Low- to middle-latitude X-ray emission from Jupiter

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111,, doi: /2006ja011792, 2006 Low- to middle-latitude X-ray emission from Jupiter Anil Bhardwaj, 1 Ronald F. Elsner, 2 G. Randall Gladstone, 3 J. Hunter Waite Jr., 3 Graziella Branduardi-Raymont, 4 Thomas E. Cravens, 5 and Peter G. Ford 6 Received 18 April 2006; revised 24 July 2006; accepted 31 August 2006; published 22 November [1] The Chandra X-ray Observatory (CXO) observed Jupiter during the period February 2003 for 40 hours (4 Jupiter rotations), using both the spectroscopy array of the Advanced CCD Imaging Spectrometer (ACIS-S) and the imaging array of the High-Resolution Camera (HRC-I). Two ACIS-S exposures, each 8.5 hours long, were separated by an HRC-I exposure of 20 hours. The low- to middle-latitude nonauroral disk X-ray emission is much more spatially uniform than the auroral emission. However, the low- to middle-latitude X-ray count rate shows a small but statistically significant hour angle dependence and depends on surface magnetic field strength. In addition, the X-ray spectra from regions corresponding to 3 5 gauss and 5 7 gauss surface fields show significant differences in the energy band kev, perhaps partly due to line emission occurring in the 3 5 gauss region but not the 5 7 gauss region. A similar correlation of surface magnetic field strength with count rate is found for the 18 December 2000 HRC-I data, at a time when solar activity was high. The low- to middle-latitude disk X-ray count rate observed by the HRC-I in the February 2003 observation is about 50% of that observed in December 2000, roughly consistent with a decrease in the solar activity index (F10.7 cm flux) by a similar amount over the same time period. The low- to middle-latitude X-ray emission does not show any oscillations similar to the 45 min oscillations sometimes seen from the northern auroral zone. The temporal variation in Jupiter s nonauroral X-ray emission exhibits similarities to variations in solar X-ray flux observed by GOES and TIMED/SEE. The two ACIS-S kev low- to middle-latitude X-ray spectra are harder than the auroral spectrum and are different from each other at energies above 0.7 kev, showing variability in Jupiter s nonauroral X-ray emission on a timescale of a day. The kev X-ray power emitted at low to middle latitudes is 0.21 GW and 0.39 GW for the first and second ACIS-S exposures, respectively. We suggest that X-ray emission from Jupiter s disk may be largely generated by the scattering and fluorescence of solar X rays in its upper atmosphere, especially at times of high incident solar X-ray flux. However, the dependence of count rate on surface magnetic-field strength may indicate the presence of some secondary component, possibly ion precipitation from radiation belts close to the planet. Citation: Bhardwaj, A., R. F. Elsner, G. R. Gladstone, J. H. Waite Jr., G. Branduardi-Raymont, T. E. Cravens, and P. G. Ford (2006), Low- to middle-latitude X-ray emission from Jupiter, J. Geophys. Res., 111,, doi: /2006ja Introduction [2] X-ray emission from Jupiter was first unambiguously observed nearly 2 1/2 decades ago by the Earth-orbiting 1 Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum Kerala, India. 2 Space Science Branch, NASA Marshall Space Flight Center, Huntsville, Alabama, USA. 3 Southwest Research Institute, San Antonio, Texas, USA. 4 Mullard Space Science Laboratory, University College London, Surrey, UK. 5 Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas, USA. 6 Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. Copyright 2006 by the American Geophysical Union /06/2006JA Einstein observatory [Metzger et al., 1983] (see Bhardwaj and Gladstone [2000] for a review of earlier searches for X-ray emission from Jupiter). These initial observations were followed about a decade later by a series of ROSAT observations spanning a period of about 6 years [Waite et al., 1994, 1995, 1997; Gladstone et al., 1998]. More recently, both X-ray cameras on the Chandra X-ray Observatory (CXO), the spectroscopy array of the Advanced CCD Imaging Spectrometer (ACIS-S) and the imaging array of the High-Resolution Camera (HRC-I), have observed Jupiter [Gladstone et al., 2002; Elsner et al., 2002, 2005a, 2005b, 2005c], as has the XMM-Newton X-ray Observatory [Branduardi-Raymont et al., 2004, 2006a, 2006b; Bhardwaj et al., 2005a]. These observations have stimulated theoretical studies of X-ray emission from Jupiter [Metzger et al., 1983; Barbosa, 1990; Waite, 1991; Singhal et al., 1992; 1of16

