scholarly journals IMF dependence of Saturn's auroras: modelling study of HST and Cassini data from 12–15 February 2008

2010 ◽  
Vol 28 (8) ◽  
pp. 1559-1570 ◽  
Author(s):  
E. S. Belenkaya ◽  
I. I. Alexeev ◽  
M. S. Blokhina ◽  
E. J. Bunce ◽  
S. W. H. Cowley ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Hubble Space Telescope ◽  
Auroral Oval ◽  
Closed Field ◽  
Propagation Delays ◽  
Magnetic Field Model ◽  
Closed Field Line ◽  
Orbit Insertion ◽  
The Imf

Abstract. To gain better understanding of auroral processes in Saturn's magnetosphere, we compare ultraviolet (UV) auroral images obtained by the Hubble Space Telescope (HST) with the position of the open-closed field line boundary in the ionosphere calculated using a magnetic field model that employs Cassini measurements of the interplanetary magnetic field (IMF) as input. Following earlier related studies of pre-orbit insertion data from January 2004 when Cassini was located ~ 1300 Saturn radii away from the planet, here we investigate the interval 12–15 February 2008, when UV images of Saturn's southern dayside aurora were obtained by the HST while the Cassini spacecraft measured the IMF in the solar wind just upstream of the dayside bow shock. This configuration thus provides an opportunity, unique to date, to determine the IMF impinging on Saturn's magnetosphere during imaging observations, without the need to take account of extended and uncertain interplanetary propagation delays. The paraboloid model of Saturn's magnetosphere is then employed to calculate the magnetospheric magnetic field structure and ionospheric open-closed field line boundary for averaged IMF vectors that correspond, with appropriate response delays, to four HST images. We show that the IMF-dependent open field region calculated from the model agrees reasonably well with the area lying poleward of the UV emissions, thus supporting the view that the poleward boundary of Saturn's auroral oval in the dayside ionosphere lies adjacent to the open-closed field line boundary.

scholarly journals Dependence of the open-closed field line boundary in Saturn's ionosphere on both the IMF and solar wind dynamic pressure: comparison with the UV auroral oval observed by the HST

2008 ◽  
Vol 26 (1) ◽  
pp. 159-166 ◽  
Author(s):  
E. S. Belenkaya ◽  
S. W. H. Cowley ◽  
S. V. Badman ◽  
M. S. Blokhina ◽  
V.V. Kalegaev
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Field Line ◽  
Open Field ◽  
Dynamic Pressure ◽  
Auroral Oval ◽  
Closed Field ◽  
Field Region ◽  
Closed Field Line ◽  
The Imf

Abstract. We model the open magnetic field region in Saturn's southern polar ionosphere during two compression regions observed by the Cassini spacecraft upstream of Saturn in January 2004, and compare these with the auroral ovals observed simultaneously in ultraviolet images obtained by the Hubble Space Telescope. The modelling employs the paraboloid model of Saturn's magnetospheric magnetic field, whose parameters are varied according to the observed values of both the solar wind dynamic pressure and the interplanetary magnetic field (IMF) vector. It is shown that the open field area responds strongly to the IMF vector for both expanded and compressed magnetic models, corresponding to low and high dynamic pressure, respectively. It is also shown that the computed open field region agrees with the poleward boundary of the auroras as well as or better than those derived previously from a model in which only the variation of the IMF vector was taken into account. The results again support the hypothesis that the auroral oval at Saturn is associated with the open-closed field line boundary and hence with the solar wind interaction.

scholarly journals IMF dependence of the open-closed field line boundary in Saturn's ionosphere, and its relation to the UV auroral oval observed by the Hubble Space Telescope

2007 ◽  
Vol 25 (5) ◽  
pp. 1215-1226 ◽  
Author(s):  
E. S. Belenkaya ◽  
I. I. Alexeev ◽  
M. S. Blokhina ◽  
S. W. H. Cowley ◽  
S. V. Badman ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Hubble Space Telescope ◽  
Auroral Oval ◽  
Field Structure ◽  
Closed Field ◽  
Space Telescope ◽  
Magnetic Field Structure ◽  
Open Field Line ◽  
Closed Field Line

