Scott McIntosh

A Magnetohydrodynamic Mechanism for the Formation of Solar Polar Vortices

whawkins — Wed, 13 Nov 2024 16:03:34 +0000

A Magnetohydrodynamic Mechanism for the Formation of Solar Polar Vortices whawkins Wed, 11/13/2024 - 09:03

Author:

whawkins

Nov 13, 2024

PNAS (Proceedings of National Academy of Sciences): Polar vortices are ubiquitous features of planetary atmospheric flows, from the Earth-like rocky planets to Jupiter- and Saturn-like gas giant planets. Very little is known about their existence or dynamics on the Sun. What should be expected near the Sun’s pole for the upcoming solar multi-viewpoint and polar missions? Here we report the first magnetohydrodynamic nonlinear simulations for the formation and evolution of solar polar vortices using a near-surface magnetohydrodynamic shallow-water model. Our findings indicate that the rush-to-the-poles, the migration of magnetic fields towards the pole following the Sun’s magnetic cycle, can positively contribute to the formation of polar vortices. The mechanism proposed here for the formation of polar vortices is the first one to involve the role of magnetic fields and may be relevant to any star with a magnetic cycle. The Sun’s polar vortices resulting from this mechanism are predominantly magnetohydrodynamic, consisting of a tight pair of cyclonic and anticyclonic swirls. This mechanism is likely to operate during all solar cycle phases except the peak, when the polar field reverses. Polar vortices can impact dynamical evolution of global flows and polar fields, which seed the next activity cycle, hence better knowledge of physics of polar regions may lead to improved solar cycle and space weather forecasts. See: NSF-NCAR news website.

Simulations of magnetohydrodynamically governed polar vortices for a 30-degree inclined view (left) and for a polar view (right). Magnetohydrodynamic simulations show that the initial mid-latitude perturbations tend to drift and cluster around the forming multiple complex cyclonic and anticyclonic swirls. Following the polar rush, these swirls ultimately lead to an evolved configuration consisting of a pair of tight swirls with m=1 pattern.

Evolution Of Amplitude And Longitude Phase Of Tachocline Rossby Waves Diffusing To The Photosphere

whawkins — Tue, 29 Oct 2024 19:38:58 +0000

Evolution Of Amplitude And Longitude Phase Of Tachocline Rossby Waves Diffusing To The Photosphere whawkins Tue, 10/29/2024 - 13:38

Author:

whawkins

Oct 29, 2024

MNRAS— (Published: 05 November 2024) Physics of MHD Rossby waves in the tachocline-layer were studied by Dikpati, Gilman, Chatterjee et al. (2020) using a fluid-particle-trajectory approach along with solving vorticity and induction equations. By extending the 2020 MHD Rossby wave model to include a hydrodynamic turbulent convection zone (CZ), they were able to examine how MHD Rossby waves generated in the tachocline might diffuse upward through the CZ to the solar surface. Major findings from this current study include (i) pure hydrodynamic Rossby wave amplitudes decline with height due to viscous diffusion at a rate that is independent of viscosity and increases with longitude wavenumber, (ii) fast MHD Rossby waves amplitude declines faster with height for increasing toroidal field, due to their longitude-phase shifting with height, which increases dissipation of kinetic energy in the wave velocities, (iii) slow MHD Rossby waves decline even faster with height because their longitude-phase shifts more rapidly with height, due to their slow phase speed. Furthermore, it was also found that low wavenumber HD and fast MHD Rossby waves, originating in the tachocline, might be detected at the photosphere, but slow MHD Rossby waves should be virtually impossible to detect. An inference from fluid particle trajectories that wave amplitudes declining with height and longitude phase shifting with height associated with decline, implies a powerful mechanism for tangling of magnetic fields, distinct from convective turbulence effects.

Amplitude decline with height of fast and slow MHD Rossby waves at the top of the channel, centered at 45 degrees latitude, compared to the bottom as a function of toroidal field strength, in kiloGauss, for longitudinal wave numbers m = 1 − 10. Left frame: fast waves; right frame: slow waves. Waves for which amplitude at top is smaller than 10−5 compared to the bottom are not plotted. Amplitude decline with height of the zero kG slow wave is included as a limiting case, even though it does not actually exist in that limit. In the limit of zero toroidal field, the fast wave becomes the hydrodynamic Rossby wave.

