Modeling of Jupiter s auroral curtain and upper atmospheric thermal structure

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2010ja016037, 2011 Modeling of Jupiter s auroral curtain and upper atmospheric thermal structure I. J. Cohen 1 and J. T. Clarke 1 Received 17 August 2010; revised 1 May 2011; accepted 9 May 2011; published 3 August [1] A three dimensional simulation that takes into account the altitude extent of auroral emission along the line of sight has been applied to the analysis of auroral images of Jupiter taken by the Hubble Space Telescope (HST). The simulation s input auroral emission profile with altitude was expanded by an emission scale height factor ranging from n = 1 to n = 4 representing different vertical extents of the auroral emission (i.e., one to four times greater than the theoretical emission profile from Grodent et al. (2001)). A radial cut of the auroral brightness averaged over an angle of 5 was compared between the original HST images and each of the series simulation outputs. We found that four of the five northern series showed the original image s emission profile well bounded by the n = 1 and n = 2 simulation outputs, while four of the five southern series showed the original image s auroral emission profile better correlated with the n = 3 simulation. This hemispheric difference could be the result of increased heating or a different distribution of influx electron energies at the southern hemisphere. The discovery of this hemispheric temperature difference supports predictions made by a three dimensional thermospheric wind model and makes a strong case for further study of the effects of Joule heating on the auroral regions. Citation: Cohen, I. J., and J. T. Clarke (2011), Modeling of Jupiter s auroral curtain and upper atmospheric thermal structure, J. Geophys. Res., 116,, doi: /2010ja Introduction 1 Center for Space Physics, Boston University, Boston, Massachusetts, USA. Copyright 2011 by the American Geophysical Union /11/2010JA [2] Jovian auroral emission in the ultraviolet was first detected with Voyager 1 s UV spectrometer by Broadfoot et al. [1979]. It was soon followed with more data from Voyager 2 [Sandel et al., 1979], and since then the aurora has been studied extensively by the International Ultraviolet Explorer [Clarke et al., 1989], the EUV and EUVS instruments on the Galileo spacecraft [Ajello et al., 1998], and most recently with the Hubble Space Telescope (HST) by several groups as detailed in a review chapter by Clarke et al. [2004]. Broadfoot et al. [1979] suggested a total auroral input flux on the order of W in each hemisphere, roughly 3 orders of magnitude stronger than the input flux seen on Earth. Globally, this auroral energy input exceeds the solar UV flux absorbed by the upper atmosphere across the planet by a factor of depending on solar activity. Thus, the Jovian upper atmosphere is energetically driven by the aurora rather than by absorbed sunlight, as on Earth [Clarke et al., 2004]. [3] Unlike the terrestrial aurora, which is modulated by the solar wind, the Jovian aurora consists of three auroral emission regions (Figure 1) which are physically separated from each other and vary independently, implying independent processes driving those emissions [Clarke et al., 2004]. The emission in the main auroral oval is driven by the enforcement of corotation of ions originating from the Io plasma torus. As the plasma diffuses radially away from the planet, its angular velocity drops due to conservation of angular momentum, and a large scale system of currents is set up which tends to accelerate the plasma back to corotation. It is the upward component of this current system, which is associated with downward precipitating electrons, that is thought to drive the emission [Cowley and Bunce, 2001; Hill, 2001; Nichols and Cowley, 2003]. The variable polar emission that is found in the region poleward of the main auroral oval is more likely mapping to the openclosed field line boundary and open field region [Clarke et al., 1980]. Finally, the satellite footprint emissions and trails seen outside the main oval are caused by atmospheric collisions of energetic particles accelerated along magnetic field lines connected to Galilean satellites. [4] In addition to the auroral emission, this energy input results in heating and ionization of the Jovian thermosphere. Energetic electrons traveling along the planet s magnetic field lines transfer their energy to H 2 molecules in the upper atmosphere through collisions. Ajello et al. [2001] suggest an electron distribution in the auroral atmosphere with three components: (1) a low energy tail resembling a power law from 0.01 to 0.2 kev, (2) a primary Maxwellian distribution with characteristic energy of kev, and (3) a highenergy tail in the form of a distribution as measured by the Galileo energetic particle detector (EPD) [Mauk et al., 1of8