2 Table 1. Details of the CXO Jupiter Observations in 2003 February Instrument/Parameter Start Time a /Value b Stop Time a /Value b Chandra ACIS-S OBSID February :13 25 February :00 Chandra HRC-I OBSID February :23 25 February :56 Chandra ACIS-S OBSID February :15 c 26 February :23 R.A., hhmm:ss 0851: :07.71 Dec, deg:mm:ss +18:31: :34:23.3 Sun distance, AU Earth distance, AU Diameter, d arcsec Elongation, e degree Phase, f degree Apparent visible magnitude a 2003 February date hhmm:ss in UT. b Start value of parameters on 24 February 2003 at 1559 and corresponding stop value on 26 February at c After removing the first ks, during which the planet overlapped the bump in the bias frame. d Projected equatorial diameter of Jupiter. Jupiter s equatorial radius is 71,492 km. e Solar elongation = Sun-Earth-Jupiter angle. f Phase = Sun-Jupiter-Earth angle. [5] Using data from the ROSAT/HRI, Waite et al. [1996, 1997] reported low-latitude soft X-ray emission with a brightness of about R and raised the possibility that this equatorial X-ray emission might be due to the precipitation of energetic sulfur or oxygen ions into the atmosphere from Jupiter s inner radiation belts. Evidence Cravens et al., 1995, 2003, 2006; Kharchenko et al., 1998; Liu and Schultz, 1999; Maurellis et al., 2000; Bhardwaj et al., 2002; Bhardwaj, 2003, 2006; Bunce et al., 2004]. [3] X-ray emission from Jupiter separates spatially into two categories: (1) the high-latitude auroral (or polar) emission, and (2) the low- to middle-latitude disk (nonauroral) emission. As we show in this paper, these two categories have different morphology, temporal behavior, and spectra. [4] Previous papers on the Chandra and XMM-Newton X-ray observations of Jupiter have concentrated on the auroral emissions [Gladstone et al., 2002; Elsner et al., 2005a; Branduardi-Raymont et al., 2004, 2006a, 2006b; Bhardwaj et al., 2005a]. The Chandra observations have shown that most of Jupiter s northern auroral X-rays come from a hot spot that is fixed in System III latitude and longitude and located significantly poleward (on field lines connecting to equatorial regions in excess of 30 R J from the planet, where R J is Jupiter s equatorial radius) of the latitudes connected to the inner magnetosphere, and that the auroral X-rays sometimes, but not always, vary with a 45-min quasi-periodicity, similar to that reported for highlatitude radio and energetic electron bursts observed by near-jupiter spacecraft. The CXO/ACIS-S [Elsner et al., 2005a] and XMM-Newton [Branduardi-Raymont et al., 2004, 2006a, 2006b] observations provided soft ( kev) X-ray spectra of Jupiter s aurora, which are consistent with high-charge states of precipitating heavy (C, O, S) ions. Such a spectral interpretation suggests energetic ion precipitation from the outer magnetosphere or the solar wind or a mixture of both, the ions then undergoing a large acceleration to attain energies of >1 MeV/nucleon before impacting Jupiter s upper atmosphere [Cravens et al., 2003; Elsner et al., 2005a; Branduardi-Raymont et al., 2004, 2006a, 2006b; Bunce et al., 2004] (see Bhardwaj et al. [2006] for review). Figure 1. Color-coded two-dimensional histograms of Chandra events from observations on 18 December 2000 (HRC-I upper right panel) and February 2003 (HRC-I upper left panel, ACIS-S first exposure lower left, second exposure lower right; see Table 1) as seen in a frame moving across the sky with Jupiter. The histograms were smoothed with two-dimensional Gaussians with sigmas of for the HRC-I data and for the ACIS-S data. The scale bar in the lower right of each panel represents 5 00, and the small circles near the center represent the sub-earth and subsolar points. The superimposed graticules show latitude and longitude at intervals of 30. Note that Jupiter s angular diameter was 7% larger in December 2000 than in February The color scale is clipped at 1 Rayleigh (R) in both panels in order to emphasize the disk emissions. The auroral emissions are overexposed (maximum brightness goes to about 6 R in auroral regions) in all panels; the disk and auroral emissions are easily separated spatially. The low- to middle-latitude X-ray spectrum was extracted from the region inside both the white rectangle and the white circle (with radius 1.05 R J ). Auroral spectra (north and south) were extracted from the regions inside the white circle but outside the white rectangle. The conversions to Rayleighs was carried out using effective areas of 54.5 cm 2 for the HRC-I data and 76.3 cm 2 for the ACIS-S data. These effective areas were calculated as averages over the nominal energies of Jupiter events for the February 2003 ACIS-S data. 2of16

3 Figure 2. Distribution of counts in latitude measured from the east-west line through the middle of the planet as seen from Chandra (not in System III). The bin size is 10 and the distribution is counts per latitudinal bin; hence the expected distribution for a uniform disk varies as the square of the cosine of the angle from the equator. The data points are summed over all longitudes measured from the north-south line through the middle of the planet as seen from Chandra. There is no need to correct for exposure time as this distribution is over the visible disk. (top) Distribution for all on-planet 2003 ACIS-S ( kev) and HRC-I events. The black curve shows the best fit cos 2 q distribution over the latitude interval ( 45,+45 ). The value of c 2 for this fit is 9.6 for 9 degrees of freedom, with probability of chance occurrence of 38%, indicating an acceptable fit. Deviations from cos 2 q at higher latitudes are due to auroral emission. (bottom) Distributions for 2003 ACIS-S events (black), 2003 HRC-I events (blue), and 2000 HRC-I events (red). The corresponding curves show the best fit cos 2 q distributions over the latitude interval ( 45,+45 ). The values of c 2 for these fits are (2003 ACIS-S 4.7, 2003 HRC-I 14.4, 2000 HRC-I 28.5), with probabilities of chance occurrence (86, 11, 0.08)%. The fits from 2003 are acceptable, that for the 2000 HRC-I observation is not. 3of16

4 Figure 3. (top) Distribution of rates (c/ks in each 0.5 hour bin) versus HA as defined by equation (1) and the best fit to equation (2). The data points include the HRC-I and ACIS-S observations on February Latitudes are restricted to the interval ( 45, +45 ), as measured from the System III equator. (bottom) Residuals to the best fit. for a correlation between low-latitude regions of low magnetic field strength and enhanced X-ray emission [Gladstone et al., 1998] lent additional support to this mechanism, since the loss cone for precipitating particles ought to be larger over regions with weaker surface magnetic field strength. However, Maurellis et al. [2000] showed that elastic scattering of solar X rays by atmospheric neutrals and fluorescent scattering of carbon K-shell X rays 4of16

