The Experts below are selected from a list of 324 Experts worldwide ranked by ideXlab platform
S S Zilitinkevich - One of the best experts on this subject based on the ideXlab platform.
-
similarity theory and calculation of turbulent Fluxes at the Surface for the stably stratified atmospheric boundary layer
Boundary-Layer Meteorology, 2007Co-Authors: S S Zilitinkevich, Igor EsauAbstract:In this paper we revise the similarity theory for the stably stratified atmospheric boundary layer (ABL), formulate analytical approximations for the wind velocity and potential temperature profiles over the entire ABL, validate them against large-eddy simulation and observational data, and develop an improved Surface Flux calculation technique for use in operational models.
Lisa Upton - One of the best experts on this subject based on the ideXlab platform.
-
predicting the amplitude and hemispheric asymmetry of solar cycle 25 with Surface Flux transport
arXiv: Solar and Stellar Astrophysics, 2016Co-Authors: David H Hathaway, Lisa UptonAbstract:Evidence strongly indicates that the strength of the Sun's polar fields near the time of a sunspot cycle minimum determines the strength of the following solar activity cycle. We use our Advective Flux Transport (AFT) code, with flows well constrained by observations, to simulate the evolution of the Sun's polar magnetic fields from early 2016 to the end of 2019 --- near the expected time of Cycle 24/25 minimum. We run a series of simulations in which the uncertain conditions (convective motion details, active region tilt, and meridional flow profile) are varied within expected ranges. We find that the average strength of the polar fields near the end of Cycle 24 will be similar to that measured near the end of Cycle 23, indicating that Cycle 25 will be similar in strength to the current cycle. In all cases the polar fields are asymmetric with fields in the south stronger than those in the north. This asymmetry would be more pronounced if not for the predicted weakening of the southern polar fields in late 2016 and through 2017. After just four years of simulation the variability across our ensemble indicates an accumulated uncertainty of about 15\%. This accumulated uncertainty arises from stochastic variations in the convective motion details, the active region tilt, and changes in the meridional flow profile. These variations limit the ultimate predictability of the solar cycle.
-
magnetic Flux transport at the solar Surface
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: J Jiang, R H Cameron, Lisa Upton, David H Hathaway, S K Solanki, L GizonAbstract:After emerging to the solar Surface, the Sun's magnetic field displays a complex and intricate evolution. The evolution of the Surface field is important for several reasons. One is that the Surface field, and its dynamics, sets the boundary condition for the coronal and heliospheric magnetic fields. Another is that the Surface evolution gives us insight into the dynamo process. In particular, it plays an essential role in the Babcock-Leighton model of the solar dynamo. Describing this evolution is the aim of the Surface Flux transport model. The model starts from the emergence of magnetic bipoles. Thereafter, the model is based on the induction equation and the fact that after emergence the magnetic field is observed to evolve as if it were purely radial. The induction equation then describes how the Surface flows -- differential rotation, meridional circulation, granular, supergranular flows, and active region inflows -- determine the evolution of the field (now taken to be purely radial). In this paper, we review the modeling of the various processes that determine the evolution of the Surface field. We restrict our attention to their role in the Surface Flux transport model. We also discuss the success of the model and some of the results that have been obtained using this model.
-
predicting the sun s polar magnetic fields with a Surface Flux transport model
The Astrophysical Journal, 2013Co-Authors: Lisa Upton, David H HathawayAbstract:The Sun's polar magnetic fields are directly related to solar cycle variability. The strength of the polar fields at the start (minimum) of a cycle determine the subsequent amplitude of that cycle. In addition, the polar field reversals at cycle maximum alter the propagation of galactic cosmic rays throughout the heliosphere in fundamental ways. We describe a Surface magnetic Flux transport model that advects the magnetic Flux emerging in active regions (sunspots) using detailed observations of the near-Surface flows that transport the magnetic elements. These flows include the axisymmetric differential rotation and meridional flow and the non-axisymmetric cellular convective flows (supergranules), all of which vary in time in the model as indicated by direct observations. We use this model with data assimilated from full-disk magnetograms to produce full Surface maps of the Sun's magnetic field at 15?minute intervals from 1996 May to 2013 July (all of sunspot cycle 23 and the rise to maximum of cycle 24). We tested the predictability of this model using these maps as initial conditions, but with daily sunspot area data used to give the sources of new magnetic Flux. We find that the strength of the polar fields at cycle minimum and the polar field reversals at cycle maximum can be reliably predicted up to 3?yr in advance. We include a prediction for the cycle 24 polar field reversal.
