DEFINING THE “BLIND SPOT” OF HINODE EIS AND XRT TEMPERATURE MEASUREMENTS

Abstract

Observing high-temperature, low emission measure plasma is key to unlocking the coronal heating problem. With current instrumentation, a combination of EUV spectral data from Hinode Extreme-ultraviolet Imaging Spectrometer (EIS; sensitive to temperatures up to 4 MK) and broadband filter data from Hinode X-ray Telescope (XRT; sensitive to higher temperatures) is typically used to diagnose the temperature structure of the observed plasma. In this Letter, we demonstrate that a "blind spot" exists in temperature-emission measure space for combined Hinode EIS and XRT observations. For a typical active region core with significant emission at 3-4 MK, Hinode EIS and XRT are insensitive to plasma with temperatures greater than ∼6 MK and emission measures less than ∼10 27 cm −5 . We then demonstrate that the temperature and emission measure limits of this blind spot depend upon the temperature distribution of the plasma along the line of sight by considering a hypothetical emission measure distribution sharply peaked at 1 MK. For this emission measure distribution, we find that EIS and XRT are insensitive to plasma with emission measures less than ∼10 26 cm −5 . We suggest that a spatially and spectrally resolved 6-24 Å spectrum would improve the sensitivity to these high-temperature, low emission measure plasma.

Inference of Heating Properties From “Hot” Non-Flaring Plasmas in Active Region Cores. I. Single Nanoflares

“…This has been identified from Hinode and SDO data (Reale et al 2009;Schmelz et al 2009;Testa & Reale 2012), and retrospectively from data obtained by the X-Ray Polychrometer instrument flown on the Solar Maximum Mission (Del Zanna & Mason 2014). While characterizing this emission is difficult (e.g., Testa et al 2011;Winebarger et al 2012), a similar scaling, EM∝T −b has been claimed (e.g., Warren et al 2012), with b of order 7-10, though Del Zanna & Mason find larger values. Warren et al quote typical errors of±2.5-3 on these values due to the very limited data available above T m and Winebarger et al have noted that the paucity of data from Hinode at these temperatures could be missing significant quantities of plasma with T>T m .…”

Measurements of Non-Thermal Line Widths in Solar Active Regions

“…See, e.g., Reale et al (2009aReale et al ( , 2009b, and Schmelz et al (2009) for initial reports of detections, Reale (2014) for a review of subsequent efforts, and O' Dwyer et al (2011) andWinebarger et al (2012) for reports of negative results and the difficulties of observing the appropriate temperature range. Notwithstanding these problems, measurements of non-thermal velocities at high temperatures (and any trend with temperature) are clearly significant for establishing whether the nanoflare-heated corona model is viable.…”

Inference of Heating Properties From “Hot” Non-Flaring Plasmas in Active Region Cores. Ii. Nanoflare Trains

“…While many workers (Reale et al 2009;Schmelz et al 2009Schmelz et al , 2015Miceli et al 2012;Del Zanna & Mason 2014;Petralia et al 2014) have claimed evidence of this hot, faint component of the emission measure, poor spectral resolution (Testa et al 2011;Winebarger et al 2012) and non-equilibrium ionization (NEI; Bradshaw & Cargill 2006;Reale & Orlando 2008) may make a positive detection of nanoflare heating difficult. However, Brosius et al (2014) used observations from the EUNIS-13 sounding rocket to identify relatively faint emission from Fe XIX in a non-flaring AR, suggesting temperatures of ∼8.9 MK.…”