E. Riihonen

Scanning an interplanetary magnetic cloud using high-energy protons

We applied a numerical modeling of the solar energetic particle (SEP) event inside a magnetic cloud in the solar wind to analyze the 17-22 MeV proton flux anisotropy observed on 1998 May 2 with the Energetic and Relativistic Nuclei and Electron (ERNE) instrument on the Solar and Heliospheric Observatory (SOHO), when SOHO was inside a magnetic cloud associated with a previous coronal mass ejection (CME). The analysis revealed a strong intermittency of the SEP transport parameters when different magnetic tubes were convected past the spacecraft. The estimated mean free path value varies over 1 order of magnitude, from ~2 to ~20 AU. The SEP event has been modeled with a prolonged injection of particles from a new CME into the previous ejecta comprising a set of magnetic loops. Both the prompt, direct proton flux and the delayed, counterstreaming flux were observed in the beginning and maximum phase of the event, but then the counterstreaming flux waned. The lack of counterstreaming protons can be explained either by the very fast escape of high-energy protons from the magnetic cloud, before they could complete one bounce in a narrow loop, or by the proton injection predominantly into one leg of a wide loop. An imprint of the magnetic compression at the leading part of the CME can be also found in the proton flux anisotropy data of SOHO/ERNE. These findings illustrate how high-precision anisotropy measurements and a numerical modeling can provide a kind of probe for the CME structure.

Catalogue of 55–80 MeV solar proton events extending through solar cycles 23 and 24

We present a new catalogue of solar energetic particle events near the Earth, covering solar cycle 23 and the majority of solar cycle 24 (1996–2016), based on the 55–80xa0MeV proton intensity data gathered by the Solar and Heliospheric Observatory/the Energetic and Relativistic Nuclei and Electron experiment (SOHO/ERNE). In addition to ERNE proton and heavy ion observations, data from the Advanced Composition Explorer/Electron, Proton and Alpha Monitor (ACE/EPAM) (near-relativistic electrons), SOHO/EPHIN (Electron Proton Helium Instrument) (relativistic electrons), SOHO/LASCO (Large Angle and Spectrometric Coronagraph) (coronal mass ejections, CMEs) and Geostationary Operational Environmental Satellite (GOES) soft X-ray experiments are also considered and the associations between the particle and CME/X-ray events deduced to obtain a better understanding of each event. A total of 176 solar energetic particle (SEP) events have been identified as having occurred during the time period of interest; their onset and solar release times have been estimated using both velocity dispersion analysis (VDA) and time-shifting analysis (TSA) for protons, as well as TSA for near-relativistic electrons. Additionally, a brief statistical analysis was performed on the VDA and TSA results, as well as the X-rays and CMEs associated with the proton/electron events, both to test the viability of the VDA and to investigate possible differences between the two solar cycles. We find, in confirmation of a number of previous studies, that VDA results for protons that yield an apparent path length of 1xa0AUxa0 xa0≾xa03xa0AU seem to be useful, but those outside this range are probably unreliable, as evidenced by the anticorrelation between apparent path length and release time estimated from the X-ray activity. It also appears that even the first-arriving energetic protons apparently undergo significant pitch angle scattering in the interplanetary medium, with the resulting apparent path length being on average about twice the length of the spiral magnetic field. The analysis indicates an increase in high-energy SEP events originating from the far-eastern solar hemisphere; for instance, such an event with a well-established associated GOES flare has so far occurred three times during cycle 24 but possibly not at all during cycle 23. The generally lower level of solar activity during cycle 24, as opposed to cycle 23, has probably caused a significant decrease in total ambient pressure in the interplanetary space, leading to a larger proportion of SEP-associated halo-type CMEs. Taken together, these observations point to a qualitative difference between the two solar cycles.

A silicon microstrip tracker in space: Experience with the AMS silicon tracker on STS-91

SummaryThe Alpha Magnetic Spectrometer (AMS) is designed as an independent module for installation on the International Space Station (ISS) in the year 2003 for an operational period of three years. The principal scientific objectives include the searches for antimatter and dark matter in cosmic rays. The AMS tracker uses silicon microstrip sensors to reconstruct charged-particle trajectories. A first version of the AMS, equipped with 2.1 m2 of silicon sensors and a permanent magnet, was flown on the NASA space shuttle Discovery duringJune 2–12, 1998. In this contribution, we describe the detector and present results of the tracker performance duringthe flight.

Great Storm Particle Event on 2000 August 11 Observed by SOHO ERNE

We report on an energetic storm particle event on 2000 August 11 observed by ERNE on board SOHO. A large-intensity enhancement started simultaneously in all energy channels between 2 and 50 MeV at 11 UT. The concurrence of the onsets indicates the local nature of the particle event. Intensities increased 100-fold from the background level before 11 UT to the maximum of the event around 15:30 UT. Three hours after the maximum, at 18:19 UT, the SOHO CELIAS Proton Monitor observed the passage of an interplanetary shock. Our analysis of the proton energy spectrum during the event indicates that the interplanetary shock was capable of accelerating particles up to energies of an order of 50 MeV. Assuming that the energy spectrum prevailing in the event describes the seed particle population for shock reacceleration, we find that the energy spectrum during major phases of the energetic storm particle event matches well with the assumption of a constant energy conversion power by the shock over the whole energy range from 2 to 50 MeV. We analyze the evolution of anisotropy of the proton flux in the 16-20 MeV channel of ERNE. The distribution of the directional intensity was observed to be relatively flat in the beginning of the event. Then the upstream distribution developed into a loss-cone-like distribution, and the intensity along the magnetic field relative to the intensity in the perpendicular direction was systematically reduced. At the shock passage, the distribution resembled a pancake.

Anomalous Cosmic-Ray Helium, Nitrogen and Oxygen in 1996 – Measurements of the ERNE Instrument on-board SOHO

The energy spectra of the anomalous components of helium, nitrogen and oxygen have been measured by the ERNE experiment on board the SOHO spacecraft. During February 28–April 30, 1996, the maximum intensity of anomalous helium was found to be 3.8 × 10-5 cm-2 sr-1 s-1 (MeV nucl-1)-1 in the energy range 10–15 MeV nucl-1. During the period January 26–April 30, 1996, the maximum oxygen intensity was 1.2 × 10-5 cm-2 sr-1 s-1 (MeV nucl-1)-1 at 4–7 MeV nucl-1, and the maximum nitrogen intensity 1.7 × 10-6 cm-2 sr-1 s-1 (MeV nucl-1)-1 at 4–9 MeV nucl-1. These peak intensities are at the same level as two solar cycles ago in 1977, but significantly higher than in 1986. This gives observational evidence for a 22-year solar modulation cycle. A noteworthy point is that the spectra of anomalous nitrogen and oxygen appear to be somewhat broader than in 1977.