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The day to night absorption ratio in auroral zone riometer measurements
Planet.
Space Sci., Vol.
25, pp. 1193 to 1198. Peqamon
THE DAY
Press,
1977. Printed
in Northern
IreIand
TO NIGHT ABSORPTION RATIO IN AURORAL ZONE RIOMETER MEASUREMENTS RICFIARD 1.ARMSTRONG* Department of.Physics, University of Tromsti, Troms0, Norway F. TOM BERKEY Department of Physics, University of Calgary, Calgary, Alberta, Canada and TORE MELBYE
Department of Physics, University of Troms#,Troms#,
Norway
(Received 18 February 1977) day to night ratio in ionospheric radio wave absorption has been studied for College, Alaska using a narrow beam riometer array and computer assisted programming of the data. For the
Abstrmt-The
period of 6 November 1967 to 17 April 1968 no significant departure from a ratio of unity was found. 1. INTRODUCl’ION The
use of the riometer has been well described by Hargreaves (1969). The instrument has given valuable information on the precipitation of energetic particles with more than about 10 keV energy and in consequence has yielded valuable insight into the development of the aurora1 substorm (see, e.g. Berkey et al., 1974). Because it is an integral technique, it is not easy to determine the height of particle precipitation responsible for the absorption of radiowaves. Indeed, it is very difficult to be precise on any characteristic of the incoming particle flux. However, certain types of events can be identified by their temporal development. For example, polar cap absorption events (and perhaps also relativistic electron precipitation events) are identifiable under most conditions. The PCA event is clearly distinguished by a marked increase in absorption at sunrise and a decrease at sunset. This is believed to be due to the removal of electrons from negative ions by associative detachment and photodetachment. Present thinking is that the former process predominates; with detachment being produced by atomic oxygen generated by solar dissociation of O2 and 0,. PCA events are characterized apparently by relatively constant particle precipitation over long periods of time. Therefore, it is easy to see that the variation due, as hypothesized, to atmospheric effects changes at zenith angles around 90”. * On leave for the year 1976 at UCLA, Los Angeles, CA, U.S.A.
The question has been put; should the same type of variation be seen on normal aurora1 substorm absorption? The typical duration of an aurora1 substorm is about 3 h and this certainly is longer than the duration of the day to night transition under most conditions. However, the aurora1 substorm can produce a very variable absorption and a statistical analysis is necessary to see if differences occur in absorption between day and night. Analyses of this type have been carried out by Holt and Landmark (1963) and by Hultqvist (1963). Holt and Landmark find a value of about 1.6 for the ratio of day to night absorption whereas Hultqvist finds the ratio to be about 1.1. This investigation is intended to examine the ratio again using more sophisticated methods of analysis and larger data samples. It is appropriate at this point to discuss the remarks of Branscomb (1964) who argued that riometers have limited spatial resolution and cannot resolve the fine structure seen in visual aurora. This may indeed be true and we can refer to a rocket flight where it is believed the transition from dusk to dawn was directly investigated (Armstrong et al., 1970). However, the improved resolution of the equipment here used (7 deg beamwidth) is a significant improvement on earlier ground based studies and we think merits a re-examination of the problem. 2. METHOD Riometer data from College, Alaska (64.65”N, 256.56”E geomagnetic) for the period 6 November 1967-17 April 1968 was used for this analysis. This
1193
1194
R. J.ARMWRONO, F. T. BERICEY and T. MELBYE
data was obtained at a frequency of 35.7 MHz and scaled at 10 min intervals. A narrow beam antenna of 7 deg half-power beamwidth was used (Ansari, 1965) with automatic daily calibration of the riometer. As emphasized by Hargreaves (l969), the obtaining of a “quiet day curve” is an important and tedious part of the riometer technique. It is possible that here error is introduced into the measurements. We have used computer techniques to determine the quiet day curve and this is carried out as described below. Once the quiet day curve is obtained, it is a straightforward calculation to find the absorption at a given time. The times for which measurements are available are then grouped as to whether the time is by day or by night. The mean absorption for all the day values is then calculated, and similarly for the night values. Thence the ratio of mean day to mean night absorption is obtained. The boundary between day and night is not easy to define because the rays of the Sun aluminate high altitudes even when the solar zenith angles exceeds 90”. However, we do not know which altitude is most pertinent to the absorption. It is probable that most of the absorption occurs below 90 km. altitude (~~~rathy et al., 1963). In addition there are time constants involved due to the complicated chemistry of the ionosphere, so that even when the Sun’s rays cease to activate the region there can still be ionization production on a scale comparable to solar ionization in full day. To make a partial check on this effect we have TABLE
1.
