Saturday, 20 May 2017

Submarine communications on VLF protecting Earth from space radiation

We briefly chatted about Very Low Frequency (VLF) communications and how such comms may travel long distances, go through a bit of soil and water, and are used for communication to submarines in "Lines, radios, and cables - oh my."

Well, interesting news on that front this week. NASA is reporting submarine communications are having a positive effect by creating a somewhat protective bubble around planet Earth.

The effect is noted as small in the paper, "Anthropogenic Space Weather" [Gombosi, T.I., Baker, D.N., Balogh, A. et al. Space Sci Rev (2017)], at least from what I can parse. I found some of the communications history very interesting in this paper, so I'd thought I'd share some of that history verbatim here for the curiously like-minded curious people.



Excerpts from "Anthropogenic Space Weather" [Gombosi, T.I., Baker, D.N., Balogh, A. et al. Space Sci Rev (2017)]

8 Space Weather Effects of Anthropogenic VLF Transmissions

8.1 Brief History of VLF Transmitters

By the end of World War 1, the United States military began use of very low frequency radio transmissions (VLF; 3–30 kHz) for long-distance shore to surface ship communications (Gebhard 1979). Since very high power can be radiated from large shore-based antenna complexes, worldwide VLF communication coverage was feasible, and along with LF and HF systems (300–30 MHz) these bands carried the major portion of naval communications traffic before later higher frequency systems came online. Early experiments also showed that VLF could penetrate seawater to a limited depth, a fact realized by the British Royal Navy during World War I (Wait 1977). Given this realization, when the modern Polaris nuclear submarine era began in the 1950s, the US Naval Research Laboratory conducted a series of thorough radio propagation programs at VLF frequencies to refine underwater communications practices (Gebhard 1979). Subsequent upgrades in transmission facilities led to the current operational US Navy VLF communications network, and other countries followed suit at various times. For example, Soviet naval communication systems were likely brought online in the late 1920s and 1930s during the interwar expansion period, and high power VLF transmitters were later established in the late 1940s and 1950s for submarine communications and time signals. These included Goliath, a rebuilt 1000 kW station first online in 1952 which partly used materials from a captured German 1940s era megawatt class VLF station operating at 16.55 kHz (Klawitter et al. 2000).

Table 2 of Clilverd et al. (2009) lists a variety of active modern VLF transmitter stations at distributed locations with power levels ranging from 25 to 1000 kW. These transmissions typically have narrow bandwidths (<50 Hz) and employ minimum shift keying (Koons et al. 1981). Along with these communications signals, a separate VLF navigation network (named Omega in the US and Alpha in the USSR) uses transmissions in the 10 kW range or higher (Inan et al. 1984, e.g. Table 1 of) with longer key-down modulation envelopes of up to 1 second duration.

8.2 VLF Transmitters as Probing Signals

Beginning in the first half of the 20th century, a vigorous research field emerged to study the properties of VLF natural emissions such as whistlers, with attention paid as well to information these emissions could yield on ionospheric and magnetospheric dynamics. Due to the high power and worldwide propagation of VLF transmissions, the geophysical research field was well poised to use these signals as convenient fixed frequency transmissions for monitoring of VLF propagation dynamics into the ionosphere and beyond into the magnetosphere (e.g. Chap. 2 of Helliwell 1965; Carpenter 1966). This was especially true since VLF transmissions had controllable characteristics as opposed to unpredictable characteristics of natural lightning, another ubiquitous VLF source. Beginning in the 1960s and continuing toT.I. Gombosi et al. the present, a vast amount of work was undertaken by the Stanford radio wave group and others (e.g. Yu. Alpert in the former USSR) on VLF wave properties, including transmitter reception using both ground-based and orbiting satellite receivers. These latter experiments occurred both with high power communications and/or navigation signals and with lower power (∼100 W), controllable, research grade transmitter signals.

The transmitter at Siple Station in Antarctica (Helliwell 1988) is worthy of particular mention, as the installation lasted over a decade (1973–1988) and is arguably the largest and widest ranging active and anthropogenic origin VLF experiment series. Two different VLF transmitter setups were employed at Siple covering 1 to ∼6 kHz frequency, with reception occurring both in-situ on satellites and on the ground in the conjugate northern hemisphere within the province of Quebec. Of particular note, the second Siple “Jupiter” transmitter, placed in service in 1979, had the unique property of having flexible high power modulation on two independent frequencies. This allowed targeted investigations of VLF propagation, stimulated emissions, and energetic particle precipitation with a large experimental program employing a vast number of different signal characteristics not available from Navy transmitter operations. These included varying transmission lengths, different modulation
patterns (e.g. AM, SSB), polarization diversity, and unique beat frequency experiments employing two closely tuned VLF transmissions. Furthermore, the ability to repeat these experiments at will, dependent on ambient conditions, allowed assembly of statistics on propagation and triggered effects. These led to significant insights that were not possible for studies that relied on stimulation from natural waves (e.g. chorus) that are inherently quite variable.

