Recovery Phase Of Magnetic Storm Research Articles

The relativistic electron response in the outer radiation belt during magnetic storms

Abstract. The relativistic electron response in the outer radiation belt during magnetic storms has been studied in relation to solar wind and geomagnetic parameters during the first six months of 1995, a period in which there were a number of recurrent fast solar wind streams. The relativistic electron population was measured by instruments on board the two microsatellites, STRV-1a and STRV-1b, which traversed the radiation belt four times per day from L ~ 1 out to L ~ 7 on highly elliptical, near-equatorial orbits. Variations in the E > 750 keV and E > 1 MeV electrons during the main phase and recovery phase of 17 magnetic storms have been compared with the solar wind speed, interplanetary magnetic field z-component, Bz , the solar wind dynamic pressure and Dst *. Three different types of electron responses are identified, with outcomes that strongly depend on the solar wind speed and interplanetary magnetic field orientation during the magnetic storm recovery phase. Observations also confirm that the L-shell, at which the peak enhancement in the electron count rate occurs has a dependence on Dst *.Key words. Magnetospheric physics (energetic particles, trapped; storms and substorms) – Space plasma physics (charged particle motion and accelerations)

Polarization jet events and excitation of weak SAR arcs

Polarization Jet (PJ), also known as Sub‐Auroral Ion Drift (SAID), events are supersonic westward plasma drifts on the equatorward edge of the diffuse aurora in the evening and nighttime sector. Their optical F‐region signatures are weak 630.0 nm red arcs colocated with regions of fast convection. These weak arcs resemble Stable Auroral Red (SAR) arcs observed during the recovery phase of magnetic storms, but have lower intensities, shorter lifetimes, and occur without a significant heat flux from the magnetosphere. Previous model studies underestimated the brightness of weak SAR arcs. We present calculations showing that ion‐neutral collisional heating and ion composition changes during PJ events may be an additional source of 630.0 nm emission, and propose experimental tests that could verify our modeling results.

Dynamic model of the magnetosphere: Case study for January 9–12, 1997

The dynamics of the magnetospheric current systems are studied in the course of the specific magnetospheric disturbance on January 9–12, 1997, caused by the interaction of the Earth's magnetosphere with a dense solar wind plasma cloud. To estimate the contribution of the different sources of the magnetospheric magnetic field to the disturbance ground measured, a dynamic paraboloid model of the magnetosphere is used. The model input parameters are defined by the solar wind density and velocity, by the strength and direction of the interplanetary magnetic field, and by the auroral AL index. The total energy of the ring current particles is calculated from the energy balance equation, where the injection function is determined by the value of the solar wind electric field. New analytical relations describing the dynamics of the different magnetospheric magnetic field sources dependent on the model input parameters are obtained. The analysis of the magnetic disturbances during the January 9–12, 1997, event shows that in the course of the main phase of the magnetic storm the contribution of the ring current, the currents on the magnetopause, and the currents in the magnetotail are approximately equal to each other by an order of magnitude. Nevertheless, in some periods one of the current systems becomes dominant. For example, an intense Dst positive enhancement (up to +50 nT) in the course of the magnetic storm recovery phase in the first hours on January 11, 1997, is associated with a significant increase of the currents on the magnetopause, while the ring current and the magnetotail current remain at a quiet level. A comparison of the calculated Dst variation with measurements indicates good agreement. The root mean square deviation is ∼ 8.7 nT in the course of the storm.

Which magnetic storms produce relativistic electrons at geosynchronous orbit?

