(2) Atmospheric and Space Physics Group Australian Antarctic Division Kingston TAS 7050 Australia
ABSTRACT
Current research utilises the recently installed Southern Hemisphere Imaging Riometer (SHIRE) located at Australia’s Davis Antarctic station to study riometer signatures associated with cusp-latitude magnetic impulse events (MIEs) detected in co-located fluxgate magnetometer data. Such observations enable further refinement of the characterisation of MIEs by observing the spatial scale, morphology and dynamics of associated patches of locally enhanced Cosmic Noise Absorption (CNA). The multiple narrow beams which comprise the field of view of an imaging riometer allow us to obtain two-dimensional images of the localised absorption patches as they evolve over time, facilitating study of the structure and motion of MIE-associated absorption events. Of those dayside MIEs with a detectable riometer signature, two distinct morphologies have emerged. The first of these has many characteristics of the morning sector absorption spikes reported by Stauning and Rosenberg (1996), yet with a previously unreported spatial and temporal bipolarity in the riometer signature. The second and more common signature observed in association with magnetic impulses is a widespread increase in absorption of spatial scale larger than the 200×200 km2 field of view of the imaging riometer, but with generally smaller amplitude and consistent antisunward and magnetic poleward convection. In this paper we present two case studies illustrating both types of events.
1 INTRODUCTION
Magnetic Impulse Events (MIEs) observed in high latitude magnetograms are perturbations in the geomagnetic field which manifest themselves as a half to a full cycle of a 300 - 800 second period pulsation. Such events have previously been explained in terms of the ionospheric response to transient magnetic reconnection at the magnetopause, commonly termed Flux Transfer Events (FTEs) (Russell and Elphic, 1979; Glassmeier, 1984; Lanzerotti et al, 1986). However a number of other magnetospheric phenomena have also been suggested as mechanisms likely to invoke a comparable ionospheric response. Such mechanisms include impulsive penetration of plasma into the magnetosphere (Heikkila et al, 1989); abrupt changes in the solar wind dynamic pressure (Sibeck, 1989; 1990); and a Kelvin-Helmholz instability acting at the low latitude boundary layers (McHenry et al, 1990). The investigation of these processes is a key element of high latitude studies, as they provide insight into the mechanisms by which solar wind energy is coupled into the Earth's magnetosphere and subsequently transferred to ionospheric altitudes.
It is generally accepted that communication to the ionosphere of a magnetospheric disturbance such as a FTE, is through intermediate field aligned currents which close in the ionospheric E region (Southwood, 1987; McHenry and Clauer, 1987). When reconnection commences at the magnetopause, solar wind plasma is linked to the ionosphere via convecting flux tubes which carry these field aligned currents. The exact nature of the field aligned current systems, and their closure currents in the ionosphere, is not well understood. However, the characteristic magnetic perturbations generated in response to the Hall component of the closure current have been extensively modeled, most recently by Chaston et al. (1993), Zhu et al. (1997), and Zhu et al. (1998).
Further studies have shown that in addition to a characteristic magnetic perturbation, there is commonly a concurrent signature in collocated riometers. A study by Korotova et al. (1997) using magnetometer and riometer data from the South Pole (-74.2° MLAT), suggests up to 80% of those impulsive magnetic events which are detected have an associated riometer signature. Two possible mechanisms may explain the enhanced radiowave absorption observed in association with magnetic impulses. The first is thought to be the most commonly observed type of absorption event and is due to enhanced ionization resulting from the precipitation of energetic electrons. Such energetic electrons (with typical energies of 30 - 300 keV) originate from populations of quasi-trapped substorm-produced electrons drifting around to the dayside. Their precipitation results in the enhancement of D-region (60 - 90 km) electron densities that subsequently enhance the level of radiowave absorption in the ionosphere. Alternatively, the large horizontal electric fields associated with currents in the ionosphere can raise the electron temperature of the intervening plasma sufficiently through non-deviative collisional heating that significant enhancements of E-region electron collision frequency may occur (typically at ~120 km altitude). Such an enhancement of electron - neutral collision frequency in a region of sufficiently high localized electron density (such as the daytime profile) can result in a cosmic noise absorption (CNA) enhancement detectable by a riometer (Stauning, 1984; Stauning et al., 1985).
