The magnetic field topology associated to two M flares

On 27 October, 2003, two GOES M-class flares occurred in the lapse of three hours in active region NOAA 10486. The two flares were confined and their associated brightenings appeared at the same location, displaying a very similar shape both at the chromospheric and coronal levels. We focus on the analysis of magnetic field (SOHO/MDI), chromospheric (HASTA, Kanzelhoehe Solar Observatory, TRACE) and coronal (TRACE) observations. By combining our data analysis with a model of the coronal magnetic field, we compute the magnetic field topology associated to the two M flares. We find that both events can be explained in terms of a localized magnetic reconnection process occurring at a coronal magnetic null point. This null point is also present at the same location one day later, on 28 October, 2003. Magnetic energy release at this null point was proposed as the origin of a localized event that occurred independently with a large X17 flare on 28 October, 2003, at 11:01 UT. The three events, those on 27 October and the one on 28 October, are homologous. Our results show that coronal null points can be stable topological structures where energy release via magnetic reconnection can happen, as proposed by classical magnetic reconnection models.Comment: 14 pages, 7 figure

Homologous Flares and Magnetic Field Topology in Active Region NOAA 10501 on 20 November 2003

We present and interpret observations of two morphologically homologous flares that occurred in active region (AR) NOAA 10501 on 20 November 2003. Both flares displayed four homologous H-alpha ribbons and were both accompanied by coronal mass ejections (CMEs). The central flare ribbons were located at the site of an emerging bipole in the center of the active region. The negative polarity of this bipole fragmented in two main pieces, one rotating around the positive polarity by ~ 110 deg within 32 hours. We model the coronal magnetic field and compute its topology, using as boundary condition the magnetogram closest in time to each flare. In particular, we calculate the location of quasiseparatrix layers (QSLs) in order to understand the connectivity between the flare ribbons. Though several polarities were present in AR 10501, the global magnetic field topology corresponds to a quadrupolar magnetic field distribution without magnetic null points. For both flares, the photospheric traces of QSLs are similar and match well the locations of the four H-alpha ribbons. This globally unchanged topology and the continuous shearing by the rotating bipole are two key factors responsible for the flare homology. However, our analyses also indicate that different magnetic connectivity domains of the quadrupolar configuration become unstable during each flare, so that magnetic reconnection proceeds differently in both events.Comment: 24 pages, 10 figures, Solar Physics (accepted

Observed flux density enhancement at submillimeter wavelengths during an X-class flare

We analyse the 30 October, 2004, X1.2/SF solar event that occurred in AR 10691 (N13 W18) at around 11:44 UT. Observations at 212 and 405 GHz of the Solar Submillimeter Telescope (SST), with high time resolution (5 ms), show an intense impulsive burst followed by a long-lasting thermal phase. EUV images from the Extreme Ultraviolet Imaging Telescope (SOHO/EIT) are used to identify the possible emitting sources. Data from the Radio Solar Telescope Network (RSTN) complement our spectral observations below 15 GHz. During the impulsive phase the turnover frequency is above 15.4 GHz. The long-lasting phase is analysed in terms of thermal emission and compared with GOES observations. From the ratio between the two GOES soft X-ray bands, we derive the temperature and emission measure, which is used to estimate the free-free submillimeter flux density. Good temporal agreement is found between the estimated and observed profiles, however the former is larger than the latter.Comment: 13 pages, 7 figure

Balance energético durante la fase impulsiva de una fulguración solar

Se analiza la fulguración solar del 21 de Mayo de 1980 en base a los datos obtenidos por el Hard X-ray Image Spectrometer (HXIS) a bordo del satélite Solar Maximum Mission (SMM). A partir de los parámetros físicos deducidos de las observaciones se estudia el balance energético de los "loops" coronales durante la fase impulsiva de la fulguración. Se tienen en cuenta los términos de variación intrínseca de la energía, pérdidas por conducción y radiación y el flujo de energía por evaporación cromosférica. Este análisis se complementa con datos provenientes de otros instrumentos del satélite como el Bent Crystal Spectrometer (BCS) y el Hard X-ray Bust Spectrometer (HXRBS).Asociación Argentina de Astronomí

Nu Carinae, una binaria visual ABI + B

Espectrogramas obtenidos con el espectrógrafo con supresor de cielo en el telescopio de 1 m del Observatorio Interamericano de Cerro Tololo muestran que la componente más débil de la binaria visual Nu Carinae tiene espectro del tipo B temprano y no F0 como figura en las publicaciones. Se comparan los parámetros del sistema binario Nu Car con otros sistemas similares y se discute su estado evolutivo.Asociación Argentina de Astronomí

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

Estudio en rayos X de las fulguraciones de noviembre de 1980

Se analizan dos fulguraciones del 6 de noviembre de 1980 con datos de rayos X obtenidos con el Hard X-Ray Imaging Spectrometer a bordo del SMM. Se hace un estudio comparativo de la morfología y de la variación temporal de los parámetros físicos de ambas fulguraciones.Asociación Argentina de Astronomí

Progressive transformation of a flux rope to an ICME

The solar wind conditions at one astronomical unit (AU) can be strongly disturbed by the interplanetary coronal mass ejections (ICMEs). A subset, called magnetic clouds (MCs), is formed by twisted flux ropes that transport an important amount of magnetic flux and helicity which is released in CMEs. At 1 AU from the Sun, the magnetic structure of MCs is generally modeled neglecting their expansion during the spacecraft crossing. However, in some cases, MCs present a significant expansion. We present here an analysis of the huge and significantly expanding MC observed by the Wind spacecraft during 9 and 10 November, 2004. After determining an approximated orientation for the flux rope using the minimum variance method, we precise the orientation of the cloud axis relating its front and rear magnetic discontinuities using a direct method. This method takes into account the conservation of the azimuthal magnetic flux between the in- and out-bound branches, and is valid for a finite impact parameter (i.e., not necessarily a small distance between the spacecraft trajectory and the cloud axis). Moreover, using the direct method, we find that the ICME is formed by a flux rope (MC) followed by an extended coherent magnetic region. These observations are interpreted considering the existence of a previous larger flux rope, which partially reconnected with its environment in the front. These findings imply that the ejected flux rope is progressively peeled by reconnection and transformed to the observed ICME (with a remnant flux rope in the front part).Comment: Solar Physics (in press

Zones with quasi-discontinuous magnetic connections in the photosphere and solar flares

The topological structure of active regions is related with flare brightening. (Mandrini et a!, 1991, 1993; Démoulin et ai, 1992, 1993, 1994). In these works, we modeled the observed longitudinal magnetic field by means of a discrete number of sub-photospheric magnetic poles, and derived the magnetic topology taking into account the connections between these poles (source method, MF). We present here a new method (quasi-discontinuity method, MQD). It finds the regions above the photosphere where the connectivity of field lines changes drastically.Asociación Argentina de Astronomí

HD 94546, un sistema múltiple?

HD 94546 es un sistema binario que espectroscópicamente aparece compuesto por una WN4+O. Niemela (1980) determinó la órbita preliminar del mismo. Con el objeto de mejorar los parámetros orbitales de HD 94546 se obtuvieron 46 espectrogramas adicionales, entre 1981 y 1984, en el Observatorio Interamericano de Cerro Tololo con el telescopio de 1 m. El análisis de las variaciones de velocidad radial de las líneas de emisión de la estrella WN y de las absorciones confirma la existencia de un período de 4,385 días y muestra que la velocidad del centro de masa del sistema varía de época en época de observación, sugiriendo que se trata de un sistema múltiple. Se analizan los elementos orbitales de este sistema.Asociación Argentina de Astronomí