1 00:00:03,400 --> 00:00:06,500 This is geology bites with all of us trampled. 2 00:00:08,500 --> 00:00:11,200 The Earth's magnetic field changes over time. 3 00:00:11,400 --> 00:00:14,200 In fact, there have been frequent reversals in the 4 00:00:14,200 --> 00:00:18,000 direction of the field with the North and South magnetic poles 5 00:00:18,000 --> 00:00:21,000 switching places. We tend to think of these 6 00:00:21,000 --> 00:00:23,400 changes as applying to the whole globe. 7 00:00:24,000 --> 00:00:27,700 But in addition to this temporal element to the magnetic fields 8 00:00:27,700 --> 00:00:31,700 variability, there is also a spatial element with the field 9 00:00:31,700 --> 00:00:34,800 changing very differently in different places across the 10 00:00:34,800 --> 00:00:38,700 planet. Cathy Constable is a professor 11 00:00:38,700 --> 00:00:41,300 at the Scripps institution of oceanography. 12 00:00:42,100 --> 00:00:45,100 She measures the remnant magnetism in rocks and human 13 00:00:45,100 --> 00:00:47,900 artifacts around the globe to reconstruct. 14 00:00:47,900 --> 00:00:50,900 Not just the variations of the Earth's magnetic field as a 15 00:00:50,908 --> 00:00:54,900 whole but also its spatial variations across the globe over 16 00:00:54,900 --> 00:00:59,500 geological time Kathy Constable. Welcome to geology bites. 17 00:00:59,700 --> 00:01:01,000 Hello. It's nice to be here. 18 00:01:01,800 --> 00:01:04,300 It seems hard enough to reconstruct the history of the 19 00:01:04,300 --> 00:01:06,100 Earth's magnetic field in one dimension. 20 00:01:06,400 --> 00:01:09,500 The temporal one but I'm covering the spatial 21 00:01:09,500 --> 00:01:12,800 distribution of the field over time seems really challenging. 22 00:01:13,400 --> 00:01:17,200 Are you really able to create a spatial map of the field at 23 00:01:17,200 --> 00:01:21,700 different times over the past? Yes, but the map is a little bit 24 00:01:21,700 --> 00:01:24,500 crude the temporal Dimension as you describe. 25 00:01:24,500 --> 00:01:28,900 It really only reflects the fuel that we might expect from a 26 00:01:28,900 --> 00:01:32,600 magnetic dipole. If we Imagine The Familiar 27 00:01:33,100 --> 00:01:35,800 permanent bar magnet at the center of the Earth. 28 00:01:36,500 --> 00:01:39,000 But we really know that the field is more complicated 29 00:01:39,000 --> 00:01:41,400 because it's driven by convection in the liquid outer 30 00:01:41,400 --> 00:01:44,400 core. So if we have better mapping in 31 00:01:44,400 --> 00:01:47,900 both space and time that's actually essential to getting 32 00:01:47,900 --> 00:01:50,800 better acquainted with what's going on in the core and we've 33 00:01:50,800 --> 00:01:56,200 been successful in building more detailed time-varying spatial 34 00:01:56,200 --> 00:01:59,500 maps for the past 10,000 and the past hundred thousand years. 35 00:02:00,500 --> 00:02:03,400 The temporal resolution is better for the more recent times 36 00:02:03,400 --> 00:02:07,700 and in both cases as I said the spatial It's a bit crude. 37 00:02:07,700 --> 00:02:10,100 It's still only on rather large scales. 38 00:02:10,699 --> 00:02:15,100 So what are the materials you analyze to do this work? 39 00:02:15,500 --> 00:02:18,800 There's a broad range of materials actually human 40 00:02:18,800 --> 00:02:22,200 artifacts that have been heated to a high enough temperature to 41 00:02:22,200 --> 00:02:25,300 reset the magnetization as they cool in place. 42 00:02:25,900 --> 00:02:30,000 That would be things like fired Pottery bricks Hearth. 43 00:02:30,000 --> 00:02:31,400 Swear. They've been high temperature 44 00:02:31,400 --> 00:02:35,200 fires Kilns and there's a decreasing number of these human 45 00:02:35,200 --> 00:02:39,200 artifacts as one goes back in time and Beyond about 3,000 46 00:02:39,200 --> 00:02:41,400 years ago. There aren't that many of them 47 00:02:41,800 --> 00:02:48,400 and the spatial distribution is limited by human activity rocks. 48 00:02:48,400 --> 00:02:52,700 In fact cover a much broader temporal range and if we think 49 00:02:52,700 --> 00:02:57,600 about lava flows, they solidify and cool and then they preserve 50 00:02:57,600 --> 00:03:01,300 an instantaneous geologically speaking spot record of the 51 00:03:01,308 --> 00:03:05,700 field like and Marine sediments. Give a more continuous record. 52 00:03:06,400 --> 00:03:10,000 One that tends to be smoothed In Time by an amount that's 53 00:03:10,000 --> 00:03:12,100 dependent on the sedimentation rate. 54 00:03:13,000 --> 00:03:16,000 So all of these things contribute to our spatial 55 00:03:16,000 --> 00:03:21,300 reconstructions so are such suitable materials conveniently 56 00:03:21,300 --> 00:03:25,400 distributed across the globe for you to collect now that lost all 57 00:03:25,400 --> 00:03:30,400 convenient as you know, there's about 70% of the earth's surface 58 00:03:30,400 --> 00:03:32,700 is ocean. There's a lot more ocean in the 59 00:03:32,708 --> 00:03:35,000 southern hemisphere than in the northern hemisphere. 