Coronal loops: new insights from EIS observations

1. Coronal loops: new insights from EIS observations G. Del Zanna1,2, S. Bradshaw3, 1 MSSL, University College London, UK 2 DAMTP, University of Cambridge, UK 3 Imperial College, UK Test of radiative cooling in quiescent active region loops. Abstract Multi-instrument observations of coronal loops of different active regions have been analysed. The general features discussed in Del Zanna (2003) and Del Zanna and Mason (2003) based on SOHO/CDS are confirmed. The pattern of dopplershifts and non-thermal widths found for the first time in NOAA 10926 (cf. Del Zanna 2007, 2008), is actually a common feature in all active regions. The observations suggest that the majority of 'cool' (0.5-1 MK) loops are observed during their radiatively cooling phase. Hinode/EIS high-cadence observations show how dynamic loops are at all temperatures. This clearly reflect the fast changes in the photospheric magnetic fields measured by SOT over a minute timescale. It is well-known that when a coronal loop undergoes radiative cooling with no heat input to partially balance the loss of energy, the temperature and density are related by T ~ N2 (Serio et al., 1991; Sylwester et al., 1993; Bradshaw & Cargill, 2005). Accurate observations of T,N will reveal extremely valuable information regarding the cooling phase of coronal loops; for example,whether there is residual energy input during this period and, if so, how much and its decay time. Furthermore, it will also provide a means to validate the physics upon which existing numerical models are based. We designed an observational sequence, with the main EIS study being: Earlier new results from Hinode/EIS (Del Zanna 2007,2008) • At T < 1MK, red-shifts in loops are ubiquitous in both legs, and are larger at lower T. • Blue-shifts are present in a sharp boundary around the hot (3 MK) loops, with a large spatial expansion. Densities are low, about 5 108 cm-3 at Fe XIII, XIV temperatures (log T=6.3), almost a factor of 10 lower than densities of the hot loops. • The boundary is located where the strongest magnetic fields are, and where both the short hot (3MK) and the over-lying long cool (1MK) loops are present. • Blue-shifts keep increasing in spatial extent and velocity in lines formed at T >1 MK and always have associated large (>50 km/s) non-thermal broadenings. • The pattern of Dopplershifts and line widths follows the changes of the underlying magnetic fields and is generally long-lasting. • GDZ_DENS_20x240_ARL1 • 2” slit - 10s exposures 20”x240” ‘sit-and stare’ • Very high (2min) cadence - About 40 lines, from transition region to flare temperatures. Unfortunately, our observational sequence was only run once and improperly [an active region near the limb was observed for about 1h]. However, some interesting results are worth noting. The main feature being the large variability in particular on the cooler and hotter lines. Intensity Doppler Velocity (+/- 20km/s) Moss Non-thermal widths (km/s) Density (Fe XII, XIII) All active regions show similar characteristics 1 MK Loop EIS Fe XII NOAA 10938 17 Jan 2007 30min cadence 4x4’ 5s exp. 1”slit Loop The challenge NOAA 10956 - 18 May 2007 13min cadence 4’x4’ 5s exp. 2”slit Spectroscopic observations with EIS show large variations in Doppler motions and intensities in active region loops, suggesting that the real changes occur on timescales much shorter than 2 minutes. This is not surprising, given that SOT/NFI/FGV images show restructuring of photospheric magnetic fields on timescales shorter than 1minute. 2-min cadence EIS spectra provide abundant complexities which need to be understood, in order to explain the large-scale features which give the false appearance of a quasi-static corona. This will require much higher cadence observations. Changes in single spectral lines are larger than the observed changes in broad-band imaging instruments such as TRACE, Hinode/XRT, which have multi-temperature components. Loop characteristics found by SOHO/CDS are confirmed. The new EIS observations suggest that all cooler loops are observed during their radiative cooling, and that heating occurs at very low heights where large outflows in hotter lines are present. Despite various limitations (large variations in pointing ad wavelength scale, low telemetry, eclipses, etc.), the EIS instrument has provided new important insights (and challenges) into the physics of coronal loops. NOAA 10961 - 1 Jul 2007 13min cadence 4’x4’ 5s exp. 2”slit A possible interpretation Bradshaw (2008) derived an equation for the critical velocity needed in order for an enthalpy flux to balance the transition region radiative losses: The table shows the critical down-flow speeds needed to balance the transition region radiative losses given a loop of apex temperature Ta and transition region density n. The down-flow speeds are weakly dependent on Ta and strongly dependent on n. However, the speeds are also quite modest (a few km s-1) even for relatively high density (n = 1010) . The down-flows given by the expression for Vc are in good agreement with the observationally measured Doppler velocities, which provides strong evidence that the majority of observed 1 MK loops are radiatively cooling and their transition regions are supported by an enthalpy flux rather than by thermal conduction. References: Bradshaw, S.J., & Cargill, P.J., 2005, A&A, 437, 311 Bradshaw,S.J., 2008, A&A, 486, L5 Del Zanna, G., proc. 1st Hinode meeting,Aug 2007, ASPC 397, 87; 2008, A&A, 481, L49 Serio, S., Reale, F., Jakimiec, J., Sylwester, B., & Sylwester, J.,1991, A&A, 241, 197 Sylwester, B., & Sylwester, J., Serio, S., et al., 1993, A&A, 267, 586 We acknowledge support from STFC (UK).