the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jan 2026
Flapping motion of onset aurora
Abstract. Onset aurora is meso-scale auroral forms (100s km) of spiral aurora arising out of the equatorward arc in association with the initial pulse of Pi2 pulsation, i.e., onset of field line dipolarization. Following the auroral onset, the head of the equatorward arc rapidly turns in a clockwise direction with expansion poleward, and during the second event of onset aurora, the whole of the arc rotates clockwise as viewed along the field lines. We model the deformation of onset aurora as an ExB drift of negatively charged solitary potential area (ion hole) in the polar ionosphere. It is suggested that twist motion of the onset aurora is analogous to the flapping motion of transmission belt in a factory driven by rotating line shaft. Like fluctuations in the line-of-sight velocity of the belt, non-uniform plasma flows in the ion hole trigger flapping motion of the arc.
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Status: closed
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RC1: 'Comment on egusphere-2026-11', Anonymous Referee #1, 31 Mar 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-11/egusphere-2026-11-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2026-11-RC1 -
AC1: 'Reply on RC1', Osuke Saka, 06 Apr 2026
Reply to referee comments RC#1
Flapping motion proposed here can be interpreted in terms of mechanical motion of the auroral arc, or torque imposed on the principal axis of the auroral arc.
It may also be related to Maxwell stress/Poynting flux. This possibility has not as yet been fully studied.
Relevant sentences have been changed in accordance with Referee comments as listed below. The changes are highlighted by bold.
LINES 31-32
Possible location of peak power for the high-latitude Pi2s is suggested to be at the Harang Discontinuity (HD) [Rostoker and Samson, 1981], and at Westward Traveling Surge (WTS) [Samson and Rostoker, 1983]. The onset aurora is a meso-scale form of discrete aurora related to substorm enhanced electrojet.
LINE 37
The former type (CCW) is caused by wave polarizations of current carrying Alfven waves generated in the magnetosphere by the plasma instability [Forsyth et al., 2020]. Alternatively, CCW rotations are generated in the ionosphere by shear flow instability of flux tubes containing upward field-aligned currents [Hallinan, 1976; Lysak and Song, 1996; Partamies et al., 2001; Keiling et al., 2009].
LINE 42
Second type (CW) is called the S-aurora, where auroral vorticities rotate in clockwise directions that are counter to the first type [Oguti, 1975].
LINES 43-44
The onset aurora, showing the second type of rotation, is a meso-scale auroral form of the S-aurora.
LINES 44-48
Generally, quasi-neutral conditions are possible in the polar ionosphere. To monitor ionospheric potential, vertical electric fields observed on the ground (atmospheric electric field) are used. There are two factors in ionospheric potential that contribute to the vertical component of magnetic fields Bz. The first is space charge potential in the ionosphere and the second is convection through the conductivity inhomogeneities. Observed correlations between atmospheric electricity and Bz component during substorm suggest the space charge potential contributes to the atmospheric electricity [Minamoto and Kadokura, 2011; Saka, 2021]. Negatively charged ion hole hypothesis can be applied to the auroral arc.
The K-H instability excites shear flow instabilities in the ion hole [Hallinan and Davis, PSS, 1970; Hallinan, JGR, 1976; Lysak and Song, JGR, 1996]. However, K-H instabilities do not explain deformation of the onset aurora. New idea (flapping motion) is proposed.
Reference:
Minamoto, Y., and Kadokura, A., 2011: Extracting fair-weather data from atmospheric electric-field observations at Syowa Station, Antarctica, Polar Science, 5, 313-318
Saka, O., Effects of auroral Ionosphere on atmospheric electricity, PEM11-P06, Abstract presented in JpGU2021,2021.
LINE 48
Those ion holes are not solitary waves caused by plasma kinetics in the collisionless plasmas [Temerin et al., 1982; McFadden et al., 2003] but rather solitary structures generated in the collisional ionosphere by precipitating energetic electrons.
