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Diagram of comet C/2014 Q2 (Lovejoy) for May 1, 2015 at 21:00 UT. This is based on image posted by Fritz Helmut Hemmerich on Facebook
In this diagram North is up and east is left. Position angles are measured from north counterclockwise.
The comet displays a radial dust fan tail, an ion tail, and large coma with light reflected by dust plus gas emission from CN and C2 Swan bands which give the coma the characteristic blue green glow. The ion tail points away from the sun and this fact makes it simple to rotate a comet image to align with calculated position angle of extended sun-comet radius vector or the parameter PsAng in JPL Horizons ephemeris. The dust fan is short in length because of the small plane angle between earth and the comet plane (22 degrees). and small sun-comet-earth phase angle (29 degrees). If the earth was directly above or below the comet, we would be able to see the full extent of its dust and ion tails. Instead we see an approximate sin theta projection. The plane angle will continue to decrease and reach zero on June 27, 2015 when the earth passes through the comet orbit plane. At that time, the comet's tails and trail if it can be seen will be edge on, the dust tail will be thicker than a trail. The tail/trail could wrap around and appear to be emating from the comet head as an apparent anti tail, depending on the viewing geometry. A trail consists of larger dust particles which are affected the least by solar radiation pressure. The dust tail shows that there is a wide range of dust particle sizes. The affect of solar radiation pressure on dust particles is inversely proportional to particle radius. That may seem counter intuitive since small particles have smaller cross sectional area. In the fan, the largest particles lie closest to the position angle of the negative velocity vector or parameter PsAMV. The smaller particles curve away from the anti solar direction in the sky. This is difficult to understand and may seem counter intuitive when the position angles of the -v vector (PsAMV) and anti solar radius vector (PsAng) are so far apart. A Finson-Probstein analyis or model could be fit to comet images to show these characteristics.
The green glow from C2 emission has been diminishing and is expected to fade when the comet recedes from the sun beyond 2 AU. This comet was 1.858 AU from the sun on date/time of observation image. CN emission will decrease and is not expected beyond 3 AU. In SOHO/Swan images, the comet's hydrogen coma has decreased in size but still visible. SWAN images capture Lyman-alpha hydrogen emission in the far ultraviolet at 121.6 nm wavelength. That is emission from the first excited state of hydrogen atoms predicted by Quantum Mechanics over a 100 years ago and very well understood in physics today. You have to have a spacecraft above earth atmosphere to see that deep into far ultraviolet spectrum. The SOHO spacecraft is in a halo orbit about the earth-sun L1 Lagrangian point.
In this diagram North is up and east is left. Position angles are measured from north counterclockwise.
The comet displays a radial dust fan tail, an ion tail, and large coma with light reflected by dust plus gas emission from CN and C2 Swan bands which give the coma the characteristic blue green glow. The ion tail points away from the sun and this fact makes it simple to rotate a comet image to align with calculated position angle of extended sun-comet radius vector or the parameter PsAng in JPL Horizons ephemeris. The dust fan is short in length because of the small plane angle between earth and the comet plane (22 degrees). and small sun-comet-earth phase angle (29 degrees). If the earth was directly above or below the comet, we would be able to see the full extent of its dust and ion tails. Instead we see an approximate sin theta projection. The plane angle will continue to decrease and reach zero on June 27, 2015 when the earth passes through the comet orbit plane. At that time, the comet's tails and trail if it can be seen will be edge on, the dust tail will be thicker than a trail. The tail/trail could wrap around and appear to be emating from the comet head as an apparent anti tail, depending on the viewing geometry. A trail consists of larger dust particles which are affected the least by solar radiation pressure. The dust tail shows that there is a wide range of dust particle sizes. The affect of solar radiation pressure on dust particles is inversely proportional to particle radius. That may seem counter intuitive since small particles have smaller cross sectional area. In the fan, the largest particles lie closest to the position angle of the negative velocity vector or parameter PsAMV. The smaller particles curve away from the anti solar direction in the sky. This is difficult to understand and may seem counter intuitive when the position angles of the -v vector (PsAMV) and anti solar radius vector (PsAng) are so far apart. A Finson-Probstein analyis or model could be fit to comet images to show these characteristics.
The green glow from C2 emission has been diminishing and is expected to fade when the comet recedes from the sun beyond 2 AU. This comet was 1.858 AU from the sun on date/time of observation image. CN emission will decrease and is not expected beyond 3 AU. In SOHO/Swan images, the comet's hydrogen coma has decreased in size but still visible. SWAN images capture Lyman-alpha hydrogen emission in the far ultraviolet at 121.6 nm wavelength. That is emission from the first excited state of hydrogen atoms predicted by Quantum Mechanics over a 100 years ago and very well understood in physics today. You have to have a spacecraft above earth atmosphere to see that deep into far ultraviolet spectrum. The SOHO spacecraft is in a halo orbit about the earth-sun L1 Lagrangian point.
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