Applying an electrical field to a polar polymer may induce a large change in the dipolar ordering, and if the associated entropy changes are large, they can be explored in cooling applications. With the use of the Maxwell relation between the pyroelectric coefficient and the electrocaloric effect (ECE), it was determined that a large ECE can be realized in the ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer at temperatures above the ferroelectric-paraelectric transition (above 70 degrees C), where an isothermal entropy change of more than 55 joules per kilogram per kelvin degree and adiabatic temperature change of more than 12 degrees C were observed. We further showed that a similar level of ECE near room temperature can be achieved by working with the relaxor ferroelectric polymer of P(VDF-TrFE-chlorofluoroethylene).

In this paper, we report our multiwavelength observations of the C4.2 circularribbon flare in active region (AR) 12434 on 2015 October 16. The short-lived flare was associated with positive magnetic polarities and a negative polarity inside, as revealed by the photospheric line-of-sight magnetograms. Such magnetic pattern is strongly indicative of a magnetic null point and spine-fan configuration in the corona. The flare was triggered by the eruption of a mini-filament residing in the AR, which produced the inner flare ribbon (IFR) and the southern part of a closed circular flare ribbon (CFR). When the eruptive filament reached the null point, it triggered null point magnetic reconnection with the ambient open field and generated the bright CFR and a blowout jet. Raster observations of the Interface Region Imaging Spectrograph (IRIS ) show plasma upflow at speed of 35−120 km s −1 in the Fe xxi 1354.09Å line (log T ≈ 7.05) and downflow at speed of 10−60 km s −1 in the Si iv 1393.77Å line (log T ≈ 4.8) at certain locations of the CFR and IFR during the impulsive phase of flare, indicating explosive chromospheric evaporation. Coincidence of the single HXR source at 12−25 keV with the IFR and calculation based on the thick-target model suggest that the explosive evaporation was most probably driven by nonthermal electrons.

We perform two-dimensional resistive magnetohydrodynamic simulations of coronal jets driven by flux emergence along the lower boundary. The reconnection layers are susceptible the formation of blobs that are ejected in the jet. Our simulation with low plasma β (Case I) shows that magnetic islands form easily and propagate upwards in the jet. These islands are multithermal and thus are predicted to show up in hot channels (335 Å and 211 Å) and the cool channel (304 Å) in observations by the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory. The islands have maximum temperatures of 8 MK, lifetimes of 120 s, diameters of 6 Mm, and velocities of 200 km s −1 . These parameters are similar to the properties of blobs observed in EUV jets by AIA. The Kelvin-Helmholtz instability develops in our simulation with moderately high plasma β (Case II), and leads to the formation of bright vortex-like blobs above the multiple high magneto-sonic Mach number regions that appear along the jet. These vortex-like blobs can also be identified in the AIA channels. However, they eventually move downward and disappear after the high magneto-sonic Mach number regions disappear. In the lower plasma β case, the lifetime for the jet is shorter, the jet and magnetic islands are formed with higher velocities and arXiv:1705.00180v1 [astro-ph.SR] 29 Apr 2017 -2temperatures, the current sheet fragments are more chaotic, and more magnetic islands are generated. Our results show that the plasmoid instability and Kelvin-Helmholtz instability along the jet are both possible causes of the formation of blobs observed at extreme ultraviolet (EUV) wavelengths.