scholarly journals Multi-instrument probing of the polar ionosphere under steady northward IMF

2000 ◽  
Vol 18 (1) ◽  
pp. 90-98 ◽  
Author(s):  
S. E. Pryse ◽  
A. M. Smith ◽  
L. Kersley ◽  
I. K. Walker ◽  
C. N. Mitchell ◽  
...  
Keyword(s):  
Closed Field ◽  
Polar Ionosphere ◽  
Plasma Convection ◽  
Polar Cap ◽  
Particle Detection ◽  
Plasma Parameters ◽  
Coherent Scatter ◽  
Comprehensive Picture ◽  
Field Lines ◽  
Lobe Reconnection

Abstract. Observations are presented of the polar ionosphere under steady, northward IMF. The measurements, made by six complementary experimental techniques, including radio tomography, all-sky and meridian scanning photometer optical imaging, incoherent and coherent scatter radars and satellite particle detection, reveal plasma parameters consistent with ionospheric signatures of lobe reconnection. The optical green-line footprint of the reconnection site is seen to lie in the sunward plasma convection of the lobe cells. Downstream in the region of softer precipitation the reverse energy dispersion of the incoming ions can be identified. A steep latitudinal density gradient at the equatorward edge of the precipitation identifies the general location of an adiaroic boundary, separating the open field lines of polar lobe cells from the closed field of viscous-driven cells. Enhancements in plasma density to the south of the gradient are interpreted as ionisation being reconfigured as it is thrust against the boundary by the antisunward flow of the viscous cells near noon. Each of the instruments individually provides valuable information on certain aspects of the ionosphere, but the paper demonstrates that taken together the different experiments complement each other to give a consistent and comprehensive picture of the dayside polar ionosphere..Key words. Ionosphere (polar ionosphere) · Magnetospheric physics (magnetosphere-ionosphere interactions; polar cap phenomena)

scholarly journals Polar observations of ion/electron bursts at the pre-dawn polar cap boundary: evidence for internal reconnection of overdraped lobe flux

2008 ◽  
Vol 26 (8) ◽  
pp. 2191-2202
Author(s):  
P. E. Sandholt ◽  
C. J. Farrugia
Keyword(s):  
Temporal Evolution ◽  
Dynamic Pressure ◽  
Closed Field ◽  
Polar Cap ◽  
Sheath Region ◽  
Field Lines ◽  
The Individual ◽  
Lobe Reconnection ◽  
Very High ◽  
Dipole Tilt

Abstract. Observations made by Polar of ion-electron bursts on the dawn side of the polar cap are presented. They occurred when conditions external to the magnetosphere corresponded to that of the sheath region of a magnetic cloud, which was characterized by very high densities/dynamic pressure and a magnetic field which was strong in all components and which was tilted antisunward (Bx<0) and northward (Bz>0) with its clock angle lying between 20 and 90° (By: 8–15 nT). A clear temporal development in the energy range spanned by the individual ion bursts (from 0.2–2 keV to 1–10 keV) was present. We relate this to a corresponding temporal evolution in the cloud sheath field and plasma. We analyze the solar wind-magnetosphere aspects of the observations using the concepts of (i) (i) overdraped lobe flux, (ii) Bx- and By-regulated sequential reconnections in opposite hemispheres (magnetopause and internal modes), and (iii) newly-closed magnetic flux. In particular, we find that the most energetic ion bursts (accompanied by bi-directionally streaming electrons at 1–10 keV and intense magnetosheath-origin fluxes) are located on newly closed field lines generated by internal reconnection occurring between overdraped lobe field lines and the closed geomagnetic field. This result corroborates a topology of lobe reconnection under conditions of dipole tilt and/or nonzero IMF Bx component advanced by Watanabe et al. (2006), which in our case is adapted to nonzero IMF By conditions.

scholarly journals Observations of significant flux closure by dual lobe reconnection

