SOHO and Solar update

A couple of interesting tidbits from NASA’s


Spotless Days

Current Stretch: 31 days

2009 total: 173 days (78%)

Since 2004: 684 days

Typical Solar Min: 485 days

Second: The Solar and Heliospheric Observatory is passing through a telemetry keyhole. Daily sun images will be intermittently delayed until routine contact is established later this week.

What is a ‘keyhole’, anyway?

SOHO's HGA Keyhole

Schematic of SOHO’s orbit, indicating the geometry of the now parked High Gain Antenna

Print media: High-res TIFF version

Also: Closeup showing Low Gain and High Gain Antennae, high-res TIFF version.

You may at times notice something unusual about the SOHO Near Real Time Images: They don’t update the way they “always” do, and EIT images are replaced by signs saying “CCD Bakeout”. All of this, and the only explanation is that “SOHO will be in a telemetry `keyhole'”?

First things first – all is well with SOHO and its instruments. In fact, those interested enough to read our calendar or the minutes of our planning meetings (see the Operations page), will have realized that the lack of fresh images came as no surprise to the SOHO science community. And they may have stumbled across the rather cryptic terms “keyhole”, as in “keyhole period” or “keyhole operations”.

The term “keyhole”, in antenna terminology, stems from a characteristic feature of aerial navigation maps showing the coverage of e.g. a radar antenna: A circular area near the antenna is shaded due to too-short range. In addition, any obstructions (hilltops, buildings) near the antenna will cause wedges to be shaded due to lack of coverage. Taken together, the appearance of the shaded area is often that of a keyhole. The term has since been generalised to refer to any area not covered by an antenna.

And what does this have to do with SOHO?

In order to get a good downlink signal, SOHO’s High Gain Antenna (HGA) needs to be pointed correctly as the spacecraft traverses its highly elliptical orbit around the L1 Lagrangian point, some 1.5 million kilometres sunward of the Earth. In early May 2003, the HGA started showing signs of trouble. Its East-West pointing mechanism started missing steps, eventually triggering an on-board protective action that reconfigured the spacecraft to use its omnidirectional Low Gain Antenna (LGA – see closeup). This configuration, which gives a much weaker signal at Earth, was only meant to transmit low rate data, sufficient for health monitoring and engineering but containing no science data.

Since the HGA was not far off the intended pointing, high rate science telemetry could soon be restored, but a long period of troubleshooting and testing started, while the antenna pointing mechanism performance seemed to degrade (see links below for details). In the end, it was concluded that an undetermined mechanical problem was indeed the cause of the problems, and the antenna appeared to be stuck. However, it could still be moved using the primary and backup drive motors at the same time, doubling the torque. The problem is, nobody could say how long that would last! So, a strategic decision was made to park the antenna at the “sweet spot”, which maximises the time during one half-orbit when the signal is strong enough for normal use. By turning the spacecraft up-side-down during the other half of the orbit, the coverage is doubled!

But when SOHO is in the “middle” parts of its orbit (see figure), there is no way it can point at the Sun while also pointing its HGA towards Earth. SOHO has “keyhole periods”, occurring twice per orbit. Each orbit around L1 takes about half a year, so keyholes affect SOHO operations roughly every 3 months. In late June 2003, as the first keyhole period approached, instruments prepared to shut down in various safe modes for a two-week period with only low rate telemetry.

However, a number of discoveries about the spacecraft’s and the Deep Space Network (DSN) stations’ capabilities were made: Using larger 34-meter stations with better receivers instead of the standard 26-meter stations, SOHO could keep beaming high rate data through the HGA for a few days extra on each side of the keyhole. When the HGA finally had to be abandoned for the LGA, it turned out that 34-meter stations could actually pick up medium rate telemetry, not just low rate telemetry.

During normal operations, SOHO always sends high rate telemetry to Earth when in contact with a ground station. While SOHO is not in contact, data from all experiments is recorded on board at what is called medium rate. The recorder is dumped at 3 times the recording speed using the extra capacity of the high rate signal. When the medium rate recording has been secured, the extra capacity is used by the MDI instrument to send higher resolution magnetograms, Dopplergrams and white light images, which do not fit in the medium rate signal.

So while the medium rate telemetry through 34-meter stations was very good news, it still meant there would be gaps whenever SOHO was not in contact. With no capacity to dump the recorders, the total capacity of 16 hours would fill up quickly.

