Sun Quakes and Solar Structure (by sarah)
I had another really engaging lecture tonight - one that had me salivating for more and which has probably resulted in me alienating myself from the rest of the MRes group.
The lecture was by Vincent Tong who it turns out has been at Imperial where I studied as an Undergraduate!
First off we had to think about what our goal was, what were we actually trying to acheive - to see inside the Earth of other body and see of what and how it is made. Then with lots of analogies which steadily become more rediculous as the lecture went on.
We then decided we couldn't break the Earth (or Sun) open like an egg to see its internal structure but had to view it as a presant and shake it to see whats inside. The shaking equals eathquakes and these we record and measure. We had a quick run through of wave types which was cool as I found I had forgotten one of them!
I ended up having to answer why s-waves which can't travel through liquid are improtant along with many other things - I don't know if they others were just being shy. I found I had remembered alot of stuff and to my suprise I am finding it far easier to infer things from information I am given. I did find the periods of time when we were working with each other to work stuff out frustrating as everyone else just kept saying they didn't understand (bar the guy at the front who kept asking really really techniqual questions about the software used).
We ran through the standard this is how you look at the inner earth but then looked at the fact that the velocity of the earthquake waves are dependent on direction and temperature and their interactions with faults and other subsurface structures. This was interesting as I discovered you can basically use the arrival times of the waves to work out the velcocity of the waves which is temperature dependent - meaning that you can tell if it has had to pass through realitively hot or cold areas. If you then have sequences of these you can see how they change - meaning you've added time and can see how things are moving or flowing - this is so useful I can not even begin to tell you.
The implications to my mind are stagering and I thought seismics where boring!
We ran through (quickly) how imaging of sections works - these are generally done by man made seismic waves (big thumbing lorries or air guns at sea). Now most of the siesmic sections I looked at during my undergraduate (if not all) were of reflected waves that were bouncing off of say bedding plans and the such like. But now there is this thing called Tomography which (I think) involves refracted waves - so these are waves that travel through the rock and are slowed down by different substances etc... sort of a minni version of how we use earthquake waves. I think it is a method stolen from medical imaging but I am not sure.
This again opens a whole new world and means you can see things that are not picked up by the reflective waves. There was somehting involwing time and depths and seeing flow rates again with this but I need to look that up some more.
We also looked at the restrictions off all of this - basically we could really do with a global network of siesmometers and though we are getting there - most of the planet is covered in water - I suggested that they could put the detectors along the cable trunks that carry say the internet - this apparently has already been done. Plus they have detectors that onces having detected an earthquake bobb back to the surface adn tell the sattalites what they heard - I personally wondered what sort of fail rate they have for such instramentation.
I guessed correctly that time would be involved and the problems of refernce and angle etc... which I was impressed with myself for. Also another problem is that the earthquakes on the earth are not actually evenly spread out - they are concentrated along the plate boundaries and the such like giving certain biases to what we are finding out. Then the earthquakes are intermittent and we have no idea what strength they will be etc... meaning that it is quite - if not not impossible - to correlate the data.
Oh - one of things was that the waves travel faster along faults that are parralel to the direction of motion and go slower through ones that are perpendicular. And this effect is sort of amplyfied if there are say lots of parrellel fualts such as along the mid ocean ridges. Measuring flow there is quiet important as there is hydrothermal perculation and stuff through the newely formed sea floor as it is hot and stuff - this affects mineral exchange and the such like. But during the working things out time he gave us I came up with the concept of using the earthquakes to tell if you where in a back arc basin, or near an accretionary prism, ie looking at the big big structures as they have different minerals and things in them (stolen straight from Wednesdays lecture). The girl I was sitting with said she had thought the same thing - the lecturer said we were coming up with good ideas.
Later on he said I was asking the right sort of questions too - though as the lecture ended up over running what with me getting excited over the way in which the earth or spheres in general can distort with motions and the like (yes I asked more questions) I think the rest of the class where about to kill me.
We got on to the heliosiesmics and wow! I think i actually got how they are detecting the sun-quakes. Now on the earth we have seismometers on the surface but the sun is a) humungous and b) too far a way and c) uber hot. But we have images in all sorts of wave lengths and the suns surface is in continous motion. The sun is a plasma which moves as a fluid and you can see the convection cells on the surface. You can take images and see how they change - think about it - surface that is further away from you such as that in a trough or valley (earthquakes are waves with peaks and troughs) will have a slight red shift to it as the wave gets stretched on route were as the peaks will be like mountains and nearer the earth or satallite depending were the detector is and be blue shifted compared to the base level surface colour (we are assuming constaint composition of the sun here.
This is cool as you can basically do the tomography tequnic and look at flow!
We also looked at solar flares and sunspots which has me a bit disturbed - is it me or does the behavour of sunspots becoming solar flares seem to suggest that the sunspots individually are magnetic poles? But that would make them a magnetic monopole which I thought could not exist?
Am I missing something here?
Anyway to my suprise I am actually now considering writing my second essay (its due in december) on helioseismics! I must be mad - but I found this really really interesting!
Mon 6th Oct 2008 11:40 pm 
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