Several excellent summaries of the literature on VLF transmission related subjects are available with extensive references, including the landmark work of Helliwell (1965) as well as the recent Stanford VLF group history by Carpenter (2015). As it is another effect of anthropogenic cause, we mention briefly here that a number of studies in the 1960s also examined impulsive large amplitude VLF wave events in the ionosphere and magnetosphere caused by above-ground nuclear explosions (e.g. Zmuda et al. 1963; Helliwell 1965).

Observations of VLF transmissions included as a subset those VLF signals that propagated through the Earth-ionosphere waveguide, sometimes continuing into the magnetosphere and beyond to the conjugate hemisphere along ducted paths (Helliwell and Gehrels 1958; Smith 1961). Ground based VLF observations (Helliwell 1965) and in-situ satellite observations of trans-ionospheric and magnetospheric propagating VLF transmissions were extensively used as diagnostics. For example, VLF signals of human origin were observed and characterized in the topside ionosphere and magnetosphere for a variety of scientific and technical investigations with LOFTI-1 (Leiphart et al. 1962), OGO-2 and OGO-4 (Heyborne et al. 1969; Scarabucci 1969), ISIS 1, ISIS 2, and ISEE 1 (Bell et al. 1983), Explorer VI and Imp 6 (Inan et al. 1977), DE-1 (Inan and Helliwell 1982; Inan et al. 1984; Sonwalkar and Inan 1986; Rastani et al. 1985), DEMETER (Molchanov et al. 2006; Sauvaud et al. 2008), IMAGE (Green et al. 2005), and COSMOS 1809 (Sonwalkar et al. 1994). VLF low Earth orbital reception of ground transmissions have been used also to produce worldwide VLF maps in order to gauge the strength of transionospheric signals (Parrot 1990).


9 High Frequency Radiowave Heating

Modification of the ionosphere using high power radio waves has been an important tool for understanding the complex physical processes associated with high-power wave interactions with plasmas. There are a number of ionospheric heating facilities around the world today that operate in the frequency range ∼2–12 MHz. The most prominent is the High Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska. HAARP is the most powerful radio wave heater in the world; it consists of 180 cross dipole antennas with a total radiated power of up to 3.6 MW and a maximum effective radiated power (EFR) of ∼4 GW. The other major heating facilities are EISCAT, SURA, and Arecibo. EISCAT isT.I. Gombosi et al. near Tromso, Norway and has an EFR of ∼1 GW. SURA is near Nizhniy Novgorod, Russia and is capable of transmitting ∼190 MW ERP. A new heater has recently been completed at Arecibo, Puerto Rico with ∼100 MW ERP. There was a heating facility at Arecibo that was operational in the 1980s and 1990s but it was destroyed by a hurricane in 1999. The science investigations carried out at heating facilities span a broad range of plasma physics topics involving ionospheric heating, nonlinear wave generation, ducted wave propagation, and ELF/VLF wave generation to name a few.

During experiments using the original Arecibo heating facility, Bernhardt et al. (1988) observed a dynamic interaction between the heater wave and the heated plasma in the 630 nm airglow: the location of HF heating region changed as a function of time. The heated region drifted eastward or westward, depending on the direction of the zonal neutral wind, but eventually “snapped back” to the original heating location. This was independently validated using the Arecibo incoherent scatter radar for plasma drift measurements (Bernhardt et al. 1989). They suggested that when the density depletion was significantly transported in longitude, the density gradients would no longer refract the heater ray and the ray would snap back, thereby resulting in a snapback of the heating location as well. However, a recent simulation study using a self-consistent first principles ionosphere model found that the heater ray did not snap back but rather the heating location snapped back because of the evolution of the heated density cavity (Zawdie et al. 2015).

The subject of ELF wave generation is relevant to communications with submarines because these waves penetrate sea water. It has been suggested that these waves can be produced by modulating the ionospheric current system via radio wave heating (Papadopoulos and Chang 1989). Experiments carried out at HAARP (Moore et al. 2007) demonstrated this by sinusoidal modulation of the auroral electrojet under nighttime conditions. ELF waves were detected in the Earth’s ionosphere waveguide over 4000 km away from the HAARP facility.

VLF whistler wave generation and propagation have also been studied with the HAARP facility. This is important because whistler waves can interact with high-energy radiation belt electrons. Specifically, they can pitch-angle scatter energetic electrons into the loss cone and precipitate them into the ionosphere (Inan et al. 2003). One interesting finding is that the whistler waves generated in the ionosphere by the heater can be amplified by specifying the frequency-time format of the heater, as opposed to using a constant frequency (Streltsovet al. 2010).

New observations were made at HAARP when it began operating at its maximum radiated power 3.6 MW. Specifically, impact ionization of the neutral atmosphere by heater-generated suprathermal electrons can generate artificial aurora observable to the naked eye (Pedersen and Gerken 2005) and a long-lasting, secondary ionization layer below the F peak (Pedersen et al. 2009). The artificial aurora is reported to have a “bulls-eye” pattern which is a refraction effect and is consistent with ionization inside the heater beam. This phenomenon was never observed at other heating facilities with lower power (e.g., EISCAT, SURA).

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