164 research outputs found
Examination of Separator Reconnection Rates in a Series of Adjacent Emerging/Existing Active Region Pairs
No full textNo abstract availabl
MHD modeling of a geoeffective interplanetary CME with the magnetic topology informed by in-situ observations
No full textVariations of the magnetic field within solar coronal mass ejections (CMEs)
in the heliosphere depend on the CME`s magnetic structure as it leaves the
solar corona and its subsequent evolution through interplanetary space. To
account for this evolution, we developed a new numerical model of the inner
heliosphere that simulates the propagation of a CME through a realistic
background solar wind and allows various CME magnetic topologies. To this end,
we incorporate the Gibson-Low CME model within our global MHD model of the
inner heliosphere, GAMERA-Helio. We apply the model to study the propagation of
the geoeffective CME that erupted on 3 April, 2010 with the aim to reproduce
the temporal variations of the magnetic field vector during the CME passage by
Earth. Parameters of the Gibson-Low CME are informed by STEREO white-light
observations near the Sun. The magnetic topology for this CME - the tethered
flux rope - is informed by in-situ magnetic field observations near Earth. We
performed two simulations testing different CME propagation directions. For an
in-ecliptic direction, the simulation shows a rotation of all three magnetic
field components within the CME, as seen at Earth, similar to that observed.
With a southward propagation direction, suggested by coronal imaging
observations, the modeled By and Bz components are consistent with the ACE
data, but the Bx component lacks the observed change from negative to positive.
In both cases, the model favors the East-West orientation of the CME flux rope,
consistent with the orientation previously inferred from the STEREO/HI
heliospheric images.Comment: 20 pages, 11 figures, 1 table. Submitted to Astrophysical Journa
Relationship between photospheric currents and coronal magnetic helicity for force-free bipolar fields
No full textAims.
The origin and evolution of the magnetic helicity in the solar corona are not well understood. For instance, the magnetic helicity of an active region is often about 1042 Mx2 (1026 Wb2), but the observed processes whereby it is thought to be injected into the corona do not yet provide an accurate estimate of the resulting magnetic helicity budget or time evolution. The variation in magnetic helicity is important for understanding the physics of flares, coronal mass ejections, and their associated magnetic clouds. To shed light on this topic, we investigate here the changes in magnetic helicity due to electric currents in the corona for a single twisted flux tube that may model characteristic coronal structures such as active region filaments, sigmoids, or coronal loops.
Methods.
For a bipolar photospheric magnetic field and several distributions of current, we extrapolated the coronal field as a nonlinear force-free field. We then computed the relative magnetic helicity, as well as the self and mutual helicities.
Results.
Starting from a magnetic configuration with a moderate amount of current, the amount of magnetic helicity can increase by 2 orders of magnitude when the maximum current strength is increased by a factor of 2. The high sensitivity of magnetic helicity to the current density can partially explain discrepancies between measured values on the photosphere, in the corona, and in magnetic clouds. Our conclusion is that the magnetic helicity strongly depends on both the strength of the current density and also on its distribution.
Conclusions.
Only improved measurements of current density at the photospheric level will advance our knowledge of the magnetic helicity content in the solar atmosphere
Chemodynamics of the Milky Way I. The first year of APOGEE data
No full textContext. The Apache Point Observatory Galactic Evolution Experiment (APOGEE) features the first multi-object high-resolution fiber spectrograph in the near-infrared ever built, thus making the survey unique in its capabilities: APOGEE is able to peer through the dust that obscures stars in the Galactic disc and bulge in the optical wavelength range. Here we explore the APOGEE data included as part of the Sloan Digital Sky Survey's 10th data release (SDSS DR10). Aims. The goal of this paper is to a) investigate the chemo-kinematic properties of the Milky Way disc by exploring the first year of APOGEE data; and b) to compare our results to smaller optical high-resolution samples in the literature, as well as results from lower resolution surveys such as the Geneva-Copenhagen Survey (GCS) and the RAdial Velocity Experiment (RAVE). Methods. We select a high-quality (HQ) sample in terms of chemistry (amounting to around 20 000 stars) and, after computing distances and orbital parameters for this sample, we employ a number of useful subsets to formulate constraints on Galactic chemical and chemodynamical evolution processes in the solar neighbourhood and beyond (e.g., metallicity distributions - MDFs, [α/Fe] vs. [Fe/H] diagrams, and abundance gradients). Results. Our red giant sample spans distances as large as 10 kpc from the Sun. Given our chemical quality requirements, most of the stars are located between 1 and 6 kpc from the Sun, increasing by at least a factor of eight the studied volume with respect to the most recent chemodynamical studies based on the two largest samples obtained from RAVE and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We find remarkable agreement between the MDF of the recently published local (d < 100 pc) high-resolution high-S/N HARPS sample and our local HQ sample (d < 1 kpc). The local MDF peaks slightly below solar metallicity, and exhibits an extended tail towards [Fe/H] = -1, whereas a sharper cutoff is seen at larger metallicities (the APOGEE sample shows a slight overabundance of stars with metallicities larger than =±0.3 with respect to the HARPS sample). Both samples also compare extremely well in an [α/Fe] vs. [Fe/H] diagram. The APOGEE data also confirm the existence of a gap in the abundance diagram. When expanding our sample to cover three different Galactocentric distance bins (inner disc, solar vicinity and outer disc), we find the high-[α/Fe] stars to be rare towards the outer zones (implying a shorter scale-length of the thick disc with respect to the thin disc), as previously suggested in the literature. Finally, we measure the gradients in [Fe/H] and [α/Fe], and their respective MDFs, over a range of 6 < R < 11 kpc in Galactocentric distance, and a 0 < z < 3 kpc range of distance from the Galactic plane. We find a good agreement with the gradients traced by the GCS and RAVE dwarf samples. For stars with 1.5 < z < 3 kpc (not present in the previous samples), we find a positive metallicity gradient and a negative gradient in [α/Fe]
Cataclysmic Variables and Gamma-Ray Sources
No full textThe cataclysmic variables are associated with high energy events and probably many of
them could be potential gamma-ray sources. Up to now Fermi-LAT detected 3 gamma-ray
transients, which belong to CVs and related objects: V407 Cyg, N Sco 2012 and N Mon 2012 =
V959 Mon. We present the first multicolour observations of the slow classical nova V959
Mon
Estrelas magnéticas quimicamente peculiares: Evolução, oscilações e Imageamento Doppler
No full textTese (doutorado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas - Programa de Pós-Graduação em Física.Apresentamos nesta tese os resultados de três estudos observacionais relacionados com as estrelas Ap, assim como a sub-classe das estrelas rapidamente oscilantes (roAp). No primeiro estudo, investigamos o estado evolutivo das estrelas Ap visando responder a seguinte pergunta: as estrelas nascem peculiares, ou desenvolvem suas peculiaridades ao longo de sua vida na seqüência principal? Para isso, observamos e classificamos 470 estrelas pertencentes a 18 aglomerados com idades entre 6.9 < log t < 8.1 e estudamos a freqüência de ocorrência de estrelas Ap em função da idade dos aglomerados
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