1 00:00:00,000 --> 00:00:05,786 If the Sun has an atmosphere then it has weather and understanding solar weather 2 00:00:05,786 --> 00:00:10,286 is going to be fun, because it involves interesting physics. 3 00:00:10,286 --> 00:00:15,858 It's going to be important because it actually effects life on earth so let's 4 00:00:15,858 --> 00:00:21,572 try to understand how, in what way solar weather is very, very different from the 5 00:00:21,572 --> 00:00:25,884 weather here on earth. And so the indication of solar weather 6 00:00:25,884 --> 00:00:28,885 are these blemishes, these spots on the sun. 7 00:00:28,885 --> 00:00:34,679 first known recording of these is in the writings of Chinese astronomer whose name 8 00:00:34,679 --> 00:00:39,635 I can only pronounce as Gan De. Galileo observed the sun spots and used 9 00:00:39,635 --> 00:00:42,986 them to find the rotation period around the sun, 10 00:00:42,986 --> 00:00:47,652 the sun rotates about its axis moving from west to east, every 25.4 days. 11 00:00:47,652 --> 00:00:52,045 This is a medieval depiction of two sun spots on the surface of the sun. 12 00:00:52,045 --> 00:00:55,158 these sun spots appear as dark spots on the sun. 13 00:00:55,158 --> 00:00:59,612 Here, you see a modern image of the sun with a few regions of sunspots. 14 00:00:59,612 --> 00:01:04,189 They appear dark, as we said, because they're cooler than the surrounding at 15 00:01:04,189 --> 00:01:08,522 the center of a sunspot, the temperature can dip as low as 4,000 Kelvin. 16 00:01:08,522 --> 00:01:10,231 Because of T4, to the fourth they emit less. 17 00:01:10,231 --> 00:01:14,930 And if you remember that the sun is approximately 100 times the radius of 18 00:01:14,930 --> 00:01:17,615 Earth. Then the typical size of a sunspot is 19 00:01:17,615 --> 00:01:21,581 about the size of Earth. And it was the observation of Wilson in 20 00:01:21,581 --> 00:01:26,333 1769 that in fact, by watching the behavior of sunspots as they approach the 21 00:01:26,333 --> 00:01:31,086 edge of the visible disc of the sun. That they're in fact depressions in the 22 00:01:31,086 --> 00:01:34,400 solar surface. This is what a sunspot looks like when 23 00:01:34,400 --> 00:01:37,402 magnified. This is an Earth like region which is 24 00:01:37,402 --> 00:01:42,154 cooler than the surrounding, the center of, part of the sunspot is called the 25 00:01:42,154 --> 00:01:45,078 umbra. The less dark exteriors called the 26 00:01:45,078 --> 00:01:48,728 penumbra of those channels have nothing to do with it, 27 00:01:48,728 --> 00:01:53,797 the words are completely borrowed. So, what are these things and hints to 28 00:01:53,797 --> 00:01:59,339 the fact to the origin of these things come by observing the patterns of their 29 00:01:59,339 --> 00:02:02,448 formation. It turns out that some spark number 30 00:02:02,448 --> 00:02:07,653 varies with a very nice eleven year cyclicity, this famous butterfly graph 31 00:02:07,653 --> 00:02:12,520 here on the right indicates, well on the bottom you see the number of, 32 00:02:12,520 --> 00:02:17,809 sunspots or the total fraction of the solar disk's area covered by sunspots as 33 00:02:17,809 --> 00:02:23,037 a function of years from somewhere in the eighteen hundreds all the way through the 34 00:02:23,037 --> 00:02:26,460 to, the present day. And we see there's this periodic 35 00:02:26,460 --> 00:02:29,633 oscillation. Lot's of sunspots there, none, and lot's 36 00:02:29,633 --> 00:02:33,741 of sunspots there, none. And when you look more closely at where 37 00:02:33,741 --> 00:02:38,408 along the sun they're generated that's what