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Cuntz, Manfred

Publications and source records attributed to Cuntz, Manfred.

11 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2006

The Supergranule Super-Rotation Illusion

Peculiar aspects of the rotation rate of the supergranules have been noted for over 20 years now. This has culminated in recent reports suggesting that the supergranules have wave-like characteristics and propagate prograde at a rate that exceeds that of the plasma anywhere below the surface. We have simulated supergranules that rotate at a rate that is independent of position or size and find that they appear to rotate at a more rapid rate. This super-rotation of the supergranules is seen in both cross-correlation and Fourier analyses of the Doppler velocity pattern. The amplitude of the rotation excess as a function of-size matches that seen in the Fourier analyses of MDI data. The source of this rotation excess is identified with the effect of projecting velocity signals into the line-of-sight. We conclude that supergranules are merely advected by the flow in the near-surface shear layer and that their apparent super-rotation does not indicate wave-like properties.

Hathaway, David H.↗
NASA NTRS2001

Shock Formation of Slow Magnetosonic Waves in Coronal Plumes

We investigate the height of shock formation in coroner plumes for slow magnetosonic waves. The models take into account plume geometric spreading, heat conduction and radiative damping. The wave parameters as well as the spreading functions of the plumes and the base magnetic field strengths are given by empirical constraints mostly from Solar and Heliospheric Observatory/Ultraviolet Coronagraph Spectrometer (SOHO/UVCS). Our models show that shock formation occurs at low coronal heights, i.e., within 1.3 solar radius, depending on the model parameters. The shock formation is calculated using the well-established wave breaking condition given by the intersection of C+ characteristics in the space-time plane. Our models show that shock heating by slow magnetosonic waves is expected to be relevant at most heights in solar coronal plumes, although slow magnetosonic waves are most likely not a solely operating energy supply mechanism.

Cuntz, Manfred↗
NASA NTRS2000

Shock Formation of Slow Magnetosonic Waves in Coronal Plumes

We investigate the height of shock formation in coronal plumes for slow magnetosonic waves. The models take into account plume geometric spreading, heat conduction, and radiative damping. The wave parameters as well as the spreading functions of the plumes and the base magnetic field strengths are given by empirical constraints mostly from Solar and Heliospheric Observatory (SOHO)/ Ultraviolet Coronograph Spectrometer (UVCS), Extreme Ultraviolet Imaging Telescope (EIT), Michelson Doppler Imager (MDI), and Large Angle Spectrometric Coronagraph (LASCO). Our models show that shock formation occurs at relatively low coronal heights, typically within 1.2 RsuN, depending on the model parameters. The shock formation is calculated using the well-established wave breaking criterion given by the intersection of C+ characteristics in the space-time plane. Our models show that shock heating by slow magnetosonic waves is expected to be relevant at most heights in solar coronal plumes, although such waves are probably not the main energy supply mechanism.

Cuntz, Manfred↗
NASA NTRS1996

Short-Term Chromospheric Variability in alpha Tauri (K5 III): Results from IUE Time Series Observations

We evaluate time series observations of chromospheric lines (Mg II, Mg I, and C II) for the K giant alpha Tau obtained using the IUE LWP camera at high dispersion. These observations cover a time span of about 2 weeks in 1994 February-March and were designed to resolve variations occurring within hours, days, and weeks. We consider the observational results in relation to theoretical acoustic heating models, motivated by the fact that alpha Tau may exhibit a basal (i.e., minimum) level of chromospheric activity. The data reveal flux variations between the extremes of 8% in Mg II h+k and 15% in each emission component. These variations occur on timescales as short as 8 hr but not on timescales longer than approx.3 days. For the h and k components, flux variations occurring on a timescale as short as 1.5 hr are also found. These changes are often not correlated (and are sometimes even anticorrelated), leading to remarkable differences in the h/k ratios. We argue that these results are consistent with the presence of strong acoustic shocks, which can lead to variable Mg II line emission when only a small number of strong shocks are propagating through the atmosphere. We deduce the electron density in the C II lambda 2325 line formation region to be log(base e) of N. approx. equals 9.0, in agreement with previous studies. Our data provide evidence that the Mg II basal flux limit for K giants might be a factor of 4 higher than suggested by Rutten et al.