Abstract. We study the dependence of Saturn's magnetospheric magnetic field structure on the interplanetary magnetic field (IMF), together with the corresponding variations of the open-closed field line boundary in the ionosphere. Specifically we investigate the interval from 8 to 30 January 2004, when UV images of Saturn's southern aurora were obtained by the Hubble Space Telescope (HST), and simultaneous interplanetary measurements were provided by the Cassini spacecraft located near the ecliptic ~0.2 AU upstream of Saturn and ~0.5 AU off the planet-Sun line towards dawn. Using the paraboloid model of Saturn's magnetosphere, we calculate the magnetospheric magnetic field structure for several values of the IMF vector representative of interplanetary compression regions. Variations in the magnetic structure lead to different shapes and areas of the open field line region in the ionosphere. Comparison with the HST auroral images shows that the area of the computed open flux region is generally comparable to that enclosed by the auroral oval, and sometimes agrees in detail with its poleward boundary, though more typically being displaced by a few degrees in the tailward direction.

scholarly journals The auroral and ionospheric flow signatures of dual lobe reconnection

2006 ◽  
Vol 24 (11) ◽  
pp. 3115-3129 ◽  
Author(s):  
S. M. Imber ◽  
S. E. Milan ◽  
B. Hubert
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Field Line ◽  
Radar Data ◽  
Auroral Oval ◽  
Closed Field ◽  
Closed Field Line ◽  
Field Lines ◽  
Lobe Reconnection ◽  
Northern And Southern Hemispheres

Abstract. We present the first substantial evidence for the occurrence of dual lobe reconnection from ionospheric flows and auroral signatures. The process of dual lobe reconnection refers to an interplanetary magnetic field line reconnecting with lobe field lines in both the northern and southern hemispheres. Two bursts of sunward plasma flow across the noon portion of the open/closed field line boundary (OCB), indicating magnetic flux closure at the dayside, were observed in SuperDARN radar data during a period of strongly northward IMF. The OCB is identified from spacecraft, radar backscatter, and auroral observations. In order for dual lobe reconnection to take place, we estimate that the interplanetary magnetic field clock angle must be within ±10° of zero (North). The total flux crossing the OCB during each burst is small (1.8% and 0.6% of the flux contained within the polar cap for the two flows). A brightening of the noon portion of the northern auroral oval was observed as the clock angle passed through zero, and is thought to be due to enhanced precipitating particle fluxes due to the occurrence of reconnection at two locations along the field line. The number of solar wind protons captured by the flux closure process was estimated to be ~2.5×1030 (4 tonnes by mass), sufficient to populate the cold, dense plasma sheet observed following this interval.

scholarly journals Interplanetary magnetic field control of Saturn's polar cusp aurora

2005 ◽  
Vol 23 (4) ◽  
pp. 1405-1431 ◽  
Author(s):  
E. J. Bunce ◽  
S. W. H. Cowley ◽  
S. E. Milan
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Interplanetary Magnetic Field ◽  
Field Line ◽  
High Latitude ◽  
Closed Field ◽  
Vortical Flows ◽  
Dayside Magnetopause ◽  
Closed Field Line ◽  
Lobe Reconnection

Abstract. Dayside UV emissions in Saturn's polar ionosphere have been suggested to be the first observational evidence of the kronian "cusp" (Gérard et al., 2004). The emission has two distinct states. The first is a bright arc-like feature located in the pre-noon sector, and the second is a more diffuse "spot" of aurora which lies poleward of the general location of the main auroral oval, which may be related to different upstream interplanetary magnetic field (IMF) orientations. Here we take up the suggestion that these emissions correspond to the cusp. However, direct precipitation of electrons in the cusp regions is not capable of producing significant UV aurora. We have therefore investigated the possibility that the observed UV emissions are associated with reconnection occurring at the dayside magnetopause, possibly pulsed, akin to flux transfer events seen at the Earth. We devise a conceptual model of pulsed reconnection at the low-latitude dayside magnetopause for the case of northwards IMF which will give rise to pulsed twin-vortical flows in the magnetosphere and ionosphere in the vicinity of the open-closed field-line boundary, and hence to bi-polar field-aligned currents centred in the vortical flows. During intervals of high-latitude lobe reconnection for southward IMF, we also expect to have pulsed twin-vortical flows and corresponding bi-polar field-aligned currents. The vortical flows in this case, however, are displaced poleward of the open-closed field line boundary, and are reversed in sense, such that the field-aligned currents are also reversed. For both cases of northward and southward IMF we have also for the first time included the effects associated with the IMF By effect. We also include the modulation introduced by the structured nature of the solar wind and IMF at Saturn's orbit by developing "slow" and "fast" flow models corresponding to intermediate and high strength IMF respectively. We then consider the conditions under which the plasma populations appropriate to either sub-solar reconnection or high-latitude lobe reconnection can carry the currents indicated. We have estimated the field-aligned voltages required, the resulting precipitating particle energy fluxes, and the consequent auroral output. Overall our model of pulsed reconnection under conditions of northwards and southwards IMF, and for varying orientations of IMF By, is found to produce a range of UV emission intensities and geometries which is in good agreement with the data presented by Gérard et al. (2004). The recent HST-Cassini solar wind campaign provides a unique opportunity to test the theoretical ideas presented here.

