Transient Flashes in Saturn’s UV Aurora: An Analysis of Hubble Space Telescope 2013–2017 Campaigns and Cassini Magnetic Field Measurements

Round 1

Reviewer 1 Report

This study has  determined the lifetime and recurrence period of Saturn's Auroral flash with observations from Hubble. Combined with magnetic field observations from Cassini, they found that the flashes have a good correspondence with the periodic fluctuations of the magnetic field, proving that the flash is a phenomenon that occurs locally in closed magnetic field line region. The paper is written smoothly and logically. The data are well processed and well organized. 

My only suggestion is to make a more concise conclusion for this whole study at the end of the Abstract.

Due to my personal limitations on this study field, I can't give more suggestions for further revision. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Review of

TRANSIENT FLASHES IN SATURN’S UV AURORA: AN ANALYSIS OF HUBBLE SPACE TELESCOPE 2013-2017 CAMPAIGNS AND CASSINI MAGNETIC FIELD MEASUREMENTS

 by Qin et al.

The paper presents remote UV images of 29 Saturn’s auroral ‘flashes’, which were collected by the Hubble Space Telescope during 2013-2017. The authors statistically analyze a number of parameters of the flashes. Further, they present corresponding in situ magnetic field observations during the flashes, when Cassini orbiter footprints were located at or near the flashes. The authors clearly find that the flashes are accompanied by the sudden magnetic field reconfiguration to a more southward field due to a localized driving process in the duskside.

The study is well-written, sound and supported by multiple observations, bringing new interesting knowledge about the nature of Saturn aurora and magnetospheric dynamics. I recommend this paper for publication after some revision, which I assume to be minor.

 Comments:

 Multiple flashes on the same days:

 Lines 320-322: The authors have observations of two or three flashes during the same days by the HST on several occasions (Table 1). First, one of the recurrent events with two flashes is presented in Figure 4. It is not clear why/how the two flashes were associated with the magnetic field disturbances at Cassini, as the duration of the flash denoted by the yellow arrows lasted from 1936 to 2142, with the observation at the Cassini footprint around 2130. An intuitive temptation would be to assign this flash to the magnetic disturbance, which indicated by the blue arrow in Figure 4d. Isn’t it that the magnetic pulse from the other flash (blue arrows in Figures 4abc) should have been observed by Cassini earlier, as the two flashes seem to move toward noon? Same problem with other examples, when the flashes could be associated with different magnetic pulses or with several ones due to long lifetime and large size of the flashes.

 The events with three flashes were observed on 2013-140 and 2016-181. It would actually be nice to make an additional analysis of the recurrent flash events from Table 1 and draw some conclusions from them for this specific subgroup of events. For instance, whether the flashes have the same recurrence period as magnetic pulses observed by Cassini, and whether flash size and brightness correlate with the strength of the magnetic pulse.

Table 1:

Have I understood correctly, that the flash may move in both azimuthal directions, as in the events in Figures 1 and 3 the flashes moved toward midnight? Could this information be added in Table 1? Some other information could be added in Table 1, e.g, which is already presented as a single number in Lines 183 (flash lifetime).

 Discussion:

 I see that the authors tempt to interpret the magnetic pulses in accord with previous works, in terms of reconnection or in terms of some MHD mode. Can this be done more specifically, using e.g., some particle data, if such data are available? Are the observed magnetic pulses analogous to the dipolarization fronts in the current sheets of planetary magnetotails, cf. https://doi.org/10.1029/2002GL015763, https://www.science.org/doi/10.1126/science.280.5366.1061, https://doi.org/10.1029/2012JA017756 ?

 In Line 368 the authors use the argument of asymmetric sawtooth vs sinusoidal shapes of the magnetic signal. In fact, Figure 1 of Yates et al., shows also asymmetry in B_theta and density at doy > 341. Thus, this statement may be contradictive. Actually, both symmetric and asymmetric shapes may manifest the same process, but at different stages of development (linear vs non-linear). For example, in the Earth’s magnetosphere, there have been found signatures of the kinetic ballooning-interchange instability. In its linear stage, the field oscillations are sinusoidal (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL051668). In the non-linear stage, the field oscillations are less regular (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL083070) and lead to localized auroral pseudobreakups. The two latter references are just to show the magnetic signatures and not a suggestion to cite.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf