Movie: Rising Buoyant Loops

This of a 3D rendering of magnetic field lines in buoyant magnetic loops (see my papers on these loops here and here). I have taken care not to render other field lines to visual clarity. The coloring gives the magnitude of the magnetic field. The black surfaces represent the inner and outer boundaries at 0.72 and 0.96 R_sun, respectively. The view is looking south along the rotation axis at a region roughly from the equator to 30 degree north latitude and about 30 degree in longitudinal extent. The movie covers 18 simulated days. You can download a copy of this movie by going to the Vimeo hosting site. If you use it please remember to acknowledge its source.

My CV

Click here to download the PDF.

Papers: Buoyant Magnetic Loops Generated by Global Convective Dynamo Action

Find in ADS 
Find in the arXiv

• Nelson, Nicholas J., Brown, Benjamin P., Brun, Allan Sacha, Miesch, Mark S., & Toomre, Juri, 2013, "Buoyant magnetic loops generated by global convective dynamo action", Solar Physics, 289, 441

This paper looks in greater detail at a large sample of buoyant loops from case S3 - a solar-like convective dynamo simulation. Below are three different views on a single buoyant loop - (a) looks south along the rotation axis, (b) looks radially inward, and (c) looks west along the axis of the loop.

Previous work has focused on small numbers of loops which were identified by visually inspecting 3D renderings of magnetic field line. Using an automated pattern recognition algorithm, I was able to locate 150+ buoyant loops in a systematic search of this simulation. The process is extremely data-intensive so I was only able to to a complete search for one magnetic activity cycle, however this provides enough loops to provide a statistical look at the properties of these loops. Here's a time-latitude map of the longitudinally-averaged magnetic field strength (red shows positive polarity, blue shows negative polarity) with wreaths of opposite polarity in each hemisphere. The southern wreath is clearly much stronger than the northern one. Over-laid are the times and latitudinal locations of each of the 138 buoyant magnetic loops located in this activity cycle, with red squares showing positive polarity loops and green diamonds showing negative polarity loops.

With a large sample of loops, we can compare properties of our simulated loops with observed properties of solar active regions.  For example, we find that our loops show similar latitudinal tilts to those proscribed by Joy's Law. We can also compare the twist of the loops with previous simulations and observations. Previous simulations have indicated that loops must have a certain amount of negative (left-handed) twist or they will break apart and dissipate as they rise. Observations show a wide variety of levels of twist at the solar surface, but a preference for left-handed twists. Our simulations show a slight preference for negative twists, but a wide variety of levels of twist are seen.

Papers: Magnetic Wreaths and Cycles in Convective Dynamos

Find in ADS
Find in the arXiv

• Nelson, Nicholas J., Brown, Benjamin P., Brun, Allan Sacha, Miesch, Mark S., & Toomre, Juri. 2013. "Magnetic Wreaths and Cycles in Convective Dynamos", The Astrophysical Journal, 762, 73

This paper discussed a series of 3D dynamo simulations with ASH of sun-like stars. These dynamos achieved magnetic wreaths - bands of primarily longitudinal magnetic field in each hemisphere. Ben Brown's work had previously shown that persistent wreaths could become cyclic with increased rotation rate.  Here Ben and I used a series of simulations to show that cycles can also occur when simulations are made more turbulent at a fixed rotation rate.

Three ASH simulations which were the focus of this paper: D3 (left), D3a (center), and D3b (right). The simulations are identical except in their diffusion. D3a is about twice as turbulent as D3, and D3b is about twice again as turbulent. From top to bottom, the panels show radial velocities near the top of each simulation, longitudinal magnetic fields at mid-convection zone, and 3D volume renderings of magnetic field lines near the equator colored by longitudinal magnetic field.

In addition to the onset of cycles, we also showed that as these simulations become more turbulent a number of fundamental balances change. The transport of angular momentum which supports differential rotation in D3 is a balance between Reynold's stresses and viscous diffusion, but in D3b diffusion has been replaced by magnetic stresses.  The wreaths in D3 are dissipated primarily by resistive diffusion, but in D3b resolved turbulence has assumed the primary role.  Finally, we showed that the cycles themselves are caused by a breakdown in the balance between turbulent correlations and diffusion in maintaining the poloidal magnetic field.

Cartoon diagram of how cycles operate in case D3b. Magnetic wreaths (a) lead to an electromotive force (b) via an alpha-like effect, which generates poloidal magnetic field (c) through induction, which generates wreaths of the opposite polarity (d) through the Omega effect.

Finally, we showed that the so-called "alpha" effect which closes the loop on our cycles has the same timescale as convection.

Papers: Buoyant Magnetic Loops in a Global Dynamo Simulation of a Young Sun

Find in ADS
Find in the arXiv

• Nelson, Nicholas J., Brown, Benjamin P., Brun, Allan Sacha, Miesch, Mark S., & Toomre, Juri. 2011. "Buoyant Magnetic Loops in a Global Dynamo Simulation of a Young Sun", The Astrophysical Journal Letters, Volume 739, Issue 2, L38

Longitudinal magnetic field as a function of radius and latitude at successive times. Two buoyant magnetic structures are captured here,

 This letter introduced a dynamo simulation of a sun-like star which produced buoyant magnetic loops.  The simulation, which we call case S3, uses an improved treatment of diffusion which allowing the simulation to be much more turbulent than was previously possible.

Papers: Global magnetic cycles in rapidly rotating younger suns

Find in ADS
Find in the arXiv

• Nelson, Nicholas J., Brown, Benjamin P., Browning, Matthew K., Brun, Allan Sacha, Miesch, Mark S., & Toomre, Juri. 2011, "Global magnetic cycles in rapidly rotating younger suns", The Physics of Sun and Star Spots, Proceedings of the International Astronomical Union, IAU Symposium, Volume 273, p. 272-275

This paper was based on a conference talk given at the IAU Symposium #273: The Physics of Sun and Star Spots in Ventura, California in August 2010. I ran and analysed all but one of the simulations here.  Ben Brown was instrumental in developing many of the analysis techniques used. Mark, Sacha, and Juri played key roles in the formulation and analysis of the simulations. This was the first presentation of buoyant magnetic loops in ASH simulations.

Papers: Strong Dynamo Action in Rapidly Rotating Suns

Find in ADS
Find in the arXiv

• Brown, Benjamin P., Browning, Matthew K., Brun, Allan Sacha, Miesch, Mark S., Nelson, Nicholas J., & Toomre, Juri, 2007, "Strong Dynamo Action in Rapidly Rotating Suns", in UNSOLVED PROBLEMS IN STELLAR PHYSICS: A Conference in Honor of Douglas Gough. AIP Conference Proceedings, Volume 948, pp. 271-27

My contribution was in running and analysing one of the three simulations presented here.