LINES 50-51
Although flow shears in the ion hole develop winding auroras of the first type with CCW rotations [Lysak and Song, 1996], we suggest flapping instability of the ion hole deforms the onset aurora with an opposite sense of rotation.
LINE 69
Rotational features observed in the onset aurora are common features of the S-aurora [Oguti, 1975].
LINE 70
Onset aurora is a meso-scale auroral form of S-aurora.
LINE 95, 100: Equation (1), (2)
References are added according to the Referee comment. Capital V is used for particle velocity. Changed part is highlighted by underlines.
Under the assumption of gyrotropy, parallel (T-para) and perpendicular (T-perp) temperatures in eV are calculated by the following equation [Birn et al., 1997],
Equation (1)
Integration occurs over the velocity space ( ) occupied by the trapped electrons or ions; me and mi denote electron and hydrogen mass, respectively. Here, f(V-perp, V-para; Φ) is the Maxwell distribution function for velocity distributions of ions and electrons with Φ representing field line potential [Knight, 1973]. Parallel and perpendicular velocity component with respect to the background magnetic fields are denoted by V-para and V-perp. The velocity distribution function of ions/electrons is given by,
Equation (2)
LINES 103-104
perpendicular temperature anisotropy of electrons becomes larger. While for ions, parallel anisotropy becomes larger than the case without a parallel potential (Figure 4).
LINES 119-120
we obtain ion drift velocities on the order of 5.9x10^1 m/s for electric fields of the order of 0.1 V/m
LINES 120-121
Those drifting ions carry Pedersen current densities of the order of 1.0μAm^-2
LINE 127: Equation (4)
Field-aligned current densities above the ion hole
LINE 168
Reference is added according to the Referee comment.
[Oguti, 2010; Rees, 1989; Tohmatsu, 1990]
LINE 180
Here, ε0 is permittivity in free space
LINE 234
shown in Figure 1 and to splitting motion of S-aurora referred to as “peeling-off” [Oguti, 1981]
LINE 234: SUMMARY
New section (section 7) is added instead of adding little more details in Summary.
- Ion hole as auroral driver
In atmospheric ionization processes, the electron charge satisfies the continuity equation when rate of production and loss are balanced [Tohmatsu, 1990]. In the case of collisional excitation by the incident energetic electrons, the continuity equation may be given as,
dne/dt = q-α*ne^2=0.
Here, ne is electron density, q is ionization rate (m^-3s^-1) due to incident primaries, α denotes recombination coefficient (m^3 s^-1) due to dissociative recombination (α=10^-7 m^3s^-1).
In the ion hole, ionospheric ions are transported from the surrounding ionosphere as target ions to the electron rich regions by the ion drift defined by
U_perp =Ωi/ (B*νin) E.
Here, E represents polarization electric fields in the ionosphere converging to the center of the ion hole. Those ion flows supply target ions to compensate for the recombination loss in the ion hole.
When electron precipitation stops (q=0), electron density initially at n0 decays with time t as,
ne(t)=n0/ (n0*α*t+1).
For n0=10^12 m-3, electron density halves in about 10s or less.
During flapping motion of the arc, the auroral arc drifts in the polar ionosphere. The drifting arc may decay in a few seconds unless auroral drivers follow the motion of the arc. Ion hole should act alone as an auroral driver.
FIGURE 1
Proton auroras are expected to lie equatorward of the arc (electron aurora). Black peaks (dark aurora) at the bottom of keogram may suggest that boundaries of electron and proton aurora move poleward.
FIGURE 1 AND 2
All-sky image (SHM) superimposed with geographic longitude and latitude lines is added in Figure 1. Geomagnetic N-S line passing through optical station SHM is also added. New Figure 1 can be found in Supplement.
LINES 383-384: CAPTIONS FOR FIGURE’S 1 AND 2
Original All-sky images are recorded from the ground. In the northern/southern hemisphere, we see auroras in the directions anti-parallel and parallel to the field lines. If all-sky images taken at the northern countries are flipped horizontally (Figure 1 and 2), auroras parallel to the field lines are invariably seen.
