the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Nov 2023
Are drivers of northern lights in the ionosphere?
Abstract. It is reported that transverse electric fields penetrating the ionosphere either from distant space or from the upper atmosphere produce charge separations along the field lines by building up positive charges immediately above the ionosphere. Those parallel electric fields produced by the charge separation deserve auroral driver. This paper discusses why and how these internal drivers in the polar ionosphere are applied for the formation of spiral auroras in the dusk sector.
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RC1: 'Comment on angeo-2023-32', Anonymous Referee #1, 06 Feb 2024
Discussion of preprint angeo-2023-32
"Are drivers of northern lights in the ionosphere?"
by Osuke SakaThe preprint discusses "... how these internal drivers in the polar ionosphere are applied for the formation of spiral auroras in the dusk sector." Unfortunately the presented work is confused and lacks reference to basic physics, especially electrostatics and electrodynamics. In my opinion, though perhaps written in good faith, the preprint does not reach scientific standards that would be required for acceptance in a journal like Annales Geophysicae.
The author in some very qualititive sense applies electrostatic concepts by using a "charge sheet model". Figure 1 shows the potential of a "negative charge sheet" in the ionosphere. It seems to be rather the model of a charged bar than a sheet. As it does not extend upward, there is an upward electric field directly above the ionosphere, pointing away from the negative charges. Is this the E_|| depicted in Figure 2? The statement "... charge separations along the field lines by building up positive charges immediately above the ionosphere" suggests that this is the basic idea of the author.
This idea has already been published in Ann. Geophys. (Saka, 2023), after 4 reviewers had tried to rectify this for better agreement with numerous observations and accepted models of auroral particle acceleration, in my opinion with only limited success.
Figure 3 seems to prepare for a discussion of a new idea about observed dynamic winding of optical auroral forms. It is rather confused: does "Amp" stand for Ampére? If yes why and how is there an electric current? Otherwise the curve seems to show the static electric potential, field, and space charge of a 1-d well. The X coordinate is perhaps not meant to be vertical as in Figure 1, rather horizontal? An "ion hole" is pointed out, it seems to be around the center where the aurora is visible? All available scientific evidence tells us that auroral emissions occur from excited atoms and molecules. They are excited by collisions with energetic precipitating electrons which also ionize. Therefore the visible aurora typically goes together with strongly enhanced density, both ion and electron, preserving quasi-neutrality. How comes an "ion hole" into this?
Figures 4 and 5 leave me entirely mystified. What is plotted, how were the curves obtained? How comes dynamics into the so far static picture?
Falsely the static "charge sheet model" is attributed to previous works on the subject. For example, Hallinan and Davis (1970) rather demonstrated that a hydrodynamic Kelvin-Helmholtz instability in the sheared ExB flow at auroral arcs would be consistent with their pioneering observations of auroral curls of a few km scale sizes. But lines 28-30 in the preprint suggest that a "charge sheet model" was used by Hallinan and Davis (1970) and also Oguti (1974). It was not.
In lines 61-63, "Discharge may occur locally and intermittently accompanying auroral precipitation. Such correlations are reported in the paper titled “Hiss emitting auroral activity” [Oguti, 1975a]. Rather, Oguti (1975a) reports correlation between observations, "... cross correlation analysis between the temporal variations in luminosity of aurora1 structures and the temporal variations of hiss intensities." A "discharge" in aurora is an invention by the author of the preprint, and the term is not used by Oguti (1975).
In proper scientific writing the results of other works are described as original as possible, and possibly the own interpretation is clearly marked as such. Other works should not be presented in a distorted fashion.
In summary, the discussion is confused and in my opinion unphysical. Another version of the (unfortunately) already published "charge sheet model" is presented. The here additional consideration of observed dynamic aspects of aurora is not usable and lacks reference to basic physics. Other published work on the subject is presented in a distorting way. I recommend to reject the preprint for publication in Ann. Geophys.
Citation: https://doi.org/10.5194/angeo-2023-32-RC1 -
AC1: 'Reply on RC1', Osuke Saka, 12 Feb 2024
Author Comment to Referee 1
AC to RC1 (vertical and parallel electric fields in Figure 1 and Figure 2)
Figures 1 and 2 are perhaps somewhat confusing because the electric potentials increase with altitudes in Figure 1 while decreasing with altitudes in Figure 2. These are characteristic features of the ionospheric driver scenario [Saka, 2023].
