Science Programs at Antarctic Stations
The projects that operate from instrumentation at South Pole Station, and at the Arrival Heights Laboratory at McMurdo Station, examine natural phenomena occuring in the earth's atmosphere and magnetosphere. The broad focus of these science programs is toward improved understanding of the mechanisms that couple solar processes into the terrestrial environment; these include investigations of phenomena associated with short-term environmental effects (auroras, induced electrical currents, radiowave communications interference), as well as those associated with longer-term effects (changes in the ozone layer, atmospheric composition studies, stratospheric winds, weather, and even climate). Instruments for these tasks include optical and radio devices for remote sensing, as well as sensors that monitor changes in the electric and magnetic fields at the station; the instruments that measure local fields (including VLF receivers) are sensitive to perturbations that propagate from remote generation regions.
For many investigations, South Pole represents a unique site for studying environmental phenomena. In the case of atmospheric studies this must be obvious, because of the location remote from contaminating sources; in addition to monitoring current atmospheric parameters, ice-coring experiments provide information about past atmospheres. However, the station is also uniquely situated for investigations associated with magnetospheric phenomena. An important region of the earth's magnetosphere separates processes associated with polar cap environments, from those normally related with auroral activity. Polar cap effects are associated with magnetic field lines that connect almost directly with the solar magnetic field on the dayside (cusp/cleft), and with the deep tail of the magnetosphere on the nightside. The boundary between the polar cap and those regions of the magnetosphere associated with closed magnetic field lines is coincident with the auroral oval, so-named because of the frequent occurrence of auroral arcs inside a narrow band around the magnetic pole. The 'polar cap' designation is normally applied to phenomena that occur poleward of the oval, so that the dayside cusp/cleft is regarded as a separate region with properties distinct from either the polar cap or auroral/sub-auroral regions.
McMurdo Station, at invariant magnetic latitude of about 80 degrees, lies inside the polar cap at all local times. Since the magnetic pole is displaced from the geographic pole, South Pole Station is located at an invariant latitude (74.25 degrees) that places the station equatorward of the nominal undisturbed auroral oval on the dayside, and poleward of the oval (in the polar cap) on the nightside; consequently at dawn and dusk the station passes underneath the oval. Under disturbed magnetic conditions the characteristics of the oval, and indeed the entire magnetosphere, experience dramatic changes, so that this simplistic picture does not hold during the most interesting times for scientific investigation.
The variety of observational details during disturbed magnetospheric conditions ranges across not only different instrument regimes, but also encompasses time scales ranging from seconds to hours, and spatial scales from meters to nearly global. Consequently the investigation of the environment during disturbed times involves coordinated observations using instruments at widely spaced fixed observatories on the earth, as well as satellite instruments in the solar wind and inside the magnetosphere. At the Antarctic observatories, the signals from many of the instruments are recorded together on a common data logger, and the data are shared among the Principal Investigators (PI's) providing the data, as well as with colleagues operating elsewhere. Since the beginning of the IGY (International Geophysical Year - 1957-1958), the collaboration in Antarctic research has been on an international scale.
The brief discussions presented below are intended to describe the basic objectives of the science experiments involved in studies of solar/magnetospheric/ionospheric coupling processes. More detailed discussions should be sought from the PI's responsible for the science programs.
Digital Data Acquisition SystemThe scientific instruments operated at the Antarctic observatories provide analog signals that are digitized and recorded by the Digital Data Acquisition Systems (DAS). These were designed and built at the University of Maryland, and have been in continuous use at the stations since December, 1981. The original DASes recorded to 9-track magnetic tape, and operated nearly flawlessly until 1994 at South Pole, and 1995 at McMurdo. These were replaced with PC-based systems, recording to magneto-optical disk. The data acquisition systems are operated as station facilities, and record data from many experiments, including,
- Riometers, Siena College, University of Maryland
- Photometers, Siena College, University of Maryland
- Searchcoil Magnetometer, Augsburg College, University of New Hampshire
- Fluxgate Magnetometer, NJIT
- Electric Field, Stanford University
- Very-Low-Frequency (VLF) Radiowaves, Stanford University
- Cosmic Rays, Bartol Research Foundation
- Wind Speed, Stanford University
Riometer ProgramRiometers are usually operated with broad-beam antennas, with beamwidth on the order of 60 degrees. These integrate absorption activity over a large portion of the ionosphere, so small scale details of the actual physical distribution of electron density enhancements are lost. Narrow beam antennas, with beamwidths of 10 to 20 degrees, are sensitive to smaller scale features of ionization, but are generally limited in the ionospheric area which can be examined at one time. The data from several riometers operated at different frequencies, but examining the same sky with broadbeam antennas, show effects which have been interpreted as being due to small scale spatial structure in the electron precipitation region which does not completely fill the antenna beam. Results analyzed from narrow beam and multiple frequency riometer data, in conjunction with optical data from all-sky cameras and photometers, indicate that the spatial scale of ionospheric electron density perturbations can be very small, on the order of a few kilometers.
