Surveying pulsating auroras

The early morning auroral oval is dominated by pulsating auroras. This category of aurora has ::::: These :::::: auroras ::::: have often been discussed as if it is just ::: they ::: are one phenomenon, but it is :::: they ::: are not. Pulsating auroras are separable based on the extent of their pulsation and structuring into at least three subcategories. This study surveyed 10 years of all-sky camera data to determine the occurrence probability for each type of pulsating aurora in magnetic local time and magnetic latitude. Amorphous pulsating aurora ::::::: Pulsating :::::: Aurora :::::: (APA) is found to be a nearly ubiquitous early morning aurora, and :::::: having :: an ::::: 86 % :::::: chance 5 :: of ::::::::: occurrence :: at ::: its ::::: peak. :::::: Patchy :::::::: Pulsating ::::::: Aurora ::::: (PPA) :::: and :::::: Patchy :::::: Aurora ::::: (PA) ::: are :::: less :::::::: common, ::::::: peaking :: at ::::: 21 % :::: and :::: 29 %, ::::::::::: respectively. :::::: Before ::::: local :::::::: midnight, : pulsating aurora is almost exclusively amorphous pre-midnight ::: APA. Occurrence distributions for each type of pulsating aurora :: of ::::: APA, ::::: PPA, ::: and ::: PA : are mapped into the magnetosphere to approximately determine the location of :::::::: equatorial ::::: plane :: to :::::::::::: approximately ::::: locate : their source regions. The patchy and patchy pulsating aurora :: PA :::: and :::: PPA : distributions primarily map to locations approximately between 4 and 9 RE, while some amorphous pulsating 10 aurora :::: APA maps to farther distances. : , ::::::::: suggesting :::: that ::: the ::::::::: mechanism :::::: which ::::::::: structures :::: PPA ::: and ::: PA :: is :::::::::: constrained ::: to ::: the :::: inner ::::::::::::: magnetosphere. :::: This :: is :: in ::::::::: agreement :::: with ::::::::::::::::::::::: Grono and Donovan (2019), ::::: which ::::::: located :::: these ::::::: auroras :::::: relative :: to ::: the :::::: proton


2011;
, but events lasting upwards of 15 hours have been observed (Jones et al., 2013). It is unknown exactly how long pulsating aurora events can persist for. Measurements of pulsating aurora event durations are conservative 35 because ground-based cameras, our primary tool for optically observing the aurora, cannot operate past sunrise (Partamies et al., 2017). The lifetimes of individual structures are known to range from a few seconds to tens of minutes (e.g., Grono et al., 2017;Grono and Donovan, 2018).
Pulsating auroral features exhibit diverse characteristics, varying in terms of shape, size, brightness, altitude, spatial stability, modulation, lifespan, and velocity, yet little effort has been spent on differentiating them. Historically, pulsating aurora was 40 subcategorized by Royrvik and Davis (1977) into patches, arcs, and arc segments, but modern literature generally only refers to :::::::: "pulsating ::::::: aurora" ::: and : "pulsating aurora and pulsating auroral patches (Yang et al., 2019;Partamies et al., 2019;Ozaki et al., 2019) : " ::::::::::::::::::::::::::::::::::::::::::::::::::::: (e.g., Yang et al., 2019;Partamies et al., 2019;Ozaki et al., 2019) and would not consider the "streaming arc" of Royrvik and Davis (1977) to be a type of pulsating aurora. Grono and Donovan (2018) recently used all-sky camera data to define criteria for differentiating pulsating aurora based on their phenomenology. They identified three types of pulsating aurora which were 45 separable based on their pulsation and structure. Amorphous Pulsating Aurora (APA) evolves so rapidly in both shape and brightness that it is usually difficult-::: -and often impossible-::: -to uniquely identify structures between successive images at a 3 : second cadence. Patchy Pulsating Aurora (PPA) consists of highly structured patches which can persist for tens of minutes and pulsate over much of their area. Patchy Aurora (PA) structures are similar to PPA but do not oscillate in brightness. While it may be oxymoronic to describe a non-pulsating feature as pulsating aurora, PA and PPA are clearly closely related in terms 50 of the underlying scattering mechanism responsible for the precipitation. Based on their appearance in the ionosphere, these two auroras seem to be differentiated only by the existence of a modulating mechanism in the magnetospheric source region.
Herein we use the term pulsating aurora :::::::: "pulsating ::::::: aurora" to collectively refer to APA, PPA, and PA, and the acronyms will be used to identify them individually.
Pulsating aurora has been shown to be pervasive in the morning sector (Jones et al., 2011;Partamies et al., 2017), but we 55 believe those studies conflated at least APA and PPA, while possibly ignoring PA altogether due to its relative lack of pulsation (Grono and Donovan, 2018). Nishimura et al. (2010Nishimura et al. ( , 2011 connected specific examples of APA and PPA, without differentiating them, with specific chorus elements in the equatorial magnetosphere. Yang et al. (2015Yang et al. ( , 2017 related the individual motion of PPA and PA patches to convection in the ionosphere, and their source regions to convection in the magnetosphere. locating the latitude boundaries of pulsating auroras relative to the proton aurora, Grono and Donovan (2019) discovered that they occur either within or equatorward of the proton aurora. PPA and PA were observed to occur predominantly equatorward of the optical b2i (Donovan et al., 2003), which is the ionospheric counterpart to the isotropy boundary for plasma sheet protons and marks the inner boundary of the thin current sheet. APA also occurred there, but in addition, it regularly extended into the transition region where the band of proton aurora luminosity originates from and the magnetic field is stretched.
Within each keogram, the upper and lower latitude boundaries as well as the start and end times of pulsating aurora events were ::::::: identified :::::: by-eye :::: and recorded. One spatial dimension does not necessarily provide enough information to accurately define the boundaries of pulsating aurora, but it provides a reasonable estimate when the alternative is to define the boundaries for hundreds of thousands of individual ASI images. This simple method of defining the event boundaries is often imprecise 95 since the size of the region which pulsating auroras cover can change, in addition to its location. Multiple sets of boundaries were often used to better define where pulsating aurora was occurring within the keograms in order to compensate. Despite this limitation, more precise and accurate methods of defining the event boundaries are prohibitively time consuming for a dataset of this size.
Based on this, it is generally straightforward to uniquely identify each type of pulsating aurora within keograms (Grono and Donovan, 2018), but certain events can be ambiguous and in these instances the full all-sky images were inspected.
An example of a keogram used to approximately define pulsating aurora occurrence. The regions where amorphous pulsating aurora (APA, red), patchy pulsating aurora (PPA, blue), and patchy aurora (PA, yellow) can be identified in the keogram are 125 marked with rectangles.

