Polar Rain Electrons Research Articles

Spatial structures in solar wind superthermal electrons and polar rain aurora

We report a special polar rain aurora case around 11:24 UT on October 27, 2003, where intense polar rain electrons produced observable polar rain auroral emission with the shape of a roughly dawn-dusk aligned bar. Associated solar wind speed and density observations during the event were around 450 km/s and 2.5 cm−3 respectively. The interplanetary magnetic field (IMF) components Bx, By, and Bz were ~5, −3, and 5 nT respectively. The negative By condition likely caused the dawnside shift and slight tilt of the polar rain aurora bar. Furthermore, although Kelvin-Helmholtz waves on the high latitude magnetopause have been previously reported to induce dawn-dusk aligned auroral bars (Zhang et al., 2007), the solar wind and IMF conditions of the event are not favorable for generating them (Zhang et al., 2013) and are therefore not a likely cause. Instead, coincident observations by the Geotail satellite show enhanced anti-sunward flux in the solar wind superthermal electrons (7 eV–42 keV) around the time of the auroral bar. The solar wind superthermal electron spatial size, when mapped into the polar ionosphere, is consistent with the width of the auroral bar, confirming a connection between the two.

Formation of the Potential Jump Over the Geomagnetically Quiet Sunlit Polar Cap Region

AbstractWe consider the formation of a potential drop over the Earth's polar cap during geomagnetically quiet daytime. The observed potential drop is primarily defined by the hydrogen, photoelectron, and polar rain fluxes ratios and depends strongly on the energy distribution of the photoelectron flux. Polar rain is an essential component of the model required for plasma quasi‐neutrality. The potential distribution along the magnetic field line has two regions, with a small, gradual, potential drop of 3–4 V and a potential jump. The value of the potential jump depends on the hydrogen ion to photoelectron flux ratio and is also controlled by polar rain electrons. With quasi‐neutrality required at its upper boundary, the jump only occurs in the presence of polar rain and its location depends on the polar rain flux. Model predictions compare well with FAST observations presented by Kitamura et al. (2012, https://doi.org/10.1029/2011JA017459).

Simultaneous Remote Observations of Intense Reconnection Effects by DMSP and MMS Spacecraft During a Storm Time Substorm.

During a magnetic storm on 23 June 2015, several very intense substorms took place, with signatures observed by multiple spacecraft including DMSP and Magnetospheric Multiscale (MMS). At the time of interest, DMSP F18 crossed inbound through a poleward expanding auroral bulge boundary at 23.5 h magnetic local time (MLT), while MMS was located duskward of 22 h MLT during an inward crossing of the expanding plasma sheet boundary. The two spacecraft observed a consistent set of signatures as they simultaneously crossed the reconnection separatrix layer during this very intense reconnection event. These include (1) energy dispersion of the energetic ions and electrons traveling earthward, accompanied with high electron energies in the vicinity of the separatrix; (2) energy dispersion of polar rain electrons, with a high‐energy cutoff; and (3) intense inward convection of the magnetic field lines at the MMS location. The high temporal resolution measurements by MMS provide unprecedented observations of the outermost electron boundary layer. We discuss the relevance of the energy dispersion of the electrons, and their pitch angle distribution, to the spatial and temporal evolution of the boundary layer. The results indicate that the underlying magnetotail magnetic reconnection process was an intrinsically impulsive and the active X‐line was located relatively close to the Earth, approximately at 16–18 RE.

On the field‐aligned electric field in the polar cap

AbstractThe Johns Hopkins University Applied Physics Laboratory open‐field line particle precipitation model predicts downward field‐aligned electric field to maintain charge quasi‐neutrality. Previous studies confirmed the existence of such electric fields. However, the present study shows that upward field‐aligned electric field can be found within upward field‐aligned current (FAC) region. In the upward FAC region, upward electric field that accelerates electron downward is seen with the occurrence rates of 82%–96%. In contrast, the occurrence rates in the downward FAC regions are 3%–11%. Polar rain electrons located in the upward FAC region adjacent to closed field lines often show a ramping up of energy with increasing latitude before reaching a plateau. This plateau may be attributed to the magnetosheath electrons that progressively have higher antisunward velocity and lower density with increasing distance from the subsolar point before they asymptotically reach the solar wind values.

