Network


Latest external collaboration on country level. Dive into details by clicking on the dots.

Hotspot


Dive into the research topics where Wolfgang Kalkofen is active.

Publication


Featured researches published by Wolfgang Kalkofen.


Solar Physics | 1971

The Harvard-Smithsonian reference atmosphere

Owen Gingerich; R. W. Noyes; Wolfgang Kalkofen; Y. Cuny

We present a model of the solar atmosphere in the optical depth range from τ5000 = 10−8 to 25. It combines an improved model of the photosphere that incorporates recent EUV observations with a new model of the quiet lower chromosphere. The latter is based on OSO 4 observations of the Lyman continuum, on infrared observations, and on eclipse electron densities.Our model differs from the Bilderberg Continuum Atmosphere (BCA) in the low chromosphere (τ5000 < 10−4), where deviations from local thermodynamic equilibrium in hydrogen and carbon have been taken into account. It also differs in the transition region between the chromosphere and the photosphere (10−4 < τ5000 < 10−2), where the temperature is lower than in the BCA, and in the convective region (τ5000 ≳ 2), where the temperature is higher than in the BCA.


The Astrophysical Journal | 1993

Dynamics of the solar chromosphere. I - Long-period network oscillations

Bruce W. Lites; Robert J. Rutten; Wolfgang Kalkofen

We analyze differences in solar oscillations between the chromospheric network and internetwork regions from a 1 hr sequence of spectrograms of a quiet region near disk center. The spectrograms contain Ca II H, Ca I 422.7 nm, and various Fe I blends in the Ca II H wing. They permit vertical tracing of oscillations throughout the photosphere and into the low chromosphere. We find that the rms amplitude of Ca II H line center Doppler fluctuations is ∼1.5 km s -1 for both network and internetwork, but that the character of the oscillations differs markedly in these two regions. Within internetwork areas the chromospheric velocity power spectrum is dominated by oscillations with frequencies at and above the acoustic cutoff frequency


The Astrophysical Journal | 1999

Excitation of Oscillations in Photospheric Flux Tubes through Buffeting by External Granules

Syed Hasan; Wolfgang Kalkofen

We examine the excitation of transverse (kink) and longitudinal (sausage) waves in magnetic flux tubes by granules in the solar photosphere. The investigation is motivated by the interpretation of network oscillations in terms of flux tube waves. We model the interaction between a granule, with a specified transverse velocity, and a vertical flux tube in terms of the Klein-Gordon equation, which we solve analytically as an initial value problem for both wave modes, assuming the same external impulse. The calculations show that for magnetic field strengths typical of the network, the energy flux in transverse waves is higher than in longitudinal waves by an order of magnitude, in agreement with the chromospheric power spectrum of network oscillations observed by Lites, Rutten, & Kalkofen. But for weaker fields, such as those that might be found in internetwork regions, the energy fluxes in the two modes are comparable. This result implies that if there are internetwork oscillations in magnetic flux tubes, they must show the cutoff periods of both longitudinal and transverse modes at 3 minutes and at 7 minutes or longer. We also find that granules with speeds of about 2 km s-1 can efficiently excite transverse oscillations in frequent short-duration (typically 1 minute) bursts that can heat the corona.


The Astrophysical Journal | 2005

Dynamics of the Solar Magnetic Network: Two-dimensional MHD Simulations

S. S. Hasan; A. A. van Ballegooijen; Wolfgang Kalkofen; O. Steiner

The aim of this work is to identify the physical processes that occur in the network and contribute to its dynamics and heating. We model the network as consisting of individual flux tubes, each with a nonpotential field structure, that are located in intergranular lanes. With a typical horizontal size of about 150 km at the base of the photosphere, they expand upward and merge with their neighbors at a height of about 600 km. Above a height of approximately 1000 km the magnetic field starts to become uniform. Waves are excited in this medium by means of motions at the lower boundary. We focus on transverse driving, which generates both fast and slow waves within a flux tube and acoustic waves at the interface of the tube and the ambient medium. The acoustic waves at the interface are due to compression of the gas on one side of the flux tube and expansion on the other. These longitudinal waves are guided upward along field lines at the two sides of the flux tube, and their amplitude increases with height due to the density stratification. Being acoustic in nature, they produce a compression and significant shock heating of the plasma in the chromospheric part of the flux tube. For impulsive excitation with a time constant of 120 s, we find that a dominant feature of our simulations is the creation of vortical motions that propagate upward. We have identified an efficient mechanism for the generation of acoustic waves at the tube edge, which is a consequence of the sharp interface of the flux concentration. We examine some broad implications of our results.


