High-latitude mapping of ULF activity, field-aligned currents, and DMSP-based dayside magnetospheric domains

V. A. Martines-Bedenko,¹ V. A. Pilipenko,¹ V. O. Papitashvili,² M. J. Engebretson,³ J. F. Watermann,⁴ and P. T. Newell⁵

¹ Institute of the Physics of the Earth, Moscow, Russia
² Space Physics Research Laboratory, University of Michigan, USA
³ Augsburg College, Minneapolis, USA
⁴ Danish Meteorological Institute, Copenhagen, Denmark
⁵ Applied Physics Laboratory, Johns Hopkins University, Laurel, USA

Contents

Abstract

Introduction

The interaction of solar wind plasma flow with the Earth's magnetosphere can be considered as a giant, natural magnetohydrodynamic (MHD) generator, which produces the large-scale, quasi-steady 3D current systems at high latitudes. These systems comprise of the field-aligned currents (FAC) in the magnetosphere, coupled with the closure (horizontal) currents in the ionosphere. The intensity and spatial distributions of these current systems to a considerable extent is controlled by the interplanetary magnetic field (IMF). The transfer of energy and momentum from the solar wind through the magnetosheath into the magnetosphere occurs at the dayside magnetospheric boundary region. Mapping of the boundary layer regions to the low altitudes is not quite certain because of the still uncertain topology of the entire magnetosphere, although it is clear that these spatially vast regions map into a very limited area around the low altitude cusps. This mapping can be studied either by using the advanced magnetic field models, or by low-altitude measurements of charged-particle precipitation, visible auroral emissions, radar observations, etc. Charged-particle precipitation characteristics seem to be the best low-altitude means to categorize the boundary layers [Newell and Meng, 1988].

The global magnetosphere-ionosphere current system, as they are constructed from magnetic measurements either on the ground or on low-altitude satellites, can be decomposed into several sub-systems. In the dawn/dusk sectors, the ionospheric DP1/2 (or field-aligned R1/R2) current system dominates. This current system consists of two longitudinally elongated current sheets with downward (upward) FAC in the poleward sheet (R1) and more wide upward (downward) FAC in the equatorward sheet (R2) at dawn (dusk). This system is intensified under conditions of southward IMF ( B_z < 0 ).

In the dayside cusp/cleft region, the DPY current system dominates [Troshichev et al., 1997]. This current system is driven by the IMF changes through the quasi-steady viscous interaction and magnetic reconnection at the dayside magnetopause. In its simple form, the DPY current system comprises two sheets of field-aligned currents coupled via the ionosphere, which drive the east-west Hall current, producing ground magnetic disturbances. The position and intensity of this current system are controlled by the IMF, especially by the azimuthal component B_y.

Poleward of the cusp, the NBZ current system resides, most evident when B_z > 0. During periods of northward IMF, the NBZ current system is located near noon at geomagnetic latitudes higher than 80^o, with the upward pre-noon FACs and downward post-noon FACs.

The energy transfer from the solar wind plasma into the magnetosphere and ionosphere has a turbulent character. Thus, it might be expected that in the key regions of the solar wind-magnetosphere, the electromagnetic noise can be generated. The occurrence of natural magnetospheric MHD waveguides and resonators may result in the noise's partial filtering producing quasi-periodic pulsations. Indeed, at high latitudes, the intense quasi-periodic ULF (ultra-low-frequency) pulsations of the geomagnetic field in the Pc5 range (1-10 mHz) are commonly observed, but the exact physical mechanism for these ULF disturbances has not yet been established. The common view is that the main source of dayside Pc5 waves is the Kelvin-Helmholtz (KH) instability at the flanks of the magnetosphere, excited by the solar wind flow. The velocity shear may exist at interfaces between other magnetospheric boundary domains, thus being the probable source of the KH-generated disturbances.

Inside the magnetosphere, these disturbances are transformed into more regular, quasi-monochromatic Pc5 pulsations under the influence of the magnetospheric resonance effects [Clauer et al., 1997]. The position of the resonance is determined by the match between the local Alfvén frequency, and the frequency f of an external source, irrelevant to a particular source mechanism. According to this notion, the latitude of maximal ULF intensity is determined by the features of the magnetospheric plasma distribution.

