By Tomas Hood, NW7US
We are in the time of year when the Northern Hemisphere experiences the longest day of darkness (the Winter Solstice), when the Sun will be at its lowest point in the sky < http://nw7us.us/1z2FIXA >. This marks the peak of the DX window on the shortwave bands, as well as on the medium wave band. With very short daylight periods, the maximum usable frequencies are generally lower across propagation paths over dark areas in the Northern Hemisphere, making for quiet higher shortwave bands. At the same time, shortwave propagation on the mid-shortwave bands, like 31 meters, often is staying active around the clock. Across daylight regions, ionospheric propagation paths exhibit higher MUFs due to the seasonal position of the Earth at this point in its orbit around the Sun.
With short daylight days, the openings on many paths are short, though possibly strong, on the higher HF frequencies. In general, paths on 31 through 19 meters are now in their seasonal peak, especially between North America and Europe in the morning, and between North America and Asia during the late afternoon hours. 19 and 22 meters are probably the best daytime DX band, opening for DX just before sunrise and remaining open from all directions through out the day, with a peak in the afternoon. Nighttime conditions will be short and weak, and mostly north/south in orientation since the Southern Hemisphere has long daylight hours.
The best band for around the clock DX will be 31 and 25 meters. Twenty-five meters continues to be an excellent band for medium distance (500 to 1,500 miles) reception during the daylight hours, with longer distancereception (up to 2000 to 3000 miles) should be possible for an hour or two after local sunrise, and again during the late afternoon and early evening. 31 and 40 meters provides medium distance daytime reception ranging between 400 and 1200 miles, and beyond 3000 miles during the hours of darkness until two to three hours after local sunrise.
Look for 75 through 120 meters as stable nighttime bands; you can expect great nighttime DX conditions, especially with the decrease in seasonal noise, and the longer hours. Look for Europe and Africa around sunset until the middle of the night, and then Asia, the Pacific, and the South Pacific as morning approaches. Signals below 120 meters are also greatly improved. Tropical and regional stations are easier to hear, with stronger openings late into night and through early morning hours. For live solar and geophysical images and data, be sure to check out the NW7US space weather and radio resource page at < http://SunSpotWatch.com >
Tuesday, December 9, 2014
Monday, October 6, 2014
HF Propagation for October
By Tomas Hood, NW7US
A change in propagation conditions in the Northern Hemisphere can be observed as we move away from the long sunlit days of summer into the longer hours of winter’s darkness. With the shorter period of sunlight each day, the ionosphere has more time during the dark hours to lose the energy created during daylight hours. This affects the propagation of radio signals by lowering the Maximum Usable Frequency (MUF) over many areas of the Earth. However, the change in the length of daily darkness is not the only influence on the propagation of radio waves through the atmosphere. The amount and strength of radiation arriving and passing through our atmosphere varies from season to season, as well as from the solar cycle minimum to the solar cycle maximum.
During the Northern Hemisphere’s winter months, the Earth is closer to the sun than during any other time of its orbit. This makes the daytime ionization more intense than that of summer daytimes. In turn, this higher-level energy during the day causes the average MUF to increase slightly as compared to the same time of day during the summer season, over the same radio signal path.
Then, with the longer winter hours of darkness, the ionosphere has more time to lose its electrical charge. This causes the MUF to dip lower at night than during the summer months.
These conditions cause a wide daily variation in the maximum frequency that can be propagated by refraction of the radio waves by the wintertime ionosphere. Many radio enthusiasts celebrate the arrival of the winter shortwave season for these reasons.
Signals below 120 meters are improving, with nighttime paths growing larger in the Northern Hemisphere. Seasonal static, which makes it difficult to hear weak DX signals, is starting to decrease as we move into winter. Expect a few DX openings during the hours of darkness and into the sunrise period. These openings will often be weak due to the relatively high signal absorption during the expected elevated geomagnetic storminess through the rest of this year. Look for openings from Europe and the south if you are listening in the eastern half of the United States, and from the south, the Far East, Australasia, and the South Pacific if you are in the western half of the country. The best propagation aid is a set of sunrise and sunset curves, since DX signals tend to peak when it is local sunrise at the easterly end of the path in question. A good Internet web site featuring a grey line map display is found at < http://www.fourmilab.to/earthview/ >. Follow the link, "map of the Earth" showing the day and night regions.
