Units and Coordinates in SunPy

This section of the guide will talk about representing physical units and physical coordinates in SunPy. SunPy makes use of Astropy for both these tasks.

Units in SunPy

All functions in SunPy that accept or return numbers associated with physcial quantities accept and return Quantity objects. These objects represent a number (or an array of numbers) and a unit. This means SunPy is always explicit about the units associated with a value. Quantities and units are powerful tools for keeping track of variables with a physical meaning and make it straightforward to convert the same physical quantity into different units.

In this section of the guide we will give a quick introduction to astropy.units and then demostrate how to use units with SunPy.

To use units we must first import them from Astropy. To save on typing we usually import units as u:

>>> import astropy.units as u

Once we have imported units we can create a quantity by multiplying a number by a unit:

>>> length = 10 * u.meter
>>> length
<Quantity 10.0 m>

A Quantity has both a .unit and a .value attribute:

>>> length.value
10.0

>>> length.unit
Unit("m")

These Quantity objects can also be converted to other units, or unit systems:

>>> length.to(u.km)
<Quantity 0.01 km>

>>> length.cgs
<Quantity 1000.0 cm>

Probably most usefully, Quantity objects will propogate units through arithmetic operations when appropriate:

>>> distance_start = 10 * u.mm
>>> distance_end = 23 * u.km
>>> length = distance_end - distance_start
>>> length
<Quantity 22.99999 km>

>>> time = 15 * u.minute
>>> speed = length / time
>>> speed
<Quantity 1.5333326666666667 km / min>

However, operations which do not make physical sense for the units specified will cause an error:

>>> length + time
...
astropy.units.core.UnitConversionError: Can only apply 'add' function to quantities with compatible dimensions

Quantities as function arguments

An extremely useful addition to the base functionality of Quanitities is the @u.quantity_input decorator. This allows you to specify required units for function arguments to ensure that the calculation within that function always make physical sense. For instance, if we defined a function to calculate speed as above, we might want the distance and time as inputs:

def speed(length, time):
    return length / time

However, this requires that length and time both have the appropriate units. We therefore want to use quantity_input to enforce this, here we use function annotations to specify the units (this is a Python 3.5+ feature, see the quantity_input documentation for more details and Python 2 instructions):

@u.quantity_input
def speed(length: u.m, time: u.s):
    return length / time

Now, when this function is called, if the units of length and time are not convertible to the units specified, an error will be raised stating that the units are incorrect or missing:

>>> speed(1*u.m, 10*u.m)
...
astropy.units.core.UnitsError: Argument 'time' to function 'speed' must be in units convertible to 's'.

>>> speed(1*u.m, 10)
...
TypeError: Argument 'time' to function 'speed' has no 'unit' attribute. You may want to pass in an astropy Quantity instead.

Note that the units of the inputs do not have to be exactly the same as those in the function definition, as long as they can be converted to those units. So for instance, passing in a time in minutes still works even though we specified time: u.s:

>>> speed(1*u.m, 1*u.minute)
<Quantity 1.0 m / min>

This may still not be quite as we want it, since we wanted the input time in seconds but the output is in m/min. We can correct this by defining the function with an additional annotation:

@u.quantity_input
def speed(length: u.m, time: u.s) -> u.m/u.s:
    return length / time

This will force the output of the function to be converted to m/s before returning, so that you will always have the same units on the output from this function:

>>> speed(1*u.m, 1*u.minute)
<Quantity 0.016666666666666666 m / s>

Physical Coordinates in SunPy

In much the same way as units are used for representing physical quantities, SunPy uses astropy.coordinates to represent points in physical space. This applies to both points in 3D space and projected coordinates in images.