2007 ◽  
Vol 25 (7) ◽  
pp. 1617-1627 ◽  
Author(s):  
S. M. Imber ◽  
S. E. Milan ◽  
B. Hubert
Keyword(s):  
Plasma Sheet ◽  
Dense Plasma ◽  
Convection Flow ◽  
Closed Field ◽  
High Latitudes ◽  
Polar Cap ◽  
Closed Field Line ◽  
Field Lines ◽  
Lobe Reconnection ◽  
Significant Flux

Abstract. We present an interval of dual lobe reconnection during which interplanetary magnetic field lines are captured by the magnetosphere by reconnecting at high latitudes in both the Northern and the Southern Hemispheres. This event was identified using measurements of the ionospheric convection flow and observations of the aurora using the SuperDARN radars and the IMAGE spacecraft. A cusp spot, characteristic of northward IMF, is clearly visible for a 30 min period enabling the ionospheric footprint of the Northern Hemisphere merging gap to be accurately determined. During the interval a strong burst of sunward flow across the dayside open/closed field line boundary (OCB) is observed, which we interpret as the reconfiguration of the magnetosphere following a burst of reconnection. Noon-midnight and dawn-dusk keograms of the aurora show that the polar cap shrinks during the interval indicating that a large amount of flux was closed by the reconnection. Using the SuperDARN potential maps it is possible to calculate that the amount of flux closed during the interval is 0.13 GWb which represents approximately 10% of the pre-existing polar cap. The number of ions captured by the burst of dual lobe reconnection was calculated to be ~2.2×1031, more than sufficient to populate a cold, dense plasma sheet. That a dense plasma sheet was not subsequently observed is discussed in terms of subsequent changes in the IMF.

scholarly journals A numerical method to compute Euler potentials for non dipolar magnetic fields

1999 ◽  
Vol 17 (3) ◽  
pp. 328-337 ◽  
Author(s):  
C. Peymirat ◽  
D. Fontaine
Keyword(s):  
Magnetic Fields ◽  
Numerical Method ◽  
Radial Distance ◽  
Magnetic Activity ◽  
Closed Field ◽  
Plasma Convection ◽  
Polar Cap ◽  
The Earth ◽  
Modeling And Forecasting ◽  
Field Lines

Abstract. The magnetospheric magnetic field may be conveniently described by two scalar functions (α, β), known as the Euler potentials. They are not uniquely defined, and they may be difficult to derive for configuration more complex than a simple dipole. We propose here a simple numerical method to compute one possible pair (α, β). In magnetospheric regions of closed field lines, α can be chosen as a function of the tube volume of unit magnetic flux. The method can be applied to a wide class of magnetic fields which describe the magnetospheric domain of closed field lines and the conjugated ionosphere. Here, it is used with the T87 Tsyganenko model. The results coincide with the dipolar potentials at close distances from the Earth. At larger distances, they display an increasing distortion with the radial distance (or the invariant latitude in the ionosphere) and the magnetic activity. In the magnetosphere, the contours of α and β are stretched towards the nightside. In the ionosphere, they also extend towards the nightside and present major distortions in a narrow ring at the polar cap boundary, which maps distant boundary layers in the magnetosphere.Key words. Ionosphere (ionosphere-magnetosphere interactions; modeling and forecasting). Magnetospheric physics (plasma convection).

scholarly journals Plasma flows, Birkeland currents and auroral forms in relation to the Svalgaard-Mansurov effect

2012 ◽  
Vol 30 (5) ◽  
pp. 817-830 ◽  
Author(s):  
P. E. Sandholt ◽  
C. J. Farrugia
Keyword(s):  
Magnetic Field ◽  
Open Field ◽  
Central Element ◽  
Flow Channel ◽  
Plasma Convection ◽  
Polar Cap ◽  
Flow Channels ◽  
Field Lines ◽  
Current Systems ◽  
Birkeland Currents

