B.S.Tate and E.A.Essex
Cooperative Research Centre for Satellite Systems
Department of Physics
La Trobe University
Bundoora Victoria 3083
Ph: 03 9479 2645
During the period 1993-1996, transmissions from the US Navy Navigational Satellite System (NNSS) polar orbiting satellites, received at Casey base (66.28° S, 110.5° E geographic) in Antarctica, were used to investigate the amplitude scintillations at 150 MHz and the Total Electron Content (TEC) obtained from the differential phase technique. Investigations of ionospheric polar patches, and polar holes were carried out. The phase and amplitude data were recorded using a JMR-1 satellite receiver system, with all measurements logged on a PC. In order to locate the patches and polar holes, the TEC data were mapped in magnetic local time (MLT) vs. invariant latitude (L) magnetic coordinates. Patches were observed in the polar cap region, at various locations and times during the April-August period of the years 1994 and 1995. The polar hole, a feature of the 0-3 MLT region at -75° to -80° L, was also observed at various times through the winter months. Average monthly TEC values were calculated, and compared to those obtained from the Parametrized Ionospheric Model (PIM) version 1.4, Daniell et al. (1995).
The polar cap station Casey is the ideal location to investigate the occurrence of ionospheric holes, patches and associated radio scintillations in the Southern high latitude ionosphere. TEC was derived using measurements from the constellation of NNSS polar orbiting satellites. The NNSS satellite constellation consists of four active satellites transmitting on 150 and 400 MHz, with several others on standby at any given time. Completing approximately thirteen orbits per day, each satellite is visible two or three times a day above an elevation of 45° at any given fixed site.
TEC (Eq. 1) is defined as the number of electrons in a volume of one square meter in cross-section, extending along a ray path from a satellite to a ground receiver.
where: N(s) is the electron density.
In terms of the differential phase experiment, Eq. 1 can be modified to:
where:A is a constant = 0.02056 × 1015 (SI units), and
F is the beat frequency observed between two frequencies f and mf.
Vertical TEC is obtained using the integration constant D, which is derived from ionosonde obtained foF2 data as shown in equation 3:
where: t is the ionospheric slab thickness, in km
A linear plot of vertical TEC calculated from equation (2) versus vertical TEC calculated from equation (3) intersects the "equation (2)" axis at a point, A × D, from which D can be determined. The value of D can then be inserted into equation (3) to obtain absolute vertical TEC values. A complete derivation of the above equations can be found in, Breed (1992).
The TEC values are plotted using magnetic co-ordinates (MLT vs. L) and geographic co-ordinates (UT vs. geog. latitude).
Variations in amplitude, phase or angle of arrival of radio signals are known as scintillations. These effects result when a radio wave propagating through the ionosphere is diffracted by irregularities that are moving relative to the raypath. Amplitude scintillations are characterised by the scintillation index S4 (defined in Equation 4), which provides a "level of scintillation activity".
where: P is the received power
is the mean received power
casey station (formerly wilkes) was the third permanent anare station to be established on the antarctic mainland. Casey is situated at the geographic location of 66.3° S, 110.5° E, and at the invariant latitude of -80.4°. Its position, poleward of both the ionospheric cusp and auroral oval region, makes it an ideal place to study polar patches and to monitor the polar hole.
Phase and amplitude data from nnss satellite signals were recorded using equipment located at casey. The equipment consisted of a jmr satellite receiver, a 24 mhz amplitude receiver and an IBM-compatible PC. The pc was used to control the jmr receiver, and was also used to record the differential phase and signal strengths to hard disk. High quality signals were obtained from the nnss satellites due to the use of an antenna that consists of two, short helical coils tuned to 150 mhz and 400 mhz respectively. These coils have polar patterns that enable high gain signals at high elevations to be received. The jmr receiver allows amplitude variations to be recorded, together with differential phase. Scintillation measurements were taken using a receiver, attached to the 24 mhz if output of the jmr. Fof2 data, used for tec baseline calibration (eq. 3) Were collected using the dps-4 ionospheric sounder located at casey, provided courtesy of atmospheric and space physics, australian antarctic division. Data were collected for approximately 15 days per month, consisting of 5 days of scintillations and tec at 50 hz resolution and 10 days of tec only at 10 hz resolution being recorded. Monthly samples of both differential phase and scintillations were sent to la trobe university via ftp once a month. The bulk of the data was archived and returned to australia at the end of the year.
Large scale plasma irregularities in the F-region ionosphere are produced near or equatorward of the dayside auroral zone and convect across the polar cap in the antisunward direction, Weber et al. (1984). Plasma enters the polar cap as a tongue of ionisation which provides the source for enhanced polar cap F-region plasmas, or 'patches' which are observed at high polar latitudes propagating away from noon. These patches are localised regions of increased F-region density (up to 5 to 10 times above background level), extending up to 1000 km across. Patches have been found to occur most often in April and August, and less likely to occur in January and July, Rodger and Graham (1996). Patches are most likely to occur when the Bz component of the interplanetary magnetic field (IMF) is southward, or when planetary magnetic index (Kp) > 4. It is possible to track these patches using the method of differential phase determination of columnar TEC, and hence determine some of the dynamics of the polar cap ionosphere Beggs et al. (1994). An example of a TEC plot showing a polar patch on day 241, 1995, is seen in Figure 1. Also shown in Figure 1 are the associated amplitude variations and S4 index recorded during the pass. For the patch observed on day 241 (shown in Figure 1), a "burst" of strong scintillation is observed during the patch event. S4 levels rise as high as 0.7 during this period.
