scholarly journals Excitation of twin-vortex flow in the nightside high-latitude ionosphere during an isolated substorm

2002 ◽  
Vol 20 (10) ◽  
pp. 1577-1601 ◽  
Author(s):  
A. Grocott ◽  
S. W. H. Cowley ◽  
J. B. Sigwarth ◽  
J. F. Watermann ◽  
T. K. Yeoman
Keyword(s):  
Flow Pattern ◽  
High Latitude ◽  
Large Scale ◽  
Local Times ◽  
Closed Field ◽  
Plasma Convection ◽  
Expansion Phase ◽  
Ionospheric Conductivity ◽  
Large Scale Flow ◽  
Open Flux

Abstract. We present SuperDARN radar observations of the ionospheric flow during a well-observed high-latitude substorm which occurred during steady northward IMF conditions on 2 December 1999. These data clearly demonstrate the excitation of large-scale flow associated with the substorm expansion phase, with enhanced equatorward flows being observed in the pre-midnight local time sector of the expansion phase auroral bulge and westward electrojet, and enhanced return sunward flows being present at local times on either side, extending into the dayside sector. The flow pattern excited was thus of twin-vortex form, with foci located at either end of the substorm auroral bulge, as imaged by the Polar VIS UV imager. Estimated total transpolar voltages were ~40 kV prior to expansion phase onset, grew to ~80 kV over a ~15 min interval during the expansion phase, and then decayed to ~35 kV over ~10 min during recovery. The excitation of the large-scale flow pattern resulted in the development of magnetic disturbances which extended well outside of the region directly disturbed by the substorm, depending upon the change in the flow and the local ionospheric conductivity. It is estimated that the nightside reconnection rate averaged over the 24-min interval of the substorm was ~65– 75 kV, compared with continuing dayside reconnection rates of ~30–45 kV. The net closure of open flux during the sub-storm was thus ~0.4–0.6 × 108 Wb, representing ~15–20% of the open flux present at onset, and corresponding to an overall contraction of the open-closed field line boundary by ~1° latitude.Key words. Ionosphere (auroral ionosphere; ionosphere-magnetosphere interactions; plasma convection)

scholarly journals Plasma convection jets near the poleward boundary of the nightside auroral oval and their relation to Pedersen conductivity gradients

2010 ◽  
Vol 28 (4) ◽  
pp. 969-976 ◽  
Author(s):  
H. Wang ◽  
H. Lühr ◽  
A. J. Ridley
Keyword(s):  
Auroral Oval ◽  
Flow Speed ◽  
Local Times ◽  
Closed Field ◽  
Plasma Convection ◽  
Polar Cap ◽  
Ionospheric Conductivity ◽  
Ion Flow ◽  
One Year ◽  
Conductivity Gradient

Abstract. In this work, we have shown that the ionospheric azimuthal plasma velocity jets near the open-closed field line boundary on the nightside can be associated with the peak in the ionospheric conductivity gradient. Both model and DMSP observations have been utilized to conduct this investigation. The model tests show that when the gradient of conductivity in the poleward boundary becomes sharper, convection peaks appear around the poleward edge of the aurora. The model results have been confirmed by DMSP observations. Hundreds of large ion flow events are identified from one year DMSP observations, with flow speed larger than 500 m/s that occurred poleward of the aurora. Among them, 280 (74%) events are found to be associated with conductivity gradient peaks. Most of the convection jets occur in winter when conductivity gradients are expected to be large. The convection jets tend to occur at later local times (21:00–22:00 MLT) at 70°–72° MLat. These events are preceded by increasing of the merging electric field suggesting that they occur after the expansion of the polar cap. Both observation and model results show that the conductivity gradient at the polar cap boundary are one of the important elements in establishing the convection jets.

scholarly journals Variations in the polar cap area during two substorm cycles

2003 ◽  
Vol 21 (5) ◽  
pp. 1121-1140 ◽  
Author(s):  
S. E. Milan ◽  
M. Lester ◽  
S. W. H. Cowley ◽  
K. Oksavik ◽  
M. Brittnacher ◽  
...  
Keyword(s):  
Hf Radar ◽  
Local Times ◽  
Convection Flow ◽  
Closed Field ◽  
Plasma Convection ◽  
Polar Cap ◽  
Ionosphere Plasma ◽  
Dayside Magnetopause ◽  
Magnetospheric Configuration And Dynamics ◽  
Open Flux

