Quantum field theory on a growing lattice

We construct the classical and canonically quantized theories of a massless scalar field on a background lattice in which the number of points--and hence the number of modes--may grow in time. To obtain a well-defined theory certain restrictions must be imposed on the lattice. Growth-induced particle creation is studied in a two-dimensional example. The results suggest that local mode birth of this sort injects too much energy into the vacuum to be a viable model of cosmological mode birth.Comment: 28 pages, 2 figures; v.2: added comments on defining energy, and reference

Fractal Reconnection in Solar and Stellar Environments

Recent space based observations of the Sun revealed that magnetic reconnection is ubiquitous in the solar atmosphere, ranging from small scale reconnection (observed as nanoflares) to large scale one (observed as long duration flares or giant arcades). Often the magnetic reconnection events are associated with mass ejections or jets, which seem to be closely related to multiple plasmoid ejections from fractal current sheet. The bursty radio and hard X-ray emissions from flares also suggest the fractal reconnection and associated particle acceleration. We shall discuss recent observations and theories related to the plasmoid-induced-reconnection and the fractal reconnection in solar flares, and their implication to reconnection physics and particle acceleration. Recent findings of many superflares on solar type stars that has extended the applicability of the fractal reconnection model of solar flares to much a wider parameter space suitable for stellar flares are also discussed.Comment: Invited chapter to appear in "Magnetic Reconnection: Concepts and Applications", Springer-Verlag, W. D. Gonzalez and E. N. Parker, eds. (2016), 33 pages, 18 figure

Magnetic Reconnection in Extreme Astrophysical Environments

Magnetic reconnection is a basic plasma process of dramatic rearrangement of magnetic topology, often leading to a violent release of magnetic energy. It is important in magnetic fusion and in space and solar physics --- areas that have so far provided the context for most of reconnection research. Importantly, these environments consist just of electrons and ions and the dissipated energy always stays with the plasma. In contrast, in this paper I introduce a new direction of research, motivated by several important problems in high-energy astrophysics --- reconnection in high energy density (HED) radiative plasmas, where radiation pressure and radiative cooling become dominant factors in the pressure and energy balance. I identify the key processes distinguishing HED reconnection: special-relativistic effects; radiative effects (radiative cooling, radiation pressure, and Compton resistivity); and, at the most extreme end, QED effects, including pair creation. I then discuss the main astrophysical applications --- situations with magnetar-strength fields (exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares and magnetically-powered central engines and jets of GRBs. Here, magnetic energy density is so high that its dissipation heats the plasma to MeV temperatures. Electron-positron pairs are then copiously produced, making the reconnection layer highly collisional and dressing it in a thick pair coat that traps radiation. The pressure is dominated by radiation and pairs. Yet, radiation diffusion across the layer may be faster than the global Alfv\'en transit time; then, radiative cooling governs the thermodynamics and reconnection becomes a radiative transfer problem, greatly affected by the ultra-strong magnetic field. This overall picture is very different from our traditional picture of reconnection and thus represents a new frontier in reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic reconnection). Article is based on an invited review talk at the Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA; February 8-12, 2010). 30 pages, no figure

Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics

We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions

Pulsar Wind Nebulae with Bow Shocks: Non-thermal Radiation and Cosmic Ray Leptons

Pulsars with high spin-down power produce relativistic winds radiating a non-negligible fraction of this power over the whole electromagnetic range from radio to gamma-rays in the pulsar wind nebulae (PWNe). The rest of the power is dissipated in the interactions of the PWNe with the ambient interstellar medium (ISM). Some of the PWNe are moving relative to the ambient ISM with supersonic speeds producing bow shocks. In this case, the ultrarelativistic particles accelerated at the termination surface of the pulsar wind may undergo reacceleration in the converging flow system formed by the plasma outflowing from the wind termination shock and the plasma inflowing from the bow shock. The presence of magnetic perturbations in the flow, produced by instabilities induced by the accelerated particles themselves, is essential for the process to work. A generic outcome of this type of reacceleration is the creation of particle distributions with very hard spectra, such as are indeed required to explain the observed spectra of synchrotron radiation with photon indices Γ≲ 1.5. The presence of this hard spectral component is specific to PWNe with bow shocks (BSPWNe). The accelerated particles, mainly electrons and positrons, may end up containing a substantial fraction of the shock ram pressure. In addition, for typical ISM and pulsar parameters, the e+ released by these systems in the Galaxy are numerous enough to contribute a substantial fraction of the positrons detected as cosmic ray (CR) particles above few tens of GeV and up to several hundred GeV. The escape of ultrarelativistic particles from a BSPWN—and hence, its appearance in the far-UV and X-ray bands—is determined by the relative directions of the interstellar magnetic field, the velocity of the astrosphere and the pulsar rotation axis. In this respect we review the observed appearance and multiwavelength spectra of three different types of BSPWNe: PSR J0437-4715, the Guitar and Lighthouse nebulae, and Vela-like objects. We argue that high resolution imaging of such objects provides unique information both on pulsar winds and on the ISM. We discuss the interpretation of imaging observations in the context of the model outlined above and estimate the BSPWN contribution to the positron flux observed at the Earth

A valiação clínica da eficácia do cloridrato de bupropiona e da terapêutica de substituição de nicotina na cessação tabágica

