A Discussion on the physics of the solar atmosphere - The quiet Sun in the extreme ultraviolet

Observations of the quiet Sun with the Harvard extreme ultraviolet spectrometer on the Skylab mission are reported for the chromosphere, transition region, and corona. The changing structure of the network is examined over the temperature range from 10 4 to 1.5 x 10 6 K, and the distribution of intensities in the cell interiors and the network examined from the standpoint of creating characteristic models. Observations of traces of periodic oscillations at 300 s in the cells for T ⩽ 2 x 10 4 is reported together with the absence of any periodic contributions at higher temperatures or periodic effects at any height in the network elements. Frequent non-periodic brightenings are observed, however, and their characteristics discussed. Observations of the increased thickness of the transition region in coronal holes, as well as other properties are discussed in limited detail. Observations of the centre-to-limb behaviour of transition region and coronal lines are used to construct coronal models, and the assumptions of spherical symmetry evaluated from the intensity data. The effects of spicules on the limb and disk data are discussed in relation to the observations.

Download Full-text

TRACE Emission Heights Estimated from TRACE Limb Observations

Symposium - International Astronomical Union ◽

10.1017/s0074180900219736 ◽

2001 ◽

Vol 203 ◽

pp. 431-433

Author(s):

M. Zhang

Keyword(s):

Transition Region ◽

Coronal Holes ◽

Active Regions ◽

Height Variation ◽

Quiet Sun ◽

Trace Data ◽

Coronal Structures

While TRACE data have provided us much information of transition region and coronal structures, many TRACE data users would like to have a knowledge of emission heights of TRACE bands. By analyzing TRACE limb observations, we give an average estimation of emission heights of TRACE 171, 195 and 1216 bands for different features like quiet Sun regions, active regions and coronal holes. Average emission heights over the limb are also discussed. Previous equator-to-pole height variation is further confirmed by TRACE data when averaging on quiet Sun regions. If averaging for all fluxes, a reverse equator-to-pole height variation is shown.

Download Full-text

Sizes of transition-region structures in coronal holes and in the quiet Sun

Astronomy and Astrophysics ◽

10.1051/0004-6361:20079235 ◽

2008 ◽

Vol 482 (1) ◽

pp. 267-272 ◽

Cited By ~ 24

Author(s):

H. Tian ◽

E. Marsch ◽

C.-Y. Tu ◽

L.-D. Xia ◽

J.-S. He

Keyword(s):

Transition Region ◽

Coronal Holes ◽

Quiet Sun

Download Full-text

An Analysis of Spikes in Atmospheric Imaging Assembly (AIA) Data

Solar Physics ◽

10.1007/s11207-021-01929-8 ◽

2021 ◽

Vol 296 (12) ◽

Author(s):

Peter R. Young ◽

Nicholeen M. Viall ◽

Michael S. Kirk ◽

Emily I. Mason ◽

Lakshmi Pradeep Chitta

Keyword(s):

Solar Atmosphere ◽

Extreme Ultraviolet ◽

Solar Surface ◽

Coronal Holes ◽

Active Regions ◽

Detection Algorithm ◽

Solar Dynamics Observatory ◽

Processing Pipeline ◽

High Resolution Images ◽

Solar Dynamics

AbstractThe Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) returns high-resolution images of the solar atmosphere in seven extreme ultraviolet (EUV) wavelength channels. The images are processed on the ground to remove intensity spikes arising from energetic particles hitting the instrument, and the despiked images are provided to the community. In this article, a three-hour series of images from the 171 Å channel obtained on 28 February 2017 was studied to investigate how often the despiking algorithm gave false positives caused by compact brightenings in the solar atmosphere. The latter were identified through spikes appearing in the same detector pixel for three consecutive frames. 1096 examples were found from the 900 image frames. These “three-spikes” were assigned to 126 dynamic solar features, and it is estimated that the three-spike method identifies 19% of the total number of features affected by despiking. For any ten-minute sequence of AIA 171 Å images there are around 37 solar features that have their intensity modified by despiking. The features are found in active regions, quiet Sun, and coronal holes and, in relation to solar surface area, there is a greater proportion within coronal holes. In 96% of the cases, the despiked structure is a compact brightening with a size of two arcsec or less, and the remaining 4% have narrow, elongated structures. By applying an EUV burst detection algorithm, we found that 96% of the events could be classified as EUV bursts. None of the spike events are rendered invisible by the AIA processing pipeline, but the total intensity over an event’s lifetime can be reduced by up to 67%. Users are recommended to always restore the original intensities in AIA data when studying short-lived or rapidly evolving features that exhibit fine-scale structure.

