DENSITY FROM THE RINGS THROUGH INNER MAGNETOSPHERE

Transcription

1 O 2 AND O 2 DENSITY FROM THE RINGS THROUGH INNER MAGNETOSPHERE M.K. Elrod 1, R.E. Johnson 1, T. A. Cassidy 1, R. J. Wilson 2, R. L. Tokar 2, W. L. Tseng 3, W.H. Ip 3 1 University of Virginia, Charlottesville, VA Space Science and Applications, Los Alamos National Laboratory, MS D466, Los Alamos, NM, Institute of Astronomy, National Central University, Chung Li 320, Taiwan The main rings and the ice grains in the tenuous F and G rings are a source of O 2 ions for the inner magnetosphere (Tokar et. al. 2005). These ions are formed from neutral O 2 produced by the decomposition of ice by incident radiation (Johnson et. al. 2006). Since the principal source of O 2 ions is from the ionization of the neutral O 2 molecules through photo-ionization and electron interactions, O 2 becomes a marker for the radiation-induced decomposition of ice and the presence of O 2 neutrals. Recently, Martens et al (2008) described O 2 beyond the orbit of Enceladus, noting the possibility that Rhea is a source. Here we focus on O 2 inside the orbit of Enceladus. Through simulations of the neutral cloud created by photo-induced decomposition of the ice in the main rings and the tenuous F and G rings (Johnson et. al. 2006, Tseng et. al. 2008), it is possible to calculate the column density of the neutrals and the O 2 source rate in the inner magnetosphere. Using the Cassini Plasma Spectrometer (CAPS) data, we describe the density of the O 2 ions from the rings out to the orbit of Enceladus. The largest source of O 2 neutrals is expected to be the main rings. However, here we examine whether or not the energetic ion irradiation of grains in the F and G rings are significant sources of O 2 and if ion-neutral reactions in the Enceladus plume are a possible source. INTRODUCTION: The main rings of Saturn are primarily composed of icy grains ranging in size from microns to meters. When UV sunlight interacts with these ice grains, it causes the water in the ice to photo-disassociate. The disassociated species can react to produce H 2 and O 2 molecules which diffuse out of the ice. H 2 molecules, being much lighter in mass, tend to escape from the system much easier than the heavier O 2 molecules. As a result the O 2 molecules collect over the rings forming a tenuous O 2 atmosphere that extends into the inner magnetosphere. The plasma from the inner magnetosphere of Saturn and UV photons interacts with this neutral cloud of O 2 molecules, creating O 2 ions. Photo-chemistry of oxygen has shown the following reactions for molecular oxygen: O 2 hν -> O 2 e (1) O 2 hν -> O O e (2) O 2 hν -> O O (3) And for atomic oxygen: O hν -> O e (4) O O -> O O (charge exchange) (5) Thus the most effective method for the formation of a molecular oxygen ion over the main rings is through ionization of neutral molecular oxygen. When the Elrod Page 1

2 Cassini Space craft entered Saturn s orbit in 2004 in order to brake properly and enter a stable orbit for the mission, it passed very close to Saturn directly over the rings. This was the only pass over the rings that has occurred thus far in the mission. The spacecraft passed over the B-ring and crossed the ring plane just outside the F ring between the F and G rings. It was during this pass that the Cassini Plasma Spectrometer (CAPS) instrument detected ionized molecular oxygen (Tokar et. al. 2005, Johnson et. al., 2006). During subsequent orbits of Saturn the CAPS instrument detected O 2 ions beyond the rings but in significantly lower concentrations (Sittler et al, 2006). The fact that there is no other point where O 2 ions have such a high concentration indicates that the main rings are the primary source for O 2 ions throughout the inner magnetosphere. Our model of the neutral cloud from the rings show that the highest column density of molecular oxygen is over the A and B ring and trails off outward as seen in figure 1. The broad distribution of neutrals outside the rings is due to low energy, ion-neutral collision and charge exchange. This model also indicates that the inclination of the sun with the respect to the rings will affect the column density on the north side versus the south side of the rings (or lit and unlit sides) (Tseng et.al. 2009). The model used to obtain the result in figure 1 is a particle tracking simulation describing the neutral cloud where loss occurs through ionization processes and ion-neutral interactions. The model results shown in figure 1 demonstrate both the trailing edge over the ring, and the impact of the angle of inclination of the sun on the rings. Figure1. The neutral O 2 column density (molecules/cm 2 ) in four situations: red: 24 north of the ring plane; green: 24 north; blue: 14 north; pink: 4 north. (Tseng et.al. 2009) Having modeled the CAPS data over the rings, the subsequent goals of this study is to analyze the CAPS data of O 2 ions from the rings to just inside the orbit of Enceladus at around 4 Rs where Rs is the Saturn Radius (Rs = km). CAPS DATA ANALYSIS Early morning of July 1, 2004 the Cassini craft entered orbit around Saturn. On this initial pass over the rings, Cassini passed approximately 1.79 Rs over the main part of the B ring. On this initial pass starting at around 2.2 Rs until the ring crossing between the F and G ring at around 2.6 Rs, the CAPS instrument detected an increase in the ion density. Figure 2 shows a diagram of the trajectory of the Cassini spacecraft s trajectory as it entered the Saturn system. Elrod Page 2

