|
My THEMIS pages
Nadège MEUNIER |
Warning : This page contains characteristics obtained with my own data,
at a given time (June 2000, P.I. J. Arnaud, November 2000,
and July 2001),
with the MTR mode (multi-line spectro-polarimetry).
They are not official pages
from THEMIS. The characteristics of the instruments may evolve :
for technical information about THEMIS status, should contact the
THEMIS team. These pages are not official
pages from BASS2000 as well.
Observations made in 2001:
Mission Report July 2001
. The mission report is available here.
These observations were coordinated with MDI on SOHO as part of the Joint
Observating Program 134 (see the JOP 134 on the
JOP pages).
Title: High-Resolution Study of Supergranulation: Plasma Flows and Magnetic
Field Structures , Participants: Nadège Meunier, Thierry Roudier
(THEMIS), Alexander Kosovichev, Tom Duvall, Laurent Gizon, Junwei Zhao and
John Beck (MDI). This report contains results obtained from a
lot of measurements
of the solar co-spatiality during the run.
NEW Mission Report September 2001 . The mission report is available here. Our objective was to observe plages (and in particular emerging magnetic fields) next to the limb using both the Hanle and Zeeman effects.
Observations made in 2000:
Exposure time and acquisition time. Only a small amount (typically less than 25% of the time for several cameras) of the acquisition time is due to the exposure time. It is useful to know this behavior when preparing your observations, especially if you are interested in a high temporal cadence. The temporal
cadence obtained during our June observations with the MTR mode are here,
for different types of sequences. It took on average 2 seconds per Stokes
parameter measurement (one simultaneous measure of I+Stokes and I-Stokes) for
7 cameras and time exposure between 0.3 and 0.7 seconds. In November 2000, it
took approx. 2.5 seconds per Stokes parameter measurement for 9 cameras
and time exposure around 0.8 seconds.
Mission Report June 2000
. This report from the October 9, 2000
concerns our observations made with THEMIS using the MTR mode
from June 23 to June 29 (2000). The P.I. was J. Arnaud. I diffuse
this report hoping that this will contribute to improve the
observating conditions at THEMIS so that its capacities can be fully
exploited.
Image motions.
An analysis of the preslit field of view
to study the influence of the seeing and scanning characteristics on
the resulting images. This analysis shows that when one measure
a sequence of Stokes parameters, the observed region on the Sun is
never the same through this sequence of 3 or 4 measurement, which
makes difficult the interpretation of a vector magnetic field derived from
these data.
Mission Report November 2000
. The mission report is available here.
These observations were coordinated with MDI on SOHO as part of the Joint
Observating Program 134 (see the JOP 134 on the
JOP pages).
Title: High-Resolution Study of Supergranulation: Plasma Flows and Magnetic
Field Structures , Participants: Nadège Meunier, Thierry Roudier
(THEMIS), Alexander Kosovichev, Tom Duvall, Laurent Gizon, Junwei Zhao and
John Beck (MDI). Only 2 days of observations have been obtained and
saved (the other 2 days of observations have been lost by the THEMIS team).
We also met some adjustment problems concerning the co-spatiality between the 2
slits (see below). This program will be reconducted during next campaign.
Co-spatiality between the 2 slits
of the MTR mode .
This document (in french!)
was written for the science advisory board of THEMIS of February 2001.
Its purpose is to present the bad adjustments that were made
during our November 2000 observations concerning the 2 slits of the
MTR mode. Some work should be devoted to this problem in the future, first
concerning the estimation of the limitation of this adjustment
(this is not yet known) and second concerning the solutions to
lower the limit. This problem combined with the image
motions makes observations of very weak magnetic fields with a spatial
resolution very difficult (see the poster below).
Attempt to observe intranetwork magnetic
fields : our poster presented at the Rome conference in
March 2001 (THEMIS and the new frontiers of solar atmosphere dynamics)
is available as a word file. The paper submitted to "Il Nuovo Cimento C" is available as a postscript file.
Title: Observation of intranetwork magnetic fields with THEMIS
Authors: Nadège Meunier, Jean Arnaud, Jean Vigneau
Abstract :
It is now well-known that weak magnetic fields inside network cells are an important component of the solar magnetic field beside the strong magnetic fields concentrated in active regions and network. These weak fields have been observed by a number a groups in the past few years (among others Keller et al. 1994, Lin 1995, Wang 1995, Lites et al. 1996, Meunier et al. 1998, Lin and Rimmele 1999, Stolpe and Kneer 2000), which yielded important clues to the behavior of the intranetwork magnetic fields.
However it is important to confirm these different results and to state with greater precision the properties of intranetwork magnetic features (vector magnetic field, lifetime...).In this poster we describe some of the spectro-polarimetric observations made with the THEMIS telescope using the multi-line mode in June 2000 in order to study the intranetwork magnetic fields. We highlight the data processing and the limitations we met when performing this study.
Conclusion :
In this paper we attempted to observe intranetwork magnetic fields with THEMIS. The analysis of the biases and the comparison between the signals provided by a line sensitive to the magnetic field and a line which is not sensitive to it show that it is impossible to detect these weak fields in the quiet sun at the present time. We estimate that the main problem is due to the combination of a bad co-spatiality between the 2 slits and the seeing. This problem should be very important for all observations requiring both high spatial resolution and high polarization sensibility. Also important are the fringes, which remains even when averaging the data, and the photon noise. The signal to noise ratio could be increased if it was possible to average the data. However, this proved to be difficult because the actual spatial resolution is then degraded to 1.5-2 arcseconds in the best conditions we had (this is due to the fact that there is no image stabilization). An attempt to average a large number of scans together (30 scans over 45 minutes) shows that here again there is no signal outside the network except due to velocity residuals.
It is therefore necessary to improve the image quality at THEMIS (and to install an image stabilization as a first step as soon as possible) and to estimate the limits of the adjustments that can be made between the 2 slits if one want to observe these intranetwork magnetic fields with THEMIS. Because the detection threshold we get for the polarization is quite high, these observations did not provide any strong constraints on the intranetwork magnetic fields.
The official THEMIS site
Our THEMIS page on the BASS2000 site : you will find there a lot of useful links (including some help to prepare the observations - Lande factors, line depth formation,...- and some links to other personal THEMIS pages).