Subjects -> AERONAUTICS AND SPACE FLIGHT (Total: 124 journals)
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- Cross-field transport and pitch-angle anisotropy of solar energetic
particles in MHD turbulence Abstract: Cross-field transport and pitch-angle anisotropy of solar energetic particles in MHD turbulence F. Fraschetti ASTRA Proc., 2, 63-65, https://doi.org/10.5194/ap-2-63-2016, 2016 Recent measurements of solar energetic particles (SEPs) by multi-spacecraft fleet at 1 AU in various energy ranges are compatible with a not-vanishing first-order anisotropy in pitch-angle of the particles phase-space distribution. By using an analytic model for the time-dependent perpendicular transport, we calculate the effect of an inhomogeneous magnetic turbulence on pitch-angle anisotropy. We find that the transport coefficient scales differently from previous transport models. PubDate: 2016-01-15T07:11:21+01:00
- Gamma-ray emitting supernova remnants as the origin of Galactic cosmic
rays Abstract: Gamma-ray emitting supernova remnants as the origin of Galactic cosmic rays M. Kroll, J. Becker Tjus, B. Eichmann, and N. Nierstenhöfer ASTRA Proc., 2, 57-62, https://doi.org/10.5194/ap-2-57-2015, 2015 It is generally believed that the cosmic ray spectrum below the knee is ofGalactic origin, although the exact sources making up the entire cosmic rayenergy budget are still unknown. Including effects of magnetic amplification,Supernova Remnants (SNR) could be capable of accelerating cosmic rays up to afew PeV and they represent the only source class with a sufficientnon-thermal energy budget to explain the cosmic ray spectrum up to the knee.Now, gamma-ray measurements of SNRs for the first time allow to derive thecosmic ray spectrum at the source, giving us a first idea of the concrete,possible individual contributions to the total cosmic ray spectrum. In thiscontribution, we use these features as input parameters for propagatingcosmic rays from its origin to Earth using GALPROP in order to investigate ifthese supernova remnants reproduce the cosmic ray spectrum and if supernovaremnants in general can be responsible for the observed energy budget. PubDate: 2015-11-12T07:11:21+01:00
- A solution to the cosmic ray anisotropy problem
Abstract: A solution to the cosmic ray anisotropy problem P. Mertsch and S. Funk ASTRA Proc., 2, 51-55, https://doi.org/10.5194/ap-2-51-2015, 2015 Observations of the cosmic ray (CR) anisotropy are widely advertised as ameans of finding nearby sources. This idea has recently gained currency afterthe discovery of a rise in the positron fraction and is the goal of currentexperimental efforts, e.g., with AMS-02 on the International Space Station.Yet, even the anisotropy observed for hadronic CRs is notunderstood, in the sense that isotropic diffusion models overpredictthe dipole anisotropy in the TeV–PeV range by almost two orders ofmagnitude. Here, we consider two additional effects normally not consideredin isotropic diffusion models: anisotropic diffusion due to the presence of abackground magnetic field and intermittency effects of the turbulent magneticfields. We numerically explore these effect by tracking test-particlesthrough individual realisations of the turbulent field. We conclude that alarge misalignment between the CR gradient and the background field canexplain the observed low level of anisotropy. PubDate: 2015-10-08T07:11:21+02:00
- The power spectrum of cosmic ray arrival directions
Abstract: The power spectrum of cosmic ray arrival directions M. Ahlers ASTRA Proc., 2, 45-49, https://doi.org/10.5194/ap-2-45-2015, 2015 Various experiments show that the arrival directions of multi-TeV cosmic raysshow significant anisotropies at small angular scales. It was recently arguedthat this small scale structure may arise naturally by cosmic ray diffusionin a large-scale cosmic ray gradient in combination with deflections in localturbulent magnetic fields. We show via analytical and numerical methods thatthe non-trivial power spectrum in this setup is a direct consequence ofLiouville's theorem and can be related to properties of relative diffusion. PubDate: 2015-10-07T07:11:21+02:00