60 00:03:35,600 --> 00:03:38,500 So there's a Each challenge in the distribution between this in 61 00:03:38,500 --> 00:03:42,100 the Southern Hemisphere and it's also true that there's less 62 00:03:42,100 --> 00:03:46,700 human activity at high latitudes and fewer human artifacts there 63 00:03:47,300 --> 00:03:51,100 and if we look at Marine sediments, which can be 64 00:03:51,100 --> 00:03:53,700 recovered from the ocean. They tend to have lower 65 00:03:53,700 --> 00:03:56,700 accumulation rates than the things that you get on land when 66 00:03:56,700 --> 00:03:59,300 Lake sediments. So the temporal resolution isn't 67 00:03:59,300 --> 00:04:04,400 as good it seems as if we can get pretty good record of 68 00:04:04,400 --> 00:04:09,000 reversals from Many many magnetic stripes that are frozen 69 00:04:09,000 --> 00:04:12,900 into the ocean crust as it's formed and spreads on either 70 00:04:12,900 --> 00:04:16,500 side of the mid-ocean ridge. Are these some of your best 71 00:04:16,500 --> 00:04:20,700 sources there are very good on some time scales. 72 00:04:21,200 --> 00:04:24,300 What happens is that the magnetic stripes are limited by 73 00:04:24,300 --> 00:04:26,600 the processes forming the ocean crust. 74 00:04:26,600 --> 00:04:29,500 And also by the fact that measurements are usually made at 75 00:04:29,500 --> 00:04:32,800 the ocean surface several kilometers from the source. 76 00:04:33,700 --> 00:04:38,500 So what happens is that at the temporal resolution is actually 77 00:04:38,500 --> 00:04:42,100 a lot better in lava flows and sediments with high accumulation 78 00:04:42,100 --> 00:04:47,200 rate and they do a better job. And so there are some fortunate 79 00:04:47,200 --> 00:04:51,900 geological circumstances where for example and steen's mountain 80 00:04:51,900 --> 00:04:55,200 in Western North America 15 million years ago. 81 00:04:55,200 --> 00:04:58,200 There were a series of lava flow eruptions that provided a 82 00:04:58,207 --> 00:05:02,800 wonderful record of magnetic reversal and you can actually 83 00:05:02,800 --> 00:05:06,400 see the evolution of this magnetic reversal through A 84 00:05:06,400 --> 00:05:10,800 record preserved in these lava flows and they're also records 85 00:05:10,800 --> 00:05:13,100 and sediments as an example from Italian segments. 86 00:05:13,100 --> 00:05:18,000 And these two records are very interesting because even though 87 00:05:18,000 --> 00:05:23,000 they are somewhat controversial they preserve very high rates of 88 00:05:23,000 --> 00:05:26,200 change or appear to preserve very high rates of change in the 89 00:05:26,200 --> 00:05:31,300 magnetic field by matter of degrees per year in terms of the 90 00:05:31,300 --> 00:05:34,500 direction and that's something that is not necessarily 91 00:05:34,500 --> 00:05:37,500 completely expected. And we're exploring a lot 92 00:05:37,500 --> 00:05:40,600 further as part of the work that we're doing where we build a 93 00:05:40,600 --> 00:05:43,400 global view of what's going on in four more recent time. 94 00:05:44,500 --> 00:05:48,500 These things can really give us very good temporal resolution. 95 00:05:48,900 --> 00:05:52,300 Tell us a bit more about why we are so interested in 96 00:05:52,300 --> 00:05:55,300 reconstructing the spatial distribution of the field. 97 00:05:55,400 --> 00:06:00,400 So my interest is purely in the geomagnetic record and getting a 98 00:06:00,407 --> 00:06:04,300 better understanding of how the magnetic field is produced in 99 00:06:04,300 --> 00:06:08,700 the Earth's core and what Rolls, its variability if we think 100 00:06:08,700 --> 00:06:12,700 about the limitations of the seafloor Stripes, they tell us 101 00:06:12,700 --> 00:06:17,000 about normal or reverse polarity protection rather than the 102 00:06:17,000 --> 00:06:19,600 details of field strength variations. 103 00:06:19,900 --> 00:06:23,100 And the resolution is typically no better than about 20 to 40 104 00:06:23,100 --> 00:06:26,300 thousand years. They also only give us the 105 00:06:26,300 --> 00:06:29,300 polarity of the field. They don't give us the details 106 00:06:29,300 --> 00:06:33,200 of directional variability or even feel strength variability 107 00:06:33,200 --> 00:06:36,000 around the globe and the spatial. 108 00:06:36,200 --> 00:06:39,900 Structure of the field does vary with time and the rates of 109 00:06:39,900 --> 00:06:43,000 change in the field also depend on location and strength. 110 00:06:43,000 --> 00:06:46,000 So we have to go to a broader understanding of the spatial 111 00:06:46,000 --> 00:06:50,900 scales to get this now numerical simulations of the core field 112 00:06:50,900 --> 00:06:53,800 provide another way to look at how the field might vary in time 113 00:06:53,800 --> 00:06:57,100 and space but they also have limitations we know the 114 00:06:57,100 --> 00:07:01,000 equations to use but it's not currently possible to do the 115 00:07:01,000 --> 00:07:04,400 computations with the right values for core properties. 