VIRGO ftp address ftp://ftp.estec.esa.nl/pub/loitenerife/virgo.html
The Luminosity Oscillation Imager (LOI) is part of the VIRGO instrument aboard SOHO. LOI's scientific objective is to identify and characterise solar pressure and internal gravity oscillations by observing the radiance variations. LOI is a low resolution imager, with 12 pixels, for measuring the radiance distribution over the solar disc at 500 nm. The spatial information provided helps to detect individual modes of oscillation using a spherical harmonic decomposition. The solar image is stabilised on the detector using piezoelectric actuators acting on the telescope's secondary mirror. The low resolution capability of the instrument allows the identification of individual azimuthal orders for l=0-7, without suffering the mixing that affects the integrated solar disc instruments.
LOI was developed by the Solar System Division as part of the VIRGO investigation (Fröhlich et al., 1997). The SSD lead scientist is T. Appourchaux; the project engineer was U. Telljohann.
LOI has been performing extremely well since cover opening on 27 March 1996 (Appourchaux et al., 1997). LOI's most critical subsystem, guidance, is performing as expected; its range and 1 sigma precision are about ±7.5 arcmin and 0.01 arcsec, respectively. The noise of the instrument for the p-modes is limited only by solar noise. With an integration of 120 days we can detect fluctuations at the 1 sigma level of about 0.1 ppm. In comparison, the instrumental noise mainly due to the electronic chain is about 0.01 ppm. Although LOI was not designed for measuring the apparent diameter of the Sun, preliminary analysis shows that variation of this diameter greater than 1 milliarcsec can be detected over 1 day.
LOI has already started to provide a wealth of information on the Sun. Inversion of the LOI p-mode frequencies obtained with a 6-month time series show no evidence that the Sun is significantly different from the latest available standard solar model (Fig. 4.2.2.1/1) (Appourchaux et al., 1996a). Thanks to the unique capability of LOI for detecting individual l, m modes (Fig. 4.2.2.1/2), there is also some indication that the core does not rotate faster than the surface (Appourchaux et al., 1996b). This result is very preliminary as longer observation times are needed. Using a time-frequency analysis, Leifsen et al. (1996) showed that the modes within an l, n multiplet are independently excited. The linewidth of the p-modes was also measured with LOI. Together with the VIRGO/SPM data, a decrease in the linewidth of the mode was measured around 3000 Hz; this is confirmation of a theoretical resonance effect with convection predicted a few years ago.
Figure 4.2.2.1/1: The relative difference in the Sun's squared
isothermal speed of sound and that of the solar reference model,
inferred from the LOI+LOWL frequencies from a 6-month data set.
(LOWL is a ground-based High Altitude Observatory instrument for
observing low degree p-modes.)
Figure 4.2.2.1/2: The main advantage of LOI over other
helioseismology instruments is its capability for isolating the
different components of an l, n mode. Here we show three
different echelle diagrams for the l=1 degree. (An echelle
diagram is obtained by cutting a power spectrum into pieces 136
Hz long and piling them on top of each other.) The two top
diagrams are obtained with LOI, the bottom one by an instrument
observing the Sun as a star. Thanks to solar rotation, the modes
with m=-1 and m=1 have different mode frequencies
(i.e. analogous to the Zeeman effect). With LOI, the frequency
separation is clearly seen by eye; the separation is about 0.8
Hz. With a full-disc integrated instrument, the modes are
unresolved. Since the frequency separation is an estimate of the
internal rotation of the Sun, LOI is in a better position than
any other non-imaging instrument for measuring the rotation of
the solar core.
Appourchaux & Toutain (1996) also reported on the detection of the p-modes in the LOI guidance signals. Unexpectedly, the p-modes could be detected as intensity fluctuations with an amplitude three times greater than in full disc, and as pointing fluctuations with amplitudes of a few milliarcsec! These are unexpected results, as the guidance system was not designed for detecting such small effects.
Other results related to the signature of low frequency solar noise were reported by Andersen et al. (1996), showing that the signature of mesogranulation in intensity could not be detected. The effects of sunspots and faculae passing over the solar disc were also detected by LOI and correlated with the other VIRGO instruments (Appourchaux et al., 1997; Fröhlich et al., 1997); these measurements are needed to understand how irradiance fluctuations arise.
References
Andersen, B.N., Appourchaux, T. et al.
