Deuteron whistler and trans-equatorial propagation of the ion cyclotron whistler

New ion cyclotron whistlers which have the asymptotic frequency of one half the local proton gyrofrequency, $\text{G}{}_{\mathrm{p}}\text{2}$, and the minimum (or equatorial) proton gyrofrequency, G_pm, along the geomagnetic field line passing through the satellite have been found in the low-latitude topside ionosphere from the spectrum analysis of ISIS VLF electric field data received at Kashima, Japan. Ion cyclotron whistlers with asymptotic frequency of G_pm or $\text{G}{}_{\mathrm{pm}}\text{2}$ are observed only in the region of $\text{B}{}_{\mathrm{m}}\text{>}\text{B}\text{2}$ or rarely $\text{B}{}_{\mathrm{m}}\text{>}\text{B}\text{4}$, where B is the local magnetic field and B_m is the mini magnetic field along the geomagnetic field line passing through the satellite.The particles with one half the proton gyrofrequency may be the deuteron or alpha particle. Theoretical spectrograms of the electron whistlers (R-mode) and the ion cyclotron whistlers (L-mode) propagating along the geomagnetic field lines are computed for the appropriate distributions of the electron density and the ionic composition, and compared with the observed spectrograms.The result shows that the ion cyclotron whistler with the asymptotic frequency of $\text{G}{}_{\mathrm{p}}\text{2}$ is the deuteron whistler, and that the ion cyclotron whistlers with the asymptotic frequency of G_pm or $\text{G}{}_{\mathrm{pm}}\text{2}$ are caused by the trans-equatorial propagation of the proton or deuteron whistler from the other hemisphere.     

Ionospheric and magnetospheric “plasmapauses”

During August 1972, Explorer 45 orbiting near the equatorial plane with an apogee of ～5.2 R_e traversed magnetic field lines in close proximity to those simultaneously traversed by the topside ionospheric satellite ISIS 2 near dusk in the L range 2.0–5.4. The locations of the Explorer 45 plasmapause crossings (determined by the saturation of the d.c. electric field double probe) during this month were compared to the latitudinal decreases of the H⁺ density observed on ISIS 2 (by the magnetic ion mass spectrometer) near the same magnetic field lines. The equatorially determined plasmapause field lines typically passed through or poleward of the minimum of the ionospheric light ion trough, with coincident satellite passes occurring for which the L separation between the plasmapause and trough field lines was between 1 and 2. Hence, the abruptly decreasing H⁺ density on the low latitude side of the ionospheric trough is not a near earth signature of the equatorial plasmapause. Vertical flows of the H⁺ ions in the light ion trough as detected by the magnetic ion mass spectrometer on ISIS were directed upward with velocities between 1 and 2 km s^?1 near dusk on these passes. These velocities decreased to lower values on the low latitude side of the H⁺ trough but did not show any noticeable change across the field lines corresponding to the magnetospheric plasmapause. The existence of upward accelerated H⁺ flows to possibly supersonic speeds during the refilling of magnetic flux tubes in the outer plasmasphere could produce an equatorial plasmapause whose field lines map into the ionosphere at latitudes which are poleward of the H⁺ density decrease.     

Non-ducted two-hop whistlers in the inner magnetosphere deduced from rocket measurement

The non-ducted whistler propagation in the inner magnetosphere is discussed using the broad-band VLF measurement on board the K-9M-26 rocket launched at 1703 hr JST on 24 August 1969 from Kagoshima Space Center (geomagnetic lat 20°N). A large number of whistlers which seemed to be two-hop whistlers originating in the northern hemisphere were observed. The main features of these whistlers are summarized: (1) their dispersion value is widely scattered in the range $\text{55–75}\text{sec}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, (2) their frequency spectra show a broad maximum in the frequency range 2–5 kHz and higher frequency components are likely to disappear. Attempts are made to interpret these properties in terms of ducted or non-ducted propagation. It is then found from the ray tracing studies that the measurements are satisfactorily explained by non-ducted propagation in the inner magnetospheric model with latitudinal density gradient such as the equatorial anomaly.     

