scholarly journals How Good Is the Bipolar Approximation of Active Regions for Surface Flux Transport?

Solar Physics ◽  
2020 ◽  
Vol 295 (9) ◽  
Author(s):  
Anthony R. Yeates
Keyword(s):  
Dipole Moment ◽  
Active Region ◽  
Transport Model ◽  
Active Regions ◽  
Surface Flux ◽  
Flux Transport ◽  
Transport Models ◽  
Magnetic Structures ◽  
Axial Dipole ◽  
Flow Parameters

Abstract We investigate how representing active regions with bipolar magnetic regions (BMRs) affects the end-of-cycle polar field predicted by the surface flux transport model. Our study is based on a new database of BMRs derived from the SDO/HMI active region patch data between 2010 and 2020. An automated code is developed for fitting each active region patch with a BMR, matching both the magnetic flux and axial dipole moment of the region and removing repeat observations of the same region. By comparing the predicted evolution of each of the 1090 BMRs with the predicted evolution of their original active region patches, we show that the bipolar approximation leads to a 24% overestimate of the net axial dipole moment, given the same flow parameters. This is caused by neglecting the more complex multipolar and/or asymmetric magnetic structures of many of the real active regions, and may explain why previous flux transport models had to reduce BMR tilt angles to obtain realistic polar fields. Our BMR database and the Python code to extract it are freely available.

scholarly journals The need for active region disconnection in 3D kinematic dynamo simulations

2019 ◽  
Vol 627 ◽  
pp. A168 ◽  
Author(s):  
T. Whitbread ◽  
A. R. Yeates ◽  
A. Muñoz-Jaramillo
Keyword(s):  
Solar Cycle ◽  
Active Region ◽  
Convection Zone ◽  
Transport Model ◽  
Active Regions ◽  
Surface Flux ◽  
Dynamo Model ◽  
Flux Transport ◽  
Kinematic Dynamo ◽  
The Difference

In this paper we address a discrepancy between the surface flux evolution in a 3D kinematic dynamo model and a 2D surface flux transport model that has been closely calibrated to the real Sun. We demonstrate that the difference is due to the connectivity of active regions to the toroidal field at the base of the convection zone, which is not accounted for in the surface-only model. Initially, we consider the decay of a single active region, firstly in a simplified Cartesian 2D model and subsequently the full 3D model. By varying the turbulent diffusivity profile in the convection zone, we find that increasing the diffusivity – so that active regions are more rapidly disconnected from the base of the convection zone – improves the evolution of the surface field. However, if we simulate a full solar cycle, we find that the dynamo is unable to sustain itself under such an enhanced diffusivity. This suggests that in order to accurately model the solar cycle, we must find an alternative way to disconnect emerging active regions, whilst conserving magnetic flux.

scholarly journals Towards an algebraic method of solar cycle prediction

2020 ◽  
Vol 10 ◽  
pp. 50 ◽  
Author(s):  
Kristóf Petrovay ◽  
Melinda Nagy ◽  
Anthony R. Yeates
Keyword(s):  
Dipole Moment ◽  
Solar Cycle ◽  
Active Regions ◽  
Surface Flux ◽  
Algebraic Method ◽  
Meridional Flow ◽  
Activity Level ◽  
Dynamo Model ◽  
Flux Transport ◽  
Axial Dipole

We discuss the potential use of an algebraic method to compute the value of the solar axial dipole moment at solar minimum, widely considered to be the most reliable precursor of the activity level in the next solar cycle. The method consists of summing up the ultimate contributions of individual active regions to the solar axial dipole moment at the end of the cycle. A potential limitation of the approach is its dependence on the underlying surface flux transport (SFT) model details. We demonstrate by both analytical and numerical methods that the factor relating the initial and ultimate dipole moment contributions of an active region displays a Gaussian dependence on latitude with parameters that only depend on details of the SFT model through the parameter η/Δu where η is supergranular diffusivity and Δu is the divergence of the meridional flow on the equator. In a comparison with cycles simulated in the 2 × 2D dynamo model we further demonstrate that the inaccuracies associated with the algebraic method are minor and the method may be able to reproduce the dipole moment values in a large majority of cycles.