The existence of specific ULF signatures of boundary phenomena is still not resolved problem. In some studies, the broadband disturbances in the period range of 3-15 min (named as "Irregular Pulsations at Cusp Latitudes," IPCL by Troitskaya [1985]) or broad-band Pc5 pulsation by Engebretson et al. [1995] were claimed to be a typical feature of the dayside boundary phenomena. In early studies [Olson, 1986; Rostoker et al., 1972; Troitskaya and Bol'shakova, 1977, 1988] it was believed that a probable source of the dayside high-latitude long-period pulsations was related to the cusp - the region of direct penetration of turbulent magnetosheath plasma into the magnetosphere/ionosphere. In other studies [Lanzerotti et al., 1999; McHarg et al., 1995] it was assumed that quasi-monochromatic Pc5 pulsation at dayside is a signature of the near-cusp closed field line and can be used as cusp discriminator. Statistical characteristics of the ground-based pulsations have been suggested for monitoring of the dynamics of the cusp/cleft region [Bolshakova, 1988; Kleimenova et al., 1985; McHenry et al., 1990]. However, later experimental observations at MACCS cusp-oriented array questioned this hypothesis [Engebretson et al., 1995]. Various hypotheses have been suggested for interpretation of the cusp-related ULF disturbances, including a fluctuating component of FACs or precipitating electrons [Engebretson et al., 1991]; fluctuations of the cusp-related current system [Olson, 1986], the KH instability in the region of the convection reversal boundary, geomagnetically conjugate with inner part of LLBL [Clauer et al., 1997].

To put the ULF study in a more global magnetosphere/ionosphere context, we developed an approach to study simultaneously the ULF global pattern together with some proxies of ionospheric electrodynamics and identification (not always available) of ionospheric projections of the dayside magnetospheric domains. The problem addressed in this paper is how the Pc5 ULF pulsations observed at the dayside of the polar region are relevant to the magnetospheric domains and global (R1/R2, DPY, and NBZ) current systems.

Empirical-Analytical Model of High-Latitude Global Electrodynamics

Long-term experimental observations at high-latitude magnetic observatories established a reliable connection between the IMF and ionospheric current systems, which resulted in several empirical-analytical models. One of the approaches (Linear Modeling of Ionospheric Electrodynamics, LiMIE), developed at the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN) [Feldstein and Levitin, 1986; Papitashvili et al., 1994], is utilized in this study. The IZMIRAN Electrodynamics Model (IZMEM) utilizes a linear regression relationship between the IMF and ground-based geomagnetic disturbances. The applied approach provides a parameterization of observed geomagnetic variations at high latitudes by the IMF components' strength and direction. Then the ionospheric electrodynamic parameters can be calculated and mapped over both the northern and southern polar regions using any statistical model of ionospheric conductivity [e.g., Robinson and Vondrak, 1984; Wallis and Budzinski, 1981].

The IZMEM model does not require collection of in situ ground-based geomagnetic data for the event under investigation or selection of a magnetically quiet period to calculate geomagnetic disturbances. These distinguish the IZMEM from other similar algorithms such as the "magnetogram inversion technique" [Mishin et al., 1980], the KRM method [Kamide et al., 1981], and the AMIE technique [Richmond and Kamide, 1988]. The IZMEM model has recently been recalibrated utilizing the DMSP electrostatic potentials [Papitashvili et al., 1999]; the Web-based interface to the IZMEM model has been made available at http://www.sprl.umich.edu/MIST/limie.html. In fact, this model allows the user to obtain instantaneous distributions of ionospheric electric potentials or FACs for a given IMF during time interval under investigation. However, we should remember that the IZMEM is an empirical model, so physical mechanisms of predicted 3D systems and identification of the basic current elements within the magnetospheric domains have not been considered so far in this model.

Ground-Based Experimental Facilities

The experimental basis of our research is the data from global networks of magnetic stations at high latitudes, shown in Figure 1:

- CANOPUS (http://www.dan.sp-agency.ca/www/canopus-home.html), a network of 13 automatic stations deployed over the West-Central Canada with a time sampling period of 5 s. The network forms two meridian arrays along 315^o and 335^o corrected geomagnetic (CGM) longitude;

- MACCS (http://space.augsburg.edu/space/MaccsHome.html), a network with 12 identical fluxgate magnetometers with 0.5-s sampling deployed in the Canadian Arctic, which includes one station in the polar cap and longitudinal profiles along geomagnetic latitudes ~79^o N and ~75^o N. Together with CANOPUS, these two arrays form 3 meridian profiles: 15^o (MC-East), ~335^o (MC-Center), and ~315^o (MC-West).

