Meteors after midnight

[jambouk]

Does anyone have a link to a good graphic or animation which demonstrates why meteors are better after midnight with the Earth spinning into the meteoroid stream? I can't find one and to be honest I'm struggling to get my head around it.

James

[IanL]

Check out this guide what I did write with my own fair hand, including the diagram you require:

https://www.northessexastro.co.uk/free-astronomy-resources/

 

[jambouk]

Thanks. I'm still not there. Sorry.

1. If the Earth was not orbiting the Sun, but still spinning on its axis, would the after midnight effect still happen? As in what is most important for this effect, Earth's orbit, or the Earths rotation on its axis?

2. Your illustration seems to suggest the Earth is catching up with slower meteoroids after midnight rather than the Earth then rotating into incoming meteoroids. This is a new concept to me. 

James

 

[LukeSkywatcher]

Earths rotation on its own axis is what matters. Most (if not all) meteor showers originate in the NW sky from our perspective. At midnight, the Earth is pretty much face on with meteors. We get the best views.

[IanL]

5 hours ago, jambouk said:

Thanks. I'm still not there. Sorry.

1. If the Earth was not orbiting the Sun, but still spinning on its axis, would the after midnight effect still happen? As in what is most important for this effect, Earth's orbit, or the Earths rotation on its axis?

2. Your illustration seems to suggest the Earth is catching up with slower meteoroids after midnight rather than the Earth then rotating into incoming meteoroids. This is a new concept to me. 

James

 

1. No. Meteors can come from any direction, e.g. directly opposite the direction of rotation on the Earth's axis (head on), in exactly the same direction (rear end) as it or in any direction between. Whilst there will be some significant bias towards motion in the plane of Earth's rotation (which roughly aligns with the ecliptic, i.e. the orbits of the majority of large solar system bodies), the overall effect of the atmosphere rotating towards incoming meteors ("head on collision") vs. away from them ("rear end collision") will net off when averaged across many meteors assuming a random distribution of approach angles.

The Earth is spinning on its axis at about 1,000 mph at the surface (speed at the top of the atmosphere is similar but harder to predict as it is not part of the solid body). Given the typical speed of meteors (relative to the Earth) varies between 25,000 mph and 160,000 mph. So the maximum ("head on") addition to relative speed due to rotation is going to be between +0.625% to +4% of those figures.

2. Yes. The Earth travels at an average of 70,000 mph on it's orbit. Obviously that is an average, fastest at perihelion and slowest at apehelion. The maximum addition to speed is going to be somewhere between +43.75% and +280% of the figures above.

So you can see effect 1 is negligible compared to effect 2.

* The percentages are terrible guesstimates, as actually we should be finding the average speeds of metors not including effects 1. and 2. and calculating the percentages from there, but it's late and this illustrates the point that the biggest factor by a large margin is the Earth's orbital velocity.

** The predominant directions of meteors will depend on a lot of factors, there is not an even distribution across all possible approach angles. Again it's not hugely material.

Just to make the point, using my radar meteor detector, the diurnal rate variation of sporadic meteors over two weeks is easily detectable:

 

[jambouk]

I need to think about this more (and I need to go to work).

I just noted down the altitude of the Perseid radiant from Stellarium over a 24 hour period, in degrees, and re-plotted your 24 hour data from the last four weeks (when the Perseids have been most active), and there is a striking correlation. Therefore, I wonder if pure numbers of recorded (or observed) events may be more closely linked with height of the radiant. [between 19:00 and 03:00 when the correlation is less marked, I wonder if these could be another annual shower, like the kappa-Cygnids where its radiant is highest between 19:00 and 02:00...]

Do you have a diurnal plot of meteor events over a whole year?

Thanks, I am getting there I am sure...

James

[jambouk]

I've been thinking about it all some more. Some more of my musings...

