Engineering topics

Leake, J. E.

Publications and source records attributed to Leake, J. E..

4 records

DOE OSTI2017

Magnetic Flux Emergence and Decay Rates for Preceder and Follower Sunspots Observed with HMI

We quantify the emergence and decay rates of preceder (p) and follower (f) sunspots within 10 active regions from 2010 to 2014 using Space-weather Helioseismic Magnetic Imager Active Region Patch data. The sunspots are small to mid-sized regions and contain a signed flux within a single polarity sunspot of (1.1--6.5)×10{sup 21} Mx. The net unsigned flux within the regions, including plage, ranges from (5.1--20)×10{sup 21} Mx. Rates are calculated with and without intensity contours to differentiate between sunspot formation and flux emergence. Signed flux emergence rates, calculated with intensity contours, for the p (f) spots average 6.8(4.9)×10{sup 19} Mx hr{sup −1}, while decay rates are −1.9(−3.4)×10{sup 19} Mx hr{sup −1}. The mean, signed flux emergence rate of the regions, including plage, is 7.1×10{sup 19} Mx hr{sup −1}, for a mean peak flux of 5.9×10{sup 21} Mx. Using a synthesis of these results and others reported previously, there is a clear trend for larger flux regions to emerge faster than smaller ones. Observed emergence rates (dϕ/dt, Mx hr{sup −1}) scale with total signed peak flux, ϕ-tilde {sub max}, as a power law with an exponent of 0.36, i.e., dϕ/dt=A ϕ-tilde {sub max}{sup 0.36}. The observed rates may assist in constraining the boundary and initial conditions in simulations which already demonstrate increased rates for flux tubes with higher buoyancy and twist, or in the presence of a strong upflow. Overall, the observed emergence rates are smaller than those in simulations, which may indicate a slower rise of the flux in the interior than what is captured in simulations.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

SUNQUAKE GENERATION BY CORONAL MAGNETIC RESTRUCTURING

Sunquakes are the surface signatures of acoustic waves in the Sun’s interior that are produced by some but not all flares and coronal mass ejections (CMEs). This paper explores a mechanism for sunquake generation by the changes in magnetic field that occur during flares and CMEs, using MHD simulations with a semiempirical FAL-C atmosphere to demonstrate the generation of acoustic waves in the interior in response to changing magnetic tilt in the corona. We find that Alfvén–sound resonance combined with the ponderomotive force produces acoustic waves in the interior with sufficient energy to match sunquake observations when the magnetic field angle changes of the order of 10° in a region where the coronal field strength is a few hundred gauss or more. The most energetic sunquakes are produced when the coronal field is strong, while the variation of magnetic field strength with height and the timescale of the change in tilt are of secondary importance.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

THE CONTRIBUTION OF CORONAL JETS TO THE SOLAR WIND

Transient collimated plasma eruptions in the solar corona, commonly known as coronal (or X-ray) jets, are among the most interesting manifestations of solar activity. It has been suggested that these events contribute to the mass and energy content of the corona and solar wind, but the extent of these contributions remains uncertain. We have recently modeled the formation and evolution of coronal jets using a three-dimensional (3D) magnetohydrodynamic (MHD) code with thermodynamics in a large spherical domain that includes the solar wind. Our model is coupled to 3D MHD flux-emergence simulations, i.e., we use boundary conditions provided by such simulations to drive a time-dependent coronal evolution. The model includes parametric coronal heating, radiative losses, and thermal conduction, which enables us to simulate the dynamics and plasma properties of coronal jets in a more realistic manner than done so far. Here, we employ these simulations to calculate the amount of mass and energy transported by coronal jets into the outer corona and inner heliosphere. Based on observed jet-occurrence rates, we then estimate the total contribution of coronal jets to the mass and energy content of the solar wind to (0.4–3.0)% and (0.3–1.0)%, respectively. Our results are largely consistent with the few previous rough estimates obtained from observations, supporting the conjecture that coronal jets provide only a small amount of mass and energy to the solar wind. We emphasize, however, that more advanced observations and simulations (including parametric studies) are needed to substantiate this conjecture.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

DISTRIBUTION OF ELECTRIC CURRENTS IN SOLAR ACTIVE REGIONS

There has been a long-standing debate on the question of whether or not electric currents in solar active regions are neutralized. That is, whether or not the main (or direct) coronal currents connecting the active region polarities are surrounded by shielding (or return) currents of equal total value and opposite direction. Both theory and observations are not yet fully conclusive regarding this question, and numerical simulations have, surprisingly, barely been used to address it. Here we quantify the evolution of electric currents during the formation of a bipolar active region by considering a three-dimensional magnetohydrodynamic simulation of the emergence of a sub-photospheric, current-neutralized magnetic flux rope into the solar atmosphere. We find that a strong deviation from current neutralization develops simultaneously with the onset of significant flux emergence into the corona, accompanied by the development of substantial magnetic shear along the active region's polarity inversion line. After the region has formed and flux emergence has ceased, the strong magnetic fields in the region's center are connected solely by direct currents, and the total direct current is several times larger than the total return current. These results suggest that active regions, the main sources of coronal mass ejections and flares, are born with substantial net currents, in agreement with recent observations. Furthermore, they support eruption models that employ pre-eruption magnetic fields containing such currents.

79 ASTRONOMY AND ASTROPHYSICS↗