Abstract. The SOHO mission has achieved important results in the physics of solar wind and coronal mass ejection acceleration, but most of the processes that drive this acceleration have not yet been explained. The Advanced Spectroscopic and Coronagraphic Explorer (ASCE) mission will carry on-board spectroscopic and polarimetric instrumentation of new generation that is designed to address the fundamental questions on this processes. Following a brief description of ASCE scientific objectives and instrumentation, the EUV polarimetric channel is described. The EUV Polarimeter (EUVP) is designed to measure for the first time the magnetic field vector in the extended corona through the Hanle effect, and the anisotropy of the ion velocity. The EUVP represents the contribution of the Italian solar physics community to the ASCE mission

A Ciaravella

E Pace

G Corti

G Noci

G Tondello

J L Kohl

L D Gardner

M Romoli

M Uslenghi

S Fineschi

CiteSeerX

Mem. S.A.It. Vol. 74, 835
c© SAIt 2003 Memorie della
The ASCE EUV Polarimeter
M. Romoli1, S. Fineschi2, M. Uslenghi3, G. Corti1, E. Pace1, A. Ciaravella4,
G. Tondello5, G. Noci1, L.D. Gardner6, and J.L. Kohl6
1 Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo
E. Fermi 2, 50125 Firenze e-mail: romoli@arcetri.astro.it
2 INAF, Osservatorio Astronomico di Torino, Via Osservatorio 20, 10025 Pino
Torinese (TO)
3 Istituto di Astrofisica Spaziale e Fisica Cosmica “G. Occhialini”, IASF-CNR,
Via Bassini 15, 20133 Milano
4 INAF, Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134
Palermo
5 Dip. di Elettronica e Informatica, Università di Padova, Via Gradenigo 6/B,
35131 Padova
6 Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA
02138, USA
Abstract. The SOHO mission has achieved important results in the physics of
solar wind and coronal mass ejection acceleration, but most of the processes that
drive this acceleration have not yet been explained. The Advanced Spectroscopic
and Coronagraphic Explorer (ASCE) mission will carry on-board spectroscopic
and polarimetric instrumentation of new generation that is designed to address
the fundamental questions on this processes. Following a brief description of ASCE
scientific objectives and instrumentation, the EUV polarimetric channel is de-
scribed. The EUV Polarimeter (EUVP) is designed to measure for the first time
the magnetic field vector in the extended corona through the Hanle effect, and
the anisotropy of the ion velocity. The EUVP represents the contribution of the
Italian solar physics community to the ASCE mission.
Key words. EUV spectropolarimetry – solar corona – solar wind and CME accel-
eration
1. Introduction
The energy that heats the corona and accel-
erates the solar wind and the coronal mass
ejections (CMEs) originates in the sub-
photospheric convective motions, but how
Send offprint requests to: Marco Romoli
Correspondence to: Largo E. Fermi 2, 50125
Firenze
this energy is transported into the solar
corona and converted in thermal, kinetic,
and magnetic energy is not known. The
Solar Heliospheric Observatory (SOHO)
has greatly advanced our knowledge about
coronal heating, solar wind acceleration,
and CMEs (Fleck & Svestka (1997)) but
many key questions remain unanswered.
836 M. Romoli et al.: The ASCE EUV Polarimeter
The proposed Advanced Spectroscopic
and Coronagraphic Explorer (ASCE)
(Gardner et al. (2002)) will address
fundamental questions on how the primary
and secondary plasma components of the
fast and slow solar wind are heated and
accelerated, and on how the CMEs are
heated and accelarated, and which is their
role in the evolution of the solar mag-
netic field. Spectroscopy and polarimetry
provide the empirical description of the
plasma that is needed to identify the
basic physical processes. Also, a detailed
understanding of the physical processes
is fundamental to learn how and when to
perform space weather predictions. ASCE
payload consists of three instruments, a
EUV coronagraph spectrometer (AUVCS),
a visible light coronagraph (ALASCO),
and a solar disk spectrometer (ASDS),
which derive their optical design from
the SOHO instruments but with greatly
improved performances. These instruments
will provide:
– intensities of EUV spectral lines, in
chromosphere, transition region and low
corona, with high spatial resolution (1”)
and high cadence (10s) (ASDS);
– spectroscopic and polarimetric di-
agnostics of the extended corona
(1.15-10R¯), with performances up
to 400 times better that those of
SOHO/UVCS, through the measure-
ment of intensities and profiles of EUV
spectral lines, including, for the first
time, the HeII 30.4nm line (AUVCS);
– visible light imaging and polarimetry of
the solar corona (1.15-10R¯) with high
cadence (1s) (ALASCO).
The most important feature that will en-
hance the performances of the corona-
graphs will be a 13 m deployable boom that
will bring the external occulter 14.7 m away
from the telescope mirrors, providing an in-
creased mirror area, decreased stray light,
and allowing observations at lower helio-
centric heights.
The Italian solar physics community
will contribute to ASCE with a EUV
Polarimeter (EUVP) that will be mounted
on the focal plane of the EUV coronagraph
spectrometer (EUVSP) forming the EUV
Polarimetric Path (EUVPP). The EUVP
is designed to measure the linearly polar-
ized brightness and the orientation of the
polarization vector of the EUV lines from
the Lyman continuum to Lyα in the outer
solar corona (r > 1.2R¯).
2. EUVP scientific objectives