2 Figure 1. Log scaled image showing the three emission regions of Jovian aurora seen in an image from the 2007 HST auroral imaging campaign. 1996, 1999] with a characteristic energy of 15 kev. Yelle and Miller [2004] suggest that the hard or high energy electrons produce auroral emissions from relatively deep levels, near the base of the thermosphere. The peak of the UV emission was found to be just above the homopause, 245 km from the 1 bar pressure level [Vasavada et al., 1999; Ingersoll et al., 1998]. [5] Grodent et al. [2001] summarized measurements of rovibrational H 2 temperatures determined from observation of high resolution ( Å) UV spectra obtained with the Goddard High Resolution Spectrograph (GHRS) on board HST [Clarke et al., 2004;Trafton et al., 1994, 1998; Liu and Dalgarno, 1996; Kim et al., 1997; Dols et al., 2000]. The H 2 temperature reflected the temperature along the line of sight weighted by the auroral volume emission rate. It was found to vary between 300 and 700 K with little correlation to the H 2 emission brightness. The Galileo Plasma Science (PLS) instrument has observed precipitation of electrons with energies of 0.1 to 10 kev between 6.8 R J and R J toward both hemispheres [Frank and Paterson, 2002], corresponding to a precipitated power about W/m 2 in the auroral atmosphere. Such a large energy input in narrow auroral regions along the main oval can also strongly modify the upper atmosphere and ionosphere with consequences for the ionospheric conductance and magnetosphere ionosphere coupling processes [Millward et al., 2002; Nichols and Cowley, 2004]. [6] The 2007 auroral imaging campaign consisted of 128 HST orbits of observations of auroral emission on the gas giants Jupiter and Saturn. This survey has yielded important results [Bunce et al., 2008; Clarke et al., 2009; Nichols et al., 2009; Gérard et al., 2009; Wannawichian et al., 2010]; however, the analysis of the auroral emissions has been limited by their two dimensionality. Previous analysis of the two dimensional images did not take into account the third dimension of depth along the line of sight and therefore did not allow accurate interpretation of where the auroral emission actually occurs on the three dimensional planet. A new simulation presented in this paper allows the application of a theoretical auroral emission profile that takes into account both altitude above the limb and depth along the line of sight of the HST images. This new dimension of altitude allows the simulation to more accurately reproduce the electrons degradation of energy and subsequent emission falloff with altitude (emission curtain). Successful mapping of the auroral emission profile with altitude can reveal information about the thermal structure of the upper atmosphere and the energy distribution of incoming particles in the auroral regions. Gaining an accurate auroral emission profile can allow a better understanding of the distribution of the precipitating particles and the extent to which the atmosphere is heated via collision of the primary particles or Joule heating. A previous, but unpublished version, of the simulation was updated to allow the application of different emission scale height factors (representing different thermal structures) in the atmosphere. This study focuses on the auroral emission scale height above the peak, whose altitude is not determined with sufficient accuracy to determine the characteristic energy of the incoming particles. The thermal structure of the upper atmosphere is then the main factor affecting the emission altitude profile, with an added contribution from the particle energy distribution. We interpret the simulation output with the best fit to the original HST image as most accurately reflecting the upper atmospheric thermal structure, and have searched for extended high altitude emissions beyond the model as evidence for a soft particle population. The energy distribution of these soft particles is not well constrained. 2. Simulation [7] First, ten time series (five of each hemisphere) were selected with the number of images in each ranging from 8 to 30. The number of images in each time series varied substantially because of the geometry requirement. All of the image series were selected because they best presented the emission of the main auroral oval running parallel to the limb of the planet. This orientation was desirable because it ensures the thinnest emission depth along the line of sight, limiting complications from the spherical geometry. For each series of images, lat/long projections of each individual image were overlaid with each other and averaged, essentially eliminating the significance of the number of images in each series. The theoretical altitude emission profile for Jovian auroral ultraviolet emission created by Grodent et al. [2001] (see Figure 2) was input into the simulation routine and used to calculate the emission brightness at every point in three dimensions. This simulation includes a parameterization of the Grodent model of emission versus altitude based on an input emission scale height factor. The simulation routine algorithmically calculates angles to the line of sight and integrates the brightness along each line of sight 2of8