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The number of hours of observation of each type of pulsating aurora ::: data ::: of :::: each :::::::: pulsating ::::: aurora :::: type : that went into panels reduces :::::: Figure :::: 4a-c : to two separate one-dimensional histograms in MLT and MLAT, allowing the occurrence distributions to be more easily compared. Panel3a ::: 5a shows that APA covers a larger range of latitudes than PPA and PA, extending farther poleward than both. PPA appears to develop nearly as farther :: far : equatorward as APA, although PA does not. Furthermore, the peak occurrence of APA appears to be 1 to 2 : degrees MLAT poleward of PPA and PA. In panel3b ::: 5b, PPA and PA have similar MLT distributions whose peaks ::::::::::: approximately : align with a local maximum of APA that is ∼3 : hours later than its peak.

4 Discussion and conclusions
The latitude and temporal boundaries of pulsating auroras that were recorded during the survey provide sets of coordinates which can be traced into the equatorial plane of the magnetosphere to estimate the location of their source regions. In this context, a set of boundaries refers to any individual rectangular region used to define the occurrence of pulsating aurora within a keogram, such as those seen in Figure1 : 2. Since a set of boundaries can cover long periods of time and therefore correspond 160 to a large region in the equatorial plane, we split each set into 1 : minute slices to more accurately map the shape of its source region. The start and end times of each set were rounded down to the nearest minute, and the latitude boundaries were mapped into the equatorial plane at each minute in-between.
To describe how Figure4 was created, two sets of bins must be defined. The total bins are what are shown in Figure 4, these bins count ::::: Figure :: 6 ::::: shows ::::::::::: distributions :::::::: counting the number of sets of boundaries that mapped to a particular region of the 165 equatorial plane. In addition, each set of boundaries has their own set of bins in XY GSM coordinates, matching the grid seen in Figure4, which we will call the event bins. The event bins are used to record where the slices of each set map to before being added to the total bins.
The XY GSM coordinates of the poleward and equatorward latitude boundaries of a slice form a line in the equatorial plane.
However, each set is only allowed to count toward the total once, so after every slice has been mapped and added to the event bins , the event bins must be clamped between 0 and 1. Thus, when the event bins are added to the total bins, they contribute only 1. Repeating this for every set of boundaries produced the distributions shown in Figure 4.

(a) Amorphous pulsating aurora (444 hours) (b) Patchy pulsating aurora (43 hours) (c) Patchy aurora (56 hours)
Pulsating aurora occurrence mapped to the equatorial plane  Figure 6. Pulsating aurora occurrence mapped to the equatorial plane. Pulsating aurora time and latitude boundaries were mapped using the T89 model (Tsyganenko, 1989) given OMNI Kp and :: the solar wind velocity GSE X component. This figure is based on the same set of events as shown in Figure2 :: 4, excluding those with poor OMNI :::: solar :::: wind data.
opposed to accurately tracing single ::::: rather :::: than :::::::: accurately ::::::: tracing :::::::: individual : events. To this end, the decreased computation time of the T89 model was deemed more valuable than an increase in accuracy which had little impact on the distributions. These distributions are in agreement with Grono and Donovan (2019), which reported PPA and PA being constrained to more equatorward latitudes than APA relative to the proton aurora. The bright band of auroral luminosity created by proton precipitation is known as : "the proton aurora". Proton precipitation occurs when the pitch angles of magnetically trapped protons are scattered as the the particles pass through tight magnetic field curvature in the equatorial plane (Tsyganenko, 1982;Sergeev 195 et al., 1983). The Earthward limit of this stochastic scattering mechanism is the isotropy boundary (e.g., Sergeev et al., 1983), which is located where the magnetic field transitions from being stretched to mostly dipolar. There is an equivalent boundary in the ionosphere, called the optical b2i (Donovan et al., 2003), which marks the rapid decrease of downgoing proton fluxes. Grono and Donovan (2019) found that all pulsating aurora occurred either within or equatorward of the proton aurora. PPA and PA occurred predominantly equatorward of the optical b2i, indicating that they originate from a region where magnetic field 200 topology is mostly dipolar. APA was seen poleward of the optical b2i, but still within the proton aurora.
Global distributions of lower-band whistler-mode chorus (Li et al., 2011), a primary driver of pulsating aurora (e.g., Nishimura et al., 2010Nishimura et al., , 2011, also indicate that these are realistic distributions of the pulsating aurora source regions. Li et al. (2011) re-210 ported lower-band chorus occurrence primarily between 5 and ∼8 : R E near magnetic midnight, and a wider occurrence region post-midnight between 5 and 10 R E . They only surveyed events between 5 and 10 R E , the most dominant region for lower-band chorus.

