Observations of the seasonal dependence of the thermal plasma density in the southern hemisphere auroral zone and polar cap at 1 Re

M. T. Johnson and J. R. Wygant, School of Physics and Astronomy, University of Minnesota, Minneapolis, MN

F. S. Mozer and M. Temerin, Space Science Laboratory, University of California, Berkeley, CA

J. Scudder, Department of Physics and Astronomy, University of Iowa, Iowa City, IA

Abstract

Average maps of the thermal plasma density in the auroral zone and polar cap around 1 Re altitude are produced from measurements of Polar spacecraft floating potential during 1 year of perigee passes. These measurements provide the first comprehensive maps of the thermal plasma density over the entire polar cap and auroral oval. There are clear variations in plasma density due to solar illumination of the ionosphere in the polar cap and auroral oval. Number density increases by a factor of 5 for illuminated verses non-illuminated ionospheric conditions over the entire auroral oval and polar cap. The data provide evidence for a large scale density cavity in the auroral zone near 70 +/- 5 degrees invariant latitude.

Introduction

The existence of a large scale density depletion on auroral magnetic field lines was first demonstrated by Calvert [1981] using measurement of the upper hybrid line as the density diagnostic. This study showed that a density cavity existed over an altitude range of 1 - 3 Re on auroral field lines with a spatial extent of about 6 degrees invariant latitude. Persoon et al. [1988] provided a map of the auroral density cavity in the afternoon to midnight local time sector using densities determined from the frequency of the upper hybrid emissions observed by the DE-1 spacecraft. This work demonstrated that the cavity was strongest on the night-side and the densities could fall as low as 0.1 cm^-3. Other local time sectors were not explored due to the lack of orbital coverage by the DE-1 wave instrument. In addition, these studies could not determine density variations throughout the entire latitudinal extent of the cavity because auroral hiss is typically only present at its pole-ward edge. The study presented here extends the local time coverage to the wayside and the early morning local time sector. The spatial and seasonal variations in plasma density present throughout the polar cap and auroral zone may also be observed because of the extensive coverage of the Polar data set.

Thermal plasma structure near 1 Re altitude on auroral magnetic field lines is of particular interest because experimental evidence from the S3-3 spacecraft [Mozer et al., 1977; Temerin, 1984], the Swedish Viking spacecraft [Bostrom, 1988; Block and Falthammar, 1991], and now the Polar [Mozer et al., 1997] and FAST [Ergun et al., 1998] spacecrafts indicates that intense, non-linear electric field structures exist at this altitude. These electric fields can accelerate electron beams downward, generating aurora, and accelerate ion beams away from the earth. Intense waves in this region are responsible for the generation of ion conics which also accelerated away from the earth [Shelley, 1995]. The plasma density structure is important in these precesses because it affects the propagation and generation of waves and the availability of current carrying particles. Thus, there is the possibility of a variety of non-linear mechanisms involving feedback between the electric fields that accelerate the plasma, the creation of density depletions, and the modification of the electric fields as they interact with the density cavity.

Data Analysis

The measurement of number density in rarefied plasmas is a difficult task and several complementary methods have been used to provide information on the structure of the thermal plasma in the terrestrial magnetosphere. The spacecraft potential, measured using the Polar Electric Field Instrument (EFI) [Harvey et al., 1995], is used as a plasma density diagnostic for this study. The Polar EFI measures the spacecraft potential by finding the potential difference between the surface of the spacecraft and an electric field probe biased to float near the local plasma potential. This technique can provide almost routine continuous information about the thermal plasma density for several years. Coverage of the Polar data set allows seasonal variations of the entire auroral oval and polar cap to be observed. Reviews of the technique, comparison to other techniques, scientific results, and references may be found in Pederson, [1995] and Escoubet et al. [1997]

The spacecraft potential is primarily determined by the balance between the thermal electron current to and the electron photocurrent from the spacecraft surface. The spacecraft potential is effected by electron temperature and density because of the electron thermal current. It has been shown for debye lengths much larger than the spacecraft radius that the floating potential of the spacecraft is an indicator of electron plasma density [Pedersen, 1995]. This is caused by the "focusing effect," which is due to low energy electrons drawn to the spacecraft diminishing the effect of the electron temperature on the spacecraft potential. Previous studies calculate the spacecraft floating potential assuming an infinite debye length. Plasmas in the polar cap and auroral oval at 1 Re with typical densities (0.1 cm^-3 to 1000.0 cm^-3) and temperatures (0.3 to 5.0 eV) [Kletzing et al., 1998] have debye lengths ranging from about 0.3 to 30 m which is on the order of the spacecraft radius (1.2 m). Therefore, the assumption of infinite debye length is not approprate. In order to ensure the accuracy of the spacecraft potential as a density indicator, we use the more general expression [Mott-Smith and Langmuir, 1926; Schott, 1968, 1995] to represent the flux of thermal electrons on the spacecraft. This expression effectively limits the focusing effect by excluding particles with impact parameters more than 1 Debye length away from the spacecraft surface. The photo-emission current as a function of spacecraft potential for the Polar spacecraft has been derived by analysis of spacecraft potential and the plasma measurements from the HYDRA instrument [Scudder and Cao, unpublished manuscript]. Scudder and Cao [unpublished manuscript] have pointed out that this curve varies seasonally due to the variation on the geometric area of the spacecraft illuminated by the sun as the orbital plane of the spacecraft processes through the year. These effects have been included in the curve used here.

