1 limit is a direct function of the equatorial cyclotron fre1uency,

as suggested by the cyclotron damping mechanism , then conditions exist for obtaining high frequencies during the nighttime �nd low frequencies during the daytime.

1 5.�.5 Emission Occurrence

_t � s convenient tv de.1ne the region of intersection of the emission propagation path with the ionosphere as the ionospheric source. The ionospheric source may be considered as the region, in or above the ionosphere, in which the ion cyclotron wave packets are reflected. The locuo of the ionospheric source is identical to that of the emission source shown in Figure 13.2.

It is of interest to investigate the dependeLce of diurnal emission occurrence on the location of the ionospheric source. The diurnal variatinn� � � emission occurrence , t fourteen stations,

ran$ing in geomagnet c latitude from .?.8° to 77° _{( L}

₌

with latitude described �n section 8 . 5 is readily appare t. The times of peak occurrence �t these stations are tabulated 1� .'able 13.1.

table.

In addition, data for �eyKjavik have been in luded in the A plot of the local time of peak occurrence against the

station latitude is shown in Figure 13.4 and it is apparent that

the pattern of occurrence peaks follows the locus of the ionospheric

source as deduced from the Christchurch data. Thus maximum emission occurrence is observed at a given station at times when the station is nearest the ionospheric source.

It is important to note that different ordinates scales are used in Figures and 13. -r. The data in Figure 13.4 suggest that the emission source is located in the vicinity of the �-� field line during the nighttime and the L-8 field line during the daytime. On the other ha , the lo11er;t and hifhest calculated positions of the source are _.8 and 1�6.5 respectively, bile

the average poo.tion waries between I 4.0 and I 5.4. The

discrepancy in daytime L values may be �nterpreted in terms of the knee magnetospheric plasma density distribution shown in Figure 13.1. For example, if the distribution changes from the normal curve to the knee curve at approximately L �, then the outermost position

of the emission source is extended to the ,=9 field line for an unperturbed dipole field. Under these conditions the average source posi tion ranges from L= 4 at nighttime to L=6.5 in the daytime. The do tted curve in Figure 13.2 indicates the diurnal variation in the average ionospheric source po sition with the knee plasma included.

If the ionosphe ric source follows the path indicate d , then stations situated be tween the L= 4 to 6 field line s may be expected to cross the path twice in a given day, and thus show diurnal

variations in emission occurrence with two peaks. Meanook is the only station exhibiting two such peaks. Howeve r, Heaco ck ( 1 963 b ) has noted that although Colle ge shows a single peak, the re are

many days on which distinc t emi sion events oc cur both in the late morning and early afternoon. Thus the absence of the wo expe cted peaks at the se latitudes doe s not ne ce ssarily indi cate that the station doe s not cross the source path twice a day. It may,

however, indicate that emissions are pre ferentially produced during the morning and early afternoon hours .

The variation in average emission frequency with latitude, as de termined by Heacock and He ssler ( 1 962 ) and Gendrin ( 1963a,c), and shown in Figure 1 0.6, may also be interpre ted in te rms of the

above occurrence pattern. Since high latitude stations observe

pe ak emission occurrence during the daytime, the average emission

fre quency will be lower than the average frequency at middle latitude

stations , which exhibit predominantly nocturnal oc currence.

After the work reported in this se ction had essentially been comple ted , the writer became aware of an unpublished pape r by Campbell and Stiltner ( 1 964 ) which considers diurnal variations in hm

emissions. As in the present work, it is sugge sted that the <lurnal

variations in emission parame te rs re sult from a diurnal variation in the location of the source in the magne tosphere, and that this

provide s simple explanations for the variations in average emission frequency and diurnal oc currence with latitude . However, no

quanticative estimations of the location of the emission source, in

1 09.