5 Table 2. Goodness of Fit of Equation (2) to Hour Angle Distribution Data Set d.o.f. c 2 Confidence, % 2003 All e ACIS HRC-I HRC-I from methane molecules located below Jupiter s homopause are also potential sources. This model predicts an X-ray brightness that agrees within a factor of two with the bulk of the low-latitude ROSAT measurements, suggesting that scattering and fluorescence of solar X rays may account for a significant fraction of Jupiter s nonauroral X-ray emission. This solar X-ray scattering mechanism is also supported by correlations of Jupiter s nonauroral X-ray emission with the F10.7 cm solar flux and of the X-ray limb with the bright visible limb [Gladstone et al., 1998]. [6] Bhardwaj et al. [2005a] studied the variability of Jupiter s nonauroral X-ray emission, comparing it to variations in the solar X-ray flux. They noted a solar X-ray flare that matched a feature in Jupiter s low- to middle-latitude X-ray light curve and suggested that the X-ray emission from Jupiter s disk is primarily due to scattered solar X rays. A recent study [Bhardwaj et al., 2005b] of X-ray emission from Saturn, the planet that has an atmospheric composition most similar to Jupiter s, provided convincing evidence for the strong influence of a solar X-ray flare on the X-ray emission from that planet s disk. Cravens et al. [2006] demonstrated that the spectrum measured by CXO at low latitudes is consistent with scattering and fluorescence of solar photons, at least for photon energies below about 900 ev. The modeled intensities were lower than the measured intensities by about a factor of 2 for higher energies, which Cravens et al. [2006] attributed to problems with the solar irradiance model used for these energies and/or the presence of another source intrinsic to the Jupiter. [7] In this paper we present an analysis of the February 2003 high spatial resolution ( half-power radius for the energies of interest for Jupiter) CXO observation of Jupiter focusing on the nonauroral emission. Elsner et al. [2005a] have previously described the highlatitude auroral X-ray emission observed at that time. We describe the morphology and temporal behavior of Jupiter s low- to middle-latitude X-ray emission and compare with the previous CXO/HRC-I 10-hour observation of 18 December We find that the spectrum of Jupiter s nonauroral X-ray emission is distinctly different from that of its X-ray aurora and compare the nonauroral spectrum with a thermal emission model spectrum. 2. Chandra Observations and Data Reduction [8] The CXO observed Jupiter during February 2003 for four rotations of the planet, using both the ACIS-S and HRC-I X-ray cameras. Table 1 provides details of these observations. During the two ACIS-S exposures, separated by the HRC-I exposure, the telescope focus and the planet were placed on the back-illuminated CCD, designated S3, in order to take advantage of this CCD s sensitivity to lowenergy X-rays. Each CCD has four readout nodes, 256 columns of pixels per node. Since the response varies somewhat from node to node, the spacecraft was oriented to allow the planet to move along a single CCD node. The ACIS data were taken using the standard ACIS frame time of s. No repointings were necessary during any of the individual CXO exposures, although the pointings for each exposure were different. The response of the ACIS optical blocking filter has an interference peak in its transmission near 9000 Å (1.4 ev) that can affect ACIS S3 observations of optically bright solar system objects [Elsner et al., 2002]. The steps taken to minimize the effects on the X-ray data from Jupiter are described by Elsner et al. [2005a]. Owing to this necessary procedure, we employ a low-energy cutoff at 0.3 kevand use the standard response matrix for our spectral analyses, keeping in mind that there may still be a tendency to undercount the higher grades events at the low end of our band. Higher-grade events occur when the absorption of a single photon produces charge in more than one pixel. Charge-transfer-inefficiency (CTI) effects are minimal for the back-illuminated S3 CCD, and no CTI corrections were made. For spectral modeling it is necessary to take into account the time-dependent contamination layer on the ACIS optical blocking filter [Plucinsky et al., 2003]. We do this by multiplying the ACIS S3 effective area by an energy-dependent correction factor calculated using the CIAO tool acisabs. As a check, we also carried out the spectral analysis correcting for contamination by using the energy-dependent effective area correction factor calculated using the contamarf tool (H. L. Marshall, private communications, 2003). The spectral modeling results were indistinguishable for these two correction methods. [9] The Very Faint mode used in the ACIS-S observations effectively suppresses the background (see Elsner et al. [2005a] for more details). Therefore we do not subtract background for the ACIS-S exposures. However, for the HRC-I exposures the background was higher and must be taken into account for determination of the planet s count rate. Cosmic rays and radioactive decay within the microchannel plate of HRC-I are the principal sources of HRC-I background and thus are not blocked by the planet. For this we assumed a circular region of 1.2 R J to 5 R J around Jupiter and calculated the count rate (which is 14 counts per ks, c/ks, for the same area as the planet s disk) per unit area. This was then subtracted from the count rate of Jupiter s disk (including the auroral zones) to derive the background subtracted count rate of 45 c/ks. The HRC-I count rate in the nonauroral disk region (see Figure 1) was 28 c/ks. On 18 December 2000 the disk rate was 68 c/ks (82 c/ks for the whole planet including the auroral zones), while the equivalent background rate was 13 c/ks. Thus the background rates are very similar for the two HRC-I observations, but Jupiter s disk was significantly brighter in soft X rays in December 2000 than in February [10] The ACIS S3 and HRC-I data were transformed into a frame of reference centered on Jupiter using appropriate ephemeris data obtained from the JPL HORIZONS ephemeris generator and Chandra orbit ancillary data provided in the data products from the Chandra X-ray Center (CXC). 3. Morphology [11] Figure 1 shows the Chandra HRC-I and ACIS-S X-ray images of Jupiter from February 2003, and, 5of16