Transverse electric fields transmitted from distant space or those arising from the neutral wind activate the polar ionosphere. They produce charge accumulations in the ionosphere (negative in one side and positive in other side) due to mobility difference of electrons and ions. Vertical electric fields due to local charges (Figure 1) instantaneously displace the mirror height of electrons bouncing along the field lines. Mirror height difference between ions and electrons built up a charge separation along the field lines. Parallel electric fields produced by the charge separation persist as quasi-neutral equilibrium solution of parallel electric field (Figure 2).
References,
Saka, O., Parallel electric fields produced by ionospheric injection, Ann. Geophys., 41, 369-373, 2023.
AC to RC2, 3 (ion hole)
It is assumed that precipitating electrons caused by a discharge in the auroral acceleration region produce auroral emissions that peak at 110km in the polar ionosphere. It is reasonable to assume that electron densities increased simultaneously in the E-layer. The auroral emission lifespan is short enough to follow auroral deformations that occur in less than one second.
Enhanced electron density expands horizontally in the E-layer and is somewhat elongated in east-west directions. At the longitudes where auroral emission is maximum, electron density may also peak. The converging potential region appears in the polar ionosphere.
Electron Larmor motions (rL=2.1m for 1keV) drift to ExB directions with E/B velocity. In the axisymmetric converging potential region, there occurs no residuals in the ExB drift. If converging potential region is elongated in east-west directions (arc alignment, principal axis), residuals appear perpendicular to the principal axis. They rotate arc alignment (see Figure A attached). Figure 3 gives potential profile along the principal axis. To localize converging potential region in the ionosphere, solitary potential profile resembling ion hole is assumed.
Figures 4 and 5 depict temporal development of the principal axis.
AC to RC4, 6 (charge sheet model)
There are two models for the spiral auroras, namely, charge sheet and current sheet.
Hallinan and Davis (1970) interpreted the deformation process of the auroral arc in terms of the KH instability of flow shears that produced small-scale auroras with wavelength of the order of 1skm. This interpretation is qualitatively consistent with the sheet beam distortions.
Oguti (1975) applied the negative charge excess to account for auroral deformation without considering scale size from 1skm to 100skm. This is because deformation processes, rotational symmetry (clockwise viewed along the field lines), and similarity of deformation speed, all manifest no matter the scale size.
The former case is a charge sheet model applied to small-scale aurora, while the latter case covers independent of the scale size.
References
Hallinan, T.J., and Davis, T.N., Small-scale auroral arc distortions, Planet. Space Sci., 1735-1744, 1970.
Oguti, T., Similarity between global auroral deformations in DAPP photographs and small scale deformations observed by a TV camera, J. Atmos. Terr. Phys., 37, 1413-1418, 1975.
AC to RC5 (hiss emissions)
Hiss emission recorded at the ground station would be VLF signals from electrons precipitating along the field lines and/or from the magnetosphere where the discharging processes occurred. For either case, observed correlation of Hiss signals and auroral activities by Oguti (1975) may indicate that VLF signals would be generated in low altitudes regions as opposed to distant space.
References
Oguti, T., Hiss emitting auroral activity, J. Atmo. Terr. Phys., 37, 761-768, 1975.
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AC1: 'Reply on RC1', Osuke Saka, 12 Feb 2024
-
RC2: 'Comment on angeo-2023-32', Anonymous Referee #2, 19 Mar 2024
Comments on “Are drivers of northern lights in the ionosphere?” by O. Saka
A general comment
The physical processes of electron excess charge accumulation in the ionosphere, how it is produced, maintained and relaxed, are of fundamental importance for the ionosphere-magnetosphere physics. The accumulation takes place wherever there is auroral electron precipitation, but it has not as yet been fully understood.
The present paper is based on Saka (2023) that proposed a scenario of a production process of an electron excess charge accumulation and its effect on physical parameters, particularly the field-aligned electric field in and above the ionosphere, that is, possible formation of the downward directed transient electric field due to negative charge accumulation and the steady-state upward directed electric field due to positive charge accumulation produced by the upward shift of the mirror height of electrons by the electric field due to the negative charge accumulation. The validness of the present paper thus requires that of Saka (2023). The reviewer wonders, however, if this scenario can work in and above the ionosphere, since it does not consider various other physical processes which interact each other in the ionosphere and the energetics.