fioC%JXJRE ADOF93D
ABSORPIION(S%
FOR
2.1 De~~itio~ of the quiet day curve The quiet day curve was obtained in the following manner (illustrated in Table 1). Each data point with its appropriate UT is fed into the computer. The appropriate sidereal time (ST) is calculated and the data grouped into 10 min intervals in sidereal time. When all the data is read in, the data points are then distributed in 144 boxes, in sidereal time. The interval of IO min was chosen because this was the sampling interval. The quiet day curve was determined anew for each month’s readings. (The one exception to this procedure was in the second period which was 41 days long, and was necessitated due to changes in the impedance of the ground beneath the antenna which altered the impedance of the antenna.) The quiet day noise signal for each box was defined as the signal value which
CALCULATING
THE
DAY
TO
NIGHT
RATIO
i,
Catcutate ST
11,
Divide into 144 ST)
b
boxes of IO min duration
F
Refer bock to UT
(A,
ST) I ‘1, ,
. (A, UT1
*
OF
UT= UNIVERSAL TIME;ST=SIDEREALTIME
CRITERION);
Diode current (I. UT 1
calculated the day-night ratio for five different assignments of the day-night boundary, namely zenith angles of 87, 90, 93, 96 and 99 deg. Of course, the boundary cannot be absolutely defined by a zenith angle, but the calculations represent an attempt to lessen the unce~~nty in the method. The computer program used (QDAY) is available on request. The program includes an option for real-time, on-line TV presentation of the quiet day curve. ON the Nord 1 system at the University of Trams@, (16 bit 64 K), the processing of a month’s data, read in on cards, required a total time of about 250 s. This includes the printing of the quiet day curve in sidereal time.
Divide UT into doy or night
(A, night 1 (A, day)
Calculates (A,
&Y),,,,
(A, night) m,on
.y.* S’ 1
l
2 26,$6
n
I
1
1’:1’
1 f 11
I 111 rzt 1 11 .12 I 21
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and symbols in the figure. A number 3 for example means three readings were obtained for the given current in the appropriate ‘box’ of sidereal time. UnfortunateIy, numbers greater than 3 are illegible in this reproduction. X denotes the point of the quiet day curve determined for the given box by exercise of the percentage criterion. A, B, etc. denote l,2, etc., observations of the appropriate current and sidereal time which have value or values greater than that determined for the quiet day curve at that sidereal time.
by a vertical line of numbers
l
. . ; . . .
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fly
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: *.***
l
. *
l
.l
~~~~Q~DAY~~~~~~V~~ FIG.~. PARTOF~COMPUTERO~OFTHEPROORAMUSEDFORD The y-ordinaterepresents riometer diode current in ma and the x-ordinate sidereal time. The experimental ObserWiOnS at 10’ inteds are separated into 144 ‘boxes’ of 10’ intervals of sidereal time. Thus there are six boxes in every hour of sidereal time-each box being represent&
f p6*9c
L
1
ii’ .
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. 11. 11 J 111 1111 I 1 .11112 x A . . ; 1 2. .r : ..,. I . . . . . . . . * . . 1 :.. l . (I t , . , , 11 111,212 12222x4 l. . . . ..l*l.**I...... *l..lllll:I:....lI..I222I..xA* . . . . . . . ..r**......... . ..* L. *..-..*......~...*.....~*...**.*. . . 11 1 I IIllI 2 l I a2111x e A . . . t t l 2181 1 2 1 . ?
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0.9 d.1 0.2 a.3 9.4 9.5 6.6 9.7 9.6 I I I I I I I f I **~*CI**e~fL~~t**l**~*~~~~~~~~~~~~~~*4**~~~~~****~~~~~~~~~~~~~**~~~~~~~**~~**~~~~~~~*~~~****~***************~* t XI 1 l 1 1 I 11. . 11 la 1 921 YA b l .’ 1
Ir
1196
R. J.
ARMSTRONG,
F. T.
separated 10% of the data points in each box from the other 90%. These 10% of the points had the maximum signal strength for the data points in the box concerned. When this process is repeated for each box, the quiet day curve is obtained in sidereal time. For each point z (measured in diode current ma) the absorption is then given as 10 log,, (y/z) dB, where y is the quiet day signal for the box in which z is placed. The readings with z 2 y are discarded. We believe in this way that the absorption is consistent with the best estimate we can make. Hook (1968) used a graphical presentation of all data points, as we have done here. However, he employed a visual judgment to determine the quiet day curve, where we have used a percentage criterion. The percentage of the criterion is not critical. To prove this, we have also calculated quiet day curves, and then night to day ratios of absorption, for 3% and 5% criteria. The results (described below) do not show much variation from those using a 10% criterion. This is not true for the period of 6 December 1967-7 January 1968. The number of data points for this time interval is not as great as for the other intervals. Inspection of the quiet day curves for the 3% and 5% criteria shows that they are irregular and show considerable deviation from the quiet day curves for the preceding and succeeding intervals. The quiet day curve derived using the 10% criterion is, however, in general agreement with those for the preceding and succeeding intervals and does not show irregular variation. In Fig. 1 part of the computer calculated curve for a single month of data is shown. An improved criterion for determining the quiet day curve could be developed on the basis of the distribution of data values in each box. Instead of taking an arbitrary percentage of the points as done here, the form of the distribution, N(I), could be used. As Fig. 2 shows, the point where d2N/d12 = 0 would perhaps be a reasonable definition for the No OF DATA POINTS
CURRENT
I
FIG. 2. SCHEMATIC REPRESENTATION OF DISTRIBUTION OF DATAPOlNTSWITHCURRENTFORASPECIFICSIDEREALTIME.
larger value of Z determined by d2N/dZ2 = 0 could be used to determine the quiet day current reading.