3 Figure 2. Theoretical auroral emission curtain (Chapman profile) from Grodent et al. [2001] that will be used as the primary input for our three dimensional simulation (solid line) and the same profile after being convolved using the 125 nm point spread function (dashed line). assuming optically thin emission and an optically thick absorption at and below the homopause. This thick homopause creates a planetary disk on top of which the auroral emission curtain is seen in the output image. The Grodent volume emission profile used in this work results from the loss of energy of the primary particles that enter the top of the atmosphere, and most of the emission is produced by secondary electron collisions [Grodent et al., 2001]. [8] After the line of sight integration is complete, the resultant two dimensional output image is oriented to match the angle of view of the original HST image. The simulation output takes into account emission extent in altitude (mirroring the theoretical profile) including emission above the limb of the planet. Figures 3a and 3c show HST images from a series taken on February 24, 2007 of the northern and southern aurora respectively. Figure 3b shows the simulation output for the HST image shown in Figure 3a with an emission scale height factor (explained in section 3) of n =1. Figure 3d shows the simulation output for the image shown in Figure 3c with an emission scale height of n = 2. Note the difference in emission extent between the n = 1 simulation output in Figure 3b and the n = 2 output in Figure 3d. 3. Analysis [9] The main objective in creating this simulation is to obtain an observation based altitude profile of the brightness of the auroral emission. The simulation output s emission profile can then be compared to the emission profile of the original HST image to find the most accurate emission scale height. The emission scale height is the vertical distance over which the emission strength falls by 1/e, comparable to the atmospheric scale height and expressed similarly as HðÞ¼kT z = Mg where k is the Boltzmann constant, T is the temperature in K, M is the mean molecular mass of the atmospheric particles, and g is the acceleration due to gravity. Knowing the correct auroral emission scale height (of the order of 500 km) will allow us to reach conclusions about the atmosphere s thermal structure and the energy distribution of the incident particles. [10] Analysis of the simulation model began with a convolution to simulate HST s imaging response. Only images ð1þ taken with HST s 125 nm filter (band pass: nm) were selected for simulation to avoid bright Lyman a emission, and so that H 2 could be isolated as the optically thin emission. A 0.08 arcsec point spread function for the 125 nm filter was created from an HST field of reference stars and tested on a simulated planetary disk of uniform brightness without any auroral emission. A small fractional base level of light ( 10 4 ) remained at the wings of the point spread function, but was subtracted off so that the convolved test disk reproduced the brightness profile from a nonauroral region of an actual HST image. This removes the ambiguity between light from the planet disk scattered in the ACS instrument and faint auroral emissions at high altitudes. [11] All of the series included images where the auroral curtain of the main oval was near the limb of the planet; this allowed for easy determination of the brightness falloff in the atmosphere above the homopause (i.e., the region of optically thin atmosphere directly above the planet s optically thick disk in the images). Preferentially selecting Figure 3. (a) A log scaled HST original image of the northern aurora. The white wedge indicates the 5 auroral region of interest. (b) The simulation output of the same HST image seen in Figure 3a with an emission scale height factor of n =1,or1 Grodent et al. s [2001] theoretical profile, as indicated by equation (2). (c) A log scaled HST original image of the southern aurora. Again, the white wedge indicates the 5 auroral region of interest. (d) The simulation output of the same HST image as Figure 3c with an emission scale height factor of n =3, or 3 Grodent et al. s theoretical profile as indicated by equation (2). Note how the atmospheric extent of the auroral emission changes when the scale height factor is changed between Figures 3b and 3d. 3of8

