WEBVTT NOTE The Sun, and the Numbers That Describe It 1 00:00:00.000 --> 00:00:16.557 Why the Sun decides which band is open. HF long-distance propagation depends on the ionosphere, and the ionosphere exists because solar ultraviolet and X-ray radiation ionise the upper atmosphere. 2 00:00:16.557 --> 00:00:34.973 More radiation means more ionisation, which means the ionosphere refracts higher frequencies back to Earth instead of letting them escape. That is the entire chain, and every number below is a proxy for one part of it. 3 00:00:34.973 --> 00:00:36.240 Sunspot number. 4 00:00:36.240 --> 00:00:42.640 Question. How does a higher sunspot number affect HF propagation? 5 00:00:43.540 --> 00:00:53.420 Answer. Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies. 6 00:00:54.020 --> 00:01:08.660 Note the specific threshold. The bands below 20 MHz work reasonably at any point in the solar cycle; it is 15, 12, and 10 metres that live or die by it. 7 00:01:08.660 --> 00:01:18.780 Question. Which of the following are the least reliable bands for long-distance communications during periods of low solar activity? 8 00:01:19.680 --> 00:01:26.360 Answer. 15 meters, 12 meters, and 10 meters. 9 00:01:26.960 --> 00:01:31.106 And the contrast that makes 20 metres the workhorse band: 10 00:01:31.106 --> 00:01:41.986 Question. At what point in the solar cycle does the 20-meter band usually support worldwide propagation during daylight hours? 11 00:01:42.886 --> 00:01:46.099 Answer. At any point. 12 00:01:46.699 --> 00:01:58.326 If you learn one band's behaviour, learn that one: 20 metres is open somewhere, almost always. Solar flux index. 13 00:01:58.326 --> 00:02:03.046 Question. What is the solar flux index? 14 00:02:03.946 --> 00:02:11.312 Answer. A measure of solar radiation with a wavelength of 10.7 centimeters. 15 00:02:11.912 --> 00:02:29.106 Measured at 10.7 cm, 2800 MHz, because that emission tracks the ionising radiation well and, unlike sunspot counts, can be measured through cloud by an instrument rather than counted by a person. 16 00:02:29.106 --> 00:02:45.682 Rough reading: below about 70 is a dead solar minimum; above 150 the high bands open reliably; above 200 is exceptional. The two geomagnetic indices. Solar radiation builds the ionosphere. 17 00:02:45.682 --> 00:02:52.912 Charged particles from the Sun disturb it, and two indices track that disturbance. 18 00:02:52.912 --> 00:02:56.992 Question. What does the K-index measure? 19 00:02:57.892 --> 00:03:03.798 Answer. The short-term stability of Earth's geomagnetic field. 20 00:03:04.398 --> 00:03:08.718 Question. What does the A-index measure? 21 00:03:09.618 --> 00:03:15.464 Answer. The long-term stability of Earth's geomagnetic field. 22 00:03:16.064 --> 00:03:31.847 K is short-term, A is long-term. Both go up when conditions are disturbed, which is the opposite direction from solar flux, high flux is good news, high A and K are bad. 23 00:03:31.847 --> 00:03:48.190 K runs 0 to 9 on a three-hourly basis; A is a daily figure derived from the K values. A quiet day is K of 0 to 2 and A under about 10. Disturbances, and how fast each arrives. 24 00:03:48.190 --> 00:04:02.852 Three kinds of solar event, and the timing is the exam's favourite detail. Sudden ionospheric disturbance (SID), a solar flare's ultraviolet and X-ray burst. 25 00:04:02.852 --> 00:04:14.652 Question. Approximately how long does it take the increased ultraviolet and X-ray radiation from a solar flare to affect radio propagation on Earth? 26 00:04:15.552 --> 00:04:18.194 Answer. 8 minutes. 27 00:04:18.794 --> 00:04:30.834 Eight minutes, because that radiation travels at the speed of light. There is no warning: by the time you see the flare, the effect has already arrived. 