The EUVP is designed to perform spectro-
polarimetry in the 90 - 130 nm wavelength
range where many of the brightest coro-
nal lines lies. The major objective of the
EUVPis the determination of the mag-
netic field intensity in the outer corona
through the measurement of the Hanle ef-
fect, that produces depolarization and ro-
tation of the polarization vector in the HI
lines. The magnetic field diagnostics, de-
scribed in Fineschi et al. (1999), is based
on the measurement of the linear polar-
ization and the intensity of the hydrogen
Lyman series lines (Lyα 121.6 nm, Lyβ
102.5 nm, and Lyδ 95.1 nm, obtained with
EUVP) and of the FeXIV 530.3 nm (ob-
served by ALASCO). This technique will
be applied between 1.2 and 1.5 R¯ and will
be able to determine magnetic fields inten-
sities above 1 gauss. The secondary objec-
tive is the measurement of the polarization
state of the atomic and ion lines (among
the brightest the O+5 lines). The polariza-
tion of the spectral lines gives an additional
piece of information for the understanding
of the solar wind velocity field, and the
ion kinetic velocity distribution. This rep-
resents the first attempt to measure the po-
larization state of spectral lines in the outer
corona. The velocity vector and the velocity
distribution anisotropy will be determined
for the ion O+5 from the measurement of
the intensity, and the linear polarization of
the OVI 103.2/103.7 nm doublet (Fineschi
(2001)).
M. Romoli et al.: The ASCE EUV Polarimeter 837
Disk light
Light trap
Coronal light
Telescope
Ly−alpha and OVI
Detectors
Rowland circle
Spectrometer
Grating
Zero order
Slit Aperture
EUVP
Fig. 1. EUVPP optical diagram.
Fig. 2. EUVP mechanical and optical layout.
3. EUVP technical description
The EUVP is designed to perform spectro-
polarimetry in the 90 - 130 nm wavelength
range. Since no transmission polarizers are
available in the 90-130 nm spectral range,
a single reflecting surface placed at about
the pseudo-Brewster angle with respect to
the incidence beam will perform the best in
terms of efficiency and polarization proper-
ties. The EUVP is mounted on the focal
plane of the spectrometer of the AUVCS
EUV spectroscopic path (EUVSP) form-
ing with the EUVSP the EUVP (Fig.1).
The radiation from the selected line is fo-
cused by the EUV spectrometer grating
onto the EUVP entrance pinhole. The EUV
polarimetry path (EUVPP) is selected by
adjusting the rotation of the EUV grating
to focus the desired wavelength onto the
entrance pinhole. The EUVP optical as-
sembly consists of a rectangular pinhole, a
rotating polarizer, and a detector (Fig.2),
whose specifications are given in Tab.1. The
polarizer is a CaF2 reflecting flat. A pre-
liminary study (Corti & Romoli (2003))
based on the reflectance properties and the
polarization properties of several materials
(metals, semiconductors, and dielectrics),
and on a laboratory characterization of se-
lected materials have returned the CaF2 as
the best candidate. The front and back sur-
faces of the fluoride polarizer form a wedge
to prevent the back surface reflection of
Lyα radiation from illuminating the detec-
838 M. Romoli et al.: The ASCE EUV Polarimeter
Table 1. EUVP specifications.
Entrance Rectangular: 300µm× 2.9mm(±20µm)
pinhole: Spectral resolution 0.2 nm
Spatial resolution 24” (radial) x 84” (tangential)
Distance pinhole - detector: 60 mm
Polarizer: Size (diameter) 25.0mm± 0.1mm
Thickness (center) 7.0mm± 0.1mm
Wedge angle 17◦ ± 0.5◦
Substrate CaF2 (uncoated)
Surface figure and finish Flat λ/10
Surface microroughness 0.8 nm rms
Polarizer angle of incidence: 70◦ ± 0.5◦
Rotation Step size 1◦ ± 0.1◦
mechanism: Range ±100◦
Coalignment with optical axis ±0.5◦
Detector: Microchannel plate Z-stack MCP
Photocathode KBr (> 35% at HI Lyman lines)
Read-out Discrete anode array in bull’s eye pattern
Size of anode array 25.0mm± 0.1mm
Size of central anode 12.5mm± 0.1mm
tor surface. Lyα is the only line in the wave-
length range of interest that is transmitted
through the material. The detector is a KBr
coated MCP with discrete anodes. The po-
larizer and the detector are assembled on
a rigid mount that rotates around an axis
that is co-aligned with the optical axis of
the incident beam. The entire polarimeter
is enclosed in a housing that can be evacu-
ated to preserve the functionality and the
performances of the detector. An aperture
door, which will be opened at the begin-
ning of the mission and closed at the end,
seals the subsystem. A EUV transmissive
window is incorporated into the door to al-
low alignment verifications on the ground
while the system is in vacuum.
4. Conclusions
ASCE has been selected by NASA for
the feasibility study within the MIDEX
(Medium Class Explorer) program and will
undergo final selection in the spring 2003.
ASCE is scheduled for launch at the begin-
ning of 2007.
Acknowledgements. We are thankful to Galileo
Avionica, Firenze, and Laben, Milano, for the
fundamental technical support. This work has
been partially supported by ASI (Italian Space
Agency).
References
Corti G., and Romoli M., 2003, Appl. Opt.,
in press;
S. Fineschi, A. van Ballegooijen, J. L. Kohl,
1999, 8th SOHO Workshop, ESA SP-
446, 317;
S. Fineschi, 2001, ASP Conf. Proc., eds.
G. Mathys, S. K. Solanki, and D. T.
Wickramasinghe, 248, 597;
Fleck B., and Svestka Z., eds., 1997, The
First Results from SOHO, (Dordrecht:
Kluwer);
L. D. Gardner, et al., 2002, SPIE, 4843, in
press.


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