Observations of Solar Coronal Rain in Null Point Topologies

Abstract

Coronal rain is the well-known phenomenon in which hot plasma high in the Sun's corona undergoes rapid cooling (from > 10^6 K to < 10^4 K), condenses, and falls to the surface. Coronal rain appears frequently in active region coronal loops and is very common in post-flare loops. This Letter presents discovery observations, which show that coronal rain is ubiquitous in the embedded bipole very near a coronal hole boundary. Our observed structures formed when the photospheric decay of active region leading sunspots resulted in a large parasitic polarity embedded in a background unipolar region. We observe coronal rain to appear within the legs of closed loops well under the fan surface, as well as preferentially near separatrices of the resulting coronal topology: the spine lines, null point, and fan surface. We analyze 3 events using SDO Atmospheric Imaging Assembly (AIA) observations in the 304, 171, and 211 {/AA} channels, as well as SDO Helioseismic and Magnetic Imager (HMI) magnetograms. The frequency of rain formation and the ease with which it is observed strongly suggests that this phenomenon is generally present in null-point topologies of this size scale. We argue that these rain events could be explained by the classic process of thermal nonequilibrium or via interchange reconnection at the null; it is also possible that both mechanisms are present. Further studies with higher spatial resolution data and MHD simulations will be required to determine the exact mechanism(s).

10 pages, 5 figures

References in corpus (9)

• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: SHARPs -- Space-weather HMI Active Region Patches
• Solar Coronal Jets: Observations, Theory, and Modeling
• JHelioviewer - Time-dependent 3D visualisation of solar and heliospheric data
• The multi-thermal and multi-stranded nature of coronal rain
• On the 2012 October 23 circular ribbon flare: emission features and magnetic topology
• Evidence for polar jets as precursors of polar plume formation
• Long-Period Intensity Pulsations in Coronal Loops Explained by Thermal Non-Equilibrium Cycles
• Unresolved fine-scale structure in solar coronal loop-tops
• Observing the formation of flare-driven coronal rain

Cited by in corpus (3)

• Multi-scale observations of thermal non-equilibrium cycles in coronal loops
• Self-consistent 3D radiative MHD simulations of coronal rain formation and evolution
• Two-dimensional simulations of coronal rain dynamics. I. Model with vertical magnetic field and an unbounded atmosphere