76 In addition , emission fine structure bands of uniform
79 The correlation coefficient is 0 640 and is significant at the
0.1% level. An example of changing m with f and T within a m
specific e�ission event is given in Table 10.2. The event considered is the Christchurch emis�ion shown in Figure 10 .9 and the time intervals in Table 10.2 are indicated by horizontal bars on the time axis in the figure.
Time Interval hr (UT) 0932 - 0940 1043 - 1049 1 1 10 - 1 1 15 1313 - 1318 Midband Frequency f (cm ps.) 0.55 + 0.04 0.73 0.80 1.33 Band Spacing T (min ) 1.9
-
+ 0.04 1.5 + 0. 02 1. 3 + 0.05 1. 2 + 0.02Mean Band Slope
m ( cps/min)
0.08
-
+ 0.010.2
-
+ 0.030.25
-
+ 0.03 0 . 30-
+ 0.04TABLE 10.2. Fine structure details for specific time intervals
in the April 4, 1964 emission event shown in Figure 10.9. 10.6 HM EMISSION OCCURRENCE AND GEOMAGNETIC CTIV TY.
It is of interest to investigate the association of hm emission occurrence with both local and planetary geomagnetic activity as indicated respectively by the local K index (Amberley, KAM ) and
The distributions of twenty minute hm emis on
occurrence with K�M and KP over the eight month recording period are shown in the upper graphs of Figure 10.8. In order to
indicate the relationshipsmore accurately it is necessary to normalize the emission occurrences. The distributions of K AM and Kr over the same time interval are shown in the middle grap h5
of Figure 10.8 and the normalized hm emission occureence distributions
appear in the lower graphs. In the normalized distributions it is found that emissions are predominantly associated with geomagnetically quiet conditions. The largest group o emissions are observed
under very quiet local conditions (K A� 0 ). The occurrence
distribution with K p is uniformly distributed for K p -S. 2 and is similar to that reported by Tepley (1962 ) for hm emissions in the
0.5 - 1 .0 cps band.
10. 7 S IMULTANEOUS OBSERVATIONS OF HM EMI 3 S IONS AT WIDELY SPACED STATIONS .
The close similarity in frequency-time characteristics of hm emissions recorded simultaneously at middle and low latitudes has been shown in the results reported by Tepley et al (1963 ) and Tepley (1964a). In addition, Tepley (1964b) presented a single emission event which was recorded simultaneously in the northern hesisphere at College, Palo Alto, and Kauai. This emission event was also observed at Christchurch and sonagrams from all four stations are shown in Figure 10.9. The fine structure pattern of the Christchurch emis0ion is very similiar to the patterns
observed in the northern hemisphere. Sonagrams from the above four stations for the emission event of April 5, 1964, shown in Figure 10.2, also indicate similar frequency-time patterns. The multi-station sonagrams for this event are illustrated in Tepley et al. (1965 ).
Furthermore, three randomly selected emissions recorded at College (R.R. Heacock, private communication) were also observed at Christchurch with similar frequency-time characteristics. Two of these emissions occurred during local night-time and one was a low frequency daytime emission.
It is therefore clear that hm emissions occasionally exhibit identical frequency-time characteristics over great latitudinal extent. However, it is unlikely that all hm emissions are
observed over such wide ranges of latitude. This is evident from the fact that middle and high latitude stations exhibit different diurnal variations in emission occurrence. Since College, Palo Alto, Kauai and Christchurch all lie within 40° geomagnetic longitude of each other it is not possible to de termine the
longitudinal extent of simultaneous emission occurrence with these stations.
81. 10. 8 FRE iuENCY-TIME C'.T R CTF.RI'>TIC..i OF .ST'1R��-TIME 1 c1 -i;;xr.:;.;roK.s.
Occasionally, discrete emissions, which do not exhibit the regularly spaced repetitive fine structure pattern characteristic of hm emissions, were observed in the Pc1 band. Some of the emissions contained an irregularly spaced fine structure while in others no fine structure was evident . �hese emi&sions comprised
11% of the Pc1 emissions recorded over the eight month period. Detailed analysis of the sonagram d�ta showed that non-structured and irregularly structured emissions occurred predominantly during geomagnetically active periods and may be conveniently classified, according to bandwidt , into broad-band and narrow-band emissions. Broad-band emissions may be identified with emissions reported by
other workers. On the other hand, narrow-band emissions cannot be
described in terms of previously defined emission categories.
10.8.1 'arrow-b�nd £missions
Narrow-band emissions comprised approxLmutely 90� of the storm-time emissions recorded over the ei�ht month period. Three
examples of this type of emis ion are shown in Figure 10.10. The
signal bandwidths are generally less than one octave and of the same order as those of hm emis�ion events. However, the upper
frequencv , � mit seldom exceeds 1 cps.
