A case study of electron precipitation in the late substorm growth phase on and nearby a preonset arc

Abstract. We follow the electron precipitation characteristics on and nearby a preonset arc using the high resolution Freja TESP instrument. Our data coverage extends from about 10 min before onset up to 1 min before onset. The arc is the most equatorward one (around MLAT 62°) of a system of growth phase arcs, and it was close to the radiation belt precipitation. Within the preonset arc, inverted-V type precipitation dominates. Poleward of the arc we also find some precipitation regions, and here there is systematically a cold electron population superposed with a warm population. Using single and double Maxwellian fits to the measured electron spectra we find the ionosphere-magnetosphere coupling parameters (field-aligned conductance K and the parallel potential drop V) as well as the effective source plasma properties (density and temperature) during the event. Compared to typical expansion phase features, the preonset parallel potential drop is smaller by a factor of ten, the electron temperature is smaller by a factor of at least five, and the field-aligned conductance is about the same or larger. The fact that there are two isotropic superposed electron populations on the poleward side of the preonset arc suggests that the distance between warm trapped electrons on dipolar field lines and colder electrons on open field lines has become so small near the onset that mixing e.g. due to finite electron Larmor radius effects can take place.Key words. Ionosphere · (ionosophere-magnetosphere interactions) · Magnetospheric physics (auroral phenomena; storms and substorms).

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Auroral streamers: characteristics of associated precipitation,convection and field-aligned currents

Annales Geophysicae ◽

10.5194/angeo-22-537-2004 ◽

2004 ◽

Vol 22 (2) ◽

pp. 537-548 ◽

Cited By ~ 62

Author(s):

V. A. Sergeev ◽

K. Liou ◽

P. T. Newell ◽

S.-I. Ohtani ◽

M. R. Hairston ◽

...

Keyword(s):

Potential Drop ◽

Auroral Oval ◽

Electron Precipitation ◽

Flux Transport ◽

Plasma Bubbles ◽

Long Duration ◽

Proton Precipitation ◽

Field Lines ◽

Steady Convection ◽

Convection Stream

Abstract. During the long-duration steady convection activity on 11 December 1998, the development of a few dozen auroral streamers was monitored by Polar UVI instrument in the dark northern nightside ionosphere. On many occasions the DMSP spacecraft crossed the streamer-conjugate regions over the sunlit southern auroral oval, permitting the investigation of the characteristics of ion and electron precipitation, ionospheric convection and field-aligned currents associated with the streamers. We confirm the conjugacy of streamer-associated precipitation, as well as their association with ionospheric plasma streams having a substantial equatorward convection component. The observations display two basic types of streamer-associated precipitation. In its polewardmost half, the streamer-associated (field-aligned) accelerated electron precipitation coincides with the strong (≥2–7μA/m2) upward field-aligned currents on the westward flank of the convection stream, sometimes accompanied by enhanced proton precipitation in the adjacent region. In the equatorward portion of the streamer, the enhanced precipitation includes both electrons and protons, often without indication of field-aligned acceleration. Most of these characteristics are consistent with the model describing the generation of the streamer by the narrow plasma bubbles (bursty bulk flows) which are contained on dipolarized field lines in the plasma sheet, although the mapping is strongly distorted which makes it difficult to quantitatively interprete the ionospheric image. The convective streams in the ionosphere, when well-resolved, had the maximal convection speeds ∼0.5–1km/s, total field-aligned currents of a few tenths of MA, thicknesses of a few hundreds km and a potential drop of a few kV across the stream. However, this might represent only a small part of the associated flux transport in the equatorial plasma sheet.Key words. Ionosphere (electric fiels and currents). Magnetospheric physics (aurroal phenomena; energetic particles, precipitating)

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Transition from substorm growth to substorm expansion phase as observed with a radial configuration of ISTP and Cluster spacecraft

Annales Geophysicae ◽

10.5194/angeo-23-2183-2005 ◽

2005 ◽

Vol 23 (6) ◽

pp. 2183-2198 ◽

Cited By ~ 22

Author(s):

V. A. Sergeev ◽

M. V. Kubyshkina ◽

W. Baumjohann ◽

R. Nakamura ◽

O. Amm ◽

...