6 Figure 4. (top) Rate map for the February 2003 Chandra data, summed over both ACIS-S exposures and the HRC-I exposure, in SIII coordinates, convolved with a two-dimensional Gaussian with s = 10. The white-line contours display the surface magnetic field strength, and black lines crossing the plot from 360 to 0 in the northern and southern hemispheres denote the magnetic footprints of the Io flux tube (R J ) = 5.9), as defined by the VIP4 model of Connerney et al. [1998]. The color bar of the figure is in counts per kilosecond per square degree. Since the assignment of System III longitude is most uncertain near the limb, only events more than 30 longitude from the limb are included. In addition the intensity scale has been clipped at 0.15 times the maximum auroral rate in order to emphasize any variations at low to middle latitudes. (bottom) Map of Jupiter s surface magnetic field strength in System III coordinates using the VIP4 model of Connerney et al. [1998]. Again the white-line contours display the surface magnetic field strength, and black lines crossing the plot from 360 to 0 in the Northern and Southern Hemispheres denote the magnetic footprints of the Io flux tube (R J = 5.9). 6of16

7 Figure 5. Distribution of count rates (c/ks-sq.deg-gauss) versus surface magnetic field strength (gauss; in half gauss bins) for (top) the February 2003 data (includes both HRC-I and ACIS-S) and (bottom) the 18 December 2000 data. The black points are for the full disk, blue points for System III latitudes in the range ( 45, +45 ), and red points for System III latitudes in the range ( 30, +30 ). The green points are for uniform disk emission (with no aurora) assuming the same number of counts. Only events more than 30 in longitude from the limb are included in this analysis. on the same scale, the corresponding HRC-I image on 18 December All images show pronounced X-rays from the north and south auroral regions. After accounting for the 7% decrease in Jupiter s angular diameter, the low- to middle-latitude X-ray emission on 25 February 2003 is dimmer by about 50% compared to that on 18 December The solar F10.7 cm flux (indicator of solar activity) was 192 on 18 December 2000 and 96 on 25 February Thus 7of16

8 Table 3. Count Rates, c/ks-sq.deg-gauss, Versus Surface Magnetic Field Strength, gauss Field Strength Gauss All Latitudes Latitudes in Range ( 45, +45 ) Latitudes in Range ( 30, +30 ) February 2003 HRC-I ACIS-S Data ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± December 2000 HRC-I Data ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± the decrease in Jupiter s low- to middle-latitude disk X-ray emission from December 2000 to February 2003 is consistent with a corresponding decrease in solar activity. [12] The low- to middle-latitude X-ray emission from Jupiter appears relatively uniform to the eye. Figure 2 shows the distributions of counts versus latitude as measured from the east-west line running through the middle of the planet. The low- to middle-latitude points are consistent with the cosine-squared dependence expected from a disk of uniform surface brightness. Departures from the cosinesquared law at higher latitudes are due to the auroral X-ray emission. [13] Gladstone et al. [1998] found a dependence of X-ray intensity on hour angle in ROSAT data. The hour angle (HA) of a point on the projected disk is defined by HA ¼ ½l III ðsunþ l III ðpointþš=15:0 þ 12:0; ð1þ where l III is system III longitude. If Jupiter s rotation axis were not tilted with respect to the sky plane and the subsolar point were identical with the subobserver point, then the distribution in HA would be identical with the azimuthal distribution as measured from the north-south line through the center of the planet, even though the planet is rotating. For a disk of uniform surface brightness, the expected azimuthal distribution varies as the cosine of azimuth. Because of the small tilt of Jupiter s rotation axis with respect to the sky plane (with an inclination to the ecliptic of deg) and the small offset of the subsolar point from the subobserver point (0.08 R J ), the hour angle distribution is not quite the same as such an azimuthal distribution; however, these effects are small enough that for uniform surface brightness the hour angle distribution should be fairly close to that distribution fðhaþ ¼ cos½15ðha 12ÞŠ: ð2þ [14] Table 2 and Figure 3 show, for the combined HRC-I and ACIS-S data from February 2003, the results of fitting the observed HA distribution for latitudes in the range ( 45, +45 ) to equation (2), as measured from the System III equator. The distribution is in count rate space, as it is necessary to correct for the variation of exposure time with HA. The confidence levels listed in Table 2 indicate the probability (in percent) of a good fit, based on statistical errors only. None of the listed fits are particularly good, but this may be due the geometrical effects discussed above. [15] The Chandra data are time-tagged and hence each X-ray photon can be mapped into System III coordinates (latitude and longitude). The top of Figure 4 shows the rate map for the February 2003 data, including both HRC-I and ACIS-S data, in System III coordinates. In this representation, the location of the X-ray aurora near the north and south magnetic poles is readily apparent. The bottom of Figure 4 shows the map of Jupiter s surface magnetic field strength in System III coordinates predicted by the VIP4 model [Connerney et al., 1998]. Comparison of these two maps suggests to us the possibility of a relation between X-ray intensity and surface magnetic field strength at low to middle latitudes. Also, using ROSAT data, Gladstone et al. [1998] found that the low-latitude X-ray intensity from Jupiter was higher in regions of low surface magnetic field strength. [16] In order to explore this possibility further, we bin the data in surface magnetic field strength bins according to the magnetic field magnitude predicted by the VIP4 model at the System III longitude and latitude corresponding to each event. After corrections for exposure time and projected area on the sky, Figure 5 and Table 3 show the results of this procedure for both the February 2003 data and the 18 December 2000 data. Both distributions show significant structure at magnetic field strengths less than 8 gauss, which according to the magnetic field strength map (bottom panel of Figure 4) are not associated with auroral emissions. The overall envelope for these distributions is roughly a rise in 8of16

