Drivers of space weather

The term "space weather" refers to a vast array of phenomena that can disturb the interplanetary medium and/or affect the Earth and near-Earth environment. This includes recurrent structures in the solar wind (fast solar wind streams, co-rotating interaction regions), the ionizing radiation and hard particle radiations from flares, radio noise from the Sun, coronal mass ejections, and shock-accelerated particles. These drivers result in geomagnetic storms, changes in the ionosphere, and atmospheric heating which can, in turn, result in a large variety of effects that are of practical concern to our technological society: ground-level currents in pipelines and electrical power grids, disruption of civilian and military communication, spacecraft charging, enhanced atmospheric drag on spacecraft, etc. A historical perspective of solar and solar radio effects on technologies is presented by Lanzerotti in Chapter 1. The drivers of space weather—fast and slow solar wind streams, flares, and coronal mass ejections—are solar in origin. An understanding of space weather phenomena lies, in part, in gaining a fundamental understanding the origin of these drivers.

Interest in coronal mass ejections (CMEs) has been particularly strong because they are associated with the largest geo-effective events and the largest solar energetic particle (SEP) events. With the detection of synchrotron radiation from CMEs (Bastian et al. 2001) a new tool has become available to detect, image, and diagnose the properties of CMEs. Fits of a simple synchrotron model to two- and three-point spectra at various locations in the source yield not only the magnetic field in the CME, but the ambient density of the thermal plasma as well. Radio CMEs may be significantly linearly polarized by the time they propagate to several solar radii from the Sun. Detection of linearly polarized radiation from radio CMEs would provide additional leverage on the magnetic field in CMEs. CMEs can be detected by other means (Bastian & Gary 1997). Using the Clark Lake Radio Observatory, Gopalswamy & Kundu (1993) report observations of thermal radiation signatures of a CME near the plasma level at 38.5, 50, and 73.8 MHz. More recently, thermal emission from CMEs (Kathiravan et al. 2002), and coronal dimmings resulting from the launch of a CME (Ramesh and Sastry 2000) have been reported in observations made by the Gauribidanur Radioheliograph between 50-65 MHz. Vourlidas provides a more comprehensive overview of radio signatures of CMEs in Chapter 11.

Coronal waves, possible analogs to chromospheric Moreton waves, were discovered by the SOHO/EIT instrument (Thompson et al. 1999; 2000; Biesecker et al. 2002) although examples have since been discovered in SXR (Khan & Aurass 2002). They represent the dynamical response of the corona to a flare and/or an associated CME. An associated phenomenon is a coronal dimming, observed in SXR (e.g., Sterling & Hudson 1997) and EUV (Harra & Sterling 2001), believed to result from the removal of coronal material due to the lift off of a CME. A radio counterpart to an "EIT wave" was recently detected by the NoRH at 17 GHz (White & Thompson 2004). Observations of radio counterparts to EIT waves and of coronal dimmings mentioned above, suggests that FASR will provide a rather complete view of chromospheric and coronal waves, dimmings, and the interaction of waves with surrounding structures such as active regions (e.g., Ofman and Thompson 2002) and filaments.

It is generally accepted that type II radio bursts are a tracer of fast MHD shocks. The shocks that produce coronal type II radio bursts may be driven by fast ejecta (Gopalswamy et al. 1997), by a blast wave (Uchida 1974, Cane & Reames 1988), or by a CME (Cliver et al. 1999; Classen & Aurass 2002). Fast ejecta and/or a blast wave are produced by a flare; a CME produces a piston-driven shock wave. The relationship between these shocks, their radio-spectroscopic signature, and other phenomena such as Moreton waves and "EIT waves" remain matters of considerable interest and controversy. Gopalswamy reviews interplanetary type II radio bursts and their relation to interplanetary shocks and CMEs in Chapter 15. With its unique ability to perform imaging spectroscopy, FASR will in some cases be able to simultaneously image the basic shock driver (flare or CME), the response of the atmosphere to the driver (chromospheric and coronal waves and coronal dimmings), and shocks which may form due to the flare and/or the CME. The emphasis placed on FASR's ability to provide an integrated picture of flare phenomena in §3.2 applies equally to CMEs and associated phenomena (type II radio bursts, EIT and Moreton waves, filament eruptions). As an instrument that images coronal energy release and particle acceleration in the middle corona, tracers of coronal shocks, and the onset and ejection of certain coronal mass ejections, simultaneously, FASR will also provide key observations that will help resolve the important and controversial problem of the origin of solar energetic particles in the interplanetary medium.

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