The occurrence of narrow-band eLisrions is si�nific-:i.ntly related to both AM md K �he analysis procedure is similar to
th t adopted in � t1on 1 .6, and the results are shown in Figure , v.11. The normal; .. e� occ .... rrences indicate maxima for 1\.p ind hAM- 4 and show that narrow-b .d emissions occur principally d�ring
geomagnetically act�ve periods. Although the majority of emissions in the normalized representation occur for KAM and l\p 4, 5 there
are t 11 as ma f not more, hm emissions with regular fine
structure occurr-.• 0 at equally active times. For ex-:i.mple, the
un-normalized data in Figure 1J.8 indicate thirty-ei6�t twenty
minute hm emisE,ion occurrences associated with K :... 4,5 while the
un-normalized data in Figure 10. 1 1 indic�te thirty-one equivalent occurrences of narrow-band st r�-time emissions. Similarly, nine occurrences of each type of emission are associated ::ith tC ,,,.= ,l , 1
'
.
The diurnal variatJon in the occurrence o f narrow-band emissio ns
is shown in Figure 1 0.1 2. The crosses repre 5ent the twenty minute
occurrence rates and the solid curve indicate s the smoothed rates obtained by averaging the twenty minut e values in slid i ng �roups o f
three . �ctivity is confine d to the local noon-midnight pe riod
and is completely absent during the early morning hours when hm emission activity pe aks (see Fi�ure 1 1.1 J.
It is important to differentiate be tween n�rrow-band sta r$- time emissions and the sub-ELF gurglers described in sec tion
e.7.
Gurglers have bandwidths of between one and three octaves and are therefore not strictly ide ntical to t�e emissions under cons ideration here although t�ey also occur predominantly du ring the loc�l afternoon( Tepley and Amundsen, 1 96�).
1 0.8.2 Broa -band Emissi� ns.
Broad-band emissions with irregularly sp�ced fine struc ture h�ve been observed in association wit� the 27 day re current geomagne tic storms . These emii,�ions are extremely broad in spe ctral energy distribution and their upper frequency limlt often exceeds the upper limit of the 3onagraph (2.� cps ). Fifteen e missions were recorded over the eight month period and twelve occurred �t time s whe n KAM � 3. A typical emi�sion is shown in Figure 1 0.1 3. O blique irregularly spaced fine struct ure band5 are observable be tween
0705 hr and 0720 hr. UT. The slope of the se bands ie approximately 1.25 cps/min. In general, the slopes of fine structure b ands in
both broad-band and n�rrow-band storm-time emissions are greater than those of hm e�issions in the same freque ncy range.
In addition, broad-band non-struc tured ac tiv ity is apparent
in Figure 1 0.1 3. This also occurs only at t imes of geomagnetic storms and generally in conjunc t ion with the irregularly structured emissions described above.
Duffus e t al ( 1 960) have shown sonagrams of geomagne tically disturbed periods which contain irregul�rly struc tured emissions associated with broad-band noise similar to that shown in Figure 1 0.13. Also, the irregularly structured emis , ions conside red here
and Amundsen ( 1 964) while the bro�d-band noise appears similar to the noise bursts de scribed by Tepley ( 1 962 ) and Tepley And Nentworth
( 1 962b ). 1 0.9 ,3UMM RY.
In this chapter two distinc t categorie s o f �c1 emissions have
been identifie d. Most c ommon are hm emissions which exhibit a
regularly spaced fine struc ture and are de tected princ ipally during
geomagnetically quie t periods. Le ss fre quently, and at times
of geomagnetic disturb ance, narrow-band anc broad-band irregularly structured and non-structured emissions are observe d.
The problem o f c lassification o f storm-time emi� sions •ith re spec t to hm emissions is important . Storm-time emi ssions are predominantly narrow-oand with bandwidths comparable to hm exissions, and gene rally oc cur be twe en local noon and midnight. There fore,
the y will have a significant influence on the re sults o f statistical studies concerned with temporal distributions o f hm emission
oc currence rates and emission frequencie s if they are included in the same category as hm emissions. If the y are generated by the same source mechanism and propagate in the same manner as hm emissions then their inclusion in such studie o may be justified. tlowever, at pre sent little is known about their or-'gin and the existence o f
a common sour ce me chanism cannot be assumed. The re fore, in the
statistical studies in the following chapters storm-time rc1 emissions will be exclude d and it will be assumed that all fine
CH TER A.t DIUR AL VARIATIONS I