Observations of Active Region Loops with the EUV Imaging Spectrometer on
                    <i>Hinode</i>

Warren, H. P.; Ugarte-Urra, Ignacio; Doschek, G. A.; Brooks, David H.; Williams, David R.

doi:10.1086/592960

Cited by 102 publications

(124 citation statements)

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“…arxiv we analysed are either cool loop structures or loop footpoints, which are often observed to be dominated by plasma with a very narrow thermal distribution (e.g. Del Zanna 2003; Warren et al 2008;Schmelz et al 2007Schmelz et al , 2014. We also emphasize that we are estimating average values of temperature and density for the emitting plasma.…”

Section: Discussion and Summarymentioning

confidence: 99%

“…Given the lack of suitable spectral lines to produce a DEM distribution, we start by assuming that the O iv and S iv plasma are nearly isothermal. This approximation can be justified by the fact that AR loops are often observed to show a very narrow distribution in their crosssection (Del Zanna 2003;Warren et al 2008;Brooks et al 2012;Schmelz et al 2007). This seem to be particularly true for cooler loops which tend to have narrower DEM distributions than hotter loops (Schmelz et al 2014(Schmelz et al , 2009).…”

Section: Density Diagnostics In the Ar Noaa 12356mentioning

confidence: 99%

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Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS

Polito

Zanna

Dudík

et al. 2016

A&A

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The intensity of the O iv 2s 2 2p 2 P-2s2p 2 4 P and S iv 3 s 2 3p 2 P-3s 3p 2 4 P intercombination lines around 1400 Å observed with the Interface Region Imaging Spectrograph (IRIS) provide a useful tool to diagnose the electron number density (N e ) in the solar transition region plasma. We measure the electron number density in a variety of solar features observed by IRIS, including an active region (AR) loop, plage and brightening, and the ribbon of the 22-June-2015 M 6.5 class flare. By using the emissivity ratios of O iv and S iv lines, we find that our observations are consistent with the emitting plasma being near isothermal (logT [K] ≈ 5) and iso-density (N e ≈ 10 10.6 cm −3 ) in the AR loop. Moreover, high electron number densities (N e ≈ 10 13 cm −3 ) are obtained during the impulsive phase of the flare by using the S iv line ratio. We note that the S iv lines provide a higher range of density sensitivity than the O iv lines. Finally, we investigate the effects of high densities (N e 10 11 cm −3 ) on the ionization balance. In particular, the fractional ion abundances are found to be shifted towards lower temperatures for high densities compared to the low density case. We also explored the effects of a non-Maxwellian electron distribution on our diagnostic method.

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Section: Discussion and Summarymentioning

confidence: 99%

Section: Density Diagnostics In the Ar Noaa 12356mentioning

confidence: 99%

Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS

Polito

Zanna

Dudík

et al. 2016

A&A

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“…In some of the weaker ARs a shallower temperature distribution was observed, and this highlights the need to adopt some method of quantitatively determining if the emission comes from the hot core loops or not. In our previous EM studies of warm coronal loops (Warren et al 2008a;Brooks et al 2012Brooks et al , 2013 we have adopted the criterion that the cross-field intensity profiles must be highly correlated with those of the spectral line with which they were identified (usually Fe XII 195.119 Å). Here we impose the condition that the emission within the boxed region for each spectral line must be spatially correlated (r>0.6) with that of the highest temperature line (Ca XIV 193.874 Å).…”

Section: Observationsmentioning

confidence: 99%

Measurements of Non-Thermal Line Widths in Solar Active Regions

Brooks

Warren

2016

ApJ

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Spectral line widths are often observed to be larger than can be accounted for by thermal and instrumental broadening alone. This excess broadening is a key observational constraint for both nanoflare and wave dissipation models of coronal heating. Here we present a survey of non-thermal velocities measured in the high temperature loops (1-4 MK) often found in the cores of solar active regions. This survey of Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) observations covers 15 non-flaring active regions that span a wide range of solar conditions. We find relatively small non-thermal velocities, with a mean value of 17.6 ± 5.3 km s −1 , and no significant trend with temperature or active region magnetic flux. These measurements appear to be inconsistent with those expected from reconnection jets in the corona, chromospheric evaporation induced by coronal nanoflares, and Alfvén wave turbulence models. Furthermore, because the observed non-thermal widths are generally small, such measurements are difficult and susceptible to systematic effects.

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“…For a few examples, see e.g. Warren et al (2008), Tripathi et al (2009), Aschwanden & Boerner (2011), Schmelz et al (2013), although some loops show some departures (Schmelz et al 2009(Schmelz et al , 2010, and the issue is still debated.…”

Section: Introductionmentioning

confidence: 99%

The multi-thermal emission in solar active regions

Zanna¹

2013

A&A

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We present simultaneous SDO AIA and Hinode EIS observations of the hot cores of active regions (ARs) and assess the dominant contributions to the AIA EUV bands. This is an extension of our previous work. We find good agreement between SDO AIA, EVE and EIS observations, using our new EIS calibration and the latest EVE v.3 data. We find that all the AIA bands are multi-thermal, with the exception of the 171 and 335 Å, and provide ways to roughly estimate the main contributions directly from the AIA data. We present and discuss new atomic data for the AIA bands, showing that they are now sufficiently complete to obtain temperature information in the cores of ARs, with the exception of the 211 Å band. We found that the newly identified Fe xiv 93.61 Å line is the dominant contribution to the 94 Å band, whenever Fe xviii is not present. Three methods to estimate the Fe xviii emission in this band are presented, two using EIS and one directly from the AIA data. Fe xviii emission is often present in the cores of ARs, but we found cases where it is formed at 3 MK and not 7 MK, the temperature of peak ion abundance in equilibrium. The best EIS lines for elemental abundance determination and differential emission measure (DEM) analysis are discussed. A new set of abundances for many elements are obtained from EIS observations of hot 3 MK loops. The abundances of the elements with low first ionisation potential (FIP), relative to those of the high-FIP elements, are found to be enhanced by about a factor of three, compared to the photospheric values. A measurement of the path length implies that the absolute abundances of the low-FIP elements are higher than the photospheric values by at least a factor of three. We present a new DEM method customised for the AIA bands, to study the thermal structure of ARs at 1 resolution. This was tested on a few ARs, including one observed during the Hi-C rocket flight. We found excellent agreement between predicted and observed AIA count rates and EIS radiances. Overall we found few differences between the AIA and Hi-C 193 Å images of coronal structures, despite the higher Hi-C resolution (0.25 ). The Hi-C images and the AIA DEM modelling suggest that some of the cooler loops (below 1 MK) are already resolved by AIA, while the hotter (1.5-2.5 MK) "background" emission is in most places still unresolved even at the Hi-C resolution. This unresolved emission is significantly lower than previously observed with TRACE, the SOHO CDS, and Hinode EIS spectrometers. Its enhancement appears to be mostly due to increased iron abundance. We find an ubiquitous presence of emission at different temperatures that is not co-spatial, and suggest that future high-resolution imaging is carried out with isothermal bands.

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Observations of Active Region Loops with the EUV Imaging Spectrometer on Hinode

Cited by 102 publications

References 20 publications

Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS

Density diagnostics derived from the O iv and S iv intercombination lines observed by IRIS

Measurements of Non-Thermal Line Widths in Solar Active Regions

The multi-thermal emission in solar active regions

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