Author name code: dejager ADS astronomy entries on 2022-09-14 =author:"De Jager, Cornelis" OR =author:"De Jager, C." ------------------------------------------------------------------------ Title: Solar magnetic variability and climate Authors: de Jager, Cornelis; Duhau, Silvia; Nieuwenhuizen, Adrianus Bibcode: 2020smvc.book.....D Altcode: When Kees de Jager returned to his birth ground Texel, he was asked by the director of Royal Institute for Sea Research, Prof. Dr. J. de Leeuw, to investigate the relationship between solar activity and climate. This research, done over a period of 15 years, generated about 10 papers with remarkable new insights. In 2019 Kees decided to compile the papers into a scientific book. This book covers:

The equatorial and poloidal magnetic fields and their relation to the Earth's NH temperature.

The notions phase diagram of the solar dynamo, Transition Point and Grand Episode.

The granular scale magnetic fields or 'bright spots'.

Telescopes for large-field high-resolution imaging of the sun.

The expectations for future solar activity derived from the decomposition of the solar dynamo into 8 'modes'. This leads to prediction of the sunspot maxima till 2130.

The statistical start of the modern heating and solar variability as part of the modern heating curve.

The non-linear time difference between the solar magnetic variations and terrestrial warming. Title: Pulsations, eruptions, and evolution of four yellow hypergiants Authors: van Genderen, A. M.; Lobel, A.; Nieuwenhuijzen, H.; Henry, G. W.; de Jager, C.; Blown, E.; Di Scala, G.; van Ballegoij, E. J. Bibcode: 2019A&A...631A..48V Altcode: 2019arXiv191002460V
Aims: We aim to explore the variable photometric and stellar properties of four yellow hypergiants (YHGs), HR 8752, HR 5171A, ρ Cas, and HD 179821, and their pulsations of hundreds of days, and long-term variations (LTVs) of years. We also aim to explore light and colour curves for characteristics betraying evolutionary loops and eruptive episodes and to investigate trends of quasi-periods and the possible need for distance revisions.
Methods: We tackled multi-colour and visual photometric data sets, looked for photometric indications betraying eruptions or enhanced mass-loss episodes, calculated stellar properties mainly using a previously published temperature calibration, and investigated the nature of LTVs and their influence on quasi-periods and stellar properties.
Results: Based on driven one-zone stellar oscillation models, the pulsations can be characterised as "weakly chaotic". The BV photometry revealed a high-opacity layer in the atmospheres. When the temperature rises the mass loss increases as well, consequently, as the density of the high-opacity layer. As a result, the absorption in B and V grow. The absorption in B, presumably of the order of one to a few 0.m 1, is always higher than in V. This difference renders redder and variable (B - V) colour indexes, but the absorption law is unknown. This property of YHGs is unpredictable and explains why spectroscopic temperatures (reddening independent) are always higher than photometric ones, but the difference decreases with the temperature. A new (weak) eruption of ρ Cas has been identified. We propose shorter distances for ρ Cas and HR 5171A than the accepted ones. Therefore, a correction to decrease the blue luminescence of HR 5171A by polycyclic aromatic hydrocarbon (PAH) molecules is necessary, and HR 5171A would no longer be a member of the cluster Gum48d. HR 5171A is only subject to one source of light variation, not by two as the literature suggests. Eruptive episodes (lasting one to two years), of YHGs prefer relatively cool circumstances when a red evolutionary loop (RL) has shifted the star to the red on the HR diagram. After the eruption, a blue loop evolution (BL) is triggered lasting one to a few decades. We claim that in addition to HR 8752, also the other three YHGs have shown similar cycles over the last 70 years. This supports the suspicion that HD 179821 might be a YHG (with a possible eruptive episode between 1925 and 1960). The range in temperature of these cyclic Teff variations is 3000 K-4000 K. LTVs mainly consist of such BL and RL evolutions, which are responsible for a decrease and increase, respectively, of the quasi-periods. The reddening episode of HR 5171A between 1960 and 1974 was most likely due to a red loop evolution, and the reddening after the 1975 eruption was likely due to a shell ejection, taking place simultaneously with a blue loop evolution.

Full Tables M.1 and M.2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A48 Title: VizieR Online Data Catalog: rho Cas differential BVRI photometry (van Genderen+, 2019) Authors: van Genderen, A. M.; Lobel, A.; Nieuwenhuijzen, H.; Henry, G. W.; de Jager, C.; Blown, E.; di Scala, G.; van Ballegoij, E. J. Bibcode: 2019yCat..36310048V Altcode: Tablem1 contains the full T2 APT VRI photometric dataset of rho Cas, 1986-2001. Magnitudes (mag) of variable (Var) and check (Chk) star are relative to the comparison star HD 223173. Tablem2 contains the full T3 APT BV photometric dataset of rho Cas, 2003-2018.

(2 data files). Title: The relation between the average northern hemisphere ground temperature and solar equatorial and polar magnetic activity Authors: De Jager, Cornelis; Nieuwenhuizen, A. C. T.; Nieuwenhuizen, H.; Duhau, S. Bibcode: 2018PAIJ....2..175D Altcode: 2018PAIJ....2..175J No abstract at ADS Title: A Remarkable Recent Transition in the Solar Dynamo Authors: de Jager, C.; Akasofu, S. -I.; Duhau, S.; Livingston, W. C.; Nieuwenhuijzen, H.; Potgieter, M. S. Bibcode: 2016SSRv..201..109D Altcode: 2016SSRv..tmp...64D We summarize the major aspects of the remarkable, fairly long lasting period (∼ 2005 to ∼ 2010) of low solar activity, that we will call the Transition. It is the transitional stage between the Grand Maximum of the 20th century and a forthcoming (most probably Regular) episode of solar activity. The various kinds of activity in the functioning of the equatorial components of the solar dynamo before and during the Transition are summarized. While the behavior of unipolar magnetic regions and their rest-latitudes already gave very early indications - mid 20th century - of the forthcoming Transition, more such indications became available around 1995 and the main part of it occurred between 2005 and 2010. Some of the inferences are discussed. We submit the hypothesis that the solar tachocline undergoes pulsations and we present some helioseismic evidences. In that scenario we find that its equatorial part has moved downward over a fairly small semi-amplitude (∼ 0.03 solar radii) during the time of the Transition. There are several indications, apart from this `pulsation', that the tachocline may even be pulsating with still smaller amplitudes in more modes. We speculate about the physical mechanism(s). Title: Yellow Hypergiants: A Comparative Study of HR 5171A, Rho Cas, and HR 8752 Authors: Lobel, A.; de Jager, C.; Nieuwenhuijzen, H.; van Genderen, A. M.; Oudmaijer, R. Bibcode: 2015EAS....71..279L Altcode: We present a comparative study of the detailed spectroscopic variability of selected Fe i absorption lines observed between 1991 and 1995 in the optical spectra of the bright yellow hypergiants HR 5171A and Rho Cas. We also compare with the high-resolution spectrum of HR 8752. The three cool hypergiants reveal broad lines with very similar shapes. The variations with time in the line shapes and depths of HR 5171A and Rho Cas are very similar. The spectroscopic variability is caused by quasi-periodic atmospheric pulsations that strongly correlates with the observed radial velocity and V-brightness changes. Title: Reply to “The influence of planetary attractions on the solar tachocline” by N. Scafetta, O. Humlum, J.E. Solheim, K. Stordahl Authors: Callebaut, D.; de Jager, C.; Duhau, S. Bibcode: 2013JASTP.102..372C Altcode: There are some evidences that sunspot cycle and solar inertial force variability are related.It is known that differential rotation drives sunspot cycle.Solar inertial force is too weak to drive differential rotation.A mechanism able to amplify solar inertial force to its required strength is still missing. Title: The Utrecht Astronomical Institute, 1632 to 2012 Authors: de Jager, C. Bibcode: 2013ASPC..470...15D Altcode: Utrecht University was founded in 1636. In 1642 the university's astronomical observatory was founded. It was, after that of Leyden, the second university observatory in the world. The observatory was initially located at the Smeetoren, a fortification dating from 1145. In 1854 it moved to Sonnenborgh, a stronghold dating from 1554. In 1987 the personnel moved again, this time to the university campus, the Uithof. We describe the development of research and education in Utrecht over the centuries, with emphasis on the period after 1920. In 2009 the faculty formulated a very favorable advice to the university, viz. that in view of its good potentials astronomical research and education should be strengthened in Utrecht. This decision was accentuated by a likely positive advice of an international visitation committee. But briefly after that the university decided to terminate astronomical education and research in Utrecht, an incomprehensible decision. Title: The hypergiant HR 8752 evolving through the yellow evolutionary void Authors: Nieuwenhuijzen, H.; De Jager, C.; Kolka, I.; Israelian, G.; Lobel, A.; Zsoldos, E.; Maeder, A.; Meynet, G. Bibcode: 2012A&A...546A.105N Altcode: Context. We study the time history of the yellow hypergiant HR 8752 based on high-resolution spectra (1973-2005), the observed MK spectral classification data, B - V- and V-observations (1918-1996) and yet earlier V-observations (1840-1918).
Aims: Our local thermal equilibrium analysis of the spectra yields accurate values of the effective temperature (Teff), the acceleration of gravity (g), and the turbulent velocity (vt) for 26 spectra. The standard deviations average are 82 K for Teff, 0.23 for log g, and 1.1 km s-1 for vt.
Methods: A comparison of B - V observations, MK spectral types, and Teff-data yields E(B - V), "intrinsic" B - V, Teff, absorption AV, and the bolometric correction BC. With the additional information from simultaneous values of B - V, V, and an estimated value of R, the ratio of specific absorption to the interstellar absorption parameter E(B - V), the "unreddened" bolometric magnitude mbol,0 can be determined. With Hipparcos distance measurements of HR 8752, the absolute bolometric magnitude Mbol,0 can be determined.
Results: Over the period of our study, the value of Teff gradually increased during a number of downward excursions that were observable over the period of sufficient time coverage. These observations, together with those of the effective acceleration g and the turbulent velocity vt, suggest that the star underwent a number of successive gas ejections. During each ejection, a pseudo photosphere was produced of increasingly smaller g and higher vt values. After the dispersion into space of the ejected shells and after the restructuring of the star's atmosphere, a hotter and more compact photosphere became visible. From the B - V and V observations, the basic stellar parameters, Teff, log M/M⊙, log L/L⊙, and log R/R⊙ are determined for each of the observational points. The results show the variation in these basic stellar parameters over the past near-century.
Conclusions: We show that the atmospheric instability region in the HR-diagram that we baptize the yellow evolutionary void actually consists of two parts. We claim that the present observations show that HR 8752 is presently climbing out of the "first" instability region and that it is on its way to stability, but in the course of its future evolution it still has to go through the second potential unstable region.

Appendix A is available in electronic form at http://www.aanda.orgTables A.x and B.x are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/546/A105, and at the external site http://www.aai.ee/HR~8752 Title: Solar Forcing of Climate Authors: de Jager, C. Bibcode: 2012SGeo...33..445D Altcode: 2012SGeo..tmp...26D Solar activity is evident both in the equatorial activity centres and in the polar magnetic field variations. The total solar irradiance variation is due to the former component. During the extraordinarily long minimum of activity between sunspot cycles 23 and 24, the variations related to the equatorial field components reached their minimum values in the first half of 2008, while those related to the polar field variations had their extreme values rather at the end of 2009 and the first half of 2010. The explanation of this delay is another challenge for dynamo theories. The role of the open solar flux has so far been grossly underestimated in discussions of Sun-climate relations. The gradual increase in the average terrestrial ground temperature since 1610 is related both to the equatorial and polar field variations. The main component (0.077 K/century) is due to the variation of the total solar irradiance. The second component (0.040 K/century) waits for an explanation. The smoothed residual increase, presumably antropogenic, obtained after subtraction of the known components from the total increase was 0.31 K in 1999. Title: VizieR Online Data Catalog: HR8752 evolving through the void (Nieuwenhuijzen+ 2012) Authors: Nieuwenhuijzen, H.; de Jager, C.; Kolka, I.; Israelian, G.; Lobel, A.; Zsoldos, E.; Maeder, A.; Meynet, G. Bibcode: 2012yCat..35460105N Altcode: 2012yCat..35469105N The main paper on the hypergiant HR8752 evolving through the void combines observational data from various sources: new spectral data, new and already published (B-V), V data. it also describes a new three-dimensional chebychev interpolating code, and gives information on the temperature calibration for hypergiants, and adds some extra Tables on colour excess and historical context (cf. below).

The electronic appendix contains a number of tables, too large for publishing in the paper. In the text and in the appendix, they are referred to as Table Axx, resp. Cxx. We note, however, that tables A1.1 to A1.6 in Section 2 of the main paper are here referred to as tables appena11 to appena16.

The observational tables appena11 to appena16, appena2, appena3, appena4 all refer to Section 2 in the main paper, while tables appena5, appena6, appena7, appena8 are used in Section 3 of the main paper. The technical table appenc1 describes the three-dimensional chebychev interpolation method used in Section 2 of the main paper. The second technical table appenc2 describes the (hypergiant) temperature calibration and transformation codes between Mk, Teff, and (B-V) data, used in Section 3 of the main paper. Table appena9 discusses some measured colour excesses (Section 3.3 of the main paper). Table appena10 discusses the possibility of historical context (Section 5.11 of the main paper).

(20 data files). Title: Sudden transitions and grand variations in the solar dynamo, past and future</xref> Authors: De Jager, Cornelis; Duhau, Silvia Bibcode: 2012JSWSC...2A..07D Altcode: The solar dynamo is the exotic dance of the sun's two major magnetic field components, the poloidal and the toroidal, interacting in anti-phase. On the basis of new data on the geomagnetic aa index, we improve our previous forecast of the properties of the current Schwabe cycle #24. Its maximum will occur in 2013.5 and the maximum sunspot number Rmax will then be 62 ± 12, which is within the bounds of our earlier forecasts. The subsequent analysis, based on a phase diagram, which is a diagram showing the relation between maximum sunspot numbers and minimum geomagnetic aa index values leads to the conclusion that a new Grand Episode in solar activity has started in 2008. From the study of the natural oscillations in the sunspot number time series, as found by an analysis based on suitable wavelet base functions, we predict that this Grand Episode will be of the Regular Oscillations type, which is the kind of oscillations that also occurred between 1724 and 1924. Previous expectations of a Grand (Maunder-type) Minimum of solar activity cannot be supported. We stress the significance of the Hallstatt periodicity for determining the character of the forthcoming Grand Episodes. No Grand Minimum is expected to occur during the millennium that has just started.

