E. Tandberg-Hanssen

The Lyman-alpha line in various solar features. I: Observations

Lyman-alpha line observations of the solar atmosphere obtained by the SMM ultraviolet spectrometer and polarimeter are presented. High spectral and spatial resolution data and broadband spectroheliograms show that, on the disk, the central reversal of the line is highly variable in depth and is frequently shifted, leading to asymmetric profiles. It is suggested that a dynamic layer overlying the limb may be responsible for distorting the background profile of the line and producing the observed asymmetric profiles. Narrow and extended prominences reaching velocities of several hundred km/sec are also noted.

Steady flows in the solar transition region observed with SMM

Steady flows in the quiet solar transition region have been observed with the Ultraviolet Spectrometer and Polarimeter (UVSP) experiment on the Solar Maximum Mission (SMM) satellite. The persistent vertical motions seen at disk center have spatial rms amplitudes of 1.4 km s/sup -1/ in the C II line, 3.9 km s/sup -1/ in Si IV, and 4.2 km s/sup -1/ in C IV. The amplitudes of the more horizontal flows seen toward the limb tend to be somewhat higher. Plots of steady vertical velocity versus intensity seen at disk center in Si IV and C IV show two distinct branches.

Vertical gradients of sunspot magnetic fields

In this paper, we describe results of a Solar Maximum Mission (SMM) guest investigation to determine vertical gradients of sunspot magnetic fields for the first time from coordinated observations of photospheric and transition-region fields. Both the photospheric vector field of a sunspot, derived from observations using the NASA Marshall Space Flight Center vector magnetograph, and the line-of-sight component in the transition region, obtained from the SMM Ultraviolet Spectrometer and Polarimeter instrument, are described. From these data, vertical gradients of the line-of-sight magnetic field component are calculated using three methods. (1) The vertical gradient is derived directly from the observations assuming a height difference of 2000 km between the photosphere and transition region. (2) Using the observed transverse photospheric field, the initial gradient (ΔBz/Δz)z = 0, is calculated from the condition ▽ · B = 0. (3) Using the photospheric line-of-sight component as the boundary condition in a potential-field calculation, the extrapolated potential field at different heights is compared to the observed transition-region field; from these comparisons, an average height difference is derived and used to calculate the average vertical gradient (ΔBz/Δz). Comparisons of gradients derived from these three methods show consistent results for methods (2) and (3). Deviations of the calculated potential transverse field at z = 0 from the observed transverse component are investigated to assess the validity of gradient calculations using method (3). Since the field is shown to be very close to a potential distribution, we conclude that the vertical gradient of Bz is lower than values from previous studies and the transition-region field occurs at a height of ≈ 4000–6000 km above the photosphere.

Observations of the longitudinal magnetic field in the transition region and photosphere of a sunspot

The Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission spacecraft has observed for the first time the longitudinal component of the magnetic field by means of the Zeeman effect in the transition region above a sunspot. The data presented here were obtained on three days in one sunspot, have spatial resolutions of 10 arc sec and 3 arc sec, and yield maximum field strengths greater than 1000 G above the umbrae in the spot. The method of analysis, including a line-width calibration feature used during some of the observations, is described in some detail in an appendix; the line width is required for the determination of the longitudinal magnetic field from the observed circular polarization.The transition region data for one day are compared with photospheric magnetograms from the Marshall Space Flight Center. Vertical gradients of the magnetic field are computed from the two sets of data; the maximum gradients of 0.41 to 0.62 G km−1 occur above the umbra and agree with or are smaller than values observed previously in the photosphere and low chromosphere.

The role of condensation and heat conduction in the formation of prominences - An MHD simulation

In this paper we investigate the effects of condensation and thermal conduction on the formation of Kippenhahn-Schlüter (K-S) type prominences in quiet regions (QP) due to symmetric mass injection. To implement this investigation, a self-consistent, two-dimensional, non-planar, time-dependent magneto-hydrodynamic (MHD) simulation model is developed. In the model, we use various values of the injection velocity, density, and magnetic field strength to determine the most favorable conditions for the QP formation. Based on these simulation results, we find that the formation of a K-S-type field configuration should be considered as a dynamic process, which needs both condensation and mass injection to supply enough mass to maintain such a configuration to complete the formation process of quiescent prominence.

