Decametric N Burst: A Consequence of the Interaction of Two Coronal Mass Ejections   总被引：1，自引：0，他引：1  

P. Démoulin  K.-L. Klein  C. P. Goff  L. van Driel-Gesztelyi  J. L. Culhane  C. H. Mandrini  S. A. Matthews  L. K. Harra 《Solar physics》2007,240(2):301-313

Radio emissions of electron beams in the solar corona and interplanetary space are tracers of the underlying magnetic configuration and of its evolution. We analyse radio observations from the Culgoora and WIND/WAVES spectrographs, in combination with SOHO/LASCO and SOHO/MDI data, to understand the origin of a type N burst originating from NOAA AR 10540 on January 20, 2004, and its relationship with type II and type III emissions. All bursts are related to the flares and the CME analysed in a previous paper (Goff et al., 2007). A very unusual feature of this event was a decametric type N burst, where a type III-like burst, drifting towards low frequencies (negative drift), changes drift first to positive, then again to negative. At metre wavelengths, i.e., heliocentric distances ≲1.5R _⊙, these bursts are ascribed to electron beams bouncing in a closed loop. Neither U nor N bursts are expected at decametric wavelengths because closed quasi-static loops are not thought to extend to distances ≫1.5R _⊙. We take the opportunity of the good multi-instrument coverage of this event to analyse the origin of type N bursts in the high corona. Reconnection of the expanding ejecta with the magnetic structure of a previous CME, launched about 8 hours earlier, injects electrons in the same manner as with type III bursts but into open field lines having a local dip and apex. The latter shape was created by magnetic reconnection between the expanding CME and neighbouring (open) streamer field lines. This particular flux tube shape in the high corona, between 5R _⊙ and 10R _⊙, explains the observed type N burst. Since the required magnetic configuration is only a transient phenomenon formed by reconnection, severe timing and topological constraints are present to form the observed decametric N burst. They are therefore expected to be rare features.  相似文献   

Expected in Situ Velocities from a Hierarchical Model for Expanding Interplanetary Coronal Mass Ejections     

P. Démoulin  M. S. Nakwacki  S. Dasso  C. H. Mandrini 《Solar physics》2008,250(2):347-374

In situ data provide only a one-dimensional sample of the plasma velocity along the spacecraft trajectory crossing an interplanetary coronal mass ejection (ICME). Then, to understand the dynamics of ICMEs it is necessary to consider some models to describe it. We derive a series of equations in a hierarchical order, from more general to more specific cases, to provide a general theoretical basis for the interpretation of in situ observations, extending and generalizing previous studies. The main hypothesis is a self-similar expansion, but with the freedom of possible different expansion rates in three orthogonal directions. The most detailed application of the equations is though for a subset of ICMEs, magnetic clouds (MCs), where a magnetic flux rope can be identified. The main conclusions are the following ones. First, we obtain theoretical expressions showing that the observed velocity gradient within an ICME is not a direct characteristic of its expansion, but that it depends also on other physical quantities such as its global velocity and acceleration. The derived equations quantify these dependencies for the three components of the velocity. Second, using three different types of data we show that the global acceleration of ICMEs has, at most, a small contribution to the in situ measurements of the velocity. This eliminates practically one contribution to the observed velocity gradient within ICMEs. Third, we provide a method to quantify the expansion rate from velocity data. We apply it to a set of 26 MCs observed by Wind or ACE spacecrafts. They are typical MCs, and their main physical parameters cover the typical range observed in MCs in previous statistical studies. Though the velocity difference between their front and back includes a broad range of values, we find a narrow range for the determined dimensionless expansion rate. This implies that MCs are expanding at a comparable rate, independently of their size or field strength, despite very different magnitudes in their velocity profiles. Furthermore, the equations derived provide a base to further analyze the dynamics of MCs/ICMEs.  相似文献   

The Recovery of CME-Related Dimmings and the ICME’s Enduring Magnetic Connection to the Sun   总被引：1，自引：1，他引：0  

G. D. R. Attrill  L. van Driel-Gesztelyi  P. Démoulin  A. N. Zhukov  K. Steed  L. K. Harra  C. H. Mandrini  J. Linker 《Solar physics》2008,252(2):349-372

It is generally accepted that transient coronal holes (TCHs, dimmings) correspond to the magnetic footpoints of CMEs that remain rooted in the Sun as the CME expands out into the interplanetary space. However, the observation that the average intensity of the 12 May 1997 dimmings recover to their pre-eruption intensity in SOHO/EIT data within 48 hours, whilst suprathermal unidirectional electron heat fluxes are observed at 1 AU in the related ICME more than 70 hours after the eruption, leads us to question why and how the dimmings disappear whilst the magnetic connectivity is maintained. We also examine two other CME-related dimming events: 13 May 2005 and 6 July 2006. We study the morphology of the dimmings and how they recover. We find that, far from exhibiting a uniform intensity, dimmings observed in SOHO/EIT data have a deep central core and a more shallow extended dimming area. The dimmings recover not only by shrinking of their outer boundaries but also by internal brightenings. We quantitatively demonstrate that the model developed by Fisk and Schwadron (Astrophys. J. 560, 425, 2001) of interchange reconnections between “open” magnetic field and small coronal loops is a strong candidate for the mechanism facilitating the recovery of the dimmings. This process disperses the concentration of  “open” magnetic field (forming the dimming) out into the surrounding quiet Sun, thus recovering the intensity of the dimmings whilst still maintaining the magnetic connectivity to the Sun. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.  相似文献   

Automatic Solar Flare Detection Using Neural Network Techniques     

Fernandez Borda  Roberto A.  Mininni  Pablo D.  Mandrini  Cristina H.  Gómez  Daniel O.  Bauer  Otto H.  Rovira  Marta G. 《Solar physics》2002,206(2):347-357

We present a new method for automatic detection of flare events from images in the optical range. The method uses neural networks for pattern recognition and is conceived to be applied to full-disk Himages. Images are analyzed in real time, which allows for the design of automatic patrol processes able to detect and record flare events with the best time resolution available without human assistance. We use a neural network consisting of two layers, a hidden layer of nonlinear neurodes and an output layer of one linear neurode. The network was trained using a back-propagation algorithm and a set of full-disk solar images obtained by HASTA (HSolar Telescope for Argentina), which is located at the Estación de Altura Ulrico Cesco of OAFA (Observatorio Astronómico Félix Aguilar), El Leoncito, San Juan, Argentina. This method is appropriate for the detection of solar flares in the complete optical classification, being portable to any Hinstrument and providing unique criteria for flare detection independent of the observer.  相似文献