For an example, when a negative excess charge accumulation is yielded in the ionosphere, surrounding positively charged ions move towards the negative charge accumulation by the electric field produced by the negative charges and tend to decrease the net excess charges, although complete neutralization cannot be attained. The place where these ions had been located got negatively charged, then ambient excess electrons there move along the field lines to the magnetosphere as field-aligned currents (FACs), resulting in the depletion of the densities of both ions and electrons (e.g., Karlsson, et al. (2007), arXiv preprint, arXiv:0704.1610). The neglect of this process and other possible processes in Saka (2023) brings about the very strong field-aligned electric field, a few 100 [mV/m] (Fig.3 of Saka (2023) ), which inevitably drives FACs. The parallel electric conductivity which is more than 10-100 [S/m] in and above the ionosphere (e.g., Kelley (1989), The Earth’s Ionosphere, Fig. B.4) gives the density of the corresponding FAC, 10-3~10-2 A/m2, which is at least 100-1000 times larger than that of intense FACs associated with substorms. This intense FAC would dissipate a huge amount of energy that the magnetosphere most probably cannot provide. The scenario proposed by Saka (2023) hence is likely to be impracticable. Since the validity of the scenario in Saka (2023) is a necessary condition for the acceptance of the present paper, the referee encourages the author first to check if the scenario of Saka (2023) is realizable in the ionosphere and to make necessary revisions of the scenario and the paper accordingly.
Specific comments
1) Conditions for the occurrence of the processes
The necessary condition for the rotational motion of the aurora mentioned explicitly in the paper seems to be only the existence of an electric field provided from the magnetosphere to the ionosphere. The electric field would then yield negative charge accumulation in the ionosphere which could move the mirror height of electrons to a higher altitude. On the other hand, electric fields are almost always provided from the magnetosphere to the ionosphere, nevertheless we observe S-shaped structure auroras only occasionally. What are additional necessary conditions for the occurrence of the S-shaped structure auroras?
2) The origin of electrons for the auroral precipitation
The paper argues that " Auroral precipitations are due to discharge of the steady-state parallel electric fields, occurring locally and intermittently in the inverted-V". Does this mean that the ambient electrons just above the ionosphere are accelerated and produce the aurora? Once the excess positive charges along a magnetic field line are discharged by the precipitating electrons, the perpendicular electric field is likely to be diminished and thereby the deformation process may terminate. How can the S-shaped structure auroras continue to be developed after the discharge?
3) The electric field required for the deformation
Show the estimated amplitude of the electric field required for the rotational symmetrical deformation of small- and large-scale S-shaped spirals at a constant speed (6~8 km/s).
4) Clarification of figures
Fig. 1: Is the negative charge (ε0divE<0) accumulation in green located at X=0 outside of the frame? Show where the positive charge accumulation is (outside of the figure?). Is the scale of the axis X same as that of the axes Y and Z? Note the approximate value of the unit, about 128 km? same as Saka 2023.
Fig. 2: Show where the negative and positive charge accumulation regions are, along with approximate heights.
Fig. 3: Use the spatial scales same as those in Fig. 1 particularly for the horizontal axis X. Is the axis X in this figure the axis Y in Fig. 1? Why is the ni-ne = 0 at the center of the accumulation, X=0?
Citation: https://doi.org/10.5194/angeo-2023-32-RC2 -
AC2: 'Reply on RC2', Osuke Saka, 26 Mar 2024
Author Comment to Referee 2
General comments (Intense FACs)
As demonstrated by the Ionospheric injection scenario [Saka, 2023], the polar ionosphere produces parallel electric fields of the order of 100 micro_V/m by internal processes. This value of amplitude may be acceptable during auroral precipitation [e.g., Chiu and Schulz, 1978]. Meanwhile, field-aligned currents (FACs) are calculated using velocity distribution functions of electrons modified by parallel potentials [Knight, 1973]. FACs estimated in this manner close via Pedersen currents in the ionosphere (1.0 micro_A/m3) [Saka, 2021].