Global and Local Dynamics of X-flare Producing Active Regions During Solar Cycle 25 Peak-Phase

whawkins — Fri, 06 Sep 2024 16:54:25 +0000

Global and Local Dynamics of X-flare Producing Active Regions During Solar Cycle 25 Peak-Phase whawkins Fri, 09/06/2024 - 10:54

Author:

whawkins

Sep 6, 2024

Astronomy & Astrophysics: As solar cycle (SC) 25 approaches its peak, a number of significant (X-class) flares have been produced. Here, we investigate the circumstances under which two of the most flare-prolific active regions of solar cycle 25, namely ARs 13590 and 13514, flared. Two aspects of the evolution of these active regions are investigated: the global-scale magnetic toroid configuration and small-scale magnetic field morphology and topology, prior, during and after the onset of major flares.

All active regions numbered by NOAA are spotted and marked on 21st February 2024 synoptic map constructed from SDO/HMI daily fits files. The double-arrowed top bar indicates which active regions are on the front side. Here, active regions resulting in X-class Flares are highlighted in red, while active regions resulting in M-class flares are highlighted in orange. Blue and red curves represent, respectively, the Northern and Southern Hemisphere magnetic toroid.

HMI Science Nuggets features: Rossby waves and the organization of photospheric magnetic fields

whawkins — Fri, 06 Oct 2023 19:06:26 +0000

HMI Science Nuggets features: Rossby waves and the organization of photospheric magnetic fields whawkins Fri, 10/06/2023 - 13:06

Author:

whawkins

Oct 6, 2023

Comparison of the butterfly diagram based on the Shannon Entropy (top) and the butterfly diagram based on the Magnetic Field Strength (bottom), showing the evolution of level of longitudinal organization of the photospheric magnetic fields as a function of latitude and time.

HMI Science Nuggets: In recent years an increasing amount of evidence points to the crucial role of magnetically-modified Rossby waves in several solar phenomena. Rossby waves are large scale vortical oscillations that arise in rotating fluid/plasma systems as a result of the Coriolis force, being one of the most fundamental physical mechanisms in the understanding of Earths’s climate and weather. In Earth’s atmosphere one of their effects is to organize the spatial temporal formation of clouds and consequently rainfall and storms. Rays (paths) along which Rossby waves propagate are the origin of atmospheric stormtracks and teleconnection patterns.

Deciphering Pre-solar-storm Features Of September-2017 Storm From Global And Local Dynamics

whawkins — Tue, 01 Aug 2023 18:47:28 +0000

Deciphering Pre-solar-storm Features Of September-2017 Storm From Global And Local Dynamics whawkins Tue, 08/01/2023 - 12:47

Author:

whawkins

Aug 1, 2023

Evolution of the magnetic fields (left) and helicity density (right) for AR 12673.

We investigate how global toroid patterns and local magnetic field topology of solar active region AR12673 together can hindcast occurrence of the biggest X-flare of cycle 24. Magnetic toroid patterns (narrow latitude-belts warped in longitude, in which active regions are tightly stringed) derived from surface distribution of active regions, prior/during AR12673 emergence, reveal that the portions of South-toroid containing AR12673 was not tipped-away from its North-toroid counterpart at that longitude, unlike the 2003 Halloween storms scenario. During minimum-phase there were too few emergences to determine multi-mode warped toroid patterns in longitude. A new emergence within AR12673 produced a complex/non-potential structure, which led to rapid build-up of helicity and winding that triggered the biggest X-flare of cycle 24. Such a minimum-phase storm can be forecast with only hours' lead-time. However, global patterns and local dynamics for a peak-phase storm, such as that from AR11263, behaved like 2003 Halloween storms, producing the second biggest X-flare of cycle 24. AR11263 was present at the longitude where the North and South toroids tipped-away from each other. Both toroids were warped in longitude due to higher wave numbers and were slowly evolving. While global toroid patterns indicate that pre-storm features can be forecast with a lead-time of a few Carrington Rotations, observed complex/non-potential field structure development hours before the storm can improve the forecast further. We infer that minimum-phase storms can be forecast only hours ahead, while flare-prone active regions in peak-phase can be anticipated at least a month ahead from global toroid patterns.