The full text of all quoted papers by the two authors of this paper can be consulted at www.cdejager.com/sun-earth-publications/. Title: The influence of planetary attractions on the solar tachocline Authors: Callebaut, Dirk K.; de Jager, Cornelis; Duhau, Silvia Bibcode: 2012JASTP..80...73C Altcode: We present a physical analysis of the occasionally forwarded hypothesis that solar variability, as shown in the various photospheric and outer solar layer activities, might be due to the Newtonian attraction by the planets. We calculate the planetary forces exerted on the tachocline and thereby not only include the immediate forces but we also take into account that these planetary or dynamo actions occur during some time, which demands integration. As an improvement to earlier research on this topic we reconsider the internal convective velocities and we examine several other effects, in particular those due to magnetic buoyancy and to the Coriolis force. The main conclusion is that in its essence: planetary influences are too small to be more than a small modulation of the solar cycle. We do not exclude the possibility that the long term combined action of the planets may induce small internal motions in the sun, which may have indirectly an effect on the solar dynamo after a long time. Title: Solar Forcing of Climate Authors: de Jager, C. Bibcode: 2012omee.book..113D Altcode: No abstract at ADS Title: Quantifying and specifying the solar influence on terrestrial surface temperature Authors: de Jager, C.; Duhau, S.; van Geel, B. Bibcode: 2010JASTP..72..926D Altcode: This investigation is a follow-up of a paper in which we showed that both major magnetic components of the solar dynamo, viz. the toroidal and the poloidal ones, are correlated with average terrestrial surface temperatures. Here, we quantify, improve and specify that result and search for their causes. We studied seven recent temperature files. They were smoothed in order to eliminate the Schwabe-type (11 years) variations. While the total temperature gradient over the period of investigation (1610-1970) is 0.087 °C/century; a gradient of 0.077 °C/century is correlated with the equatorial (toroidal) magnetic field component. Half of it is explained by the increase of the Total Solar Irradiance over the period of investigation, while the other half is due to feedback by evaporated water vapour. A yet unexplained gradient of -0.040 °C/century is correlated with the polar (poloidal) magnetic field. The residual temperature increase over that period, not correlated with solar variability, is 0.051 °C/century. It is ascribed to climatologic forcings and internal modes of variation. We used these results to study present terrestrial surface warming. By subtracting the above-mentioned components from the observed temperatures we found a residual excess of 0.31° in 1999, this being the triangularly weighted residual over the period 1990-2008. We show that solar forcing of the ground temperature associated with significant feedback is a regularly occurring feature, by describing some well observed events during the Holocene. Title: Solar Gleissberg periodicities in relation to grand episodes Authors: Duhau, S.; de Jager, C. Bibcode: 2009EGUGA..11.6131D Altcode: The solar Gleissberg cycle is not restricted to one single periodicity, as was assumed by its discoverer. Later, it was shown that there are at least two components, each of which dominating in another period of time. We connect this observation to an earlier finding, viz. that the occurrence of the Grand Episodes is related to the behaviour of the solar dynamo in connection to its so-called Transition Point: another Grand Episode starts when the curve defined by the proxies of the poloidal and toroidal magnetic field components has crossed or passed along the Transition Point's coordinates. Here, we present evidence that each Grand Episode is related to one or more specific components of the Gleissberg cycle; each of them with its own period length. This finding must have implications for our understanding of the dynamo mechanism Title: Forecasting the parameters of sunspot cycle 24 and beyond Authors: de Jager, C.; Duhau, S. Bibcode: 2009JASTP..71..239D Altcode: Solar variability is controlled by the internal dynamo which is a non-linear system. We develop a physical-statistical method for forecasting solar activity that takes into account the non-linear character of the solar dynamo. The method is based on the generally accepted mechanisms of the dynamo and on recently found systematic properties of the long-term solar variability. The amplitude modulation of the Schwabe cycle in dynamo's magnetic field components can be decomposed in an invariant transition level and three types of oscillations around it. The regularities that we observe in the behaviour of these oscillations during the last millennium enable us to forecast solar activity. We find that the system is presently undergoing a transition from the recent Grand Maximum to another regime. This transition started in 2000 and it is expected to end around the maximum of cycle 24, foreseen for 2014, with a maximum sunspot number Rmax=68±17. At that time a period of lower solar activity will start. That period will be one of regular oscillations, as occurred between 1730 and 1923. The first of these oscillations may even turn out to be as strongly negative as around 1810, in which case a short Grand Minimum similar to the Dalton one might develop. This moderate-to-low-activity episode is expected to last for at least one Gleissberg cycle (60-100 years). Title: Episodes of relative global warming Authors: de Jager, C.; Duhau, S. Bibcode: 2009JASTP..71..194D Altcode: Solar activity is regulated by the solar dynamo. The dynamo is a non-linear interplay between the equatorial and polar magnetic field components. So far, in Sun-climate studies, only the equatorial component has been considered as a possible driver of tropospheric temperature variations. We show that, next to this, there is a significant contribution of the polar component. Based on direct observations of proxy data for the two main solar magnetic fields components since 1844, we derive an empirical relation between tropospheric temperature variation and those of the solar equatorial and polar activities. When applying that relation to the period 1610-1995, we find some quasi-regular episodes of residual temperature increases and decreases, with semi-amplitudes up to ~0.3 °C. The present period of global warming is one of them. Title: The Solar Dynamo and Its Phase Transitions during the Last Millennium Authors: Duhau, S.; de Jager, C. Bibcode: 2008SoPh..250....1D Altcode: 2008SoPh..tmp..112D We analyze the variation of the solar-dynamo magnetic-field components during the last millennium through a study of their proxy data. We introduce a phase diagram with as abscissa and ordinate the proxies of the values of the toroidal and poloidal magnetic field components. In this diagram the dynamo system appears to regularly cross a well-defined point, which we call the Transition Point. Such crossings occurred five times during the past millennium. Each of these crossings preceded a Grand Episode, either a Minimum or a Maximum one. In addition to these two types of quasiperiodic behavior, a third type consisting of weaker quasiregular oscillations (R) around the Transition Point's coordinates is identified. These periods appear to last one or two times the Gleissberg cycle length. Between the various types of episodes there are brief phase transitions. We identify two types of such phase transitions. Title: A long-term relationship between solar magnetic field, geomagnetic field and Earth's rotation rate. Authors: Duhau, S.; de Jager, C. Bibcode: 2008AGUSMGP31C..06D Altcode: All solar and solar terrestrial variables observables at ground level undergo time changes that have their origins - some of them only partially - in solar magnetic field variations. We have found that solar magnetic field amplitude cycle may be well described as the superposition of a attractor constant level, and oscillations around it that are well represented by the Gleissberg cycle - this last related to Grand Maxima and Grand Minima occurrence - and a decadal oscillation that contains the odd-even rule. In longer term scales a millennial oscillation must be also considered. Here we compare that signals in sunspot number, geomagnetic Mayaud index, sudden storm commencement (SI) index, length of day variation and the velocity of the westward drift and dipolar intensity of the geomagnetic field. Very well defined relationships that depend on the variable are found. In particular we need to divide the decadal variation in a Hale cycle and a semi-secular oscillation since the Hale oscillation in the Earth's rotation rate and in Geomagnetic field are not related and the semi secular oscillation is strongly amplified in these two variables. The origin of the so found relationship, that may help to reconstruct shorter time series of the related variables from other known since longer, is discussed. Title: On the Physical Origin of the Cycles in Long-term Modulation of Solar Activity Authors: Duhau, S.; de Jager, C. Bibcode: 2007AGUSMSH23B..03D Altcode: By a Morlet wavelet analysis in sub harmonics of the 11 year fundamental frequency of sunspot number as a proxy for the toroidal component of solar dynamo magnetic field the evolution for the last 400 years of four well defined cycles- a decadal, a semi-secular, the Gleissberg and the Suess ones - in the modulation of this component of the solar dynamo field is found. The properties of these cycles as seen in geomagnetic index aa and Si as proxy data for polar dynamo field and CME's frequency and intensity, respectively, are described . From this procedure and by analyzing longer proxy time series in the light of the known non-linear properties of solar dynamo system, the meaning of the four cycles and its variability are discussed. Title: On possible drivers of Sun-induced climate changes Authors: de Jager, Cornelis; Usoskin, Ilya Bibcode: 2006JASTP..68.2053D Altcode: 2006JATP...68.2053D We tested the validity of two current hypotheses on the dependence of climate change on solar activity. One of them states that variations in the tropospheric temperature are caused directly by changes of the solar radiance (total or spectral). The other suggests that cosmic ray (CR) fluctuations, caused by the solar/heliospheric modulation, affect the climate via cloud formation. Confronting these hypotheses with seven different sets of the global/hemispheric temperature reconstructions for the last 400 years, we found that the former mechanism is in general more prominent than the latter. Therefore, we can conclude that in so far as the Sun climate connection is concerned tropospheric temperatures are more likely affected by variations in the UV radiation flux rather than by those in the CR flux. Title: Solar Forcing of Climate. 1: Solar Variability Authors: de Jager, C. Bibcode: 2005SSRv..120..197D Altcode: We describe the properties of the Sun, those of its Active Regions (Centres of Activity, ARs or CAs) and the 11 and 22-year cycles as observed via the variable numbers of sunspots. We describe the variations with time of the solar irradiance and of the flux of ejected magnetised plasma. We discuss the probable cause of solar variability. Planetary influences are ruled out; the variability is intrinsic and is described by the solar dynamo. The dynamo is characterised by internal toroidal and more superficial poloidal fields, interchanging and alternating in a 22-year periodicity. From these two components in the solar magnetic fields emanate two possible scenarios for the Sun-climate interaction. Solar irradiance variations are related to those in the solar toroidal magnetic fields. The fraction of the solar irradiance that reaches the Earth's ground level and low troposphere is emitted by the solar photosphere. That fraction does not significantly vary since the quiet photosphere does not significantly vary during the cycle. The variable part of the solar radiation flux is mainly emitted by the chromospheric parts of the CAs. That radiation component does not reach the Earth's troposphere since it is absorbed in the higher, stratospheric terrestrial layers. Tropospheric solar-driven variations should therefore be due to stratosphere-troposphere coupling. The Group Sunspot number R Gs is a proxy for the variable irradiance component and for the toroidal field variations. Ejected solar plasma clouds such as the Coronal Mass Ejections (CMEs) and plasma ejected from Ephemeral Solar Regions and from the polar facular regions are related to variations in the poloidal magnetic fields. On the average they have their maximum intensity about a year after the maximum number of spots: we call this interval the Energetic Emissions Delay. These gas clouds fill the heliosphere with magnetised plasma. Thus, by emitting magnetised plasma, the Sun influences the Earth's atmosphere indirectly, by heliospheric modulation of the component of the galactic cosmic radiation (CR) that reaches tropospheric levels. Modulation is only important for cosmic ray particles with energies below about 50 GeV. Cosmic ray ionisation plays a minor role at ground level but it is the predominant ionising agent in higher atmospheric layers, already above a few kilometres. The amplitudes of the CR variations depend on those of the solar cycle. The atmospheric rate of ionisation varies with CR-intensity. A current hypothesis is that the variable ionisation may affect the degree of cloudiness. Cosmogenic radionuclides such as 10Be are proxies for this influence and for the poloidal field variations. The R G and cosmogenic radionuclide proxies, although loosely correlated, refer to the two different aspects of the solar dynamo with their different terrestrial effects; they do not reach maximum intensity simultaneously and should therefore neither be confused nor be interchanged. Cases have occurred in which the one varied strongly while the other did hardly or not at all. The explanation must be intrinsic in dynamo theory. There is a significant solar signal in the troposphere; it depends on latitude and longitude. A physical investigation of the cause of the Sun-climate relationship based on one unique Δ T(time)-curve, assumed valid for the whole Earth's surface, is therefore basically incorrect. Never during the past 10,000 years has the Sun been as active in ejecting magnetised plasma as during the past few decades. Estimates suggest that the level of solar activity may recently have passed its maximum and that it may decrease in coming decades. Solar activity is variable with six well-determined quasi-periodicities. Attempts to theoretically describe the solar dynamo have so far succeeded only in explaining the qualitative aspects. They fail in a numerical description and notably in one that would permit one to forecast solar activity with acceptable precision. This is so because the solar dynamo is a non-linear system that occasionally shows phase catastrophes. It is a quasi-periodic engine with the properties of deterministic chaos. “The future of such a chaotic system is intrinsically unpredictable”. Title: Do Planetary Motions Drive Solar Variability? Authors: De Jager, Cornelis; Versteegh, Gerard J. M. Bibcode: 2005SoPh..229..175D Altcode: We examine the occasionally forwarded hypothesis that solar activity originates by planetary Newtonian attraction on the Sun. We do this by comparing three accelerations working on solar matter at the tachocline level: Those due to planetary tidal forces, to the motion of the Sun around the planetary system's centre of gravity, and the observed accelerations at that level. We find that the latter are by a factor of about 1000 larger than the former two and therefore cannot be caused by planetary attractions. We conclude that the cause of the dynamo is purely solar. Title: Popularization of Astronomy in the Netherlands Authors: de Jager, C.; Drummen, M. Bibcode: 2004ASSL..310..233D Altcode: 2004osa5.book..233D No abstract at ADS Title: Rapid variations of narrow absorption line components in the spectrum of P Cygni Authors: de Jager, Cornelis; Israelian, Garik Bibcode: 2003NewA....8..475D Altcode: 2003astro.ph..4357D We report the detection of rapid wavelength and intensity variations of narrow components in absorption line profiles of the hot galactic supergiant P Cygni. During most of the time, in 1 week of observations, there were two such components present. Although the period of observation is too short for firm conclusions, the velocity curves do not contradict the photometric period of 17.3 days. These curves may be interpreted as a double wave, 'swinging in counterphase'. This observation would suggest the presence of one long wave over the stellar surface. The waves are possibly gravity waves with wavelengths of the order of half the stellar radius. Title: High-Resolution Spectroscopy of the Yellow Hypergiant ρ Cassiopeiae from 1993 through the Outburst of 2000-2001 Authors: Lobel, A.; Dupree, A. K.; Stefanik, R. P.; Torres, G.; Israelian, G.; Morrison, N.; de Jager, C.; Nieuwenhuijzen, H.; Ilyin, I.; Musaev, F. Bibcode: 2003ApJ...583..923L Altcode: 2003astro.ph..1238L We present an overview of the spectral variability of the peculiar F-type hypergiant ρ Cas, obtained from our long-term monitoring campaigns over the past 8.5 yr with four spectrographs in the northern hemisphere. Between 2000 June and September an exceptional variability phase occurred when the V brightness dimmed by about a full magnitude. The star recovered from this deep minimum by 2001 April. It is the third outburst of ρ Cas on record in the last century. We observe TiO absorption bands in high-resolution near-IR spectra obtained with the Utrecht Echelle Spectrograph during the summer of 2000. TiO formation in the outer atmosphere occurred before the deep brightness minimum. Atmospheric models reveal that the effective temperature decreases by at least 3000 K, and the TiO shell is driven supersonically with M~=5.4×10-2 Msolar yr-1. Strong episodic mass loss and TiO have also been observed during the outbursts of 1945-1947 and 1985-1986. A detailed analysis of the exceptional outburst spectra is provided, by comparing with high-resolution optical spectra of the early M-type supergiants μ Cep (Ia) and Betelgeuse (Iab). During the outburst, central emission appears above the local continuum level in the split Na D lines. A prominent optical emission line spectrum appears in variability phases of fast wind expansion. The radial velocity curves of Hα and of photospheric metal absorption lines signal a very extended and velocity-stratified dynamic atmosphere. The outburst spectra indicate the formation of a low-temperature, optically thick circumstellar gas shell of 3×10-2 Msolar during 200 days, caused by dynamic instability of the upper atmosphere of this pulsating massive supergiant near the Eddington luminosity limit. We observe that the mass-loss rate during the outburst is of the same order of magnitude as has been proposed for the outbursts of η Carinae. We present calculations that correctly predict the outburst timescale, whereby the shell ejection is driven by the release of hydrogen ionization recombination energy. Title: The Millennium Outburst of the Cool Hypergiant rho Cassiopeiae: Spectroscopy and Modeling Authors: Lobel, A.; Dupree, A. K.; Stefanik, R. P.; Torres, G.; Israelian, G.; Morrison, N.; de Jager, C.; Nieuwenhuijzen, H.; Ilyin, I.; Musaev, F. Bibcode: 2003IAUS..210P.F10L Altcode: 2002astro.ph.11508L Between 2000 June and September an exceptional variability phase occurred in the peculiar F-type hypergiant Rho Cas, when the V-brightness dimmed by at least a full magnitude. The star recovered from this deep minimum by 2001 April. It is the third outburst of Rho Cas on record in the last century. We observe TiO absorption bands in high-resolution optical and near-IR spectra obtained with the Utrecht Echelle Spectrograph during the summer of 2000. TiO formation in the outer atmosphere occurred before the deep brightness minimum.Atmospheric models reveal that the effective temperature decreases by at least 3000 K, and the TiO shell is driven supersonically with Mdot =~5.4 10^-2 Msun/yr. Strong episodic mass loss and TiO have also been observed during the outbursts of 1945-47 and 1985-86. An analysis of the exceptional outburst spectra of 2000-01 is provided, by comparing with high-resolution optical spectra of the early M-type supergiants Mu Cep (Ia) and Betelgeuse (Iab). During the outburst, central emission appears above the local continuum level in the split Na D lines. The outburst spectra indicate the formation of a low- temperature, optically thick circumstellar gas shell of 3 10^-2 Msun during \~200 d, caused by dynamic instability of the upper atmosphere of this pulsating massive supergiant near the Eddington luminosity limit. We present an equation that correctly predicts the outburst time-scale, whereby the shell ejection is driven by the release of hydrogen recombination energy. We observe that the mass-loss rate during the outburst is of the same order of magnitude as has been proposed for the outbursts of Eta Carinae. The research results in this paper are described in further detail in Lobel et al. (2003). A spectral movie sequence of the outburst is available at http://cfa-www.harvard.edu/~alobel/ Title: The Millenium Outburst of the Yellow Hypergiant ρ Cassiopeiae Authors: Lobel, A.; Dupree, A. K.; Stefanik, R. P.; Torres, G.; Israelian, G.; Morrison, N.; Ilyin, I.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 2002AAS...201.4909L Altcode: 2002BAAS...34.1185L We report the largest mass-loss rate of ~5 % of the solar mass per year, directly observed in any stellar object so far during the recent outburst of the Yellow Hypergiant ρ Cas. In the fall of 2000, the enigmatic cool luminous supergiant dimmed by more than a full visual magnitude, thereby changing its spectral type form early F- to early M-type. Our spectral monitoring reveals that the effective temperature decreased by more than 3000 K during the event, from above 7000 K to below 4000 K in less than ~200 days. It is the third outburst of ρ Cas on record in the last century. The optical spectrum becomes comparable to that of the red supergiant Betelgeuse, and reveals strongly blue-shifted molecular absorption bands of titanium-oxide (TiO). We determine from the newly formed TiO bands a gas mass-loss rate of the same order of magnitude as has been proposed for the giant outbursts of the Luminous Blue Variable η Carinae. The outburst of ρ Cas produces an outward propagating circumstellar shock wave, driven by recombination of hydrogen gas in its wake, resulting in a tremendous cooling of the entire outer atmosphere. Over the past 18 months since the outburst we observe a very prominent inverse P Cygni profile in Balmer Hα . Strong emission in this line has not before been observed in ρ Cas over this long period of time. The optical spectroscopic monitoring signals an unusal strong collapse of the upper Hα atmosphere, which we also observed in the months before the outburst event. The recent observations may therefore signal that a new and stronger outburst of ρ Cas is imminent. This reseach has been supported in part by a Space Telescope Science Institute grant to the Smithsonian Astrophysical Observatory. Title: Early solar space research Authors: de Jager, C. Bibcode: 2002css1.book..203D Altcode: No abstract at ADS Title: Spectroscopy of the Rho Cas Millennium Outburst Authors: Lobel, A.; Dupree, A. K.; Stefanik, R. P.; Torres, G.; Israelian, G.; de Jager, C.; Nieuwenhuijzen, H.; Ilyin, I. Bibcode: 2001AAS...19915802L Altcode: 2002BAAS...34..567L An exceptional variability phase occurred in the peculiar F-type hypergiant ρ Cas (Ia+) when the V brightness dimmed by about a full magnitude between June and September 2000. The star recovered from this deep minimum by April 2001. It is the third outburst of ρ Cas on record in the last century. We detect the formation of TiO bands in high-resolution spectra obtained from our long-term monitoring campaigns over the past decade with the Utrecht Echelle Spectrograph on the William Herschel Telescope, and the Sofin spectrograph of the Nordic Optical Telescope (La Palma). Optical and near-IR TiO bandheads, i.e. from the γ -system 0-0 transition at 7050 Å, develop in the summer of 2000. TiO formed before the deep V minimum occurred, and a first analysis shows that during the descending portion of the light curve, the spectrum is a peculiar mixture of a F-type supergiant, with TiO bands that characterize the spectra of M-type stars. This phenomenon has also been observed during the outbursts of 1945-47 and 1985-86. A preliminary analysis of these exceptional spectra is presented, by comparing them with high-resolution optical spectra of the early M-type supergiants μ Cep (Ia) and Betelgeuse (Iab). Central emission is observed above the local continuum level in the split Na D lines. An analysis of ρ Cas' optical emission line spectrum is provided in Lobel 1997, Pulsation and Atmospherical Instability of Luminous F- and G-type Stars, Maastricht: Shaker. We propose the formation of a low-temperature, optically thick, mass shell, caused by instability of the upper atmosphere of this pulsating massive supergiant near the Eddington luminosity limit (Lobel 2001, ApJ, 558, 780). A review is at http://xxx.lanl.gov/format/astro-ph/0108358 This research is supported in part by STScI grant GO-08280.02-97A to the SAO. Title: Instability regions in the upper HR diagram Authors: de Jager, Cornelis; Lobel, Alex; Nieuwenhuijzen, Hans; Stothers, Richard Bibcode: 2001MNRAS.327..452D Altcode: The following instability regions for blueward evolving-supergiants are outlined and compared. (1) Areas in the Hertzsprung-Russell (HR) diagram where stars are dynamically unstable. (2) Areas where the effective acceleration in the upper part of the photospheres is negative, hence directed outward. (3) Areas where the sonic points of the stellar winds (where vwind=vsound) are situated inside the photospheres, at a level deeper than τRoss=0.01. We compare the results with the positions of actual stars in the HR diagram and we find evidence that the recent strong contraction of the yellow hypergiant HR 8752 was initiated in a period during which <geff><0, whereupon the star became dynamically unstable. The instability and extreme shells around IRC+10420 are suggested to be related to three factors: <geff><0 the sonic point is situated inside the photosphere; and the star is dynamically unstable. Title: The photosphere of P Cygni Authors: de Jager, C. Bibcode: 2001ASPC..233..215D Altcode: 2001pcyg.conf..215D Investigations of P Cygni during the last few decades have yielded a fairly consistent set of data for the photospheric parameters. I use T eff = 18600; log(L/Lo) = 5.8; vturb = 20 km/s; log(Mdot)= -4.7; X = 0.35. The resulting photospheric model has negative values of geff in the higher parts of the photosphere (optical depths below 0.7). In addition, the sonic point is reached already at a Rosseland optical depth of 0.18. These properties explain the outward motions in the atmosphere and the observed microturbulent velocities. While the body of the star has a <Gamma1>-value of about 4/3, the average atmospheric <Gamma1> = 1.338 is just above this limit. Hence the star is marginally dynamically stable. The strong stellar wind is a consequence of the outward-directed radiation pressure. The luminosity variations of relative short periods (smaller than about 500 d) are pressure waves with wavelengths of tens of millions of kilometers. The brightness variations of longer periods, between 1500 and 3700 d, are ascribed to large-scale gravity waves (nonradial pulsations) with a quasi recurrence time of that order. For the evolution I suggest the sequence Blue supergiant - Red supergiant - Yellow hypergiant - S Dor star (LBV) - WR star or supernova. Title: Will HR8752 become a P Cygni type star? Authors: de Jager, C.; Lobel, A.; Israelian, G.; Nieuwenhuijzen, H. Bibcode: 2001ASPC..233..191D Altcode: 2001pcyg.conf..191D As long as the yellow hypergiant HR8752 has been observed spectroscopically it has shown erratic and significant fluctuations in its effective temperature. But an impressive and hitherto never observed rise in its temperature started around 1985. Since that time Teff has risen from 4600 K to 7900 K. Regular further observations are needed to see if and when this rise will stop, and what will happen thereafter. The instability is related to the fact that in its evolution the star has entered the Yellow Evolutionary Void, a region in the Hertzsprung-Russell diagram where blueward-evolving supergiants have unstable atmospheres and where the stars are also dynamically unstable. The rise in temperature started after a period of enhanced mass loss and at a time when the photospheric acceleration was directed outward. Recently (1998) the photospheric effective acceleration has again decreased to below zero. This star is the first in which dynamic instability can be studied observationally in great detail. We hypothesise that HR8752 will traverse the Void and thereafter, after a period of stability, will enter the "blue'' region of dynamic instability, and then may become a star like P Cygni. Title: Book Review: Plasma turbulence and energetic particles in astrophysics / Astronomical Observatory of the Jagiellonian University, 1999 Authors: De Jager, Cornelis Bibcode: 2000SSRv...92..613D Altcode: No abstract at ADS Title: De novae van deze eeuw. Authors: de Jager, C. Bibcode: 2000Zenit..27..154D Altcode: No abstract at ADS Title: J. Andersen (ed.), Transactions of the International Astronomical Union, Volume XXIIIB Authors: de Jager, Cornelis Bibcode: 2000SSRv...91..718D Altcode: No abstract at ADS Title: Checking the yellow evolutionary void. Three evolutionary critical Hypergiants: HD 33579, HR 8752 & IRC +10420 Authors: Nieuwenhuijzen, H.; de Jager, C. Bibcode: 2000A&A...353..163N Altcode: We have checked the reality of the yellow evolutionary void (which is an area in the Hertzsprung-Russell diagram where atmospheres of blueward evolving super- and hypergiants are moderately unstable), by comparing one star inside the void: HD 33579 (= R76), and two at the low-temperature border of it: HR 8752 (= HD 217476, V509 Cas) and IRC+10420. We found that the first star has a large mass and a fairly stable behaviour over time. These aspects suggest, together with abundance determinations by others, that it is a fairly young, still redward-evolving supergiant. For such a star the void is not forbidden. The two other stars, HR 8752 resp. IRC+10420, have low masses which places them in the post-red blueward loop. They show indications of the expected bouncing effect for blueward returning red supergiants: when approaching the void they eject mass, resulting in a sudden reduction of T_eff and a decrease of the atmospheric acceleration to g_eff =~ zero. Thereafter T_eff and g_eff increase again. For HR 8752 two such recent `bounces' have been identified. The photometric variations of HR 8752 and of HD 33579 are due to high-l gravity-wave pulsations. Based on observations at the La Palma Observatory and the ESO Observatory in Chili. Title: Dzidek Niemirowicz 1928 - 2000 Authors: de Jager, Cornelis Bibcode: 2000CIBu..147....2D Altcode: No abstract at ADS Title: De versnelde uitdijing van het heelal. Authors: de Jager, C. Bibcode: 1999Zenit..26..502D Altcode: No abstract at ADS Title: Periodieke inslagen op aarde? Authors: de Jager, C. Bibcode: 1999Zenit..26..427D Altcode: No abstract at ADS Title: De laatste "zonsverduistering". Authors: de Jager, C. Bibcode: 1999Zenit..26..340D Altcode: No abstract at ADS Title: M. Livio, S. M. Fall and P. Madau (eds.), The Hubble Deep Field, Space Telescope Conference Series 11 Authors: de Jager, Cornelis Bibcode: 1999SSRv...89..693D Altcode: The proceedings of a conference dealing with only one piece of observational activity: the deepest optical image obtained by the HST during a 150 orbit observational program. The main result, presented in several papers, is that we are now able to witness the period of galaxy formation. Very readable too is Allan Sandage's historical review, being the introductory paper to the meeting. Title: Wat de kleintjes ons leren. Restanten van het ontstaan van het zonnenstelsels. Authors: de Jager, C. Bibcode: 1999Zenit..26..214D Altcode: No abstract at ADS Title: C. C. Petersen and J. C. Brandt, Hubble Vision, Early Adventures with the Hubble Space Telescope Authors: de Jager, Cornelis Bibcode: 1999SSRv...88..608D Altcode: No abstract at ADS Title: Stuart Ross Taylor, Destiny or Chance; our Solar System and its Place in the Cosmos Authors: de Jager, Cornelis Bibcode: 1999SSRv...88..609D Altcode: No abstract at ADS Title: Erratum: "A generalized version of the Rankine-Hugoniot relations including ionization, dissociation, radiation and related phenomena" [Astron. Astrophys., Vol. 280, No. 1, p. 195 - 200 (Dec 1993)]. Authors: Nieuwenhuijzen, H.; de Jager, C.; Cuntz, M.; Lobel, A.; Achmad, L. Bibcode: 1999A&A...343..661N Altcode: No abstract at ADS Title: Waarom kwamen de Leoniden te vroeg? Authors: de Jager, C.; van Woerden, H. Bibcode: 1999Zenit..26..110D Altcode: No abstract at ADS Title: De onstuimige jeugd van het heelal. Authors: de Jager, C. Bibcode: 1999Zenit..26...52D Altcode: No abstract at ADS Title: Onzichtbare massa is wellicht heel "gewoon". Authors: de Jager, C. Bibcode: 1999Zenit..26...13D Altcode: No abstract at ADS Title: Flare dynamics. Authors: Antonucci, E.; Alexander, D.; Culhane, J. L.; de Jager, C.; MacNeice, P.; Somov, B. V.; Zarro, D. M. Bibcode: 1999mfs..conf..331A Altcode: The following topics were dealt with: results from soft X-ray spectra, chromospheric evaporation, nature of nonthermal line broadening, flare modelling. Title: Announcement Space Science Reviews Provides an Opportunity to Debate: Challenges to Long-Standing Unsolved Space Physics Problems in the 20th Century Authors: de Jager, Cornelis; Akasofu, Syun Bibcode: 1999SSRv...87..551D Altcode: No abstract at ADS Title: G. Haskell and M. Rycroft (eds.), New Space Markets, Proceedings of the International Symposium held in Strasbourg, 26-28 May, 1997 Authors: de Jager, Cornelis Bibcode: 1999SSRv...87R.546D Altcode: No abstract at ADS Title: Drie vragen over de corona. Authors: de Jager, C. Bibcode: 1998Zenit..25..310D Altcode: No abstract at ADS Title: Leon Golub and Jay M. Pasachoff, The Solar Corona Authors: de Jager, Cornelis Bibcode: 1998SSRv...85..549D Altcode: No abstract at ADS Title: De veranderlijke zon en het klimaat. Authors: de Jager, C. Bibcode: 1998Zenit..25..148D Altcode: No abstract at ADS Title: Een loslopende neutronenster. Authors: de Jager, C. Bibcode: 1998Zenit..25..117D Altcode: No abstract at ADS Title: Toch een planetoide in Chicxulub? Authors: de Jager, C. Bibcode: 1998Zenit..25...82D Altcode: No abstract at ADS Title: The spectral variability of the cool hypergiant rho Cassiopeiae Authors: Lobel, A.; Israelian, G.; de Jager, C.; Musaev, F.; Parker, J. Wm.; Mavrogiorgou, A. Bibcode: 1998A&A...330..659L Altcode: We checked the change of the effective temperature with the atmospherical pulsation of rho Cas from combined analyses of optical spectra of 1993-95 and IUE spectra of 1979-81. We find an upper range for bigtriangleup T_eff =~ 750 K over a period of 16-17 months. We present a thorough study of the related absorption line profile changes from which a velocity stratification with excitation energy could not be detected for the metallic lines. The distinct evolution of Hα displays variable distortions of filling-in by emission with the pulsation. Our theoretical line profile fits yield a value for vsini of 25 kms(-1) . We report the finding of two different causes for the splitting of absorption line cores in the spectra of rho Cas. For the phase near highest effective temperature we have analyzed the development of very far shortward extended line wings assuming velocities up to 150 kms(-1) . These violet absorptions originate about 2.5 R_{*} above the photosphere in a cool and variable supersonic wind from which we determine an upper value of the mass-loss rate of 9.2 10(-5) M_⊙y(-1) . We also report the finding of an increase of the supersonic wind velocity by ~ 15 kms(-1) per electronvolt decrease, measured from the maximum velocities of these violet absorptions for Fei lines. A separate analysis of the IUE spectra reveals tremendous changes of the Balmer continuum flux by a factor 4 to 5 in only 26 hours, clearly distinct from the long-term variations of Hα . Partly based on observations taken at the La Palma Observatory, the Special Astrophysical Observatory and with the International Ultraviolet Explorer Title: The yellow hypergiants Authors: de Jager, Cornelis Bibcode: 1998A&ARv...8..145D Altcode: We list the main stellar data of known hypergiants and similarly luminous stars, and then concentrate on a review of the yellow hypergiants. These stars are post-red supergiants evolving along blueward loops in the Hertzsprung-Russell diagram. Their properties, their location in the Hertzsprung-Russell diagram and their occasional mass ejections are related to a region of atmospheric instability in the H-R diagram, the Yellow Void. The `bouncing against the border of the Void' of three objects: ρ Cas, HR8752 and IRC+10420, is described. The apparent atmospheric instability of yellow hypergiants is related to the atmospheric pulsations. There are indications that the approach to the Void is associated with an increased amplitude of the pulsation and with enhanced mass loss. The observed small-scale motion field is only apparently strongly supersonic; the observed large stochastic velocities are the quasi-stochastically varying thermal motions in the many hot sheets that occur in the wakes of many small shocks, while the real hydrodynamic velocity component is small and subsonic. This shock-wave field is also responsible for the observed rate of mass loss and for emission in the wings of Hα . Most yellow hypergiants have envelopes containing gas and dust, but a thick extended envelope, presumably dissipating and showing bipolar outflow, is only known around IRC+10420. At the interface of the bipolar wind and the interstellar medium one or more stationary shocks may develop as is observed in the case of IRC+10420 and suspected with ρ Cas. Title: I knew Edith for nearly half a century Authors: de Jager, Cornelis Bibcode: 1998ASSL..222...21D Altcode: 1998ream.conf...21D No abstract at ADS Title: