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Proposal ID Macpherson_129

Date of Proposal



University of Oxford

Dept of Physics (Theoretical Physics)
Oxford University
1 Keble Road
Oxford OX1 3NP


(44) 1865 273983

(44) 1865 273947

Authors:Carole Jordan and Keith Macpherson (Oxford), Richard Harrison (CDS)
Bob Stern (EIT), Julia Saba (MDI), Philippe Lemaire (SUMER)

To investigate the systematic behaviour of He I and He II line fluxes
in comparison with atmospheric models determined from other transition
region lines.

Scientific case:
The overall scientific case is as set out in our original proposal
HELEN which has appeared in the CDS Blue Book, and as study
(POP 22) in one version of the SUMER Red Book. Briefly, Jordan (1975,
MNRAS, 170, 429), found that lines of He I and He II appear to have
higher fluxes than expected from models made using other transition
region lines formed at similar temperatures, and proposed that this
enhancement could arise through excitation by electrons with
temperatures higher than those expected in ionization equilibrium. The
enhancement factor was found to be less in coronal holes than in the
quiet Sun. The first objective is to examine this behaviour at the
higher spatial resolution of the CDS and SUMER instruments, by
observing several quiet Sun regions, from Sun-centre to near the limb,
and a coronal hole. With the benefit of the actual CDS spectra, we
propose to use the NIS spectra rather then the GIS spectra. To obtain
the same results, the NIS spectra require far shorter exposure times,
cover the same range of transition region temperature, provide the
He II (304 A) line in second order, as well as the He I lines. The
SUMER spectra required are the same. I.e. we need to obtain spectra
around the wavelengths of the Si IV lines, which are transition region
lines formed around the same temperature as He II under `normal'
circumstances. Such spectra also include the O IV] lines from which the
electron density can be measured. The latter are essential for
quantitative modelling.

In a later paper Jordan (1980, Phil. Trans, Roy. Soc. London A297,
541) discussed how the enhancements might depend on measurable
transition region parameters (electron pressure, temperature gradient,
non-thermal velocities) and stressed that dynamic effects must be taken
into account. The second aim is to test the correlations proposed by
using also the widths of the Si IV and O IV] lines observed with SUMER.

Observations with the EIT will give the type of structure in and
around the field of view to be observed. Coincident MDI spectrograms
would give information on the magnetic field which has so far not been
available in the context of the helium lines.

Operating Details:

See full details under JOP 62 proposal on SOHO JOP WWW page

Required EIT data:

JOP 62 was run on April 25th and 27th 1997. We request synoptic EIT images
for these dates, primarily in He II 304 although the other images would be
useful as well. This will allow us to correlate the EIT data with the CDS
and SUMER observations to observe the general structure and type of region
under study. MDI magnetogram data for the same regions is also under

EIT Team Member:

Bob Stern at Lockheed has agreed to be our EIT Contact.

Bob Stern at Lockheed has agreed to be our EIT Contact.





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