Doctoral Thesis: Magnetic Wreaths, Cycles, and Buoyant Loops in Convective Dynamos

I successfully defended my doctoral thesis on August 5th, 2013. You can download a PDF copy here or get the "official" copy from the JILA web database.

My thesis included the results from my previously published papers as well as chapters on the validation of the dynamic Smagorinsky subgrid-scale model and a new upper boundary condition which applies small-scale plumes on the top of the simulation in order to mimc the effects of near-surface convection.

Sample snapshot of the upper boundary condition for case P in Mollweide projection. (a) Radial velocity field applied at the outer boundary. (b) Pressure field implicitly applied by a boundary condition on the plume opening angles. The pressure is generally negative in the cold downflows. (c) Entropy field for the same plumes with low entropy in the downflows and high entropy in the upflows. 

Conferences: Flux Emergence Worshop 2013

From April 15th through the 18th I attended the 5th edition of the Flux Emergence Workshop in Nice, France. I also attended the 4th edition of FEW at the Space Sciences Lab in Berkeley, California in 2011. FEW is a fairly unique conference compared to others I have attended in that it is topically very focused and registration is capped at about 40 people. This allows every attendee to give a 35 minute presentation. Presentations are generally much more interactive than in other conferences. The audience members regularly interrupt talks with questions which often result in some extended discussion involving the speaker and several other people. Because the meeting is so small these tend to be very productive and focused discussions.

I presented a talk entitled "Modeling the Origin of Tilt, Twist, Active Longitudes, and More: Buoyant Loops in Global Convective Dynamos", which was focused on the statistical properties of the ensemble of buoyant magnetic loops from my convective dynamo simulation (see papers here and here). One of the very interesting topics discussed at length concerns twist in sunspots. Previous simulations have indicated that buoyant magnetic loops need to be twisted in order to coherently rise through the solar convection zone. Measuring the twist in real sunspots is difficult and requires vector magnetograms which are only now becoming widely available, but observations seem to indicate that sunspots have an order of magnitude less twist on average than the minimum required by previous simulations. My simulated buoyant loops show a wide variety of amounts of twist and, on average, even slightly less than what is observed at the solar surface.

The histogram above shows the distribution of twists in my convective dynamo simulation (case S3) with a best-fit Gaussian distribution over-plotted in the black dashed curve. The vertical red dashed line shows the average twist observed in sunspots. Previous simulations have required a minimum twist rate on the order of +/-3 in these units. If the question is "Is twist required for the coherent rise of buoyant magnetic structures, or is it merely acquired from the dynamo generation mechanism?", the answer appears to be the latter.

It was a great conference and at a particularly beautiful location. The venue was Masion du Seminaires in Nice. As you can see from the pictures below, it was a spectacular venue.

View of Masion du Seminaires in Nice, France from the Mediterranean See.

View of Nice and the Mediterranean See from a room like mine at the conference hotel.

The next FEW will be in Boulder in 2015 and I am already excited to attend.

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.