Slingshot prominences above stellar X-ray coronae

We present a new model for the coronal structure of rapidly rotating solar-type stars. The presence of prominences trapped in co-rotation 2 to 5 stellar radii above the stellar surface has been taken as evidence that the coronae of these stars must be very extended. The observed surface magnetic fields, however, cannot contain X-ray emitting gas out to these distances. We present an alternative model: that these prominences are trapped in long thin loops embedded not in the X-ray corona, but in the wind. Above coronal helmet streamers, oppositely-directed wind-bearing field lines reconnect to form closed loops which then fill up with gas that was originally part of the wind. We demonstrate that static equilibria exist for these loops at a range of pressures and temperatures. The maximum loop height falls as the rotation rate increases, but rises as the loop temperature decreases. For a solar-mass star with rotation period 0.5 days, whose X-ray corona extends 1stellar radius above the surface, loops at temperatures of 10, 000 K can extend out to 5 stellar radii.Comment: 9 pages, 8 figure

On the Relationship Between Photospheric Footpoint Motions and Coronal Heating in Solar Active Regions

PublishedArticleCoronal heating theories can be classified as either direct current (DC) or alternating current (AC) mechanisms, depending on whether the coronal magnetic field responds quasi-statically or dynamically to the photospheric footpoint motions. In this paper we investigate whether photospheric footpoint motions with velocities of 1-2 km s–1 can heat the corona in active regions, and whether the corona responds quasi-statically or dynamically to such motions (DC versus AC heating). We construct three-dimensional magnetohydrodynamic models for the Alfvén waves and quasi-static perturbations generated within a coronal loop. We find that in models where the effects of the lower atmosphere are neglected, the corona responds quasi-statically to the footpoint motions (DC heating), but the energy flux into the corona is too low compared to observational requirements. In more realistic models that include the lower atmosphere, the corona responds more dynamically to the footpoint motions (AC heating) and the predicted heating rates due to Alfvén wave turbulence are sufficient to explain the observed hot loops. The higher heating rates are due to the amplification of Alfvén waves in the lower atmosphere. We conclude that magnetic braiding is a highly dynamic process

The heating of solar coronal loops by Alfvèn wave turbulence

This project was supported under contract NNM07AB07C from NASA to the Smithsonian Astrophysical Observatory (SAO) and contract SP02H1701R from Lockheed Martin Space and Astrophysics Laboratory (LMSAL) to SAO.In this paper we further develop a model for the heating of coronal loops by Alfvèn wave turbulence (AWT). The Alfvèn waves are assumed to be launched from a collection of kilogauss flux tubes in the photosphere at the two ends of the loop. Using a three-dimensional magneto-hydrodynamic (MHD) model for an active-region loop, we investigate how the waves from neighboring flux tubes interact in the chromosphere and corona. For a particular combination of model parameters we find that AWT can produce enough heat to maintain a peak temperature of about 2.5 MK, somewhat lower than the temperatures of 3 – 4 MK observed in the cores of active regions. The heating rates vary strongly in space and time, but the simulated heating events have durations less than 1 minute and are unlikely to reproduce the observed broad Differential Emission Measure distributions of active regions. The simulated spectral line non-thermal widths are predicted to be about 27 km s−1, which is high compared to the observed values. Therefore, the present AWT model does not satisfy the observational constraints. An alternative “magnetic braiding” model is considered in which the coronal field lines are subject to slow random footpoint motions, but we find that such long period motions produce much less heating than the shorter period waves launched within the flux tubes. We discuss several possibilities for resolving the problem of producing sufficiently hot loops in active regions.PostprintPeer reviewe

Effects and Costs of Cervical Cancer Screening

The objective of this thesis is to evaluate cervical eaueer screening by assessing its various effects and costs. Questions to be addressed concern the rednetion in incidence and mortality, the unfavourablc effects and the cost-effectiveness of different screening polides (age range and interval combinations). To this end, we used acervical eaueer version of MISCAN, a simulation model that was designed and programmed to evaluate cancer screening. This cervical eaueer screening model. had already been tested against screening data sets from British Columbia (Canada) and from Dutch pilot screeningprojectsin 1976- 1985. The potential impact of new developments in cervical eaueer screening, such as auto~1ated cytological evaluation and HPV-testing, has been investigated. The ultimate questions are: should ccrvical caoeer screening be conti. nued, and what changes should be recommended in order to iinprove its efficiency? The relationship between benefits and unfavourable effects was stuclied for three efficient cervical cancer screening strategies incorporating 5, 10 and 25 smears per woman (Chapter 2). It was concluded that wlien the screening frequency was increased, the ratio of favourable to unfavourable effects became worse

The management of cervical intra-epithelial neoplasia (CIN): Extensiveness and costs in The Netherlands

In order to provide greater insight into both the extensiveness and the medical costs of the diagnosis and treatment of screen-detected cervical intra-epithelial neoplasia (CIN) in general medical practice in The Netherlands, data from national registries and gynaecology departments were retrieved, and experts were interviewed. Of the 5060 women diagnosed with CIN in 1988, more than 50% were treated in hospital with conisation or hysterectomy, which on average took 5.5 days stay per admission. The assessed average duration of the total pre- and post-treatment period is 4.6 years. The average total medical costs in women with detected CIN III are Dfl 3700 per woman. The diagnosis of CIN I and II involves more medical procedures and time than CIN III, but fewer women have conisation or hysterectomy, resulting in lower total medical costs (Dfl 2572). The overall extent and costs of the management of CIN should be accounted for when balancing the benefits, unfavourable effects and costs of cervical cancer screening

Modeling the Sun's open magnetic flux and the heliospheric current sheet

By coupling a solar surface flux transport model with an extrapolation of the heliospheric field, we simulate the evolution of the Sun's open magnetic flux and the heliospheric current sheet (HCS) based on observational data of sunspot groups since 1976. The results are consistent with measurements of the interplanetary magnetic field near Earth and with the tilt angle of the HCS as derived from extrapolation of the observed solar surface field. This opens the possibility for an improved reconstruction of the Sun's open flux and the HCS into the past on the basis of empirical sunspot data.Comment: 16 pages, 5 figures, Accepted for publication in Ap