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ROME AIR DEVELOPMENT CENTER 6RIFFISS AFS NY F/G 20/14 

EFFECT OF ENERGETIC PARTICLE EVENTS ON VLF/LF PROPAGATION PARAM—ETC(U) 
OCT 80 J P TURTLE» J E RASMUSSEN 
RADC-TR-80-307 


NL 
































EFFECTS OF ENERGETIC PARTICLE 
EVENTS ON VLF/LF PROPAGATION 
PARAMETERS, 1974-1977 


John P. Turrto 
John E. Rasmusson 
Way no I. Kiomotti 


oT 

Y st ? 2 




AFPtOVID FOR PUMIC RCLCASC; OtSTRISUTfON UNUMITCO 


ROME AIR DEVELOPMENT CENTER 

Air Fore* Systems Command 

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.EFFECTS OFIENERGETIC PARTICLE EVENTS 
ON VU/LF PROPAGATION PARAMETERS, 
1974-1977,' " * 

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10. SUPPLEMENTARY NOTES 

♦Megapulse Inc., Bedford, MA 


VLF propagation 

LF propagation 

Ionospheric disturbances 

Polar cap absorption events 


10. ABSTRACT (Continue on roverae aide ft neceaeary and Identity by block number) 

This report provides a summary of disturbance effects of energetic par¬ 
ticle events on VLF/LF propagation parameters as observed by the USAF 

High Resolution VLF/LF Ionosounder in Northern Greenland. Disturbance 
effects on ionospheric reflectivity parameters, including reflection heights 
and coefficients, are presented along with data from a riometer, a magnetom¬ 
eter, and satellite particle detectors. 

__ 


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SECURITY CLASSIFICATION OF THIS PAGE (When Hat* Entered* 


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Preface 


The authors thank Royce C. Kahler and Duane Marshall for help with the in¬ 
strumentation which made the measurements possible, and Jens Ostergaard and 
Bjarne Ebbesen for the outstanding operation in Qanaq, Greenland. 

Appreciation is also extended to the Danish Commission for Scientific Research 
in Greenland for allowing these measurements to be conducted, and to Jorgen 
Taagholt and V. Neble Jensen of the Danish Meteorological Institute's Ionospheric 
Laboratory for their continued cooperation in this program. 







I 

i 

I 


Contents 


1. INTRODUCTION 7 

2. EVENT DATA 8 

2.1 Observed Waveforms 13 

2.2 Quantitative Reflection Parameters 14 

2.2.1 Reflection Heights 14 

2. 2. 2 Reflection Coefficients 14 


2.3 Polarization Ellipses for the Down-Coming Skywaves 15 

3. SUPPLEMENTARY DATA 15 

4. DISTURBANCE CHARACTERISTICS 17 

REFERENCES 67 


Illustrations 


1. lonosounder Propagation Path, Thule AB - Qanaq, Greenland 9 

2. Transmitting Antenna, Thule AB - Qanaq, Greenland 10 

3. Basic Ionosounding Experiment 10 

4. Orthogonal Receiving Antennas, Qanaq, Greenland 11 

5. Example of Parallel and Perpendicular Waveforms 


12 





6. Fourier Amplitude Spectrum of Transmitted Pulse 12 

7. Conversion Curve, Groundwave-Skywave Arrival Time 

Difference to Reflection Height 13 

8. VLF/LF Ionospheric Reflectivity Data for 5 November 1974 

(DOY 309) Solar Particle Event 19 

9. VLF/LF Ionospheric Reflectivity Data for 30 April 1976 

(DOY 121) Solar Particle Event 27 

10. VLF/LF Ionospheric Reflectivity Data for 22 August 1976 

(DOY 235) Solar Particle Event 35 

11. VLF/LF Ionospheric Reflectivity Data for 26 July 1977 

(DOY 207) Solar Particle Event 43 

12. VLF/LF Ionospheric Reflectivity Data for 24 September 1977 

(DOY 267) Solar Particle Event 51 

13. VLF/LF Ionospheric Reflectivity Data for 22 November 1977 

(DOY 326) Solar Particle Event 59 

Tables 

1. Disturbance Event Data 16 


6 













y-i\ 


Effects of Energetic Particle Events 
on VLF/LF Propagation Parameters, 1974—1977 


I. INTRODUCTION 

A compilation of data on the VLF/LF reflectivity of the polar ionosphere from 
1975 to 1977 has been published in previous technical reports. 1 10 In this re¬ 
port, the data for specific periods are expanded in order to give a more detailed 
presentation of the effects of energetic particle events on VLF/LF propagation 
parameters. These periods have been chosen to show disturbance effects for 
events in which the 13.7 to 25.2 - MeV proton flux recorded by the IMP 7/8 satel- 
lites exceeded 10 particles/cm" sec sr MeV. The propagation data were ob¬ 
tained by the USAF High Resolution VLF/LF Ionosounder^ which provides 
direct measurements of ionospheric reflection height and the reflection coefficient 

i 3 

matrix elements M R M and n R : . Also included are data on particle flux 
density, HF riometer absorption, and geomagnetic field intensity. 