LINE 412: CAPTION FOR FIGURE 5
Upward current density in μAm^-2
-
AC1: 'Reply on RC1', Osuke Saka, 06 Apr 2026
-
RC2: 'Comment on egusphere-2026-11', Anonymous Referee #2, 01 Apr 2026
Review of "Flapping motion of onset aurora" by Saka-san
This study seeks to explain the deformation and spiraling of aurora occurring during field line dipolarization.
This study unfortunately strikes me as rather hand-wavy, with huge leaps being taken in the chains of reasoning. This is pointedly illustrated in Section 3, where an entire scenario is constructed on the basis of assumptions that have no clear root in observations. Two of the three references in this section are to the authors' own work – Saka (2019) and Saka (2023). I note that one of the reviewers of Saka (2019) stated, "Although it will undoubtedly be somewhat controversial in its overall focus, sufficient caveats have been added to help the reader decide for themselves." The first reviewer also pointed to major flaws, but the paper was accepted without further comment from this reviewer. In the second round of revisions for Saka (2023), one reviewer recommended that the paper be rejected because of major flaws in reasoning, while the second stated, "Basically the manuscript as it is lacks a good connection to the reality."
The sentiments from these reviewers are basically my sentiments about this paper. To give a single concrete illustration of what I am talking about, the author states on Line 74 that they assume horizontal plasma flows in the ionosphere are caused by incident westward electric fields. This is already controversial, and should be clearly motivated by more than it being a convenient starting point. Next, the author assumes that these westward electric fields lead to charge separation in the polar ionosphere that are negative in the lowere latitudes and positive in the higher latitudes. To me these are huge, wildly speculative assumptions. And these are the starting point!
Later on qualitative and highly speculative statements such as the following are made: "Meanwhile, some of trapped electrons above the positively charged area do not return to the magnetosphere because they were drawn into the ionosphere. The magnetosphere may become an electron void region. Negative charges immediately above the ionosphere and an electron void region in the magnetosphere initiate charge separation."
I did not read the entire paper, because the theoretical premise of the paper is too weak. The author does present observations of spiraling aurora, but these observations have no obvious connection to the many large assumptions that the author makes to arrive at an explanation.
I suggest that the paper be rejected. I encourage the author to be far more quantitative, and this may very well mean attempting to quantitatively model the magnetosphere-ionosphere system with some relevant set of equations and concrete, quantitative assumptions that are themselves motivated by actual observations of conditions in the magnetosphere-ionosphere-thermosphere system.
Below I have a number of minor comments that the author may consider if they wish to resubmit at some point.
Line 42: It would be very helpful for readers like me if the author explained a bit more what defines S-aurora. The letter "S" would seem to suggest both clockwise and counterclockwise rotation occurring together, so it's not clear why S-aurora refers only to clockwise spirals.
Lines 45–48: Here the author first states that in this study the author regards auroral arcs as negatively charged solitary areas, or an "ion hole". The author then states that ion holes (i.e., auroral arcs) "are not solitary waves caused by plasma kinetics, but rather solitary structures generated in the collisional ionosphere by precipitating energetic electrons."
What does the author mean that an ion hole (which I understand means "auroral arc" in this study) does not arise from plasma kinetics? Is this intended as a statement of fact, or an expression of opinion, or an assumption for the paper? Can the author please clarify?
Lines 61–70: The author states on Lines 64–65 that "This event demonstrates a clockwise twist with splitting arc at the poleward boundary." What does "This event" refer to? Only the Figure 2 event, or both Figure 1 and Figure 2 events? The reason I ask is that later on (Line 67) the author says that Figure 1 is an example of a clockward spiraling arc that splits at the poleward boundary, while Figure 2 is a "rotational type of spirals".
Lines 67–68: What does the author mean by "rotational type of spirals"? Does this just mean the entire arc system rotates together, as opposed to the case shown in Figure 1, where only the poleward arc rotates clockwise?