A Cold War in Space Research Authors: de Jager, Cornelis Bibcode: 1998fyc..conf...25D Altcode: No abstract at ADS Title: The Variable Spectrum of the Yellow Hypergiant rho Cassiopeiae Authors: Israelian, G.; Lobel, A.; de Jager, C.; Musaev, F. Bibcode: 1998ASPC..154.1601I Altcode: 1998csss...10.1601I We have analyzed high-resolution optical spectra of the cool hypergiant rho Cas covering a period 2 years. The aim of this analysis is to check the range of the effective temperature variations with the pulsation of the atmosphere, in order to study variable emission components in Hα, to investigate the splitting of metallic absorption lines and to compute abundances of Na, Fe and other elements. We found an upper range of Delta T_eff ~= 750 K over a period 17 months, whereas the effective temperature change within a single pulsation period remained limited to 400 K. We discuss the notorious splitting of low excitation metallic absorption lines observed for rho Cas. The allowed emission reversals in the cores of these low energy lines emerge from cool and static shells in a bipolar stellar wind. Furthermore, variable absorption in the supersonic stellar wind recurrently produces far violet extended line wings with the atmospherical pulsations, from which we have derived the mass-loss rate and wind extension above the photosphere. The emission components of Hα suggest the presence of a thermally excited outer atmospherical region (a variable quasi-chromosphere). Several observed parameters (like the mass-loss rate) can be derived theoretically if we assume that the observed ``microturbulent'' line broadening is not caused by stochastic small-scale turbulent motions (the classical notion of microturbulence) but by thermal motions in stochastically distributed high-temperature sheets behind the many shocks. Title: Bruine dwergen: de kleinste "sterren" in het heelal. Authors: de Jager, C. Bibcode: 1997Zenit..24..404D Altcode: No abstract at ADS Title: Turbulence, mass loss and Hα emission by stochastic shocks in the hypergiant ρ Cassiopeiae. Authors: de Jager, C.; Lobel, A.; Israelian, G. Bibcode: 1997A&A...325..714D Altcode: The hypergiant ρ Cas is known for its variable rate of mass loss, with an average value of about 10^-5^Msun_/yr, and the supersonic value for the line-of-sight component of the microturbulent velocity, about 11km/s. Emission components in Hα suggest the presence of a thermally excited outer atmospheric region. Since hydrodynamical turbulence in a stellar atmosphere turns rapidly into a field of shock waves, and shock waves are known to be able to initiate a stellar wind and heat stellar atmospheric layers, we have tried to predict the rate of mass loss, the microturbulent velocity component and the observed Hα profile by assuming a stochastic field of shock waves. To that end we adopted a Kolmogoroffian spectrum of shock waves, characterized by only one parameter: the maximum Mach number in front of the shocks: M_1,max_. Behind every shock a thin hot region originates. Spectroscopically, the thermal motions in these sheetlike regions cannot be distinguished from the stochastic hydrodynamic (shock wave) motion component, and therefore these hot regions add to the line broadening and will also contribute to the observed 'microturbulence'. We find that it is indeed possible to explain the observed rate of mass loss (we derived log˙(M)=~-5(Msun_/yr)), as well as the high value for the quasi-microturbulence (we calculated =~12km/s). The hot sheets behind the shocks appear to be responsible for the observed 'microturbulence'; this thermal contribution is much larger than that of the hydrodynamic (shock) motions, which is only 0.4 to 0.5km/s. Non-LTE calculations of the Hα line profile show that the shocks, in association with the observed time-dependent variation of T_eff_ can reproduce aspects of the variable emission in Hα. These three aspects of this star, viz. the observed rate of mass loss, the observed supersonic 'microturbulence', as well as the Hα line profile can be simulated by one parameter only: viz. M_1,max_=1.06 to 1.08, a value that characterizes a fairly weak shock-wave field. Title: An obstacle to the late evolution of massive stars Authors: de Jager, Cornelis; Nieuwenhuijzen, Hans Bibcode: 1997MNRAS.290L..50D Altcode: During their evolution, massive stars first swell and thus become red supergiants, after which they shrink again to evolve to the blue supergiant phase. When, during this blueward loop, the effective photospheric temperature T_eff has increased to values of about 7000 K, the main part of the photosphere becomes moderately unstable. Observations of the yellow hypergiant star HD 217476=HR 8752 show that the approach to this instability region may lead to phases of enhanced mass loss and consequent reduced surface temperature, after which T_eff slowly increases until it again `bounces' against the atmospheric instability region. Two such bounces have been observed in the last 30 yr and a third seems imminent; frequent observations of this star are therefore useful. Other `yellow hypergiants', like rhoCas and IRC+10420, seem to show the same behaviour. Repeated observations of these stars may help us to understand how massive stars pass the instability region during their evolution towards the blue supergiant phase. Title: Tien jaar supernova 1987A. Authors: de Jager, C. Bibcode: 1997Zenit..24..335D Altcode: No abstract at ADS Title: 3-D MHD Simulation of X-Type Coalescence of Two Current-Loops Authors: Sakai, Jun-ichi; de Jager, Cornelis Bibcode: 1997SoPh..173..347S Altcode: We present simulation results of X-type coalescence of two current-loops, by using a 3-D resistive MHD code. The results are compared with the limb flare observed by Yohkoh on 2 August, 1993 at 08:30 UT, which is a good example of partial X-type coalescence. It is shown that the maximum temperature enhancement near the cross-point of the two loops, obtained from the simulation, agrees well with the observations when the plasma β is 0.08. Title: K. Madders, A New Force at a New Frontier, Europe's Development in the Space Field of Its Main Actors, Policies, Law and Activities from its Beginnings up to the Present Authors: de Jager, C. Bibcode: 1997SSRv...81..199D Altcode: No abstract at ADS Title: Presolaire insluitsels in meteorieten. Authors: van Dijk, T.; de Jager, C. Bibcode: 1997Zenit..24..290V Altcode: No abstract at ADS Title: A White and a Red Star|(Special Historical Review) Authors: De Jager, Cornelis Bibcode: 1996SoPh..169..443D Altcode: No abstract at ADS Title: Book reviews Authors: Shore, Steven N.; van der Kruit, P. C.; Achterberg, A.; Hoyng, P.; de Jager, Cornelis Bibcode: 1996SSRv...78..557S Altcode: No abstract at ADS Title: Book reviews Authors: Marx, G.; Jaffe, W.; Maltby, P.; Grevesse, N.; Wu, S. T.; Lüst, R.; Kleczek, J.; Chupp, E. L.; Bloemen, Hans; de Jager, Cornelis; van Teeseling, A.; Shea, M. A.; Butcher, H.; van der Kruit, P. C.; Nieuwenhuijzen, H.; Steeghs, Danny; Fälthammar, Carl-Günne; Greenberg, J. M.; Rasool, S. I. Bibcode: 1996SSRv...77..369M Altcode: No abstract at ADS Title: Solar Flares and Collisions Between Current-Carrying Loops Types and Mechanisms of Solar Flares and Coronal Loop Heating Authors: Sakai, Jun-Ichi; de Jager, Cornelis Bibcode: 1996SSRv...77....1S Altcode: This paper deals with the temporal dynamics of solar flares. It gives a systematic description of solar flare models and tries to link the observations to results of simulations. After a review of the development of ideas on flare structure and on theories on current-loop interaction in flares since the pioneering work by Gold and Hoyle (1960), this paper gives first a synthesis of present-days observationally based views on solar flares, essentially describing the developments since the review by de Jager (1986). We distinguish between confined/impulsive and eruptive/dynamic flares (briefly: confined and eruptive). The main difference between these two types is one of field-line topology: ‘closed’ or ‘open’. The ‘grand instability’ in a field-line system opening to space is basic to the relation between eruptive flares, filament instability, and Coronal Mass Ejections. A fair part of the paper deals with the developments in our understanding of the physical processes during collisions between current-carrying loops. After work by Tajima et al. (1982), who introduced the concept of current-loop coalescence in solar flares, using results obtained from two-dimensional particle simulation, it became clear that the current-loop interaction process includes a rich variety of physical mechanisms associated with rapid magnetic energy conversion through partial or complete magnetic reconnection, such as prompt high-energy particle acceleration, plasma heating, shock formation, plasma-jet formation and plasma radiation. This part of the paper concentrates on the developments since the review by Sakai and Ohsawa (1987), dealing with particle acceleration by magnetic reconnection and shocks during current-loop coalescence in solar flares. Theoretical research performed since the above review paper refers to magnetic reconnection, shock formation, particle acceleration and plasma emission during collisions between current-carrying loops. These theoretical developments are compared with observations. The Yohkoh spacecraft, launched August 30, 1991, observed many evidences of two-loops interaction in active regions and a growing number of examples of flares due to coalescence of loops. These observational evidences are reviewed in a systematic way (Section 6), by relating them to the theoretical studies described before. First we describe flares due to interactions of the Y-, X-, and I-types. These can be either confined or eruptive. A new observation is the occurrence of ‘ongoing partial reconnection’ in a flare. Thereafter, we discuss flares due to single-loop instability (these are confined flares), followed by the newly discovered ‘cusp-type’ flares. These are eruptive flares. We end with a description of a typical eruptive flare. In Section 7 we summarize observational and theoretical evidences for our earlier proposed scheme of three different phases of acceleration in flares. The summary in the last section compares flare observations with flare simulations; it deals briefly with the three phases of acceleration, and outlines the present status and prospects of simulation studies and related theoretical research. Title: Book reviews Authors: Humphreys, R. M.; Kemp, S.; Savonije, G.; van der Hucht, K. A.; van der Kruit, P. C.; Miley, G.; Bumba, V.; van Nieuwkoop, J.; van Hoolst, T.; Cox, A.; Rutten, R. J.; Kleczek, J.; de Jager, Cornelis; Jerzykiewicz, M.; Zwaan, C.; Poedts, S.; Sakai, Jun-Ichi; Pecker, J. -C.; Heikkila, W.; de Jong, T.; Wilson, P. R.; Müller, E. A.; Hoyng, P.; Icke, V.; Shore, S. N.; Achterberg, A.; Lucchin, F.; Butcher, H.; Ne'Eman, Y.; Heidmann, J.; Belton, M. J. S.; de Graauw, Th.; Waters, L. B. F. M.; Pacini, F.; Hultqvist, B.; Akasofu, S. -I.; Vial, J. -C.; Schatzman, E.; van der Laan, H.; Cole, K. D.; Vanbeveren, D.; Southwood, D.; van der Klis, M.; Katgert, Peter Bibcode: 1996SSRv...76..339H Altcode: No abstract at ADS Title: Near-unstable stars: the yellow-white hypergiants. Authors: de Jager, C. Bibcode: 1996PINSA..62..461D Altcode: The yellow-white hypergiants are stars of extreme luminosities. Atmospheric abundances and mass determinations suggest that the stars are evolved objects, moving blueward in their evolution. Three well-studied objects are described (Rho Cas, HR 8752, and HD 33579). Methods of spectroscopic diagnosis allows for a determination of the various accelerations acting on the photospheric gas. In the Hertzsprung-Russell diagram the yellow hypergiants are situated at the low-temperature side of a region of atmospheric instability: the Yellow Evolutionary Void. This suggests a relation: since these stars are evolving in the HR diagram from the red supergiant region towards the blue, it is natural that the atmospheres become unstable, with a consequent increased rate of mass loss, when the star is nearing the Void. Title: Atmospheric accelerations and the stability of dynamic supergiant atmospheres. Authors: Nieuwenhuijzen, H.; de Jager, C. Bibcode: 1995A&A...302..811N Altcode: The goal of this paper is to study instability regions in the HR diagram, through a calculation of the atmospheric accelerations for spherically symmetric stars, in dynamic equilibrium, without using detailed atmospheric models. The input data are five primary data, viz.: the stellar luminosity L, the effective temperature T_eff_, the mass M, the rate of mass loss ˙(M), and the microturbulent velocity component ζmu_, while we assume the temperature for a reference atmospheric layer, an assumption that appears not to be critical. An iterative solution of the momentum equation, simultaneous with some other equations, yields values for the various accelerations acting on a stellar atmosphere and their algebraic sum g_eff_', the predicted effective acceleration. In the first part of the paper we compare this latter quantity with the g_eff_-value derived observationally from spectral studies of nine program stars and we find overall fair agreement. This supports the method as well as the values of the five input data. In part 2 we determine g'_eff_ in same way for the whole upper part of the Hertzsprung-Russell diagram by using statistical primary data on the mass (based on evolutionary calculations), on mass-loss and on microturbulence (shock-strengths). We find as a fairly general rule that, as stars move along their evolutionary track, and for time scales longer than the dynamic time scale of the atmosphere, the atmosphere continuously adapts to the new (L,T_eff_)-values and essentially remains stable. Current practice of determining the stability limit of stellar atmospheres by extrapolating hydrostatic models to the Eddington limit is not justified by this study. There is one exception: we find a small area around T_eff_=8300K and log(L/Lsun_)=5.7, where no solution is possible for evolved stars on their blueward evolutionary track; the stars in this area have in any case effective accelerations <1mm/s^2^: the "Yellow Evolutionary Void". In the third part we estimate approximately the pulsational (in-)stability of stars in the upper part of the Hertzsprung-Russell diagram, by comparing the g'_eff_ values determined in part 2 with the average outward pulsational acceleration. We thus confirm the 'Yellow Evolutionary Void' for blueward evolving stars, and also find an instability region for blueward evolving stars in the area occupied by the Wolf-Rayet stars. This seems to agree with the observations that the low-temperature boundary of the Yellow Evolutionary Void appears to coincide with the region where the yellow hypergiants are clustering. The yellow hypergiants are therefore interpreted as blueward moving stars with ZAMS masses of about 25-40Msun_, and actual masses between 15 and 25Msun_. For our galaxy it is found that only a few stars are situated within the 'Yellow Evolutionary Void', in accordance with our expectation. Title: Book-Review - Isolated Pulsars Authors: Vauclair, G.; Achterberg, A.; Narlikar, Jayant; Lub, J.; van der Laan, H.; Sakai, J. I.; Schrijver, C. J.; de Jager, Cornelis; Léna, P.; Vanbeveren, D.; Audouze, J.; van den Heuvel, E. P. J. Bibcode: 1995SSRv...73..435V Altcode: No abstract at ADS Title: Book Review: A view of the universe / Cambridge U Press, 1993 Authors: de Jager, C. Bibcode: 1995SSRv...73..440D Altcode: 1995SSRv...73..440M No abstract at ADS Title: Ongoing Partial Reconnection in a Limb Flare Authors: de Jager, Cornelis; Inda-Koide, Mika; Koide, Shinji; Sakai, Jun-Ichi Bibcode: 1995SoPh..158..391D Altcode: The limb flare on August 2, 1993 08:30 UT, consisting of two crossed loops, shows evidence of partial X-type reconnection. The observations suggest that partial reconnection was an ongoing process during about 10 min until flare ignition. Title: Book reviews Authors: Mewe, R.; de Jager, Cornelis; Kleczek, J.; Icke, Vincent; van Bueren, H. G.; Jones, C.; Schatzman, E.; Meynet, G.; van der Kruit, P. C.; van der Hucht, K. A.; Raadu, M. A.; Sterken, C. Bibcode: 1995SSRv...72..679M Altcode: No abstract at ADS Title: Determining Stellar Masses from a Study of Atmospheric Accelerations Authors: Nieuwenhuijzen, H.; de Jager, C. Bibcode: 1995LIACo..32..239N Altcode: 1995sews.book..239N No abstract at ADS Title: Atmospheric Instability Regions in the HR-Diagram and the Yellow Evolutionary Void Authors: de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1995LIACo..32..459D Altcode: 1995sews.book..459D No abstract at ADS Title: Pulsation of the yellow hypergiant ρ Cassiopeiae in 1970. Authors: Lobel, A.; de Jager, C.; Nieuwenhuijzen, H.; Smolinski, J.; Gesicki, K. Bibcode: 1994A&A...291..226L Altcode: We checked the hypothesis that the yellow hypergiant ρ Cas is pulsating radially, by the application of an improved Baade-Wesselink test, in which the effective temperature during an interval of 178 days in 1970 is determined from an analysis of five stellar spectra. We assume that the changing atmospherical circumstances can be represented by various static Kurucz models over the pulsation period. The changing effective temperature, combined with the radial velocity and luminosity data, yields that radial pulsations are only possible if the star would have a radius of 2000+/-200Rsun_, while the best value is rather 400Rsun_. The conclusion is supported by the study of allowed wave motions, which also excludes radial pulsations. An investigation of the evolution of splitted absorption profiles during this pulsation cycle combined with the radial velocity curve reveals that line doubling in ρ Cas is produced by a narrow emission line superimposed on a single broad absorption line instead of being produced by radially propagating shock waves. We conclude that ρ Cas was pulsating non-radially in 1970. Byproducts of the investigation are determinations of the variation of the microturbulent velocity (ranging between 11.1 and 13.5km/s) and of the effective acceleration of gravity. Title: Book-Review - I.A.U. TRANS.21B - 21ST General Assembly Authors: Bergeron, J.; de Jager, C. Bibcode: 1994SSRv...69..209B Altcode: No abstract at ADS Title: Book Review: To a rocky moon: a geologist's history of lunar exploration / U Arizona Press, 1993 Authors: Williams, D. E.; de Jager, C. Bibcode: 1994SSRv...69..213W Altcode: No abstract at ADS Title: The shock strength in super- and hypergiant atmospheres derived from microturbulence data Authors: Nieuwenhuijzen, H.; de Jager, C.; Cuntz, M. Bibcode: 1994A&A...285..595N Altcode: This study is based on evidence that small-scale motions, observed as microturbulent line broadening, in the atmospheres of super- and hypergiants can possibly be interpreted as fields of shock waves moving outwards through the atmosphere. We derive values for the average shock strength M_1_ for a number of well-observed stars. We found that the value of M_1_ increases strongly with stellar luminosity, suggesting that shock wave pressure is relevant for the stability limit of most evolved stars (Humphreys-Davidson Limit) and for stellar evolution calculations. Our results however apply only to those stars in which photospheric shock wave patterns exist. For some stars we studied the dependence of M_1'_ on the Rosseland optical depths τ_R_ and we found that M_1'_(τ_R_) does not change very much. We compared the energy fluxes associated with the shocks with acoustic energy fluxes given by Bohn and with radiative energy losses from the chromospheres (only possible for cool giants and supergiants). We found that these fluxes are compatible for cool stars. The energy fluxes of Bohn cannot be used for super- and hypergiants with T_eff_ > 6500 K as all these stars are lacking convection zones. Title: Book reviews Authors: Burlaga, L. F.; Kleczek, J.; Schatzman, E.; Adams, D. J.; Rutten, R. J.; van der Kruit, P. C.; de Jager, Cornelis; Trams, N. R.; Righini, Alberto; Ergma, E.; Grün, Eberhard; Icke, Vincent Bibcode: 1994SSRv...67..223B Altcode: No abstract at ADS Title: Massive stars: setting the stage. Authors: de Jager, C. Bibcode: 1994ems..conf....7D Altcode: The paper gives a summary of the situation mid-1993 of theory and observations regarding massive stars. The author describes: stellar mass loss and its implications, pre-main-sequence evolution, the main sequence, problems of atmospheric instability, Luminous Blue Supergiants, Yellow Hypergiants, Wolf-Rayet stars and supernovae. Title: Current-carrying loops and their interaction in solar flares. Workshop summary. Authors: de Jager, C. Bibcode: 1994clis.conf....5D Altcode: The contributions to the Workshop are placed in the context of present-days views on the origin, structure and development of flares, by discussing: flare classification; the study of gradual flares; the Impulsive Phase Explosion; observations of loop interactions and flare trigger; physics of single loops and loop reconnnection; the three phases of acceleration in flares. At some places one goes beyond the contents of the Workshop presentation in order to place the research in the broader context of present-days views on flares. Title: Multiple X-type reconnection in a small solar limb flare observed by YOHKOH. Authors: de Jager, C.; Inda-Koide, M.; Koide, S.; Sakai, J. -I. Bibcode: 1994clis.conf..149D Altcode: The limb flare on August 2, 1993 08:30 UT appears to be a schoolclass example of X-type reconnection. Its study shows that reconnection is a multiple process that gradually leads to flaring. The authors compared observations with predictions from X-type reconnection theory, while also including a comparison of the radiated energy with the magnetic energy content of the azimuthal field component. Title: Book Review: Solar system evolution: a new perspective / Cambridge U Press, 1992 Authors: de Jager, C. Bibcode: 1994SSRv...67..228D Altcode: 1994SSRv...67..228T No abstract at ADS Title: A generalized version of the Rankine-Hugoniot relations including ionization, dissociation and related phenomena Authors: Nieuwenhuijzen, H.; de Jager, C.; Cuntz, M.; Lobel, A.; Achmad, L. Bibcode: 1993A&A...280..195N Altcode: For purposes of computing shocks in stellars atmospheres and winds we have developed a generalized version of the Rankine-Hugoniot relations including ionization, dissociation, radiation and related phenomena such as excitation, rotation and vibration of molecules. The new equations are given in analytical form. They are valid as long as the internal energy E, the total pressure P, and the first adiabatic coefficient gamma1 can be evaluated. However, we have not treated shock structures. In the case of non-LTE we have to employ an approximation for gamma1 because in that case no definition exists. Our new version of the Rankine-Hugoniot relations can easily be used for many purposes including ab-initio modeling. In our derivation we introduce a parameter gammaH, which is definded as the ratio of the enthalpy H (sometimes called heat function w) to the internal energy E (sometimes called U). Using this parameter we solve the equations for changing mu and (d(natural log P)/d(natural log rho))ad identically equal to gamma1 on both sides of the shock. Both gammaH and gamma1, and also mu are functions of pressure P and temperature T. We present: (1) the derivation, (2) examples of gamma1 (P,T) and gammaH (P,T) which include/exclude ionization and radiation, and (3) as an example the differences in post-shock parameters as function of the pre-shock temperature for the case with ionization and without radiation. Title: A statistical study of the distribution of stars in the log Teff - log gN plane. Authors: Achmad, L.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1993A&AS..100..465A Altcode: A statistical study is made of the distribution of stars in the log T(eff)-log gN plane based on the photometric data set of the Geneva system. To that end we made an analysis of the methods for deriving the atmospheric parameters T(eff) and log gN (the Newtonian acceleration of gravity) from different sets of photometric data, corrected for the influence of extinction. We found, using the Geneva photometric system, that the Newtonian gravity can only be determined with good accuracy for stars with effective temperatures below 10,000 K. We show the relations between luminosity class and gravity for stars with T(eff) values of 3.75 to 3.94 and compare these with the theoretically predicted radiative instability line of Gustafsson and Plez (1992). Title: Book-Review - Sky Catalogue 2000.0 - VOL.1 - Stars Magnitude 8.0 Authors: de Jager, C.; Hirshfeld, A.; Sinnott, R. W.; Ochsenbein, F. Bibcode: 1993SSRv...63..395D Altcode: No abstract at ADS Title: Book reviews Authors: Narlikar, J. V.; Hovenier, J. W.; Achterberg, A.; Barr, P.; de Jager, Cornelis; van der Hulst, J. M. Bibcode: 1993SSRv...63..391N Altcode: No abstract at ADS Title: Book reviews Authors: Murawski, K.; Grevesse, N.; Piteri, S.; Nieuwenhuyzen, H.; van der Hage, J. C. H.; Icke, Vincent; Hovenier, J. W.; Rutten, R. J.; De Greve, J. P.; Kaufmann, P.; Burki, G.; de Jager, Cornelis Bibcode: 1993SSRv...65..365M Altcode: No abstract at ADS Title: Massive stars: Setting the stage Authors: de Jager, C. Bibcode: 1993SSRv...66....7D Altcode: The paper gives a summary of the situation mid-1993 of theory and observations regarding massive stars. I describe: stellar mass loss and its implications, pre-main-sequence evolution, the main sequence, problems of atmospheric instability, Luminous Blue Supergiants, Yellow Hypergiants, Wolf-Rayet stars and supernovae. Title: Book reviews Authors: Rutten, R. J.; Mewe, R.; Houziaux, L.; Cheng, Chung-Chieh; van der Klis, M.; Sylwester, Janusz; Tajima, T.; Kresák, Ľ.; Minarik, S.; de Jager, Cornelis; van der Kruit, P. C. Bibcode: 1993SSRv...65..181R Altcode: No abstract at ADS Title: Gravity waves in the atmosphere of Betelgeuse? Authors: de Jager, C. Bibcode: 1993Obs...113...43D Altcode: No abstract at ADS Title: Book-Review - Reports on Astronomy Authors: McNally, D.; de Jager, C. Bibcode: 1993SSRv...62R.211M Altcode: No abstract at ADS Title: Book Review: Men and women of space / Univelt, 1992 Authors: de Jager, C. Bibcode: 1993SSRv...65..372D Altcode: 1993SSRv...65..372H No abstract at ADS Title: Editorial Authors: de Jager, Cornelis; Howard, Robert F.; švestka, Zdeněk Bibcode: 1993SoPh..143D...7D Altcode: No abstract at ADS Title: Book-Review - Mars the Red Planet Authors: Catermole, P.; de Jager, C. Bibcode: 1993SSRv...65..190C Altcode: No abstract at ADS Title: Book Review: The hour of our delight: cosmic evolution, order, and complexity / Freeman, 1991 Authors: de Jager, C. Bibcode: 1993SSRv...62Q.210D Altcode: No abstract at ADS Title: Book Review: An introduction to astrophysical hydrodynamics / Academic Press, 1992 Authors: de Jager, C. Bibcode: 1993SSRv...65..189D Altcode: 1993SSRv...65..189S No abstract at ADS Title: Book Review: The cosmic water hole / MIT Press, 1991 Authors: de Jager, C. Bibcode: 1993SSRv...62R.210D Altcode: No abstract at ADS Title: On the instability parameters of stellar atmospheres. Authors: Lobel, A.; Achmad, L.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1992A&A...264..147L Altcode: We discuss the significance of the adiabatic exponents Gamma1, Gamma2, and Gamma3 and derive expressions for their calculation in a stellar atmosphere including simultaneous single-ionization of various elements and the presence of an equilibrium radiation field. A discussion is given of the relation of Gamma1 to dynamic instability. It is shown that some parts of some Kurucz models for extreme supergiant atmosphere are dynamically instable as a result of ionization and radiation in the deeper layers. Title: Book Review: Infrared and radio astronomy and astrometry / Pergamon, 1991 Authors: de Jager, C. Bibcode: 1992SSRv...61..428K Altcode: No abstract at ADS Title: Yellow hypergiant interferometry: a clue to understanding evolutionary instability. Authors: De Jager, Cornelis; Nieuwenhuijzen, Hans Bibcode: 1992ESASP.344..109D Altcode: 1992spai.rept..109D Yellow hypergiants are stars with nearly unstable atmospheres, characterized by strong low-mode non-radial pulsations and intense mass loss. In the Hertzsprung-Russell diagram they are located at the cool side of a region where atmospheres are unstable. The atmospheric pulsations should manifest themselves as hot and cool areas on the disk. The strong mass loss will lead to a rather dense equatorial decretion disk that should be observable interferometrically. Many other types of stars may have similar properties. Title: Science, para-science, pseudo-science. Authors: de Jager, C. Bibcode: 1992ZemVs...2...98D Altcode: No abstract at ADS Title: Book reviews Authors: Blanc, M.; van der Hucht, K. A.; Heintze, J. R. W.; Kaufmann, P.; Neéman, V.; Nieuwenhuijzen, H.; Stahl, O.; de Jager, Cornelis; van der Kruit, P. C. Bibcode: 1992SSRv...62..205B Altcode: No abstract at ADS Title: Atmospheric model and dynamical state of the atmosphere of the supergiant Eta Leonis (A0 Ib). Authors: Lobel, A.; Achmad, L.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1992A&A...256..159L Altcode: The authors redetermined the atmospheric model parameters of η Leo on the basis of a set of equivalent width data of 47 Fe I and 71 Fe II lines, determined previously by B. Wolf (1971). The procedure followed is an iterative one, involving the determination of the depth variation of microturbulence. The authors find: Teff = 10200±370K; log g = 1.9±0.4 cm s-1 and Δlog Z (= logarithmic abundances compared to the solar values) = 0.14±0.10 for Fe. A further result is that the line of sight microturbulence velocity component ζμ hardly varies with depth and equals 5.4±0.7 km s-1, hence the motion field, which consists of isothermal shock waves, obeys the "limiting shockstrength regime". The observationally determined shock strenght is Msobs = 2.3. Title: Book reviews Authors: Bhatnagar, Arvind; Achterberg, A.; de Jager, Cornelis; Kaufmann, P.; van Gent, R. H.; Rodríguez, L. F. Bibcode: 1992SSRv...59..407B Altcode: No abstract at ADS Title: Areas of atmospheric instability in the Hertzsprung-Russell diagram Authors: Nieuwenhuijzen, H.; de Jager, C. Bibcode: 1992iesh.conf..171N Altcode: No abstract at ADS Title: Supergiant shock strength in relation to stellar luminosity Authors: Nieuwenhuijzen, H.; de Jager, C. Bibcode: 1992iesh.conf..127N Altcode: No abstract at ADS Title: Non-Spherical Envelopes and Flows (Invited Paper) Authors: de Jager, C. Bibcode: 1992ASPC...22..400D Altcode: 1992nvos.work..400D No abstract at ADS Title: Introduction Authors: de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1992iesh.conf....9D Altcode: No abstract at ADS Title: Spectroscopic diagnostics of a field of shock waves Authors: de Jager, C. Bibcode: 1992iesh.conf..129D Altcode: No abstract at ADS Title: Dynamics and stability of supergiant atmospheres Authors: de Jager, C. Bibcode: 1992iesh.conf...98D Altcode: No abstract at ADS Title: Editorial Authors: de Jager, Cornelis; Švestka, Zdeněk; Howard, Robert F. Bibcode: 1992SoPh..137D...5D Altcode: No abstract at ADS Title: Instabilities in evolved super- and hypergiants Authors: de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1992iesh.conf.....D Altcode: No abstract at ADS Title: The large-scale atmospheric motion field of Alpha Orionis Authors: de Jager, C.; Eriksson, K. Bibcode: 1992iesh.conf..117D Altcode: No abstract at ADS Title: Book-Review - Rotation and Mixing in Stellar Interiors Authors: Goupil, M. J.; Zahn, J. P.; de Jager, C. Bibcode: 1992SSRv...59..410G Altcode: No abstract at ADS Title: Book Review: The numerical modelling of non-linear stellar pulsation. Problems and prospects / Kluwer, 1990 Authors: de Jager, C. Bibcode: 1991SSRv...58..383D Altcode: No abstract at ADS Title: The contribution function for the depth of formation of lines in a stellar atmosphere. Authors: Achmad, L.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1991A&A...250..445A Altcode: A function describing the contribution of the various layers to the formation of spectral lines in a stellar atmosphere is derived. The function is valid for absorption lines as well as for emission lines, and is based on the separation of the line and the continuum emergent radiation. In this respect, this function differs from the conventional contribution functions, which are based on the total radiation or on the line depression. It is shown that it does not appear possible to find a unique function using the concept of line depression, in contrast to the function developed in this paper. Title: Atmospheric model parameters and shock wave field for the supergiant alpha Carinae (FOIb). Authors: Achmad, L.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1991A&A...249..192A Altcode: The depth dependence of microturbulent velocity in the atmosphere of alpha Carinae is examined by means of observational data from the 694 FeI and FeII lines. The model atmospheric parameters are expanded to include effective temperature, gravity, microturbulence, and abundance because the depth dependence is sensitive to the model atmosphere. The depth formation of the spectral lines is reviewed, and a method of linearization is applied to the analyses of both spectral lines and atmospheric parameters. The average microturbulent velocity component is 2.8 +/- 0.2 km/s, and the differences between the turbulent velocities are small for Fe lines that are neutral or singly ionized. Microturbulent velocity does not vary significantly with depth which suggests that the atmosphere of alpha Carinae obeys the limiting shock-strength regime. Title: High-Energy Flare Explosions Driven by 3-DIMENSIONAL X-Type Current Loop Coalescence Authors: Sakai, Jun-Ichi; de Jager, Cornelis Bibcode: 1991SoPh..134..329S Altcode: We present a model for high-energy solar flare explosions driven by 3-dimensional X-type current loop coalescence. The 3-dimensional X-type current loop coalescence, where two crossed flux-tubes interact at one point, is a fundamentally new process as compared to the 1-D and 2-D cases studied earlier. This process is studied by a first-order approach of the relevant variables near the point of coalescence; it appears to yield reliable information in a sufficiently large area around this point. It is shown that, following a strong plasma collapse due to the pinch effect, a point-like plasma explosion can be driven while fast magnetosonic shock waves can also be excited. We found that the conditions in the area producing the remarkable flare bursts of 21 May, 1984 were indeed such that the many flare spikes could have been due to 3-D explosive X-type current loop coalescence. We also show, by studying the conditions of shock formation in a gamma ray flare, that the time delay of γ-rays from the impulsive phase could be the time needed for the shock formation in the flaring region. Title: The luminosity of the B2 hypergiant HD 80077. Authors: Carpay, J.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1991A&A...248..475C Altcode: The extremely bright and relatively stable hypergiant HD 80077 is an enigmatic object. Since the star is assumed to have a luminosity of 2 x 10 exp 6, considerable variability and a large mass loss rate would be expected. However, its mass loss rate of (5 +10/-3) x 10 exp -6 solar mass/yr and its variability are very low with respect to its luminosity. The star is assumed to be a member of the cluster Pismis 11, and therefore to have a known distance. The above-mentioned luminosity has been derived using the apparent visual magnitude, the bolometric correction and the distance to Pismis 11. Indications that the star is not a member of Pismis 11, but a foreground star, therefore having a lower luminosity have been looked for. For this, no evidence has been found. Title: Elementary Flare Bursts Explained by Explosive Coalescence Authors: de Jager, Cornelis; Sakai, Jun-Ichi Bibcode: 1991SoPh..133..395D Altcode: The observed burst durations of Elementary Flare Bursts (5-25 s), as well as the related other flare characteristics, such as the temperature (30-50 MK), the electron density (log ne = 10 to 11) and the magnetic field strengths (100 G to 200 G) can be explained quantitatively by the mechanism of explosive coalescence. Title: The atmospheric motion field in super- and hypergiants. Authors: de Jager, C.; de Koter, A.; Carpay, J.; Nieuwenhuijzen, H. Bibcode: 1991A&A...244..131D Altcode: The paper presents an investigation of the motion fields in the outer atmospheres of a few well-studied supergiants and hypergiants for which good observational information is available from spectral studies. The following topics are examined: the existence of pressure and gravity waves; the dissipation of gravity-wave energy by radiative damping; the reduction of the radiative damping of gravity waves by layer curvature; the large-scale motion of two moderate supergiants (Alpha Per and 41 Cyg); pressure waves and their transformation into shock waves; weak shock dissipation of wave energy; the spectrum of shock-wave energy; and the gravity wave and shock wave components of motion in supergiants and hypergiants. Title: Hoe ontstaan zonnevlammen? Authors: de Jager, C. Bibcode: 1991Zenit..18...48D Altcode: No abstract at ADS Title: Book Review: Lectures on spectral-line analysis: F,G, and K stars / The Publisher, Ontario, Canada Authors: de Jager, C. Bibcode: 1990SSRv...54..451D Altcode: 1990SSRv...54..451G No abstract at ADS Title: Book Review: NGC 2000.0 : The complete new general catalogue and index catalogues of nebulae and star clusters. / Sky Publishing Corp. and CUP, 1988 Authors: Sinnnott, R. W.; de Jager, C. Bibcode: 1990SSRv...54R.190S Altcode: No abstract at ADS Title: Book-Review - Highlights of Astronomy Authors: McNally, D.; de Jager, C. Bibcode: 1990SSRv...54..450M Altcode: No abstract at ADS Title: Book reviews Authors: Haubold, Hans Joachim; Snijders, M. A. J.; van Rensbergen, W.; de Jager, Cornelis; van de Weygaert, R.; Achterberg, A.; Linssen, P. F. J.; Hoyng, P.; Kleczek, J.; Bauer, Siegfried J.; Hayakawa, S. Bibcode: 1990SSRv...54..187H Altcode: No abstract at ADS Title: Parametrization of stellar rates of mass loss as functions of the fundamental stellar parameters M, L, and R. Authors: Nieuwenhuijzen, H.; de Jager, C. Bibcode: 1990A&A...231..134N Altcode: We investigate the dependence of Mṡ on the three fundamental parameters mass M, radius R and luminosity L for a sample of 247 stars (number of independent data points with weight unity =454). "Average expected" mass-values are derived from evolutionary calculations. A simple formula, viz.

