lonosonde signatures of Pc1 pulsations

0021-‘)I6991 $3.00+ Pergamon Pma

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Ionosonde signatures of Pcl pulsations P. M. ASLIN, M. J. JARVIS and K. MORRISON British Antarctic Survey, Natural

Environment Research Council. High Cross. Madingley Cambridge CB3 OET, U.K.

Road,

Abstract-The first observations of the signatures of Pcl pulsations in ionosonde echo data are reported. Oscillations are frequently observed in Doppler velocity, echo amplitude, group range and skymap echolocation position, and are clearly associated with simultaneous Pcl geomagnetic micropulsations with a similar frequency. These oscillations, recorded at the sub-aurora1 Antarctic station of Halley (76 S. 27‘W. L N 4.2), occur in either or both E- and F-region echo time series during a wide range of ionospheric conditions. Focusing of the ionospheric echoes due to the compressional action of the hydromagnetic wave is suggested as a possible mechanism.

1. INTRODUCTION

observations of the ionospheric signature of Pcl magnetic pulsations (periods -2 s). The only previous reference to such observations in the literature is by DINGLE (1972). He ran, for 1 yr at a mid-latitude site, an experiment specifically designed to record such events by monitoring phase-path changes in a continuous wave signal transmitted via ionospheric reflection from 28 km away. On the nine nights on which easily identifiable ( > 0.05 nT peak-peak) geomagnetic micropulsations occurred while his equipment was operating correctly, no coincident phasepath fluctuations were detected. Contrary to that evidence, it is shown here that, at sub-aurora1 latitudes at least, such phase-path fluctuations are common. They are shown to have a markedly different signature from that of ionosonde echo parameter oscillations in the Pc3-5 range. For the sake of brevity, the ionospheric signature of a magnetic pulsation will be referred to as an ISP throughout this paper. Thus, for example, the ionospheric signature of a Pcl will be referred to as an

It is generally accepted that Pcl magnetic pulsations are ion-cyclotron waves generated as a consequence of proton gyroresonance instability in the equatorial region of the magnetosphere (FRASER et al.. 1984). Their wavelength is small compared with the length of the geomagnetic field lines along which they travel. The ‘slow mode’ (left-hand polarization, ordinary mode, shear Alfvirn) wave trains can ‘bounce’ between the northern and southern hemispheres at close to the Alfvbn velocity, to produce amplitude-modulated signatures at conjugate points with a modulation period of l-2min. Each time they reach the ionosphere, some of the energy may be coupled to the Alfvin ‘fast mode’ (right-hand polarization, extraordinary mode, compressional Alfvkn), and propagate away from the magnetospheric exit region to higher or lower latitudes via ducts of minimum Alfvin velocity and low absorption formed by the F2-layer peak (FRASER et al., 1984). Pc3-5 pulsations, on the other hand, are standing wave oscillations of the geomagnetic field lines, of varying complexity (e.g. NISHIDA, 1978; GOUGH and ORR, 1986; LIN et al., 1986), and with wavelengths comparable with the dimensions of the magnetosphere. [Pc2 remain relatively unknown as a distinct pulsation phenomenon (ARNOLDY et al., 1988).] Observations of PCpulsations manifested in vertical incidence ionosonde data have been reported by a number of authors (e.g DAVIES and BAKER, 1966; KLOSTERMEYERand R~~TTGER, 1976; MENK et al., 1983 ; SUTCLIFFE and POOLE, 1984 ; KOLOKOLOVand MARAKHOVSKIY, 1988; JARVIS and GOUGH, 1988; TEDD et al., 1989). All discuss pulsations in the range Pc3-5 and Pi2; these have periods from -20 s upwards. This paper discusses digital ionosonde

ISPC1.

2. INSTRUMENTATION The data were recorded at Halley (76”S, 27”W, L - 4.2), Antarctica. The digital ionosonde at Halley is a NOAA HF radar (GRUBB, 1979), referred to as the ‘Advanced Ionospheric Sounder’ (AIS). This is operated alternately in ionogram and kinesonde (WRIGHT and FEDOR, 1967) modes. The latter provides interleaved time series on a number of fixed sounding frequencies at a pre-defined sampling rate. For the data presented here, this sampling rate is 0.24s. In addition to providing frequency, time, amplitude and group range information, the phases 343

P. M.

344

ASLIN

PI

d.

of change group range.