- The large-scale anisotropy with the PAMELA calorimeter
Abstract: The large-scale anisotropy with the PAMELA calorimeter A. Karelin, O. Adriani, G. Barbarino, G. Bazilevskaya, R. Bellotti, M. Boezio, E. Bogomolov, M. Bongi, V. Bonvicini, S. Bottai, A. Bruno, F. Cafagna, D. Campana, R. Carbone, P. Carlson, M. Casolino, G. Castellini, C. De Donato, C. De Santis, N. De Simone, V. Di Felice, V. Formato, A. Galper, S. Koldashov, S. Koldobskiy, S. Krut'kov, A. Kvashnin, A. Leonov, V. Malakhov, L. Marcelli, M. Martucci, A. Mayorov, W. Menn, M. Mergé, V. Mikhailov, E. Mocchiutti, A. Monaco, N. Mori, R. Munini, G. Osteria, F. Palma, B. Panico, P. Papini, M. Pearce, P. Picozza, M. Ricci, S. Ricciarini, R. Sarkar, M. Simon, V. Scotti, R. Sparvoli, P. Spillantini, Y. Stozhkov, A. Vacchi, E. Vannuccini, G. Vasilyev, S. Voronov, Y. Yurkin, G. Zampa, and N. Zampa ASTRA Proc., 2, 35-37, https://doi.org/10.5194/ap-2-35-2015, 2015 The large-scale anisotropy (or the so-called star-diurnal wave) has beenstudied using the calorimeter of the space-born experiment PAMELA. The cosmicray anisotropy has been obtained for the Southern and Northern hemispheressimultaneously in the equatorial coordinate system for the time period2006–2014. The dipole amplitude and phase have been measured for energies1–20 TeV n-1. PubDate: 2015-10-02T07:11:21+02:00
- Cosmic ray transport and anisotropies to high energies
Abstract: Cosmic ray transport and anisotropies to high energies P. L. Biermann, L. I. Caramete, A. Meli, B. N. Nath, E.-S. Seo, V. de Souza, and J. Becker Tjus ASTRA Proc., 2, 39-44, https://doi.org/10.5194/ap-2-39-2015, 2015 Using the data from the Interstellar Medium (ISM) we propose a physical model for the spectrum of the magnetic irregularities in the ISM. With this model we discuss the Galactic Cosmic Ray (GCR) spectrum, as well as the extragalactic Ultra High Energy Cosmic Rays (UHECRs), their chemical abundances and anisotropies. UHECRs may include a proton component from many radio galaxies integrated over vast distances, visible already below 3 EeV. PubDate: 2015-10-02T07:11:21+02:00
- The cosmic ray anisotropy below 1015 eV
Abstract: The cosmic ray anisotropy below 1015 eV G. Di Sciascio ASTRA Proc., 2, 27-33, https://doi.org/10.5194/ap-2-27-2015, 2015 The measurement of the anisotropy in the cosmic ray (CR) arrival directiondistribution provides important informations on the propagation mechanismsand on the identification of their sources. In the last decade the anisotropycame back to the attention of the scientific community, thanks to several newtwo-dimensional representations of the CR arrival direction distributionwhich clearly showed the existence of anisotropies at different angularscales in both hemispheres. The origin of the observed anisotropies is stillunknown. So far, no theory of CRs in the Galaxy exists yet to explain theobservations leaving the standard model of CRs and that of the local magneticfield unchanged at the same time. In this paper the observations of GalacticCR anisotropy will be briefly summarized, with particular attention to theresults obtained by the ARGO-YBJ experiment in the Northern Hemisphere. PubDate: 2015-09-29T07:11:21+02:00
- Diffuse synchrotron emission from galactic cosmic ray electrons
Abstract: Diffuse synchrotron emission from galactic cosmic ray electrons G. Di Bernardo, D. Grasso, C. Evoli, and D. Gaggero ASTRA Proc., 2, 21-26, https://doi.org/10.5194/ap-2-21-2015, 2015 Magnetic fields permeate the interstellar medium (ISM), extending far beyond the Galactic disc. The non-thermal phenomena, like e.g. the Galactic radio emission is doubtless a viable method of observation to clearly delineate the magnetic structure of our Galaxy.In this regard, the aim addressed in this contribution is to simulate the polarized Galactic synchrotron emission, due to the diffuse radiation by charged relativistic particles, at all relevant frequencies, 10 MHz up to 100 GHz. PubDate: 2015-09-22T07:11:21+02:00
- Search for a positron anisotropy with PAMELA experiment