-Mṡ=9.6310-15 (L/Lsun)1.42 (M/Msun)0.16 (R/Rsun)0.81 Msun Msun yr-1

appears to give a good representation of observed rates of mass loss over the whole Hertzsprung-Russell diagram. For luminous stars with Teff > 5000 K the standard deviation is equal to the intrinsic error (0.37) of the rate of mass loss. The representation is comparable to or even better than that of the expressions by Reimers and Lamers, which were derived for restricted areas of the HR-diagram. Title: An explanation of the `granulation boundary' in the HR diagram Authors: de Jager, Cornelis Bibcode: 1990SoPh..126..201D Altcode: The granulation boundary is a line running in the HR diagram from about F0V stars to G1Ib. It divides the HR diagram in two regions in which the `spectral line bisector' behaves differently. To the right (cooler stars) the Fraunhofer lines in stellar spectra show the characteristics typical for convective motions. For hot stars it indicates the presence of large velocities involving a large part of the line-forming region. We give evidence that the opposite behaviour shown by spectra of hotter stars can be explained by gravity waves. Title: Science Fringe Science and Pseudo-Science Authors: de Jager, C. Bibcode: 1990QJRAS..31...31D Altcode: No abstract at ADS Title: Les étoiles les plus lumineuses. Authors: de Jager, Cornelis Bibcode: 1990CRASG...7....1D Altcode: It is pointed out that there is an upper limit to the brightness of stars, beyond which no star can exist. It is shown that this limit coincides with the brightness limit for which the effective acceleration (i.e., the sum of the effects of gravitation and forces due to thermal, radiative, and turbulent pressure directed outward) drops to a very small value. In atmospheres of the most extreme stars, gravity waves can occur with very long wavelengths and elevated periods. It is suggested that it is these waves that are responsible for the instabilities of supergiant and hypergiant stars. Title: Book Review: Data of the planetary system / Akademie Verlag, Berlin Authors: de Jager, C. Bibcode: 1990SSRv...53..168D Altcode: 1990SSRv...53..168R No abstract at ADS Title: Acceleration phases in high-energy solar flares Authors: de Jager, C. Bibcode: 1990AdSpR..10i.101D Altcode: 1990AdSpR..10..101D In high-energy solar flares, defined as flares that emit an observable gamma-ray spectrum, there can be three subsequent phases of particle acceleration. Primary acceleration of electrons to ~ 10 MeV occurs within less than 0.1 s. It is suggested that this happens in many explosive coalescences between the many fluxthreads in the flare area. One or two seconds later ions are accelerated to ~ 100 MeV in an expanding shock-wave dominated electric field system; this is the second phase. A few minutes later ions can be further accelerated (to ~ GeV energies) in a field line system opening to space. This is the third phase. Title: Book-Review - Planets and Perception - Telescopic Views and Interpretations 1609-1909 Authors: Sheehan, W.; de Jager, C. Bibcode: 1990SSRv...53..170S Altcode: No abstract at ADS Title: Book-Review - Planets and Perception - Telescopic Views and Interpretations - 1609-1909 Authors: Sheehan, W.; de Jager, C. Bibcode: 1990SSRv...53Q.170S Altcode: No abstract at ADS Title: The Solar X-Ray/Cosmic Gamma-Ray Burst Experiment Aboard the Ulysses Mission Authors: Hurley, K.; Sommer, M.; Paschmann, G.; Boer, M.; de Jager, C.; Heise, J.; van Rooijen, J.; Niel, M.; Vedrenne, G.; Cline, T.; Henoux, J. -C. Bibcode: 1990ICRC....5...74H Altcode: 1990ICRC...21e..74H; 1989ICRC....5...74H No abstract at ADS Title: Book reviews Authors: de Graaf, W.; Doom, C.; Wesselius, P. R.; de Jager, Cornelis; van der Kruit, P. C.; Achterberg, A.; de Waard, H.; van Hugo, Woerden; Kleczek, J.; Hudson, H. S.; van der Hucht, K. A. Bibcode: 1989SSRv...51..425D Altcode: No abstract at ADS Title: Acceleration Through X-Type Coalescence in a Small Energetic Flare Authors: Sakai, Jun-Ichi; de Jager, Cornelis Bibcode: 1989SoPh..123..389S Altcode: We studied the acceleration conditions in the small but fairly energetic flare of May 21, 1984 at 13∶26 UT. The most pronounced aspect of this flare was a series of 13 microwave/X-ray spikes, each lasting for about 0.1 s. A previous study has shown that each of these was due to a series of successive sudden formations of small plasma knots of high-energy particles. Each of these knots lost its energy in about 50 ms. In the present study we show that these knots can originate by the process of X-type (3-D) flux tube coalescence. The predicted rise time (30 to 50 ms) and energy are in good agreement with the observationally derived parameters. Title: Coronal Explosions as a Signature of Current Loop Coalescence in Solar Flares Authors: Sakai, Jun-Ichi; de Jager, Cornelis Bibcode: 1989SoPh..123..393S Altcode: The coronal explosions, discovered by De Jager and Boelee (1984), and interpreted by them as manifestations of plasma streaming out of the flare kernels, can also be interpreted as signatures of current loop coalescence in the flaring region. Title: Introduction Authors: de Jager, Cornelis Bibcode: 1989SSRv...50..393D Altcode: No abstract at ADS Title: Space Future Forum Authors: Sagdeev, R. Z.; Marchuk, G. I.; Raushenbakh, B. V.; de Jager, C.; Lown, B.; Ortner, J.; Keller, S.; Bonnet, R.; Dakov, M.; Dunaev, A. I.; Sagan, C. Bibcode: 1989SSRv...50..395S Altcode: 1989IAUCo.107..395S No abstract at ADS Title: Book-Review - International Directory of Astronomical Associations and Societies Authors: Heck, A.; Manfroid, J.; de Jager, C. Bibcode: 1989SSRv...50..389H Altcode: 1989IAUCo.107..389H No abstract at ADS Title: Book reviews Authors: Marov, M. Ya; Garmany, C. D.; Heck, André; Schadee, A.; McNally, D.; van der Kruit, P. C.; Pecker, J. -C.; de Jager, Cornelis; Somov, B. V. Bibcode: 1989SSRv...50..385M Altcode: No abstract at ADS Title: Mass loss rate and atmospheric turbulence of the B2 hypergiant HD 80077. Authors: Carpay, J.; de Jager, C.; Nieuwenhuijzen, H.; Moffat, A. Bibcode: 1989A&A...216..143C Altcode: Turbulence in the atmosphere of HD 80077, situated on the Humphreys-Davidson limit, has been studied in order to test whether turbulence plays a role in the instability of very hot stars. A mass loss rate of (5 +10 or -3) x 10 to the -6th solar masses/yr is obtained which is very low for stars along the upper luminosity limit in the Hertzsprung-Russell diagram. The object has a photospheric microturbulent velocity component of 23 + or - 2 km/s and an outwardly directed turbulent acceleration of 11-24 cm/sq s. Title: Book-Review - Exploring the Universe with the IUE Satellite Authors: Kondo, Y.; Wamsteker, W.; Boggess, A.; Grewing, M.; de Jager, C.; Lane, A. L.; Linksy, J. L.; Wilson, R.; Waters, R. Bibcode: 1989SSRv...49..443K Altcode: 1988SSRv...49..443K No abstract at ADS Title: Editorial Authors: de Jager, Cornelis; Švestka, Zdeněk; Howard, Robert F. Bibcode: 1989SoPh..121D...9D Altcode: No abstract at ADS Title: Coordinated observations of a large impulsive flare on UV Ceti. Authors: de Jager, C.; Heise, J.; van Genderen, A. M.; Foing, B. H.; Ilyin, I. V.; Kilkenny, D. S.; Marvridis, L.; Cutispoto, G.; Rodono, M.; Seeds, M. A.; Yuen, K. Ng.; van Driel, W.; Rabattu, X.; Zodi, A. M.; Vilas Boas, J. W. S.; Scalise, E.; Schaal, R. E.; Kaufmann, P.; Waelkens, C. Bibcode: 1989A&A...211..157D Altcode: The characteristics of the major flare observed on UV Ceti on Decemmber 1985 at 01:26 UT during an international observing campaign are described. X-ray observations were obtained with Exosat in the 0.06-0.3 keV bands and 1-6 keV bands; optical photometry was obtained in 11 wavelength bands, and spectra were recorded in the wavelength range 3500 to 7000 A. The results of the data analysis support the Impulsive Explosion Model proposed for stellar flares and disclose some differences with the solar case. Title: Book-Review - Reports of Astronomy Authors: Swings, J. P.; de Jager, C. Bibcode: 1989SSRv...51R.426S Altcode: No abstract at ADS Title: Atmospheric Dynamics of Luminous Stars Authors: de Jager, Cornelis; Carpay, Joost; de Koter, Alex; Nieuwenhuijzen, Hans; Schellekens, Erik Bibcode: 1989ASSL..157..211D Altcode: 1989IAUCo.113..211D; 1989plbv.coll..211D A review is given of data and theories on the motion fields in super and hypergiants with special reference to LBVs. It is shown that the radiative momentum flux is incapable of driving the episodical bursts of mass loss of these stars and that there are several indications that the LBV-phenomenon is hydrodynamically driven. The sum of turbulent and radiative accelerations in the atmospheres of the most luminous stars compensates the gravitational acceleration for stars near the Humphreys-Davidson limit. This explains their atmospheric near-instability. The motion field in the atmosphere of a typical LBV consists mainly of low-order gravity waves, while acoustic waves are rapidly damped. These gravitation waves may be stochastic rather than coherently ordered. These stochastic pulsations are assumed to be responsible for the LBV phenomenon. Title: Development of concepts on stellar atmospheres and envelopes. Authors: de Jager, Cornelis Bibcode: 1989mse..proc....3D Altcode: In the years before about 1940 the physical and mathematical bases were laid for the quantitative analysis of stellar atmospheres, but the years around 1940 saw a number of research results that greatly changed the face of astrophysics and were the starting-points for important further developments. The most important of these were: the discovery of the sub-nuclear energy source; that the solar and cosmic abundance ratio H/metals was about 10000; the identification of the coronal lines; the elaboration of MHD. The author describes some highlights of the periods before and around 1940, of the period around 1950, and presents a brief review of current problems in the study of stellar atmospheres. Title: Flare energetics. Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veck, N. J. Bibcode: 1989epos.conf..377W Altcode: The authors have sought to establish a comprehensive and self-consistent picture of the sources and transport of energy within a flare. To achieve this goal, they chose five flares in 1980 that were well observed with instruments on the Solar Maximum Mission, and with other space-borne and ground-based instruments. The events were chosen to represent various types of flares. Details of the observations available for them and the corresponding physical parameters derived from these data are presented. The flares were studied from two perspectives, the impulsive and gradual phases, and then the results were compared to obtain the overall picture of the energetics of these flares. The authors also discuss the role that modeling can play in estimating the total energy of a flare when the observationally determined parameters are used as the input to a numerical model. Finally, a critique of our current understanding of flare energetics and the methods used to determine various energetics terms is outlined, and possible future directions of research in this area are suggested. Title: Rates of mass loss in the upper H-R diagram. Authors: Nieuwenhuijzen, H.; de Jager, C. Bibcode: 1989ASSL..157..305N Altcode: No abstract at ADS Title: Luminous Blue Variables Need not BE Blue Authors: de Jager, C.; van Genderen, A. M. Bibcode: 1989ASSL..157..127D Altcode: 1989IAUCo.113..127D; 1989plbv.coll..127D No abstract at ADS Title: Atmospheric Parameters and Accelerations in the Outer Parts of Luminous Hot Stars Authors: Nieuwenhuijzen, H.; de Jager, C. Bibcode: 1989ASSL..157..287N Altcode: 1989plbv.coll..287N; 1989IAUCo.113..287N No abstract at ADS Title: Mass Loss Rates in the Hertzprung-Russell Diagram Authors: de Jager, C.; Nieuwenhuijden, H.; van der Hucht, K. A. Bibcode: 1988BICDS..35..141D Altcode: No abstract at ADS Title: The relation between rotational velocity and mass-loss for massive stars Authors: Nieuwenhuijzen, H.; de Jager, C. Bibcode: 1988A&A...203..355N Altcode: The dependence of stellar mass loss on rotation for nonemission-line stars is investigated, confirming numerically an earlier result showing a seemingly good correlation between the stellar mass-loss rate (MLR) and v(rot)sin(i) but showing that this correlation merely reflects the distribution of MLR and v(rot)sin(i) over the Hertzsprung-Russell diagram; it is physically not real. An attempt is made to disentangle the dependence of MLR on the three variables: Teff, L, and v(rot)sin(i) by empirically determining the relation over the HR diagram. It then appears that, for each point in the HR diagram, MLR depends only weakly on v(rot)sin(i), in agreement with theoretical predictions. For Be and shell stars the MLR for their high-latitude parts follows approximately the same rule as the one valid for non-Be stars, while the MLR from their equatorial areas is larger by a factor 100, in rough agreement with a theoretical prediction. Title: The dynamical state of the atmosphere of the supergiant alpha Scorpii(M 1.5 Iab). Authors: de Koter, A.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1988A&A...200..146D Altcode: The dynamical state of the atmosphere of α Sco A (M1.5 Iab) has been investigated on the basis of spectra in the visual spectral range. The authors find the ratio between the microturbulent velocity component and the velocity of sound to increase from 0.4 at τ5000 = 1 to 0.6 at τ5000 = 10-3. The turbulent acceleration gt is 0.15 cm s-2 which should be compared with ggrav = -1.0 cm s-2. Per scaleheight the fraction 0.95 of the mechanical energy flux is dissipated, a remarkably large value for the relatively small microturbulent velocity. The macroturbulent velocity component has the surprisingly large value of 16 km s-1. Title: Solar flares through electric current interaction Authors: de Jager, Cornelis Bibcode: 1988Ap&SS.144..311D Altcode: The fundamental hypothesis by Alfvén and Carlqvist (1967) that solar flares are related to electrical currents in the solar chromosphere and low corona is investigated in the light of modern observations. We confirm the important role of currents in solar flares. There must be tens of such current loops (‘flux threads’) in any flare, and this explains the hierarchy of bursts in flares. We summarize quantitative data on energies, numbers of particles involved and characteristic times. A special case is the high-energy flare: this one may originate in the same way as less energetic ones, but it occurs in regions with higher magnetic field strength. Because of the high particle energies involved their emission seats live only very briefly; hence the area of emission coincides virtually with the seat of the instability. These flares are therefore the best examples for studying the primary instability leading to the flare. Finally, we compare the merits of the original Alfvén-Carlqvist idea (that flares originate by current interruption) with the one that they are due to interaction (reconnection) between two or more fluxthreads. We conclude that a final decision cannot yet be made, although the observed extremely short time constants of flare bursts seem to demand a reconnection-type instability rather than interruption of a circuit. Title: The atmospheric structure, stellar wind and binary characteristics ofthe hypergiant HR 8752 (G0-5Ia+). Authors: Piters, A.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1988A&A...196..115P Altcode: The photospheric parameters of the hypergiant HR 8752 were determined on the basis of photometric and spectroscopic data. Results are T(eff) = 4200 K, log g(eff) = 0; the microturbulent velocity component is 5.9 + or - 0.4 km/s, a value close to the velocity of sound. The turbulent pressure and its gradient could be determined by making use of spectral lines originating at greatly different optical depths. From these values the (outward) turbulent acceleration is derived which appears to be comparable in value to the (inward) gravitational acceleration. This explains the strong instability of the star's photosphere. The long-standing problem of the enhancement of the ionic lines in the star's spectrum could be solved: the major parts of these lines originate in the part of the wind of the hypergiant that is ionized by a hot (B1) companion. The radio spectrum of HR 8752 is also emitted by the H(+) region and can be explained quantitatively. The rate of mass loss is about 2 x 10 to the -5th solar mass/yr. Title: The dynamical state of the atmosphere of the supergiant alpha Cygni (A2Ia). Authors: Boer, B.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1988A&A...195..218B Altcode: High resolution near-ultraviolet spectra of α Cygni (λ/Δλ = 8×104) were studied to investigate the dynamical state of the atmosphere. The microturbulent line-of-sight velocity component is found to be constant (10.3±0.7 km s-1) over a large range of optical depths (-4 < log τc < -1). The macroturbulent velocity distribution is double-peaked at velocities, both positive and negative, of 14±2 km s-1, suggesting preponderantly up- and downward moving elements (convective?). These motions may explain the non-radial pulsations found by Lucy (1976). The outward turbulent acceleration in the outer layers of the atmosphere is 7.6 cm s-2, yielding a gravitational acceleration of 26.0 cm s-2, and a stellar mass of 18 M_sun;. The authors also derive a new value for vr sin i (8±3 km s-1). Title: A differential analysis of UV photospheric lines of OBN and OBC stars. Authors: Wollaert, J. P. M.; Lamers, H. J. G. L. M.; de Jager, C. Bibcode: 1988A&A...194..197W Altcode: The paper presents measurements of the equivalent widths of several photospheric C, N, O, He, Si and Fe lines in the ultra-violet spectra of 55 OBN, OBC and normal supergiants and main-sequence stars. Seventy-two short-wavelength, high-resolution spectrograms obtained with IUE were used. The differences in equivalent widths of OBN and OBC stars relative to normal stars are interpreted as abundance differences and yield upper and lower limits to respectively under- and overabundances of the elements C, N and O. The effects of anomalous strength of CNO lines appear to be less pronounced in the UV than in the visual. The main reason for this is the saturation of the lines investigated. Different scenarios to bring products of the CNO-cycle to the stellar surface are discussed. The OBN stars can be explained in terms of abundance differences due to evolutionary effects as well as initial differences. The OBC stars may be explained by differences in atmospheric structure or initial abundances. Title: High Energy Flares Authors: de Jager, C. Bibcode: 1988BAAS...20..725D Altcode: No abstract at ADS Title: Experiences from the SMY/SMA Period Authors: de Jager, C. Bibcode: 1988BAAS...20..749D Altcode: No abstract at ADS Title: The dynamical state of the atmosphere of the supergiant Alpha Cygni (A2 Iae) derived from high-resolution ultraviolet spectra. Authors: Boer, B.; de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1988JPhys..49..383B Altcode: 1988IAUCo.102..383B In order to study the apparent near instability of supergiant atmospheres high-resolution (λ/Δλ = 8×104) BUSS (Balloon-borne Ultraviolet Stellar Spectrometer) spectra of the supergiant Alpha Cyg have been investigated. Title: Fast Variations in the Ultraviolet Resonance Lines of alpha Camelopardalis (O9.5 Ia): Evidence for Blobs in the Wind Authors: Lamers, Henry J. G. L. M.; Snow, Theodore P.; de Jager, Cornelis; Langerwerf, A. Bibcode: 1988ApJ...325..342L Altcode: The 72 IUE spectra of Alpha Cam and 19 IUE spectra of Kappa Cas, obtained during 72 hours of continuous IUE time in September 1978 were searched for variations in the profiles of the resonance lines of Si IV, C IV, and N V, and the results are discussed. The UV resonance lines in the spectra of Alpha Cam showed variations at the 2 percent level near -1800, -700, and +700 km/s. The first two variations can be explained by absorption components of outward-accelerated blobs or shells with an average acceleration of 1.5 cm/sq s. The characteristics of the blobs and shells are discussed, including the column densities and masses. No variations were found in the spectra of Kappa Cas. Title: Mass loss rates in the Hertzsprung-Russell diagram. Authors: de Jager, C.; Nieuwenhuijzen, H.; van der Hucht, K. A. Bibcode: 1988A&AS...72..259D Altcode: From the literature the authors collected values for the rate of mass loss for 271 stars, nearly all of population I and of spectral types O through M. Rates of stellar mass loss -Mdetermined according to six different methods were compared and appear to yield the same result per star within the limits of errors. The M-data can well be represented by one empirical interpolation formula, as a function of the effective temperature Teff and luminosity L. In addition the authors studied some groups of other stars: fast rotators and chemically evolved stars. The chemically evolved stars have rates of mass loss which are larger than those of "normal" stars occupying the same positions in the Hertzsprung-Russell diagram. Title: Book Review: The Soviet cosmonaut team. / Unifelt Inc., 1986 ; GRH Publications, 1990 Authors: de Jager, C. Bibcode: 1988SSRv...46..380D Altcode: 1988SSRv...46..380H No abstract at ADS Title: Stellar mass loss and atmospheric instability Authors: de Jager, Cornelis; Nieuwenhuijzen, Hans Bibcode: 1988LNP...305..101D Altcode: A review is given of rate of mass-loss values M in the upper part of the Hertzsprung-Russell diagram. Near the luminosity limit of stellar existence M = -10-4 M) yr-1. Episodical mass loss in bright variable super- and hypergiants does not significantly increase this value. For Wolf-Rayet stars the rate of mass loss is larger by a factor 140 than for non-evolved stars with the same Teff and L; for C stars this factor is ten. This can be explained qualitatively. Rotation ap pears hardly to influence the rate of mass loss except for vrot-values close to the break-up velocity. This is in accordance with theory. We suggest the existence of a Red Supergiant Branch; along that branch mass loss is virtually independent of luminosity. Stellar winds along the upper limit of stellar existence are mainly due: to radiation pressure for hot supergiants ( 10 000 K) ; to turbulent pressure for cool supergiants (3000-10 000 K), and to dust-driven and pulsation-driven winds for cooler stars. The turbulent pressure may originate in large-scale stochastic motions as observed in Alpha Cyg. Episodical mass loss, as observed in P Cyg, HR 8752 and other Very Luminous Variables may be due to occasional violent stochastic motions, resulting in a shock-driven episodical mass-loss component. Title: Energetic Phenomena in Impulsive Solar Flares Authors: de Jager, C. Bibcode: 1988ICRC....7...66D Altcode: 1987ICRC....7...66D; 1988ICRC...20g..66D No abstract at ADS Title: Stellar Mass Loss and Atmospheric Instability Authors: de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1988LNP...305..102D Altcode: 1988IAUCo.108..102D; 1988adse.conf..102D A review is given of rate of mass-loss values Min the upper part of the Hertzsprung-Russell diagram. Title: Looking Backwards - Some Successful Programmes Authors: de Jager, C. Bibcode: 1988copa.conf...17D Altcode: The author describes a few observational campaigns in which he has been involved in the past. Although these campaigns were very different in topic, scope and duration, some general experiences can be formulated which appear to have influenced the degree of success of these enterprises. Title: Low Resolution Observations of a B2 Hypergiant Authors: de Jager, C. Bibcode: 1988iue..prop.3296D Altcode: No abstract at ADS Title: Turbulence-Driven Atmospheric Instability and Largescale Motions in Super Giants and Hypergiants Authors: Boer, B.; Carpay, J.; de Koter, A.; de Jager, C.; Nieuwenhuijzen, H.; Piters, A.; Spaan, F. Bibcode: 1988LNP...305..131B Altcode: 1988adse.conf..131B; 1988IAUCo.108..131B Spectral studies of super- and hypergiants show that the (outward directed) turbulent acceleration approaches the value of the gravitational acceleration for the most luminous stars, which makes their atmospheres unstable. Title: Book-Review - Progress in Solar Physics Authors: de Jager, C.; Svestka, Z. Bibcode: 1988ApL&C..26..372D Altcode: 1988ApL....26..372D No abstract at ADS Title: Sterrenwacht "Sonnenborgh": een nieuwe fase. Authors: de Jager, C. Bibcode: 1988Zenit..15..332D Altcode: No abstract at ADS Title: Microturbulence in the upper photosphere of alpha Persei (F5 Ib) derived from ultraviolet spectral observations. Authors: Spaan, F. H. P.; de Jager, C.; Nieuwenhuijzen, H.; Kondo, Y. Bibcode: 1987A&A...185..229S Altcode: High-resolution ultraviolet spectra of the moderate supergiant α Per (F5 Ib) were studied to determine the dynamic state of its upper photosphere. It was found that the line-of-sight microturbulent velocity component ζμ in the region of origin of the UV spectrum (which is 73,000 km above that of formation of the visual spectrum) is about 5 km s-1, and is slightly smaller than the value derived from the visual spectrum. This is ascribed to dissipation of mechanical energy between the higher and lower layers where, respectively, the ultraviolet and visual light lines originate. Between these two levels, which are one scale height apart, the mechanical energy flux decreases to about 0.3 of its photospheric value. The consequent value for the (outward directed) turbulent acceleration gt is 24 cm s-2, more than one half the observationally determined effective acceleration of gravity. Title: Book Review: Transactions of the IAU, Vol. XIXB. / Reidel, 1986. Authors: de Jager, C. Bibcode: 1987SSRv...45..410D Altcode: 1987SSRv...45..410S No abstract at ADS Title: The Solar Maximum Year and its Analysis Phase Authors: De Jager, Cornelis Bibcode: 1987SoPh..114..387D Altcode: No abstract at ADS Title: A High-Energy Solar Flare Burst Complex and the Physical Properties of its Source Region Authors: de Jager, Cornelis; Kuijpers, Jan; Correia, Emilia; Kaufmann, Pierre Bibcode: 1987SoPh..110..317D Altcode: We discuss a solar flare microwave burst complex, which included a major structure consisting of some 13 spikes of 60 ms FWHM each, observed 21 May, 1984 at 90 GHz (3 mm). It was associated with a simultaneous very hard X-ray burst complex. We suggest that the individual spikes of both bursts were caused by the same electron population: the X-bursts by their bremsstrahlung, and the microwave bursts by their gyrosynchrotron emission. This latter conclusion is based on the evidence that the radio turnover frequency was ≤ 150 GHz. It follows that the emission sources were characterized by an electron density of about 1011 cm−3, a temperature of 5 × 108 K and a magnetic field of about 1400-2000 G. They had a size of about 350 km; if the energy release is caused by reconnection the sources of primary instability could have been smaller and in the form of thin sheets with reconnection speed at a fraction of the Alfvén velocity and burst-like energy injections of ≈ 1027 erg during about 50 ms each. The energized plasma knots lost their injection energy by saturated convective flux (collisionless conduction) in about 30 ms. Title: Book-Review - Progress in Solar Physics Authors: de Jager, C.; Svestka, Z.; Jordan, C. Bibcode: 1987Obs...107..130D Altcode: No abstract at ADS Title: A new determination of the statistical relations between stellar spectral and luminosity classes and stellar effective temperature and lumonosity. Authors: de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1987A&A...177..217D Altcode: From a set of 199 determinations of bolometric stellar luminosity L and 268 of stellar effective temperature Teff we derived the statistical dependances of log Teff and log (L/Lsun) respectively on spectral type and luminosity class. For computer-use we introduced continuous variables s (for spectral type) and b (for luminosity class). The relations log Teff (s, b) and log (L/Lsun) (s, b) are given in terms of Chebychev polynomials and their cross-products. A comparison is made with existing systems: there are small differences which we ascribe to the addition of new data on temperature and luminosity, not used in previous determinations. The 1σ values for determinations of unit weight of log Teff and log L respectively appear to be 0.021 and 0.164, which shows that there is a factor eight in their relative accuracies. Title: Book-Review - Progress in Solar Physics Authors: de Jager, C.; Svestka, Z. Bibcode: 1987Sci...236R1009D Altcode: No abstract at ADS Title: Rate of mass-loss in the Hertzsprung-Russell Diagram Authors: de Jager, C.; Nieuwenhuijzen, H.; van der Hucht, K. A. Bibcode: 1987ilet.work..205D Altcode: The authors have collected literature data on rates of mass-loss Mfor 264 O through M-type stars and a number of stars of other types. It appears possible to develop log (-M) into a series of Chebychev polynomia of a the first kind in log Teff and log (L/L_sun;) and their cross products. Title: The future of planetary astronomy Authors: de Jager, Cornelis Bibcode: 1987AdSpR...7l.161D Altcode: 1987AdSpR...7..161D No abstract at ADS Title: Stellar instabilities in the upper part of the Hertzsprung-Russell Diagram Authors: de Jager, Cornelis Bibcode: 1987ASSL..136..191D Altcode: 1987ilet.work..191D A review is given of some of the main topics addressed at the present workshop. The main characteristics of stars in the upper part of the HR diagram are discussed. The properties of the most massive stars and the variability of blue stars are considered. Pulsations of massive stars and photospheric and atmospheric disturbances in luminous early-type stars are examined. Title: The influence of photospheric turbulence on stellar mass loss Authors: de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1987ilet.work..267D Altcode: The authors show that the stellar rate of mass loss is positively correlated with the average microturbulent photospheric velocity, and that the energy contained in the microturbulent motions is of the same order of magnitude as the wind energies. Title: Flare energetics Authors: Wu, S. T.; De Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J. Bibcode: 1986epos.conf..5.1W Altcode: 1986epos.confE...1W In this investigation of flare energetics, researchers sought to establish a comprehensive and self-consistent picture of the sources and transport of energy within a flare. To achieve this goal, they chose five flares in 1980 that were well observed with instruments on the Solar Maximum Mission, and with other space-borne and ground-based instruments. The events were chosen to represent various types of flares. Details of the observations available for them and the corresponding physical parameters derived from these data are presented. The flares were studied from two perspectives, the impulsive and gradual phases, and then the results were compared to obtain the overall picture of the energics of these flares. The role that modeling can play in estimating the total energy of a flare when the observationally determined parameters are used as the input to a numerical model is discussed. Finally, a critique of the current understanding of flare energetics and the methods used to determine various energetics terms is outlined, and possible future directions of research in this area are suggested. Title: Characterization of the Total Flare Energy Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.41W Altcode: 1986epos.confE..41W No abstract at ADS Title: Energetics of the Impulsive Phase Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf..5.5W Altcode: 1986epos.confE...5W No abstract at ADS Title: Energetics of the Gradual Phase Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.20W Altcode: 1986epos.confE..20W No abstract at ADS Title: Review of Impulsive Phase Phenomena Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.60W Altcode: 1986epos.confE..60W No abstract at ADS Title: Flares Chosen for Energetics Study Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.47W Altcode: 1986epos.confE..47W No abstract at ADS Title: Relationships among the Phases Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.39W Altcode: 1986epos.confE..39W No abstract at ADS Title: A high energy solar flare burst complex and the physical properties of its source region Authors: De Jager, C.; Kuijpers, J.; Correia, E.; Kaufmann, P. Bibcode: 1986cospar........D Altcode: A solar flare microwave burst complex, which exhibited a major structure consisting of some 13 spikes of 60 ms FWHM each, observed 21 May 1984 at 90 GHz (3 mn) is discussed. It was associated with a simultaneous very hard X-ray burst complex. A possible explanation in which the individual spikes of both bursts were caused by the same electron population is developed: the X-ray bursts by their bremsstrahlung, and the microwave bursts by their gyro-synchrotron emission. This latter explanation is based on the assumption that the radio turnover frequency is less than 150 GHz. The emission sources were characterized by an electron density of about 10 to the 11th power/cu cm, a temperature of 5 x 10 to the 8th power K and magnetic field of about 1400 to 2000 G. They had a size of about 350 km; if the energy release is caused by reconnection the sources of primary instability could have been smaller and in the form of thin sheets with reconnection speed at a fraction of the Alfven velocity and burst-like energy injections of 10 to the 27th power erg during about 50 ms each. The energized plasma knots lost their injection energy by saturated convective flux (collisionless conduction) in about 30 ms. Title: Book-Review - Progress in Solar Physics Authors: de Jager, C.; Svestka, Z. Bibcode: 1986S&T....72R.479D Altcode: No abstract at ADS Title: New Observations of Coronal Explosions and Their Interpretation Authors: Lemmens, Andre F. P.; de Jager, Cornelis Bibcode: 1986SoPh..106..365L Altcode: We examined five flares, observed by the Hard X-Ray Imaging Spectrometer aboard the Solar Maximum Mission, for the occurrence of coronal explosions and found that these occur only if (a) the flare shows distinct single impulsive hard X-ray bursts and (b) it shows upward (convective) motions during the initial part of the impulsive phase. Coronal explosions are therefore explained as a manifestation of plasma streaming laterally out of the flare kernel(s). There is some evidence that streaming occurs into a number of cylindrical fluxtubes which spread over a larger area, thus supporting the `spaghetti-bundle model' for the flaring region. Title: Book-Review - Supernovae Authors: Murdin, P.; Murdin, L.; de Jager, C. Bibcode: 1986SSRv...44..186M Altcode: No abstract at ADS Title: Solar Flares and Particle Acceleration Authors: de Jager, C. Bibcode: 1986SSRv...44...43D Altcode: Energy release in solar flares occurs during the impulsive phase, which is a period of a few to about ten minutes, during which energy is injected into the flare region in bursts with durations of various time scales, from a few tens of seconds down to 0.1 s or even shorter. Non-thermal heating is observed during a short period, not longer than a few minutes, in the very first part of the impulsive phase; in average flares, with ambient particle densities not larger than a few times 1010 cm-3 it is due to thick-target electron beam injection, causing chromospheric ablation followed by convection. In flares with larger densities the heating is due to thermal fronts (Section 1). The average energy released in chromospheric regions is a few times 1030 erg, and an average number of 1038 electrons with E ≳ 15 keV is accelerated. In subsecond pulses these values are about 1035 electrons and about 1027 erg per subsecond pulse. The total energy released in flares is larger than these values (Section 2). Energization occurs gradually, in a series of fast non-explosive flux-thread interactions, on the average at levels about 104 km above the solar photosphere, a region permeated by a large number (≳ 10) of fluxthreads, each carrying electric currents of ≈ 1010 1011 A. The energy is fed into the flare by differential motions of magnetic fields driven by photospheric-chromospheric movements (Section 3). In contrast to these are the high-energy flares, characterized by the emission of gamma-radiation and/or very high-frequency (millimeter) radiobursts. Observations of such flares, of the flare neutron emission, as well as the observation of 3He-rich interplanetary plasma clouds from flares all point to a common source, identified with shortlived (∼ 0.1 s) superhot (≳ 108 K) flare knots, situated in chromospheric levels (Section 4). Pre-flare phenomena and the existence of homologous flares prove that flare energization can occur repeatedly in the same part of an active region: the consequent conclusions are that only seldom the full energy of an active region is exhausted in one flare, or that the flare energy is generated anew between homologous flares; this latter case looks more probable (Section 5). Flare energization requires the formation of direct electric fields, in value comparable with, or somewhat smaller than the Dreicer field (Section 6). Such fields originate by current-thread reconnection in a regime in which the current sheet is thin enough to let resistive instability originate (Section 7). Particle acceleration occurs ‘by fast reconnection in magnetic fields ≳ 100 G and electric fields exceeding about 0.3 times the Dreicer field at fairly low particle densities (≈ 1010 cm-3); for larger densities plasma heating is expected to occur (Section 8). Transport of accelerated particles towards interplanetary space demands a field-line configuration open to space. Such a configuration originates mainly after the gradual gamma-ray/proton flares, and particularly after two-ribbon flares; these flares belong to the dynamic flares in Sturrock and Švestka's flare classification. Acceleration to GeV energies occurs subsequently in shock waves, probably by first-order Fermi acceleration (Section 9). Title: Book reviews Authors: Gurtovenko, E. A.; Kostik, R. I.; Tlamicha, A.; Zerull, R.; Schadee, Aert; Mészáros, A.; Néeman, Y.; Giese, R. H.; Sinclair, A. T.; Kleczek, J.; Trendelenburg, E. A.; Hillebrandt, Wolfgang; Sehnal, L.; de Graaff, W.; Slottje, C.; Courvisier, T.; van Beek, H. F.; Baud, B.; de Jager, C.; Danby, J. M. A.; Somov, B. V.; Cassinelli, J.; Verbunt, F. Bibcode: 1986SSRv...44..177G Altcode: No abstract at ADS Title: Book-Review - Space Science Reviews Authors: Akasofu, S. I. .; Beynon, W. J. G.; de Jager, C.; Rasool, S. I.; Krivsky, L. Bibcode: 1986Ap&SS.122..187A Altcode: No abstract at ADS Title: La phase initiale de l'éruption solaire. Authors: de Jager, C. Bibcode: 1986LAstr.100..167D Altcode: Characteristics of the impulsive phase of a solar flare as determined from SMM data are summarized. The initial phase is marked by bursts of X and microwave radiation. The 20-30 keV X-ray photons are 10,000 times as energetic as visible energy and are accompanied by electrons traveling at 100,000 km/sec. The bursts originate in a gas heated to 20 million degrees for several minutes over a solar surface area 5000 to 10,000 km across. The two feet of a flare are sited on regions of opposite polarity thousands of kilometers apart. Flux tubes associated with the events carry currents of 10 trillion A. Gaseous clouds gathered over the feet of the flare arch in the first minutes of an X-ray burst are heated to over 50 million degrees, a temperature associated with high thermal conductivity and a decrease by a half within minutes. The 3-4 km thick clouds form just above the surface and rapidly convect into the chromosphere where, exposed to repeated burst events, they end in a coronal explosion. More data is needed to identify the source of accompanying gamma ray photons which have energies one to two orders of magnitude higher than those of the X-rays. Title: The flare kernel in the impulsive phase Authors: De Jager, C. Bibcode: 1986sfcp.nasa...29D Altcode: The impulsive phase of a flare is characterized by impulsive bursts of X-ray and microwave radiation, related to impulsive footpoint heating up to 50 or 60 MK, by upward gas velocities (150 to 400 km/sec) and by a gradual increase of the flare's thermal energy content. These phenomena, as well as non-thermal effects, are all related to the impulsive energy injection into the flare. The available observations are also quantitatively consistent with a model in which energy is injected into the flare by beams of energetic electrons, causing ablation of chromospheric gas, followed by convective rise of gas. Thus, a hole is burned into the chromosphere; at the end of impulsive phase of an average flare the lower part of that hole is situated about 1800 km above the photosphere. H alpha and other optical and UV line emission is radiated by a thin layer (approx. 