Abstract. This study employs observations from several sources to determine the location of the polar cap boundary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified. These data sources include global auroral images from the Ultraviolet Imager (UVI) instrument on board the Polar spacecraft, SuperDARN HF radar measurements of the convection flow, and low altitude particle measurements from Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST) spacecraft. Changes in the open flux content of the magnetosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, allowing us to estimate the day- and nightside reconnection voltages during two substorm cycles. Specifically, increases in the polar cap area are found to be consistent with open flux being created when the IMF is oriented southwards and low-latitude magnetopause reconnection is ongoing, and decreases in area correspond to open flux being destroyed at substorm breakup. The polar cap area can continue to decrease for 100 min following the onset of substorm breakup, continuing even after substorm-associated auroral features have died away. An estimate of the dayside reconnection voltage, determined from plasma drift measurements in the ionosphere, indicates that reconnection can take place at all local times along the dayside portion of the polar cap boundary, and hence presumably across the majority of the dayside magnetopause. The observation of ionospheric signatures of bursty reconnection over a wide extent of local times supports this finding.Key words. Ionosphere (plasma convection; polar ionosphere) – Magnetospheric physics (magnetospheric configuration and dynamics)

scholarly journals The role of crystal fabric in flow near an ice divide

2007 ◽  
Vol 53 (181) ◽  
pp. 277-288 ◽  
Author(s):  
Erin C. Pettit ◽  
Throstur Thorsteinsson ◽  
H. Paul Jacobson ◽  
Edwin D. Waddington
Keyword(s):  
Flow Pattern ◽  
Large Scale ◽  
Cone Angle ◽  
West Antarctica ◽  
Crystal Preferred Orientation ◽  
Siple Dome ◽  
Polycrystalline Ice ◽  
Measured Profile ◽  
Large Scale Flow

AbstractPolycrystalline ice near an ice divide typically shows a crystal fabric (crystal preferred orientation) with c axes clustered vertically. We explore the effect of this fabric on the large-scale flow pattern near an ice divide. We incorporate an analytical formulation for anisotropy into a non-linear flow law within a finite-element ice-sheet flow model. With four different depth profiles of crystal fabric, we find that the effect of fabric is significant only when a profile has a minimum cone angle of less than ~25º. For a steady-state divide, the shape and size of the isochrone arch can depend as much on the crystal fabric as it does on the non-linearity of ice flow. A vertically oriented fabric tends to increase the size of the isochrone arch, never to reduce it. Also, non-random fabric has little effect on the ice-divide-flow pattern when ice is modeled as a linear (Newtonian) fluid. Finally, when we use a crystal-fabric profile that closely approximates the measured profile for Siple Dome, West Antarctica, the model predicts concentrated bed-parallel shearing 300 m above the bed.

scholarly journals Deep rotating convection generates the polar hexagon on Saturn

2020 ◽  
Vol 117 (25) ◽  
pp. 13991-13996 ◽  
Author(s):  
Rakesh K. Yadav ◽  
Jeremy Bloxham
Keyword(s):  
Flow Pattern ◽  
High Latitude ◽  
Large Scale ◽  
Three Dimensional ◽  
Giant Planets ◽  
Zonal Flows ◽  
Self Organized ◽  
Consistent Model ◽  
Self Consistent ◽  
Polygonal Pattern

Numerous land- and space-based observations have established that Saturn has a persistent hexagonal flow pattern near its north pole. While observations abound, the physics behind its formation is still uncertain. Although several phenomenological models have been able to reproduce this feature, a self-consistent model for how such a large-scale polygonal jet forms in the highly turbulent atmosphere of Saturn is lacking. Here, we present a three-dimensional (3D) fully nonlinear anelastic simulation of deep thermal convection in the outer layers of gas giant planets that spontaneously generates giant polar cyclones, fierce alternating zonal flows, and a high-latitude eastward jet with a polygonal pattern. The analysis of the simulation suggests that self-organized turbulence in the form of giant vortices pinches the eastward jet, forming polygonal shapes. We argue that a similar mechanism is responsible for exciting Saturn’s hexagonal flow pattern.

scholarly journals The accuracy of using the spectral width boundary measured in off-meridional SuperDARN HF radar beams as a proxy for the open-closed field line boundary

2005 ◽  
Vol 23 (7) ◽  
pp. 2599-2604 ◽  
Author(s):  
G. Chisham ◽  
M. P. Freeman ◽  
T. Sotirelis ◽  
R. A. Greenwald
Keyword(s):  
Field Line ◽  
Large Scale ◽  
Spectral Width ◽  
Hf Radar ◽  
Closed Field ◽  
Plasma Convection ◽  
Meridional Direction ◽  
Closed Field Line ◽  
Good Proxy ◽  
Automated Algorithms

