AP - recent articles
https://ap.copernicus.org/articles/
Recent articles of the journal ASTRA ProceedingsCross-field transport and pitch-angle anisotropy of solar energetic particles in MHD turbulence
https://doi.org/10.5194/ap-2-63-2016
<b>Cross-field transport and pitch-angle anisotropy of solar energetic particles in MHD turbulence</b><br>
F. Fraschetti<br>
ASTRA Proc., 2, 63–65, https://doi.org/10.5194/ap-2-63-2016, 2016<br>
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.
2016-01-15T09:44:53+01:00Gamma-ray emitting supernova remnants as the origin of Galactic cosmic rays
https://doi.org/10.5194/ap-2-57-2015
<b>Gamma-ray emitting supernova remnants as the origin of Galactic cosmic rays</b><br>
M. Kroll, J. Becker Tjus, B. Eichmann, and N. Nierstenhöfer<br>
ASTRA Proc., 2, 57–62, https://doi.org/10.5194/ap-2-57-2015, 2015<br>
It is generally believed that the cosmic ray spectrum below the knee is of Galactic origin, although the exact sources making up the entire cosmic ray energy budget are still unknown. Including effects of magnetic amplification, Supernova Remnants (SNR) could be capable of accelerating cosmic rays up to a few PeV and they represent the only source class with a sufficient non-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 the cosmic ray spectrum at the source, giving us a first idea of the concrete, possible individual contributions to the total cosmic ray spectrum. In this contribution, we use these features as input parameters for propagating cosmic rays from its origin to Earth using GALPROP in order to investigate if these supernova remnants reproduce the cosmic ray spectrum and if supernova remnants in general can be responsible for the observed energy budget.
2015-11-12T09:44:53+01:00A solution to the cosmic ray anisotropy problem
https://doi.org/10.5194/ap-2-51-2015
<b>A solution to the cosmic ray anisotropy problem</b><br>
P. Mertsch and S. Funk<br>
ASTRA Proc., 2, 51–55, https://doi.org/10.5194/ap-2-51-2015, 2015<br>
Observations of the cosmic ray (CR) anisotropy are widely advertised as a means of finding nearby sources. This idea has recently gained currency after the discovery of a rise in the positron fraction and is the goal of current experimental efforts, e.g., with AMS-02 on the International Space Station. Yet, even the anisotropy observed for hadronic CRs is not understood, in the sense that isotropic diffusion models overpredict the dipole anisotropy in the TeV–PeV range by almost two orders of magnitude. Here, we consider two additional effects normally not considered in isotropic diffusion models: anisotropic diffusion due to the presence of a background magnetic field and intermittency effects of the turbulent magnetic fields. We numerically explore these effect by tracking test-particles through individual realisations of the turbulent field. We conclude that a large misalignment between the CR gradient and the background field can explain the observed low level of anisotropy.
2015-10-08T09:44:53+02:00The power spectrum of cosmic ray arrival directions
https://doi.org/10.5194/ap-2-45-2015
<b>The power spectrum of cosmic ray arrival directions</b><br>
M. Ahlers<br>
ASTRA Proc., 2, 45–49, https://doi.org/10.5194/ap-2-45-2015, 2015<br>
Various experiments show that the arrival directions of multi-TeV cosmic rays show significant anisotropies at small angular scales. It was recently argued that this small scale structure may arise naturally by cosmic ray diffusion in a large-scale cosmic ray gradient in combination with deflections in local turbulent magnetic fields. We show via analytical and numerical methods that the non-trivial power spectrum in this setup is a direct consequence of Liouville's theorem and can be related to properties of relative diffusion.
2015-10-07T09:44:53+02:00Cosmic ray transport and anisotropies to high energies
https://doi.org/10.5194/ap-2-39-2015
<b>Cosmic ray transport and anisotropies to high energies</b><br>
P. L. Biermann, L. I. Caramete, A. Meli, B. N. Nath, E.-S. Seo, V. de Souza, and J. Becker Tjus<br>
ASTRA Proc., 2, 39–44, https://doi.org/10.5194/ap-2-39-2015, 2015<br>
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.