Relativistic electrons appear in the geosynchronous environment following some, but not all, geomagnetic storms. The ability to identify which storms produce these electrons would bring us much closer to explaining the mechanism responsible for their appearance, and it would provide the space weather community with a means to anticipate the electron hazard to geosynchronous spacecraft. We apply a recently developed statistical technique to produce an hourly time series of relativistic electron conditions at local noon along geosynchronous orbit using several geosynchronous monitors. We use a cross‐correlation analysis to determine what parameters in the solar wind and magnetosphere might influence the flux of relativistic electrons. We then perform a superposed epoch analysis to compare storms with and storms without the appearance of these electrons. We investigate a number of solar wind and magnetospheric parameters for these two sets of storms at 1‐hour resolution. In particular, sustained solar wind velocity in excess of 450 km s−1 is a strong external indicator of the subsequent appearance of relativistic electrons. In the magnetosphere, long‐duration elevated Pc 5 ULF wave power during the recovery phase of magnetic storms appears to discriminate best between those storms that do and do not produce relativistic electrons.

Reversed Plasma Convection Flow in the Polar Ionosphere from Eiscat Radar Observation

AbstractAn abnormal anti‐sunward intense flow of plasma convection is observed by EISCAT radar in the high latitude ionosphere near the polar cap boundary. It occurred in the morning sector of MLT under conditions of southward IMF Bz during the recovery phase of a moderate magnetic storm. A synthetic analysis of radar data and in‐situ satellite observations is made in combination with a tracing of magnetic field lines by means of a magnetospheric field model on this event. It is evident that the abnormally reversed westward high flow of plasma convection, the plasma structure and properties are probably the ionospheric signatures of magnetic reconnection taking place during the dayside magnetopause.

Electron acceleration by magnetic pumping on the tail magnetopause

Based on the quasi-linear kinetic theory of statistical Alfvén acceleration a quantitative investigation of the possibility of relativistic electron acceleration by magnetic pumping on the tail magnetopause is considered in the paper. This theory is used for the explanation of long duration electron flux enhancements with assumption that the magnetic pumping is activated by the geomagnetic substorm activity during the recovery phase of magnetic storms. An analytical expression for average acceleration time is obtained. It is shown that the magnetic pumping with period ∼1 hour in the outer magnetosphere provides the gradual (several days) electron acceleration up to energy ∼10 MeV. The energetic spectrum of the accelerated electrons is calculated with an assumption that the electrons escape from the acceleration region. The dependence of the electron energetic spectrum shape on the relation between escaping and acceleration times is studied. Relativistic electron enhancements in the magnetosphere in March–June 1994 from CORONAS—I data is presented. The dynamics of L-distribution of the relativistic electron intensity is superimposed to the solar wind and magnetosphere variations. Comparison of the experimental data with theoretical results is performed.

Responses of the polar ionosphere to the Bastille Day solar event

Simultaneous observations at Zhongshan Station, Antarctica, are presented for the interval of 13–17 July 2000 to show responses of the polar ionosphere to the Bastille Day (14 July 2000) solar event. The polar ionosphere was highly disturbed, as shown by frequently large deviations of the geomagnetic H-component, large riometer absorption events and strong ULF waves. Associated with the huge solar proton event produced by the X5/3B flare, a polar cap absorption (PCA) was observed. It began at ∼ 10:40 UT on 14 July and ended at ∼ 19:40 UT on 17 July. Superposed on it, there was a large absorption event with a peak of 26 dB, starting at ∼ 03:00 UT and ending at ∼ 11:10 UT on 15 July. This kind of absorption was probably produced by an intense `cloud of energetic electrons' during an auroral substorm. The ULF waves were very intense during the main phase and the recovery phase of the severe magnetic storm on 15 and 16 July. The ionospheric absorption was so strong that the digisonde signal was blacked out most of the time. The ionosphere returned to normal in the afternoon on 17 July.

EISCAT observation of a high‐latitude ionization trough associated with a reversed westward plasma flow

Trough‐like plasma structures in the high‐latitude ionosphere near the polar cap boundary are investigated with EISCAT CP3 data and in‐situ satellite observations. Emphasis is placed on one interesting event occurring in the morning sector during the recovery phase of a moderate magnetic storm. The event features are probably ionospheric signatures of cusp processes caused by magnetic reconnection on the dayside magnetopause.