Keyword(s):

Transition Region ◽

Growth Phase ◽

Plasma Sheet ◽

Magnetic Energy ◽

Global Scale ◽

Substorm Onset ◽

Plasma Injection ◽

Storms And Substorms ◽

Auroral Phenomena ◽

Inner Region

Abstract. Transition from the growth phase to the substorm expansion during a well-isolated substorm with a strong growth phase is investigated using a unique radial (THEMIS-like) spacecraft constellation near midnight, including the probing of the tail current at ~16 RE with Cluster, of the transition region at ~9 RE with Geotail and Polar, and of the inner region at 6.6 RE with two LANL spacecraft. The activity development on both a global scale and near the spacecraft footpoints was monitored with global auroral images (from the IMAGE spacecraft) and the ground network. Magnetospheric models, tuned using in-situ observations, indicated a strong tail stretching and plasma sheet thinning, which included the growth of the near-Earth current (approaching 30 nA/m2) and possible formation of a local B minimum in the neutral sheet (~5 nT) at ~10–12 RE near the substorm onset. However, there were no indications that the substorm onset was initiated just in this region. We emphasize the rather weak magnetic and plasma flow perturbations observed outside the thinned plasma sheet at Cluster, which could be interpreted as the effects of localized earthward-contracting newly-reconnected plasma tubes produced by the impulsive reconnection in the midtail plasma sheet. In that case the time delays around the distinct substorm onset are consistent with the activity propagation from the midtail to the inner magnetosphere. A peculiar feature of this substorm was that 12min prior to this distinct onset, a clear soft plasma injection to the GEO orbit was recorded which has little associated effects both in the ionosphere and in the transition region at ~9 RE. This pseudo-breakup was probably due to either a localized ballooning-type activity or due to the braking of a very narrow BBF whose signatures were also recorded by Cluster. This event manifested the (previously unknown) phenomenon, a strong tail overloading (excessive storage of magnetic energy) contrasted to the modest energy dissipation and plasma acceleration, which are both discussed and interpreted as the consequences of cold/dense and thick pre-substorm plasma sheet which often occurs after the long quiet period. The lessons of using the radial spacecraft configurations in substorm onset studies are also discussed. Keywords. Magnetospheric physics (Auroral phenomena, plasma sheet, storms and substorms)

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Statistics of a parallel Poynting vector in the auroral zone as a function of altitude using Polar EFI and MFE data and Astrid-2 EMMA data

Annales Geophysicae ◽

10.5194/angeo-23-1797-2005 ◽

2005 ◽

Vol 23 (5) ◽

pp. 1797-1806 ◽

Cited By ~ 5

Author(s):

P. Janhunen ◽

A. Olsson ◽

N. A. Tsyganenko ◽

C. T. Russell ◽

H. Laakso ◽

...

Keyword(s):

Poynting Vector ◽

Radial Distance ◽

Auroral Zone ◽

Electron Precipitation ◽

Alfven Wave ◽

Altitude Range ◽

Altitude Profile ◽

Morning Sector ◽

Field Lines ◽

Auroral Phenomena

Abstract. We study the wave-related (AC) and static (DC) parallel Poynting vector (Poynting energy flux) as a function of altitude in auroral field lines using Polar EFI and MFE data. The study is statistical and contains 5 years of data in the altitude range 5000–30000 km. We verify the low altitude part of the results by comparison with earlier Astrid-2 EMMA Poynting vector statistics at 1000 km altitude. The EMMA data are also used to statistically compensate the Polar results for the missing zonal electric field component. We compare the Poynting vector with previous statistical DMSP satellite data concerning the electron precipitation power. We find that the AC Poynting vector (Alfvén-wave related Poynting vector) is statistically not sufficient to power auroral electron precipitation, although it may, for Kp>2, power 25–50% of it. The statistical AC Poynting vector also has a stepwise transition at R=4 RE, so that its amplitude increases with increasing altitude. We suggest that this corresponds to Alfvén waves being in Landau resonance with electrons, so that wave-induced electron acceleration takes place at this altitude range, which was earlier named the Alfvén Resonosphere (ARS). The DC Poynting vector is ~3 times larger than electron precipitation and corresponds mainly to ionospheric Joule heating. In the morning sector (02:00–06:00 MLT) we find that the DC Poynting vector has a nontrivial altitude profile such that it decreases by a factor of ~2 when moving upward from 3 to 4 RE radial distance. In other nightside MLT sectors the altitude profile is more uniform. The morning sector nontrivial altitude profile may be due to divergence of the perpendicular Poynting vector field at R=3–4 RE. Keywords. Magnetospheric physics (Auroral phenomena; Magnetosphere-ionosphere interactions) – Space plasma physics (Wave-particle interactions)