Analyzing longitude distribution of photospheric magnetic fields from MDI/HMI synoptic maps by using information theory

whawkins — Thu, 18 May 2023 19:58:20 +0000

Analyzing longitude distribution of photospheric magnetic fields from MDI/HMI synoptic maps by using information theory whawkins Thu, 05/18/2023 - 13:58

Evidence for Rossby waves

Author:

whawkins

May 18, 2023

The Astrophysical Journal: Much of the research on the magnetic activity of the Sun has been focused on its asymmetric component, however the longitudinal complexity plays a fundamental role in the solar magnetic activity. Rossby waves have recently been proposed as a fundamental mechanism regarding the non-asymmetric of the solar magnetic fields. Here, we use HMI and MDI magnetic field synoptic maps to evaluate the magnetic field structures (mainly active regions) organization and propagation as a function of time and latitude. We first demonstrate using information theory that the organization of longitudinal structures observed on synopitc maps is proportional to the level of activity at a given latitude. We further show that this organization on the longitudinal structures is persisent and due to long-lived features. The drift velocity of these long lived photospheric features is inferred, and is shown to significantly vary with latitude and is compatible with the phase speed of tachocline magnetic-Rossby waves with a toroidal field in the range of 5-10 kG. Our results suggest that Rossby waves contribute to the organization and propagation of photospheric magnetic features on the timescale of several months and beyond.

Magnetohydrodynamics Instabilities of Double Magnetic Bands in a Shallow-water Tachocline Model: I Cross-equatorial Interactions of Bands

whawkins — Thu, 19 Jan 2023 22:16:14 +0000

Magnetohydrodynamics Instabilities of Double Magnetic Bands in a Shallow-water Tachocline Model: I Cross-equatorial Interactions of Bands whawkins Thu, 01/19/2023 - 15:16

Author:

whawkins

Jan 19, 2023

For low effectiive gravity (G = 0.5) growth rate contours for m = 1 modes are displayed in the field-strength space, in which x-axis denotes the strength of the high-latitude band and the y-axis that of the low-latitude band. Left and right panels are respectively for antisymmetric (m = 1,A) and symmetric (m = 1,S) modes. As the band-system migrates from high latitudes towards the equator, four rows from top to the bottom show how the instability features change respectively for bands at 60◦ − 30◦ (aa,ab), 50◦ − 20◦ (ba,bb), 40◦ − 10◦ (ca,cb) and 35◦ − 5◦.

Astrophysical Journal: Along with the ”butterfly diagram” of sunspots, combined observational studies of ephemeral active regions, X-ray and EUV brightpoints, plage, filaments, faculae and prominences demonstrate a pattern, which is known as the Extended Solar Cycle (ESC). This pattern indicates the wings of the sunspot butterfly could be extended to much higher latitudes (up to ∼ 60 degrees), to earlier time than the start of a sunspot cycle, hence yielding a strong overlap between cycles. Thus during the ongoing cycle’s activity near 30-degrees latitude in each hemisphere, the next cycle kicks off at around 60- degrees. By representing these epochs of overlaps by oppositely-directed double magnetic bands in each hemisphere, we compute the unstable eigenmodes for MHD Rossby waves at the base of the convection zone and study how the properties of these energetically active Rossby waves change as these band-pairs migrate equatorward. We find that in each hemisphere the low-latitude band interacts with the high-latitude band and drive the MHD instability as the solar activity progresses from 35◦ to 15◦ latitude, which is essentially the rising phase. When the activity proceeds further equatorward from 15-degrees, interaction between low- and high-latitude bands weakens, and the cross-equatorial interaction between two low-latitude bands in each hemisphere starts. The eigenmodes in latitude- longitude planforms also reflect such changes in their pattern as the bend of the the active cycle moves below 15-degree latitude.

