ION-CYCLOTRON WAVES AT IO

C. T. Russell1 and D. E. Huddleston2

Adv. Space Res., 26(10), 1505-1511, 2000.

 

1Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA

2 Hughes Space and Communication Company, El Segundo, CA 90245, USA

 

ABSTRACT

The massloading of the near Io torus by sulfur dioxide leads to the generation of intense ion cyclotron waves, near the singly ionized sulfur dioxide ion gyro frequency, that have been detected by the Galileo magnetometer. While other ions are produced near Io, the Io torus plasma contains nearly isotropic Maxwellian distributions of these ions that damp any waves near their ion cyclotron frequencies. Since SO2+ dissociates rapidly in the Io torus environment, there is no background of damping ions at the SO2+ ion gyro frequency. To zeroth order these waves grow at the ion cyclotron frequency, are left-hand circularly polarized and propagate along the magnetic field. In actuality the waves grow over a finite bandwidth, propagate at an angle to the field and are elliptically polarized. At the upper edge of the band of wave growth the waves propagate at their greatest angles to the magnetic field, and are more elliptically polarized. In fact in some regions the waves become slightly right-handed. These polarizations may arise due to the propagation of the waves in an increasing field strength in a multi ion environment. Finally, we examine the data obtained by the Voyager spacecraft along the "same" field line as Galileo but 16.5 years earlier and 12 RIo southward of Io. No ion cyclotron waves are seen here or anywhere else along the Voyager Io passage. If we assume that ion cyclotron waves were generated at the same strength in 1979, then the waves must have been significantly attenuated by propagation or absorption.

INTRODUCTION

The ion pick-up process at Io produces a ring distribution of ions in the near-Io torus very similar to that produced in the cometary environment. However, the magnetic field at Io is strong and the plasma beta is low in contrast to the situation at a comet. Moreover, the plasma in the Io torus recirculates while at the comet the ions are added to new undisturbed solar wind. Thus while we expect similar processes to occur at Io and comets, the characteristics of these processes should also have their differences [Huddleston et al., 1998; 1999]. Voyager 1 had flown by Io at the discreet distance of 11 RIo in 1979, crossing the magnetic field line that had the field been undisturbed would have intersected Io. A disturbance in the direction of the magnetic field, called an Alfven wing because it travels at the Alfven velocity and bends but does not compress the field; was observed and the plasma detector responded as expected for an Alfven wave . However, no waves other than the Alfven wing itself were reported in the vicinity of Io.

The Galileo flyby of Io was designed to use Io’s gravitational field to assist with the orbit insertion process so Galileo flew much closer to Io, within 1000 km of the surface, and through the wake in the corotational flow. The low resolution data revealed a depressed field near Io’s equatorial plane, with an enhanced field, that partially recovered from the depression, in the center of the corotational wake . Higher resolution data later became available revealing ion cyclotron waves on either side of Io and mirror mode waves at the edge of the wake . These ion cyclotron waves signal the presence of SO2+ gyrating at right angles to the magnetic field. The waves arise as the SO2+ scatters in pitch angle. The waves’ amplitude has been used to estimate the strength of the SO2+ source [Huddleston et al., 1997]. These waves can also transport energy from the SO2+ to the other species if the waves have properties that allow this interaction. To a first approximation the ion cyclotron waves were at the SO2+ gyro frequency, propagating parallel to the local magnetic field and left-hand circularly polarized. However, when examined more carefully the behavior of these waves is much more complex. It is the purpose of this paper to examine this complexity using dynamic spectral analysis to explore the power in the waves, their ellipticity and their direction of propagation as a function of frequency and time. We also examine the difference in wave power seen by Galileo and Voyager at different locations on the same field line in the Io frame, albeit 16.5 years apart at different local times, magnetic latitudes, and plasma conditions.

