||Precision VUV Spectro-Polarimetry for Solar Chromospheric Magnetic Field Measurements
||489, Solar Polarization 7
||Ishikawa, R.; Bando, T.; Hara, H.; Ishikawa, S.; Kano, R.; Kubo, M.; Katsukawa, Y.; Kobiki, T.; Narukage, N.; Suematsu, Y.; Tsuneta, S.; Aoki, K.; Miyagawa, K.; Ichimoto, K.; Kobayashi, K.; Auchère, F.; CLASP team
||The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is
a VUV spectro-polarimeter optimized for measuring the linear polarization
of the Lyman-α line (121.6 nm)
to be launched in 2015 with NASA's sounding rocket (Ishikawa et al. 2011; Narukage et al. 2011; Kano et al. 2012; Kobayashi et al. 2012).
With this experiment, we aim to (1) observe the scattering polarization
in the Lyman-α line, (2) detect the Hanle effect, and
(3) assess the magnetic fields in the upper chromosphere and transition
region for the first time.
The polarization measurement error consists of scale error δ a (error in amplitude
of linear polarization), azimuth error Δφ (error in the direction of linear polarization),
and spurious polarization ε
(false linear polarization signals).
The error ε should be suppressed below 0.1% in the Lyman-α core
(121.567 nm ±0.02 nm), and 0.5% in the Lyman-α wing
(121.567 nm ±0.05 nm), based on our scientific requirements shown in Table 2 of Kubo et al. (2014).
From scientific justification,
we adopt Δ φ<2° and δ a<10% as the instrument requirements.
The spectro-polarimeter features a continuously rotating MgF2 waveplate (Ishikawa et al. 2013),
a dual-beam spectrograph with
a spherical grating working also as a beam splitter, and
two polarization analyzers (Bridou et al. 2011), which are mounted at 90 degree from each other to measure
two orthogonal polarization simultaneously.
For the optical layout of the CLASP instrument, see Figure 3 in Kubo et al. (2014).
Considering the continuous rotation of the half-waveplate, the modulation efficiency is 0.64 both for Stokes Q and U.
All the raw data are returned and demodulation
(successive addition or subtraction of images) is done on the ground.
We control the CLASP polarization performance in the following three steps.
First, we evaluate the throughput and polarization properties
of each optical component in the Lyman-α line,
using the Ultraviolet
Synchrotron ORbital Radiation Facility (UVSOR)
at the Institute for Molecular Science.
The second step is polarization calibration of the spectro-polarimeter
Since the spurious polarization caused by
the axisymmetric telescope is estimated to be negligibly small because of the symmetry (Ishikawa et al. 2014),
we do not perform end-to-end polarization calibration.
As the final step, before the scientific observation near the limb, we make a short observation at the Sun
center and verify the polarization sensitivity, because the scattering polarization is expected to be close to zero at the Sun center due to symmetric geometry.
In order to clarify whether we will be able to achieve the required
polarization sensitivity and accuracy via these steps, we exercise
polarization error budget,
by investigating all the possible causes and their magnitudes of polarization
errors, all of which are not necessarily verified by the polarization calibration.
Based on these error budgets, we conclude that a polarization sensitivity of 0.1% in the line core,
δ a<10% and
Δ φ<2° can be achieved
combined with the polarization calibration of the spectro-polarimeter
and the onboard calibration at the Sun center(refer to Ishikawa et al. 2014, for the detail).
We are currently conducting verification tests of the flight components
and development of the UV light source for the polarization calibration.
From 2014 spring, we will begin the integration, alignment, and calibration.
We will update the error budgets throughout the course of these tests.