(1996). IAU 181
Symposium, Nice.
Appourchaux, T. & Toutain, T. (1996). IAU 181
Symposium, Nice.
Appourchaux, T., Sekii, T. et al. (1996a). IAU 181
Symposium, Nice.
Appourchaux, T., Gough, D.O., Sekii, T. & Toutain, T.
(1996b). IAU 181
Symposium, Nice.
Appourchaux, T., Andersen, B.N., Fröhlich, C.,
Telljohann, U. et al. (1997). Sol. Phys. 170, 27.
Fröhlich, C., Andersen, B.N., Appourchaux, T. and the
VIRGO team. (1997). Sol. Phys. 170, 1.
Leifsen, T., Andersen, B.N., Appourchaux, T. et al. (1996).
IAU 181 Symposium, Nice.
Solar core rotation From time series of the IRIS ground-based network developed by the Nice group, L. Gizon et al. (1997) derived new constraints on the solar core rotation. From the improved measurements of the low degree (l=1,2,3) rotational splitting frequencies, and assuming the rotation law to be known in the outer layers, the authors find that a core rotating slightly faster than the radiative envelope provides the best fit to the data.
Effect of active structures on helioseismological
measurements
The effect of rotational modulation due to active
structures on irradiance, global velocity and helioseismological
measurements was simulated (Fig. 4.2.2.2). Foing's (1996) semi-
empirical determination of temperature contrast in sunspots,
penumbrae and faculae with realistic centre-to-limb coefficients
at different wavelengths were used to compute the expected
modulation. These simulations allow one to model the variations
observed by SOHO's instruments. We also show that different
higher harmonics of the rotational period are selected in
intensity and velocity variations, and that they can contribute
significantly (and therefore must be modelled and corrected) to
the power background at the temporal frequencies where one
searches for the elusive g-modes.
Figure 4.2.2.2: Calculated intensity modulation at 500 nm due
to the combined effect of a sunspot group and associated faculae.
The strong centre-to-limb brightening of the faculae creates the
double peaked extra emission every rotation. Also, the interplay
of shorter time scales for sunspot evolution with the more stable
facular contribution can lead to different cases for the global
irradiance variations. The correspond-ing intensity variations
with amplitude of 50 ppm at the current solar minimum are
accompanied by global velocity variations of 1 m/s with 13-day
period mostly due to the facular contribution, and by a
significant power in the higher harmonics of the rotational
period.
Reference
Foing, B.H. (1996). IAU 181 Symposium,
Nice. Gizon, L. et al. (1997). A&A 317, L71.
To study shock wave propagation in the upper layers of the solar atmosphere, a SOHO Joint Observing Programme (JOP) has been designed and led by B. Fleck, involving SUMER, CDS and EIT on SOHO, coordinated with ground-based observations at the Vacuum Tower Telescope at Izana, Tenerife. There were two successful observing campaigns, in mid-June and mid-September 1996. Fig. 4.2.2.3 shows CDS and SUMER raster scans, together with a Ca K filtergramme observed at the VTT. One can easily recognise the supergranular network in all SUMER lines and most of the CDS lines. Details on these observations (JOP026: The Signature of Ca II K Grains in the Transition Region and Corona), including a list of the selected lines, exposure times, etc can be found on the SOHO pages on the Web at: http://sohowww.nascom.nasa.gov/operations/JOPs.
Figure 4.2.2.3: Ca filtergramme as observed at the VTT at Izana, Tenerife, on 16 June 1996 at disc centre,
together with CDS and SUMER raster scans in a variety of EUV lines, formed at a wide range of temperature
from about 6000 K to more than a million degrees. One can easily recognise the same supergranular
network pattern in Ca K and in all SUMER lines, as well as in the cooler CDS lines.
These raster scans were performed before the actual observing run for alignment purposes.
To achieve the required high temporal resolution of 10-15 s, the actual observations were done with a fixed slit.
The aim of the present investigation is to observe how the 3 min shock waves causing the Ca K grains in the middle chromosphere propagate into the higher layers of the solar atmosphere, how they impact the transition region and what signature they leave in the lower corona. How much of the energy is reflected at the steep temperature gradient of the transition region? How much of the energy is dissipated and where? High resolution UV emission line observations combined with simultaneous and co-spatial ground-based observations, thus covering all 'layers' of the solar atmosphere, from the photosphere through the chromosphere up into the transition region and corona, should give us some answers to these questions, as well as to the question of whether a non-magnetic chromosphere exists or not.