Resonant enhancement of relativistic electron fluxes during geomagnetically active periods

Abstract. The strong increase in the flux of relativistic electrons during the recovery phase of magnetic storms and during other active periods is investigated with the help of Hamiltonian formalism and simulations of test electrons which interact with whistler waves. The intensity of the whistler waves is enhanced significantly due to injection of 10-100 keV electrons during the substorm. Electrons which drift in the gradient and curvature of the magnetic field generate the rising tones of VLF whistler chorus. The seed population of relativistic electrons which bounce along the inhomogeneous magnetic field, interacts resonantly with the whistler waves. Whistler wave propagating obliquely to the magnetic field can interact with energetic electrons through Landau, cyclotron, and higher harmonic resonant interactions when the Doppler-shifted wave frequency equals any (positive or negative) integer multiple of the local relativistic gyrofrequency. Because the gyroradius of a relativistic electron may be the order of or greater than the perpendicular wavelength, numerous cyclotron, harmonics can contribute to the resonant interaction which breaks down the adiabatic invariant. A similar process diffuses the pitch angle leading to electron precipitation. The irreversible changes in the adiabatic invariant depend on the relative phase between the wave and the electron, and successive resonant interactions result in electrons undergoing a random walk in energy and pitch angle. This resonant process may contribute to the 10-100 fold increase of the relativistic electron flux in the outer radiation belt, and constitute an interesting relation between substorm-generated waves and enhancements in fluxes of relativistic electrons during geomagnetic storms and other active periods.Key words. Magnetospheric physics (energetic particles · trapped; plasma waves and instabilities; storms and substorms)

Resonant enhancement of relativistic electron fluxes during geomagnetically active periods

<strong class="journal-contentHeaderColor">Abstract.</strong> The strong increase in the flux of relativistic electrons during the recovery phase of magnetic storms and during other active periods is investigated with the help of Hamiltonian formalism and simulations of test electrons which interact with whistler waves. The intensity of the whistler waves is enhanced significantly due to injection of 10-100 keV electrons during the substorm. Electrons which drift in the gradient and curvature of the magnetic field generate the rising tones of VLF whistler chorus. The seed population of relativistic electrons which bounce along the inhomogeneous magnetic field, interacts resonantly with the whistler waves. Whistler wave propagating obliquely to the magnetic field can interact with energetic electrons through Landau, cyclotron, and higher harmonic resonant interactions when the Doppler-shifted wave frequency equals any (positive or negative) integer multiple of the local relativistic gyrofrequency. Because the gyroradius of a relativistic electron may be the order of or greater than the perpendicular wavelength, numerous cyclotron, harmonics can contribute to the resonant interaction which breaks down the adiabatic invariant. A similar process diffuses the pitch angle leading to electron precipitation. The irreversible changes in the adiabatic invariant depend on the relative phase between the wave and the electron, and successive resonant interactions result in electrons undergoing a random walk in energy and pitch angle. This resonant process may contribute to the 10-100 fold increase of the relativistic electron flux in the outer radiation belt, and constitute an interesting relation between substorm-generated waves and enhancements in fluxes of relativistic electrons during geomagnetic storms and other active periods.<br><br><b>Key words.</b> Magnetospheric physics (energetic particles · trapped; plasma waves and instabilities; storms and substorms)

The role of precipitation losses in producing the rapid early recovery phase of the Great Magnetic Storm of February 1986