The astropy coordinates module is primarily used through the SkyCoord class:

>>> from astropy.coordinates import SkyCoord

To enable the use of the solar physics specific frames defined in SunPy we also need to import them:

>>> from sunpy.coordinates import frames

A SkyCoord object to represent a point on the Sun can then be created:

>>> c = SkyCoord(70*u.deg, -30*u.deg, obstime="2017-08-01",
                 frame=frames.HeliographicStonyhurst)
>>> c
<SkyCoord (HeliographicStonyhurst: obstime=None): (lon, lat, rad) in (deg, deg, km)
    (70.0, -30.0, 695508.0)>

This SkyCoord object can then be transformed to any other coordinate frame defined either in Astropy or SunPy, for example:

>>> c.transform_to(frames.Helioprojective)
<SkyCoord (Helioprojective: obstime=2017-08-01 00:00:00, rsun=695508.0 km, observer=<HeliographicStonyhurst Coordinate (obstime=2017-08-01 00:00:00): (lon, lat, radius) in (deg, deg, AU)
    ( 0.,  5.78339799,  1.01496923)>): (Tx, Ty, distance) in (arcsec, arcsec, km)
    ( 769.74997696, -498.75932128,   1.51668819e+08)>

It is also possible to convert three dimensional positions to astrophysical frames defined in Astropy, for example ICRS.

>>> c.transform_to('icrs')
<SkyCoord (ICRS): (ra, dec, distance) in (deg, deg, km)
  ( 49.85053118,  0.05723938,  1417577.02975451)>

Observer Location

Both Helioprojective and Heliocentric frames are defined based on the position of the observer. Therefore to transform either of these frames to a different frame the location of the observer must be known. The default observer is the Earth. A different observer can be specified for a coordinate object using the observer argument to SkyCoord. For SunPy to calculate the location of the Earth, it must know the time for which the coordinate is valid; this is specified with the obstime argument.

Using the observer location it is possible to convert a coordinate as seen by one observer to a coordinate seen by another:

>>> hpc1 = SkyCoord(0*u.arcsec, 0*u.arcsec, observer="earth",
                    obstime="2017-07-26",
                    frame=frames.Helioprojective)

>>> hpc1.transform_to(frames.Helioprojective(observer="venus",
                                             obstime="2017-07-26"))
<SkyCoord (Helioprojective: obstime=2017-07-26 00:00:00, rsun=695508.0 km, observer=<HeliographicStonyhurst Coordinate (obstime=2017-07-26 00:00:00): (lon, lat, radius) in (deg, deg, AU)
  ( 77.03547231,  3.17032536,  0.72510629)>): (Tx, Ty, distance) in (arcsec, arcsec, km)
  (-1285.11970265,  106.17983302,   1.08317783e+08)>

Using Coordinates with SunPy Map

SunPy Map uses coordinates to specify locations on the image, and to plot overlays on plots of maps. When a Map is created, a coordinate frame is constructed from the header information. This can be accessed using .coordinate_frame:

>>> import sunpy.map
>>> from sunpy.data.sample import AIA_171_IMAGE

>>> m = sunpy.map.Map(AIA_171_IMAGE)
>>> m.coordinate_frame
<Helioprojective Frame (obstime=2011-06-07 06:33:02.770000, rsun=696000000.0 m, observer=<HeliographicStonyhurst Coordinate (obstime=None): (lon, lat, radius) in (deg, deg, m)
  ( 0.,  0.048591,   1.51846026e+11)>)>

This can be used when creating a SkyCoord object to set the coordinate system to that image:

>>> c = SkyCoord(100 * u.arcsec, 10*u.arcsec, frame=m.coordinate_frame)
<SkyCoord (Helioprojective: obstime=2011-06-07 06:33:02.770000, rsun=696000000.0 m, observer=<HeliographicStonyhurst Coordinate (obstime=None): (lon, lat, radius) in (deg, deg, m)
  ( 0.,  0.048591,   1.51846026e+11)>): (Tx, Ty) in arcsec
  ( 100.,  10.)>

This SkyCoord object could then be used to plot a point on top of the map:

>>> import matplotlib.pyplot as plt

>>> ax = plt.subplot(projection=m)
>>> m.plot()
>>> ax.plot_coord(c, 'o')

For more information on coordinates see SunPy coordinates section of the Code Reference.