Spectroscopic Observationsof Fe XVIII in

Solar Active Regions

Luca Teriaca,H. Warren, W. Curdt

Teriaca et al. 2012, ApJL (accepted)

The hot (>6 MK), out of flare, emission of active regions

• The amount of hot plasma in ARs is a fundamental information to understand the heating mechanism.

• Impulsive heating (nanoflares) in unresolved loop strands would produce significant emission at T~10 MK (e.g., Klimchuk & Cargill 2001, Cargill & Klimchuk 2004).

• Significant emission near 10 MK is found in X-ray broad-band imaging (e.g., Reale et al. 2009; Schmelz et al. 2009).

• XRT broad temperature response makes it difficult to identify high temperature, low emission measure plasma if there is cooler emission along the line of sight.

Reale et al. 2009

The hot (>6 MK), out of flare, emission of active regions

• Somehow contradictory results are coming from spectroscopy (DEMs peak around 4 MK: e.g., Warren et al. 2011, 2012).

5.6 MK

The hot (>6 MK), out of flare, emission of active regions

• Strong, unblended spectral lines formed above 6 MK are needed.

• Fe XVIII 974.86 (Tf=7.1 MK) is arguably the best, out of flare, line in the VUV spectrum.

NOAA 11339 – 8th Nov. 2011

SUMER status and performance

• Only detector B is operational.• Only the uncoated areas can

detect signal. • Bright lines produce an

electronic ghost 2.11 Åredward.

– H I Ly γ at 972.54 produces a ghost around 974.65.

• Likely loss of sensitivity.– Sensitivity loss and lower

voltage mitigates the electronic ghosts.

• Unreliable (but traceable) scanning mechanism.

– Data were re-sampled to a final 3.04″ resolution.

– Final FOV 211″×281″.

~30 Å

Ca XIV 943.63 Å Fe XVIII 976.86 Å

SUMER observations of hot plasma in a non flaring AR

Ghost and blends assessment: QS spectra from November 10th

The Si VIII 974.58 line - Off limb spectra

Spectra at top of bright loops

Spectra at foot of bright loops

Spectra at top of higher loops

SUMER observations of hot plasma in a non flaring AR

Method to subtract the warm component from AIA 94 images. Warren et al. 2012

AIA 94 vs. SUMER Fe XVIII 974.86

and Ca XIV 943.63

The Fe XVIII 974.86 line, is strong and unblended.

Significant signal from Fe XVIII in the AIA 94 channel.

Temperature analysis from Fe XVIII

974.86 to Ca XIV 943.63 line ratio.

• It seems unlikely that the relative calibration has changed across the small 943 to 975 Åspectral range.

• Similar first ionization potentials and charge-over-mass ratios. No reasons to expect substantial variations of their abundance ratio with respect to the value measured in the corona and in the photosphere.

• Isothermal approximation. No significant emission above 4 MK

Summary

• The Fe XVIII 974.86 line, is strong and unblended.

• The Ca XIV 943, although weaker, appears also substantially umblended. It probes cooler (about 4 MK) plasma.

• The observed ratio between the two lines indicates (isothermal approximation) that most of the plasma in the brighter Fe XVIII active region loops is at temperatures between 3.5 and 4 MK.

• We find that is possible to remove the contaminating blends and form relatively pure Fe XVIII images that are consistent with the spectroscopic observations from SUMER.

Solar orbiter: what we will see with SPICE?

• With SUMER (bare detector), we observed 2.15 count/s over the line at loop top in Fe XVIII and about 0.4 count/s in Ca XIV.

• SPICE (7mm2 at 100 nm), when considering the 2” resolution, is about as sensitive as SUMER bare detector.

• It should be possible to acquire good line profiles with about 25s exposures.

• SPICE will not be affected by electronic ghosting like SUMER.

Solar C: what we will see with LEMUR?

• With SUMER (bare detector), we do observe 2.15 count/s over the line at loop top in Fe XVIII and about 0.4 count/s in Ca XIV.

• LEMUR (7.2 cm2 at 95 nm), is about 100 times more efficient (at comparable resolution). There reason to believe that SUMER has lost sensitivity since 2007, so the efficiency gain is likely higher.

• It allows few second cadence observations of line radiances, profiles, and Doppler flows down to 2 km/s in hot (7 MK) plasma.

• LEMUR will not be affected by electronic ghosting like SUMER.