Line 74: Does "horizontal plasma flows" refer specifically to north-south flows during substorm onset?
Citation: https://doi.org/10.5194/egusphere-2026-11-RC2 -
AC2: 'Reply on RC2', Osuke Saka, 06 Apr 2026
Reply to referee comments RC#2
- General comments
The onset aurora is a meso-scale form of S aurora generated in association with the initial pulse of Pi2 pulsations. Relevant plasma dynamics in the midnight magnetosphere could be found in the first 10 min intervals of Pi2 onset [Saka et al., 2010].
At the initial pulse of Pi2 pulsation (first one-min-interval of Pi2 onset), field line reconfigurations in the nightside geosynchronous altitudes occur in all three components starting with the increase of the inclination angle (dipolarization), decrease of field magnitudes, and field line expansions to the dayside sector (Figure A in supplement, adapted from Saka et al., 2000). Such reconfigurations expanding from dawn to dusk sectors can be explained by the transport of magnetic flux B as well as plasma pressure P from nightside to the dayside sector through shear flows. The shear flows last for about 10 min following the Pi2 onset [Saka et al., 2010].
We suppose that the shear flows are not extensions of the BBF from the tail. Shear flow causes boundary perturbations (surface waves) in the equatorial plane through K-H instability. Surface wave perturbations produce non-current carrying slip motions in the nightside equatorial plane rather than current carrying twist motions [Saka et al., 2007]. Consequently, we propose an “ionospheric driver scenario” as a hypothesis in these 10 min intervals for generating field-aligned currents as well as parallel electric fields in the inner magnetosphere. We can assume that an ionospheric driver initiates onset aurora.
References
Saka, O., Akaki, H., Reeves, G.D., and Baker, D.N. Magnetic fields and particle signatures in the vicinity of nightside geosynchronous altitudes in the first one-minute-interval of Pi2 onset: a case study. J. Atmos. Solar Terr. Phys. 62, 17-30, 2000.
Saka, O., D. Koga, and K. Hayashi, A plasma bulk motion in the midnight magnetosphere during auroral breakup inferred from all-sky image and magnetic field observations at geosynchronous altitudes. J.Atmos. Solar Terr.Phys., 69, 1063, 2007.
Saka, O., K. Hayashi, and M. Thomsen, First 10 min intervals of Pi2 onset as geosynchronous altitudes during the expansion of energetic ion regions in the nighttime sector. J. Atmos. Solar Terr. Phys., 72, 1100, 2010.
- Minor comments
Line 42
For S-aurora, auroras unfold the auroral pleat in a clockwise direction. For windup-auroras, auroras fold the sheet in a counterclockwise direction. Both S- and Windup-auroras show the S pattern when fully developed.
Lines 45-49
The scale size of the ion hole (auroral arc) is meso-scale of the order of 100s km. Ion holes observed by satellites in acceleration regions are in kinetic scale less than km [Temerin et al., Phys. Rew. Lett.,1982; Hasegawa and Sato, Phys. Fluids, 1982]. We assume that auroral arc is an “ion hole structure”.
Lines 61-70
“This event” refers to the event portrayed in Figure 2.
Lines 67-68
When an entire arc system rotates simultaneously, it is called “rotational type”.
Line 74
“Horizontal plasma flows” refers to N to S flows.
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AC3: 'Reply on RC2', Osuke Saka, 07 Apr 2026
Additional reply comments to RC2#
Regarding the excitation of surface waves, we are reminded of field line resonance by solar wind interactions with the magnetosphere [Chen and Hasegawa, JGR, 1974]. In the present case for the nighttime magnetosphere, we can assume that there were no jumps in densities and Alfven velocities but rather a jump in flow velocities. In these conditions, dispersion relation of the surface waves caused by the K-H instability is,
ω=U*k_perp.
