coronal mass ejections
The Solar and Heliospheric Observatory (SOHO) [Domingo et al., 1995] is the first mission that includes helioseismology, optical, and particle instruments on the same platform. The spacecraft is located near the Langragian Point L1, between the Sun and the Earth. The 3-axis stabilization enables an uninterrupted observation of the interior of the Sun, its surface, and the corona, and in-situ analysis of the solar wind including its suprathermal particle population..SOHO forms part of the International Solar Terrestrial Physics (ISTP) program. This efficient system provides the tool to observe coronal mass ejections on their way from the Sun to the Earth's magnetosphere.
The solar activity is correlated with the averaged sunspot number that varies within an 11-year period. As the activity of the Sun reaches its maximum, often so-called flares are observed. Flares are defined as a sudden, rapid, and intense variation in brightness. They produce energetic particles (protons and electrons) and radiation from radio waves to X-rays and gamma-rays. Solar flares are classified according to their X-ray brightness in the wavelength range 1 to 8 Å: X-class (Intensity ≥ 10-4 W/m2), M-class (10-5 ≤ Intensity < 10-4 W/m2, and C-class (10-6 ≤ Intensity < 10-5 W/m2).
Intense flares are often accompanied by coronal mass ejections. A closed magnetic structure containing up to 10 billion tons of charged particles (mainly protons and electrons) is ejected from the Sun into interplanetary space.In the vicinity of the Sun CMEs reach speeds as high as 2000 km/s. As they move much faster than the solar wind, shock waves are generated. They expand and propagate through the interplanetary space accelerating different species of particles, especially solar wind ions, interstellar pick-up ions, remnant flare particles. The frequency of CMEs also varies with the sunspot cycle. At solar minimum one CME a week is observed, and near solar maximum an average of 2 to 3 CMEs per day. Only a small fraction of the CMEs is directly heading for Earth. As the expanding cloud of an Earth-directed CME looms larger and larger it appears to envelop the Sun, forming a halo. Therefore they are called halo CMEs.
Large halo events, e.g. during July 14-16, 2000 (Bastille Day Event), can cause intense geomagnetic storms. During these storms aurorae are visible in middle latitudes, such as Central Europe. Technologies such as spacecraft operations, power systems, radio communications, navigation signals can be affected [Bamert and Hofer, 2003].
Figure 1: The Bastille Day Event: The X-flare observed by SOHO/EIT, and the evolution of the halo CME observed by SOHO/LASCO. Shortly after the eruptions numerous sensors i.a. on SOHO, ACE and Wind, were saturated.
In this work coronal mass ejection events are analyzed using observations of remote-sensing optical and in-situ particle instruments on SOHO, and in addition data of the magnetometer MAG [Smith et al.,1998] on the Advanced Composition Explorer (ACE) [Stone et al., 1998], which is located near SOHO.
The Extreme-Ultraviolet Imaging Telescope (EIT) is able to image the solar transition region and inner corona in four, selected bandpasses in the extreme ultraviolet (EUV): Fe IX/X (171 Å), Fe XII (195 Å), Fe XV (284 Å), and He II (304 Å). EIT can image active regions, e.g. filaments and prominences, and flares. For details of the experimental methods we refer to Delaboudiniere et al. [1995].
The Large Angle Spectroscopic Coronagraph (LASCO) [Brückner et al., 1995] observes the corona from 1.1 to 32 solar radii. One of the scientific objectives of LASCO is to understand why coronal mass ejections occur and what initiates their release.
The SOHO/LASCO CME-catalog contains a list of all CMEs identified by LASCO and also a number of measurements that characterize the CMEs: time of first appearance in the C2 coronagraph field of view, central position angle, angular width, speed and acceleration from height-time measurements.
The information about observations during the analyzed events (if they were available) are listed in Table 1. The Roman numbers (I-IV) correspond to the four events: (I) The Bastille Day Event, July 14-16, 2000,(II) April 2-5, 2001, (III) April 9-12, 2001, and (IV) September 24-26, 2001.
The times of the shock arrival are taken from the CELIAS/PM website An incomplete list of possible Interplanetary Shocks observed by the PM. Shocks cannot be identified using only solar windproton data. One needs electron and minor ion measurements in addition to magnetic field measurements (there is no magnetometer on SOHO). Nevertheless, a reasonable system of identification of at least the larger shocks is possible usingonly the PM data, which basically give the proton solar wind speed, and the kinetic temperature of the protons. Developing automated procedures for identifying various interplanetary structures, as the Shockspotter program, would be a useful addition to a space weather warning system.
Additional shock parameters (V_sh, Theta_Bn, and M_A) given in Table 1 are also taken from the website List of transients and disturbances. Vsh is the velocity of the shock in the spacecraft frame, ΘBn is the angle between then upstream magnetic field vector and the shock normal, and MA is the upstream Alfvén Mach Number MA = vup/VA with Alfvén speed VA = B/√(μ0 mp Np).
The particle instruments on ACE and SOHO cover the entire energy range from solar wind to solar energetic particles. The (Highly) Suprathermal Time-Of-Flight ((H)STOF) sensor on SOHO (see Section 5 for description of the sensor and data analysis) plays an important role in this respect, because it bridges the gap between the bulk solar wind ions and the low-energy solar energetic particles. The (H)STOF sensor offers one of the first opportunities to verify the detailed spatial and temporal evolution of ions in the energy range between 35 keV/amu and a few MeV/amu. In this energy range, the injection into first-order Fermi acceleration is believed to occur.
3 Acceleration of suprathermal ions at a strong interplanetary shock