116 00:07:04,800 --> 00:07:09,200 And why is that The reason is that the core properties are 117 00:07:09,200 --> 00:07:12,100 just very challenging for numerical simulations the 118 00:07:12,100 --> 00:07:16,500 viscosity in the core turns out to be very low what that means 119 00:07:16,500 --> 00:07:19,900 is that the numerical simulations would ideally have 120 00:07:20,000 --> 00:07:24,200 very high spatial resolution. They take very short time steps 121 00:07:24,200 --> 00:07:28,300 in order to resolve what's going on on very small spatial scales. 122 00:07:28,300 --> 00:07:30,200 And that's very numerically challenging. 123 00:07:31,200 --> 00:07:34,900 The viscosity is basically expressed in the dimensionless 124 00:07:34,900 --> 00:07:38,500 Ekman number. And it's effectively orders of 125 00:07:38,500 --> 00:07:42,400 magnitude too high in the numerical simulations that were 126 00:07:42,400 --> 00:07:46,200 currently using. It's too hard to compute if we 127 00:07:46,200 --> 00:07:51,000 use the correct numbers and what happens then is that there are 128 00:07:51,000 --> 00:07:55,300 issues in converting the non dimensional numbers that are 129 00:07:55,300 --> 00:07:58,200 used in these simulations in the equations and these simulations 130 00:07:58,200 --> 00:08:01,500 to actual dimensional time scales for the Earth. 131 00:08:02,300 --> 00:08:06,200 Usually we have to choose between scaling according to an 132 00:08:06,200 --> 00:08:09,200 advection or a diffusion time scale depending on what we're 133 00:08:09,200 --> 00:08:14,000 interested in and the relative importance of advection versus 134 00:08:14,000 --> 00:08:17,000 diffusion in the processes. We're looking at so diffusion is 135 00:08:17,000 --> 00:08:21,700 basically what happens as a result of ohmic heat losses in 136 00:08:21,700 --> 00:08:24,600 the core. So electrical losses lose energy 137 00:08:24,600 --> 00:08:27,900 in the magnetic field because of the finite conductivity of the 138 00:08:27,900 --> 00:08:34,799 core the advection basically is a product of the convection that 139 00:08:34,799 --> 00:08:38,900 drives the magnetic field in the core and with advection what 140 00:08:38,900 --> 00:08:43,400 happens is that on short time scales and large length scales 141 00:08:43,400 --> 00:08:47,900 the advection carries the field around with the fluid and we can 142 00:08:47,900 --> 00:08:52,300 see this actually in maps of the modern-day field where you can 143 00:08:52,300 --> 00:08:56,400 see some features of the field that are basically drifting 144 00:08:56,400 --> 00:09:01,600 Westward or moving around the polar region, and the advection 145 00:09:01,600 --> 00:09:05,500 is Anton short time scales, whereas on longer timescales the 146 00:09:05,500 --> 00:09:08,700 diffusion becomes important and that's one of the things that 147 00:09:08,700 --> 00:09:12,200 has to play into the loss of energy in the dipole part of the 148 00:09:12,200 --> 00:09:16,600 field when you get an Excursion or a reversal just to come back 149 00:09:16,600 --> 00:09:19,800 to the problem of getting the right viscosity into the models 150 00:09:20,200 --> 00:09:22,400 roughly. What is the viscosity of the 151 00:09:22,400 --> 00:09:24,500 core? Can you compare it to something? 152 00:09:24,500 --> 00:09:27,600 We're familiar with the viscosity of the core is really 153 00:09:27,600 --> 00:09:31,800 like that of water. And you might contrast that with 154 00:09:31,900 --> 00:09:36,900 the silicate mantle of the earth, which has a viscosity 155 00:09:37,100 --> 00:09:40,900 that is orders of magnitude higher and therefore the time 156 00:09:40,900 --> 00:09:45,500 scales for convection in the mantle are much much longer than 157 00:09:45,500 --> 00:09:47,600 they are in the core. If you think about the 158 00:09:47,800 --> 00:09:51,000 convection advection timescales and the core there of the order 159 00:09:51,000 --> 00:09:55,900 of hundreds of years versus We Believe millions of years for 160 00:09:55,900 --> 00:09:58,000 most of the processes that are going on in the man. 161 00:09:59,000 --> 00:10:03,300 And so we're trying to mimic the viscosity in the core the best 162 00:10:03,300 --> 00:10:06,900 numerical simulations right now are using this dimensionless 163 00:10:06,900 --> 00:10:10,000 Eggman number that I mentioned of the order of 10 to the minus 164 00:10:10,000 --> 00:10:14,100 6 or 10 to the minus 7, and we believe that the correct number 165 00:10:14,100 --> 00:10:16,900 for the core is somewhere between 10 to the minus 10 and 166 00:10:16,900 --> 00:10:20,700 10 to the minus 15. Wow, not even close not even 167 00:10:20,700 --> 00:10:24,900 close and that's why we have to be very careful about drawing 168 