4 Figure 4. (a e) Auroral emission profiles for five northern hemisphere image series. The solid line represents the emission profile of the original HST image, while the dashed lines represent the simulation outputs with n = 1 through n = 4. In Figures 4b 4e, note that the original image lies within the boundaries of n =1andn = 2 for four of the five northern series. The HST image s emission profile in Figure 4a shows better correlation with the n = 3 simulation emission. images where the emission curtain was clearly displayed near the limb of the planet helped us to limit the region from which the emission occurred and avoid emission that might be beyond the limb of the planet that would otherwise have added to the line of sight intensity. [12] We applied the point spread function convolution to the three dimensional simulation output image that came from each series lat/long composite projection, and then took a radial profile of brightness from the center of the planet outwards averaging over an angular width of 5 (see Figure 3). Background residuals from the original HST image were removed by subtracting off the radial profile of a region of the planet without aurora. 4of8

5 Figure 5. (a e) Auroral emission profiles for five southern hemisphere image series. The solid line represents the emission profile of the original HST image, while the dashed lines represent the simulation outputs for n =1throughn = 4. Note that only one of the series (Figure 5d) shows the HST image between the n = 1 and n = 2 boundaries that was seen for the northern hemisphere in Figure 4. The other four images (Figures 5a 5c and 5e) are composed of two separate components: a very hot lower component which correlates to an emission scale height of n = 4 or higher from 247 to 1000 km and a hot upper component which correlates to an emission scale height of n = 3 at altitudes above 1000 km. [13] The parameterization of added heating in the upper atmosphere proceeded as follows. The several hundred km altitude resolution of the data, combined with the long path lengths through the auroral emission regions, do not permit us to determine the fine scale altitude structure of the auroral atmosphere. We can characterize the general altitude extent of the auroral emission at high altitudes, and assign a corresponding characteristic temperature. We have done this in 5of8

6 Figure 6. Parameterized temperature profiles characteristic of the northern pole (dotted line), which has an emission scale height factor of n = 1.5, and the southern pole (dashed line), which has a factor of n = 3. Each is compared to the input Grodent et al. [2001] theoretical profile at n = 1 (solid line) via equation (3). comparison with the Grodent et al. [2001] model of the UV auroral emission versus altitude, which is tied to an assumed altitude profile of the atmospheric temperature. The parameterization is based on the requirement that incident particles of a given energy stop after passing through a constant column of atmosphere, while the entire atmosphere expands with altitude as the temperature increases. Since the vertical column equals the number density times the neutral scale height, the column is directly proportional to the scale height and thus the temperature. The emission profile with altitude will thus scale linearly with temperature. [14] Simulated profiles of emission versus altitude were thus created by multiplying the Grodent profile of emission versus altitude by a constant IðÞ¼I z 0 Fz ðþwhere FðÞ¼Grodent z distribution n where I(z) is the intensity of photons per volume (equivalent to the emission intensity), I 0 is the peak emission intensity, and n is our emission scale height factor. This parameterization does not take into account the detailed changes in the thermal structure of the upper atmosphere that would result from the deposition of heat from different energy distributions of incident particles. Such a treatment would be a logical next step for the analysis but is beyond the scope of this work. 4. Results [15] Figure 4 shows a comparison of the emission profile of the original HST image (solid line) from the peak auroral emission near the homopause ( 240 km) to 3000 km and the simulation outputs with emission scale height factor values of n = 1 through n = 4 (dashed lines) for five northern hemisphere image series. Figure 5 shows the original HST image compared with outputs of n = 1 through n = 4 for five southern hemisphere series over the same range. [16] The comparison in Figure 4 shows that the original HST image lies well within the boundaries of the n = 1 and n = 2 simulation outputs for four of the five northern series, ð2þ suggesting that the appropriate emission scale height value is between n = 1 and n = 2. This implies that actual temperatures at the northern pole may be slightly higher than predicted by Grodent et