But DSN also has 70-meter stations – and they could in fact lock onto the high rate signal while using the LGA. We were back in business! With enough support from 70-meter stations, SOHO could have lost the HGA entirely and continued as before.

But the larger DSN stations do not stand in line at your service all the time! Although SOHO’s second keyhole (fall 2003) had so many 70-meter passes lined up that the data loss was negligible, this was highly unusual. With the Mars and Stardust missions competing in a different league of priority, SOHO has had to do with almost no 70-meter stations during subsequent keyholes (and there’s not much relief in sight).

Although the amount of observing time lost is not all that great, considering the length of the mission, it matters a lot to a few instruments: MDI, GOLF and VIRGO have long-term helioseismology programs that are severely affected by gaps in the data. VIRGO also has important calibrations of its Total Solar Irradiance observations 3 times every day. These can be secured by stretching the 16-hour recording capacity to its maximum, recording those calibrations that are not captured by real time contacts. Other instruments that typically analyse their data in time-series are also affected, of course.

Continuous data sets are in fact so important to GOLF and VIRGO that they routinely send 3 copies of each measurement: one real-time copy and two delayed copies. Thus in theory, if you control the times of the telemetry capture, only one third of the telemetry is needed for a continuous sequence. This fact, plus a few 70-meter stations to dump the recorder every now and then, makes it possible to preserve the continuity of their data during some keyholes (assuming not too many things go wrong with the ground stations!).

As a good illustration of the data coverage, the CELIAS/MTOF Proton Monitor tracks the solar wind at L1. A plot covering most of the winter 2003 keyhole (and a few days prior for comparison) is shown below:

Two-week plot of CELIAS/MTOF PM data during winter keyhole

MDI is also hit hard by keyholes: They normally use the high rate time that is left after dumping the recorder for dowlinking all the MDI images that are shown on our real-time pages. And since there’s no such time left over during a typical keyhole, they are faced with an almost total blackout. They do have some telemetry coming down in medium rate, though, and they have been able to redirect the 96 minute cadence magnetograms to be downlinked in this portion of their telemetry, but this is not possible for the MDI Continuum images.

And what about EIT’s CCD bakeout?

Over time, many types of CCDs (Charge-Coupled Devices) used to detect EUV radiation are degraded by contamination of heavy ions from cosmic/solar particle radiation (interfering with the doping of the chip), deposits and polymerisation of deposits on the surface and, finally, trapped charges (again interfering with the doping of the substrate). These forms of degradation reduce the sensitivity of the detector. Fortunately, by heating the CCD over an extended time period, some of these effects can be reduced. Heating the CCD to improve its performance is called “baking out” the CCD. Since EIT would be unable to do “business as usual” during keyholes, they schedule their bakeouts during periods when data loss is inevitable.

50 thoughts on “SOHO and Solar update

  1. Fascinating.
    A wonderful example of how good science and good technologists can work together to remedy what is at first glance a crippling breakdown. The SOHO team should get an award for its heroics, rescuing a unique mission several times over.

  2. Very nice explanation and a tribute to the kind of difficulty they are overcoming. I do have a small peeve, however. The % spotless days is NOT 78%. Space Weather rounds up the percentage as soon as it clears a whole number. It’s 77.5% now and won’t be 78% until Sunday, September 16th – five days from now. I’d be happier if they would do their rounding at the .5% point: 77.5 = 77, 77.51 = 78. /peeve off

  3. It’s a hostile environment up there. Getting reliably calibrated results over long periods is fraught with difficulty. Hats off to the SOHO team and their efforts.

  4. “As above so below”, but -for some- why to bother watching that Sun or Sol or whatever, if it has nothing to do with us?….
    My guess is: Those who deny the Sun above have some trouble with their Sun within…
    Instead they are apt to proclaim and salute their prince of dark CO2 or even Soot perhaps.

  5. “”” rephelan (08:43:42) :
    Very nice explanation and a tribute to the kind of difficulty they are overcoming. I do have a small peeve, however. The % spotless days is NOT 78%. Space Weather rounds up the percentage as soon as it clears a whole number. It’s 77.5% now and won’t be 78% until Sunday, September 16th – five days from now. I’d be happier if they would do their rounding at the .5% point: 77.5 = 77, 77.51 = 78. /peeve off. “””
    Well, I have a small peeve of my own; since next sunday is my birthday; and I object strongly to having a whole month removed from my life time on earth just to give you a nice round number percentage of spotless days.
    So Phoo on you !

