AD-762 279

THULE RIOMETER OBSERVATIONS OF POLAR CAP ABSORPTION EVENTS (1962-1972)

Raymond J. Cormier

Air Force Cambridge Research Laboratories L. G. Hanscom Field, Massachusetts

30 January 1973

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AFCRL-TR.73.0060 30 JANUARY 1973 AIR FORCE SURVEY IN GEOPHYSICS, NO. 255

AIR FORCE CAMBRIDGE RESEARCH LABORATORIES L. C. HANSCOM FIELD. BEDPORD. MASSACHUSETTS

Thule Riometer Observations of Polar Cap Absorption Events (1962-1972)

RAYMOND J. CORMIER

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AFCRL-TR.73-0060 30 JANUARY 1973 AIR FORCE SURVEY IN GEOPHYSICS, NO. 255

IONOSPHERIC PHYSICS LABORATORY PROJECT 5631

AIR FORCE CAMBRIDGE RESEARCH LABORATORIES L. C. HANSCOM FIELD, BEDFORD, MASSACHUSETTS

Thule Riometer Observations of Polar Cap Absorption Events (1962-1972)

RAYMOND J. CORMIER

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AIR FORCE SYSTEMS COMMAND United States Air Force

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Air Force Cambridge Research Laboratories (LID L. G. Hanscom Field Bedford, Massachusetts 01730

20. REPONT SICURITV CLAIIIPICkTIOII

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J RtPOKT TITLE

THULE RIOMETER OBSERVATIONS OF POLAR CAP ABSORPTION EVENTS (1962-1972)

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Raymond J. Cormier

«. «E»0«T DATE

30 January 1973 • a CONTNACT OR SRANT NO.

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11. ABSTRACT

Polar Cap Absorption (PCA) events have been observed at Thule, Greenland, (geographic N 76° 33', W 68° 40') using a ground-based riometer for the past decade. The riometer was operated at a fixed frequency of 30 MHz, and utilized a two-element Yagi antenna directed toward the zenith. This report provides information for operational systems personnel and systems designers concerning the mean behavior of PCAs and other aspects of the phenomenon; for example, the extreme cases in duration and magnitude In addition, included as an appendix are plots of dB absorption and proton flux (obtained from satellites) versus time for each event. The total number of events is 29.

DO '0"" 1473 "'' I NOV U '""

Unclassified Security CI«i»i(icatioa

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Unclassified Security Classification

Polar cap absorption events Mean behavior of PCAs Ionospheric absorption Riometers Solar proton events

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Unclassified Security Classification

- -^

Abstract

Polar Cap Absorption (PCA) events have been observed at Thule, Greenland,

(geographic N 76° 33', W 68° 40') using a ground-based riometer for the past

decade. The riometer was operated at a fixed frequency of 30 MHz, and utilized

a two-element Yagi antenna directed toward the zenith. This report provides

information for operational systems personnel and systems designers concerning

the mean behavior of PCAs and other aspects of the phenomenon; for example, the

extreme cases in duration and magnitude. In addition, included as an appendix

are plots of dB absorption and proton flux (obtained from satellites) versus time

for each event. The total number of events is 29.

iii

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Contents

1. INTRODUCTION

2. MEAN BEHAVIOR OF PCAs AT THULE

ACKNOWLEDGMENTS

REFERENCES

APPENDIX: Twenty-nine PC A Events Used to Obtain Mean Behavior

1

2

5

5

7

Illustrations

1. Hours of Absorption Observed on 30-MHz Riometer at Thule, Greenland, for Selected Levels

1. List of PCA Events

Preceding page blank

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Thule Riometer Observations of Polar Cap Absorption

Events(1962-1972)

1. INTRODUCTION

For the past 10 years a 30-MHz riometer was operated on a continuous basis

at Thule, Greenland. During that time, the riometer was used to observe polar

cap absorption (PCA) events. These events were also observed by other experi-

ments such as VLF propagation paths, HF propagation paths, and increases in

fmin observed on an ionosonder located in Greenland.

The purpose of this report is to provide information to systems designers and

operational systems personnel concerning the behavior of PCA events in such a way

as to place this phenomenon in proper perspective with other ionospheric phenom-

ena and geomagnetic disturbances. Provided for reference is a catalog of PCAs

observed at Thule, from January 1962 through August 1972. It is important at

this time to state that the events presented do not include all possible PCAs ob-

served by other means during that period, nor certainly all the solar proton events

observed on various satellites (Shea and Smart, private communication).

(Received for publication 30 January 1973)

.

MAtfa

The criteria used in selecting PCAs was that the observed vertical absorption

on the 30-MHz riometer must equal 0.5 dB above the diurnal and seasonal varia-

tion. This is important because the diurnal and seasonal variation in absorption

observed at Thule can be 0.8 dB (Cormier, 1970), which exceeds the PCA lower

limit of 0.5 dB.