al. s [2001] theoretical model. Additionally, emission seen as high as 3000 km ( 10 kr) seems to agree with atmospheric data obtained from the model presented by Chaufray et al. [2010] that predicts a sufficient density of H 2 at that altitude ( cm 3 ) to allow for collisions to create the faint emission seen at the higher altitudes. However, due to the low number of counts that can be obtained at this altitude, we cannot draw further conclusions about the contribution of a soft particle distribution. [17] Figure 5 shows that the emission profiles of the HST images of the southern pole are not bounded by the n =1 and n = 2 simulation outputs as the northern hemisphere images were. Four of the five southern series display a twocomponent profile composed of a very hot lower component, which resembles an emission scale height of n =4or higher from km, and a hot upper component which resembles an emission scale height of n = 3 at altitudes above 1000 km. This resemblance with higher scale height factors implies that temperatures at the southern pole are higher than those at the northern pole. Unfortunately, we still lack the imaging ability to fully resolve the emission curtain and interpret the thermal structure of the low altitude emission below about 1000 km and thus cannot make a strong conclusion about this very hot lower component. However, based on the high altitude emissions seen in the HST images, it does appear that the emission scale height factor in the southern auroral region is one to three times higher than its northern counterpart in the region z km. Figure 6 shows a first order parameterization of the resultant temperature profiles of the northern and southern poles (dotted and dashed lines, respectively) compared to the discrete thermal profile from Grodent et al. [2001] (solid line) by TðÞ¼T z 0 Gz ðþwhere GðzÞ ¼ Grodent thermal profile n where T(z) is the atmospheric temperature as a function of altitude, T 0 is the peak atmospheric temperature, and n is again the emission scale height factor. As with the emission scale height factor, this preserves the column of molecules through which an incoming particle passes before stopping, but does not take into account the added heat deposited in the atmosphere. We approximated the northern hemispheric value of n = 1.5 since a precise value of the emission scale height factor is not known. Figure 6 shows the northern temperatures roughly 500 K higher near 800 km and the southern temperatures roughly 1700 K higher than the discrete auroral cases proposed by Grodent et al. [2001]. The southern hemispheric results are also significantly higher than the temperature profiles created by Melin et al. [2006], who used their own scaling factor of the Grodentetal. [2001] thermal profile to model an auroral heating event observed in the IR. [18] We have also investigated the nature of the high altitude extent of the auroral curtain. The slopes of the HST images emission profiles increase just above 1000 km and take on slopes characteristic of higher temperature emission. ð3þ 6of8

7 This high altitude emission could be a superthermal component, in addition to the known upper atmospheric thermal excess recounted by Yelle and Miller [2004]. Count rates are sufficient for the region from 1300 to 3000 km to allow us to conclude that this increase in slope is real; however, due to our inability to obtain sufficient count rates at altitudes greater than 3000 km we can present this evidence for, but not the specific nature of, a superthermal component. 5. Discussion [19] The hemispheric temperature variation seen in our study could be explained by two possible phenomena. The first could be an increase in atmospheric heating in the southern polar region, or inversely that the northern polar region is relatively cooler because of either a lower heating rate or more rapid loss of heat. The second possibility is that the southern polar region receives a different influx of electron energies than the northern polar region. The southern auroral region could see more soft particles heating the upper atmosphere, or a softer overall energy distribution. [20] The circulation model of Jupiter s thermosphere used by Majeed et al. [2009] found a global asymmetry in the planet s thermospheric wind system that blows hydrogen to a preferential position in the northern hemisphere (see Bougher et al. [2005] for details). This wind driven realignment of H 2 molecules lessens Joule heating (heat produced by the nonemission causing electrons with energies below 10 ev) in the northern hemisphere. The obvious difference in temperature between the two hemispheres found by analysis of the simulation outputs could be attributable to the influence of this restraint of Joule heating in the northern hemisphere. Whether or not this is the correct explanation, the results do indicate