2015-10-02T09:44:53+02:00The large-scale anisotropy with the PAMELA calorimeter
https://doi.org/10.5194/ap-2-35-2015
<b>The large-scale anisotropy with the PAMELA calorimeter</b><br>
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<br>
ASTRA Proc., 2, 35–37, https://doi.org/10.5194/ap-2-35-2015, 2015<br>
The large-scale anisotropy (or the so-called star-diurnal wave) has been studied using the calorimeter of the space-born experiment PAMELA. The cosmic ray anisotropy has been obtained for the Southern and Northern hemispheres simultaneously in the equatorial coordinate system for the time period 2006–2014. The dipole amplitude and phase have been measured for energies 1–20 TeV n-1.
2015-10-02T09:44:53+02:00The cosmic ray anisotropy below 1015 eV
https://doi.org/10.5194/ap-2-27-2015
<b>The cosmic ray anisotropy below 1015 eV</b><br>
G. Di Sciascio<br>
ASTRA Proc., 2, 27–33, https://doi.org/10.5194/ap-2-27-2015, 2015<br>
The measurement of the anisotropy in the cosmic ray (CR) arrival direction distribution provides important informations on the propagation mechanisms and on the identification of their sources. In the last decade the anisotropy came back to the attention of the scientific community, thanks to several new two-dimensional representations of the CR arrival direction distribution which clearly showed the existence of anisotropies at different angular scales in both hemispheres. The origin of the observed anisotropies is still unknown. So far, no theory of CRs in the Galaxy exists yet to explain the observations leaving the standard model of CRs and that of the local magnetic field unchanged at the same time. In this paper the observations of Galactic CR anisotropy will be briefly summarized, with particular attention to the results obtained by the ARGO-YBJ experiment in the Northern Hemisphere.
2015-09-29T09:44:53+02:00Diffuse synchrotron emission from galactic cosmic ray electrons
https://doi.org/10.5194/ap-2-21-2015
<b>Diffuse synchrotron emission from galactic cosmic ray electrons</b><br>
G. Di Bernardo, D. Grasso, C. Evoli, and D. Gaggero<br>
ASTRA Proc., 2, 21–26, https://doi.org/10.5194/ap-2-21-2015, 2015<br>
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.
2015-09-22T09:44:53+02:00Search for a positron anisotropy with PAMELA experiment
https://doi.org/10.5194/ap-2-17-2015
<b>Search for a positron anisotropy with PAMELA experiment</b><br>
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<br>
ASTRA Proc., 2, 17–20, https://doi.org/10.5194/ap-2-17-2015, 2015<br>
The PAMELA experiment has been collecting data since 2006; its results indicate a rise in the positron fraction with respect to the sum of electrons and positrons in the cosmic-ray (CR) spectrum above 10 GeV. This excess can be due to additional sources, as SNRs or pulsars, which can lead to an anisotropy in the local CR positron, detectable from current experiments. We report on the analysis on spatial distributions of positron events collected by PAMELA, taking into account also the geomagnetic field effects. No significant deviation from the isotropy has been observed.
2015-09-09T09:44:53+02:00A Consistent Scenario for the IBEX Ribbon, Anisotropies in TeV Cosmic Rays, and the Local Interstellar Medium
https://doi.org/10.5194/ap-2-9-2015
<b>A Consistent Scenario for the IBEX Ribbon, Anisotropies in TeV Cosmic Rays, and the Local Interstellar Medium</b><br>
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<br>
ASTRA Proc., 2, 9–16, https://doi.org/10.5194/ap-2-9-2015, 2015<br>
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.
2015-09-08T09:44:53+02:00Observation of the Cosmic-Ray Shadow of the Moon and Sun with IceCube
https://doi.org/10.5194/ap-2-5-2015
<b>Observation of the Cosmic-Ray Shadow of the Moon and Sun with IceCube</b><br>
F. Bos, F. Tenholt, J. Becker Tjus, and S. Westerhoff<br>
ASTRA Proc., 2, 5–8, https://doi.org/10.5194/ap-2-5-2015, 2015<br>
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 in different detector configurations. The cosmic-ray Moon shadow was observed with high statistical significance (> 6σ) in previous analyses when the IceCube detector operated in a smaller configuration before it was completed in December 2010. This work shows first analyses of the cosmic-ray Sun shadow in IceCube. A binned analysis in one-dimension is used to measure the Moon and Sun shadow with high statistical significance greater than 12σ.