00:10:24,900 --> 00:10:27,700 reliable physical conclusions about what's going on in the 169 00:10:27,708 --> 00:10:31,000 core from Um these numerical simulations, it's not that 170 00:10:31,000 --> 00:10:34,600 they're wrong in what they do. It's that they're just not 171 00:10:34,600 --> 00:10:37,800 necessarily completely appropriate for what we want to 172 00:10:37,800 --> 00:10:40,000 use them for when we're comparing with the geomagnetic 173 00:10:40,000 --> 00:10:42,900 field. So we're mapping the field here 174 00:10:42,900 --> 00:10:46,300 on the Earth's surface with the goal of improving our 175 00:10:46,300 --> 00:10:51,200 understanding of what's going on in the core, but between us and 176 00:10:51,200 --> 00:10:56,500 the core lie 29 hundred kilometers of mantle Is the 177 00:10:56,500 --> 00:10:59,500 mantle completely transparent to the magnetic field? 178 00:11:00,500 --> 00:11:04,700 No, it's not but it's is to a good approximation for what we 179 00:11:04,700 --> 00:11:11,500 do the mantle is basically made of silicates and it has a finite 180 00:11:11,500 --> 00:11:15,800 but low electrical conductivity the conductivity and the core is 181 00:11:15,800 --> 00:11:18,700 much much higher. In fact studying. 182 00:11:18,700 --> 00:11:22,400 The conductivity of the mantle is another field entirely in its 183 00:11:22,400 --> 00:11:24,000 own. Right and that's done using 184 00:11:24,000 --> 00:11:25,900 magnetic and electrical data as well. 185 00:11:27,300 --> 00:11:29,900 So the effect of the mantle when we're looking at the signal 186 00:11:29,900 --> 00:11:34,000 that's coming out of the corps is basically to smooth things 187 00:11:34,000 --> 00:11:35,600 out. That's what we could call a 188 00:11:35,600 --> 00:11:39,700 temporal damping in the signal so that everything that comes to 189 00:11:39,700 --> 00:11:43,300 us is smoother over a period of a few months so we can't detect 190 00:11:43,300 --> 00:11:47,000 very short-term variations that might be going on in the core 191 00:11:47,800 --> 00:11:51,200 and that's the main effect and it doesn't matter for the 192 00:11:51,200 --> 00:11:56,500 purpose of looking at paleo field structure because The 193 00:11:56,500 --> 00:12:00,000 temporal resolution that we have in the best of circumstances 194 00:12:00,000 --> 00:12:02,700 might be a decade to a hundred years. 195 00:12:03,000 --> 00:12:06,300 What kind of things are. We looking to pin down better 196 00:12:06,400 --> 00:12:11,400 using your spatial measurements? Can you constrain the values for 197 00:12:11,400 --> 00:12:14,300 example of the various parameters that you mentioned 198 00:12:14,300 --> 00:12:17,600 the viscosity in the Ekman number and there's also the 199 00:12:18,000 --> 00:12:21,300 rally number which defines the amount of energy in the 200 00:12:21,308 --> 00:12:26,600 convecting core. not directly the best estimates that we have 201 00:12:26,600 --> 00:12:30,900 for these parameters basically come from mineral physics and 202 00:12:30,900 --> 00:12:34,000 petrology and our understanding of the materials in the core and 203 00:12:34,000 --> 00:12:41,300 the temperature of the court but in the spatial representations, 204 00:12:41,300 --> 00:12:44,000 we had we can look at representative properties of the 205 00:12:44,000 --> 00:12:47,300 Earth's field and we can compare them with the results from 206 00:12:47,300 --> 00:12:50,500 simulations to decide if the simulations are doing a good job 207 00:12:51,300 --> 00:12:56,100 and one issue that we Address, which is an interesting one is 208 00:12:56,100 --> 00:12:59,000 whether there are long-term Geographic variations in the 209 00:12:59,000 --> 00:13:01,800 spatial structure that reflects boundary conditions at the 210 00:13:01,800 --> 00:13:05,400 core-mantle boundary. For example, there's some 211 00:13:05,500 --> 00:13:08,700 evidence that the field appears to be more active in the 212 00:13:08,700 --> 00:13:12,000 Southern Hemisphere and also more active in the Atlantic than 213 00:13:12,000 --> 00:13:17,200 in the Pacific region and this Builds on a bigger picture that 214 00:13:17,200 --> 00:13:21,600 we get from seismic tomography which supports differences in 215 00:13:21,600 --> 00:13:23,300 the region around the core mantle. 216 00:13:23,400 --> 00:13:25,400 A boundary. So that's something that we can 217 00:13:25,400 --> 00:13:29,500 actually see the consequence of in the magnetic measurements and 218 00:13:29,500 --> 00:13:33,100 we can then go back to the numerical simulations and say 219 00:13:33,100 --> 00:13:37,700 well actually if you want to get a realistic representation of 220 00:13:37,700 --> 00:13:41,900 the geomagnetic field, you need to include these spatially 221 00:13:41,900 --> 00:13:46,500 varying boundary conditions in the numerical simulations, and 222 00:13:46,500 --> 00:13:48,900 that's one part of it. And then there are other things 223 00:13:48,908 --> 00:13:53,400 that are harder to deal with in terms of the constraints. 