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Modeling of nonstationary electron precipitation by the whistler cyclotron instability

Annales Geophysicae ◽

10.1007/s00585-998-1455-1 ◽

1998 ◽

Vol 16 (11) ◽

pp. 1455-1460 ◽

Cited By ~ 1

Author(s):

A. G. Demekhov ◽

A. A. Lyubchich ◽

V. Y. Trakhtengerts ◽

E. E. Titova ◽

J. Manninen ◽

...

Keyword(s):

Linear Equations ◽

Energetic Electron ◽

Electron Precipitation ◽

Plasma Parameters ◽

Wave Interactions ◽

Cyclotron Maser ◽

Consistent Model ◽

Storms And Substorms ◽

Auroral Phenomena

Abstract. We study a simple self-consistent model of a whistler cyclotron maser derived from the full set of quasi-linear equations. We employ numerical calculations to demonstrate dependencies of pulsation regimes of whistler-mode wave interactions with energetic electrons on plasma parameters. Possible temporal evolution of those regimes in real conditions is discussed; calculations are compared with case-study experimental data on energetic electron precipitation pulsations. A reasonable agreement of the model results and the observations has been found.Key words. Magnetospheric physics (Auroral phenomena; Energetic particles · precipitating; Storms and substorms)

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Simultaneous optical, CUTLASS HF radar, and FAST spacecraft observations: signatures of boundary layer processes in the cusp

Annales Geophysicae ◽

10.5194/angeo-22-511-2004 ◽

2004 ◽

Vol 22 (2) ◽

pp. 511-525 ◽

Cited By ~ 12

Author(s):

K. Oksavik ◽

F. Søraas ◽

J. Moen ◽

R. Pfaff ◽

J. A. Davies ◽

...

Keyword(s):

Electric Field ◽

Electric Fields ◽

Broad Band ◽

Spectral Width ◽

Hf Radar ◽

Optical Data ◽

Low Energy Electrons ◽

Field Lines ◽

Auroral Phenomena ◽

Electric Fields And Currents

Abstract. In this paper we discuss counterstreaming electrons, electric field turbulence, HF radar spectral width enhancements, and field-aligned currents in the southward IMF cusp region. Electric field and particle observations from the FAST spacecraft are compared with CUTLASS Finland spectral width enhancements and ground-based optical data from Svalbard during a meridional crossing of the cusp. The observed 630nm rayed arc (Type-1 cusp aurora) is associated with stepped cusp ion signatures. Simultaneous counterstreaming low-energy electrons on open magnetic field lines lead us to propose that such electrons may be an important source for rayed red arcs through pitch angle scattering in collisions with the upper atmosphere. The observed particle precipitation and electric field turbulence are found to be nearly collocated with the equatorward edge of the optical cusp, in a region where CUTLASS Finland also observed enhanced spectral width. The electric field turbulence is observed to extend far poleward of the optical cusp. The broad-band electric field turbulence corresponds to spatial scale lengths down to 5m. Therefore, we suggest that electric field irregularities are directly responsible for the formation of HF radar backscatter targets and may also explain the observed wide spectra. FAST also encountered two narrow highly structured field-aligned current pairs flowing near the edges of cusp ion steps. Key words. Ionosphere (electric fields and currents). Magnetosphere physics (magnetopause, cusp, and boundary layers; auroral phenomena)

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Middle-energy electron anisotropies in the auroral region