Localized auroral disturbance in the morning sector of topside ionosphere as a standing electromagnetic wave

An intense, localized auroral disturbance observed by Intercosmos-Bulgaria-1300 satellite in the morning sector at the altitude 850 km is analyzed in detail. The disturbance is characterized by strong “jumps” of electric and magnetic fields reaching ～ 80 mV/m and ～ 100 nT, fluctuations of ion density (Δn/n ～ 70%) and bursts of downward and upward energetic electron fluxes. Electric and magnetic disturbances display a distinct spatial-temporal relationship typical for the standing quasi-monochromatic wave ($\text{? ～ 1}\text{Hz}\text{, λ}{}_{\mathrm{x}}\text{～ 10}\text{km}$). The ratio of amplitudes of electric and magnetic fluctuations is equal to Alfvén velocity (ΔE/ΔB ～ v_A/c). However, a strong parallel component of the electric field (～ 30 mV/m) and large ion density fluctuations indicate significant changes of plasma properties (the effects of anomalous resistivity are possible).     

Conjugate measurements of VLF transmitter signals at middle latitude (L = 1.93)

Whistler-mode signals were measured at the conjugate point (L = 1.93) in South Australia of the VLF transmitter in Eastern U.S.S.R. The intensity of whistler-mode signals increased usually at sunset and occasionally at sunrise, and was moderate during night-time. By means of the calculation of the wave attenuation in the lower ionosphere, the result may be interpreted in terms of different mechanisms of propagation in the magnetosphere and transmission out of the ionosphere: spatially coherent propagation in a field-aligned duct and transmission out of the duet onto the ground at sunset and sunrise; and spatially incoherent propagation and transmission at night. A high correlation was found between the occurrence probability of the 14.881 kHz whistler-mode signals and the occurrence number of whistlers, which may suggest that the occurrence of whistlers is controlled predominantly by propagation conditions.     

An analysis of the upper atmospheric wind observed by LOGACS

Wind velocities at 140–200 km altitude were observed by a Low-G Accelerometer Calibration System (LOGACS) flown on an Agena satellite during a geomagnetic storm. An interesting wind reversal observed by the satellite at auroral latitudes is satisfactorily explained by the neutral air motion caused by the $\text{E × B}$ drift deduced from the ground-based geomagnetic data recorded at stations near the meridian of the satellite orbit.     

Magnetospheric chorus: Occurrence patterns and normalized frequency

New characteristics of VLF chorus in the outer magnetosphere are reported. The study is based on more than 400 hours of broadband (0.3–12.5 kHz) data collected by the Stanford University/Stanford Research Institute VLF experiment on OGO 3 during 1966–1967. Bandlimited emissions constitute the dominant form of whistler-mode radiation in the region $\text{4? L? 10}$. Magnetospheric chorus occurs mainly from 0300 to 1500 LT, at higher L at noon than at dawn, and moves to lower L during geomagnetic disturbance, in accord with ground observations of VLF chorus. Occurrence is moderate near the equator, lower near 15°, and maximum at high latitudes (far down the field lines). The centre frequency ? of the chorus band varies as L^?3> and at low latitudes is closely related to the electron gyrofrequency on the dipole field line through the satellite. Based on the measured local gyrofrequency $\text{?}{}_{\mathrm{H}}$, the normalized frequency distribution of chorus observed within 10° of the dipole equator shows two peaks, at $\text{?}\text{?}{}_{\mathrm{H}}\text{? 0.53}$ and $\text{?}\text{?}{}_{\mathrm{H}}\text{? 0.34}$. This bimodal distribution is a persistent statistical feature of near equatorial chorus, independent of L, LT and K_p. However there is considerable variability in individual events, with chorus often observed above, below, and between these statistical peaks; in particular, it is not unusual for single emissions to cross $\text{?}\text{?}{}_{\mathrm{H}}\text{= 0.50}$. When two bands are simultaneously present individual emission elements only rarely show one-to-one correlation between bands. For low K_p the median bandwidth of the upper band, gap and lower band are all ～16% of their centre frequencies, independent of L; for higher K_p the bandwidth of the lower band increases. Bandwidth also increases with latitude beyond ~10°. Starting frequencies of narrowband emissions range throughout the band. The majority of the emissions rise in frequency at a rate between 0.2 and 2.0 kHz/sec; this rate increases with K_p and decreases with L. Falling tones are rarely observed at dipole latitudes <2.5°. The observations are interpreted in terms of whistler-mode propagation theory and a gyroresonant feedback interaction model. An exact expression is derived for the critical frequency, $\text{?}\text{?}{}_{\mathrm{H}}\text{? 0.5}$, at which the curvature of the refractive index surface vanishes at zero wave normal angle. Near this frequency rays with initial wave normal angles between 0° and ?20° are focused along the initial field line for thousands of km, enhancing the phase-bunching of incoming gyroresonant electrons. The upper peak in the bimodal normalized frequency distribution is attributed to this enhancement near the critical frequency, at latitudes of ~5°. Slightly below the critical frequency interference between modes with different ray velocities may contribute to the dip in the bimodal distribution. The lower peak may reflect a corresponding peak in the resonant electron distribution, or guiding in field-aligned density irregularities. The observations are consistent with gyroresonant generation of emissions near the equator, followed by spreading of the radiation over a range of L shells farther down the field lines.     