Rate

of

Rate of change

of

Doppler

velocity.

ULF D component

0

10

20

30

40

SECONDS

Fig. 1. A typical example of pulsations in a fixed frequency ( 1.5MHz) ionospheric sounding simultaneous with Pel pulsations of a similar period (day 105, 1986, 0804UT).

of each echo detected in an array of four dipole antennae are processed to provide echo parameters including ‘skymap’ echolocation, line-of-sight Doppler velocity and magneto-ionic mode (WRIGHT and PITTEWAY, 1979; JARVISand DUDENEY,1986). The Pel observations were made using a ULF/VLF correlator and data logger, referred to as the ‘Realtime Antarctic Logger Facility (RALF). The RALF measures the H, D and vertical components of the geomagnetic field over the frequency range 0.02IO Hz, with a sensitivity of -0.001 nT at I Hz and at a sampling rate of 0.05s. Further details of the oxperimen~al system arc given by GOUtiH o’fN/. ( 1987).

A typical example of ionosonde ISPCI pulsations observed at Halley is shown in Fig. I. The group range has been plotted as the first derivative with time because this should relate more directly to the Doppler velocity. Echo amplitude has also been plotted as the first derivative of amplitude with time as a means of removing the longer-period trend. Pulsations in all parameters are clearly visible and are particularly obvious in the signal amplitude, with similar “pearl’ modulation to that seen in the ULF waves. The ISPcI studied here have a markedly different signature from that of ISPc3--5 (JARVISand G~LJ~H, 1988): (I) Both produce oscillations in the Doppler velocity (i.e. rate of change of phase) of the reflected

ionospheric signal at a similar frequency to one of the geomagnetic PC spectral components, although not necessarily the strongest. These oscillations are typically - 80” per second peak-to-peak for ISPcl (typically I.5 MHz sounding frequency) and _ 140’ per second peak-to-peak for ISPc3-5 (typically 4 MHz sounding frequency)----equivalent to - 22 m s _ ’ nT_ I and _ 3 m s ’ nT ‘, respectively. (2) There are ~(~c.z.)G oscillations in the group range (from time-of-flight) at the same frequency as the Doppler velocity oscillations during fSPc1 events. These are typically 2 km peak-to-peak. In contrast, they are rare and, if present, hardly discernible during JSPc3-5 events. (3) There are alirnj~ large oscitlations in echo amplitude at the same frequency as the Doppler velocity oscillations during ZSPrl events. These arc typically 6 dB peak-to-trough. Such oscillations have not been observed at Halley during ISPc3-5 events. (4) There are qftrn oscillations in the ‘skymap’ echolocation position present during ISPrl events. These are typically +_lOkm, but can be as large as 50km, peak-to-peak. They are seldom observed during ISPt.3-5 events. (5) ISPCI are observed in either E- or /Gegion echo data, and occasionally in both simultaneously. At Halley. ISPc3-5 have only ever been observed in Fregion data. It should be noted, however, that TEDD et t-11.(1989) have recently reported the first reliable evidence of ISPc34 in E-region echoes : their results arc from a mid-latitude station (f, = 2. I).

33s

Ionosonde signatures of Pcl pulsations The fSPcl also show the following characteristics : (I) In a sample of 20 ISPcl events over 7 different days, the group range and amplitude oscillations are closely in phase 70% of the time; otherwise they are closely in anti-phase. They remain either in phase or in anti-phase over periods of I h or more. When they are in anti-phase, the signature tends to bc less like a sine wave and more noisy. (2) The Doppler velocity oscillations are more variable but are usually either approximately in phase or approximately in anti-phase with the amplitude oscillations. Both phase relationships are equally common and show no apparent pattern of occurrence. Abrupt switching between in phase and in anti-phase can often occur several times during a 2-min sounding. POOLE et al. (1988) have shown theoretically that the relative phases of the Doppler velocity oscillations and the oscillations in the H, D and Z components of the geomagnetic field are highly variable and dependent upon the relative influences of the various mechanisms. This is borne out by observational measurements in the Pc3-4 period range (e.g. JARVIS and GOUGH, 1988 ; TEDD et nl., 1989). This variability is also observed in the relative phases of the geomagnetic and ionosonde pulsations at Pcl frequencies. This may be due, in part at least, to the fact that the pulsation frequency observed in the ionosonde data is often (but not always) close to the minimum frequency of a band of geomagnetic Pcl activity; this means that the peak power in the two records is at slightly different frequencies.

