MHD

The Coronal Veil

kolinski — Mon, 11 Jul 2022 20:15:00 +0000

The Coronal Veil kolinski Mon, 07/11/2022 - 14:15

Publication: Astrophysical Journal; Authors: Malanushenko, A.; Cheung, M. C. M.; DeForest, C. E.; Klimchuk, J. A.; Rempel, M.

Author:

kolinski

Jul 11, 2022

3D view of the simulated corona; bottom panel: magnetogram; back panel: synthetic AIA 211A coronal image; middle vertical plane: volumetric emissivity. Field lines are included for reference.

Coronal loops, seen in solar coronal images, are believed to represent emission from magnetic flux tubes with compact cross sections. We examine the 3D structure of plasma above an active region in a radiative magnetohydrodynamic simulation to locate volume counterparts for coronal loops. In many cases, a loop cannot be linked to an individual thin strand in the volume. While many thin loops are present in the synthetic images, the bright structures in the volume are fewer and of complex shape. We demonstrate that this complexity can form impressions of thin bright loops, even in the absence of thin bright plasma strands. We demonstrate the difficulty of discerning from observations whether a particular loop corresponds to a strand in the volume, or a projection artifact. We demonstrate how apparently isolated loops could deceive observers, even when observations from multiple viewing angles are available. While we base our analysis on a simulation, the main findings are independent from a particular simulation setup and illustrate the intrinsic complexity involved in interpreting observations resulting from line-of-sight integration in an optically thin plasma. We propose alternative interpretation for strands seen in Extreme Ultraviolet images of the corona. The "coronal veil" hypothesis is mathematically more generic, and naturally explains properties of loops that are difficult to address otherwise-such as their constant cross section and anomalously high density scale height. We challenge the paradigm of coronal loops as thin magnetic flux tubes, offering new understanding of solar corona, and by extension, of other magnetically confined bright hot plasmas.

Acoustic-gravity wave propagation characteristics in three-dimensional radiation hydrodynamic simulations of the solar atmosphere

kolinski — Thu, 03 Mar 2022 21:49:36 +0000

Acoustic-gravity wave propagation characteristics in three-dimensional radiation hydrodynamic simulations of the solar atmosphere kolinski Thu, 03/03/2022 - 14:49

Publication: Philosophical Transactions of the Royal Society; Authors: B. Fleck; M. Carlsson; E. Khomenko; M. Rempel; O. Steiner; G. Vigeesh

Author:

kolinski

Mar 3, 2022

ν − z phase diagrams (cuts at constant horizontal wavenumber at approx. 2 and 4 Mm−1 through the stacks of the layer-by-layer two-dimensional k − ω phase difference diagrams) for various MURaM models. There is a dramatic difference between the subphotospheric layers (the convection zone) and the solar atmosphere above z = 0 km (τ5000 = 1) layer. In the atmosphere (z > 0 km), we see in large parts the expected behaviour: green and red areas at the lowest frequencies in the gravity wave regime, followed by a transition into propagating acoustic waves with increasingly negative phase differences (purple and blue regions).

There has been tremendous progress in the degree of realism of three-dimensional radiation magneto-hydrodynamic simulations of the solar atmosphere in the past decades. Four of the most frequently used numerical codes are Bifrost, CO5BOLD, MANCHA3D and MURaM. Here we test and compare the wave propagation characteristics in model runs from these four codes by measuring the dispersion relation of acoustic-gravity waves at various heights. We find considerable differences between the various models. The height dependence of wave power, in particular of high-frequency waves, varies by up to two orders of magnitude between the models, and the phase difference spectra of several models show unexpected features, including ±180° phase jumps.

This article is part of the Theo Murphy meeting issue 'High-resolution wave dynamics in the lower solar atmosphere'.

Mausumi Dikpati Publication is Featured by AAS Nova

kolinski — Mon, 22 Nov 2021 18:08:14 +0000

Mausumi Dikpati Publication is Featured by AAS Nova kolinski Mon, 11/22/2021 - 11:08

Author:

kolinski

Nov 22, 2021

Synoptic magnetograms for three successive Carrington rotations (CRs 2152, 2153, and 2154) show some longitudinal patterns of evolving active regions; in each hemisphere the two circled active regions are evolving in such a way that the distance between them is increasing, as roughly depicted by the magenta and aquamarine lines connecting active region centroids in the left three panels (a). The northern hemisphere active region shows prograde propagation in longitude while the southern hemisphere one shows retrograde propagation. (b) The right panels (gray-scale magnetogram images) show images in a Carrington grid and the full disk. What causes such pattern changes and what are the implications for space weather?