Seventy-five through 120 meters are coming alive in late October. Expect long-range DX on the low bands, starting close in right after sunset, and extending farther as the night develops. Signals here should peak from Europe and from a generally easterly direction around midnight. DX paths will move farther west through the night. By morning, openings from Asia should be common. For openings in a generally western direction, expect a peak just after sunrise. The band should remain open from the south throughout most of the night. Propagation in this band is quite similar to that expected on 41 meters, except that signals will be somewhat weaker on the average, noise levels will be a bit higher, and the period for band openings in a particular direction will be a bit shorter.
Forty-one meters should be the hottest DX band during the dark hours as the seasonal static levels are lower than they were during the summer. The band should be open first for European DX in the eastern United States during the late afternoon. Signals should increase in intensity as darkness approaches. During the hours of darkness, expect good DX openings from most areas of the world. Signals should peak from an easterly direction about midnight, and from a westerly direction just after sunrise. Excellent openings toward the south should be possible throughout most of the nighttime period.
Paths on 31 through 19 meters are becoming ever more reliable between North America and Europe in the morning and between North America and Asia during the late afternoon hours. The strongest openings occur for a few hours after sunrise and during the sunset hours.
Thirty-one and 25 meters will often remain open into many areas late into the night and will open early in the morning, especially when part of the propagation path moves through sunlit regions. However, these bands are crowded and signals are usually very strong and steady. Twenty-five meters is expected to be an excellent band for medium distance (500 to 1500 miles) reception during the daylight hours. Longer distance reception (up to 2000 to 3000 miles) should be possible for an hour or two after local sunrise, and again during the late afternoon and early evening. Thirty-one meters will provide medium distance daytime reception ranging between 400 and 1200 miles.
Twenty-two through 19 meters compete with 16 for the best daytime DX band during October. They will open for DX just before sunrise and should remain open from all directions throughout the day, with a peak in the afternoon. Nighttime conditions will favor openings from the south and tropical areas. Since the Southern Hemisphere has long daylight hours, DX paths on these bands from stations in the south will be common.
Sixteen through 13 meters will occasionally open through October when flux levels reach above 100. Paths from Europe and the South Pacific as well as from Asia, at least during days of higher solar flux levels, are common, especially on 16 meters. Look for best conditions from Europe and the northeast before noon and from the rest of the world during the afternoon hours. Reception from the South Pacific, Australia, New Zealand, and the Far East should be possible well into the early evening.
Conditions during October should include moderate levels of trans-equatorial propagation (TE) in which stations in the southern states and parts of the Caribbean will be able to work into the northern areas of South America during the late afternoon. During peak years of a solar cycle, October is one of the best months for TE activity, especially later in the month. Since we are in a rather moderate solar cycle (that’s a liberal perspective), Sunspot Cycle 24, these openings may not occur often, but it is possible an exciting opening might occur from time to time; look for them by getting on the air and trying.
While sporadic-E activity is sparse during October in the northern Temperate Zone (where much of the U.S. is located), there is some possibility of extended tropospheric propagation conditions during October because of the changing weather patterns. Higher VHF is the best frequency range to watch for this.
Tuesday, August 5, 2014
By Tomas Hood, NW7US
It is that time of year, again, when propagation gets a bit tricky. Conditions up in the ionosphere can vary drastically from one day to the next. One day, typical summertime conditions rule, while the next we may well see conditions resembling those typical of autumn. This is the month where we see less and less summer-like conditions, and improvements signaling the arrival of the autumn DX season.
Autumn DX conditions feature somewhat higher daytime usable frequencies and somewhat lower nighttime usable frequencies. When you add equinoctial conditions that can begin as early as late August, we often experience optimum openings between the northern and southern hemispheres on the one hand, but periods of active to stormy conditions on the other.
Despite Solar Cycle 24’s low to moderate activity level, during the daylight hours good DX conditions should be possible on 17 and 20 meters. Expect signals on these bands to peak approximately during the two-hour window immediately following sunrise and again during the late afternoon. These two bands will see openings for DX throughout the daylight hours. Fairly good DX openings should occur along an arc extending across central Africa, Latin America, and into the far Pacific area. Peak conditions should occur during the afternoon hours, but an increasing number of earlier openings should be possible by early September.
Between sundown and sunrise 20 meters is expected to be the best DX band. Openings might be possible to many areas of the world, some with surprisingly strong signal levels, especially when using digital and CW modes. Until good DX conditions should be found for openings toward Latin America, the far Pacific, and into Asia. You might even catch some activity on 17 or even 15. Fairly good conditions are also expected on 30, 40, 60, and 80 meters despite the high static level at times. Openings should be possible before along an arc extending from northern Europe, through Africa, and into Latin America, the far Pacific, and Asia after .