DYNAMIC SPECTRAL ANALYSIS

To explore the detailed behavior of the waves we have calculated Fast Fourier Transform spectra over 256, 0.25-second samples, shifting by 32 points and then repeating. Frequency estimates were then averaged over 7 values. Each of the three components (radial, corotational and southward) as well as the total field were analyzed. The compressional power was subtracted from the sum of the powers of each of the components to produce the transverse power. The full real and imaginary cross spectra were calculated at each frequency. Since the waves are mainly nearly circularly polarized, the technique of was used to calculate the ellipticity and direction of propagation at each frequency. Ellipticity is defined over a range of 1 to -1 where 1 is right-hand circular, -1 left-handed circular and 0 is linearly polarized. The direction of propagation can vary from 0o (parallel to the field) to 90o (perpendicular). The coherence of the waves in the radial and corotational directions roughly transverse to the field direction was calculated at each frequency and used as a mask in plotting the dynamic spectra shown here. Only values are plotted for which the coherence of the radial and azimuthal components i.e. the BR and Bf components, was greater than 0.4. Since these two components are nearly perpendicular to the main field, their high coherence occurs for transverse waves.

DYNAMIC SPECTRA

We have divided the encounters into four analysis periods: 1705-1720; 1720-1735; 1735-1750, and 1750-1805 UT on December 7, 1995. Figure 1 shows the transverse and compressional powers, the ellipticity and the direction of propagation for the first period of time. Table 1 lists points along the trajectory. The ion cyclotron waves begin at 1707 UT with a series of bursts about 2 minutes apart. The waves have a finite compressional component with a power about an order of magnitude less than the transverse power. Beginning about 1717 UT the power appears in two separate branches. The third panel from the top shows the ellipticity of the waves. The main branch is left-hand, nearly circularly polarized but the upper band near 1719 includes a short burst of linear, slightly right-hand polarized waves. The bottom panel shows that the propagation is generally nearly parallel to the field but that these linearly polarized right-handed waves are propagating nearly perpendicular (60o) to the magnetic field.

Figure 1. Dynamic spectra of the waves seen near Io by the Galileo spacecraft from 1705-1720 UT on 5 December, 1995. Top Transverse power. Upper middle Compressional power. Lower middle Ellipticity. Bottom Direction of propagation. All spectra have been masked by blocking out portions of the spectrum whose coherence between the two transverse components to the field (BR and B) was less than 0.4. This ensures that the waves have a significant transverse component.

Figure 2. Dynamic spectra of the waves seen by Galileo from 1720 to 1735 UT. The BR component is radially outward. The B direction is parallel to corotation. For all dynamic spectra the data were analyzed in blocks of 256 points and then shifted by 32 points. The transverse power is calculated by summing the power over all three components and subtracting the power in the total field. See the caption of Figure 1 for more details.

As Io is approached the waves continue to grow and the color bar is shifted upward a half decade in Figure 2 that covers the interval 1720-1735 UT. The upper frequency band that split off at 1717 UT rejoins the main frequency band at 1722 UT. The transverse power is close to two orders of magnitude more intense than the compressional power. The waves are left-hand elliptically polarized everywhere but the ellipticity varies with frequency and time. The direction of propagation is nearly along the field except in the separate upper frequency band from 1717-1722. In this band the waves propagate at a large angle to the magnetic field.

Figure 3. Dynamic spectra of the waves seen by Galileo from 1735 to 1750 UT. See the captions of Figures 1 and 2 for more details. Figure 4. Dynamic spectra of the waves seen by Galileo from 1750 to 1805 UT. See the captions of Figures 1 and 2 for more details.