If we adopt kinetic approach using velocity distribution functions, ionospheric injection scenario may partly explain M-I coupling.
References
Knight, S., Parallel electric fields, Planet. Space Sci., 21, 741-750, 1973.
Chiu, Y. T. and Schultz, M., Self-consistent particle and parallel electrostatic field distributions in the magnetospheric-ionospheric auroral region, J. Geophys. Res., 83, 629-642, 1978.
Saka, O., Ionospheric control of space weather, Ann. Geophys., 39, 455-460, 2021.
Specific comments
- Conditions for the occurrence of the process
The converging potential region may persist above the polar ionosphere during substorms. In contrast, S-auroras are observed less often because the discharge process can occur only occasionally in the converging potential region. Discharge generates auroral electrons in the upward electric field regions. Such electrons are accelerated by the parallel electric fields in the localized flux tube and thereby yield auroral arc (see Figure A). For a discussion of discharge, refer to specific comment 2.
- Origin of electrons for auroral precipitation
In upward electric field regions, electrons having perpendicular temperature anisotropy (T_para/T_perp >1) are a potential source of auroral electrons. If temperature anisotropy exceeds the upper limit somewhere in the upward electric field region, pitch angle diffusions initiated by VLF wave fill the loss cone by escaping electrons. Parallel electric fields remaining in the flux tube accelerate escaping electrons as auroral precipitation. Referred to as “discharge”, this process is analogous to a lightning flash during a thunderstorm.
- Electric fields required for deformation
In the polar ionosphere, electric fields of the order of 300-400mV/m yield the twist motion at 6-8 km/s. Meanwhile, discharge expands auroral acceleration region above the ionosphere at 6-8 km/s. Auroral deformation proceeds in the ionosphere and in the acceleration region.
- Clarification of figures
Figure 1
It is assumed that westward electric fields localized in a north-south direction are transmitted from distant space. They produce negative charge accumulations in the south side of ionosphere and positive in the north because of the mobility difference of electrons and ions. Figure B shows potential profiles in a vertical plane produced by two charge sheets in the ionosphere running parallel in an east-west direction. Charge sheet is narrow in north-south direction (128km for example) and wide in east-west direction (1280km). The positive charge sheet in the north and the negative sheet in the south are separated by 128km, a distance identical to the charge sheet width. As shown by a green bar, the electrostatic potential increases with increasing altitude above negative charge sheet while decreasing with altitudes above the positive charge sheet. The paired charge sheets generate converging and diverging potential regions above them in a complementary manner. Auroral precipitation occurs above the negative charge sheet.
Figure 2
A positive charge was distributed between 100km and 3000km with a peak at 1500km. Negative charge (electrons) occurred beyond 3000km. These charge separations do not match the converging potential structure. To resolve such inconsistencies, we need additional processes described below.
Hot plasmas bounce back and forth in mirror geometry. Bouncing plasmas maintain parallel potentials by changing their pitch-angle as a solution of quasi-neutral equilibrium [Stern, 1981]. When the potential decreases along field lines with altitudes in the upward electric field, pitch-angle simultaneously increases with altitudes for electrons and decreases for ions [Persson, 1966]. Consequently, parallel velocities of electrons become slower and those of ions become faster as the altitudes increase. Average space charge, dQ=q*ds/(t*v_para) (Alfven and Falthammar, 1963), is yielded accordingly. Plasma distribution in Figure C showing electron rich regions at the center top are a solution of quasi-neutral equilibrium.
Reference
Alfven, H., and Falthammar, C-G., Cosmical Electrodynamics, Oxford University Press, London, 1963.
Persson, H., Electric field parallel to the magnetic field in a low-density plasma, Phys. Pluid, 9, 1090-1098, 1966.
Stern, .P., One-dimensional models of quasi-neutral parallel electric fields, J. Geophys. Res., 86, 5839-5860, 1981.
Figure 3
Potential profile in Figure 3 is produced by electrons precipitated in the polar ionosphere. Scale size of the auroral arc varies in X from 1 km to 100s km. Figure D shows clockwise motion of auroras. (A) ni-ne has a peak at the center (X=0) whereas (B) ni-ne has no peaks. The deformation pattern differs according to the profile of ni-ne as exemplified by jetting water from sprinkler (A) and rotating wheel (B).
