21 research outputs found
On the Exchange of Kinetic and Magnetic Energy Between Clouds and the Interstellar Medium
We investigate, through 2D MHD numerical simulations, the interaction of a
uniform magnetic field oblique to a moving interstellar cloud. In particular we
explore the transformation of cloud kinetic energy into magnetic energy as a
result of field line stretching. Some previous simulations have emphasized the
possible dynamical importance of a ``magnetic shield'' formed around clouds
when the magnetic field is perpendicular to the cloud motion (Jones et al.
1996, Miniati et al. 1998). It was not clear, however, how dependent those
findings were to the assumed field configuration and cloud properties. To
expand our understanding of this effect, we examine several new cases by varing
the magnetic field orientation angle with respect to the cloud motion (\theta),
the cloud-background density contrast, and the cloud Mach number.
We show that in 2D and with \theta large enough, the magnetic field tension
can become dominant in the dynamics of the motion of high density contrast, low
Mach number clouds. In such cases a significant fraction of cloud kinetic
energy can be transformed into magnetic energy with the magnetic pressure at
the cloud nose exceeding the ram pressure of the impinging flow. We derive a
characteristic timescale for this process of energy ``conversion''. We find
also that unless the cloud motion is highly aligned to the magnetic field,
reconnection through tearing mode instabilities in the cloud wake limit the
formation of a strong flux rope feature following the cloud. Finally we attempt
to interpret some observational properties of the magnetic field in view of our
results.Comment: 24 pages in aaspp4 Latex and 7 figures. Accepted for publication in
The Astrophysical Journa
Radio observations of candidate magnetic O stars
Context: Some O stars are suspected to have to have (weak) magnetic fields
because of the observed cyclical variability in their UV wind-lines. However,
direct detections of these magnetic fields using optical spectropolarimetry
have proven to be very difficult. Aims: Non-thermal radio emission in these
objects would most likely be due to synchrotron radiation. As a magnetic field
is required for the production of synchrotron radiation, this would be strong
evidence for the presence of a magnetic field. Such non-thermal emission has
already been observed from the strongly magnetic Ap/Bp stars. Methods: We have
performed 6 & 21 cm observations using the WSRT and use these, in combination
with archival VLA data at 3.6 cm and results from the literature, to study the
radio emission of 5 selected candidate magnetic O stars. Results: Out of our
five targets, we have detected three: Per, which shows a non-thermal
radio spectrum, and Cam and Cep, which show no evidence of a
non-thermal spectrum. In general we find that the observed free-free (thermal)
flux of the stellar wind is lower than expected. This is in agreement with
recent findings that the mass-loss rates from O stars as derived from the
H line are overestimated because of clumping in the inner part of the
stellar wind.Comment: Published in A&
The ionosphere and radio interferometry
This paper reviews the effects of the ionosphere on radio astronomjcal observations, what we can learn about the ionosphere from radio interferometry, and a procedure to correct for these effects. This study analyzes the results obtained from observations of celestial point soUl.ces with the Westerbork Synthesis Radio Telescope, WSRT, in the Netherlands from the period 1970-1991. The main conc1usions are: 1) A1though seasona1 effects are c1ear, the occurrence and "strength" of ionospheric irregu1arities show no dependence on solar activity. 2) Assuming that the frequency of occurrence of ionospheric disturbances in Spring and Autumn are similar, Ihe "ionospheric" Winter starts on day 348 ± 3 and ali seasons last for three months. 3) Travelling ionospheric disturbances, TIDs, occur most frequently during daytime in Winter periods. 4) The propagation parameters of these travelling ionospheric irregularities and their periods indicate that these belong main1y to the c1ass of medium sca]e TIDs. 5) Radio interferometry is a powerful tool to locate irregularities causing scintillation and to determine their dimensions. 6) The occurrence of non-periodic irregu1arities is, however, not a function of time of day. 7) The daily variation in the amplitude and frequency of occurrence of the TIDs suggest that the generation of gravity waves may be caused by winds and tides in the lower thermosphere/mesosphere. On the basis of the availab1e data, a definition of a "disturbance measure" indicating to what extent the ionosphere is "quiet" is proposed. Procedures to correct for ionospheric effects and an eva1uation of the different methods to obtain information on the ionospheric e1ectron content are reviewed in sections 8 and 9, respectively
Refraction of transatmospheric signals in geodesy : proceedings of the symposium /
Proceedings of the symposium The Hague, The Nederlands, May 19 - 22, 199