The
BERKEY
and T.
MELRYE
TABLE 2. DAY TO NIGHT RATIOS FOR FIVE PERIODS FOR VARIOUSVALUESOF(1)THESOLARZENITHANGLECHOSEN TO SEPARATE! DAY AND NIGHT AND (2)l-HE PERCENTAGE CRITERION USEDTODETERMINETHEQUIETDAYCURVE,AS DESCRIBED IN SECTION 2.1 OF THE TEXT IN ADDITION. THE MAXIMUM VALUES OF ABSORPTION FOR THE FIVB PERIODS ARE GIVEN
Zenith angle
87 90 93 96 99
87 90 t: 99
Criterion 5% 10%
3%
6 Nov.-S Dec. 1.471 1.473 1.20 1.208 1.12 1.122 1.06 1.059 1.02 1.024
6 Dec. 1967-7 Jan. 1968 1.631 1.442 1.004 1.653 1.438 ‘1.008 1.629 1.474 1.016 1.609 1.473 1.009 8 Jan.-l0
87 ;: 96 99
1967 1.48 1.223 1.135 1.065 1.026
1.056 0.945 0.902 0.878 0.861
Feb. 1968
1.056 0.925 0.902 0.878 0.861
Maximum absorption (dB) 11.1
7.782
10.669
1.0 0.882 0.828 0.796 0.774 7.597
;: 99
11 Feb.-l3 Mar. 1968 1.061 1.061 0.992 1.034 1.034 0.972 1.005 1.005 0.948 0.983 0.983 0.927 0.96 0.903 0.96
6.214
K 93 96 99
14 Mar.-l7 Apr. 1968 0.831 0.832 0.823 0.802 0.802 0.786 0.789 0.789 0.774 0.786 0.786 0.761 0.764 0.765 0.733
87 90
quiet day value of the current. A technique to implement this definition is currently being developed at the University of Calgary. For the calculations used in Table 2, all values of the absorption using the criterion specified were included. It is possible that some of the absorptions thus used relate to relatively high altitude (E and F regions) ionization events where the ion chemistry is radically different from that in the D-region. It is plausible that these events could have lower absorption than D-region events. Therefore, a test was made to see if the day to night ratio was sensitive to the presence of data with low values of
Day to
TABLE 3. DAY TO NIGHT RATIOS FOR FIVE PJXRIODSFOR FIVE DI-NT VALUES GF THE MINIMUM ABSORFrIoN USED FOR SELFXXING DATA. AS IN TABLE 2 FIVe DIFFERRNT VALUES OF SOLAR ZENlTH ANGLJ3 ARE USED TO SEPARAm DAY AND
NIGHT.
ARE
0,
%R
VALURS
OF MlNIhWM
ABSORFTION
USED
0.2, 0.3, 0.4 AND0.5 dB. TEiEPRRCRNTAGE CRITERION. AS DBFINED IN SECTION 2.1 IS 10% 0.1,
Zenith angle
Minimum abso6pt$n (dB) 0.1 0.2 . 0.5
0 1.48 1.223 1.135 1.065 1.026
6 Nov.-5 Dec. 1.338 1.277 1.167 1.131 1.099 1.069 1.037 1.015 1.0 0.983
1967 1.207 1.102 1.046 0.998 0.973
1.151 1.054 1.019 0.981 0.969
absorption. Tables 3 and 4 show the results of this study. Day to night ratios of absorption were calculated for the same time intervals as before. However, here all measurements of absorption less than given values were excluded. These given values were 0.1, 0.2, 0.3 and 0.5 dB for Table 3. For Table 4 they were I, 1 S, 2, 2.5, 3 and 3.5 dB. The general result appears to be a decrease in the day to night ratio as the minimum value of absorption is raised. However, this decrease is slight and there are small increases.