that the Grodent et al. [2001] model of the auroral thermal structure falls a factor of two or three short in its prediction of the correct atmospheric temperature profile when applied to Jupiter s southern auroral hemisphere. [21] If the difference in altitudinal emission distribution seen between the two hemispheres is simply a result of a larger number of incident electron collisions, either hard or soft, then assumptions made about the global thermal structure may still be correct. The increased heat seen at the southern pole might be a direct result of higher or more concentrated auroral emission. Yelle and Miller [2004] agree that low energy electron precipitation such as this would provide a source for the upper atmospheric thermal excess, but there is no explanation for why this increased electron population would exist only at the southern pole. [22] The derived high temperatures are not consistent with recent H 3 + results from Lystrup et al. [2008] nor those found by Melin et al. [2006] which both agree with the thermal profile found by Grodent et al. [2001], the latter treating cases only differing from Grodent et al. by about 20%. One possible explanation lies in the differences between the UV and IR observations. Imaging in the UV is sensitive to layered emission curtains where the emission is being produced by incident electrons. These separate emission curtains are washed out when viewed from Earth, but they dominate the signal in the observed UV images. UV images thus sample the regions of the atmosphere where the energy is being deposited, and reflect those higher temperatures. By contrast, the H 3 + images are sensitive to number density and temperature of the ionosphere, and present the average temperature over the entirety of the auroral ionosphere, including both the auroral emission curtains and the neighboring ionosphere that is only indirectly heated by the incident auroral particles. This could explain in part the difference in apparent thermal profiles in the UV and IR. 6. Conclusion [23] We have created a three dimensional simulation that allows a further analysis of HST images of Jovian auroral emissions. Using this simulation, we find that the emission profile of the northern aurora demonstrates a thermal structure similar to the profile theorized by Grodentetal. [2001], while the southern aurora has a structure which shows temperatures a factor of three larger than Grodent et al. predict. Majeed et al. [2009] predicted this hemispheric temperature difference using a three dimensional thermospheric model. [24] A difference in temperature between the two hemispheres could be caused by preferential alignment of H 2 molecules at the northern hemisphere that lessens Joule heating (as suggested by Majeed et al. [2009]) or by an unexplained larger influx of incident electrons at the southern hemisphere which would increase local heating (as explained by Yelle and Miller [2004]). The increased slope at high altitudes suggests a superthermal component in the high altitude atmosphere, but low count rates at high altitudes limit us from reaching specific conclusions about its nature. [25] The first three dimensional analysis of HST images of the Jovian aurora agrees with predictions made by threedimensional models of the Jovian thermosphere. The northsouth temperature difference found by our HST simulation makes a strong case for future study of the effects of collisional heating on auroral regions by soft non emission causing electrons. The thermal variation also serves as motivation to develop updates and advances to models of auroral thermal structure that allow response to atmospheric heating. [26] Acknowledgments. This work is based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the AURA Inc. for NASA. We acknowledge the contributions of Juwhan Kim and Dan Golembeski to earlier versions of the simulation program and Denis Grodent for model data contributed to this analysis. This research was supported by NASA grants HST GO A and HST GO A from the Space Telescope Science Institute to Boston University. [27] Masaki Fujimoto thanks Takeshi Sakanoi and two other reviewers for their assistance in evaluating this paper. References Ajello, J., et al. (1998), Galileo orbiter ultraviolet observations of Jupiter aurora, J. Geophys. Res., 103(E9), 20,125 20,148. Ajello, J. M., D. E. Shemansky, W. R. Pryor, A. I. Stewart, K. E. Simmons, T. Majeed, J. H. Waite, G. R. Gladstone, and D. 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Miller (2004), Jupiter s thermosphere and ionosphere, in Jupiter: The Planet, Satellites and Magnetosphere, edited by F. Bagenal, T. Dowling, and W. McKinnon, pp , Cambridge Univ. Press, Cambridge, U. K. J. T. Clarke and I. J. Cohen, Center for Space Physics, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA. (icohen121@gmail.com) 8of8