Post by tmcewan on Jun 5, 2008 14:29:26 GMT
Hi,
I'm sure most of you have already looked at the online data available from the AIM spacecraft (lasp.colorado.edu/aim/cips/data/index.php). After examining the 'daisy' images from May 2007 and May 2008 I started to wonder if the visual reports made during May were genuine; the AIM data certainly suggests that little, if any, NLC activity has occurred during May 07 & 08. So, I have spent the last week or so looking at the online NLC records from May (www.nlcnet.co.uk/archive.htm) and the photographs submitted over 2006, 2007 and 2008.
In my opinion, very few of the photographs look like NLC and most have, unfortunately, been captured in poor observing conditions of haze and/or low cloud. Admittedly, observers are seeing something and I know there are dangers in examining photographic records like this, but, weighing up the available evidence I don’t think this is NLC. Before I go any further, please note that I am not criticising or singling out any observers here - the reports I have identified with potential problems have come from different observers and countries.
There are other worrying facts. May reports tend to describe vague features (veils with possible diffuse bands is the typical appearance) and I can find no photographic record showing sharp band, billow or whirl structure – the classic NLC features. May sightings also tend to be isolated (with the single exception of the 2006, May 30-31 event) or confined to local regions where similar meteorological conditions will prevail . There is a notable lack of confirmation from observers with clear skies. In some cases negative reports clash with positives.
I am now of the opinion that the majority of May NLC reports are the result of misidentifying low, semi-transparent streaks of haze and/or cloud, draped over the twilight arch. It is also possible that the brightest part of the twilight arch is sometimes mistaken as an NLC veil. I have seen such phenomena many times myself and know how convincing they can be.
Clearly, if erroneous reports are being made, we have to do something about it and I’d now like to see an open discussion about it here on the Forum. I’d like to hear from observers who have experienced this problem and how they have dealt with it. I'd also like to see photographs illustrating the haze/cloud NLC illusion. My hope is that this will then lead to solutions and more accurate, reliable observing and reporting from us all.
To start things off, I’d like to propose that in addition to standard observing procedures the following checks are made by observers throughout the season when they are faced with faint and diffuse ‘NLC’:
1) Check carefully with binoculars. This is often the only check necessary as true NLC bears magnification.
2) Check the western and eastern ends of the twilight arch for evidence of haze and/or thin streaks of cloud (in some circumstances I find it easier to spot haze/cloud against a slightly darker background sky).
3) If possible, take a sequence of digital photographs at, say, 30 second intervals, or, make a video record. NLC should then be easy to spot when animations are run. Please don’t be tempted to tweak digital images.
4) If still unsure observers should simply mark reports as ‘Suspect – requiring confirmation’.
5) Submit all negative observations. Ken Kennedy at the BAA Aurora Section is actively encouraging this and suggests that negative observations of as little as 10 minutes duration are useful.
I look forward to follow-up postings...
Regards,
Tom
I'm sure most of you have already looked at the online data available from the AIM spacecraft (lasp.colorado.edu/aim/cips/data/index.php). After examining the 'daisy' images from May 2007 and May 2008 I started to wonder if the visual reports made during May were genuine; the AIM data certainly suggests that little, if any, NLC activity has occurred during May 07 & 08. So, I have spent the last week or so looking at the online NLC records from May (www.nlcnet.co.uk/archive.htm) and the photographs submitted over 2006, 2007 and 2008.
In my opinion, very few of the photographs look like NLC and most have, unfortunately, been captured in poor observing conditions of haze and/or low cloud. Admittedly, observers are seeing something and I know there are dangers in examining photographic records like this, but, weighing up the available evidence I don’t think this is NLC. Before I go any further, please note that I am not criticising or singling out any observers here - the reports I have identified with potential problems have come from different observers and countries.
There are other worrying facts. May reports tend to describe vague features (veils with possible diffuse bands is the typical appearance) and I can find no photographic record showing sharp band, billow or whirl structure – the classic NLC features. May sightings also tend to be isolated (with the single exception of the 2006, May 30-31 event) or confined to local regions where similar meteorological conditions will prevail . There is a notable lack of confirmation from observers with clear skies. In some cases negative reports clash with positives.
I am now of the opinion that the majority of May NLC reports are the result of misidentifying low, semi-transparent streaks of haze and/or cloud, draped over the twilight arch. It is also possible that the brightest part of the twilight arch is sometimes mistaken as an NLC veil. I have seen such phenomena many times myself and know how convincing they can be.
Clearly, if erroneous reports are being made, we have to do something about it and I’d now like to see an open discussion about it here on the Forum. I’d like to hear from observers who have experienced this problem and how they have dealt with it. I'd also like to see photographs illustrating the haze/cloud NLC illusion. My hope is that this will then lead to solutions and more accurate, reliable observing and reporting from us all.
To start things off, I’d like to propose that in addition to standard observing procedures the following checks are made by observers throughout the season when they are faced with faint and diffuse ‘NLC’:
1) Check carefully with binoculars. This is often the only check necessary as true NLC bears magnification.
2) Check the western and eastern ends of the twilight arch for evidence of haze and/or thin streaks of cloud (in some circumstances I find it easier to spot haze/cloud against a slightly darker background sky).
3) If possible, take a sequence of digital photographs at, say, 30 second intervals, or, make a video record. NLC should then be easy to spot when animations are run. Please don’t be tempted to tweak digital images.
4) If still unsure observers should simply mark reports as ‘Suspect – requiring confirmation’.
5) Submit all negative observations. Ken Kennedy at the BAA Aurora Section is actively encouraging this and suggests that negative observations of as little as 10 minutes duration are useful.
I look forward to follow-up postings...
Regards,
Tom