3 singles mode for the instrument is designed to simply count the number of strikes made in a single sweep. Depending on the telemetry mode there can be 1-16 sweeps/ Acycle. Each Acycle lasts 4 seconds long, so in the highest telemetry mode, like that used during the 2004 entry pass, the highest amount of data will be collected. In all other telemetry modes, the counts will be summed up. Figure 2. Schematic of the Cassini trajectory near the Rings. Red regions indicate when CAPS rotated into the direction of plasma flow and ion densities were enhanced. As this trajectory shows, the spacecraft began at an altitude of approximately 0.25Rs north of the ring to about 0.15 Rs when the spacecraft rotated in the first area of interest, and then crossed the ring plane in the second area of interest. To determine the column density of the O 2 ions, it is necessary to determine the ion temperature, velocity relative to the space craft, and density at its position. It is also necessary to separate the different ion types entering the detector into the different species to correctly determine the temperature density and velocities. Since O 2 is twice the mass of the water group (also a dominate ion in the inner magnetosphere) and the O ion, it is much easier to determine the peak from the O peak by mass. Using Maxwell distribution to fit to the curve, it is possible to determine the moments of the phase space i.e., the temperature, density and velocity of the individual ions. The singles detector, used for this study, is part of the CAPS instrument on Cassini. It has 63 different energy bins or settings. As the ion enters the instrument, the charge/mass ratio will cause the trajectory of the ion to change and it will strike one of the different bins. The Since the ions have a temperature, the counts need to be converted to phase space and then fitted to a Maxwellian to determine T i. The counts are plotted vs energy. This flux F(E) is calculated from the Maxwellian fit to the counts vs energy bin: (6) Here eff(e) is the efficiency of the instrument as a function of ion energy E, G(E) is the geometrical factor of the instrument, v is the velocity of the particle. The key piece of this equation comes from the f(e) which is the Maxwellian fitting curve: Here n is the number density, m is the ion mass, T is the ion temperature in ev, and u is the velocity of the ion relative to the spacecraft. (7) This simple analysis gives a one dimensional approximation to the ion phase space distribution. When the spacecraft passed over the rings, the CAPS instrument was not actuating, meaning that the detector was pointing in one direction during the entire pass. To get a complete three dimensional analysis, the detector needs to be actuating or scanning across the field Elrod Page 3

4 both in and out of the line of view of the plasma, the peak of ion density. CAPS DATA Figure 3 shows a sample of the flux and fitting process for when the detector was over the B ring and pointed in the mean plasma flow direction. The higher peak in this graph is the O 2 ions while the lower peak is the O. The heavier ions will have the higher energy while the lighter mass ions will be at the lower energy. Figure 4. Density versus Rs of O 2 and O ions. Red line is O 2 and blue is O. Each anode that has a measurable peak, is individually analyzed, at each point. Then the anodes are averaged together to create one density per radial point. This curve was smoothed using a three point averaging to remove sharp jump in the density. Figure 3. Time 03:46:17 The blue line is the actual data, the red line is the O 2 flux fit and the green line is the O flux fit line with the black line the sum of the two. This is a snap shot measurement made near 1.92 Rs over the main rings specifically the B-ring. There are eight anodes on the detector. To accumulate the single densities from all anodes per Acycle, the densities are summed up to the Acycle resolution, then the anodes are averaged together for each point of measurement. Figure 4 is a graph of the densities at the space craft location. Figure 5 is the ion temperatures at the spacecraft location. Near the rings the plasma is moving close to the rotational velocity of Saturn s Magnetosphere, ~ km/s depending on position over the rings. These higher velocities of ions, as compared to further out in the magnetosphere make for the steep narrow curves seen in figure 3. Figure 5. Ion Temperature ev vs Rs.Red line is O 2 and blue is O. Similar to the density the anodes with measurable peaks are averaged together at each point to get the temperature of the O 2 and O. O 2 has lower temperature due to the fact that more energy is released in the O 2 reaction. This curve was also smoothed using a three point averaging. In order to determine the column density of the ions, we calculate the scale height H, a function of the temperature and ion mass. The projected density at the magnetic equator n o, depends on the altitude and the scale height. Elrod Page 4

5 (8) (9) indicates that there is also a slight increase in O 2 ions near Rhea indicating that Rhea might be a second source, though much less so than the rings (Martens et. al. 2007). (10) Here n = local density, z = altitude above the magnetic equator, q = ion charge, and m i = mass of the ion. Figure 6 shows the projected equatorial density for the region near the rings. Figure 6 shows the column density over the rings. Figure8. Column density of O 2 from the rings to Rhea. Diamonds indicate the density over the rings and squares out to Rhea (Tokar et al 2005, Martens et.al., 2007) DISCUSSION: Figure 6. Projected density of O 2 to the magnetic equator. The relatively high column density of O 2 over the rings indicates that the rings are by far the largest source of O 2 and, thus, O 2 ions in the inner magnetosphere of Saturn. While the Cassini spacecraft has passed several times near the plume of Enceladus, the source of the E-ring and the source of water ice grains, there has been little to no evidence for a strong O 2 source found in the plume or near Enceladus as yet. At present we are analyzing the CAPS data between Enceladus and the main rings. In this region very energetic ions interact with the tenuous F, G and E ring and might be and additional source of O 2 and consequently O 2. REFERENCES Figure 7. Column density of the region over the ring of O 2. Figure 8 compares the column density of O 2 from the ring out to 10Rs. This figure Bouhram, M.; R. E. Johnson; J.-J. Berthelier; J.- M. Illiano; R. L. Tokar; D. T. Young; F. J. Crary (2006), A test-particle model of the atmosphere/ionosphere system of Saturn's main rings, Geophys. Res. Lett., 33, L05016 Elrod Page 5

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