By late August it should be possible to work some DX on 160 meters during the hours of darkness. Conditions on this band, as well as on 40, 60 and 80 meters, will tend to peak just as the sun begins to rise on the light, or easternmost, terminal of a path.
For short-skip openings during August and early September, try 80 meters during the day for distances less than 250 miles, with 60 and 40 meters also usable. During the hours of darkness both 80 and 160 meters should provide excellent communications over this distance. For openings between 250 and 750 miles use 30 and 40 meters during the day for distances up to 500 miles, and 20 and 17 meters between 500 and 750 miles. At night, 40 and 30 meters should be the best bands for this distance until , with 80 meters optimum from to sunrise. Try 60 meters, as well. For openings between 750 and 1300 miles, try 20 and 17 meters, as they should provide optimum propagation during the hours of daylight. Optimum conditions should continue on these bands for this distance range after sundown and until . Between and sunrise the best band should be 40 meters, but check 60 meters, too. For openings between 1300 miles and the one-hop short-skip limit of approximately 2300 miles try 20 and 17 meters during the day, with 15 meters also usable. After sundown try 30, 40 and 60 meters, with 80 meters also providing good propagation conditions for this distance range.
Sporadic-E propagation usually begins to taper off during August, but it should continue to occur fairly frequently. Some 6-Meter sporadic-E openings are expected during the month over distances of approximately 750 to 1300 miles. During periods of intense and widespread sporadic-E ionization, two-hop openings may be possible considerably beyond this range. Also check the 2-Meter band for an occasional sporadic-E short-skip opening between approximately 1200 to 1400 miles. While sporadic-E short-skip openings may occur at any time, there is a tendency for them to peak between 8 AM and noon, and again between local daylight time.
The Perseids meteor shower covers the period of late July to late August. The peak is expected to occur mid-August and will be most observable in the Northern Hemisphere. The maximum hourly visual rate could reach 100.
Aurora? Check for aurora-scatter type openings, on both 6 and 2 meters, which can range from a few hundred up to about a thousand miles, and they are usually characterized by very rapid flutter and Doppler shift on SSB signals. These aurora events may occur when the K-index numbers rise above 5.
For the very patient, check the six meter band for possible trans-equatorial (TE) openings between local daylight time. This type of propagation favors openings from the southern tier states into South America, with the signal path crossing the magnetic equator at a right angle. TE openings during August are rare, but they can occur. Very weak signals and severe flutter fading usually characterize them.
Wednesday, July 2, 2014
The NW7US PROPAGATION FORECAST
By Tomas Hood, NW7US
Solar activity is roughly the same this year as last year. This results in maximum usable frequencies staying consistent with the same period, last year. We expect good openings into most areas of the world through out the day on 22, 19, and 16 meters. Through the summer, expect a lot of propagation between north and south regions during the daylight hours. Nineteen and 16 meters will be the strong daytime bands, with 19 remaining a popular band throughout the year. Reception of stations located in tropical or equatorial areas may be possible well into the hours of darkness. For distances between 800 to several thousand miles, expect exceptionally strong signals. Multi-hop signals will be prevalent. Late afternoon and early evening broadcasts will likely congest the band.
Twenty-five and 22 meters will remain open from just before sunrise to a few hours past sunset. From late afternoon to well into darkness, expect these bands to offer worldwide coverage.
Thirty-one meters is a year-round power band with outstanding domestic and international paths, around the clock. During periods of low geomagnetic activity this summer, this band may offer long distance DX all through the night.
Forty-one and 49 meters offer domestic propagation during daylight hours and somewhat during the night. Geomagnetic storms will wipe it out, however. The tropical bands (60, 75, 90, and 120 meters) are not noticeably affected by the solar flux, but are degraded during geomagnetic storminess. Through the summer, expect these bands to be more challenging.
Overall, daytime bands will open just before sunlight, and last a few hours after dark. Look higher in frequency during the day, as these frequencies will be less affected by any solar storms occurring, and more broadcasters have transmissions in these upper bands.
July is one of the noticeably more active months of the year for VHF propagation between stations from about 500 to about 3000 miles apart. Strong signals appear on the lower VHF spectrum, and then quickly fade away. Experienced VHF DXers know this season as the Sporadic-E season, and July is in the very peak of the yearly season that begins in May, and ends by September.
