Title: Saturn's aurora observed by the Cassini camera at visible wavelengths



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Ms. Ref. No.:  ICARUS-13474

Title: Saturn's aurora observed by the Cassini camera at visible wavelengths.

Icarus

Response to Reviewers' comments:


I thank both reviewers for detailed suggestions on the manuscript clarity. I appreciate the extensive notes of Reviewer #2 improving the manuscript style and the suggested references. Although I do not agree with some strong claims of Reviewer #2 on controversial issues, I definitely appreciate his/her involvement in our study.
I type my answers to the reviewers’ comments below in the lines starting with (*).
Reviewer #1: Report on the manuscript on 'Saturn's aurora observed by the Cassini camera at visible wavelengths', by Dyudina et al.
The manuscript on Saturn's visible aurora is the only study discussing observations of visible aurora at Saturn, which are indeed very interesting. However, I believe that some sections (especially section 2 and 3) deserve to be further developed before publication. Please find below some general comments which could improve the manuscript.
The abstract and the introduction should be modified according to the comments below.
Section 2: I suggest replace the title 'geometry' with something like 'morphology'.

* changed to “Auroral Morphology and Location”


This section reports the observations of a couple of features such as arcs, spots, spiral shape, vortices etc but their discussion is very poor. There is very brief comparison with one or two studies in UV or IR without mentioning the physical origin of the observed features. A couple of attempts to explain some features ('plasma tongues', 'polar rain') are poorly documented. What is the 'plasma tongue'? Do the authors mean 'injected plasma'?
*Discussion of auroral features, comparison with other observations, discussion of physical origins of aurora and comparison of different models had been added to Section 2.
The auroral morphology at Saturn has been quite well studied over the last years, based both on theoretical studies and observations, and many features have been detailed discussed especially with UVIS and VIMS observations (see UV-IR literature on Saturn). I suggest that the authors should present a more detailed comparison with the UV and IR aurora which includes possible interpretation of the observed features in visible.
*Comparison with UV and IR aurora is added to Section 2
In addition, it is important to include a discussion about what we learn from the visible about the aurora at Saturn.
*A new section “Conclusuions” discusses this
Section 3: The title of this section should also be changed. One possibility is to replace it with 'characterist timescales or periodicities'.
*I changed the title to “Temporal Variations and Periodicities”
I find this section hard to follow and not very convincing. The criteria used to determine the periodicity are not well justified. For example it is not clear how the brightness 'the brightest part of the auroral arc' can be used as a criterion. The brightness of the auroral at Saturn might change with time as the authors point out later in the text. More arguments are required to justify the criteria used.
* I have added the following clarification to description of Supplementary Movies 5 and 6:

* “Brightness is the most prominent feature of the aurora, and it is clear in the movies that one side of the auroral oval is always bright, and the other side is dark. For example in Supplementary Movies S5 at longitudes ∼180-360◦ aurora is seen on all rotations, but at latitudes ∼0-120◦ aurora is not detectable on any ro- tations. However the exact location of the brightness maximum in longitude is harder to determine compared to that of the low-latitude extremum.”


In their methods the authors should also consider that not all auroral components rotate with the same velocity. At least that is the case in UV. Their velocity depends on which magnetospheric processes they are related to.
*I have added the following clarification to the description of Supplementary Movies 5 and 6:

* “Aurora changes brightness with time. Auroral features change speed from su- perrotation to subrogation (see Supplementary Movie S3). Because of that, small-scale features can not be used to determine auroral corotation period. However, the average auroral structure, as described by the three criteria above, persists from rotation to rotation.”


Section 4: The spectrum and vertical structure is better analysed than the previous too. The authors include a more detailed discussion of their observations, comparing with models and UV observations.
There is no mention of the visible aurora at Jupiter. I believe the author should include a discussion of the visible aurora at Jupiter (Vasavada et al., 1999, Ingersoll et al., 1998 etc) and even a comparison with the one at Saturn if it is appropriate.
* I have added this reference to the introduction. This is the first paper to report visible aurora on Saturn, as I clarified in the abstract.