-
predicting the sun s polar magnetic fields with a Surface Flux transport model
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: Lisa Upton, David H HathawayAbstract:The Sun's polar magnetic fields are directly related to solar cycle variability. The strength of the polar fields at the start (minimum) of a cycle determine the subsequent amplitude of that cycle. In addition, the polar field reversals at cycle maximum alter the propagation of galactic cosmic rays throughout the heliosphere in fundamental ways. We describe a Surface magnetic Flux transport model that advects the magnetic Flux emerging in active regions (sunspots) using detailed observations of the near-Surface flows that transport the magnetic elements. These flows include the axisymmetric differential rotation and meridional flow and the non-axisymmetric cellular convective flows (supergranules) all of which vary in time in the model as indicated by direct observations. We use this model with data assimilated from full-disk magnetograms to produce full Surface maps of the Sun's magnetic field at 15-minute intervals from 1996 May to 2013 July (all of sunspot cycle 23 and the rise to maximum of cycle 24). We tested the predictability of this model using these maps as initial conditions, but with daily sunspot area data used to give the sources of new magnetic Flux. We find that the strength of the polar fields at cycle minimum and the polar field reversals at cycle maximum can be reliably predicted up to three years in advance. We include a prediction for the cycle 24 polar field reversal.
A R Yeates - One of the best experts on this subject based on the ideXlab platform.
-
a double ring algorithm for modeling solar active regions unifying kinematic dynamo models and Surface Flux transport simulations
The Astrophysical Journal, 2010Co-Authors: Andres Munozjaramillo, Dibyendu Nandy, P C H Martens, A R YeatesAbstract:The emergence of tilted bipolar active regions (ARs) and the dispersal of their Flux, mediated via processes such as diffusion, differential rotation, and meridional circulation, is believed to be responsible for the reversal of the Sun's polar field. This process (commonly known as the Babcock-Leighton mechanism) is usually modeled as a near-Surface, spatially distributed α-effect in kinematic mean-field dynamo models. However, this formulation leads to a relationship between polar field strength and meridional flow speed which is opposite to that suggested by physical insight and predicted by Surface Flux-transport simulations. With this in mind, we present an improved double-ring algorithm for modeling the Babcock-Leighton mechanism based on AR eruption, within the framework of an axisymmetric dynamo model. Using Surface Flux-transport simulations, we first show that an axisymmetric formulation—which is usually invoked in kinematic dynamo models—can reasonably approximate the Surface Flux dynamics. Finally, we demonstrate that our treatment of the Babcock-Leighton mechanism through double-ring eruption leads to an inverse relationship between polar field strength and meridional flow speed as expected, reconciling the discrepancy between Surface Flux-transport simulations and kinematic dynamo models.
-
a double ring algorithm for modeling solar active regions unifying kinematic dynamo models and Surface Flux transport simulations
arXiv: Solar and Stellar Astrophysics, 2010Co-Authors: Andres Munozjaramillo, Dibyendu Nandy, P C H Martens, A R YeatesAbstract:The emergence of tilted bipolar active regions and the dispersal of their Flux, mediated via processes such as diffusion, differential rotation and meridional circulation is believed to be responsible for the reversal of the Sun's polar field. This process (commonly known as the Babcock-Leighton mechanism) is usually modeled as a near-Surface, spatially distributed $\alpha$-effect in kinematic mean-field dynamo models. However, this formulation leads to a relationship between polar field strength and meridional flow speed which is opposite to that suggested by physical insight and predicted by Surface Flux-transport simulations. With this in mind, we present an improved double-ring algorithm for modeling the Babcock-Leighton mechanism based on active region eruption, within the framework of an axisymmetric dynamo model. Using Surface Flux-transport simulations we first show that an axisymmetric formulation -- which is usually invoked in kinematic dynamo models -- can reasonably approximate the Surface Flux dynamics. Finally, we demonstrate that our treatment of the Babcock-Leighton mechanism through double-ring eruption leads to an inverse relationship between polar field strength and meridional flow speed as expected, reconciling the discrepancy between Surface Flux-transport simulations and kinematic dynamo models.