Simulating Solar Near-surface Rossby Waves By Inverse-cascade From Supergranule Energy

whawkins — Wed, 26 Oct 2022 18:19:17 +0000

Simulating Solar Near-surface Rossby Waves By Inverse-cascade From Supergranule Energy whawkins Wed, 10/26/2022 - 12:19

Author:

whawkins

Oct 26, 2022

Astrophysical Journal—M. Dikpati, P. A. Gilman, G. A. Guerrero, A. G. Kosovichev, S. W. McIntosh, K. R. Sreenivasan, J. Warnecke, T. V. Zaqarashvili

Three snapshots during evolution of kinetic energy spectra in spectral space, for initial solid rotation, in a T42 model.

Rossby waves are found at several levels in the Sun, most recently in its supergranule layer. We show that Rossby waves in the supergranule layer can be excited by an inverse cascade of kinetic energy from the nearly horizontal motions in supergranules. We illustrate how this excitation occurs using a hydrodynamic shallow-water model for a 3D thin rotating spherical shell. We find that initial kinetic energy at small spatial scales inverse-cascades quickly to global scales, exciting Rossby waves whose phase velocities are similar to linear Rossby waves on the sphere originally derived by Haurwitz. Modest departures from the Haurwitz formula originate from nonlinear finite amplitude effects and/or the presence of differential rotation. Like supergranules, the initial small scale motions in our model contain very
little vorticity compared to their horizontal divergence, but the resulting Rossby waves are almost all vortical motions. Supergranule kinetic energy could have gone mainly into gravity waves, but we find that most energy inverse-cascades to global Rossby waves. Since kinetic energy in supergranules is three or four orders of magnitude larger than that of the observed Rossby waves in the supergranule layer, there is plenty of energy available to drive the inverse-cascade mechanism. Tachocline Rossby waves were previously shown to play crucial roles in causing "seasons" of space weather through their nonlinear interactions with global flows and magnetic fields. We discuss briefly how various Rossby waves in tachocline, convection zone, supergranule layer and corona can be reconciled in a unified framework.

Global Maps of the Magnetic Field in the Solar Corona

kolinski — Tue, 23 Nov 2021 16:05:28 +0000

Global Maps of the Magnetic Field in the Solar Corona kolinski Tue, 11/23/2021 - 09:05

Author:

kolinski

Nov 23, 2021

An international team led by solar physicists from Peking University, China and National Center for Atmospheric Research (NCAR), USA, has recently measured the global magnetic field of the solar corona for the first time. The team used observations from the Coronal Multi-channel Polarimeter, an instrument operated by NCAR’s High Altitude Observatory. Their results have been published in the Science magazine on August 7, 2020.

Magnetic field of the Sun calculated from the potential field source surface model (Yang, Tian, Tomczyk et al. 2020, Sci China Tech Sci).

The Sun is a magnetized star, and its magnetic field plays a critical role in shaping the solar atmosphere. The 11-year solar cycle, the spectacular solar eruptions and the million-degree solar corona are all driven or governed by the evolution of the solar magnetic field. Due to the magnetic coupling of different atmospheric layers, information on the magnetic field of the whole atmosphere is required to study the interplay between the solar plasma and magnetic field. However, routine measurements of the solar magnetic field have only been achieved at the photospheric level (solar surface). More than one century has passed since the first measurement of the solar magnetic field, we still do not have a precise knowledge of the magnetic field in the upper solar atmosphere, especially the corona, which impedes our complete understanding of the solar magnetism and its interaction with the solar plasma.

More than 20 years ago, a technique called coronal seismology or magnetoseismology has been introduced for coronal magnetic field diagnostics. This method makes use of magnetohydrodynamic (MHD) oscillations or waves that are observed in coronal loops or other coronal structures. From the MHD theory, the observed wave parameters can be used to infer the average magnitudes of the magnetic field in the oscillating structures. However, these oscillations/waves are just occasionally observed in small regions of the corona, and thus their potential for magnetic field diagnostics is limited.

A coronal image (left) and the corresponding magnetic field map (right) (Yang, Bethge, Tian, Tomczyk, et al. 2020, Science).