The comparison between the characteristic signatures of ISPcI and ZSPc3-5 has been summarized

in Table 1. The reference point of the relative phases is arbitrary for each class of pulsation periods. 4. OCCURRENCE RATES The AIS and RALF are operated all year long. However, the RALF uses real-time analysis of the magnetic activity designed to record those events

judged to be of interest, offering a saving on data handling and storage. Similarly, the AIS only sounds in the kinesonde mode for about one-twentieth of the year, but gives coverage fairly evenly across it. During 1986, special kinesonde soundings were often started when the RALF indicated that Pcl pulsations were present. Pulsations are most easily detected in the AIS data in the time series of the rate of change of phase (Doppler velocity) between temporally adjacent echoes from the same range on the same frequency. On some occasions it would be impossible to detect pulsations in the AIS data. This is either due to the lack of echoes resulting from total ionospheric absorption of signals at the sounding frequency, or due to excessively ‘spread’ conditions where echoes are received quasi-randomly from many heights and geographical positions, making detection of a coherent time series of echoes from the same reflection region impossible. The occurrence statistics for Pcl oscillations detected in the ionosonde data at Halley for 1986 are shown in Fig. 2. In this respect, detection is achieved by visual inspection of the Doppler velocity records. An oscillation is ‘detectable’ if it has a peak-to-peak amplitude of more than -40” per second. The proportion of time when ionospheric conditions were not suitable for observing pulsations, due to blackout or spread, are shown by the stippled area ; these times are excluded from the following discussion. ISPcI were observed in a high proportion of the ionosonde data recorded specifically because the ULF logger was detecting ULF activity. The disparity between the number of these specially initiated soundings on the dayside and nightside does not reflect the distribution of ULF activity, merely that of the physical presence of an operator. Of these specially initiated soundings, 42% contained ZSPcl on the dayside and 25% on the nightside. During soundings made randomly for other purposes, but still suitable for detecting LSPel, the occurrence rate was 12% on the dayside and 3% on the nightside. ISPcl are therefore a relatively common feature at Halley. They are more likely to occur on the dayside

Table 1. A comparison of ISPcl and ISPr3-5 as observed at Halley (this table gives typical values of the magnitudes and relative phases of the PC and simultaneous ISPc pulsations) PC1

Pc3- 5

--Doppler velocity Echo amplitude Group range Skymap Geomagnetic data

Ainplitude

Relative phase (degrees)

Amplitude

Relative phase (degrees)

11m/s 6 dB (peak~peak) I km 5 km 0.5 nT

Oor IX0 0 0 ‘? Variable

8 mls

0

2.5 nT

Variable

346

P.

M.

&UN

et

al.

likely to hold the key to an explanation because of their large amplitude, their bimodal phase relationship with the other ionospheric parameters and the fact that they are absent in Pc3-5 ionospheric pulsations. A number of processes are considered below, with particular regard to the echo amplitude signature and its phase. 5.1. Particle precipitation