224 00:13:54,100 --> 00:13:57,000 Which we don't really have a systematic way of addressing. 225 00:13:57,000 --> 00:14:00,800 You said the actually doesn't generally conform to a simple 226 00:14:01,000 --> 00:14:03,500 dipole. But can you describe what your 227 00:14:03,500 --> 00:14:07,500 spatial-temporal map of the Earth field actually does look 228 00:14:07,500 --> 00:14:14,400 like Yes, it depends on the time interval that we look at if we 229 00:14:14,400 --> 00:14:18,900 look at long-term averages over say a million years. 230 00:14:19,000 --> 00:14:23,100 It does look a lot like a simple dipole with Direction varying 231 00:14:23,100 --> 00:14:25,800 around the earth as you might expect for a simple bar magnet 232 00:14:25,800 --> 00:14:29,200 at the center of the Earth. But over time the direction and 233 00:14:29,200 --> 00:14:32,100 the magnitude can vary a lot and it doesn't always look like a 234 00:14:32,100 --> 00:14:35,500 dipole. So as I mentioned on average 235 00:14:35,500 --> 00:14:39,000 there are indications that you see more activity. 236 00:14:39,100 --> 00:14:42,400 In the field variations in the southern hemisphere than in the 237 00:14:42,400 --> 00:14:46,300 north and also in the Atlantic hemisphere, the piece of the 238 00:14:46,300 --> 00:14:50,500 earth that includes the Atlantic and the Indian Ocean versus the 239 00:14:50,500 --> 00:14:55,500 Pacific Hemisphere and the Lush amp Excursion, which occurred 240 00:14:55,600 --> 00:14:59,600 about 41,000 years ago is one example where the dipole and 241 00:14:59,600 --> 00:15:02,500 overall field decays and strength and structures really 242 00:15:02,500 --> 00:15:07,200 get quite complicated rates of change in the field can also be 243 00:15:07,200 --> 00:15:11,400 quite variable over. Time and also with position and 244 00:15:11,400 --> 00:15:14,400 we can actually estimate these from our time varying 245 00:15:14,400 --> 00:15:17,900 representations. So we find the fastest 246 00:15:17,900 --> 00:15:21,100 directional changes in Equatorial to mid-latitudes. 247 00:15:21,600 --> 00:15:24,000 We find the fastest field strength changes at high 248 00:15:24,000 --> 00:15:28,500 latitudes and the most rapid directional changes occur when 249 00:15:28,500 --> 00:15:30,900 the field strength is lower. You mentioned the LaShawn 250 00:15:30,900 --> 00:15:33,300 Excursion. What exactly is that? 251 00:15:33,300 --> 00:15:36,600 Is that a very special occasion or was it just the most recent 252 00:15:36,700 --> 00:15:41,300 such event? So we think that excursions are 253 00:15:41,300 --> 00:15:47,100 basically failed reversals that they occur when the field 254 00:15:47,100 --> 00:15:52,900 strength drops and in principle, the magnetic field could reverse 255 00:15:52,900 --> 00:15:56,200 but something prevents that from happening and one possible 256 00:15:56,200 --> 00:16:02,100 reason that might be is that the reverse field isn't able to 257 00:16:02,200 --> 00:16:05,500 establish a hold in Earth's solid inner core. 258 00:16:05,800 --> 00:16:08,300 And so it's just kind of moving around. 259 00:16:08,600 --> 00:16:11,500 In the outer core and then the inner core which retains the 260 00:16:11,500 --> 00:16:16,100 original polarity allows the original dipole polarity to 261 00:16:16,200 --> 00:16:21,000 reassert itself over time. So there are lots of excursions 262 00:16:21,000 --> 00:16:24,100 that have been found in the paleomagnetic record. 263 00:16:24,100 --> 00:16:26,400 The Le sharp Excursion is the best documented one. 264 00:16:26,400 --> 00:16:30,500 It's also one of the more recent ones are our suggestions of a 265 00:16:30,500 --> 00:16:34,600 couple more that occurred since them, but we don't find a global 266 00:16:34,600 --> 00:16:37,800 extent for these so maybe those are things that didn't quite 267 00:16:37,800 --> 00:16:40,500 even make it. To the Excursion status I should 268 00:16:40,500 --> 00:16:47,300 have said that the overall strength decays and once the 269 00:16:47,300 --> 00:16:52,200 dipole strength decays, then the pieces of the field that I'm not 270 00:16:52,200 --> 00:16:55,800 represented through the dipole become more dominant, you could 271 00:16:55,800 --> 00:16:58,200 think of them as being background noise and they 272 00:16:58,200 --> 00:17:01,500 basically make the field look much more complicated during an 273 00:17:01,500 --> 00:17:03,900 Excursion. Do you have a qualitative 274 00:17:03,900 --> 00:17:07,800 understanding of what's going on in the core that makes this 275 00:17:07,800 --> 00:17:10,400 happen? The global picture that we have 276 00:17:10,400 --> 00:17:14,200 no of excursions suggest that they're mostly related to the 277 00:17:14,200 --> 00:17:18,900 loss of dipole strength the highest field strength occurs at 278 00:17:18,900 --> 00:17:22,300 high latitudes. So we get rapid changes there as 279 00:17:22,300 --> 00:17:26,800 a dipole strength drops, and the other thing that we think is 280 00:17:26,800 --> 00:17:31,800 that excursions may be initiated at low latitudes by the growth 281 00:17:31,800 --> 00:17:36,100 of reverse flux patches or reverse magnetic field at or 282 00:17:36,100 --> 00:17:41,000 near the core surface and these And my great pole words and 283 00:17:41,000 --> 00:17:43,800 result in a full-scale polarity reversal. 