scholarly journals On large plasmoid formation in a global magnetohydrodynamic simulation

2011 ◽  
Vol 29 (1) ◽  
pp. 167-179 ◽  
Author(s):  
I. Honkonen ◽  
M. Palmroth ◽  
T. I. Pulkkinen ◽  
P. Janhunen ◽  
A. Aikio
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Operational Definition ◽  
Closed Field ◽  
Ae Index ◽  
Angular Change ◽  
Line Region ◽  
Field Lines ◽  
Fully Connected ◽  
The Imf

Abstract. We investigate plasmoid formation in the magnetotail using the global magnetohydrodynamic (MHD) simulation GUMICS-4. Here a plasmoid implies a major reconfiguration of the magnetotail where a part of the tail plasma sheet is ejected downstream, in contrast to small Earthward-propagating plasmoids. We define a plasmoid based solely on the structure of the closed (connected to the Earth at both ends) magnetic field line region. In this definition a plasmoid is partly separated from the ordinary closed field line region by lobe field lines or interplanetary field lines resulting from lobe reconnection. We simulate an event that occurred on 18 February 2004 during which four intensifications of the auroral electroject (AE) index occurred in 8 h. Plasmoids form in the simulation for two of the four AE intensifications. Each plasmoid forms as a result of two consecutive large and fast rotations of the interplanetary magnetic field (IMF). In both cases the IMF rotates 180 degrees at 10 degrees per minute, first from southward to northward and some 15 min later from northward to southward. The other two AE intencifications however are not associated with a plasmoid formation. A plasmoid does not form if either the IMF rotation speed or the angular change of the rotation are small. We also present an operational definition for these fully connected plasmoids that enables their automatic detection in simulations. Finally, we show mappings of the plasmoid footpoints in the ionosphere, where they perturb the polar cap boundary in both hemispheres.

Effects of the interplanetary magnetic field on the location of the open-closed field line boundary

2016 ◽  
Vol 121 (7) ◽  
pp. 6341-6352 ◽  
Author(s):  
C. Wang ◽  
J. Y. Wang ◽  
R. E. Lopez ◽  
L. Q. Zhang ◽  
B. B. Tang ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Field Line ◽  
Closed Field ◽  
Closed Field Line

scholarly journals Magnetospheric magnetic field modelling for the 2011 and 2012 HST Saturn aurora campaigns – implications for auroral source regions

2014 ◽  
Vol 32 (6) ◽  
pp. 689-704 ◽  
Author(s):  
E. S. Belenkaya ◽  
S. W. H. Cowley ◽  
C. J. Meredith ◽  
J. D. Nichols ◽  
V. V. Kalegaev ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Ring Current ◽  
Hubble Space Telescope ◽  
Current System ◽  
Outer Edge ◽  
Local Times ◽  
Closed Field ◽  
Principal Connection ◽  
Magnetic Field Model ◽  
Field Lines