- Greenland Coastal Array (http://www.dmi.dk/projects/chain/), two latitudinal arrays of 21 magnetic stations with 20-s sampling deployed along the West (~40^o) and East (~95^o) Coasts of Greenland. This network is augmented by MAGIC (http://www.sprl.umich.edu/MIST/) consisting of the array on the Greenland ice cap (~60^o);

- IMAGE (http://www.geo.fmi.fi/image/), an auroral and sub-auroral network of 24 magnetometers with 10-s sampling at stations along the Scandinavian meridian (~105^o).

Later we plan to augment our technique's database with the data from CPMN (Circum-pan Pacific Magnetometer Network, formerly 210^o Magnetic Meridian, http://stdb2.stelab.nagoya-u.ac.jp/mm210/), which currently comprises 46 magnetic stations with fluxgate magnetometers with 1-s sampling.

Automated Identification of Ionospheric Projections of the Magnetospheric Domains From DMSP Data

In this study the observational results of DMSP satellites F10-F12 are used. The automated dayside region identification program is based on fine features of energetic particle spectra in the range 30-eV-30-KeV. This algorithm explicitly separates the basic magnetospheric signatures at altitude of the DMSP satellites (~800 km) [Newell et al., 1991a, 1991b]. Classification of the boundary regions is described on the Web site http://sd-www.jhuapl.edu/Aurora/index.html and given in the Appendix. The region identification database is given as a set of files with the results of equatorward and poleward universal time (UT) crossings of each boundary, as well as labeling the crossing of equatorward boundary by the geomagnetic latitude/magnetic local time in the geocentric coordinates.

Visualization and Mapping Technique

We develop a technique for simultaneous mapping (as a sequence of 2D "snap-shots") of the ionospheric electrodynamic pattern, predicted by the IZMEM model, and of the ULF spectral power. We consider it as a first step in providing the dynamical 2D analysis of ULF pulsations.

The program decimates the original geomagnetic data to a common sampling period of 20 s. Then, a FFT technique is used to estimate the spectral power in a selected frequency band within a moving window for each station. These data are used to construct the 2D spatial distribution of ULF power for a particular time interval. In subsequent analysis, the 1-hour window and 1.5-8.0 mHz frequency band are used. The time interval is indicated by its onset, that is, "10 UT" denotes "1000-1100 UT." On each plot, a normalized ULF power is indicated. This relative magnitude of the power peak for a given time interval is calculated according to the maximum throughout the whole day, which may be used as an indicator of ULF intensity in each snap-shot.

For the calculation of 2D spatial distributions of FACs over the high-latitude ionosphere as predicted by the IZMEM model, the necessary data are taken automatically from the OMNI database of 1-hour means of the IMF/solar wind parameters. Thus, the semi-empirical model IZMEM driven by the IMF parameters produces the snap-shots of 2D polar plots with the spatial structure of FACs throughout the dayside high-latitude ionosphere. The upward (negative, denoted by green color) FACs are assumed to be transported by precipitating electrons, whereas the downward (positive, denoted with red) FACs are carried by the upward flow of ionospheric electrons.

To establish a correspondence between the spatial distribution of ULF intensities, FACs, and magnetospheric boundaries, we overlay on the plots available DMSP satellite tracks with the results of automated identification of magnetospheric boundaries. The ground track of each orbit is plotted in geomagnetic coordinates where parts of the track are colored according to the color scheme adopted for the DMSP-detected domains. Thus, this track clearly indicates in different colors the position of ionospheric projection of each magnetospheric region.

This technique is used to analyze the relationship between the large-scale current systems, the intensity of low-frequency ULF waves, and the correspondence of both parameters to the dayside magnetospheric boundaries. The study scheme addresses the following questions:

- What is the correspondence between the basic FAC regions predicted by the IZMEM model and dayside magnetospheric domains?

- What is the correspondence between the position of spatial peak in the ULF Pc5 power distribution and FACs?

- In which magnetospheric domains probable sources of the ULF activity are located?

Special attention is paid to the noon and morning MLT sector, as this is the region of the most intense ULF activity in the Pc5 band. Three days have been selected (rather arbitrarily) for analysis:

- December 26, 1995 (day # 360). The IMF B_z is slightly negative (  -(1-2) nT) during the day. Northward B_z excursions at ~0400 UT and ~1700 UT stimulated substorms observed at ~0400 UT (GCA, MACCS, and CANOPUS), and at ~1800 UT (IMAGE).

