ANNUAL MEETING OF THE SOCIETY OF AMATEUR RADIO ASTRONOMERS AT GREEN BANK, WV, JULY 15, 1996 .

COMETARY ORIGIN OF SPORADIC-E: QUESTIONS & RESPONSES

by G. Neil Spokes, Ph.D.

I HAVE SUBJECTED THE QUESTIONS TO SOME INTERPRETATION (INFORMATION IN [ ] BRACKETS). RESPONSES INCLUDE SOME ADDITIONAL INFORMATION BEYOND THAT GIVEN DURING THE TALK

My thanks to Hal Braschwitz for noting down the questions. I also thank the questioners who put such smart questions to me. If questioners care to self-identify, I will endeavor to extend credit appropriately at a future date. I can be contacted at

Many of the sporadic-E occurrences referred to in the questions and comments that follow are related to the mid-latitude sporadic-E events described by Pocock and Dyer in their March 1992 QST article ( ARRL, Newington CT, 1992.)

Q1. [WHAT WERE THE] TIME INTERVALS ON THE [FIGURES IN THE ARTICLE ON THE GREAT SPORADIC-E] 1987 EVENT? [REF: PAPER OF DR. MICHAEL R. OWEN IN QST MAY, 1988 -- MOSTLY ON 144 MHz SPORADIC-E.]]

A1. Dr. Owen presented the ham-to-ham contact (QSO) information in 10 min. increments. It was noted that the QSO contacts were, typically, never repeated (during a VHF contest, hams "wasting time" making duplicate contacts forfeit the opportunity of additional point scores associated with making new contacts).

Q2. HOW [IS IT POSSIBLE] TO TUNE INTO THE 88 MHz [SPORADIC-E] SIGNALS?

A2. Pat Dyer started his time recording based on making observations of the presence of sporadic-E stations just above 88 MHz in the FM broadcast band, avoiding local signals. The recordings were started after finding progressive evidence of sporadic-E at lower frequencies. The lower frequency events were noted on a 30 - 50 MHz FM scanner and a channel 2-6 TV receiver(54-88 MHz), using various antennas. Dyer's records noted the duration of the 88 MHz event in intervals of 5 min.

Q3. CLARIFY [HOW YOU KNOW THAT THE] PERIODICITY [WAS] NOT DOMINATED BY HIGH VALUES DURING ONE YEAR. [HOW WOULD] THE JULY 15 TOTALS BE AFFECTED BY LEAP YEAR?

A3. There were 4,600 to 10,500 sporadic-E minutes per year during the observation interval, with an average of 7,512 minutes per year. The 11 year accumulated data show six major peaks in excess of 1,500 minutes at particular dates in the June-July time period. The absolute maximum number of minutes in a single 17 hour time period in a day is 60 times 17 or 1020, however, I suspect that a "big sporadic-E day" might occupy less than one third of this amount of time or less than about 350 minutes, suggesting that one year's big event will not swamp the general picture. The original data are not available. Pocock and Dyer in footnote 8 point out that in the June-July time period, the larger peaks were accumulated during repeat occurrences on those dates in many of the years. This led to a larger accumulation of data in those months and the smaller peaks may have been smaller in part because of the fewer number of occurrences on those dates.

Unless I am able to persuade someone else to do this in the near future, I plan to find and examine specific-date vertical ionosonde data at 12 MHz. I need help and anyone interested is invited to help with location and access to the information.

Leap year adjustments do not affect the positioning of the peaks much. The error for any one year is generally not more than 12 hours on either side of the average, and, maximum error for the main June-July period (which is >1/3 year removed from February 29) probably does not exceed 9 hours. The very low number of minutes for July 15 (<1/10th of the adjacent peaks of July 14th and 17th) is indicative of a true trough value which, with sidereal correction could be even lower.

Q4. [WHAT IS THE] VARIANCE OF [THE NUMBER OF ACCUMULATED MINUTES ON] INDIVIDUAL DAYS [WITH THE YEAR AS VARIABLE]?

A4. This is another version of question 3. I do not have the specific data needed to answer that question. Pocock subjected the data to many statistical treatments. The results are in the March 1992 QST paper. The correlations tested were based on calendar date minutes of sporadic-E. Not only was the peak/trough date-specificity validated but a series of 5-day intervals was identified from peak to peak and from trough to trough.

