Umran S. Inan

 

Outline

A. INTRODUCTION

B. SCIENTIFIC BACKGROUND AND QUESTIONS

     1. Relativistic Electron Precipitation

     2. Solar Particle Events (SPEs) 

     3. Magnetosphere-ionosphere coupling at high latitudes/polar cap regions

C. THE RESEARCH

     1. Measurements of the VLF Beacon Signal

     2. System Description, Development and Operation

Bibiography

 

A. INTRODUCTION

The three year program will investigate key scientific questions concerning lower ionospheric and mesospheric signatures of 
(i) relativistic electron precipitation from the Earth�s
outer radiation belts
(ii) solar proton events
(iii) energetic particle precipitation, Joule heating and
other magnetrosphere-ionopshere coupling processes occurring at high latitudes and polar cap regions

The ionospheric signatures of both steady and burst precipitation are to be measured via their effects on a very low frequency (VLF) beacon signal transmitted from South Pole and received at various Antarctic locations (Figure 1). A key component of the proposed investigation is thus the establishment of a VLF beacon transmitter facility at South Pole. 

The study is
(i) aimed at understanding how
corpuscular energy from the sun is transferred through the magnetosphere into the middle and upper atmosphere, dynamically coupling these regions
(ii) relevant to the National Space Weather Program
(iii) complementary to a number of ongoing ground- and satellite-based programs
(iv) timely since the occurrence rate and intensity of the primary phenomena targeted for investigation
peaks during the few years following solar maxima [Reeves et al., 2000]

 

Fig. 1. Propagation paths from a South PoleVLF beacon to various Antarctic sites. Of particular importance are the observations atPA,HBA, CF, and SNA the Beacon signal to which crosses the relativistic electron precipitation regions. The dark blue shading represents the typical extent of the enhancement regions whereas the lighter shaded areas are those affected in more intense events where precipitation extends to lower latitudes (see Figure 2). Observations at SYO, DVS, CSY and MCM allow the measurement of energetic electron precipitation in the polar cap regions currently monitored by HF radars (Section B.3) and Solar Particle Events (Section B.2), and also provide �calibration� so that signal changes due to relativistic electron precipitation can be clearly identified among other variations. Observations at Campbell (CI) and Macquarie (MI) islands, and other sites in New Zealand (NZ) and Australia (AU) can provide further information on the local time variation of relativistic electron precipitation.


The fluxes of relativistic (>1 MeV) electrons in the outer magnetosphere at subauroral latitudes (4.5<L<7) have been known to undergo pronounced increases and decreases [e.g., Baker et al., 1986; Nagai, 1988]. High resolution and high sensitivity measurements of relativistic electrons are currently being collected at high altitudes with the POLAR satellite [Blake et al., 1995] while the associated precipitation of these energetic electrons into the atmosphere is extensively documented with data from the SAMPEX [Baker et al., 1993a; 1994; 1998; XinLin Li et al., 1999]. At geosynchronous orbit, where large numbers of high-energy electrons are present and exhibit great variability in flux [e.g., Baker et al., 1979], the highly penetrating relativistic particles have deleterious effects on spacecraft subsystems [e.g., Reagan et al., 1983; Baker, 1985; Gussenhoven et al., 1987]. When they precipitate into the lower ionosphere and mesosphere, these highly energetic electrons penetrate to altitudes as low as 40-60 km with an energy flux which is 3-4 orders of magnitude greater than the galactic cosmic ray or solar EUV deposition [Reagan, 1977; Baker et al., 1987; 1993b; Gaines et al., 1994], and may well affect the neutral and ion chemistry of the middle atmosphere [Spear et al., 1984; Rusch et al., 1981; Solomon et al., 1981; Callis et al., 1991; 1996; 1997; 1998]. The rate of energy deposition into the upper atmosphere during such enhancements may be as high as ~ 1019 - 1020 ergs/day [Imhof and Gaines, 1993]. Examples of global distributions of relativistic electron precipitation regions measured on the SAMPEX satellite are shown in Figure 2.