The Korotova et al. (1997) study, however, was made using a broadbeam riometer which integrates the received signals from a wide field of view and thus provides no information on the spatial scale or motion of the absorption event. In addition, a broadbeam riometer cannot distinguish spatial variations in cosmic noise absorption levels from temporal variations. On the other hand, observations with an imaging riometer, with its multiple narrow beams and superior spatial resolution, allows us to produce two dimensional "images" of the patches of enhanced CNA as they evolve over time. Thus observations can be made of the spatial scale, morphology and dynamics of MIE–associated absorption patches. This enables further refinement of the characterisation of magnetic impulse events by assisting in distinguishing events with comparable magnetic signatures such as FTEs, precipitation spike events (Stauning and Rosenberg, 1996), and Travelling Convection Vortices (TCV’s) (Glassmeier, 1992).
The motion of the ionospheric signatures of these events is particularly characteristic. TCVs propagate tailward typically with speeds of 2 - 10 km/s, with occurrence peaking at about 09 and 14 MLT. There is very little poleward motion of these signatures (Glassmeier, 1992). Alternatively, FTEs, or the ionospheric signatures of reconnection as modeled by Chaston et al., (1993), show a significant poleward component of motion of the footprint of the reconnected flux tube and an east-west component of motion that is dependent on IMF-By. Typical velocities through the ionosphere are 0.5 - 3 km/s with occurrence peaking within a few hours about local magnetic noon.
The imaging riometer used to study the CNA signatures of cusp latitude impulsive magnetic events was the recently installed Southern Hemisphere Imaging Riometer (SHIRE) located at Australia's Davis Antarctic station. Events were initially identified using collocated three component fluxgate magnetometer data.
2 THE SOUTHERN HEMISPHERE IMAGING RIOMETER EXPERIMENT
Over the 1996/97 Austral summer, installation of the SHIRE imaging riometer at Davis, Antarctica (-74.5°S magnetic latitude) was completed. The project is a collaboration between the Space Physics Group at the University of Newcastle, the Institute for Physical Science and Technology (IPST) at the University of Maryland, and the Australian Antarctic Division.
Figure 1. Antenna array of the Southern Hemisphere Imaging Riometer Experiment (SHIRE), located at Davis, Antarctica (-74.5° MLAT)
The SHIRE imaging riometer (see Figure 1), one of the Imaging Riometer for Ionospheric Studies (IRIS) systems (Detrick and Rosenberg, 1990), consists of a phased array of circularly polarised antennas in a square 8×8 element configuration. The array has its major axes aligned with the compass magnetic N - S and E - W directions with half-wavelength spacing between individual antenna elements. Signals received by each of the 64 crossed dipole ("turnstile") antennas are fed into an eight port Butler matrix which is used as the beam forming device (Butler and Lowe, 1961). The standard Butler matrix has been modified to produce a zenith pointing beam resulting in the sacrifice of one of the output ports of the matrix. The result of such phasing is the formation of 49 conical beams each with a -3 dB beamwidth of approximately 13° and oriented in a unique direction forming a network of partially overlapping beams. Figure 2 shows the horizontal planar cross-section at ionospheric altitudes (assumed to be 90 km) of the –3 dB power level for all beams of the IRIS system. This is then the effective field of view of the instrument at 90 km altitude, which is the assumed average altitude for ionospheric absorption of cosmic radio waves (Detrick and Rosenberg, 1990).
Figure 2. Projection of the -3 dB contours of the 49 IRIS beams onto the ionosphere at 90 km altitude. The dashed circle in the centre represents the nominal (-3 dB) viewing area of a conventional broad-beam antenna (from Detrick and Rosenberg, 1990).
SHIRE operates at 38.2 MHz with a 250 KHz bandwidth, and logs the 49 channels continuously at 1 Hz by sweeping sequentially across each row of detectors. The data logged from each beam is normalised to remove the diurnal variation in received cosmic noise power, and can be viewed as individual time series plots in a similar manner to the output of a broadbeam riometer, or as a time series of two dimensional "images" of patches of enhanced CNA. The multiple narrow beam configuration of SHIRE also results in a wider area coverage (~200×200 km2 at 90 km altitude), at significantly enhanced temporal and spatial resolution (1 second, and up to 20 km, respectively).