(Received for publication 6 October 1980.) 

One to the number of references cited on this page, the reader is directed to 
pages 67 and 68 for references 1 through 10. 

II. Lewis, E. A., Rasmussen, J. E., and Kossey, P. A. (1973) Measurements 

of ionospheric reflectivity from 6 to 35 kHz, J. Geophys, Res . 78:19. 

12. Kossey, P. A., Rasmussen, J. E., and Lewis, E. A. (1974) VLF pulse iono- 

sounder measurements of the reflection properties of the lower ionosphere, 
Akademie Verlag , COSPAR. 

13. Rudden, K. G. (1961) Radio Waves in the Ionosphere , Cambridge University 

Press, London, p. ffWi 


7 










The VLF/LF Ionosounding Transmitter (Figure 1) is located at Thule Air Base, 
Greenland (76° 33' N Lat., 68° 40' W Long.), and the receiving site is 106 km 
north at the Danish Meteorological Institute's Ionospheric Observatory in Qanaq, 
Greenland (77° 24' N Lat., 69° 20' W Long., Geomagnetic Lat. 89° 06' N). The 
ionosounding transmissions consist of a series of extremely short (approximately 
100 usee) VLF pulses, precisely controlled in time, and radiated from the 130-m 
vertical antenna (Figure 2). At the receiver, the radiated signal arrives first by 
groundwave propagation (Figure 3). Due to the extremely short pulse length, this 
signal has passed the receiver before the arrival of the ionospherically reflected 
skywave pulse, providing independent groundwave and skywave data. Orthogonal 
loop antennas (Figure 4) are used to receive the two polarization components of 
the ionospherically reflected skywave signal. One loop, oriented in the plane of 
propagation, senses the groundwave and the unconverted or "parallel" (II) compo¬ 
nent of the down-coming skywave; the second loop, nulled on the groundwave, 
senses the converted or perpendicular (i) skywave component. The signal from 
each of the antennas is digitally averaged to improve the signal-to-noise ratio of 
the individual received waveforms before they are recorded on magnetic tape. An 
example of the observed waveforms is given in Figure 6, where the parallel wave¬ 
form (a) consists of a groundwave propagated pulse, a quiet interval containing 
low level, off path groundwave reflections, followed by the first-hop parallel sky- 
wave component; the perpendicular waveform (b) is also shown. Each of these 
waveforms is comprised of 266 digitally averaged points, spaced 2 usee apart. 
Ionospheric reflection parameters are derived by computer processing of the 
ground and ionospherically reflected waveforms, with allowance made for factors 
such as ground conductivity and antenna patterns (see Section 2.2). 

On 25 September 1975, and on 1 May 1977, the transmitted waveform was 
O'- nged slightly; Figure 6 shows the resulting effect on the Fourier amplitude 
.-pectrum of the received groundwave signal. Although the data presented are 
generally limited to frequencies in the first, or principal lobe of the spectrum, 
information at higher frequencies can be used when sufficient signal-to-noise con¬ 
ditions exist. There is, however, a frequency range around each spectral null 
where insufficient signal exists for measurements. 


2. F.VKNT DATA 

The data are presented for each disturbance event in three general formats: 
first, the observed waveforms are shown in a synthetic three-dimensional display 
which starts approximately two days prior to the event and covers a fourteen-dav 
period; second, the data are presented in the frequency domain with reflection 


8 









WEST LONGITUDE 



Figure 1. Ionosounder Propagation Path, Thule AB - Qanaq, Greenland 


9 


NORTH LATITUDE 








Figure 2. Transmitting Antenna, 
Thule AB - Qanaq, Greenland 


- ' W « i f 



Figure 3. Basic Ionosounding Experiment 


10 










Orthogonal Receiving Antennas, Qanaq, Greenland 


! 


1 

'i 

i 









3 

1 


-GROUNDWAVE 


-PARALLEL SKY WAVE 
V COMPONENT 


w a. “PARALLEL" WAVEFORM SENSED BY THE LOOP ANTENNA ORIENTED 


IN THE PLANE OF PROPAGATION 


PERPENDICULAR SKYWAVE 
\ COMPONENT 


b. "PERPENDICULAR" WAVEFORM SENSED BY THE LOOP ANTENNA 
ORIENTED PERPENDICULAR TO THE PLANE OF PROPAGATION 


TIME - MICROSECONDS 


Figure 5. Example of Parallel and Perpendicular Waveforms 


PRE 25 SEP 1975 

25 SEP 1975 TO I MAY 1977 

I MAY 1977 TO END OF 1977 



w// V*. 