284 00:17:44,800 --> 00:17:49,500 Are there any other qualitative findings you see in your Palio 285 00:17:49,500 --> 00:17:54,300 spatial map? Sorry over time varying paleo 286 00:17:54,300 --> 00:17:56,900 field models. I think is what we call of but 287 00:17:56,900 --> 00:18:01,200 you know one person's model is somebody else's idea of reality. 288 00:18:01,600 --> 00:18:06,100 So if we look over the past two million years one of the things 289 00:18:06,100 --> 00:18:11,300 that we see is that on average the dipole decays more slowly 290 00:18:11,700 --> 00:18:14,800 basically on diffusion time scales, which are of the order. 291 00:18:14,800 --> 00:18:20,700 For 30,000 years then it bounces back and this is a fairly subtle 292 00:18:20,800 --> 00:18:24,500 result in the sense that there's a lot of variability in the 293 00:18:24,508 --> 00:18:27,500 dipole moment where it goes up and down anyway, but when you 294 00:18:27,500 --> 00:18:30,500 look at it on long time scales, it's decaying more slowly than 295 00:18:30,500 --> 00:18:35,000 it comes back which suggests that in bouncing back the 296 00:18:35,000 --> 00:18:38,600 convection or the advection of the field to produce new field 297 00:18:38,600 --> 00:18:43,000 is a more important property. The other thing that we see is 298 00:18:43,000 --> 00:18:48,400 that if we look at the long-term decay of field strength going 299 00:18:48,400 --> 00:18:54,000 into excursions or reversals that those Decay time scales are 300 00:18:54,000 --> 00:18:57,900 of the order of 10 to 15 thousand years, but the 301 00:18:57,900 --> 00:19:01,000 directional changes can be much more rapid and they can be as 302 00:19:01,000 --> 00:19:05,400 large as one two, maybe even 10 degrees per year. 303 00:19:05,900 --> 00:19:10,700 These are things that we would expect to be able to find in the 304 00:19:10,900 --> 00:19:14,600 Core models the numerical models that people make about the core. 305 00:19:16,000 --> 00:19:19,800 Do these observations sort out the models? 306 00:19:19,800 --> 00:19:24,200 You said it wasn't specifically good enough to tell you what the 307 00:19:24,200 --> 00:19:26,500 Ahmed number should be or what the rally number should be. 308 00:19:26,500 --> 00:19:30,800 But does it nonetheless sift out some models from others infant 309 00:19:30,800 --> 00:19:31,400 full? It does. 310 00:19:31,800 --> 00:19:36,700 Yes, the simulations are numerically precise and they can 311 00:19:36,700 --> 00:19:41,200 be used to predict anything that we could observe and make into 312 00:19:41,200 --> 00:19:44,400 our paleomagnetic time-varying model. 313 00:19:44,900 --> 00:19:47,500 But even though they're new. Actually precisely not yet, 314 00:19:47,500 --> 00:19:51,400 universally successful. And of course paleo field data 315 00:19:51,400 --> 00:19:53,800 are messy. So we need to be careful about 316 00:19:53,800 --> 00:19:58,500 demanding precise agreement, but many simulations do not show 317 00:19:58,500 --> 00:20:02,300 this asymmetric growth and Decay rate for the dipole. 318 00:20:03,100 --> 00:20:07,000 We don't get understand a systemic reason for this in fact 319 00:20:07,000 --> 00:20:10,700 because we would like to be able to find a family of simulations 320 00:20:10,700 --> 00:20:14,900 that showed it and be able to say, ah, yes these simulations 321 00:20:14,900 --> 00:20:18,800 have an Ahmed number that's small enough or energy 322 00:20:18,800 --> 00:20:20,900 distribution. That's the right size. 323 00:20:21,100 --> 00:20:25,000 We don't yet have a systemic reason for this in the numerical 324 00:20:25,000 --> 00:20:28,200 simulations reversals are sometimes produced by just 325 00:20:28,200 --> 00:20:31,100 turning up the Rayleigh number which is effectively a mechanism 326 00:20:31,100 --> 00:20:36,100 for increasing the amount of energy in the system and tipping 327 00:20:36,100 --> 00:20:38,800 the balance away from dipole dominance. 328 00:20:39,900 --> 00:20:44,600 What we see in the real field at the earth's surface is a 329 00:20:44,600 --> 00:20:48,100 decrease in the dipole strength, but not necessarily an in any 330 00:20:48,100 --> 00:20:50,600 increase in energy in the rest of the field. 331 00:20:51,000 --> 00:20:54,100 I think the short answer to your question is this is an area 332 00:20:54,100 --> 00:20:57,800 where we can expect interesting developments as the numerical 333 00:20:57,800 --> 00:21:00,500 model as find a path to more Earth-like conditions. 