Modulation of low-energy galactic cosmic rays over solar maximum (Cycle 20)

Abstract. Ulysses, launched in October 1990, began its second out-of-ecliptic orbit in December 1997, and its second fast latitude scan in September 2000. In contrast to the first fast latitude scan in 1994/1995, during the second fast latitude scan solar activity was close to maximum. The solar magnetic field reversed its polarity around July 2000. While the first latitude scan mainly gave a snapshot of the spatial distribution of galactic cosmic rays, the second one is dominated by temporal variations. Solar particle increases are observed at all heliographic latitudes, including events that produce >250 MeV protons and 50 MeV electrons. Using observations from the University of Chicago’s instrument on board IMP8 at Earth, we find that most solar particle events are observed at both high and low latitudes, indicating either acceleration of these particles over a broad latitude range or an efficient latitudinal transport. The latter is supported by "quiet time" variations in the MeV electron background, if interpreted as Jovian electrons. No latitudinal gradient was found for >106 MeV galactic cosmic ray protons, during the solar maximum fast latitude scan. The electron to proton ratio remains constant and has practically the same value as in the previous solar maximum. Both results indicate that drift is of minor importance. It was expected that, with the reversal of the solar magnetic field and in the declining phase of the solar cycle, this ratio should increase. This was, however, not observed, probably because the transition to the new magnetic cycle was not completely terminated within the heliosphere, as indicated by the Ulysses magnetic field and solar wind measurements. We argue that the new A<0-solar magnetic modulation epoch will establish itself once both polar coronal holes have developed.Key words. Interplanetary physics (cosmic rays; energetic particles; interplanetary magnetic fields)

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Latitudinal and radial variation of >2 GeV/n protons and alpha-particles at solar maximum: ULYSSES COSPIN/KET and neutron monitor network observations

Annales Geophysicae ◽

10.5194/angeo-21-1295-2003 ◽

2003 ◽

Vol 21 (6) ◽

pp. 1295-1302 ◽

Cited By ~ 9

Author(s):

A. V. Belov ◽

E. A. Eroshenko ◽

B. Heber ◽

V. G. Yanke ◽

A. Raviart ◽

...

Keyword(s):

Magnetic Field ◽

Cosmic Rays ◽

Neutron Monitor ◽

Solar Minimum ◽

Latitudinal Gradient ◽

Solar Maximum ◽

Cosmic Ray ◽

Galactic Cosmic Rays ◽

Alpha Particles ◽

Polar Regions

Abstract. Ulysses, launched in October 1990, began its second out-of-ecliptic orbit in September 1997. In 2000/2001 the spacecraft passed from the south to the north polar regions of the Sun in the inner heliosphere. In contrast to the first rapid pole to pole passage in 1994/1995 close to solar minimum, Ulysses experiences now solar maximum conditions. The Kiel Electron Telescope (KET) measures also protons and alpha-particles in the energy range from 5 MeV/n to >2 GeV/n. To derive radial and latitudinal gradients for >2 GeV/n protons and alpha-particles, data from the Chicago instrument on board IMP-8 and the neutron monitor network have been used to determine the corresponding time profiles at Earth. We obtain a spatial distribution at solar maximum which differs greatly from the solar minimum distribution. A steady-state approximation, which was characterized by a small radial and significant latitudinal gradient at solar minimum, was interchanged with a highly variable one with a large radial and a small – consistent with zero – latitudinal gradient. A significant deviation from a spherically symmetric cosmic ray distribution following the reversal of the solar magnetic field in 2000/2001 has not been observed yet. A small deviation has only been observed at northern polar regions, showing an excess of particles instead of the expected depression. This indicates that the reconfiguration of the heliospheric magnetic field, caused by the reappearance of the northern polar coronal hole, starts dominating the modulation of galactic cosmic rays already at solar maximum.Key words. Interplanetary physics (cosmic rays; energetic particles) – Space plasma physics (charged particle motion and acceleration)

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Heliospheric Magnetic Field Configuration at Solar Maximum Conditions: Consequences for Galactic Cosmic Rays

10.1063/1.1809501 ◽

2004 ◽

Cited By ~ 1

Author(s):

T. H. Zurbuchen

Keyword(s):

Magnetic Field ◽

Cosmic Rays ◽

Solar Maximum ◽

Galactic Cosmic Rays ◽

Field Configuration ◽

Magnetic Field Configuration ◽

Heliospheric Magnetic Field

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Low-Energy Break in the Spectrum of Galactic Cosmic Rays

Physical Review Letters ◽

10.1103/physrevlett.108.051105 ◽

2012 ◽

Vol 108 (5) ◽

Cited By ~ 44

Author(s):

A. Neronov ◽

D. V. Semikoz ◽

A. M. Taylor

Keyword(s):

Cosmic Rays ◽

Galactic Cosmic Rays ◽

Low Energy

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Radial and latitudinal gradients of galactic cosmic rays in the heliosphere at solar maximum

Advances in Space Research ◽

10.1016/j.asr.2010.06.033 ◽

2010 ◽

Vol 46 (10) ◽

pp. 1313-1317 ◽

Cited By ~ 6

Author(s):

O.G. Morales-Olivares ◽

R.A. Caballero-Lopez

Keyword(s):

Cosmic Rays ◽

Solar Maximum ◽

Galactic Cosmic Rays ◽

Latitudinal Gradients

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Production of atomic hydrogen by cosmic rays in dark clouds

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834008 ◽

2018 ◽

Vol 619 ◽

pp. A144 ◽

Cited By ~ 12

Author(s):

Marco Padovani ◽

Daniele Galli ◽

Alexei V. Ivlev ◽

Paola Caselli ◽

Andrea Ferrara

Keyword(s):

Cosmic Rays ◽

Atomic Hydrogen ◽

Molecular Hydrogen ◽

Molecular Clouds ◽

Cosmic Ray ◽

Galactic Cosmic Rays ◽

Dissociation Rate ◽

Low Energy ◽

Hydrogen Atoms ◽

Dark Clouds

Context. Small amounts of atomic hydrogen, detected as absorption dips in the 21 cm line spectrum, are a well-known characteristic of dark clouds. The abundance of hydrogen atoms measured in the densest regions of molecular clouds can only be explained by the dissociation of H2 by cosmic rays. Aims. We wish to assess the role of Galactic cosmic rays in the formation of atomic hydrogen, for which we use recent developments in the characterisation of the low-energy spectra of cosmic rays and advances in the modelling of their propagation in molecular clouds. Methods. We modelled the attenuation of the interstellar cosmic rays that enter a cloud and computed the dissociation rate of molecular hydrogen that is due to collisions with cosmic-ray protons and electrons as well as fast hydrogen atoms. We compared our results with the available observations. Results. The cosmic-ray dissociation rate is entirely determined by secondary electrons produced in primary ionisation collisions. These secondary particles constitute the only source of atomic hydrogen at column densities above ~1021 cm−2. We also find that the dissociation rate decreases with column density, while the ratio between the dissociation and ionisation rates varies between about 0.6 and 0.7. From comparison with observations, we conclude that a relatively flat spectrum of interstellar cosmic-ray protons, such as suggested by the most recent Voyager 1 data, can only provide a lower bound for the observed atomic hydrogen fraction. An enhanced spectrum of low-energy protons is needed to explain most of the observations. Conclusions. Our findings show that a careful description of molecular hydrogen dissociation by cosmic rays can explain the abundance of atomic hydrogen in dark clouds. An accurate characterisation of this process at high densities is crucial for understanding the chemical evolution of star-forming regions.

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