The possible role of precipitation losses in eroding stormtime ring current is subject to debate. To explore this controversy, the recovery phase of the February 6–10, 1986, great magnetic storm is examined, when intense ion precipitation was observed at midlatitudes by NOAA‐6 and DMSP satellites. This storm period is particularly interesting because the ring current exhibits distinctive two‐phase decay as seen in the Dst index, the early rapid timescale decay corresponding to the intense ion precipitation period described above. Hamilton et al. [1988] concluded, from close agreement between the observed timescale for ring current decay and the theoretical timescale for O+ charge exchange loss, that rapid early recovery phase of this storm resulted from the charge exchange loss of high energy O+; the second and longer decay phase was equated with H+ charge exchange loss. A model of the ring current evolution during this great magnetic storm [Fok et al., 1995] failed to reproduce the observed ring current decay rates, a puzzling result because charge exchange losses were well represented in the ring current model and initial and boundary conditions were taken from the same data set used in the Hamilton et al. [1988] study. A simple energy balance calculation for the global ring current is carried out using (1) either an energy input predicted from upstream solar wind parameters or one calculated from the drift‐loss model output, (2) collisional loss timescales extracted from the drift‐loss model, and (3) precipitation losses estimated from NOAA‐6 and DMSP observations. The energy balance model replicates the evolution of the ring current energy content derived from Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) observations when ion precipitation losses are included and model energy input function is reduced to agree with predictions based upon upstream solar wind parameters. The O+ charge exchange losses and observed global precipitation losses were of equal magnitude in early recovery of the ring current during this great magnetic storm. Later longer decay timescales in the model resulted from a combination of O+ and H+ charge exchange losses; O+ charge exchange losses remained important throughout the model time interval. The present model produces agreement with the AMPTE/CCE estimates of ring current kinetic energy content versus time. Disagreement between the Dst* inferred from the AMPTE/CCE particle measurements and the observed Dst* is an interesting issue needing further explanation.

Strong localized variations of the low-altitude energetic electron fluxes in the evening sector near the plasmapause

Abstract. Specific type of energetic electron precipitation accompanied by a sharp increase in trapped energetic electron flux are found in the data obtained from low-altitude NOAA satellites. These strongly localized variations of the trapped and precipitated energetic electron flux have been observed in the evening sector near the plasmapause during recovery phase of magnetic storms. Statistical characteristics of these structures as well as the results of comparison with proton precipitation are described. We demonstrate the spatial coincidence of localized electron precipitation with cold plasma gradient and whistler wave intensification measured on board the DE-1 and Aureol-3 satellites. A simultaneous localized sharp increase in both trapped and precipitating electron flux could be a result of significant pitch-angle isotropization of drifting electrons due to their interaction via cyclotron instability with the region of sharp increase in background plasma density.Key words. Ionosphere (particle precipitation; wave-particle interaction) Magnetospheric Physics (plasmasphere)

Strong localized variations of the low-altitude energetic electron fluxes in the evening sector near the plasmapause

Specific type of energetic electron precipitation accompanied by a sharp increase in trapped energetic electron flux are found in the data obtained from low-altitude NOAA satellites. These strongly localized variations of the trapped and precipitated energetic electron flux have been observed in the evening sector near the plasmapause during recovery phase of magnetic storms. Statistical characteristics of these structures as well as the results of comparison with proton precipitation are described. We demonstrate the spatial coincidence of localized electron precipitation with cold plasma gradient and whistler wave intensification measured on board the DE-1 and Aureol-3 satellites. A simultaneous localized sharp increase in both trapped and precipitating electron flux could be a result of significant pitch-angle isotropization of drifting electrons due to their interaction via cyclotron instability with the region of sharp increase in background plasma density.Key words. Ionosphere (particle precipitation; wave-particle interaction) Magnetospheric Physics (plasmasphere)

Collisional losses of ring current ions

The time evolution of the ring current population during the recovery phase of a typical moderate magnetic storm is studied, using a newly developed kinetic model for H+, He+ and O+ ions which includes nonequatorially mirroring particles. The bounce‐averaged distribution function is defined for variables that are accessible to direct measurement, and some useful formulas for calculating the total energy and number density of the ring current are derived. The bounce‐averaged kinetic equation is solved, including losses due to charge exchange with neutral hydrogen and Coulomb collisions with thermal plasma along ion drift paths. Time‐dependent magnetospheric electric fields and anisotropic initial pitch angle distributions are considered. The generation of ion precipitating fluxes is addressed, a process that is still not completely understood. It is shown that both the decrease of the distribution function due to charge exchange losses and the buildup of a low‐energy population caused by Coulomb collisions proceed faster for particles with smaller pitch angles. The maximum of the equatorial precipitating fluxes occurs on the nightside during the early recovery phase and is found to be of the order of 104–105 cm−2sr−1s−1keV−1. The mechanisms considered in this paper indicate that magnetospheric convection plays the predominant role in causing ion precipitation; Coulomb scattering contributes significantly to the low‐energy ion precipitation inside the plasmasphere.