Sporadic E propagation (abbreviated as Es or Es) affects the highest frequencies of the shortwave spectrum, as well as the lower to (sometimes) the mid-VHF spectrum. It occurs most frequently during late spring and early summer. Sporadic E propagation does not typically last very long, but the openings can be quite strong.
The summertime Sporadic-E (Es) season for the Northern Hemisphere should still be active through July. Usually these Es openings are single-hop events with paths up to 1000 miles, but July's Es events, like June's, can be double-hop. Look for HF openings on the higher frequencies, as well as on low-VHF, throughout the day but especially in the afternoon.
Of course, with the increased geomagnetic storminess, there is a chance of Aurora and the related Aurora-mode propagation. Check out the latest conditions at my propagation page, < http://SunSpotWatch.com/ >.
I'd like to hear from you
I welcome your thoughts, questions, and experiences regarding this fascinating science of propagation. You may e-mail me, write me a letter, or catch me on the HF Amateur bands. On Twitter, please follow @NW7US (and if you wish to have an hourly automated update on space weather conditions and other radio propagation-related updates, follow @hfradiospacewx). I invite you to visit my online propagation resource at < http://SunSpotWatch.com/ >, where you can get the latest space data, forecasts, and more, all in an organized manner. If you are on FaceBook.com, check out < https://www.facebook.com/spacewx.hfradio > and < https://www.facebook.com/NW7US >.
Until next month,
73, Tomas, NW7US
PO Box 27654
Omaha, NE 68127
Web site: firstname.lastname@example.org
Twitter: @NW7US ( http://Twitter.com/NW7US )
Twitter: @hfradiospacewx ( http://Twitter.com/hfradiospacewx )
Monday, February 3, 2014
High-frequency Propagation This Month
Sunspot Cycle 24 has been quite tame compared with recent cycles. Some are speculating that we’ve already seen the peak of this cycle—but time will tell. How alive can the higher frequencies be with long-distance propagation? It is always a surprise to the casual Amateur Radio Operator and SWLer when they get on a band like ten meters during the solar minimum, and discover that there is still some life on the band, beyond Short-Skip distances. This can be especially true during periods when massive sunspots occur and raise the daily 10.7-cm flux levels enough to wake up the higher frequencies. However, the low solar activity of recent months just does not support world-wide Dxing on the highest HF bands for any significant length of days. The lower HF bands can become real players, though, as veteran HF operators know.
We are starting to approach the end of the winter season. The period of darkness is growing shorter, causing a rise of the average daily maximum usable frequency (MUF) on any given radio propagation path that traverses the ionosphere in the Northern Hemisphere. That’s helpful in overcoming the increasing geomagnetic activity expected. Additionally, noise levels are still low, so reliable DX is possible. The solar activity is finally high enough to provide HF ionospheric propagation on higher frequencies (even some low-VHF F-region propagation has been observed since late 2011). General conditions are expected to be good to excellent for HF propagation throughout February.
Specifically, during the first three months of the year the earth is at perigee with the sun. This causes long winter nights, which in turn allows the ions of lower layers to drift upward and add to the F2 region. The F2 region contains the maximum ion density (foF2), which usually defines the maximum usable frequency (MUF) for DX paths.
Throughout these winter months, the foF2 increases slowly day-by-day until it reaches the highest monthly average of the year sometime during this quarter.
On the shortwave bands above 22 meters, expect paths to open shortly after sunrise, and will remain open until early to late evening. Morning and evening DX openings between some areas in the Northern Hemisphere on these bands are very short, because the band in question closes on one end of the path before it opens on the opposite end.
Paths on 31 through 22 meters remain in their seasonal peak much like in January, but with longer openings. Continue to look for great openings between North America and Europe in the morning and between North America and Asia during the late afternoon hours. Twenty-two meters will often be the best daytime DX band, with 31 and 25 running a close second.
Ninety through 41 meters will be useful almost 24 hours a day. Daytime conditions will resemble those of 25 meters, but skip and signal strength may decrease during midday on days with high solar flux values. Nighttime will be good except after days of very high MUF conditions. Generally, the usable distance is expected to be somewhat greater on the higher of these bands than on 90. DX activity tends to increase later in the evening toward midnight. Look for Africa and South Pacific (Australia, Papua New Guinea, and so on) on 90 through 60 meters throughout the night. On 41, 49 and 60 meters, long path DX is possible along the gray line.