Low-frequency upstream wave as a probable source of low-latitude Pc 3–4 magnetic pulsations

Daytime Pc 3–4 pulsation activities observed at globally coordinated low-latitude stations [SGC (L = 1.8,λ = 118.0°W), EWA(1.15,158.1°W), ONW(1.3,141.5°E)] are evidently controlled by the cone angle θ_XB of the IMF observed at ISEE 3. Moreover, the Pc 3–4 frequencies ($\text{?}$) at the low latitudes and high latitude (COL; L = 5.6 and λ = 147.9°W) on the ground and that of compressional waves at geosynchronous orbit (GOES 2; L = 6.67 and λ = 106.7°W) are also correlated with the IMFmagnitude(B_IMF).The correlation of $\text{?}$ of the compressional Pc 3–4 waves at GOES 2 against B_IMF is higher than those of the Pc 3–4 pulsations at the globally coordinated ground stations, i.e., γ = 0.70 at GOES 2, and (0.36,0.60,0.66,0.54) at (COL, SGC, EWA, ONW), respectively. The standard deviation ($\text{σ}{}_{\mathrm{n}}\text{= ± Δ?}\text{mHz}$) of the observed frequencies from the form $\text{?}$ (mHz) = 6.0 × B_IMF (nT) is larger at the ground stations than at GOES 2, i.e., $\text{Δ? = ± 6.6}\text{mHz at}$GOES 2, and ±(13.9, 9.1, 10.7, 12.1) mHz at (COL, SGC, EWA, ONW), respectively. The correlations between the IMF magnitude B_IMF and Pc 3–4 frequencies at the low latitudes are higher than that at the high latitude on the ground, which can be interpreted by a “filtering action” of the magnetosphere for daytime Pc 3–4 magnetic pulsations. The scatter plots of pulsation frequency $\text{?}$ against the IMF magnitude B_IMF for the compressional Pc 3–4 waves at GOES 2 are restricted within the forms $\text{? = 4.5 × B}{}_{\mathrm{<mtext>IMF</mtext>}}\text{and}\text{? = 7.5 × B}{}_{\mathrm{<mtext>IMF</mtext>}}$. The frequency distribution is in excellent agreement with the speculation ($\text{<ω}{}_{\mathrm{sc}}\text{Ω}{}_{\mathrm{i}}\text{= 0.3 ～ 0.5}$) of the spacecraft frame frequency of the magnetosonic right-hand waves excited by the anomalous ion cyclotron resonance with reflected ion beams with V₆ = 650 ～ 1150 km s^?1 in the solar wind frame observed by the ISEE satellite in the Earth's foreshock. These observational results suggest that the magnetosonic right-handed waves excited by the reflected ion beams in the Earth's foreshock are convected through the magnetosheath to the magnetopause, transmitted into the magnetosphere without significant changes in spectra, and then couple with various HM waves in the Pc 3–4 frequency range at various locations in the magnetosphere.     

Damping of geomagnetic pulsations by the ionosphere

A significant sink of geomagnetic pulsation energy is due to Joule dissipation in the ionosphere. To investigate this we have computed the damping experienced by standing Alfvén waves in a dipole magnetic field. Both the uncoupled poloidal and toroidal modes are considered with Joule dissipation being introduced through a boundary condition which relates the electric and magnetic field strengths at the ionosphere, viz: $\text{4πΣ}{}_{\mathrm{p}}\text{E}\text{c}\text{= b,}\text{where}\text{Σ}{}_{\mathrm{p}}$ is the height integrated Pederson conductivity. The damping rates are strongly dependent on the ionospheric conductivity and we find that typically the normalized damping rate, $\text{γ}\text{ω}\text{,}\text{is}\text{～0.1}$ for nightside values of conductivity and ～0.01 for the dayside. This would account for the observed scale of bandwidths in pulsation signals. Away from regions of extreme damping we find γ ∝ L^?1Σ_p^?1.     