Studies of sunspot penumbrae surface magnetic fields
A
3-D force-free model of the structure of the magnetic field in
'fluted' sunspots, mentioned in the previous report, has
unexpectedly resulted in a possible explanation of the 'anemone'
structure frequently observed in soft X-ray loops overlying
relatively isolated sunspots, as well as in a new model for the
penumbral Evershed flow as coherent short scale motion rather
than a large scale physical flow (Martens et al., 1996). A
follow-up study of fully non-linear 3-D force-free models for the
structure of penumbrae (Neukirch & Martens, 1997) has
resulted in the development of a new class of cylindrical slab-
type force-free solutions, which were previously thought to exist
only in Cartesian and spherical geometries (Fig. 4.2.2.4).
Figure 4.2.2.4: A scenario for Evershed flow in the penumbra
of a fluted sunspot. The neutral line in a fluted sunspot zigzags
between the penumbral boundaries, and is mostly directed along
the radial. The observed flow consists of siphon flows along the
series of short and flat loops that bridge the neutral line. The
net result is the appearance of a radially outward-directed flow
if observed with limited resolution.
Dynamic phenomena in the solar corona
The
identification of the source of solar type-U radio bursts with
travelling soft X-ray fronts, discovered by Aurass et al. in
1994, has opened up a new avenue for identifying the complex
topological connections in the solar corona by connecting
particle acceleration sites with particle impact sites in the
chromosphere through X-ray loops. Investigations are ongoing
(Aurass et al., 1997) into flares where there appears to be a
sudden change in magnetic topology as the flare progresses,
probably due to reconnection, as modelled in Martens et al.
(1990).
References
Aurass, H., Klein, K.-L. & Martens,
P.C.H. (1997). PASP, in press.
Aurass, H., Klein, K.-L. & Martens, P.C.H. (1994).
Sol. Phys. Letters 155, 203.
Martens, P.C.H. et al. (1990). In Plasma Phenomena in the
Solar Atmosphere, p.153, Dubois, M.A. et al. (Eds.).
Martens, P.C.H. et al. (1996). ApJ 463, 372.
Neukirch, T. & Martens, P.C.H. (1997). In preparation.
Observational and theoretical investigations of impulsive (possibly magnetic reconnection) phenomena have been made, using numerical simulations of particle acceleration and transport, and observations of small impulsive events with SOHO.
The numerical work has concentrated on modelling the acceleration and transport of charged particles in and around magnetic neutral points, using orbit integration and stochastic techniques for simulating the trajectories of individual particles. It has been found (Fletcher & Petkaki, 1996), that power-law distributions of relativistic electrons can be generated by stochastic acceleration and transport in an X-type neutral structure, on timescales as small as 0.1 s, rendering the process a good candidate for impulsive solar flare energisation. Hard X-rays and visible signatures of accelerated electrons also formed the focus of theoretical work (Fletcher & Martens, 1996), in which a model for the generation of looptop impulsive hard X-ray sources - the major discovery by the Hard X-ray Telescope aboard the Japanese Yohkoh satellite (Masuda et al., 1995) - is developed. The model, in which these sources are caused by the magnetic trapping and Coulomb scattering of non- thermal electrons in the magnetic field associated with a flare current sheet, qualitatively explains observed behaviour of looptop hard X-ray sources. Quantitative comparisons for individual sources is at present underway.
Moving to a smaller scale, three of the coronal instruments (SUMER/CDS/EIT) on SOHO were used in a Joint Observing Programme (JOP) of impulsive events, namely coronal jets and explosive bright points, along with SOHO/MDI and Yohkoh/SXT. SOHO has revealed that the solar transition region and corona is varying at all resolvable scales, with constant reorganisation of the magnetic field resulting in brightening and expulsions of material. The aim of the observing programme (SOHO JOP 033) was to use the advanced spatial, temporal and spectral capabilities of the SOHO instruments to investigate possible connections between explosive events and jets previously observed by, for example, HRTS in UV transition region lines (Dere et al., 1989) and jets seen by the Yohkoh SXT (Shibata et al., 1992), and to search for evidence of reconnection as the jet driver. In the campaign, several high velocity and explosive events near bright points were seen by SUMER, while CDS, Yohkoh and EIT showed brightness and topological variations on timescales of a few minutes (Fig. 4.2.2.5). Analysis of the data, involving L. Fletcher, is in progress.