U and k_perp denote shear flow velocity and perpendicular wave vector, respectively [Sake et al., 2010]. This relation is interpreted by a passage of structure with the characteristic wavelength (2π/k_perp) perpendicular to the field lines. Propagating boundary perturbations at U (Pi2 pulsation) produces slip motions [Saka et al., 2007].
Citation: https://doi.org/10.5194/egusphere-2026-11-AC3
-
AC2: 'Reply on RC2', Osuke Saka, 06 Apr 2026
Status: closed
-
RC1: 'Comment on egusphere-2026-11', Anonymous Referee #1, 31 Mar 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-11/egusphere-2026-11-RC1-supplement.pdf
-
AC1: 'Reply on RC1', Osuke Saka, 06 Apr 2026
Reply to referee comments RC#1
Flapping motion proposed here can be interpreted in terms of mechanical motion of the auroral arc, or torque imposed on the principal axis of the auroral arc.
It may also be related to Maxwell stress/Poynting flux. This possibility has not as yet been fully studied.
Relevant sentences have been changed in accordance with Referee comments as listed below. The changes are highlighted by bold.
LINES 31-32
Possible location of peak power for the high-latitude Pi2s is suggested to be at the Harang Discontinuity (HD) [Rostoker and Samson, 1981], and at Westward Traveling Surge (WTS) [Samson and Rostoker, 1983]. The onset aurora is a meso-scale form of discrete aurora related to substorm enhanced electrojet.
LINE 37
The former type (CCW) is caused by wave polarizations of current carrying Alfven waves generated in the magnetosphere by the plasma instability [Forsyth et al., 2020]. Alternatively, CCW rotations are generated in the ionosphere by shear flow instability of flux tubes containing upward field-aligned currents [Hallinan, 1976; Lysak and Song, 1996; Partamies et al., 2001; Keiling et al., 2009].
LINE 42
Second type (CW) is called the S-aurora, where auroral vorticities rotate in clockwise directions that are counter to the first type [Oguti, 1975].
LINES 43-44
The onset aurora, showing the second type of rotation, is a meso-scale auroral form of the S-aurora.
LINES 44-48
Generally, quasi-neutral conditions are possible in the polar ionosphere. To monitor ionospheric potential, vertical electric fields observed on the ground (atmospheric electric field) are used. There are two factors in ionospheric potential that contribute to the vertical component of magnetic fields Bz. The first is space charge potential in the ionosphere and the second is convection through the conductivity inhomogeneities. Observed correlations between atmospheric electricity and Bz component during substorm suggest the space charge potential contributes to the atmospheric electricity [Minamoto and Kadokura, 2011; Saka, 2021]. Negatively charged ion hole hypothesis can be applied to the auroral arc.
The K-H instability excites shear flow instabilities in the ion hole [Hallinan and Davis, PSS, 1970; Hallinan, JGR, 1976; Lysak and Song, JGR, 1996]. However, K-H instabilities do not explain deformation of the onset aurora. New idea (flapping motion) is proposed.
Reference:
Minamoto, Y., and Kadokura, A., 2011: Extracting fair-weather data from atmospheric electric-field observations at Syowa Station, Antarctica, Polar Science, 5, 313-318
Saka, O., Effects of auroral Ionosphere on atmospheric electricity, PEM11-P06, Abstract presented in JpGU2021,2021.
LINE 48
Those ion holes are not solitary waves caused by plasma kinetics in the collisionless plasmas [Temerin et al., 1982; McFadden et al., 2003] but rather solitary structures generated in the collisional ionosphere by precipitating energetic electrons.
LINES 50-51
Although flow shears in the ion hole develop winding auroras of the first type with CCW rotations [Lysak and Song, 1996], we suggest flapping instability of the ion hole deforms the onset aurora with an opposite sense of rotation.
LINE 69
Rotational features observed in the onset aurora are common features of the S-aurora [Oguti, 1975].
LINE 70
Onset aurora is a meso-scale auroral form of S-aurora.
