Dynamics of day and night aurora during substorms

The motion of auroral forms on the day- and nightside of the Earth has been studied during different substorm phases by means of all-sky camera films. A substorm is characterized by a shift of the luminescence region towards the equator at noon and mainly towards the pole at midnight. However, individual forms drift predominantly toward the pole on the dayside and towards the equator on the nightside. The velocity of the poleward motion at noon is largest during the expansive phase of a substorm and amounts on the average to 330 $\text{m}\text{sec}$ but even during relatively quiet magnetic conditions a poleward motion is observed.     

Three-dimensional current system in different phases of a substorm

It is assumed that the three-dimensional current system of a substorm passes three successive stages. (1) When a dawn-to-dusk magnetospheric electric field appears, a current system with field-aligned currents at the poleward boundary of the auroral zone arises. An equivalent ionospheric current system calculated, taking into account a day-night asymmetry of ionospheric conductivity, looks like the well-known DP-2 system including an eastward low-latitude current and a greater magnitude of the dusk vortex in comparison with the dawn one. (2) An electric drift of plasma towards the Earth leads to the appearance of a westward partial ring current increasing in time. This current is closed by field-aligned currents at the equatorward boundary of the auroral zone. The calculated equivalent current system is similar to the well-known one of the precursory phase. (3) An increase of the auroral ionospheric conductivity during the expansive phase produces an increase of all currents and a turning of field-aligned currents at the equatorward boundary of the auroral zone relative to those at the poleward one. The calculated equivalent current system is similar to the DP-1 system.     

A uniform belt of diffuse auroral emission seen by the ISIS-2 scanning photometer

One of the most striking and persistent features in high latitude regions as seen by the ISIS-2 scanning auroral photometer is a fairly uniform belt of diffuse auroral emission extending along the auroral oval. Indications are that this region follows, contributes to, and may in a sense actually define the auroral oval during quiet times.The diffuse belt is sharply defined at its equatorward edge, which is located at an invariant latitude of about 65° in the midnight sector during relatively low magnetic activity (K_p = 1?3). The poleward edge of the region is not as sharply defined but is typically at about 68°. Discrete auroras (arcs and bands) are located, in general, near the poleward boundary of the diffuse aurora. The position of the belt appears to be relatively unaffected by the occurrence of individual substorms, even when discrete forms have moved well poleward. Representative intensities at 5577 Å are 1–2 kR (corrected for albedo) at quiet times and may reach 5 kR during an auroral substorm.It appears that the mantle aurora and proton aurora constitute this diffuse aurora in the midnight sector. Precipitating protons and electrons both contribute to the emissions in this region.     

Precipitation of > 115 kev protons in the evening and forenoon sectors in relation to the magnetic activity

Data from a low altitude polar orbiting satellite, on auroral protons >115 keV in the evening and forenoon sectors, are presented.In the forenoon sector there is a weak but fairly steady precipitation at Λ ≈ 75° during quiet conditions. This precipitation is situated at higher invariant latitudes at local noon than at local dawn and can probably be ascribed to the high energy tail of the polar cleft protons. During moderately disturbed conditions, especially during the recovery phase of geomagnetic storms, there are some seemingly more “impulsive” precipitation events at Λ ≈ 65°. During very disturbed conditions these two precipitation zones in the forenoon sector seem to merge.In the evening sector a rather sharp equatorward boundary of the main precipitation, at Λ ≈ 69° during quiet conditions, varies fairly smoothly from pass to pass. South of this boundary, at invariant latitudes around 62°, there is a steady weak drizzle from the radiation belt. Due to a longitudinal effect this drizzle, as recorded by the satellite, shows a diurnal variation.The equatorward boundaries of the main precipitation at both local times move equatorward with increasing ring current strength. When D_st gets less than about — 100nT, the poleward boundaries are found to move equatorward too. From an attempt to reveal some of the substorm-dependent changes of the precipitation it is found that an equatorward shift of the precipitation areas takes place during, or just prior to, the substorm expansive phase, accompanied by a large intensity increase in the evening sector, whereas the recovery phase is linked with a poleward expansion of the precipitation at both local times.     