20 km) at the bottom of the flare kernel. The upward rising and outward streaming gas cools down by conduction in about 45 s. The non-thermal effects in the initial phase are due to curtailing of the energy distribution function by escape of energetic electrons. The single flux tube model of a flare does not fit with these observations; instead we propose the spaghetti-bundle model. Microwave and gamma-ray observations suggest the occurrence of dense flare knots of approx. 800 km diameter, and of high temperature. Future observations should concentrate on locating the microwave/gamma-ray sources, and on determining the kernel's fine structure and the related multi-loop structure of the flaring area. Title: Editorial Authors: de Jager, Cornelis; Švestka, Zdeněk Bibcode: 1986SoPh..107..200D Altcode: 1987SoPh..107..200D No abstract at ADS Title: Simultaneous optical and X-ray observations of a flare on BY Draconis. Authors: de Jager, C.; Heise, J.; Avgoloupis, S.; Cutispoto, G.; Kieboom, K.; Herr, R. B.; Landini, M.; Langerwerf, A. F.; Mavridis, L. N.; Melkonian, A. S.; Molenaar, R.; Monsignori-Fossi, B. C.; Nations, H. L.; Pallavicini, R.; Piirola, V.; Rodono, M.; Seeds, M. A.; van den Oord, G. H. J.; Vilhu, O.; Waelkens, C. Bibcode: 1986A&A...156...95D Altcode: The authors present a first report on a campaign for simultaneous observations in the visual, radio and X-ray ranges of stellar flares on the binary BY Dra. During two nights of observations they observed one significant flare, simultaneously in soft X-rays and visible wavelengths, and one or two smaller bursts. The main flare impulsively heated an area of ⪉2×107km2 of the stellar photosphere to a brightness temperature ⪆25000K, during about 5 min, and generated hot plasma, emitting soft X-rays. This emission reached maximum after the impulsive burst and lasted for about an hour; it covered a much larger area on the star than the optical emission. There was no significant hard X-ray component. The authors ascribe the optical continuous emission to the flare's impulsive phase, and the soft X-ray emission to the gradual phase. Title: Stellar Atmospheric Instability in the Upper Part of the Hertzsprung-Russell Diagram Authors: de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1986IAUS..116..255D Altcode: The authors list evidences supporting the validity of their instability criterion and the consequent relations (cf. de Jager 1978, 1980, 1984). Title: The Dependence of the Stellar Rate of Mass Loss on Effective Temperature and Velocity Authors: de Jager, C.; Nieuwenhuijzen, H.; van der Hucht, K. A. Bibcode: 1986IAUS..116..109D Altcode: From the existing literature data have been collected on the stellar rate of mass loss for 189 stars with known or derived values of the effective temperature Teff and luminosity L. It appears that Mdepends only on Teff and L for the O- through M-type stars brighter that about 3×103L_sun;. Title: Acceleration in a high-energy flare. Authors: de Jager, C.; Correia, E.; Kaufmann, P. Bibcode: 1986AdSpR...6f.187D Altcode: 1986AdSpR...6R.187D The authors describe a well-studied flare (21 May, 1984) for which the emission area is a plasma knot of a few hundred km diameter, with a temperature of 5×108K and a magnetic field between 1400 and 2000 G. The authors show that this region coincides virtually with the area of primary energization of the flare. The reconnection area has a diameter not exceeding about 50 km. Title: The physics of solar flares. Proceedings of Symposium 5 of the COSPARTwenty-sixth Plenary Meeting held in Toulouse, France, 30th June - 11th July 1986. Authors: de Jager, C.; Svestka, Z. Bibcode: 1986AdSpR...6f....D Altcode: 1986AdSpR...6.....D Observations and theoretical models of solar-flare phenomena are discussed in reviews and reports presented at the symposium formally marking the end of the international Solar-Maximum-Year/Solar-Maximum-Analysis project. Topics examined include flare buildup, precursors, and onset; the impulsive phase; the gradual phase; and coronal and interplanetary phenomena. Title: Flare energetics. Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veck, N. J. Bibcode: 1986NASCP2439....5W Altcode: In this investigation of flare energetics, the authors establish a comprehensive and self-consistent picture of the sources and transport of energy within a flare. They chose five flares in 1980 that were well observed with instruments on the SMM, and with other space-borne and ground-based instruments. Details of the observations available for them and the corresponding physical parameters derived from these data are presented. The flares were studied from two perspectives, the impulsive and gradual phases, and then the results were compared to obtain the overall picture of the energetics of these flares. The authors also discuss the role that modeling can play in estimating the total energy of a flare when the observationally determined parameters are used as the input to a numerical model. Title: Het begin van een zonnevlam. Authors: de Jager, C. Bibcode: 1986Zenit..13..214D Altcode: No abstract at ADS Title: Progress in solar physics : review papers invited to celebrate the centennial volume of Solar physics Authors: de Jager, Cornelis; Svestka, Zdenek Bibcode: 1986pspr.book.....D Altcode: 1986QB521.6.P76.... No abstract at ADS Title: The flare kernel in the impulsive phase. Authors: de Jager, C. Bibcode: 1986NASCP2421...29D Altcode: The impulsive phase of a flare is characterized by impulsive bursts of X-ray and microwave radiation, related to impulsive footpoint heating up to 50 or 60 MK, by upward gas velocities (150 to 400 km s-1) and by a gradual increase of the flare's thermal energy content. A hole is "burned" into the chromosphere; at the end of the impulsive phase of an average flare the lower part of that "hole" is situated about 1800 km above the photosphere. Hα and other optical and UV line emission is radiated by a thin layer (≈20 km) at the bottom of the flare kernel. The single flux tube model of a flare does not fit with these observations; instead the spaghetti-bundle model is proposed. Microwave and gamma-ray observations suggest the occurrence of dense flare knots of ≡800 km diameter, and of high temperature. Future observations should concentrate on locating the microwave/gamma-ray sources, and on determining the kernel's fine structure and the related multi-loop structure of the flaring area. Title: The solar maximum year and its analysis phase Authors: de Jager, Cornelis Bibcode: 1986AdSpR...6f.353D Altcode: 1986AdSpR...6..353D This paper concludes the Toulouse symposium on the Solar Maximum Analysis, and marks the end of a period which started 1973, and was devoted to the preparations and the observational phase of the Solar Maximum Year (SMY, 1979 - 1981) and the subsequent analysis phase (SMA). The author describes the history of SMY-SMA and briefly summarizes the highlight discoveries of this fascinating period during which the knowledge of solar flares has fundamentally changed. Title: 21 May 1980 flare review Authors: de Jager, C.; Svestka, Z. Bibcode: 1985SoPh..100..435D Altcode: A review is given of observations and theories relevant to the solar flare of 21 May, 1980, 20 ∶ 50 UT, the best studied flare on record. For more than 30 hr before the flare there was filament activation and plasma heating to above 10 MK. A flare precursor was present ≥6 min before the flare onset. The flare started with filament activation (20 ∶ 50 UT), followed by thick-target heating of two footpoints and subsequent ablation and convective evaporation involving energies of 1 to 2 × 1031 erg. Coronal explosions occurred at 20 ∶ 57 UT (possibly associated with a type-II burst) and at 21 ∶ 04 UT (associated with an Hα spray?). Post-flare loops were first seen at 20 ∶ 57 UT, and their upward motion is interpreted as a manifestation of successive field-line reconnections. A type-IV radio burst which later changed into a type-I noise storm was related to a giant coronal arch located just below the radio noise storm region. Some implications and difficulties these observations present to current flare theories are mentioned. Title: Book-Review - Reports on Astronomy Authors: West, R. M.; De Jager, C. Bibcode: 1985SSRv...41..403W Altcode: No abstract at ADS Title: Book-Review - Sky Catalogue 2000.0 Authors: Hirshfeld, A.; Sinnott, R. W.; De Jager, C. Bibcode: 1985SSRv...41..399H Altcode: No abstract at ADS Title: Book-Review - Supernovae as Distance Indicators Authors: Bartel, N.; De Jager, C. Bibcode: 1985SSRv...41..404B Altcode: No abstract at ADS Title: Kernel Heating and Ablation in the Impulsive Phase of Two Solar Flares Authors: de Jager, C. Bibcode: 1985SoPh...98..267D Altcode: At the very start of the impulsive phase of two solar flares the temperature derived from medium-energy (≈ 16 keV) X-ray countrates was observed to rise abruptly, by several times 107 K above the temperature derived from low-energy X-ray (≈ 7 keV) countrates. The difference between the two temperatures relaxed to zero thereafter, quasi-exponentially, with a characteristic time of ≈ 1.5 min. This differential temperature variation appears to mimique the differences between the ionic kinetic and the electron temperatures derived from spectral observations (Figures 1 and 2). Title: Book-Review - Astrophysics and Space Science Reviews Authors: Syunyaev, R. A.; De Jager, C. Bibcode: 1985SSRv...41Q.400S Altcode: No abstract at ADS Title: Book reviews Authors: Mészáros, Attila; Eviatar, A.; de Jager, C.; Ruttien, Robert J.; Hultqvist, B.; Jakimiec, J.; Paerels, F.; Storm, Richard G.; van der Hucht, K. A.; Riedler, W.; Nieuwstadt, F. T. M. Bibcode: 1985SSRv...41..393M Altcode: No abstract at ADS Title: Book-Review - les Objets de Messier Reperage Observation Photographie Authors: Guillaud-Saumur, B.; Rethore, O.; de Jager, C. Bibcode: 1985SSRv...41..394G Altcode: No abstract at ADS Title: Coronal Explosions Authors: de Jager, C. Bibcode: 1985SoPh...96..143D Altcode: We searched for a new phenomenon, called `coronal explosions', in three solar flares, and found them in all three. A coronal explosion is the propagation of a density wave through the flaring area. The wave emerges from one or two small areas (the `sources') which are close to, but not identical with the sources of hard X-ray burst emission. In all three cases the explosion starts at the end of the impulsive phase, during or after the last hard (≳ 20 keV) X-ray burst. The velocities of propagation range between 1800 and a few tens of km s-1, and tend to decrease with time. We suggest that the bursts are magneto-hydrodynamical (shock) waves moving downward into denser regions. Title: The identification of massive supernova progenitors (I) Authors: de Jager, C. Bibcode: 1985ASSL..120..135D Altcode: 1985bems.symp..135D The average observational characteristics of supernovae of types II and V are summarized. The ZAMS-mass of supernova progenitors is derived, using their rate of occurrence and theoretical considerations on stellar evolution (the latter only for type II supernovae). The preexplosion masses are derived from the lightcurves (types II and V) and from the absolute magnitude before the explosion (only for type V's). Mass ranges of 6 to 16 solar masses, average 9.5 solar masses (type II) and greater than 100 solar masses (type V) were found as well as preexplosion masses of 7 + or - 1 solar mass (type II) and approximately 2000 solar masses (1961v). The location of presupernovae in the Hertzsprung-Russell diagram is now known with a fair accuracy. It is suggested that Mu-Carinae was not a supernova. Title: Solar Physics and Interplanetary Travelling Phenomena Authors: de Jager, Cornelis; Chen, Biao Bibcode: 1985spit.conf.....D Altcode: No abstract at ADS Title: The Impulsive Phase and Coronal Explosions in Solar X-Ray Flares Authors: de Jager, C. Bibcode: 1985spit.conf..917D Altcode: No abstract at ADS Title: Introductory Talk Authors: de Jager, C. Bibcode: 1985spit.conf....7D Altcode: No abstract at ADS Title: The impulsive phase of solar flares. Authors: de Jager, C. Bibcode: 1985BuAst..10...81D Altcode: The impulsive phase of a solar flare is a period of a few minutes duration, characterized by impulsive release of energy, particularly of high energy photons. Observations made in 1980 with instrumentation aboard the Solar Maximum Mission have shown that the phenomena occurring during the impulsive phase are due to reconnection of interacting field line (or electrical current) systems, leading to acceleration of beams of energetic electrons, causing heating and subsequent ablation (evaporation) of localized areas of the upper chromosphere ("footpoint heating") followed by convective ascent of heated plasma. The upward and lateral spreading of this hot gas characterizes the next, the gradual, phase. Title: Evidence for a supersonic turbulent velocity gradient in the outer photosphere of the supergiant alpha Cygni (A2Ia). Authors: de Jager, C.; Mulder, P. S.; Kondo, Y. Bibcode: 1984A&A...141..304D Altcode: Two high-resolution near-ultraviolet spectra of the supergiant α Cyg have been analyzed with the aim of determining the average microturbulent line-of-sight velocity component which was found to be 15.0±0.5 km s-1. This value is close to the local sound velocity (13.7 km s-1). This result, together with previous determinations in the visual spectral region, is consistent with the picture that shock waves are the dominant structure in the outer photosphere of Deneb. The macroturbulent velocity component is ≈10 km s-1, a factor two to four smaller than earlier communicated values for the line of sight components of the stochastic macroturbulent velocities. Title: Solar magnetohydrodynamics; Symposium review. Authors: De Jager, C. Bibcode: 1984ESASP.220..287D Altcode: 1984ESPM....4..287D The overall structure of the Sun, solar magnetic fields, and the corona are reviewed. Radial variation of solar density and angular rotational velocity; internal mixing in the Sun; solar granulation; magnetic convection; umbral oscillations in sunspots; solar cycle variations; magnetic flux tubes; dynamo theory; coronal heating; coronal mass ejection; and solar activity are covered. Title: The stability limit of hypergiant photospheres. Authors: de Jager, C. Bibcode: 1984A&A...138..246D Altcode: The author examines the consequences of the hypothesis that the stability limit of the atmospheres of hypergiants is reached when the effective acceleration of gravity geff = ggrav(1 - Γ)-gturb ≈ 0, where ggrav = GM/R2, gturb is the outward acceleration due to the gradient of turbulent pressure, and Γ is the Eddington ratio |grad/ggrav|. In stars close to the stability limit dissipation of mechanical energy builds up a field of supersonic turbulent motions in the photosphere which tends to destabilize the atmosphere. The larger the absolute luminosity, the deeper in the photosphere dissipation becomes important. This manifests itself in a rate of mass-loss, increasing with L. For near-unstable stars with Te ≈ 104K the rate of mass-loss is predicted to be nearly by a factor ten larger for a star brighter by one magnitude. This effect greatly changes the evolutionary tracks and explains why brighter stars have not been detected. Title: The Envelope of the Over-contact Binary AW UMa Authors: de Jager, C. Bibcode: 1984iue..prop.2017D Altcode: No abstract at ADS Title: Spatial Development of X-Ray Emission during the Impulsive Phase of a Solar Flare Authors: de Jager, C.; Boelee, A.; Rust, D. M. Bibcode: 1984SoPh...92..245D Altcode: The flare of 11 November, 1980, 17∶25 UT occurred in a magnetically complex region. It was preceded by some ten minutes by a gradual flare originating over the magnetic inversion line, close to a small sunspot. This seems to have triggered the main flare (at 70 000 km distance) which originated between a large sunspot and the inversion line. The main flare started at 17∶23∶20 UT with a slight enhancement of hard X-rays (E > 30 keV) accompanied by the formation of a dark loop between two Hα bright ribbons. In 3-8 keV X-rays a southward expansion started at the same time, with ν ∼- 500 km s −1. At the same time a surge-like expansion started. It was observable slightly later in Hα, with southward velocities of ≳ 200 km s−1. The dark Hα loop dissolved at ∼ 17∶24 UT at which time several impulsive phenomena started such as a complex of hard X-ray bursts localized in a small area. At the end of the impulsive phase at 17∶25∶40 UT, a coronal explosion occurred directed southward with an initial expansion velocity of ∼ 1800 km s−1, decreasing in 40 s to ∼500 km s−1. Title: Impulsive Phase Heating and a Coronal Explosion in a Solar Flare Authors: de Jager, C.; Boelee, A. Bibcode: 1984SoPh...92..227D Altcode: The flare of 12 November 1980, 02∶50 UT, in Active Region 2779 (NOAA classification) was studied by using X-ray images obtained with the Hard X-Ray Imaging Spectrometer aboard NASA's Solar Maximum Mission. In a ten-minute period, between about 02∶44 and 02∶54 UT, some five short-lived impulsive bursts occurred. We found that the so-called `hard' bursts (≳ 15 keV) are also detectable in low energy images. During that 10 min period - the impulsive phase - the heat input into the flare and the total number of energetic electrons increased practically exponentially, to reach their maximum values at 02∶54 UT. At the end of that period, when the thermal energy content of the flare was largest, a burst was observed, for the first time, to spread in a broad southern direction from an initially small area with a speed of about 50 km s−1. We have called this phenomenon a coronal explosion. Title: Book reviews Authors: van der Hucht, K. A.; Pedersen, A.; Kesák, Ľ.; Zwaan, C.; Fárník, F.; Kovalevsky, J.; van Beek, H. F.; Mewe, R.; Page, D. E.; van Genderen, A. M.; Koch-Miramond, L.; de Jager, C.; Ooms, G.; Pedersen, Arne; de Jager, C.; Hultqvist, Bengt; Waters, R.; Schrijver, J. Bibcode: 1984SSRv...38..179V Altcode: No abstract at ADS Title: Investigating the Internal Structure and Evolution of Massive Stars by Space Techniques Authors: de Jager, C. Bibcode: 1984srps.conf...47D Altcode: All massive stars (⪆15 M_sun;) are variable in luminosity (Δm ≈ 10-2mag) and radial velocity (Δv ≈ 1 km s-1), and have a large rate of mass loss. A precise determination of the spectrum of the vR- and m-variations with noise levels of ca. 10-3km s-1 and 10-5mag would allow for a direct determination of the internal structure and (hence) evolutionary status of stars with M ⪆ 5 M_sun;. If the above accuracy can be reached in one hour integration time, then a four-years observational period would yield the following results: a precise spectrum of pulsations for some 20 stars, allowing for an accurate determination of the internal structure and evolutionary phase; general information on the internal structure of about 400 stars. A lifetime of the Observatory of 12 years is suggested, which would yield detailed information on 60 and general data on 1200 stars. Title: The brightest stars. Authors: de Jager, C. Bibcode: 1984brst.book.....D Altcode: No abstract at ADS Title: Book-Review - Astrophotography - Techniques for the Amateur Authors: Martinez, P.; de Jager, C. Bibcode: 1984SSRv...38..184M Altcode: No abstract at ADS Title: The Accuracy Limit of Groundbased Stellar Photometry Authors: Heintze, J. R. W.; de Jager, C.; van der Veen, W. Bibcode: 1984srps.conf..157H Altcode: The ultimate accuracy that can be reached in ground-based stellar photometry depends primarily on the fluctuations of the atmosphere causing scintillations and variations of transparency, and besides on the instrumental stability and available integration time. Fluctuations in instrumental stability can be made negligibly small by basing the optical system and observational methods strictly on differential techniques. The integration time constitutes, together with variations in the atmosphere, the real limiting factor. Considerations based on available atmospheric and photometric observational data, yield that the present-days practical limiting accuracy of ground-based photometry is of the order of (3 - 7)×10-4mag for the observation of a 7th mag star by a one-meter telescope at an altitude of 4000 m and an integration time of 103s. Title: Shock-driven mass loss of stars. Authors: de Jager, C.; van den Oord, G. H. J. Bibcode: 1984mlao.conf...59D Altcode: The authors suggest that shocks, fed by multi-mode pulsational motion of the photospheres of supergiants are the basic mechanism for the mass loss of cool and medium type supergiants. They review the present theoretical situation, and describe the implementations of the observed transient mass loss component of the pulsating Beta Cephei stars BW Vul and σ Sco. Title: Energy input in solar flares and coronal explosions Authors: de Jager, C. Bibcode: 1984AdSpR...4g.303D Altcode: 1984AdSpR...4..303D A coronal explosion is a density wave observed in X-ray images of solar flares. The wave occurs at the end of the impulsive phase, which is the time at which the flare's thermal energy content has reached its maximum value. It starts in a small area from where it spreads out, mainly into one hemisphere, with velocities that tend to rapidly decrease with time, and which are between ~ 103 and a few tens of km s-1. We interpret them as magneto-hydrodynamic waves that (mainly) move downward from the low corona into denser regions. Title: Book-Review - a Field Guide to the Stars and Planets Authors: Menzel, D. H.; Pasachoff, J. M.; de Jager, C. Bibcode: 1984SSRv...38..185M Altcode: No abstract at ADS Title: Book-Review - 8TH International Conference on Numerical Methods in Fluid Dynamics Authors: Krause, E.; de Jager, C. Bibcode: 1983SSRv...36..429K Altcode: No abstract at ADS Title: Book-Review - Spatial Telecommunications - Part Two - Spatial Sector Authors: de Jager, C. Bibcode: 1983SSRv...36..423D Altcode: No abstract at ADS Title: Book-Review - Computational Methods for Fluid Flow Authors: Peyret, R.; Taylor, T. D.; de Jager, C. Bibcode: 1983SSRv...36..430P Altcode: No abstract at ADS Title: Book-Review - Landolt-Bornstein Numerical Data and Functional Relationship in Science and Technology Authors: Schaifers, K.; Voigt, H. H.; de Jager, C. Bibcode: 1983SSRv...36..420S Altcode: No abstract at ADS Title: Book-Review - I.A.U. General Assembly - 18TH - Proc. Authors: West, R. M.; de Jager, C. Bibcode: 1983SSRv...36..430W Altcode: No abstract at ADS Title: Enhanced X-Ray Emission above 3.5-KEV in Active Regions in the Absence of Flares Authors: Schadee, A.; de Jager, C.; Svestka, Z. Bibcode: 1983SoPh...89..287S Altcode: We demonstrate that even in the absence of flares there are very often volumes of hot plasma in the corona above active regions with temperatures in excess of 10 million degrees. Characteristics of this hot plasma and its time variations seem to be different in active regions of different phase of development. These hot plasma regions are sources of very weak, but clearly recognizable, X-ray emission above 3.5 keV. Long-lived X-ray brightenings, 104 times weaker than a flare, but lasting up to 10 hr occur predominantly along the H∥ = 0 line, apparently low in the corona. After major flares, long-lived X-ray emission is also radiated from tops of arches extending high into the corona. Some other long-lived sources, far from the H∥ = 0 line, may be associated with newly emerging flux. Short-lived X-ray sources, with fluxes ranging from subflare levels to 10−3 times the flare flux, last for 2 to more than 30 min and are probably microflares. They seem to be most frequent in growing young active regions and appear often in areas with newly emerging flux. Title: Book-Review - Spatial Telecommunications - Part Three - Earth Sector Systems of Telecommunications by Satellite Authors: de Jager, C. Bibcode: 1983SSRv...36..429D Altcode: No abstract at ADS Title: Book-Review - Numerical and Physical Aspects of Aerodynamic Flows Authors: Cebeci, T.; de Jager, C. Bibcode: 1983SSRv...36..430C Altcode: No abstract at ADS Title: Book reviews Authors: Kleczek, J.; Nussbaumer, H.; van der Hucht, K. A.; De Greve, J. P.; Ooms, G.; Rutten, R. J.; van der Laan, H.; Jäger, F. W.; Reijnen, G. C. M.; Bijleveld, W.; Kistemaker, J.; de Jager, C.; Mustel, E. R.; Ne'Eman, Y.; Priest, E. R.; Stiller, H.; Seifert, W.; Namba, O.; Kuperus, M.; Hoekstra, Roel; Stumpers, F. L. H. M.; Frank, S.; Zimmerman, J. T. F.; De Loore, C.; Gendrin, R.; Schrijver, J.; Mulder, P. S.; Pounds, K. A.; Young, R. S.; Houziaux, L.; Engvold, O.; Bok, B. J.; de Graaff, W. Bibcode: 1983SSRv...36..415K Altcode: No abstract at ADS Title: Radio, X-ray, and optical observations of the flare of June 13, 1980, at 6h22m UT Authors: Kattenberg, A.; Allaart, M.; de Jager, C.; Schadee, A.; Schrijver, J.; Shibasaki, K.; Švestka, Z.; van Tend, W. Bibcode: 1983SoPh...88..315K Altcode: A subflare of importance Sf was observed on June 13, 1980 simultaneously by instruments aboard the Solar Maximum Mission (SMM) and various ground based observatories. We describe and compare different kinds of observations, with emphasis on the Hard X-Ray Imaging Spectrometer (HXIS) images and spectra, and on the one-dimensional microwave images with high time and spatial resolution, obtained with the Westerbork Synthesis Radio Telescope (WSRT). The fast electrons causing the X-ray and microwave impulsive bursts had a common acceleration source, but the burst were produced at the opposite footpoints of the loops involved, with microwaves emitted near to a sunspot penumbra. The flare (of a `compact' type) was probably triggered by an emerging flux, and two possible interpretations of this process are briefly discussed. Title: The BUSS spectrum of beta Lyrae. Authors: Hack, M.; Sahade, J.; de Jager, C.; Kondo, Y. Bibcode: 1983A&A...126..115H Altcode: The spectrum of Beta Lyrae from about 1975 to 3010 A taken with the Balloon-borne ultraviolet Stellar Spectrograph experiment in May 1976 at phase 0.61 P is analyzed. Results show the presence of N II semi-forbidden emission and provide evidence for about the same location, in the outer envelope of the system, of the layers responsible for the resonance Mg II doublet emissions and for the "narrow" H-alpha emission. In addition, three sets of absorption lines, P Cygni profiles of Fe III and broad Beals Type III emissions of Mg II, are found to be present. Title: Book reviews Authors: Namba, O.; Kuijpers, Jan; De Loore, C.; Roody, R. M.; Marlborough, J. M.; Rasool, S. I.; Kovalesky, J.; van der Kruit, P. C.; Kleczek, J.; de Jager, C.; Lewis, A.; Hovenier, J. W.; Grewing, M. Bibcode: 1983SSRv...35..293N Altcode: No abstract at ADS Title: Development of flare morphology in X-rays, and the flare scenario Authors: de Jager, C. Bibcode: 1983SoPh...86...21D Altcode: We define the impulsive phase of a flare as its first phase, characterized by: X-ray bursts of short (seconds to tens of seconds) duration, a patchy X-ray morphology, and injection of energy. It lasts some five to ten minutes. The gradual or diffuse phase starts virtually at the same time as the impulsive one and is characterized by a gradually varying X-ray flux from a larger, diffuse, area situated higher than the sources of the impulsive X-ray bursts. The diffuse cloud is initially (during the first five minutes) hotter by a few million degrees than the sources of the impulsive phase bursts and is assumed to be caused by convective motions with upward velocities of a few hundred km s−1. It contains about the same number of energetic electrons as the impulsive burst patches contained initially. It cools gradually down by radiative and conductive losses, a process that may last for about an hour. Title: The flares of April 1980. A case for flares caused by interacting field structures. Authors: Machado, M. E.; Somov, B. V.; Rovira, M. G.; de Jager, C. Bibcode: 1983SoPh...85..157M Altcode: We discuss the spatial and temporal characteristics of X-ray flares occurring in the active region NOAA2372 from April 6 to 13, 1980. The flares are seen to extend in most cases across the whole active complex, involving several magnetic features. They originate in an intermediate bipole, between the two main sunspots of the active region, where high magnetic shear was detected. A rapid expansion is seen in some cases, in conjunction with the start of the impulsive hard X-ray bursts. We also detect, in the late phases of some of the events, a large soft X-ray structure overlying the whole active region, which also shows up as a noise storm region at metric wavelengths. These large loops cool by heat conduction but, in some cases, Hα condensations seem to appear, probably as a result of magnetic compression and a condensation mode of the thermal instability. The topological aspects of the field configuration are discussed, in the context of flare models invoking magnetic reconnection at the site of the primary energy release. In such a model, the intermediate bipole is the natural site of initial magnetic reconnection, particle acceleration and heating. In one particular case of a flare observed at the limb, we find possible evidence of particle acceleration in a neutral sheet at the boundary between two clearly defined magnetic structures. Title: The Queen's Flare - its Structure and Development - Precursors Pre-Flare Brightenings and Aftermaths Authors: de Jager, Cornelis; Machado, Marcos E.; Schadee, Aert; Strong, Keith T.; Švestka, Zdeněk; Woodgate, Bruce E.; van Tend, W. Bibcode: 1983SoPh...84..205D Altcode: We continue previous research on the limb flare of 30 April, 1980, 20:20 UT, observed in X-rays by several instruments aboard the Solar Maximum Mission (SMM). It is shown quantitatively that the flare originated in an emerging magnetically confined kernel (diameter ∼ 20″) which existed for about ten to fifteen minutes, and from which energetic electrons streamed, in at least two injections, into a previously existing complicated magnetic loop system thus forming a less bright but extended and long-lived tongue. The tongue had a length of ∼ 35 000 km and lasted ∼ 90 min in X-rays (∼ 10 keV); at lower energies (∼ 0.7 keV) it was larger (∼ 80 000 km) and lasted longer. The total number of energetic electrons (≈ 1037) initially present in the kernel is of the same order as the number present in the tongue after the kernel's decline. This gives evidence that the energetic electrons in the tongue originated mainly in the kernel. The electron number densities in the kernel and tongue at maximum brightness were ∼ 4.5 × 1011 and ∼ 1 × 1011 cm#X2212;3, respectively. During the first eight minutes of its existence the tongue was hotter than the kernel, but it cooled off gradually. Its decline in intensity and temperature was exponential; energy was lost by radiation and by conduction through the footpoints of the loop system. These footpoints have a cross-section of only ∼ 3 × 106 km2. This small value, as well as photographs in a CIV UV emission line, suggests a highly filamentary structure of the system; this is further supported by the finding that the tongue had a `filling factor' of ∼ 10#X2212;2. Several faint X-ray brightenings (≲ 0.005 of the flare's maximum intensity) were observed at various locations along the solar limb for several hours before and after the flare. At ∼ 30 min before the flare's onset a faint (≲ 0.02) flare precursor occurred, coinciding in place and shape with the flare. First the kernel precursor was brightest but the tongue precursor increased continuously in brightness and was the brightest part of the precursor some 10-15 min after the first visibility of the kernel precursor, until the start of the main flare. This suggests (weak) continuous electron acceleration in the tongue during a period of at least 30 min. The main flare was caused by strong emergence of magnetic field followed by two consecutive field line reconnections and accelerations in a small loop system, causing footpoint heating. Subsequently plasma streamed (convectively) into a pre-existing system of larger loops, forming the tongue. Title: 3.5 keV X-ray Emission in Absence of Flares Authors: Schadee, A.; de Jager, C.; Svestka, Z. Bibcode: 1983BAAS...15R.704S Altcode: No abstract at ADS Title: Corrigendum: "The flares of April 1980. A case for flares caused by interacting field structures" [Sol. Phys., Vol. 85, No. 1, p. 157 - 184(1983)]. Authors: Machado, M. E.; Somov, B. V.; Rovira, M. G.; de Jager, C. Bibcode: 1983SoPh...89..233M Altcode: No abstract at ADS Title: Book-Review - Astrophysics from Spacelab Authors: Bernacca, P. L.; Ruffini, R.; de Jager, C. Bibcode: 1983SSRv...35R.297B Altcode: No abstract at ADS Title: The ISPM solar flare X-ray and cosmic gamma-ray burst experiment. Authors: Cotin, F.; de Jager, C.; Henoux, J. -C.; Heise, J.; Hilhorst, M.; Hurley, K.; Niel, M.; Paschmann, G.; Sommer, M.; van Rooijen, J.; Vedrenne, G. Bibcode: 1983ESASP1050..209C Altcode: The ISPM solar flare X-ray and cosmic gamma-ray burst instrument (designated the HUS experiment) consists of a pair of Cs I scintillators and a pair of Si surface barrier detectors for measuring X-rays in the ranges 15 - 150 and 5 - 15 keV, respectively. High-time-resolution X-ray measurements (up to 8 ms) will be used to study X- and gamma-ray bursts of solar and cosmic origin. Title: Book-Review - Reports on Astronomy Authors: Waymen, P. A.; de Jager, C. Bibcode: 1983SSRv...35..297W Altcode: No abstract at ADS Title: Mass Loss from astronomical objects; a summary. Authors: de Jager, C. Bibcode: 1983HiA.....6..603D Altcode: A summary of recent developments in the following research fields is given: (1) stellar mass loss data and possible mechanisms; (2) mass loss for special types of stars; (3) interaction with the interstellar medium; (4) mass loss from quasars. Title: Space observations of solar flares Authors: De Jager, C. Bibcode: 1983ebro.conf...63D Altcode: Space observations of flares are shown to help in establishing current theories on solar flares. The hard X-ray imaging spectrometer used in the Solar Maximum Mission is described. Observations with the instrument determine the location, morphology and energy spectra of high energy emissions which helps to solve the problem of flare models and mechanisms. Title: Origin and development of solar flares. Authors: de Jager, C. Bibcode: 1983MeBel..45....1D Altcode: No abstract at ADS Title: Space observations of solar flares. Authors: de Jager, C. Bibcode: 1983POEM...14...63D Altcode: A decision between current theories on solar flares can probably be made by X-ray observations of the morphology and spectrum of flares. The author summarizes how space observations of flares have helped to establish current theories on solar flares and describes the instrument HXIS (Hard X-ray Imaging Spectrometer) in the Solar Maximum Mission, launched February 14, 1980. Title: Origin and development of solar flares Authors: de Jager, C. Bibcode: 1983HiA.....6...53D Altcode: A review is given of knowledge on solar flares with particular emphasis on progress made during the international Solar Maximum Year (1979-1981). The pre-flare structure is described by a flux-tube or circuit model. The instability leading to a flare may occur by a disturbance of the field topology (mostly field emergence) followed by field-line reconnection (circuit coupling). In the first (impulsive) phase of a flare, this causes jets of energetic electrons originating near the top of the flux tube to bombard lower chromospheric regions: footpoint heating. In the second (gradual or diffuse) phase, heated gas from the footpoints ascends convectively, producing a large cloud of hot gas. Consequent shock-wave phenomena cause moving fronts and associated waves in the high parts of the corona; these show up in coronagraphic or radio-observations. Several hours after a large flare, extended loop-like structures can appear, with temperatures greater than 6 MK, emitting a very faint X-ray flux. They are the basic