9 X-ray events in the two ACIS-S observations to allow this more extensive analysis. We therefore compare spectra in the magnetic field strength intervals 3 5 gauss and 5 7 gauss, as shown in Figure 6. Allowing for a multiplicative scale factor, the two spectra are not consistent with each other with 99.9% confidence. Adding a gaussian line at 1.32 kev produces a somewhat better fit, acceptable with 8.18% confidence. The existence of such a line (or blend) at the lowest magnetic field strengths is not unambiguously established by these data. Assuming it is real, examination of the X-ray spectral line databases CHIANTI V4.2 [Dere et al., 1997; Young et al., 2003] and ISIS [Houck and DeNicola, 2000] reveals the most common lines within ±90 ev of 1.32 kev to be those of highly ionized Cr, Fe, Ni, and Co. If we may exclude lines from these heavy atoms, then other possibilities are lines from Ne X, Na XI, and Mg X and XI. These databases do not list any lines of O, C, or S within ±90 evof 1.32 kev. More detailed analysis of these spectral differences requires additional, longer ACIS-S observations of Jupiter. Figure 6. (top) X-ray spectra (c/ks-sq deg-kev), summed over the two ACIS-S observations taken over February 2003, for the magnetic field intervals 3 5 gauss (blue) and 5 7 gauss (red), with events restricted to latitudes in the range ( 45, +45 ) and more than 30 in longitude from the limb. For both spectra, each energy bin contains at least 10 events. Testing whether the two spectra are consistent with each other, except for a multiplicative scale factor, leads to a value for c 2 = for 18 degrees of freedom. Thus we can reject this hypothesis with 99.90% confidence. (middle) The difference spectrum divided by its errors (chi). Note the relatively large excursions (in two cases greater than 3s) between 1.0 and 1.5 kev. (bottom) The difference spectrum divided by its errors (chi), but with a Gaussian line added to the 5 7 gauss spectrum. The line energy determined by the new fit is 1.3 kev with line width comparable to the energy resolution (120 ev) of the ACIS-S. Now we find an improved value for c 2 = for 15 degrees of freedom, acceptable with 8.18% confidence. X-ray intensity with magnetic field strength, which is of course to be associated with the auroral X-ray emission. However, both distributions show a striking drop at field strengths 6 gauss, with a much narrower dip at 12 gauss. The depth and statistical significance of these dips depend on the chosen latitude range. While the numerology may suggest some sort of resonant or harmonic phenomena, the physical origin of these dips is not presently understood by us. These results indicate that the low- to middle-latitude X-ray intensity does correlate with surface magnetic field strength but in a more complicated manner than the simple anticorrelation found in the ROSAT data by Gladstone et al. [1998]. [17] Additional clues might be provided by examining and comparing high-quality X-ray spectra for each magnetic field interval. Unfortunately, there are not enough 4. Temporal Variability and Relation to Solar X-Ray Flux [18] In view of the surprising finding of min quasi-periodic oscillations in the 18 December 2000 Chandra HRC-I data for the northern auroral zone [Gladstone et al., 2002], we searched the February 2003 ACIS-S and HRC-I data for time variability in the low- to middlelatitude X-ray emission. For this analysis we imposed no constraints on System III longitude. The upper left panel of Figure 7 shows the combined ACIS-S and HRC-I X-ray light curve, using 4-min bins, for February 2003 for latitudes in the range ( 60, +60. There is a noticeable rise and fall near the beginning of the HRC-I data. The same behavior is seen in the upper right panel of Figure 7, showing the light curve for the HRC-I background, computed as described in section 2. Presumably, this rise and fall in HRC-I background is due to a disturbance in the solar wind passing by Chandra at this time. We elected to excise the first 197 min of the HRC-I data in order to avoid introducing effects due to this rise and fall into the power spectral density (PSD). The lower left panel of Figure 7 shows the X-ray light curve with the excised HRC-I data removed. [19] Fourier analysis of the unsmoothed time series then produced the PSD shown in the lower right panel of Figure 7. There are no statistically significant peaks for periods from 100 min down to 8 min. In order to understand the structure appearing in the PSD at periods longer than 100 min, we fit a linear trend to the observed light curve as shown in the upper left panel of Figure 8. We carried out 100 Monte Carlo simulations adding Poisson noise about this trend and removing data in the two gaps we have introduced in the X-ray time series, leading to the average simulation light curve shown in the lower left panel of Figure 8. The only time dependences in the simulated light curves arise from the linear trend, the presence of gaps, and Poisson noise. For each simulated light curve, we calculated the corresponding PSD. The lower right panel of Figure 8 shows the average of these 100 PSDs, which can be compared with 9of16