RESUMO: O tabagismo representa, na actualidade, um impo1tante problema de Saude Pública, sendo inequfvoco o seu papel na patogénese de inumeras doenr;as, nomeadamente, na patologia do foro pneumológico. Assim, toma-se primordial a prevenrção do infcio dos habitos tabágicos e, no caso dos fumadores, o apoio médico especializado na sua desabituarção.Todos os anos, aproximadamente 20 em 50 milhões de fumadores nos Estados Unidos da América abandonam os habitos tabagicos, mas apenas 6% destes indivfduos mantcm a abstinencia nicotfnica a Iongo prazo.Nas ultimas décadas têm surgido múltiplas formas de intervcnr;ao farmacológica e nao farmacol6gica na cessarção tabágica, adquirindo uma relevância inquestiom'ivel na mesma.O principal objective do presente estudo foi comparar a efidcia, na desabituação tabagica, da terapeutica de subst ituirção de Nicotina - Sistema Transdénnico (TSN) com cloridrato de bupropiona, utilizado quer lsoladamente quer associado. Foi efectuado um estudo prospecti ve, multicêntrico. randomizado, duplamente cego, envolvendo um grupo paralelo controlado com placebo e abrangendo utn total de 893 indivfduos fumadores: 244 submetidos a TSN; 244 medicados com bupropiona; 245 com TSN e bupropiona; 160 sob placebo (oral: sistema transdém1ico).Os critérios de inclusão foram os segu intes: idadeâ¥18 anos; 11° de cigarros/diaâ¥15; peso corporalâ¥4 5. 5 Kg: motivarção para ab:111donar os hábitos tábagicos; apenas um fumador por agregado familiar. Foram exclufdos indi vfduos com depressão nervosa.O tratamento consistia em 9 semanas de bupropiona (150 mg/dia nos primeiros 3 dias e. posterionnente, 300 mg em duas tomas diárias) ou placebo, bem como, 8 semanas de TSN (início no dia 8: 21 mg/dia entre as semanas 2 e 7, 19 mg/dia na semana 8, e 7 mg/dia na semana 9) ou placebo. 0 dia alvo para dei xar de fumar foi, usualmente, o dia 8.A taxa de cessarção aos 12 meses foi de 15,6% no grupo placebo, de 16,4% no grupo com TSN, de 30,3% no grupo com bupropiona e de 35,5% no grupo em que se associou TSN e bupropiona. Na semana 7, os indi vfduos do grupo placebo tinham urn ganho ponderal de 2, I Kg comparado com 1,6 Kg no grupo TSN, I,7 Kg no grupo bupropiona e I, I Kg no grupo medicado com a associação Urn total de 3 11 indivfduos (34,8%) suspenderam uma ou ambas as medicações destes 177 abandonaram o estudo sem dar qualquer informação adicional, enquanto que 134, embora tenham suspendido a terapeutica, participaram nofollow-up.O grupo placebo apresentou uma maior percentagem de abandono da terapeutica (48,8%), seguido pelos grupos medicados com TSN (35,7%), bupropiona (3 1,9%) e, por último, com a associação TSN/bupropfona. O perfodo de follow-up foi cerca de 12 meses. 17,9% dos indivíduos suspenderam a terapêutica devido a efeitos adversos: 6 no grupo placebo (3,8%), 16 no grupo TSN (6,6%), 29 no grupo bupropiona (11,9%) e 28 no grupo combinado (II,9%). Os efeitos secundários mais frequentes foram as insónias e cefaleias.O tratamento com cloridrato de bupropiona isolado ou em combinação com o sistema transdérmico de nicotina traduz-se numa maior taxa de cessação tabagica quando comparada com o uso apenas de substituto nicotfnico ou de placebo. A taxa de abandono do tabagismo foi superior na terapêutica combinada do que na utili zação isolada de bupropiona, mas a diferença não foi estatisticamente significativa. COMENTARIO: A cessacção tábagica, conhecidos que são os efeitos nefastos do tabagismo, constitui, hoje urn dos objectivos principais de Saúde Pública. Com esta finalidade foram e têm sido utilizados numerosos métodos não farmacológicos e farmacológicos com níveis variáveis de eficácia. No primeiro grupo estão englobados a hipnose, acupunctura, programas de auto-aj uda, terapia de grupo e terapia comportamental. As intervençõoes farmacológicas actuais incluem os substitutos de nicotina e os antidepressivos (entre o quais seencontra a bupropiona). Esta última vertente terapeutica revela-se pertinente, pois, numerosos estudos cientfficos tern evidenciado uma elevada prevalencia de depressão nervosa entre os fumadores apresentando os indivíduos deprimidos uma maior dificuldade em abandonar os hábitos tabágicos.O cloridrato de bupropiona é urn inibidor selectivo da recepção de catecolaminas (noradrenalina e dopamina) e urn inibidor relativamente fraco de recepcção de serotonina. Muito embora o mecanismo de accção deste fármaco no processo de desabituacçãao nicotínica seja desconhecido, pensa-se que mediado por mecanismos dopaminèrgicos e/ou noradrenérgicos a nível cerebral. Esta accção imita os efeitos de nicotina e reduz os seus fveis de necessidade associados à depêndencia.O presente estudo revela que o tratamento com bupropiona isolada ou associada com o sistema transdérmico de nicotina resulta, a Iongo prazo, numa mxa de abandono do tabagismo superior a obtida com a utilização isolada de TSN ou placebo. Os sintomas de abstinência sao inferiores quando se utilizam os substitutos nicotfnicos e/ou bupropiona.Outros estudos demonstraram, também, que ambos diminuem o ganho ponderal relacionado com a cessação tabágica. A associac;ao de bupropiona e TSN produziu a taxa de abstinencia contínua mais elevada, muito embora não fosse estatisticamente diferente, de modo significativo, da obtida com o referido antideRressivo em monoterapia.Assim, o cloridrato de bupropiona apresenta-se como uma promissora arma terapeutica na desabituação tabágica estando os seus benefícios amplamente demonstrados. Palavras Chave: Cessacção tabágica, antidepressivo bupropiona, Sistema Transdérmico de Nicotin