structures of a more extended configuration, visible on metric radio waves, and pointing out from the area where the flare occurred. They may extend to distances of 10 to the 6th km or more from the solar surface. Title: Book-Review - Revealing the Universe Authors: Cornell, J.; Lightman, A. P.; de Jager, C. Bibcode: 1983SSRv...35..297C Altcode: No abstract at ADS Title: Book reviews Authors: Ne'Eman, Y.; Zahn, J. -P.; Habing, H. J.; Wittenberg, H.; Zwaan, C.; Murray, C. A.; de Jager, Cornelis; Kresák, L. Bibcode: 1982SSRv...33..459N Altcode: No abstract at ADS Title: Book reviews Authors: de Jager, C.; Sehnal, L.; Kleczek, Josip; Vanysek, V.; Quenby, J. J.; van Duinen, R. J.; van Gent, R. H.; Kaastra, J. S.; Ringeard, G.; Mewe, R.; Stiller, H.; Wäsch, R.; Wenzel, K. -P.; Vrijer, A.; Ceplecha, Z.; Hellings, P.; Houziaux, L. Bibcode: 1982SSRv...31..453D Altcode: No abstract at ADS Title: The Photospheric Velocity Field of Procyon Authors: de Jager, C.; Neven, L. Bibcode: 1982Ap&SS..84..297D Altcode: BUSS observations of the profiles of two well observed spectral lines in the ultraviolet spectrum of αCMi (Procyon; F5 IV V) are analysed with a Fourier transform method in order to determine values of various parameters of the velocity field of the upper photosphere. We find a microturbulent line-of-sight velocity componentL μ = 0.9 ± 0.4 km s-1, a macroturbulent velocity componentL M = 5.3 ± 0.2 km s-1, and a rotational velocity componentv R sini=10.0±1.2 km s-1. In these calculations a single-moded sinusoidal isotropic macroturbulent velocity function was assumed. The result appears to be sensitive to the assumed shape of the macroturbulence function: for an assumed Gaussian shape the observations can be described withv R sini=4 km s-1 andL M = 11.6 ± 2.7 km s-1. A comparison is made with other results and theoretical predictions. Title: Review of galactic ultraviolet astronomy. Authors: de Jager, C. Bibcode: 1982ESASP.176...43D Altcode: 1982IUE3r.......43D; 1982iue..conf...43D Results of IUE investigations of: chromospheres, coronas and winds of cool stars; hot stars and supernovae; binaries and novae; mass loss and the late evolution of single massive stars, are summarized. Stars in the pre-main sequence phase; interstellar matter; and the galactic halo, were also studied. Title: The pulsation of the outer layers of the BET CEP star SIG Sco. Authors: Burger, M.; de Jager, C.; van den Oord, G. H. J. Bibcode: 1982A&A...109..289B Altcode: The pulsation of the outer layers of the Beta Cephei-type variable σ Sco is investigated using 17 ultraviolet spectrograms (1200-2000 A) obtained during seven hours of observations with the International Ultraviolet Explorer. A pulsational model established earlier for BW Vul is confirmed. According to this model, after a period of photospheric rest the outer layers expand suddenly. The acceleration for the outermost layers, investigated with the resonance lines of C IV and Si IV, is found to be larger than for the photosphere. During the phase of deceleration that follows, no falling down of the C IV absorbing layers is observed; their velocity component remains directed outward. A transient stellar wind component sets in just before the star has its minimum radius (phase, approximately 0.9); it is strongest at a phase of approximately 0.2. The observations are interpreted as suggesting the mechanism of pulsational driven mass loss. Title: Stellar Atmospheric Velocity Fields - the Beta-Cephei Variables Gamma-Pegasi and Beta-Cephei Authors: de Jager, C.; Sato, N.; Burger, M.; Neven, L. Bibcode: 1982Ap&SS..83..411D Altcode: The shape parameters of a number of selected ultraviolet lines in BUSS-spectra of the Beta Cephei stars γ Peg and β Cep have been analyzed to determine the principal parameters of the atmospheric velocity field. We find for both stars a fairly high value (∼5 km s-1) for the microturbulent line-of-sight velocity component, which confirms an earlier result based on lower resolution UV spectra. Macroturbulent and rotational velocities are virtually zero in the atmosphere of γ Peg; for β Cep we findv rotsini=40 km s-1. Title: Comets Authors: Brandt, John C.; de Jager, C. Bibcode: 1982SSRv...31..119B Altcode: No abstract at ADS Title: The pulsation of the outer layers of the BET Cep-type variable BW Vul. Authors: Burger, M.; de Jager, C.; van den Oord, G. H. J.; Sato, N. Bibcode: 1982A&A...107..320B Altcode: Eleven high-resolution ultraviolet spectrograms of BW Vul were obtained by means of the IUE over 1.6 periods to compare the pulsation of this star as observed in the strong far-UV resonance lines with observations of the UV lines of photospheric origin. The radial velocity curve of the C IV lines is different from the photospheric one, showing that both the photosphere and the C IV layers are accelerated upwards impulsively. After about one hour, the acceleration decreases to zero, and then the atmosphere falls with a nearly constant downward acceleration of 2100 cm/sq s. The varying asymmetry indicates mass loss due to the pulsation of the atmosphere, superimposed on the continuous mass loss of the star. Title: Book Reviews Authors: Shaviv, G.; de Jager, C.; De Loore, C.; Fricke, W.; Kleczek, J.; van Zolingen, R. J. C.; Vardya, M. S.; Wöhl, H.; Tauber, G.; de Jong, T.; de Graaff, W. Bibcode: 1982SSRv...31..119S Altcode: No abstract at ADS Title: Space Science Comes of Age Authors: Hanle, P.; Chamberlain, V. D.; de Jager, C. Bibcode: 1982SSRv...31..119H Altcode: No abstract at ADS Title: Book-Review - the Brightest Stars Authors: de Jager, C.; Hutchings, J. B. Bibcode: 1982Natur.295..632D Altcode: No abstract at ADS Title: Book-Review - Stars and Star Clusters Authors: Voigt, H. H.; Schaifers, K.; de Jager, C. Bibcode: 1982SSRv...33R.461V Altcode: No abstract at ADS Title: Observations of a POST Flare Radio Burst in X-Rays Authors: Svestka, Z.; Hoyng, P.; van Tend, W.; Boelee, A.; de Jager, C.; Stewart, R. T.; Acton, L. W.; Bruner, E. C.; Gabriel, A. H.; Rapley, C. G.; de Jager, C.; LaFleur, H.; Nelson, G.; Simnett, G. M.; van Beek, H. F.; Wagner, W. J. Bibcode: 1982SoPh...75..305S Altcode: More than six hours after the two-ribbon flare of 21 May 1980, the hard X-ray spectrometer aboard the SMM imaged an extensive arch above the flare region which proved to be the lowest part of a stationary post-flare noise storm recorded at the same time at Culgoora. The X-ray arch extended over 3 or more arc minutes to a projected distance of 95 000 km, and its real altitude was most probably between 110 000 and 180 000 km. The mean electron density in the cloud was close to 109 cm−3 and its temperature stayed for many hours at a fairly constant value of about 6.5 × 106 K. The bent crystal spectrometer aboard the SMM confirms that the arch emission was basically thermal. Variations in brightness and energy spectrum at one of the supposed footpoints of the arch seem to correlate in time with radio brightness suggesting that suprathermal particles from the radio noise regions dumped in variable quantities into the low corona and transition layer; these particles may have contributed to the population of the arch, after being trapped and thermalized. The arch extended along the H∥ = 0 line thus apparently hindering any upward movement of the upper loops reconnected in the flare process. There is evidence from Culgoora that this obstacle may have been present above the flare since 15-30 min after its onset. Title: Shock-driven Mass Loss of Stars Authors: de Jager, C.; van den Oord, G. H. J. Bibcode: 1982mlao.conf...59D Altcode: No abstract at ADS Title: The Brightest Stars Authors: de Jager, C.; Vardya, M. S. Bibcode: 1982SSRv...31..121D Altcode: No abstract at ADS Title: Looking ahead after the termination of the solar Authors: de Jager, Cornelis Bibcode: 1982AdSpR...2k...1D Altcode: 1982AdSpR...2....1D No abstract at ADS Title: Detection of Hydrodynamical Flow in and Around Algol Binaries Authors: de Jager, C. Bibcode: 1982iue..prop.1300D Altcode: No abstract at ADS Title: Book Review: Atlas of Photomicrographs of the Surface Structures of Lunar Regolith Particles Authors: Rode, O.; Ivanov, A.; Nazarov, M.; Cimbálniková, A.; Jurek, K.; Hejl, V.; de Jager, C. Bibcode: 1982SSRv...31..453R Altcode: No abstract at ADS Title: Book-Review - Landolt-Bornstein Numerical Data in Science and Technology Authors: Schaifers, K.; Voigt, H. H.; de Jager, C. Bibcode: 1982SSRv...32..464S Altcode: No abstract at ADS Title: Book-Review - the Brightest Stars Authors: de Jager, C.; Marlborough, J. M. Bibcode: 1981Obs...101..217D Altcode: No abstract at ADS Title: Book-Review - the Brightest Stars Authors: de Jager, C.; Tutukov, A. V. Bibcode: 1981SvA....25..750D Altcode: No abstract at ADS Title: Reflections on Solar Variability Authors: de Jager, C. Bibcode: 1981SoPh...74...11D Altcode: The Sun is a variable star in many respects: there are secular varitions related to the general solar evolution, and - as discovered in recent years - there are many short period variations of which the 5 min and 160 min pulsations are the most important ones. Magnetic fields in the outer convective mantle play a dominant role in the 22-years cycle. The origin of solar magnetic variability is a key problem in astrophysics. The relation between solar variability and changes in the Earth's climate is a crucial element in climatology. Title: Origin and Location of the Hard X-Ray Emission in a Two-Ribbon Flare Authors: Hoyng, P.; Duijveman, A.; Machado, M. E.; Rust, D. M.; Svestka, Z.; Boelee, A.; de Jager, C.; Frost, K. T.; Lafleur, H.; Simnett, G. M.; van Beek, H. F.; Woodgate, B. E. Bibcode: 1981ApJ...246L.155H Altcode: No abstract at ADS Title: The limb flare of 1980 April 30 as seen by the hard X-ray imaging spectrometer Authors: van Beek, H. F.; de Jager, C.; Schadee, A.; Svestka, Z.; Boelee, A.; Duijveman, A.; Galama, M.; Hoekstra, R.; Hoyng, P.; Fryer, R.; Simnett, G. M.; Imhof, J. P.; LaFleur, H.; Maseland, H. V. A. M.; Mels, W. M.; Schrijver, J.; van der Laan, J. J. M.; van Rens, P.; van Tend, W.; Werkhoven, F.; Willmore, A. P.; Wilson, J. W. G.; Machado, M. E.; Zandee, W. Bibcode: 1981ApJ...244L.157V Altcode: X-ray imaging of the limb event of 1980 April 30 shows that the flaring involved two distinct components: a pointlike component, which was the source of the initial hard X-ray burst and an extensive tongue reaching some 30,000 km above the limb. The tongue had a higher temperature than the other parts of the structure and seemed to be enhanced by energetic electrons that derived their energy from the initial source. Title: Multicolor linear polarimetry of Betelgeuse and Antares Authors: Tinbergen, J.; Greenberg, J. M.; de Jager, C. Bibcode: 1981A&A....95..215T Altcode: Multicolor linear polarimetry of Alpha Ori and Sco confirms in a qualitative way the ideas about large-scale moving elements in these stellar atmospheres, as suggested by Schwarzschild (1975). For one well-observed scattering element, a relation between the degree of ionization and the fraction of silicon condensed in grains is deduced. Sizable residuals remain at short and long wavelengths. Time variations of the short-wavelength residuals suggest time variations in the dust formed in the stellar atmospheres. Title: Hard X-ray imaging of two flares in active region 2372 Authors: Hoyng, P.; Duijveman, A.; Boelee, A.; de Jager, C.; Galama, M.; Hoekstra, R.; Imhof, J.; Lafleur, H.; Machado, M. E.; Fryer, R. Bibcode: 1981ApJ...244L.153H Altcode: Hard X-ray images of two flares observed by the Hard X-ray Imaging Spectrometer (HXIS) aboard SMM on 1980 April 7 and 10 are discussed. A comparison with H-alpha images and the photospheric magnetic field maps shows that the emission originates in (arcades of) loops which differ greatly in the hardness of the X-ray spectra. On April 7 the hardest X-ray emission coincided with the brightest H-alpha patch. On April 10 the most intense X-ray emission appeared to be concentrated in a looplike structure with a softer spectrum at the top and a harder spectrum in the legs. Temperature estimates from flux ratios in different energy bands tend to confirm that small, hot components are embedded in more extensive, cooler flaring regions. Title: Book reviews Authors: Page, D. E.; Pick, M.; Kresák, L.; Reijnen, G. C. M.; Cameron, A. G. W.; Schuurmans, Cornelius J. E.; Ya Marov, Mikhail; Goldberg, B.; Kaiser, T. R.; Lemaire, J.; Sehnal, L.; Elsässer, H.; Valniček, B.; de Graaff, W.; Pacini, Franco; Rohlfs, K.; de Landtsheer, A. C.; Zhongolovich, I. D.; de Jager, C.; Greve, A.; Ruprecht, J.; Tayler, R. J.; Kockarts, G.; Axford, W. I.; Tabor, H.; Bolle, H. J.; Kaldis, E.; Reijnen, Gijsbertha C. M.; De Loore, C.; Kleczek, J.; Murray, C. A.; Wallenquist, Åke Bibcode: 1981SSRv...28..105P Altcode: No abstract at ADS Title: Introduction Authors: De Jager, Cornelis; Švestka, Zdeněk Bibcode: 1981SoPh...72D...7D Altcode: No abstract at ADS Title: Book-Review - the Brightest Stars Authors: de Jager, C. Bibcode: 1981Natur.289Q.733D Altcode: No abstract at ADS Title: Book Review - Transactions of the IAU V.17B Authors: Wayman, P. A.; de Jager, C. Bibcode: 1981SSRv...28..118W Altcode: No abstract at ADS Title: Book Review - Journal of Astrophysics and Astronomy Authors: Ramaeshan, S.; de Jager, C. Bibcode: 1981SSRv...28..112R Altcode: No abstract at ADS Title: Book-Review - the Brightest Stars Authors: de Jager, C.; Smak, J. Bibcode: 1981AcA....31..393D Altcode: No abstract at ADS Title: Book Review - Nonradial and Nonlinear Stellar Pulsations Authors: Hill, H. A.; Dziembowski, W. A.; de Jager, C. Bibcode: 1981SSRv...28Q.112H Altcode: No abstract at ADS Title: Book-Review - the Brightest Stars Authors: de Jager, C.; Kriz, S. Bibcode: 1981BAICz..32..318D Altcode: No abstract at ADS Title: Structural development of the X-ray limb flare of 30 April 1980 Authors: de Jager, C.; Hoyng, P.; Lafleur, H.; Schadee, A.; Svestka, Z.; van Beek, H. F.; van Tend, W.; Fryer, R.; Simnett, G. M. Bibcode: 1981AdSpR...1m.251D Altcode: 1981AdSpR...1..251D We describe the development of the limb flare of 30 April 1980, 20:20 UT, as observed by the Hard X-ray Imaging Spectrometer (HXIS) aboard the Solar Maximum Mission (SMM). It consisted of a short-lived bright nucleus (FWHM < 10,000 km), just inside the Sun's limb; a longer lasting tongue, extending to a height of ~ 30,000 km, and a more complicated feature, approximately situated at the Sun's limb. The tongue was a pre-existing magnetic structure that started emitting X-rays only a few seconds after the bright nucleus, and which had a slightly higher temperature than the nucleus; its X-ray emission may be caused by electrons escaped from the nucleus. Title: Solar maximum mission experiment: Early results of the hard X-ray imaging experiment Authors: Boelee, A.; de Jager, C.; Duijveman, A.; Galama, M.; Hoekstra, R.; Hoyng, P.; Imhof, J. P.; Lafleur, H.; Maseland, H. V. A. M.; Mels, W. A.; Schadee, A.; Schrijver, J.; Svestka, Z.; van Beek, H. F.; van Rens, P.; van der Laan, J. J. M.; van Tend, W.; Werkhoven, F.; Wiersma, G.; Zandee, W.; Simnett, G. M.; Charlton, C. P.; Fryer, R.; Willmore, A. P.; Wilson, J. W. G.; Machado, M. E. Bibcode: 1981AdSpR...1m.255B Altcode: 1981AdSpR...1R.255B We have selected four widely different flares from the early period of operations of the Hard X-Ray Imaging Spectrometer (HXIS) on SMM to illustrate the characteristic imaging properties of this experiment. For the small flare of April 4, 1980, we demonstrate the instrument's capability for locating a compact source. In the weak, but extensive, flare of April 6 we show how well the instrument can display spatial structure, and also the low level of the instrument background. In the 1B flare of April 7 we are able to locate positions of the X-ray emission in the soft and hard channels, and estimate the positional variations of the emission patches. Finally, in the IN flare of April 10, which produced the strongest hard X-ray burst we have seen so far, we repeat some of the studies made for the April 7 event, and also demonstrate the capability of the HXIS instrument to study the development, with high time resolution, of individual 8'' × 8'' elements of the flare. Title: A relation between solar activity and winter temperatures in Holland between 1634 and 1975. Authors: de Jager, C. Bibcode: 1981PRNAA..84..457D Altcode: 1981RNAAS..84..457D No abstract at ADS Title: Book-Review - the Brightest Stars Authors: de Jager, C.; Mihalas, D. Bibcode: 1981S&T....61..432D Altcode: No abstract at ADS Title: Solar Maximum Mission experiment - Early results of the hard X-ray imaging experiment Authors: Simnett, G. M.; Charlton, C. P.; Fryer, R.; Boelee, A.; de Jager, C.; Duijveman, A.; Galama, M.; Hoekstra, R.; Hoyng, P.; Imhof, J. P. Bibcode: 1981AdSpR...1m.255S Altcode: 1981AdSpR...1..255S Four widely different flares from the early period of operations of the Hard X-ray Imaging Spectrometer (HXIS) on SMM have been selected to illustrate the characteristic imaging properties of this experiment. For the small flare of April 4, 1980, the instrument's capability for locating a compact source is demonstrated. In the weak, but extensive, flare of April 6, the ability of the instrument to display spatial structure, and also the low level of the instrument background, are shown. In the 1B flare of April 7, positions of the X-ray emission in the soft and hard channels are capable of being located, and the positional variations of the emission patches can be estimated. Finally, in the 1N flare of April 10, which produced the strongest hard X-ray burst seen so far, some of the studies made for the April 7 event are repeated, and the capability of the HXIS instrument to study the development, with high time resolution, of individual 8 x 8 arcsec elements of the flare is also demonstrated. Title: Two years of observation with the international ultraviolet explorer Authors: de Jager, C.; Lamers, H. J. G. L. M. Bibcode: 1981AdSpR...1m.185D Altcode: 1981AdSpR...1..185D The launching of I.U.E., in January 1978, has provided the astronomical community with a powerful observatory-type spacecraft enabling one a practically continuous observation of ultra-violet spectra of celestial bodies. Two years of operation have led to many new developments, in which a large number of astronomers participated. This review summarizes a part of these developments. While refraining from a description of the results obtained in solar-system and extra-galactic research we discuss: early-type and Wolf-Rayet stars, medium- and late-type stars, planetary nebulae, novae and dwarf-novae, X-ray binaries, supernovae, the interstellar medium and the galactic halo. Title: Mass Loss from Massive Stars Authors: de Jager, C. Bibcode: 1981mms..conf...67D Altcode: 1982mms..conf...67D No abstract at ADS Title: Book Review - Solar and Interplanetary Dynamics IAU SYMP.#91 Authors: Dryer, M.; Tandberg-Hanssen, E.; de Jager, C. Bibcode: 1981SSRv...28..114D Altcode: No abstract at ADS Title: Observation of the Dynamical State of the Outer Atmospheres of Beta Cephei Stars Authors: de Jager, C. Bibcode: 1981iue..prop..894D Altcode: No abstract at ADS Title: Book Review - Space Art Authors: Muller, R.; de Jager, C. Bibcode: 1981SSRv...28..117M Altcode: No abstract at ADS Title: Book Review - Space Research V.20 Authors: Rycroft, M. J.; de Jager, C. Bibcode: 1981SSRv...28..111R Altcode: No abstract at ADS Title: Book-Review - the Brightest Stars Authors: de Jager, C.; Scholz, G. Bibcode: 1981AN....302..320D Altcode: No abstract at ADS Title: The Outer Layers of the Beta-Cephei Type Variables Bw-Vulpeculae and Omicron-Scorpii Authors: Burger, M.; de Jager, C.; van den Oord, G. H. J.; Groupe Etoiles Variables de L'Observatoire de Nice Bibcode: 1981pbs..work..181B Altcode: No abstract at ADS Title: Book Review - Stellar Turbulence IAU Colloquium no. Authors: Gray, D. F.; Linsky, J. L.; de Jager, C. Bibcode: 1981SSRv...28..113G Altcode: No abstract at ADS Title: Structural development of the X-ray limb flare of 30 April 1980. Authors: de Jager, C.; Fryer, R.; Hoyng, P.; Lafleur, H.; Schadee, A.; Simnett, G. M.; Svestka, Z.; van Beek, H. F.; van Tend, W. Bibcode: 1981hea..conf..251D Altcode: No abstract at ADS Title: Solar Maximum Mission experiment: early results of the hard X-ray imaging experiment. Authors: Simnett, G. M.; Boelee, A.; Charlton, C. P.; de Jager, C.; Duijveman, A.; Fryer, R.; Galama, M.; Hoekstra, R.; Hoyng, P.; Imhof, J. P.; Lafleur, H.; Machado, M. E.; Maseland, H. V. A. M.; Mels, W. A.; Schadee, A.; Schrijver, J.; Svestka, Z.; van Beek, H. F.; van Rens, P.; van der Laan, J. J. M.; van Tend, W.; Werkhoven, F.; Wiersma, G.; Willmore, A. P.; Wilson, J. W. G.; Zandee, W. Bibcode: 1981hea..conf..255S Altcode: No abstract at ADS Title: De zon een opmerkelijke ster. Authors: de Jager, C. Bibcode: 1981Zenit...8..242D Altcode: No abstract at ADS Title: Two years of observation with the International Ultraviolet Explorer. Authors: de Jager, C.; Lamers, H. J. G. L. M. Bibcode: 1981hea..conf..185D Altcode: No abstract at ADS Title: The ultraviolet spectrum of BET CMa stars. Authors: Burger, M.; de Jager, C.; Kamperman, T. M.; Neven, L. Bibcode: 1980A&A....90..170B Altcode: Ultraviolet spectra of 15 beta Canis Majoris stars in wavelength bands of approximately 100 A around 2100, 2500, and 2800 A (resolution 1.8 A), obtained with the Ultraviolet Stellar Spectrophotometer S 59 on board the ESRO TD-1A satellite are discussed. In general the spectra are similar to those of 'normal' stars, only the star alpha Vir has He I, C II, and Mg II lines slightly weaker than normal. Comparison with theoretical computations shows that the Fe abundance in the beta CMa stars is solar and that the average microturbulent velocity is about 4 km/sec. The UV spectral lines of beta Cep do not show any significant variations in equivalent width with phase. Title: Book reviews Authors: Guest, J. E.; Malin, Michael C.; Melchior, P.; Muller, C.; Namba, O.; Hultqvist, B.; Stoker, P. H.; Kleczek, Josip; Balder, E. J.; van der Sluis, A.; van de Stadt, H.; Larsson-Leander, Gunnar; Fiszdon, W.; de Groot, S. R.; de Boer, W. P. H.; Tennekes, H.; de Jager, Cornelis; Kleczek, J. Bibcode: 1980SSRv...26..447G Altcode: No abstract at ADS Title: The Solar Maximum Year and Related Space Experiments Authors: Svestka, Z.; de Jager, C. Bibcode: 1980SSRv...26..317S Altcode: The Solar Maximum Year is a world-wide cooperative project to gain more insight in certain aspects of solar flares. It consists of three sub-programs: The Flare Build-up Study (FBS), the Study of Energy Release from Flares (SERF), and the Study of Travelling Interplanetary Phenomena (STIP). These programs are described. We also describe space observations to be performed during SMY, particularly the Solar Maximum Mission Satellite. Title: Pulsation of the outer atmosphere of BW Vul. Authors: Burger, M.; de Jager, C. Bibcode: 1980ESASP.157...47B Altcode: 1980IUE2n.......47B; 1980iue..conf...47B Observations of the far UV spectrum of the beta Cephei variable BW Vul show that the pulsation consists of three phases: a phase of photospheric rest; a sudden upward acceleration of the photosphere and outer atmosphere (the maximum acceleration being approximately equal to the gravitational value); descent of the whole atmosphere with downward acceleration 1/4 of the gravitational value. At the end of the second phase a temporal stellar wind occurs with maximum velocity of 500 km s(-1). Title: Book reviews Authors: de Groot, T.; Vesseur, H. J. A.; Heintze, J. R. W.; de Jager, C.; Kleczek, J.; Tuominen, Jaakko; Bogus, K. Bibcode: 1980SSRv...25..431D Altcode: No abstract at ADS Title: Book-Review - the Sun Our Future Energy Source Authors: McDaniels, D. K.; De Jager, C. Bibcode: 1980SSRv...26R.453M Altcode: No abstract at ADS Title: Book-Review - Mass Loss and Evolution of O-Type Stars IAU SYMP.NO. 83 Vancouver-Island 1978JUN5-9 Authors: Conti, P. S.; De Loore, C. W. H.; de Jager, C. Bibcode: 1980SSRv...25...84C Altcode: No abstract at ADS Title: Book reviews Authors: Henrichs, H. F.; Szebehely, V.; Ponnamperuma, Cyril; de Jager, C.; Pfotzer, G.; Kovalevsky, J.; Habing, H. J. Bibcode: 1980SSRv...25...83H Altcode: No abstract at ADS Title: Book-Review - New Horizons in Astronomy ed. Authors: Brandt, J. C.; Maran, S. P.; de Jager, C. Bibcode: 1980SSRv...25Q..85B Altcode: No abstract at ADS Title: The brightest stars Authors: de Jager, C. Bibcode: 1980GAM....19.....D Altcode: Luminous stars of the upper part of the Hertzsprung Russell diagram are examined. The evolutionary relation between bright stars is discussed and emphasis is placed on the unity of the stars, notwithstanding the seeming diversity. The study deals with stars brighter than about 1000 solar luminosity and the structure of very tenuous stellar atmospheres is investigated. Stellar observations with high-resolution ultraviolet stellar spectroscopy are reported for features of near-instable stars such as stellar chromospheres, coronas, and extended shells. Title: Near-unstable supergiants. Authors: de Jager, C. Bibcode: 1980ESAJ....4..123D Altcode: The instability of massive supergiant stars is discussed in light of the absence of massive luminous cool supergiants. Consideration is given to the Eddington limit to stellar luminosities and the dissipation of turbulent energy by massive stars, and it is shown that these processes are insufficient to explain in the absence of red supergiants larger than about 25 solar masses. It is suggested that in the course of evolution towards low temperatures, massive supergiants retrace their steps on the Hertzsprung-Russel diagram and move to the region of higher photospheric temperatures. Such stars would become unstable as the ratio of specific heats approaches 4/3, and would begin to pulsate, as is observed in all stars more than approximately 10,000 times brighter than the sun. It is suggested that these instabilities would most likely originate in the convection layers just below the stellar surface, and would lead to the formation of stellar coronas as detected by ANS and the HEAO-2 satellites. Title: Book-Review - Catalogue of the Universe Authors: Murdin, P.; Allen, D.; Malin, D.; de Jager, C. Bibcode: 1980SSRv...25..432M Altcode: No abstract at ADS Title: Book-Review - Nonlinear Methods of Spectral Analysis Authors: Haykin, S.; De Jager, C. Bibcode: 1980SSRv...26..454H Altcode: No abstract at ADS Title: Grote Europese sterrenwacht in wording. Authors: de Jager, C. Bibcode: 1980Zenit...7..354D Altcode: No abstract at ADS Title: Uitzonderlijke vliegtuigwaarnemingen van de zonnekorona. Authors: de Jager, C. Bibcode: 1980Zenit...7..226D Altcode: No abstract at ADS Title: Wringende trillingen van het zonsoppervlak. Authors: de Jager, C. Bibcode: 1980Zenit...7..468D Altcode: No abstract at ADS Title: On the Seats of Elementary Flare Bursts Authors: de Jager, C. Bibcode: 1979SoPh...64..135D Altcode: We suggest to identify the elementary flare bursts with the excitation of the small kernels that occur in flare loops that are observed in soft X-ray pictures of flares. We stress the need of simultaneous observations of spatial structure and time variations of hard X-ray bursts sources in various wavelength regions. Title: Short time changes in the terminal velocity of the stellar wind of alpha Cam ((O9.5 Ia). Authors: de Jager, C.; Lamers, H. J. G. L. M.; Macchetto, F.; Snow, T. P. Bibcode: 1979A&A....79L..28D Altcode: The UV spectrum of the star Alpha Cam (09.5 Ia) was observed for 72 hours continuously in September 1978, with IUE. During these three days the star shows gradual short-term changes of the edge velocity of the C IV and N V resonance lines. The change is largest for the lines formed at large distance from the star, and may be due to either UV-flux variations of the star or variations in the mass loss rate (puffs). Evidence is also found for erratic very-short terms variations of the edge velocities. These are probably correlated with the ionization equilibrium: the ions with the highest ionization potential showing the largest variations. Title: Book-Review - Vistas in Astronomy V.22:3 Authors: Beer, A.; Beer, P.; de Jager, C. Bibcode: 1979SSRv...24..367B Altcode: No abstract at ADS Title: Book reviews Authors: Ooms, G.; Wittenberg, H.; de Jager, C.; Pinkau, K.; Lehr, A.; Rinner, K. Bibcode: 1979SSRv...24..367O Altcode: No abstract at ADS Title: Book reviews Authors: Gendrin, R.; Namba, O.; van Kampen, N. G.; de Jager, Cornelis; Forbes, E. G.; Kleczek, J.; Reijnen, G. C. M.; Genzel, R.; Smets, G.; Radelaar, R. Bibcode: 1979SSRv...24..253G Altcode: No abstract at ADS Title: Balloon-borne Ultraviolet Stellar Spectrograph. II. Highlights of first observational results. Authors: de Jager, C.; Kondo, Y.; Hoekstra, R.; van der Hucht, K. A.; Kamperman, T. M.; Lamers, H. J. G. L. M.; Modisette, J. L.; Morgan, T. H. Bibcode: 1979ApJ...230..534D Altcode: We describe a few of the most important features, visible in a first inspection of the high- resolution (0.1 A) mid-ultraviolet spectra ( A) of 33 stars obtained in two BUSS flights. The profiles of the Mg ii lines in early-type (B8-A2) supergiants show the existence of considerable mass flow, partly in irregular "puffs." The features in Mg ii in Betelgeuse are due to a cool expanding outer shell above a hotter chromospheric region. Emission features in the shell star Tau indicate infalling material, while the Be star 4) Per has a mass outflow. We have detected some 80 emission lines of Fe I, Fe ii, and Fe iii in spectra of late-type giants and supergiants. The composite spectrum of the binary a Sco (M1.5 lab + B2.5 V) is described, with particular reference to circumstellar lines. Subject headings: line identifications - stars: Be - stars: mass loss - stars: supergiants - ultraviolet: spectra Title: Balloon-borne Ultraviolet Stellar Spectrograph. I. Instrumentation and observation. Authors: Kondo, Y.; de Jager, C.; Hoekstra, R.; van der Hucht, K. A.; Kamperman, T. M.; Lamers, H. J. G. L. M.; Modisette, J. L.; Morgan, T. H. Bibcode: 1979ApJ...230..526K Altcode: A dual star-tracking system and a system including a telescope, an echelle spectrograph, and a SEC vidicon are the chief components of the Balloon-borne Ultraviolet Stellar Spectrograph (BUSS), which has flown four successful missions. The BUSS missions have yielded 81 spectra for 56 stars, recorded with a resolution of 0.1 A in the wavelength range from 2200 to 3400 A. BUSS observations include: profiles of Mg II lines indicating considerable mass flow in early-type supergiants; Mg II features suggesting a cool expanding outer shell above a hotter chromosphere; emission features in Zeta Tau (a shell star) indicating infalling material; and emission features of the Be star Phi Per suggesting mass outflow. Title: Book reviews Authors: Lequeux, J.; Reijnen, G. C. M.; Kleczek, Josip; Gray, D. F.; Dommanget, J.; Namba, O.; Verbunt, Frank; Savonije, Gertian; de Jager, Cornelis; van Bueren, H. G.; Hovenier, J. W.; Fokker, A. D.; Hoekstra, Roel; Hultqvist, Bengt; Kresák, L. Bibcode: 1979SSRv...23..683L Altcode: No abstract at ADS Title: Large Turbulent Elements in Supergiant Photospheres Authors: de Jager, C.; Vermue, J. Bibcode: 1979Ap&SS..62..245D Altcode: During the ‘cool phase’ of the super-supergiant HR 8752, which happened around 1973, when the star's spectral type was K2...K5 Ia+, the most probable vertical extent of the main turbulent elements in the star's photosphere was about 6 times the density scale height, which is about half the stellar radius. In early-type photospheres (class Ia) it is about 10 times the atmospheric density scale height (about 0.25 of the stellar radius), while in less extreme (luminosity class Ib) medium-type supergiants the most probable vertical extent of the elements is approx. 8 times the density scale height (≈0.05R). Large turbulent elements are apparently a common feature in supergiant photospheres; the more extreme the supergiant the larger the relative size of the eddies. Title: Macro- and Micro-Turbulent Filter Functions for Weak Lines in Stellar Atmospheres Authors: Vermue, J.; de Jager, C. Bibcode: 1979Ap&SS..61..129V Altcode: Following a similar discussion given earlier for the solar case (De Jager, 1972) we compute in this paper spectral line profiles for the spatial wavelengths in which a stellar motion field can be decomposed, and thereafter the macro-and micro-turbulent filter functionsf M(θk) andf μ(θk), where θ is the optical scale height andf 2(k) dk the fraction of the energy of the turbulent motions between wavenumbersk andk+dk of the spectrum of turbulence that contributes to either kind of turbulence. If micro-and macro-turbulent velocity components are known for a certain star, and if the spectrum of turbulence is sharp enough, the ratiof M/f μ would enable one to derive the average size of the turbulent elements in the star's atmosphere. The computations apply to weak lines in idealized stellar atmospheres, and refer to two cases: isotropic turbulence, and radial pulsations. These filters can be suitably used in a diagnostic method for the analysis of the motion field in the solar and stellar atmospheres. Some examples of applications to stars of very different kinds are given. Title: K. O. Kiepenheuer, 1910 - 1975. Authors: de Jager, C. Bibcode: 1979ssms.conf....1D Altcode: No abstract at ADS Title: Beta CMA Stars Authors: de Jager, C. Bibcode: 1979iue..prop..306D Altcode: No abstract at ADS Title: Stars and their coronae. Authors: de Jager, C. Bibcode: 1979xrgr.conf...17D Altcode: No abstract at ADS Title: Space Research, Vol._XIX. Proceedings of the open meetings of the working groups on physical sciences of the twenty-first plenary meeting of COSPAR, Innsbruck, Austria, 29_May_- 10_June 1978. Authors: Rycroft, M. J.; de Jager, C. Bibcode: 1979srvp.book.....R Altcode: No abstract at ADS Title: Het ruimte-onderzoek. Authors: de Jager, C. Bibcode: 1979plan.book...92D Altcode: No abstract at ADS Title: Nauwelijks stabiele reuzensterren. Authors: de Jager, C. Bibcode: 1979PKNAW..88..120D Altcode: No abstract at ADS Title: Mass Loss from Late Supergiant B-Stars and A-Stars Authors: Lamers, H. J. G. L. M.; Macchetto, F.; de Jager, C. Bibcode: 1979IUE1.symp..382L Altcode: No abstract at ADS Title: Short-Time Variations in P-Cygni Profiles of Early-Type Supergiants Authors: Lamers, H. J. G. L. M.; Snow, T. P.; Macchetto, F.; de Jager, C. Bibcode: 1979IUE1.symp..381L Altcode: No abstract at ADS Title: Global development of space research, 1977-1978 Authors: de Jager, C. Bibcode: 1979spre.conf....3D Altcode: 1979spre.proc....3D Various aspects of space research in the 1977-1978 period are reviewed. Attention is given to the following fields: material sciences, life sciences, geophysical measurements, remote sensing of earth resources, weather and climate, the magnetosphere, the solar wind and its interactions, solar, planetary, and stellar physics, and high energy astrophysics. Title: The Velocity Field in the Atmosphere of δ Cephei Authors: de Jager, C. Bibcode: 1978Ap&SS..59..165D Altcode: Observations related to the photospheric velocity field of δ Cephei can be interpreted as follows: during the whole cycle of pulsations the only motion form in the atmosphere is a wave motion with a nearly constant full amplitude of approximately 15 km s-1, and a wavelength of about 106 km (which are quantities, about equal to the amplitudes of pulsational velocity and radius of the star). There are no significant small-scale ‘turbulent’ velocity components. The ‘microturbulent’ and ‘macroturbulent’ velocities, as derived from spectral line observations, are fully compatible with this picture. Title: E. N. Parker first recipient of the George Ellery Hale Prize. Authors: De Jager, Cornelis; Švestka, Zdeněk Bibcode: 1978SoPh...60....3D Altcode: 1978SoPh...60....3. No abstract at ADS Title: Low-energy protons: Gradients in interplanetary space and distribution in the solar corona Authors: Stevens, G. A.; Vanrooijen, J. J.; Svestka, Z.; De Jager, C. Bibcode: 1978clus.nasa..234S Altcode: First and second order anisotropy measurements are proposed as a tool for studying the coronal source function and interplanetary propagation of low energy protons. Optimum orbit and attitude requirements are suggested for a three telescope system. Some limitations with regard to the lower energy limit for a feasible set-up are discussed. Title: Book reviews Authors: Roberts, Paul H.; Scholer, Manfred; de Jager, C.; Ceplecha, Zdeněk; Grewing, M.; Kresák, L.; Bumba, Václav; Pottasch, S. R.; Sehnal, L.; Pagel, Bernard; Reijnen, G. C. M.; Ness, Norman F.; de Jager, Cornelis Bibcode: 1978SSRv...22..213R Altcode: No abstract at ADS Title: Properties of elementary flare bursts. Authors: de Jager, C.; de Jonge, G. Bibcode: 1978SoPh...58..127D Altcode: From a study of eight hard X-ray flares, all with durations of less than five minutes, it is found that these flares can be completely decomposed into short-lived bursts, called Elementary Flare Bursts (EFB). For each one flare the individual EFB's have approximatively the same Full Width at Half Maximum (FWHM); average values range between 4 s (± 1 s) and 24 s (± 5 s) for the different flares. Yet there are significant differences between the FWHM's for the individual EFB's of a flare. The EFB's are slightly asymmetric; rise time is approx. 0.9 of the decay time. Their half-widths decrease with increasing photon energy, proportional to E-0.69±0.05. Title: Book reviews Authors: de Jager, Cornelis; Takens, R. J. Bibcode: 1978SSRv...22..115D Altcode: No abstract at ADS Title: Excited Cosmic Plasmas (Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978) Authors: de Jager, C. Bibcode: 1978Ap&SS..55..147D Altcode: We describe some aspects of the energetic radiations of high-energy cosmical plasmas in stellar environments, mainly stellar chromospheres and coronae, and solar and stellar flare-type phenomena. As far as possible we discuss the morphology and physics of these plasmas, and we speculate on their origin. This paper is a review, partly of a historical character, describing particularly some contributions from the Astronomical Institute at Utrecht to this field of astrophysical research. Title: Analysis of spectra of α CMi and α Cen A observed with the orbiting stellar ultraviolet spectrophotometer S59 in ESRO's TD1A satellite Authors: de Jager, C.; Kamperman, T. M.; Neven, L. Bibcode: 1978Ap&SS..54..343D Altcode: Ultraviolet spectra of α CMi and α Cen A taken with moderate spectral resolution (approx. 