Cuntz, Manfred↗
NASA NTRS1996

Chromospheric Heating in Late-Type Stars: Evidence for Magnetic and Nonmagnetic Surface Structure

The aim of this paper is to evaluate recent observational and theoretical results concerning the physics of chromospheric heating as inferred from IUE, HST-GHRS and ROSAT data. These results are discussed in conjunction with theoretical model calculations based on acoustic and magnetic heating to infer some conclusions about the magnetic and non-magnetic surface structure of cool luminous stars. I find that most types of stars may exhibit both magnetic and nonmagnetic structures. Candidates for pure nonmagnetic surface structure include M-type giants and super-giants. M-type supergiants are also ideal candidates for identifying direct links between the appearance of hot spots on the stellar surface (perhaps caused by large convective bubbles) and temporarily increased chromospheric heating and emission.

Cuntz, Manfred↗
NASA NTRS1992

Stochastic stellar wind models - Evolved stars

Observational, semiempirical, and theoretical results are presented for individual cool giants and supergiants which show that stochastic stellar wind flows are an extremely important feature in these stars. It is shown that the status of the existing theoretical models is very uncertain, due to the fact that appropriate models must simultaneously take into account a broad variety of strongly coupled physical processes. The potentially important physical processes include global oscillation modes; convection; magnetic fields; the generation, propagation, and interaction of stochastic waves; three-dimensional radiative transfer effects; time-dependent hydrogen ionization; thermal bifurcation and instabilities; and nonequilibrium chemistry, particularly the formation and destruction of molecules and dust.

Cuntz, Manfred↗
NASA NTRS1990

Stochastic shock waves as a candidate mechanism for the formation of the He I 10830-A line in cool giant stars

The question of whether strong shocks produced in time-dependent stochastic wave models can explain the formation of the He I 10830-A line in cool giant stars is examined. The research is based on the ab initio chromosphere model for Arcturus by Cuntz (1987), showing that a stochastic distribution of wave periods leads to overtaking and merging of shocks, which occasionally produce very strong shocks with temperatures larger than 40,000 K in the postshock regions. These temperatures can easily produce a significant population in the 2s 3S state by electron collisional excitation. The 10830-A line occurs in absorption when the densities in the shocks exceed 10 million/cu cm.

Cuntz, Manfred↗
NASA NTRS1990

On the generation of mass loss in cool giant stars due to propagating shock waves

This paper investigates mass loss in late-type giant stars produced by propagating shock waves, assuming both monochromatic waves and acoustic frequency spectra. In monochromatic wave computations, the wave period is varied by a factor of 50. The wave models show that short-period waves produced in stellar convective zones do not play any direct role in generating the observed mass loss, except for episodic mass-loss events. However, these waves are important for heating stellar chromospheres. Continuous mass-loss solutions are obtained when the wavelengths are comparable to or larger than a stellar radius. In the case of Arcturus, a mass-loss rate between 10 to the -10th and 10 to the -11th solar mass/yr is found, and the final flow speed of the wind is larger than 40 km/s, which are close to the observed values.

Cuntz, Manfred↗
NASA NTRS1990

Chromospheric extents predicted by time-dependent acoustic wave models

Theoretical models for chromospheric structures of late-type giant stars are computed, including the time-dependent propagation of acoustic waves. Models with short-period monochromatic shock waves as well as a spectrum of acoustic waves are discussed, and the method is applied to the stars Arcturus, Aldebaran, and Betelgeuse. Chromospheric extent, defined as the monotonic decrease with height of the time-averaged electron densities, are found to be 1.12, 1.13, and 1.22 stellar radii for the three stars, respectively; this corresponds to a time-averaged electron density of 10 to the 7th/cu cm. Predictions of the extended chromospheric obtained using a simple scaling law agree well with those obtained by the time-dependent wave models; thus, the chromospheres of all stars for which the scaling law is valid consist of the same number of pressure scale heights.

Cuntz, Manfred↗
NASA NTRS1990

The He I 10830 line in Arcturus produced by stochastic shocks

An investigation is conducted as to whether strong shocks produced in time-dependent stochastic wave models can explain the formation of the He I 10830 line in a cool giant star like Arcturus. The exploratory research is based on the work of Cuntz (1987) who found that stochastic waves lead to overtaking and merging of shocks producing occasionally very strong shocks with temperatures larger than 40,000 K in the postshock regions.

Cuntz, Manfred↗