Author: Megan Hammond

ESA’s magnetic field mission Swarm
Image: ESA

Today’s (7/7/2014) Metro newspaper has a nice article in the Metrocosm section by Ben Gilliland entitled “Portends of magnetogeddon” . Apart from the slight alarmism, it’s (as usual with Ben) pretty good, and I’m pleased to see him talk up the magnetic field even more than I usually do! It seems to be driven by the current ESA satellite mission Swarm, which is measuring the near-Earth magnetic field with the best ever satellite instruments on three separate spacecraft, with a view to separating magnetic sources external to the Earth (like the sun, auroral currents, and the magnetosphere) and those internal (from the core, and from magnetised material at Earth’s surface).  However, much of the article is focussed on more general information, such as magnetic reversals (well known to readers of this blog!) and the possible effects of weakening of the field. That the field is dropping in magnitude is not a new result – we’ve known this for a century or so, and now have evidence that this has been happening for at least 400 years (and probably longer) but it gets a lot of publicity every time we have new measurements – I remember something similar happening when the Ørsted mission was launched in 1999. There was a nice documentary on this over 10 years ago, joint between WGBH Nova for PBS in America, and Channel 4 in the UK called “Magnetic Storm” (“Magnetic flip” in the UK) from which most of the information is still accurate. If you are interested, you can find it online. The issue is that the magnetic field provides us with a shield from harmful solar and cosmic radiation, and so if it weakens (and potentially reverses), this shield would be weakened. However, the answer to the initial question is

Don’t Panic!

(Readers of a certain age may recognise the source of both this response and the slight misquote of the title…..) The answer from the documentary is that the field will weaken, but not completely disappear, it isn’t going to happen for the next 1000 years or so if it does then, and even if it does, the protective measures required will be not to lie on the beach in Florida, and to wear a large floppy hat!

Whether the field is reversing is not known, and (to an extent) not knowable. We have models made from observational data and palaeomagnetic measurements (some created here at Liverpool) which show that in the last 10,000 years, the magnetic field has both decayed more rapidly than it is at present, and been substantially weaker than it is now, but then recovered. Therefore, the current fall in the magnetic field could be simply part of its normal variation. Things are a little more complicated, as there is a strong and growing feature of the “wrong” polarity at the surface of the Earth’s core under the South Atlantic, giving rise to the weaker area of field that we see as the “South Atlantic Anomaly” – which causes occasional havoc in the instruments of satellites flying overhead. This feature could be a signature of a major change in the field, but it could also simply reflect rearrangement of magnetic field at the core – if this becomes more complicated, it could look weaker from the surface without the field itself actually getting any weaker.

What Swarm is really for is getting a much more detailed picture of small-scale structures in the magnetic field, and its rapid changes on time scales of years – so magnetic “weather”, rather than the “climate” of a field reversal. Hopefully, this will give us a much better idea of these smaller variations – my interest is particularly in comparing these variations with changes in Earth rotation.  If this work is successful, we may yet also have something to say about the longer term decline in the field, although I can guarantee that this won’t affect anyone for many lifetimes to come!

Richard Holme

Today’s post was guest written by Richard Holme.   Richard Holme is Professor of Geomagnetism at the University of Liverpool. His research interests are in the behaviour of the magnetic fields of the Earth and other planets on timescales of milliseconds to billions of years, and variations in Earth rotation and their link to Earth’s core. He is involved with the ESA geomagnetic mission Swarm, is looking forward to July 2016 for Juno to arrive at Jupiter, 2017 for Cassini to begin close orbits of Saturn, and 2033 (?!) for the arrival of Juice at Ganymede.

Philippe and Valerie taking archaeomagnetic samples from a Roman aged achaeological site in France

For regular readers of this blog, the term “archaeomagnetism” will have been seen a number of times, most frequently in posts by me or Andy Herries!

Archaeomagnetism is the study of burnt material found on archaeological sites. This can include everything from hearths, fireplaces and kilns through to tiles, bricks and pottery. Basically anything that has been subjected to heat at some point, either deliberately (e.g. the firing of a pot) or accidently (e.g. if a fire burns down a building, its foundations and walls become suitable for archaeomagnetic study as a consequence).