Density profile with ni-ne =0at X=0 may more commonly occur because electrons often have a single pair of ions in the region X<0 or in X>0.
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AC2: 'Reply on RC2', Osuke Saka, 26 Mar 2024
Status: closed
-
RC1: 'Comment on angeo-2023-32', Anonymous Referee #1, 06 Feb 2024
Discussion of preprint angeo-2023-32
"Are drivers of northern lights in the ionosphere?"
by Osuke SakaThe preprint discusses "... how these internal drivers in the polar ionosphere are applied for the formation of spiral auroras in the dusk sector." Unfortunately the presented work is confused and lacks reference to basic physics, especially electrostatics and electrodynamics. In my opinion, though perhaps written in good faith, the preprint does not reach scientific standards that would be required for acceptance in a journal like Annales Geophysicae.
The author in some very qualititive sense applies electrostatic concepts by using a "charge sheet model". Figure 1 shows the potential of a "negative charge sheet" in the ionosphere. It seems to be rather the model of a charged bar than a sheet. As it does not extend upward, there is an upward electric field directly above the ionosphere, pointing away from the negative charges. Is this the E_|| depicted in Figure 2? The statement "... charge separations along the field lines by building up positive charges immediately above the ionosphere" suggests that this is the basic idea of the author.
This idea has already been published in Ann. Geophys. (Saka, 2023), after 4 reviewers had tried to rectify this for better agreement with numerous observations and accepted models of auroral particle acceleration, in my opinion with only limited success.
Figure 3 seems to prepare for a discussion of a new idea about observed dynamic winding of optical auroral forms. It is rather confused: does "Amp" stand for Ampére? If yes why and how is there an electric current? Otherwise the curve seems to show the static electric potential, field, and space charge of a 1-d well. The X coordinate is perhaps not meant to be vertical as in Figure 1, rather horizontal? An "ion hole" is pointed out, it seems to be around the center where the aurora is visible? All available scientific evidence tells us that auroral emissions occur from excited atoms and molecules. They are excited by collisions with energetic precipitating electrons which also ionize. Therefore the visible aurora typically goes together with strongly enhanced density, both ion and electron, preserving quasi-neutrality. How comes an "ion hole" into this?
Figures 4 and 5 leave me entirely mystified. What is plotted, how were the curves obtained? How comes dynamics into the so far static picture?
Falsely the static "charge sheet model" is attributed to previous works on the subject. For example, Hallinan and Davis (1970) rather demonstrated that a hydrodynamic Kelvin-Helmholtz instability in the sheared ExB flow at auroral arcs would be consistent with their pioneering observations of auroral curls of a few km scale sizes. But lines 28-30 in the preprint suggest that a "charge sheet model" was used by Hallinan and Davis (1970) and also Oguti (1974). It was not.
In lines 61-63, "Discharge may occur locally and intermittently accompanying auroral precipitation. Such correlations are reported in the paper titled “Hiss emitting auroral activity” [Oguti, 1975a]. Rather, Oguti (1975a) reports correlation between observations, "... cross correlation analysis between the temporal variations in luminosity of aurora1 structures and the temporal variations of hiss intensities." A "discharge" in aurora is an invention by the author of the preprint, and the term is not used by Oguti (1975).
In proper scientific writing the results of other works are described as original as possible, and possibly the own interpretation is clearly marked as such. Other works should not be presented in a distorted fashion.
In summary, the discussion is confused and in my opinion unphysical. Another version of the (unfortunately) already published "charge sheet model" is presented. The here additional consideration of observed dynamic aspects of aurora is not usable and lacks reference to basic physics. Other published work on the subject is presented in a distorting way. I recommend to reject the preprint for publication in Ann. Geophys.
Citation: https://doi.org/10.5194/angeo-2023-32-RC1 -
AC1: 'Reply on RC1', Osuke Saka, 12 Feb 2024
Author Comment to Referee 1
AC to RC1 (vertical and parallel electric fields in Figure 1 and Figure 2)
Figures 1 and 2 are perhaps somewhat confusing because the electric potentials increase with altitudes in Figure 1 while decreasing with altitudes in Figure 2. These are characteristic features of the ionospheric driver scenario [Saka, 2023].
