Imaging Riometers

The University of Maryland has provided 38.2 MHz imaging riometers for each PENGUIn AGO. A riometer (Relative Ionospheric Opacity meter) is an instrument that measures the opacity of the earth's atmosphere to cosmic radio noise, which is used as a constant background against which small changes in the electron density of the ionosphere can be examined. The ionospheric plasma attenuates high-frequency (HF) radiowave energy that passes through it, so the riometer operates at frequencies particularly susceptible to this attenuation, in the range from 20 to 50 MHz.

The electron density changes that the riometer is used to examine are primarily due to the precipitation of energetic electrons from the magnetosphere into the atmosphere. The riometer is most sensitive to incident electrons that deposit energy at 55 km altitude; an electron needs energy on the order of one MeV to reach this low in the atmosphere. However, the sensitivity of the riometer enables it to measure the effects of electrons below 10 keV energy, corresponding to altitudes above 110 km. The auroral precipitation events of most interest to riometry generally have electron energies in the few tens of keV, so the most significant effects occur near 90 km altitude.

Although zenith-viewing riometers were the first to be used, the use of antenna arrays producing multiple narrow beams is necessary in order to examine the spatial scale of regions of energetic electron precipitation, which are coincident with cosmic radio noise absorption activity, and to resolve time development from spatial motion. The imaging riometer provides good spatial and temporal resolution for examining auroral precipitation events. It complements the optical all-sky-camera, and operates year-round since it is not affected by sunlight and cloud cover. The lower spatial resolution permits it to operate with higher time resolution than the all-sky-camera without taxing the data recording capability of the AGO.

The PENGUIn imaging riometer system employs two riometers for redundancy and to double the rate of data recording. The 12-bit analog-to-digital converter produces 24 bytes of data for one complete scan (in 12 s) of the 16 beams for each riometer, resulting in a complete riometer image of the radio sky every 6 s.

Fluxgate Magnetometer

Bell Laboratories - Lucent Technologies has provided fluxgate magnetometers for each PENGUIn AGO. Each instrument measures and records the three DC vector components of the geomagnetic field at 1-s intervals. Each component of these systems has a noise level of typically 0.01 nT rms between 0 and 1 Hz. Similar magnetometers are deployed at South Pole Station and McMurdo, Antarctica.

Magnetic field values and variations are considered some of the most basic of diagnostics for ground-based observations of ionospheric and magnetospheric processes. Although they are now complemented in the PENGUIn AGOs and elsewhere by other types of instruments, they remain a basic and necessary component of most studies of these phenomena.

Search Coil Magnetometer

Tohoku University has provided search coil magnetometers for each PENGUIn AGO. Variations in magnetic fields are measured along three orthogonal components (N-S, E-W, and vertical) by individual coils which are buried at a depth of 1 m from the snow surface with a distance of at least 5 m between sensors. Each search coil has a linear frequency response from ~0.001 Hz to 2 Hz, and is equipped with a low pass filter with a cutoff frequency of 2 Hz. The output signals of each coil are sampled simultaneously at 0.5-s intervals and are digitized using a 12-bit A/D converter, providing a dynamic range of +/- 1.6 pT to +/- 3.2 nT at 1 Hz and +/- 160 pT to +/- 320 nT at 0.01 Hz.

All Sky Auroral Imaging

Lockheed Palo Alto Research Laboratories has developed special purpose low light level auroral all sky imagers for use in the AGOs. Each imager, which incorporates a two-channel all-sky intensified CCD camera with a single all-sky optical channel and a single detector, is capable of acquiring images in two different wavelengths (630.0 +/- 3.0 nm and 427.8 +/- 5.0 nm) simultaneously, and is optically identical to the imager installed at South Pole Station. Imager data, recorded at a rate of one image every 2 minutes, have 10 km geographic resolution over most of the field of view, ranging to 30 km at the edges. Imager sensitivity is 20 Rayleighs in an 8-s exposure; with bracketing, the dynamic range varies from 20 Rayleighs to 10000 Rayleighs. Images are digitized using an 8-bit quasi-logarithmic scheme and compressed to conserve AGO data storage space.

Monochromatic all sky imagers are a powerful tool for observing and monitoring the optical aurora. The PENGUIn all sky imagers are being used to a) Determine the polar cap auroral morphology under varied conditions of IMF, solar wind pressure and internal magnetospheric conditions; b) Compare auroral dynamics to ionospheric plasma convection and magnetic variations; and c) Intercompare the optical aurora observed from the ground with satellite based observations.

Individual all-sky images can be produced for analysis, but in addition may be combined from contiguous AGO sites (and South Pole) or may be processed to generate clock-dial keograms and/or latitude-UT keograms.