1-0 43.3 60.0 

JL'tNC Y ' KH L j 


Figure G. Fourier Amplitude 
Spectrum of Transmitted Pulse 













heights and coefficients plotted as a function of frequency over the range from 
approximately 5 to 30 kHz; third, the data are presented as a function of time-of- 
day. In addition to reflection information, this section contains data on ionospheric 
absorption, geomagnetic field data activity, and solar proton fluxes. 

2.1 Observed Waveforms 

A three-dimensional waveform display is presented for a 2-week period con¬ 
taining each disturbance event, together with a display of the same 2-week period 
from a year in which it was not disturbed. For each display, the waveforms were 
stacked one behind the other in linear time, progressing from bottom to top. Each 
individual waveform is a 30-min average of approximately 10,000 pulses. The 
horizontal scale for these plots is linear in time (microseconds), measured from 
the start oi the groundwave. This scale can be used to calculate an effective 
height of reflection by attributing the time delay between the start of the ground- 
wave and the start of the skywave to a difference in travel distance, assuming a 
sharply bounded, mirror-like ionosphere. Figure 7 gives a conversion curve for 
this calculation based on simple geometry and the specific Thule AB - Qanaq, 
Greenland separation of 10G km. For the disturbance periods, fixed local ground 
clutter, amounting to only 2 percent of the groundwave amplitude, was removed to 
avoid interference with the skywave and improve the appearance of the waveforms. 


Figure 7. Conversion Curve, 
Groundwave-Skywave Arrival 
Time Difference to lleflection 
Height 



GROUNDWAVE- SKYWAVE ARRIVAL TIME DIFFERENCE - MSEC 


13 






The three-dimensional displays of the disturbed and normal parallel wave¬ 
forms are given for each event in Parts A and B of Figures 8 through 13. A plot 
of the diurnal variation in solar zenith angle for the midpoint of the path appears 
in Part C. The perpendicular waveform displays are shown in Parts D and E. 

The time of maximum particle flux is indicated on the disturbance plots. 

2.2 Quantitative Parallel and Perpendicular Reflection Parameters 

For each event individual parallel and perpendicular waveforms were selected 
in order to show the effects of the disturbance on the ionospheric reflection height 
and reflection coefficients as a function of frequency. The selected waveforms 
from the disturbance period are shown in Part F of the data figures, whereas the 
corresponding undisturbed waveforms are shown in Part G. 

2.2.1 REFLECTION HEIGHTS 

The group mirror height (GMH) of reflection was obtained by determining the 
group delay of the skywave relative to the groundwave and attributing this differ¬ 
ence to a difference in the propagation distance. The group delay can be defined 
as the rate of change of phase with frequency as discussed in Lewis et al. 11 For 
the GMH data presented in this report, a finite frequency difference of 1.0 kHz was 
used, and the corresponding phase difference as a function of frequency for the 
groundwave and both skywave signals was obtained by Fourier analysis of the re¬ 
spective pulses. The GMH calculations took into account ground conductivity 
(10 3 mho/m is assumed), with the Wait and Howe 14 corrections applied. Group 
mirror heights for the parallel and perpendicular waveforms are plotted as a func¬ 
tion of frequency in Parts H and I of Figures 8 through 13 for both normal and dis¬ 
turbed conditions. The GMHs are also presented as a function of time-of-day for 
the average frequency of 16.5 kHz. In Figures 8 through 13, Parts L and O, paral¬ 
lel and perpendicular reflection height information is given based on two-hour 
averaged data for the two-week period; Parts V and W show the 24-hour period of 
the event onset in greater detail, based on 5-min averaged data. These parts in¬ 
clude a normal reflection height curve for reference purposes. Each point of the 
reference height curve is an average, by two-hour time blocks, for the 14-day 
normal period indicated. 

2.2.2 REFLECTION COEFFICIENTS 

Assuming that the ionosphere acts as a "mirror" at the GMH, we obtained 

13 

plane wave reflection coefficients by comparing the ratio oHhe skywave Fourier 
amplitude at a specific frequency to that of the groundwave, taking into account the 

14. Wait, J. R. and Howe, H. H. (1956) Amplitude and Phase Curves for Ground- 
wave Propagation in the Band 200 Cycles per Second to 500 Kilocycles. 

Natl. Bureau of Standards, U. S. Circ. No. 574. 

14 




w 






E 






- ■■■■ . *-’*«'• - 


wave spreading, earth curvature, ground conductivity, path lengths, and antenna 
patterns including ground image effects. 

