Arcetri Solar Physics Group

Corona and Solar Wind
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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.

Figure 1

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.

Figure 2 Figure 3

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.

Figure 4

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, 2000).

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, 2002), while fast wind acceleration has been studied in a research which focussed on the behavior of wind protons (Cuseri et al., ApJ, 514, 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., ApJ, 529, 2000).

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., ApJ, 574, 2002). Observational estimates of the energy input to protons and oxygen VI ions have also been made (see Zangrilli et al., ESA-SP, 508, 2002).

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, 2002). 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