Interplanetary origin of geomagnetic activity in the declining phase of the solar cycle

Interplanetary magnetic field (IMF) and plasma data are compared with ground‐based geomagnetic Dst and AE indices to determine the causes of magnetic storms, substorms, and quiet during the descending phase of the solar cycle. In this paper we focus primarily on 1974 when the AE index is anomalously high . This year is characterized by the presence of two long‐lasting corotating streams associated with coronal holes. The corotating streams interact with the upstream low‐velocity (300–350 km s−1), high‐density heliospheric current sheet (HCS) plasma sheet, which leads to field compression and ∼ 15‐ to 25‐nT hourly average values. Although the Bz component in this corotating interaction region (CIR) is often &lt; −10 nT, typically the field directionality is highly variable, and large southward components have durations less than 3 hours. Thus the corotating stream/HCS plasma sheet interaction region can cause recurring moderate (−100 nT ≤ Dst ≤ −50 nT) to weak (−50 nT ≤ Dst ≤ −25 nT) storms, and sometimes no significant ring current activity at all (Dst &gt; −25 nT). Storms of major (Dst ≤ −100 nT) intensities were not associated with CIRs. Solar wind energy is transferred to the magnetosphere via magnetic reconnection during the weak and moderate storms. Because the Bz component in the interaction region is typically highly fluctuating, the corresponding storm main phase profile is highly irregular. Reverse shocks are sometimes present at the sunward edge of the CIR. Because these events cause sharp decreases in field magnitude, they can be responsible for storm recovery phase onsets. The initial phases of these corotating stream‐related storms are caused by the increased ram pressure associated with the HCS plasma sheet and the further density enhancement from the stream‐stream compression. Although the solar wind speed is generally low in this region of space, the densities can be well over an order of magnitude higher than the average value, leading to significant positive Dst values. Since there are typically no forward shocks at 1 AU associated with the stream‐stream interactions, the initial phases have gradual onsets. The most dramatic geomagnetic response to the corotating streams are chains of consecutive substorms caused by the southward components of large‐amplitude Alfvén waves within the body of the corotating streams. This auroral activity has been previously named high‐intensity long‐duration continuous AE activity (HILDCAAs). The substorm activity is generally most intense near the peak speed of the stream where the Alfvén wave amplitudes are greatest, and it decreases with decreasing wave amplitudes and stream speed. Each of the 27‐day recurring HILDCAA events can last 10 days or more, and the presence of two events per solar rotation is the cause of the exceptionally high AE average for 1974 (higher than 1979). HILDCAAs often occur during the recovery phase of magnetic storms, and the fresh (and sporadic) injection of substorm energy leads to unusually long storm recovery phases as noted in Dst. In the far trailing edge of the corotating stream, the IMF amplitudes become low, &lt;3 nT, and there is an absence of large‐amplitude fluctuations (Alfvén waves). This is related to and causes geomagnetic quiet. There were three major (Dst ≤ −100 nT) storms that occurred in 1974. Each was caused by a nonrecurring moderate speed stream led by a fast forward shock. The mechanisms for generating the intense interplanetary Bs which were responsible for the subsequent intense magnetic storms was shock compression of preexisting southwardly directed Bz (Bs) for the two largest events and a magnetic cloud for the third (weakest) event. Each of the three streams occurred near a HCS crossing with no obvious solar optical or X ray signatures. It is speculated that these events may be associated with flux openings associated with coronal hole expansions. In conclusion, we present a model of geomagnetic activity during the descending phase of the solar cycle. It incorporates storm initial phases, main phases, HILDCAAs, and geomagnetic quiet. It uses some of the recent Ulysses results. We feel that this model is sufficiently developed that it may be used for predictions, and we encourage testing during the current phase of the solar cycle.