In certain parts of the world (for specific time periods), it is possible to date archaeological samples by comparing the declination, inclination and intensity values recorded in the archaeomagnetic samples (these 3 values describe the geomagnetic field vector) with the known changes in the geomagnetic field. One of the great things about archaeomagnetism (in my opinion) is the variety of ways in which you can use it. Associate Professor at Liverpool, Andy Herries, focuses on dating Hominid sites in South Africa by dating speolotherms (speolotherms are also known as flow stones and are created through the deposition of carbonate through time. Stalactites and stalagmites form in the same way.) see http://www.ncbi.nlm.nih.gov/pubmed/21392817.   It is worth noting that in speoltherms the record of the magnetic field is preserved as a chemical remanant magnetisation rather than a thermal remament magnetisation.

I myself am focused on trying to gather well-dated archeointensity data from Turkey for the dual purpose of enabling archaeomagnetic dating in the future as well as allowing us to accurately reconstruct changes in the geomagnetic field. This work is very interesting because there have been a number of recent papers linking sudden changes in the magnetic field with climate change and civilisation collapse e.g. http://www.sciencedirect.com/science/article/pii/S0012821X06002792.  I’m investigating whether I can see any evidence for this.

In this post I have only briefly mentioned some of the applications of archaeomagnetism: it is a fascinating field with new methods and ideas being tested every day.

The International Association of Geomagnetism and Aeronomy meets every 4 years in different international locations.  The most recent of these was in Merida, Mexico and 4 members of the Liverpool geomagnetism laboratory went along to present their most recent findings, hear about current developments in the field and catch up with other scientists interested in palaeomagnetism and geomagnetism based around the world.   It was a fascinating experience for me as it was my first international conference as well as being the biggest I have attended so far!  There were between 500 and 550 delegates at the meeting including Neil Sutie, Andreas Nilsson, John Shaw and myself from the University of Liverpool.  We tended to attend the same sessions and heard a range of talks from environmental magnetic proxies to magnetic anisotropy to tectonic plate reconstructions to archaeomagnetism.

The most interesting talk for me personally was given by Elina Aidona from the Aristole University of Thessaloniki, Greece who had conducted a study on archaeological ovens and found that the errors on the archaeomagnetically assigned age was lower than the errors on the thermoluminescence dates.  This is highly unusual and was a very interesting case study.

Andreas and I visiting the local Mayan site of Uxmal

In addition to attending the most relevant sessions it was nice to hear about how others view and study the magnetic field.  I particularly enjoyed a talk by Ciaran Beggan of the British Geological Survey detailing the potential threat posed to the national grid from magnetic storms.  It was comforting to hear that as the UK’s grid is highly interconnected, in the event of a magnetic storm, the excess electricity generated by the interaction of the Sun’s magnetic field with the Earth’s magnetic field and transported along the grid, should dissipate relatively quickly.  For more detail about this see here: http://nora.nerc.ac.uk/502627/

In the most recent Magnetic Personalities meeting we looked at a paper published in Quaternary Science Reviews in 2005 entitled “Geomagnetic Field intensity during the last 60,000 years based on ¹⁰Be and ³⁶Cl from the Summit ice cores and ¹⁴C” by the authors Raimund Muscheler et al. I chose this paper as I thought it would be interesting to discuss a paper that approaches the problem of geomagnetic field reconstruction in a very different way.

The authors reconstructed past changes in the geomagnetic field intensity based on ¹⁰Be, ³⁶Cl and ¹⁴C records under the assumption that radionuclide records are mainly influenced by changes in their production rates.  Most reconstructions of the geomagnetic field intensity are based on the natural remanent magnetisation acquired by ceramics, volcanic rocks or lacustrine and marine sediments.   

Cosmogenic radionuclides (e.g. ¹⁰Be, ³⁶Cl and ¹⁴C) are produced in the Earth’s atmosphere by the interaction of galactic cosmic rays with the atoms of the atmosphere.  The connection between cosmogenic radionuclide production and the Earth’s magnetic field is as follows: the higher the geomagnetic field intensity, the stronger the deflection of the primary cosmic ray particles that consist mainly of protons and α-particles.  As a consequence, less cosmogenic radionuclides are produced during periods of high geomagnetic field intensity and more are produced in low geomagnetic field intensities.  There are a number of other factors that could influence cosmogenic radionuclide which the authors detail in the paper.

