arXiv subjects

Lucas A. Tarr

Publications and source records attributed to Lucas A. Tarr.

24 records · Page 2Linked to original sources

arXiv2017

Science Objective: Understanding Energy Transport by Alfvénic Waves in Solar Flares

Solar flares are driven by the release of magnetic energy from reconnection events in the solar corona, whereafter energy is transported to the chromosphere, heating the plasma and causing the characteristic radiative losses. In the collisional thick-target model, electrons accelerated to energies exceeding 10 keV traverse the corona and impact the chromosphere, where they deposit their energy through collisions with the much denser plasma in the lower atmosphere. While there are undoubtedly high energy non-thermal electrons accelerated in flares, it is unclear whether these electron beams are the sole mechanism of energy transport, or whether they only dominate in certain phases of the flare's evolution. Alfvénic waves are generated during the post-reconnection relaxation of magnetic field lines, so it is important to examine their role in energy transport.

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arXiv2016

Magnetoacoustic Waves in a Stratified Atmosphere with a Magnetic Null Point

We perform nonlinear MHD simulations to study the propagation of magnetoacoustic waves from the photosphere to the low corona. We focus on a 2D system with a gravitationally stratified atmosphere and three photospheric concentrations of magnetic flux that give rise to a magnetic null point with a magnetic dome topology. We find that a single wavepacket introduced at the lower boundary splits into multiple secondary wavepackets. A portion of the packet refracts towards the null due to the varying Alfvén speed. Waves incident on the equipartition contour surrounding the null, where the sound and Alfvén speeds coincide, partially transmit, reflect, and mode convert between branches of the local dispersion relation. Approximately $15.5\%$ of the wavepacket's initial energy ($E_{input}$) converges on the null, mostly as a fast magnetoacoustic wave. Conversion is very efficient: $70\%$ of the energy incident on the null is converted to slow modes propagating away from the null, $7\%$ leaves as a fast wave, and the remaining $23\%$ (0.036$E_{input}$) is locally dissipated. The acoustic energy leaving the null is strongly concentrated along field lines near each of the null's four separatrices. The portion of the wavepacket that refracts towards the null, and the amount of current accumulation, depends on the vertical and horizontal wavenumbers and the centroid position of the wavepacket as it crosses the photosphere. Regions that refract towards or away from the null do not simply coincide with regions of open versus closed magnetic field or the local field orientation. We also modeled wavepacket propagation using a WKB method and found that it agrees qualitatively, though not quantitatively, with the results of the numerical simulation.

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arXiv2015

Relating magnetic reconnection to coronal heating

It is clear that the solar corona is being heated and that coronal magnetic fields undergo reconnection all the time. Here we attempt to show that these two facts are in fact related - i.e. coronal reconnection generates heat. This attempt must address the fact that topological change of field lines does not automatically generate heat. We present one case of flux emergence where we have measured the rate of coronal magnetic reconnection and the rate of energy dissipation in the corona. The ratio of these two, $P/\dotΦ$, is a current comparable to the amount of current expected to flow along the boundary separating the emerged flux from the pre-existing flux overlying it. We can generalize this relation to the overall corona in quiet Sun or in active regions. Doing so yields estimates for the contribution to corona heating from magnetic reconnection. These estimated rates are comparable to the amount required to maintain the corona at its observed temperature.

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arXiv2013

Quiescent Reconnection Rate Between Emerging Active Regions and Preexisting Field, with Associated Heating: NOAA AR11112

When magnetic flux emerges from beneath the photosphere it displaces the preexisting field in the corona, and a current sheet generally forms at the boundary between the old and new magnetic domains. Reconnection in the current sheet relaxes this highly stressed configuration to a lower energy state. This scenario is most familiar, and most often studied, in flares, where the flux transfer is rapid. We present here a study of steady, quiescent flux transfer occurring at a rate three orders of magnitude below that in a large flare. In particular we quantify the reconnection rate, and related energy release, occurring as new polarity emerges to form Active Region 11112 (2010-10-16T00:S18W14) within a region of preexisting flux. A bright, low lying kernel of coronal loops above the emerging polarity, observed with AIA onboard SDO and XRT onboard Hinode, originally shows magnetic connectivity only between regions of newly emerged flux when overlaid on magnetograms from HMI. Over the course of several days, this bright kernel advances into the preexisting flux. The advancement of an easily visible boundary into the old flux regions allows measurement of the rate of reconnection between old and new magnetic domains. We compare the reconnection rate to the inferred heating of the coronal plasma. To our knowledge, this is the first measurement of steady, quiescent heating related to reconnection. We determine that the newly emerged flux reconnects at a fairly steady rate of 0.38e16 Mx/s over two days, while the radiated power varies between 2~8e25 erg/s over the same time. We find that as much as 40% of the total emerged flux at any given time may have reconnected. The total amount of transferred flux (1e21 Mx) and radiated energy (7.2e30 ergs) are comparable to that of a large M- or small X-class flare, but are stretched out over 45 hours.

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arXiv2013

Energetic consequences of flux emergence

When magnetic field in the solar convection zone buoyantly rises to pierce the visible solar surface (photosphere), the atmosphere (corona) above this surface must respond in some way. One response of the coronal field to photospheric forcing is the creation of stress in the magnetic field, generating large currents and storing magnetic free energy. Using a topological model of the coronal magnetic field we will quantify this free energy. We find the free energy just prior to major flares in active regions to be between 30% and 50% of the potential field energy. In a second way, the coronal field may topologically restructure to form new magnetic connections with newly emerged fields. We use our topological model to quantify the rapid restructuring in the case of solar flare and coronal mass ejections, finding that between 1% and 10% of total active region flux is exchanged. Finally, we use observational data to quantify the slow, quiescent reconnection with preexisting field, and find that for small active regions between 20% and 40% of the total emerged flux may have reconnected at any given time.

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arXiv2013

Calculating Separate Magnetic Free Energy Estimates for Active Regions Producing Multiple Flares: NOAA AR11158

It is well known that photospheric flux emergence is an important process for stressing coronal fields and storing magnetic free energy, which may then be released during a flare. The \emph{Helioseismic and Magnetic Imager} (HMI) onboard the \emph{Solar Dynamics Observatory} (SDO) captured the entire emergence of NOAA AR 11158. This region emerged as two distinct bipoles, possibly connected underneath the photosphere, yet characterized by different photospheric field evolutions and fluxes. The combined active region complex produced 15 GOES C--class, 2 M--class, and the X2.2 Valentine's Day Flare during the four days after initial emergence on February 12th, 2011. The M and X class flares are of particular interest because they are nonhomologous, involving different subregions of the active region. We use a Magnetic Charge Topology together with the Minimum Current Corona model of the coronal field to model field evolution of the complex. Combining this with observations of flare ribbons in the 1600Å channel of the \emph{Atmospheric Imaging Assembly} (AIA) onboard SDO, we propose a minimization algorithm for estimating the amount of reconnected flux and resulting drop in magnetic free energy during a flare. For the M6.6, M2.2, and X2.2 flares, we find a flux exchange of $4.2\times 10^{20}\unit{Mx},\ 2.0 \times 10^{20}\unit{Mx}, \hbox{and} 21.0 \times 10^{20}\unit{Mx}$, respectively, resulting in free energy drops of $3.89\times 10^{30}\unit{ergs}, 2.62\times 10^{30}\unit{ergs}, \hbox{and} 1.68\times 10^{32}\unit{ergs}$.

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