  6. George E. Smith (09:44:35):
    I’d be happier if they would do their rounding at the .5% point: 77.5 = 77, 77.51 = 78. /peeve off
    173 spotless days / 223 days in year to date = 77.57%
    Which, by your own “peeve” should be rounded to 78%… which is what they’ve done. So what’s the problem?

  7. Whoops … the quote in my previous post should be attributed to “rephelan (08:43:42)” NOT “George E. Smith (09:44:35)”. Sorry!

  8. Time to remember David Archibald’d forecast::
    Archibald (2006) predicted that climate during the forthcoming Solar Cycles 24 and 25 would be significantly cold. As at late 2008, the progression of the current 23/24 solar minimum indicates that a severe cool period is now inevitable, similar to that of the Dalton Minimum. A decline in average annual temperature of 2.2° C is here predicted for the mid-latitude regions over Solar Cycle 24. The result will be an equator-ward shift in continental climatic conditions in the mid-latitudes of the order of 300 km, with consequent severe effects on world agricultural productivity.

  9. Well to me, one of the most interesting parts of this informative story is that weird position of SOHO (a London suburb).
    Just what the hell is an orbit doing sitting out there in left field, and not going around the earth at all.
    Does anybody have a nice link to a quick explanation of this Lagrangian orbit.
    I gather that the L-point is gravitationally equally spaced from sun and earth; but actually quite close to earth relatively speaking. If it weren’t for the earth, then the L-point would orbit the sun faster than the earth does.
    So this must be one of those mathematically ingenious three body problems that actually has a solution somewhat akin to the Trojan orbits out in Jupiter land.
    I did learn some general orbital mechanics half a century ago, including non inverse square law gravitational laws; but I can’t say I ever heard of this Lagrangian orbit before.
    Pretty damn cute though, and this is the sort of stuff that the NASA folks do, including JPL etc, that occasionally gives us the idea that there are some NASA folks who know what they are doing; they aren’t all like Dr Hansen.

  10. Not so fast.
    In looking at SOHO’s same period last year, I see some days missing, but not in a continuous band. A hopscotch pattern emerges.
    2008 August 10, 14, 16, 19, 20, 22, 24 and 26 are missing.
    Data search (multiple images in days are deleted for brevity- actual # of images returned is well over 100): results for the period 2008-07-27 to 2008-09-28:
    The keyhole, if repeating, means that July 29 through August 6 are missing, as well as August 7 through 11
    Vetting this story, it appears that there is an MDI problem well beyond simple keyhole and bakeouts.
    Why not just say it, makes some headlines, and show the public how much the Solar Hubble is being missed? The groundwork is already laid down with the press release of Deep Solar Minimum and how science is keeping an extra sharp eye on the Sun.
    NASA, you are not making any sense here.

  11. Way to go SOHO people. Other NASA, indeed all government people, take note. A rational, factual, intelligent explanation is very welcome. It must be tough to do that when most governmental actions defy logic from the moment of their inception. Recall that the locus of inception for all govermental programs is the congress.

  12. George E. Smith (09:44:35) :
    In case I miss it, let me wish you a happy birthday now, George, and wish you many more. In fact, it is my hope that you will be buried in a coffin made from 100 year old, which I may remember to plant next week.
    PSU-EMS-Alum (09:54:01) :
    I just knew someone would call me on that. The peeve is that they’ve been calling the spotless days 78% for the last four or five days and didn’t just round at +7.5% today. I didn’t make myself clear and was justifiably called to account for it. Here at Anthony’s shop precision and accuracy are important. I’ll take this lesson and try with greater diligence to live up to the high standards so many practice here.

  13. ah, George, that was supposed to be 100 year old oak. Gotta learn to proofread before hitting “send”, too. Best wishes. Seriously.

  14. AlanG (11:58:51) :
    OT, but this is worth reading about why cap and trade isn’t

    If I live enough I will see Maunder II and becoming 4th. empoverished world. We have already seen becoming the new, the more intelligent becoming idiots and the poor ones rich..