butterfly graph above shows us. 38 00:02:38,408 --> 00:02:42,605 This is sort of a picture of The, the vertical axis is latitude along 39 00:02:42,605 --> 00:02:44,920 the sun. And you see that early in a cycle 40 00:02:44,920 --> 00:02:49,055 sunspots are created at mid latitudes, and then as the cycle proceeds their 41 00:02:49,055 --> 00:02:53,575 formed closer and closer to the equator. Then there's a whole bunch of then at the 42 00:02:53,575 --> 00:02:58,043 equator, then they all disappear. And then there's a break, and then some 43 00:02:58,043 --> 00:03:01,925 more sun spots. Form at mid latitudes, and slowly they 44 00:03:01,925 --> 00:03:05,322 move towards the equator. And the pattern repeats. 45 00:03:05,322 --> 00:03:10,383 it's hard to see in this graph. But in fact 2010 was the end of a very 46 00:03:10,383 --> 00:03:14,751 abnormally long solar minimum in which there were no sun spots. 47 00:03:14,751 --> 00:03:20,297 And nobody understands exactly, as far as I can tell what the cause was for this 48 00:03:20,297 --> 00:03:21,060 reason. Now. 49 00:03:21,060 --> 00:03:27,292 An important a discovery is that you can associate sunspots to magnetic free 50 00:03:27,292 --> 00:03:31,074 regions of strong vertical magnetic fields of the sun. 51 00:03:31,074 --> 00:03:36,256 So magnetic field poking out of the surface of the sun and the way you 52 00:03:36,256 --> 00:03:42,069 measure this is because it turns out that a magnetic field has an effect on the 53 00:03:42,069 --> 00:03:45,511 spectrum of Atoms in the presence of a magnetic 54 00:03:45,511 --> 00:03:48,103 field. You can measure the shift in the 55 00:03:48,103 --> 00:03:51,491 absorption lines that we saw in the Sun's spectrum. 56 00:03:51,491 --> 00:03:55,212 It's all by looking. And we can therefore measure the 57 00:03:55,212 --> 00:03:59,398 magnitude of a magnetic field. And this is a magnetic field map. 58 00:03:59,398 --> 00:04:03,632 Again we have the lines of latitude. And longitude along the sun. 59 00:04:03,632 --> 00:04:08,632 As, as a function of time, we see that when there are lots of sun spots, there 60 00:04:08,632 --> 00:04:11,921 are a lot of these regions of high magnetic field. 61 00:04:11,921 --> 00:04:16,263 Notice that yellow is one polarity and blue, the opposite polarity. 62 00:04:16,263 --> 00:04:19,487 So magnetic field pointing in and out of the sun. 63 00:04:19,487 --> 00:04:23,764 And sunspots appear in pairs. You see regions of blue preceded by 64 00:04:23,764 --> 00:04:26,330 yellow in this rotation. So there is 65 00:04:26,330 --> 00:04:31,214 blue dot and then to the west of it, a yellow dot in the northern hemisphere and 66 00:04:31,214 --> 00:04:34,389 the polarity is reversed in the southern hemisphere. 67 00:04:34,389 --> 00:04:39,091 And this is consistent throughout a cycle and would then have in the, between 68 00:04:39,091 --> 00:04:43,915 cycles the polarity reverses so that if the blue dots, we don't have two cycles 69 00:04:43,915 --> 00:04:48,616 here, but if the blue dots were ahead of the yellow dots in the north in this 70 00:04:48,616 --> 00:04:53,440 cycle and the yellow ahead of the blue in the north, in the south in this cycle 71 00:04:53,440 --> 00:04:56,310 this would reverse from one cycle to the other. 