Download Full-text

Spectroscopic Techniques for Determining Electron Densities in the Solar Atmosphere

International Astronomical Union Colloquium ◽

10.1017/s025292110010733x ◽

1988 ◽

Vol 102 ◽

pp. 13-23

Author(s):

H.E. Mason

Keyword(s):

Transition Region ◽

Solar Atmosphere ◽

Coronal Holes ◽

Active Regions ◽

Impulsive Phase ◽

Homogeneous Layer ◽

Spectroscopic Techniques ◽

Electron Densities ◽

Coronal Emission

AbstractThe determination of electron density in the solar atmosphere using diagnostic line ratios has been a field of intense activity over the past ten years. These spectroscopic techniques have given us an insight into the physical conditions of such diverse phenomena as flares, active regions, sunspots, coronal holes and the quiet Sun. In this paper, an overview will be given of the methods used for determining electron densities in the solar atmosphere. This will include a discussion of the accuracy of the atomic parameters required in such analyses. Several different approximations are used to calculate electron scattering cross-sections. These will be outlined and their accuracy for individual ions will be assessed.The use of these techniques have led to some fundamental discoveries about the nature of the solar atmosphere. The transition region was conventionally envisaged as a homogeneous layer between the low temperature chromosphere and the high temperature corona. We now know that the transition region has unresolved filamentary structures with very small “filling factors” at low temperatures. In contrast, the coronal emission seems to be more homogeneously distributed. A lot of effort has gone into the determination of the electron densites in solar flares, particularily during the impulsive phase. Such studies are crucial to distinguish between various theoretical flare models. These problems will be discussed in relation to analyses of spectral data from SKYLAB, HRTS, SMM, SOLEX and XSST and with a view to future projects such as SOHO.

Download Full-text

Extreme-ultraviolet bursts and nanoflares in the quiet-Sun transition region and corona

Astronomy and Astrophysics ◽

10.1051/0004-6361/202039969 ◽

2021 ◽

Vol 647 ◽

pp. A159

Author(s):

L. P. Chitta ◽

H. Peter ◽

P. R. Young

Keyword(s):

High Resolution ◽

Solar Atmosphere ◽

Significant Role ◽

Extreme Ultraviolet ◽

Coronal Heating ◽

Solar Dynamics Observatory ◽

Quiet Sun ◽

Solar Dynamics ◽

Event Based ◽

Open Question

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.

Download Full-text

A small-scale EUV jet in the quiet Sun region

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313003207 ◽

2012 ◽

Vol 8 (S294) ◽

pp. 555-559

Author(s):

Junchao Hong ◽

Yunchun Jiang ◽

Ruisheng Zheng ◽

Yi Bi

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Magnetic Reconnection ◽

Solar Atmosphere ◽

Extreme Ultraviolet ◽

Small Scale ◽

Quiet Sun ◽

Filament Channel ◽

Field Lines ◽

Using Data

AbstractSolar jets are typical proxies of small-scale magnetic reconnection events in the solar atmosphere. In this paper, we observe a small-scale jet in a solar quiet region, using data from SDO/Atmospheric Imaging Assembly (AIA), Helioseismic and Magnetic Imager (HMI), with supplemental data from STEREO/EUVI. From HMI magnetograms and calculated photospheric flows, we find that the jet is related to the interaction between unipolar network fields and emerging internetwork bipoles at the boundary of a supergranular cell. In AIA extreme-ultraviolet images, the jet actually includes two successive plasma ejections along different directions. The first ejection follows a distorted path which guides plasma into a small filament channel nearby. However, the second one shot straight along another direction that is parallel with extrapolated potential magnetic field lines on the local. According to these observations, we advocate that during the jet eruption new emerging magnetic fields are reconnecting at the edge of the supergranular cell with different kinds of ambient fields from low (magnetic canopy) to high (high-reaching loops) to allow the occurrence of successive ejections along different directions.

Download Full-text