Using the above conditions as a guide, patches were located on several occasions during the period of April 1994 through to August 1995. The patches from 1995 are plotted in Figure 2 in MLT vs. invariant latitude polar plots. All patches shown were observed during periods of IMF Bz< 0. Planetary Index K p levels varied from 0.3 (0+) up to 7.0 (7). Approximately 50% of the patches occurred during periods with Kp > 3.7 (4-). These results tend to agree with the accepted conditions for patch behaviour. foF2 records were also analysed in an attempt to verify that the patches observed in TEC records could also be seen in fo F2 records. This could not be verified with any level of certainty, however, as the foF2 records for the days in question contained many omissions of data. Amplitude scintillation measurements were taken using the method developed by Kersley et al. (1988) and were used to identify scintillation producing patches. Not all patches identified were observed to produce scintillation. The S4 index was observed to vary over the course of a patch in all scintillation-producing patches.
The ionisation 'hole' poleward of the nightside auroral zone first became evident during the late 1970's, Brinton et al. (1978) and Crowley et al. (1993). It arises due to the long transport time of ionisation from the dayside across the dark polar cap, also from plasma stagnation (see Kelley (1989) for a detailed description). The polar hole is expected to be located in the post-midnight sector (00-03 MLT) when the IMF Bzis southward with a positive By, whilst it is expected to be found in the pre-midnight sector of the polar cap (21-24 MLT) when the IMF Bzis southward with a negative By , Beggs et al. (1994). It is also expected that the polar hole ionisation density is lowest during winter with a minimum density in the southern hemisphere at 20.3 UT for all seasons. This hole can also be located using differential phase TEC measurements. An example of a hole seen in TEC measurements on day 146, 1995, along with the associated amplitude variations and S4 index is shown in Figure 3. As seen in Figure 3, scintillation levels slowly rise during the hole event as S4 rises from 0.2 up to a peak of 0.4 in the 3.0 to 2.5 hours MLT region. A large proportion of passes showing polar holes exhibited similar scintillation behaviour.
The hole was located at various times, during the period of April to August 1995, for
both IMF Bz, By< 0, and for IMF Bz < 0, By
> 0, as shown in Figure 4. As expected, the hole was only observed during conditions of
southward IMF (Bz < 0), and seen to be situated in the -70° to
-80° invariant latitude (L ) region, for all IMF By. The hole was also seen to be located in the 21-24 MLT region for IMF By < 0 and in the 00-03 MLT region for IMF By > 0.
Selected TEC passes were compared to results from the Parametrized Ionospheric Model
(PIM) version 1.4, Daniell et al. (1995). Figure 5 shows a comparison of experimental data,
with theoretical data, both plotted in geographic co-ordinates. Also shown is the
experimental data, plotted in magnetic co-ordinates. The results obtained from PIM show
only a large-scale correlation to the experimental data. PIM does not give the correct
background levels of TEC
for most of the passes analysed, although the monthly average values are close to that expected, if only on a large scale. The average background TEC levels predicted by PIM were accurate to within 2-3 TEC units (where one TEC unit is equal to 1 × 1016 electrons.m-2) to that recorded experimentally. PIM is not able to predict the position of polar holes or patches, as these are dependent on the IMF parameters.
Patches were observed in the -80° to -90° invariant latitude region, predominantly between 12-24 MLT, on several days throughout the periods of April to November 1994, and April to August 1995. Patches were only observed during periods of southward interplanetary magnetic field (IMF Bz < 0) conditions. November 1994 and April 1995 were the times when patches were most often observed. TEC levels rose by a factor of 2 times to 4 times to that of background levels during the event of a patch. Strong scintillation was observed on several occasions when TEC levels rose significantly compared to background levels. The ionisation 'hole' was observed on several days during the months of April to August 1995. The hole was predominantly situated between -70° to -80° invariant latitude, between 21-24 MLT (By < 0), and 00-03 MLT (B y > 0). As predicted, holes were found in two regions, dependent on whether IMF B y was positive or negative. This study shows that the method of differential phase determination of total electron content can be used to locate patches and holes in the polar cap ionosphere.
Special thanks go to Australian Antarctic Division personnel Lloyd Symons, Dale Siver, Dr. Didier Monselesan and Dr. Darryn Schneider, for assistance with data collection. Thanks also to Dr. Anthony Breed, from the Australian Antarctic Division, for assistance with data analysis software. Also thanks to Mr. Robert Polglase of La Trobe University for help with technical matters, and to Dr. Paul Smith for providing foF2 and IMF data. IMF data obtained from the WIND satellite, courtesy of the National Space Science Data Centre. This research is supported by an Antarctic Science Advisory Council (ASAC) grant, and by logistic support from the Australian Antarctic Division.
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