Effects of interplanetary magnetic field and magnetospheric substorm variations on the dayside aurora

Photometric observations of dayside auroras are compared with simultaneous measurements of geomagnetic disturbances from meridian chains of stations on the dayside and on the nightside to document the dynamics of dayside auroras in relation to local and global disturbances. These observations are related to measurements of the interplanetary magnetic field (IMF) from the satellites ISEE-1 and 3. It is shown that the dayside auroral zone shifts equatorward and poleward with the growth and decay of the circum-oval/polar cap geomagnetic disturbance and with negative and positive changes in the north-south component of the interplanetary magnetic field (B_z). The geomagnetic disturbance associated with the auroral shift is identified as the DP2 mode. In the post-noon sector the horizontal disturbance vector of the geomagnetic field changes from southward to northward with decreasing latitude, thereby changing sign near the center of the oval precipitation region. Discrete auroral forms are observed close to or equatorward of the ΔH = 0 line which separates positive and negative H-component deflections. This reversal moves in latitude with the aurora and it probably reflects a transition of the electric field direction at the polar cap boundary. Thus, the discrete auroral forms observed on the dayside are in the region of sunward-convecting field lines. A model is proposed to explain the equatorward and poleward movement of the dayside oval in terms of a dayside current system which is intensified by a southward movement of the IMF vector. According to this model, the Pedersen component of the ionospheric current is connected with the magnetopause boundary layer via field-aligned current (FAC) sheets. Enhanced current intensity, corresponding to southward auroral shift, is consistent with increased energy extraction from the solar wind. In this way the observed association of DP2 current system variations and auroral oval expansion/contraction is explained as an effect of a global, ‘direct’ response of the electromagnetic state of the magnetosphere due to the influence of the solar wind magnetic field. Estimates of electric field, current, and the rate of Joule heat dissipation in the polar cap ionosphere are obtained from the model.     

The development of the substorm in auroral radio absorption

Observations with a meridional chain of riometers in the auroral zone show that the onset of an auroral absorption substorm is usually preceded by weak activity. This activity sometimes takes the form of a weak bay-like event, moving slowly equatorward. Its velocity (typically 200–500 m/sec) is consistent with the inward convection of energetic plasma in the magnetosphere— that is, with E × B drift in an electric field of about 1.3 mV/m. The bay intensifies as it moves, indicating energization of the plasma during convection.     

Observed connections between apparent polar cap features and the instantaneous diffuse auroral oval

Images of the instantaneous nightside auroral distribution reveal that at times the orientation of auroral oval arcs changes to become characteristic of polar cap arcs. These connecting arcs all terminate in the diffuse aurora in the midnight sector, and their separation from the equatorward boundary of the diffuse aurora generally increases away from the midnight termination. The occurrence of these features requires a northward interplanetary magnetic field (positive B_z) as well as low magnetic activity. The existence of connecting arcs and the observation that they are at times the poleward boundary of weak diffuse emission indicate that the poleward boundary of auroral emissions can be significantly modified during non-substorm periods. Such a distortion implies that there can be a modification of the standard convection pattern in the magnetosphere during periods of positive B_z to produce expanded regions of sunward convection in the high latitude ionosphere.     

Evidence for a poleward meridional flow on the sun

We define for observational study two subsets of all polar zone filaments, which we call polemost filaments and polar filament bands. The behavior of the mean latitude of both the polemost filaments and the polar filament bands is examined and compared with the evolution of the polar magnetic field over an activity cycle as recently distilled by Howard and LaBonte (1981) from the past 13 years of Mt. Wilson full-disk magnetograms. The magnetic data reveal that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions that originate in active region latitudes and subsequently drift to the poles. After leaving the active-region latitudes, these unipolar f-polarity regions do not spread equatorward even though there is less net flux equatorward; this indicates that the f-polarity regions are carried poleward by a meridional flow, rather than by diffusion. The polar zone filaments are an independent tracer which confirms both the episodic polar field formation and the meridional flow. We find:

1. The mean latitude of the polemost filaments tracks the boundary of the polar field cap and undergoes an equatorward dip during each arrival of additional polar field.
2. Polar filament bands track the boundary latitudes of the unipolar regions, drifting poleward with the regions at about 10 m s^-1.
3. The Mt. Wilson magnetic data, combined with a simple model calculation, show that the filament drift expected from diffusion alone would be slower than observed, and in some cases would be equatorward rather than poleward.
4. The observation that filaments drift poleward along with the magnetic regions shows that fields of both polarities are carried by the meridional flow, as would be expected, rather than only the f-polarity flux which dominates the strength. This leads to the prediction that in the mid-latitudes during intervals between the passage of f-polarity regions, both polarities are present in nearly equal amounts. This prediction is confirmed by the magnetic data.

     

A synoptic investigation of particle precipitation dynamics for 60 substorms in IQSY (1964–1965) and IASY (1969)

Sixty auroral absorption substorms (30 in IQSY and 30 in IASY) have been analysed on the basis of riometer-recordings taken at some 40 stations distributed over auroral, subauroral and polar cap latitudes. Synoptic maps showing isoabsorption curves have been produced every 15 min (sometimes every 5 min) of the 60 substorms; 705 maps altogether.Some of the results of the analysis are as follows.Initiation of a substorm most frequently occurs near midnight but may occur anywhere between early evening and late morning. The time of onset becomes earlier and the latitude of onset moves equatorward as the level of magnetic activity increases.The longitude expansion velocities are contained in the range 0.7–7 km/sec except for a few extreme values which exceed 20 km/sec.The auroral absorption eastward expansion velocity is smaller than the corresponding velocity of the boundary of the region of activation of the visual aurora after break up by a factor $\text{14}\text{?12}$.The expansion velocity corresponds, in general, to drift velocities of electrons of energies in the range 50–300 keV but, for the extreme speeds, electron energies around 1 MeV are needed.Expansion of the absorption in the westward direction was seen in about half of the substorms studied. In about half of these, expansion along the auroral oval could be indentified, but in almost all of these cases some expansion in the auroral zone latitudes was also seen. In about an equal number of events, expansion was confined primarily to the auroral zone.The velocity of the westward expansion was about 1 km/sec along the auroral oval (i.e. approximately equal with the speed of the westward travelling surge) but about 2 km/sec along the auroral zone.The meridional expansion velocities found agree well with those measured for visual aurora (? 1 km/sec).The variability of the behaviour of different substorms is very large. To illuminate this the following may be mentioned, in addition to what has been stated above about the statistics.Although the absorption maximum practically always moves eastward from the initiation region, exceptions have been seen in which the maximum started moving west and in a later phase went eastward.Sometimes the absorption maximum stays in the injection area or very close to it, although in most cases it moves eastward into the dayside. In extreme eases it has been found to move more than 270° in the eastward direction.There are auroral absorption substorms in which injection seems to take place in more than one area simultaneously.The observations cannot all be understood in terms of gradient and curvature drift of electrons from a small area of injection only. A broad intrusion of hot plasma from the tail into the inner magnetosphere seems to be needed.No strong dependence of particle precipitation on the illumination of the upper ionosphere by sunlight was seen. The results do, therefore, not support the hypothesis of Brice and Lucas (1971) that cold plasma density increases, originating in the ionosphere, significantly increase the precipitation rate of energetic trapped particles.     

Modeling the midlatitude F-region ionospheric storm using east-west drift and a meridional wind

Under magnetically quiet conditions, ionospheric plasma in the midlatitude F-region corotates with the Earth and relative east-west drifts are small compared to the corotation velocity. During magnetic storms, however, the enhanced dawn-to-dusk magnetospheric convection electric field often penetrates into the midlatitude region, where it maps into the ionosphere as a poleward electric field in the 18:00 LT sector, producing a strong westward plasma drift. To evaluate the ionospheric response to this east-west drift, the time-dependent O⁺ continuity equation is solved numerically, including the effects of production by photoionization, loss by charge exchange and transport by diffusion, neutral wind and $\text{E}\text{×}\text{B}$ drift. In this investigation only the neutral wind's meridional component and east-west $\text{E}\text{×}\text{B}$ drift are included. It is found that an enhanced equatorward wind coupled with westward drift produces an enhancement in the peak electron density (NMAX(F2)) and in the electron content (up to 1000 km) in the afternoon sector and a subsequent greater-than-normal decay in ionization after 18:00 LT. These results agree in general with midlatitude F-region ionospheric storm observations of NMAX(F2) and electron content which show an afternoon enhancement over quiet-time values followed by an abrupt transition to lower-than-normal values. Westward drift appears to be a sufficient mechanism in bringing about this sharp transition.     