Comparing the calculated results above to a curve produced from the analysis of HYDRA electron density [ Scudder and Cao, unpublished manuscript] show agreement to at least factor of two. Therefore, the experimental calibration provided by HYDRA should be good to a factor of two for densities from 0.1 to 100.0 cm^-3 in the polar cap and auroral oval. The Polar spacecraft has a 0.8 x 8.0 Re orbit with perigee in the southern hemisphere and an inclination of 85 degrees. The data used to create the Average maps are from the Polar key parameter files from April 15, 1996 to April 15, 1997. The data is converted to plasma number density and then thirty second averages are then binned and averaged in magnetic local time and invariant latitude.

Results

Four maps (Figure 1) of the entire southern hemisphere are made by averaging data from ~ 240 Polar perigee passes over the southern hemisphere. Data from April 15, 1996 to April 15, 1997 is plotted in invariant latitude to show the average number density variation due to seasonal and ionospheric illumination. The color scale saturates at 100 cm-3. Regions where there is an absence of data on a given map are white.

Figures 1a and 1b show data from April 15, 1996 to October 15, 1996 and October 15, 1996 to April 15, 1997 respectively. These time intervals roughly correspond to winter and summer in the southern hemisphere. The map during the winter months (Figure a1) shows a clear, large scale plasma depletion in the night-side auroral zone at 70 +/- 5 degrees invariant latitude. The density at mid-latitudes and in the polar cap ranges from 20 to 100 cm^-3. The average magnitude of the density in the cavity is less than 5 cm^-3 with minimum average densities less than 1 cm^-3 around 20 MLT. There is a clear enhancement of the plasma density by a factor of 5 over the entire polar cap and auroral zone during the summer (Figure 1b). Individual line plots before averaging (not shown) indicate that lowest densities of about 0.1 cm^-3 are observed near dusk.

Figures 1c and 1d show plots for which the data is divided into cases where the footpoint of the field line on which the measurements is made is non-illuminated or illuminated, respectively. The data are divided based on the X GSE position of the field line footpoint with a positive (negative) value of X GSE indicating a field line whose foot point is illuminated (non-illuminated). No distinction is made between the different incident angles of solar illumination on the ionosphere. Figure 1c shows the density on field lines whose footpoints are in darkness. The average depth of the depletion is less than 5 cm^-3 with the lowest average depletion of a few tenths cm^-3 in the pre-midnight sector centered on 72 degrees invariant latitude. The density cavity in the auroral zone now extends almost to noon compared as opposed to Figure 1a. Figure 1d shows plasma densities for field lines whose footpoints are illuminated. The plasma depletion in the auroral oval is not as prominent as in the non-illuminated case. Figure 1d reveals an overall plasma depletion to exist around 72 degrees with a density in the depletion of about 20 cm^-3. These maps provide strong evidence that the effect seen in Figures 1a and 1b are due to seasonal variations in ionospheric illumination.

Summary

Average maps of the thermal plasma density in the polar cap and auroral zone an altitude of ~ 1 Re provide evidence for the following conclusions: (1) The thermal plasma density in the polar cap and auroral oval at ~ 1 Re altitude depends strongly on the illumination of the ionosphere. Overall average plasma density increased by a factor of 5 for illuminated versus non-illuminated ionospheric conditions. (2) The density cavity observed in this study seems to correspond to that seen by Calvert [1981] and Persoon et al. [1988]. Additional coverage shows that the location of the auroral density cavity extends to all MLT for both the summer (illuminated) and winter (non-illuminated) times. The average density under non-illuminated conditions is around 5 to 10 cm^-3 at all MLT in the auroral oval. The lowest average plasma density of a few tenths cm^-3 occurs near dusk and in the pre-midnight sector at 20 MLT at 72 degrees invariant latitude. A density depletion is seen at all MLT in the auroral zone except for a few hours around noon. (3) The plasma depletion in the auroral acceleration region is not as prominent under illuminated conditions and has an average plasma density of around 20 cm^-3 or and increase by a factor of 2 or 3 compared to non-illuminated conditions. Similar density patterns and effects are seen in plots generated for the following year from April 15, 1997 to April 15, 1998.

Studies show a maximum probability of observing auroral electrons [Newell et al., 1996], intense UV auroral emissions [Liou et al., 1997], and a high probability of observing ion beams [Redsun et al., 1985] occur in the duskside auroral oval. This corresponds to the lowest plasma densities seen in the auroral oval for this study. Density depletions are also observed in the post-midnight sector where there is a lower probability of seeing auroral electrons. This suggests that the mechanism which produces plasma density cavities is not only associated with auroral acceleration, but also due to other mechanisms. Ion conics are seen [Redsun et al., 1985] at all MLT indicating that transverse ion heating maybe responsible for plasma depletions. Composite auroral UVI images [Liou et al., 1997], ion beams [Collin et al., 1998], and electrostatic shocks [Bennett et al., 1983] show seasonal effects on discrete aurora. It has been suggested that increasing the ion scale height increases above the auroral acceleration region and the density cavity to satellite altitudes. The increase in the ionospheric conductivity may decrease large and small scale electric fields if the magnetospheric driver is a current source.

Acknowledgments This research was supported by NASA Grant NAG5-3182.

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