High-resolution photography of the solar chromosphere. XVII Geometry of H-alpha flare loops observed on the disk

Romano

et al. 2011

A&A

Context. Photospheric shear plasma flows in active regions may be responsible for the magnetic helicity injection in the solar corona not only during the energy storage process before a solar eruption, but also during and after the release of the free magnetic energy caused by the eruption. Indeed, after a filament eruption or expansion the magnetic torque imbalance can induce shear flows that can be responsible for yet another injection of magnetic helicity into the corona. Aims. We investigated the magnetic helicity balance in an active region where a confined solar eruption occurred. This was done to verify a possible relationship between the filament expansion and the helicity transport at its footpoints. We aimed to verify if this variation in the helicity transport rate could be interpreted as a consequence of the magnetic torque imbalance caused by the tube expansion, as proposed by Chae et al. (2003, J. Kor. Astron. Soc., 36, 33). Methods. We used 171 Å TRACE data to measure some geometrical parameters of the new magnetic system produced by a filament eruption that occurred on 2001 November 1 in active region NOAA 9682. We used MDI full disk line-of-sight magnetogram data to measure the accumulation of magnetic helicity in the corona before and after the event.Results. From the measured expansion factor in the magnetic arcade, visible at 171 Å during the eruption, we estimated that the resulting torque imbalance at the photosphere ought to lead to the injection of negative helicity following the eruption. We compared this with measurements of the helicity injection using photospheric velocity and magnetogram data. Conclusions. In contradiction to the expectations from the Chae et al. model, the helicity injection after the eruption was positive. We offer the alternative interpretation that the helicity injection resulted from torque of the opposite sign, generated as the filament lost its negative helicity through magnetic reconnection with its surroundings.

Section: Discussionmentioning

confidence: 99%

Magnetic helicity balance during a filament eruption that occurred in active region NOAA 9682

Romano

et al. 2011

A&A

“…To obtain values of the deprojected loop length we conducted a loop axis reconstruction using a geometrical technique of Loughhead et al (1983). This technique is based on coordinates transformation between three natural coordinates systems: observer's (or image) system, heliographic system, and loop system aligned with the loop plane.…”

Section: Estimate Of the Loop Parameters And Calculation Of Periodsmentioning

confidence: 99%

Flare quasi-periodic pulsations with growing periodicity

Reznikova

Shibasaki

2010

A&A

We conducted a wavelet analysis of the flare intensity variations for the long duration flare on 2005 August 22 observed with the Nobeyama Radioheliograph at frequencies 17 and 34 GHz and with the Ramaty High Energy Solar Spectroscopic Imager at 25-50 keV. We found that the signals contain a well-pronounced periodicity in which the oscillation period grows from 2.5 to 5 min. An analysis of the loop length and plasma temperature evolution during the flare allowed us to interpret the quasi-periodic pulsations in terms of the second standing harmonics of the slow magnetoacoustic mode. This mode can be generated by the initial impulsive energy release and work as a trigger for the repeated energy releases.

“…The azimuthal angle γ is defined as the angle between the footpoint baseline and the circle of latitude parallel to the equator and passing through the eastern footpoints. We have used the IDL procedure Divergingloop (Contarino et al 2006), based on the method developed by Loughhead et al (1983) and on the assumptions that the loop footpoints can be determined, the loop central axis lies in a plane and the loop is symmetrical about an axis perpendicular to its baseline. We have found a relative inclination of ∼12…”

Section: Magnetograms Analysismentioning

confidence: 99%

A C-level flare observed in an arch filament system: reconnection between pre-existing and emerging field lines?

Zuccarello¹,

Battiato

Contarino

et al. 2008

A&A

Context. Observations show that solar flares are often caused by the emergence of new magnetic flux from subphotospheric layers and by the interaction of the rising field lines with the ambient magnetic field. In this framework, recent theoretical models of magnetic flux emergence have investigated the effectiveness of magnetic reconnection as a function of the old and new flux system relative orientations. Aims. We aim to compare phenomena that occurred in an active region, before and during a small flare, with the effects of magnetic reconnection between nearly parallel magnetic field lines, foreseen by these models. Methods. We analyzed high resolution photospheric and chromospheric data acquired during a coordinated observational campaign performed with the THEMIS telescope in IPM mode, as well as MDI magnetograms and TRACE 1600 and 171 Å images, to investigate the dynamics and the magnetic configuration of the active region hosting the flare.Results. An emerging arch filament system (AFS) was observed in the area between the two main sunspots: it showed typical upward motion at the arch tops and plasma downward motion at the footpoints. A C-level flare, characterized by a factor of 3 peak enhancement in the GOES X-ray emission with respect to the pre-event background, occurred in this zone, where the configuration of the emerging magnetic field lines showed a small (∼12• ) relative inclination with respect to the old flux system. Conclusions. In an active region (age ≥6 days) a new magnetic flux bundle emerged between the two main polarity spots. It gave rise to the formation of pores in the photosphere and to an AFS in the chromosphere. The interaction between the new and the pre-existing field lines, characterized by a small relative inclination, might have caused a weak reconnection process and given rise to the C-level flare. This result is in broad agreement with numerical simulations predicting very limited reconnection when the two flux systems have an almost parallel orientation.