The structure of the polar ionosphere during exceptionally quiet periods

Knowledge of the structure of the polar ionosphere during exceptionally quiet periods is basic for studying complicated ionospheric behaviors during disturbances. On the basis of data from an airborne ionosonde as well as a meridian chain of ground-basedionosondes, the circumpolar structure of the E,-and F-regions is elucidated. There are two circumpolar zones of E-region ionization with differing characteristics. The first is an auroral E,-layer and/or retarded type sporadic E-band that has previously (Whalen et al., 1971) been found to be identical with the continuous aurora. The second is a zone of non-retarded type spora die E located poleward of the former band. In general, discrete auroras are co-located with the latter. The main trough, a prominent feature of the night sector F-region, is most pronounced in the early morning. The main trough is bounded on the poleward side by a well defined ‘wall’ of F-region ionization. The night sector poleward trough wall is located approximately three degrees of latitude equatorward of the auroral oval. A ‘plateau’ of F-region ionization extends from the poleward trough wall to the auroral oval.     

Auroral displays near the ‘foot’ of the field line of the ATS-5 satellite

This paper presents a brief summary of an extensive correlative study of ATS-5 particle and magnetic field data with all-sky photographs from Great Whale River which is near the ‘foot’ of the field lines passing through the ATS-5 satellite. In particular, an effort is made to identify specific particle features with specific auroral displays during substorms, such as a westward travelling surge, poleward expansive motion and drifting patches. Some of the important findings are (i) in early evening hours, the first encounter of ATS-5 with hot plasma is associated with the equatorward shift of the diffuse aurora, but not necessarily with westward travelling surges (even when the satellite is embedded in the plasma sheet.) (ii) In the midnight sector, an injection corresponds very well to the initial brightening of an auroral arc. (iii) Specific features of morning sector auroras (for example, drifting patches) are difficult to correlate with specific particle features (gross features, but not specific).Comparing these results with particle data from low-latitude polar orbiting satellites, it is concluded that the plasma sheet near the earthward edge (consisting of plasmas injected during earlier substorms) is little affected during substorms.     

Topside optical view of the dayside cleft aurora

Photometers on the ISIS-II spacecraft provide a view of the atomic oxygen 5577 and 6300 Å emissions and the $\text{N}{}_2{}^{\mathrm{+}}\text{3914}\text{A}\text{?}$ emission detected as dayside aurora in the magnetospheric cleft region. The 6300 Å emission forms a continuous and permanent band across the noon sector, at about 78° invariant latitude, with a defined region of maximum intensity that is never less than 2kR (uncorrected for albedo), and is centred near magnetic noon. There are significant differences in the intensity patterns on either side of noon and their responses to geomagnetic activity. Discrete 3914 Å auroral forms appear within this region, at preferred locations that cannot be precisely specified, but which tend to the poleward edge of the 6300 Å emission in the evening, and the equatorward edge in the morning where the difference between the two emissions is greatest. It is concluded that the discrete auroras observed by all-sky cameras in the day sector do follow the 6300 Å emission through the cleft region, though a definite cleft boundary is not defined. Substantial 6300 Å emission having a peak intensity near noon is also seen in the low latitude “outer auroral belt”, while the diffuse 3914 Å emission tends to show a relative minimum near noon. On the morning side the 3914 Å intensity is displaced to lower latitude and earlier local times, compared to the 6300 Å emission.     