2015-08-05T09:44:53+02:00The Local Bubble as a cosmic-ray isotropizer
https://doi.org/10.5194/ap-2-1-2015
<b>The Local Bubble as a cosmic-ray isotropizer</b><br>
I. Gebauer, M. Weinreuter, S. Kunz, and D. Gaggero<br>
ASTRA Proc., 2, 1–3, https://doi.org/10.5194/ap-2-1-2015, 2015<br>
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.
2015-07-31T09:44:53+02:00TeV Cosmic Ray Anisotropy and the Heliospheric Magnetic Field
https://doi.org/10.5194/ap-1-65-2014
<b>TeV Cosmic Ray Anisotropy and the Heliospheric Magnetic Field</b><br>
P. Desiati and A. Lazarian<br>
ASTRA Proc., 1, 65–71, https://doi.org/10.5194/ap-1-65-2014, 2014<br>
Cosmic rays are observed to possess a small non uniform distribution in arrival direction. Such anisotropy appears to have a roughly consistent topology between tens of GeV and hundreds of TeV, with a smooth energy dependency on phase and amplitude. Above a few hundreds of TeV a sudden change in the topology of the anisotropy is observed. The distribution of cosmic ray sources in the Milky Way is expected to inject anisotropy on the cosmic ray flux. The nearest and most recent sources, in particular, are expected to contribute more significantly than others. Moreover the interstellar medium is expected to have different characteristics throughout the Galaxy, with different turbulent properties and injection scales. Propagation effects in the interstellar magnetic field can shape the cosmic ray particle distribution as well. In particular, in the 1–10 TeV energy range, they have a gyroradius comparable to the size of the Heliosphere, assuming a typical interstellar magnetic field strength of 3 μG. Therefore they are expected to be strongly affected by the Heliosphere in a manner ordered by the direction of the local interstellar magnetic field and of the heliotail. In this paper we discuss on the possibility that TeV cosmic rays arrival distribution might be significantly redistributed as they propagate through the Heliosphere.
2014-10-31T09:44:53+01:00Effects of stellar evolution and ionizing radiation on the environments of massive stars
https://doi.org/10.5194/ap-1-61-2014
<b>Effects of stellar evolution and ionizing radiation on the environments of massive stars</b><br>
J. Mackey, N. Langer, S. Mohamed, V. V. Gvaramadze, H. R. Neilson, and D. M.-A. Meyer<br>
ASTRA Proc., 1, 61–63, https://doi.org/10.5194/ap-1-61-2014, 2014<br>
We discuss two important effects for the astrospheres of runaway stars: the propagation of ionizing photons far beyond the astropause, and the rapid evolution of massive stars (and their winds) near the end of their lives. Hot stars emit ionizing photons with associated photoheating that has a significant dynamical effect on their surroundings. 3-D simulations show that H ii regions around runaway O stars drive expanding conical shells and leave underdense wakes in the medium they pass through. For late O stars this feedback to the interstellar medium is more important than that from stellar winds. Late in life, O stars evolve to cool red supergiants more rapidly than their environment can react, producing transient circumstellar structures such as double bow shocks. This provides an explanation for the bow shock and linear bar-shaped structure observed around Betelgeuse.
2014-09-11T09:44:53+02:00MHD flows at astropauses and in astrotails
https://doi.org/10.5194/ap-1-51-2014
<b>MHD flows at astropauses and in astrotails</b><br>
D. H. Nickeler, T. Wiegelmann, M. Karlický, and M. Kraus<br>
ASTRA Proc., 1, 51–60, https://doi.org/10.5194/ap-1-51-2014, 2014<br>
The geometrical shapes and the physical properties of stellar wind – interstellar medium interaction regions form an important stage for studying stellar winds and their embedded magnetic fields as well as cosmic ray modulation. Our goal is to provide a proper representation and classification of counter-flow configurations and counter-flow interfaces in the frame of fluid theory. In addition we calculate flows and large-scale electromagnetic fields based on which the large-scale dynamics and its role as possible background for particle acceleration, e.g., in the form of anomalous cosmic rays, can be studied. We find that for the definition of the boundaries, which are determining the astropause shape, the number and location of magnetic null points and stagnation points is essential. Multiple separatrices can exist, forming a highly complex environment for the interstellar and stellar plasma. Furthermore, the formation of extended tail structures occur naturally, and their stretched field and streamlines provide surroundings and mechanisms for the acceleration of particles by field-aligned electric fields.