Abstract: Search for a positron anisotropy with PAMELA experiment B. Panico, O. Adriani, G. C. Barbarino, G. A. Bazilevskaya, R. Bellotti, M. Boezio, E. A. Bogomolov, M. Bongi, V. Bonvicini, S. Bottai, A. Bruno, F. Cafagna, D. Campana, P. Carlson, M. Casolino, G. Castellini, C. De Donato, C. De Santis, N. De Simone, V. Di Felice, V. Formato, A. M. Galper, U. Giaccari, A. V. Karelin, S. V. Koldashov, S. Koldobskiy, S. Y. Krutkov, A. N. Kvashnin, A. Leonov, V. Malakhov, L. Marcelli, M. Martucci, A. G. Mayorov, W. Menn, M. Mergé, V. V. Mikhailov, E. Mocchiutti, A. Monaco, N. Mori, R. Munini, G. Osteria, F. Palma, M. Pearce, P. Picozza, M. Ricci, S. B. Ricciarini, R. Sarkar, V. Scotti, M. Simon, R. Sparvoli, P. Spillantini, Y. I. Stozhkov, A. Vacchi, E. Vannuccini, G. I. Vasilyev, S. A. Voronov, Y. T. Yurkin, G. Zampa, and N. Zampa ASTRA Proc., 2, 17-20, https://doi.org/10.5194/ap-2-17-2015, 2015 The PAMELA experiment has been collecting data since 2006; its resultsindicate a rise in the positron fraction with respect to the sum of electronsand positrons in the cosmic-ray (CR) spectrum above 10 GeV. This excess canbe due to additional sources, as SNRs or pulsars, which can lead to ananisotropy in the local CR positron, detectable from current experiments. Wereport on the analysis on spatial distributions of positron events collectedby PAMELA, taking into account also the geomagnetic field effects. Nosignificant deviation from the isotropy has been observed. PubDate: 2015-09-09T07:11:21+02:00
- A Consistent Scenario for the IBEX Ribbon, Anisotropies in TeV Cosmic
Rays, and the Local Interstellar Medium Abstract: A Consistent Scenario for the IBEX Ribbon, Anisotropies in TeV Cosmic Rays, and the Local Interstellar Medium N. A. Schwadron, P. Frisch, F. C. Adams, E. R. Christian, P. Desiati, H. O. Funsten, J. R. Jokipii, D. J. McComas, E. Moebius, and G. Zank ASTRA Proc., 2, 9-16, https://doi.org/10.5194/ap-2-9-2015, 2015 We develop a simple diffusive model of the propagation of cosmic rays and the associated cosmic ray anisotropy due to cosmic ray streaming against the local interstellar flow. We show that the local plasma and field conditions sampled by IBEX provide characteristics that consistently explain TeV cosmic ray anisotropies. These results support models that place the interstellar magnetic field direction near the center of the IBEX ribbon. PubDate: 2015-09-08T07:11:21+02:00
- Observation of the Cosmic-Ray Shadow of the Moon and Sun with IceCube
Abstract: Observation of the Cosmic-Ray Shadow of the Moon and Sun with IceCube F. Bos, F. Tenholt, J. Becker Tjus, and S. Westerhoff ASTRA Proc., 2, 5-8, https://doi.org/10.5194/ap-2-5-2015, 2015 Moon shadow analyses are standard methods to calibrate cosmic-ray detectors.We report on a three-year observation of cosmic-ray Moon and Sun shadows indifferent detector configurations. The cosmic-ray Moon shadow was observedwith high statistical significance (> 6σ) in previous analyses when theIceCube detector operated in a smaller configuration before it was completedin December 2010. This work shows first analyses of the cosmic-ray Sun shadowin IceCube. A binned analysis in one-dimension is used to measure the Moonand Sun shadow with high statistical significance greater than 12σ. PubDate: 2015-08-05T07:11:21+02:00
- The Local Bubble as a cosmic-ray isotropizer
Abstract: The Local Bubble as a cosmic-ray isotropizer I. Gebauer, M. Weinreuter, S. Kunz, and D. Gaggero ASTRA Proc., 2, 1-3, https://doi.org/10.5194/ap-2-1-2015, 2015 The Sun resides in the so-called Local Bubble, an underdense region, embedded in a dense wall of molecular clouds. This structure is expected to act as an efficient cosmic-ray isotropizer. Using realistic assumptions on the impact of the Local Bubble on cosmic-ray diffusion, we demonstrate that the Local Bubble can dilute the directional information of energetic positrons and electrons in cosmic rays. PubDate: 2015-07-31T07:11:21+02:00
- TeV Cosmic Ray Anisotropy and the Heliospheric Magnetic Field
