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NuSTAR Observations of Small-Scale Phenomena in the Quiet Sun

Sarah Paterson1

1University of Glasgow, UK

E-mail: s.paterson.5@research.gla.ac.uk

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I. G. Hannah1, B. W. Grefenstette2, H. Hudson1,3, S. Krucker3,4, L. Glesener5, D. M. Smith6, S. M. White7

2Caltech, 3UC Berkeley, 4FNHW, 5Minnesota, 6UC Santa Cruz, 7AFRL

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Introduction

  • The Nuclear Spectroscopic Telescope ARray (NuSTAR): a sensitive HXR focusing telescope capable of observing the Sun above 2.5 keV (Grefenstette et al., 2016)
  • NuSTAR is an imaging spectrometer
    • Can perform spectroscopy on very faint solar HXR sources to investigate their contribution to coronal heating
  • During the recent solar minimum, NuSTAR observed the quiet Sun several times, capturing many small-scale HXR sources

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The NuSTAR Quiet Sun Observations

  • NuSTAR has observed the Sun in 2 modes: mosaics and dwells
  • Mosaic example:

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25 x 100s pointings in ~ 1 hour orbit

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28 September 2018 Mosaics

  • 2 NuSTAR orbits
  • Several sources present in NuSTAR, as well as SDO/AIA and Hinode/XRT:
    • An emerging flux region (EFR), bright points, a jet, loops, a bright region on the limb, diffuse emission
  • Pointings overlap so that a source may be observed 4 times in an orbit

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Emerging Flux Region

  • This went on to become AR 12723β a few days later
  • Captured in 8 pointings over 2 orbits (P12, 13, 18, 19)
  • Lightcurves show that SDO/AIA 211Å and Hinode/XRT Be-thin agree in orbit 1, but are out of sync in orbit 2
    • Though the relative change in 211Å in tiny compared to XRT
    • Look at NuSTAR spectra to investigate

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Green shaded areas show where NuSTAR captured the EFR

Yellow NuSTAR 2.2-4.0 keV contours on Hinode/XRT Be-thin images

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EFR: NuSTAR Spectra

  • All EFR NuSTAR spectra are well fit by a single thermal model
  • Spectra from pairs of consecutive pointing are fit simultaneously to improve SNR
  • Fits indicate T ~ 2.5 MK throughout, EM varies between 1.9 – 6.2 x 1044 cm-3, following similar pattern to Hinode/XRT lightcurve

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EFR: DEMs

  • The DEM of the EFR was reconstructed using the regularized inversion approach of Hannah and Kontar, 2012
  • No significant change in DEM between the NuSTAR times in orbit 1, DEM in orbit 2 higher between logT ~ 6.3 – 6.5, in agreement with the Hinode/XRT lightcurve
  • Using Hinode/XRT and particularly NuSTAR data in the DEM helps to constrain the solution, showing that there is virtually no emission above 4 MK.

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Jet

  • Captured in first NuSTAR orbit
  • NuSTAR and Hinode/XRT lightcurves in agreement, but not SDO/AIA 211Å
  • NuSTAR spectral fits give T = 2.6 MK (slightly above peak in 211Å sensitivity), EM = 8.9 x 1043 cm-3
  • From testing non-thermal upper limits range, would need a very steep, almost mono-energetic, spectrum between 3 and 4 keV to power heating

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Summary

  • NuSTAR has observed a variety of small-scale features on the quiet Sun
  • NuSTAR spectral fits of all these features investigated so far give temperatures < 3.5 MK, no high T or non-thermal
  • Including Hinode/XRT and NuSTAR as well as SDO/AIA to DEM calculation helps to constrain the solution at high T, showing virtually no emission > 4MK
  • Work on the NuSTAR QS observations is ongoing
    • Future work will focus on the dwell observations, allowing the temporal evolution of QS sources to be investigated in more detail
    • A summary of all NuSTAR solar observations can be found at http:/ianan.github.io/nsovr

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