10 Figure 7. Timing results for February 2003 X-ray events in the latitude range ( 60, +60 ). For all light curves the time origin corresponds to UT 1558:06 on 24 February The light curves have been created by 12-min boxcar smoothing of a 4-min binning of the data. The black vertical lines from top to bottom mark the boundaries of excised data (see text). (upper left) X-ray light curve for ACIS-S and HRC-I data. (upper right) Off-planet HRC-I background rescaled to the size of Jupiter, showing the sharp rise and fall near the beginning of the HRC-I exposure. (lower left) Same as upper left but with the anomalous HRC-I interval removed. (lower right) Power spectral density (PSD) versus period (in min), computed from the unsmoothed 4-min binning of the data. The horizontal solid line shows the expectation value for a steady source with Poisson statistics. The dotted lines show the single period probabilities of chance occurrence as labeled on the right. Figure 8. Comparison of observed and simulated light curves and PSDs. (upper left) Observed X-ray light curve with best-fit linear trend. (upper right) PSD for the observed light curve. (lower left) The average simulated light curve (100 trials). (lower right) The average simulated PSD (100 trials). 10 of 16

11 Figure 9. Time evolution of solar X-ray EUV fluxes during February 2003 at 1 AU compared to the Jupiter s low- to middle-latitude X-ray flux measured by Chandra: (a) solar 1 8 Å ( kev) flux measured by GOES 10 with 30 min time bins; (b) solar Å ( kev) and Å ( kev) flux measured by SOHO/SEM with 5 min bins (note the break in the y-axis); (c) solar 5 65 Å ( kev) and 5 25 Å ( kev) measured by TIMED/SEE with 3-min observation-averaged flux obtained every orbit for 15 measurements per day (note the break in the y-axis); and (d) Jupiter s low- to middle-latitude X-ray measured by Chandra with 30 min bins. The Chandra time series is shifted by 4353 s ( in Earth day) in order to correct for light travel time differences from the Sun to Earth and from the Sun to Jupiter to Earth. Also shown in Figure 9d is the daily averaged solar flux in 1 70 Å ( kev) measured by TIMED/SEE (vertical scale on the right). The solid vertical line at 0.95 day marks the time of exposure transition from ACIS-S to HRC-I and that at 1.8 day the transition from HRC-I to ACIS-S. A gap day appears because we expunge ks data taken at the beginning of the second ACIS-S exposure when Jupiter overlapped its location in the second bias frame. the solar X-ray flux incident on the planet. In Figure 9 we display the solar X-ray/EUV flux measured by several Sunobserving satellites during the February 2003 CXO observations, along with the X-ray light curve from Jupiter s low to middle latitudes. These near-earth satellites were facing the same solar hemisphere as Jupiter within 22 ; the Sun-Earth-Jupiter angle was 153 with Jupiter trailing. The solar X-ray flux clearly rises on average throughout the period February The trend for the CXO observations is less obvious, in part because the middle 20 hours of data are from the HRC-I and the first and last 8.5 hours are from the ACIS-S, and the two cameras have different responses. However, it is clear that the X-ray flux in the second ACIS-S exposure is stronger than in the first ACIS-S exposure, showing an increase 60% This is similar to the increases in the GOES 10 data (80%) and the TIMED/SEE data (40%). However, the SOHO/SEM Å and Å fluxes show increases of only 5%. On the other hand, in these bands the solar X-ray flux is dominated by emission at lower energies where variability is less. Solar activity was at a low level during February 2003, and no flare in the C class or greater was observed. As then expected for scattering and fluorescence of solar X-rays, no large variation in Jupiter s low- to middle-latitude X-ray emission was observed. Such variations in response to solar X-ray flares have been reported at other times for Jupiter [Bhardwaj et al., 2005a] and for Saturn [Bhardwaj et al., 2005b]. [22] Figure 10 compares light curves at 60 min binning for Jupiter s low- to middle-latitude X-ray emission on 18 December 2000 with the corresponding GOES 10 solar flux data in the 1 8 Å and Å bands. At this time, the GOES 10 satellite was viewing the same hemisphere of the Sun as Jupiter within an angle of 19 (the Sun-Earth-Jupiter angle was with the Jupiter trailing, and the Sun- the observed PSD repeated in the upper right panel. We conclude that the structure in the observed PSD for periods longer than 100 min is due to the rising linear trend interacting with the two gaps we introduced into the data. [20] A similar procedure applied to the 18 December 2000 HRC-I data also found no evidence for periodic variability in the range min. These null results would certainly be expected for emission due to scattering and fluorescence of solar X-rays. [21] According to Bhardwaj et al. [2005a], longer timescale variability in the X-ray emission measured from Jupiter s low to middle latitudes during the November 2003 XMM-Newton observations was similar to that in Figure 10. Light curves (60-min binned) for the Chandra HRC-I observation of Jupiter disk X-rays and for GOES 1 8 Å and Å solar fluxes on 18 December The GOES data are shown at the midpoint of the 1-hour bin. The time of Jovian X-rays is shifted by 4083 s to account for light travel time delay between Sun-Jupiter-Earth and Sun-Earth. Note that the GOES Å solar flux is plotted after multiplying by a factor of of 16

