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Broadening of SI VIII lines observed in the solar polar coronal holes
Doyle, J. G.; Wilhelm, K.; Teriaca, L. +1 more
We study the variation of the line width and electron density as a function of height above two coronal holes from forbidden spectral lines of Si viii. The spectra were obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer flown on the Solar and Heliospheric Observatory spacecraft. The observations concentrate on the d…
An analytic solar magnetic field model
McKenzie, J. F.; Banaszkiewicz, M.; Axford, W. I.
We describe a simple analytic model for the magnetic field in the solar corona and interplanetary space which is appropriate to solar minimum conditions. The model combines an azimuthal current sheet in the equatorial plane with an axisymmetric multipole field representing the internal magnetic field of the Sun. The radial component of the field f…
Measurement of the electron temperature gradient in a solar coronal hole
Wilhelm, K.; Fludra, A.; Lemaire, P. +3 more
It has long been established that the high speed solar wind streams observed at 1 A.U. originate from the coronal hole regions of the Sun. Theoretical modelling of the acceleration mechanism depends critically on the value of the maximum of temperature existing close to the Sun. Measurements of the temperature in coronal holes prior to SOHO are un…
Quiet Sun EUV transient brightenings and turbulence. A panoramic view by EIT on board SOHO
Berghmans, D.; Clette, F.; Moses, D.
Since January 1996, the Extreme-Ultraviolet Imaging Telescope (EIT, onboard SOHO) has produced unique image sequences covering a wide field of view with a high temporal resolution, in the He Ii transition region line and in several Fe coronal emission lines. Using two sequences acquired with cadences of the order of one minute and with durations r…
Solar irradiances and radiances of UV and EUV lines during the minimum of sunspot activity in 1996
Wilhelm, K.; Curdt, W.; Lemaire, P. +6 more
Full Sun observations in UV and EUV emission lines were performed by SUMER (Solar Ultraviolet Measurements of Emitted Radiation) on SOHO (Solar and Heliospheric Observatory) in 1996. The radiometric pre-flight calibration of SUMER is traceable to a primary radiometric source standard - the electron storage ring BESSY. Based on this calibration, th…
A model of solar total and spectral irradiance variations
Solanki, S. K.; Unruh, Y. C.; Fligge, M. +2 more
We model solar spectral irradiance variations under the assumption that they are produced by sunspots and faculae alone. The model is based on three components, i.e. quiet Sun, assumed to be temporally invariant, sunspots and faculae whose temporal variations are mimicked using time series of sunspot areas and Mg II core-to-wing ratios. The detail…
Properties of the quiet Sun EUV network
Keenan, F. P.; Phillips, K. J. H.; Gallagher, P. T. +1 more
Observations of the quiet Sun network in a small region at Sun centre taken with the Coronal Diagnostic Spectrometer (CDS) on board SOHO are reported for EUV lines with T_e between ~ 10(4) and 10(6) K. The changing structure of the network in the upper chromosphere, transition region, up to the corona was examined using intensity distributions whi…
Solar H i Lyman alpha full disk profile obtained with the SUMER/SOHO spectrometer
Wilhelm, K.; Curdt, W.; Lemaire, P. +2 more
An uncalibrated solar hydrogen Lyman alpha profile was obtained with the use of the spectrometer on SOHO. The collection of the light scattered by the telescope permitted to average the profile over the full solar disk. The profile taken at the L1 Sun-Earth Lagrangian position is free of the central geocoronal absorption. Then, taking a…
Center-to-limb line width measurements of solar chromospheric, transition region and coronal lines
Doyle, J. G.; Wilhelm, K.; Erdelyi, R. +1 more
Line widths derived from observational data obtained by SUMER onboard SOHO in August 1996 at disk center and at the limb are used to confront those derived via assuming Alfven and/or magneto-acoustic wave heating. The observational data clearly shows a center-to-limb variation in the upper chromospheric and transition region lines, with only a mar…
Oscillations of sunspot magnetic fields
Solanki, S. K.; Scherrer, P. H.; Stenflo, J. O. +2 more
We report on velocity and magnetic field oscillations observed in sunspots using the MDI instrument onboard SOHO. In addition to the well-known velocity oscillations, the data clearly show highly localised oscillations of the magnetogram signal in different parts of the sunspots. We show that only oscillations of the magnetic field vector can prod…