Sounding Rocket Observations of Active Region Soft X-Ray Spectra Between 0.5 and 2.5 nm Using a Modified SDO/EVE Instrument

Seth Wieman,Andrew Jones,Thomas Woods,Leonid Didkovsky,Christopher Moore

doi:10.1007/s11207-016-0999-6

Abstract

Spectrally resolved measurements of individual solar active regions (ARs) in the soft X-ray (SXR) range are important for studying dynamic processes in the solar corona and their associated effects on the Earth’s upper atmosphere. They are also a means of evaluating atomic data and elemental abundances used in physics-based solar spectral models. However, very few such measurements are available. We present spectral measurements of two individual ARs in the 0.5 to 2.5 nm range obtained on the NASA 36.290 sounding rocket flight of 21 October 2013 (at about 18:30 UT) using the Solar Aspect Monitor (SAM), a channel of the Extreme Ultaviolet Variability Experiment (EVE) payload designed for underflight calibrations of the orbital EVE on the Solar Dynamics Observatory (SDO). The EVE rocket instrument is a duplicate of the EVE on SDO, except the SAM channel on the rocket version was modified in 2012 to include a freestanding transmission grating to provide spectrally resolved images of the solar disk with the best signal to noise ratio for the brightest features, such as ARs. Calibrations of the EVE sounding rocket instrument at the National Institute of Standards and Technology Synchrotron Ultraviolet Radiation Facility (NIST/SURF) have provided a measurement of the SAM absolute spectral response function and a mapping of wavelength separation in the grating diffraction pattern. We discuss techniques (incorporating the NIST/SURF data) for determining SXR spectra from the dispersed AR images as well as the resulting spectra for NOAA ARs 11877 and 11875 observed on the 2013 rocket flight. In comparisons with physics-based spectral models using the CHIANTI v8 atomic database we find that both AR spectra are in good agreement with isothermal spectra (4 MK), as well as spectra based on an AR differential emission measure (DEM) included with the CHIANTI distribution, with the exception of the relative intensities of strong Fe xvii lines associated with $2p^{6}$ – $2p^{5}3{s}$ and $2p^{6}$ – $2p^{5}3{d}$ transitions at about 1.7 nm and 1.5 nm, respectively. The ratio of the Fe xvii lines suggests that the AR 11877 is hotter than the AR 11875. This result is confirmed with analysis of the active regions imaged by X-ray Telescope (XRT) onboard Hinode.

Highlights

Solar soft X-ray (SXR) (0.1 – 10 nm) emissions originate from high-temperature plasma in the solar corona and vary dramatically with solar activity both in terms of band-integrated intensity and spectral distribution (Neupert, 2006)
In comparisons with physics-based spectral models using the CHIANTI v8 atomic database we find that both active regions (ARs) spectra are in good agreement with isothermal spectra (4 MK), as well as spectra based on an AR differential emission measure (DEM) included with the CHIANTI distribution, with the exception of the relative intensities of strong Fe xvii lines associated with 2p6–2p53s and 2p6–2p53d transitions at about 1.7 nm and 1.5 nm, respectively
This result is confirmed with analysis of the active regions imaged by X-ray Telescope (XRT) onboard Hinode

Summary

Introduction

Solar soft X-ray (SXR) (0.1 – 10 nm) emissions originate from high-temperature plasma in the solar corona and vary dramatically with solar activity both in terms of band-integrated intensity and spectral distribution (Neupert, 2006). The literature concerning ions emitting in the SXR range shows vast disagreement between spectral line intensities from (the limited number of) solar, stellar, and laboratory plasma observations, and those modeled based on available atomic data are often reported (Brown et al, 1998; Laming et al, 2000; Del Zanna, 2011) Such discrepancies are common for the Fe xvii ion with emissions that are dominant in the solar measurements reported in this work. This article reports solar SXR spectral measurements of two individual active regions (ARs), NOAA numbers 11877 and 11875, observed on a 2013 sounding rocket flight with the EVE/SAM pinhole camera which, for the sounding rocket version of the instrument, includes a freestanding transmission grating that allows spectra to be determined for bright features on the disk with sufficient spatial separation, such as ARs. We describe the analyses required to determine spectra from the wavelength-diffracted images and compare resulting spectra to CHIANTI spectral models and to AR temperature and emission measure maps determined from concurrent X-ray Telescope (XRT) measurements. We discuss the potential significance where differences in these comparisons occur

Instrumentation

Observations and Analysis

Findings

Summary and Conclusions