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It is unknown which specific mechanisms and conditions are involved in each of these types of pulsating aurora, but structural similarity between PA and PPA (Grono and Donovan, 2018) ::::::::::::::::::::::: (Grono and Donovan, 2018) indicates that they are differentiated only by the existence of modulating processes in the source region. This suggests that pulsation and structuring are the two fundamental aspects of pulsating aurora phenomenology. APA can begin to appear much earlier than PPA and PA, occurrence peaks earlier, and it seems to be the only type that can constitute an entire pulsating auroral event on its own (Grono and 245 Donovan, 2018) ::::::::::::::::::::::: (Grono and Donovan, 2018 One cannot totally exclude the possibility, but it is generally possible to distinguish them. I identified events using keograms and in many cases where I felt there was ambiguity, I reviewed the full images. In no cases did I find I had confused discrete and diffuse auroras. I find misidentifying the type of pulsating aurora is the larger concern since there seems to be some gradation between the types. The presence of multiple pathlines having similar trajectories during a single event can be a helpful indicator when recognizing PA pathlines. Also, amorphous pulsating aurora (APA) seems to be part of every pulsating aurora event (Grono and Donovan, 2018), another helpful clue. A new figure has been added to help address this.

Figure 2:
What are the features appearing in the RANK keogram north of 72 degrees, that appear to be pulsating? Perhaps it would be helpful to also show a keogram with no pulsating aurora, to contrast with Figure 2?
If I understand correctly which features you are referring to, these seem to be arcs. We have added a new figure to help address this.

Fixed.
Line 136: "nearly as farther" should be "nearly as far".

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Line 138: The local maximum in the APA MLT histogram seems like it could be a single isolated MLT bin with a larger number of events, but there is actually a slight dip for this bin in the PA and PPA histograms. Could it be that some PA and PPA was mis-labeled as APA at this time?
We were careful in our classifications, often reviewing videos of the ASI data to ensure we were accurate. As for the chance of misidentification, I do not see a mechanism which could be responsible for so many mislabelled events at this specific time. This would seem to correspond to tens of days worth of data being mislabelled at this particular MLT.
I have added "approximately" to this sentence to reflect the fact that they do not exactly align.
In general you don't consider the possibility, likelihood or effect of mis-identification when discussing your results. Is there any way you could quantitatively estimate uncertainties on your occurrence percentages?
We discuss ways in which classifications are ambiguous but cannot define a likelihood for misidentification to occur. We believe it is low due to the regular review of keogram data in movies of the full ASI images.

Discussion and conclusions:
Line 149: GSM is used here without the acronym being defined. Although this is a common acronym, it is worth defining for clarity. Similarly for GSE later.

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Line 147: I realise explaining the method used is quite complicated, but this paragraph and the following one are difficult to follow. I think you are describing the exact process you use in your computer code, but probably the terminology could be reduced in the paper and the explanation simplified. Instead of using the terms "total bins" and "event bins", can you just say the 1 RE x 1 RE bins shown in Figure 5 count the number of events (i.e. rectangles on the keograms) that intersect that bin, when mapped using T89? Is this correct? You could include the detail that the events are mapped at a 1-minute resolution to determine intersection with the equatorial bins.

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Line 158: I think one or two words are missing here around "passed". The sentence doesn't seem complete.

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Line 183: "spacecraft" is plural, it doesn't need an s on the end.

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Line 185: "This agrees with our observations." -Could you be more specific here? You are not measuring the proton aurora, right?

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Line 194 and 195: Is "develop" the right word here? Perhaps "extends" on line 194 and "exist" or "is found" on line 195? To me "develop" implies a location of initial formation, which I don't think is what you mean.

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Line 203: Do you mean Figure 4a rather than 3a?
3a is what was intended. This section is comparing the probability of occurrence between the two studies, which Figure 4 does not contain. While Jones et al. (2011) does not feature a latitude distribution and ours is in MLAT and MLT, we think Figure 3 is the more insightful comparison, as opposed to Figure 4b.