12 Figure 11. Jupiter s auroral X-ray spectra compared to the low- to middle-latitude X-ray spectrum observed during the two Chandra ACIS-S exposures on 24 February 2003 and February 2003: (a) north auroral spectrum; (b) south auroral spectrum; and (c) low- to middle-latitude spectrum. Note the different scales on the vertical axes. The three regions for spectral analysis are shown in the bottom panels of Figure 1. The two ACIS-S exposures were of nearly the same duration and taken 1 day apart (see Table 1). For the displayed spectra, ACIS-S energy channels were grouped together so that each point represents at least 10 events. Jupiter s low- to middle-latitude X-ray spectrum is noticeably harder than the auroral spectra, extending to higher energies. Jupiter-Earth angle was 5.3 ). The errors for the Jupiter X-ray light curve are large, but it may show a similar trend to that in the solar data. 5. Jupiter s Low- to Middle-Latitude X-Ray Spectrum [23] Figure 11 shows Jupiter s low- to middle-latitude X-ray spectra obtained during each of the two ACIS-S exposures (separated by about a day and each of 8.5 hour duration), together with the spectra from the north and south auroral zones. The ACIS-S background rate derived from the kev events located outside 1.2 Jupiter radii and scaled to the area of the planetary disk is less than 3% of the emission from the total disk. In addition, the background contribution from true X-rays from beyond Jupiter s orbit is blocked by the planet. We therefore neglect the background in our spectral analysis of the ACIS data. [24] Jupiter s low- to middle-latitude X-ray spectra peak at and extend to higher energies than the auroral spectra, raising the possibility that different physical mechanisms are responsible for the X-ray emission in each case. In addition, the two low- to middle-latitude X-ray spectra, separated by about 1 day, are different from each other, with increased X-ray emission above 0.6 kev in the second exposure. From Figure 9 we note that the TIMED/SEE 5 25 Å and Å solar fluxes increased by 40% and 20%, respectively, between 24 and 26 February The increase in the hardness of the nonauroral X-ray emission could thus be due to an increase in the hardness of the incident solar X-ray flux, as suggested by Figure 9, where the degree of flux increase between beginning and end of the observations is larger at higher energies. Because of this change in spectral shape, we fit model spectra to the two exposures separately. In order to obtain reasonable fits without invoking overly complicated models, with questionable physical applicability, we reduced the energy range for spectral analysis, covering 0.5 to 1.5 kev for single temperature fits and 0.3 to 1.5 kev for two temperature fits. There is very little flux measured above 1.5 kev, although there is measurable flux at least down to 0.3 kev, below which the data are affected by the procedures described in section 2. We use the X-ray astronomy spectral fitting code XSPEC [Arnaud, 1996] to fit model X-ray spectra to the spectral data. [25] A simple thermal bremsstrahlung model in XSPEC gives a very poor fit to the kev nonauroral X-ray 2 spectra, with values of c red (c 2 per degree of freedom) 10 for each exposure. We therefore turned to the MEKAL model ( xspec/manual/xsmodelmekal.html) based on model calculations of X-ray emission from a hot, optically thin plasma in ionization equilibrium [Mewe et al., 1985; Mewe et al., 1986; Kaastra, 1992; Liedahl et al., 1995] (see also Arnaud and Rothenflug [1985] and Arnaud and Raymond [1992] for the adopted ionization balance). The MEKAL model, which allows some choices for abundances and cross sections, is widely used in X-ray astronomy to fit the X-ray spectra from stellar coronae, and we expect it to provide a reasonably close approximation to the solar X-ray spectrum incident on Jupiter. To the extent that Jupiter s low- to middle-latitude X-ray spectrum arises from scattering and fluorescence of solar X rays in its atmosphere and to the extent that X-ray interactions in Jupiter s atmosphere preserve the incident spectral shape, the MEKAL model would be a reasonable choice for fitting Jupiter s low- to middle-latitude X-ray spectrum (but see Cravens et al. [2006] for discussion of X-ray albedos for the outer planets). In our case, using default (solar) abundances and cross sections, the MEKAL model does provide fits much improved over those using a simple thermal bremsstrahlung model. Including a gaussian line with line center fixed at 1.35 kev and zero intrinsic width gives even better results. Table 4 lists the parameters of the best-fit models, while Figure 12 compares the best-fit models (including the line) with the spectral data. For comparison, Table 4 includes the best-fit parameters for the April 2003 XMM-Newton data [Branduardi-Raymont et al., 2004]. [26] The kev low- to middle-latitude energy fluxes implied by these fits are and erg/s cm 2 for the first and second exposures, respectively. The corresponding X-ray luminosities of 12 of 16

13 Figure 12. Fit of model X-ray spectrum (MEKAL plus gaussian line at 1.35 kev, see text), with residuals, to Jupiter s low- to middle-latitude X-ray spectra measured by Chandra ACIS-S on 24 February 2003 (top) and February 2003 (bottom). Each spectral point represents at least 20 events. Best-fit parameters are given in Table of 16