  15. I was thrown by this: “ the spacecraft traverses its highly elliptical orbit around the L1 Lagrangian point..”
    I thought, “how can you orbit something that has no mass?” Turns out you can, SOHO is in a Halo orbit around the Lagrange point.
    Yew learn sumtin knew every day…

    Dr. John Holdren, do you agree with James Hansen’s statement that the CEO’s of large energy companies are guilty of should be tried (sic) for crimes against humanity?
    Holdren replied:
    I couldn’t really say. I’m not qualified to assess what the heads of oil companies, past or present, have done in this domain. My understanding is that Exxon, in particular, did fund a variety of small think tanks to generate what amounts to propaganda against understanding of what climate change was doing and the human role in causing it. Whether that sort of activity really constitutes crimes against humanity is something for those more embedded in the legal system than I to judge …

  17. Lots of stuff I didn’t know. Very interesting.
    That schematic begs the question, what is the SOHO orbiting? It’s not obvious in my first (quick) pass of the material.

  18. Great post, and I actually understand it thanks to the JPL Basics of Space Flight tutorial (pointed out in another post on WUWT).
    So, “George E. Smith (09:57:14) : Does anybody have a nice link to a quick explanation of this Lagrangian orbit.”
    That JPL link has wonderful descriptions of all types of orbits. Well worth the time to go through it, IMHO.
    Great job by everyone involved in SOHO to keep the mission alive.

  19. AlanG (11:58:51) :
    OT, but this is worth reading about why cap and trade isn’t:

    It’s what wasn’t said.
    The why of omission is far more noteworthy than the lengthy jargon of
    past problems, orbital mechanics, etc.

  20. Harold Vance (14:08:28) :
    By the same source, we must get more than 92 days in a row to beat the longest record.

  21. Well actually, it was having September the 16th fall in just five days that has me concerned; I was hoping it would be August 16th instead.
    The 77.5 I can live with; just give me back the rest of the month of August and early September.

  22. “”” PSU-EMS-Alum (09:54:01) :
    George E. Smith (09:44:35):
    I’d be happier if they would do their rounding at the .5% point: 77.5 = 77, 77.51 = 78. /peeve off
    173 spotless days / 223 days in year to date = 77.57%
    Which, by your own “peeve” should be rounded to 78%… which is what they’ve done. So what’s the problem? “””
    The problem is that five days from now (then) will hopefully be August 16th and not September 16th as asserted in rephelan’s post.

  23. George E. Smith (14:39:05) :
    Ouch! I really gotta renew that supply of ginko-whatever it is. Ummm… I don’t suppose you’d happen to know which YEAR this is… and doesn’t September come some time after July?

  24. rephelan (15:18:31) :

    and doesn’t September come some time after July?

    As my year starts in August, no.
    However, if my name were Henry, I’d spell it Hen3ry 😉

  25. Harold Vance (14:08:28) :
    The photoheliograms from 1913 show small penumbral only spots that were not recorded in the Wolf # system. 07-02, 06-01 and maybe 04-09.
    1913 4 6 96.415 1 11 11 82 0 5(u) 44(p) 0
    1913 7 9 190.329 1 8 2 10 95 1(u) 6(p) 66
    2008 07 19 11 56 48 GYUL 6 19 3(u) 10(p)
    (9 days with penumbra only small spots)
    2008 09 22 10 48 35 GYUL 12 52 7(u) 29(p)
    So, by 1913 standards, we have had a 64 day run in 2008, and a 61(est.) day run in 2009 so far.
    Hopefully, this will give you a more detailed depiction of where things lie as far as comparisons with 1913 go. They just didn’t pay a whole lot of never-mind to wispy spots in 1913, and it’s one more factor to consider.

  26. Harold Vance (14:08:28) :
    According to solaemon, the current record for SC24 for longest spotless streak is 31 days (21-JUL-08 to 20-AUG-08), so the current streak has a fighting chance to beat it.

    Well, SIDC reported sunspots on 7/23 and 7/30, so unless those get revised away, the streak from 2008 will stand as far as solaemon is concerned.

  27. Basil (15:53:59) :
    As far as counting penumbra-only spots goes, there will never be another 1913 streak, even if we fell face-first into Maunder Minimum II. The uncontrolled use of high-technology guarantees this result. When SDO picks hordes of pores, it’s game over.

  28. rbateman (15:44:37) :
    So, by 1913 standards, we have had a 64 day run in 2008, and a 61(est.) day run in 2009 so far
    You´ll make some gwrs, conveniently desguised as deniers, jump up!…
    Then the next step will be something like this:” Due to the lack of ecological responsability of many countries we are now having the terrible consequence that CO2 has increased so much as to provoke an unsuspected effect: An Extended Global Cooling (EGB) .
    Solar activity has nothing to do with this phenomena as it has proceeded as predicted by Nasa. Current cycle has reached its peak with a SSN count of 120.