 

Fig. 2. Daily averaged global images of > 400 keV electron precipitation as measured on the low altitude SAMPEX satellite [Baker et al., 1995]. The three insets show the data from different days, as indicated (e.g., 93216 as day 216 of 1993), with the images constructed by averaging between 16-orbits per day. The sequence illustrates a relativistic electron enhancement event which peaks on day 93220. The data from day 93216 shows a typical �quiet� period. The persistent intense precipitation over South America is due to the South Atlantic Anomaly. The lower right inset shows the configuration of the proposed experiment, illustrating that a VLF beacon at South Pole would be uniquely positioned to study this phenomena, especially with receptions of the beacon signal at Palmer Station (PA). While SAMPEX provides global averages and samples each ionospheric region once per day, the proposed beacon experiment will measure the temporal variations and also assess the local time distribution via measurements on a set of distributed paths as shown.

 

VLF sounding (i.e., the measurement of the amplitude and phase of subionospheric signals) is a sensitive tool for the measurement of ionospheric conductivity (i.e., electron density and temperature), especially at altitudes below 90 km [e.g., Sechrist, 1974], and some of the early work on relativistic electron precipitation events has indeed relied on subionospheric VLF measurements [e.g., Thorne and Larsen, 1976]. In recent years, the VLF remote sensing method has been extensively utilized to study a variety of lower ionospheric disturbances, including those associated with lightning discharges [e.g., Inan et al., 1993; Burgess and Inan, 1993], heating by HF [Barr et al., 1985; Bell et al., 1993] and VLF waves [Rodriguez and Inan, 1994; Rodriguez et al., 1994], the auroral electrojet [Kikuchi and Evans, 1983; Cummer et al., 1994; 1997], and relativistic electron precipitation enhancements [Demirkol et al., 1999]. Computer-based models of VLF propagation and scattering are now available [Poulsen et al., 1990; 1993; Smith and Cotton, 1990] so that the VLF method can now be quantitatively used to interpret ionospheric signatures of relativistic electron precipitation in terms of their spatial extent and the altitude profiles of ionization [Demirkol et al., 1999].

South Pole is an ideal location for the VLF beacon transmitter for several important reasons. The ~3000 m thick ice sheet allows the use of an �elevated� horizontal antenna providing a radiation efficiency of ~10% at ~20 kHz frequency (as opposed to only 0.1% for the same antenna on conducting ground elsewhere on earth) [Raghuram et al., 1974; Helliwell and Katsufrakis, 1974]. The location of South Pole poleward of the relativistic electron enhancement region and the fact that most of the signals observed at Antarctic sites (Figure 1) propagate on relatively short (< 3000 m) paths minimize possibly confusing effects of other ionospheric disturbances. During austral winter, with much of the Antarctic ionosphere in dark, local time variations in the magnitude and spatial (i.e., L-shell) extent of relativitic electron precipitation can be determined with observations at regular intervals (1-min out of every 15-min is the planned duty cycle; see section C.3.c) over the course of a 24-hour period. The reception of the beacon signal is aided by the relatively low radio frequency interference environment at the various Antarctic sites. During the 1-minute transmission period, the beacon signal amplitude and phase can be measured with high-time resolution (<1 ms), allowing for detection of both quasi-stationary variations and burst precipitation effects.

The VLF method quantitatively determines the ionospheric signatures of relativistic electron precipitation, by interpreting data in the light of theoretical models of VLF propagation [Cotton and Smith, 1991; Poulsen et al., 1993; Rodriguez et al., 1994; Cummer et al., 1997]. The predicted signature of a typical enhancement event as would be observed with the beacon signal received at Palmer Station is shown in Figure 3. We note that the signal amplitude changes by ~10 dB in response to a relativistic electron precipitation event, as opposed to typical ~2 dB variability (at night) due to changes in the ambient lower ionospheric density. Experimental verification of unusually large signal changes produced by relativistic electron precipitation (as observed on a northern hemisphere VLF path) was recently presented [Demirkol et al., 1999], demonstrating that they are unambigiously detectable and can be used (via the use of VLF propagation models) to infer the altitude profiles of lower ionospheric electron density.

