HOP 0155

abstract of observational proposal

Science goals: The study of helium line formation in the EUV solar spectrum can provide understanding of the physics of the lower transition region. Helium is one of the few elements that exhibits strong emission lines formed in the upper chromosphere/lower transition region and it is the second most abundant element in the Sun. However, analysing helium lines in the solar atmosphere is a particular challenging task, because of their optical thickness and the unusual behaviour of their intensities. Such a study requires spatially and spectrally resolved observations in the EUV range as well as sophisticated atomic modelling. The work focuses on to what extent differential emission measure (DEM) distributions can reproduce the observed fluxes of the He I and He II lines, as observed by the Hinode/EIS and SoHO/CDS. It will also examine what is the mechanism that enhances the helium line intensities compared to other transition region lines.

How the objective will be achieved: Hinode EIS observes a strong resonance line of He II at 256 Å. This will be combined with SoHO/CDS observations of all resonance lines of He I and the line of He II at 303.8 Å in 2nd order. The peculiar behaviour of He lines in the solar upper atmosphere will be investigated using DEM and the line ratios between the resonance lines of He I an He II, concentrating on the analysis of active regions. To achieve this, we request simultaneous observations using SoHO/CDS and Hinode/EIS for active region targets.

request to EIS

Slits: 1×256 arc sec2 2×256 arc sec2
Raster area: 60×256 arc sec2
Emission lines: Helium line: 256.3 Å.
Lines for co-alignment between instruments: O VI 184.1 Å, O V 192.9 Å, Si VII 275.3 Å, Si VII 275.7 Å, O IV 279.6 Å, O IV 279.9 Å.
Lines at high temperature (for DEM analysis): Fe X 184.5 Å, Fe XII 186.9 Å, Fe VIII 186.6 Å, Fe XI 192.8 Å, Fe XII 193.5 Å, Fe XVI 262.9 Å.

The following raster has been already validated and tested:

ral_he_int_2slit
Data rate: 23 kbits/s
Data volume: 16 Mbits
Cadence: 11 minutes, 51 seconds

remarks

Targets: Active regions on disk (preferably within +-5 arcmin from disk centre, but other disk locations will also be acceptable).

Duration of observations:
Active region (4 hours/day):
Raster: ral_he_int_2slit, repeat at least 10 times (2 hours) at two different pointings in the active region (2 hours at each pointing), point to bright transition region emission or strong magnetic field concentrations (but preferably not sunspot) (apply solar rotation tracking).
If possible, run on three different days or different active regions.