Plasma dynamics in solar macrospicules from high-cadence extreme-UV observations

Abstract

Macrospicules are relatively large spicule-like formations found mainly over the polar coronal holes when observing in the transition region spectral lines. In this study, we took advantage of the two short series of observations in the He ii 304 Å line obtained by the TESIS solar observatory with a cadence of up to 3.5 s to study the dynamics of macrospicules in unprecedented detail. We used a one-dimensional hydrodynamic method based on the assumption of their axial symmetry and on a simple radiative transfer model to reconstruct the evolution of the internal velocity field of 18 macrospicules from this dataset. Besides the internal dynamics, we studied the motion of the apparent end points of the same 18 macrospicules and found 15 of them to follow parabolic trajectories with high precision which correspond closely to the obtained velocity fields. We found that in a clear, unperturbed case these macrospicules move with a constant deceleration inconsistent with a purely ballistic motion and have roughly the same velocity along their entire axis, with the obtained decelerations typically ranging from 160 to 230 m s −2 , and initial velocities from 80 to 130 km s −1 . We also found a propagating acoustic wave for one of the macrospicules and a clear linear correlation between the initial velocities of the macrospicules and their decelerations, which indicates that they may be driven by magneto-acoustic shocks. Finally, we inverted our previous method by taking velocities from the parabolic fits to give rough estimates of the percentage of mass lost by 12 of the macrospicules. We found that typically from 10 to 30% of their observed mass fades out of the line (presumably being heated to higher coronal temperatures) with three exceptions of 50% and one of 80%.

What is a Macrospicule?

“…4a). For the seemingly linear dependence α = kφ 0 , the fitted coefficient is k = 1.01 ± 0.02, which is close to that obtained by Loboda & Bogachev (2017) for a smaller set of 36 coronal-hole jets. This suggests a strong influence of the poloidal global magnetic field in coronal holes on the jets' orientations.…”

“…Moreover, the above estimates do not account for the fact that, in these works, certain parts of the limb were effectively blocked by large-scale solar activity, thus attenuating the calculated birth rates. Similarly, the results of Loboda & Bogachev (2017) imply a much higher birth rate of up to 0.2 s −1 for the coronal-hole macrospicules, since in this work all kinds of the jets were considered irrespective of their structure and dynamics, some of which were arguably polar surges rather than macrospicules. However, this rate is still significantly lower than the results of Wang (1998).…”

“…Further processing steps mainly followed the data reduction method as described in Loboda & Bogachev (2017), although with several modifications. First, to determine the position of the jet's axis, a maximum intensity map M [l, h] = max t C was built, an example of which is shown in Fig.…”

“…What is more important, however, is that these two studies have identified jets as long as 60.4 Mm and ∼ 65 Mm respectively, while in the present study the maximum length was found to be 44.6 Mm despite the larger, or at least comparable, number of jets examined. In our previous study, jets of a similar length were qualified as polar surges rather than macrospicules, having much more complex dynamics and consequently non-parabolic trajectories (Loboda & Bogachev 2017).…”

“…However, as the orbits of both STEREO spacecrafts are close to the ecliptic plane, such observations would be effective only for the two small regions around the solar poles, where the relatively lowlying jets can be seen off limb from both viewpoints, and moreover would require a reliable cross-identification of these small features. Additionally, both periods during which the STEREO spacecraft were located at an angle of 90 • to each other or to the Earth coincided with the previous minimum of solar activity, and therefore the approximate symmetry of the poloidal magnetic field would likely result in generally predictable inclinations of the polar jets, which are presumably guided by the open magnetic field lines (Loboda & Bogachev 2017).…”