72 00:04:56,310 --> 00:05:00,381 And this is our hint. That what drives these sunspots is in 73 00:05:00,381 --> 00:05:04,829 fact the magnetic field. It turns out that what makes them cool is 74 00:05:04,829 --> 00:05:09,884 that, remember we talked about, when we talked about the Van Allen radiation 75 00:05:09,884 --> 00:05:15,209 belts, we said that regions of strong magnetic field repel charged particles, 76 00:05:15,209 --> 00:05:19,437 the [INAUDIBLE] gas coming up to the photosphere from the 77 00:05:19,437 --> 00:05:24,349 interior regions is charged. Magnetic field intense magnetic fields, 78 00:05:24,349 --> 00:05:27,857 like in the sunspots choke off this convection. 79 00:05:27,857 --> 00:05:33,261 And for that reason, the interior of a sunspot is cooler than the surrounding 80 00:05:33,261 --> 00:05:36,559 part of the sun. Down, as I said, to 4,000 Kelvin. 81 00:05:36,559 --> 00:05:39,375 And. So we understand sun spots is a magnetic 82 00:05:39,375 --> 00:05:42,839 phenomenon. And the fact that the particles on the in 83 00:05:42,839 --> 00:05:47,601 the atmosphere are charged and therefore interact strongly with the magnetic 84 00:05:47,601 --> 00:05:50,384 field. As are, by the way, all of the particles 85 00:05:50,384 --> 00:05:55,332 in the interior of the sun plays an important role in understanding what the 86 00:05:55,332 --> 00:05:59,300 causes of this phenomenon and some of its consequences. 87 00:05:59,300 --> 00:06:03,198 So we're led to look to the solar magnetic field to try to understand that 88 00:06:03,198 --> 00:06:07,149 and see how that drives the weather. the magnetic field of the sun is not 89 00:06:07,149 --> 00:06:09,800 quite as that of the Earth. It's not well described. 90 00:06:09,800 --> 00:06:13,567 Most times. Like a bar man in the North and the South 91 00:06:13,567 --> 00:06:16,623 pole. Though at any time, there is roughly a 92 00:06:16,623 --> 00:06:21,102 north and a south pole. And it is roughly aligned with the axis 93 00:06:21,102 --> 00:06:24,371 of rotation. The big difference, is that in the 94 00:06:24,371 --> 00:06:28,992 interior of the sun, the roiling convecting fluid that presumably 95 00:06:28,992 --> 00:06:33,754 generates the magnetic field. Is not just conducting but is charged. 96 00:06:33,754 --> 00:06:37,252 It's a plasma. And you recall, when we talked about the 97 00:06:37,252 --> 00:06:41,132 interaction of the solar wind with the Earth's magnetic field. 98 00:06:41,132 --> 00:06:46,012 That I said that because a stream of charge particles both is influenced by a 99 00:06:46,012 --> 00:06:49,078 magnetic field but also creates a magnetic field. 100 00:06:49,078 --> 00:06:53,771 You can imagine that we have a situation where magnetic fields and charged 101 00:06:53,771 --> 00:06:56,650 particles are strongly interacting. And we can. 102 00:06:56,650 --> 00:07:01,616 Model this by imagining that the magnetic field lines and the particles are tied to 103 00:07:01,616 --> 00:07:04,690 each other. So if we have a weak current that we had 104 00:07:04,690 --> 00:07:08,296 in our tube, the current is trapped around the magnetic field. 105 00:07:08,296 --> 00:07:13,085 But if we have a stronger current, it can drag the magnetic field around, which is 106 00:07:13,085 --> 00:07:17,520 what the solar wind did to Earth's magnetic field and is what the rotating. 107 00:07:17,520 --> 00:07:21,475 Plasma in the interior of the Sun does to the magnetic field. 108 00:07:21,475 --> 00:07:26,597 And a crucial ingredient in this is that the rotation period of the Sun is not 109 00:07:26,597 --> 00:07:31,632 uniform like in, in the case of Jupiter. The Sun experiences a fluid body, that 110 00:07:31,632 --> 00:07:36,370 experiences differential rotation. The sun's rotation period is 25 and a bit 111 00:07:36,370 --> 00:07:41,280 days at the equator, but 29 days at the pole so that the equator rotates faster 112 00:07:41,280 --> 00:07:44,573 than the pole. This has the following effect upon the 113 00:07:44,573 --> 00:07:47,867 sun's magnetic field. So here's the rotating sun, it's 114 00:07:47,867 --> 00:07:52,529 undergoing differential rotation. You start with sort of a dipolar magnetic 115 00:07:52,529 --> 00:07:57,314 field, but the magnetic field lines are trapped by the plasma, and because the 116 00:07:57,314 --> 00:08:02,100 equator rotates faster than the poles, the magnetic field lines are stretched. 