Coronal studies have a long tradition in Arcetri as they started in the 60's,
when signals from the first X-ray satellites operating in this area and giving
information on the global X-ray emission of the corona, were received
at the Observatory. These pioneering studies continued over the years, in close
collaboration with scientists from different countries. Scientists in Arcetri
have collaborated to coronal missions like Skylab and SMM and have an active role
in the SOHO mission, operational at present (2003). The corona, and its outer
extension into the interplanetary space, offer many challenges to reaserchers,
as they pose questions which still wait for a solution: what is
the mechanism that heats the corona? where does solar wind originate? which
mechanism is responsible for solar wind acceleration? Space observations
offer invaluable clues to solve these problems.
Information on the outer solar atmosphere, i.e. the corona and the solar wind,
comes mainly from space observations, which either provide data in XUV
radiation or measure plasma properties with in situ instrumentation. The
analysis and interpretation of space observations is an area where extensive
work is being done at the Arcetri Observatory, in collaboration with personnel
from several Institutions abroad (e. g., Center for Astrophysics, Cambridge, USA,
Marshall Space Flight Center, Huntsville, AL, USA) and
from the Department of Astronomy and Space Science of the University of Firenze.
In the last few years, the activity in this area focussed on the analysis and
interpretation of data from SOHO and, in particular, from SOHO/UVCS, whose co-PI
is Giancarlo Noci, from the Department of Astronomy. This experiment provided
the solar community with new
tools, which allowed us to learn more about the corona at distances of a few
solar radii. This region had been little explored, both because of the problems
met by instrumentation, which has to cope with very dim radiation and because
spectroscopic techniques for data interpretation have been developed recently.
The UVCS experiment is an ultraviolet coronagraph spectrometer, with a channel
working in the range 1145 to 1287 Angstroms and a second channel operating in \
the range 984 to 1080 Angstroms
(for a full description of the
UVCS instrument, see Kohl et al. 1995, Solar Physics, 162, 313). The field of
view of UVCS is 40 arcmin long and has a width set by the slit width. Data
are acquired at altitudes from 1.5 to 10 solar radii with the slit tangent
to the solar radius. Rotating the slit positions around the Sun allows
observations of the 360 degree corona. Figure 1 above
gives a composite image of the inner corona as seen by SOHO/EIT
in the Fe XV radiation surrounded by the SOHO/UVCS corona in the oxygen VI
radiation, on July 15/16, 2000. A visible light polarimeter measures polarized
radiation in the 4500 to 6000 Angstrom range, in one
point only of the UVCS slit. White light images of the entire corona, at
altitudes from 2 to 30 solar radii, are provided by the LASCO coronagraphs
and help setting UVCS observations in the wider context. Figures 2 and 3 show
the white light corona as seen by LASCO C2 and C3 coronagraphs
on November 11, 2000. LASCO C2 covers altitudes from 2 to 6 solar radii,
LASCO C3 gets out to 30 solar radii.
Diverse problems can be tackled by analyzing UVCS data. Some examples of the issues that are currently under investigation include:
- where does the solar wind originate from?
- where is the solar wind accelerated?
- is the same acceleration mechanism working on protons and heavier ions?
- which are the physical conditions of streamers?
- how do coronal and in situ plasma parameters correlate?
Fast solar wind (with speed around 800 km/s) is known to originate from coronal
holes, the dark areas that, at the phase of minimum solar activity, extend around
and downwards from the solar poles. Slow solar wind (with speed around 400 km/s)
probably originates from the edges of streamers, those large structures,
made up of closed loops eventually opening towards the
interplanetary space, which at minimum activity cluster in a low latitude belt.
The image at right shows a streamer observed by UVCS in the oxygen VI 1032
Angstrom radiation on July 11, 1996.
Data have been taken from 1.6 to 4 solar radii; isophotes give intensity
levels and highlight the two-peaked distribution of the OVI intensity observed
in streamers at the phase of minimum activity of the solar cycle.
However, there is no conclusive evidence about the site where wind originates.
Within coronal holes, fast wind may originate in plumes or interplumes (see
Teriaca et al., Ap J., 2003, in press). In order to check the hypothesis that
slow wind originates from streamers edges, one looks for a correlation between
elemental abundances in the streamers' edges and the values of abundances
measured in situ in low speed streams. However, no correlation has been
uncontroversially established, yet
(see Parenti et al., A & A, 363, 2000, Bemporad et al., ApJ, submitted). One of the
motivations to explore the physical parameters in and across streamers is
provided by this unsolved problem.
A good opportunity to follow the evolution of plasma from the corona
out to the interplanetary medium is offered by a geometrical configuration
which occurs twice per year, the SOHO-Sun-Ulysses quadrature. In this
situation the SOHO-Sun-Ulysses angle is 90 degrees and plasma parcels remotely
observed by SOHO are later sampled by
in situ instrumentation. Several observational campaigns have been, and will be,
run at the time of quadratures, under the leadership of scientists from
the Arcetri Observatory and the Marshall Space Flight Center
(see, Suess et al.
, JGR 105
The objectives of the campaigns are dictated by the
position of the Ulysses spacecraft: if the radial through Ulysses and the Sun
center crosses through low latitudes, research will focus on slow wind problems,
if the radial through Ulysses and the Sun center crosses through high latitudes,
research will focus on fast wind problems. Because the nascent solar wind has a
few km/s speed, while the wind speed at 1 AU is on the order of several hundred
km/s, there must be a region in between where the wind is accelerated.
Quadratures observations have provided information on the region where slow wind
is accelerated (Poletto et al.
, JGR 107
while fast wind acceleration has been studied in a research which focussed
on the behavior of wind protons (Cuseri et al.
, i999). Most of the
acceleration mechanisms imply the presence of waves: the white light UVCS
channel has provided evidence for the presence of compressional waves in coronal
holes (see e. g. Ofman et al.
One of the most interesting and intriguing results obtained by UVCS revealed
heavy ions to have a higher outflow speed than protons, both in fast and slow
wind. This result raised further
questions on the nature of the mechanism by which heavy ions and protons are
accelerated and on the latitude distribution of the speed of the nascent solar
wind particles. UVCS observations at several latitudes and
altitudes allowed semi-empirical two dimensional models to be developed
(see Zangrilli et al.
Observational estimates of the energy input to protons and oxygen VI ions
have also been made (see Zangrilli et al.
, ESA-SP, 508
Work has been initiated recently to cover areas to which little attention has
been dedicated in the past. Among these, data taken at the time comets
entered the UVCS field of view are now being analyzed. These objects
are clearly visible as an enhancement of the Lyman alpha radiation of neutral
hydrogen measured by UVCS. We
plan to derive the gas production rate and the size of the comet nucleus
(Bemporad et al.
, in preparation). UVCS quadrature campaigns are known to include
data sets where Coronal Mass Ejections (CMEs) have been observed
(Aznar Cuadrado et al.
, ESA-SP, 508
We look forward to finding out how CME coronal data correlate with in situ
data from the same events.
This brief report is meant to give an overview of the activity of people working
in coronal and solar wind. Hopefully, we have been able to convey and share
the interest and excitement these studies give us. We enjoy, and feel privileged,
to work in this area.
Last Updated: 03 June, 2003