Abstract
The quiet solar corona consists of myriads of loop-like features, with magnetic fields originating from network and internetwork regions on the solar surface. The continuous interaction between these different magnetic patches leads to transient brightenings or bursts that might contribute to the heating of the solar atmosphere. The literature on a variety of such burst phenomena in the solar atmosphere is rich. However, it remains unclear whether such transients, which are mostly observed in the extreme ultraviolet (EUV), play a significant role in atmospheric heating. We revisit the open question of these bursts as a prelude to the new high-resolution EUV imagery expected from the recently launched Solar Orbiter. We use EUV image sequences recorded by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) to investigate statistical properties of the bursts. We detect the bursts in the 171 Å filter images of AIA in an automated way through a pixel-wise analysis by imposing different intensity thresholds. By exploiting the high cadence (12 s) of the AIA observations, we find that the distribution of lifetimes of these events peaks at about 120 s. However, a significant number of events also have lifetimes shorter than 60 s. The sizes of the detected bursts are limited by the spatial resolution, which indicates that a larger number of events might be hidden in the AIA data. We estimate that about 100 new bursts appear per second on the whole Sun. The detected bursts have nanoflare-like energies of 1024 erg per event. Based on this, we estimate that at least 100 times more events of a similar nature would be required to account for the energy that is required to heat the corona. When AIA observations are considered alone, the EUV bursts discussed here therefore play no significant role in the coronal heating of the quiet Sun. If the coronal heating of the quiet Sun is mainly bursty, then the high-resolution EUV observations from Solar Orbiter may be able to reduce the deficit in the number of EUV bursts seen with SDO/AIA at least partly by detecting more such events.
Highlights
Coronal emission from hot plasma in the quiet Sun outside active regions is mostly observed in the form of a diffuse background that is loosely related to the magnetic concentrations in the chromospheric network
On scales of over 10 Mm, coronal bright points exhibit flaring activity (Krucker et al 1997), which might subsequently explode as the underlying photospheric magnetic flux converges (Priest et al 1994), and launch the hot plasma jets that are observed in soft X-ray images (Moore et al 2018; Madjarska 2019)
The event statistics we present might be of interest to gain further insights into extreme ultraviolet (EUV) bursts that will be observed with the recently launched Solar Orbiter (Müller et al 2020)
Summary
Coronal emission from hot plasma in the quiet Sun outside active regions is mostly observed in the form of a diffuse background that is loosely related to the magnetic concentrations in the chromospheric network. On scales of over 10 Mm, coronal bright points exhibit flaring activity (Krucker et al 1997), which might subsequently explode as the underlying photospheric magnetic flux converges (Priest et al 1994), and launch the hot plasma jets that are observed in soft X-ray images (Moore et al 2018; Madjarska 2019) These explosive events are common throughout different layers of the quiet-Sun atmosphere. Blinkers (Harrison 1997), EUV transients (Berghmans et al 1998), and microand nanoflares or microevents (Krucker & Benz 1998; Parnell & Jupp 2000; Aschwanden et al 2000a,b; Benz & Krucker 2002) have been extensively studied using SOHO2 and TRACE3 observations This diverse classification might arise because different diagnostics (spectroscopic vs imagery), different instruments, and different methods of event detection were employed (Harrison et al 2003). The event statistics we present might be of interest to gain further insights into EUV bursts that will be observed with the recently launched Solar Orbiter (Müller et al 2020)
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