D. Passos

Tissue phantom for optical diagnostics based on a suspension of microspheres with a fractal size distribution

We demonstrate experimentally the possibility of reproducing the phase function, absorption, and scattering coefficients of a real biological tissue (adult brain white matter and liver) using a suspension of polystyrene microspheres with a fractal size distribution. The design of a light scattering goniometer with a cylindrical cell in air is discussed, and phase function measurements using the device are described. The scattering coefficient is measured using transmission spectrophotometry and the absorption and reduced scattering coefficients are measured using a time-resolved method. A good match between real tissue and phantom parameters is demonstrated.

Characteristics of magnetic solar-like cycles in a 3D MHD simulation of solar convection

We analyse the statistical properties of the stable magnetic cycle unfolding in an extended 3D magnetohydrodynamic simulation of solar convection produced with the EULAG-MHD code. The millennium simulation spans over 1650 years, in the course of which forty polarity reversals take place on a regular ∼40 yr cadence, remaining well-synchronized across solar hemispheres. In order to characterize this cycle and facilitate its comparison with measures typically used to represent solar activity, we build two proxies for the magnetic field in the simulation mimicking the solar toroidal field and the polar radial field. Several quantities that characterize the cycle are measured (period, amplitudes, etc.) and correlations between them are computed. These are then compared with their observational analogs. From the typical Gnevyshev-Ohl pattern, to hints of Gleissberg modulation, the simulated cycles share many of the characteristics of their observational analogs even though the simulation lacks poloidal field regeneration through active region decay, a mechanism nowadays often considered an essential component of the solar dynamo. Some significant discrepancies are also identified, most notably the in-phase variation of the simulated poloidal and toroidal large-scale magnetic components, and the low degree of hemispheric coupling at the level of hemispheric cycle amplitudes. Possible causes underlying these discrepancies are discussed.

A solar dynamo model driven by mean-field alpha and Babcock-Leighton sources: fluctuations, grand-minima-maxima, and hemispheric asymmetry in sunspot cycles

Extreme solar activity fluctuations and the occurrence of solar grand minima and maxima episodes, are well established, observed features of the solar cycle. Nevertheless, such extreme activity fluctuations and the dynamics of the solar cycle during Maunder minima-like episodes remain ill-understood. We explore the origin of such extreme solar activity fluctuations and the role of dual poloidal field sources, namely the Babcock-Leighton mechanism and the mean-field alpha effect in the dynamics of the solar cycle. We mainly concentrate on entry and recovery from grand minima episodes such as the Maunder minimum and the dynamics of the solar cycle. We use a kinematic solar dynamo model with a novel set-up in which stochastic perturbations force two distinct poloidal field alpha effects. We explore different regimes of operation of these poloidal sources with distinct operating thresholds, to identify the importance of each. The perturbations are implemented independently in both hemispheres which allows one to study the level of hemispheric coupling and hemispheric asymmetry in the emergence of sunspots. From the simulations performed we identify a few different ways in which the dynamo can enter a grand minima episode. While fluctuations in any of the

A LOW-ORDER SOLAR DYNAMO MODEL: INFERRED MERIDIONAL CIRCULATION VARIATIONS SINCE 1750

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A Stochastically Forced Time Delay Solar Dynamo Model: Self-consistent Recovery from a Maunder-like Grand Minimum Necessitates a Mean-field Alpha Effect

effects can trigger intermittency we find that the mean-field alpha effect is crucial for the recovery of the solar cycle from a grand minima episode which a Babcock-Leighton source alone, fails to achieve. Our simulations also demonstrate other cycle dynamics. We conclude that stochastic fluctuations in two interacting poloidal field sources working with distinct operating thresholds is a viable candidate for triggering episodes of extreme solar activity and that the mean-field alpha effect capable of working on weak, sub-equipartition fields is critical to the recovery of the solar cycle following an extended solar minimum.

Near‐Earth heliospheric magnetic field intensity since 1750: 1. Sunspot and geomagnetic reconstructions

In this work we present the possible variations that the meridional circulation of the Sun might have undergone duringthelast 250years. Inordertodothis,wereducean � -dynamotoalow-ordersystemthat focusesonthetime evolution of one of the solar magnetic field components. Afterward we used a method based on the analysis of phase spaceof thesuperficialtoroidalmagneticfieldtoinfer changes inthesuperficialmeridionalcirculation. Weused sunspotnumberstobuildatimeseriesthatapproximatelyrepresentsthemagneticfieldbehavior.Afterreconstructingthe time series’ phase space we assume equilibrium solutions for each solar cycle and we fit them to our model. The resultingfitparametersareshowntodependonbackgroundquantitiesof thetheoreticalmodel,suchasmagneticdiffusivity, differential rotation, meridional circulation, etc. The methodology presented here allows one to extract information about the meridional circulation average behavior, and possibly other parameters, from more that 250 years of sunspot number observations.