The nightside magnetic field line open–closed boundary and polar rain electron energy-latitude dispersion

Abstract. The polar rain electrons near the open–closed field line boundary on the nightside often exhibit energy-latitude dispersion, in which the energy decreases with decreasing latitude. The solar wind electrons from the last open-field line would E × B drift equatorward as they move toward the ionosphere, resulting in the observed dispersion. This process is modeled successfully by an open-field line particle precipitation model. The existing method for determining the magnetotail X line distance from the electron dispersion underestimates the electron path length from the X line to the ionosphere by at least 33%. The best estimate of the path length comes from using the two highest energy electrons in the dispersion region. The magnetic field line open–closed boundary is located poleward of the highest energy electrons in the dispersion region, which in turn is located poleward of Defense Meteorological Satellite Program (DMSP) b6, b5e, and b5i boundaries. In the four events examined, b6 is located at least 0.7–1.5° equatorward of the magnetic field line open–closed boundary. The energy-latitude dispersion seen in the electron overhang may result from the plasma sheet electron curvature and gradient drifts into the newly closed field line.

Nightside polar rain aurora boundary gap and its applications for magnetotail reconnection

[1] Polar rain electrons with energy around 1 keV occasionally fill the polar cap ionosphere. Their energy flux is high enough to produce detectable auroral emissions, called polar rain aurora (PRA). We report a PRA event on 2 August 2002 and the estimation of the magnetotail reconnection location and size from the PRA boundary. The distinguishing feature of this event identified from the TIMED/GUVI data is an emission depletion gap between PRA and auroral oval. The size of the gap is about 2° in latitude and extended from the dusk sector (17:00 MLT) to the premidnight sector (22:00 MLT). This gap coincides with the electron flux gap in the DMSP electron energy flux data. The electron flux data show the existence of energy dispersion at the gap. These GUVI and DMSP observations support the idea that the gap is very likely due to a magnetic reconnection in the magnetotail that blocks the solar wind electron entry into the polar ionosphere. Based on the electron energy dispersion, the nightside X line is estimated to be at X = −50 to −60 RE, which is consistent with results from other studies. The gap extends from the nightside to the duskside and presumably to the dawnside. This new information suggests that the extension of the tail X line over the entire plasma sheet in the dawn-dusk direction under a strongly southward IMF (Bz ∼ −10 nT) and the curved nature of the X line.

Does the polar cap disappear under an extended strong northward IMF?

The suggestion that the polar cap can completely disappear under certain northward IMF conditions is still controversial. We know that the size of the polar cap is strongly controlled by the interplanetary magnetic field (IMF). Under a southward IMF, the polar cap is usually large and filled with weak diffuse polar rain electrons. The polar cap shrinks under a northward IMF. Here we use the global auroral images and coincident particle measurements on May 15, 2005 to show that the discrete arcs (due to precipitation of both electrons and ions) expanded from the dayside oval to the nightside oval and filled the whole polar ionosphere after a long (8 h) and strong (∼5–30 nT) northward IMF B z, The observations suggested that the polar cap disappeared under a closed magnetosphere.

Polar rain gradients and field‐aligned polar cap potentials

ACE SWEPAM measurements of solar wind field‐aligned electrons have been compared with simultaneous measurements of polar rain electrons precipitating over the polar cap and detected by DMSP spacecraft. Such comparisons allow investigation of cross‐polar‐cap gradients in the intensity of otherwise‐steady polar rain. The generally good agreement of the distribution functions, f, from the two data sources confirms that direct entry of solar electrons along open field lines is indeed the cause of polar rain. The agreement between the data sets is typically best on the side of the polar cap with most intense polar rain but the DMSP f's in less intense regions can be brought into agreement with ACE measurements by shifting all energies by a fixed amounts that range from tens to several hundred eV. In most cases these shifts are positive which implies that field‐aligned potentials of these amounts exist on polar cap field lines which tend to retard the entry of electrons and produce the observed gradients. These retarding potentials undoubtedly appear in order to prevent the entry of low‐energy electrons and maintain charge quasi‐neutrality that would otherwise be violated since most tailward flowing magnetosheath ions are unable to follow polar rain electrons down to the polar cap. In more limited regions near the boundary of the polar cap there is sometimes evidence for field‐aligned potentials of the opposite sign that accelerate polar rain electrons. A solar electron burst is also studied and it is concluded that electrons from such bursts can enter the magnetotail and precipitate in the same manner as polar rain.

Polar rain aurora

Global FUV auroral imagers (IMAGE/SI‐13 and DMSP/SSUSI) observed, for the first time, two similar auroral events in the southern polar cap due to intense (keV) polar rain electrons from the solar wind as observed by DMSP and Geotail. Such an aurora is called polar rain aurora. The polar rain aurora could fill the dayside polar cap initially and developed a dawn‐dusk alignment while they moved anti‐sunward. The associated IMF Bz was mostly southward. The IMF Bx changed from negative to positive for the first event and stayed positive for the second event. The strong IMF By was associated with the two events. The dawn‐dusk alignment of polar rain aurora might be due to the dawn‐dusk aligned magnetic flux tubes in the magnetosheath caused by the dominant IMF By and modulation of the keV electrons by the nonoscillatory drift mirror waves and pitch angle diffusion via the electron cyclotron instability.

