14 research outputs found
Magnetic Flux of EUV Arcade and Dimming Regions as a Relevant Parameter for Early Diagnostics of Solar Eruptions - Sources of Non-Recurrent Geomagnetic Storms and Forbush Decreases
This study aims at the early diagnostics of geoeffectiveness of coronal mass
ejections (CMEs) from quantitative parameters of the accompanying EUV dimming
and arcade events. We study events of the 23th solar cycle, in which major
non-recurrent geomagnetic storms (GMS) with Dst <-100 nT are sufficiently
reliably identified with their solar sources in the central part of the disk.
Using the SOHO/EIT 195 A images and MDI magnetograms, we select significant
dimming and arcade areas and calculate summarized unsigned magnetic fluxes in
these regions at the photospheric level. The high relevance of this eruption
parameter is displayed by its pronounced correlation with the Forbush decrease
(FD) magnitude, which, unlike GMSs, does not depend on the sign of the Bz
component but is determined by global characteristics of ICMEs. Correlations
with the same magnetic flux in the solar source region are found for the GMS
intensity (at the first step, without taking into account factors determining
the Bz component near the Earth), as well as for the temporal intervals between
the solar eruptions and the GMS onset and peak times. The larger the magnetic
flux, the stronger the FD and GMS intensities are and the shorter the ICME
transit time is. The revealed correlations indicate that the main quantitative
characteristics of major non-recurrent space weather disturbances are largely
determined by measurable parameters of solar eruptions, in particular, by the
magnetic flux in dimming areas and arcades, and can be tentatively estimated in
advance with a lead time from 1 to 4 days. For GMS intensity, the revealed
dependencies allow one to estimate a possible value, which can be expected if
the Bz component is negative.Comment: 27 pages, 5 figures. Accepted for publication in Solar Physic
Coronal Dimmings and the Early Phase of a CME Observed with STEREO and Hinode/EIS
We investigate the early phase of the 13 February 2009 coronal mass ejection
(CME). Observations with the twin STEREO spacecraft in quadrature allow us to
compare for the first time in one and the same event the temporal evolution of
coronal EUV dimmings, observed simultaneously on-disk and above the limb. We
find that these dimmings are synchronized and appear during the impulsive
acceleration phase of the CME, with the highest EUV intensity drop occurring a
few minutes after the maximum CME acceleration. During the propagation phase
two confined, bipolar dimming regions, appearing near the footpoints of a
pre-flare sigmoid structure, show an apparent migration away from the site of
the CME-associated flare. Additionally, they rotate around the 'center' of the
flare site, i.e., the configuration of the dimmings exhibits the same
'sheared-to-potential' evolution as the postflare loops. We conclude that the
motion pattern of the twin dimmings reflects not only the eruption of the flux
rope, but also the ensuing stretching of the overlying arcade. Finally, we find
that: (1) the global-scale dimmings, expanding from the source region of the
eruption, propagate with a speed similar to that of the leaving CME front; (2)
the mass loss occurs mainly during the period of strongest CME acceleration.
Two hours after the eruption Hinode/EIS observations show no substantial plasma
outflow, originating from the 'open' field twin dimming regions.Comment: accepted for publication in Solar Physic
The Origin, Early Evolution and Predictability of Solar Eruptions
Coronal mass ejections (CMEs) were discovered in the early 1970s when space-borne coronagraphs revealed that eruptions of plasma are ejected from the Sun. Today, it is known that the Sun produces eruptive flares, filament eruptions, coronal mass ejections and failed eruptions; all thought to be due to a release of energy stored in the coronal magnetic field during its drastic reconfiguration. This review discusses the observations and physical mechanisms behind this eruptive activity, with a view to making an assessment of the current capability of forecasting these events for space weather risk and impact mitigation. Whilst a wealth of observations exist, and detailed models have been developed, there still exists a need to draw these approaches together. In particular more realistic models are encouraged in order to asses the full range of complexity of the solar atmosphere and the criteria for which an eruption is formed. From the observational side, a more detailed understanding of the role of photospheric flows and reconnection is needed in order to identify the evolutionary path that ultimately means a magnetic structure will erupt
An Observational Overview of Solar Flares
We present an overview of solar flares and associated phenomena, drawing upon
a wide range of observational data primarily from the RHESSI era. Following an
introductory discussion and overview of the status of observational
capabilities, the article is split into topical sections which deal with
different areas of flare phenomena (footpoints and ribbons, coronal sources,
relationship to coronal mass ejections) and their interconnections. We also
discuss flare soft X-ray spectroscopy and the energetics of the process. The
emphasis is to describe the observations from multiple points of view, while
bearing in mind the models that link them to each other and to theory. The
present theoretical and observational understanding of solar flares is far from
complete, so we conclude with a brief discussion of models, and a list of
missing but important observations.Comment: This is an article for a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011