By combining the data from the GRIP and GISP2 ice cores from Central Greenland, it is possible to obtain a continuous record of the deposition of cosmogenic radionuclides in Greenland for the last 60,000 years.  As a group we discussed the results and generally were more convinced by the trends seen in the ¹⁰Be results than the other radionuclides.  We also discussed how well the Laschamp geomagnetic excursion showed up in the data, an event which was first reported in 1969 by Bonhommet and Zähringer.

All in all, it was a very interesting paper to study and its always good to sit in the sunshine and discuss a paper that tackles a problem in an interesting way!

In August/ September last year I was privileged to be invited to visit a French archaeological site in the midi Pyrenees in the company of Professor Philippe Lanos, director of archaeomagnetism research at the University of Rennes 1.  The archaeological site was a Roman age staging post where travellers could rest their horses and enjoy a bath.  As the bath area was both hot and generally made from fired material (like tiles) we heavily sampled the bath area.

 

Valerie and I taking samples. On the left I am using a magnetic compass to measure the orientation of the sample with respect to magnetic North. On the right, Valerie is using a sun compass to measure the location with respect to the sun. We also recorded the exact time and date of measurement.

We speculated that the tiles supporting the floor of the bath (the hypocaust) may have had two magnetic components – one from their original firing when they were created as tiles and a second lower temperature component from their proximity to the fire (the praefurnium or furnace room) that was heating the bath area.  We discussed whether there might be a temperature gradient with distance from the praefurnium.   Hopefully the later lab experiments will help answer these questions

With the overall general aim of providing the archaeologists with potential dates depending on the magnetic signature recorded by the samples, over 200 individual samples were taken.   (For more information of archaeomagnetic dating see: http://www.english-heritage.org.uk/publications/archaeomagnetic-dating-guidelines)

All my samples! They are a mixture of cores and cubes depending on the strength of the sample material

So when I returned to France in April of this year to begin the measuring I was faced with a lot of samples to measure.  I had 6 weeks in which to do it but initially I wasn’t sure it would be enough time!

In the first 2 weeks of my trip to the Rennes 1 geomagnetism laboratory, I was able to carry out two experiments.  The first was a palaeointensity experiment on 45 cores which involved repeated heating steps to increasing temperature.

In addition to the palaeointensity experiment, I also conducted a demagnetisation experiment to see if we could find evidence of two heating components in the samples from the hypocaust.   When the demagnetisation experiment was completed, I carried out an anisotropy experiment to check that the samples weren’t significantly anisotropic as this can affect their ability to record magnetisation.

In this picture I am preparing to measure the positive y component of the magnetic vector in the samples as part of the anisotropy experiment.

In the next four weeks, I intend to carry out at least two more palaeointensity experiments and look at tiles which lined the drains from the baths.  If all goes well, I should have time at the end to discuss my results with Philippe and others at Rennes 1.

Its really good to visit another lab and carry out experiments because invariably they have different instruments to those at Liverpool, different techniques are favoured and its always good to have fresh ears to discuss your work with!  I am finding it a very rewarding experience and sometimes I even think my French might be improving!

When not conducting experiments, carrying out fieldwork, attending conferences, reading papers or writing up, it’s nice to learn new skills and meet other PhD students and early career researchers. Laura and I achieved this last Friday by attending “Standing up for Science Media Workshop” organised by the Sense About Science charity. In an increasingly interconnected world it’s important that scientists are able to communicate in a way that’s understood by all. After all, you don’t have to be an expert to be interested in a subject! In addition to Laura and myself there were students from a number of UK universities: Edinburgh, Manchester, Nottingham etc, from a number of different disciplines; Biology, Chemistry, Earth Sciences, Veterinary medicine and so on. In total there were 45 attendees which demonstrated how passionate scientists from all backgrounds are about engaging with the public regarding their science in an accurate and enjoyable way.

