Whistler-mode Wave Studies at Palmer Station, Antarctica


Useful Links

10-second spectrograms from Palmer, updated every minute.

24-hour summary spectrograms, updated daily.

Online data availability from recent and past years.

Download raw and narrowband data from Palmer station, and other Stanford ELF/VLF stations.

Past trip reports from students who have performed annual maintenance at Palmer station.

Palmer Station Receiver and Data

Palmer Station, shown in Figure 1, is located on Anvers Island, near the tip of the Antarctic peninsula, at 64.77$ ^\circ$S, 64.05$ ^\circ$W, with IGRF geomagnetic parameters of L = 2.4, 50$ ^\circ$S geomagnetic latitude. The Palmer VLF receiver records broadband VLF data at 100 kilosamples per second using two cross-loop magnetic field antennas, with 96 dB of dynamic range sensitivity, and is capable of measuring signals on the order of a femtotesla ($ 10^{-15}$ tesla). Due to its remoteness from anthropogenic electromagnetic noise sources, Palmer Station is one of the most electromagnetically quiet ELF/VLF receiving sites in the world.

Figure 1: Palmer Station is one of the most electromagnetically quiet ELF/VLF receiving sites in the world. (Left) Palmer antenna located atop the Anvers Island glacier on the Antarctic peninsula. (Right) Palmer and associated stations Commandante Ferraz (Brazil), Vernadsky (Ukraine), Rothera (UK) can be used to locate the ionospheric exits points of magnetospheric emissions.
Image palmer_map_and_ant

A large amount of data is recorded at Palmer annually. Palmer records data 24 hours per day, 7 days per week, at two bytes per sample, 100 kilosamples per second, on two orthogonal channels. The total annual data volume is 12.6 terabytes (TB). Because of limited telemetry from Palmer, this data cannot be transfered back to Stanford via the Internet. Instead, external USB hard drives of size 1 TB or greater are shipped back and forth via Stanford and Palmer. The data is then loaded onto the Stanford VLF data server, where it is made available to the worldwide community, along with data from many other remote sites, at http://vlf.stanford.edu.

The Palmer receiver is maintained annually by students from the Stanford VLF group (Figure 2). Students are responsible for calibrating the receiver, replacing old equipment, and ensuring that the antenna is remains accessible and operational during changing antarctic conditions.

Figure 2: The Stanford receiver at Palmer Station is maintained by students in the VLF group. (Left) Graduate student Daniel Golden and Palmer Science Technician Neal Scheibe search for a potential new site for the Palmer antenna. (Right) Other graduate students at Deception Island, en route to Palmer from Punta Arenas, Chile.
Image dan_and_pinguinos_2

Science at Palmer Station

One of the major outstanding scientific problems in the field of heliophysics is understanding and quantifying the dominant source and loss processes that contribute to the variability of relativistic electron populations in the Earth's radiation belts. Whistler mode waves play a major role in controlling the dynamic evolution of these particle populations. These waves are regularly able to penetrate the ionosphere to be detected at ground-based stations like Palmer.

Three distinct types of magnetospheric emissions are observed at Palmer: chorus, hiss, and whistlers. Examples of these emissions, received at Palmer, are shown in Figure 3. Chorus is generated in the Earth's magnetosphere, beyond the plasmasphere, and can influence both the acceleration [Baker et al., 2004; Horne et al., 2005; Spasojevic and Inan, 2005] and loss [O'Brien et al., 2004; Inan et al., 1992; Lorentzen et al., 2001] of energetic particles. Hiss is generated within the plasmasphere, and is one of the main scattering agents that maintains the slot region between the inner and outer radiation belts [Abel and Thorne, 1998; Lyons and Thorne, 1973]. Whistlers are generated from terrestrial lightning near the surface of the Earth, and are also important contributors to energetic electron scattering in the slot region of the radiation belts [Abel and Thorne, 1998; Bortnik et al., 2002].

Figure 3: Whistler-mode emissions observed at Palmer play critical roles in controlling the structure of the radiation belts. Chorus (top panel) is an important source and loss mechanism for energetic particles, while hiss (middle panel) and whistlers (bottom panel) are important loss mechanisms.
Image emissions

Chorus and whistlers have separately been proposed to be important sources of hiss. The original suggestions that lightning was a source of hiss, made some time ago by Sonwalkar and Inan [1989] and Draganov et al. [1992], have recently sparked competing ideas [Green et al., 2005; Meredith et al., 2006] about the frequency range and significance of lightning control over plasmaspheric hiss. Palmer's conjugate point near the East coast of the United States, shown in the left panel of Figure 4, allows Palmer the unique opportunity to observe hiss that is generated from North American lightning, one of the major sources of lightning worldwide. By searching for correlations between (1) lightning in the vicinity of Palmer's conjugate point and (2) hiss observed at Palmer, one can determine which populations of lightning contribute to hiss, and under what geomagnetic circumstances. The possibility of chorus as a source of hiss [Bortnik et al., 2008; Santolík et al., 2006; Parrot et al., 2004] may also be tested using Palmer data. The preponderance of simultaneous observations of chorus and hiss at Palmer [Golden et al., 2009] allows examination of the relationship between these two emissions.

Figure 4: Palmer data can be used to study the effects of lightning-generated whistler waves. (Left) Palmer's magnetic conjugate point is in the vicinity of the peak of North American lightning activity, and Palmer regularly observes whistlers from this source. (Right) Raytracing of 1 kHz chorus ray paths beginning at the equatorial chorus source region and terminating within the ionospheric transmission cone near Palmer (green triangle).
Image lightning_rays

Ground-based measurements of magnetospheric emissions have several significant advantages over space-based measurements, including cost, data throughput, and the controllability of the measurement location. In particular, the dearth of space-based wave measurements in the inner magnetosphere during the previous peak of the solar cycle underscores the importance of long-term ground-based observations. However, the main disadvantage of ground-based measurements is that they are limited to observing the subset of magnetospheric emissions that penetrate through the ionosphere to the ground [e.g. Sonwalkar, 1995, pp 424-425]. Thus in order to make inferences about the implications of Palmer observations, it is critical to understand which wave populations are observable at Palmer. This can be accomplished by triangulating the ionospheric penetration points of the waves using measurements from other stations on the Antarctic peninsula, such as Rothera (UK), Ferraz (Brazil) and Vernadsky (Ukraine) (Figure 1). Once the ionospheric penetration points are known, one can then perform reverse raytracing on rays that begin at that point, such as in the right panel of Figure 4. This procedure allows us to interpret received waves in the context of their generation region, and the regions in which they interact with energetic particles.

Acknowledgements

ELF/VLF research at Palmer Station is supported by the National Science Foundation Office of Polar Programs.

Bibliography

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