- February 18, 1995 (day # 049). At this day the IMF B_z was slightly northward and B_y varied from +5 nT to -4 nT at ~3-4 UT, then remained near zero.

- November 14, 1995 (day # 328). The OMNI IMF data indicate distinct periods with B_y < 0 (up to -5 nT) for different B_z conditions. Southward deviations of B_z cause substorms at 1545 UT (CPMN) and 1900 (IMAGE).

The IMF B_y changes during two latter days should produce variations of the DPY current system, thus a possible coupling of dayside Pc5 activity and DPY current intensification could be examined.

Identification of FAC Regions as Derived From the IZMEM Model and Their Comparison With the Magnetospheric Boundaries

Figure 2 (event February 18, 1995, day # 049) presents a typical pattern of FAC as derived from the IZMEM model: the occurrence of the NBZ current system near noon at latitudes higher than 80^o CGM latitude, with the upward pre-noon and downward post-noon FACs, and below 80^o, typical R1 structure with downward on the dawn side and upward on the dusk side. At 1600 UT in the pre-noon hours (~1000 MLT), the upward R2 current coincides with the CPS projection, whereas the most intense part of the R1 downward FAC is mainly in the LLBL, with some admixture of the BPS projections. The NBZ system is clearly seen because of IMF B_z > 0 where the upward NBZ current is located in the mantle. Similar regularities have been observed in prevailing number of other events.

Correspondence Between ULF Activity and FAC

The typical spatial ULF structure in the Pc5 band can be seen in Figure 3 for the event of December 26, 1995 (day # 360) at 1300 UT. Simultaneous occurrence of the near-noon (at East GCA) and morning (at MACCS) maximums of ULF power is observed. This observation shows that at least two sources in the Pc5 band may operate simultaneously. The overlaying of FAC pattern shows that the first Pc5 source is located in the morning sector, equatorward of the Region 1 FACs. As the examination of magnetograms (not shown) indicates, this source generates quasi-monochromatic Pc5 pulsations. The other source is located near noon. The magnetograms show that this ULF activity is more long periodical and irregular, as compared with the morning Pc5. This second source of broadband Pc5 (IPCL) was often attributed to the cusp ionospheric projection.

The Figure 3 shows that morning Pc5 are excited not in the same region as the Region 1 current, but equatorward of it. Thus, these pulsations may be related to the auroral electrojet flowing between the R1 and R2 current sheets.

For the analyzed days, we found no clear correspondence between near-noon Pc5 peak and high-latitude current system features (such as DPY).

Identification of Source Regions of ULF Activity

Simultaneous mapping of the ULF power distribution snapshots with the DMSP tracks makes it possible to identify the probable source region of ULF activity. Some examples are given below:

At December 26, 1995 (day # 360) 1800 UT (Figure 4) the maximum of monochromatic Pc5 power in the early morning hours at ~72 ^o is equatorward of the R1 current zone, as in the previous event, December 26, 1995 (Figure 3). The DMSP magnetospheric domain identification algorithm indicates that the center of Pc5 power is located inside the CPS region. At the same time, weaker near-noon and evening spatial maximums can be seen. However, there are no DMSP passes over these centers of the ULF activity to identify the magnetospheric region of their source.

In another event, at December 26, 1995 (day # 360) at 1400 UT, the morning Pc5 pulsation maximum (MC-Center) is near the CPS/BPS boundary ~72 ^o (Figure 5). The weaker spatial local maximum can also be seen near noon and in the afternoon hours.

During the November 24, 1995 (day # 328) 1800 UT event (Figure 6), the DMSP passes through the near-noon maximum of ULF activity. The magnetograms examination (not shown) indicates that this ULF activity is the broadband Pc5, commonly named cusp-related Pc5 or IPCL. The latitude of the spatial distribution peak, F 78^o, is higher than typical latitudes of morning Pc5. As DMSP data indicate, the source region of the near-noon, broadband Pc5 for this event is not located in the cusp, but coincides with the equatorward boundary of the LLBL.