(Photo courtesy of T.J. Rosenberg)
A recent trend in riometry is toward the use of antennas providing several narrow beams, which examine different parts of the sky. Some of these systems have been constructed to provide several fixed beams, and others scan the ionosphere in a linear path with one or more steerable beams. The IRIS system was designed to operate as a fast-scan multiple-beam instrument to examine the entire ionospheric sky, out to about 45 degrees from the zenith. The IRIS antenna is a sophisticated phased-array which produces 49 narrow beams, on the order of 12 degrees beamwidth, all of which are sampled once a second. This system is capable of examining ionospheric electron density perturbations in fine time scale, as well as small spatial scale.
At South Pole Station, the University of Maryland operates broadbeam riometers at 20.5, 30, 38.2, and 51.4 MHz. The signals from these instruments are digitized and recorded at 1-Hz resolution by the station data acquisition system. The broadbeam antennas are located in a group about 1-km from the Cusp Lab. For the 20.5, 30 and 51.4 MHz systems, the broadbeam antenna comprises two adjacent parallel horizontal dipoles; the antenna for the 38.2 MHz system is a single element identical to those used with the IRIS phased array, including a chicken-wire ground plane located a quarter-wavelength below the horizontal dipole element plane. The antenna beam projects a nearly circular -3 dB locus onto the ionosphere, where the absorption of cosmic radio noise occurs; the -3 dB beamwidth is approximately 60-degrees, so the diameter of the typical ionospheric absorption region at 90 km altitude is about 100 km.
MagnetometersBoth the sun and the earth have magnetic fields, generated by electrical currents flowing inside the respective bodies. In addition to electromagnetic radiation (e.g., light, radiowaves, x-rays), the sun emits an electrically-charged gas (solar wind plasma) which carries the 'frozen-in' solar magnetic field along with it as it streams away from the sun. The plasma is composed almost entirely of electrons and protons. Perturbations in the plasma density/magnetic field propagate as Alfven waves. At 1-AU (astronomical unit - the mean distance of the earth from the sun) the solar wind plasma density is about ten particles per cubic centimeter, the solar wind speed is about 400 km/s, and the magnitude of the interplanetary magnetic field (IMF) is about 10 nT.
(Graphic courtesy of NASA)
If there were no solar wind, the earth's magnetic field would be dipolar, but the pressure of the solar wind causes the simultaneous distortion of the earth's magnetic field, and deflection of the solar wind around the earth. Not surprisingly, the steady-state configuration resembles the 'bow shock' around the prow of a moving boat, so that designation has been used in the identification of the magnetospheric analog. The complicated steady-state configuration contains several distinct regions, as shown in the figure. This picture represents merely an 'average' view of the conditions that prevail in globally-defined regions during quiet conditions. The steady-state environment represents an equilibrium balance between the energy input from the solar wind, and processes which transfer energy within the magnetosphere, including energy deposition into the atmosphere. Because of the tilt of the earth's magnetic dipole axis (about 11.5 degrees), the magnetic pole performs a diurnal rotation around the geographic pole. This causes distortions in the steady-state magnetosphere, and these effects are accentuated by the seasonal tilt of the rotational axis with respect to the solar ecliptic plane (about 23 degrees). The picture during disturbed times is extremely dynamic, and the physical processes involved in the transfer of energy under these conditions are not well understood.