Abstract. Determining reliable proxies for the ionospheric signature of the open-closed field line boundary (OCB) is crucial for making accurate measurements of magnetic reconnection. This study compares the latitudes of spectral width boundaries (SWBs) measured by different beams of the Goose Bay radar of the Super Dual Auroral Radar Network (SuperDARN), with the latitudes of OCBs determined using the low-altitude Defense Meteorological Satellite Program (DMSP) spacecraft, in order to determine whether the accuracy of the SWB as a proxy for the ionospheric projection of the OCB depends on the line-of-sight direction of the radar beam. The latitudes of SWBs and OCBs were identified using automated algorithms applied to 5 years (1997–2001) of data measured in the 1000–1400 magnetic local time (MLT) range. Six different Goose Bay radar beams were used, ranging from those aligned in the geomagnetic meridional direction to those aligned in an almost zonal direction. The results show that the SWB is a good proxy for the OCB in near-meridionally-aligned beams but becomes progressively more unreliable for beams greater than 4 beams away from the meridional direction. We propose that SWBs are identified at latitudes lower than the OCB in the off-meridional beams due to the presence of high spectral width values that result from changes in the orientation of the beams with respect to the gradient in the large-scale ionospheric convection pattern. Keywords. Ionosphere (Instruments and techniques; 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 The thermospheric effects of a rapid polar cap expansion

1998 ◽  
Vol 16 (10) ◽  
pp. 1380-1391 ◽  
Author(s):  
D. W. Idenden
Keyword(s):  
Large Scale ◽  
Plasma Convection ◽  
Polar Cap ◽  
Wind Fields ◽  
Expansion Phase ◽  
Ion Drifts ◽  
Modeling And Forecasting ◽  
Interaction Modeling ◽  
Atmosphere Interaction ◽  
Energy And Momentum

Abstract. In a previous publication we used results from a coupled thermosphere-ionosphere-plasmasphere model to illustrate a new mechanism for the formation of a large-scale patch of ionisation arising from a rapid polar cap expansion. Here we describe the thermospheric response to that polar cap expansion, and to the ionospheric structure produced. The response is dominated by the energy and momentum input at the dayside throat during the expansion phase itself. These inputs give rise to a large-scale travelling atmospheric disturbance (TAD) that propagates both antisunward across the polar cap and equatorward at speeds much greater than both the ion drifts and the neutral winds. We concentrate only on the initially poleward travelling disturbance. The disturbance is manifested in the neutral temperature and wind fields, the height of the pressure level surfaces and in the neutral density at fixed heights. The thermospheric effects caused by the ionospheric structure produced during the expansion are hard to discern due to the dominating effects of the TAD.Key words. Ionosphere (ionosphere · atmosphere interaction; modeling and forecasting; plasma convection).

scholarly journals A multipoint study of a substorm occurring on 7 December, 1992, and its theoretical implications

1999 ◽  
Vol 17 (11) ◽  
pp. 1369-1384 ◽  
Author(s):  
N. J. Fox ◽  
S. W. H. Cowley ◽  
V. N. Davda ◽  
G. Enno ◽  
E. Friis-Christensen ◽  
...  
Keyword(s):  
Substorm Onset ◽  
Hf Radar ◽  
Closed Field ◽  
Plasma Convection ◽  
Ionosphere Plasma ◽  
Initial Onset ◽  
Field Lines ◽  
Open Flux ◽  
Eiscat Radar ◽  
Geosynchronous Satellites

Abstract. On 7 December 1992, a moderate substorm was observed by a variety of satellites and ground-based instruments. Ionospheric flows were monitored near dusk by the Goose Bay HF radar and near midnight by the EISCAT radar. The observed flows are compared here with magnetometer observations by the IMAGE array in Scandinavia and the two Greenland chains, the auroral distribution observed by Freja and the substorm cycle observations by the SABRE radar, the SAMNET magnetometer array and LANL geosynchronous satellites. Data from Galileo Earth-encounter II are used to estimate the IMF Bz component. The data presented show that the substorm onset electrojet at midnight was confined to closed field lines equatorward of the pre-existing convection reversal boundaries observed in the dusk and midnight regions. No evidence of substantial closure of open flux was detected following this substorm onset. Indeed the convection reversal boundary on the duskside continued to expand equatorward after onset due to the continued presence of strong southward IMF, such that growth and expansion phase features were simultaneously present. Clear indications of closure of open flux were not observed until a subsequent substorm intensification 25 min after the initial onset. After this time, the substorm auroral bulge in the nightside hours propagated well poleward of the pre-existing convection reversal boundary, and strong flow perturbations were observed by the Goose Bay radar, indicative of flows driven by reconnection in the tail.Key words. Ionosphere (auroral ionosphere; plasma convection) · Magnetospheric physics (storms and substorms)

scholarly journals Multi-instrument mapping of the small-scale flow dynamics related to a cusp auroral transient