The Astrophysical Journal | 1999

DOES THE SUN HAVE A FULL-TIME CHROMOSPHERE?

Wolfgang Kalkofen; Peter Ulmschneider; Eugene H. Avrett

The successful modeling of the dynamics of H2v bright points in the nonmagnetic chromosphere by Carlsson & Stein gave as a by-product a part-time chromosphere lacking the persistent outward temperature increase of time-average empirical models, which is needed to explain observations of UV emission lines and continua. We discuss the failure of the dynamical model to account for most of the observed chromospheric emission, arguing that their model uses only about 1% of the acoustic energy supplied to the medium. Chromospheric heating requires an additional source of energy in the form of acoustic waves of short period (P < 2 minutes), which form shocks and produce the persistent outward temperature increase that can account for the UV emission lines and continua.


The Astrophysical Journal | 2003

KINK AND LONGITUDINAL OSCILLATIONS IN THE MAGNETIC NETWORK ON THE SUN: NONLINEAR EFFECTS AND MODE TRANSFORMATION

S. S. Hasan; Wolfgang Kalkofen; A. A. van Ballegooijen; Peter Ulmschneider

We examine the propagation of kink and longitudinal waves in the solar magnetic network. Previously, we investigated the excitation of network oscillations in vertical magnetic flux tubes through buffeting by granules and found that footpoint motions of the tubes can generate sufficient wave energy for chromospheric heating. We assumed that the kink and longitudinal waves are decoupled and linear. We overcome these limitations by treating the nonlinear MHD equations for coupled kink and longitudinal waves in a thin flux tube. For the parameters we have chosen, the thin tube approximation is valid up to the layers of formation of the emission features in the H and K lines of Ca II, at a height of about 1 Mm. By solving the nonlinear, time-dependent MHD equations we are able to study the onset of wave coupling, which occurs when the Mach number of the kink waves is of the order of 0.3. We also investigate the transfer of energy from the kink to the longitudinal waves, which is important for the dissipation of the wave energy in shocks. We find that kink waves excited by footpoint motions of a flux tube generate longitudinal modes by mode coupling. For subsonic velocities, the amplitude of a longitudinal wave increases as the square of the amplitude of the transverse wave, and for amplitudes near Mach number unity, the coupling saturates and becomes linear when the energy is nearly evenly divided between the two modes.


The Astrophysical Journal | 1997

Oscillations in Chromospheric Network Bright Points

Wolfgang Kalkofen

Intensity oscillations observed in the H and K lines of Ca II in network bright points in the quiet Sun are interpreted in terms of transverse and longitudinal magnetoacoustic waves propagating upward inside magnetic flux tubes. It is supposed that the waves are generated impulsively in the photosphere as transverse waves. As they propagate upward, their velocity amplitude increases exponentially until they become nonlinear in the chromosphere, where they transfer power to longitudinal waves. The impulsive generation produces waves at the cutoff frequency of transverse waves. On the assumption that this frequency signature is transferred to the longitudinal waves, the magnetic field strength implied by the period observed in the chromosphere is consistent with the Zeeman effect observed in the photosphere.