CoMP is a coronagraph with a 20-cm aperture. Using the Fe XIII 1074.7 nm and 1079.8 nm infrared spectral lines, it can observe the solar corona in the range of about 1.05 to 1.35 solar radii from the solar center through imaging spectroscopy and spectropolarimetry. The Doppler image sequence obtained from CoMP observations often reveal the prevalence of propagating periodic disturbances, indicating the ubiquitous presence of transverse MHD waves in the corona. The team has successfully applied the magnetoseismology method to these pervasive waves. They have extended the previously developed wave-tracking technique to the whole field of view, and obtained the global distribution of the wave phase speed. The intensity ratio of the two Fe XIII lines is sensitive to the electron density, thus has been used to derive the global map of coronal electron density. Combing the wave-tracking and density diagnostic results, they have successfully mapped the magnetic field in the global corona.

This is the first time that a global map of the coronal magnetic field has been obtained through actual coronal observations, thus marking a leap towards solving the problem of coronal magnetic field measurements. In principle, with this technique, global coronal magnetic field maps could now be routinely obtained, filling in the missing part of the measurements of the Sun’s global magnetism. Together with simultaneously measured photospheric magnetograms, these synoptic coronal magnetograms will provide critical information to advance our understanding of the magnetic coupling between different atmospheric layers as well as the physical mechanisms responsible for solar eruptions and solar cycle.

See write-ups in Science, New Scientist, Science News

Global maps of the magnetic field in the solar corona

Detecting the Chromospheric Footpoints of the Solar Wind

kolinski — Tue, 16 Nov 2021 21:53:48 +0000

Detecting the Chromospheric Footpoints of the Solar Wind kolinski Tue, 11/16/2021 - 14:53

Publication: ApJL; Authors: Paul Bryans, Scott McIntosh, David Brooks, Bart De Pontieu

Author:

kolinski

Nov 16, 2021

Coronal Holes present the source of the fast solar wind. However, the fast solar wind is not unimodal—there are discrete, but subtle, compositional, velocity, and density structures that differentiate different coronal holes as well as wind streams that originate within one coronal hole. In this Letter we exploit full-disk observational “mosaics” performed by the Interface Region Imaging Spectrograph (IRIS) spacecraft to demonstrate that significant spectral contrast exists within the chromospheric plasma of coronal holes.

SDO and IRIS full-disk observations from 2016 Feb 22. The HMI and AIA Frankenmaps (panels A and B) are constructed of sub-images with the same fields of view and timing of the IRIS rasters that comprise the mosaic. The CHs in the northern hemisphere and at the south pole are most evident in the AIA intensity image (B), but not in Mg II intensity (C). The CHs are evident, however, in the Mg II peak separation map (D). After removing the center-to-limb variation, this is even clearer (F).

The spectral contrast seen delineates topological differences within the globally open structure. In particular, we show that the “peak separation” of the Mg II h line at 2803 A illustrates changes in what appear to be open magnetic features within a coronal hole. These observations point to a chromospheric source for the inhomogeneities found in the fast solar wind. These chromospheric signatures can provide additional constraints on magnetic field extrapolations close to the source, potentially on spatial scales smaller than from traditional coronal hole detection methods based on intensity thresholding in the corona. This is of increased importance with the advent of Parker Solar Probe and Solar Orbiter and the ability to accurately establish the connectivity between their in situ measurements and remote sensing observations of the solar atmosphere.

Scott McIntosh

A Magnetohydrodynamic Mechanism for the Formation of Solar Polar Vortices

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Evolution Of Amplitude And Longitude Phase Of Tachocline Rossby Waves Diffusing To The Photosphere

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HMI Science Nuggets features: Rossby waves and the organization of photospheric magnetic fields

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Deciphering Pre-solar-storm Features Of September-2017 Storm From Global And Local Dynamics

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Analyzing longitude distribution of photospheric magnetic fields from MDI/HMI synoptic maps by using information theory

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Magnetohydrodynamics Instabilities of Double Magnetic Bands in a Shallow-water Tachocline Model: I Cross-equatorial Interactions of Bands

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Simulating Solar Near-surface Rossby Waves By Inverse-cascade From Supergranule Energy

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Global Maps of the Magnetic Field in the Solar Corona

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Detecting the Chromospheric Footpoints of the Solar Wind

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