The topside equatorial ionosphere during daytime: Elevated plasma temperatures,the transequatorial O+ breeze and the dynamics of minor ions

Steady-state calculations are performed for the daytime equatorial F2-region and topside ionosphere. Values are calculated of the electron and ion temperatures and the concentrations and field-aligned velocities of the ions O⁺, H⁺ and He⁺. Account is taken of upward $\text{E × B}$ drift, a summer-winter horizontal neutral air wind and heating of the electron gas by thermalization of fast photoelectrons.The calculated plasma temperatures are in accord with experiment: at the equator there is an isothermal region from about 400–550 km altitude, with temperatures of about 2400 K around 800 km altitude. The transequatorial O⁺ breeze flux from summer to winter in the topside ionosphere is not greatly affected by the elevated plasma temperatures. The field-aligned velocities of H⁺ and He⁺ depend strongly on the O⁺ field-aligned velocity and on the presence of large temperature gradients. For the minor ions, ion-ion drag with O⁺ cannot be neglected for the topside ionosphere.     

Plasmaspheric hiss observed in the topside ionosphere at mid- and low-latitudes

Latitudinal characteristics of ELF hiss in mid- and low-latitudes have been statistically studied by using ELF/VLF electric field spectra (50 Hz-30 kHz) from ISIS-1 and -2 received at Kashima station, Japan from 1973 to 1977. Most ISIS ELF/VLF data observed in mid- and low-latitude include ELF hiss at frequencies below a few kHz. The ELF hiss has the strongest intensity among VLF phenomena observed by the ISIS electric dipole antenna in mid- and low-latitudes, but the ELF hiss has no rising structure like the chorus in the detailed frequency-time spectrum. The ELF hiss is classified into the steady ELF hiss whose upper frequency limit is approximately constant with latitude and the ELF hiss whose upper frequency limit increases with latitude. These two types of ELF hiss occur often in medium or quiet geomagnetic activities. Sometimes there occurs a partial or complete lack of ELF hiss along an ISIS pass.Spectral shape and bandwidth of ELF hiss in the topside ionosphere are very similar to those of plasmaspheric hiss and of inner zone hiss. The occurrence rate of steady ELF hiss is about 0.3 near the geomagnetic equator and decreases rapidly with latitude around L = 3. Hence it seems likely that ELF hiss is generated by cyclotron resonant instability with electrons of several tens of keV in the equatorial outer plasmasphere beyond L = 3.Thirty-seven per cent of ELF hiss events received at Kashima station occurred during storm times and 63% of them occurred in non-storm or quiet periods. Sixty-seven per cent of 82 ELF hiss events during storm times were observed in the recovery phase of geomagnetic storms. This agrees with the previous satellite observations of ELF hiss by search coil magnetometers. The electric field of ELF hiss becomes very weak every 10 s, which is the satellite spin period, in mid- and low-latitudes, but not near the geomagnetic equator. Ray tracing results suggest that waves of ELF hiss generated in the equatorial outer plasmasphere propagate down in the electrostatic whistler mode towards the equatorial ionosphere, bouncing between the LHR reflection points in both the plasmaspheric hemispheres.     

Characteristics of whistler ducts recorded at Gulmarg

Morphological features of whistlers recorded at low latitude ground station Gulmarg (geomag. lat., 24 26N) are studied to deduce information about ducts. The morphological characteristics of low latitude whistlers are discussed and compared with the characteristics of middle and high latitude whistlers. The maximum electron density (N _m ) at the height of the ionosphere obtained from whistler dispersion comes out to be higher than that of the background, which is in accordance with the characteristics of the whistler duct. The equivalent width of the whistler duct at the maximum height of its path is found to be close to the value obtained from satellite observations. The characteristics of whistler ducts in low latitude ionosphere are similar to those in middle and high latitude ionosphere. The width of ducts estimated from the diffuseness of the whistler track observed during magnetic storm is found to lie in the range of 50–200 km.     