Dynamic evolution of recurrent mass ejections observed in Hα and Civ lines

During a coordinated SMY program, the consecutive formation of two new active centers merging together within AR 2646 was observed from 28 August, to 5 September, 1980. The two preceding spots compressed an inverse polarity spot on 1 September 1980, causing recurrent ejecta of matter with time intervals around 10 min. The observations of the MSDP spectrograph operating in Hα at the Meudon Solar tower and of the UVSP spectrometer on SMM in the Civ 1548 Å line show that cold and hot material had the same projection, although the upward Civ velocity structure was more extended than the Hα one. We present evidence that observed contrasts of the Hα absorbing structure can be interpreted in terms of a dynamic ‘cloud model’ overlying the chromosphere. Hα matter follows a magnetic channel with upward velocity around 20–30 km s−1 in the first phase of the event and with downward velocity (∼ - 40 km s−1) in the second phase. The stored energy is not sufficient to trigger a flare, nor even to propulse matter along the full length of an arch, because of the periodic reorganisation of the magnetic field.

Evidence for systematic flows in the transition region around prominences

The solar transition region in the neighbourhood of prominences has been studied from observations with the Ultraviolet Spectrometer and Polarimeter of NASAs Solar Maximum Mission satellite. Dopplergrams from observations of the transition-region lines Civ λ 1548 Å and Siiv λ 1393 Å, which are formed at about 105 K, give velocity amplitudes typically in the range ± 15 km s-1. Prominences are found to be located very close to dividing lines between areas of up- and down-draughts in the transition-region. The observed pattern suggests that the 105 K gas flows take place within arcades of magnetic loops, which most likely are part of the supporting magnetic structure for the prominence matter. An additional band of blue-ward Doppler shifts is frequently seen close to quiescent prominences. This may be the source of outward flowing matter along the helmet streamers above filament channels.

Photospheric electric current and transition region brightness within an active region

Distributions of vertical electric current density (Jz) calculated from vector measurements of the photospheric magnetic field are compared with ultraviolet spectroheliograms to investigate whether resistive heating is an important source of enhanced emission in the transition region. The photospheric magnetic fields in Active Region 2372 were measured on 6 and 7 April, 1980 with the MSFC vector magnetograph; ultraviolet wavelength spectroheliograms (Lα and Nv 1239 Å) were obtained with the UVSP experiment aboard the Solar Maximum Mission satellite. Spatial registration of the Jz (5 arc sec resolution) and UV (3 arc sec resolution) maps indicates that the maximum current density is cospatial with a minor but persistent UV enhancement, but there is little detected current associated with other nearby bright areas. We conclude that although resistive heating may be important in the transition region, the currents responsible for the heating are largely unresolved in our measurements and have no simple correlation with the residual current measured on 5 arc sec scales.

Preliminary observations and results obtained with the ultraviolet spectrometer and polarimeter

New observation with the Ultraviolet Spectrometer and Polarimeter (UVSP) of a number of manifestations of solar activity obtained during the first three months of Solar Maximum Mission operations are presented. Attention is given to polarimetry in sunspots, oscillations above sunspots, density diagnostics of transition-zone plasmas in active regions, and the eruptive prominence - coronal transient link.

Magnetohydrodynamic models of coronal transients in the meridional plane. IV - Effect of the solar wind

A two-dimensional, time-dependent magnetohydrodynamic model in the meridional plane with and without an ambient solar wind in an ambient radial magnetic field has been used to investigate mass motions associated with coronal transients. It is shown that the solar wind does not significantly affect the general dynamic characteristics of the mass motion. The ambient solar wind, however, increases the velocity of the mass motion and produces a moderate change in the thermodynamic properties of the coronal plasma.