effects

A direct consequence of Pcl waves produced by proton gyroresonance interaction in the region of the equatorial plane will be proton precipitation into the ionosphere and associated secondary electron enhancement. On the assumption that this secondary electron enhancement in the lower ionosphere would produce radio wave absorption, then it might be expected that the baseline amplitude of the ionospheric echoes would oscillate with the same frequency as the NIGHTSIDE DAYSIDE (06ow18WLT) f1800-0600LT) ULF modulation envelope, although not necessarily in phase with it. This does not appear to be the case; Blackout or ‘spread conditions - making detection of pulsations in the ionospheric there is no evidence of any relationship between the data impassible. baseline echo amplitude and the ULF modulation Fixed-frequency soundings run specifically because PC1 pulsations were observed in the envelope. The envelope of the echo amplitude ISPcl geomagn#tic data. is, however, often modulated in a very similar way to Pcl signatures found in ionosonde data the amplitude of the ULF pulsations. This implies Fig. 2. Occurrence frequency of lSPcl activity in suitable that the echo amplitude pulsations are more closely fixed frequency soundings throughout 1986 (total 938h). related to the variations in the local geomagnetic field The majority of soundings were gathered routinely. A small than they are to the equatorial proton gyroresonance proportion (hatched) were initiated by the operator during generation mechanism. The modulation of the echo Pcl events. ainplitude ISPcl can be clearly seen in the example given in Fig. I, although, as is often the case, there is not an exact one-to-one correlation between its than on the nightside, even when ULF activity is modulation envelope and that of the Pcl. It has been proposed (e.g. BELL, 1976) that PC] present. It is noticeable, however, that they occur under a large range of ionospheric conditions. Exampulsations might be generated within the ionosphere as a consequence of repeated electron precipitation ples were noted during both quiet and active ionospheric conditions, during a passage of the mid-latibursts at their bounce period along the geomagnetic field line. This precipitation would enhance the Etude trough over Halley, concurrent with high density sporadic-E and over a range of magnetic activity from region conductivity with the same repetitiveness as the electron bounces and the oscillating currents thereby Kp - 0 and -4. They also occur on occasions when the Pcl pulsations exhibit linear, right-handed, or leftinduced would, in turn, induce slow mode Alfvin handed polarization [as defined by SUTCLIFFE(198H)]. waves along the geomagnetic field line. Under this scenario, the echo amplitude envelope would be largest when the strongest Pcl pulsations are produced, 5. INTERPRETATION OF THE /‘cl IONOSPHERIC which is generally what is seen in the data. It is, SIGNATURE however, ditficult to interpret the echo amplitude oscillations in terms of absorption. Firstly. they are The fact that IS&l have different characteristics to generally observed in phase with the group range iSPc3-5 and also that Pcl have a different causal oscillations. Electron enhancement along the echo mechanism to Pc3-5 means that their interpretation path, below the reflection height, would increase the in terms of the dynamic ionospheric processes which group range at the same time as decreasing the echo they reflect should, in the first instance, be considered amplitude through absorption, and thus the group separately. Relating ISPcl to a causal mechanism has range and amplitude oscillations would be expected proved difficult. The echo amplitude oscillations are

Ionosonde signatures of R-1 pulsations to be in anti-phase. Secondly, on the few occasions when oscillations are seen on more than one sounding frequency, the strongest echo amplitude oscillations tend to be seen on the highest frequency. Absorption would produce the opposite effect, the absorption being approximately proportional to the inverse square of the sounding frequency. It is possible to explain the phase match between echo amplitude and group range if there is an electron enhancement over a wide height regime, from the Dregion up through to the reflection height. Then it is possible for the reduction in height of the reflecting plasma density contour to reduce the group range at the same time as the echo amplitude increases due to absorption. Such a balance, between the increased group range due to the precipitation along the path and the decreased group range due to a lowering in height of the electron density contour would explain the presence of group range pulsations both in phase and in anti-phase with the echo amplitude oscillations. It would also explain the fact that either phase relationship held over relatively long periods of time, presumably dependent on the prevailing ionospheric electron density profile and the particle precipitation energy spectrum and flux. However, many of the pulsations are observed in F-region echoes, and the electron enhancement would therefore have to exist from the D-region right up through the F-region. In the F-region, where the ionospheric sluggishness is tens of minutes, electron enhancements would be cumulative, leading to a ‘staircase’ reduction, rather than a quasi-sinusoidal oscillation, in group range. Only under conditions where the precipitation region was spatially small and the enhancement was removed by some transport mechanism might a 2-s period quasi-sinusoidal signature in the F-region group range be observed. Further evidence that particle precipitation has little role to play is the fact that the minimum frequency of reflected echoes cf,,,) during many of the events is very low (e.g. 1.1 MHz), implying little or no enhancement to the D-region. 5.2. Direct action of’ the hydromagnetic wat’e The requjrement for a periodic electron density enhancement from the D-region through to the Fregion may be fulfilled by changes induced as a direct effect of the hydromagnetic wave which permeates the whole ionosphere. With particular reference to the Pc3 and Pi2 period range at mid-latitudes, POOLE et al. (1988) have compared the contributions to the Doppler velocity oscillations of three main processes. These are the Doppler