 

Table 1. Trajectory of the Galileo spacecraft in Io-centered radially outward, southward and corotational components

Time

Radial

Southward

Corotational

1705

20.0 RIo

2.9

-0.8 RIo

1720

12.7

1.9

-0.5

1735

5.5

1.0

0.5

1750

-1.6

-0.2

1.9

1805

-8.7

-1.0

3.7

Figure 3 includes the entry into and passage through the wake region. The compressional wave power increases rapidly from 1735-1744 UT so that it significantly exceeds the transverse power from 1742-1744 UT and is roughly equal from 1739 to 1742 UT. The ellipticity shows that all the ion cyclotron waves, even when there are strong compressional waves present, are left-hand, nearly circularly polarized. The upper frequencies from 1740-1742 UT are propagating at about 50o to the field and are more linearly polarized than those at lower frequencies. We recall that we are using the Means [1972] technique for defining the ellipticity and the direction of propagation. This technique is sensitive to circularly polarized and elliptically polarized signals, not linearly polarized because it uses only the imaginary part of the spectral matrix. The burst of wave power at 1749 UT signals the emergence of the spacecraft into the ion cyclotron wave region on the other side of Io. The waves here are again slightly more compressional than transverse. Figure 4 extends the analysis from 1750-1805 UT. The color bar now drops half an order of magnitude from the previous two intervals. At the beginning of the interval the waves are only slightly stronger in the transverse power but soon the compressional power drops rapidly while the transverse components stays relatively strong. The ellipticity shows that almost everywhere the waves are left-hand elliptically polarized and propagating roughly along the field except for the upper frequency band near 1758 UT. Here the waves become linearly polarized and propagate at a large angle to the field. At 1800 UT the waves effectively cease.

VOYAGER-GALILEO COMPARISON

As shown by Russell [1998] the Voyager and Galileo trajectories crossed each other radially outward from Io and slightly downstream at a location of (3.6, 0.7, 0.8) RIo in radial, southward and co-rotational Io-centered coordinates. Figure 5 shows power spectra of the transverse and compressional components at Voyager from 1505 to 1511:30 UT on 5 March 1979 and at Galileo from 1734 to 1740:15 on 7 December 1995 on the same scale as Voyager and Galileo each passed this common field line. The wave power at the SO2+ gyrofrequency peak at Galileo is 4 orders of magnitude greater than the power at Voyager. There is no evidence at Voyager here or at other locations along the Io encounter trajectory of SO2+ or other ion cyclotron waves. We note however that the two encounters occurred at different magnetic latitudes of Io, different local times and plasma densities (see e.g. Bagenal et al., 1997).

SUMMARY AND CONCLUSIONS

We can understand why the waves at Io due to the picked up ions in the Io torus are predominantly at the SO2+ gyrofrequency. The nearly isotropic Maxwellian distribution of the background plasma suppresses wave growth at the gyrofrequencies of the other ions but because SO2+ dissociates well before it returns to Io there is no such damping for SO2+ [Huddleston, 1997; 1998; 1999]. We have not yet conducted a theoretical study of the effect of these multi-ion species on the propagation of the ion cyclotron waves generated near the SO2+ gyrofrequency but expect that these other species do cause some of the "unusual" behavior in the ellipticity and the direction of propagation.

Figure 5. Transverse and compressional power spectra as seen by Galileo and Voyager as they crossed the "same" field line sixteen and one half years apart. The "same" field line is defined with respect to an Io-centered reference frame.

For example, a left-handed wave generated at a frequency between the gyrofrequencies of two significant ion components of the plasma can be "converted" to a right-handed wave as it propagates into a higher field strength and drops in frequency relative to the local gyrofrequencies [e.g. Fraser et al., 1992]. These affects may also alter the direction of propagation. Off-angle propagation leads to more elliptical polarization that can allow the waves to resonate with harmonics of the gyrofrequency. In this way the SO2+ waves could be damped by the S+ ion distribution. Such possibilities need to be checked. Perhaps these effects can explain why the waves seem unable to propagate from Io down the field line only about 20,000 km to Voyager, if as we expect, the waves were present at Io in 1979 in strengths similar to those observed in 1995.

ACKNOWLEDGMENTS

This work was supported by the National Aeronautics and Space Administration through a grant through the Jet Propulsion Laboratory.

REFERENCES

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