High-precision timing observations of the millisecond pulsar PSR 1821-24 have been conducted on 305 individual dates at the Nancay radiotelescope at 1.4 GHz between March 2 1989 and July 21 1993. The Time Of Arrival residuals after the standard fit of the pulsar parameters are characterised by a rms of 2.78 mu s. This dense and precise timing series has allowed the first determination of the apparent second period derivative P for PSR 1821-24 and of its proper motion. We find that the observed quasi-cubic variation (i.e. the apparent P) of the post-fit residuals matches the level of low-frequency noise predicted by the empirical relationship of Arzoumanian et al. (1994) for the rotation irregularities of classical pulsars. We conclude that the millisecond pulsar PSR 1821-24, similarly to PSR 1937+21, exhibits instrinsic rotation irregularities, unless P is a jerk (it) induced during a close encounter in the globular cluster M28. This pulsar is the youngest known millisecond pulsar according to a characteristic age (1/2 P-0/P-0) of 30 x 10(6) yr. Relatively young millisecond pulsars, like PSR 1821-24 and PSR 1937+21, might be prone to rotation irregularities while old millisecond pulsars like PSR 1855+09 an more stable.We find a discrepancy between the optical proper motion of M28 and our timing proper motion of PSR 1821-24 larger than the pulsar escape velocity in the cluster. Reconciling these two proper motions is important for the kinematics of M28 and the study of the gravitational potential of the Galactic disk and bulge.Finally, we present the daily timing observations of PSR 1821-24 conducted every year since 1989 when the solar corona intervenes between the pulsar and the Earth at Christmas time. We have used these timing observations along a single cut through the corona to fit a spherically symmetric model of its electron density n(e) = n(0)(r/r(0))(-alpha). With a slightly more sophisticated model for the coronal electron density, precise timing observations of PSR 1821-24 during these periods over a complete solar cycle could monitor the global flattening expected for the coronal plasma at the solar minimum.

Observations of the millisecond pulsar PSR 1937+21 at Nancay Radio Observatory provide the densest series of times of arrival (TOA) and of flux densities on this pulsar. They were measured on 525 individual dates over 4.5 years from 1988 December, 22. Comparison between the astrometric parameters fitted to the Nancay and Arecibo independent timing series shows that they are consistent at the level of 4 times the formal uncertainty (sigma) for all parameters except the period (7 sigma) but that is likely caused by the different TAI time scales used in the 2 data sets. The post-fit TOA residuals of the two series are characterized by the same rms, 0.36 mu s at 1.4 GHz, if the times of arrival with the highest flux densities (> 345 mJy) are selected at Nancay. We find also that there is a high correlation between the TOA residuals and the flux density of this pulsar as expected if refractive scintillation is important in the ionized interstellar medium. Finally, when the second period derivative is not solved for in the Nancay data, the post-fit TOA residuals are dominated by a low-frequency noise as already discovered in the Arecibo data on PSR 1937+21. In this context, we study the effects of the large minor planets recently discovered at the edge of the Solar System (Chiron, Phollus, 1992 QB1) and not included in the ephemerides used for the analysis. We conclude that they are negligible. However, the hypothetical more massive tenth Planet (Planet X) might already be partially responsible for the low-frequency noise in PSR 1937+21 residuals. We show that, if this planet exists, its signature will be large over 20 years of high-precision timing of the millisecond pulsars PSR 1937+21, PSR 1855+09 and PSR 1821-24

LARGE fluctuations in the radio emissions from the quasar 0954+658 (ref. 1) attest to the existence of large-scale inhomogeneities in the ionized interstellar medium. These fluctuations, termed extreme scattering events, are caused by the refractive focusing of the radio waves by discrete plasma structures. In principle, the radio emissions of pulsars should also be a sensitive probe of such phenomena2-5, which should be manifest as fluctuations in the flux density and increased delays in the timing measurements. Here we report the detection of an extreme scattering event, lasting about 15 days, in timing observations of the millisecond pulsar 1937+21. This event provides independent evidence for the existence of large-scale structure in the interstellar medium, and allows us to constrain the properties of the lensing structure (distance, velocity, linear dimension and electron density) more tightly than has been possible from detections of lensed extragalactic radio sources. Of more practical importance, such events could be the dominant source of timing noise in millisecond pulsars at approximately 1 GHz, and will need to be considered when searching for the subtle signature of gravitational waves in pulsar timing measurements.

One of us (J.H.) proposed a scheme to select preferred radio frequencies for SETI by using the rotational frequencies of pulsars and to test it in the field. The conversion from pulsar rotational frequencies to SETI frequencies in the 1–10 GHz galactic radio window is made in a very precise quasi-unique way by use of remarkable mathematical numbers. The selection of specific pulsars is made according to the types of SETI targets. We present the status of experimental observations made with the large Nançay (France) decimetric radiotelescope applied to the following typical searches: (1) target stars closer than 25 parsecs; (2) unknown targets in a globular cluster; (3) unknown targets anywhere in our galaxy closer than 2.5 kpc. In addition to stars with planet-like candidate companions, special emphasis was given to stars with candidate disks older than two billion years as these may have provided enough time for advanced life to appear. This experiment is a new step in a project of collaboration between Paris Observatory and NASA SETI Program Office started in 1981. It gives new insight to faint RFI relevant to SETI.