Transverse electric fields transmitted from distant space or those arising from the neutral wind activate the polar ionosphere. They produce charge accumulations in the ionosphere (negative in one side and positive in other side) due to mobility difference of electrons and ions. Vertical electric fields due to local charges (Figure 1) instantaneously displace the mirror height of electrons bouncing along the field lines. Mirror height difference between ions and electrons built up a charge separation along the field lines. Parallel electric fields produced by the charge separation persist as quasi-neutral equilibrium solution of parallel electric field (Figure 2).
References,
Saka, O., Parallel electric fields produced by ionospheric injection, Ann. Geophys., 41, 369-373, 2023.
AC to RC2, 3 (ion hole)
It is assumed that precipitating electrons caused by a discharge in the auroral acceleration region produce auroral emissions that peak at 110km in the polar ionosphere. It is reasonable to assume that electron densities increased simultaneously in the E-layer. The auroral emission lifespan is short enough to follow auroral deformations that occur in less than one second.
Enhanced electron density expands horizontally in the E-layer and is somewhat elongated in east-west directions. At the longitudes where auroral emission is maximum, electron density may also peak. The converging potential region appears in the polar ionosphere.
Electron Larmor motions (rL=2.1m for 1keV) drift to ExB directions with E/B velocity. In the axisymmetric converging potential region, there occurs no residuals in the ExB drift. If converging potential region is elongated in east-west directions (arc alignment, principal axis), residuals appear perpendicular to the principal axis. They rotate arc alignment (see Figure A attached). Figure 3 gives potential profile along the principal axis. To localize converging potential region in the ionosphere, solitary potential profile resembling ion hole is assumed.
Figures 4 and 5 depict temporal development of the principal axis.
AC to RC4, 6 (charge sheet model)
There are two models for the spiral auroras, namely, charge sheet and current sheet.
Hallinan and Davis (1970) interpreted the deformation process of the auroral arc in terms of the KH instability of flow shears that produced small-scale auroras with wavelength of the order of 1skm. This interpretation is qualitatively consistent with the sheet beam distortions.
Oguti (1975) applied the negative charge excess to account for auroral deformation without considering scale size from 1skm to 100skm. This is because deformation processes, rotational symmetry (clockwise viewed along the field lines), and similarity of deformation speed, all manifest no matter the scale size.
The former case is a charge sheet model applied to small-scale aurora, while the latter case covers independent of the scale size.
References
Hallinan, T.J., and Davis, T.N., Small-scale auroral arc distortions, Planet. Space Sci., 1735-1744, 1970.
Oguti, T., Similarity between global auroral deformations in DAPP photographs and small scale deformations observed by a TV camera, J. Atmos. Terr. Phys., 37, 1413-1418, 1975.
AC to RC5 (hiss emissions)
Hiss emission recorded at the ground station would be VLF signals from electrons precipitating along the field lines and/or from the magnetosphere where the discharging processes occurred. For either case, observed correlation of Hiss signals and auroral activities by Oguti (1975) may indicate that VLF signals would be generated in low altitudes regions as opposed to distant space.
References
Oguti, T., Hiss emitting auroral activity, J. Atmo. Terr. Phys., 37, 761-768, 1975.
-
AC1: 'Reply on RC1', Osuke Saka, 12 Feb 2024
-
RC2: 'Comment on angeo-2023-32', Anonymous Referee #2, 19 Mar 2024
Comments on “Are drivers of northern lights in the ionosphere?” by O. Saka
A general comment
The physical processes of electron excess charge accumulation in the ionosphere, how it is produced, maintained and relaxed, are of fundamental importance for the ionosphere-magnetosphere physics. The accumulation takes place wherever there is auroral electron precipitation, but it has not as yet been fully understood.
The present paper is based on Saka (2023) that proposed a scenario of a production process of an electron excess charge accumulation and its effect on physical parameters, particularly the field-aligned electric field in and above the ionosphere, that is, possible formation of the downward directed transient electric field due to negative charge accumulation and the steady-state upward directed electric field due to positive charge accumulation produced by the upward shift of the mirror height of electrons by the electric field due to the negative charge accumulation. The validness of the present paper thus requires that of Saka (2023). The reviewer wonders, however, if this scenario can work in and above the ionosphere, since it does not consider various other physical processes which interact each other in the ionosphere and the energetics.
