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Spectroscopic observations at extreme and far ultraviolet wavelengths have revealed systematic upflows in the solar transition region and corona. These upflows are best seen in the network structures of the quiet Sun and coronal holes, boundaries of active regions, and dimming regions associated with coronal mass ejections. They have been intensively studied in the past two decades because they are highly likely to be closely related to the formation of the solar wind and heating of the upper solar atmosphere. We present an overview of the characteristics of these upflows, introduce their possible formation mechanisms, and discuss their potential roles in the mass and energy transport in the solar atmosphere. Though past investigations have greatly improved our understanding of these upflows, they have left us with several outstanding questions and unresolved issues that should be addressed in the future. New observations from the Solar Orbiter mission, the Daniel K. Inouye Solar Telescope and the Parker Solar Probe will likely provide critical information to advance our understanding of the generation, propagation and energization of these upflows.
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The magnetic activity of the Sun directly impacts the Earth and human life. Likewise, other stars will have an impact on the habitability of planets orbiting these host stars. The lack of information on the magnetic field in the higher atmospheric layers hampers our progress in understanding solar magnetic activity. Overcoming this limitation would allow us to address four paramount long-standing questions: (1) How does the magnetic field couple the different layers of the atmosphere, and how does it transport energy? (2) How does the magnetic field structure, drive and interact with the plasma in the chromosphere and upper atmosphere? (3) How does the magnetic field destabilise the outer solar atmosphere and thus affect the interplanetary environment? (4) How do magnetic processes accelerate particles to high energies? New ground-breaking observations are needed to address these science questions. We suggest a suite of three instruments that far exceed current capabilities in terms of spatial resolution, light-gathering power, and polarimetric performance: (a) A large-aperture UV-to-IR telescope of the 1-3 m class aimed mainly to measure the magnetic field in the chromosphere by combining high spatial resolution and high sensitivity. (b) An extreme-UV-to-IR coronagraph that is designed to measure the large-scale magnetic field in the corona with an aperture of about 40 cm. (c) An extreme-UV imaging polarimeter based on a 30 cm telescope that combines high throughput in the extreme UV with polarimetry to connect the magnetic measurements of the other two instruments. This mission to measure the magnetic field will unlock the driver of the dynamics in the outer solar atmosphere and thereby greatly advance our understanding of the Sun and the heliosphere.
The specification of the upper atmosphere strongly relies on solar proxies that can properly reproduce the solar energetic input in the UV. Whilst the microwave flux at 10.7 cm (also called F10.7 index) has been routinely used as a solar proxy, we show that the radio flux at other wavelengths provides valuable complementary information that enhances their value for upper atmospheric modelling. We merged daily observations from various observatories into a single homogeneous data set of fluxes at wavelengths of 30, 15, 10.7, 8 and 3.2 cm, spanning from 1957 to today. Using blind source separation (BSS), we show that their rotational modulation contains three contributions, which can be interpreted in terms of thermal bremsstrahlung and gyro-resonance emissions. The latter account for 90% of the rotational variability in the F10.7 index. Most solar proxies, such as the MgII index, are remarkably well reconstructed by simple linear combination of radio fluxes at various wavelengths. The flux at 30 cm stands out as an excellent proxy and is better suited than the F10.7 index for the modelling the thermosphere-ionosphere system, most probably because it receives a stronger contribution from thermal bremsstrahlung. This better performance is illustrated here through comparison between the observed thermospheric density, and reconstructions by the Drag Temperature Model.
Solar chromosphere and coronal heating is a big question for astrophysics. Daily measurement of 985 solar spectral irradiances (SSIs) at the spectral intervals 1-39 nm and 116-2416 nm during March 1 2003 to October 28 2017 is utilized to investigate phase relation respectively with daily sunspot number, the Mount Wilson Sunspot Index, and the Magnetic Plage Strength Index. All SSIs which form in the whole heated region: the upper photosphere, chromosphere, transition region, and corona are found to be significantly more correlated to weak magnetic activity than to strong magnetic activity, and to dance in step with weak magnetic activity. All SSIs which form in the low photosphere (the unheated region), which indicate the energy leaked from the solar subsurface are found to be more related to strong magnetic activity instead and in anti-phase with weak magnetic activity. In the upper photosphere and chromosphere, strong magnetic activity should lead SSI by about a solar rotation, also displaying that weak magnetic activity should take effect on heating there. It is thus small-scale weak magnetic activity that effectively heats the upper solar atmosphere.
We performed a systematic study of 12 active regions (ARs) with a broad range of areas, magnetic flux and associated solar activity in order to determine whether there are upflows present at the AR boundaries and if these upflows exist, whether there is a high speed asymmetric blue wing component present in the upflows. To identify the presence and locations of the AR upflows we derive relative Doppler velocity maps by fitting a Gaussian function to {it Hinode}/EIS Fe XII 192.394,AA line profiles. To determine whether there is a high speed asymmetric component present in the AR upflows we fit a double Gaussian function to the Fe XII 192.394,AA mean spectrum that is computed in a region of interest situated in the AR upflows. Upflows are observed at both the east and west boundaries of all ARs in our sample with average upflow velocities ranging between -5 to -26~km s$^{-1}$. A blue wing asymmetry is present in every line profile. The intensity ratio between the minor high speed asymmetric Gaussian component compared to the main component is relatively small for the majority of regions however, in a minority of cases (8/30) the ratios are large and range between 20 to 56~%. These results suggest that upflows and the high speed asymmetric blue wing component are a common feature of all ARs.
The structure and dynamics of the outer solar atmosphere are reviewed with emphasis on the role played by the magnetic field. Contemporary observations that focus on high resolution imaging over a range of temperatures, as well as UV, EUV and hard X-ray spectroscopy, demonstrate the presence of a vast range of temporal and spatial scales, mass motions, and particle energies present. By focussing on recent developments in the chromosphere, corona and solar wind, it is shown that small scale processes, in particular magnetic reconnection, play a central role in determining the large-scale structure and properties of all regions. This coupling of scales is central to understanding the atmosphere, yet poses formidable challenges for theoretical models.
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