SHIRE is situated at a magnetic latitude of -74.5° and passes through local magnetic noon at approximately 0950 UT. Such a location generally positions the riometer just equatorward of the footprint of the cusp, near the last closed field line. At this latitude, magnetic field lines regularly map to the outermost regions of the magnetosphere, such as the dayside magnetopause where the dynamic interactions of the solar wind and the Earth's magnetosphere take place.
3 CASE STUDIES
The selection criteria used for the detection of MIEs in fluxgate magnetometer data was based on that of Lanzerotti et al (1991). The identification routine stipulate the magnetic deflection satisfy a minimum amplitude criteria (taken as ∆ > 40nT, and #8710; X, ∆ 20nT), and that it is of a temporal duration of 300 - 900s. It also requires the deflection to be distinguishable from noisy magnetic records by limiting the number of extrema in a temporal window surrounding the event, and requiring ∆ Z to be above background fluctuations. Although a necessary selection criteria, the requirement of an event free from magnetic noise is not ideal as it effectively removes events occurring during magnetically active times when Davis moves toward open field lines, and to a lesser extent events around magnetic noon where average dayside magnetic activity peaks.
In a preliminary investigation, a number of impulsive events were identified in 1997 fluxgate magnetometer data recorded at Davis, and their associated riometer signatures were studied. Among those events with a detectable riometer signature, some distinct morphologies have been observed, namely short duration localized absorption events detected in only a small number of beams of the array and often showing a bipolar structure, and secondly (and more commonly) a widespread increase in CNA observed in all beams of the array indicating an event of spatial scale larger than the 200×200 km2 field of view of the instrument.
We now examine two representative examples satisfying these exclusive criteria.
3.1 Event of 0730 UT, 18 September, 1997
Figure 3 shows an event of the first type – an MIE with an associated riometer signature consisting of a short duration spike. A distinct event in the magnetogram, this magnetic impulse occurred in the local morning at Davis (~0930 MLT) during a period of moderate magnetic activity (Kp = 3+). The figure shows the relative timing and magnitude of the fluxgate magnetometer signature (bottom three traces) and the riometer signature observed in the (1,0) beam of the SHIRE array (top trace). Taking the time of event occurrence as the point at which the signal reaches 90% of it’s peak value, the magnetic signature and the riometer signature – a bipolar modulation of local radiowave absorption levels of considerably shorter period – can be seen to occur simultaneously when the relative fields of view of the instruments are taken into consideration.
Figure 3. Time series of received signal intensity in one beam of the SHIRE array, and the associated magnetic signature as measured by a collocated three-component fluxgate magnetometer on 18 September, 1997. A bipolar spike in absorption can be seen associated with the longer period magnetic impulse at 0738 UT. A positive deflection in the riometer trace indicates an increase in radiowave absorption levels.
Figure 4. Time series of absorption images (L to R, T to B) for the interval 0736-0740 UT on 18 September, 1997 showing the progression of a MIE-associated absorption event. Each image represents an ~200´ 200 km2 view of the ionosphere, with an ~6 second time difference between adjacent panels. The orientation of the images is indicated at lower right. The absorption scale is centered on the average taken as zero so that both positive and negative deflections in absorption intensity may be observed.
To observe the spatial and temporal structure of this absorption event, all 49 beams of the array were combined using an interpolation routine to produce a time series of absorption 'images' (see Figure 4). Each square in this figure represents a 200×200 km area of the ionosphere with the time series progressing left to right, top to bottom. Absorption intensities are color coded according to the scale on the bottom of the figure. The absorption scale is deliberately centered around an average taken to be the background level of absorption and set to zero so that both positive and negative deflections in absorption levels can be observed.
The convection or phase velocity of the event can be estimated through a cross correlation technique along specific rows of the array using the data shown in Figure 4. This technique assumes the event has the form of a steadily propagating front resulting in constant time delays between detection in successive beams along each row of the array. After converting to a geomagnetic coordinate system, this yields a westward (i.e., antisunward) phase velocity of ~600 – 700 m/s.
Figure 5. Time evolution of absorption intensities along one row and one column of the SHIRE array for the event at 0738 UT on 18 September, 1997. A westward progression of the signature can be seen in the lower panel.