The 120-meter band continues to remain stable, with very low noise levels. Throughout the winter season, high noise may occur during regional snowstorms. The band opens just before sunset and lasts until the sun comes up on the path of interest. Except for daytime short-skip signal strengths, high solar activity has little impact. Continue to look for Europe and Africa around sunset until the middle of the night, and then Asia, the Pacific, and the South Pacific as morning approaches.
Signals below 120 meters will remain strong and exciting, except during times of regional storms and high geomagnetic activity. Medium Wave DX is still quite hot throughout February.
Trans-equatorial (TE) scatter propagation tends to increase during the equinoctial period and some 6-meter openings may be possible between 7 and 10 PM local time. The best bet for such openings is between the southern tier states and South America for paths approximately at right angles to the equator. An occasional TE opening may also be possible on 2 meters. Unlike F2-layer or sporadic-E openings on 6 meters, TE openings are characterized by very weak signals with considerable flutter fading.
If you use Twitter.com – you can follow @hfradiospacewx for hourly updates that include the K index numbers (and, follow this columnist – @nw7us). You can also check the numbers at < http://SunSpotWatch.com >.
CURRENT SOLAR CYCLE PROGRESS
The Royal Observatory of Belgium, the world’s official keeper of sunspot records, reports a monthly mean sunspot number of 90.3 for December, 2013, up from November’s 77.6 and from October’s 85.6. The low for the month was 65 on December 7. The high of 136 occurred on December 10. The mean value for December results in a 12-month running smoothed sunspot number of 62.6 centered on June 2013. Following the curve of the 13-month running smoothed values, a smoothed sunspot level of 79 is expected for February 2014, plus or minus 14 points.
Canada's Dominion Radio Astrophysical Observatory at Penticton, British Columbia reports a 10.7-cm observed monthly mean solar flux of 147.7 for December, 2013, about the same as November’s 148.4. This indicates a clear resurgence of solar activity. This provided a lot of great DX and activity even on the Ten-meter band. The twelve-month smoothed 10.7-cm flux centered on June 2013 is 120.9, up from May’s 118.1. A smoothed 10.7-cm solar flux of about 134 is predicted for February 2014.
The geomagnetic activity as measured by the planetary-A index (Ap) for December 2013 is 5. The twelve-month smoothed Ap index centered on June 2013 is a steady 7.1. Geomagnetic activity should be much the same as we have had during January. Refer to the Last Minute Forecast for the outlook on what days that this might occur (remember that you can get an up-to-the-day Last Minute Forecast at < http://SunSpotWatch.com > on the main page).
I'd like to hear from you
I welcome your thoughts, questions, and experiences regarding this fascinating science of propagation. You may e-mail me, write me a letter, or catch me on the HF Amateur bands. On Twitter, please follow @NW7US (and if you wish to have an hourly automated update on space weather conditions and other radio propagation-related updates, follow @hfradiospacewx). I invite you to visit my online propagation resource at < http://sunspotwatch.com/ >, where you can get the latest space data, forecasts, and more, all in an organized manner. If you are on Facebook, check out < http://www.facebook.com/spacewx.hfradio > and < http://www.facebook.com/NW7US >.
Until next month,
73, Tomas, NW7US
PO Box 27654
Omaha, NE 68127
The Need for a Smoothed Solar Index – Characterizing a Solar Cycle
This is Part I of K9LA's series on the value of determining a smoothed solar index. It appeared in the final edition of WorldRadio Online magazine, January 2013. - KI6SN
BY CARL LUETZELSCHWAB, K9LA*
Sunspots have been observed for more than 2000 years. The invention of the telescope in the early 1600s allowed permanent records of sunspot activity to be made. Around the middle of the 18th century European astronomers began keeping records on a regular basis. From these records scientists put together the familiar sunspot cycle plot that started with Cycle 1 in 1755, and continues with the present Cycle 24.
The raw data is a daily sunspot number. The data in the early cycles is sometimes sparse but by 1850, around the peak of Cycle 9, the data was very reliable.
(BACKGROUND: My “Propagation” column in the November 2011 edition of WorldRadio Online, Photo A, discusses the possibility of missing an early solar cycle due to insufficient data. The gaps were shown in Figure 1 of that column. – K9LA.)
After World War II – 1947, to be exact – we began measuring 10.7 cm solar flux. The 10.7 cm solar flux is objective: it’s a measurement, assuming a calibrated set-up. Visually counting sunspots, on the other hand, is subjective. In other words, human interpretation is required. Just like sunspots, the raw data is a daily 10.7 cm solar flux value.