The observations of the VLF beacon signal at Palmer Station will provide the core dataset with which the scientific questions discussed in the next section will be investigated. The unique disposition of the South Pole�Palmer baseline with respect to the ionospheric regions affected by relativistic electron precipitation is illustrated in Figures 1 and 2. As of 1999, the Stanford VLF observation system at Palmer Station has been fully modernized and interfaced with the Internet so that raw or reduced VLF data can be brought to Stanford in near-real time. In view of the relatively low duty cycle for the VLF beacon operations (1-min out of every 15-min) all of the VLF beacon phase and amplitude data acquired at Palmer will be brought back in near-real-time and will be made available to the scientific community over the World Wide Web.

Continuous ground-based VLF measurements of ionospheric signatures will strongly complement the extensive measurements of relativistic electron populations on several geosynchronous satellites [Baker et al., 1986], on the POLAR spacecraft [Blake et al., 1995], and on DMSP,SAMPEXandUARSsatellites at lowearth orbit [e.g., Gussenhoven et al., 1987; Baker et al., 1994; 1998; Gaines et al., 1994]. Furthermore, the VLF beacon operations will also be closely coordinated with specific low altitude satellite passes to calibrate the VLF technique and to resolve important questions concerning spatial versus temporal variations [Imhof et al., 1992; 1993b; Blake et al., 1993].

 

Fig. 3. VLF signature of a relativistic electron enhancement event. (a) VLF signal path from SPA to PA with respect to the disturbed ionospheric region (shaded). (b) crossectional view depicting the perturbation of the earth-ionosphere waveguide mode structure of the VLF signal due to the electron precipitation. (c) ionization profiles resulting from relativistic electron precipitation fluxes corresponding to the different levels (1,2,3,4) as shown in (e), under relatively tenous ambient Dregion conditions. (d) same as (c) except for dense ambient Dregion conditions. (e) a typical relativistic electron precipitation enhancement, shown here rising and falling over 7 days. The flux levels and energy spectra of the precipitation was taken to be as given by Gaines et al. [1994], based on measurements on the UARS satellite. (f) the calculated (see Section C.3) VLF signal amplitude variation as observed at Palmer.

 


Systematic measurements of the VLF beacon signal both at coastal stations (Figure 1) and at Automatic
Geophysical Observatories (Figure 5) will also provide a unique data base to investigate (i) the ionospheric and mesospheric effects of Solar Proton Events (SPEs) [e.g., Reeves et al., 1992], and (ii) energetic electron precipitaton and Joule heating components of high latitude/polar cap magnetosphere-ionosphere coupling processes. SPEs occur less frequently than relativistic electron enhancements but cover the entire polar cap down to~60. magnetic latitude [Reagan et al., 1981; Collis et al., 1998] and are knownto cause significant depletion of mesospheric ozone [e.g., Solomon et al., 1983; Reid et al., 1991]. The altitude profile of enhanced ionization produced by SPEs can be derived from measurements on short and wholly ice-based VLF propagation paths as shown in Figure 5 so that the various atmospheric effects of SPEs [e.g., Johnson et al., 1993] can be better assessed. Measurement of conductivity enhancements over a distributed set of paths can also be used to map the ionospheric areas that are affected by particle precipitation occuring due to a variety of physical processes and representing a significicant form of energy transport (and thus coupling) between the magnetosphere and the ionosphere. In the latter connection, the proposed program is highly complementary to and synergistically enhances other Antarctic Upper Atmospheric research efforts, such as the Automatic Geophysical Observatory (AGO) programs of NSF/OPP and British Antarctic Survey (BAS), and the SUPERDARN coherent HF radar networks [Greenwald et al., 1995] (see Figure 5 and section D).

The proposed program is directly relevant to the objectives of the National Space Weather Program, in particular addressing the program goals of (i) investigation of the coupling between the solar wind and the magnetosphere, and (ii) improved ionospheric specification, forecast and nowcast of the evolution of ionospheric disturbances, with particular emphasis on those processes affecting navigation andcommunication systems. The objectives of the proposed program also address the recommendations of the National Academy of Sciences report titled Solar Influences on Global Change, to "monitor continuously the energetic particle inputs to the Earth�s atmosphere" and "to understand, the relationship between space-based measurements to the energy spectrum and fluxes of both solar and galactic energetic particles reaching different altitudes in the Earth�s atmosphere". In this connection, it is important to note that relativistic elecrtron precipitation is a primary phenomena manifested by major Space Weather events [Baker et al., 1998; Reeves et al., 1998], and that the time variation of ionospheric currents as measured at Antarctic ground-based sites has been shown to be very similar to that speccarft-charging/disruption episodes measured in situ [Lanzerotti et al., 1998]. The VLF beacon transmitter at South Pole will be established in a most cost effective manner by using VLF transmitter (specifically power amplifiers) equipment removed from Siple Station just before its closure (see section C). 