117 00:08:02,100 --> 00:08:06,544 So that you get first you started with a north south magnetic field, then you 118 00:08:06,544 --> 00:08:08,877 ended up with an east west magnetic field. 119 00:08:08,877 --> 00:08:13,210 And these field lines are stretched and these stretch field lines are kind of 120 00:08:13,210 --> 00:08:17,137 like rubber bands. They contain large amounts of magnetic 121 00:08:17,137 --> 00:08:22,992 energy and this releases by the field sort of reconnecting, popping out of the 122 00:08:22,992 --> 00:08:26,070 sun's surface, creating all these bubbles. 123 00:08:26,070 --> 00:08:30,593 And these might remind you some of the structures we saw when we looked at 124 00:08:30,593 --> 00:08:35,538 Coronagraph remember that if magnetic field lines poke out of the sun then the 125 00:08:35,538 --> 00:08:40,604 charged particles trapped along them will spiral along the magnetic fields and form 126 00:08:40,604 --> 00:08:43,680 loops of charged particles and these are the 127 00:08:43,680 --> 00:08:48,811 prominences that we saw and what happens is that this process of winding up the 128 00:08:48,811 --> 00:08:53,510 magnetic field until its energetically favorable to reconnect and the sun's 129 00:08:53,510 --> 00:08:57,282 field becomes a mess for a while takes about eleven years. 130 00:08:57,282 --> 00:09:01,115 And so every eleven years the sun's polarity in fact reverses. 131 00:09:01,115 --> 00:09:05,999 You start with the nice polar magnetic field that we started with and then you 132 00:09:05,999 --> 00:09:09,680 wind it up. You produce a mess, and 133 00:09:09,680 --> 00:09:14,634 the mass reinstates a polar magnetic field with the opposite North South 134 00:09:14,634 --> 00:09:18,871 polarity after about eleven years and the pattern repeats itself. 135 00:09:18,871 --> 00:09:24,217 This is the cause of the magnetic cycle, and of the creation of the sunspots. 136 00:09:24,217 --> 00:09:29,497 So sunspots are these regions where the magnetic field lines poke out of the sun 137 00:09:29,497 --> 00:09:34,777 and in and their orientation in each hemisphere is due to the direction of 138 00:09:34,777 --> 00:09:38,950 rotation relative to the direction of the magnetic field and 139 00:09:38,950 --> 00:09:44,366 This reverses between cycles because the polarity reverses and we get a big 140 00:09:44,366 --> 00:09:50,210 vertical component where these arches poke out and at the bottom of the two 141 00:09:50,210 --> 00:09:53,916 base points of each of these arches sits a sun spot. 142 00:09:53,916 --> 00:09:59,903 So the most active times in the sun's magnetic history are the times when the 143 00:09:59,903 --> 00:10:04,916 field is in the process of reversing. These reconnection events release the 144 00:10:04,916 --> 00:10:08,105 magnetic energy stored in these magnetic fields. 145 00:10:08,105 --> 00:10:11,420 I told you that you could treat them as rubber bands. 146 00:10:11,420 --> 00:10:14,359 They snap back. They release a lot of energy 147 00:10:14,359 --> 00:10:17,270 the local release of energy in a recollection event. 148 00:10:17,270 --> 00:10:19,965 Can be on the order of ten to the 25 joules. 