Subsurface Meridional Flow from HMI Using the Ring-Diagram Pipeline

We have determined the meridional flows in subsurface layers for 18 Carrington rotations (CR 2097 to 2114) analyzing high-resolution Dopplergrams obtained with the Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO). We are especially interested in flows at high latitudes up to 75^° in order to address the question whether the meridional flow remains poleward or reverses direction (so-called counter cells). The flows have been determined in depth from near-surface layers to about 16 Mm using the HMI ring-diagram pipeline. The measured meridional flows show systematic effects, such as a variation with the B ₀-angle and a variation with central meridian distance (CMD). These variations have been taken into account to lead to more reliable flow estimates at high latitudes. The corrected average meridional flow is poleward at most depths and latitudes with a maximum amplitude of about $20~\mathrm{m\,s}^{-1}$ near 37.5^° latitude. The flows are more poleward on the equatorward side of the mean latitude of magnetic activity at 22^° and less poleward on the poleward side, which can be interpreted as convergent flows near the mean latitude of activity. The corrected meridional flow is poleward at all depths within ±?67.5^° latitude. The corrected flow is equatorward only at 75^° latitude in the southern hemisphere at depths between about 4 and 8 Mm and at 75^° latitude in the northern hemisphere only when the B ₀ angle is barely large enough to measure flows at this latitude. These counter cells are most likely the remains of an insufficiently corrected B ₀-angle variation and not of solar origin. Flow measurements and B ₀-angle corrections are difficult at the highest latitude because these flows are only determined during limited periods when the B ₀ angle is sufficiently large.     

A comparison between simultaneous I.P.D.P. groundbased observations and observations of energetic protons obtained by satellites

A study of simultaneous groundbased observations of I.P.D.P. (intervals of pulsation of diminishing period) magnetic field fluctuation events and satellite observations of energetic protons have been performed. Some of our results are as follows. (1) The region of I.P.D.P. occurrence is always located equatorward of the isotropic proton precipitation. (2) The I.P.D.P. generation is not connected with the poleward leap of the aurora and the poleward expansion of the precipitating protons. (3) In the evening to afternoon sector enhanced pitch angle scattering is found near L = 4 during I.P.D.P. events, earlier shown to be associated with ion cyclotron resonance. (4) I.P.D.P. events seem to be associated with increased fluxes of (40–60) keV protons injected during substorms near the plasmapause in the equatorial plane.In order to explain the observations we invoke the following model: at substorm onset ring current protons are injected deep into the nightside magnetosphere covering a certain region in L and L.T., with the inner edge of the proton population following McIlwain's injection boundary. The protons drift azimuthally westward and generate ion cyclotron waves in a certain L interval at or inside the plasmapause. By taking into account the shape and position of the plasmapause and the injection boundary, the exterrt and position of the wave generating region can be determined. The frequency-time dispersion of the I.P.D.P. is largely attributed to the L-dependent drift velocity of protons in a narrow energy band. The model is able to explain the observations during several individual events. Also, the model predicts the general trends that have been found by statistical analysis of I.P.D.P. events and accounts for the constant frequency observed by satellites during I.P.D.P. events.     

First results of micropulsation activity observed by sabre

Between September 1981 and March 1982 many Pc5 micropulsation events have been observed by the Wick component of the new SABRE auroral radar. A feature of these observations is that in addition to the characteristic poleward drifting regions of radar backscatter, equatorward moving regions are frequently observed. A consistent diurnal pattern in the direction of drift emerges from a statistical analysis of the data.     

The Auroral electrojet and field-aligned current

The location of field-aligned currents in the evening sector with respect to the auroral electrojets is examined. The tri-axial TRIAD satellite data and the simultaneous ground magnetometer data from along the Alaska meridian are analysed. It is shown that an intense upward fieldaligned current flows out from the region of the westward electrojet where discrete auroras are located. The downward flowing current exists in the region further equatorward, namely in the region of the eastward electrojet. However, the downward current is present even when there is no eastward electrojet. The boundary between the upward and the downward currents coincides, in most cases, with the boundary between the westward and the eastward auroral electrojets. Thus, the Harang discontinuity, a narrow area separating the positive and negative H bays, is the region where there is no field-aligned current.     