I don't have a solid explanation for the 5-day interval. My best suggestion is that at each point in the comet's orbit, the emitted debris trails trace sheet like paths extending out from the comet's orbit. The trails are driven directly out by radiation pressure and solar wind. Consequently, the material that ablates off during the comet's approach to the sun is pushed back and therefore retarded compared with the comet's path. After perigee the ablated material is accelerated ahead of the comet track. This difference in time of origin may give the comet tail a curved shape. Throughout their traverse in orbit as the particles leave the region of the sun and move toward apogee, the tail particles continue tracing out sheet-like paths. The trails will further spread out ahead of and behind the comet. The effects of this broadening but not necessarily thickening of the particle paths will be repeated following each encounter with the sun. In this way, the particle debris sheet-field is supplemented with every perigee event. Each trail debris particle is in its own orbit around the sun. These particle cloud sheet fields are encountered by the earth as it passes through them on its orbit about the sun.

In the May-August period of about 120 days there are about 2 dozen such particle cloud sheets, each associated with a different comet. I hypothesize, that these sheets are each, naturally, no more than 2-3 days thick (i.e. less than or of the order of about 4,000,000 miles thick) and at the position of the earth's orbit those peaks are separated by about 5 days (8,000,000 miles). The trail orbit sheets will certainly intersect one another. Possibly, the trails have self-adjusted through the millennia so that they do not occupy the same position. The place where they intersect will be a region wherein there are particle-particle interactions of one trail with another. The encounters are electrostatic or mechanical in nature. Quite minor "collisions" will lead to the emptying of an area. The crossing points are the sporadic-E trough locations because the particles have scattered one another out. In a sense, then, the battles have been fought and in the course of time the impacting trails can no longer be found in the regions where they interact with one another. The bigger trail or trail thread or knot may be expected to "win the battle" for that particular component of the orbit.

A key point to my thinking is that while comet photos show the particles dramatically streaming away into outer space, the fact is that the particles just do not vanish. Each comet's trail material is simply located in a sheet-like orbit that travels all the way out to apogee and back and the debris trail material continues in that sheet, ready to do battle again with any other particulate trails on the next perigee event and over and over again thereafter.

A related question to that concerning the daily variance is "How can these sporadic-E peaks be so thin (a mere 4,000,000 miles thick), recognizing that visual meteor showers may be up to several weeks thick (i.e. as thick as 30,000,000 - 50,000,000 miles)?" If the particles are as small as I suspect, then their ability to remain participants for millennia (i.e. for dozens, or, maybe, hundreds of comet orbit revolutions) is of special interest. Do the particles recondense on the main comet or on one another as they follow the comet toward apogee in the frozen, weakly illuminated vastnesses of the outer solar system only to be re-ablated for another release when they return to perigee? Far away from the sun, the solid particles will be both positively and negatively charged. Electrons will occasionally neutralize a positively charged particle, but may often come to rest on neutral or weakly negatively charged particles to make up a host of negatively charged particles. Consequently the electrostatic forces will exercise a cohering role (far stronger than gravity) as particles travel through the solar system's outer perimeter. The exploration of sporadic-E may therefore constitute an astronomical probe of the fate of cometary particulates and therefore presents a fascinating and very inexpensive source of new astronomical data if my interpretation is correct. The inexpensive aspect is that mere review of date-specific 12 MHz vertical ionosonde data from around the world for the last 50 years could provide a useful basis set of information.

Q5. [HOW WOULD] THE DATA [APPEAR IF PRESENTED IN THE FORM OF MINUTES AT EACH DATE ] OVER AN 11 YEAR PERIOD [IN A SEQUENTIAL RECORD]?

A5. I would very much like to have seen a date-specific correlation study applied to the Pocock/Dyer data presented in the form of a string. However, this was not done, so there has been no test for a 365.25 day interval correlation. I do believe however, that the data in the Pocock Dyer paper represent an important astronomical record. The data were submitted to the statisticians in the date-specific form and there was, therefore, a de facto

assumption of calendar date-specificity.

Q6. [DO] ALL THE PARTICLES HAVE THE SAME SIZE?

A6. Not really, but I expect the earth's magnetic field to exert some filtering of particles so that closely adjacent particles may be expected to have the same or very similar mass/charge ratios since the incoming particles have a single velocity and are usually grouped in a packet of small horizontal extent (100 - 200 km across, for example) making the earth's field like a great mass spectrometer.

Q7. [IS THERE] ANY CORRELATION WITH AURORA?

A7. No, Pocock and Dyer found what is, in effect, a negative correlation since aurora occurs primarily in the equinoctial months whereas, the sporadic-E is almost absent in those months.

Q8. [IS THERE ANY CORRELATION OF SPORADIC-E WITH TELEPHONE] MICROWAVE LINK [PROBLEMS. THE MICROWAVE LINK PROBLEMS ARE ASSOCIATED WITH] ATMOSPHERIC DUCTING? THE TELEPHONE COMPANY DATA [IS A MATTER OF PUBLIC RECORD ON ACCOUNT OF THE PHONE COMPANY'S LEGAL OBLIGATION TO REPORT THIS INFORMATION].