3. RYkwLlY3
These
0.926 0.921 0.87 0.935 0.935 0.876 0.947 0.933 0.871 0.951 0.936 0.874
:::82 0.828 0.796 0.774
0 943 0:834 0.789 0.759 0.745
night
0.941 0.907 0.891 0.887
8 Jan.-l0 Feb. 1968 0 937 0:840 0.801 0.77 0.758
0 921 0:SSl 0.828 0.803 0.79
0.828 0.788 0.786 0.79 0.791
11 Feb.-12 Mar. 1968 0.992 0.972 0.948 0.927 0.903
0.983 0.954 0.93 0.905 0.883
0.953 0.918 0.889 0.871 0.861
0.939 0.9 0.879 0.868 0.859
0.913 0.902 0.895 0.899 0.892
14 Mar.-17 Apr. 1968 0.823 0.786 0.774 0.761 0.733
0.850 0.822 0.805 0.784 0.752
0.848 0.829 0.818 0.801 0.762
0.874 0.855 0.845 0.839 0.808
0.866 0.837 0.824 0.823 0.81
4. DAY
NOVJZMRER-5
DECEMBER 1967
6 Nov.-5 Dec. 1967 Minimum absorption (dL3) 2 2.5 3 1.5
ratios
summarized
of absorption
in Table for
2.
roughly
4. CONCJAMONS
TO NIGHT RATIOS FOR ONE PERIOD 6 FOR SIX VALUES OF MINIMUM ALLOWED ABSORPTION. THSSE VALUES ARE 1, 1.5, 2, 2.5, 3 AND 3.5dB. FIVE VALUES OF THE SOLAR ZENITH ANGLE! USED TO SEPARATE DAY AND NIGHT ARE USED AS IN TABLES 2 AND 3. A 10% PERCENTAGE CRITRRION IS USED TABLE
are
The day to monthly intervals are shown for three values of the parameter determining the quiet day curve (QDC) and for five zenith angles determining the day to night transition. The changes with the QDC parameter are minimal, except as discussed above, and we feel that the QDC’s determined are relatively accurate. The day to night ratio in only one case (zenith angle equal to 87) exceeds 1.25 and in only one case is less than 0.75. The number of cases using the 10% criterion with a day to night ratio greater or equal to unity is 10. The number less than unity is 14. In view of observational and measuring errors and of possible latent errors we suggest that there is no evidence for a variation of absorption from day to night for the riometer observations considered here.
6 Dec. 1967-7 Jan. 1968 1.004 1.008 1.016 1.009
1197
night absorption ratio in aurora1 zone
Zenith angle
1
87
1.063
0.998
0.96
0.934
0.943
0.888
Z! 96 99
0.97 1.001 0.961 0.953
0.933 0.952 0.924 0.925
0.914 0.928 0.904 0.901
0.911 0.893 0.887 0.891
0.907 0.922 0.904 0.912
0.874 0.863 0.858 0.862
3.5
The results of this study support the measurements of Hultqvist (1963), and would indicate that statistically the difference from day to night absorption can be small. We would stress that the antenna system we have used has much better resolution than that of other workers. Further refinements of the technique of analysis are possible but it is believed that the results would not be substantially altered. However, the geomagnetic position of College vis-6-G Scandinavia may have bearing on the results. It is well known for example that differences in longitude affect the absorption statistical pattern (Rapoport, 1970, Waite, 1965, Berkey, 1973). Furthermore, only 5 months winter data were availabfe and this represents a limitation to the results. For example, a day to night variation in summer might well exist.
R. J.
1198
hhSSTRONI3,
F. T. BERKBYand T. MaLea
Acknowledgements-The cooperation of the computer centre at the University of Tromsb is gratefully acknowledged. Financial support for one of the authors (F. T. B.) was provided by the Royal Norwegian Council for Scientific and Industrial Research and by the National Research Council of Canada (Grant A 3131). REmRENcEs
Ansari, Z. A. (1965). A narrow-beam antenna array for radio wave absorption studies in the aurora1 zone. Proc. IEEE 53,530. Armstrong, R. J., Folkestad, K. and Troim, J. (1970). A D-region sunrise aurora1 rocket flight. J. atios. ten. Phys. 32, 1305. Berkey, F. T. (1973). A comparison of the latitudinal variation of aurora1 absorption at different longitudes. J. atmos. ten. Phys. 35, 1881. Berkey, F. T., Driatskiy, V. M., Henriksen, K., Hultqvist. B., Jelly, D. H., Shchuka, T. I., Theander. A. and Yliniemi, J. (1974). A synoptic investigation of particle precipitation dynamics for 60 substorms in IQSY (1964-65) and IASY (1969) Planet. Space Sci. 22,255. Branscomb, L. M. (1964). A review of photodetachment
and related negative ion processes relevant to aeronomy. Ann. Geophys. 20, 105. Hargreaves, J. K. (1969). Aurora1 absorption of HF radio waves in the ionosphere, a review of results from the first decade of riometrv. Proc. IEEE 57. 1348. Holt, 0. and Landmark,-B. (1963). On thk solar control of aurora1 type absorption. Radioastronomical and Satellite Studies of the Alomosphere (Ed. J. Aarons), pp. 251-255. North-Holland, Amsterdam. Hook, J. L. (1968). Morphology of aurora1 zone radiowave absorption in the Alaska sector. J. armos. ten. Phys. 30, 1341. Hultqvist, B. (1963). On the height distribution of the ratio of negative ion and electron densities in the lower ionosphere. 3. ahnos. ten. Phys. 25, 225. Parthasarthy, R., Lerfald, G. M. and Little, C. G. (1963). Derivation of electron-density profiles in the lower ionosphere using radio absorption measurements at multiple frequencies. J. geophys. Res. 68, 358. Rapoport, Z. TS. (1970). Comparison of the variability of aurora1 radio wave absorption in the ionosphere at Loparskaya and College. Geomagn. Aeron. 10,58. Waite, C. W. (1965). Aurora1 phonomena in an integralinvariant coordinate system. Can. J. Phys. 43,2319.