Annales Geophysicae ◽

10.5194/angeo-22-237-2004 ◽

2004 ◽

Vol 22 (1) ◽

pp. 237-249 ◽

Cited By ~ 4

Author(s):

P. Janhunen ◽

A. Olsson ◽

H. Laakso ◽

A. Vaivads

Keyword(s):

Electron Distribution ◽

Electron Flow ◽

High Mobility ◽

High Energy ◽

Auroral Oval ◽

Distribution Functions ◽

Particle Interactions ◽

Latitude Distribution ◽

Field Lines ◽

Auroral Phenomena

Abstract. Field-aligned anisotropic electron distribution functions of T∥ > T⊥ type are observed on auroral field lines at both low and high altitudes. We show that typically the anisotropy is limited to a certain range of energies, often below 1keV, although sometimes extending to slightly higher energies as well. Almost always there is simultaneously an isotropic electron distribution at higher energies. Often the anisotropies are up/down symmetrical, although cases with net upward or downward electron flow also occur. For a statistical analysis of the anisotropies we divide the energy range into low (below 100eV), middle (100eV–1keV) and high (above 1keV) energies and develop a measure of anisotropy expressed in density units. The statistical magnetic local time and invariant latitude distribution of the middle-energy anisotropies obeys that of the average auroral oval, whereas the distributions of the low and high energy anisotropies are more irregular. This suggests that it is specifically the middle-energy anisotropies that have something to do with auroral processes. The anisotropy magnitude decreases monotonically with altitude, as one would expect, because electrons have high mobility along the magnetic field and thus, the anisotropy properties spread rapidly to different altitudes. Key words. Magnetospheric physics (auroral phenomena). Space plasma physics (wave-particle interactions; changed particle motion and acceleration)

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A linear auroral current-voltage relation in fluid theory

Annales Geophysicae ◽

10.5194/angeo-22-1719-2004 ◽

2004 ◽

Vol 22 (5) ◽

pp. 1719-1728 ◽

Cited By ~ 11

Author(s):

J. Vedin ◽

K. Rönnmark

Keyword(s):

Kinetic Model ◽

Large Scale ◽

Equations Of State ◽

Potential Drop ◽

Fluid Models ◽

Linear Current ◽

Current Voltage ◽

Auroral Phenomena ◽

Auroral Current ◽

Current Voltage Relation

Abstract. Progress in our understanding of auroral currents and auroral electron acceleration has for decades been hampered by an apparent incompatibility between kinetic and fluid models of the physics involved. A well established kinetic model predicts that steady upward field-aligned currents should be linearly related to the potential drop along the field line, but collisionless fluid models that reproduce this linear current-voltage relation have not been found. Using temperatures calculated from the kinetic model in the presence of an upward auroral current, we construct here approximants for the parallel and perpendicular temperatures. Although our model is rather simplified, we find that the fluid equations predict a realistic large-scale parallel electric field and a linear current-voltage relation when these approximants are employed as nonlocal equations of state. This suggests that the concepts we introduce can be applied to the development of accurate equations of state for fluid simulations of auroral flux tubes.Key words. Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions) – Space plasma physics (kinetic and MHD theory)

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Parallel electric fields: variations in space and time on auroral field lines

Journal of Plasma Physics ◽

10.1017/s0022377807006538 ◽

2008 ◽

Vol 74 (1) ◽

pp. 53-64 ◽

Cited By ~ 2

Author(s):

J. VEDIN ◽

K. RÖNNMARK

Keyword(s):

Electric Fields ◽

Potential Drop ◽

Fluid Simulation ◽

Ion Dynamics ◽

Space And Time ◽

Auroral Electrons ◽

Field Lines

AbstractWe present results from a particle–fluid simulation of auroral electrons and discuss the distribution of parallel electric fields along auroral field lines and the processes occurring during the build up of these electric fields. Neglecting field-aligned ion dynamics, the main potential drop has a width of about 5000, km and is centered at an altitude of roughly 5000, km. We find that the gradient in the potential becomes steeper and the build up of the potential drop becomes faster if the temperature of the magnetospheric electrons is lower.

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