1.8 Å) are used to analyse whether a determination of stellar chemical abundances of Fe and Cr and of the photospheric parameters is possible. For αCMi, for which good spectral data are available, we findT eff=7660±110 K; logg eff=3.05±0.1. Further, Δlogɛ(Fe)=-0.06±0.09; Δ log ɛ(Cr)=-0.01±0.09 with regard to ‘standard’ (solar) abundances. For α Cen A the resulting data — particularly the photospheric ones — are less certain, but it seems that the Fe abundance may be smaller than the ‘standard’ value. Title: Book reviews Authors: van Diggelen, J.; Ne'Eman, Yuval; Nyboer, B. R. A.; Kleczek, J.; Dobrzycki, Jerzy; Hartmann, G.; Swider, W.; Hempenius, S. A.; de Jager, C.; 't Hooft, G.; Radelaar, S.; Wisse, J. A.; de Jong, T.; Houtgast, J.; Bezemer, J. Bibcode: 1978SSRv...21..703V Altcode: No abstract at ADS Title: Book reviews Authors: Plavec, Miroslav; Hoekstra, R.; de Jager, C.; Grygar, Jiři; Otterman, J.; van den Dool, H. M.; Namba, O.; Gunsing, C. J. Th.; Pecker, Jean-Claude; Kwee, K. K.; Perek, L.; Callebaut, D.; Kuijpers, Jan; de Graaff, W.; Reijnen, G. C. M.; Swanenburg, B.; Grevesse, N.; Kleczek, J.; Piquet, P.; Fokker, A. D.; van Bueren, H. G.; Page, D. Edgar; van Duinen, R. J.; Pacini, Franco Bibcode: 1978SSRv...21..469P Altcode: No abstract at ADS Title: An imaging soft X-ray telescope for Spacelab. Authors: Catura, R. C.; Culhane, J. L.; de Jager, C.; Garmire, G. P.; Novick, R. Bibcode: 1978nisa.symp..247C Altcode: 1978nisa.conf..247C No abstract at ADS Title: Editorial Authors: De Jager, Cornelis; Švestka, Zdeněk Bibcode: 1978SoPh...56....3D Altcode: 1978SoPh...56....3. No abstract at ADS Title: A review of space research, 1976 - 1977. Authors: de Jager, C. Bibcode: 1978spre.conf....3D Altcode: 1978spre.proc....3D Various remote sensing projects are discussed noting the Fourier spectrometer and a high-resolution multispectral scanner camera. Projects involving the determination of crop species and measurements of atmospheric absorption lines are discussed. Studies of the upper atmosphere are considered including the ozone layer, the influx of energetic solar protons, and density and pressure profiles. Rocket sounding for ionospheric and magnetospheric observations is described along with determinations of the total solar radiation flux and X-ray emissions from solar flares. Data concerning the solar wind are assessed and the Viking and Luna-24 missions are outlined. The U.S.-Netherlands balloon-borne ultraviolet stellar spectrograph device and its applications are reviewed. Title: Improved values for the solar micro- and macro-turbulent filter functions Authors: de Jager, C.; Vermue, J. Bibcode: 1977SoPh...54..313D Altcode: The contributions of any arbitrary photospheric velocity field to (macroturbulent) line displacement, and to (microturbulent) line broadening can be expressed by the macro- and micro-turbulent filters fM(k) and ft(k), where k is the wavenumber of the energy spectrum in which the line-of-sight component of the velocity field can be decomposed. As a correction to a previous computation of fM and ft we give in this Note improved values for the filter functions for weak lines in LTE. An example of the way to use the filter functions is given. Title: An imaging soft X-ray telescope for Spacelab Authors: Catura, R. C.; Culhane, J. L.; De Jager, C.; Garmire, G. P.; Novick, R. Bibcode: 1977cosp.meetR....C Altcode: A grazing-incidence soft X-ray telescope (SXT) of Wolter Type I design is described which has been proposed for use in Spacelab missions. The SXT consists of a four-mirror nested array of confocal paraboloid-hyperboloid elements and is intended to have optimal response in the wavelength range from 6 to 100 A (it also covers the range between 100 and 1000 A). Basic elements of the SXT are reviewed together with a design study of two different hyperboloid lengths. An objective grating for studying point-source spectra is examined, and several focal-plane detectors are discussed, including an image-sensitive proportional counter, a channel electron multiplier array, and a gas scintillation proportional counter. SXT sensitivity is estimated for eight wavelength ranges between 8 and 1100 A. Title: BUSS Observations II. Extended Atmospheres of Betelgeuse. Authors: Kondo, Y.; Modisette, J. L.; Morgan, T. H.; de Jager, C.; Lamers, H. J.; van der Hucht, K. A. Bibcode: 1977BAAS....9..365K Altcode: No abstract at ADS Title: Book reviews Authors: Reijnen, G. C. M.; Kleczek, J.; Millman, Peter M.; Vesseur, H. J. A.; Bar-Nun, Akiva; de Jager, C.; van Albada, T. S.; Rawer, K.; Hinze, J. O.; Trümper, J.; de Jager, Cornelis; Müller, O.; Kovalevsky, J.; Hammerschlag, R. H.; Hoyng, Peter Bibcode: 1977SSRv...20..235R Altcode: No abstract at ADS Title: Image Processing Techniques in Astronomy (Book Review) Authors: de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1977ApL....18..177D Altcode: No abstract at ADS Title: The Spectroscopy and Polarimetry of Flare X-Radiation Authors: de Jager, C. Bibcode: 1977uxsa.collE...6D Altcode: 1977IAUCo..43S....D No abstract at ADS Title: Book reviews Authors: Reijnen, G. C. M.; Rycroft, M. J.; de Jager, C.; Page, D. E.; Lyttleton, R. A.; Jongen, H. F.; Ne'Eman, Y.; Herbig, G. H.; Kleczek, J.; Hartmann, William K.; Delbouille, L.; Stoker, P. H. Bibcode: 1977SSRv...20..115R Altcode: No abstract at ADS Title: High resolution stellar spectroscopy in the balloon ultraviolet. Authors: de Jager, C.; Kondo, Y.; van der Hucht, K. A.; Morgan, T. H. Bibcode: 1977spre.conf..741D Altcode: 1977spre.proc..741D The JSC/SRL Balloon-borne Ultraviolet Stellar Spectrometer (BUSS) comprises a 40 cm telescope, an echelle spectrograph and a SEC Vidicon detector. Operating at an altitude of 40 km this instrument provides spectra of stars with 0.1 A resolution in the 2000-3400 A region. Observations made on 19 May 1976, are discussed. Title: Space Research, Vol._XVII. Proceedings of open meetings of working groups on physical sciences of the nineteenth plenary meeting of COSPAR, Philadelphia, Pennsylvania, USA, 8 - 19 June 1976 and COSPAR/IAGA symposium on minor constituents and excited species, Philadelphia, Pennsylvania, USA, 9 - 10 June 1976. Authors: Rycroft, M. J.; Stickland, A. C.; de Jager, C. Bibcode: 1977srvp.book.....R Altcode: No abstract at ADS Title: BUSS Observations. I. The Near-Ultraviolet Spectrum of α Sco AB. Authors: van der Hucht, K. A.; Lamers, H. J.; de Jager, C.; Kondo, Y.; Morgan, T. H.; Modisette, J. L. Bibcode: 1976BAAS....8..545V Altcode: No abstract at ADS Title: Balloon-borne High Resolution Ultraviolet Stellar Spectrophotometry Utilizing Echelle Spectrograph and SEC Vidicon. II. Authors: Kondo, Y.; Modisette, J. L.; Morgan, T. H.; de Jager, C.; Lamers, H. J.; van der Hucht, K. A. Bibcode: 1976BAAS....8R.548K Altcode: No abstract at ADS Title: An investigation of the ultraviolet spectra of beta Lyrae with the TD-1A satellite. Authors: Hack, M.; van den Heuvel, E. P. J.; Hoekstra, R.; de Jager, C.; Sahade, J. Bibcode: 1976A&A....50..335H Altcode: An investigation of scans of Beta Lyrae with the S 2/68 and S 59 experiments aboard the satellite TD-1A confirms the model accepted so far with the addition that the emitting regions of C IV, and probably of Si IV, are located in the circumstellar envelope which surrounds the secondary component, and that the Fe III emission originates within the expanding outer envelope which surrounds the whole system. The violet absorption borders of the Mg II doublet and of the Fe III emissions yield velocities which agree with those derived from the lines of the triplet series of He I in the optical and near infrared regions, and reflect the velocity of expansion of the outermost layers of the thin envelope in which the stars are embedded. Title: Balloon-borne high resolution ultraviolet stellar spectrophotometry utilizing echelle spectrograph and SEC Vidicon. Authors: Kondo, Y.; de Jager, C.; Morgan, T. H.; van der Hucht, K. A.; Modisette, J. L. Bibcode: 1976BAAS....8..428K Altcode: No abstract at ADS Title: Book reviews Authors: Jacchia, L. G.; de Jager, C.; van den Broek, P. Ph.; van Diggelen, J.; Kivelson, M. G.; Russell, C. T. Bibcode: 1976SSRv...19..161J Altcode: No abstract at ADS Title: Solar Flares Authors: de Jager, C.; Kuperus, M.; Rosenberg, H. Bibcode: 1976RSPTA.281..507D Altcode: 1976RSLPT.281..507D A summary is given of some recent observational data on solar flares. Particularly we discuss the flare build-up process and the time scales involved. We suggest as a possible magnetic field configuration a multiply kinked or supertwisted flux tube. The role of plasma turbulence and the Fermi mechanism in particle acceleration is discussed. Title: Physics of the Solar Atmosphere Authors: de Jager, C.; Kuperus, M.; Rosenberg, H. Bibcode: 1976RSPTA.281..415D Altcode: 1976RSLPT.281..415D A summary is given on recent results on the physics of the quiet solar atmosphere, and active regions. This includes: solar rotation, velocity fields and waves, magnetic field concentration, the transition region, coronal magnetic field structure, and prominences. Title: Preface Authors: de Jager, Cornelis; Obayashi, Tatsuzo; Švestka, Zdeněk Bibcode: 1976SoPh...47....9D Altcode: No abstract at ADS Title: Book reviews Authors: de Jager, C.; van Rensbergen, Walter; Kuperus, M.; Falthammar, Carl-Gunne; Mewe, R.; Reunen, G. C. M.; Bruzek, A.; Swanenburg, B. N.; Kleczek, J.; Millman, Peter M.; Vesseur, H. J. A.; Pacini, F.; Monfils, A. Bibcode: 1976SSRv...18..541D Altcode: No abstract at ADS Title: Astronomical observations from outside the terrestrial atmosphere. Authors: Code, A. D.; Bonnet, R. M.; Gorenstein, P.; Harper, D. A.; York, D. G.; van Duinen, R.; Wesselius, P.; de Jager, C.; Gurzadyan, G. A.; de Graaf, W.; Kondo, Y. Bibcode: 1976IAUTA..16c.195C Altcode: No abstract at ADS Title: Instruments and techniques (Instruments et techniques). Authors: Meinel, A. B.; Baranne, A.; Baum, W. A.; Dollfus, A.; Duchesne, M.; Godoli, G.; Hunter, A.; de Jager, C.; Livingston, W. C.; Mikhel'Son, N.; Sedmak, G.; Sinvhal, S. D.; Walker, M. Bibcode: 1976IAUTA..16a..19M Altcode: No abstract at ADS Title: Het planetenstelsel. Kolleges sterrekunde voor afgestudeerden 1976. Authors: de Jager, C.; Reijnen, G. C. M. Bibcode: 1976hepl.conf.....D Altcode: 1976hpks.book.....D No abstract at ADS Title: Space research XVI. Proceedings of open meetings of Working Groups on Physical Sciences of the eighteenth plenary meeting of COSPAR, Varna, Bulgaria, 29 May - 7 June 1975 and COSPAR symposium and workshop on results from Coordinated Upper Atmosphere Measurement Programs, Varna, Bulgaria, 29_-_31 May 1975. Authors: Rycroft, M. J.; de Jager, C. Bibcode: 1976srxp.book.....R Altcode: No abstract at ADS Title: Ultraviolet observations of beta Cephei and beta Canis Majoris. Authors: Beeckmans, F.; Burger, M.; de Jager, C.; Kamperman, T. M. Bibcode: 1976mpvs.conf....3B Altcode: No abstract at ADS Title: Elementary flare bursts. Authors: van Beek, H. F.; de Feiter, L. D.; de Jager, C. Bibcode: 1976spre.conf..819V Altcode: 1976spre.proc..819V Two hard X-ray flares observed on 2 and 7 August 1972 with the hard X-ray detector of the Utrecht Space Research Laboratory aboard ESRO's TD-1A satellite are examined in detail. It is shown that both flares can be decomposed completely into a number of individual Elementary Flare Bursts (EFB's) with no residual radiation left. For one and the same flare all EFB's seem to have approximately the same time profile but these profiles are different for the two flares investigated: the full widths at half maximum (FWHM) of the EFB's are 5 and 15 s respectively. Title: Non-thermal broadening of weak lines Authors: Gurtovenko, E. A.; de Jager, C.; Lindenbergh, A.; Rutten, R. J. Bibcode: 1976pmas.conf..331G Altcode: No abstract at ADS Title: Progress in Ultraviolet Stellar Spectrophotometry with S59 Authors: de Jager, C.; Lamers, H. J. G. L. M.; van der Hucht, K. A. Bibcode: 1975Ap&SS..38..313D Altcode: 1975IAUCo..27..313D A progress report is given of investigations related to observations of stellar spectra obtained with the ultraviolet stellar spectrophotometer S59 aboard the ESRO TD-1A satellite. We describe first the processing of the observations: intensity and wavelength calibration, identification of lines, classification of spectra. Thereafter some important groups of lines are dealt with: non-LTE computation of the Mg ii lines are presented: apart from the peculiar emission line and shell stars they compare well with the observations; intensity ratios, of Fe ii and Fe iii lines are an important temperature classification criterion. Interstellar UV lines indicate large deficiencies of some metals, as compared to solar values. A new ultraviolet continuous extinction curve is determined. We discuss the strong outstreaming motions observed in α Cygni, and the consequent mass loss of this star (< 3 × 10-10 mathfrak{M}_ odot yr-1and finally we describe the composite spectrum of the (WC8+09I) bnary γ2 Velorum; the ultraviolet continuous spectrum of the WC star is about one magnitude brighter than any theory predicts. Title: Image processing techniques in astronomy Authors: de Jager, C.; Nieuwenhuijzen, H. Bibcode: 1975ASSL...54.....D Altcode: 1975ipta.proc.....D; 1975QB461.I48...... No abstract at ADS Title: Book reviews Authors: de Jager, C.; De Loore, C.; Reijnen, G. C. M. Bibcode: 1975SSRv...18..141D Altcode: No abstract at ADS Title: Two Years of Operation of the Ultraviolet Stellar Spectrophotometer S59 in E.S.R.O.'s TD1A Satellite Authors: de Jager, C.; Hoekstra, R.; van der Hucht, K. A.; Kamperman, T. M.; Lamers, H. J. G. L. M. Bibcode: 1975RSPTA.279..413D Altcode: 1975RSLPT.279..413D A progress report is given of the current state of the investigation of the ca. 3500 spectra obtained during the life time of the satellite. A summary is presented of the general types of investigation, namely those which involve a study of major parts of the observational material, and the more specific investigations, which refer to selected stars or groups of stars, or to special lines or groups of lines. Title: The Sun in the Far Infrared and Sub-mm Region Authors: de Jager, C. Bibcode: 1975SSRv...17..645D Altcode: A review is given of the solar radiation between wavelengths of approx. 5 μm. and 1 mm. After discussing the astrophysical background (Section 2), we review the brightness temperatures measured in the continuous spectrum in the disk center (Section 3), and compare them with model predictions. The observed limb darkening (or brightening) is described in Section 4, and the line spectrum in Section 5. In Section 6 considerations are given on the usefulness of infrared observations for the investigation of small structures on the Sun. Title: Some Remarks on Supergiant Photospheres Authors: de Jager, C. Bibcode: 1975RSPTA.279..421D Altcode: 1975RSLPT.279..421D Some characteristic aspects of near ultraviolet spectra of supergiant stars, observed with the Utrecht ultraviolet stellar spectrometer S59 aboard E.S.R.O.'s TD1A satellite are described. A comparison of the observed maximum brightness of supergiants with theoretical computation shows that in the brightest early-type supergiants the outward driving forces due to radiation pressure and to the turbulent pressure gradient must be about equal. Title: Supplementary Remarks to `On the Average Depth of Formation of Weak Fraunhofer Lines' by E. Gurtovenko, V. Ratnikova, and C. de Jager Authors: Gurtovenko, E.; Ratnikova, V.; de Jager, C. Bibcode: 1975SoPh...42...43G Altcode: No abstract at ADS Title: The Radiation Field in Photospheric Models for Extreme Supergiants Authors: de Jager, C.; Neven, L. Bibcode: 1975Ap&SS..33..295D Altcode: On the basis of assumed photospheric temperature models for 36 extreme supergiants (logg e -values of 1, 0.5 and 0;T e ranging from approx. 3700 33 000 K) photospheric fluxesS(τλ) were computed for 36 wavelengths ranging from 100 Å to 60 000 Å. The hot models are in perfect radiative equilibrium; the cooler show deviations up to 10%, sometimes even larger. Only in the relatively deep parts of the photospheres (τ5≳1) the radiation field at each geometrical level can be characterized by one unique radiation temperature; for smaller τ5-values there are large deviations from local thermal equilibrium. The influence of deviations from local thermodynamical equilibrium on the fluxes is briefly examined, and appears small but for the shortest wavelengths. In tables and graphs we give for these models πF(γ)-values, integrated fluxes, effective temperatures, coloursU, B andV, and the Balmer discontinuityD. Title: Hard X-rays from the sun. Authors: van Beek, H. F.; Hoyng, P.; de Jager, C.; Stevens, G. A. Bibcode: 1975NTNA...41..101V Altcode: No abstract at ADS Title: Dynamik von Sternatmosphären Authors: de Jager, C. Bibcode: 1975MitAG..36...15D Altcode: The characteristics of solar convection are considered along with aspects of hydrodynamic turbulence, waves in the solar atmosphere, the mechanical energy flux of sun and stars, and questions concerning the occurrence of stellar chromospheres and coronas. The theory of convection in stellar atmospheres, which was first conceived by Schwarzschild (1906), has been further developed. It is pointed out that this theory, in principle, provides an approach to predict the field of motion in stellar atmospheres and to compute the mass loss of the stars. A realization of these possibilities requires, however, further advances in the theory and more observational data. Title: Image processing techniques in astronomy; Proceedings of the International Conference, Utrecht, Netherlands, March 25-27, 1975 Authors: de Jager, C.; Nieuwenhuijzen, N. Bibcode: 1975STIA...7622376D Altcode: The papers deal with recent technical developments that have contributed to improvements in the performance of existing telescopes and in the design of new ones, methods for acquiring and storing astronomical images, image-processing hardware and software, and applications of these developments to astronomical problems. Topics discussed include photometry of extended images, trends in astronomical spectroscopy, photon-counting detector arrays based on microchannel array plates, vidicon systems, and automatic processing of star plates. Consideration is also given to several software techniques for image processing, aperture synthesis, echelle spectra image processing for the International Ultraviolet Explorer, an intercomparison of ground-based and satellite pictures of the sun, the removal of vignetting from Apollo low-light-level photographs, and digital image centering. Individual items are announced in this issue. Title: Coronale gaten, donkere gebieden in de zonnecorona. Authors: de Jager, C. Bibcode: 1975Zenit...2..292D Altcode: No abstract at ADS Title: Nova Cygni 1975. Authors: de Jager, C.; Reijmerink, F. Bibcode: 1975Zenit...2..324D Altcode: No abstract at ADS Title: Stellar coronas. Authors: de Jager, C. Bibcode: 1975MSRSL...9..369D Altcode: 1975apes.conf..369D; 1975assp.conf..369D The present state of knowledge about stellar coronal heating is reviewed, noting that such heating is due to mechanical-energy loss by viscous dissipation of wave energy. Wave modes in stellar photospheres are described along with the generation of mechanical-energy fluxes, early predictions of the physical parameters of stellar coronas, and more recent computations of expected soft X-ray fluxes from coronas. Results are discussed for Astronomical Netherlands Satellite observations of soft X radiation from the Sirius system and Capella as well as for UV observations of coronal lines in the spectra of several other stars. Title: Ultraviolet observations of beta-Cephei and beta-Canis Majoris Authors: Beeckmans, F.; Burger, M.; De Jager, C.; Kamperman, T. M. Bibcode: 1975STIN...7813980B Altcode: Results of ultraviolet observations obtained from onboard ESRO TD-1A satellite experiments S2/68 and S59 are presented. The Utrecht stellar spectrophotometer S59 recorded stellar fluxes in the spectral ranges 2060 to 2160, 2495 to 2595, and 2770 to 2870 A with a spectral resolution of about 1.8 A. The experiment S2/68, a combined experiment of British and Belgian groups, observed in the regions 1350 to 1750, 1750 to 2150, and 2150 to 2550 A with a resolution of 36 A. The variations of beta-Cep and beta-CMa observed are presented graphically. If a radiation temperature between 20,000 and 25,000 K is assumed, the temperature difference between minimum and maximum will be 600 to 1000 K for beta-Cep and 200 to 400 K for beta-CMa. Title: Struktuur van de atmosfeer van Venus. Authors: de Jager, C. Bibcode: 1975Zenit...2..354D Altcode: No abstract at ADS Title: Rijpvorming op de satelliet Io. Authors: de Jager, C. Bibcode: 1975Zenit...2..203D Altcode: No abstract at ADS Title: Foto's van Io uit de ruimte. Authors: de Jager, C. Bibcode: 1975Zenit...2..349D Altcode: No abstract at ADS Title: Gasring om Jupiter in baan van Io. Authors: de Jager, C. Bibcode: 1975Zenit...2..362D Altcode: No abstract at ADS Title: High energy plasmas in astrophysics Authors: de Jager, C. Bibcode: 1975xris.conf..806D Altcode: Conditions are described under which high-energy cosmic plasmas can originate. Some of these plasmas derive their high particle energies directly from the internal thermal energy of a stellar body, but most of them derive it from a cooler gas; hence, thermodynamic upgrading of energy is needed. The mechanisms by which this occurs depend on the (electromagnetic) configuration of the medium. Laboratory experiments are reviewed which were performed to examine the configurations or processes involved. The most relevant are the discharge-produced and magnetospheric types. The best studied astrophysical high-energy plasmas can be similarly classified. The essential characteristics and the upgrading processes occurring in these plasmas are reviewed. The analogy between solar flares and magnetospheric substorms is discussed. Title: Space Research XV. Proceedings of open meetings of Working Groups on Physical Sciences of the seventeenth plenary meeting of COSPAR, São Paulo, Brazil - June 1974. Authors: Rycroft, M. J.; de Jager, C. Bibcode: 1975srxp.book.....R Altcode: No abstract at ADS Title: Phenomenology of the Subflare; A Synthesis of CINOF Authors: de Jager, C. Bibcode: 1975SoPh...40..133D Altcode: During the CINOF Campaign, June 1972, more than a hundred small chromospheric brightenings were observed. The observational results of about ten of them were investigated in more detail. They show a number of regularities; these are listed in Section 7. Title: Astronomical research in space Authors: de Jager, C. Bibcode: 1975STIA...7621802D Altcode: The paper summarizes the experience the Dutch space research program has so far (1975) gathered in the area of astronomical investigations carried out in space, in particular, on orbiting platforms. The most significant accomplishments include (1) the measurement of the X-ray emission from the flare star YZ CMi, which was on the order of 10 to the 33rd erg, carried out aboard the Dutch ANS satellite; (2) solar X-ray burst measurements by the Dutch X-ray spectrometer aboard the TD-1A satellite, which are among the best as regards sensitivity, dynamic range, spectral range, and resolution; (3) the ANS discovery of permanent X-ray emission from the star Sirius, constituting the first discovery of a stellar corona; and (4) TD-1A spectra of the Wolf-Rayet component of gamma 2 Velorum. Future plans include the joint Dutch-Japanese project (LEINAX) for rocket measurements of cosmic X-ray emission, high-resolution UV measurements from a balloon in cooperation with the Johnson Space Center, and contributions to the American High Energy Astrophysical Laboratory and Solar Maximum Mission. Title: Mercurius heeft tòch een magnetosfeer. Authors: de Jager, C. Bibcode: 1975Zenit...2..345D Altcode: No abstract at ADS Title: Drie wolkentypen in atmosfeer van Venus. Authors: de Jager, C. Bibcode: 1975Zenit...2..382D Altcode: No abstract at ADS Title: An All-Sky Camera Battery for X-Ray Astronomy Authors: de Jager, C. Bibcode: 1974Ap&SS..31..417D Altcode: The increasing number of interesting variable X-ray sources asks for an instrument for continuously monitoring the brightness of the most important X-ray sources over the whole sky. We describe a system consisting of a set of identical large position-sensitive detectors, mounted on the sides of a polyhedral satellite. Each counter is illuminated through a large diaphragm, with binary shadow characteristics. By holographic methods the X-ray image of the observed part of the sky could be derived from the illumination of each detector by multiplex analysing methods. The satellite should have fairly coarse (half a degree accuracy) three-axial stabilization, and an on-board computersteered data accumulation system. A relation is derived between the number of transparent holes in the diaphragm, the total count rate, and the flux of the faintest observable source. It appears that for a diaphragm with a few hundred transparent holes (detector geometrical area ≈103 cm2) an integration time of approximately 6 min is needed in order that sources with a count rate of 0.01 cm-2 s-1 should still be observable with a confidence limit of 99%. Title: On the Determination of the Photospheric Velocity Distribution from Profiles of Weak Fraunhofer Lines Authors: Rutten, R. J.; Hoyng, P.; de Jager, C. Bibcode: 1974SoPh...38..321R Altcode: We derive the conditions under which the profile of a weak Fraunhofer line can be described as the convolution of the separate profiles of damping, thermal and non-thermal motions at the average depth of formation of the line. The average velocity distribution along the line of sight, rather than its customary chosen macro- and micro-turbulent components, is then found from the deconvolution of the observed profile with the known other contributions. Reversely, the observed profiles can be compared to predicted profiles on the basis of De Jager's (1974) theoretical turbulence broadening curves. Title: On the Average Optical Depth of Formation of Weak Fraunhofer Lines Authors: Gurtovenko, E.; Ratnikova, V.; De Jager, C. Bibcode: 1974SoPh...37...43G Altcode: Methods for the determination of the average optical depth of formation of weak Fraunhofer lines are compared, and their relative merits are discussed. Distinction should be made between the region of origin of the emergent radiation, and of the line depression. For weak or fairly weak lines the average optical depth of formation of the line depression is the relevant quantity; it should be determined by using a computational scheme based on the classical weighting functions of line formation; other methods give physically unsignificant or conflicting results. Title: Book reviews Authors: de Jager, C. Bibcode: 1974SoPh...36..239D Altcode: No abstract at ADS Title: Book reviews Authors: de Jager, C.; Hartle, James B.; Drawin, H. W.; Giovanelli, R. G.; Freudenthal, Hans; Wittenberg, H.; Tobias, Cornelius A.; van der Laan, H.; Smit, J. A. Bibcode: 1974SSRv...15..741D Altcode: No abstract at ADS Title: Report from Editorial Board meeting Authors: de Jager, C. Bibcode: 1974SoPh...34..275D Altcode: No abstract at ADS Title: Book reviews Authors: de Jager, C.; Bruzek, A. Bibcode: 1974SoPh...34..507D Altcode: No abstract at ADS Title: The Influence of a Photospheric spectrum of Turbulence on the Profiles of Weak Fraunhofer Lines Authors: De Jager, Cornelis Bibcode: 1974SoPh...34...91D Altcode: We assume that the motion field in the solar photosphere is described by a spectrum of turbulence, defined by suitably chosen parameters. For various values of the spectral parameters we compute average (i.e. averaged over a sufficiently large part of the photosphere) profiles of weak Fraunhofer lines. The resulting profiles which represent the distribution function of line-of-sight velocity components as modified by the transfer of radiation through the atmosphere, are thereupon still broadened by a function representing the influence of the distribution function of the granulation cell sizes. The resulting functions should be compared with the distribution function of line-of-sight velocity components as derived from observations, in order to arrive at an observational derivation of the parameters of the photospheric spectrum of turbulence. Title: The outer layers of novae and supernovae. Joint discussion at the XVth general assembly of the I.A.U., Sydney 1973. Authors: de Jager, C. Bibcode: 1974HiA.....3..499D Altcode: No abstract at ADS Title: Time Profiles and Photon Spectra of Solar Hard X-rays Authors: van Beek, H. F.; de Feiter, L. D.; de Jager, C. Bibcode: 1974ASSL...42..533V Altcode: 1974cimo.symp..533V With a spectrometer aboard the ESRO TD-1A satellite, time profiles and photon spectra of solar X-rays have been observed in the energy range above 24 keV. Most of the X-ray bursts studied thus far appear to consist of numerous short-lived spikes with rise and decay times of a few seconds. Such spikes, called 'Elementary Flare Bursts' (EFB's) cluster together in large numbers and constitute the conventional high energy flare. The form of the photon spectra could be determined very accurately and was in all cases of the power-law type. For the Aug. 4, 1972 flare, the photon spectra appear to be combinations of two power-law spectra; for the lower energies the spectral coefficient is smaller than for the higher energies. The transition energy is about 60 keV. Title: Supernova anno 1667 is door niemand gezien. Authors: de Jager, C. Bibcode: 1974Zenit...1R...2D Altcode: No abstract at ADS Title: The outer layers of novae and supernovae Authors: de Jager, C. Bibcode: 1974olns.conf.....D Altcode: No abstract at ADS Title: Sterren in wording. Authors: de Jager, C. Bibcode: 1974Zenit...1S...2D Altcode: No abstract at ADS Title: Observation of cosmic X-ray sources with the Netherlands astronomical satellite (ANS). Authors: Brinkman, A. C.; Heise, J.; de Jager, C. Bibcode: 1974PhiTR..34...43B Altcode: No abstract at ADS Title: Is de Gum-nevel te groot om te kunnen worden gezien? Authors: de Jager, C. Bibcode: 1974Zenit...1R..16D Altcode: No abstract at ADS Title: The Orbiting Stellar Ultraviolet Spectrophotometer S59 in ESRO's TD-1A Satellite Authors: de Jager, C.; Hoekstra, R.; van der Hucht, K. A.; Kamperman, T. M.; Lamers, H. J.; Hammerschlag, A.; Werner, W.; Emming, J. G. Bibcode: 1974Ap&SS..26..207D Altcode: The ultraviolet stellar spectrophotometer S59 of the Utrecht Astronomical Institute uses the stabilization properties of the ESRO TD-1A satellite. This spacecraft scans the sky along eliptic meridians with an orbital precession of one degree per day, thus covering the whole celestial sphere in half a year. This property is combined with a tracking system which points the spectrophotometer during four minutes at stars of sufficient brightness. During this time interval the ultraviolet stellar spectrum is scanned with a resolution of 1.7 Å in three bands of about 100 Å, around 2110, 2545 and 2825 Å. The optical, mechanical and electronic properties of the instrument and its tracking system are described in some detail, as well as the optical and technical performance in laboratory tests and in orbit. Some results obtained during the first half year of operation are briefly described. Title: Satelliet kan poollicht van boven bekijken. Authors: de Jager, C. Bibcode: 1974Zenit...1Q...2D Altcode: No abstract at ADS Title: Apollo-experiment brengt aan het licht: toch een atmosfeer rondom de maan. Authors: de Jager, C. Bibcode: 1974Zenit...1...14D Altcode: No abstract at ADS Title: Titan [satelliet met een dampkring] geeft ons nog vele raadsels op. Authors: de Jager, C. Bibcode: 1974Zenit...1V...2D Altcode: No abstract at ADS Title: Stoten gammastraling uit het heelal. Authors: de Jager, C. Bibcode: 1974Zenit...1...13D Altcode: No abstract at ADS Title: Nieuwe zonnecyclus dient zich an. Authors: de Jager, C. Bibcode: 1974Zenit...1....6D Altcode: No abstract at ADS Title: Hard X-ray observations of elementary flare bursts, and their interpretation. Authors: van Beek, H. F.; de Feiter, L. D.; de Jager, C. Bibcode: 1974spre.conf..447V Altcode: 1974spre.meet..447V Recent observations of solar hard X-ray bursts, performed with high time resolution aboard the ESRO TD-1A satellite, show that often these bursts consist of numerous short-lived spikes with rise and decay times of the order of a few seconds. It is argued that these spikes, which are called elementary flare bursts (EFB), are the essential physical phenomena, which, clustered together in large numbers, constitute the conventional high-energy flare. For some cases studied it is found that the rise and decay times can be as low as 1 to 2 seconds; individual points deviate only slightly from this average value. These times do not depend on the photon energy; this seems to indicate that the EFBs occur at a fairly low level in the chromosphere (less than or equal to 1500 km above the photosphere). For one particular case studied this conclusion is corroborated by the observation of short-lived flashes as observed optically with an interference filter centered at 3835 A. In this region a considerable part of the ambient electrons are accelerated to energies of several tens of kiloelectron volts. Title: Book reviews Authors: Tauber, G. E.; Ceplecha, Zdenek; Pace, Nello; Peters, Rudolph A.; de Graaff, W.; Vishniac, Wolf V.; Rees, M. J.; Kwee, K. K.; Rawer, K.; de Jager, C.; Brandt, John C.; Reeves, H.; Kleczek, J.; Fokker, A. D.; Seaton, M. J.; Öpik, E. J. Bibcode: 1973SSRv...15..147T Altcode: No abstract at ADS Title: Review of Publications: Proceedings of the Fourteenth General Assembly Brighton, 1970 Authors: de Jager, C.; Jappel, A. Bibcode: 1973JRASC..67...42D Altcode: No abstract at ADS Title: Solar Flares, Suprathermal Plasma Nodules, and Loop Prominences Authors: de Feiter, L. D.; de Jager, C. Bibcode: 1973sari.conf...93D Altcode: No abstract at ADS Title: Ontstaan en levensloop van sterren. Authors: de Jager, C.; van den Heuvel, E. P. J. Bibcode: 1973oelv.book.....D Altcode: No abstract at ADS Title: Space Research XIII. Proceedings of open meetings of Working Groups on Physical Sciences of the fifteenth plenary meeting of COSPAR, Madrid, Spain, 10_-_24 May, 1972. Vol._1,_2. Authors: Rycroft, M. J.; Runcorn, S. K.; de Jager, C. Bibcode: 1973srxp.book.....R Altcode: No abstract at ADS Title: Superthermal Plasma Nodules and Their Relation to Solar Flares Authors: De Feiter, L. D.; De Jager, C. Bibcode: 1973SoPh...28..183D Altcode: We define superthermal plasma nodules as bright points (diameter ≲ 20″), visible on high resolution X-ray heliograms. Flares appear to show a strong tendency to occur at the places of these nodules. There are indications that (part of) the hot plasma produced by consecutive flares is accumulated and confined in the superthermal plasma nodules, and that with increasing energy content of a nodule the probability for a drastic change of its magnetic structure increases, thus reducing the possibility for more flares to occur. Title: Ultraviolet stellar spectra obtained with the Utrecht orbiting stellar spectrophotometer S 59 aboard the ESRO TD-1 A satellite. Authors: Hoekstra, R.; van der Hucht, K. A.; de Jager, C.; Kamperman, T.; Lamers, H. J. Bibcode: 1973spre.conf..871H Altcode: No abstract at ADS Title: Western European Cooperation in Space Astronomy Authors: de Jager, C. Bibcode: 1973sari.conf..156D Altcode: No abstract at ADS Title: International Astronomical Union, Information Bulletin. No._29. Authors: de Jager, C. Bibcode: 1973IAUIB..29.....D Altcode: No abstract at ADS Title: Book reviews Authors: Sagan, Carl; Daniel, R. R.; de Jager, C.; Bowell, Edward L. G. Bibcode: 1972SSRv...14..175S Altcode: No abstract at ADS Title: Review of Publications- Highlights of Astronomy, Volume 2 Authors: de Jager, C. Bibcode: 1972JRASC..66..280D Altcode: No abstract at ADS Title: Stellar Winds and Mass Loss of a Rotating Star Authors: de Grève, J. P.; De Loore, C.; de Jager, C. Bibcode: 1972Ap&SS..18..128D Altcode: The mass loss to be expected from the corona of a rotating F2-star is calculated. The rotation is supposed to be rigid up to a certain distances, as if it were maintained by a strong magnetic field. Dependent on the values of the rotational velocity the mass loss can increase to 26 40% forv rot up to 200 km s-1. Title: The empirical determination of line source functions, β L -values, and the microturbulent and convective velocity components as functions of depth in the photosphere-chromosphere transition region Authors: De Jager, C.; Neven, L. Bibcode: 1972SoPh...25..277D Altcode: An empirical method for determining line source functions, previously applied by us to the cores of infrared lines has now been extended to the whole line profile and was applied to centre-limb observations of sixteen lines of five infrared multiplets, mainly of high excitation potential (Table I). The present investigation was performed in two steps. In the first part of the paper approximate values are derived for the depth dependence of the four functions named in the title of this paper, where βL is the ratio between the actual and the LTE population of the lower level of the transitions involved. In the second part of the paper we use these empirically derived functions to compute the line profiles. From the remaining differences between observed and computed profiles, corrections are derived to the four functions. The main results are: (a) Convective velocities: see Table IV.