Space Sci., Vol.
25, pp. 1193 to 1198. Peqamon
THE DAY
Press,
1977. Printed
in Northern
IreIand
TO NIGHT ABSORPTION RATIO IN AURORAL ZONE RIOMETER MEASUREMENTS RICFIARD 1.ARMSTRONG* Department of.Physics, University of Tromsti, Troms0, Norway F. TOM BERKEY Department of Physics, University of Calgary, Calgary, Alberta, Canada and TORE MELBYE
Department of Physics, University of Troms#,Troms#,
Norway
(Received 18 February 1977) day to night ratio in ionospheric radio wave absorption has been studied for College, Alaska using a narrow beam riometer array and computer assisted programming of the data. For the
Abstrmt-The
period of 6 November 1967 to 17 April 1968 no significant departure from a ratio of unity was found. 1. INTRODUCl’ION The
use of the riometer has been well described by Hargreaves (1969). The instrument has given valuable information on the precipitation of energetic particles with more than about 10 keV energy and in consequence has yielded valuable insight into the development of the aurora1 substorm (see, e.g. Berkey et al., 1974). Because it is an integral technique, it is not easy to determine the height of particle precipitation responsible for the absorption of radiowaves. Indeed, it is very difficult to be precise on any characteristic of the incoming particle flux. However, certain types of events can be identified by their temporal development. For example, polar cap absorption events (and perhaps also relativistic electron precipitation events) are identifiable under most conditions. The PCA event is clearly distinguished by a marked increase in absorption at sunrise and a decrease at sunset. This is believed to be due to the removal of electrons from negative ions by associative detachment and photodetachment. Present thinking is that the former process predominates; with detachment being produced by atomic oxygen generated by solar dissociation of O2 and 0,. PCA events are characterized apparently by relatively constant particle precipitation over long periods of time. Therefore, it is easy to see that the variation due, as hypothesized, to atmospheric effects changes at zenith angles around 90”. * On leave for the year 1976 at UCLA, Los Angeles, CA, U.S.A.
The question has been put; should the same type of variation be seen on normal aurora1 substorm absorption? The typical duration of an aurora1 substorm is about 3 h and this certainly is longer than the duration of the day to night transition under most conditions. However, the aurora1 substorm can produce a very variable absorption and a statistical analysis is necessary to see if differences occur in absorption between day and night. Analyses of this type have been carried out by Holt and Landmark (1963) and by Hultqvist (1963). Holt and Landmark find a value of about 1.6 for the ratio of day to night absorption whereas Hultqvist finds the ratio to be about 1.1. This investigation is intended to examine the ratio again using more sophisticated methods of analysis and larger data samples. It is appropriate at this point to discuss the remarks of Branscomb (1964) who argued that riometers have limited spatial resolution and cannot resolve the fine structure seen in visual aurora. This may indeed be true and we can refer to a rocket flight where it is believed the transition from dusk to dawn was directly investigated (Armstrong et al., 1970). However, the improved resolution of the equipment here used (7 deg beamwidth) is a significant improvement on earlier ground based studies and we think merits a re-examination of the problem. 2. METHOD Riometer data from College, Alaska (64.65”N, 256.56”E geomagnetic) for the period 6 November 1967-17 April 1968 was used for this analysis. This
1193
1194
R. J.ARMWRONO, F. T. BERICEY and T. MELBYE
data was obtained at a frequency of 35.7 MHz and scaled at 10 min intervals. A narrow beam antenna of 7 deg half-power beamwidth was used (Ansari, 1965) with automatic daily calibration of the riometer. As emphasized by Hargreaves (l969), the obtaining of a “quiet day curve” is an important and tedious part of the riometer technique. It is possible that here error is introduced into the measurements. We have used computer techniques to determine the quiet day curve and this is carried out as described below. Once the quiet day curve is obtained, it is a straightforward calculation to find the absorption at a given time. The times for which measurements are available are then grouped as to whether the time is by day or by night. The mean absorption for all the day values is then calculated, and similarly for the night values. Thence the ratio of mean day to mean night absorption is obtained. The boundary between day and night is not easy to define because the rays of the Sun aluminate high altitudes even when the solar zenith angles exceeds 90”. However, we do not know which altitude is most pertinent to the absorption. It is probable that most of the absorption occurs below 90 km. altitude (~~~rathy et al., 1963). In addition there are time constants involved due to the complicated chemistry of the ionosphere, so that even when the Sun’s rays cease to activate the region there can still be ionization production on a scale comparable to solar ionization in full day. To make a partial check on this effect we have TABLE
1.