A recent publication by Mausumi Dikpati was featured by AAS Nova in November 2020. The AAS Nova website is designed to highlight some of the most interesting recent results being published in AAS journals. Congratulations to Mausumi for her outstanding publication titled: Physics of Magnetohydrodynamic Rossby Waves in the Sun.

Space Weather Challenge & Forecasting Implications Of Rossby Waves

kolinski — Thu, 18 Nov 2021 17:34:26 +0000

Space Weather Challenge & Forecasting Implications Of Rossby Waves kolinski Thu, 11/18/2021 - 10:34

Publication Name: Space Weather; First HAO Author's Name: Mausumi Dikpati

Author:

kolinski

Nov 18, 2021

Rossby waves arise in thin layers within fluid regions of stars and planets. These global wave-like patterns occur due to the variation in Coriolis forces with latitude. In the past several years observational evidence has indicated that there are also Rossby waves in the Sun. Although Rossby waves have been detected in the Sun’s photosphere and corona, they most likely originate in the solar tachocline, the sharp shear layer at the base of the solar convection zone, where the differential rotation driven by convection transitions to the solidly rotating radiative interior. These waves differ from their Earth’s counterparts by being strongly modified by toroidal magnetic fields in the solar tachocline.

Recent simulations of magnetohydrodynamics (MHD) of tachocline Rossby waves and magnetic fields are demonstrated to produce strong “Tachocline Nonlinear Oscillations” (or TNOs), which have periods similar to those observed in the solar atmosphere - enhanced periods of solar activity, or “seasons” -- occurring at intervals between 6 months and two years. These seasonal/sub-seasonal bursts produce the strongest eruptive space weather events. Thus, a key to forecasting the timing, amplitude and location of future activity bursts, and hence space weather events, could lie in our ability to forecast the phase and amplitude of Rossby waves and associated TNOs. Accurately forecasting the properties of solar Rossby waves and their impact on space weather will require linking surface activity observations to the MHD of tachocline Rossby waves, using modern data assimilation techniques. Both short-term (hours to days) and long-term (decadal to millennial) forecasts of space weather and climate are now being made. We highlight in this article the potential of solar Rossby waves for forecasting space weather on intermediate time-scales, of several weeks to months up to a few years ahead.

Physics Of MHD Rossby Waves In The Sun

kolinski — Wed, 17 Nov 2021 17:32:52 +0000

Physics Of MHD Rossby Waves In The Sun kolinski Wed, 11/17/2021 - 10:32

Publication Name: Astrophysical Journal; First HAO Author's Name: Mausumi Dikpati

Author:

kolinski

Nov 17, 2021

Evidence of the existence of hydrodynamic and MHD Rossby waves in the Sun is accumulating rapidly. We employ an MHD Rossby wave model for the Sun in simplified Cartesian geometry, with a uniform toroidal field and no differential rotation, to analyze the role of each force that contributes to Rossby wave dynamics, and compute fluid particle trajectories followed in these waves. This analysis goes well beyond the traditional formulation of Rossby waves in terms of conservation of vorticity.

Top: Particle trajectories for HD and MHD retrograde Rossby waves of same wavelength in x and same stream function amplitude, for both clockwise and anticlockwise flow starting points. Retrograde MHD Rossby waves (solid curves) are faster compared to corresponding HD Rossby waves (dashed curves). Bottom: Slow prograde MHD Rossby waves, which do not have HD counterparts.

Hydrodynamic Rossby waves propagate retrograde relative to the rotation of the reference frame, while MHD Rossby waves can be both prograde and retrograde. Fluid particle trajectories are either clockwise or counterclockwise spirals, depending on where in the wave pattern they are initiated, that track generally in the direction of wave propagation. Retrograde propagating MHD Rossby waves move faster than their hydrodynamic counterparts of the same wavelength, becoming Alfven waves at very high field strengths. Prograde MHD Rossby waves, which have no hydrodynamic counterpart, move more slowly eastward than retrograde MHD Rossby waves for the same toroidal field, but with a speed that increases with toroidal field, in the high field limit again becoming Alfven waves. The longitude and latitude structures of all these waves, as seen in their velocity streamlines and perturbation field lines as well as fluid particle trajectories, are remarkably similar for different toroidal fields, rotation, longitudinal wavelength, and direction of propagation.

MHD

The Coronal Veil

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Acoustic-gravity wave propagation characteristics in three-dimensional radiation hydrodynamic simulations of the solar atmosphere

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Mausumi Dikpati Publication is Featured by AAS Nova

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Space Weather Challenge & Forecasting Implications Of Rossby Waves

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Physics Of MHD Rossby Waves In The Sun

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