Remote sensing of a magnetotail reconnection X‐line using polar rain electrons

We report on electron phase space distributions (PSDs) observed near the plasma sheet (PS) boundary layer (PSBL) by the Cluster electron spectrometers when the northern lobe was occupied by significant fluxes of polar rain (PR) electrons. These observations reveal the spatial structure of the electron transition layer (TL) between the polar rain electrons and the PSBL electron population accelerated during reconnection. This TL comprises overlapping spatial dispersion signatures in both energy and pitch angle, which are caused by convection of flux tubes across the magnetic separatrix during the electron time‐of‐flight (TOF) from the X‐line combined with acceleration in the reconnection region. Analysis of this structure allows us to estimate the location of the X‐line. By assuming the PSBL population arises through acceleration of the PR electrons, comparison of their PSD indicates that the electrons gain energy proportional to their initial energy.

Accelerated polar rain electrons as the source of Sun‐aligned arcs in the polar cap during northward interplanetary magnetic field conditions

Identifying the source of electrons producing polar cap arcs (P‐C arcs), ubiquitous under northward interplanetary magnetic field (IMF) conditions, should be a notable step toward solving the long‐standing problem of determining the topology of the magnetosphere for northward IMF. We calculate several electrodynamics limits that follow from a kinetic theory perspective of electrons precipitating into the polar cap from the electron reservoir in the distant magnetospheric tail, for currents sustainable without electron acceleration. We then calculate mutual relationships between several electrodynamics parameters for currents beyond these limits, if electron acceleration processes were to draw the additional current‐carrying electrons from this same reservoir. We compare these theoretical limits and mutual relationships with those actually observed in the polar cap. We conclude that for typical P‐C arc conditions, there is no need to look further than the magnetospheric tail for the source reservoir of electrons needed to provide observed fluxes in either polar rain or P‐C arcs. We close by putting this into the context of relevant models.

Entry process of low‐energy electrons into the magnetosphere along open field lines: Polar rain electrons as field line tracers

The strahl component of solar wind electrons, which constitutes field‐aligned electron heat flux running away from the Sun, is a strong candidate for the origin of the polar rain. We investigate the entry process of the strahl electrons into the distant‐tail magnetosphere and discuss topologies of Earth's field lines in this paper. The Geotail satellite has often observed either gradual or abrupt transitions from the magnetosheath electrons to the bidirectional lobe electrons at the magnetopause. In some cases, the strahl flux of the sheath electrons flowing tailward gradually turned near the magnetopause and finally became the earthward flux of the bidirectional lobe electrons. This transition is accompanied by the rotation of the magnetic field direction and indicates the direct entry of the strahl electrons along open field lines. On the basis of the data of 38 magnetopause crossings by Geotail, we investigate variation of density of the strahl (polar rain) electrons and that of ion density near the magnetopause. It is shown that the strahl electrons decrease upon crossing the magnetopause, and the decrease is correlated with that of ions, although, quantitatively, the strahl electrons do not decrease so much as ions. It is suggested that the strahl electrons enter the magnetosphere along open field lines more freely than ions, but their entry is under the influence of charge neutrality with ions. It remains as a problem why the Geotail data do not show the presence of electrons escaping from the magnetosphere along open field lines.

Observations of bidirectional electrons in the distant tail lobes: GEOTAIL results

ISEE‐3 demonstrated the relatively frequent occurrence of low‐energy (50 ∼ 500 eV) electron flux enhancements within the distant magnetotail lobes. These electrons were found to be “bidirectional” in the sense that intensities were highest parallel (and antiparallel) to the lobe magnetic field lines. It was concluded that these electrons must enter the tail lobes along open field lines at the distant magnetopause and that this population would constitute the source for polar rain electrons at low altitudes in the polar cap regions. Similar plasma electron features have been found using the Low Energy Particle (LEP) sensor system onboard the GEOTAIL spacecraft. Remarkably close correspondences of low‐energy (≲ 2 keV) ion tailward flow enhancements are found to occur when bidirectional electron fluxes suddenly intensify. Such ion plasma measurements were not available with ISEE‐3 and, thus, a new aspect of solar wind entry and overall plasma dynamics is revealed by these GEOTAIL measurements in the distant tail. Other ISTP data (e.g., IMP‐8 and CANOPUS) are used to analyze selected periods.

