation) by studying simultaneous Doppler
and vector magnetic field data.

THEMIS OBSERVATIONS

We propose to observe simultaneously the magnetic field and the velocity
fields related to the supergranules, using full Stokes
parameters measurement with the THEMIS multiline spectrograph.
The velocity field will be determined from the Doppler effect
(line-of-sight velocity) as well as by tracking granules if the spatial
resolution allows it (horizontal velocities). The multiline approach
will allow us to observe at different altitude in the solar photosphere
and will lead to a better estimation of the vector magnetic field.
The spectral domains will be : Fe I 6301 A, Ni 6768 A, Fe I 5691 A,
Fe I 5250 A, Fe I 5576 A, Fe I 5636 A (Note : for practical reason
in the camera setting, the domains 5250 A and 5636 A cannot be observed
simultaneously. Therefore, a mask will allow to select either of these
domain, all other lines being observed simultaneously).
Our purpose is to estimate the differences in velocities between
the different structures while looking at different scales and using
different methods, while the observations has been made simultaneously.
Tracking of the same region for at least several hours will be performed.
The use of 2 cameras for each spectral domain will allow to
have a field of view of 2' in the slit direction.
The time step between two consecutive scans will depend on the
acquisition time due to the telescope, which is still a little slow.
For a 2' x 2' field of view, we estimate it to be
15-20 minutes (4 stokes, 0.5 sec exposure time).

MDI COORDINATED OBSERVATIONS

The supergranulation studies require sufficiently high spatial resolution and
good tracking.  However the tracking precision of THEMIS is not very good
because the whole telescope is moved. Also, the tip-tilt mirror has not been
yet implemented. Therefore it will be important to have simultaneous
observations
providing a good reference and a test of the precision of the observed
displacements.
MDI observations are very important for this purpose because:
  - they are seeing free (although we expect a higher resolution
	from THEMIS);
  - one of the line we will observe with THEMIS is the Ni
	line used by MDI (this is also important for testing MDI calibrations).
  - MDI images are obtained without any scanning and with a high cadence.
We will need both magnetograms and Dopplergrams in the high resolution field
of view, as well as intensity.
The MDI Dopplergrams will be also used for determining flows near the surface
and
in the interior by the time-distance technique. Therefore, the MDI campaigns
hr_fe_me_ve or hr_fe_me_v2 are the most appropriate for our study.

TARGETS

We will make sequences in the MDI high-resolution field of view for both
quiet and active regions. The observing time for joint observations
is from 07:00 to 17:00 UT, November 21-27, 2000.