(b)(Micro-)turbulent velocities: see Figure 8. Between τ5 = 10-4 and 10-1: «υτ» ≈ 1.4 km s-1, which is an upper limit since an unknown contribution of macroscopic motions could not be separated, (c) Line source functions: see Figures 9, 15 and 16. The source functions are close to the black-body function for τ5≳ 10-3, slight deviations occur in higher levels. The interesting behaviour of the CaII source function near τ5 = 10-5 should be noted. (d) Non LTE-functions: first approximations for the functions log βL (τ5) were derived empirically in the first part, and are shown in Figure 10; the second approximation shows them to be too large and the real values seem to be closer to one-half or one-third of these functions. Title: Editorial Authors: De Jager, Cornelis; Švestka, Zdeněk Bibcode: 1972SoPh...25....3D Altcode: 1972SoPh...25....3. No abstract at ADS Title: Micro- and Macroturbulent Motions and the Velocity Spectrum of the Solar Photosphere Authors: De Jager, C. Bibcode: 1972SoPh...25...71D Altcode: A given motion field in a stellar atmosphere is usually observed through `filters' defined by line shifts and -broadenings and conventionally called macroturbulence and microturbulence. Title: Book reviews Authors: Schuiling, R. D.; Vesseur, H. J. A.; Wilhelm, Klaus; Hassan, H.; van Sabben, D.; de Jager, C.; van Kampen, N. G. Bibcode: 1972SSRv...13..190S Altcode: No abstract at ADS Title: Book reviews Authors: de Jager, C.; Quenby, J. J.; Hutchinson, G. W.; Allan, H. R. Bibcode: 1972SoPh...22..240D Altcode: 1972SoPh...22..240. No abstract at ADS Title: Solar Energy Sources Authors: de Jager, C. Bibcode: 1972ASSL...29....1D Altcode: 1972sun..conf....1D No abstract at ADS Title: On the Applicability of Goldberg and Unno's Method to the Determination of Microturbulent Velocities in an Atmosphere with Convection Authors: De Jager, C.; Neven, L. Bibcode: 1972SoPh...22...49D Altcode: The method of Goldberg and Unno for the determination of microturbulent velocities in a stellar atmosphere is only applicable if there are no macroturbulent or convective motions. Title: Structuur en dynamica van de zonnefotosfeer. Authors: de Jager, C. Bibcode: 1972KNAW...79..154D Altcode: No abstract at ADS Title: De suksesvolle lancering van ESRO's Thor-Delta 1A satelliet. Authors: van Beek, H. F.; de Jager, C.; Lamers, H. Bibcode: 1972HemD...70..171V Altcode: No abstract at ADS Title: Solar-terrestrial physics 1970; proceedings of the International Symposium on Solar-Terrestrial Physics held in Leningrad, U.S.S.R. 12-19 May 1970. Authors: de Jager, C.; Dyer, E. R. Bibcode: 1972ASSL...29.....D Altcode: 1972sun..conf.....D No abstract at ADS Title: The sun: Part I of solar-terrestrial physics/1970. Proceedings of the international symposium on solar-terrestrial physics held in Leningrad, U.S.S.R., 12 - 19 May 1970. Authors: de Jager, C. Bibcode: 1972sstp.conf.....D Altcode: 1972sun..confQ....D No abstract at ADS Title: International Astronomical Union, Information Bulletin. Nos._27,_28. Authors: de Jager, C. Bibcode: 1972IAUIB..27.....D Altcode: No abstract at ADS Title: Solar energy sources. Authors: de Jager, C. Bibcode: 1972sstp.conf....1D Altcode: No abstract at ADS Title: Editorial Authors: De Jager, Cornelis; Švestka, Zdeněk Bibcode: 1971SoPh...21....3D Altcode: 1971SoPh...21....3. No abstract at ADS Title: Book reviews Authors: Chup, E. L.; Koelbloed, D.; de Jager, C.; Quenby, J. J.; van Kampen, N. G.; Pettengill, G. H.; Schuurmans, C. J. E.; de Graaff, W.; Oort, J. H.; Sagan, Carl; van der Kerk, G. J. M.; Pecker, Jean-Claude; Szebehely, V.; Hutchinson, G. W.; Allan, H. R. Bibcode: 1971SSRv...12..526C Altcode: No abstract at ADS Title: The Production of Solar and Stellar Chromospheres and Coronae Authors: de Jager, C. Bibcode: 1971RSPTA.270..175D Altcode: Knowledge of the detailed field of turbulence in the solar granulation, and of the consequent photospheric mechanical flux, is one of the basic elements for understanding the solar chromosphere and corona. The other element is constituted by the structure and magnetic fields of the supergranular network, since the coarse mottles at the supergranular boundaries seem to yield a mechanical flux nearly an order of magnitude larger than that of the supergranular cell regions. In the non-magnetic solar regions the upper photospheric mechanical flux is about 0.1 J cm-2 s-1. This flux is equal to that emerging from the low-chromospheric vibrations; it seems not to be related to the observed microturbulent motions. Above such regions the chromosphere may be fairly thin, not exceeding 1000 or 2000 km. The precise thickness and detailed structure of the chromospheric layers can only be determined from a discussion of the interplay of the downward conductive flux with the dissipational losses of the up- and downward mechanical fluxes, and the radiative losses of the chromospheric matter. The greater apparent thickness of the chromosphere at the limb is due to the accumulated influence of the spicules seen from the side. Lack of knowledge of stellar photospheric inhomogeneities and fields handicaps a reliable prediction of stellar chromospheres and coronae. Yet a few attempts have been made, and predicted X-ray fluxes from coronae of some nearby stars are given (table 3). Title: Editorial Authors: de Jager, C. Bibcode: 1971SSRv...12..135D Altcode: No abstract at ADS Title: Coronae of Cool Stars Authors: de Jager, C.; De Loore, C. Bibcode: 1971Ap&SS..11..284D Altcode: With the hypothesis generally adopted as responsible for the heating of the solar corona an attempt was made to predict the coronal temperature for a few cool stars. Stars with effective temperatures between 2500K and 3700K, with logg-values of 3 or 4, appear to have coronal temperatures only 600 to 25000K higher thanT eff. In order to determine the possibility of coronal heating caused by sound waves we made a comparison between the frequency of the sound waves and the cut-off frequencies. Title: Book reviews Authors: Hinze, J. O.; van Diggelen, J.; de Jager, C.; Hutchinson, G. W. Bibcode: 1971SSRv...12..131H Altcode: No abstract at ADS Title: Editorial Authors: de Jager, Cornelis; Švestka, Zdeněk Bibcode: 1971SoPh...17..287D Altcode: 1971SoPh...17..287. No abstract at ADS Title: M. G. J. Minnaert, 1893 February 12 - 1970 October 26. Authors: de Jager, C. Bibcode: 1971QJRAS..12..338D Altcode: No abstract at ADS Title: Marcel Gilles Josef Minnaert Authors: de Jager, C. Bibcode: 1971Ap&SS..10..182D Altcode: No abstract at ADS Title: International Astronomical Union, Information Bulletin. No._26. Authors: de Jager, C. Bibcode: 1971IAUIB..26.....D Altcode: No abstract at ADS Title: Instrumentation for high-resolution stellar UV-spectrophotometry. Authors: de Jager, C.; Hammerschlag, A.; Werner, W. Bibcode: 1971IAUS...41..349D Altcode: No abstract at ADS Title: Joint Organization for Solar Observations. Annual report 1970. Authors: de Jager, C.; Kiepenheuer, K. O.; Michard, R.; Maltby, P.; Rösch, J.; Brandt, P. N. Bibcode: 1971joso.book.....D Altcode: No abstract at ADS Title: Solar Physics at the Utrecht Astronomical INstitute Authors: Fokker, A. D.; Houtgast, J.; De Jager, C. Bibcode: 1971SoPh...16..227F Altcode: No abstract at ADS Title: Atomic data of importance for ultraviolet and X-ray astronomy Authors: de Jager, C. Bibcode: 1971adiu.conf.....D Altcode: No abstract at ADS Title: High angular resolution astrophysical observations from space. Authors: de Jager, C. Bibcode: 1971EExSc...1..243D Altcode: No abstract at ADS Title: Reports on Astronomy. Authors: de Jager, C.; Jappel, A. Bibcode: 1971IAUTB..14.....D Altcode: No abstract at ADS Title: Highlights of Astronomy, Volume_2. presented at the XIVth general assembly of the I.A.U. 1970. Authors: de Jager, C. Bibcode: 1971havp.book.....D Altcode: No abstract at ADS Title: M. G. J. Minnaert, 1893 February 12 - 1970 October 16. Authors: de Jager, C. Bibcode: 1971UtrOv.160.....D Altcode: No abstract at ADS Title: Book reviews Authors: Zagar, F.; Schmidt, F. H.; Robe, H.; de Jager, C.; Kleczek, J. Bibcode: 1970SSRv...11..453Z Altcode: No abstract at ADS Title: Book review Authors: de Jager, C. Bibcode: 1970SSRv...11..152D Altcode: No abstract at ADS Title: On the Ratio Between the Mechanical Fluxes in- and outside the Solar Chromospheric Mottles Authors: De Jager, C.; De Loore, C. Bibcode: 1970SoPh...13..126D Altcode: Available information about the relative areas on the `quiet' and `active' parts of the sun covered with magnetic elements, together with theoretical results on the relation between the photospheric mechanical flux and the consequent coronal electron density, allows one to conclude that the mechanical flux generated in the photospheric magnetic elements is about seven times as large as the flux generated in non-magnetic regions. Title: Book reviews Authors: de Jager, C.; King-Hele, D. G.; Heise, John; ĹEna, P.; Somoza, R.; Lindblad, Per Olof; Ritchey, H. W.; de Feiter, L. D. Bibcode: 1970SSRv...10..737D Altcode: No abstract at ADS Title: Space and Ground-Based Stellar Spectrophotometry : a Summary Authors: de Jager, C. Bibcode: 1970IAUS...36..355D Altcode: No abstract at ADS Title: Predicted X-Ray Fluxes of Stellar Coronas Authors: De Loore, C.; de Jager, C. Bibcode: 1970IAUS...37..238D Altcode: No abstract at ADS Title: High Energy Flare Radiation Authors: de Jager, C. Bibcode: 1970ASSL...19...25D Altcode: 1970iso..conf...25D No abstract at ADS Title: International Astronomical Union, Information Bulletin. No._25. Authors: de Jager, C. Bibcode: 1970IAUIB..25.....D Altcode: No abstract at ADS Title: Damping Constants for Infrared Fraunhofer Lines Authors: De Jager, C.; Neven, L. Bibcode: 1970SoPh...11....3D Altcode: Empirical values of solar damping constants and their variation with optical depth were derived according to a method developed earlier by the authors. The damping constants refer to six infrared multiplets (24 lines). The average optical depths range from τ0 = 0.5 to 2.2. Corresponding theoretical damping constants were computed, mainly on the basis of Van der Waals damping, and with the help of detailed computations of the mean square radii of the atomic levels by Van Rensbergen. The empirical values are systematically larger than the theoretical ones, with factors ranging between 1.8 and 4.9. Some speculations about the source of this discrepancy are given. Title: Editorial Authors: De Jager, Cornelis; Švestka, Zdeněk Bibcode: 1969SoPh...10..243D Altcode: 1969SoPh...10..243. No abstract at ADS Title: Book reviews Authors: de Jager, C.; Janssens, Paul Bibcode: 1969SSRv...10..455D Altcode: No abstract at ADS Title: Book reviews Authors: Nicolet, M.; Müller, Edith A.; de Jager, C.; Rutten, H. S.; Millman, P. M.; Fielder, G. Bibcode: 1969SSRv...10..314N Altcode: No abstract at ADS Title: Book reviews Authors: Veldkamp, J.; Kovalevesky, J.; Plaut, L.; Bondi, H.; Mewe, R.; de Jager, C. Bibcode: 1969SSRv....9..872V Altcode: No abstract at ADS Title: Book reviews Authors: de Jager, C.; Ollongren, A.; de Graaf, Tj.; Levin, B. J.; Tucker, M. Bibcode: 1969SSRv....9..740D Altcode: No abstract at ADS Title: Editorial Authors: Švestka, Z.; De Jager, C. Bibcode: 1969SoPh....8....3S Altcode: 1969SoPh....8....3. No abstract at ADS Title: Book reviews Authors: de Jager, C.; di Benedetto, F.; de Groot, T.; van Diggelen, J.; van Diggelen, J.; Herczeg, T.; Lamers, H. J.; van Albada, G. B. Bibcode: 1969SSRv....9..617D Altcode: No abstract at ADS Title: Book reviews Authors: Fokker, A. D.; Clark, D. D.; Kutzer, Ants; de Jager, C. Bibcode: 1969SSRv....9..419F Altcode: No abstract at ADS Title: Solar flares; properties and problems Authors: de Jager, C. Bibcode: 1969sfsr.conf....1D Altcode: No abstract at ADS Title: Solar flares and space research Authors: de Jager, C.; Svestka, Z. Bibcode: 1969sfsr.conf.....D Altcode: 1969sfsr.book.....D No abstract at ADS Title: Sterrenkunde. Authors: de Jager, Cornelis; van Albada, G. B. Bibcode: 1969ster.book.....D Altcode: 1969QB43.J34....... No abstract at ADS Title: Het ruimte-onderzoek. Authors: de Jager, C. Bibcode: 1969HemD...67..134D Altcode: No abstract at ADS Title: Editorial Authors: De Jager, C.; Švestka, Z. Bibcode: 1968SoPh....5..259D Altcode: 1968SoPh....5..259. No abstract at ADS Title: Book reviews Authors: de Graaff, W.; de Jager, C.; Emming, J. G.; van Bueren, H. G. Bibcode: 1968SSRv....8..591D Altcode: No abstract at ADS Title: On the Occurrence of Convective Motions in the Upper Photosphere Authors: De Jager, C.; Neven, L. Bibcode: 1968SoPh....4..379D Altcode: We have examined whether the motion field in the photosphere in the range of optical depths 0.25< τ0< 0.6 is dominated by thermal convection or by vibrations. The observed asymmetries of infrared Fraunhofer lines indicate the presence of motions, and the fact that the asymmetry is zero for lines of low excitation and increases with the excitation potential shows that these motions are chiefly convective in this part of the photosphere: upward moving elements appear to be hotter than downward moving ones. Title: Book reviews Authors: de Jager, C.; Kleczek, J. Bibcode: 1968SoPh....4..375D Altcode: 1968SoPh....4..375. No abstract at ADS Title: Book reviews Authors: de Feiter, L. D.; de Jager, C. Bibcode: 1968SoPh....3..623D Altcode: 1968SoPh....3..623. No abstract at ADS Title: Book reviews Authors: de Jager, C.; Hinze, J. O.; Wittenberg, H. Bibcode: 1968SSRv....8..135D Altcode: No abstract at ADS Title: Source Functions in the Cores of Infrared Fraunhofer Lines Authors: De Jager, C.; Neven, L. Bibcode: 1968SoPh....3..159D Altcode: No abstract at ADS Title: The Bilderberg conference on the structure of the quiet photosphere Authors: De Jager, C. Bibcode: 1968SoPh....3....4D Altcode: 1968SoPh....3....4. No abstract at ADS Title: Editorial Authors: De Jager, C.; Švestka, Z. Bibcode: 1968SoPh....3....3D Altcode: 1968SoPh....3....3. No abstract at ADS Title: The High-Energy Flare Plasma Authors: de Jager, C. Bibcode: 1968IAUS...35..480D Altcode: No abstract at ADS Title: Hard Solar X-ray Bursts Authors: de Jager, C. Bibcode: 1968HiA.....1..541D Altcode: No abstract at ADS Title: The Development and Structure of an Active Region Authors: de Jager, C. Bibcode: 1968IAUS...35..602D Altcode: No abstract at ADS Title: New Techniques in X-Ray Astronomy Authors: de Jager, C. Bibcode: 1968HiA.....1..108D Altcode: No abstract at ADS Title: Acceleration processes in solar flares, Introductory lecture Authors: de Jager, C. Bibcode: 1968mmsf.conf..171D Altcode: No abstract at ADS Title: The Coordination of Ground-Based and Space Observations: Summary Authors: de Jager, C. Bibcode: 1968HiA.....1..547D Altcode: No abstract at ADS Title: The Bilderberg Model of the Photosphere and Low Chromosphere Authors: Gingerich, O.; De Jager, C. Bibcode: 1968SoPh....3....5G Altcode: From 17 through 21 April 1967, an international study week was held in the `Bilderberg' near Arnhem, Netherlands, with the aim of obtaining an internationally acceptable model of the solar photosphere and low chromosphere. It was found that such a model, based on observed intensities and center-to-limb observations of the solar continuous spectrum, could indeed be established. This model, henceforth called the Bilderberg Continuum Atmosphere (BCA), is shown in Table I, which gives the temperature, gas and electron pressures, and other data as functions of the continuous optical depth at 5000 Å between τ5000 = 10−7 and 25. The model is characterized by a flat temperature minimum of 4600 °K between τ5000 ≈ 10−2 to 10−4. The model is homogeneous, and in hydrostatic equilibrium. A hydrogen-helium ratio of 10 has been assumed. Title: Book reviews Authors: van Gils, J. N.; van Diggelen, J.; de Jager, C.; Brussaard, P. J. Bibcode: 1967SSRv....7..534V Altcode: No abstract at ADS Title: The Hard Solar X-Ray Burst of 18 September 1963 Authors: De Jager, C. Bibcode: 1967SoPh....2..327D Altcode: A hard solar X-ray burst was observed by J-P. Legrand on 18 September 1963, 13:56 UT, at balloon altitude. It lasted a few minutes; a steep increase was followed by an exponential decay. During its declining phase a weak radio burst was observed on 3 and 10 cm, not on longer wavelengths. Title: Note on Solar Hard X-Ray Bursts Authors: De Jager, C. Bibcode: 1967SoPh....2..347D Altcode: Observationally solar X bursts fall into three different categories : soft X bursts (E < 10 keV), deka-keV bursts (10-150 keV), and very hard X bursts or deci-MeV bursts (200-1000 keV). The first kind is quasi-thermal, the last kind is non-thermal. The real existence of the third kind of burst looks probable but has not yet been proved by direct observations. The difference between deci-MeV and deka-keV bursts may mainly be a matter of geometry of the emitting plasma. Title: Book reviews Authors: van Wijngaarden, L.; Zandbergen, P. J.; Slachmuylders, E.; de Graaff, W.; Vandenkerckhove, J.; van Herk, G.; de Jager, C. Bibcode: 1967SSRv....7..396V Altcode: No abstract at ADS Title: Auroral and associated magnetospheric phenomena at very high latitudes Authors: de Jager, C. Bibcode: 1967SSRv....7D...3D Altcode: No abstract at ADS Title: Geodesy and satellites Authors: de Jager, C. Bibcode: 1967SSRv....7....3D Altcode: No abstract at ADS Title: Reviews Authors: de Jager, C.; Hinze, J. O.; Frantzen, A. J. Bibcode: 1967SSRv....7..135D Altcode: No abstract at ADS Title: Editorial Authors: de Jager, C.; Švestka, Z. Bibcode: 1967SoPh....1..303D Altcode: 1967SoPh....1..303. No abstract at ADS Title: Reviews Authors: Jongbloed, J.; van Diggelen, J.; de Jager, C.; Busch, H.; Vos, J. J. Bibcode: 1967SSRv....6..574J Altcode: No abstract at ADS Title: A Systematic Method for the Analysis of High-Resolution Fraunhofer Line Profiles Authors: De Jager, C.; Neven, L. Bibcode: 1967SoPh....1...27D Altcode: A fraunhofer line profile depends on various parameters, partly related to the photospheric structure (T, Pg, Pe, vconv, vturb), partly to the atom or ion involved (such as oscillator strength, energy levels), partly also resulting from the interaction of the relevant kind of particles with the photosphere, and the photospheric radiation field. In this paper we shall mainly pay attention to the determination of: the macroturbulent (convective) velocities, vconv (τ); the damping constant γ (τ); the abundance, Ael; the distribution function ϕ(vconv, τ) of the convective velocities at each depth τ; the source function, S (τ); the microturbulent velocities, vturb (τ). Title: Editorial Authors: de Jager, C.; Švestka, Z. Bibcode: 1967SoPh....1....3D Altcode: 1967SoPh....1....3. No abstract at ADS Title: Observation of solar line profiles. I. The centre-to-limb variation of profiles of fifty lines in the near infrared solar spectrum Authors: de Jager, C.; Neven, L. Bibcode: 1967BANS....1..325D Altcode: 1966BANS....1..325N; 1966BANS....1..325D No abstract at ADS Title: Buchbesprechungen über: Solar Physics. Ed. C. DE JAGER, and Z. ŠVESTKA. (Ref. R. O. REDMAN) Authors: de Jager, C.; Švestka, Z. Bibcode: 1967ZA.....67...95D Altcode: No abstract at ADS Title: Theoretical Aspects of Solar X-Radiation Authors: de Jager, C. Bibcode: 1967ASSL....9..101D Altcode: 1967ers..conf..101D No abstract at ADS Title: Spectroscopic data for sixty new model photospheres Authors: de Jager, C.; Neven, L. Bibcode: 1967BANS....2..125D Altcode: No abstract at ADS Title: The structure of the quiet photosphere and the low chromosphere Authors: de Jager, C. Bibcode: 1967sqpl.conf.....D Altcode: 1968QB528.S77...... No abstract at ADS Title: Observation of solar line profiles. I. The centre-to-limb variation of profiles of fifty lines in the near infrared solar spectrum (Abstract) Authors: de Jager, C.; Neven, L. Bibcode: 1967BAN....19..160D Altcode: No abstract at ADS Title: Spectroscopic data for sixty new model photospheres Authors: de Jager, Cornelis; Neven, L. Bibcode: 1967sdsn.book.....D Altcode: 1967QB1.B94v2n4.... No abstract at ADS Title: Book reviews Authors: de Jager, C.; van den Heuvel, E. P. J.; Capart, J. J.; Jongbloed, J.; Minnaert, M. Bibcode: 1966SSRv....6..273D Altcode: No abstract at ADS Title: Note on the influence of the apparatus profile on spectral line profiles Authors: de Jager, C.; Neven, L. Bibcode: 1966BAN....18..306D Altcode: No abstract at ADS Title: Théorie des Atmospheres Stellaires (Theory of Stellar Atmospheres) Authors: de Jager, C. Bibcode: 1966IAUTB..12Q.334D Altcode: No abstract at ADS Title: Deviations from LTE in the Catom and the Abundance of C in the Solar Photosphere Authors: de Jager, C.; Neven, L. Bibcode: 1966IAUS...26..149D Altcode: No abstract at ADS Title: The profile of the infrared He I lines over the undisturbed solar disk Authors: de Jager, C.; Namba, O.; Neven, L. Bibcode: 1966BAN....18..128D Altcode: No abstract at ADS Title: Recommendations concerning notations to be used in transfer theory Authors: de Jager, C. Bibcode: 1966IAUTB..12R.334D Altcode: No abstract at ADS Title: Sunspots. R. J. BRAY and R. E. LOUGHEAD: Chapman and Hall, London, 1964. xvi + 302pp. 84 s Authors: de Jager, C. Bibcode: 1965P&SS...13..869D Altcode: No abstract at ADS Title: Book reviews Authors: de Jager, C. Bibcode: 1965SSRv....4..262D Altcode: No abstract at ADS Title: Solar X radiation Authors: de Jager, C. Bibcode: 1965AnAp...28..125D Altcode: No abstract at ADS Title: Editorial Authors: de Jager, C. Bibcode: 1965SSRv....4....3D Altcode: No abstract at ADS Title: Review paper : Solar X radiation Authors: de Jager, C. Bibcode: 1965IAUS...23...45D Altcode: No abstract at ADS Title: The Solar Center of Activity Authors: de Jager, C. Bibcode: 1965ASSL....2...55D Altcode: 1965istr.conf...55D No abstract at ADS Title: Optical Solar Flares Authors: de Jager, C. Bibcode: 1965ASSL....2...78D Altcode: 1965istr.conf...78D No abstract at ADS Title: Radio and X-Ray Emission Associated with Solar Flares Authors: de Jager, C. Bibcode: 1965ASSL....2...86D Altcode: 1965istr.conf...86D No abstract at ADS Title: The solar spectrum Authors: de Jager, Cornelis Bibcode: 1965ASSL....1.....D Altcode: 1965sosp.conf.....D No abstract at ADS Title: Particle Emission Associated with Solar Flares. Model of Flare Events Authors: de Jager, C. Bibcode: 1965ASSL....2...96D Altcode: 1965istr.conf...96D No abstract at ADS Title: The Solar Photosphere Authors: de Jager, C. Bibcode: 1965ASSL....2...13D Altcode: 1965istr.conf...13D No abstract at ADS Title: The Quiet Chromosphere and Corona Authors: de Jager, C. Bibcode: 1965ASSL....2...26D Altcode: 1965istr.conf...26D No abstract at ADS Title: The Computation of Spectroscopic Data for 60 New Model Photopheres Authors: de Jager, C.; Neven, L. Bibcode: 1964SAOSR.167..270D Altcode: No abstract at ADS Title: The Utrecht Reference Model of the Photosphere Authors: Heintze, J. R. W.; Hubenet, H.; de Jager, C. Bibcode: 1964SAOSR.167..240H Altcode: No abstract at ADS Title: Book reviews Authors: van Bueren, H. G.; van den Heuvel, E.; de Jager, C.; Kallmann-Bijl, H. Bibcode: 1964SSRv....3..856V Altcode: No abstract at ADS Title: A reference model of the solar photosphere and low chromosphere Authors: Heintze, J. R. W.; Hubenet, H.; de Jager, C. Bibcode: 1964BAN....17..442H Altcode: No abstract at ADS Title: The structure of a sunspot Authors: de Jager, C. Bibcode: 1964susp.conf..242D Altcode: No abstract at ADS Title: Solar Ultraviolet and X-Ray Radiation Authors: de Jager, C. Bibcode: 1964rege....1....1D Altcode: No abstract at ADS Title: Book reviews Authors: Jaarsma, F.; Veldkamp, J.; Underhill, Anne B.; de Jager, C. Bibcode: 1963SSRv....2..871J Altcode: 1963SSRv....2..871. No abstract at ADS Title: Energy transport and "turbulence" in a sunspot Authors: de Jager, C. Bibcode: 1963BAN....17..253D Altcode: No abstract at ADS Title: Combined light-, colour- and radial-velocity measurements of the β Cephei-type variable 12 (DD) Lacertae Authors: de Jager, C. Bibcode: 1963BAN....17....1D Altcode: 1964BAN....17....1D No abstract at ADS Title: Editorial Note Authors: de Jager, C. Bibcode: 1963SSRv....1..619D Altcode: 1963SSRv....1..619J No abstract at ADS Title: The Sun as a Source of Interplanetary Gas Authors: de Jager, C. Bibcode: 1963SSRv....1..487D Altcode: 1963SSRv....1..487J No abstract at ADS Title: The temperature minimum in the upper solar photosphere Authors: de Jager, C. Bibcode: 1963BAN....17..209D Altcode: No abstract at ADS Title: A Note on Bursts of Radio Emission and High Energy (>20 keV) X-Rays from Solar Flares Authors: De Jager, C.; Kundu, M. R. Bibcode: 1963spre.conf..836D Altcode: No abstract at ADS Title: Emission of Gas From the Sun Authors: De Jager, C. Bibcode: 1963spre.conf..491D Altcode: No abstract at ADS Title: Review of Publications- Space Science Reviews, vol. 1, no. 1, editor-in-chief Authors: de Jager, C. Bibcode: 1962JRASC..56..269D Altcode: No abstract at ADS Title: The abundance of lead in the Sun Authors: de Jager, C.; Neven, L. Bibcode: 1962BAN....16..307D Altcode: No abstract at ADS Title: Upper Atmospheric Turbulence Determined by Means of Rockets Authors: Blamont, J. E.; de Jager, C. Bibcode: 1962JGR....67.3113B Altcode: The motion field of the upper atmosphere was studied with the aid of sodium trails brought to the 100-km level by means of a Véronique rocket. Below 102 km the trail shows globular distortions in the form of elements having an average diameter of 0.5 km; above that height the trail is completely smooth, though highly curved, showing no small irregularities. The turbulence state of the upper atmosphere was studied by investigating diffusion of the sodium atoms. Above 102 km the broadening of the trail can be completely accounted for by molecular diffusion; below that height the observed rate of increase of the globule diameters can be explained only by assuming a combination of molecular and turbulent diffusion. This information and the increasing chemical differentiation above 102 km strongly indicate that the 100-km level marks the difference between the turbulent and the nonturbulent parts of the upper atmosphere. The absence of turbulence in the upper atmosphere is bound to be due to the increasing relative importance of the friction forces there. The Reynolds number rapidly decreases with increasing atmospheric height. The absence or occurrence of turbulence cannot be due to shear effects: the Richardson number is too great in the region studied by us, which shows that shear turbulence is not to be expected in this part of the earth's atmosphere. Since the energy fed into the turbulence field is rather small, turbulence cannot be fully developed; in fact, it is predicted that the turbulent element can only have diameters smaller than about 2 km, which is not in disagreement with the observed element diameter of 0.5 km. The smallest visible wavelength of the motion field is of the order 50 to 100 meters, in agreement with ionospheric investigations (20 to 60 meters) and theoretical predictions (60 meters). Title: Introduction Authors: de Jager, C. Bibcode: 1962SSRv....1....5D Altcode: 1962SSRv....1....5J No abstract at ADS Title: Atmospheric turbulence near the 100 km level Authors: Blamont, J. E.; de Jager, C. Bibcode: 1962sose.conf...94B Altcode: No abstract at ADS Title: Tables and graphs enabling the computation of the profiles of H [gamma] and H [delta] in stellar spectra Authors: de Jager, Cornelis; Neven, L. Bibcode: 1962AnOB....9.....D Altcode: 1962AnBel...9.....D No abstract at ADS Title: The acoustic energy flux of the Sun and the formation of the corona Authors: de Jager, C.; Kuperus, M. Bibcode: 1961BAN....16...71D Altcode: No abstract at ADS Title: Satellite Photography by Means of Small Schmidt Cameras Authors: De Jager, C. Bibcode: 1961spre.conf...47D Altcode: No abstract at ADS Title: Space Research II Authors: van de Hulst, H. C.; de Jager, C.; Moore, A. F. Bibcode: 1961spre.conf.....V Altcode: No abstract at ADS Title: Non-Thermal Solar X-Rays and Radio Emission Authors: de Jager, C. Bibcode: 1961LIACo..10..635D Altcode: 1961LIACo..10..631D; 1961MSRSL...4..631D No abstract at ADS Title: The Ultra-Violet and X-Ray Spectra of the Stars Authors: de Jager, C.; Neven, L. Bibcode: 1961LIACo..10..552D Altcode: 1961MSRSL...4..551D; 1961LIACo..10..551D No abstract at ADS Title: The development of a solar centre of activity Authors: De Jager, C. Bibcode: 1961VA......4..143D Altcode: No abstract at ADS Title: Observations of infrared solar line profiles Authors: Delbouille, L.; de Jager, C.; Neven, L. Bibcode: 1960AnAp...23..949D Altcode: No abstract at ADS Title: Asymptotic widening parameters for the hydrogen lines Authors: de Jager, C. Bibcode: 1960AnAp...23..889D Altcode: No abstract at ADS Title: The computation of partition functions in a stellar atmosphere Authors: de Jager, C.; Neven, L. Bibcode: 1960BAN....15...55D Altcode: No abstract at ADS Title: A program for the computation of bn tables for H and He Authors: de Jager, C.; Kanno, M.; Neven, L. Bibcode: 1960AnAp...23..843D Altcode: No abstract at ADS Title: Concluding remarks Authors: de Jager, C. Bibcode: 1960AnAp...23..976D Altcode: No abstract at ADS Title: The X-ray radiation of the sun Authors: De Jager, C. Bibcode: 1960spre.conf..628D Altcode: No abstract at ADS Title: Interpretation of solar radio-observations Authors: de Jager, C. Bibcode: 1960raso.conf..313D Altcode: No abstract at ADS Title: Saturation effects in very faint Fraunhofer lines Authors: de Jager, C.; Neven, L. Bibcode: 1959Obs....79..102D Altcode: No abstract at ADS Title: Book Reviews: Discussion of Short-lived Solar Radio Transients Authors: de Jager, C.; van't Veer, F. Bibcode: 1959IrAJ....5..158D Altcode: No abstract at ADS Title: 36. The Influence of the Metal Abundance on the Continuous Stellar Spectra Authors: De Jager, C.; Neven, L. Bibcode: 1959LIACo...9..413D Altcode: 1959MSRSL...3..413D No abstract at ADS Title: The structure of the chromosphere and the low corona Authors: de Jager, C. Bibcode: 1959IAUS....9...89D Altcode: No abstract at ADS Title: Structure and Dynamics of the Solar Atmosphere. Authors: de Jager, Cornelis Bibcode: 1959HDP....52...80D Altcode: The undisturbed photosphere and chromosphere The undisturbed photosphere Granulation Temperature and density inhomogeneities in the photosphere Micro- and macro-turbulence in the photosphere Theory of convection and turbulence in the solar atmosphere The chromosphere The chromosphere at the limb The chromosphere on the disk Structure and dynamics of the chromosphere; transition to - the corona The disturbed parts of the photosphere and chromosphere Sunspots The individual spots Group of spots Photospheric and chromospheric faculae The faculae proper The "centre of activity" Flares and associated phenomena Monochromatic observations Spectrographic observations Dynamical phenomena associated with flares Filaments and prominences The quiescent prominences Moving prominences The corona Optical observations The quiet minimum corona; photometry, ionization and excitation The structure of the quiet minimum corona; temperatures and - densities The active parts of the corona and the "maximum corona" Radio emission from the Sun The quiet Sun Thermal radiation from centres of activity Non-thermal radiation: type III and type U bursts Other non-thermal radiophenomena, often connected with flares Solar rotation and the solar cycle The solar cycle Solar rotation and the Sun's general magnetic field Solar and terrestrial relationship General references Title: 27. Loss of Matter by Stars Authors: De Jager, C. Bibcode: 1959LIACo...9..280D Altcode: 1959MSRSL...3..280D No abstract at ADS Title: A discussion of short-lived solar radio transients at 200 Mc/s Authors: de Jager, Cornelis; van't Veer, F. Bibcode: 1958dsls.book.....D Altcode: 1958QB4.U8l14p1.... No abstract at ADS Title: 18. The Chromospheres of G-Type Stars Authors: De Jager, C. Bibcode: 1958LIACo...8..172D Altcode: No abstract at ADS Title: A discussion of short-lived solar radio transients at 200 Mc/s Authors: de Jager, C.; van't Veer, F. Bibcode: 1958VKAWA..21....1D Altcode: No abstract at ADS Title: The helium spectrum of the chromosphere Authors: de Jager, C.; de Groot, B. H. M. Bibcode: 1957BAN....14...21D Altcode: No abstract at ADS Title: Temperture-variation and turbulence in the low chromosphere Authors: de Jager, C. Bibcode: 1957BAN....13..275D Altcode: No abstract at ADS Title: The interpretation of hydrogen spectroheliograms Authors: de Jager, C. Bibcode: 1957BAN....13..133D Altcode: No abstract at ADS Title: Note on the complex light- and velocity-curves of DD Lacertae Authors: de Jager, C. Bibcode: 1957BAN....13..149D Altcode: 1956BAN....13..149D No abstract at ADS Title: Spectroscopic data for 50 model photospherese Authors: de Jager, Cornelis; Neven, L. Bibcode: 1957sdfm.book.....D Altcode: 1957QB4.U8l13p4.... No abstract at ADS Title: Spectroscopic data for 50 model photospheres Authors: de Jager, Cornelis; Neven, L. Bibcode: 1957AnOB....8.....D Altcode: 1957AnBel...8.....D No abstract at ADS Title: The Abundances of Diatomic Molecules of H, C, N, and O and of H2O in Stellar Atmospheres Authors: de Jager, C.; Neven, L. Bibcode: 1957LIACo...7..357D Altcode: No abstract at ADS Title: Some properties of solar radio-transients on fast 200 Mc./s. records Authors: de Jager, C.; van't Veer, F. Bibcode: 1957IAUS....4..366D Altcode: No abstract at ADS Title: A model of the upper photosphere from eclipse photometry of the solar limb Authors: Hubenet, H.; de Jager, C. Bibcode: 1956BAN....13...43H Altcode: No abstract at ADS Title: The occurrence of technetium in the Sun Authors: Greenstein, Jesse L.; de Jager, C. Bibcode: 1956BAN....13...13G Altcode: No abstract at ADS Title: The molecular absorption coefficient of the A00 band of O2 Authors: de Jager, C. Bibcode: 1956BAN....13....9D Altcode: No abstract at ADS Title: The distribution of Lyα radiation over the solar disk Authors: de Jager, C. Bibcode: 1956sei..conf..174D Altcode: No abstract at ADS Title: The profile of the Brackett α line in the solar spectrum Authors: de Jager, C.; Migeotte, M.; Neven, L. Bibcode: 1956AnAp...19....9D Altcode: No abstract at ADS Title: Ontstaan EN levensloop van sterren EN planeten. Authors: de Jager, Cornelis Bibcode: 1955olvs.book.....D Altcode: 1955QB801.J22...... No abstract at ADS Title: On the Influence of Temperature Inhomogeneities in the Solar Photosphere on the Profiles of the Hydrogen Lines; with Special Reference to Hα Authors: de Jager, C. Bibcode: 1955stat.conf..108D Altcode: 1954stat.conf..108D No abstract at ADS Title: The Capture of Zodiacal Dust by the Earth Authors: de Jager, C. Bibcode: 1955LIACo...6..174D Altcode: No abstract at ADS Title: High-energy Microturbulence in the Solar Photosphere Authors: de Jager, C. Bibcode: 1954Natur.173..680D Altcode: 1954Natur.173..680J THE Balmer, Paschen and Brackett lines of hydrogen in the solar spectrum are much wider than they should be according to computations based on the classical widening mechanism of the hydrogen lines1-the statistical Stark effect of the ions surrounding the emitting atoms. The amount of widening is greater for the Pasohen than for the Balmer lines; it is greatest for the Brackett lines. It seems impossible to ascribe this widening to atomic processes, since Jürgens2 has shown in elegant laboratory experiments that the profile of Hβ is explained by the statistical Stark effect of the ions. Hence, the abnormal widening of the hydrogen lines must be explained by a mechanism which is active in the solar photosphere but not in laboratory experiments. It is difficult to ascribe it to super-excitation since the wings of the hydrogen lines are formed in rather deep parts of the solar photosphere, where deviations from thermodynamical equilibrium are not very likely to occur. It can be shown that the widening is probably explained by the microturbulence of the solar photosphere, a mechanism suggested to me by Prof. A. Unsöld, of Kiel. Title: Observational models and helium abundances of the atmospheres of four B-type stars Authors: Neven, L.; de Jager, C. Bibcode: 1954BAN....12..103N Altcode: No abstract at ADS Title: The Uranium Content of a Stone Meteorite Authors: van Dijk, Th.; de Metter, J.; de Jager, C. Bibcode: 1954LIACo...5..495V Altcode: 1954MSRSL...1..495V; 1954pna..conf..495V No abstract at ADS Title: A New Determination of the Abundance of Deuterium in the Solar Atmosphere Authors: de Jager, C. Bibcode: 1954LIACo...5..460D Altcode: 1954MSRSL...1..460D; 1954pna..conf..460D No abstract at ADS Title: Observations of five stars suspected to belong to the class of β Canis Majoris stars Authors: de Jager, C. Bibcode: 1953BAN....12...91D Altcode: No abstract at ADS Title: Note on the pulsation of v Eridani and 16 Lacertae Authors: de Jager, C. Bibcode: 1953BAN....12...88D Altcode: No abstract at ADS Title: The photo-electric star photometer of the Utrecht Observatory Authors: de Jager, C. Bibcode: 1953BAN....12...93D Altcode: No abstract at ADS Title: The variation of light and radial velocity of DD Lacertae Authors: de Jager, C. Bibcode: 1953BAN....12...81D Altcode: No abstract at ADS Title: The interpretation of the observed hydrogen lines in the infrared solar spectrum II. Authors: de Jager, C.; Neven, L. Bibcode: 1952PRNAA..55..165D Altcode: 1952RNAAS..55..165D No abstract at ADS Title: The hydrogen spectrum of the sun Authors: de Jager, Cornelis Bibcode: 1952hss..book.....D Altcode: 1952QB4.U8l13p1.... No abstract at ADS Title: The hydrogen spectrum of the sun Authors: de Jager, Cornelis Bibcode: 1952RAOU....1.....D Altcode: No abstract at ADS Title: The Spectrum of Turbulence in the Earth's Upper Atmosphere Authors: de Jager, C. Bibcode: 1952LIACo...3..223D Altcode: No abstract at ADS Title: The Structure of the Metal Meteorites and the Problem of their Origin Authors: de Jager, C. Bibcode: 1952LIACo...4..289D Altcode: No abstract at ADS Title: The hydrogen spectrum of the sun Authors: de Jager, Cornelis Bibcode: 1952PhDT........29D Altcode: No abstract at ADS Title: Absence of Hydrogen Radiation of Wavelength 21 cm. in the Sun Authors: de Jager, C.; Minnaert, M.; Muller, C. A. Bibcode: 1951Natur.168..391D Altcode: OBSERVATIONS have been made at the Kootwijk Station of the Dutch Organization for Radio Astronomy, in order to ascertain whether hydrogen radiation of 21 cm. wave-length, detected in the galaxy, would be found also in the sun. The 7.50metre mirror and the receiver technique were the same as used for the investigations of the Galaxy*. No indication of a spectral line was found. The apparatus was sufficiently sensitive to detect a variation of 1/260 in the continuous radio spectrum. Title: The model of the solar atmosphere and the continuous absorption coefficient in the infrared Authors: Neven, L.; de Jager, C. Bibcode: 1951BAN....11..291N Altcode: No abstract at ADS Title: Interprétation des Mesures de Vitesses Radiales dans les Granules Solaires Authors: de Jager, C.; Pecker, J. C. Bibcode: 1951CRASB.232.1645D Altcode: No abstract at ADS Title: The profiles of the lines of the Paschen and Brackett series of hydrogen in the solar spectrum. Authors: de Jager, C.; Neven, L. Bibcode: 1950PRNAA..53..157D Altcode: 1950RNAAS..53..157D No abstract at ADS Title: Estimates of AZ Centauri Authors: de Jager, C. Bibcode: 1947BAN....10..248D Altcode: No abstract at ADS Title: Note on the distribution of light on Jupiter's bright satellites Authors: de Jager, C. Bibcode: 1946BAN....10...81D Altcode: No abstract at ADS