arXiv subjects

S. V. H. Haugan

Publications and source records attributed to S. V. H. Haugan.

239 records · Page 14Linked to original sources

arXiv1998

Flows in Sunspot Plumes Detected with SOHO

Bright EUV sunspot plumes have been observed in eight out of eleven different sunspot regions with the Coronal Diagnostic Spectrometer -- CDS on SOHO. From wavelength shifts we derive the line-of-sight velocity, relative to the average velocity in the rastered area, 120 arcsec x 120 arcsec. In sunspot plumes we find that the motion is directed away from the observer and increases with increasing line formation temperature, reaches a maximum between 15 and 41 km~s$^{-1}$ close to log T $\approx$ 5.5, then decreases abruptly. The flow field in the corona is not well correlated with the flow in the transition region and we discuss briefly the implication of this finding.

astro-ph↗

arXiv1998

EUV Sunspot Plumes Observed with SOHO

Bright EUV sunspot plumes have been observed in five out of nine sunspot regions with the Coronal Diagnostic Spectrometer -- CDS on SOHO. In the other four regions the brightest line emissions may appear inside the sunspot but are mainly concentrated in small regions outside the sunspot areas. These results are in contrast to those obtained during the Solar Maximum Mission, but are compatible with the Skylab mission results. The present observations show that sunspot plumes are formed in the upper part of the transition region, occur both in magnetic unipolar-- and bipolar regions, and may extend from the umbra into the penumbra.

astro-ph↗

arXiv1995

Simulation of microlensing lightcurves by combining contouring and rayshooting

The contouring methods described by Lewis et al. (1993) and Witt (1993) are very efficient and elegant for obtaining the magnification of a point source moving along a straight track in the source plane. The method is, however, not very efficient for extended sources, because the amplification needs to be computed for numerous parallel tracks and then convolved with the source profile. Rayshooting is an efficient algorithm for relatively large sources, but the computing time increases with the inverse of the source area for a given noise level. This poster presents a hybrid method, using the contouring method in order to find only those parts of the lens area that contribute to the light curve through the rayshooting. Calculations show that this method has the potential to be $10$--$10^5$ times more efficient than crude rayshooting techniques.

astro-ph↗

arXiv1995

Separating intrinsic and microlensing variability using parallax measurements

In gravitational lens systems with 3 or more resolved images of a quasar, the intrinsic variability may be unambiguously separated from the microlensing variability through parallax measurements from 3 observers when there is no relative motion of the lens masses (Refsdal 1993). In systems with fewer than 3 resolved images, however, this separation is not straightforward. A general approach that may be used for this purpose is presented. For simplicity, only the one-dimensional case is considered in detail: Given a well-sampled time series of the observed flux at two points in space with a known separation, choosing a velocity $v_{\perp}$ of the observers perpendicular to the line of sight determines the microlensing magnification history, and thereby also the intrinsic variability. The velocity is chosen by minimizing some measure ($χ^2$) of the residual intrinsic variability. In many cases this gives a close approximation to the true magnification. In cases where the relative motion of the lensing point masses is important, only a partial separation will be possible.

astro-ph↗

arXiv1995

The microlensing events in Q2237+0305A: No case against small masses/large sources

It is demonstrated that the 1988-90 microlensing events in image A of Q2237+0305 reported by Racine (1992) do not exclude microlensing models with very low average mass, making the source radius larger than the projected Einstein radius $η_0$ (Refsdal and Stabell 1991, 1993). This is contrary to what has been claimed by Witt and Mao (1994). Since these events are the best resolved microlensing events recorded in Q2237+0305, further work should not exclude the possibility of a large source when interpreting lightcurve data.

astro-ph↗