LINE 95, 100: Equation (1), (2)
References are added according to the Referee comment. Capital V is used for particle velocity. Changed part is highlighted by underlines.
Under the assumption of gyrotropy, parallel (T-para) and perpendicular (T-perp) temperatures in eV are calculated by the following equation [Birn et al., 1997],
Equation (1)
Integration occurs over the velocity space ( ) occupied by the trapped electrons or ions; me and mi denote electron and hydrogen mass, respectively. Here, f(V-perp, V-para; Φ) is the Maxwell distribution function for velocity distributions of ions and electrons with Φ representing field line potential [Knight, 1973]. Parallel and perpendicular velocity component with respect to the background magnetic fields are denoted by V-para and V-perp. The velocity distribution function of ions/electrons is given by,
Equation (2)
LINES 103-104
perpendicular temperature anisotropy of electrons becomes larger. While for ions, parallel anisotropy becomes larger than the case without a parallel potential (Figure 4).
LINES 119-120
we obtain ion drift velocities on the order of 5.9x10^1 m/s for electric fields of the order of 0.1 V/m
LINES 120-121
Those drifting ions carry Pedersen current densities of the order of 1.0μAm^-2
LINE 127: Equation (4)
Field-aligned current densities above the ion hole
LINE 168
Reference is added according to the Referee comment.
[Oguti, 2010; Rees, 1989; Tohmatsu, 1990]
LINE 180
Here, ε0 is permittivity in free space
LINE 234
shown in Figure 1 and to splitting motion of S-aurora referred to as “peeling-off” [Oguti, 1981]
LINE 234: SUMMARY
New section (section 7) is added instead of adding little more details in Summary.
- Ion hole as auroral driver
In atmospheric ionization processes, the electron charge satisfies the continuity equation when rate of production and loss are balanced [Tohmatsu, 1990]. In the case of collisional excitation by the incident energetic electrons, the continuity equation may be given as,
dne/dt = q-α*ne^2=0.
Here, ne is electron density, q is ionization rate (m^-3s^-1) due to incident primaries, α denotes recombination coefficient (m^3 s^-1) due to dissociative recombination (α=10^-7 m^3s^-1).
In the ion hole, ionospheric ions are transported from the surrounding ionosphere as target ions to the electron rich regions by the ion drift defined by
U_perp =Ωi/ (B*νin) E.
Here, E represents polarization electric fields in the ionosphere converging to the center of the ion hole. Those ion flows supply target ions to compensate for the recombination loss in the ion hole.
When electron precipitation stops (q=0), electron density initially at n0 decays with time t as,
ne(t)=n0/ (n0*α*t+1).
For n0=10^12 m-3, electron density halves in about 10s or less.
During flapping motion of the arc, the auroral arc drifts in the polar ionosphere. The drifting arc may decay in a few seconds unless auroral drivers follow the motion of the arc. Ion hole should act alone as an auroral driver.
FIGURE 1
Proton auroras are expected to lie equatorward of the arc (electron aurora). Black peaks (dark aurora) at the bottom of keogram may suggest that boundaries of electron and proton aurora move poleward.
FIGURE 1 AND 2
All-sky image (SHM) superimposed with geographic longitude and latitude lines is added in Figure 1. Geomagnetic N-S line passing through optical station SHM is also added. New Figure 1 can be found in Supplement.
LINES 383-384: CAPTIONS FOR FIGURE’S 1 AND 2
Original All-sky images are recorded from the ground. In the northern/southern hemisphere, we see auroras in the directions anti-parallel and parallel to the field lines. If all-sky images taken at the northern countries are flipped horizontally (Figure 1 and 2), auroras parallel to the field lines are invariably seen.
LINE 412: CAPTION FOR FIGURE 5
Upward current density in μAm^-2
-
AC1: 'Reply on RC1', Osuke Saka, 06 Apr 2026
-
RC2: 'Comment on egusphere-2026-11', Anonymous Referee #2, 01 Apr 2026
Review of "Flapping motion of onset aurora" by Saka-san
This study seeks to explain the deformation and spiraling of aurora occurring during field line dipolarization.