Fig. 4. Great circle propagation paths from the South Pole VLF beacon transmitter to different Automatic Geophysical Observatory (AGO) sites. As part of a science team headed by University of Maryland, Stanford participates in a program involving magnetometer riometer, all sky camera, and ULF/ELF/VLF wave measurements at six different locations in the Antarctic plateu (AP1, AP2, AP3, AP4, AP5, and AP6). The Stanford-built receivers atMCMand on theAGOs are already designed to detect the VLF beacon signal. Other potential observation sites include Davis (DVS) and Vostok (VOS) stations as shown. Also shown for comparison is the fields-of-view of the SUPERDARN coherent HF radars located at HBA, SNA, and SYO [Greenwald et al., 1995].

B. SCIENTIFIC BACKGROUND AND QUESTIONS

1. Relativistic Electron Precipitation

The Earth�s outer magnetosphere is often populated to a surprising degree by relativistic electrons [Paulikas and Blake, 1979; Baker et al., 1979]. The origin of the multi-MeV electrons observed at geosynchronous orbit and the source of the pronounced fluctuations in their intensity are not known [Baker et al., 1987], although they are generally correlated with the onset of substorm activity [Bailey, 1968; Thorne and Larsen, 1976; Nagai, 1988]. Enhancements occur with relatively regular 27-day periodicity and are well associated with solar wind stream structures [Baker et al., 1986]. The phenomena occurs over a variety of time scales; the highly relativistic component (3-10 MeV) exhibits enhancements which typically rise on a 2- to 3-day time scale and decay on a 3- to 4-day scale [Baker et al., 1986], while the so-called relativistic electron precipitation (REP) events, rise to a maximum in 20-30 minutes and decay over 1-5 hours [Rosenberg et al., 1972]. Further, extreme decreases in particle flux at geosynchronous orbit lasting 10-30 minutes that occur during substorm growth phases [Baker and McPherron, 1990] and drifting holes in >300 keV electron data lasting 1-7 minutes and following substorm onset [Sergeev et al., 1992] have recently been observed. While much of the systematic data on relativistic electron enhancements was from geosynchronous orbit [e.g., Baker et al., 1979; 1986], the associated precipitation of these energetic particles into the upper atmosphere was measured from the ground using HF [Bailey, 1968] and VLF [Thorne and Larsen, 1976] techniques, riometer and VHF scatter [Rosenberg et al., 1972] methods, from balloon-based x-ray detectors [e.g., Parks et al., 1979], and from rocket-based platforms [e.g., Herraro et al., 1991]. Data from the SAMPEX [Baker et al., 1993a; 1994; 1998] mission have facilitated systematic measurements of relativistic electron precipitation. In addition to the relatively steady enhancements which rise and fall over many days, intense short duration (0.1-10 sec) bursts (narrow spikes) of >1 MeV electrons have been observed [Imhof et al., 1991; 1992; Blake et al., 1996; Lorentzsen et al., 2000]. Understanding the circumstances (magnitude and spatial distribution) of this precipitation is also important from the standpoint of the associated effects in the lower ionosphere and mesosphere. The possible role of the precipitating high energy populations in coupling the magnetosphere to the mesosphere and in affecting the chemistry of the middle atmosphere is somewhat speculative but is of great potential significance [Baker et al., 1987; 1993b]. There is evidence which suggests that relativistic electron precipitation events may induce significant (10 - 20%) ozone depletions at high latitudes [Spear et al., 1984; Callis et al., 1991; 1998a,b]. Simultaneous measurements of associated secondary ionization profiles (as would be facilitated by the proposed program) would thus be very important. If such a connection exists, precipitation associated with relativistic electron enhancements could impose a modulating effect (27-day and 11-year cycles of solar wind and magnetospheric variability) on the lower D-region ionization (the 27-day periodicity was recently shown Demirkol et al. [1999] to be the case) and, possibly, on the upper level ozone chemistry [Baker et al., 1987]. We note, however, that the 27-day periodicity is itself solar-cycle dependent. Some of the outstanding scientific questions concerning relativistic electron enhancements and our approach to their resolution can be summarized as follows:

Questions: 

What are the ionospheric signatures of relativistic electron enhancements? What fraction of the electrons observed at geosynchronous orbit ultimately precipitate into the earth�s atmosphere? Where does such precipitation occur? To what degree are the pronounced fluctuations in intensity and extreme dropouts reflected in the precipitating electrons? Is there a 27-day periodicity in the precipitating component? Are the intensifications of the precipitating component related to substorm onset in the same way as the enhancements at geosynchronous altitude? What is the local time distribution of relativistic electron precipitation? What are the ionospheric signatures of short duration (< 10-s) relativistic precipitation bursts observed on low altitude satellites? What is the relationship of relativistic electron precipitation to the trapping boundary?

Approach: 

The propagation paths between the proposed South Pole VLF beacon transmitter and receivers located at lower geomagnetic latitudes (e.g., Palmer, Halley Bay) are ideally suited for the measurement of ionospheric effects of relativistic electron enhancements Figure 1. Systematic amplitude and phase data of the VLF beacon signal measured at, for example, Palmer Station will be interpreted in the context of theoretical models ofVLFpropagation and scattering to infer the magnitude of the disturbance (i.e., the ionization profile; see Figure 3) and spatial (i.e., L-shell) extent of the affected ionospheric regions. Measurements over a 24-hour period will determine local time variations. With synoptic operations (e.g., 1-minute out of every 15-minutes), the longer term enhancements (i.e., days) as well as bursty precipitation (i.e., 0.1-10 sec) can be measured. Comparisons with geosynchronous satellite data and other ground magnetometer data will be required in addressing some of the questions. If enhancements observed on satellites as short duration events are spatially confined to the trapping boundary they would not appear bursty in ground-based VLF data. If, on the other hand, they are bursty in nature, they would appear to have rapid onsets and relatively slow decays corresponding to recovery of secondary ionization, which may be interpreted in the context of D-region chemistry models to ascertain the energy spectrum of the precipitation [Glukhov et al., 1992].

Questions: 

What are the effects in the lower ionosphere of relativistic electron precipitation? What are the profiles of enhanced electron density in the mesosphere and D-region during these events? Under what conditions can relativistic electrons cause a significant reduction in atmospheric ozone at high altitudes?

Approach: 

The fact that the VLF propagation paths are short and wholly ice-based allows for easy isolation of the effects of relativistic electron enhancements and quantitative interpretation of signal amplitude and phase variations in terms of the altitude profile of ionization using available VLF propagation models [Poulsen et al., 1993; Cotton and Smith, 1991]. As can be seen from Figure 3, altitude profiles of ionization expected to be produced by relativistic electron enhancements can cause large and distinctly identifiable changes in the signal amplitude and phase (not shown). We can thus expect to determine the resultant ionospheric density profiles by interpreting data (e.g., from Palmer Station) on the amplitude and phase of the beacon signal in the light of VLF propagation models. The accuracy of such a determination would increase if the L-shell extent of the enhancements are separately known, for example via lowaltitude satellite data (e.g., SAMPEX). Once the altitude profiles of the associated ionization enhancements are determined, the resultant effects on the production and loss of stratospheric odd nitrogen and ozone can be evaluated [Callis et al., 1998a,b].