fioC%JXJRE ADOF93D
ABSORPIION(S%
FOR
2.1 De~~itio~ of the quiet day curve The quiet day curve was obtained in the following manner (illustrated in Table 1). Each data point with its appropriate UT is fed into the computer. The appropriate sidereal time (ST) is calculated and the data grouped into 10 min intervals in sidereal time. When all the data is read in, the data points are then distributed in 144 boxes, in sidereal time. The interval of IO min was chosen because this was the sampling interval. The quiet day curve was determined anew for each month’s readings. (The one exception to this procedure was in the second period which was 41 days long, and was necessitated due to changes in the impedance of the ground beneath the antenna which altered the impedance of the antenna.) The quiet day noise signal for each box was defined as the signal value which
CALCULATING
THE
DAY
TO
NIGHT
RATIO
i,
Catcutate ST
11,
Divide into 144 ST)
b
boxes of IO min duration
F
Refer bock to UT
(A,
ST) I ‘1, ,
. (A, UT1
*
OF
UT= UNIVERSAL TIME;ST=SIDEREALTIME
CRITERION);
Diode current (I. UT 1
calculated the day-night ratio for five different assignments of the day-night boundary, namely zenith angles of 87, 90, 93, 96 and 99 deg. Of course, the boundary cannot be absolutely defined by a zenith angle, but the calculations represent an attempt to lessen the unce~~nty in the method. The computer program used (QDAY) is available on request. The program includes an option for real-time, on-line TV presentation of the quiet day curve. ON the Nord 1 system at the University of Trams@, (16 bit 64 K), the processing of a month’s data, read in on cards, required a total time of about 250 s. This includes the printing of the quiet day curve in sidereal time.
Divide UT into doy or night
(A, night 1 (A, day)
Calculates (A,
&Y),,,,
(A, night) m,on
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and symbols in the figure. A number 3 for example means three readings were obtained for the given current in the appropriate ‘box’ of sidereal time. UnfortunateIy, numbers greater than 3 are illegible in this reproduction. X denotes the point of the quiet day curve determined for the given box by exercise of the percentage criterion. A, B, etc. denote l,2, etc., observations of the appropriate current and sidereal time which have value or values greater than that determined for the quiet day curve at that sidereal time.
by a vertical line of numbers
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~~~~Q~DAY~~~~~~V~~ FIG.~. PARTOF~COMPUTERO~OFTHEPROORAMUSEDFORD The y-ordinaterepresents riometer diode current in ma and the x-ordinate sidereal time. The experimental ObserWiOnS at 10’ inteds are separated into 144 ‘boxes’ of 10’ intervals of sidereal time. Thus there are six boxes in every hour of sidereal time-each box being represent&
f p6*9c
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0.9 d.1 0.2 a.3 9.4 9.5 6.6 9.7 9.6 I I I I I I I f I **~*CI**e~fL~~t**l**~*~~~~~~~~~~~~~~*4**~~~~~****~~~~~~~~~~~~~**~~~~~~~**~~**~~~~~~~*~~~****~***************~* t XI 1 l 1 1 I 11. . 11 la 1 921 YA b l .’ 1
Ir
1196
R. J.
ARMSTRONG,
F. T.
separated 10% of the data points in each box from the other 90%. These 10% of the points had the maximum signal strength for the data points in the box concerned. When this process is repeated for each box, the quiet day curve is obtained in sidereal time. For each point z (measured in diode current ma) the absorption is then given as 10 log,, (y/z) dB, where y is the quiet day signal for the box in which z is placed. The readings with z 2 y are discarded. We believe in this way that the absorption is consistent with the best estimate we can make. Hook (1968) used a graphical presentation of all data points, as we have done here. However, he employed a visual judgment to determine the quiet day curve, where we have used a percentage criterion. The percentage of the criterion is not critical. To prove this, we have also calculated quiet day curves, and then night to day ratios of absorption, for 3% and 5% criteria. The results (described below) do not show much variation from those using a 10% criterion. This is not true for the period of 6 December 1967-7 January 1968. The number of data points for this time interval is not as great as for the other intervals. Inspection of the quiet day curves for the 3% and 5% criteria shows that they are irregular and show considerable deviation from the quiet day curves for the preceding and succeeding intervals. The quiet day curve derived using the 10% criterion is, however, in general agreement with those for the preceding and succeeding intervals and does not show irregular variation. In Fig. 1 part of the computer calculated curve for a single month of data is shown. An improved criterion for determining the quiet day curve could be developed on the basis of the distribution of data values in each box. Instead of taking an arbitrary percentage of the points as done here, the form of the distribution, N(I), could be used. As Fig. 2 shows, the point where d2N/d12 = 0 would perhaps be a reasonable definition for the No OF DATA POINTS
CURRENT
I
FIG. 2. SCHEMATIC REPRESENTATION OF DISTRIBUTION OF DATAPOlNTSWITHCURRENTFORASPECIFICSIDEREALTIME.
larger value of Z determined by d2N/dZ2 = 0 could be used to determine the quiet day current reading.
