M. Teittinen

Injection of 10 MeV Protons in Association with a Coronal Moreton Wave

We report extreme-UV observations of the coronal Moreton wave and concurrent observations of ~10-100 MeV protons. Observations are carried out with the Extreme-UV Imaging Telescope and the Energetic and Relativistic Nuclei and Electron instrument on board the SOHO spacecraft. We study the proton events associated with coronal mass ejections (CMEs) centered near the central meridian. Observations reveal the initial injection of 10 MeV protons during the period when the coronal Moreton wave was traversing the western hemisphere of the Sun, this being an early signature of the CME launch. Acceleration of the CME-associated protons starts during the CME liftoff, while the main proton production occurs several hours later, when the CME expands in the interplanetary medium. Between the first proton production and the maximum intensity time, a spectral softening is observed. We analyze in detail the 1997 September 24 event. Development of the event indicates that the spectral softening may be due to a change in the acceleration regime, so the proton production starts with the less intensive but hard-spectrum injection and then moves to the more intensive but soft-spectrum injection farther from the Sun.

Energetic (∼ 1 to 50 MeV) protons associated with Earth‐directed coronal mass ejections

During the period from January through mid-May, 1997, four large Earth-directed CMEs were observed by the Large Angle Spectroscopic Coronograph (LASCO). These CMEs were associated with long-lasting fluxes of >1.6 MeV protons detected by the Energetic and Relativistic Nuclei and Electron instrument (ERNE). However, the magnitudes of energetic proton events differed dramatically on different occasions. In strong proton events, production of 10-50 MeV protons started during expansion of the coronal Moreton wave in the western hemisphere of the Sun. The new SOHO observations suggest that potentialities of CMEs to produce energetic particles in the interplanetary medium crucially depend on the previous evolution of the explosion below ∼2R⊙. Forecasting of the near-Earth >10 MeV proton intensity requires multiwavelength observations of the early phase of an event particularly the Extreme-ultraviolet Imaging Telescope (EIT) observations.

Energetic and relativistic nuclei and electron experiment of the SOHO mission

Abstract The design of the ERNE experiment of the ESA and NASA collaborative SOHO mission is described. ERNE will investigate the sun by measuring energetic particles. Starting from the design objectives, as determined by the scientific goals of the experiment, and from the adopted basic solutions, the design and structure of the instrument are presented in detail. The fundamental technical aspects encountered in building a space instrument are briefly considered. The methods of implementation of scientifically the most important parts of the instrument, the sensors for measuring energetic particles and the associated electronics, are thoroughly explained. Both hardware and software are examined. The pre-flight calibrations of the instrument are described and the performance of the instrument in space is demonstrated.

SOHO/Energetic and Relativistic Nucleon and Electron Experiment Measurements of Energetic H, He, O, and Fe Fluxes during the 1997 November 6 Solar Event

A brilliant solar X-ray and Hα flare and a coronal mass ejection (CME) on 1997 November 6 were associated with high particle fluxes in interplanetary space at energies above MeV. The CME had an exceptionally high leading edge velocity (1560 km s-1) as observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO). The Energetic and Relativistic Nucleon and Electron experiment (ERNE), also on SOHO, measured high H, He, O, and Fe fluxes in several energy channels from 3 to 200 MeV nucleon-1. The oxygen energy spectrum in energy range 3-200 MeV nucleon-1 was of a broken-power-law form with a break around 50 MeV nucleon-1. In addition, ERNE observed abrupt changes in the intensity, elemental composition, and anisotropy of high-energy particles, which may indicate two energetic particle sources during several hours after the solar flare eruption. The observational results lead us to conclude that, most of the time, the O and Fe nuclei were injected by the interplanetary shock associated with the coronal mass ejection emitted around 12 UT on November 6. However, during two time periods the injection source might have been different or complementary. The first period was in the very beginning of the event, 13:20-13:40 UT, when the particle streaming showed very strong anisotropy with the maximum intensity from the direction of the Sun. The second was between 17 UT November 6 and 5 UT November 7, when particle fluxes were dominated by a particle population with a different elemental composition and a different spectral shape of O as compared with the particle population prevailing in the beginning and during the long decay phase of the event. We propose that the source of these particles was associated with a coronal shock wave traveling in the low solar atmosphere.

Observations of galactic cosmic ray modulation during Earth‐directed coronal mass ejections

We describe three cosmic-ray decreases observed by the energetic particle telescope ERNE on board SOHO spacecraft during the period January–May, 1997. The decreases in cosmic-ray proton intensities in the energy range above 50 MeV were observed after the passage of interplanetary shocks on February 9, April 10, and May 15. All the shocks were produced by Earth-directed coronal mass ejections. The amplitudes of the decreases compared to the counting rates just before the shock arrival were 8.5%, 14%, and 7%, respectively. The observed recovery times were 1, 4.2, and 5 days, respectively. After the February and May events the pre-shock levels were reached, but in the April event the recovery was only to 3% below this level. In each case, there were also clear indications of the passage of the ejecta. The energy dependences of the cosmic-ray helium flux decreases at 12–100 MeV/n were similar to each other in the April and May events, while in the February event the decrease was less pronounced.

Erne observations of energetic particle fluxes

Abstract The observations of the ERNE instrument on-board the SOHO 1 spacecraft are analysed. The main period under study is from the beginning of May till mid-October 1996. Only very small energetic particle events were observed. A comparison of the ERNE measurements with energetic particle observations during the two previous solar minima indicates that the flux levels of protons during the first half of 1996 are exceptionally low. The daily counting rates of quiet-time protons and 4He in the energy range 13–100 MeV/nucleon show statistically significant variations with time. Following two energetic particle events in July and August, anticorrelations were found between enhancements of low energy protons in the range 1.5–8 MeV, possibly accelerated in a corotating interaction region, and the modulation of high energy protons and helium above 13 MeV/nucleon. The period of the observed anticorrelations coincide with the solar rotation period.

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.

Specialized detector techniques and electronics of the ERNE instrument onboard SOHO

ERNE is designed to study the composition and energy spectra of particles encountered in interplanetary space in the energy range from 1 MeV/n to well beyond 500 MeV/n. Several innovative ideas had to be incorporated in the design of the instrument in order to fulfill the scientific requirements. Position-sensitive strip detectors are used in the High Energy Detector (HED) for determining particle trajectories. Integrated interstrip capacitances were designed to allow for a very simple read-out technique from the two edges of each detector. The event recognition, signal multiplexing and control of the pulse height analysis are based on a gate array technique. The versatility of the gate array also allowed an elegant realization of a number of other functions and instrument control tasks. By using the gate array a very compact structure of the digital control electronics was achieved.