347

velocity produced by changes in the refractive index through its dependence on the magnetic field intensity, an E x B drift term producing a vertical velocity component, and a combination of compressional terms due to direct action of the hydromagnetic wave. They con&de that, as far as the Doppler velocity is concerned, direct refractive index changes have negligible effect, being an order ofmagnitude smaller than either of the other two terms. Although the vertical component of the bulk plasma motion due to the E x B drift plays an important role, the dominant mechanism is, in many cases, compression and rarefa~tion of plasma frozen into the field lines as they oscillate under the action of the meld-aligned component associated with the magnetic pulsation (they neglect rapid production or loss processes such as particle precipitation). Of the three mechanisms which they consider, the combined compressional term decreases with decreasing pulsation frequency towards a frequency-independent ‘gradient’ component of two orders of magnitude smaller. For Pc3 frequencies and above, the ‘gradient’ component is negligible (in their example) and the compressional term becomes approximately proportional to frequency. The direct refractive index changes (their V,,, and V,r) are also proportional to frequency, but the E x B term is frequency independent. The example given by POOLE et al. (1988) (their table 1) is for a pulsation period of 20s (a frequency of 50 mHz). The linear dependence on frequency suggested by the results of POOLE etal. (1988) may not be valid when extrapolated to Pcl frequencies. However, it suggests that, at a typical Pcl frequency of 0.5 Hz and for identical ionospheric conditions to those used by POOLE et al. (1988), the frequency-dependent Doppler velocity components may increase by an order of magnitude. The ionospheric parameters used in their calculations are not atypical of Halley. The dip angle at Halley is -64 : that used by Poole et al. is -60”. Thus the ‘compressional’ component in their compressional term (their Vj,.) would dominate. The calculations by Poole et al. for a Pc3 event under typical ionospheric conditions give Doppler velocity values of between I and 3m/s (depending on reflection height) for a ground-level pulsation with a b, component of 1 nT and a sounding frequency of 7.3 MHz. The observed values for Pei and Pc3 pulsations at Halley are typically 22m/s per nT and - 3 m/s per nT, respectively (Table 1). Thus, not only are the Pc3 Doppler velocity values in extremely good agreement with the values of POOLE et al. (1988), but the amplitude of the Doppler velocity pulsations increases significantly with pulsation frequency as

348

P. M. ASLIN et al.

suggested by their mathematical expressions for the ‘compressional’ component. While an explanation of the Doppler velocity signature therefore seems clear, there are a number of outstanding inconsistencies : (1) If the mechanism responsible for pulsations in both the Pcl and Pc3 period ranges is similar, it is puzzling that large amplitude and group range oscillations are observed in ISPcl but not ZSPc3-5. It is unlikely that the lack of any group range and echo amplitude pulsations in the ZSPc3--5 data is linked to the ratio of the ‘compressional’ terms at each pulsation period. Based upon the above calculations this ratio will be _ 12 : 1 for ISPcl : iSPc3-5. If a typical 0.5 nT Pcl produces a group range oscillation of 1 km amplitude, then these figures predict that a typical 2.5nT Pc3, as measured on the ground, would produce a group range oscillation of -0.5 km ; thus iSPr1 and ISPcF5 group range oscillations would be of a similar order of magnitude. (2) POOLEet ul. (1988) show oscillations below the E-region peak to be relatively small, with the ‘compressional’ term in particular falling off rapidly with decreasing height. It is difficult to reconcile this with the fact that some of the strongest ISPcl Doppler velocity oscillations observed at Halley are from the E-region, often at sounding frequencies reflecting well below the E-region peak. (3) E-region pulsation signatures are commonly observed at Halley for Pcl but never for Pc3-5. The ‘in phase’ group range and amplitude pulsations may be due to focusing and defocusing of the HF waves by the periodic electron density changes induced by the compression and rarefaction of the ionospheric plasma. Compressions will lower the reflection height of a particular HF frequency, while at the same time defocusing the waves as they are refracted away from the region of highest electron density. Conversely, rarefaction will increase the reflection height and the HE waves will be focused. WRIGHT (1974) has studied the required relationship between the amplitude (i.e. height variation) and sinusoidal horizontal wavelength of a rippled ionospheric surface necessary for focusing and defocusing to occur; both theoretical and observational results are presented. The observations show that, for an amplitude of 1 km, the optimum wavelength for focusing is typically - I20 km. The scale of localization of the incident slow mode Alfven wave propagating along the field from the magnetosphere is given by FUJITA and TAMAO (1988) as - lOOkm, based on statistical observations of PrI pulsations and defined

