Title of the publication
Properties of ubiquitous magnetic reconnection events in the lower solar atmosphere
Journal: Astronomy & Astrophysics
1st Author: Jayant Joshi
Position: Guest researcher
Co-authors from RoCS:
- Luc Rouppe van der Voort
Short summary by the author:
Magnetic reconnection in the deep solar atmosphere can give rise to enhanced emission in the Balmer hydrogen lines, a phenomenon referred to as Ellerman bombs. Recent high quality Hββ observations indicate that Ellerman bombs are more common than previously thought and it was estimated that at any time about half a million Ellerman bombs are present in the quiet Sun. We performed an extensive statistical characterization of the quiet Sun Ellerman bombs (QSEBs) in these new Hββ observations. We analyzed a 1 h dataset of quiet Sun observed with the Swedish 1-m Solar Telescope that consists of spectral imaging in the Hββ and Hααlines, as well as spectropolarimetric imaging in Fe I 617.3 nm. We detected a total of 2809 QSEBs. The lifetime varies between 9 s and 20.5 min with a median of 1.14 min. The maximum area ranges between 0.0016 and 0.2603 Mm22 with a median of 0.018 Mm22. A subset (14%) of the QSEBs display enhancement of the Hββline core. On average, the line core brightening appears 0.88 min after the onset of brightening in the wings, and the distance between these brightenings is 243 km. This gives rise to an apparent propagation speed ranging between −−14.3 and +23.5 km s−1−1, with an average that is upward propagating at +4.4 km −1−1. The average orientation is nearly parallel to the limbward direction. QSEBs are nearly uniformly distributed over the field of view but we find empty areas with the size of mesogranulation. QSEBs are located more frequent near the magnetic network where they are often bigger, longer lived and brighter. We conclude that QSEBs are ubiquitous in quiet Sun and appear everywhere except in areas of mesogranular size with weakest magnetic field (BLOS≲50BLOS≲50~G). Our observations support the interpretation of reconnection along vertically extended current sheets.
Title of the publication
Journal: Astronomy & Astrophysics
1st Author: Nancy Narang
Position: Postdoctoral Fellow
Co-authors from RoCS:
- Kalugodu Chandrashekhar
-
Shahin Jafarzadeh
-
Mikołaj Szydlarski
- Sven Wedemeyer
Short summary by the author:
We present a statistical analysis of power distribution of oscillations in a plage region in active region NOAA AR12651, observed jointly with ALMA (Atacama Large Millimeter/Submillimeter Array), IRIS (Interface Region Imaging Spectrograph), and SDO (Solar Dynamics Observatory). We employ coordinated ALMA Band-6 (1.25 mm) brightness temperature maps, IRIS Slit-Jaw Images in 2796 Å passband, and observations in six passbands (1600 Å, 304 Å, 131 Å, 171 Å, 193 Å and 211 Å) of AIA (Atmospheric Imaging Assembly) onboard SDO. We perform Lomb-Scargle transforms to study the distribution of oscillation power over the observed region by means of dominant period maps and power maps. We study spatial association of oscillations through the atmosphere mapped by the different passbands, with focus on the correlation of power distribution of ALMA oscillations with others. We do not observe any significant association of ALMA oscillations with IRIS and AIA oscillations. While the global behavior of ALMA dominant oscillations shows similarity with that of transition region and coronal passbands of AIA, the ALMA dominant period maps and power maps do not show any correlation with those from the other passbands. The spatial distribution of dominant periods and power in different period intervals of ALMA oscillations is uncorrelated with any other passband. We speculate the non-association of ALMA oscillations with those of IRIS and AIA be due to significant variations in the height of formation of the millimeter continuum observed by ALMA. Additionally, the fact that ALMA directly maps the brightness temperature, in contrast to the intensity observations by IRIS and AIA, can result in the very different intrinsic nature of the ALMA oscillations compared to the IRIS and AIA oscillations.
Title of the publication
The magnetic topology of the inverse Evershed flow
Journal: Astronomy & Astrophysics
1st Author: Avijeet Prasad
Position: Postdoctoral Fellow
Short summary by the author:
Context. The inverse Evershed flow (IEF) is a mass motion towards sunspots at chromospheric heights.
Aims. We combined high-resolution observations of NOAA 12418 from the Dunn Solar Telescope and vector magnetic field mea- surements from the Helioseismic and Magnetic Imager (HMI) to determine the driver of the IEF.
Methods. We derived chromospheric line-of-sight (LOS) velocities from spectra of Hα and Ca ii IR. The HMI data were used in a non-force-free magnetic field extrapolation to track closed field lines near the sunspot in the active region. We determined their length and height, located their inner and outer foot points, and derived flow velocities along them.
Results. The magnetic field lines related to the IEF reach on average a height of 3 Mm over a length of 13 Mm. The inner (outer) foot points are located at 1.2 (1.9) sunspot radii. The average field strength difference ∆B between inner and outer foot points is +400 G. The temperature difference ∆T is anti-correlated with ∆B with an average value of -100 K. The pressure difference ∆p is dominated by ∆B and is primarily positive with a driving force towards the inner foot points of 1.7 kPa on average. The velocities predicted from ∆ p reproduce the LOS velocities of 2–10 km s−1 with a square-root dependence.
Conclusions. We find that the IEF is driven along magnetic field lines connecting network elements with the outer penumbra by a gas pressure difference that results from a difference in field strength as predicted by the classical siphon flow scenario.