The reflection coefficient n R n , obtained from analysis of the parallel sky- 
wave component, is plotted as a function of frequency for both normal and disturbed 
conditions in Part H. From the corresponding perpendicular skywave pulses, the 
coefficient n R t was obtained; it appears as a function of frequency in Part I. 

The || R|| coefficient for 16 kHz is plotted as a function of time-of-day in Part M 
along with the averaged normal coefficient. As with the reflection heights, a more 
detailed ( | R j| coefficient plot, based on 5-min averaged data is shown in Part V. 
To show the variation in reflectivity as a function of frequency during the event, 
the reflection coefficients were calculated at 8 kHz, 16 kHz, and 22 kHz and are 
plotted in Part N as a function of time for the 14-day period. The corresponding 
reflection coefficient plots for ||R i are given in Parts P, Q, and W. 

For certain coefficient data points, plotted as asterisks, the reflection co¬ 
efficient appears without a corresponding GMH. For these particular data, only 
the skywave-groundwave ratios could be obtained since the skywaves were too weak 
to provide reliable group delay information. The reflection coefficients were esti¬ 
mated using a nominal GMH of 80 km in the calculations. These estimated coeffi¬ 
cient values are included in the averages presented in Parts M, N, P, and Q, but 
the assumed heights are not used in the GMH averages. 

2.3 Polarization Ellipses for the Down-Coming Skywaves 

As described by Rasmussen et al, 15 the polarization ellipse of the skywave 
can be determined from the amplitudes of the parallel and perpendicular compo¬ 
nents and their phase difference. Each ellipse represents the locus of the tip of 
the rotation field vector as seen when looking in the direction of propagation of the 
down-coming skywave. The ellipses are drawn to a scale in which the incident 
wave amplitude is unity, and each division on the axis is 0.1. The direction 
of rotation is indicated by an arrow. Parts J and K of Figures 8 through 13 
presefent polarization ellipse data as a function of frequency at 5 kHz intervals 
based on the selected disturbed and normal waveforms of Parts F and G, 
respectively. 


3. SUPPLEMENTARY DATA 


In order to interpret the effects of ionospheric disturbances on the VLF'LF 
ionosounding data, information from several geophysical sensors is presented. 
Parts R and S of Figures 8 through 13 present data from a magnetometer and a 

15. Rasmussen, J. E., et al., (1975) Low Frequency Wave-Reflection Properties 
of the Equatorial Ionosphere , AFCRL-TR-75-0615. 


15 







1 1 1 III! I .1 WW.J« |w wid g mu BCr 

'•>- . .. : ^ gam**** m - igamam* ■*• ■■■>--— i * “* -^— | —- 

*i 

i 


30-MHz riometer operated by RADC at Thule Air Base. The riometer, the con¬ 
ventional monitor of ionospheric disturbances, measures the signal level of cosmic -j 

radio noise passing through the ionosphere. The cyclical diurnal variation seen in 
the riometer signal level during undisturbed periods is caused primarily by the 
earth's daily rotation with respect to the extraterrestrial noise sources. A de¬ 
crease in the received noise level represents an increase in the absorption result- ( 

ing from enhanced ionization due to energetic particles. The absorption effects of 
energetic particle events are seen as an abrupt decrease in the riometer signal 
level followed by a gradual recovery to normal over a period of several days. The 
magnetometer data plotted are the horizontal (H) component of the polar magnetic 
field determined by a 3-axis fluxgate magnetometer at Thule Air Base. The mag¬ 
netometer responds to the effects of polar ionospheric current systems related to 
disturbance events. 

In addition to the information from the ground-based monitors, particle flux 
data are presented from the Applied Physics Laboratory of Johns Hopkins Univer¬ 
sity experiments aboard the IMP 7 and 8 satellites. * These satellites are in 
roughly circular orbits at about 35 earth radii. The data presented in Parts T and 
U are hourly averages of differential flux levels for protons in two energy ranges: 

0.97 to 1.85 MeV and 13.7 to 25.2 MeV. These particle data are most important 
for relating the VLF/LF ionosounder effects to the size of a particular disturbance. 

The supplementary data are summarized in Table 1. 


