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Active region upflows: 2. Data driven MHD modeling  

Kamalam Vanninathan   Submitted: 2015-09-21 02:21

Context. Observations of many active regions show a slow systematic outflow/upflow from their edges lasting from hours to days. At present no physical explanation has been proven, while several suggestions have been put forward. Aims. This paper investigates one possible method for maintaining these upflows assuming that convective motions drive the magnetic field to initiate them through magnetic reconnection. Methods. We use Helioseismic and Magnetic Imager (HMI) data to provide an initial potential three dimensional magnetic field of the active region NOAA 11123 on 2010 November 13 where the characteristic upflow velocities are observed. A simple one-dimensional hydrostatic atmospheric model covering the region from the photosphere to the corona is derived. Local Correlation Tracking of the magnetic features in the HMI data is used to derive a proxy for the time dependent velocity field. The time dependent evolution of the system is solved using a resistive three-dimensional MagnetoHydro-Dynamic code. Results. The magnetic field contains several null points located well above the photosphere, with their fan planes dividing the magnetic field into independent open and closed flux domains. The stressing of the interfaces between the different flux domains is expected to provide locations where magnetic reconnection can take place and drive systematic flows. In this case, the region between the closed and open flux is identified as the region where observations find the systematic upflows. Conclusions. In the present experiment, the driving only initiates magneto-acoustic waves, without driving any systematic upflows at any of the flux interfaces.

Authors: K. Galsgaard, M. S. Madjarska, K. Vanninathan, Z. Huang, M. Presmann
Projects: None

Publication Status: Accepted for publication in A&A
Last Modified: 2015-09-23 13:31
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Active region upflows: 1. Multi-instrument observations  

Kamalam Vanninathan   Submitted: 2015-09-21 02:19

Upflows at the edges of active regions (ARs) are studied by spatially and temporally combining multi-instrument observations obtained with EIS/Hinode, AIA and HMI/SDO and IBIS/NSO, to derive their plasma parameters. This information is used for benchmarking data-driven modelling of the upflows (Galsgaard et al., 2015). The studied AR upflow displays blueshifted emission of 5-20 km s-1 in Fe XII and Fe XIII and its average electron density is 1.8x109 cm^3 at 1 MK. The time variation of the density shows no significant change (in a 3sigma error). The plasma density along a single loop drops by 50% over a distance of 20000 km. We find a second velocity component in the blue wing of the Fe XII and Fe XIII lines at 105 km s-1. This component is persistent during the whole observing period of 3.5 hours with variations of only 15 km s-1. We also study the evolution of the photospheric magnetic field and find that magnetic flux diffusion is responsible for the formation of the upflow region. High cadence Hα observations of the chromosphere at the footpoints of the upflow region show no significant jet-like (spicule/rapid blue excursion) activity to account for several hours/days of plasma upflow. Using an image enhancement technique, we show that the coronal structures seen in the AIA 193A channel is comparable to the EIS Fe XII images, while images in the AIA 171A channel reveals additional loops that are a result of contribution from cooler emission to this channel. Our results suggest that at chromospheric heights there are no signatures that support the possible contribution of spicules to AR upflows. We suggest that magnetic flux diffusion is responsible for the formation of the coronal upflows. The existence of two velocity components possibly indicate the presence of two different flows which are produced by two different physical mechanisms, e.g. magnetic reconnection and pressure-driven.

Authors: K. Vanninathan, M.S. Madjarska, K. Galsgaard, Z.Huang, J.G. Doyle
Projects: None

Publication Status: Accepted for publication in A&A
Last Modified: 2015-09-23 13:31
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Coronal response to an EUV wave from DEM analysis  

Kamalam Vanninathan   Submitted: 2015-09-18 01:56

EUV (Extreme-Ultraviolet) waves are globally propagating disturbances that have been observed since the era of the SoHO/EIT instrument. Although the kinematics of the wave front and secondary wave components have been widely studied, there is not much known about the generation and plasma properties of the wave. In this paper we discuss the effect of an EUV wave on the local plasma as it passes through the corona. We studied the EUV wave, generated during the 2011 February 15 X-class flare/CME event, using Differential Emission Measure diagnostics. We analyzed regions on the path of the EUV wave and investigated the local density and temperature changes. From our study we have quantitatively confirmed previous results that during wave passage the plasma visible in the Atmospheric Imaging Assembly (AIA) 171A channel is getting heated to higher temperatures corresponding to AIA 193A and 211A channels. We have calculated an increase of 6 - 9% in density and 5 - 6% in temperature during the passage of the EUV wave. We have compared the variation in temperature with the adiabatic relationship and have quantitatively demonstrated the phenomenon of heating due to adiabatic compression at the wave front. However, the cooling phase does not follow adiabatic relaxation but shows slow decay indicating slow energy release being triggered by the wave passage. We have also identified that heating is taking place at the front of the wave pulse rather than at the rear. Our results provide support for the case that the event under study here is a compressive fast-mode wave or a shock.

Authors: K. Vanninathan, A.M. Veronig, K. Dissauer, M.S. Madjarska, I.G. Hannah, E.P. Kontar
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2015-09-23 13:30
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Off-limb (spicule) DEM distribution from SoHO/SUMER observations  

Kamalam Vanninathan   Submitted: 2012-03-28 08:23

In the present work we derive a Differential Emission Measure (DEM) distribution from a region dominated by spicules. We use spectral data from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on-board the Solar Heliospheric Observatory (SoHO) covering the entire SUMER wavelength range taken off-limb in the Northern polar coronal hole to construct this DEM distribution using the CHIANTI atomic database. This distribution is then used to study the thermal properties of the emission contributing to the 171 Å channel in the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO). From our off-limb DEM we found that the radiance in the AIA 171 Å channel is dominated by emission from the Fe IX 171.07 A line and has sparingly little contribution from other lines. The product of the Fe IX 171.07 A line contribution function with the off-limb DEM was found to have a maximum at logTmax (K) = 5.8 indicating that during spicule observations the emission in this line comes from plasma at transition region temperatures rather than coronal. For comparison, the same product with a quiet Sun and prominence DEM were found to have a maximum at logTmax (K) = 5.9 and logTmax (K) = 5.7, respectively. We point out that the interpretation of data obtained from the AIA 171 Å filter should be done with foreknowledge of the thermal nature of the observed phenomenon. For example, with an off-limb DEM we find that only 3.6% of the plasma is above a million degrees, whereas using a quiet Sun DEM, this contribution rises to 15%.

Authors: K. Vanninathan, M. S. Madjarska, E. Scullion, J. G. Doyle
Projects: SDO-AIA,SoHO-SUMER

Publication Status: accepted for publication in Solar Physics
Last Modified: 2012-03-28 12:44
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Abstracts by Author
Active region upflows: 2. Data driven MHD modeling
Active region upflows: 1. Multi-instrument observations
Coronal response to an EUV wave from DEM analysis
Off-limb (spicule) DEM distribution from SoHO/SUMER observations

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