Abstract. The traditional explanation of the polar cap magnetic deflections, referred to as the Svalgaard-Mansurov effect, is in terms of currents associated with ionospheric flow resulting from the release of magnetic tension on newly open magnetic field lines. In this study, we aim at an updated description of the sources of the Svalgaard-Mansurov effect based on recent observations of configurations of plasma flow channels, Birkeland current systems and aurorae in the magnetosphere-ionosphere system. Central to our description is the distinction between two different flow channels (FC 1 and FC 2) corresponding to two consecutive stages in the evolution of open field lines in Dungey cell convection, with FC 1 on newly open, and FC 2 on old open, field lines. Flow channel FC 1 is the result of ionospheric Pedersen current closure of Birkeland currents flowing along newly open field lines. During intervals of nonzero interplanetary magnetic field By component FC 1 is observed on either side of noon and it is accompanied by poleward moving auroral forms (PMAFs/prenoon and PMAFs/postnoon). In such cases the next convection stage, in the form of flow channel FC 2 on the periphery of the polar cap, is particularly important for establishing an IMF By-related convection asymmetry along the dawn-dusk meridian, which is a central element causing the Svalgaard-Mansurov effect. FC 2 flows are excited by the ionospheric Pedersen current closure of the northernmost pair of Birkeland currents in the four-sheet current system, which is coupled to the tail magnetopause and flank low-latitude boundary layer. This study is based on a review of recent statistical and event studies of central parameters relating to the magnetosphere-ionosphere current systems mentioned above. Temporal-spatial structure in the current systems is obtained by ground-satellite conjunction studies. On this point we emphasize the important information derived from the continuous ground monitoring of the dynamical behaviour of aurora and plasma convection during intervals of well-organised solar wind plasma and magnetic field conditions in interplanetary coronal mass ejections (ICMEs) during their Earth passage.

Convection For Northward Interplanetary Field

1996 ◽  
Author(s):  
Charles F. Kennel
Keyword(s):  
Field Line ◽  
Dynamic Pressure ◽  
Field Conditions ◽  
Closed Field ◽  
Flow State ◽  
Dayside Magnetopause ◽  
Statistical Studies ◽  
Closed Field Line ◽  
Field Lines ◽  
Lobe Reconnection

Besides common sense, a number of results suggest that we can learn more about the slow “viscous” flow state by studying the magnetosphere during northward interplanetary field conditions. In particular, statistical studies have consistently identified a “residual” state of magnetospheric and ionospheric convection in northward field conditions. The integrated potential across the high latitudeionosphere does not drop below a certain resting value of about 20 kV even when the interplanetary field has been due north for several hours. There appears to be a similar residual component of geomagnetic activity that is independent of the direction of the interplanetary field (Scurry and Russell, 1991). Its correlation with the dynamic pressure of the solar wind strengthens our suspicion that it is related to viscosity. Will we be able to prove the convection in this residual state is driven by viscosity? Does the flow in northward field conditions resemble the underlying irregular flow state of the plasma sheet found at other times? Does the magnetosphere approach the teardrop configuration during prolonged intervals of northward interplanetary field? These are but a few of the questions that whet our interest in convection during northward field conditions. One does not arrive at the state of pure viscous convection immediately after the interplanetary field swings northward. Dungey (1963) was the first of many to argue that a northward magnetosheath field line will reconnect with an open tail lobe field line to create one that is connected to the ionosphere at one end and draped over the dayside magnetopause at the other. The sudden reconfiguration of stress will lead to sunward convection on the newly reconnected field lines. In the ionosphere, this superposes a “reverse” two-cell convection pattern in the central polar cap upon the two “direct” convection cells. If and when the draped reconnected field line finds a partner in the opposite tail lobe with which to reconnect, a newly closed field line will form. Dungey had imagined that the same magnetosheath field line would reconnect simultaneously with both tail lobes, in which case the rate at which open magnetic flux is closed depends upon the rate of tail-lobe reconnection.

scholarly journals Overlapping ion structures in the mid-altitude cusp under northward IMF: signature of dual lobe reconnection?