334 00:21:01,900 --> 00:21:06,000 So the record that you found shows that there were periods 335 00:21:06,200 --> 00:21:10,000 maybe even extended periods when the Earth's field more or less 336 00:21:10,000 --> 00:21:12,700 vanished. Does that have an impact on 337 00:21:12,700 --> 00:21:15,500 life? We might imagine that it would 338 00:21:16,000 --> 00:21:19,600 because it's widely believed that a magnetic field is an 339 00:21:19,600 --> 00:21:23,500 important ingredient for life. But actually it seems that we're 340 00:21:23,500 --> 00:21:27,500 OK even during reversals development of the human species 341 00:21:27,500 --> 00:21:31,200 has actually occurred over the past, you know few. 342 00:21:31,700 --> 00:21:36,400 Use and it doesn't seem to have been detrimental to that 343 00:21:36,400 --> 00:21:38,900 Evolution. What happens is that the Earth's 344 00:21:38,900 --> 00:21:42,800 field creates the magnetosphere which holds off the solar wind 345 00:21:42,800 --> 00:21:47,300 and cosmic rays, which would normally be considered damaging 346 00:21:47,300 --> 00:21:52,400 and potentially carcinogenic and that's good for the pet that the 347 00:21:52,400 --> 00:21:55,600 field existed is was good for the original development and 348 00:21:55,600 --> 00:22:00,400 existence of the atmosphere which is essential to Life as we 349 00:22:00,400 --> 00:22:03,900 know it if we Turn down the field today as we would during a 350 00:22:03,908 --> 00:22:07,000 reversal the current atmosphere would still be present. 351 00:22:07,000 --> 00:22:10,900 Even if the magnetosphere was very much compressed and smaller 352 00:22:11,500 --> 00:22:14,400 and the atmosphere actually also plays a strong role now and 353 00:22:14,400 --> 00:22:17,700 protecting us from cosmic rays. So that would survive certainly 354 00:22:17,700 --> 00:22:18,900 for long enough that it would be. 355 00:22:18,900 --> 00:22:21,300 Okay. It's not like on Mars for 356 00:22:21,300 --> 00:22:24,900 example where the Dynamo died out long ago and there is 357 00:22:24,900 --> 00:22:26,500 essentially no remaining atmosphere. 358 00:22:27,000 --> 00:22:30,700 So you're saying the impact of a disappearing field might have 359 00:22:30,700 --> 00:22:33,600 been more severe. At the outset when life was just 360 00:22:33,600 --> 00:22:37,100 emerging then it would be now. Yes, I think that's true. 361 00:22:37,500 --> 00:22:40,900 Do we know if the F already had a magnetic field billions of 362 00:22:40,900 --> 00:22:46,000 years ago when in fact life was just getting going so the 363 00:22:46,000 --> 00:22:49,500 earliest widely accepted magnetic paleomagnetic field 364 00:22:49,500 --> 00:22:53,900 records are from about three and a half billion years ago, and 365 00:22:53,900 --> 00:22:56,900 the field may have been in existence very early in Earth's 366 00:22:56,900 --> 00:22:58,700 four and a half billion year history. 367 00:22:58,700 --> 00:23:01,500 There are ongoing attempts to recover. 368 00:23:01,600 --> 00:23:05,000 Our records from the world's oldest rocks. 369 00:23:05,700 --> 00:23:08,200 It's widely believed that a magnetic field may be an 370 00:23:08,200 --> 00:23:12,800 important ingredient for Life an interesting thing that is in 371 00:23:12,800 --> 00:23:16,700 emerging just recently is that the sequencing of some 372 00:23:17,000 --> 00:23:20,600 modern-day magnetic bacteria so bacteria that use the magnetic 373 00:23:20,600 --> 00:23:25,000 field grow magnetite within their bodies and use the 374 00:23:25,000 --> 00:23:29,400 magnetic field for navigation and things like this suggests 375 00:23:29,400 --> 00:23:31,500 that their genes go back to the arc. 376 00:23:31,700 --> 00:23:34,300 And so the archean is two-and-a-half to 377 00:23:34,300 --> 00:23:36,000 three-and-a-half billion years ago. 378 00:23:36,300 --> 00:23:40,000 So if they were using magnetite for the same purpose at that 379 00:23:40,000 --> 00:23:44,400 time that suggests that studying of bacteria could actually also 380 00:23:44,400 --> 00:23:47,700 provide a mechanism for studying the existence of the magnetic 381 00:23:47,700 --> 00:23:51,000 field in the past or that's a provide supporting evidence for 382 00:23:51,000 --> 00:23:53,200 the existence of the magnetic field in the past. 383 00:23:54,200 --> 00:23:58,000 For my last question if you had unlimited funds for your 384 00:23:58,000 --> 00:24:00,500 research, what would you use them for? 385 00:24:01,200 --> 00:24:04,500 Okay, I'd like to mention three important issues that really 386 00:24:04,500 --> 00:24:07,700 limit understanding of long-term field variations. 387 00:24:08,400 --> 00:24:10,900 The number one thing I would like to deal with would be 388 00:24:10,900 --> 00:24:13,200 improving chronological constraints. 