Figure 6. Absorption intensities (in dB) in all 7 * 7 beams of the SHIRE array during the interval 0736-0742 UT on 18 September, 1997. A positive deflection indicates an increase in radiowave absorption levels in this figure. The frames are arranged in the order that the beams project onto the ionosphere as viewed from below. Magnetic south is to the top of the page.
Figure 7. Time series of received signal intensity in one beam of the SHIRE array, and the associated magnetic signature as measured by a co-located three-component fluxgate magnetometer on 22 August, 1997. An absorption signature of comparable period can be seen to be associated with the magnetic impulse commencing at 0808 UT. A positive deflection in the riometer trace denotes an increase in radiowave absorption.
Figure 8. Absorption intensities (in dB) in all beams of the SHIRE array during the interval 0800-0818 UT on 22 August, 1997. The frames are arranged in the order that the beams project onto the ionosphere as viewed from below. Magnetic south is to the top of the page. A positive deflection in the riometer trace denotes an increase in radiowave absorption.
Figure 9. Time series of absorption images (L to R, T to B) for the interval 0805-0813 UT on August 22, 1997 showing the progression of the MIE-associated absorption event. Each image represents a ~200*200 km2 view of the ionosphere.
Figure 5 shows the evolution of absorption intensities over time along one column and one row of the SHIRE array. Again the absorption scale is centered around an average taken as zero, and a clear westward progression of the absorption feature may be observed. Plotting data from other rows in the same manner reveals a substantial (~200 m/s) southward (i.e., poleward) component of motion of the main peak in intensity, in addition to the predominant westward progression.
Figure 6 shows the time series absorption data from the full SHIRE array with the frames oriented on the page in the same order that the beams project onto the ionosphere as viewed from below. A positive deflection in this figure indicates an increase in the level of radiowave absorption (in dB). At any one instant, the absorption spike can be seen to be restricted to a small number of beams. The size of the event is no more than 70 (40) km in N–S (E–W) extent, and 50–150 seconds in total duration.
IMF and solar wind conditions (as measured by the WIND satellite located 88 Re upstream) during and immediately prior to the impulse event were relatively stable. IMF Bx, By, and Bz were all strongly negative during the event (-4 nT, -10 nT, and -7 nT respectively), circumstances which favour the occurrence of FTEs.
In summary for this event the features of note are:
1. A spatially and temporally restricted absorption event, no more than 70 (40) km in N–S (E–W) extent, and 50–150 s in total duration. 2. Bipolar structure in absorption signature.
The imaging riometer response here is representative of the more commonly observed absorption events associated with magnetic impulse events. Enhanced absorption is seen in Figure 8 to occur in all beams of the array over the duration of the event which commenced at 0808 UT (~1000 MLT) and finished at 0812 UT. Unlike the previous example, this event has a temporally unipolar riometer signature of comparable period to the magnetic impulse. The CNA patch can be seen in Figure 9 to form close to the eastern boundary of the instrument field of view and expand into the field of view over a time scale of ~1 minute. The patch is then seen to move west and south out of the field of view of the imaging riometer. Total absorption event duration is just under five minutes. Using the same cross correlation technique described above, the phase velocity was determined to be ~2 km/s in the geomagnetic westward direction, and ~1 km/s in southward (i.e., antisunward).
Magnetic activity during the event was relatively quiet (Kp=1+), and again the inferred solar wind and IMF conditions at the magnetopause just prior to the event were stable. However, this time IMF Bz was marginally northward (+1 nT), and IMF By slightly positive (+2 nT). Under these conditions, we can reasonably expect Davis to be situated equatorward of the cusp and cleft on closed magnetic field lines. This is confirmed by an inspection of the dynamic power and phase spectra for this day which reveals the presence of strong and consistent Pc5 field-line resonance signatures in the magnetic field at Davis, as observed by a collocated induction magnetometer. Such magnetic field-line resonance signatures - the appearance of a "Pc5 arch" in cross-power and the shift in relative phase for Pc5 pulsations between longitudinal stations about noon (Ables et al, 1998) - confirm the supposition that Davis is mapped to closed magnetic field lines on this day.
Although there is no evidence for a solar wind dynamic pressure variation impinging on the magnetosphere at the start of this event, the bipolar deflection of Z, the large spatial scale of the absorption event and its antisunward motion on closed magnetic field lines at ~2 km/s, are all consistent with the signature of a Travelling Convection Vortex (TCV) as reported by Friis-Christensen et al. (1988).