This study unfortunately strikes me as rather hand-wavy, with huge leaps being taken in the chains of reasoning. This is pointedly illustrated in Section 3, where an entire scenario is constructed on the basis of assumptions that have no clear root in observations. Two of the three references in this section are to the authors' own work – Saka (2019) and Saka (2023). I note that one of the reviewers of Saka (2019) stated, "Although it will undoubtedly be somewhat controversial in its overall focus, sufficient caveats have been added to help the reader decide for themselves." The first reviewer also pointed to major flaws, but the paper was accepted without further comment from this reviewer. In the second round of revisions for Saka (2023), one reviewer recommended that the paper be rejected because of major flaws in reasoning, while the second stated, "Basically the manuscript as it is lacks a good connection to the reality."
The sentiments from these reviewers are basically my sentiments about this paper. To give a single concrete illustration of what I am talking about, the author states on Line 74 that they assume horizontal plasma flows in the ionosphere are caused by incident westward electric fields. This is already controversial, and should be clearly motivated by more than it being a convenient starting point. Next, the author assumes that these westward electric fields lead to charge separation in the polar ionosphere that are negative in the lowere latitudes and positive in the higher latitudes. To me these are huge, wildly speculative assumptions. And these are the starting point!
Later on qualitative and highly speculative statements such as the following are made: "Meanwhile, some of trapped electrons above the positively charged area do not return to the magnetosphere because they were drawn into the ionosphere. The magnetosphere may become an electron void region. Negative charges immediately above the ionosphere and an electron void region in the magnetosphere initiate charge separation."
I did not read the entire paper, because the theoretical premise of the paper is too weak. The author does present observations of spiraling aurora, but these observations have no obvious connection to the many large assumptions that the author makes to arrive at an explanation.
I suggest that the paper be rejected. I encourage the author to be far more quantitative, and this may very well mean attempting to quantitatively model the magnetosphere-ionosphere system with some relevant set of equations and concrete, quantitative assumptions that are themselves motivated by actual observations of conditions in the magnetosphere-ionosphere-thermosphere system.
Below I have a number of minor comments that the author may consider if they wish to resubmit at some point.
Line 42: It would be very helpful for readers like me if the author explained a bit more what defines S-aurora. The letter "S" would seem to suggest both clockwise and counterclockwise rotation occurring together, so it's not clear why S-aurora refers only to clockwise spirals.
Lines 45–48: Here the author first states that in this study the author regards auroral arcs as negatively charged solitary areas, or an "ion hole". The author then states that ion holes (i.e., auroral arcs) "are not solitary waves caused by plasma kinetics, but rather solitary structures generated in the collisional ionosphere by precipitating energetic electrons."
What does the author mean that an ion hole (which I understand means "auroral arc" in this study) does not arise from plasma kinetics? Is this intended as a statement of fact, or an expression of opinion, or an assumption for the paper? Can the author please clarify?
Lines 61–70: The author states on Lines 64–65 that "This event demonstrates a clockwise twist with splitting arc at the poleward boundary." What does "This event" refer to? Only the Figure 2 event, or both Figure 1 and Figure 2 events? The reason I ask is that later on (Line 67) the author says that Figure 1 is an example of a clockward spiraling arc that splits at the poleward boundary, while Figure 2 is a "rotational type of spirals".
Lines 67–68: What does the author mean by "rotational type of spirals"? Does this just mean the entire arc system rotates together, as opposed to the case shown in Figure 1, where only the poleward arc rotates clockwise?
Line 74: Does "horizontal plasma flows" refer specifically to north-south flows during substorm onset?
Citation: https://doi.org/10.5194/egusphere-2026-11-RC2 -
AC2: 'Reply on RC2', Osuke Saka, 06 Apr 2026
Reply to referee comments RC#2
- General comments
The onset aurora is a meso-scale form of S aurora generated in association with the initial pulse of Pi2 pulsations. Relevant plasma dynamics in the midnight magnetosphere could be found in the first 10 min intervals of Pi2 onset [Saka et al., 2010].