Abstract: TeV Cosmic Ray Anisotropy and the Heliospheric Magnetic Field P. Desiati and A. Lazarian ASTRA Proc., 1, 65-71, https://doi.org/10.5194/ap-1-65-2014, 2014 Cosmic rays are observed to possess a small non uniform distribution in arrival direction. Such anisotropyappears to have a roughly consistent topology between tens of GeV and hundreds of TeV, with asmooth energy dependency on phase and amplitude.Above a few hundreds of TeV a sudden change in the topology of the anisotropyis observed. The distribution of cosmic ray sources in the Milky Way isexpected to inject anisotropy on the cosmic ray flux. The nearest and mostrecent sources, in particular, are expected to contribute more significantlythan others. Moreover the interstellar medium is expected to have differentcharacteristics throughout the Galaxy, with different turbulent propertiesand injection scales. Propagation effects in the interstellar magnetic fieldcan shape the cosmic ray particle distribution as well. In particular, in the1–10 TeV energy range, they have a gyroradius comparable to the size of theHeliosphere, assuming a typical interstellar magnetic field strength of 3 μG.Therefore they are expected to be strongly affected by theHeliosphere in a manner ordered by the direction of the local interstellarmagnetic field and of the heliotail. In this paper we discuss on thepossibility that TeV cosmic rays arrival distribution might be significantlyredistributed as they propagate through the Heliosphere. PubDate: 2014-10-31T07:11:21+01:00
- Effects of stellar evolution and ionizing radiation on the environments of
massive stars Abstract: Effects of stellar evolution and ionizing radiation on the environments of massive stars J. Mackey, N. Langer, S. Mohamed, V. V. Gvaramadze, H. R. Neilson, and D. M.-A. Meyer ASTRA Proc., 1, 61-63, https://doi.org/10.5194/ap-1-61-2014, 2014 We discuss two important effects for the astrospheres of runaway stars: thepropagation of ionizing photons far beyond the astropause, and the rapidevolution of massive stars (and their winds) near the end of their lives. Hotstars emit ionizing photons with associated photoheating that has asignificant dynamical effect on their surroundings. 3-D simulations show thatH ii regions around runaway O stars drive expanding conical shellsand leave underdense wakes in the medium they pass through. For late O starsthis feedback to the interstellar medium is more important than that fromstellar winds. Late in life, O stars evolve to cool red supergiants morerapidly than their environment can react, producing transient circumstellarstructures such as double bow shocks. This provides an explanation for thebow shock and linear bar-shaped structure observed around Betelgeuse. PubDate: 2014-09-11T07:11:21+02:00
- MHD flows at astropauses and in astrotails
Abstract: MHD flows at astropauses and in astrotails D. H. Nickeler, T. Wiegelmann, M. Karlický, and M. Kraus ASTRA Proc., 1, 51-60, https://doi.org/10.5194/ap-1-51-2014, 2014 The geometrical shapes and the physical properties of stellar wind –interstellar medium interaction regions form an important stage for studyingstellar winds and their embedded magnetic fields as well as cosmic raymodulation. Our goal is to provide a proper representation and classificationof counter-flow configurations and counter-flow interfaces in the frame offluid theory. In addition we calculate flows and large-scale electromagneticfields based on which the large-scale dynamics and its role as possiblebackground for particle acceleration, e.g., in the form of anomalous cosmicrays, can be studied. We find that for the definition of the boundaries,which are determining the astropause shape, the number and location ofmagnetic null points and stagnation points is essential. Multipleseparatrices can exist, forming a highly complex environment for theinterstellar and stellar plasma. Furthermore, the formation of extended tailstructures occur naturally, and their stretched field and streamlines providesurroundings and mechanisms for the acceleration of particles byfield-aligned electric fields. PubDate: 2014-09-05T07:11:21+02:00
- Lyman-α observations of astrospheres