Three‐Dimensional Ring Current Decay Model

This work is an extension of a previous ring current decay model. In the previous work, a two‐dimensional kinetic model was constructed to study the temporal variations of the equatorially mirroring ring current ions, considering charge exchange and Coulomb drag losses along drift paths in a magnetic dipole field. In this work, particles with arbitrary pitch angle are considered. By bounce averaging the kinetic equation of the phase space density, information along magnetic field lines can be inferred from the equator. The three‐dimensional model is used to simulate the recovery phase of a model great magnetic storm, similar to that which occurred in early February 1986. The initial distribution of ring current ions (at the minimum Dst) is extrapolated to all local times from AMPTE/CCE spacecraft observations on the dawnside and duskside of the inner magnetosphere spanning the L value range L = 2.25 to 6.75. Observations by AMPTE/CCE of ring current distributions over subsequent orbits during the storm recovery phase are compared to model outputs. In general, the calculated ion fluxes are consistent with observations, except for H+ fluxes at tens of keV, which are always overestimated. A newly invented visualization idea, designated as a chromogram, is used to display the spatial and energy dependence of the ring current ion differential flux. Important features of storm time ring current, such as day‐night asymmetry during injection and drift hole on the dayside at low energies (&lt;10 keV), are manifested in the chromogram representation. The pitch angle distribution is well fit by the function, jo(1 + Ayn), where y is sine of the equatorial pitch angle. The evolution of the index n is a combined effect of charge exchange loss and particle drift. At low energies (&lt;30 keV), both drift dispersion and charge exchange are important in determining n.

The equatorial electrojet and the magnetosphere-ionosphere coupling—I. Analysis of low-latitude geomagnetic data

Ground magnetic data of three low-latitude magnetic stations in China (Qiongzhong, Guangzhou and Quanzhou) are used to study the equatorial electrojet during the main phase and the initial stage of the recovery phase of strong magnetic storms. The analysis of the Z-component demonstrates that corresponding to the development of the ring current, there is a negative disturbance of ΔZ (i.e. an additional eastward electrojet), while during the recovery of D st there is a positive disturbance (a westward electrojet). This tendency can be seen at all local times, but the average amplitude varies with the local time. The effect becomes insignificant at the later stage of a storm, or in case of a storm that closely follows another one. Superposed epoch analysis, correlation analysis, X 2-analysis and analysis of variance are employed to test the results. Data from Indian stations are included to test the universality of the finding. The influence of the polar electrojet is also discussed. The research indicates the importance of distinguishing between the features at different storm phases and between effects of dynamic coupling and those of electrodynamic coupling in studying the equatorial electrojet of disturbed days.

Studies on Plasma and Plasma Waves in the Plasmasphere and Auroral Particle Acceleration Region, by PWS on board the EXOS-D (Akebono) Satellite