2012 ◽  
Vol 30 (3) ◽  
pp. 489-501 ◽  
Author(s):  
F. Pitout ◽  
C. P. Escoubet ◽  
M. G. G. T. Taylor ◽  
J. Berchem ◽  
A. P. Walsh
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Low Energy ◽  
Closed Field ◽  
Opposite Hemisphere ◽  
Cluster Ion ◽  
Low Energy Tail ◽  
Field Lines ◽  
Lobe Reconnection ◽  
Cooling Model

Abstract. On some rare occasions, data from the Cluster Ion Spectrometer (CIS) in the mid-altitude cusp reveal overlapping ion populations under northward interplanetary magnetic field (IMF). While the poleward part of the cusp exhibits the expected reverse dispersion due to lobe reconnection, its equatorward part shows a second ion population at higher-energy that coexists with the low energy tail of the dispersion. This second population is either dispersionless or slightly dispersed with energies increasing with increasing latitudes, indicative of lobe reconnection as well. Our analysis of a case that occurred 3 September 2002 when the IMF stayed northward for more than two hours suggests that the second population comes from the opposite hemisphere and is very likely on newly-closed field lines. We interpret this overlap of cusp populations as a clear mid-altitude signature of re-closed magnetic field lines by double lobe reconnection (reconnection in both hemispheres) under northward IMF. This interpretation is supported by modelling performed with the Cooling model and an MHD model.

scholarly journals Ionospheric signatures of the low-latitude boundary layer under conditions of northward IMF and small clock angle

2006 ◽  
Vol 24 (8) ◽  
pp. 2169-2178 ◽  
Author(s):  
S. E. Pryse ◽  
R. W. Sims ◽  
J. Moen ◽  
K. Oksavik
Keyword(s):  
Boundary Layer ◽  
Particle Energy ◽  
Polar Cap ◽  
Low Latitude ◽  
Central Plasma ◽  
High Latitude Ionosphere ◽  
Field Lines ◽  
Lobe Reconnection ◽  
Low Latitude Boundary Layer

Abstract. A case study is presented that concerns the footprints of the low-latitude boundary layer in the high-latitude ionosphere. The measurements were made near local magnetic noon in summertime under conditions of Bz>0 and small clock angle. Of particular interest are particle fluxes measured in the region by the NOAA-12 satellite that revealed energetic (>30 keV) electrons, characteristic of trapped particles, together with a population of softer precipitating magnetosheath particles. The particle energy-distribution was distinct from those identifying the central plasma sheet at lower latitudes. On its poleward side the layer extended to at least the latitude of the polar cap boundary as identified in ion flows and electron densities measured by the EISCAT Svalbard radar. It is proposed that the particles of the low-latitude boundary layer occurred on newly-closed magnetic field lines, which were formed by the closure of open polar cap field by lobe reconnection in both Northern and Southern Hemispheres.

scholarly journals Intermittent thermal plasma acceleration linked to sporadic motions of the magnetopause, first Cluster results

2001 ◽  
Vol 19 (10/12) ◽  
pp. 1523-1532 ◽  
Author(s):  
J.-A. Sauvaud ◽  
R. Lundin ◽  
H. Rème ◽  
J. P. McFadden ◽  
C. Carlson ◽  
...  
Keyword(s):  
Local Density ◽  
Magnetic Activity ◽  
Magnetospheric Plasma ◽  
Local Magnetic Field ◽  
Closed Field ◽  
Plasma Convection ◽  
Ionosphere Plasma ◽  
Planetary Ionosphere ◽  
Field Lines ◽  
Pressure Changes

Abstract. This paper presents the first observations with Cluster of a very dense population of thermal ionospheric ions (H+, He+, O+) locally "accelerated" perpendicularly to the local magnetic field in a region adjacent to the magnetopause and on its magnetospheric side. The observation periods follow a long period of very weak magnetic activity. Recurrent motions of the magnetopause are, in the presented cases, unexpectedly associated with the appearance inside closed field lines of recurrent energy structures of ionospheric ions with energies in the 5 eV to  ~1000 eV range. The heaviest ions were detected with the highest energies. Here, the ion behaviour is interpreted as resulting from local electric field enhancements/decreases which adiabatically enhance/lower the bulk energy of a local dense thermal ion population. This drift effect, which is directly linked to magnetopause motions caused by pressure changes, allows for the thermal ions to overcome the satellite potential and be detected by the suprathermal CIS Cluster experiment. When fast flowing, i.e. when detectable, the density (~ 1 cm-3) of these ions from a terrestrial origin is (in the cases presented here) largely higher than the local density of ions from magnetospheric/plasma sheet origin which poses again the question of the relative importance of solar and ionospheric sources for the magnetospheric plasma even during very quiet magnetic conditions.Key words. Ionosphere (planetary ionosphere; plasma convection) Magnetospheric physics (magnetopause, cusp and boundary layers)