The
BERKEY
and T.
MELRYE
TABLE 2. DAY TO NIGHT RATIOS FOR FIVE PERIODS FOR VARIOUSVALUESOF(1)THESOLARZENITHANGLECHOSEN TO SEPARATE! DAY AND NIGHT AND (2)l-HE PERCENTAGE CRITERION USEDTODETERMINETHEQUIETDAYCURVE,AS DESCRIBED IN SECTION 2.1 OF THE TEXT IN ADDITION. THE MAXIMUM VALUES OF ABSORPTION FOR THE FIVB PERIODS ARE GIVEN
Zenith angle
87 90 93 96 99
87 90 t: 99
Criterion 5% 10%
3%
6 Nov.-S Dec. 1.471 1.473 1.20 1.208 1.12 1.122 1.06 1.059 1.02 1.024
6 Dec. 1967-7 Jan. 1968 1.631 1.442 1.004 1.653 1.438 ‘1.008 1.629 1.474 1.016 1.609 1.473 1.009 8 Jan.-l0
87 ;: 96 99
1967 1.48 1.223 1.135 1.065 1.026
1.056 0.945 0.902 0.878 0.861
Feb. 1968
1.056 0.925 0.902 0.878 0.861
Maximum absorption (dB) 11.1
7.782
10.669
1.0 0.882 0.828 0.796 0.774 7.597
;: 99
11 Feb.-l3 Mar. 1968 1.061 1.061 0.992 1.034 1.034 0.972 1.005 1.005 0.948 0.983 0.983 0.927 0.96 0.903 0.96
6.214
K 93 96 99
14 Mar.-l7 Apr. 1968 0.831 0.832 0.823 0.802 0.802 0.786 0.789 0.789 0.774 0.786 0.786 0.761 0.764 0.765 0.733
87 90
quiet day value of the current. A technique to implement this definition is currently being developed at the University of Calgary. For the calculations used in Table 2, all values of the absorption using the criterion specified were included. It is possible that some of the absorptions thus used relate to relatively high altitude (E and F regions) ionization events where the ion chemistry is radically different from that in the D-region. It is plausible that these events could have lower absorption than D-region events. Therefore, a test was made to see if the day to night ratio was sensitive to the presence of data with low values of
Day to
TABLE 3. DAY TO NIGHT RATIOS FOR FIVE PJXRIODSFOR FIVE DI-NT VALUES GF THE MINIMUM ABSORFrIoN USED FOR SELFXXING DATA. AS IN TABLE 2 FIVe DIFFERRNT VALUES OF SOLAR ZENlTH ANGLJ3 ARE USED TO SEPARAm DAY AND
NIGHT.
ARE
0,
%R
VALURS
OF MlNIhWM
ABSORFTION
USED
0.2, 0.3, 0.4 AND0.5 dB. TEiEPRRCRNTAGE CRITERION. AS DBFINED IN SECTION 2.1 IS 10% 0.1,
Zenith angle
Minimum abso6pt$n (dB) 0.1 0.2 . 0.5
0 1.48 1.223 1.135 1.065 1.026
6 Nov.-5 Dec. 1.338 1.277 1.167 1.131 1.099 1.069 1.037 1.015 1.0 0.983
1967 1.207 1.102 1.046 0.998 0.973
1.151 1.054 1.019 0.981 0.969
absorption. Tables 3 and 4 show the results of this study. Day to night ratios of absorption were calculated for the same time intervals as before. However, here all measurements of absorption less than given values were excluded. These given values were 0.1, 0.2, 0.3 and 0.5 dB for Table 3. For Table 4 they were I, 1 S, 2, 2.5, 3 and 3.5 dB. The general result appears to be a decrease in the day to night ratio as the minimum value of absorption is raised. However, this decrease is slight and there are small increases.
3. RYkwLlY3
These
0.926 0.921 0.87 0.935 0.935 0.876 0.947 0.933 0.871 0.951 0.936 0.874
:::82 0.828 0.796 0.774
0 943 0:834 0.789 0.759 0.745
night
0.941 0.907 0.891 0.887
8 Jan.-l0 Feb. 1968 0 937 0:840 0.801 0.77 0.758
0 921 0:SSl 0.828 0.803 0.79
0.828 0.788 0.786 0.79 0.791
11 Feb.-12 Mar. 1968 0.992 0.972 0.948 0.927 0.903
0.983 0.954 0.93 0.905 0.883
0.953 0.918 0.889 0.871 0.861
0.939 0.9 0.879 0.868 0.859
0.913 0.902 0.895 0.899 0.892
14 Mar.-17 Apr. 1968 0.823 0.786 0.774 0.761 0.733
0.850 0.822 0.805 0.784 0.752
0.848 0.829 0.818 0.801 0.762
0.874 0.855 0.845 0.839 0.808
0.866 0.837 0.824 0.823 0.81
4. DAY
NOVJZMRER-5
DECEMBER 1967
6 Nov.-5 Dec. 1967 Minimum absorption (dL3) 2 2.5 3 1.5
ratios
summarized
of absorption
in Table for
2.