Accounting for small bipolar magnetic regions in solar irradiance reconstructions

2021 ◽  
Author(s):  
Bernhard Hofer ◽  
Natalie A. Krivova ◽  
Sami K. Solanki ◽  
Robert Cameron ◽  
Chi-Ju Wu ◽  
...  
Keyword(s):  
Time Scales ◽  
Solar Irradiance ◽  
Climate Models ◽  
Transport Model ◽  
Active Regions ◽  
Surface Flux ◽  
Realistic Model ◽  
Maunder Minimum ◽  
Flux Transport ◽  
Reasonable Description

<p>Historical solar irradiance is a critical input to climate models. As no direct measurements are available before 1978, reconstructions of past irradiance changes are employed instead. Such reconstructions are based on the knowledge that solar irradiance on time scales of interest to climate studies is modulated by the evolution of the solar surface magnetic structures, such as sunspots and faculae. This calls for historical records or proxies of such features. The longest direct, and thus mostly used, record is the sunspot number. It allows a reasonable description of the emergence and evolution of active regions, which are larger magnetic regions containing sunspots. At the same time, a significant amount of the magnetic flux on the Sun emerges in the form of the so-called ephemeral magnetic regions, which are weaker short-lived bipolar regions that do not contain sunspots. Due to their high frequency, ephemeral regions are an important source of the irradiance variability, especially on time scales longer than the solar cycle. Difficulties in their proper accounting are a main reason for the high uncertainty in the secular irradiance variability. Existing models either do not account for their evolution at all or link them linearly to active regions. We use a new, more realistic model of the ephemeral region emergence, relying on recent independent solar observations, as input to a surface flux transport model (SFTM) to simulate the evolution of the magnetic field in such regions. The latter can then be used to reconstruct the solar irradiance since the Maunder minimum.</p>

scholarly journals Hemispheric injection of magnetic helicity by surface flux transport

2019 ◽  
Vol 631 ◽  
pp. A138 ◽  
Author(s):  
G. Hawkes ◽  
A. R. Yeates
Keyword(s):  
Magnetic Field ◽  
Solar Cycle ◽  
Solar Rotation ◽  
Surface Flux ◽  
Magnetic Helicity ◽  
Solar Cycles ◽  
Flux Transport ◽  
Transport Models ◽  
Radial Magnetic Field ◽  
Helicity Injection

Aims. We estimate the injection of relative magnetic helicity into the solar atmosphere by surface flux transport over 27 solar cycles (1700–2009). Methods. We determine the radial magnetic field evolution using two separate surface flux transport models: one driven by magnetogram inputs and another by statistical active region insertion guided by the sunspot number record. The injection of relative magnetic helicity is then computed from this radial magnetic field together with the known electric field in the flux transport models. Results. Neglecting flux emergence, solar rotation is the dominant contributor to the helicity injection. At high latitudes, the injection is always negative/positive in the northern/southern hemisphere, while at low latitudes the injection tends to have the opposite sign when integrated over the full solar cycle. The overall helicity injection in a given solar cycle depends on the balance between these two contributions. This net injected helicity correlates well with the end-of-cycle axial dipole moment.

scholarly journals Towards an algebraic method of solar cycle prediction

2020 ◽  
Vol 10 ◽  
pp. 46 ◽  
Author(s):  
Melinda Nagy ◽  
Kristóf Petrovay ◽  
Alexandre Lemerle ◽  
Paul Charbonneau
Keyword(s):  
Dipole Moment ◽  
Solar Cycle ◽  
Active Region ◽  
Input Data ◽  
Active Regions ◽  
Algebraic Method ◽  
Dynamo Model ◽  
Good Reproduction ◽  
Detailed Statistical Analysis ◽  
Activity Cycles

An algebraic method for the reconstruction and potentially prediction of the solar dipole moment value at sunspot minimum (known to be a good predictor of the amplitude of the next solar cycle) was suggested in the first paper in this series. The method sums up the ultimate dipole moment contributions of individual active regions in a solar cycle: for this, detailed and reliable input data would in principle be needed for thousands of active regions in a solar cycle. To reduce the need for detailed input data, here we propose a new active region descriptor called ARDoR (Active Region Degree of Rogueness). In a detailed statistical analysis of a large number of activity cycles simulated with the 2 × 2D dynamo model we demonstrate that ranking active regions by decreasing ARDoR, for a good reproduction of the solar dipole moment at the end of the cycle it is sufficient to consider the top N regions on this list explicitly, where N is a relatively low number, while for the other regions the ARDoR value may be set to zero. For example, with N = 5 the fraction of cycles where the dipole moment is reproduced with an error exceeding ±30% is only 12%, significantly reduced with respect to the case N = 0, i.e. ARDoR set to zero for all active regions, where this fraction is 26%. This indicates that stochastic effects on the intercycle variations of solar activity are dominated by the effect of a low number of large “rogue” active regions, rather than the combined effect of numerous small ARs. The method has a potential for future use in solar cycle prediction.