Reviewer #2: Comments on the Paper by Dyudina et al Submitted to Icarus


This submission is potentially an important paper being the first to overview Saturn's auroral emissions in the visible band, obtained on the nightside using the Cassini/ISS. Unfortunately, the paper as submitted has a number of gross deficiencies that make it unsuitable for publication in its present form. I hope the authors will be able to improve the paper sufficiently that a positive recommendation can be made after revision, but they should be in no doubt that this will require a significant overhaul. To aid in this process I have written some very detailed and explicit comments into the typescript, which should have been forwarded to them. Comments follow that emphasize some main points.
* I made most of the stylistic corrections suggested in the marked-up manuscript.
a) Referencing and reporting of previous work is generally inadequate and often inaccurate.
* I added references to the introduction section and throughout the text.
A particularly bad example occurs on p16 where it is claimed that the slow variation of the SKR periods was discovered using Cassini data. It was not.
*The sentence with Cassini reference is changed to:

*“However, from Ulyssis (Galopeau and Lecacheux, 2000) and later observations it became clear that the SKR period changes with time, and that there are different SKR periods in the North and South hemispheres.”


It also seems rather extraordinary that no mention is made of similar observations of visible band auroras at Jupiter using the Galileo imager.
* The Vasavada et al. reference added to the introduction;
* Answering the reviewer’s note on the manuscript discussing Fig. 1 grid lines, i.e., “Actually these can hardly be seen and need to be enhanced prior to publication”

* While experimenting with grid weights I decided to keep grid weight the same as in the original manuscript because otherwise it distract the reader from the low-brightness aurora.


b) A particular feature of the paper is the generation of "longitude" maps from the movie data (e.g., Figs 2, 3, 5, and 6). However, no clear statement is made in the paper about what rotation period is used to generate the maps, and it is only in the figure caption on p13 that it becomes clear that the period concerned is the IAU period based on the Voyager NH SKR period from 1981! If an SKR period is used to order these data, then why not use a relevant "SKR" period for the intervals concerned - detailed values for which have been published from both the radio data directly, and from magnetic oscillation data (see particularly Provan et al. (JGR, 118, 3243, 2013) covering the specific intervals discussed in Fig 7).
*As clarified now in the description of Fig. 1,

* “As will be discussed later, after filtering out the local-time-dependent aurora, nearly-corotating auroral structures are seen in the visible movies, which makes it useful to connect them to standard System III longitude. System III coordinates assume planetocentric latitude and west longitude with 10.656222-hour SKR rotation measured by Voyager (Desch and Kaiser, 1981).”


* The usage of non-standard coordinates would complicate cross-comparison with aurora observations by other instruments (ultraviolet and infrared images).
c) The impression is given in the discussion that Saturn's auroras consist of spiral structures that rotate around the poles at the 'planetary' rotation period, though evidence in the present paper appears in my estimation to be tenuous at best.

*Supplementary Movie S1 is added to clarify the nature of the spiral structures we report, as described after Figure 1 discussion.


Spirals have indeed sometimes been reported (e.g., Grodent et al (JGR, 110, A07215, 2005),
*I added the discussion of Grodent et al.’s 2005 spirals in Figure 2 description
but they do not behave in the manner indicated here.
* In description of Figure 2 an explanation is added: “The ultraviolet observations did not have temporal resolution and time coverage to separate the corotation of the spirals from local time effect, as we do here. Corotation was not detected in ultraviolet aurora until recently (Lamy et al., 2013).”
Indeed, Saturn's auroras exhibit a strong tendency for LT organization, with e.g. a bright near-stationary arc in the dawn sector, and higher-latitude more diffuse emissions post-noon, as observed from Earth.
*We report some local time effects that our visible observations are sensitive to. See the details in our discussion of Fig 4.
The idea suggested here that one can really unravel the overall longitude structure by watching what happens with time at midnight is fictional.
* As added in the discussion of Fig. 1 “As will be discussed later, after filtering out the local-time-dependent aurora, nearly-corotating auroral structures are seen in the visible movies, which makes it useful to connect them to standard System III longitude.”

* We believe that structure that visibly corotates and reappears on consecutive rotations is steady enough to consider it “frozen” into System III coordinates.

