Auroral riometer absorptions and the F-region disturbances observed over a wide range of latitudes

JournalofAtmosphericandTerrestrialPhysics, Vol. 45, No. 2/3, pp. 175-179, 1983.

0021 9169/83/0201754)5 $03.00/0 © 1983 Pergamon Press Ltd.

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Auroral riometer absorptions and the F-region disturbances observed over a wide range of latitudes L. A. HAJKOWICZ Department of Physics, University of Queensland, St. Lucia, Queensland 4067, Australia (Received in final form 14 October 1982) Abstract-- Standard riometer data from a southern auroral station werecompared with ionograms obtained at fivestations positioned from sub-auroral to equatorial latitudes. The rapid onset in riometer absorption, during intense substorm activities in an equinoctial period, was associated with a sequential propagation of ionospheric disturbances deduced from the F-region parameters h'F and range spread-F. The time shift between absorption maxima and extrapolated commencementtimes of the disturbances was consistent with the presence of large-scale travelling ionospheric disturbances (TIDs), propagating equatorwards with velocitieslying typicallyin the range 6(K~900m s- t, and with a median velocity of 720 m s- 1. It is suggested that the onset of TIDs is associated with high-energyparticle precipitation, manifested by the occurrence of auroral absorption events.Similarityof absorption increases at the southern and northern conjugate points, found from a previous riometer study, would indicate that large-scaleTIDs are simultaneouslygenerated in both hemispheres. 1. I N T R O D U C T I O N

The presence of ionospheric disturbances, associated with geomagnetic storms, has been observed on a number of occasions. For example, BOW~aAN and SHRESTHA (1966) reported a progressive rise in the virtual height (h'F) of the F-region for three Australian stations: Canberra, Brisbane and Townsville. The disturbance was generated during enhanced geomagnetic activity and propagated equatorwards. THOM~ (1968) observed ionospheric disturbances over a wide range of northern latitudes. The disturbances appeared to originate at high latitudes during the onset of magnetic substorms. He concluded that the progressive changes in the ionosphere were associated with largescale trains of travelling waves, moving equatorwards. BOWMAN (1978) performed statistical analysis of ionosonde data for low and mid-latitude stations and suggested that increases in the ionospheric parameter h'F are related to the onset times of magnetic substorms in the polar region. The height increases were recorded for several hours after the onset times and seem to be associated with the simultaneous occurrence ofspreadF. In particular, his Fig. 6 shows that a sudden depression of the horizontal component of the earth's magnetic field in polar latitudes leads to a rise in h'F at mid-latitude and equatorial stations. The rise in h'F was also accompanied by onsets of pronounced spread-F at these stations. The disturbance appeared to propagate equatorwards, with a velocity of 740 m s - ~. The occurrence of a height rise of the F2-region, following magnetic substorms, has been well documented (el. BOWMAN,1977). It appears, however, that little is known either about the precise location of the source of

the disturbance or about its accurate commencement time. For example, THOME (1968) determined the presence of an equatorwards-propagating disturbance but could not define its origin both in time and in the geographical coordinates. BOWMAN(1978) determined the onset time of a negative bay of the magnetic Hcomponent with an accuracy of 15 min, preceding the occurrence of an ionospheric disturbance. He could define only an approximate area where the source was located. The presence of overhead auroral surges, characteristic of magnetic substorms, can be detected from the cosmic noise absorption in the D-region, using standard riometer techniques. It is well known that the various stages of visual auroral substorms, such as the brightening of auroral bands, the break-up stage, etc. can be identified from riometer records. Examples of such close association between the absorption increases, and the photometer and all-sky camera observations ofaurora, have been given by HAJKOWICZ (1968, 1969) for the southern and northern auroral zones. An extensive study of the signature of large-scale auroral structure in riometer absorption has been conducted by BERKEYet al. (1980), using a meridional chain of riometer stations. It became evident that the particle precipitation, associated with the visual auroral surge and auroral absorption, was limited to several degrees of latitude in the northern auroral zone. An earlier study by PARTHASARATHYand BERKEY (1965) indicated that sudden-onset absorption events were elongated in the east-west direction but were limited to a distance less than 250 km in the northsouth direction. HARTZ and BRICE (1967) concluded that absorption increases during geomagnetic sub-

175

L. A. HA~KOWICZ

t76

storms are associated with the 'hard' or energetic particle precipitation whose locus is aligned along a constant geomagnetic latitude. Nr~LSEN and AXFORD (1977) estimated the latitudinal width of the absorption region to be about 50 kin. Finally, NIELSEN (1980) concluded, from the data derived from the riometer and from the auroral radar system Stare, that there is a close relationship in space dynamics between particle precipitation, riometer absorption, and ionospheric electric fields during the substorm expansion phase. It is evident that the spatial and temporal characteristics of particle precipitation, associated with the generation of magnetic substorms, can be inferred from the auroral absorption in the D-region. The present results show that the times of absorption maxima could give accurate high-latitude reference points for tracing ionospheric disturbances over a wide range of latitudes.

h'F Sr 350 - -3 (kin) 250-

2. METHODS AND RESULTS

Vertical-incidence ionosonde data were analysed for an equinoctial period: March-April 1979, for which exceptionally large and frequent magnetic disturbances were observed. Ionograms, obtained at 15-rain intervals, were examined for the parameters characteristic of the F-region : virtual height (h'F) and range spread (St). The intensity of St was assigned values from 1 (range spread 10-20 km) to 3 (range spread >/30 km). The recording period was limited to night-time where it is difficult to obtain reliable values of the critical frequency of the F-region (foF2) and consequently the latter parameter was not used in the analysis. Ionograms were obtained at the following stations : Hobart (42.9°S and 147.2°E geog., 51.6°S geom.), Canberra (35.3°S and 149.0°E geog., 43.9°S geom.), Brisbane (27.5°S and 152.5 ° geog., 35.6°S geom.),

4th APRIL,1979

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LT)~

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Fig. l. An example of an absorption peak, at the commencement of an intense substorm activity, followed by enhancements in the F-region parameters at various ionospheric stations (time: 150°EST).

177

Auroral riometer absorptions over a wide range of latitudes Townsville (19.3°S and 146.7°E geog., 28.4°S geom.) and Vanimo (2.7°S and 141.3°E geog., 12.5°S geom.). The standard riometer, operating at a frequency of 30 MHz, was situated at Macquarie Island (54.5°S and 158.9°E geog., 61.0° geom., L-value = 5.3) in the southern auroral zone. The riometer antenna was pointed upwards and consisted of a linearly polarized array of dipoles, giving a beamwidth of 60° between half-power points. Figure 1 shows an example of a rapid absorption onset during a period (on the night 3-4 April 1979) characterized by a sharp change in the magnetic disturbance level (local magnetic K-index, derived from the Macquarie Island magnetograms, increased from K = ! at 2200~100 LT to K = 8 at 0 1 ~ 0 0 LT). This type of absorption event, classified as a sudden absorption increase (SAI), is well correlated with the presence of visual auroral surges in the field of view of the riometer (cf. for example, HAJKOW]CZ,1968, 1969). A sudden onset in large auroral substorm activities (which continued till about 0800 LT) can be inferred from the riometer record. The initial, small variations in absorption from 0145 to 0227 LT were followed by a sharp, impulse-like increase in absorption which reached a level of 2.4 dB at 0230 LT; the onset time At (i.e. from the starting time at 0227 LT to the time of the first absorption maximum at 0230 LT) was here only 3 min. The entire disturbed period was associated with occurrences of consecutive absorption peaks, exceeding 3 dB, and rapidly following each other. Figure t shows also the corresponding ionospheric data derived from ionograms obtained at five sounding stations. It is evident that there was a consistent time delay in the onset times of the ionospheric disturbance as inferred from the variations in the parameter h'F and Sr at each station, following the first major absorption peak at Macquarie Island. At Hobart, the disturbance was first noted at 0315 LT; there were increases in the range spread-F (from Sr = 1 at 0300 LT to Sr = 2 at 0315 LT) and in the virtual height for this station. Similar effects were also observed at Canberra and Brisbane, where the disturbance was first noted at 0330 and 0400 LT, respectively. There was a large increase in h'F for Townsville, first noted at 0415 LT, but no spread-F was evident for this station. Strong equatorial spread-F at Vanimo preceded the substorm commencement, terminating at 0315 LT. There was an enhancement in Sr at this station at 0445 LT, following a gradual increase in h'F. It should be noted that the initial enhancements in Sr and h'F at Hobart, Canberra, Brisbane and Townsville remained constant or tended to increase over a period of time. This was different at Vanimo where the enhancement in Sr had a short lifetime.

The sequential pattern in the occurrence of ionospheric enhancements shown in Fig. 1, suggests the presence of a disturbance propagating from auroral to equatorial latitudes. The riometer records make it possible to define the disturbance onset time with an accuracy of + 30 s. Since the ionosonde records were obtained at 15 rain intervals it was not possible to define the disturbance onset time at any single ionospheric station with a comparable accuracy. Linear regression analysis was used to study the commencement times of the disturbance for various stations, followingthe onset of the absorption maximum at Macquarie Island (Fig. 2). The error bars for each station are the intervals between soundings. The sloping line corresponds to a velocity of 630 m s - 1 for the disturbance inferred from Fig. 1. The uncertainty in the velocity is about +100m s-L It was possible to detect 27 absorption peaks which were followed by sequential ionospheric disturbances (cf. HmKOWICZ, 1982b for more examples). Three quarters of all the events were recorded during highly disturbed geomagnetic conditions, with K-index ranging from 6 to 8. The majority of absorption events had maximum absorption between 1-4 dB, and the onset time At from 3 to 6 min. The prevailingabsorption type was classified as SAI (sudden absorption increase), signified by a rapid onset and slow decay of absorption with several peaks during one event (ANsAa~I,1965). In every case it was possible to detect a sequential disturbance pattern after the first major peak at the

L,th APRIL, 1979. Abs. max 0230 LT 10'

7

v

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I I 1 TIME DELAY (HOURS)

1 2

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Fig. 2. Derivation of the velocity of a disturbance from riometer and ionosonde records. The disturbance is assumed to commence at the latitude of Macquarie Island (M).

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L. A. HAJKOWICZ

Table 1. Percentage occurrence of ionospheric enhancements Type of enhancement Station

h'F and Sr

h'F only

Sr only

Hobart Canberra Brisbane Townsville Vanimo

35 24 22 4 29

19 40 39 52 38

46 36 39 44 33

(%)

(%)

(%)

3. DISCUSSION AND CONCLUSIONS

start of a complex absorption event (asin Fig. 1) or after an isolated absorption increase (HAJKOWlCZ, 1982b). On occasions, the presence of several substorms during one night could be inferred from the riometer records ; the disturbances appeared to originate after the first major absorption peak for each substorm. Table 1 shows the percentage occurrences of ionospheric enhancements at each station. In a large number of cases enhancements were noted either in the virtual height (h'F) or in the range spread-F (Sr). Simultaneous enhancements in h'F and Sr were relativelyfrequent at Hobart and Vanimo but were rare at Townsville. Enhancements in Sr only were relatively common for all stations. The sequential disturbances were usually traced to equatorial latitudes. On two occasions the disturbance appeared to terminate before Townsville, and on four occasions the disturbance did not reach Vanimo although it was still recorded at Townsville. Figure 3 shows the occurrence number of velocities (in the indicated velocity intervals) of all the sequential disturbances. Velocities were computed using linear

B

36 QC

O

500

(500

700 8(30 900 VELOCITY RANGE Irn s-l)

extrapolation of commencement times of the disturbance for each station (cf. Fig. 2). The disturbances propagated with a range of velocities, mainly between 600-900 m s - 1. The median velocity was 720 m s - 1.

I000

Fig. 3. The velocity occurrence number of sequential ionospheric disturbances.

It is evident that absorption maxima, associated with the onset of magnetic substorms in the auroral region, are followed by sequential disturbances of the ionosphere over a wide range of latitudes. It appears that the disturbances originate close to the area in which absorption increases are recorded. This can be explained in terms of a particle precipitation pattern whose spatial position can be interpreted from the riometer data. The energy input associated with auroral particle influx could be sufficient to cause the disturbances to be observable for ranges of a few thousand kilometres (cf. HtlNSUCI~R, 1982). The large distance of propagation of the disturbance, observed in the present study, appears to indicate a linear rather than point source of generation. For example, FRANCIS (1975) suggested that gravity waves (whose ionospheric manifestations are TIDs) from a linear source would decrease in amplitude less rapidly than gravity waves from a point source by a full power of the horizontal range from the source. The linearity of source also can be expected from the large longitudinal extent in the auroral riometer absorption and associated particle precipitation, as discussed above. The computed velocities of the sequential disturbances (Fig. 3) are in the same range as those characteristic of large-scale TIDs. This type of TID is generated during intense geomagnetic disturbances which was the case for the majority of the disturbances found in the present study. Relatively frequent occurrences of the enhanced range spread-F, at the time when sequential disturbances were present (Table 1), appears to be yet another proof of the existence of such TIDs. BOWMAN(1981) conducted an extensive study on the structure of irregularities contributing to spread-F in southern mid-latitudes. He concluded that the range spreading variety of spread-F results primarily from specular reflection from large-scale irregularities which can be regarded as TIDs. He suggested that the smallscale ionization structures, which are also present, contribute to the diffuse appearance ofionograms. This appears to be supported by HUNSUCKER (1982) who noted that both large- and medium-scale TIDs occur at mid-latitudes, following the onset of auroral substorms. Finally, the presence of large-scale TIDs has been recently linked with simultaneous occurrences of smallscale ionospheric inhomogeneities, contributing to spread-F and scintillations in radio-satellite trans-

Auroral riometer absorptions over a wide range of latitudes missions (SINNO and KAN, 1980; KERSLEY et al., 1980; HAJKOWlCZ, 1982a; HAJKOWICZ et al., 1976). It is of considerable interest that occurrences of auroral absorption events (as depicted in Fig. 1) are not limited to one hemisphere only. HAJKOWICZ (1968, 1969, 1970a, b) conducted an extensive study on the simultaneous occurrences of auroral absorptions at Macquarie Island and at its close conjugate station in Alaska, Kotzebue. His examples of the development of magnetic substorms, based on riometer, all-sky camera and photometer data, indicate that the major phases of substorms are similar at the opposite ends of the earth's magnetic field hnes. In particular, the rapid onset of

179

auroral absorptions (SAI events) associated with visual auroral surges, has close similarity at both conjugate stations (HAJKOWICZ, 1968, 1969). F r o m this it can be envisaged that the generation of large-scale TIDs is simultaneous at both auroral zones, following the onset of absorption events. Acknowledgements--The ionosonde and riometer records

were supplied by the Ionospheric Prediction Service (IPS), Sydney and the Antarctic Division of the Department of Science and Technology, Kingston, respectively. K-indices were supplied by the Bureau of Mineral Resources, Geology and Geophysics, Canberra. I am grateful to Mrs D. J. DEAROENfor her assistance with the data analysis.

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1982b

Ionospheric Research Section Report No. 10, Department of Physics, University of Queensland.

Reference is also made to the following unpublished material:

HAIKOWICZL.A.