2. Solar Particle Events (SPEs) 

In contrast to relativistic electron precipitation events, SPEs occur less frequently but cover the entire polar cap regions down to ~60. magnetic latitude [e.g., Lanzerotti, 1971; Potemra and Lanzerotti, 1971; Reagan, 1977]. The occurrence, intensity and duration of these events are highly variable [Shea and Smart, 1990]. Events occur much more frequently during solar maximum conditions and typically last for a few to several days. Recent data indicates that the present solar cycle, which started with the solar minimum in October 1986, has exhibited an unprecedented number of SPEs with characteristic risetimes of ~1 hour and decay times of 3 to 12 hours [Reeves et al., 1992]. Further, brief increases of the proton flux associated with sudden commencements were observed. It is well established that major SPEs can cause large (70%) depletions of ozone at high latitudes [Solomon et al., 1983]. The energy input from the moderate to large SPEs can not only greatly increase the ionization but also can significantly affect the neutral chemistry causing large (20-70%) depletions of ozone at high latitudes and mesospheric altitudes [Reagan et al., 1981; Solomon et al., 1983; Reid et al., 1991; Shumilov et al., 1992; Jackman et al., 1993], altering neutral winds [Johnson and Luhmann, 1993], and leading to a cooling of the atmosphere at 45-65 km altitudes [Zadorozhnyy et al., 1992]. Some of the outstanding scientific questions concerning Solar Proton Events and our proposed approach to their resolution can be summarized as follows:

Questions: 

What are the ionospheric signatures of Solar Particle Events? How do they differ from those of relativistic electron precipitation events? How do the ionospheric signatures (i.e., ionization profiles) vary across the polar cap?

Approach: 

The proposed South Pole VLF beacon will provide us with the means to systematically monitor the associated ionospheric effects of SPEs. In this connection, we note that early work on SPEswas carried out via subionospheric VLF remote sensing [Potemra and Zmuda, 1970; Potemra and Rosenberg, 1973]. Measurements of the beacon signal over a distributed set of short, homogeneous (ice-based) VLF paths shown in Figure 5 will allow the determination of both the altitude profiles of associated ionization and its spatial variation. The former is particularly straightforward since (i) many of the paths are short enough for the signal propagation to be described by a single- or few-hop ray model [Budden, 1985] and (ii) the entire length of most paths will be under the influence of the SPE since these events typically cover the entire polar cap down to ~ 60. geomagnetic latitude [Reagan et al., 1981]. The latter is simply facilitated by the distribution of available paths.

 

3. Magnetosphere-ionosphere coupling at high latitudes/polar cap regions

The subject of magnetosphere-ionosphere coupling deals with the mechanisms of energy transport into the upper atmosphere, in the form of either particle precipitation or Joule heating (which results from ionospheric currents driven by magnetospheric electric fields). The subionospheric VLF technique is particularly suited for the measurement of ionospheric conductivity profile at altitudes< 100km[Sechrist, 1974; Cummer et al., 1997], thus complementing other measurements such as with imaging riometers [Detrick and Rosenberg, 1990]. When used together with data on ionospheric current distribution (inferred either with optical imagers or from HF radars) the conductivity profile measurements can, in principle, determine both the direct precipitation and Joule heating contributions. For this purpose, the very short and distributed set of paths from South Pole to Automatic Geophysical Observatories (Figure 5) are well suited, both in terms of allowing quantitative interpretation and providing simultaneous coverage of large distributed regions. Note that because particle precipitation occurs due to a variety of physical processes, it is important to map the ionospheric areas that are affected by particular classes of events. In terms of the availability of the HF radar and optical data, it suffices to mention that the VLF paths shown in Figure 5 lie under the coverage area of three different coherent HF radars [Greenwald t al., 1995] and that the Automatic Geophysical Observatories are all equipped with all-sky cameras.

Questions: 

What are the energetic particle precipitation patterns in the polar cap during substorms and how do they relate to electric field and convection patterns?

Approach: 

Substorms have dramatic manifestations in the polar cap, involving expansion of intense auroral emissions deep into the polar cap, and unique phenomena such as the so-called theta auroras. Simulatenous observations of the convective patterns (HF radars), auroral emissions (optical) and energetic particle precipitation (riometers and VLF beacons) wil lbe used to help understand the topology of the magnetospheric substorm, which is of fundamental importance to understanding the solar wind/geospace system.