Table 1. Disturbance Event Data 


Date 

Maximum 

13. 7-25. 2 MeV 
Protons/cm 2 
sr sec MeV 

Minimum 

16 kHz 11 
Reflection 
Height (km) 

30 MHz 

Riometer (dB) 
Absorption 

Illumination 

Conditions 

5 Nov 74 

Day 309 

1.3 

63 

<0. 5 

nighttime 

30 Apr 76 

Day 121 

6 

58 

3 

daytime 

22 Aug 76 

Day 235 

0. 6 

60 

1.7 

daytime 

26 July 77 

Day 207 

0.02 

70 

<0. 5 

daytime 

24 Sep 77 

Day 267 

2 

57 

2 

daytime 

22 Nov 77 

Day 326 

14 

64 

0. 75 

nighttime 


’■'Particle data obtained from the National Space Science Data Center, Greenbelt, MD 



16 







4. DISTURBANCE CHARACTERISTICS 


From the events included in this report, the following VLF/LF disturbance 
patterns can be seen in the ionosounding data. The reflection heights for both the 
parallel and perpendicular components drop coincident with the influx of energetic 
particles. The level to which the reflection heights drop depends first, upon the 
magnitude of the particle flux, and second, upon the presence or absence of solar 
illumination during the event. The lowest reflection heights for a given particle 
flux are attained during a daytiriie event. As seen in Table 1, the 22 Aug 76 
(DOY 235) polar daytime event with a 13 to 25 MeV proton flux of 0. 6 particles 
resulted in a 16-kHz reflection height of 60 km; whereas the more energetic 
22 Nov 77 (DOY 326) polar nighttime event with flux of 14 particles caused a drop 
in reflection height to only 64 km. During polar daytime events with continuous 
D-region illumination, there is little if any diurnal height variation even if there 
are variations before and after the event. During polar nighttime events, a com¬ 
bination of particle ionization and weak noontime solar illumination can produce a 
diurnal variation during the event when none was present before or even after. 

For day-night events with the sun rising and setting, the disturbed reflection 
heights show a strong diurnal pattern, sometimes with larger variations than 
either before or after the event. 

Reflection coefficients during energetic particle disturbances also behave dif¬ 
ferently, depending upon the solar illumination conditions. During a normal polar 
daytime period, with the sun continually above the horizon, reflection coefficients 
are quite variable and much lower than during a normal nighttime period. During 
a daytime particle event, reflection coefficients can actually increase with respect 
to normal conditions, particularly for 8 and 16 kHz. The coefficients show less 
diurnal variation during the event since the effects of particle ionization appear to 
override the effects of varying solar zenith angle. For the two strong daytime 
events, 30 Apr 76 (DOY 122) and 22 Aug 76 (DOY 235), several days after event 
maximum, the ionospherically reflected pulse became very weak for a period of 
approximately one day, resulting in reduced reflection coefficients before return¬ 
ing to normal conditions. This behavior is not associated with additional particle 
precipitation; it may be a part of the daytime recovery process for the particular 
geometry of the Thule AQ - Qanaq path. Reflection coefficient behavior for night¬ 
time and day-night disturbances is less complex, and similar in pattern to the 
reflection heights. During a polar nighttime event, the reflection coefficients 
decrease coincident with the particle influx and then recover steadily to normal. 
During day-night disturbed conditions, the reflection coefficients show an enhanced 
diurnal variation compared with variations prior to and following the event. 


17 






5 November 1974 Solar Particle Event 


Date: 5 November 1974 DOY: 309 
Report Figure: 8 

Related Solar Flare: 5 November 1530 UT 

Start of Ionospheric Effects: 1600 UT 

Time of Maximum 13-25 MeV Proton Flux: 2200 UT 

n 

Maximum Flux: 1.3 particles/cm sec sr MeV 

Length of Particle Event: 3 days 

Lowest Reflection Height (16 kHz): 63 km 

Time of Lowest Height: 1730 UT 

30 MHz Riometer Absorption: 0. 5 dB 

Solar Zenith Angle Range: 93°-121° 

Illumination Conditions: nighttime 

Typical of polar nighttime conditions, before the event the M R u 16 kHz iono¬ 
spheric reflection coefficient was large (0. 5) and the reflection height high (85 km). 
The disturbance effects on the propagation parameters were quite clear: a rapid 
decrease followed by a gradual return to normal. As seen in Parts L-Q, before 
the event there was little diurnal variation in the parameters due to insufficient 
noontime ionizing radiation. During the event, however, an enhanced variation is 
noted; this was probably due to a combination of particle ionization, and a small 
amount of noontime solar radiation. 

It is probable that a small particle event occurred between 12 and 14 November 
(DOY 316-318). This can be seen in the reflection height and coefficient data in 
Parts L-Q, as well as in the particle and magnetometer data in Parts R and U. 


