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تسجيل مستخدم جديدAlfven waves in simulations of solar photospheric vortices
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Using advanced numerical magneto-hydrodynamic simulations of the magnetised solar photosphere, including non-grey radiative transport and a non-ideal equation of state, we analyse plasma motions in photospheric magnetic vortices. We demonstrate that apparent vortex-like motions in photospheric magnetic field concentrations do not exhibit tornado-like behaviour or a bath-tub effect. While at each time instance the velocity field lines in the upper layers of the solar photosphere show swirls, the test particles moving with the time-dependent velocity field do not demonstrate such structures. Instead, they move in a wave-like fashion with rapidly changing and oscillating velocity field, determined mainly by magnetic tension in the magnetised intergranular downflows. Using time-distance diagrams, we identify horizontal motions in the magnetic flux tubes as torsional Alfven perturbations propagating along the nearly vertical magnetic field lines with local Alfven speed.
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Using numerical simulations of the magnetised solar photosphere carried out with the radiative magneto-hydrodynamic code, MURaM, and detailed spectro-polarimetric diagnostics of the simulated photospheric 6302A FeI line, spectro-polarimetric signatur
es of Alfven waves in magnetised intergranular lanes of the simulated solar photosphere were analysed at different positions at the solar disk. The torsional Alfven waves in the intergranular lanes are horizontal plasma motions, which do not have a thermal perturbation counterpart. We find signatures of Alfven waves as small-scale line profile Doppler shifts and Stokes-V area asymmetry enhancements in the simulated off-disk centre observations. These photospheric features disappear when the simulated observations are degraded with a telescope PSF similar to the one of Hinode. We analyse the possibilities for direct observations and confirmation of Alfven wave presence in the solar photosphere.
We report the detection of oscillatory phenomena associated with a large bright-point group that is 430,000 square kilometers in area and located near the solar disk center. Wavelet analysis reveals full-width half-maximum oscillations with periodici
ties ranging from 126 to 700 seconds originating above the bright point and significance levels exceeding 99%. These oscillations, 2.6 kilometers per second in amplitude, are coupled with chromospheric line-of-sight Doppler velocities with an average blue shift of 23 kilometers per second. A lack of cospatial intensity oscillations and transversal displacements rules out the presence of magneto-acoustic wave modes. The oscillations are a signature of Alfven waves produced by a torsional twist of +/-22 degrees. A phase shift of 180 degrees across the diameter of the bright point suggests that these torsional Alfven oscillations are induced globally throughout the entire brightening. The energy flux associated with this wave mode is sufficient to heat the solar corona.
Context. Small-scale bright features in the photosphere of the Sun, such as faculae or G-band bright points, appear in connection with small-scale magnetic flux concentrations. Aims. Here we report on a new class of photospheric bright points that
are free of magnetic fields. So far, these are visible in numerical simulations only. We explore conditions required for their observational detection. Methods. Numerical radiation (magneto-)hydrodynamic simulations of the near-surface layers of the Sun were carried out. The magnetic field-free simulations show tiny bright points, reminiscent of magnetic bright points, only smaller. A simple toy model for these non-magnetic bright points (nMBPs) was established that serves as a base for the development of an algorithm for their automatic detection. Basic physical properties of 357 detected nMBPs were extracted and statistically evaluated. We produced synthetic intensity maps that mimic observations with various solar telescopes to obtain hints on their detectability. Results. The nMBPs of the simulations show a mean bolometric intensity contrast with respect to their intergranular surroundings of approximately 20%, a size of 60-80 km, and the isosurface of optical depth unity is at their location depressed by 80-100 km. They are caused by swirling downdrafts that provide, by means of the centripetal force, the necessary pressure gradient for the formation of a funnel of reduced mass density that reaches from the subsurface layers into the photosphere. Similar, frequently occurring funnels that do not reach into the photosphere, do not produce bright points. Conclusions. Non-magnetic bright points are the observable manifestation of vertically extending vortices (vortex tubes) in the photosphere. The resolving power of 4-m-class telescopes, such as the DKIST, is needed for an unambiguous detection of them.
A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfven-wave (AW) energy flux. AWs energize the solar wind via two mechanisms: heating and work. We use high-resolution direct numerical simulations of reflecti
on-driven AW turbulence (RDAWT) in a fast-solar-wind stream emanating from a coronal hole to investigate both mechanisms. In particular, we compute the fraction of the AW power at the coronal base ($P_{rm AWb}$) that is transferred to solar-wind particles via heating between the coronal base and heliocentric distance $r$, which we denote $chi_{rm H}(r)$, and the fraction that is transferred via work, which we denote $chi_{rm W}(r)$. We find that $chi_{rm W}(r_{rm A})$ ranges from 0.15 to 0.3, where $r_{rm A}$ is the Alfven critical point. This value is small compared to~one because the Alfven speed $v_{rm A} $ exceeds the outflow velocity $U$ at $r<r_{rm A}$, so the AWs race through the plasma without doing much work. At $r>r_{rm A}$, where $v_{rm A} < U$, the AWs are in an approximate sense stuck to the plasma, which helps them do pressure work as the plasma expands. However, much of the AW power has dissipated by the time the AWs reach $r=r_{rm A}$, so the total rate at which AWs do work on the plasma at $r>r_{rm A}$ is a modest fraction of $P_{rm AWb}$. We find that heating is more effective than work at $r<r_{rm A}$, with $chi_{rm H}(r_{rm A})$ ranging from 0.5 to 0.7. The reason that $chi_{rm H} geq 0.5$ in our simulations is that an appreciable fraction of the local AW power dissipates within each Alfven-speed scale height in RDAWT, and there are a few Alfven-speed scale heights between the coronal base and $r_{rm A}$.
Bright points (BPs) in the solar photosphere are radiative signatures of magnetic elements described by slender flux tubes located in the darker intergranular lanes. They contribute to the ultraviolet (UV) flux variations over the solar cycle and hen
ce may influence the Earths climate. Here we combine high-resolution UV and spectro-polarimetric observations of BPs by the SUNRISE observatory with 3D radiation MHD simulations. Full spectral line syntheses are performed with the MHD data and a careful degradation is applied to take into account all relevant instrumental effects of the observations. It is demonstrated that the MHD simulations reproduce the measured distributions of intensity at multiple wavelengths, line-of-sight velocity, spectral line width, and polarization degree rather well. Furthermore, the properties of observed BPs are compared with synthetic ones. These match also relatively well, except that the observations display a tail of large and strongly polarized BPs not found in the simulations. The higher spatial resolution of the simulations has a significant effect, leading to smaller and more numerous BPs. The observation that most BPs are weakly polarized is explained mainly by the spatial degradation, the stray light contamination, and the temperature sensitivity of the Fe I line at 5250.2 AA{}. The Stokes $V$ asymmetries of the BPs increase with the distance to their center in both observations and simulations, consistent with the classical picture of a production of the asymmetry in the canopy. This is the first time that this has been found also in the internetwork. Almost vertical kilo-Gauss fields are found for 98 % of the synthetic BPs. At the continuum formation height, the simulated BPs are on average 190 K hotter than the mean quiet Sun, their mean BP field strength is 1750 G, supporting the flux-tube paradigm to describe BPs.
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