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Register a new userUbiquitous quiet-Sun jets
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Added by
Valentin Martinez Pillet
Publication date
2011
fields
Physics
and research's language is
English
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IMaX/Sunrise has recently reported the temporal evolution of highly dynamic and strongly Doppler shifted Stokes V signals in the quiet Sun. We attempt to identify the same quiet-Sun jets in the Hinode spectropolarimeter (SP) data set. We generate combinations of linear polarization magnetograms with blue- and redshifted far-wing circular polarization magnetograms to allow an easy identification of the quiet-Sun jets. The jets are identified in the Hinode data where both red- and blueshifted cases are often found in pairs. They appear next to regions of transverse fields that exhibit quiet-Sun neutral lines. They also have a clear tendency to occur in the outer boundary of the granules. These regions always display highly displaced and anomalous Stokes V profiles. The quiet Sun is pervaded with jets formed when new field regions emerge at granular scales loaded with horizontal field lines that interact with their surroundings. This interaction is suggestive of some form of reconnection of the involved field lines that generates the observed high speed flows.
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We investigate the fine structure of magnetic fields in the atmosphere of the quiet Sun. We use photospheric magnetic field measurements from {sc Sunrise}/IMaX with unprecedented spatial resolution to extrapolate the photospheric magnetic field into higher layers of the solar atmosphere with the help of potential and force-free extrapolation techniques. We find that most magnetic loops which reach into the chromosphere or higher have one foot point in relatively strong magnetic field regions in the photosphere. $91%$ of the magnetic energy in the mid chromosphere (at a height of 1 Mm) is in field lines, whose stronger foot point has a strength of more than 300 G, i.e. above the equipartition field strength with convection. The loops reaching into the chromosphere and corona are also found to be asymmetric in the sense that the weaker foot point has a strength $B < 300$ G and is located in the internetwork. Such loops are expected to be strongly dynamic and have short lifetimes, as dictated by the properties of the internetwork fields.
We present a visual determination of the number of bright points (BPs) existing in the quiet Sun, which are structures though to trace intense kG magnetic concentrations. The measurement is based on a 0.1 arcsec angular resolution G-band movie obtained with the Swedish Solar Telescope at the solar disk center. We find 0.97 BPs/Mm^2, which is a factor three larger than any previous estimate. It corresponds to 1.2 BPs per solar granule. Depending on the details of the segmentation, the BPs cover between 0.9% and 2.2% of the solar surface. Assuming their field strength to be 1.5 kG, the detected BPs contribute to the solar magnetic flux with an unsigned flux density between 13 G and 33 G. If network and inter-network regions are counted separately, they contain 2.2 BPs/Mm^2 and 0.85 BPs/Mm^2, respectively.
This work reviews our understanding of the magnetic fields observed in the quiet Sun. The subject has undergone a major change during the last decade (quiet revolution), and it will remain changing since the techniques of diagnostic employed so far are known to be severely biased. Keeping these caveats in mind, our work covers the main observational properties of the quiet Sun magnetic fields: magnetic field strengths, unsigned magnetic flux densities, magnetic field inclinations, as well as the temporal evolution on short time-scales (loop emergence), and long time-scales (solar cycle). We also summarize the main theoretical ideas put forward to explain the origin of the quiet Sun magnetism. A final prospective section points out various areas of solar physics where the quiet Sun magnetism may have an important physical role to play (chromospheric and coronal structure, solar wind acceleration, and solar elemental abundances).
While the quiet Sun magnetic field shows only little variation with the solar cycle, long-term variations cannot be completely ruled out from first principles. We investigate the potential effect of quiet Sun magnetism on spectral solar irradiance through a series of small-scale dynamo simulations with zero vertical flux imbalance ($langle B_zrangle=0$) and varying levels of small-scale magnetic field strength, and one weak network case with an additional flux imbalance corresponding to a flux density of $langle B_zrangle=100$ G. From these setups we compute the dependence of the outgoing radiative energy flux on the mean vertical magnetic field strength in the photosphere at continuum optical depth $tau=1$ ($langle vert B_zvertrangle_{tau=1}$). We find that a quiet Sun setup with a mean vertical field strength of $langle vert B_zvertrangle_{tau=1}=69$ G is about $0.6~%$ brighter than a non-magnetic reference case. We find a linear dependence of the outgoing radiative energy flux on the mean field strength $langle vert B_zvertrangle_{tau=1}$ with a relative slope of $1.4cdot 10^{-4}$ G$^{-1}$. With this sensitivity, only a moderate change of the quiet Sun field strength by $10%$ would lead to a total solar irradiance variation comparable to the observed solar cycle variation. While this does provide strong indirect constraints on possible quiet Sun variations during a regular solar cycle, it also emphasizes that potential variability over longer time scales could make a significant contribution to longer-term solar irradiance variations.
The quiet Sun exhibits a wealth of magnetic activities that are fundamental for our understanding of solar and astrophysical magnetism. The magnetic fields in the quiet Sun are observed to evolve coherently, interacting with each other to form distinguished structures as they are advected by the horizontal photospheric flows. We study coherent structures in photospheric flows in a region of quiet Sun consisted of supergranules. Supergranular turbulence is investigated by detecting hyperbolic and elliptic Lagrangian coherent structures (LCS) using the horizontal velocity fields derived from Hinode intensity maps. Repelling/attracting LCS are found by computing the forward/backward finite-time Lyapunov exponent (FTLE). The Lagrangian centre of a supergranular cell is given by the local maximum of the forward FTLE; the Lagrangian boundaries of supergranular cells are given by the ridges of the backward FTLE. Objective velocity vortices are found by calculating the Lagrangian-averaged vorticity deviation, and false vortices are filtered by applying a criterion given by the displacement vector. The Lagrangian centres of neighboring supergranular cells are interconnected by ridges of the repelling LCS, which provide the transport barriers that allow the formation of vortices and the concentration of strong magnetic fields in the valleys of the repelling LCS. The repelling LCS also reveal the most likely sites for magnetic reconnection. We show that the ridges of the attracting LCS expose the locations of the sinks of photospheric flows at supergranular junctions, which are the preferential sites for the formation of kG magnetic flux tubes and persistent vortices.
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