Campbell and 3tiltne r ' s work is base d on _ c 1 dat� obtained from a world-wide network of recording stations. Graphs plotted

by these a�thors, for the diurnal variat ion in emission a�plitudes , show that maximum nighttime amplitudes are observLd �t stati ons on the L 11 field line, ctnd tho.t maximum d�ytime am rlitu1e s oc _{,.J r} at

stations on tne �=6 field line. These figures are in Jgreement

with the day and night loc ations o f the emission scurce calcu lated in the present study (L 4 and 6.5 ).

1 3.2.4 Energetic �article �istribution�

I f the theories fo r the �enerot,._ (,n ul tlm e ·3issi ons revie wed in Chapter IX are corre ct, then the diurnal movement of the emission source may oe associate d with 7. s imil3r mve:ne nt i r. the distribution

of ene rge tic protons ·:.it:iin the magne tos;he re . ,esults from s�tel lite

studies show tnat .;;uch a variation doe s exiGt in the inter..s . ties of �eomagnetically trapped elec trans (Fr 9.nk et al . , 1 J63; ( 'Brien, 1 963 ; Frank et al. , 1 964 a). It is not kno ,vn at the- pre ...;en t. time wht t h er a similar variation exists in prot ons. 1owever, it is of

interest to brie fly inve stig.;:..te the pos.s iLle causes of day to night asymmetry in energetic p3rtic le dis t rloution in order to �scertain whether this asymmetry is t� be 8 xpected for th� particular types of particles suggeste d for hm emission generat�on. The relevant particles are 1 00-500 keV protons , s11 5c::e -sted oy Jbaya�ni ( 1 96·1 ) and Cornwall ( 1 965) , and 3-1 i _.,;..eV protons , suggeste d by lient orth and Tepley ( 1 964).

The departure of trappe d particle drift p�ths from rotation�l symmetry may be accounte d for either oy inhomogemtie s in the geomagnetic field or by exte rnal electri� fields. uince gradient induced drift, _{resulting from an inhomo6eneous m�c,etic field}, is

energy dependent w�ereas electric field ind aced drift is not (opltzer,

, 962 ) , the former will provide the gre�test contribution to the drift of high energy particles (E � 1 00keV ) while the l at' e r drift

predo.::iinates for thermal particles (E -E- 1 keV ) . In the t ransition

energy range the drifts , ill oe of equ�l mignitude.

According to �ones ( 1 96� ), day to night asymmetry for high energy particles m�y be expected from gradients in the ge0magnetic field produced by Golar wind compression on the day side. However,

1 1 1.

There is evidence of a decrease in ionospher ic and

magnetospherdc plasma densities at times of geomagnetic disturbance (Yeh and Swenson, 1961; Hagg 1963; Carpenter, 1962). In particular Carpenter has shown, from studies of nose whistler�,that the

equatorial magnetospheric plasma density in the region L=1.7 to 5.6 may be reduced by factors of up to four during the main phase and recovery phase of very severe geomagnetic storms . However, for moderately severe storms, reductions of the order of 20% are to be expected. Since this reduction is insufficient to account for

the observed decrease in T by factors of two to three it must be concluded that the emission source moves to lower latitudes at times

of geomagnetic storms. As in the diurnal movement, this lowering of emission propagation paths is consistent with the observed variation in f . The storm-time location of the emission source

is represented by the lower dashed curve in Figure 13.2. Furthermore,

the return of the emission source to it s average position is gradual and may occur in the days following the storm.

It has also been shown that mighttime values of fine structure band spacing are negatively correlated with �K • In the absence of significant storm-time variations in the magnetospheric

plasma density, this indicates a modulation of the emission source position by geomagnetic activity. In Figure 13.5 is plotted the variation in the source position with � K • This graph was deduced from the data in Figure 12.2. The average nighttime

position of the emission source is L = 5.0 at extremely quiet periods,

and L=3.7 at distunbed periods .