Abstract: Lyman-α observations of astrospheres J. L. Linsky and B. E. Wood ASTRA Proc., 1, 43-49, https://doi.org/10.5194/ap-1-43-2014, 2014 Charge-exchange reactions between outflowing stellar wind protons andinterstellar neutral hydrogen atoms entering a stellar astrosphere produce aregion of piled-up-decelerated neutral hydrogen called the hydrogen wall.Absorption by this gas, which is observed in stellar Lyman-α emissionlines, provides the only viable technique at this time for measuring themass-loss rates of F–M dwarf stars. We describe this technique, present analternative way for understanding the relation of mass-loss rate with X-rayemission, and identify several critical issues. PubDate: 2014-08-25T07:11:21+02:00
- Clumps in stellar winds
Abstract: Clumps in stellar winds J. S. Vink ASTRA Proc., 1, 39-41, https://doi.org/10.5194/ap-1-39-2014, 2014 We discuss the origin and quantification of wind clumping and mass–lossrates (Ṁ), particularly in close proximity to the Eddington(Γ) limit, relevant for very massive stars (VMS). We present evidencethat clumping may not be the result of the line-deshadowing instability(LDI), but that clumps are already present in the stellar photosphere. PubDate: 2014-07-30T07:11:21+02:00
- Observations of the anisotropy of cosmic rays at TeV–PeV
Abstract: Observations of the anisotropy of cosmic rays at TeV–PeV S. BenZvi ASTRA Proc., 1, 33-37, https://doi.org/10.5194/ap-1-33-2014, 2014 During the past decade, multiple observatories have reported significantobservations of the anisotropy of cosmic rays in the TeV energy band. Theanisotropy has been observed at large scales and small scales in both theNorthern and Southern Hemispheres. The source of the anisotropy is notwell-understood, though both a galactic and a heliospheric origin have beensuggested. We discuss recent observations of the shape and energy dependenceof the anisotropy, with particular attention to measurements by the IceCubeNeutrino Observatory in the Southern Hemisphere and the Milagro andHigh-Altitude Water Cherenkov (HAWC) observatories in the NorthernHemisphere. PubDate: 2014-07-25T07:11:21+02:00
- Cosmic rays as regulators of molecular cloud properties
Abstract: Cosmic rays as regulators of molecular cloud properties M. Padovani, P. Hennebelle, and D. Galli ASTRA Proc., 1, 23-27, https://doi.org/10.5194/ap-1-23-2014, 2014 Cosmic rays are the main agents in controlling the chemical evolution andsetting the ambipolar diffusion time of a molecular cloud. We summarise theprocesses causing the energy degradation of cosmic rays due to theirinteraction with molecular hydrogen, focusing on the magnetic effects thatinfluence their propagation. Making use of magnetic field configurationsgenerated by numerical simulations, we show that the increase of the fieldline density in the collapse region results in a reduction of the cosmic-rayionisation rate. As a consequence the ionisation fraction decreases,facilitating the decoupling between the gas and the magnetic field. PubDate: 2014-06-27T07:11:21+02:00
- Cosmic ray particles from exploding massive stars with winds
Abstract: Cosmic ray particles from exploding massive stars with winds P. L. Biermann ASTRA Proc., 1, 29-31, https://doi.org/10.5194/ap-1-29-2014, 2014 The origin of cosmic rays is still unsettled. Many sources have been proposedover the years, and exploding stars still provide the most promisingcandidates. Here we examine one of these scenarios, and compare the resultingpredictions with data: Massive stars have winds, and when these starsexplode, the resulting shock runs through the wind. The observable phenomenonis called radio-supernova, and many have been observed in non-thermal radioemission. This emission allows to determine the magnetic field in the wind asa function of radius, and so allows to check, whether such explosions canachieve the high energies required and also explain the flux and the spectraof cosmic rays. The observations show this to be the case, and so we concludethat radio supernovae can explain the high-energy Galactic cosmic rays overthe entire energy range, and that the spectral predictions are compatiblewith observations. PubDate: 2014-06-27T07:11:21+02:00
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