3D Magnetic Reconnection at a Coronal Null Point

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 +
SDO/HMI observation were used as input for multipole magnetic field
 +
extrapolation, the results of which are shown in Figure 2.
 +
The extrapolation shows an X-shaped feature which is consistent with
 +
the EUV observations.
 +
A combination of EUV observations and extrapolation results suggests
 +
that this feature is a coronal null point.
 +
 +
[[File:329f2.png|500px|thumb|center|Figure 2:
 +
Results from multipole potential field extrapolations based on
 +
[http://hmi.stanford.edu SDO/HMI] photospheric Zeeman observations. Top: Samples
 +
of extrapolated potential magnetic field lines (white), viewed from
 +
above, overlaid on the HMI line-of-sight magnetogram produced before
 +
(left), during (middle), and after (right) the collapse of the 3D
 +
null point. Blue filled circles lie on the locations of NRH centroids,
 +
and the colours match the correlation to frequency given in Figure
 +
2. Overplotted onto each frame as a black dashed line is the location
 +
of the
 +
[https://en.wikipedia.org/wiki/Magnetic_reconnection magnetic separator]
 +
that passes closest to the radio brightness centroids. The three
 +
times were chosen to match those shown in Figure 1. Bottom: The
 +
same magnetic field lines and radio sources shown in the top row
 +
are shown again, viewed at an angle to represent the 3D structure
 +
of the field. The dashed line again refers to the separator. Radio
 +
source heights are estimated directly from the
 +
[https://en.wikipedia.org/wiki/Corona "onefold Newkirk model"] of
 +
coronal density structure.
 +
]]
 +
 +
The Nancay Radioheliograph observations show a Type 1 noise storm
 +
associated with this region, with the radio imaging
 +
matching the X-shape feature in AIA 171 Â as it rotated
 +
across the disk. The NRH observations were further analysed to
 +
obtain the brightness temperature  and longitudinal extent of
 +
the radio source see Fig 3 (bottom). Just before the collapse there
 +
is a small decrease in brightness temperature at all frequencies during the collapse
 +
the is a simultaneous sharp rise in all frequencies which is followed
 +
by a peak in emission from higher frequency channels which moves
 +
to lower frequency channels over the subsequent 10 minutes. The
 +
extent of the radio source increases just before the collapse,
 +
before decreasing and returning to a similar value as before the
 +
collapse.
 +
 +
[[File:329f3.png|500px|thumb|center|Figure 3:
 +
(a) Time-distance plots demonstrating the apparent plane-of-sky
 +
motion of brightening in the 171 Â channel along slice A (top panel)
 +
and slice B (bottom panel), which are shown in Figure 2.
 +
The horizontal dotted line denotes the location of the apparent coronal
 +
3D null point, and vertical dashed lines correspond to the three
 +
time intervals used to produce the three panels in Figure 2. (b)
 +
Timedistance plots for the 193 Â channel. Overlaid on both time-distance
 +
plots are dash-dotted lines indicating approximate velocities of
 +
the flow features from which they are offset. (c) NRH brightness
 +
temperature, averaged over the area within a 50% contour of each
 +
NRH image, for its 7 highest recorded frequencies, smoothed over 6
 +
points, and normalised to clarify local peaks. (d) NRH extent,
 +
measured as the linear distance between the centroid of the highest
 +
and lowest frequency stable sources, which were 432 and 298 MHz,
 +
respectively.
 +
]]
== References ==
== References ==
[1] [http://adsabs.harvard.edu/abs/2018arXiv180609365O "Three-dimensional magnetic reconnection in a collapsing coronal loop system"]
[1] [http://adsabs.harvard.edu/abs/2018arXiv180609365O "Three-dimensional magnetic reconnection in a collapsing coronal loop system"]

Revision as of 18:47, 22 July 2018


Nugget
Number: 329
1st Author: Shane Maloney
2nd Author: Aidan O'Flannagain
Published: 30 July 2018
Next Nugget: TBD
Previous Nugget: Multimessenger
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Introduction

Magnetic reconnection is believed to be the primary mechanism by which free energy stored in coronal magnetic fields is rapidly released during solar eruptive events. Unfortunately, owing to the small spatial scales on which reconnection is thought to occur, it is not directly observable by remote sensing of the solar corona. However, larger scale processes, such as associated inflows and outflows, plus signatures of accelerated particles, have been put forward as evidence of reconnection. Magnetic reconnection is often studied at X-points, the simplest view of the process in two dimensions.

