Abstract

Introduction

• Chorus is an intense plasma wave that permeates magnetospheric regions between the plasmapause and the magnetopause and can be observed on the ground over a range of latitudes.
• It typically consists of repeating, usually rising and often overlapping coherent tones, occurs regularly in association with disturbed magnetospheric conditions and in conjunction with microburst electron precipitation [10]  [9], and is known to be a driver of pulsating aurora and maybe even the morningside diffuse aurora [4]. Chorus is thought to be generated via cyclotron resonance with energetic electrons in the 5 to 150 keV range [11].
• Chorus that is generated within 1-2 R_E of the magnetopause may originate in local regions of minimum magnetic field strength that occur off the magnetic equator as a result of solar wind compression of the dayside magnetosphere [13] [3] and subsequently propagate to the ground in the polar regions. Consequently, polar chorus as measured on the ground is typically the most intense in the local noon sector occurring in the frequency range of 400-1500 Hz [11].
• New evidence for direct association of chorus (as observed on spacecraft) with solar wind parameters during an intense coronal mass ejection (CME) event was recently reported by Lauben et al. [6].
• In this report, we focus on results from two study periods, 23 February 1997 and 11 April 1997, the latter being a CME disturbance. On both days extremely intense chorus activity was interrupted by periods of deep quieting lasting several minutes and associated with reductions in dynamic pressure.

Data Sets Used

•  The network of Automatic Geophysical Observatories in Antarctica with coverage from the auroral zone to the polar cap is well positioned for ground-based observations of polar chorus.
• An ELF/VLF receiver system records continuous narrowband and synoptic broadband wave activity incident upon North/South and East/West crossed loop antennas [12].
• IRIS imaging riometers [1] provide a sky map of the ionospheric absorption of cosmic noise that results from energetic particle precipitation into the lower ionosphere.

Figure 1: Locations of the AGO stations P1-P4 and South Pole are shown with respect to the International Geomagnetic Reference Field (IGRF-1995). The legend shows the magnetic latitude, l, and the difference between universal and magnetic local time, UT-MLT at each site. The antenna orientation is also identified with the vertical line representing the North/South antenna and the horizontal line the East/West antenna.

• Solar wind conditions were assessed using data from the three-dimensional plasma (3DP) [8] and the magnetic field (MFI) [7] instruments aboard the WIND satellite in conjunction with similar measurements by the comprehensive plasma (CPI) [2] and magnetic field (MGF) [5] instruments on Geotail.

Case I: 23 February 1997

• Low level chorus activity began around 1000 UT at three stations, P2, P3, and SP, and gradually increased in intensity for several hours. Chorus was not detected at P1 and P4 until about noon UT and stayed at much lower levels throughout the period of interest. The peak intensity of 400 mV/m at P2 represented some of the most intense chorus measured at these sites, especially during the austral summer.

Figure 2: South Pole synoptic broadband recordings show the entire duration of chorus activity for February 23, 1997. The system records for one minute every fifteen minutes on the N/S antenna.

Starting at 1330 UT there were several sudden decreases in chorus activity that were evident at all five stations, the most dramatic occurring several minutes after 1400 UT.

During this period the WIND satellite was located at 213 R_E sunward of the Earth while Geotail was located 13.5 R_E behind the earth in the nightside magnetosheath region.

The solar wind travel time delay between the satellites was estimated to be 55 minutes, and for alignment with ground data the WIND measurements were shifted +47 minutes and the Geotail measurements -8 minutes consist with feature alignment and satellite measurements of solar wind velocity.

The chorus intensity was highly correlated with the solar wind dynamic pressure, and the deep quieting in chorus at 1400 UT, as well as the smaller drops, were associated with pressure relaxation. The interplanetary magnetic field was northward throughout most of the period although it changed to southward for a brief interval just prior to the largest chorus dropout.

Figure 3: Narrowband recordings from the 1-2 kHz E/W channel are shown with time aligned solar wind data for the February 23, 1997 case.

• AGO broadband recordings were able to capture the drop out effect during the 1405 UT 2 second synoptic period as shown in Figure 4.

Figure 4: Synoptic broadband recordings from the N/S antenna capture the chorus dropout at 1405 UT. Intense hiss was detected at P1 on the N/S channel until 1405 UT. The synoptic period is two seconds.

•  Riometer absorption data in riogram format (distance along the geomagnetic meridian vs. time) and 1-2 kHz narrowband ELF recordings for P2 and P3, are depicted in Figure 5. These two data sets show a correlation between particle precipitation and polar chorus at both sites. No measurable absorption was detected at the highest latitude sites (P1, P4). Some weaker precipitation was also detected at the intermediate location of SP (not shown).

Figure 5: Riometer absorption and 1-2 kHz narrowband ELF data recorded at P2 and P3. The riometer data are displayed in riogram format, where the ordinate is depicted as distance at a 90-km altitude range (D region) with south (north) at the bottom (top), UT time is along the abscissa, and color as given by the bar is proportional to absorption in decibels (dB).

Case II: 11 April 1997

• During a CME-induced disturbance on April 11, 1997, chorus was observed at P2, P3 and SP for over eight hours with unusually variable intensity. P1 on this day was quiet and no data were available from P4.
• Both satellites were in the interplanetary medium upstream from the Earth with WIND at 230 R_E and Geotail at 16 R_E. Figure 7 shows a high correlation between the chorus intensity and solar wind conditions over a period of several hours.
• The average time delay between WIND and Geotail throughout this period was approximately 54 minutes, with another 5 minute delay before features were seen on the ground.
• Similar to February 23, 1997, the decreases in chorus intensity at P3 and SP stations correlate well with precipitation drops particularly the sharp drop at 1500 UT. Data from P2 was inconclusive for this case, due to high levels of background noise.

Figure 6: South Pole synoptic broadband recordings show the entire duration of chorus activity for April 11, 1997. The system records for one minute every fifteen minutes on the N/S antenna.

Figure 7: Time aligned solar wind and ground-based chorus data for the April 11, 1997 case.

Summary

• Results from these two study periods indicate that ELF chorus intensity can be controlled directly by the solar wind dynamic pressure. The response of chorus was fast with respect to sudden changes in pressure.
• Directional antennas indicate that chorus penetrated to the ground over a large area when the sites were in the local noon sector.
• Riometer absorption decreased simultaneously with chorus indicating that the particle precipitation observed at the auroral stations, P2, P3 and SP, during this period was driven by chorus.
• At the high latitude stations, P1 and P4, chorus amplitudes were significantly reduced and did not correlate with precipitation. The lack of measurable absorption at P1 and P4 is consistent with the fact that these higher latitude sites are likely to lie under open field lines. Chorus is likely generated on closed field lines which connect to equatorward ionospheric regions and was observed at P1 and P4 as a result of propagation in the Earth-ionosphere waveguide.

Acknowledgments

References

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