2. MEAN BEHAVIOR OF FCAs AT THULE

During the observation period, a total of 53 PCA events was investigated. Of

this total, 11 did not produce events greater than 0.5 dB for two possible reasons:

first, the proton flux was of insufficient magnitude to produce an observable effect

on the riometer recording; and secondly, the seasonal effect (darkness during the

winter months) may have contributed to a reading of minimal absorption, the

mechanism of the nighttime recovery phenomenon (Reid, 1966; Leinbach, 1967),

Of the remaining, 13 were ambiguous for reasons such as equipment malfunction,

antenna problems, or data loss. However, the whole event period was not neces-

sarily lost. It is constructive to add at this time that the large PCA events greater

than 10 dB would certainly not be included in the questionable category, because

large PCA events are unambiguous on riometer recordings. The 29 remaining

events (see Table 1) illustrated in the Appendix are unambiguous events; they are

used to obtain the mean behavior of PCA events. In Table 1, tlie following informa-

tion is given: date of onset time; time in UT of the onset, max, and end time;

observed magnitude of dB absorption on the riometer; and proton flux observed by

satellites.

Table 1. List of PCA Events

Date

Time (UT) Riometer

Max dB

2 Protons/cm sec ster

Max Proton Flux Begin Max End

1962

2000 1 Feb

2200 9 Feb

1400 2 Feb

1100 10 Feb

2100 2 Feb

NONE REPO

NONE GREA

2.8

2.4

RTED

PER THAN

2,600E>1.4 MeV

0.35E> 10 MeV

0.5 dB

Feb 1

1963

Feb 9

1964

1965

1

Table 1. List of PCA Events (Cont)

Date

Time (UT) Riometer

Max dB

o Protons/cm sec ster

Max Proton Flux Begin Max End

1966

Mar 24 Jul 7 Aug 28 Sep 2

0900 24 Mar 0100 7 Jul 1900 28 Aug 0800 2 Sep

1100 24 Mar 1200 7 Jul 0100 20 Aug 1300 3 Sep

1800 24 Mar 2100 8 Jul 0300 3 1 Aug 2000 6 Sep

0.6 2.1 2.4

14.0

15E>20 28E> 15 16i:>20

1.300E> 15

MeV MeV MeV MeV

1967

Jan 28 Mar 11 May 24 May 28 Jun 6

1500 28 Jan 1900 11 Mar 0400 24 May 0600 28 May 0900 6 Jun

1800 29 Jan 1900 12 Mar 1400 25 May 0900 28 May 0200 7 Jun

0300 3 1 Jan 2000 12 Mar 0100 27 May 1700 30 May 2200 8 Jun

2.7 1.0 8.3 4. 1 1.8

0. 6K>15

980E> 10 115E> 10 20E> 10

MeV

MeV MeV MeV

1968

Jun 9 Oct 31 Nov 18

1000 9 Jun 1200 31 Oct 1400 18 Nov

0800 10 Jun 1900 2 Nov 1600 19 Nov

0100 12 Jun 0600 3 Nov 1900 20 Nov

6.5 2.5 1.7

354E>10 133E> 10 849E>10

MeV MeV MeV

1969

Jan 24 Feh 25 Mar 30 Apr 11 Nov 2

1400 24 Jan 1200 25 Feb 1100 30 Mar 1300 11 Apr 1200 2 Nov

1700 24 Jan 1500 25 Keb 1900 30 Alar

13 Apr 1600 2 Nov

0900 25 Jan 0100 26 Feb

0000 21 Apr 0200 5 Nov

1.2 2. 1 1.3

>16

3E>10 88E>10 26E>10

1,348E> 10 1.317E> 10

MeV MeV MeV MeV MeV

1970

1200 7 Mar 1500 28 Mar 2100 8 Mar 5.1 93E>10 MeV Mar 7

1971

0300 25 Jan 1200 6 Apr 0000 21 Apr 2200 1 Sep 0100 17 Dec

1600 26 Jan 1800 6 Apr 0800 21 Apr 1000 2 Sep 0800 17 Dec

2000 27 Jan 0200 8 Apr 1000 21 Apr 0400 5 Sep 1600 17 Dec

2.3 2.2 0.9 5.2 1.9

1, 170E>10 51E> 10 3E>10

245E>10

MeV MeV MeV MeV

Jan 25 Apr 6 Apr 21 Sep 1 Dec 17

1972

Jan 20 May 28 Aug 3 Aug 7

1500 20 Jan 2300 28 May 0000 3 Aug 1800 7 Aug

1100 21 Jan 2300 29 May

4 Aug 2300 8 Aug

0500 22 Jan 0300 1 Jun

0100 11 Aug

1.8 2.2

>16 14

39E>10 MeV

The mean behavior was obtained to help answer questions asked by many in-

vestigators, for example: How many PC As can one expect a year? Flow long does

a PCA last? How often can severe PC As be expected to occur? During what per-

centage of time can one expect to have appreciable HP absorption phenomena?

kMM^a

The mean duration of a PCA event is 63.6 hours. The mean magnitude is

4. 0 dB. The mean duration of the proton flux is 84. 3 hours. The proton flux was 2

cut off when the flux reached 1 proton/cm sec ster for energies of E> 10 MeV.