Magnetospheric electric field from low latitude whistlers during magnetic storm

An attempt has been made to estimate the east-west component (E_w) of the magnetospheric equatorial electric field near L = 1.12 during a magnetic storm period from the whistlers observed at our low latitude ground station, Nainital (geomag.lat. 19°1'N), on March 25, 1971 in the 0130–0500 IST sector. The method of measuring E_w from the observed cross L-motions of whistler ducts within the plasmasphere, indicated by changes in nose frequency of whistlers, has been outlined. The nose frequencies of non-nose whistlers under consideration have been deduced from Dowden-Allcock linear Q-technique. The variation of ($\text{?}{}_{\mathrm{n}}\text{)}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ with local time has been shown, the slope of which can be directly related to the convection electric field. The estimated equatorial electric field at L? 1.12 is in the range 0.1–0.5 mV m^?1 (in the 0130–0500 IST sector) during a storm period, which is in agreement with the results reported by earlier workers. The departure from a dipole field and the contribution of an induced electric field from the temporal changes have been discussed. The importance of an electric field study has been indicated.     

Heavy ion measurements in the synchronous altitude region

The launch of the P78-2 (SCATHA) satellite in January 1979 has provided a new opportunity to study the energetic ion composition in the high altitude equatorial regions of the earth's magnetosphere. In particular detailed pitch angle distributions were obtained as a function of ion species and energy. The energies measured range from ～ 90 to 250 keV/nucleon for Z ? 2. Data are presented which were acquired in late March and early April 1979. The relative abundance of He and CNO nuclei are found to be ～ 10^?2 and 5 × 10^?4 respectively at L ? 5.5. Only an upper limit on the relative abundance of Fe group nuclei of < 3 × 10^?7 was obtained. The angular distributions of the heavy ions was found to be very steep for $\text{B}\text{B}{}_0\text{< 1.5}$ and then to flatten markedly.     

Excitation of magnetosonic waves with discrete spectrum in the equatorial vicinity of the plasmapause

There is a magnetosonic waveguide under the arch of the plasmasphere. This channel, in the form of a ring with radius L～4, surrounds the Earth. It is shown that in this region of the magnetosphere the flute-like electromagnetic disturbances (k6 = 0) with frequencies ω = sω_p can be excited by energetic protons, with non-monotonic dependence on transverse energy ($\text{??/?ε}{}_{\mathrm{\perp }}\text{> 0}$). The interpretation of magnetic pulsations which have been observed in the equatorial vicinity of the plasmapause on the satellite OGO-3 in the frequency range ～10² cps (Russell et al., 1970) is given. In particular the origin of discrete structure of the observed spectra (narrow band spikes for a rather broad range of frequency) is discussed.     

The influence of the equatorial anomaly on the ground reception of non-ducted whistlers at low latitudes

It is the purpose of this paper to study whether the non-ducted propagation in the inner plasmasphere in the presence of the equatorial anomaly might be relevant to daytime whistlers observed on the ground at low latitudes. Realistic models of the equatorial anomaly simulating the satellite observations have been incorporated in the ray tracing computations. It is found that there are two different non-ducted modes able to penetrate through the ionosphere onto the ground; (1) whispering gallery mode around the anomaly field line which is trapped just by the outer boundary of the anomaly, and (2) pro-longitudinal (PL) mode at a latitude around 30° which is supported by the horizontal gradient in the tail of the anomaly. These modes may provide a new interpretation for some whistlers observed on the ground. The properties of these modes are examined in detail and then compared with those of ducted propagation. This study may be useful for distinguishing the propagation mode in future ground-based experiments.     