 

C. THE RESEARCH

1. Measurements of the VLF Beacon Signal

The VLF beacon signal will be measured at various Antarctic sites by collaborating investigators. Emphasis will be on the west Antarctic sector, with data being acquired at Palmer Station (by Stanford), at Commandante Ferraz (Brazil), at Halley Bay (British Antarctic Survey), at Sanae (South Africa), and at Syowa (Japan) Stations (Figure 1). The VLF paths from South Pole to these sites lie in the magnetic north direction and cross the relativistic electron precipitation regions without encountering the generally more disturbed high latitude auroral and polar cap ionospheres, thus lending themselves to easier quantitative interpretation of the observed signatures. Accordingly, the horizontal dipole antenna of the VLF beacon will be oriented in the magnetic east-west direction to maximize signal transmission in the direction of these stations.

 

Fig. 5. Reception at South Pole of signals from Siple and other transmitter signals. Dynamic spectra showing the frequency range of 0-11 kHz illustrates the reception of Siple transmitter signal at a time when the transmitterwas radiating a total power of 200 Watts. Also shown are 10.2 kHz signals from three different Omega transmitters, arriving over paths of length >10,000 km.

 


Experience with many years of operations with the Siple Station VLF transmitter [Helliwell and
Katsufrakis, 1974; Helliwell, 1988] and the observations of the Siple signal at other Antarctic sites (such as Palmer [Tkalcevic, 1983], South Pole [Carpenter et al., 1985], and Halley Bay Stations [Hurren et al., 1986; Cotton and Smith, 1991]) indicates that the signal radiated by the proposed South Pole VLF beacon can be easily detected at all of the sites shown in Figure 1 and Figure 5, in spite of the well known high propagation losses at VLF over thick ice-sheet paths [Westerlund and Reder, 1973]. To underscore this point, we show in Figure 6 sample VLF spectra observed at South Pole during a time when the Siple transmitter was operating at very low power levels, radiating a total of <200Watts (to be compared with the ~500Watts estimated radiated power of the proposed VLF beacon). We can see that the Siple signal, in this case at 2.5 kHz, is easily detectable with an intensity of 3.5 V/m. At other times (not shown), when the Siple transmitter operated at higher power levels, the third harmonic (i.e., 7.5 kHz) was clearly detectable at South Pole, even though the radiated power at this frequency (outside the bandwidth of the tuned antenna) is estimated to be <100 Watts. In fact, the unintentional third harmonic of the Siple transmitter signal was often observed at Palmer (~1400 km) and Halley Bay (~1500 km) stations with sufficient strength to allow ionospheric diagnostics [Carpenter et al., 1988]. We also note from Figure 6 that 10.2 kHz signals from three different Omega navigation transmitters (no longer operational) are visible, despite the fact that they propagated over the ice sheet to reach South Pole. The Omega transmitters nominally radiated 10 kW. Assuming a loss of 20 dB in the first 1000 km along their propagation path and ~ 2 dB/1000-km thereafter, these signals are reduced in intensity by a total of > 35 dB by the time they arrive at South Pole along propagation paths of length > 10, 000 km [Crary, 1961]. Based on this type of data, signals from a South Pole VLF beacon facility operating at 20 kHz and radiating ~ 500 Watts are fully expected to be detectable with sufficient signal-to-noise ratio at the various Antarctic sites of interest, which are at distances ranging from 1500 km (Halley Bay) to 2800 km (Palmer) from South Pole.

 

2. System Description, Development and Operation

The establishment of the VLFbeacon transmitter system at South Pole is largely facilitated by the use of unique VLF equipment (specifically highly linear audio frequency power amplifiers) retrograded from Siple Station just before its permanent closure in December 1989. To facilitate the synoptic operation of the beacon for 1-min out of every 15-minutes, a rechargeable Uninterrupted Power Supply (UPS) will be used, which will draw a continuous ~ 600Wfrom the station power supply, charging its batteries, and operating autonomously (i.e., using its own charged batteries) during the 1-min transmission time. In this way, the operation of the VLF beacon does not place any unusual peak-power demands, so that the entire system appears to the station power supply as any ordinary instrument or device with ~600 Watt power rating.

 

Fig. 6. System block diagram. The transmitter will deliver ~6 kW to the antenna terminals. With an estimated antenna efficiency of 8-10%, the radiated power is ~500W [Raghuram et al., 1974]. The antenna is a 6.25 km (tip-to-tip) dipole resonant at ~ 20 kHz. The wire is elevated from the ice (~4-5 ft) on poles at ~200 ft intervals.

 


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