The workshop was split into three discussion sessions. In the first session we met two scientists who have various media experiences: from appearing on local news, to providing expert opinions for Brian Cox’s latest documentary, to appearing on ITV’s Day Break explaining their research. They both regularly talked to journalists and they shared with us their worst and best media experiences and some advice.

After lunch we had a discussion session with 3 journalists; David Derbyshire a freelance environment journalist and science writer; Rebekah Erlam, Broadcast journalist, BBC Radio 5 Live and Morwenna Grills, University of Manchester Media Relations Officer and former Sky News and BBC journalist. It was fascinating to hear their views and made me realise the amount of pressure they’re under to write a number of different science stories in an incredibly short amount of time. A science journalist probably has the same amount of time to research and write an article from scratch as I took to write this blog post! I also learnt that journalists don’t write their own headlines, sub-editors do. I found this surprising although it did explain why you get sensationalist headlines above relatively unconfirmed data. Editors want to sell newspapers after all!

In the third and final discussion session with representatives from Sense about Science and the Voice of Young Science Network we asked all the questions we had left and found out more about the network.

It was a very well organised and interesting day and I would highly recommend it to other early career researchers. The best pieces of advice I took away and will be implementing are:
• Make your work accessible
• Say yes to opportunities
• Start a blog
• Join twitter as it is a great place to network
• Practise explaining your research in 20 seconds using as little jargon as possible
Most of all I came away with the confidence that even though we’re only at the beginnings of our career, early stage career researchers can and should regularly communicate with the public and try to tackle any scientific errors that they notice in the media.

This summer Laura Roberts and I headed across the pond to the Institute for Rock Magnetism (IRM) at the University of Minneapolis to attend the first ever Summer school in Rock Magnetism course being held there. We hoped this course would be useful as we both came to geomagnetism from Geology and have sometimes found our PhD’s a little overwhelming! The course itself was fascinating and covered a wide variety of topics: hysteresis, paleointensity, fine particle magnetism, remanent magnetisations to name but a few! The day was split into morning lectures which started at 8.30 (we didn’t even have a kettle available to make this early start more bearable!) followed by laboratory exercises in the afternoon. These were fascinating as the emphasis at the IRM is different to that at Liverpool and the IRM focuses a lot more on low temperature experiments. This meant we got to use a variety of new (to us!) instruments such as an MPMS (Magnetic Properties Measurement System) and a VSM (Vibrating Sample Magnetometer).
The trip also enabled us to meet a variety of other PhD and masters students from across the world (there were 11 different nationalities making up the 25 students attending the course). This gave us a great opportunity to find out about others’ research and to talk to those at a similar stage in their careers and learn how research is done at other labs. It was reassuring to know that we weren’t the only ones who felt a little confused by it all at times! We also learnt about the differences in how masters degrees and PhDs are awarded in other countries.
As part of the summer school, we were split into five groups of five, to undertake group projects, which, we worked on in the afternoons after lectures, in the lab. I found this part of the summer school very enjoyable. Each group worked on different sample types and it opened my eyes to the range of applications geomagnetism has. The aim was to characterise the magnetic properties of each group of samples. We worked on soil samples from a soil pit on University grounds; on dust samples collected in pollution monitors across Minneapolis; as well as on sea floor basalts from Iceland and the Mid Atlantic Ridge; and volcanic tuff samples from the Tiva Canyon (Yucca Mountain) in Nevada. After 7 days of experiments and hard work putting together and interpreting our results, each group gave a presentation to the rest of the class.
On our free weekend we went on a local fieldtrip which was really interesting and we visited some amazing, large glacial potholes in the Minnesota Interstate Park along the St Croix River. These potholes have been dated to the last glacial period when a torrent of water flowed south as the glaciers melted, carving the St Croix river. Where sand and gravel were caught in eddies or whirlpools in this fast flowing glacial meltwater they eroded the basalt host rock so quickly a vertical shaft was formed. There are more than 80 of these potholes in the Glacial Gardens in the park which is the greatest concentration of them in the world.

Laura and I next to the St Croix River

We both had a great time at the school and learnt a lot. Although unfortunately I still think we have a lot more to learn before we finish our PhD’s!

For more information and photos from the summer school see: http://www.irm.umn.edu/IRM/ssrm2011.html