TITLE: Chromospheric network structure and dynamics

AUTHORS: A. Pietarila(1), Z. Xu (1) M. Bianda(2) A. Feller(1) R.
Ramelli(2), H. Socas-Navarro(3)                 
(1)Max Planck Institute for Solar System Research
(2)Istituto Ricerche Solari Locarno
(3)High Altitude Observatory

UPDATE HISTORY:  first draft : March 12 2008 

PARTICIPATING INSTRUMENTS AND OBSERVATORIES: 
VTT
THEMIS
SST
Hinode

SCIENTIFIC JUSTIFICATION:
The chromospheric network was observed for the first time over 100 years
ago by Hale and Deslanders (1896). Its connection to the supergranular flow
field was found in the 60s (e.g. Simon & Leighton 1963, 1964). However, it
is still unclear why the network appears bright in the emission of
chromospheric lines. It is clear that the reason must be either directly or
indirectly (e.g. MHD waves) tied to the network magnetic fields.  Network
regions with strong magnetic flux, e.g. center of network patches, tend to
be brighter than regions with less flux but there is no one-to-one
correlation between the hottest regions and regions with the strongest
magnetic flux. There is no self-consistent, generally fully accepted
picture of the magnetic and thermal structure of the network at different
atmospheric heights.  An often used cartoonish picture of the network is
one where the network magnetic field expands with height as the density
decreases and eventually the field fills up the available volume forming a
so called canopy across the internetwork (Gabriel 1976). To better
understand the structure and dynamics of the network it is essential to
make observations that allow for the extraction of physical quantities such
as the temperature structure and magnetic field configuration
simultaneously at different heights. We propose to do that by combining
spectrometer data of the transition region from the SUMER instrument on
board the SOHO satellite with spectropolarimetric observations of
chromospheric spectral lines using the ZIMPOL II instrument on THEMIS and
the TIP II on the VTT. The proposed observing campaign would for the first
time provide a 3-D view of the thermal, magnetic and velocity structures of
the network in a wide range of heights from the photosphere up to the
transition region. It will also allow us to study in detail how the
atmospheric dynamics are affected by magnetic fields originating from the
network.

TARGET:
Chromospheric network structure in a quiet region.

DETAILED OBSERVING SEQUENCE PER INSTRUMENT:

SUMER:
Slit: 1"*300"
Exposure time: 50 sec
Spectral window: HeI 584 A, OI 1152 A and CIII 1175 A.


VTT & THEMIS:
High resolution spectropolarimetric observations with TIP II (mounted on
VTT) and ZIMPOL II (mounted on THEMIS).
Choice of lines
VTT : He I 10830 A, Si I 10827 A
THEMIS : Ca II 8498 A, Na I D1 5896 A, Mg I b 5172 A
The main observation modes include 2D spatial scans and high time cadence
(15 seconds) observations
with a fixed slit position.

SST: 
imaging in Ca II 8542 A using the Fabry-Perot and Ca II K imaging with a 1
A filter.

Hinode(SOT):
High spatial resolution observations with the SOT/BFI (Stokes vector
filtergrams in the MgIb and Na I D1 lines)

OBSERVATION PERIOD:
June 20 - 30, 2008.