This conceptual difficulty would be removed if one instead ordered the data by "SKR" phase and recognized that the changes being observed are those associated with the rotating perturbation system associated with the SKR modulations, as

they effect the auroras at the specific LT of observation.


*The SKR rotating system is virtually the same as System III within our error bars on auroral rotation (See Fig. 7). We consider System III close enough to current SKR rotation and keep the System III mapping in the new draft.

*As added at the end of Section 3,

*“No direct comparison of timing in visible aurora variations with SKR is performed here. Such comparison is an interesting subject for separate future research.”
d) Rather than observing spirals, I suggest that instead the authors may have rediscovered in visible auroral emissions the 'wobbling' back and forth of the center of the auroral oval at the "SKR period" that was originally discovered in UV images by Nichols et al (JGR, 2008). Further references are given on p12. Figs 5 and 6 appear to be compatible with an oscillation of the oval rather than a spiral, with the brightest auroras tending to occur when at the lowest latitudes as found previously (meaning that the field-aligned currents of the "SKR" system then point upward, as opposed to downward when at the higher latitudes).
* as added in Fig 2. Description:

*” The motion of the day side of ultraviolet auroral oval in Nichols et al. (2008) was approximated as a circle with changing radius and center. The fitted center location was found to oscillate with the period close to corotation with Saturn. Such fitting would also result from the near- corotation of the spiral-shaped ”oval”, which shape was not resolved in the ultraviolet observations..”


e) With regard to the origins of the auroras discussed on p7, the authors should note that the 'plasma tongue' hypothesized by Goldrich and Farmer does not exist. The auroras then do not form a spiral that map to a non-existent plasma feature. A rotating 'plasma cam' does exist (Burch et al (GRL, 36, L14203, 2009) which forms part of the "SKR" perturbation system moving through the more slowly rotating magnetospheric plasma, but this has no connection with the overall morphology of the auroras. More realistically, the auroras are related to regions of upward-directed field-aligned current that are principally related to sub-corotation of the plasma, but also strongly modulated by the process that also modulates the SKR emissions as mentioned above.
*our interpretation of Burch et al, 2009 paper (close to direct citations from the paper) is added to Fig. 2 description:

* “Spirals that nearly-corotate with Saturn were observed in ion data from CAPS (the Cassini Plasma Spectrometer). They project to the magnetospheric equator at 10-50 Saturn’s radii (Rs). Burch et al. (2009) suggests that the observed dense nearly corotating ”plasma cam” inside 10 Rs is responsible for plasma loading to the spirals. Two possible origins of the ”cam” were proposed: closed field line reconnection; or plasma convection inside magnetosphere, in which case the ”cam” is the extension of ”plasma tongue” from Goldreich and Farmer (2007). The exact mapping of the ”cam” and the spirals to aurora is not discussed in Burch et al. (2009).”


f) The idea floated on p11 that the auroral periods might stand as proxy for the deep rotation period of Saturn is similarly flawed. How could the aurora know the deep period when the atmosphere and the magnetosphere evidently do not? The 'planetary' periods seen in these observations are almost certainly the same as the variable planetary periods of the magnetic field and SKR modulations as indicated above. The only possibility for a connection with the deep interior might involve a small-scale field inhomogeneity that produces auroral effects (such as in the NH of Jupiter), but none such have ever been recognized e.g. in copious UV images.
*The reviewer apparently refers to our sentence “This motion raises the obvious question of whether aurora can serve as a clock for the unknown internal rotation period of Saturn.”

*We clarify this sentence to make sure it is not interpreted as a claim that aurora period is identical to the deep rotation, see the second paragraph of section “Timing”:



*”This motion raises the obvious question of whether aurora can provide some information about the unknown internal rotation period of Saturn. The distur- bances of Saturn’s magnetosphere, which the aurora represents, are expected to be connected to to Saturn’s bulk rotation (Goldreich and Farmer, 2007; Gurnett et al., 2007; Cowley and Provan, 2013).”
g) Extraordinarily, no conclusions or summary are offered.

* I added conclusions


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