Observed heating effects of conjugate photoelectrons

A gridded spherical electrostatic analyzer aboard Injun 5 has been used to measure fluxes of thermal and hyperthermal electrons at subauroral latitudes in the midnight sector of the northern ionosphere between altitudes of 2500 and 850 km. Due to the offset between the geomagnetic and geographic poles hyperthermal fluxes, consisting of energetic photoelectrons that have escaped from the sunlit southern hemisphere are observed along orbits over the Atlantic Ocean and North America but not over Asia. The ambient electron temperatures (T_e) near 2500 km have their highest values at trough latitudes for all longitudes. At altitudes near 1000 km elevated electron temperatures in the trough were not a consistent feature of the data. Equatorward of the trough, in the longitude sector to which conjugate photoelectrons have access, T_e ～ 4000 K at 2500 km and ～ 3000 K at 1000 km. For regions with the conjugate point in darkness T_e ? 2300 K over the 1000–2500 km altitude range. The effective thermal characteristics of conjugate photoelectrons are studied as functions of altitude and latitude. The observations indicate that (1) at trough latitudes elevated electron temperatures in the topside ionosphere are mostly produced by sources other than conjugate photoelectrons, and (2) at subtrough latitudes, in the Alantic Ocean-North American longitude sector, conjugate photoelectrons contribute significantly to the heating of topside electrons. Much of the conjugate photoelectron energy is deposited at altitudes >2500 km then conducted along magnetic field lines into the ionosphere.     

Upper limit on the radar cross section of the comet Kohoutek

An attempt to observe radar echoes from the comet Kohoutek was made at a radio frequency of 7840 MHz (λ ～- 3.8 cm) on 12 January 1974 using the Haystack Observatory radar in Massachusetts. A search for an echo over a range of band-widths covering 2Hz to 66kHz yielded no positive result. The upper limit on the radar cross section is therefore approximately $\text{10}{}^4\text{B}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{km}{}^2$, where B is the (unknown) bandwidth of the echo in Hertz. For $\text{B ? 100}\text{Hz}$, it follows that (i) the nucleus, if a perfect spherical reflector, must be less than 250 km in diameter, and (ii) the density of any millimeter-sized particles must be less than 1m^?3 for a coma of diameter 10⁴km.     

Coincident observations of ionospheric troughs and the equatorial plasmapause

Explorer 45 traversed the plasmapause (determined approximately via the saturation of the d.c. electric field experiment) at near-equatorial latitudes on field lines which were crossed by Ariel 4 (~600km altitude) near dusk in May 1972 and on field lines which were crossed by Isis II (~1400km altitude) near midnight in December 1971 and January 1972. Many examples were found in which the field line through the near-equatorial plasmapause was traversed by Explorer 45 within one hour local time and one hour universal time of Ariel and Isis crossings of the same L coordinate. For the coincident passes near dusk, the RF electron density probe on Ariel detected electron density depletions near the plasmapause L coordinates when Ariel was in darkness. When the Ariel passes were in sunlight, however, electron depletions were not discernable near the plasmapause field line. On the selected near-midnight passes of Isis II, electron density depressions were typically detected (via the topside sounder) near the plasmapause L coordinate. The dusk Ariel electron density profiles are observed to reflect O⁺ density variations. Even at the high altitude of Isis near midnight, O⁺ is found to be the dominant ion in the trough region whereas H⁺ is dominant at lower latitudes as is evident from the measured electron density scale heights. In neither local time sector was it possible to single out a distinctive topside ionosphere feature as an indicator of the plasmapause field line as identified near the equator. At both local times the equator-determined plasmapause L coordinate showed a tendency to lay equatorward of the trough minimum.     

Latitudinal structures of discrete arcs resulting from viscous interaction between sheared plasma flows

Latitudinal structures of discrete arcs are modelled as a consequence of the quasi-steady magnetosphere-ionosphere coupling involving viscous interaction between sunward and anti-sunward plasma flows in the magnetosphere. The quasi-steady state in the magnetosphere and ionosphere coupling is described by the magnetospheric and ionospheric current conservation and the field-aligned currentpotential relation assuming adiabatic electron motion along field lines. The upward and downward fieldaligned currents are assumed to be stably maintained by vorticity-induced space charges in the region of plasma flow reversal, where divergence of the magnetospheric electric field E_⊥ is negative and positive, respectively. By introducing the effective conductance Σ_dc arising from the anomalous viscosity, a specific relation between the dc field-aligned current density J_∥ and the magnetospheric electric field E_⊥ is derived as $\text{J}{}_{\mathrm{\parallel }}\text{=−Σ}{}_{\mathrm{dc}}\text{div}\text{E}{}_{\mathrm{\perp }}$. Sufficiently large potential drops to accelerate auroral electrons are shown to exist along the auroral field lines originating from the flow reversal region with div $\text{E}{}_{\mathrm{\perp }}\text{< 0}$. It is shown that the latitudinal structure of a discrete arc is primarily determined by the magnetospheric potential structure and the characteristic width is on the order of 10 km at the ionospheric altitude.