Discussion

Our study shows that the IZMEM-predicted location of Region 1 FACs coincides mostly with the LLBL. This is in accord with the common notion that LLBL is the driver of the Region 1 current system. This correspondence gives additional support to the physical background of the IZMEM model. At the same time, Region 2 FAC in the morning sector are situated mostly in the CPS region.

In the morning sector, the peak of ULF intensity is commonly situated at the equatorward boundary of the downward FAC (Region 1). This location corresponds to the typical position of an intense westward auroral electrojet (Hall current) located equatorward of Region 1, probably between the Region 1 and Region 2 FACs. Therefore, as it follows from the above, in the morning/dayside sector the latitudes where the AEJ intensity and the Pc5 power reach peak magnitudes are not independent and related to each other. This fact fits the early observations of Lam and Rostoker [1978], but this effect is not taken into account by modern theories of the ULF Pc5 waves, whose assume that the position of the morning Pc5 peak is determined only by the magnetospheric plasma distribution. These experimental results could be significant for developing of models that are more adequate, but such adequate interpretation may require a substantial revision or augmentation of existing Pc5 models.

Early studies of dayside ULF activity at high latitudes gave hope that long-period irregular variations were supposedly closely associated to the cusp/cap interface, and thus could be used as a simple indicator of dayside cusp position and polar cap boundary. However, further studies of high latitude broadband wave activity on the dayside [e.g. Engebretson et al., 1995] showed that it cannot be simply associated with boundary layer or cusp proximity, but, instead, shows the coordinated time dependence across several hours of local time. Attempts to find a cause for these widespread temporal variations in the solar wind, IMF, or substorm injections have so far been fruitless.

In contrast to the approach in this paper, the search for specific ULF signatures of boundary phenomena in most previous studies were based on data from isolated stations with limited latitude/longitude coverage. At sub-auroral stations the persistent occurrence of quasi-monochromatic Pc5 pulsations are observed, mostly in early morning hours during a substorm's recovery phase. At higher latitudes irregular long-period variations, named as IPCL in [Troitskaya and Bolshakova, 1988] and broad-band pulsations in [Engebretson et al., 1995], were revealed. However, some case studies with more extended arrays showed a regular transition from irregular broad-band (IPCL) pulsations at high latitudes to more intense and monochromatic Pc5 pulsations at lower latitudes [Clauer et al., 1997; Pilipenko et al., 1998]. Thus, these events indicated that Pc5 and IPCL pulsations are not separate wave phenomena, but the manifestations of the same wave process, whereas the difference in their appearance is related to the resonant amplification deeper into the magnetosphere, probably at closed dipole-like field lines. The region of possible ULF driver is hard to identify, because the secondary maximum in a resonant point is higher than the primary maximum in the source region. Thus, simultaneous occurrence of IPCL and Pc5 near-noon may signify a situation when both ULF driver and resonant response are observed on the ground. However, Kleimenova et al. [1998] presented ULF events where IPCL and Pc5 were not accompanied each other. Therefore, the problem of the IPCL/Pc5 coupling needs further investigation.

Among all the considered events, we never observed a spatial correspondence between the proper cusp and ULF activity peak. Thus, the widely used terms "cusp-related pulsation" or "cusp-associated ULF waves" are likely not adequate in studying real events; probably, the terms "near-noon high-latitude Pc5" or "broad-band ULF activity" would be more adequate. In the considered event, we found that spatial peak of ULF activity near noon maps to the LLBL inner boundary. This fact needs a further statistical evaluation.

Coincidence of the probable source region of near-noon Pc5 with the LLBL projection agrees with the simultaneous radar and magnetometer observations discussed by Clauer et al. [1997], who attributed the source of these pulsations to the reversal boundary of ionospheric convection associated with the LLBL. The suggested mechanism is the KH instability, excited at the convection reversal boundary.

Observations of morning and post-noon Pc5 led earlier workers to the conclusion that K-H instability at the magnetopause or LLBL is a likely candidate for Pc5 drivers. Later, indications were found that impulsive variations of the dynamic pressure of the solar wind and FTE constitute a possible source of Pc5 wave packets in the magnetosphere. Thus, the occurrence of Pc5 pulsations can be considered as an indicator of either internally generated turbulence or external magnetosheath turbulence. Our analysis often revealed the simultaneous occurrence of 3 regions of ULF intensification: morning sector, near-noon, and post-noon hours. They may be hardly ascribed to the same driving mechanism, such as K-H instability.