by the distance from the injection centre at which the rotational sense of polarization of horizontal magnetic field variations are reversed. Resonance widths of Pc4 and Pc5 pulsations are given by HANSONet al. ( 1979) and WALKERet ai. ( f 979) as - 300 and 100 km. respectively. Thus the scale sizes of both Prl and Pc35 pulsations are such that, at first sight at least, a group range oscillation of amplitude 1 km is consistent with a strong echo amplitude oscillation due to focusing effects. The amplitude and group range oscillations will be in phase, as is most often observed in the case of Pcl pulsations at Halley. WRIGHT (1974), however, assumes a sinusoidal reflecting surface in attempting to match theoretical and observational results with regard to travelling ionospheric wave-like disturbances. The comprcssional action of the pulsation field will produce a wave emanating from the field line with the maximum pulsation field. The echoing HF signals will therefore be subjected to both a sinusoidal reflector and lenslike compressions and rarefactions in electron density perpendicular to the field line along the propagation path. While the sphere of influence of both classes of PC within the ionosphere may be similar, the curvature of the effective reflector produced by the compressional action of the PC will be dependent on the pulsation period. The compressional wave perpendicular to the field line will have a wavelength dependent on the Alfven velocity and pulsation frequency. Typical Alfven velocities range from -2OOlOOOkm/s in the ionosphere (FUJITA, 1987). This implies typical wavelengths of 400-2000 km for Pcl (0.5 Hz). The equivalent wavelength for Pc3 (0.05 Hz) would be 400&20,000 km. Thus, while not in full agreement with the wavelength predicted by WRZHT (1974) for a 1km amplitude group range oscillation, it can be seen that Pcl pulsations will produce a wavelike plasma density contour in the ionosphere much more suited to focusing than might be produced by a Pc3 were it to be sustainable over such large distances. This may explain the presence of amplitude, group range and skymap echolocation position oscillations in ISPcl which are absent or very weak in ISPc3-5.

6. SUMMARY In summary, the first reported observations of ionospheric signatures of Pcl pulsations in ionosonde echo data have been discussed. They occur during about one-third of the periods during which Pcl events are recorded by a co-sited ULF detector. The ionospheric signature is different from that observed during Pc3-

Ionosonde

signatures of Pcl pulsations

5 events. In particular, group range and large echo amplitude oscillations are observed at Pcl pulsation frequencies which are not present at Pc3-5 frequenties. Unlike Pc3--5 ionospheric pulsations observed at Halley, these Pet pulsations are observed in both Eand F-region echo time series. It is unlikely that the Prl ionospheric signatures are produced as a result of particle precipitation processes. The magnitude of the ISPcl Doppler velocity oscillations is consistent with direct compressional action of the hydromagnetic wave. Taking the exampie given by POOLE et ul.(1988). and extrapolating the pulsation frequency to that of Pcl pulsations, suggests that compression and rarefaction of the ionospheric plasma will dominate over other components

349

contributing to the ionospheric Doppler velocity. The amplitude and group range oscillations may be explained, in part, by focusing and defocusing of the ionospheric echoes by the wave-like electron density contours and the electron density variations along the propagation path produced as a result of this compressionat action. There are inconsistencies remaining, however. It is clear that further research is necessary to understand the observational results. Case studies to investigate possible focusing effects quantitatively in terms of the horizontal propagation of the compressional PcI Alf&n wave through the ionosphere, the position of the source region and the PcI polarization are underway.

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