4 DISCUSSION
With reference to the event on 18 September (Event 1), short duration spikes in the level of cosmic noise absorption on the dayside have been observed previously (Stauning and Rosenberg, 1996). They were reported to occur preferentially in the local afternoon hours and during periods of southward IMF. However, the mechanism producing such absorption spike events in the morning sector is not yet understood. Stauning and Rosenberg (1996) proposed eastward drifting quasi-trapped energetic electrons generated by substorm activity on the nightside as the source of high energy electrons, and suggested their precipitation may be triggered by an impulsive disturbance from substorm activity or solar wind interactions. The clearly westward drift of the absorption signature in this event, however, would appear not to be consistent with such an explanation.
One further consideration of importance is the location of Davis with respect to the open/closed field line boundary. As the drift motion of the trapped population of electrons requires closed field lines, absorption events of the electron precipitation type should be restricted to regions equatorward of the cusp or cleft region. With the prevailing IMF Bz and By strongly and consistently negative during and immediately prior to the event on September 18, the footprint of the cusp would be expected to be shifted well into the post-noon sector away from Davis, and the polar cap expanded equatorward of Davis, leaving the station mapped to open field lines [Newell et al, 1989]. Confirming the supposition of open magnetic field lines is the absence of distinct Pc5 resonance signatures (such as the "Pc5 arch") in the dynamic power spectrum for this day. Although the absence of such signatures is in itself no way conclusive evidence that the station was in the open field-line regime, the combination of favourable IMF conditions and an absence of resonance signatures would suggest that this was indeed the case. In light of this, it would appear unlikely that eastward drifting populations of trapped substorm electrons were the source of energetic electrons producing the observed riometer event. Finally, WIND spacecraft measurements reveal no evidence of solar wind dynamic pressure variations impinging on the magnetopause immediately prior to the event. However, it must be noted the WIND spacecraft was located 88 Re upstream in the solar wind which is constantly evolving.
An interesting feature of the first event was the bipolar structure in the riometer signature. A depression in absorption levels such as that evident in this event, is best explained in terms of a modulation of radiowave absorption about a previously enhanced background level. Considering the two most likely mechanisms mediating the level of cosmic noise absorption, both electron precipitation and collisional heating of the lower E-region ionosphere could conceivably produce a bipolar fluctuation in CNA. However, earlier studies of CNA enhancements thought to be caused by collisional heating of the E-region (Stauning, 1984) have shown the characteristic absorption signature to be weak and slowly fluctuating, somewhat different from the rapidly changing spike event observed here.
With reference to the event on August 22 (Event 2), the appearance of distinct Pc5 resonance signatures concurrent with the event, confirms event occurrence on closed magnetic field lines. Consistent anti-sunward motion of the plasma patch then on closed magnetic field lines suggests an explanation in terms of viscous interaction of the magnetic flux tubes with solar-wind plasma flows down the flanks of the magnetosphere.
5 SUMMARY
The recently installed SHIRE imaging riometer is being used to investigate the spatio-temporal morphology of cosmic noise absorption (CNA) events associated with magnetic impulse events at high latitudes. Such observations enable further characterisation of magnetic impulses according to the dynamics and spatial scale of their associated absorption signatures. In this preliminary study, two distinct morphologies have been observed on the dayside:
1. Localized, short duration absorption spikes restricted to a small number of beams, and often showing a bipolar structure. Possibly the ionospheric signature of an FTE. And;
2. A widespread increase in absorption of spatial scale larger than the 200 - 200km2 field of view of the imaging riometer, consistent with the observed characteristics of TCVs.
Further work in this study involves a more precise identification of the location of the cusp and cleft footprints and the open/closed field line boundary in relation to Davis during each event. Use of DMSP particle data and a study of local magnetic pulsation activity during and immediately prior to each event will allow reasonable estimates of the local magnetic field line topology, so that more accurate classification of events may be made.
6 ACKNOWLEDGMENTS
This research was supported by the Australia Research Council, the University of Newcastle, the University of Maryland, the Australian Antarctic Science Advisory Committee, and the Australian Antarctic Division. We thank Lloyd Symons, and other ANARE staff for their efforts in installing SHIRE at Davis. The Cooperative Research Centre for Satellite Systems (CRCSS) is supported by the Commonwealth of Australia.
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