Figure 4.2.2.5: CDS rasters of a possible jetting bright
point, imaged in four lines whose emission curves peak at four
different temperatures, and hence image different temperature
regions of the solar atmosphere. These are He I, 584.33 (20 000
K, chromosphere), O V, 629.73 (200 000 K, transition region, low
corona), Mg IX 286.06 (1 million K, corona) and Si XII 520.67 nm
(2 million K, corona). The wavelength profiles are plotted at the
brightest pixel in the He II image (courtesy of the SOHO/CDS
instrument team).
In addition, an investigation with SOHO/UVCS into polar plume structure and evolution is underway. Observations made at a number of positions above the north polar coronal hole have revealed elongated regions of enhanced intensity in Lyman-alfa and Oxygen VI, which can be mapped to the pole itself. L. Fletcher and colleagues are using techniques such as Doppler dimming, to determine transverse speeds, and line broadening to determine temperatures, with the aim of characterising and further understanding the origin and effect of the plumes.
References
Dere, K.P., Bartoe, J.-D.F. & Brueckner, G.E. (1989).
Sol. Phys. 123, 41.
Fletcher, L. & Martens, P.C.H. (1996). A&A..
Submitted.
Fletcher,L & Petkaki, P. (1996). Sol. Phys.
Accepted.
Masuda et al. (1995). Nature 371, 495.
Shibata, K. et al. (1992). PASJ 44, L173.
The analysis of high resolution rocket UV solar filter-grams (Transition Region Camera from 1979 to 1992) was continued in a recent comparison between Lyman-alfa and H alfa images of prominences (Wiik et al., 1996). There was generally a good correlation found between the intensities of the two lines in plages, but not in filaments.
The 3 November 1994 total solar eclipse data, obtained with the Solar System Division CCD camera on a 25 cm telescope, were analysed: images, objective grating spectra and spectrophotometry of quiet corona, protuberances and polar plumes were successfully achieved. Support photographic instruments obtained fine images of the inner corona, plumes, streamers, protuberances and outer corona. From the digitised data, we could distinguish different fine structures in the closed equatorial streamers, and the open coronal holes. From the gradient of the Thomson scattered emission, we could map the heterogeneities and vertical gradient of the structures, and derive equivalent temperatures in streamers and coronal holes reliably from 10 000 km to 100 000 km above the solar limb (Fig. 4.2.2.6/1) (Duvet, 1996; Foing, Duvet et al., 1996). A similar analysis was performed on data from the 24 October 1995 total eclipse (Fig. 4.2.2.6/2) (Cravatte, 1996; Foing et al., 1996).
Figure 4.2.2.6/1: Equivalent temperature at 44 000 km above
the solar limb, derived from the 3 November 1994 solar eclipse.
The CCD frame was obtained with the SO camera and a transportable
telescope at Putre, Chile. One can see a low temperature (1
million K) with polar plumes homogeneities around the North
coronal hole (angle theta from -10° to 20°) and a
higher temperature (1.2-1.7 million K) in equatorial streamer
structures (theta 30-130°).
Figure 4.2.2.6/2: Composite of digitised coronal eclipse
images obtained during the 24 October 1995 total eclipse with
light from a 1.25 m focal telescope, showing the dipolar magnetic
field configuration at solar minimum, with polar plumes and
strong equatorial streamers enhanced with spatial filtering.
References
Cravatte, S. (1996). Magistere Thesis, Univ. Paris XI.
Duvet, L. (1996). DEA Thesis, Univ. Paris VI/Paris Obs.
Foing, B.H., Duvet, L. et al. (1996). Adv. Space Res.
Submitted.
Foing, B.H., Wiik, J.E. et al. (1996). Proc. IAU 153
Symposium.
Wiik, J.E. et al. (1996). Adv. Space Res. 17,
(4/5)105.
The solar-stellar connection is the study of solar-like magnetic phenomena on stars and the Sun. The aim is to understand the magnetic dynamo, chromosphere, corona, variability and instabilities for stars and the Sun as a whole. This topic has undergone an outstanding development in recent years, thanks to the opening, with space-based observatories, of new wavelength windows (X-ray, EUV, UV), where magnetically active stars are prominent emitters. This progress has far-reaching implications that go beyond the magnetic activity problem itself, most notably the influence of magnetic activity on Earth's environment, long- term effects on Earth's atmosphere and climate, the evolution and interiors of the Sun and stars, and the transfer of mass and angular momentum to the interplanetary and interstellar medium.