Abstract. A unique set of images of Saturn's northern polar UV aurora was obtained by the Hubble Space Telescope in 2011 and 2012 at times when the Cassini spacecraft was located in the solar wind just upstream of Saturn's bow shock. This rare situation provides an opportunity to use the Kronian paraboloid magnetic field model to examine source locations of the bright auroral features by mapping them along field lines into the magnetosphere, taking account of the interplanetary magnetic field (IMF) measured near simultaneously by Cassini. It is found that the persistent dawn arc maps to closed field lines in the dawn to noon sector, with an equatorward edge generally located in the inner part of the ring current, typically at ~ 7 Saturn radii (RS) near dawn, and a poleward edge that maps variously between the centre of the ring current and beyond its outer edge at ~ 15 RS, depending on the latitudinal width of the arc. This location, together with a lack of response in properties to the concurrent IMF, suggests a principal connection with ring-current and nightside processes. The higher-latitude patchy auroras observed intermittently near to noon and at later local times extending towards dusk are instead found to straddle the model open–closed field boundary, thus mapping along field lines to the dayside outer magnetosphere and magnetopause. These emissions, which occur preferentially for northward IMF directions, are thus likely associated with reconnection and open-flux production at the magnetopause. One image for southward IMF also exhibits a prominent patch of very high latitude emissions extending poleward of patchy dawn arc emissions in the pre-noon sector. This is found to lie centrally within the region of open model field lines, suggesting an origin in the current system associated with lobe reconnection, similar to that observed in the terrestrial magnetosphere for northward IMF.

Magnetic field perturbations in closed-field-line systems with zero toroidal magnetic field

2004 ◽  
Vol 11 (5) ◽  
pp. 2318-2321 ◽  
Author(s):  
D. D. Ryutov ◽  
J. Kesner ◽  
M. E. Mauel
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Toroidal Magnetic Field ◽  
Closed Field ◽  
Closed Field Line

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

2015 ◽  
Vol 33 (1) ◽  
pp. 39-46 ◽  
Author(s):  
S. Wing ◽  
Y. L. Zhang
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Open Field ◽  
Magnetic Field Line ◽  
Path Length ◽  
Closed Field ◽  
Open Field Line ◽  
Polar Rain ◽  
Closed Field Line ◽  
The Magnetic Field

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.

scholarly journals Estimates of magnetotail reconnection rate based on IMAGE FUV and EISCAT measurements

2005 ◽  
Vol 23 (1) ◽  
pp. 123-134 ◽  
Author(s):  
N. Østgaard ◽  
J. Moen ◽  
S. B. Mende ◽  
H. U. Frey ◽  
T. J. Immel ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Electric Field ◽  
Field Line ◽  
Oscillation Mode ◽  
Closed Field ◽  
Reconnection Rate ◽  
Magnetospheric Convection ◽  
Closed Field Line ◽  
Magnetotail Reconnection

Abstract. Dayside merging between the interplanetary and terrestrial magnetic fields couples the solar wind electric field to the Earth's magnetosphere, increases the magnetospheric convection and results in efficient transport of solar wind energy into the magnetosphere. Subsequent reconnection of the lobe magnetic field in the magnetotail transports energy into the closed magnetic field region. Combining global imaging and ground-based radar measurements, we estimate the reconnection rate in the magnetotail during two days of an EISCAT campaign in November-December 2000. Global images from the IMAGE FUV system guide us to identify ionospheric signatures of the open-closed field line boundary observed by the two EISCAT radars in Tromsø (VHF) and on Svalbard (ESR). Continuous radar and optical monitoring of the open-closed field line boundary is used to determine the location, orientation and velocity of the open-closed boundary and the ion flow velocity perpendicular to this boundary. The magnetotail reconnection electric field is found to be a bursty process that oscillates between 0mV/m and 1mV/m with ~10-15min periods. These ULF oscillations are mainly due to the motion of the open-closed boundary. In situ measurements earthward of the reconnection site in the magnetotail by Geotail show similar oscillations in the duskward electric field. We also find that bursts of increased magnetotail reconnection do not necessarily have any associated auroral signatures. Finally, we find that the reconnection rate correlates poorly with the solar wind electric field. This indicates that the magnetotail reconnection is not directly driven, but is an internal magnetospheric process. Estimates of a coupling efficiency between the solar wind electric field and magnetotail reconnection only seem to be relevant as averages over long time intervals. The oscillation mode at 1mHz corresponds to the internal cavity mode with additional lower frequencies, 0.5 and 0.8mHz, that might be modulated by solar wind pressure variations.

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