Determining the Acceleration Mechanism and Magnetic Topological Structure of Coronal Jets: Evidence for Multiple Acceleration Mechanisms
Farid, Samaiyah Islam
Solar coronal jets are small-scale, energetic eruptions, characterized by a column-like spire and a bright, dome-shaped base. Jets are often associated with notable changes in the underlying photospheric magnetic field and have been found to initiate when opposite polarity magnetic flux elements emerge, cancel, flyby, or otherwise interact. Jets observed in Extreme Ultraviolet (EUV) and X-rays are loosely grouped based on morphological features. They are considered either standard; having a narrow spire, or blowout; having abroad spire. Jet plasma parameters vary widely, EUV and Xray jet spire lengths 1−20xE4 km, widths 1−4xE4 km and plane of sky outflow velocities from 100 to 800 km/s (near the local Alfv ́en velocity). Jets have been associated with the eruption of small sigmoidal-like filaments, and often exhibit twist motion during an eruption. Because of their association with transient photospheric flux elements, jets are thought to be primarily driven by magnetic reconnection, however, models describing the relationship between initiation and plasma properties during an eruption are not well understood, and are often contradictory. This is further complicated by observations show that jets with similar initiation mechanisms can exhibit a wide range of plasma parameters with different topological features, while embedded in different coronal environments. Recent 3D and 2.5D models show that in addition to magnetic tension released during reconnection, jets may also be accelerated via chromospheric evaporation, the untwisting motion of the field lines, and/or by Alfv ́enic waves that transverse along newly reconnected field lines. Chromospheric evaporation is often associated with flaring active regions when magnetic reconnection in the corona heats and drives chromospheric material upward at velocities comparable to the local sound speed. In those cases, the tell-tell signs are enhanced blue shifts in spectroscopic lines sensitive to hot plasma, and a notable increase in plasma velocity as a function of temperature. In an untwisting jet, twisted magnetic field lines could impart energy via propagating torsional Alfv ́en waves. Each of these mechanisms could work independently or in tandem with the magnetic tension released during magnetic reconnection. Although much work has been done on jets, a comprehensive approach is needed to formulate a picture of coronal jets from initiation to acceleration. For the ﬁrst time, we investigate acceleration mechanisms of six coronal jets embedded in different environments, by combining multi-wavelength imaging observations, spectroscopic observations and 3D magnetic topological modeling. First, we use observations from Hinode’s X-ray Telescope (XRT), Solar Dynamics Observatory’s Atmospheric Imaging Assembly (SDO-AIA), and Interface Region Imaging Spectrograph (IRIS), to capture the plane of sky outﬂow velocities as a function of temperature. When available, we use IRIS spectroscopic observations of the Si IV line proﬁle to calculate line-of-sight velocity, Doppler velocity and non-thermal line broadening. We look for evidence of chromospheric evaporation by examining temperature as a function of velocity and examine spectroscopic data for regions of strong non-thermal broadening. These regions we infer as possible locations of magnetic reconnection. Next we use a Non-Linear Force-Free Field (NLFFF) model, to examine the magnetic topology of selected jets before and during their eruption and compare the model results with the evolution of the jet in EUV. In cases where a ﬁlament is observed, we employ the ﬁlament insertion method. In one jet we complete a more thorough topological analysis including the model of the quasi-separatrix layers (QSL) , and energy partition during the eruption. We ﬁnd evidence of chromospheric evaporation in at least one of the coronal jets based on the relationship between temperature and velocity. In 4 jets we ﬁnd possible evidence of chromospheric evaporation, where the correlation between velocity and temperature is not obvious, but correlations can be observed in particular temperature groups. In most of the jets, (5 of 6) a ﬁlament can be observed and including those jets that exhibit twist. We ﬁnd that the NLFFF model matches EUV observations of the jet spire well, allowing us to identify the height of the null point (region) and the upper limits of the toroidal, and poloidal ﬂux and compare these parameters to observations. These observations combined to give a unique insight into the acceleration mechanisms of coronal jets. For the ﬁrst time, we combine observations and topological modeling to characterize the reconnection region and ﬁnd evidence of different acceleration mechanisms in coronal jets.