28 00:04:30.834 --> 00:04:39.594 Question. What effect does a sudden ionospheric disturbance have on the daytime ionospheric propagation? 29 00:04:40.494 --> 00:04:47.774 Answer. It disrupts signals on lower frequencies more than those on higher frequencies. 30 00:04:48.374 --> 00:05:04.087 The flare's radiation supercharges the D region, which absorbs, and absorption is strongest at low frequencies. The low bands go silent while the high bands carry on. 31 00:05:04.087 --> 00:05:10.334 Coronal mass ejection (CME), an actual cloud of charged particles. 32 00:05:10.334 --> 00:05:19.414 Question. How long does it take a coronal mass ejection to affect radio propagation on Earth? 33 00:05:20.314 --> 00:05:24.714 Answer. 15 hours to several days. 34 00:05:25.314 --> 00:05:35.554 Particles travel far slower than light, so there is warning. This is why space weather forecasts exist and are worth reading. 35 00:05:35.554 --> 00:05:45.981 Question. How is long distance radio communication usually affected by the charged particles that reach Earth from solar coronal holes? 36 00:05:46.881 --> 00:05:51.539 Answer. HF communication is disturbed. 37 00:05:52.139 --> 00:06:01.828 Geomagnetic storm, a temporary disturbance in Earth's geomagnetic field, which is what arriving particles produce. 38 00:06:01.828 --> 00:06:07.988 Question. How can a geomagnetic storm affect HF propagation? 39 00:06:08.888 --> 00:06:13.808 Answer. Degrade high-latitude HF propagation. 40 00:06:14.408 --> 00:06:27.354 High latitude specifically: the particles funnel toward the poles along the field lines, so polar paths suffer first and worst. And the compensation: 41 00:06:27.354 --> 00:06:33.857 Question. How can high geomagnetic activity benefit radio communications? 42 00:06:34.757 --> 00:06:40.597 Answer. Creates auroras that can reflect VHF signals. 43 00:06:41.197 --> 00:06:57.757 The event that ruins HF opens VHF auroral propagation. A geomagnetic storm is bad news on 20 metres and an opportunity on 6. The 27-day cycle. 44 00:06:57.757 --> 00:07:07.113 Question. What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle? 45 00:07:08.013 --> 00:07:13.762 Answer. Rotation of the Sun's surface layers around its axis. 46 00:07:14.362 --> 00:07:34.069 The Sun rotates roughly every 27 days, so an active region that faced Earth today faces it again in about a month. If conditions were exceptional four weeks ago, it is worth being on the air. The dashboard, summarised. 47 00:07:34.069 --> 00:07:52.873 Number, Sunspot number; Measures, active regions; Good is, high. Number, Solar flux index; Measures, 10.7 cm radiation; Good is, high. Number, A-index; Measures, long-term geomagnetic stability; Good is, low. 48 00:07:52.873 --> 00:08:10.591 Number, K-index; Measures, short-term geomagnetic stability; Good is, low. Check yourself. You see a large flare on a solar imaging site. How long until the HF bands notice, and which bands first? 49 00:08:10.591 --> 00:08:22.524 K-index is 6 and solar flux is 180. Good day on 15 metres to Europe? Why do the high bands, not the low ones, track the solar cycle? 50 00:08:28.524 --> 00:08:38.700 About 8 minutes, the radiation travels at light speed, and the lower frequencies are disrupted most, because the D region absorbs. 51 00:08:38.700 --> 00:08:55.687 Probably not for a polar path: high flux is promising but K of 6 is a substantial geomagnetic storm, which degrades high-latitude propagation. A southern path may be fine. The ionosphere always refracts the low bands. 52 00:08:55.687 --> 00:09:04.924 Whether it refracts 21 or 28 MHz at all depends on how strongly it is ionised, which is what the solar cycle controls.