149 00:10:19,965 --> 00:10:24,314 This leads to rather dramatic solar flares where one region of the Sun 150 00:10:24,314 --> 00:10:29,092 suddenly becomes very bright, and amidst a lot of ultra-violet and x-ray light. 151 00:10:29,092 --> 00:10:33,931 And these beautiful ultra-violet images, you can actually trace the outlines of 152 00:10:33,931 --> 00:10:36,810 those magnetic field arches that I spoke about. 153 00:10:36,810 --> 00:10:40,924 That's not because we can see the magnetic fields, but because we see the 154 00:10:40,924 --> 00:10:45,207 ultraviolet emissions, from the charged particles that are trapped along the 155 00:10:45,207 --> 00:10:48,758 magnetic fields. It's very pre, kind of these particles to 156 00:10:48,758 --> 00:10:52,140 give such an indication of the fields. so solar flares 157 00:10:52,140 --> 00:10:56,643 Are energetic events. There are far more violent events that go 158 00:10:56,643 --> 00:10:59,789 on. they're called prominences or coronal 159 00:10:59,789 --> 00:11:04,508 mass ejections and here is a, a collection of views from many NASA 160 00:11:04,508 --> 00:11:09,655 observatories of a coronal mass ejection that occurred a few months ago. 161 00:11:09,655 --> 00:11:15,617 this is an event in which an amount of hydrogen and charged particles in the 162 00:11:15,617 --> 00:11:20,762 order of the mass of the earth is ejected at high speed as magnetic pressure 163 00:11:20,762 --> 00:11:25,775 basically builds up and ejects these charged particles at high speed. 164 00:11:25,775 --> 00:11:31,388 this one was not directed towards earth. When such events happen and are directed 165 00:11:31,388 --> 00:11:36,801 towards earth this is, these periodic sudden increases in the intensity of the 166 00:11:36,801 --> 00:11:40,210 solar wind that cause beautiful aurora but also 167 00:11:40,210 --> 00:11:45,057 can cause disruption of communications as the electronics and communication 168 00:11:45,057 --> 00:11:48,757 satellites is injured by this stream of charged particles. 169 00:11:48,757 --> 00:11:53,669 And so, this is why we have so many different observatories observing the Sun 170 00:11:53,669 --> 00:11:58,580 at many wave lengths, from many angles in order to be able to understand this 171 00:11:58,580 --> 00:12:03,620 phenomena, if possible predict them and certainly give us the three day warning. 172 00:12:03,620 --> 00:12:08,545 three days is approximately the time it takes one of these coronal mass ejections 173 00:12:08,545 --> 00:12:12,601 to get from the sun to Earth. And getting the three day warning, allows 174 00:12:12,601 --> 00:12:17,468 satellite operators to sort of shutdown operations in an orderly fashion and not 175 00:12:17,468 --> 00:12:19,960 allow damage to happen to their satellites. 176 00:12:19,960 --> 00:12:24,570 And so, we've developed a pretty decent understanding of how the sun works as a 177 00:12:24,570 --> 00:12:28,888 star both in terms of the internal energetic and energy production and, in 178 00:12:28,888 --> 00:12:33,674 terms of the structure of the sun itself. Our job now is going to be try to extend 179 00:12:33,674 --> 00:12:38,051 that to nearby starts and the first question we're going to have to ask is, 180 00:12:38,051 --> 00:12:42,311 how far are these things because anything we see needs to be interpreted 181 00:12:42,311 --> 00:12:47,155 differently depending on how far it is. And so our first chore is going to be to 182 00:12:47,155 --> 00:12:50,890 measure distances to stars. And it's to that, that we're going to 183 00:12:50,890 --> 00:12:51,474 turn next.