1 Introduction

The first steps of Sun-Earth connection science were made by Edmund Halley, who following the spectacular auroral displays in Europe in March 1716, suggested that particles moving along the Earth’s magnetic field lines were the cause of the aurora. Following that, Anders Celcius and Olav Hiorter in 1747 discovered the temporal coincidence between compass needle variations and bright auroral displays. Comparing simultaneous magnetic variations in London and in Uppsala they further realized that the phenomena they were studying were related to processes in the planetary scale. The geomagnetic activity connection to solar processes was established by mid-nineteenth century: Solar flare correlation with active, bright auroras and geomagnetic disturbances was found by Carrington in 1860, and long-term observations showed 11-year variability both in sunspot numbers and occurrence frequency of magnetic disturbances and auroras.

While the modern space era with detailed in-situ and remote sensing measurements in and from space have resolved many issues concerning the behavior of the Sun, the solar wind, and the terrestrial space environment, many of the basic physics questions concerning the Sun-Earth connection remain open. On the other hand, the utilization of space has added a new practical flavor to the academic research, because the rapid time variations in the space plasma systems pose a hazard to technological systems and humans in space as well as on Earth. The term “space weather” now refers to conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere (upper parts of the atmosphere) that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health. The adverse conditions in the space environment can cause disruption of satellite operations, communications, navigation, and electric power distribution grids on ground, leading to a variety of societal and economic losses.

The time scales interesting to studies of space weather processes are determined both by the intrinsic time scales of the processes themselves, but also on the lead time that predictions can be given. The solar and magnetospheric processes pose several timescales ranging from solar cycle and longer (long-term solar activity variations) to 27 days (recurrent solar activity), days (magnetic storms), hours (magnetospheric substorms) and even minutes and seconds (particle acceleration events, plasma instability growth times). On the other hand, before an event can be predicted, some indication of its occurrence must have been observed. Energetic particles reach the Earth within only a few, maybe up to 20 min after their release from the solar surface or interplanetary shock front giving only a very short lead time after a warning can be given. The solar wind travel time from the Sun to the Earth is of the order of 80 hours, while solar wind monitors at the first Lagrangian point (L1), 1.4 million km from the Earth, provide measurements of the incoming solar wind that reaches the outer edges of the magnetosphere within about 40 minutes to 1 hour of their detection. Thus, as our capability of predicting solar wind properties from solar observations alone is poor, we are at the moment limited to at best warnings 80 hours in advance and predictions at maximum 1 hour before the event starts.

Today’s challenge for space weather research is to (i) learn to quantitatively predict the state of the magnetosphere and ionosphere from measured solar wind and interplanetary magnetic field conditions, (ii) to extend the physical understanding also to solar processes such that predictions can be made using solar observations to gain more lead time. In addition to that, we need engineering and life sciences to evaluate the hazards and risks on a variety of technological systems and humans in space, onboard high-altitude aircraft, and on ground.

This article reviews the basic properties of the magnetosphere and open questions regarding its dynamics, the most typical solar activity events that cause space weather effects, and the effects caused by solar activity that are seen on space-borne and ground-based technological systems as well as on humans. More details of the solar processes associated with space weather phenomena can be found in a review by Schwenn (2006Jump To The Next Citation Point).


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