  29. Totally inapropos but tangentially related: When I was a Military Intelligence Officer (and yes, I do know what Rocket J. Squirrel says about MI) I heard the following reply when GEN Bonesteel, US Forces Korea Commander, asked a colleague what the North Korean Army would do in a particular situation. In a response somewhat akin to what the AGW crowd says the reply was: “They can attack, withdraw, hold in place, or reinforce.” “Ah,” General Bonesteel said, I know what they CAN do but what WILL they do.” The reply, “I don’t know, Sir.”
    And yes, it can get warmer, colder, or stay the same. But what will actually happen no one seems to really know. Yet.

  30. Nogw (17:32:51) :
    They already said that a while back.
    The Jolly Green Giant will rampage across the planet, frying, drowing, smashing, freezing, burning, withering, shaking and pouring lava on hapless manking. Please tithe generously.

  31. John Stover (17:59:55) :
    It will let you know as soon as it decides to stop doing what it has been doing:
    Cooling off in lag-step to the stubborn mule Solar Activity with attendant confusing noise.

  32. DaveE (15:43:13) :
    OK, if your year starts in August, you may be a teacher. The Hen3ry has me stumped. I KNOW I’m going to feel really dumb when you explain, but that’s been about par for today on this thread.

  33. Mike Ramsey (21:05:13) :
    There are no plans to build or launch a SOHO replacement.
    SDO was supposed to be up already (still on hold) and SOHO was supposed to be getting reading doing the calibration work for SDO.
    Now, if Walter Cronkite were still around, he would have the entire nation on the edge of their seats with this story. The big Solar Minimum, NASA keeping an eagle-eye on the Sun, the imagers down. Meanwhile, back at NASA, the engineers and techs are working feverishly to get SOHO back online for mission-critical solar watch.
    Alas, there is NO story. Total media blackout.
    Check it out. Call your local news and try to get them interested in this story.
    When the Mars Rovers had a problem or a Lander went silent, the whole world knew about it.
    All the journalistic angles are there.
    You tell me what’s up with that.

  34. rephelan (22:19:17) :
    Hen3ry, the 3’s silent you see 😉
    It’s from Tom Lehrer – An evening wasted. Intro to “We will all go together (when we go)”

  35. DaveE (01:44:29) :
    Ahhh, Thank you. I was vaguely aware of Vatican Rag but knew nothing of Tom Lehrer. It appears perhaps I should have. Still trying to work out if I should be feeling silly. Maybe I’ll do an informal survey: if two out of three of my contemporaries and classmates recognize the name or the work, I’ll feel silly. His story sounds like a page out of the Jazz Singer. Priceless. Thanks for the reference.

  36. And Space Weather is reporting 32 days without a sunspot and nary a plage in sight. DMI’s COI chart is showing Arctic temps going below 273.15K about ten days early:
    And I haven’t been able to see any Perseids because we have too d@#$ much cloud cover. Otherwise, summer came late to the shoreline, but it’s been nice for the last week or so….

  37. I usually catch my Perseids around midnight or later. Always see a few nice fireballs. Nothing this year.

  38. rbateman (16:12:21) :
    I usually catch my Perseids around midnight or later. Always see a few nice fireballs. Nothing this year.
    Couldn’t see anything down here near Baton Rouge -> Blaming it on the refinery lights overhead outside during mid-shift. And the humidity. And the clouds. And the rain covers over the repair area. Can anybody think of anything I’ve overlooked? 8<)

  39. RACookPE1978 (19:04:32) :
    Can anybody think of anything I’ve overlooked?
    CO2. You can’t see the Perseids because the higher concentration of anthropegenic CO2 is blocking the visibility, and because CO2 traps heat, the illumination of the burning meteorite is lost against the blazing atmosphere.

  40. Then the C02 surely should have blocked the Milky Way and all the 6th mag stars.
    Nah, just my luck to miss it.

  41. The Mannian Principle of Missing Perseid Showers:
    If a Perseid Shower fell in the dark through the night shift engineer’s hardhat, but there was no climate scientist there to write about it in a peer-reviewed journal, did it really happen?

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