As all the meteors of a given annual shower, say the Perseids, appear from a radiant, at any point in time, the radiant is either facing the direction of travel of the Earth around the Sun, or facing away from the direction of travel. Whichever way the radiant is facing, the Earth is still travelling at 70,000 mph around the Sun. Now in order for the meteors to only appear from the radiant suggests the stream of particles interacting with the atmosphere is relative confined, else Perseids would be emerging from any point in the sky, not just from the radiant. The radiant thus marks a distance, static, point in distant solar system from where the tube of particles are emerging. So in any 24 hour period, the Earth will only be travelling towards that point, at right angles to that point, or away from that point, it can’t do more than one of these options. The only difference will be the relative difference the rotation of the Earth introduces, where by in effect if the Earth is travelling into the stream of particles then its forward “speed” is 70,000 mph plus 1000 mph for rotation, and the side rotating away from the stream will be 70,000 mph minus 1000 mph.

So in effect, this goes back to the only difference after midnight is the speed of rotation of the Earth (and altitude of the radiant)....

I really wish there was an animation to help me get my head about this.

James

[jambouk]

I bribed a student at work, with a cornish pasty, to help me extract the altitude data for the commoner annual showers. All except the Draconids, are highest after midnight. I am sure the height of the radiant is the bigger player in the "meteors are better after midnight" and "in the pre-dawn sky" phenomenon....

[michaelmorris]

There two main components to the velocity at which a meteor travels towards the Earth's surface.

1. The velocity of the meteroid through space
2. The velocity of the Earth's surface through space

This latter can be split into two main sub-components

• The velocity of the Earth around the Sun
• The velocity of the Earth's surface as it spins on it's axis

This second sub-component is a negative figure between midday and midnight and a positive component between midnight and midday.  However, because the meteor showers are caused by the Earth ploughing into a stream of dust particles along our orbital track, the effect is only seen on the side of the Earth facing in the direction of travel of our orbit.  This means that the increased closing velocity caused by our spin is only seen between midnight and 6:00am.

 

[jambouk]

Michael,

Thanks.

If the Earth is ploughing face on into the meteor stream, then surely midnight to 6am is rotating into the stream, and 6am to midday is rotating away, as I've tried to indicate in the awful diagram.

If the meteors in an annual shower are only encountered face on, with the increased closing velocity, when would we observe meteors with the Earth rotating away from the stream other than in daylight? This is the thing I think I'm struggling with.

James

[IanL]

13 hours ago, jambouk said:

I need to think about this more (and I need to go to work).

I just noted down the altitude of the Perseid radiant from Stellarium over a 24 hour period, in degrees, and re-plotted your 24 hour data from the last four weeks (when the Perseids have been most active), and there is a striking correlation. Therefore, I wonder if pure numbers of recorded (or observed) events may be more closely linked with height of the radiant. [between 19:00 and 03:00 when the correlation is less marked, I wonder if these could be another annual shower, like the kappa-Cygnids where its radiant is highest between 19:00 and 02:00...]

Do you have a diurnal plot of meteor events over a whole year?

Thanks, I am getting there I am sure...

James

To be clear, this is a radar detector plot of a peroid months ago outside a major shower period. So it is purely sporadics and thus radiants / visual observabilty  (related to height of radiant) don't enter in to it. I don't have useful data for a longer period yet as scripts need a rewrite to get useful logs.

Bottom line is that you are overthinking this. The forward orbital velocity far outweighs any rotational effect as explained above. For a shower the direction and altitude if the radiant have an effect for visual observers,  since you will observe more and fainter meteors when favourable, but that doesnt affect the absolute number that hit the foward and backward facing sides of the Earth.

[jambouk]

I think I've got my head around it now. Neil Bone's book has helped. I'll type up my clarified thoughts at the weekend. I think the fundamental issue is the that I think the "better after midnight" saying applies principally to sporadics, and not to meteors as part of an annual shower (unless of course you also factor in height of the radiant).

Very interesting. Thanks for your contributions.

James

 

[lenscap]

Imagine there was a shower radiating from Polaris.

This would add the same number of meteoroids on average to the before midnight observer as to the after midnight one.

The sporadics would be unchanged so the relative advantage of the after-midnight would be reduced.

For a head-on radiant there would be a big addition to after-midnight sightings but the before-M crew would only see a few extra grazers. The rest would have been hoovered up by the Earth.

This would increase the relative advantage of the after-M observers.

