Solar flares and space weather

3.11 Solar flares and space weather

The various types of radiation produced during the flare process are of interest in the space weather context. The EUV radiation in particular the Lyman-alpha radiation at 121.6 nm wavelength is absorbed in the Earths atmosphere and causes its instantaneous heating and expansion. Earth-orbiting satellites cruising in low orbits may sense a sudden drag that lowers their orbits (see Figure 5 in Lean, 1991 ). Any direct damage from the X-ray and Gamma radiation dose to technical systems and humans in space has not been reported, the intensities are probably too low.

The judgment of the role of flares on space weather in terms of geomagnetic effects has changed dramatically in recent years. Since Carrington’s discovery of the apparent connection between strong flares and geomagnetic activity in 1959, this connection has been considered a cause and effect relation for many years, despite some obvious shortcomings. Only in the 1980s, it became clear that the only type of solar transient that has a unique cause and effect relation to geomagnetic activity lies in CMEs, not in flares. Schwenn (1983 ) and Sheeley Jr et al. (1983 , 1985 ) showed that every CME launched with a speed exceeding 400 km/s eventually drives a shock wave, which then can be observed in situ, provided that the observer is located within the angular span of that CME. If this shock and the often times following ejecta cloud hits the Earth, geomagnetic effects may occur (provided some conditions on the orientation of the interplanetary magnetic field are also fulfilled). In reverse, every shock wave observed in space (except the ones at corotating interaction regions) can uniquely be associated with an appropriately pointed CME at the Sun. This proves that there is a causal chain linking CMEs to geomagnetic effects. No similar statement can be made for flares. Indeed, there are many CMEs (with geoeffects) without associated flares, and there are flares without associated CMEs (and without geoeffects). The longstanding flare myth was finally abolished (see Gosling, 1993 ; Reames, 1999 ). However, for the very big and most dangerous events like the one Carrington happened to witness, strong X-ray flares and large CMEs usually occur in a close timely context (Švestka, 2001 ). It is now commonly thought that both: flares and CMEs, are just the symptoms of a common underlying “magnetic disease” of the Sun (Harrison, 2003 ).

A very different aspect in space weather issues concerns the radio bursts (not to mention here their direct effects on cell phones and the GPS system, to be addressed in the Living Reviews in Solar Physics article “Space Weather: The Terrestrial Perspective”). The type III radio bursts can be tracked through large parts of the heliosphere and help to locate their probable source regions near the Sun. The curvature of the Parker spiral outlined by the type III electrons provides information on the solar wind environment through which theses electrons are passing. Even more important are the metric and kilometric type II radio bursts in that they show the motion of interplanetary shocks from the Sun up to Earth and further out. Thus, this information can be used for practical space weather analysis and forecasting.