Energy and mass transport associated with impulsive spicular flows in solar coronal holes

Abstract

How the solar atmosphere is heated from a temperature of about $5,000-6,000$\,K in the lower atmosphere to about $1-2$\,MK in the corona has challenged the astrophysical community for about 80 years. The same puzzle exists for the stellar coronae heating as well. In this study, we present a series of findings on solar spicules and their subsequent impact on the corona within a coronal hole environment, characterized by locally open magnetic field lines, combining insights from MHD simulations with observations. We find that the convective and turbulent motions around the solar surface cause plenty of shocks and small-scale magnetic reconnection in the lower atmosphere. The combined effects of shock compression and reconnection outflows then drive the formation of groups of spicules with a quasi-period of about $300$\,s and width of $\sim 200-500$\,km. The spicule upflows provide an averaged mass flux above $10^{-9}$\,kg\,m$^{-2}$\,s$^{-1}$ in the lower corona to sustain the solar wind in coronal holes, and they continuously trigger further new local slow-mode waves and shocks. These waves supply an energy flux of $10-100$\,W\,m$^{-2}$ in the lower corona, and they are dissipated by heat conduction and compression heating to sustain the corona temperature of about $1$\,MK. The results also indicate that the upward propagating disturbances (PDs) observed in extreme ultraviolet (EUV) passbands are caused by both spicule upflows and slow-mode waves and shocks. Our findings help to understand the long standing problem of coronal heating and the origin of solar winds in coronal hole regions.

Figures (12)

• Figure 1: AIA and IRIS images taken around 01:02 UT on May 18, 2020. (A-B) show coronal holes in the Sun's south polar region, with the rectangular box marking the location of polar plumes. (C-D) present a zoomed-in view and a running difference image of AIA 171 Å, respectively. The running difference (RD) image highlights a propagating disturbance, visible as an elongated white strip within the black box. The RD images are obtained by subtracting an image taken two minutes earlier. (E-F) display the IRIS 2796 Å image and its corresponding RD image, clearly capturing a large spicule at this moment. The Y-axis of panels (C-F) starts from the bottom of the white box marked in panels (A-B). The black rectangular boxes in panels C and E mark the location of the artificial slits used to generate time-distance plots.
• Figure 2: Time-distance maps corresponding to the slits marked in panels (C-E) of Figure 1. The slanted bright ridges extending over large distances represent propagating disturbances (PDs) observed in the AIA 171 Å and 193 Å channels. The IRIS 2796 Å time-distance maps show the evolution of spicules. To highlight these evolution of several prominent spicules, we have marked spicular evolutions with white parabolic curves. These same curves are overplotted on the AIA 171 Å and 193 Å maps to illustrate the connection between spicular activity observed in the IRIS channel in (B) and (D) and PDs seen in the AIA channels in (A) and (C).
• Figure 3: The distributions of plasma parameters at different altitudes. (A) shows the distributions of initial temperature ($T_0=6530$ K), density ($rho_0=2.734 \time10^{-4}$ kg m$^{-3}$) and pressure ($P_0=11820$ N m$^{-2}$) along the Y-direction, and they are normalized by using the reference values at the bottom of photosphere. The evolutions of temperature and density at different altitudes are presented in (B) and (D), and these values are the averaged ones in the X-direction.
• Figure 4: Evolution of spicules and the lower corona above them within a cycle of spicule formation in the 2.5 D simulation with the initial magnetic field $B_0=5$ G. Distribution of logarithmic temperature (A-E), logarithmic density (F-J), and velocity in the $Y$-direction (K-O) at five different times are presented. The white solid curves in (F-J) outline the magnetic field lines. The black dashed boxes in (B) and (G) represent the zoomed in region in Figure 6(A-H). The big thick black arrow in each panel points to the top of one particular spicule with a temperature below 10$^5$ K. The black contour lines in (K-O) outline the positions having large values of $-\nabla \cdot V$, indicating the possible invoked slow mode shocks. The animations of the corresponding temperature distribution, density distribution and velocity ($V_Y$) distribution are available in MovieS1.
• Figure 5: Distributions of different variables in the zoomed lower atmosphere for the 2.5 D run with $\mathbf{B_0=5}$ G at time $\mathbf{3461.07}$ s, when a bunch of new spicules just start to appear at the altitude between $\mathbf{Y=0.5-2}$ Mm. The logarithmic temperature ($lgT$) (A), current density ($J_z$) (B), magnetic field in the Y-direction ($B_y$) (C), the negative divergence of velocity ($-\nabla \cdot V$) (D), the acceleration along the Y- direction contributed by the pressure gradient ($A_p$) (E), and the acceleration along the Y- direction contributed by the Lorentz force ($A_L$) (F) are presented. The black thin solid line in (B) represents the position where the plasma $\beta=1$.