In 1981 we began a SETI search on nearby solar type stars: in all some 344 stellar targets. Previous reports to IAF Congresses have presented work in progress. This paper presents the summary of all our observations which detected no narrowband signal from another technology but did detect several unsuspected sources of terrestrial technology and provided an excellent opportunity to evaluate the statistical nature of the observatory noise environment.

The properties of doublets of thin lenses in the Gaussian optics approximation were investigated. Two different ways for such a doublet to give strictly achromatic images of the input beam waist were found. Both solutions may be useful in a variety of applications, one being the possibility of shaping asymmetrical beams for fan beam antennas illumination. Using modes higher than the fundamental mode will allow the design of more realistic focal systems.

L'etoile variable Mira, U Herculis, a ete observee dans les 2 raies principales OH par le reseau VLBI europeen. Les points chauds les plus deplaces vers le bleu dans les 2 transitions maser sont trouves coincidents. A 1665 MHz plusieurs composantes compactes dispersees sur une aire de 0.35″ par 0.7″ ont ete detectees, certaines d'entre elles possedant une contre-partie compacte a 1667 MHz. La plus grande partie du flux OH est produit dans des regions maser de grandes etendues, soutenant ainsi un modele de structure noyau-halo pour ces masers.

S. GulkisSpace Physics and Astrophysics SectionF. Biraud and J. HeidmannObservatoire de ParisMeudon, FranceJ. TarterUniversity of California, Berkeley, CaliforniaThe SETI Institute, Los Altos, CaliforniaThe Nan_ay decimetric Radio Telescope (NRT) in Nan_ay, France, is described,and its potential use for SETI (Search for Extraterrestrial Intelligence) observationsis discussed. The conclusion reached is that the NRT is well suited for SETI obser-vations because of its large collecting area, its large sky coverage, and its widebandfrequency capability. However, a number of improvements are necessary in orderto take full advantage of the system in carrying out an efficient SETI program. Inparticular, system sensitivity should be increased. This can be achieved through aseries of improvements to the system, including lowering the ground pickup noisethrough the use of ground reflectors and more efficient feed design, and by usinglow-noise amplifer front ends.

The Nancay decimetric Radio Telescope (NRT) in Nancay, France, is described, and its potential use for Search for Extraterrestrial Intelligence (SETI) observations is discussed. The conclusion reached is that the NRT is well suited for SETI observations because of its large collecting area, its large sky coverage, and its wideband frequency capability. However, a number of improvements are necessary in order to take full advantage of the system in carrying out an efficient SETI program. In particular, system sensitivity should be increased. This can be achieved through a series of improvements to the system, including lowering the ground pickup noise through the use of ground reflectors and more efficient feed design, and by using low-noise amplifier front ends.

Results are reported of the measurement of balloon motion in the upper layers of the Venus atmosphere. The measurement of the balloon trajectories was carried out by differential very-long-baseline interferometry (VLBI) observations of the balloons relative to the fly-by spacecrafts (VEGA-1 and VEGA-2). Balloon-2, launched in the southern hemisphere of the planet led to the discovery of gas motion with average velocities of 65.3 m s-1 and 3.4 m s-1 in longitude and latitude, respectively. Balloon-1, launched in the northern hemisphere, showed that gas motions occur with an average velocity of 68.7 m s-1 in the longitudinal direction, but no wind was detected in the latitudinal direction. Pendulum motion of the balloon baskets with a period of 7.4 s was detected on many occasions. This occurred 10 times more frequently in the southern hemisphere than in the northern one. The nature of these phenomena is discussed.

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere by the VEGA spacecraft.

The GRIG-2 geodetic VLBI experiment was conducted in 1985, linking for the first time South America, Europe and Africa. At the single frequency band of 1.66 GHz which was used, the monitoring of the ionosphere is a critical aspect and several predictions of Total Electron Content (TEC) were used. One of them is derived from dual band Doppler observations of TRANSIT satellites, which were simultaneously conducted. The influence of these models on the solution is presented, with comparisons with other VLBI solutions. Decimetric accuracy has been achieved.

Detection d'une tache compacte brillante decalee vers le bleu sur la face avant de l'enveloppe en expansion de l'etoile variable de type Mira, U Her. La non-detection d'une tache decalee vers le rouge sur la face opposee de l'enveloppe suggere que la tache observee (a l'aide de technique VLBI) est peut-etre l'image amplifiee par effet maser de l'emission continue de l'etoile centrale. L'emission OH pourrait alors servir de lieu entre les systemes de reference optique et radio

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere near Venus midnight by the VEGA spacecraft on 11 and 15 June 1985. Initial analysis of only radial velocities indicates that each balloon was blown westward about 11,500 kilometers (8,000 kilometers on the night side) by zonal winds with a mean speed of about 70 meters per second. Excursions of the data from a model of constant zonal velocity were generally less than 3 meters per second; however, a much larger variation was evident near the end of the flight of the second balloon. Consistent systematic trends in the residuals for both balloons indicate the possibility of a solar-fixed atmospheric feature. Rapid variations in balloon velocity were often detected within a single transmission (330 seconds); however, they may represent not only atmospheric motions but also self-induced aerodynamic motions of the balloon.