Hot explosions in the cool atmosphere of the Sun

Abstract

The solar atmosphere was traditionally represented with a simple one-dimensional model. Over the past few decades, this paradigm shifted for the chromosphere and corona that constitute the outer atmosphere, which is now considered a dynamic structured envelope. Recent observations by IRIS (Interface Region Imaging Spectrograph) reveal that it is difficult to determine what is up and down even in the cool 6000-K photosphere just above the solar surface: this region hosts pockets of hot plasma transiently heated to almost 100,000 K. The energy to heat and accelerate the plasma requires a considerable fraction of the energy from flares, the largest solar disruptions. These IRIS observations not only confirm that the photosphere is more complex than conventionally thought, but also provide insight into the energy conversion in the process of magnetic reconnection.The energy produced in the core of the Sun by the fusion of hydrogen into helium is transported toward the surface first by radiation, and then by convection. The layer where the photons become free to escape defines the visible surface of the Sun. The atmosphere of the Sun above the surface was traditionally described as one-dimensionally stratified. Moving outward from the photosphere, the innermost layer, the temperature drops before rising again slightly in the middle layer, the chromosphere. When the outgoing energytransported by a heating mechanism that is not yet fully understood -can no longer be buffered by radiative loss and hydrogen ionization, the temperature rises steeply. This transition marks the boundary of the corona, the outermost layer, which is brilliantly visible to the naked eye in a total solar eclipse. Semi-empirical models represent this simplified one-dimensional stratification well (1). However, more advanced observations and models have established that the outer atmosphere (chromosphere and corona) is highly structured and dynamic (2,3,4). Modern models of the solar atmosphere also take

Hαfeatures with hot onsets

“…Lines of sight as A then sample hot EB tops without such blends. These blends and their small Dopplershifts, observed not only in EBs but also in FAFs (Paper III), support the conclusion of Peter et al (2014) that IBs occur at large depths. For EBs this was already established in Paper I: they have their feet in the low photosphere.…”

“…They show similar patterns, with wide peaks for the first and especially the fourth ions. In CE the Si iv presence peaks with relatively low amplitude at log (T ) ≈ 4.9 (T ≈ 80 000 K) as quoted by Peter et al (2014), but in LTE it peaks already at T ≈ 25 000 K and also with much larger amplitude. Since the Si iv lines in the EB spectra of Paper III reach only EB thickness τ ≈ 1 they may be formed on the rising branch and represent only T ≈ 15 000−20 000 K. Figure 5 shows extinction-versus-temperature diagrams that I wish to call "Cecilia Payne diagrams" although they are somewhat more elaborate than Payne's curves.…”

“…In their discovery paper Peter et al (2014) suggested that these are pockets of hot gas in the upper photosphere and may correspond to EBs in Hα. In Paper III we found that they are more likely FAFs while EBs may also show IB signatures in their aftermaths.…”

Recent advances in coronal heating

“…in minutes) short, low-lying loops at transition region temperatures, finally resolving the postulated 'unresolved fine structure' (necessary to reconcile observed emission and velocities with models of the solar atmosphere). The classical, layered view of the solar atmosphere is further challenged by Peter et al [28] who find 10 5 K plasma, heated by reconnection, low down, near the solar surface and sandwiched between cooler layers. Tian et al [29] report on small-scale jets undergoing rapid heating to transition region temperatures which could form an intermittent but persistent contribution to the solar wind.…”

What can large-scale magnetohydrodynamic numerical experiments tell us about coronal heating?

“…One example is the high densities found at transition region temperatures in Ellerman-bomb-type events [64].…”