In the case presented in this Nugget, we think instead that the reconnection occurs at a 3D coronal null point (Ref. [2]). This study employs data from many instruments: SDO/AIA, SDO/HMK, the Nançay Radiohelioaph and GOES (as well as RHESSI), in a study of the partial collapse of a coronal structure associated with a Type I radio storm (.pdf). We interpret the observations as evidence for accelerated electrons in the reconnection region responding to a gradual fall and rapid rise in electric drift velocity, in response to the inflowing and outflowing field lines at a 3D null point.

Observations

Figure 1 shows SDO/AIA observations of the passage of an X-shaped feature from the limb towards disk centre. The X-shaped feature appears to undergo a partial collapse on 6 July 2013 at approximately 09:45. Before this collapse four loop structures are visible in 171 A channel while after the collapse the northern and western segments are no longer visible

Figure 1: Overview of the collapse of the X-shaped coronal structure. Top: AIA 171 Â images covering the 90 minutes surrounding the collapse. Overlaid on the earliest image are the slices used to generate the timedistance plots shown in Figure 3. Slices A and B are 67 arcseconds and 96 arcseconds long, respectively; the arrow denotes the reference direction for the timedistance plots. Middle: Nançay Radioheliograph 430 MHz images showing the Type I storm source at the times of the above images. Overlaid is the field of view of the AIA images. Bottom: GOES 1-8 Â and RHESSI 3-6 keV light curves, showing the flares that occurred during the collapse. (from [Ref. 1]).

SDO/HMI observation were used as input for multipole magnetic field extrapolation, the results of which are shown in Figure 2. The extrapolation shows an X-shaped feature which is consistent with the EUV observations. A combination of EUV observations and extrapolation results suggests that this feature is a coronal null point.

Figure 2: Results from multipole potential field extrapolations based on SDO/HMI photospheric Zeeman observations. Top: Samples of extrapolated potential magnetic field lines (white), viewed from above, overlaid on the HMI line-of-sight magnetogram produced before (left), during (middle), and after (right) the collapse of the 3D null point. Blue filled circles lie on the locations of NRH centroids, and the colours match the correlation to frequency given in Figure 2. Overplotted onto each frame as a black dashed line is the location of the magnetic separator that passes closest to the radio brightness centroids. The three times were chosen to match those shown in Figure 1. Bottom: The same magnetic field lines and radio sources shown in the top row are shown again, viewed at an angle to represent the 3D structure of the field. The dashed line again refers to the separator. Radio source heights are estimated directly from the "onefold Newkirk model" of coronal density structure.

The Nancay Radioheliograph observations show a Type 1 noise storm associated with this region, with the radio imaging matching the X-shape feature in AIA 171 Â as it rotated across the disk. The NRH observations were further analysed to obtain the brightness temperature and longitudinal extent of the radio source see Fig 3 (bottom). Just before the collapse there is a small decrease in brightness temperature at all frequencies during the collapse the is a simultaneous sharp rise in all frequencies which is followed by a peak in emission from higher frequency channels which moves to lower frequency channels over the subsequent 10 minutes. The extent of the radio source increases just before the collapse, before decreasing and returning to a similar value as before the collapse.

Figure 3: (a) Time-distance plots demonstrating the apparent plane-of-sky motion of brightening in the 171 Â channel along slice A (top panel) and slice B (bottom panel), which are shown in Figure 2. The horizontal dotted line denotes the location of the apparent coronal 3D null point, and vertical dashed lines correspond to the three time intervals used to produce the three panels in Figure 2. (b) Timedistance plots for the 193 Â channel. Overlaid on both time-distance plots are dash-dotted lines indicating approximate velocities of the flow features from which they are offset. (c) NRH brightness temperature, averaged over the area within a 50% contour of each NRH image, for its 7 highest recorded frequencies, smoothed over 6 points, and normalised to clarify local peaks. (d) NRH extent, measured as the linear distance between the centroid of the highest and lowest frequency stable sources, which were 432 and 298 MHz, respectively.

References

[1] "Three-dimensional magnetic reconnection in a collapsing coronal loop system"

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