For an example, when a negative excess charge accumulation is yielded in the ionosphere, surrounding positively charged ions move towards the negative charge accumulation by the electric field produced by the negative charges and tend to decrease the net excess charges, although complete neutralization cannot be attained. The place where these ions had been located got negatively charged, then ambient excess electrons there move along the field lines to the magnetosphere as field-aligned currents (FACs), resulting in the depletion of the densities of both ions and electrons (e.g., Karlsson, et al. (2007), arXiv preprint, arXiv:0704.1610). The neglect of this process and other possible processes in Saka (2023) brings about the very strong field-aligned electric field, a few 100 [mV/m] (Fig.3 of Saka (2023) ), which inevitably drives FACs. The parallel electric conductivity which is more than 10-100 [S/m] in and above the ionosphere (e.g., Kelley (1989), The Earth’s Ionosphere, Fig. B.4) gives the density of the corresponding FAC, 10-3~10-2 A/m2, which is at least 100-1000 times larger than that of intense FACs associated with substorms. This intense FAC would dissipate a huge amount of energy that the magnetosphere most probably cannot provide. The scenario proposed by Saka (2023) hence is likely to be impracticable. Since the validity of the scenario in Saka (2023) is a necessary condition for the acceptance of the present paper, the referee encourages the author first to check if the scenario of Saka (2023) is realizable in the ionosphere and to make necessary revisions of the scenario and the paper accordingly.
Specific comments
1) Conditions for the occurrence of the processes
The necessary condition for the rotational motion of the aurora mentioned explicitly in the paper seems to be only the existence of an electric field provided from the magnetosphere to the ionosphere. The electric field would then yield negative charge accumulation in the ionosphere which could move the mirror height of electrons to a higher altitude. On the other hand, electric fields are almost always provided from the magnetosphere to the ionosphere, nevertheless we observe S-shaped structure auroras only occasionally. What are additional necessary conditions for the occurrence of the S-shaped structure auroras?
2) The origin of electrons for the auroral precipitation
The paper argues that " Auroral precipitations are due to discharge of the steady-state parallel electric fields, occurring locally and intermittently in the inverted-V". Does this mean that the ambient electrons just above the ionosphere are accelerated and produce the aurora? Once the excess positive charges along a magnetic field line are discharged by the precipitating electrons, the perpendicular electric field is likely to be diminished and thereby the deformation process may terminate. How can the S-shaped structure auroras continue to be developed after the discharge?
3) The electric field required for the deformation
Show the estimated amplitude of the electric field required for the rotational symmetrical deformation of small- and large-scale S-shaped spirals at a constant speed (6~8 km/s).
4) Clarification of figures
Fig. 1: Is the negative charge (ε0divE<0) accumulation in green located at X=0 outside of the frame? Show where the positive charge accumulation is (outside of the figure?). Is the scale of the axis X same as that of the axes Y and Z? Note the approximate value of the unit, about 128 km? same as Saka 2023.
Fig. 2: Show where the negative and positive charge accumulation regions are, along with approximate heights.
Fig. 3: Use the spatial scales same as those in Fig. 1 particularly for the horizontal axis X. Is the axis X in this figure the axis Y in Fig. 1? Why is the ni-ne = 0 at the center of the accumulation, X=0?
Citation: https://doi.org/10.5194/angeo-2023-32-RC2 -
AC2: 'Reply on RC2', Osuke Saka, 26 Mar 2024
Author Comment to Referee 2
General comments (Intense FACs)
As demonstrated by the Ionospheric injection scenario [Saka, 2023], the polar ionosphere produces parallel electric fields of the order of 100 micro_V/m by internal processes. This value of amplitude may be acceptable during auroral precipitation [e.g., Chiu and Schulz, 1978]. Meanwhile, field-aligned currents (FACs) are calculated using velocity distribution functions of electrons modified by parallel potentials [Knight, 1973]. FACs estimated in this manner close via Pedersen currents in the ionosphere (1.0 micro_A/m3) [Saka, 2021].
If we adopt kinetic approach using velocity distribution functions, ionospheric injection scenario may partly explain M-I coupling.
References
Knight, S., Parallel electric fields, Planet. Space Sci., 21, 741-750, 1973.