Axial dipole moments of solar active regions in cycles 21-24

2020 ◽  
Author(s):  
Iiro Virtanen ◽  
Ilpo Virtanen ◽  
Alexei Pevtsov ◽  
Kalevi Mursula
Keyword(s):  
Magnetic Field ◽  
Dipole Moment ◽  
Magnetic Flux ◽  
Opposite Sign ◽  
Dipole Moments ◽  
Active Regions ◽  
Photospheric Magnetic Field ◽  
Axial Dipole ◽  
Axial Moment ◽  
Cycle 24

<p>The axial dipole moments of emerging active regions control the evolution of the axial dipole moment of the whole photospheric magnetic field and the strength of polar fields. Hale's and Joy's laws of polarity and tilt orientation affect the sign of the axial dipole moment of an active region, determining the normal sign for each solar cycle. If both laws are valid (or both violated), the sign of the axial moment is normal. However, for some active regions, only one of the two laws is violated, and the signs of these axial dipole moments are the opposite of normal. The opposite-sign axial dipole moments can potentially have a significant effect on the evolution of the photospheric magnetic field, including the polar fields.</p><p>We determine the axial dipole moments of active regions identified from magnetographic observations and study how the axial dipole moments of normal and opposite signs are distributed in time and latitude in solar cycles 21-24.We use active regions identified from the synoptic maps of the photospheric magnetic field measured at the National Solar Observatory (NSO) Kitt Peak (KP) observatory, the Synoptic Optical Long term Investigations of the Sun (SOLIS) vector spectromagnetograph (VSM), and the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO).</p><p>We find that, typically, some 30% of active regions have opposite-sign axial dipole moments in every cycle, often making more than 20% of the total axial dipole moment. Most opposite-signed moments are small, but occasional large moments, which can affect the evolution of polar fields on their own, are observed. Active regions with such a large opposite-sign moment may include only a moderate amount of total magnetic flux. We find that in cycles 21-23 the northern hemisphere activates first and shows emergence of magnetic flux over a wider latitude range, while the southern hemisphere activates later, and emergence is concentrated to lower latitudes. We also note that cycle 24 differs from cycles 21-23 in many ways. Cycle 24 is the only cycle where the northern butterfly wing includes more active regions than the southern wing, and where axial dipole moment of normal sign emerges on average later than opposite-signed axial dipole moment. The total axial dipole moment and even the average axial moment of active regions is smaller in cycle 24 than in previous cycles.</p>

scholarly journals Very Large Array (VLA) Observations of Solar Active Regions

1980 ◽  
Vol 86 ◽  
pp. 109-117
Author(s):  
Kenneth R. Lang ◽  
Robert F. Willson
Keyword(s):  
Active Region ◽  
Solar Corona ◽  
Circular Polarization ◽  
Zeeman Effect ◽  
Active Regions ◽  
Large Array ◽  
Magnetic Dipoles ◽  
Very Large Array ◽  
Magnetic Structures ◽  
Brightness Temperatures

Very Large Array (VLA) synthesis maps of the total intensity and the circular polarization of three active regions at 6 cm wavelength are presented. The radiation from each active region is dominated by a few intense cores with angular sizes of ~ 0.5′, brightness temperatures of ~ 106 K, and degrees of circular polarization of 30 to 90%. Some of the core sources within a given active region exhibit opposite senses of circular polarization, suggesting the feet of magnetic dipoles, and the high brightness temperatures suggest that these magnetic structures belong to the low solar corona. We also present comparisons between our VLA maps of circular polarization and Zeeman effect magnetograms of the lower lying photosphere. There is an excellent correlation between the magnetic structures inferred by the two methods, indicating that synthesis maps of circular polarization at 6 cm can be used to delineate magnetic structures in the low solar corona.

Data Reduction for Radionuclide Transport Codes Used in Performance Assessments: An Example of Simplification Process

1997 ◽  
Vol 506 ◽  
Author(s):  
B. Dverstorp ◽  
B. Mendes ◽  
A. Pereira ◽  
B. Sundström
Keyword(s):  
Data Reduction ◽  
Transport Model ◽  
Computer Experiments ◽  
Simple Calculation ◽  
Radionuclide Transport ◽  
Transport Pathway ◽  
Transport Models ◽  
Transport Pathways ◽  
Flow Parameters ◽  
Spatially Varying

ABSTRACTThe input data required for transport models for use in long-term risk assessments of repositories for radioactive waste, in geological media, are intrinsic to the performance of the models. The flow parameters utilized in these models typically come from 2 or 3D hydro-geological calculations done in a prior phase of an assessment. This paper examines some of the standard simplifications introduced when hydrogeological data are reduced to ID as is often required for radionuclide transport models. Two key aspects of data reduction are the determination of average properties of fractured media between and along transport pathways. To quantify possible errors associated with these reduction procedures, two computer experiments have been done. We show that the use of effective flow parameters, representing the average properties of a set of independent transport pathways, in a 1 D radionuclide transport model can result in an underestimation of peak releases by one order of magnitude or more. This result is valid for short-lived nuclides whenever retardation is an important factor. On the other hand, averaging of spatially varying transport properties along a transport pathway may lead to unjustified conservatism. A simple calculation example using Monte Carlo technique, shows that a model that does not take into account spatially varying retardation properties along the transport pathways may overestimate peak release rates by several orders of magnitude. We conclude that more sophisticated transport models taking into account available hydrogeological information on spatial variability are needed to fully understand the potential errors associated with consequence calculations in the performance assessment

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