Spectral signatures of outer stellar atmospheres are representative of the density of surface magnetic fields. Numerous multi-line spectra were gathered with different space- and ground-based facilities to create a comprehensive database. Different approaches were applied:
We performed a detailed study for a sample of M dwarf stars with different activity levels. We developed grids of non-LTE atmospheric models for a wide range of parameters, and further attempted a global synthesis of our results with current T Tauri stars and solar models and observations so as to encompass the whole range of known 'magnetic activity' levels among stars (Houdebine et al., 1996). Numerical results and synthesised spectra were successfully compared to observations (Houdebine, Foing & Stempels, 1997).
References
Houdebine, E.R., Mathioudakis, M., Doyle, J.G. & Foing,
B.H. (1996). A&:A 305, 209.
Houdebine, E.R., Foing, B.H. & Stempels, H.C. (1997).
A&A, in press.
Analysis of data collected during large intercontinental MUSICOS campaigns in previous years was continued. Hubert, B.H. Foing and collaborators detected from 258 high S/N spectra of the He I 6678 line, collected during MUSICOS 1989, the signature of non-radial pulsations identified as the tesseral mode (absolute value of m=l - 1 = 9±2) on the Be star 48 Per (Hubert et al., 1997).
From a series of 619 spectra of the Delta Scuti-type star Theta 2 Tauri measured during MUSICOS 91, Kennelly, B.H. Foing and collaborators detected non-radial oscillation modes of low degree (0<l<3) from radial velocity variations. We also detected modes of higher degree (3<l<10) from line profile variations in the rotational profile that we could identify using a 2-D Fourier representation over time- wavelength. The detected frequencies were found in agreement with those computed from theoretical models (Kennelly et al., 1996). We could also derive azimuthal structures in the wind and chromosphere of the pre-main sequence star AB Aur (Boehm et al., 1996).
From a southern campaign (MUSICOS 94), including ESO La Silla, Cerro Tololo CTIO, South Africa SAAO and the 4 m AAT in Australia in coordination with HST and IUE, Balona, B.H. Foing and collaborators could, for the first time, detect non-radial pulsations on the star Gamma Doradus, and explain the variability of this prototype for a new class of variables. Three modes were identified as sectorial retrograde modes with spherical harmonic of low degrees (l,m) = (3,3) and (1,1) (Balona, Boehm, Foing et al., 1996). The spectral and photometric time-series of the Post-T Tauri star AB Doradus were used to obtain a Doppler image of the starspot distribution, and to trace the evolution of the circumstellar prominence system over five rotations (Collier-Cameron et al., 1996). Simultaneous time-resolved spectroscopy from the GHRS/HST allowed us to trace the coronal- prominence interface in the C IV transition region line and to monitor the associated UV variability. Finally, the proto-stellar system Beta Pictoris was observed during the MUSICOS campaign in coordination with GHRS/HST in order to diagnose comet infall and vaporisation events.
The recently completed multi-site campaign (MUSIC-OS 96; second half of November 1996), combined observations from Xinglong, McDonald, Isaac Newton Telescope and Haute Provence. The objectives included: the study of Doppler and magnetic imaging and the study of the star-disc interaction in the T Tauri star SU Aur; the study of surface and wind magnetic structures in the Herbig Ae star AB Aur and in O star Xi Per; and the observation of non-radial pulsation of the Delta Scuti star V480 Tau. The ESA-MUSICOS spectrograph installed at the 2.5 m INT (see 4.2.1.12) recorded 213 echelle spectra on the four variable targets.
References
Balona, L.A., Boehm, T., Foing, B.H. et al. (1996).
MNRAS 281, 1315.
Boehm, T. et al. (incl. B.H. Foing). (1996). A&A
Suppl. 120, 431.
Collier-Cameron et al. (incl. B.H. Foing) (1996).A&A,
submitted.
Hubert, A.M., Floquet, M., Foing B.H. et al. (1997).
A&A, in press.
Kennelly, E.J., Walker, G.A., Catala, C., Foing, B.H. et al.
(1996). A&A 313, 571.
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