19 


1 



HBCSaua Hffl* Ri tM T - HOF rilMD 







Ionospheric Reflectivity Data for 5 November 1974 (DOY :i09) Solar Particle Event (continued) 






VLF/LF Ionospheric Reflectivity Data for 5 November 1974 (DOY 309) Solar Particle Event (continued) 





















■e 8. VLF/LF Ionospheric Reflectivity Data for 5 November 1974 (DOY 309) Solar Particle Event (continued) 
































30 April 1976 Solar Particle Event 


Date: 30 April 1976 DOY: 121 
Report Figure: 9 

Related Solar Flare: 30 April 2014 UT x-ray class: X2 
Start of Ionospheric Effects: 2050 UT 

Time of Maximum 13-25 MeV Proton Flux: 1 May 0300 UT 

Maximum Flux: 6 particles/cm^ sec sr MeV 

Length of Particle Event: 5 days 

Lowest 16 kHz Reflection Height: 58 km 

Time of Lowest Reflection Height: 1 May 0015 UT 

30 MHz Riometer Absorption: 3. 5 dB 

Solar Zenith Angle Range: 63°-91° 

Illumination Conditions: daytime 

As is typical of normal daytime conditions the M R M 16 kHz reflection coeffi¬ 
cients (Part H) were much lower (0.08) than daring normal polar nighttime condi¬ 
tions (0.5). The effects of the disturbance on the reflection coefficients were less 
clearly defined than during a nighttime event. As seen in Part N, the parallel 
reflection coefficient showed an increase during the first part of the event. This 
was followed by a relatively stable period with little diurnal variation for the next 
two days. On 3 May (DOY 124), two days after event particle maximum, the re¬ 
flected skywave pulse became very weak for about a day (Parts A and D). This is 
seen in the reflection coefficients in Parts N and Q which were lower on 3 May 
than during event maximum, partii ’arly for 16 and 22 kHz. This was not related 
to an increase in particle flux, but may be part of the recovery process as asso¬ 
ciated with the particular geometry of the Thule Air Base - Qanaq path. Subse¬ 
quent to 3 May, there was a gradual recovery over the next four days. The reflec¬ 
tion heights during the event followed the usual pattern of abrupt drop followed by 
a gradual recovery; there was no diurnal height variation during this daytime 
event. The reflection heights returned to normal on 3 May (DOY 124), several 
days before the reflection coefficients. 


27 


IBBCKDD0 FAOS BUNfc-NO* TIUCD 










Ionospheric Reflectivity Data for 30 April 









J«l«S (IE! 'OKI) ‘JiGl OK JOJ 1 l ,: < I [joj[ ot.ioqdsouoi .1 r J /. i' 1 A *0 djn3 















ure 9. VLF/LF Ionospheric Renectivity Data for 30 April 1976 (DOY 121) Solar Particle Event (continued) 









VLF/LF Ionospheric Reflectivity Data for 30 April 1976 (DOY 121) Solar Particle 
















Figure 9. VLF/LF Ionospheric Reflectivity Data for 30 April 1976 (IK>Y 121) Solar Particle Event (continued) 








»«.Vi !■! iw mUm i . h jtaf i ■ i n mto - 


t 


FXZBBSmW 


22 August 197S Solar Particle Event 

Dr!11 22 August 1976 DOV : 23 > 

Report Figure: 10 

Related So1.<r Flare: 22 August 121-1 I T -.-ray cl.. M'l 

Star; of Ionospheric Effects: 1220 1 i 

Tim*' ( ,f Maximum 13-25 MeV Proton Flu*: *- : j 

. ■> 

Maximum Flux: 0.6 particles/cm sr sec Me\ 

Length of l'article Event: 3 days 
Lowest 16 kHz Reflection Height: 60 km 
Time of Lowest Reflection Height: 1600 I T 
30 MHz Riometer Absorption: 1.7 dll 
Solar Zenith Angle Range: 65°-93° 

Illumination Conditions: daytime 

As with other polar daytime disturbance events, the R reflection coeffi¬ 
cients at maximum particle flux (Part H) showed an increase for some frequencies 
and a decrease for other frequencies. After event maximum, the reflection coeffi¬ 
cients were relatively steady for the next several days as compared with the coeffi¬ 
cients before and after the event (Parts \ and Q). In the latter part of the event, 
when the ionosphere had nearly recovered, the reflected sky wave pulse became 
very weak for a period of about a day (Parts A and D). This same effect was seen 
in the April 1976 event (Figure 9). As seen in Parts N and Q on 24 August (IX >Y 
237), the reflection coefficients were below the level at event maximum, particu¬ 
larly for 16 and 22 kHz. This again appears to be part of the disturbance recovery 
process rather than the result of an enhancement in the particle event itself. The 
reflection heights showed the usual daytime event pattern of an abrupt drop fol¬ 
lowed by a gradual recovery with no diurnal variations. 