Plasma wave, and sounder experiments (PWS) on board the EROS-D (Akebono) satellite are continuously providing the wide variety of data which contribute to the studies on plasma wave phenomena associated with the wave particle interactions in the regions of the cusp, auroral particle acceleration, and magnetic equator. Observation results of the auroral kilometric radiation (AKR) with Poynting vector, polarization as well as the dynamic spectra, have indicated that the generation mechanism of AKR is not necessarily due to the cyclotron maser mechanism but majority parts of AKR's are caused by the conversion of upper hybrid waves through the Doppler conversion processes and also through irregularities of the plasma in the auroral particle acceleration regions. Propagating electromagnetic wave phenomena in the plasmasphere are 1) banded rising and falling LF waves (ERIFLE), 2) discrete HF emissions, and 3) terrestrial hectometric radiations. There are localized modes of the plasma waves such as 4) upper hybrid mode waves, 5) Z-mode waves and 6) quasi-electrostatic whistler mode waves. Reflecting the evidence of the strong wave particle interactions, in the region of the cusp and auroral particle precipitation region, 7) broad band electrostatic noises are frequently observed and these become good indicator of the particle precipitation when the low energetic particle detector makes a pause of their operation in the low latitude range. Storm time effects of the plasma waves show also unique and important features; plasmaspheric odd harmonic, (n+1/2) fc emissions which have never detected in the plasmasphere inside of the plasmapause in the regular condition of the magnetic field activities take place even in the plasmasphere inside of the plasmapause in the times of large magnetic storm.The plasma density profiles obtained from the observed upper hybrid waves provide wide variety of variation of the plasmasphere and plasmapause structure with informations of the plasma instabilities there. A review has been made for the instabilities identified in the magnetic equator whose occurrence region is circulating at the magnetic equator covering the altitude from 1, 000km to 10, 000km. In the storm time, the plasmapause has apparently been disrupted with depressed density and loosing sharp edge of the density variation. In the recovery phase of the large magnetic storm, the plasma density distribution shows large disturbances where the amplitudes of the irregular variation of the plasma density become 5 to 6 times or sometimes an order of magnitude of the background density itself. Also during recovery phase of the storm, a large scale cavity of the plasma is formed centered at the magnetic equator coinciding with the position of expected ring current. All of these results show that the PWS on board the EROS-D satellite are making effective contribution to study on the energy transfer through the plasmasphere providing data of the electron density, temperature anisotropy and informations on existing free energy of the plasma and the energetic particles.

The effects of magnetic storm phases on F layer irregularities below the auroral oval

Through the study of two periods in September and October 1981 it was possible to observe F layer irregularity development and intensity primarily over subauroral latitudes in the area of the plasmapause. Using sets of data which include in situ measurements of low‐energy (&lt; 12 eV) electron precipitation, ionosonde measurements of spread F, and a series of scintillation measurements, we observed the following phenomena: (1) the descent of the auroral irregularity region to include subauroral latitudes in the general area of the plasmapause during the main phases of a series of magnetic storms, and (2) the “subauroral or plasmapause storm” case when irregularities were noted primarily at lower latitudes, which took place during the recovery phase of magnetic storms when local magnetic activity was low or moderate. Irregularities at these latitudes appearing during both magnetic quiet and magnetically disturbed conditions make statistical modeling of intense F layer irregularities for latitudes in this region very difficult. Dynamic modeling, which would follow the phases of a magnetic storm from commencement through injection to recovery, would more likely be successful in predicting periods of spread F, scintillations, and other manifestations of F layer irregularities at subauroral latitudes. We suggest that the subauroral irregularities in the recovery phase of magnetic storms are the result of energy stored in the ring current which then is slowly released. The appearance of intense F layer irregularities at subauroral latitudes over several days in the recovery phase of magnetic storms is similar to the recovery pattern of Dst, the measure of ring current energy density.

A Sun‐aligned arc observed by DMSP and AE‐C

On May 12, 1977, 8 minutes after a DMSP satellite photographed a sun‐aligned auroral arc in the southern polar cap, AE‐C crossed over the same arc. Precipitating electrons were observed with a peak energy flux of 0.94 erg cm−2 s−1 but with no clear monoenergetic beam, and coincident ion precipitation was measured at energies of a few keV. A very sharp ion convection reversal was found coincident with the particle precipitation and embedded in a region of constant antisunward flow. Magnetic field data indicate that the arc occurred during the recovery phase of a weak magnetic storm, with the IMF in a toward sector and the Z component nearly zero. The data are consistent with a source of particles at altitudes of the order of 5–8 RE on field lines containing the plasma sheet boundary layer. The electron spectra do not indicate the existence of an electrostatic potential along the magnetic field lines projecting from the arc. The electrodynamic properties associated with the arc appear to be consistent with a simple model in which field‐aligned currents are required along any boundary where the horizontal ionospheric current diverges.