Lisa Upton

MAGNETIC FLUX TRANSPORT AND THE LONG-TERM EVOLUTION OF SOLAR ACTIVE REGIONS

With multiple vantage points around the Sun, STEREO and SDO imaging observations provide a unique opportunity to view the solar surface continuously. We use He II 304 A data from these observatories to isolate and track ten active regions and study their long-term evolution. We find that active regions typically follow a standard pattern of emergence over several days followed by a slower decay that is proportional in time to the peak intensity in the region. Since STEREO does not make direct observations of the magnetic field, we employ a flux-luminosity relationship to infer the total unsigned magnetic flux evolution. To investigate this magnetic flux decay over several rotations we use a surface flux transport model, the Advective Flux Transport (AFT) model, that simulates convective flows using a time-varying velocity field and find that the model provides realistic predictions when information about the active regions magnetic field strength and distribution at peak flux is available. Finally, we illustrate how 304 \AA\ images can be used as a proxy for magnetic flux measurements when magnetic field data is not accessible.

Global non-potential magnetic models of the solar corona during the March 2015 eclipse.

Seven different models are applied to the same problem of simulating the Sun’s coronal magnetic field during the solar eclipse on 2015 March 20. All of the models are non-potential, allowing for free magnetic energy, but the associated electric currents are developed in significantly different ways. This is not a direct comparison of the coronal modelling techniques, in that the different models also use different photospheric boundary conditions, reflecting the range of approaches currently used in the community. Despite the significant differences, the results show broad agreement in the overall magnetic topology. Among those models with significant volume currents in much of the corona, there is general agreement that the ratio of total to potential magnetic energy should be approximately 1.4. However, there are significant differences in the electric current distributions; while static extrapolations are best able to reproduce active regions, they are unable to recover sheared magnetic fields in filament channels using currently available vector magnetogram data. By contrast, time-evolving simulations can recover the filament channel fields at the expense of not matching the observed vector magnetic fields within active regions. We suggest that, at present, the best approach may be a hybrid model using static extrapolations but with additional energization informed by simplified evolution models. This is demonstrated by one of the models.

An Updated Solar Cycle 25 Prediction With AFT: The Modern Minimum

Over the last decade there has been mounting evidence that the strength of the Suns polar magnetic fields during a solar cycle minimum is the best predictor of the amplitude of the next solar cycle. Surface flux transport models can be used to extend these predictions by evolving the Suns surface magnetic field to obtain an earlier prediction for the strength of the polar fields, and thus the amplitude of the next cycle. In 2016, our Advective Flux Transport (AFT) model was used to do this, producing an early prediction for Solar Cycle 25. At that time, AFT predicted that Cycle 25 will be similar in strength to the Cycle 24, with an uncertainty of about 15% . AFT also predicted that the polar fields in the southern hemisphere would weaken in late 2016 and into 2017 before recovering. That AFT prediction was based on the magnetic field configuration at the end of January 2016. We now have 2 more years of observations. We examine the accuracy of the 2016 AFT prediction and find that the new observations track well with AFTs predictions for the last two years. We show that the southern relapse did in fact occur, though the timing was off by several months. We propose a possible cause for the southern relapse and discuss the reason for the offset in timing. Finally, we provide an updated AFT prediction for Solar Cycle 25 which includes solar observations through January of 2018.

Modeling Coronal Response in Decaying Active Regions with Magnetic Flux Transport and Steady Heating

We present new measurements of the dependence of the Extreme Ultraviolet radiance on the total magnetic flux in active regions as obtained from the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO). Using observations of nine active regions tracked along different stages of evolution, we extend the known radiance - magnetic flux power-law relationship (

Predicting the amplitude and hemispheric asymmetry of solar cycle 25 with surface flux transport

I\propto\Phi^{\alpha}

High Latitude Meridional Flow on the Sun May Explain North-South Polar Field Asymmetry

) to the AIA 335 \AA\ passband, and the Fe XVIII 93.93 \AA\ spectral line in the 94 \AA\ passband. We find that the total unsigned magnetic flux divided by the polarity separation (

Predicting the corona for the 21 August 2017 total solar eclipse

\Phi/D

Predicting the amplitude and hemispheric asymmetry of solar cycle 25 with surface flux transport: PREDICTING SOLAR CYCLE 25 WITH SURFACE FLUX TRANSPORT

) is a better indicator of radiance for the Fe XVIII line with a slope of

Observed Properties of Giant Cells

\alpha=3.22\pm0.03

Effects of the Observed Meridional Flow Variations since 1996 on the Sun's Polar Fields

. We then use these results to test our current understanding of magnetic flux evolution and coronal heating. We use magnetograms from the simulated decay of these active regions produced by the Advective Flux Transport (AFT) model as boundary conditions for potential extrapolations of the magnetic field in the corona. We then model the hydrodynamics of each individual field line with the Enthalpy-based Thermal Evolution of Loops (EBTEL) model with steady heating scaled as the ratio of the average field strength and the length (