So why do we need anything more than the daily sunspot number or daily 10.7 cm solar flux? There are two reasons.
- The first has to do with characterizing a solar cycle, which I’ll address in this month’s column.
- The second has to do with propagation predictions, which I’ll address in next month’s column.
Accompanying this column, Figure 1 shows Cycle 23 in terms of the daily sunspot number. The data is very spiky, which simply says the daily sunspot number – and daily 10.7 cm solar flux – are very dynamic. With respect to the figure, three questions to ask are:
- When did Cycle 23 start?
- When and how big was the maximum?
- When did Cycle 23 end?
Figure 1 – Cycle 23 daily sunspot numbers
When, What and Why?
As for the start of Cycle 23, it was likely sometime in 1996. The spiky data precludes pinning this down to a specific month.
As for the maximum of Cycle 23, it kind of looks like there might have been two peaks – one around the middle of 2000 when the sunspot number was almost 250 and another in early 2001 when the sunspot number was around 240.
The dip between these high sunspot numbers also hints at two peaks. But what about the sunspot number around 210 in late 1999? Could this be considered a peak? If so, couldn’t the other daily maximums in the sunspot number be considered peaks? So could Cycle 23 have had many peaks?
As for the end of Cycle 23, it’s somewhere in the 2008 time frame. Again the spiky data precludes pinning it down to a specific month.
Since we’re having trouble pinning down the start and end and the peak – or peaks – let’s average the daily values to give us monthly means. (REMEMBER: The “mean” is the same as the “average.” – K9LA.)
Figure 2 is the same data as in Figure 1, but with the monthly means added in dark blue.
Figure 2 – Cycle 23 daily and monthly mean sunspot numbers
The monthly mean data is still kind of spiky, and does not allow us to pin down the start and end of Cycle 23 to a specific month/year. But at least we can now better see that Cycle 23 appears to have had two broad peaks.
With respect to the peaks, should we take the two highest monthly means – about 170 in mid-2000 and about 150 in late-2001 – as the peaks? But what about all the other peaks in the monthly mean data? Should they figure in somehow?
Making It All Work ‘Smoothly’
Enter: the use of the smoothed value.
Now, hang on! The smoothed value for a desired month uses:
- The monthly mean from the desired month
- The monthly means from the five months before the desired month
- The monthly means from the five months after the desired month
- One-half the monthly mean from the sixth month prior to the desired month
- One-half the monthly mean from the sixth month after the desired month
Whew! Thus the smoothed value is heavily averaged. Also note that the smoothed value is six months behind the current month.
The calculation of the smoothed value requires 13 months of data, but using one-half the monthly means at both ends results in 12 full-month data points. This can cause some confusion, as I’ve seen the smoothed value called a 13-month running average (since 13 months of data is required) and a 12-month running average (since it ends up with 12 full-month data points). Regardless, the smoothed value is heavily averaged – in other words, it is smoothed.
Figure 3 is the same data as in Figure 2, but now with the smoothed values added in green.
Figure 3 – Cycle 23 daily, monthly and smoothed sunspot number
Now it’s easy to see that Cycle 23 indeed had two broad peaks – one in April 2000 at a smoothed sunspot number of 120.8 and the other in November 2011 at a smoothed sunspot number of 115.5.
These months and smoothed values easily come from the raw smoothed data that is used to produce Figure 3.
That’s More Like It
Now we can also easily determine a likely start and end for Cycle 23. We can simply use the lowest smoothed sunspot number for both.
For the start of Cycle 23, May 1996 has the lowest smoothed value (8).
For the end of Cycle 23, November 2008 and December 2008 have the lowest smoothed value (1.7).
Be advised that these numerical minimums for the start and end times are usually the starting points – the official start and end times of a solar cycle can be revised a bit based on the comparison of old versus new sunspots and other factors.
Although I went through this exercise using sunspot numbers, I could have used 10.7 cm solar flux with the same results. That is, the smoothed value allows us to better determine the start time, the end time and the peak(s) of a solar cycle. This is why the official measurement of a solar cycle is the smoothed value.
Coming Up . . .
February CQ Plus' Practical Propagation presents Part II of K9LA's series on why we need a smoothed solar index. This month he explains its value for making accurate propagation predictions. Visit <http://www.cq-amateur-radio.com> – KI6SN.