Formation of polar cap arcs

It is shown that polar cap arcs of typical electron energy ≤ 1 keV can be produced by the downward acceleration of polar rain electrons by parallel potential drops on open field lines. The formation of these arcs is the result of current response to meso‐scale velocity shear structures in the ionospheric convection that is not matched at the magnetosphere. The theory predicts the formation of a pair of meso‐scale field‐aligned currents (one upward and one downward) for each ionospheric shear structure. The theory does not require a bifurcated magnetic configuration for each polar cap arc; hence, it can account for the frequently encountered multiple polar cap arcs without cutting up the magnetosphere into a large number of open and closed magnetic field regions.

Strong electron bidirectional anisotropies in the distant tail: ISEE 3 observations of polar rain

A detailed observational treatment of bidirectional electrons (∼50 to 500 eV) in the distant magnetotail (r ≳ 100 RE) is presented. It is found that electrons in this energy range commonly exhibit strong, field‐aligned anisotropies in the tail lobes. Because of large tail motions, the ISEE 3 data provide extensive sampling of both the north and south lobes in rapid succession. These data demonstrate directly the strong asymmetries that exist between the north and south lobes at any one time. The bidirectional fluxes are found to occur predominantly in the lobe directly connected to the sunward interplanetary magnetic field in the open magnetosphere model (north lobe for away sectors and south lobe for toward sectors). Electron anisotropy and magnetic field data are presented which show the transition from unidirectional (sheath) electron populations to bidirectional (lobe) populations. Thus we demonstrate the open nature of the distant magnetopause and show that the source of the higher‐energy, bidirectional lobe electrons is the tailward directed electron heat flux population in the distant magnetosheath. Taken together, the present evidence suggests that the bidirectional electrons that we observe in the distant tail are closely related to the polar rain electrons observed previously at lower altitudes. Furthermore, these data provide strong evidence that the distant tail is composed largely of open magnetic field lines in contradistinction to some recently advanced models.

Polar rain: Solar coronal electrons in the Earth's magnetosphere

ISEE 1 low‐energy electron measurements have revealed the frequent presence of field‐aligned fluxes of a few hundred eV electrons in the geomagnetic tail lobes. In the northern tail lobe these electrons are most prominent when the interplanetary magnetic field is directed away from the sun. This characteristic helps identify the electrons as polar rain electrons that are usually identified by their uniform precipitation pattern over the earth's polar caps. Although tail lobe electrons have been seen previously at high altitudes, the ISEE data provide the first demonstration of their field‐aligned nature, and this characteristic places a stringent constraint on their origin. By mapping the tail lobe velocity distribution function into the solar wind, we have confirmed previous suggestions that the polar rain is indeed of solar wind origin and is due to the access of electrons to the magnetotail lobe. More specifically, however, it is demonstrated that the more energetic component of the polar rain is composed of electrons from the solar wind “strahl,” a field‐aligned component of the solar wind which is difficult to measure but which is thought to be caused by the collisionless transit of hundred eV electrons from the inner solar corona to 1 AU. The preferential entry of the strahl into that polar cap most directly connected magnetically to the sun explains a known north‐south asymmetry in polar rain intensity. Furthermore, it is suggested that cases of higher‐energy polar rain that have been thought to require a magnetospheric field‐aligned acceleration mechanism are more likely due to an enhanced intensity of the solar wind strahl caused by easier access of solar corona electrons to 1 AU and the polar caps. This idea is supported by the observation that previously reported cases of intense polar rain are invariably associated with tenuous or high‐speed solar wind that is expected to produce enhanced strahls. It is shown how polar rain measurements may provide a good measure of (1) the temperature and the state of statistical equilibrium in the solar corona and (2) the degree to which interplanetary field lines connect with those of the earth.

F layer ionization patches in the polar cap

Ground‐based optical and digital ionosonde measurements were conducted at Thule, Greenland to measure ionospheric structure and dynamics in the nighttime polar cap F layer. These observations showed the existence of large‐scale (800–1000 km) plasma patches drifting in the antisunward direction during a moderately disturbed (Kp ≥ 4) period. Simultaneous Dynamics Explorer (DE‐B) low‐altitude plasma instrument (LAPI) measurements show that these patches with peak densities of ∼106 el cm−3 are not locally produced by structured particle precipitation. The LAPI measurements show a uniform precipitation of polar rain electrons over the polar cap. The combined measurements provide a comprehensive description of patch structure and dynamics. They are produced near or equatorward of the dayside auroral zone and convect across the polar cap in the antisunward direction. Gradients within the large scale, drifting patches are subject to structuring by convective instabilities. UHF scintillation and spaced receiver measurements are used to map the resulting irregularity distribution within the patches.