ELF/VLF Measurements

Stanford University has developed an ELF/VLF receiver for use on the PENGUIn AGOs which consists of one digital broadband snapshot system (BBS), five narrowband channels referred to as 'hiss' filters (30 Hz-1 kHz, 1-2 kHz E-W, 1-2 kHz N-S, 2-4 kHz, and 30-40 kHz), and two additional narrowband channels tuned to the frequencies of powerful VLF transmitters [Shafer et al., 1994].

Electromagnetic and electrostatic waves play an important role in the transport and accelation of magnetospheric and ionospheric plasma, and particle precipitation driven by these waves constitutes a significant form of energy deposition into the Earth's ionosphere. Because many of these waves follow magnetic field lines down to the ionosphere, measurements of ELF/VLF waves (300 Hz - 30 kHz) at multiple sites provide a powerful means of remotely sensing magnetospheric processes. In the context of the PENGUIn program, ELF/VLF measurements complement other observations primarily targeted towards documenting the spatial and temporal distributions of precipitation activity via optical and riometer measurements, and the measurement of magnetic activity.

Waves in this frequency range play an important role in the acceleration, transport, and loss of ionospheric and magnetospheric plasmas and their measurements provide a means of remotely sensing physical processes which occur in near Earth space. Low resolution data on overall ELF/VLF activity usually consist of the recordings of the detected signal amplitude in selected bandwidth channels, sampled relatively slowly (e.g., ~ 10 Hz). However, since a diverse range of different types of waves are commonly observed, including discrete emissions such as chorus, steady and structureless emissions such as hiss, and other signals originating in lightning discharges, wideband measuremnts of the signal waveform are necessary to identify the nature of the waves. At manned stations, such recordings are most compactly made in analog form (more recently also using video or digital-auido systems) on magnetic tape, which typically accomodate up to 30 kHz bandwidth in real time.

A complicating aspect of ELF/VLF measurements on the ground is the fact that waves of magnetospheric origin which penetrate the lower ionosphere may propagate to long distances in the earth-ionosphere waveguide and be detectable at distances of many hundreds of kilometers from their points of entry. Thus, simultaneous multiple-site mesurements are necessary to determine the spatial extent of ionospheric and magnetospheric regions within which the wave activity resides.

Since the early 1980s, Stanford University has maintained and operated an ELF/VLF system at South Pole Station, Antarctica. In the context of the PENGUIn program, ELF/VLF observations at South Pole are crucially important for two reasons: (i) SP is part of the meridional and latitudinal chains constituted by the PENGUIn sites, and (ii) continuous and synoptic wideband (30 Hz-30 kHz) ELF/VLF measurements at South Pole, not possible at AGO sites due to data limitations, provide key information on the diverse range of wave types the intensity of which are registered in the narrowband channels.

The seven channels and the BBS in the PENGUIn ELF/VLF receiver system share a common power system and line receiver, and the narrowband channels each have separate detector/integrators in a common module. A separate dual-channel low-noise preamplifier unit is deployed outside near the sensors, which consist of two 1.7 x 1.7 m^2 square loop antennas deployed in orthogonal directions, magnetic N-S and E-W. The sensitivity of the ELF/VLF receiver system is 1. 89 x 10-4 microVolts m-1 Hz-1/2, and is limited by the relatively small loop antenna used at each AGO site. Two of the hiss filters cover the same frequency range (1-2 kHz) with two different antennas (N-S and E-W), to allow the extraction of direction of arrival information during post-processing. All signals other than the 1-2 kHz channel are sampled and recorded continuously at 1 Hz; the two polarizations of the 1-2 kHz signal are each recorded at half this rate.

The PENGUIn ELF/VLF receiver system incorporates a single-board computer, with operating program stored on EPROM so it can be replaced annually as requirements (frequency range, snapshot times, N-S or E-W channels or goniometer mode) change. Wideband signals from the line receiver are sampled at 25 kHz with 12-bit resolution for six seconds at the beginning of each hour; the processor can sample/record the North-South or the East-West channel, or simulate a goniometer.

LF/MF/HF Radio Receiver

Dartmouth has provided LF/MF/HF radio receivers for deployment on the PENGUIn AGOs. These receivers extend the frequency range of the AGO instrumentation to cover nearly all of the remainder of the electromagnetic emissions that are known to be produced by ionospheric and magnetic processes and can penetrate to below the ionosphere. Categories of events already observed using these receivers include auroral roars, narrowband emissions near 2 and 3 times the ionospheric electron gyrofrequency, MF bursts, and many LF auroral hiss events. Auroral hiss is known to be associated with auroral activity in the southern and northern hemispheres, and all three types of LF/MF/HF emissions have been observed together during substorm onsets at auroral latitudes in the northern hemisphere.

The LF/MF/HF swept frequency receivers analyze signals from magnetic loop antennas, providing .01 - 5.0 MHz spectra with 50 kHz frequency resolution each 10 seconds. Essentially identical instrumentation is used at South Pole and at AGO sites: a vertical magnetic loop antenna 2.5 m^2 in area and a superheterodyne receiver with 10 kHz bandwidth and 70 dB dynamic range.