scholarly journals Dayside flow bursts and high-latitude reconnection when the IMF is strongly northward

2006 ◽  
Vol 24 (8) ◽  
pp. 2227-2242 ◽  
Author(s):  
H. Hu ◽  
T. K. Yeoman ◽  
M. Lester ◽  
R. Liu ◽  
H. Yang ◽  
...  
Keyword(s):  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
High Latitude ◽  
Closed Field ◽  
Flow Data ◽  
Plasma Convection ◽  
Convection Pattern ◽  
Reconnection Rate ◽  
Northern Hemispheric ◽  
Field Lines

Abstract. The characteristics of dayside ionospheric convection are studied using Northern Hemispheric SuperDARN data and DMSP particle and flow observations when the interplanetary magnetic field (IMF) was strongly northward during 13:00–15:00 UT on 2 March 2002. Although IMF Bx was positive, which is believed to favour Southern Hemisphere high-latitude reconnection at equinox, a four-cell convection pattern was observed and lasted for more than 1.5 h in the Northern Hemisphere. The reconnection rate derived from an analysis of the Northern Hemisphere SuperDARN data illustrates that the high-latitude reconnection was quasi-periodic, with a period between 4–16 min. A sawtooth-like and reverse-dispersed ion signature was observed by DMSP-F14 in the sunward cusp convection at around 14:41 UT, confirming that the high-latitude reconnection was pulsed. Accompanying the pulsed reconnection, strong antisunward ionospheric flow bursts were observed in the post-noon LLBL region on closed field lines, propagating with the same speed as the plasma convection. DMSP flow data show that a similar flow pattern and particle precipitation occurred in the conjugate Southern Hemisphere.

scholarly journals On determining the noon polar cap boundary from SuperDARN HF radar backscatter characteristics

2000 ◽  
Vol 18 (12) ◽  
pp. 1523-1530 ◽  
Author(s):  
M. Pinnock ◽  
A. S. Rodger
Keyword(s):  
Local Time ◽  
Time Of Flight ◽  
Radar Data ◽  
Accurate Estimate ◽  
Hf Radar ◽  
Closed Field ◽  
Plasma Convection ◽  
Polar Cap ◽  
Radar Backscatter ◽  
Magnetospheric Cusp

Abstract. Previous work has shown that ionospheric HF radar backscatter in the noon sector can be used to locate the footprint of the magnetospheric cusp particle precipitation. This has enabled the radar data to be used as a proxy for the location of the polar cap boundary, and hence measure the flow of plasma across it to derive the reconnection electric field in the ionosphere. This work used only single radar data sets with a field of view limited to ~2 h of local time. In this case study using four of the SuperDARN radars, we examine the boundary determined over 6 h of magnetic local time around the noon sector and its relationship to the convection pattern. The variation with longitude of the latitude of the radar scatter with cusp characteristics shows a bay-like feature. It is shown that this feature is shaped by the variation with longitude of the poleward flow component of the ionospheric plasma and may be understood in terms of cusp ion time-of-flight effects. Using this interpretation, we derive the time-of-flight of the cusp ions and find that it is consistent with approximately 1 keV ions injected from a subsolar reconnection site. A method for deriving a more accurate estimate of the location of the open-closed field line boundary from HF radar data is described.Key words: Ionosphere (ionosphere–magnetosphere interactions; plasma convection) · Magnetospheric physics (magnetopause · cusp · and boundary layers)

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