by alter-ego » Thu Dec 12, 2019 2:51 am
sallyseaver wrote: ↑Thu Dec 05, 2019 5:31 am
I'm wondering how to interpret the image.
In this APOD image, there are lines, also, the image includes color information: hue, saturation and brightness.
SOFIA is reading
polarity polarization of the far infrared radiation. Do the lines indicate the transverse direction of the radiation? Is the transverse direction orthogonal to the magnetic field? What does the color info tell us?
The lines directly map the magnetic field lines, and since the long axis of the dust grains are perpendicular to these lines then the FIR polarization is also perpendicular to the field lines. The heated dust grains preferentially emit light polarized along the long axis of the dust grain. It's more common to reference the polarization direction instead "transverse direction of the radiation." Therefore it's more clear to say the IR polarization is orthogonal the magnetic field lines. The color represents temperature: 10K (red) → 100K (blue)
What in this image corresponds to one polarity polarization versus another and how does this help us to understand the magnetic field depicted by the image.
Within a small region, it should be clear that the measured FIR polarization direction reveals the orthogonal field line direction. Subsequently, an array of this polarization-direction data will yield a family of mostly continuous magnetic field lines. The projection of a 3-D region of space to a 2-D image containing integrated line-of-sight FIR polarizations may limit magnetic field interpretation.
Edit: Although the simple rotational energy minimum for an elongated particle is for rotation about smaller axis, the reality is complicated by radiation-induced torques/forces, magnetic field strength, and particle properties (e.g. size, aspect ratio and if it's paramagnetic). The APOD states the grains are "rotating in alignment with the local magnetic field". That could mean the long axis is alignment with the field lines. In any case, the discussion still holds except that, instead of orthogonal, the field lines are parallel (or some other predicted angle) to the polarization direction. The case for parallel alignment is possible.
Interstellar Grain Alignment
Dust Grain Alignment in the Interstellar Medium[quote=sallyseaver post_id=297689 time=1575523883 user_id=142730]
I'm wondering how to interpret the image.
In this APOD image, there are lines, also, the image includes color information: hue, saturation and brightness.
SOFIA is reading [s]polarity[/s] [color=#0000FF]polarization[/color] of the far infrared radiation. Do the lines indicate the transverse direction of the radiation? Is the transverse direction orthogonal to the magnetic field? What does the color info tell us?[/quote]
The lines directly map the magnetic field lines, and since the long axis of the dust grains are perpendicular to these lines then the FIR polarization is also perpendicular to the field lines. The heated dust grains preferentially emit light polarized along the long axis of the dust grain. It's more common to reference the polarization direction instead "transverse direction of the radiation." Therefore it's more clear to say the IR polarization is orthogonal the magnetic field lines. The color represents temperature: 10K (red) → 100K (blue)
[quote]What in this image corresponds to one [s]polarity[/s] [color=#0000FF]polarization[/color] versus another and how does this help us to understand the magnetic field depicted by the image.
[/quote]
Within a small region, it should be clear that the measured FIR polarization direction reveals the orthogonal field line direction. Subsequently, an array of this polarization-direction data will yield a family of mostly continuous magnetic field lines. The projection of a 3-D region of space to a 2-D image containing integrated line-of-sight FIR polarizations may limit magnetic field interpretation.
[i]Edit:[/i] Although the simple rotational energy minimum for an elongated particle is for rotation about smaller axis, the reality is complicated by radiation-induced torques/forces, magnetic field strength, and particle properties (e.g. size, aspect ratio and if it's paramagnetic). The APOD states the grains are "rotating in alignment with the local magnetic field". That could mean the long axis is alignment with the field lines. In any case, the discussion still holds except that, instead of orthogonal, the field lines are parallel (or some other predicted angle) to the polarization direction. The case for parallel alignment is possible.
[url=https://www.sofia.usra.edu/sites/default/files/Other/Documents/04-11-18_Andersson.pdf]Interstellar Grain Alignment[/url]
[url=http://bgandersson.net/grain-alignment]Dust Grain Alignment in the Interstellar Medium[/url]