2014-09-05T09:44:53+02:00Lyman-α observations of astrospheres
https://doi.org/10.5194/ap-1-43-2014
<b>Lyman-α observations of astrospheres</b><br>
J. L. Linsky and B. E. Wood<br>
ASTRA Proc., 1, 43–49, https://doi.org/10.5194/ap-1-43-2014, 2014<br>
Charge-exchange reactions between outflowing stellar wind protons and interstellar neutral hydrogen atoms entering a stellar astrosphere produce a region of piled-up-decelerated neutral hydrogen called the hydrogen wall. Absorption by this gas, which is observed in stellar Lyman-α emission lines, provides the only viable technique at this time for measuring the mass-loss rates of F–M dwarf stars. We describe this technique, present an alternative way for understanding the relation of mass-loss rate with X-ray emission, and identify several critical issues.
2014-08-25T09:44:53+02:00Clumps in stellar winds
https://doi.org/10.5194/ap-1-39-2014
<b>Clumps in stellar winds</b><br>
J. S. Vink<br>
ASTRA Proc., 1, 39–41, https://doi.org/10.5194/ap-1-39-2014, 2014<br>
We discuss the origin and quantification of wind clumping and mass–loss rates (Ṁ), particularly in close proximity to the Eddington (Γ) limit, relevant for very massive stars (VMS). We present evidence that clumping may not be the result of the line-deshadowing instability (LDI), but that clumps are already present in the stellar photosphere.
2014-07-30T09:44:53+02:00Observations of the anisotropy of cosmic rays at TeV–PeV
https://doi.org/10.5194/ap-1-33-2014
<b>Observations of the anisotropy of cosmic rays at TeV–PeV</b><br>
S. BenZvi<br>
ASTRA Proc., 1, 33–37, https://doi.org/10.5194/ap-1-33-2014, 2014<br>
During the past decade, multiple observatories have reported significant observations of the anisotropy of cosmic rays in the TeV energy band. The anisotropy has been observed at large scales and small scales in both the Northern and Southern Hemispheres. The source of the anisotropy is not well-understood, though both a galactic and a heliospheric origin have been suggested. We discuss recent observations of the shape and energy dependence of the anisotropy, with particular attention to measurements by the IceCube Neutrino Observatory in the Southern Hemisphere and the Milagro and High-Altitude Water Cherenkov (HAWC) observatories in the Northern Hemisphere.
2014-07-25T09:44:53+02:00Cosmic ray particles from exploding massive stars with winds
https://doi.org/10.5194/ap-1-29-2014
<b>Cosmic ray particles from exploding massive stars with winds</b><br>
P. L. Biermann<br>
ASTRA Proc., 1, 29–31, https://doi.org/10.5194/ap-1-29-2014, 2014<br>
The origin of cosmic rays is still unsettled. Many sources have been proposed over the years, and exploding stars still provide the most promising candidates. Here we examine one of these scenarios, and compare the resulting predictions with data: Massive stars have winds, and when these stars explode, the resulting shock runs through the wind. The observable phenomenon is called radio-supernova, and many have been observed in non-thermal radio emission. This emission allows to determine the magnetic field in the wind as a function of radius, and so allows to check, whether such explosions can achieve the high energies required and also explain the flux and the spectra of cosmic rays. The observations show this to be the case, and so we conclude that radio supernovae can explain the high-energy Galactic cosmic rays over the entire energy range, and that the spectral predictions are compatible with observations.
2014-06-27T09:44:53+02:00Cosmic rays as regulators of molecular cloud properties
https://doi.org/10.5194/ap-1-23-2014
<b>Cosmic rays as regulators of molecular cloud properties</b><br>
M. Padovani, P. Hennebelle, and D. Galli<br>
ASTRA Proc., 1, 23–27, https://doi.org/10.5194/ap-1-23-2014, 2014<br>
Cosmic rays are the main agents in controlling the chemical evolution and setting the ambipolar diffusion time of a molecular cloud. We summarise the processes causing the energy degradation of cosmic rays due to their interaction with molecular hydrogen, focusing on the magnetic effects that influence their propagation. Making use of magnetic field configurations generated by numerical simulations, we show that the increase of the field line density in the collapse region results in a reduction of the cosmic-ray ionisation rate. As a consequence the ionisation fraction decreases, facilitating the decoupling between the gas and the magnetic field.
2014-06-27T09:44:53+02:00