Grand minima under the light of a low order dynamo model

Fluctuations in the Suns magnetic activity, including episodes of grand minima such as the Maunder minimum have important consequences for space and planetary environments. However, the underlying dynamics of such extreme fluctuations remain ill-understood. Here, we use a novel mathematical model based on stochastically forced, non-linear delay differential equations to study solar cycle fluctuations in which time delays capture the physics of magnetic flux transport between spatially segregated dynamo source regions in the solar interior. Using this model, we explicitly demonstrate that the Babcock-Leighton poloidal field source based on dispersal of tilted bipolar sunspot flux, alone, cannot recover the sunspot cycle from a grand minimum. We find that an additional poloidal field source effective on weak fields—e.g., the mean-field α effect driven by helical turbulence—is necessary for self-consistent recovery of the sunspot cycle from grand minima episodes.

Near-Earth heliospheric magnetic field intensity since 1750: 2. Cosmogenic radionuclide reconstructions

We present two separate time series of the near-Earth heliospheric magnetic field strength (B) based on geomagnetic data and sunspot number (SSN). The geomagnetic-based B series from 1845-2013 is a weighted composite of two series that employ the interdiurnal variability index; this series is highly correlated with in situ spacecraft measurements of B (correlation coefficient, r = 0.94; mean square error, MSE = 0.16 nT2). The SSN-based estimate of B, from 1750-2013, is a weighted composite of eight time series derived from two separate reconstruction methods applied to four different SSN time series, allowing determination of the uncertainty from both the underlying sunspot records and the B-reconstruction methods. The SSN-based composite is highly correlated with direct spacecraft measurements of B and with the composite geomagnetic B time series from 1845-2013 (r = 0.91; MSE = 0.24 nT2), demonstrating that B can accurately reconstructed by both geomagnetic and sunspot-based methods. The composite sunspot and geomagnetic B time series, with uncertainties, are provided as supplementary electronic material.

Meridional Circulation Dynamics from 3D Magnetohydrodynamic Global Simulations of Solar Convection

In this work we use a low order dynamo model and study under which conditions can it reproduce solar grand minima. We begin by building the phase space of a proxy for the toroidal component of the solar magnetic field and we develop a model, derived from mean field dynamo theory, that gives the time evolution of the toroidal field. This model is characterized by a non-linear oscillator whose coefficients retain most of the physics behind dynamo theory. In the derivation of the model we also include stochastic oscillations in the α effect. We found evidences that stochastic fluctuations in α effect can trigger grand minima episodes in this model under some considerations. We also explore other ways of creating grand minima by looking into the physical mechanisms that compose the coefficients of the oscillator. The balance between meridional circulation and magnetic diffusivity as well as the field intensification by buoyancy driven instabilities, might have a crucial role in inducing grand minima.

Evolution of Solar Parameters since 1750 Based on a Truncated Dynamo Model

This is Part 2 of a study of the near-Earth heliospheric magnetic field strength, B, since 1750. Part 1 produced composite estimates of B from geomagnetic and sunspot data over the period 1750–2013. Sunspot-based reconstructions can be extended back to 1610, but the paleocosmic ray (PCR) record is the only data set capable of providing a record of solar activity on millennial timescales. The process for converting 10Be concentrations measured in ice cores to B is more complex than with geomagnetic and sunspot data, and the uncertainties in B derived from cosmogenic nuclides (~20% for any individual year) are much larger. Within this level of uncertainty, we find reasonable overall agreement between PCR-based B and the geomagnetic- and sunspot number-based series. This agreement was enhanced by excising low values in PCR-based B attributed to high-energy solar proton events. Other discordant intervals, with as yet unspecified causes remain included in our analysis. Comparison of 3 year averages centered on sunspot minimum yields reasonable agreement between the three estimates, providing a means to investigate the long-term changes in the heliospheric magnetic field into the past even without a means to remove solar proton events from the records.