2005 ◽  
Vol 23 (7) ◽  
pp. 2657-2670 ◽  
Author(s):  
K. Oksavik ◽  
J. Moen ◽  
H. C. Carlson ◽  
R. A. Greenwald ◽  
S. E. Milan ◽  
...  
Keyword(s):  
Flow Pattern ◽  
Large Scale ◽  
Flow Channel ◽  
Small Scale ◽  
Plasma Convection ◽  
Polar Cap ◽  
Data Set ◽  
Ionosphere Plasma ◽  
Combining Data ◽  
Low Altitude

Abstract. In this paper we focus on flux transfer events (FTEs) and poleward moving auroral forms (PMAFs) in the cusp region, combining data from the EISCAT Svalbard radar, SuperDARN HF radars, ground-based optics, and three low-altitude polar-orbiting spacecraft. During an interval of southward interplanetary magnetic field the EISCAT Svalbard radar tracked a train of narrow flow channels drifting into the polar cap. One 30-60 km wide flow channel surrounded by flow running in the opposite direction is studied in great detail from when it formed equatorward of the cusp aurora, near magnetic noon, until it left the field-of-view and disappeared into the polar cap. Satellite data shows that the flow channel was on open field lines. The flow pattern is consistent with field-aligned currents on the sides of the flow channel; with a downward current on the equatorward side, and an upward current on the poleward side. The poleward edge of the flow channel was coincident with a PMAF that separated from the background cusp aurora and drifted into the polar cap. A passage of the DMSP F13 spacecraft confirms that the FTE flow channel was still discernable over 15 minutes after it formed, as the spacecraft revealed a 30–40 km wide region of sunward flow within the anti-sunward background convection. From the dimensions of the flow channel we estimate that the magnetic flux contained in the event was at least 1 MWb. This data set also shows that Birkeland current filaments often seen by low-altitude spacecraft in the cusp/mantle are really associated with individual FTE events or a train of FTEs in progress. As the region 0 or cusp/mantle current represents the statistical average consistent with the large-scale flow pattern, we therefore introduce a new term – FTE currents – to denote the unique pair of Birkeland current sheets that are associated with individual meso-scale FTE flow disturbances. The poleward moving auroral forms (PMAFs), often referred to in the literature, are the optical signature of the upward FTE current. Keywords. Magnetospheric physics (Current systems; Magnetopause, cusp and boundary layers) – Ionosphere (plasma convection)

scholarly journals Spatial structure of summertime ionospheric plasma near magnetic noon

2005 ◽  
Vol 23 (1) ◽  
pp. 25-37 ◽  
Author(s):  
R. W. Sims ◽  
S. E. Pryse ◽  
W. F. Denig
Keyword(s):  
High Latitude ◽  
Ionospheric Plasma ◽  
Local Times ◽  
Soft Particle ◽  
Supporting Evidence ◽  
Plasma Convection ◽  
Polar Cap ◽  
Ionospheric Electron Density ◽  
Plasma Distribution

Abstract. Results are presented from a multi-instrument study of the spatial distribution of the summertime, polar ionospheric electron density under conditions of relatively stable IMF Bz<0. The EISCAT Svalbard radar revealed a region of enhanced densities near magnetic noon that, when comparing radar scans from different local times, appeared to be spatially confined in longitude. This was identified as the tongue-of-ionisation (TOI) that comprised photoionisation of sub-auroral origin that is drawn poleward into the polar cap by the anti-sunward flow of the high-latitude convection. The TOI was bounded in longitude by high-latitude troughs; the pre-noon trough on the morning side with a minimum near 78° N and the post-noon trough on the afternoon side with a minimum at 80° N. Complementary measurements by radio tomography, the SuperDARN radars, and a DMSP satellite, together with comparisons with earlier modelling work, provided supporting evidence for the interpretation of the density structuring, and highlighted the role of plasma convection in the formation of summertime plasma distribution. Soft particle precipitation played only a secondary role in the modulation of the large summertime densities entering the polar cap.

Sign in / Sign up

Export Citation Format

Share Document