The longest event occurred on 11 April 1969, which lasted 203 hours. The shortest

ev ;nt was 9 hours observed on 24 March 1966.

Another parameter of interest to system designers is the amount of time a

particular absorption level is exceeded, which leads to the amount of power re-

quired to account for the expected absorption. The levels chosen in this report are

> 10 dB. >5 dB, >3 dB, >2 dB, and > 1 dB. The total number of hours of PCA

events observed during the 29 events is 1853 hours or 76.8 days. The percentages

of the total time of occurrence for each level are as follows;

>10dB - 3.6%, >5dB - 11.3%

> 3 dB - 17.9%, >2dB - 29.0%

> 1 dB - 57.3%

When considering the foregoing percentages, one must note that the hours of

absorption are not evenly distributed over the report period. To illustrate this,

representative levels are shown in the Figure 1 histogram. The most striking

feature of this figure is the lack of any events during the year 1964 and 1965,

During 1964, no reference to any appreciable solar proton event was reported;

during 1965, no absorption event which exceeded 0,5 dB was detected on the riom-

eter. Large events greater than 10 dB were observed only in three distinct years;

namely, 1966, 1969, and 1972. The relationship between the proton flux curve observed on satellites and

2 the riometer absorption as reported by Judy and Adams (1969), namely, J = K A

where J = proton flux, A = riometer absorption and K is 8_^ 2 was investigated so

as to verify its reliability. The relationship was shown to be very good for the

months of March through September. The months of April and October do not

behave systematically; that is, some follow the relationship and some do not. The

winter montns show no consistent behavior. Of course, the data sample is too

small to draw definite conclusions concerning the relationship.

mm m jtm

et O I

62 63 64 65 66 67 68 69 70 71 72

Figure 1. Hours of Absorption Observed on 30-MHz Riometer at Thule, Greenland, for Selected Levels

Acknowledg ments

The author thanks Dr. S. Silverman and his staff for the operation and main-

tenance of the riometer equipment at the AFCRL Geopole Observatory at Thule

AFB, Greenland. Thanks slso to Dr. K. Toman of AFCRL for the critical reading

and the suggestions for the improvement of this paper.

References

Cormier, R.J. (1970) Riometry as an Aid to Ionospheric Forecasting. Environmental Research Papers, No. 343, AFCRL-70-0689.

Juday, R.W., and Adams, G.W. (1969) Riometer Measurements, Solar Proton Intensities and Radiation Dose Rates, Planet. Space Sei., 17:1313.

MM

Leinbach, H. (1967) Mid-day Recoveries of Polar Cap Absorption, J. Geophys. Res.. 72:5473.

Reid, G.C. (1966) Ionospheric Effects of PCA Events, Space Research VII. North Holland Publishing Co.. Amsterdam, p 864.

Smart, D. F.. and Shea. M.A., Private communication.

^4a

Appendix

Twenty-nine PCA Events Used to Obtain Mean Behavior

Included are plots for the 29 PCA events listed in Table 1 and used in the hist-

ogram of Figure 1. The graphs show plots of dB absorption versus universal time

for the 30-MHz Thule riometer. Some graphs contain plots of the square root of

the proton flux of energies > 10 MeV as observed from various satellites.* The

events prior to 24 May 1967 do not have proton flux data readily available in the

ESSA data pamphlets. The August 1972 events do not have proton flux data available

as yet in the ESSA data pamphlets of the comprehensive series.

Included on the plots is the calculated value of expected absorption (represented 2 by the letter x) using the Judy and Adams relationship of J = KA . In these cases,

the value for K is 8.

The mean behavior as discussed in the foregoing is presented as a guide to

operational personnel and, in particular, systems designers. For instance, if one

needs real-time information, a system or systems must be designed to operate in

the extreme cases, adding to the cost of the system. If the information could be

delayed a number of hours until the severe disturbance was terminatedj then one

could design the system for the mean-behavior magnitude, that is, about 4 dB.

*The proton flux data were obtained from the Solar Geophysical Data, IER-FB- Part II, 282, 292, 298, and 300; Comprehensive Reports 301, 303, 309, 313, 328, 336, 338, and 339, U.S. Department of Commerce, Environmental Sciences Services Administration.

Preceding page blank

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Plot 13 Nov. 18, 1968

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Plot 18 Nov. 2. 1969

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6 12 ia 0 6 12 is 0 • 12 MAR 7 1970 MAR 8 1970 MAR 91970

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Plot 19 Mar. 7, 1970

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Plot 20 Jan. 25, 1971

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Plot 21 Apr. 6, 1971

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6 12 18 0 DEC 18 1971

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CALCULATED dB

0 8 12 ie JAN 20 1972

6 12 18 0 JAN 21 1972

TIME-UT

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Plot 25 Jan. 20, 1972

25

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0 6 12 18 0 6 12 18 0 6 12 MAY 28 1972 MAY 29 1972 MAY 30 1972

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Plot 26 May 28. 1972

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