Magnetic field inference in active region coronal loops using coronal rain clumps

Abstract

Aims. We aim to infer information about the magnetic field in the low solar corona from coronal rain clumps using high-resolution spectropolarimetric observations in the Ca II 8542 Å line obtained with the Swedish 1 m Solar Telescope. Methods. The weak-field approximation (WFA) provides a simple tool to obtain the line-of-sight component of the magnetic field from spectropolarimetric observations. We adapted a method developed in a previous paper in order to assess the different conditions that must be satisfied in order to properly use the WFA for the data at hand. We also made use of velocity measurements in order to estimate the plane-of-the-sky magnetic field component, so that the magnetic field vector could be inferred. Results. We have inferred the magnetic field vector from a data set totalling 100 spectral scans in the Ca II 8542 Å line, containing an off-limb view of the lower portion of catastrophically cooled coronal loops in an active region. Our results, albeit limited by the cadence and signal-to-noise ratio of the data, suggest that magnetic field strengths of hundreds of Gauss, even reaching up to 1000 G, are omnipresent at coronal heights below 9 Mm from the visible limb. Our results are also compatible with the presence of larger magnetic field values such as those reported by previous works. However, for large magnetic fields, the Doppler width from coronal rain is not that much larger than the Zeeman width, thwarting the application of the WFA. Furthermore, we have determined the temperature, T, and microturbulent velocity, ξ, of coronal rain clumps and off-limb spicules present in the same data set, and we have found that the former ones have narrower T and ξ distributions, their average temperature is similar, and coronal rain has microturbulent velocities smaller than those of spicules.

Multi-Scale Variability of Coronal Loops Set by Thermal Non-Equilibrium and Instability as a Probe for Coronal Heating

“…This quantity can be used to estimate the amplitudes from transverse MHD waves (Pant and Doorsselaere, 2020), and therefore their energy flux and potential role in coronal heating. As stated in section 2.2.3, non-thermal line broadening can vary significantly depending on the event, with loops showing average values between 5-10 km s −1 only (Antolin et al, 2015;Kriginsky et al, 2021) while others show values of 8-16 km s −1 (with a tail up to 22 km s −1 ), that is, on the same order or larger than the thermal component Froment et al (2020), see Figure 10. It is interesting to note that the 15-20 km s −1 range is common in active region loops with temperatures between 1 MK and 5 MK (Chae et al, 1998;Hara and Ichimoto, 1999;Brooks and Warren, 2016;Testa et al, 2016), and also with that obtained from models based on either MHD waves (Shi et al, 2021) or field-line braiding (Pontin et al, 2020), which suggests that the same level of turbulence can be found in loops with or without TNE.…”

“…While the WFA provides the LOS component of the field, the total magnitude can be inferred from the rain's trajectory, which in turn can be known from the ratio between the POS and Doppler (LOS) velocity component. Using this, Kriginsky et al (2021) infer total field values of 500 G (with minima/maxima of 200/1,000 G) at heights of ≈ 10 Mm above the limb (probably, 15-20 Mm heights or so above the surface) in an active region (see Figure 18), consistent with the measurements of Schad et al (2016). These coronal magnetic field measurements are striking since they are a factor of 10 larger than usually assumed coronal magnetic field values in the corona.…”

“…On the low temperature end, estimates have been obtained based on spectral line widths in Hα, Ca II H and Ca II 8542 , where average temperatures oscillate between 5000 K and 33 000 K, with minima of 2000 K or less (Antolin et al, 2015). Observing with multiple lines simultaneously offers the possibility of more precise temperature measurements, along with non-thermal broadening estimates (see Figure 10), by assuming a common temperature and non-thermal line broadening between lines forming in similar conditions (Ahn et al, 2014;Froment et al, 2020;Kriginsky et al, 2021). The reported non-thermal broadening average values oscillate between 6 and 12 km s −1 , with maxima of 20 km s −1 .…”

“…A detailed study of the non-thermal velocity during TNE cycles is needed in order to confirm this. Using the non-thermal velocity measurements from coronal rain and an average magnetic field value of 500 G from Kriginsky et al (2021) (at the same height as the non-thermal broadening measurements) we can also calculate an upper limit for the Alfvén wave energy flux (assuming that the amplitude of the wave is equal to the non-thermal broadening) of 1 2 ρ(δv) 2 v A ≈ 10 7 erg cm −2 s −1 , taking an average number density for the loop of 5 × 10 9 cm −3 . If kink waves are observed instead (Antolin and Verwichte, 2011;Kohutova and Verwichte, 2016), the wave energy flux is on the order of 5 × 10 5 -10 6 erg cm −2 s −1 for a filling factor of 5% (Van Doorsselaere et al, 2014).…”

Observations of Instability-driven Nanojets in Coronal Loops

“…τ is always exceptionally large for n 1 > 10 11 cm −3 . This means that the RTI is likely to form under usual coronal rain conditions, but only for weak magnetic fields that may not be expected in active regions (Nakariakov & Ofman 2001;Kriginsky et al 2021). This aligns with the work by Moschou et al (2015) and Xia et al (2017), who have simulated coronal condensation that shows the formation of RTI using weak magnetic fields of less than 10 G at the apex of the loop.…”