BTW I believe rotation makes no difference to meteoroid numbers.

Rotation causes a slight variation in the relative velocity between the observer and the incoming shower but this is irrelevant.

It is the relative velocity between the Earth & the shower that controls frequency & this is unchanged by rotation.

If there was a polar shower they would be called the Polaroids wouldn't they? Sorry. Couldn't resist. ?

 

 

t

[IanL]

2 minutes ago, lenscap said:

Rotation causes a slight variation in the relative velocity between the observer and the incoming shower but this is irrelevant.

The net velocity (I suppose you'd call it "indicated air speed" if it was a plane) is affected by the spin of the atmosphere by c. +/- 1,000mph depending on the direction and angle of the meteor. This in turn would affect the total energy released in the meteor's trail, thus brightness and thus in some cases whether the meteor exceeds the limiting magnitude for visual observation. I imagine the net effect on observed hourly rates is fairly marginal but it can't be zero?

[lenscap]

Yes , for visual, but by the same argument  daylight detections may marginally reduce because of the reduced airspeed towards midday.

( I assume radar detects some ionisation threshhold - is that correct?)

And the 1000mph is only near the equator. Margins as you say.

[IanL]

3 minutes ago, lenscap said:

( I assume radar detects some ionisation threshhold - is that correct?)

Radar detection is complex to say the least, still trying to get my head around it. There are two elements.

Firstly the short-lived head echo (which is the reflection from the plasma immediately surrounding the fast moving meteor). This usually persists for no more than half a second and often shows a marked Doppler frequency shift due changes in radial velocity relative to the detector, i.e. a line of sight effect, plus a small component due to deceleration of the meteor head. The detectability will depend the energy of the meteor (size, net entry speed), and geometry of the meteor's track and the transmitting and receiving stations. I'm currently ploughing through some academic papers. From what I have read so far, you can derive a range of estimates for either the speed of the meteor (if you make assumptions about the distance to the point of closest approach from the detector), or the distance to the point of closest approach if you make assumptions about the speed (possible for shower-related meteors where there is data available for mean speeds).

Secondly the ionised meteor trail. This may be very short lived or non existent in the 'under-dense' regime, where the ionised particles recombine rapidly, or may be long-lived (seconds to tens of seconds) in the 'over-dense' regime. The latter seems more likely for larger meteors, but it isn't a cut-and-dried relationship to size or speed as far as I can read up. The trail is usually complex because there are diffraction effects in play - constructive and destructive interference from stronger reflections at different points along the trail's length. Also using the Graves space radar it is even more complex, as it is a scanning phased array with four separate beams each of which scans in steps through an eighth of the Southern sky. So you end up with wildly varying echos depending on all of the above, with the speed of the meteor being only one component amongst several affecting detectability.

[lenscap]

Thanks for the detailed response. Hopefully there will be a few Perseid stragglers tonight.

But preferably before midnight; I'm getting too old for an all-nighter.

 

[jambouk]

On 16/08/2018 at 15:09, lenscap said:

1. The sporadics would be unchanged so the relative advantage of the after-midnight would be reduced.

1

 

1. There are more sporadics after midnight, as the Earth is ploughing forwards at 70,000 mph and there is an unobstructed view forwards allowing collisions. Before midnight sporadics can only really strike the atmosphere by catching up with the Earth from behind which is travelling away form them at 70,000 mph so only the really fast ones interact. 

From: https://link.springer.com/chapter/10.1007/978-0-387-09461-8_2

 

 

[lenscap]

2 hours ago, jambouk said:

There are more sporadics after midnight, as the Earth is ploughing forwards at 70,000 mph and there is an unobstructed view forwards allowing collisions. Before midnight sporadics can only really strike the atmosphere by catching up with the Earth from behind which is travelling away form them at 70,000 mph so only the really fast ones interact.

I agree.

Lets say the after-mids are seeing 12 per hour & the before-mids are 4, all sporadics. Relative advantage to after mids 12/4=3

Say a polar shower adds 4 to each, ratio is now 16/8=2 so both will see more meteors but the relative advantage of the after-mids has reduced.