P C H Martens - One of the best experts on this subject based on the ideXlab platform.
-
a double ring algorithm for modeling solar active regions unifying kinematic dynamo models and Surface Flux transport simulations
The Astrophysical Journal, 2010Co-Authors: Andres Munozjaramillo, Dibyendu Nandy, P C H Martens, A R YeatesAbstract:The emergence of tilted bipolar active regions (ARs) and the dispersal of their Flux, mediated via processes such as diffusion, differential rotation, and meridional circulation, is believed to be responsible for the reversal of the Sun's polar field. This process (commonly known as the Babcock-Leighton mechanism) is usually modeled as a near-Surface, spatially distributed α-effect in kinematic mean-field dynamo models. However, this formulation leads to a relationship between polar field strength and meridional flow speed which is opposite to that suggested by physical insight and predicted by Surface Flux-transport simulations. With this in mind, we present an improved double-ring algorithm for modeling the Babcock-Leighton mechanism based on AR eruption, within the framework of an axisymmetric dynamo model. Using Surface Flux-transport simulations, we first show that an axisymmetric formulation—which is usually invoked in kinematic dynamo models—can reasonably approximate the Surface Flux dynamics. Finally, we demonstrate that our treatment of the Babcock-Leighton mechanism through double-ring eruption leads to an inverse relationship between polar field strength and meridional flow speed as expected, reconciling the discrepancy between Surface Flux-transport simulations and kinematic dynamo models.
-
a double ring algorithm for modeling solar active regions unifying kinematic dynamo models and Surface Flux transport simulations
arXiv: Solar and Stellar Astrophysics, 2010Co-Authors: Andres Munozjaramillo, Dibyendu Nandy, P C H Martens, A R YeatesAbstract:The emergence of tilted bipolar active regions and the dispersal of their Flux, mediated via processes such as diffusion, differential rotation and meridional circulation is believed to be responsible for the reversal of the Sun's polar field. This process (commonly known as the Babcock-Leighton mechanism) is usually modeled as a near-Surface, spatially distributed $\alpha$-effect in kinematic mean-field dynamo models. However, this formulation leads to a relationship between polar field strength and meridional flow speed which is opposite to that suggested by physical insight and predicted by Surface Flux-transport simulations. With this in mind, we present an improved double-ring algorithm for modeling the Babcock-Leighton mechanism based on active region eruption, within the framework of an axisymmetric dynamo model. Using Surface Flux-transport simulations we first show that an axisymmetric formulation -- which is usually invoked in kinematic dynamo models -- can reasonably approximate the Surface Flux dynamics. Finally, we demonstrate that our treatment of the Babcock-Leighton mechanism through double-ring eruption leads to an inverse relationship between polar field strength and meridional flow speed as expected, reconciling the discrepancy between Surface Flux-transport simulations and kinematic dynamo models.
Igor Esau - One of the best experts on this subject based on the ideXlab platform.
-
similarity theory and calculation of turbulent Fluxes at the Surface for the stably stratified atmospheric boundary layer
Boundary-Layer Meteorology, 2007Co-Authors: S S Zilitinkevich, Igor EsauAbstract:In this paper we revise the similarity theory for the stably stratified atmospheric boundary layer (ABL), formulate analytical approximations for the wind velocity and potential temperature profiles over the entire ABL, validate them against large-eddy simulation and observational data, and develop an improved Surface Flux calculation technique for use in operational models.
