Generating ions VDF's using mms_part_slice2d.

[tmw233]

Hi, I have a question regarding the mms_part_slice2d function.

I want to rotate the ion VDF into the frame BE described below. When I do this, the flux from the core population of ions (from the magnetosheath) appears to be removed.

    rotation: str
        Aligns the data relative to the magnetic field and/or bulk velocity.
        This is applied after the CUSTOM_ROTATION. (BV and BE are invariant
        between coordinate systems); valid options:

            'BV':  The x axis is parallel to B field; the bulk velocity defines the x-y plane
            'BE':  The x axis is parallel to B field; the B x V(bulk) vector defines the x-y plane

I can't see anywhere in the docs where it describes that this is happening. I wanted to ask if I'm correct that the core population has been removed, or am I misunderstanding something?

I am trying to attach an example slice in the BV and BE frames below.

Thanks

Tom

[jameswilburlewis]

That's interesting! To my knowledge, there aren't any automatically applied population subtractions for the ion data types.

Could you please send the code & time range you used to generate the above plots, so I can see what options you were using, and try reproducing the issue?

[tmw233]

Hi, thanks for your reply.

Here is the code that I used:

#Create a single slice
from pyspedas import mms_part_slice2d
#Time of the slice
time_start = '2019-10-15/20:15:04'

mms_part_slice2d(
probe='1',
interpolation='2d',
time=time_start,
instrument='fpi',
species='i',
rotation='bv',
data_rate='brst',
energy=False,
xrange=[-900, 900],
yrange=[-900, 900],
zrange=[1e-25, 1e-21],
)

and

mms_part_slice2d(
probe='1',
interpolation='2d',
time=time_start,
instrument='fpi',
species='i',
rotation='be',
data_rate='brst',
energy=False,
xrange=[-900, 900],
yrange=[-900, 900],
zrange=[1e-25, 1e-21],
)

I have been playing with the zdirrange option and the theta range. I'm aware that we are generating a 2D slice of a 3D population, so I'm wondering if, when we move our frame into BE, we might not be sampling much of the core population in the +/-Z direction at near X=0 Y=0.

When I try to interpolate over [-90, +90] degrees, it does improve, but still seems to appear to be 'losing' overall phase space density.

I am now thinking that it could be more to do with how the 2D interpolation is done.

Thanks

Tom