At the initial pulse of Pi2 pulsation (first one-min-interval of Pi2 onset), field line reconfigurations in the nightside geosynchronous altitudes occur in all three components starting with the increase of the inclination angle (dipolarization), decrease of field magnitudes, and field line expansions to the dayside sector (Figure A in supplement, adapted from Saka et al., 2000). Such reconfigurations expanding from dawn to dusk sectors can be explained by the transport of magnetic flux B as well as plasma pressure P from nightside to the dayside sector through shear flows. The shear flows last for about 10 min following the Pi2 onset [Saka et al., 2010].
We suppose that the shear flows are not extensions of the BBF from the tail. Shear flow causes boundary perturbations (surface waves) in the equatorial plane through K-H instability. Surface wave perturbations produce non-current carrying slip motions in the nightside equatorial plane rather than current carrying twist motions [Saka et al., 2007]. Consequently, we propose an “ionospheric driver scenario” as a hypothesis in these 10 min intervals for generating field-aligned currents as well as parallel electric fields in the inner magnetosphere. We can assume that an ionospheric driver initiates onset aurora.
References
Saka, O., Akaki, H., Reeves, G.D., and Baker, D.N. Magnetic fields and particle signatures in the vicinity of nightside geosynchronous altitudes in the first one-minute-interval of Pi2 onset: a case study. J. Atmos. Solar Terr. Phys. 62, 17-30, 2000.
Saka, O., D. Koga, and K. Hayashi, A plasma bulk motion in the midnight magnetosphere during auroral breakup inferred from all-sky image and magnetic field observations at geosynchronous altitudes. J.Atmos. Solar Terr.Phys., 69, 1063, 2007.
Saka, O., K. Hayashi, and M. Thomsen, First 10 min intervals of Pi2 onset as geosynchronous altitudes during the expansion of energetic ion regions in the nighttime sector. J. Atmos. Solar Terr. Phys., 72, 1100, 2010.
- Minor comments
Line 42
For S-aurora, auroras unfold the auroral pleat in a clockwise direction. For windup-auroras, auroras fold the sheet in a counterclockwise direction. Both S- and Windup-auroras show the S pattern when fully developed.
Lines 45-49
The scale size of the ion hole (auroral arc) is meso-scale of the order of 100s km. Ion holes observed by satellites in acceleration regions are in kinetic scale less than km [Temerin et al., Phys. Rew. Lett.,1982; Hasegawa and Sato, Phys. Fluids, 1982]. We assume that auroral arc is an “ion hole structure”.
Lines 61-70
“This event” refers to the event portrayed in Figure 2.
Lines 67-68
When an entire arc system rotates simultaneously, it is called “rotational type”.
Line 74
“Horizontal plasma flows” refers to N to S flows.
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AC3: 'Reply on RC2', Osuke Saka, 07 Apr 2026
Additional reply comments to RC2#
Regarding the excitation of surface waves, we are reminded of field line resonance by solar wind interactions with the magnetosphere [Chen and Hasegawa, JGR, 1974]. In the present case for the nighttime magnetosphere, we can assume that there were no jumps in densities and Alfven velocities but rather a jump in flow velocities. In these conditions, dispersion relation of the surface waves caused by the K-H instability is,
ω=U*k_perp.
U and k_perp denote shear flow velocity and perpendicular wave vector, respectively [Sake et al., 2010]. This relation is interpreted by a passage of structure with the characteristic wavelength (2π/k_perp) perpendicular to the field lines. Propagating boundary perturbations at U (Pi2 pulsation) produces slip motions [Saka et al., 2007].
Citation: https://doi.org/10.5194/egusphere-2026-11-AC3
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AC2: 'Reply on RC2', Osuke Saka, 06 Apr 2026
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