Solar Physics | 1970

The solar Lyman continuum and the structure of the solar chromosphere

R. W. Noyes; Wolfgang Kalkofen

Data on the spectrum and center-to-limb variation of the solar Lyman continuum have been analyzed. They show: (a) The brightness temperature of the Lyman continuum is about 6500 K, but the kinetic temperature, as deduced from the slope of the continuum, lies between 8000 and 9000 K. The difference between the kinetic temperature and the brightness temperature requires that the source function be smaller than the Planck function by a factor of several hundred. (b) The Lyman continuum exhibits slight limb darkening longward of 825 Å, and slight limb brightening shortward of 750 Å. The crossover point varies from equator to pole and with solar activity. (c) The slope d ln I(λ)/dλ of the Lyman continuum decreases toward the limb, implying that the kinetic temperature increases outward in the region of Lyman continuum formation.Using radiative transfer calculations for a plane-parallel atmosphere in hydrostatic equilibrium, we have derived a homogeneous model of the upper chromosphere that reproduces the main features of the observations. It is characterized by a temperature of 8300 K and a pressure of about 0.15 dyne/cm2 at τLyc = 1, and it has an abrupt temperature rise at a height of 1500 km above the limb. More precise agreement with the observations will require a detailed treatment of the inhomogeneous nature of the upper chromosphere.


The Astrophysical Journal | 2001

The Case against Cold, Dark Chromospheres

Wolfgang Kalkofen

Is the solar chromosphere always hot, with relatively small temperature variations (δT/T ~ 0.1), or is it cold most of the time, with temperature fluctuations that reach δT/T ~ 10 at the top of the chromosphere? Or, equivalently, is the chromosphere heated continually or only for a few seconds once every 3 minutes? Two types of empirical model, one essentially time independent and always hot, the other highly time dependent and mostly cold, come to fundamentally different conclusions. This paper analyzes the time-dependent model of the quiet, nonmagnetic chromosphere by Carlsson & Stein and shows that it predicts deep absorption lines, none of which are observed; intensity fluctuations in the Lyman continuum that are much larger than observed; and time-averaged emission that falls far short of the observed emission. The paper concludes that the solar chromosphere, while time-dependent, is never cold and dark. The same conclusion applies for stellar chromospheres. A complete, time-dependent model of the nonmagnetic chromosphere must describe two phenomena: (1) dynamics, like that modeled by Carlsson & Stein for chromospheric bright points but corrected for the geometrical properties of shocks propagating in an upward-expanding channel, and (2) the energetically more important general, sustained heating of the chromosphere, as described by current time-independent empirical models but modified in the upper photosphere for the formation of molecular absorption lines of CO in a dynamical medium. This model is always hot and, except for absorption features caused by departures from local thermodynamic equilibrium, shows chromospheric lines only in emission.


The Astrophysical Journal | 2007

Is the Solar Chromosphere Heated by Acoustic Waves

Wolfgang Kalkofen

Space observations with TRACE have measured only 10% of the energy flux required to heat the nonmagnetic part of the solar chromosphere and have thereby called into question the theory of chromospheric heating by acoustic waves. To explain the deficit in the measured flux, heating by processes related to the magnetic field and the limited spatial resolution of the space observations have been invoked. This paper argues that radiation emerging from the nonmagnetic chromosphere shows that the heating mechanism is dissipation of acoustic waves. The full energy flux required for acoustic heating of the chromosphere must therefore pass through the photosphere. The explanation of the missing flux by the limited spatial resolution of TRACE confirms the principle of the effect, but the test is preliminary since the hydrodynamic model on which the test is based has temperature fluctuations that far exceed those of the Sun. The shape of the acoustic spectrum observed with TRACE appears to support the theory of wave generation in the solar convection zone. But the low energy flux and the limited acoustic frequency range of the observations prevent a definitive conclusion.

Collaboration


Dive into the Wolfgang Kalkofen's collaboration.

Top Co-Authors

Avatar
Top Co-Authors

Avatar

S. S. Hasan

Indian Institute of Astrophysics

View shared research outputs
Top Co-Authors

Avatar
Top Co-Authors

Avatar

Bruce W. Lites

National Center for Atmospheric Research

View shared research outputs
Top Co-Authors

Avatar

David M. Rust

Johns Hopkins University Applied Physics Laboratory

View shared research outputs
Top Co-Authors

Avatar
Top Co-Authors

Avatar

R. W. Noyes

Smithsonian Astrophysical Observatory

View shared research outputs
Top Co-Authors

Avatar
Top Co-Authors

Avatar
Top Co-Authors

Avatar
Researchain Logo
Decentralizing Knowledge