roughly
4. CONCJAMONS
TO NIGHT RATIOS FOR ONE PERIOD 6 FOR SIX VALUES OF MINIMUM ALLOWED ABSORPTION. THSSE VALUES ARE 1, 1.5, 2, 2.5, 3 AND 3.5dB. FIVE VALUES OF THE SOLAR ZENITH ANGLE! USED TO SEPARATE DAY AND NIGHT ARE USED AS IN TABLES 2 AND 3. A 10% PERCENTAGE CRITRRION IS USED TABLE
are
The day to monthly intervals are shown for three values of the parameter determining the quiet day curve (QDC) and for five zenith angles determining the day to night transition. The changes with the QDC parameter are minimal, except as discussed above, and we feel that the QDC’s determined are relatively accurate. The day to night ratio in only one case (zenith angle equal to 87) exceeds 1.25 and in only one case is less than 0.75. The number of cases using the 10% criterion with a day to night ratio greater or equal to unity is 10. The number less than unity is 14. In view of observational and measuring errors and of possible latent errors we suggest that there is no evidence for a variation of absorption from day to night for the riometer observations considered here.
6 Dec. 1967-7 Jan. 1968 1.004 1.008 1.016 1.009
1197
night absorption ratio in aurora1 zone
Zenith angle
1
87
1.063
0.998
0.96
0.934
0.943
0.888
Z! 96 99
0.97 1.001 0.961 0.953
0.933 0.952 0.924 0.925
0.914 0.928 0.904 0.901
0.911 0.893 0.887 0.891
0.907 0.922 0.904 0.912
0.874 0.863 0.858 0.862
3.5
The results of this study support the measurements of Hultqvist (1963), and would indicate that statistically the difference from day to night absorption can be small. We would stress that the antenna system we have used has much better resolution than that of other workers. Further refinements of the technique of analysis are possible but it is believed that the results would not be substantially altered. However, the geomagnetic position of College vis-6-G Scandinavia may have bearing on the results. It is well known for example that differences in longitude affect the absorption statistical pattern (Rapoport, 1970, Waite, 1965, Berkey, 1973). Furthermore, only 5 months winter data were availabfe and this represents a limitation to the results. For example, a day to night variation in summer might well exist.
R. J.
1198
hhSSTRONI3,
F. T. BERKBYand T. MaLea
Acknowledgements-The cooperation of the computer centre at the University of Tromsb is gratefully acknowledged. Financial support for one of the authors (F. T. B.) was provided by the Royal Norwegian Council for Scientific and Industrial Research and by the National Research Council of Canada (Grant A 3131). REmRENcEs
Ansari, Z. A. (1965). A narrow-beam antenna array for radio wave absorption studies in the aurora1 zone. Proc. IEEE 53,530. Armstrong, R. J., Folkestad, K. and Troim, J. (1970). A D-region sunrise aurora1 rocket flight. J. atios. ten. Phys. 32, 1305. Berkey, F. T. (1973). A comparison of the latitudinal variation of aurora1 absorption at different longitudes. J. atmos. ten. Phys. 35, 1881. Berkey, F. T., Driatskiy, V. M., Henriksen, K., Hultqvist. B., Jelly, D. H., Shchuka, T. I., Theander. A. and Yliniemi, J. (1974). A synoptic investigation of particle precipitation dynamics for 60 substorms in IQSY (1964-65) and IASY (1969) Planet. Space Sci. 22,255. Branscomb, L. M. (1964). A review of photodetachment
and related negative ion processes relevant to aeronomy. Ann. Geophys. 20, 105. Hargreaves, J. K. (1969). Aurora1 absorption of HF radio waves in the ionosphere, a review of results from the first decade of riometrv. Proc. IEEE 57. 1348. Holt, 0. and Landmark,-B. (1963). On thk solar control of aurora1 type absorption. Radioastronomical and Satellite Studies of the Alomosphere (Ed. J. Aarons), pp. 251-255. North-Holland, Amsterdam. Hook, J. L. (1968). Morphology of aurora1 zone radiowave absorption in the Alaska sector. J. armos. ten. Phys. 30, 1341. Hultqvist, B. (1963). On the height distribution of the ratio of negative ion and electron densities in the lower ionosphere. 3. ahnos. ten. Phys. 25, 225. Parthasarthy, R., Lerfald, G. M. and Little, C. G. (1963). Derivation of electron-density profiles in the lower ionosphere using radio absorption measurements at multiple frequencies. J. geophys. Res. 68, 358. Rapoport, Z. TS. (1970). Comparison of the variability of aurora1 radio wave absorption in the ionosphere at Loparskaya and College. Geomagn. Aeron. 10,58. Waite, C. W. (1965). Aurora1 phonomena in an integralinvariant coordinate system. Can. J. Phys. 43,2319.