Call for Comet Tail Images

C/1995 O1 (Hale-Bopp) above UCL's Mullard Space Science Laboratory in April 1997

C/1995 O1 (Hale-Bopp) above UCL's Mullard Space Science Laboratory in April 1997

We issue an open call for images of comets’ ion tails, including, but not restricted to C/2012 S1 (ISON), and C/2013 R1 (Lovejoy).

Details on how to submit images are at the bottom of this page.


Amateur images of comets’ ion tails can provide invaluable information on the conditions of the solar wind in the comets’ vicinity. These include:

  • The solar wind’s speed and direction
  • Degree of variability in solar wind conditions
  • The location of the heliospheric current sheet, which separates regions of opposite magnetic polarity in the solar wind
  • The presence of the interplanetary counterparts of coronal mass ejecta, CMEs

With this information, we can learn a great deal more about the way in which comets interact with the solar wind, how CMEs propagate through interplanetary space, and ultimately, about how the Sun’s behaviour changes over the solar activity cycle.

How can comet images provide this information?

Comets act like giant wind socks. They travel through the very tenuous solar wind that constantly flows outwards from the Sun at hundreds of km per second. Ions from comets join the solar wind flow, and we can observe the ions remotely as the ion or plasma tail. Changes in solar wind flow speed and direction cause changes in the appearance of the tail.

Can’t we use spacecraft to get this information?

In most cases, unfortunately not. Although spacecraft solar wind data is more precise and accurate than information that we obtain from comet images, comets provide a free source of information on solar wind conditions.

Spacecraft such as SOHO and STEREO also provide valuable comet images when possible, but these facilities can’t image comets far from their usual look directions.

What exactly are we asking for?

Images! Wide-field to narrow field, from all camera types from DSLRs to dedicated CCD imagers all are welcome. Please contribute as many as you can. If you took several images in one night, please send them all in, as ion tails are constantly changing. To extract the maximum amount of information, we need as many images as possible of these comets. When comets are not circumpolar, we need data from observers located at all longitudes to attempt to keep a continuous record of solar wind conditions.

What happens to the data?

Any images contributed will not be used for any commercial gain, but purely for scientific research. The contribution of all who donate data to this study will be acknowledged in subsequent scientific publications in peer-reviewed journals.

We do plan to construct movies of tail features using contributed images, and may post some such movies on our website, with a list of those who contributed data. If you do not wish your images in such movies to be placed on a webpage, then please do state this clearly and we will of course honour your request.

How are the comet images analyzed?

First, we identify the starfield in the background of your images, using astrometry.net

This provides us with the scale and orientation of your images, allowing us to compare your data to those obtained by other observers. Once this is done, we have an automatic estimate of the time when your image was obtained.

If you have provided information on your location and time at which the images were taken, we use these to constrain parallax effects for comets near Earth, and to aid the timing of the image in the rare case when the starfield is not recognized automatically.

We then use our specially-written IDL software to map your image onto the comet’s orbital plane, and the image is then analyzed semi-automatically. From each image, we use the positions at several points along the ion tail axis to infer solar wind speeds at the comet during a period of up to several hours before your image was taken. Sequences of images from single and multiple observers help us improve the accuracy of these results.

As the data collection builds up, we gather a valuable resource detailing the solar wind conditions at the comet. These results are then compared to simulations of the solar wind based on solar data, to test how well these computer models reproduce conditions in the inner solar system. Where possible, these solar wind speeds are compared to those obtained by spacecraft nearby.

Periods when major disturbances appear in the tail, such as kinks, clouds, and disconnection events, are analyzed in more detail, using different techniques. Features are traced from image to image, and, where possible, a map of speeds and directions of tail features is built up.

What image formats would we like?

In many cases, JPEG or PNG are fine. If you have raw format images, they can of course be very useful.

If you contribute a mosaic constructed from several images, please let us know so that we can take this into account when analyzing the data.

If you are providing images from a digital camera, please ensure that your time and date settings are correct so that EXIF data can be used when possible.

FITS files in some cases can be more difficult for us to process; please email us with questions about contributing files in this format.

How do I contribute data?

For most cases, simply email the images as attachments to cometobs@gmail.com

If you have contributions of ~100Mb or more, please contact us for instructions for data transfer.

If you’re simply contributing images in the email, place the name of the comet in the subject line. If you’re emailing us for any other reason, please note this in the subject, e.g. “Query”

Don’t forget to include this essential information in the email:

- Your name

- Your location, with country

- Date(s) of observation

- If possible, if not included in the image filename, please also provide time of observation. If you don’t have the exact time, please still contribute the image – it can still be very valuable! The image time provided should ideally correspond to the start of the exposure. Please provide an additional note if this is not the case.

- If possible, the length of the exposure.

If you have your own webpage or blog, please mention that too in the email.

Image filenames

If possible, please name your image files using this format:


Where the first part is the comet designation, yyyy=year of observation, mm=month, dd=day of month, then hours, minutes, seconds. If you don’t know the time very precisely, please don’t guess – leave mm_ss etc. as they are.

Replace yourname with your name; if you have a common surname, please use your first name too to avoid possible confusion

We thank everyone in advance. We shall attempt to reply to all contributors as soon as possible, but there may be delays in responding during busy times.

Any questions? Please email cometobs@gmail.com

Geraint Jones and Yudish Ramanjooloo

Mullard Space Science Laboratory

University College London

Page last modified on 03 dec 13 14:31