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The temperature-Dependent Nature of Coronal Dimmings  

Eva Robbrecht   Submitted: 2010-08-02 07:37

The opening-up of the magnetic field during solar eruptive events is often accompanied by a dimming of the local coronal emission. From observations of filament eruptions recorded with the Extreme-Ultraviolet Imager on {it STEREO} during 2008-2009, it is evident that these dimmings are much more pronounced in 19.5~nm than in the lower-temperature line 17.1~nm, as viewed either on the disk or above the limb. We conclude that most of the cooler coronal plasma is not ejected but remains gravitationally bound when the loops open up. This result is consistent with Doppler measurements by Imada and coworkers, who found that the upflow speeds in a transient coronal hole increased dramatically above a temperature of 1 MK; it is also consistent with the quasistatic behavior of polar plumes, as compared with the hotter interplume regions that are the main source of the fast solar wind. When the open flux reconnects and closes down again, the trapped plasma is initially heated to such high temperatures that it is no longer visible at ion{Fe}{9} 17.1~nm. Correspondingly, 17.1~nm images show a dark ribbon or ``heat wave'' propagating away from the polarity inversion line and coinciding with the brightened ion{Fe}{15} 28.4~nm and ion{Fe}{12} 19.5~nm post-eruptive loops and their footpoint areas. Such dark ribbons provide a clear example of dimmings that are not caused by a density depletion. The propagation of the ``heat wave'' is driven by the closing-down, not the opening-up, of flux and can be observed both off-limb and on-disk.

Authors: E. Robbrecht & Y.-M. Wang
Projects: STEREO

Publication Status: accepted for publication in ApJ Letters
Last Modified: 2010-08-04 06:37
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On the Weakening of the Polar Magnetic Fields during Solar Cycle 23  

Eva Robbrecht   Submitted: 2009-12-08 12:39

The Sun's polar fields are currently 40% weaker than they were during the previous three sunspot minima. This weakening has been accompanied by a corresponding decrease in the interplanetary magnetic field (IMF) strength, by a ∼20% shrinkage in the polar coronal-hole areas, and by a reduction in the solar-wind mass flux over the poles. It has also been reflected in coronal streamer structure and the heliospheric current sheet, which only showed the expected flattening into the equatorial plane after sunspot numbers fell to unusually low values in mid-2008. From latitude?time plots of the photospheric field, it has long been apparent that the polar fields are formed through the transport of trailing-polarity flux from the sunspot latitudes to the poles. To address the question of why the polar fields are now so weak, we simulate the evolution of the photospheric field and radial IMF strength from 1965 to the present, employing a surface transport model that includes the effects of active region emergence, differential rotation, supergranular convection, and a poleward bulk flow. We find that the observed evolution can be reproduced if the amplitude of the surface meridional flow is varied by as little as 15% (between 14.5 and 17 m s-1), with the higher average speeds being required during the long cycles 20 and 23.

Authors: Wang, Y.-M., Robbrecht, E., Sheeley, N. R. Jr.
Projects: None

Publication Status: published in ApJ 707 (2009), 1372
Last Modified: 2009-12-09 08:07
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A broad perspective on automated CME Tracking: towards higher level space weather forecasting  

Eva Robbrecht   Submitted: 2006-02-16 07:15

We discuss our current capabilities to deliver the CME parameters required for the space weather forecasting process. The ever growing importance of space weather has lead to new requirements on the timeliness and objectiveness of CME detection. It has become indispensable to report the occurrence of Earth-directed CMEs and to predict their possible impact on the geospace environment. Early 2005, we are on the eve of a new era in space weather forecasting. We point out the restricted accuracy on the current forecasts and discuss a chance for amelioration. This invokes data-driven models (empirical and numerical), triggered by a real-time CME disturbance, simulating the propagation and interaction of the ejection with the ambient solar wind. We discuss the link between the direct observable parameters (like the CME {it projected} speed and angle around the occulter) and the required input parameters (like radial speed, direction, ...). The only way to guarantee the real-time value of the simulations is by employing software which autonomously detect CME parameters in a variety of data. This paper focusses on the automated CME detection algorithms that are currently available. Automated CME tracking is yet in its infancy, therefore this `review' will be an outlook on the potential of this field rather than looking back on already achieved milestones.

Authors: Robbrecht, E. and Berghmans, D.
Projects: None

Publication Status: in press to AGU Geophysical Monograph Series, Booktitle: Solar Eruptions and Energetic Particles
Last Modified: 2006-02-16 07:15
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Automated recognition of coronal mass ejections (CMEs) in near-real-time data  

Eva Robbrecht   Submitted: 2004-06-24 01:19

This paper presents a new method and first applications of software that we have developed to autonomously detect CMEs in image sequences from LASCO (Large Angle Spectrometric Coronagraph). The crux of the software is the detection of CMEs as bright ridges in [time,height] maps using the Hough transform. The next step employs clustering and morphological closing operations to mark out different CMEs. The output is a list of events, similar to the classic catalogs, with starting time, principle angle, angular width and velocity estimation for each CME. In addition we present a new type of CME overview map that clearly shows all detected CMEs in a [principal angle, time of occurrence] coordinate system. In contrast to catalogs assembled by human operators, these CME detections can be done without any human interference on real-time data 24 hours per day (see www.sidc.oma.be/cactus for the real-time output with data covering the last 4 days). Therefore the detection is not only more immediate, but, more importantly, also more objective. In this paper we describe the software and validate its performance by comparing its output with the SOHO LASCO CME catalog. experimental results on real-time data show that the developed technique can achieve excellent results in measuring starting time and principal angle and good results for the angular width and velocity measurement compared to the CMEs listed in the catalog. Its overall success rate is presently about 94 %. The software also reveals CMEs or other features that have not been listed in the catalog. Such unreported cases might influence CME statistics and they demonstrate that also the present catalogs do not have a 100 % success rate. This inevitably leads to a discussion on the definition of a CME. Prospects for improvement and exploitation are discussed.

Authors: E. Robbrecht, D. Berghmans
Projects: Soho-LASCO

Publication Status: A&A (in press)
Last Modified: 2004-06-24 01:22
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Abstracts by Author
The temperature-Dependent Nature of Coronal Dimmings
On the Weakening of the Polar Magnetic Fields during Solar Cycle 23
A broad perspective on automated CME Tracking: towards higher level space weather forecasting
Automated recognition of coronal mass ejections (CMEs) in near-real-time data

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