14 Table 4. Best Fit Parameters for Jupiter s Low- to Middle-Latitude X-Ray Spectra Model a c 2 red b dof c kt 1, kev d Norm 1 e M M + L M + M M + M + L OBSID 3726 g kt 2, kev d Norm 2 e h h Line Strength f M M + L M + M M + M + L OBSID 4418 i h h M+L+L k XMM j a Spectral models fitted to the data: M = MEKAL; M + L = MEKAL + gaussian line; M + M = MEKAL + MEKAL; and M + M + L = MEKAL + MEKAL + gaussian line. The energy range for fits to single and double MEKAL models were kev and kev, respectively. Thus the number of spectral bins differs for single and double MEKAL model fits. b Reduced c 2 equal to the best-fit value for c 2 divided by the number of degrees of freedom. c Number of degrees of freedom equal to the number of spectral bins minus the number of free parameters. d Temperature in kev of the MEKAL model. e XSPEC normalization of the MEKAL model in 10 6 photons/s cm 2. f Total line photon flux in 10 6 photon/s cm 2 for a gaussian line with line center fixed at 1.35 kev and with zero intrinsic width. g The first ACIS-S exposure on 24 February h Since this temperature lies well below our low energy cutoff of 0.3 kev, XSPEC was unable to set a lower error bar for this parameter. i The second ACIS-S exposure on February j The XMM observation on April 2003 [Branduardi-Raymont et al., 2006b]. k The second line in the XMM spectral model is Si XIII at 1.86 kev and 0.39 GW, respectively, show a factor of two increase in just 1 day. For comparison, the X-ray luminosity for the northern auroral zone was 0.68 GW during the first ACIS-S exposure [Elsner et al., 2005a]. Adding a second narrow Gaussian line at 1.2 kev slightly 2 improves the fit for the second exposure (c red = 1.2 for 23 degrees of freedom). For the MEKAL plus one line fits, the best-fit temperature for the second exposure is 23% higher than for the first exposure. As already pointed out, this harder spectrum appears to correlate with increased hardness of the incident solar X-ray flux. [27] According to the X-ray spectral line databases CHI- ANTI V4.2 [Dere et al., 1997; Young et al., 2003] and ISIS [Houck and DeNicola, 2000], the most common lines within ±90 ev of 1.35 kev are those of highly ionized Cr, Fe, Ni, and Co. Excluding lines from these heavy atoms, other possibilities are lines from Ne X, Mg X, and Mg XI. The line due to Mg XI ( 1 S 1 P 0 ) emission is a strong line in the solar X-ray spectrum and so scattering of photons in this line may account for the possible line emission. These databases do not list any lines of O, C, or S within ±90 ev of 1.35 kev. [28] Recently, Cravens et al. [2006] compared the low- to middle-latitude ACIS-S spectra to model spectra for scattering and fluorescence of solar X-rays from Jupiter s upper atmosphere, finding reasonable agreement in the energy band kev. However, their models fall below the data by a factor of about 2 in the kev band. One reason could be the known variability of the incident solar X-ray spectrum. A true test of their model would involve comparing the measured spectrum with the solar spectrum averaged over the same time interval as the X-ray observations (taking light travel time into account). The solar X-ray flux can change by factors of a few to a few tens over the solar cycle and it can increase by factors of hundreds to thousands during an X-ray flare [cf. Peres et al., 2000]. 6. Summary [29] The low surface brightness of Jupiter s nonauroral X-ray emission hampers our ability to determine the physical processes which are responsible for it. Our results point to scattering (and fluorescent emission from carbon in methane molecules [Cravens et al., 2006]) of the incident solar X-ray flux as an important mechanism. The nonauroral X-ray spectra in the kev energy band can be fitted by thermal emission from a hot optically thin plasma in ionization equilibrium, a model representative of the incident solar X-ray spectrum. The X-ray flux from low to middle latitudes on Jupiter varies on timescales from days to years in a manner similar to variations in the solar X-ray flux as measured by near-earth satellites and by the F10.7 cm proxy for solar activity. On the other hand, Cravens et al. [2006] found that applying a calculated albedo function to an assumed incident solar X-ray spectrum provides a bad fit at energies above 0.8 kev. We find evidence for temporal correlation between Jupiter s observed nonauroral X-ray emission and solar fluxes measured at Earth, although this result could be put on firmer ground by, for example, the observation of a large flare (such as was seen at Saturn [Bhardwaj et al., 2005b]). [30] Because of Chandra s superb spatial resolution and time-tagging of events, we were able to construct relationships between the X-ray intensity from Jupiter in System III coordinates and the surface strength of Jupiter s magnetic field. As shown in Figure 5, the correlation is more complicated than the simple anticorrelation reported from ROSAT observations [Gladstone et al., 1998]. There appears to be no reason that scattering and fluorescence of 14 of 16

15 solar X-rays should lead to such a correlation. Such behavior could result from increased particle precipitation from radiation belts closer to the planet than in other regions. This component of the low- to middle-latitude X-ray emission would then map out regions similar to the South Atlantic Anomaly in the Earth s radiation belt. There is also an apparent difference in the spectra for regions with surface fields in the 3 5 gauss range and those with surface fields in the 5 7 gauss range. The lower field regions show excess emission for several tens of ev around 1.32 kev. However, particle precipitation seems unlikely to account for all, or even most, of Jupiter s nonauroral X-ray emission as we would not expect it to correlate temporally with the incident solar X-ray flux. [31] In conclusion, we have found evidence for two mechanisms possibly being responsible for Jupiter s lowto middle-latitude nonauroral X-ray emission. We suggest that scattering and fluorescence of the incident solar X-ray flux in Jupiter s atmosphere normally predominates, producing the observed hard spectrum and temporal correlations; in addition, we propose the existence of a component from particle precipitation in regions of low surface magnetic field strength. [32] The ideal way to investigate Jupiter s nonauroral X-ray emission in more detail and resolve the outstanding questions about its origin would be simultaneous high statistical quality, spatially resolved spectral measurements of both the emission from the planet, and the solar X-ray flux incident on it which would allow an effective scattering albedo to be determined and compared with theoretically calculated albedos [i.e., Cravens et al., 2006]. [33] Acknowledgments. We thank Tom Wood for help in providing the TIMED/SEE Version 7 Data Products. The solar SEM data were taken with the CELIAS/SEM experiment on the SOHO spacecraft which is a joint ESA and NASA mission. 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