Drawing and using a Great Arc

How to define and draw a great arc on an image of the Sun, and to extract intensity values along that arc.

import matplotlib.pyplot as plt
import numpy as np

import astropy.units as u
from astropy.coordinates import SkyCoord

import sunpy.map
from sunpy.coordinates.utils import GreatArc
from sunpy.data.sample import AIA_171_IMAGE

We start with the sample data

m = sunpy.map.Map(AIA_171_IMAGE)

Let’s define the start and end coordinates of the arc.

start = SkyCoord(735 * u.arcsec, -471 * u.arcsec, frame=m.coordinate_frame)
end = SkyCoord(-100 * u.arcsec, 800 * u.arcsec, frame=m.coordinate_frame)

Create the great arc between the start and end points.

great_arc = GreatArc(start, end)

Plot the great arc on the Sun.

fig = plt.figure()
ax = plt.subplot(projection=m)
m.plot(axes=ax)
ax.plot_coord(great_arc.coordinates(), color='c')
plt.show()
AIA $171 \; \mathrm{\mathring{A}}$ 2011-06-07 06:33:02

Now we can calculate the nearest integer pixels of the data that correspond to the location of arc.

pixels = np.asarray(np.rint(m.world_to_pixel(great_arc.coordinates())), dtype=int)
x = pixels[0, :]
y = pixels[1, :]

Get the intensity along the arc from the start to the end point.

Define the angular location of each pixel along the arc from the start point to the end.

angles = great_arc.inner_angles().to(u.deg)

Plot the intensity along the arc from the start to the end point.

fig, ax = plt.subplots()
ax.plot(angles, intensity_along_arc)
ax.set_xlabel('degrees of arc from start')
ax.set_ylabel('intensity')
ax.grid(linestyle='dotted')
plt.show()
great arc example

Total running time of the script: ( 0 minutes 1.339 seconds)

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