35 


HtBCEOUO JRAOC BUMMWt IUND 







Ionospheric Reflectivity Data for 22 A 
















Ionospheric Reflectivity Data for 22 August 1976 (DOY 235) Solar Particle Event (continued) 





















10. VLF/LF Ionospheric Reflectivity Data for 22 August 1976 (DOY 235) Solar Particle Event (continued) 




10. VLF/LF Ionospheric Reflectivity Data for 22 August 1976 (DOY 235) Solar Particle Event (continued) 





F Ionospheric Reflectivity Data for 22 August 1976 (DOY 235) Solar Particle Event (continued) 















gusi 
















26 July 1977 Solar Particle Event 

Date: 26 July 1977 DOY: 207 

Report Figure: 11 

Related Solar Flare: No data 

Start of Ionospheric Effects: 1600 UT 

Time of Maximum 13-25 MeV Proton Flux: 2000 UT 

2 

Maximum Flux: 0. 02 particles/cm sec sr MeV 

Length of Particle Event: 3 days 

Lowest 16 kHz Reflection Height: 70 km 

Time of Lowest Reflection Height: 0100 'JT 27 July 

30 MHz Riometer Absorption: 0. 5 dB 

Solar Zenith Angle Range: 58°-83° 

Illumination Conditions: daytime 

This was the smallest particle event to be included in this report. Unlike 
other events reported here, the particle flux rose rather slowly to a maximum. 
As is typical of polar daytime events with continuous D-region illumination, the 
reflection height curves show a drop in height, followed by a gradual return to 
normal with no diurnal variation. The reflection coefficients which prior to the 
event were quite low and irregular, increased, and showed less variation during 
the event. The ionospheric reflection coefficients returned to normal by 29 July. 


43 


ffOCCSOUtf f*QB BUMK-MOg VUMB 























activity Datn for 20 July 1977 (DOY 207) Solar Particle Evert {• ontinuod) 











11. VLF/LF Ionospheric Reflectivity Data for 26 July 1977 (DOY 207) Solar Particle Event (continued) 










VLF/LF Ionospheric Reflectivity Data for 26 July 1977 (DOY 207) Solar Particle Event (continued) 








Figure 11. VLF/LF Ionospheric Reflectivity Data for 2G July 1977 (DOY 207) Solar Particle Event (continued) 











dm* t ' H'Wl W B i 



Ionospheric Reflectivity Data for 26 July 1977 (DOY 207) Solar Particle Event (continued) 









24 September 1977 Solar Particle Event 

Date: 24 September 1977 DOY: 267 

Report Figure: 12 

Related Solar Flare: No data 

Start of Ionospheric Effects: 0615 UT 

Time of Maximum 13-25 MeV Proton Flux: 1000 UT 

2 

Maximum Flux: 2 particles/cm sec sr MeV 
Length of Particle Event: 6 days 
Lowest 16 kHz Reflection Height: 57 km 
Time of Lowest Reflection Height: 1400 UT 
30 MHz Riometer Absorption: 2 dB 
Solar Zenith Angle Range: 75°-103° 

Illumination Conditions: Day-night 

This energetic particle event occurred during an already disturbed period; a 
series of events had occurred, beginning on 8 September (DOY 251). No data are 
presented for these events as ionosounding records are incomplete. This was a 
day-night event with the sun rising and setting. Unlike polar daytime or nighttime 
events, there was an enhanced diurnal variation of the reflection heights and coeffi¬ 
cients during the event. Solar radiation and particle ionization resulted in lower 
noontime reflection heights; the absence of solar radiation at night allowed the re¬ 
combination of electrons, and thus a partial recovery toward normal reflection 
height conditions. 


51 



































8 12. VLF/LF Ionospheric Reflectivity Data for 24 September 1977 (DOY 267) Solar Particle Event (continued) 









12. VLF/LF Ionospheric Reflectivity Data for 24 September 1977 (DOY 267) Solar Particle Event (continued) 


















LF Ionospheric Reflectivity Data for 24 September 1977 (DOY 267) Solar Particle Event (continued) 















22 November 1977 Solar Particle Event 


Date: 22 November 1977 DOY: 326 
Report Figure: 13 

Related Solar Flare: 0945 UT x-ray class: XI 

Start of Ionospheric Effects: 1030 UT 

Time of Maximum 13-25 MeV Proton Flux: 1600 UT 

2 

Maximum Flux: 14 particles/cm sec sr MeV 
Length of Particle Event: 8 days 
Lowest 16 kHz Reflection Height: 64 
Time of Lowest Reflection Height: 1700 UT 
30 MHz Riometer Absorption: 0.75 dB 
Solar Zenith Angle Range: 96°-124° 

Illumination Conditions: nighttime 

Based on the high energy proton flux, this was the strongest event during the 
period of this report. Because this was a polar nighttime event, however, the 
magnitude of the effects on the VLF/LF parameters was less than would have oc¬ 
curred had there been solar illumination. The daytime 20 August 1976 event with 

O 

only 0.6 particles/cm sec sr MeV produced a lower reflection height of about 
60 km. As is typical of a nighttime event, the disturbance effects on the reflection 
heights and coefficients (Parts L-Q) are well defined: an abrupt drop followed by 
a gradual recovery basically following the particle flux curve. 


