The association of coronal mass ejections with their effects near the Earth

Abstract

Abstract. To this day, the prediction of space weather effects near the Earth suffers from a fundamental problem: The radial propagation speed of "halo" CMEs (i.e. CMEs pointed along the Sun-Earth-line that are known to be the main drivers of space weather disturbances) towards the Earth cannot be measured directly because of the unfavorable geometry. From inspecting many limb CMEs observed by the LASCO coronagraphs on SOHO we found that there is usually a good correlation between the radial speed and the lateral expansion speed V exp of CME clouds. This latter quantity can also be determined for earthward-pointed halo CMEs. Thus, V exp may serve as a proxy for the otherwise inaccessible radial speed of halo CMEs. We studied this connection using data from both ends: solar data and interplanetary data obtained near the Earth, for a period from January 1997 to 15 April 2001. The data were primarily provided by the LASCO coronagraphs, plus additional information from the EIT instrument on SOHO. Solar wind data from the plasma instruments on the SOHO, ACE and Wind spacecraft were used to identify the arrivals of ICME signatures. Here, we use "ICME" as a generic term for all CME effects in interplanetary space, thus comprising not only ejecta themselves but also shocks as well. Among 181 front side or limb full or partial halo CMEs recorded by LASCO, on the one hand, and 187 ICME events registered near the Earth, on the other hand, we found 91 cases where CMEs were uniquely associated with ICME signatures in front of the Earth. Eighty ICMEs were associated with a shock, and for 75 of them both the halo expansion speed V exp and the travel time T tr of the shock could be determined. The function T tr =203-20.77* ln (V exp ) fits the data best. This empirical formula can be used for predicting further ICME arrivals, with a 95% error margin of about one day. Note, though, that in 15% of comparable cases, a full or partial halo CME does not cause any ICME signature at Earth at all; every fourth partial halo CME and every sixth limb halo CME does not hit the Earth (false Correspondence to: R. Schwenn (schwenn@linmpi.mpg.de) alarms). Furthermore, every fifth transient shock or ICME or isolated geomagnetic storm is not caused by an identifiable partial or full halo CME on the front side (missing alarms).

Space Weather: Physics, Effects and Predictability

“…1998) which, when hitting the Earth's magnetosphere, can trigger a geomagnetic storm. Figure 2a shows a CME event on February 27, 2000, observed by the Large Angle Spectroscopic Coronograph (LASCO C3 coronagraph) on the SOHO spacecraft (Schwenn et al 2005). The instruments aboard SOHO have recorded more than 10 3 CMEs during the 23rd solar cycle (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) with unprecedented spatial and temporal resolutions; the observed CMEs are of various sizes, angular widths and speeds (Messerotti et al 2009).…”

“…The Sun is drawn to scale. The image is a difference between two exposures taken about 10 h apart from each other (credit: Schwenn et al 2005). b Schematic of the threedimensional structure of an ICME and upstream shock, relating magnetic field, plasma and bidirectional beams of suprathermal (C100 eV) electrons (BDEs) signature (credit: Zurbuchen and Richardson 2006) SMEI is less sensitive to CMEs erupting off the dark-side of the Sun because of Thomson scattering at these large elongations and frequent multiple CMEs can merge at large distances from the Sun.…”

“…However, not every halo CME will reach the Earth. Schwenn et al (2005) studied 181 front-side halo CMEs and showed that 15% did not show any in situ discernable signature. The longitudinal location of the source is of vital importance.…”

“…Forecasting space weather is the specification of the state of the space environment at a future time; it is a major challenge Schwenn et al 2005;Weigel et al 2006). This is because, in addition to formulating and implementing a test of a model or theory, it is burdened with the complications of issuing a predictive statement with limited information and in advance of the event (Vassiliadis 2007).…”

Variability of Geomagnetic Field with Interplanetary Magnetic Field at Low, Mid and High Latitudes

“…During extreme solar events like Coronal Mass Ejection (CME), filament and prominence eruptions, huge clouds of solar plasma containing frozen in magnetic field are thrown out [7][8][9]. Their interplanetary counterparts known as Interplanetary Coronal Mass Ejections (ICMEs) and other such structures have a fast rotating magnetic field, and on reaching the near-earth environment if they cause the IMF to be oriented northward, same as the direction of the geomagnetic field, then there is almost no interaction between the two.…”