Field-aligned and ionospheric currents

Zmuda and Armstrong (1974) showed that the field-aligned currents consist of two pairs; one is located in the morning sector and the other in the evening sector. Our analysis of magnetic records from the TRIAD satellite suggests that in each pair the poleward field-aligned current is more intense than the equatorward current, a typical ratio being 2:1. This difference has a fundamental importance in understanding the coupling between the magnetosphere and the ionosphere. We demonstrate this importance by computing the ionospheric current distribution by solving the continuity equation $\text{▽ .}\text{I}\text{= j}{}_{\mathrm{\parallel }}$ using the “observed” distribution of j_∥ for several models of the ionosphere with a high conductive annular ring (simulating the auroral oval).It is shown that the actual field-aligned and ionospheric current system is neither a simple Birkeland type, Boström type nor Zmuda-Armstrong type, but is a complicated combination of them. The relative importance among them varies considerably, depending on the conductivity distribution, the location of the peak of the field-aligned currents, etc. Further, it is found that the north-south segment of ionospheric current which connects the pair of the field-aligned currents in the morning sector does not close in the same meridian and has a large westward deflection. Thus, it has an appreciable contribution to the westward electrojet. One of the model calculations shows that the entire north-south closure current contributes to the westward electrojet.     

Pi 3 magnetic pulsations associated with substorms

Using magnetic data from the North American IMS network at high latitudes, Pi 3 pulsations are analysed for a period of $\text{4}\text{1}\text{2}$ continuously-disturbed days. The data were obtained from 13 stations in the Alaska and Fort Churchill meridional chains and in the east-west chain along the auroral zone. In the past, Pi 3 pulsations associated with substorms have been classified into two sub-categories, Pi p and Ps 6. However, we find that Pi 3's which have longer periods than Pi p and which are different from Ps 6 are more commonly observed than these two special types. Power spectra, coherence and phase differences are compared among the stations. Results show that noticeable differences for latitudinal dependence of period and amplitude exist among midnight, morning and late-evening Pi 3 pulsations. Results for Pi 3 occurring near midnight indicate that the periods at which the power spectral density is a maximum are longest, and the amplitude largest, near the center of the westward auroral electrojet. On the other hand, for Pi 3 pulsations occurring in the morning, the periods at which the power spectral density is a maximum are longest, and the amplitude largest, near the poleward edge of the westward electrojet. Furthermore, for Pi 3 pulsations occurring in the late evening, their periods are longer and their amplitudes larger near both the Harang discontinuity and the poleward edge of the westward electrojet than near its center. Correlations between pairs of adjoining stations are better in the polar cap than at auroral latitudes. It is also found from hodograms that the sense of polarization often varies from one station to another for the same event, and that the time duration in which the same rotational sense is maintained is shorter near midnight than in the morning and late evening. It is suggested that the source regions of the morning and late-evening Pi 3's lie on the electrojet boundaries; that is at the Harang discontinuity (in the evening) and at the poleward edge of the westward electrojet (in the morning and evening). The generation of midnight Pi 3 pulsations, centered at a location within the westward auroral electrojet appears to be associated directly with the generation of that electrojet.     

Evolution of latitude zonal structure of the large-scale magnetic field in solar cycles

Properties of a latitude zonal component of the large-scale solar magnetic field are analyzed on the basis of H charts for 1905–1982. Poleward migration of prominences is used to determine the time of reversal of the polar magnetic field for 1870–1905. It is shown that in each hemisphere the polar, middle latitude and equatorial zones of the predominant polarity of large-scale magnetic field can be detected by calculating the average latitude of prominence samples referred to one boundary of the large-scale magnetic field. The cases of a single and three-fold polar magnetic field reversal are investigated. It is shown that prominence samples referred to one boundary of the large-scale magnetic field do not have any regular equatorward drift. They manifest a poleward migration with a variable velocity up to 30 m s^-1 depending on the phase of the cycle. The direction of migration is the same for both low-latitude and high-latitude zones. Two different time intervals of poleward migration are found. One lasts from the beginning of the cycle to the time of polar magnetic field reversal and the other lasts from the time of reversal to the time of minimum activity. The velocity of poleward migration of prominences during the first period is from 5 m s^-1 to 30 m s^-1 and the second period is devoid of regular latitude drift.