English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

The NASA STEREO mission opened up the possibility to forecast the arrivaltimes, speeds and directions of solar transients from outside the Sun-Earthline. In particular, we are interested in predicting potentially geo-effectiveInterplanetary Coronal Mass Ejections (ICMEs) from observations of densitystructures at large observation angles from the Sun (with the STEREOHeliospheric Imager instrument). We contribute to this endeavor by derivinganalytical formulas concerning a geometric correction for the ICME speed andarrival time for the technique introduced by Davies et al. (2012, ApJ, inpress) called Self-Similar Expansion Fitting (SSEF). This model assumes that acircle propagates outward, along a plane specified by a position angle (e.g.the ecliptic), with constant angular half width (lambda). This is an extensionto earlier, more simple models: Fixed-Phi-Fitting (lambda = 0 degree) andHarmonic Mean Fitting (lambda = 90 degree). This approach has the advantagethat it is possible to assess clearly, in contrast to previous models, if aparticular location in the heliosphere, such as a planet or spacecraft, mightbe expected to be hit by the ICME front. Our correction formulas are especiallysignificant for glancing hits, where small differences in the direction greatlyinfluence the expected speeds (up to 100-200 km/s) and arrival times (up to twodays later than the apex). For very wide ICMEs (2 lambda > 120 degree), thegeometric correction becomes very similar to the one derived by M\"ostl et al.(2011, ApJ, 741, id. 34) for the Harmonic Mean model. These analyticexpressions can also be used for empirical or analytical models to predict the1 AU arrival time of an ICME by correcting for effects of hits by the flankrather than the apex, if the width and direction of the ICME in a plane areknown and a circular geometry of the ICME front is assumed.

Speeds and arrival times of solar transients approximated by self-similar expanding circular fronts