Off-limb (spicule) DEM distribution from SoHO/SUMER observations

TL;DR

This study derives the DEM distribution from off-limb spicule regions using SUMER data, revealing that AIA 171 Å emission during spicule observations mainly originates from transition region temperatures, not the corona.

Contribution

First to derive an off-limb DEM for spicules using SUMER data and CHIANTI, clarifying the thermal origin of AIA 171 Å emission in these regions.

Findings

AIA 171 Å emission during spicule observations is dominated by Fe IX at transition region temperatures.

Only 3.6% of plasma is above a million degrees in spicule regions, lower than quiet Sun estimates.

The thermal interpretation of AIA 171 Å data depends on the specific DEM used.

Abstract

In the present work we derive a Differential Emission Measure (DEM) dis- tribution from a region dominated by spicules. We use spectral data from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on-board the Solar Heliospheric Observatory (SoHO) covering the entire SUMER wavelength range taken off-limb in the Northern polar coronal hole to construct this DEM distribution using the CHIANTI atomic database. This distribution is then used to study the thermal properties of the emission contributing to the 171 {\AA} channel in the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO). From our off-limb DEM we found that the radiance in the AIA 171 {\AA} channel is dominated by emission from the Fe ix 171.07 {\AA} line and has sparingly little contribution from other lines. The product of the Fe ix 171.07 {\AA} line contribution function with the off-limb DEM was found to have a maximum at logTmax (K) = 5.8 indicating that during spicule observations the emission in this line comes from plasma at transition region temperatures rather than coronal. For comparison, the same product with a quiet Sun and prominence DEM were found to have a maximum at logT max (K) = 5.9 and logTmax (K) = 5.7, respectively. We point out that the interpretation of data obtained from the AIA 171 {\AA} filter should be done with foreknowledge of the thermal nature of the observed phenomenon. For example, with an off-limb DEM we find that only 3.6% of the plasma is above a million degrees, whereas using a quiet Sun DEM, this contribution rises to 15%.