A8. I would not expect any correlation with surface weather-related effects except for elongation of the sporadic-E paths to as much as 2600 km. The possibility has not been checked. This is a nice piece of information for future use in correlation studies -- thank you for the tip.

Q9. WHAT TYPES OF METAL IONS ARE FOUND IN THE [SPORADIC-E CLOUDS] IN THE IONOSPHERE?

A9. The metal ions found in the rocket experiments reported in the 1960's include magnesium, sodium, silicon, and possibly iron. These are the species that you would expect from silicate particles of the type similar to those found in the core part of the meteoritic particles. Monatomic metal ions have a very much longer life than the normally present NO+ ions. This long life is due to the very much smaller electron-ion recombination coefficients of the metal ions. European Space Agency's Jochen Kissel found when studying the Halley dust particles that the very small <10-16 g particles were almost pure CHON, i.e. they were organic in nature. The larger particles had a progressively larger inorganic core. I note that the CHON species, being organic, are, effectively, combustible with oxygen. Flame ionization is another possibility that may be considered in that region of the ionosphere. The cloud particle's energy is its own source of ignition.

Q10. THE SOLAR WIND [MAY HAVE SOME CHARACTERISTICS THAT ARE ASSOCIATED WITH A PARTICULAR DIRECTION NOT TIED UP WITH THE ROTATION OF THE SUN. WOULD NOT THE SOLAR WIND, THEN, HAVE] DATE SPECIFIC [CHARACTERISTICS]?

A10. I do not think that solar wind events can be associated with a particular date. Only if the solar wind were locked into the celestial sphere by some means, could this be so. I don't see how this can be the case. The sun rotates with a rotation rate period of 27 days -- faster at the equator, slower at the poles. The detailed behavior of the solar wind is not to my knowledge date specific - I cannot conceive how, year after year, the solar wind would have a low intensity at the July 15th location, preceded and followed, year after year, by peaks of greater than ten times that amount in just a 2 day interval on either side.

Q11. [HAS ANYONE] VERIFIED THE DATE-SPECIFIC [FINDINGS]?

A11. No. The work of Dyer in this area is of landmark status and has not to my knowledge been repeated. It is tough to duplicate in a short time, work that requires a minimum of 11 years to accomplish. I would very much like to see that effort repeated, especially in the southern hemisphere although different peak dates would be expected since different meteor shower paths are involved.

Q12. [DO ANY EARTH-ORBITING] SATELLITES PROVIDE LARGE PARTICLE [RECORD] INFORMATION?

A12. I am sure that, aside from the Giotto and Vega data on Halley's comet that Kissel looked at, there are some available satellite data. I do not yet know where to find it. Much of the satellite data, however, is collected over a long period of time, where individual dates are of lesser importance. The grain of sand-sized or even micron diameter-sized particles have been of much greater interest on account of the damage that such particles can do to individuals and space equipment. Additionally, such particle sizes create the more spectacular visual meteor tracks and hence get more attention. I noted that one of the Shuttle missions was moved in time to avoid a well known meteor shower. The microparticle showers that I suspect are correlated with sporadic-E, capture less interest at NASA.

Q13. [IS THERE] ANY DATE-SPECIFIC INFORMATION FROM THE SOUTHERN HEMISPHERE?

A13. Not that I am aware of. Pocock and Dyer's data are the first of this type. Again, the southern hemisphere 12 MHz vertical incidence ionosonde data are probably there for the asking and it is only a matter of time before we get the needed information.

Q14. [ARE THERE ANY] CORRELATIONS OF COMET ORBITS WITH SPORADIC-E [DATES]?

A14. I have not looked explicitly at the comet orbits. However, there is an intimate tie-in of comet orbits with meteor orbits and the radiants from which the meteors appear to emanate. I view as promising, the finding in the work presented today that in June, there are seven dates for which sporadic-E peaks are preceded or followed within two days by identified minor visual meteor showers -- you will note that this covers the entire month of June.

Q15. DO YOU PERSONALLY COUNT THE SPORADIC-E [OCCURRENCES]?

A15. Not so far. I hope that some of the ideas that I get from this SARA conference will enable me to do this remotely. I cannot be at the receiver terminal for hands on data taking at this time.

Q16. [SOME OF THE] INFORMATION [SOUGHT SHOULD BE] ON INTERNET. [e.g. THE IONOSONDE DATA. ]

A16. I appreciate this suggestion -- (some specifics were given). Thank you.
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