As in the case of the diurnal variation, the movement in the emission source may be associated with a similar movement in the distribution of the energetic particles generating hm emissions. Frank et al. (1964b) have reported that the intensities of protons between L=2 . 8 - 3.6,with energies greater than 500keV, are

relatively insensitive to geomagnetic disturbances. These are the particles suggested by Cornwall (1965 ) for emission generation and the a-sence of a storm-time variation is incompat ible with the

observed emi ssion source variation s. On the other hand, the

soft electron flux (E �40keV) in the region L

>

a rapid storm-time enhancement followed by a gradual recovery over a period of a week (Frank et al., 1964b; Freeman, 1964). At the present time it is not known whether low energy protons undergo similar changes.

A more complete evaluation of the storm-time variation in the hm emi ssion source, in terms of proposed generation mechanisms,

must however await further observations on the temporal variations in the distribution of low energy trapped radiation.

113. CH ?TER XIV

CONCLUSIONS AND RECOMM1NvATIONS

The purpose of the work reported in Part B of this thesis has been to develop recording and data analysis instrumentation suitable for observing the frequency-time properties of discrete emissions in the Pc1 geomagnetic micropulsation band, and to interpret the characteristics of eight months of recorded data in order to gain insight into the origin of these emissions.

In the development of recording instrumentation, considerable attention has been devoted to the design theory of a galvanometer photocell preamplifier driven from an induc tive source. As a result, it has been found that the frequency response of a

standard laboratory galvanometer may be extended, by the application of a high degree of negative feedback, to cover a considerable

portion of the Pc1 band.

From the analysis of recorded data, two distinct categories

of Pc1 emissions have been identified. Most common are

hydromagnetic (run) emissions, which generally contain a regular and repetitive rising frequency fine structure pattern, and

occur most frequently during geomagnetically quiet periods. Less frequently, and at times of geomagnetic disturbance, irregularly structured and non-structured Pc1 emissions are observed. In the present study particular emphasis has been placed on the interpretation of run emission properties in terms of recently suggested models involving the generation and propagation of ion cyclotron hydromagnetic wave packets in the magnetosphere.

Observations from frequency-time displays have shown that most hm emissions are composed of adjacent and interrelated series of narrow-band emissions with bandwidths of the order of 0.05 - 0.1 cps. Although insufficent evidence is available to prove conclusively that emissions propagate as ion cyclotron wave packets, it has been

found that many o f the emission fine structure properties are at least consistent with this mode of propagation.

The existence of significant diurnal variations in hm emission midband frequency and fine structure band spac; ng have bee­

established. Experimental results and theoret c arguments have been prese�ted which indicate th�t these variations are not associated with the daytime absorption o f hydromagnetic waves in the ionosphere, but are associ ted with the emission source within the magnetosphere. In terms of emissi n propagation theory, the diurnal variations in emission parameters show t at the em. ssion source is located at lower latitudes during the n� httime than during the daytime. If the gyro frequency model o f wagnetospheric plasma density distribution is assumed , then the emission source is located in the vicinity of the L=4.v field line between

03 - 06 hr. LT, and the L=5.4 field line ��tween 1 2 - 1 5 hr. LT. Consideration of a knee plasma density di tribution extends the daytime source location to the L=6.5 field line.

The diurnal movement in the loc .tion of the emission source provides simple explanation for the variation in hm emission diurnal occurrence with 1 titude, and is also consistent with the vari tion in average emission frequency wit latitude.

It has been suggested that the diurnal movement in the source may be associated with a similar movement in the drift paths of trapped particles in the magnetosphere. For field lines between L=4 and 6. 5 , it is considered that magnetospheric convectio- is the most likely cause of a depai:ture of energetic particle drift paths from rotational symmetry. This being the case, the particles associated with hm emission generation must be o� relatively low energy (E � 100keV).

A preliminary analysis of the effects o f geomagnetic activity on hm emissions has shown that the emission midband frequency and fine structure band spacing undergo significant changes at times of moderately severe geomagnetic disturbance. These variations indicate a movement o f the emission source to lower latitudes at disturbed times. A modulation of the nighttime emission

1 1 .'.;}.