The investigation of solar/magnetic storm-time phenomena is the single principal purpose of much of the upper atmosphere research at South Pole Station. Magnetic substorms are responsible for the creation of various dynamic processes in the magnetosphere, including flux transfer events associated with magnetic merging regions in the boundary layer between the solar wind and the inner magnetosphere; precipitation of solar wind/boundary layer plasma directly into the atmosphere through the polar cusp/cleft; plasmoids in the magnetospheric tail which are associated with the energization and precipitation of energetic electrons into the nightside atmosphere, creating westward travelling surges, among other effects; pulsating auroras in the morningside auroral oval, caused by large-scale precipitation modulations with periods on the order of several minutes; expansion of the auroral oval to lower magnetic latitudes; mid-afternoon poleward surges of auroral structures into the polar cap from the auroral oval, etc. All of these phenomena are accompanied by perturbations in the terrestrial magnetic field; the surface magnetic field strength at auroral latitudes is about 60,000 nT, and perturbations of about 1000 nT are common during disturbed times, sometimes lasting for several days, but usually with time scales of only minutes or hours.
Searchcoil MagnetometerVariations in the magnetic field at South Pole Station are studied using two different magnetometers. Rapid fluctuations in the geomagnetic field (with periods a fraction of a second) are measured with a searchcoil magnetometer, or micropulsation detector, consisting of a mu-metal core surrounded by many thousands of turns of wire. The detector for a single axis is about 6-feet long, and about eight inches in diameter; two perpendicular axes measure the horizontal components of dB/dt at South Pole and McMurdo Stations. The project is S-102, and the PI is Dr. R.L. Arnoldy, University of New Hampshire.
Slow variations of the terrestrial magnetic field
are measured with the fluxgate magnetometer (DC to the Nyquist
frequency of 0.5 Hz, at 1-Hz sampling rate); the sensor itself comprises
three small mutually perpendicular coils of wire.
The fluxgate magnetometer can
measure fluctuations as small as 0.03 nT, although magnetic storm effects
often are associated with perturbations of 1000 nT or more.
the fluxgate magnetometer can be found in the University of Maryland's
Data Acquisition System Manual. The science project is S-101, and
the PI is Dr. L.J. Lanzerotti, Lucent Technologies/Bell Laboratories.
Electric FieldThe solar wind plasma that becomes trapped inside the magnetosphere is confined by the structure of the magnetic field to various regions, such as the Van Allen radiation belts, and even extending into the tail of the magnetosphere. In the absence of solar flare activity, the magnetospheric plasma achieves a steady-state balance between source and loss mechanisms. Some of the plasma is lost through the tail because the magnetic field lines are not believed to close on the nightside high-latitude magnetosphere; other processes include electromagnetic wave-particle interaction mechanisms which cause the precipitation of plasma into the atmosphere. During magnetic substorms, horizontal ionospheric electric currents flow in the E-region ionosphere; these currents are possible because in this altitude range (about 85-140 km) the electrons remain constrained against motion perpendicular to the magnetic field, while the protons are 'un-magnetized' because collisions with the neutral atmosphere occur with a frequency greater than the cyclotron gyrofrequency. Consequently, atmospheric tidal motions as well as electric fields lead to currents. The measurement of slowly-varying (or DC) electric fields is very difficult, and the experimental practice sometimes resembles artistry more than science. The measurement of the surface electric field includes wind-related static electricity, which is particularly bothersome in the cold, dry Antarctic environment.
Optical InstrumentsA variety of optical instruments operate at South Pole Station during the austral winter season. Instruments which examine natural phenomena in the D- and E-region ionosphere include photometers and all-sky cameras operated by the University of Maryland, Lockheed/Martin's Advanced Technology Center (formerly Palo Alto Research Laboratory), and Utah State University. The optical instruments are located on the top floor of the Skylab Building at South Pole Station, and operate through domes projecting through the roof of the building. Other optical experiments include interferometers and lidars, which examine phenomena in the neutral atmosphere as well as the ionosphere. The Science Program Plan published by USAP describes these projects briefly. Since the riometer program is directed toward ionospheric phenomena associated with the precipitation of auroral electrons into the atmosphere, the principal optical instruments that provide complementary measurements are the photometers and all-sky cameras.
At both South Pole and McMurdo Stations, photometers measure ionospheric optical emissions at 630 nm and 427.8 nm. The field of view of the photometers (60 degrees) was designed to match that of the broadbeam riometers, in order to make useful comparisons between data from the different instruments.