Chiu, Y. T. and Schultz, M., Self-consistent particle and parallel electrostatic field distributions in the magnetospheric-ionospheric auroral region, J. Geophys. Res., 83, 629-642, 1978.
Saka, O., Ionospheric control of space weather, Ann. Geophys., 39, 455-460, 2021.
Specific comments
- Conditions for the occurrence of the process
The converging potential region may persist above the polar ionosphere during substorms. In contrast, S-auroras are observed less often because the discharge process can occur only occasionally in the converging potential region. Discharge generates auroral electrons in the upward electric field regions. Such electrons are accelerated by the parallel electric fields in the localized flux tube and thereby yield auroral arc (see Figure A). For a discussion of discharge, refer to specific comment 2.
- Origin of electrons for auroral precipitation
In upward electric field regions, electrons having perpendicular temperature anisotropy (T_para/T_perp >1) are a potential source of auroral electrons. If temperature anisotropy exceeds the upper limit somewhere in the upward electric field region, pitch angle diffusions initiated by VLF wave fill the loss cone by escaping electrons. Parallel electric fields remaining in the flux tube accelerate escaping electrons as auroral precipitation. Referred to as “discharge”, this process is analogous to a lightning flash during a thunderstorm.
- Electric fields required for deformation
In the polar ionosphere, electric fields of the order of 300-400mV/m yield the twist motion at 6-8 km/s. Meanwhile, discharge expands auroral acceleration region above the ionosphere at 6-8 km/s. Auroral deformation proceeds in the ionosphere and in the acceleration region.
- Clarification of figures
Figure 1
It is assumed that westward electric fields localized in a north-south direction are transmitted from distant space. They produce negative charge accumulations in the south side of ionosphere and positive in the north because of the mobility difference of electrons and ions. Figure B shows potential profiles in a vertical plane produced by two charge sheets in the ionosphere running parallel in an east-west direction. Charge sheet is narrow in north-south direction (128km for example) and wide in east-west direction (1280km). The positive charge sheet in the north and the negative sheet in the south are separated by 128km, a distance identical to the charge sheet width. As shown by a green bar, the electrostatic potential increases with increasing altitude above negative charge sheet while decreasing with altitudes above the positive charge sheet. The paired charge sheets generate converging and diverging potential regions above them in a complementary manner. Auroral precipitation occurs above the negative charge sheet.
Figure 2
A positive charge was distributed between 100km and 3000km with a peak at 1500km. Negative charge (electrons) occurred beyond 3000km. These charge separations do not match the converging potential structure. To resolve such inconsistencies, we need additional processes described below.
Hot plasmas bounce back and forth in mirror geometry. Bouncing plasmas maintain parallel potentials by changing their pitch-angle as a solution of quasi-neutral equilibrium [Stern, 1981]. When the potential decreases along field lines with altitudes in the upward electric field, pitch-angle simultaneously increases with altitudes for electrons and decreases for ions [Persson, 1966]. Consequently, parallel velocities of electrons become slower and those of ions become faster as the altitudes increase. Average space charge, dQ=q*ds/(t*v_para) (Alfven and Falthammar, 1963), is yielded accordingly. Plasma distribution in Figure C showing electron rich regions at the center top are a solution of quasi-neutral equilibrium.
Reference
Alfven, H., and Falthammar, C-G., Cosmical Electrodynamics, Oxford University Press, London, 1963.
Persson, H., Electric field parallel to the magnetic field in a low-density plasma, Phys. Pluid, 9, 1090-1098, 1966.
Stern, .P., One-dimensional models of quasi-neutral parallel electric fields, J. Geophys. Res., 86, 5839-5860, 1981.
Figure 3
Potential profile in Figure 3 is produced by electrons precipitated in the polar ionosphere. Scale size of the auroral arc varies in X from 1 km to 100s km. Figure D shows clockwise motion of auroras. (A) ni-ne has a peak at the center (X=0) whereas (B) ni-ne has no peaks. The deformation pattern differs according to the profile of ni-ne as exemplified by jetting water from sprinkler (A) and rotating wheel (B).
Density profile with ni-ne =0at X=0 may more commonly occur because electrons often have a single pair of ions in the region X<0 or in X>0.
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AC2: 'Reply on RC2', Osuke Saka, 26 Mar 2024
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