The assumption of the K-H instability as a universal driving source of geomagnetic pulsations meets some difficulties. The classical linear theory of instabilities of a tangential discontinuity between two semi-infinite half-spaces predicts that short wave disturbances with lateral scales of an order of the boundary layer thickness d are predominantly excited. The energy transfer is in this case effective only for short wave disturbances ( kd 1 ), so, the zone of the flow turbulization and the pulsation scale should be relatively small. But, the observational data give wavelengths in the region of morning Pc5 generation not less than 10  R_E. Probably, consideration of mechanisms beyond the linear instability theory (such as non-linear convective vortexes due to the inverse turbulence cascade, multi-layer structure of boundary layers, etc.) would be necessary for adequate model of morning/evening Pc5 model.

The same K-H instability can be hardly applied to the near-noon Pc5 because in this region the velocity of the magnetosheath plasma flow is not high. The Pc5/IPCL activity at the near-noon hours could be impulsively driven pulsations that occur in response to the magnetosheath plasma discontinuities and buffeting. In line with this idea, analysis of a series of IPCL bursts under moderate geomagnetic activity showed that these signals possess rather distinctive features, typical for a system near a critical transition to a chaotic regime [Kurazkovskaya and Klain, 2000]. The authors suggested that IPCL series might be a manifestation of the development of dynamic turbulence in field-aligned currents in the cusp region. The difference in source mechanisms of the noon and morning Pc5 should reveal itself in the spatial structure (e.g. azimuthal phase velocity) of pulsations, which is to be verified in further studies.

Conclusions

In most cases in the morning sector, the region of downward FAC corresponds to the LLBL, whereas the upward FAC corresponds to the CPS. Thus, the LLBL is a driver of the R1 current system, at least in the morning sector.

Often, three regions of ULF excitation are simultaneously observed: during morning hours, near noon, and during afternoon hours, that may indicate the simultaneous occurrence of several drivers of ULF waves. In the morning sector, the peak of ULF intensity is commonly situated equatorward of the boundary of downward R1 FACs, probably in the region of the auroral electrojet.

The resonant monochromatic response to Pc5 driving is observed near the CPS/BPS interface. The latitude of the spatial distribution peak of broadband ULF pulsations in the Pc5 range near noon, which are used to be names "cusp-related pulsation," in fact coincides with the equatorward boundary of the LLBL.

Acknowledgments

Additional geomagnetic data used in this study were kindly provided by the CANOPUS and IMAGE teams, to whom we gratefully acknowledge. The help of J. Skura in obtaining the DMSP data is appreciated. The research of J.M.E. in Minnesota was supported by U.S. National Science Foundation (NSF) grant ATM-9610072, and V.O.P acknowledges the support in Michigan form the NSF award OPP-9614175. V.A.M-B and V.A.P thank the Danish Meteorological Institute for the support of their visits.

Appendix A: Identifications of Dayside Magnetospheric Domains From DMSP Data

Regions are identified as one of the following, generally moving from higher to lower latitudes:

prn - intense polar rain, the suprathermal component of solar wind electrons often observed over the polar cap;

mantle - de-energized magnetosheath ions observable poleward of the dayside oval;

cusp - is the projection of the magnetospheric exterior cusp, a region with full intensity magnetosheath ions and electrons. Although the cusp can join the open LLBL reasonably smoothly, the cusp is taken to be where the ion and electron fluxes begin to approximate magnetosheath values, which generally occurs where the ion cutoff drops to at least 1-3 keV and below;

opll - clearly open LLBL, with low-energy ion cutoffs and magnetosheath electrons at reduced fluxes, observed immediately equatorward of the cusp;

llbl - closed LLBL with no low energy ion cutoffs, and spectra closely resembling high altitude LLBL passes. Such signatures are generally observed away from noon, not immediately equatorward of the southward IMF cusp. Low-altitude LLBL is named cleft sometimes;

bps - precipitation which closely resembles the poleward portion of the nightside auroral oval. The electrons have a typical temperature of about 300 eV, somewhat higher than the LLBL;

ps - the hard zone of electron precipitation on the dayside. These consist of electrons, which have been injected into the near-Earth region on the nightside and subsequently drift around the Earth. Typical energies are above 1 keV;

uncl - unclear or unclassified: when the flux levels are clearly significant but the precipitation did not fit any of the quantitative rules for other regions;

void - meaning fluxes are generally near or below noise levels.

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