389 00:24:13,700 --> 00:24:16,500 How could we get a better measure of the age of these 390 00:24:16,500 --> 00:24:20,400 paleomagnetic samples when they're recording the field so 391 00:24:20,400 --> 00:24:23,900 that we could make that more accurate if we could do that? 392 00:24:24,100 --> 00:24:26,400 Representations of the field would be better. 393 00:24:26,600 --> 00:24:29,300 We wouldn't be putting a measurement of field strength or 394 00:24:29,300 --> 00:24:32,500 Direction at the wrong time when we reconstruct the field. 395 00:24:33,000 --> 00:24:37,000 So I would like to make everybody have to date every 396 00:24:37,000 --> 00:24:39,700 paleomagnetic sample. They take before they record 397 00:24:39,700 --> 00:24:42,700 what's going on and more importantly I would like them to 398 00:24:42,700 --> 00:24:45,700 focus on samples that can be dated very accurately. 399 00:24:46,000 --> 00:24:50,900 What would that mean in typical dating that we use? 400 00:24:50,900 --> 00:24:53,600 We're relying on radiocarbon dating. 401 00:24:54,100 --> 00:24:58,300 And for that to be a good measure of age, we need to know 402 00:24:58,300 --> 00:25:00,800 what the production rate of radio carbon is in the 403 00:25:00,800 --> 00:25:04,100 atmosphere. And that's a limiting factor one 404 00:25:04,100 --> 00:25:07,300 way of getting around that might be to use something like 405 00:25:07,300 --> 00:25:11,700 speleothems or vard records where in principle you can count 406 00:25:11,700 --> 00:25:15,600 the years and establish a record going back in time in much the 407 00:25:15,600 --> 00:25:18,300 same way as one would for a tree ring chronology. 408 00:25:18,500 --> 00:25:22,600 What a speedier thumbs involves speleothems are basically 409 00:25:22,600 --> 00:25:25,900 stalagmites or stalactites. Tights that grow in caves and if 410 00:25:25,900 --> 00:25:28,700 you've ever been inside a cave you might have seen that they 411 00:25:28,700 --> 00:25:33,500 have these annual layers related to the seasonal feeding of the 412 00:25:33,500 --> 00:25:37,700 carbonate that forms a structure and that record if you can 413 00:25:37,700 --> 00:25:42,200 establish the age and magnetic record within the stalagmite 414 00:25:42,200 --> 00:25:46,600 could allow you to get a very fine scale temporal record of 415 00:25:46,600 --> 00:25:49,600 what's going on in the magnetic field and principle. 416 00:25:49,600 --> 00:25:55,000 It could be annual resolution. Now valve records come from 417 00:25:55,000 --> 00:25:59,600 sediments with annual layering due to changing seasonal input 418 00:25:59,600 --> 00:26:03,800 to the sediment and there are especially prevalent in glacial 419 00:26:03,800 --> 00:26:07,000 or sometimes in tropical regions subtropical regions. 420 00:26:07,000 --> 00:26:11,100 Basically, you can count the layers back in time and perhaps 421 00:26:11,100 --> 00:26:15,000 get a very accurate record of the time evolution of the field 422 00:26:15,000 --> 00:26:20,000 and pair that with the magnetic record which makes for a much 423 00:26:20,000 --> 00:26:24,700 better kind of data than That is not accurately dated. 424 00:26:25,000 --> 00:26:29,000 So that would be my first thing so my second thing and being 425 00:26:29,000 --> 00:26:30,300 quite greedy here. Actually. 426 00:26:30,600 --> 00:26:34,900 I also want to have better spatial temporal sampling and 427 00:26:34,900 --> 00:26:38,600 uncertainty estimates for the records that people get so that 428 00:26:38,600 --> 00:26:41,500 needs meat more field programs. So I want to send people out to 429 00:26:41,500 --> 00:26:45,100 collect more stuff so that we have more records and better 430 00:26:45,100 --> 00:26:48,800 Global and temporal coverage than we have at the moment and 431 00:26:48,800 --> 00:26:52,200 the third thing I would put my money into would be more 432 00:26:52,200 --> 00:26:54,100 concentrated. Aunt rated effort in the 433 00:26:54,100 --> 00:27:00,400 numerical modeling to understand whether we can get close enough 434 00:27:00,400 --> 00:27:05,500 to the right parameter regime to believe that we understand 435 00:27:05,500 --> 00:27:09,000 what's going on with the dominant physical processes in 436 00:27:09,000 --> 00:27:10,800 the court. First of all, we need to improve 437 00:27:10,800 --> 00:27:13,900 the Paleo field records so that we can build a better view of 438 00:27:13,900 --> 00:27:18,500 what we want to reproduce within numerical simulations and then 439 00:27:18,500 --> 00:27:22,500 the numerical simulations have to figure out how to get across 440 00:27:22,900 --> 00:27:27,200 Several orders of magnitude difficulty in not being able to 441 00:27:27,200 --> 00:27:30,500 simulate the Ackman number Kathy Constable. 442 00:27:30,600 --> 00:27:32,500 Thank you very much. Thank you. 443 00:27:32,500 --> 00:27:35,800 It was a pleasure for more about geology bites as well as 444 00:27:35,800 --> 00:27:38,800 pictures and diagrams that illustrate this podcast. 445 00:27:39,200 --> 00:27:41,400 You can go to geology bites.com.