59 






RECEIVED PARALLEL WAVEFORM DATA 



Ionospheric Reflectivity Data for 22 November 1977 (DOY 326) Solar Particle Event (continued) 







Ionospheric Reflectivity Data for 22 November 1977 (DOY 326) Solar Particle Event (continued) 







Figure 13. VLF/LF Ionospheric Reflectivity Data for 22 November 1977 (DOY 326) Solar Particle Event (continued) 









13. VLF/LF Ionospheric Reflectivity Data for 22 November 1977 (DOY 326) Solar Particle Event (continued) 






Figure 13. VLfZlf Ionospheric Reflectivity Data for 22 November 1977 (DOY 326) Solar Particle Event (continued, 













VLF/LF Ionospheric Reflectivity Data for 22 November 













References 


1. Rasmussen, J. E,, McLain, R. J., and Turtle, J. P. (1976) VLF/LF 

Reflectivity of the Polar Ionosphere, 19 January - 2 March 197 5. AFCRL- 
TR-76-0045, AD A022674. 

2. Rasmussen, J. E., McLain, R. J., and Turtle, J. P, (1976) VLF/LF 

Reflectivity of the Polar Ionosphere, 2 March - 3 May 1976. RADC-TR-76- 
146, AD A026465. 

3. Rasmussen, J. E., McLain, R. J., Turtle, J. P., and Klemetti, W. I. (1976) 

VLF/LF Reflectivity of the Polar Ionosphere, 4 May - 5 July 1975. RADC- 
TR-76-270, AD A034023. 

4. Rasmussen, J. E., McLain, R. J., Turtle, J. P., and Klemetti, W. I. (1976) 

VLF/LF Reflectivity of the Polar Ionosphere, 20 July - 20 September 1975 . 
RADC-TR-76-327, AD A036913. 

5. Rasmussen, J. E., McLain, R. J., Turtle, J. P., and Klemetti, W. I. (1976) 

VLF/LF Reflectivity of the Polar Ionosphere, 21 September 1975 - 3 January 
1976, RADC-TR-76-378, AD A037794. 

6. Rasmussen, J. E., Turtle, J. P., Pagliarulo, R. P., and Klemetti, W. I. 

(1977) VLF/LF Reflectivity of the Polar Ionosphere, 4 Janua. y - 3 July 

1976 . RADC-TR-77-68, AD A040920. 

7. Rasmussen, J, E., Turtle, J. P,, Pagliarulo, R. P., and Klemetti, W. I. 

(1977) VLF'/LF Reflectivity of the Polar Ionosphere, 1 August 1976 - 
1 January 1977 , RADC-TR-77-141, AD A044050. ^ 

8. Rasmussen, J. E., Turtle, J. P., Pagliarulo, R. P., and Klemetti, W. I. 

(1977) VLF/LF Reflectivity of the Polar Ionosphere, 2 January - 30 April 

1977. RADC-TR-77-251, AD A047238. 

9. Rasmussen, J. E., Turtle, J. P., Pagliarulo, R. P., and Klemetti, W. I. 

(1977) VLF/LF Reflectivity of the Polar Ionosphere, 1 May - 3 September 
1977, RADC-TR-77-428, AD A053236. 


67 










10. Pagliarulo, R. P., Turtle, J. P., Rasmussen, J. E., and Klametti, W. 1. 
(1978) VLF/LF Reflectivity of the Polar Ionosphere, 4 September - 


31 December 1977 , RADC-TR-78-95, AD A060918. 

11. Lewis, E. A., Rasmussen, J. E., and Kossey, P. A. (1973) Measurements 

of ionospheric reflectivity from 6 to 35 kHz, J. Geophys. Res. 78:19. 

12. Kossey, P. A., Rasmussen, J. E., and Lewis, E. A. (1974) VLF pulse iono- 

sounder measurements of the reflection properties of the lower ionosphere, 
Akademie Verlag . COSPAR. 

13. Budden, K. G. (1961) Radio Waves in the Ionosphere, Cambridge University 

Press, London, p. fF5I 

14. Wait, J. R, and Howe, H. H. (1956) Amplitude and Phase Curves for Ground- 


wave Propagation in the 3and 200 Cycles per Secona to 500 Kilocycles, 


Natl. Bur. Stand. U. S. Circ. No. 574. 

15. Rasmussen, J. E., etal., (197 5) Low Frequency Wave-Reflection Properties 


of the Equatorial Ionosphere, AFCRL-TR-75-0615. 


68