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

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.

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

Annales Geophysicae ◽

10.5194/angeo-28-1559-2010 ◽

2010 ◽

Vol 28 (8) ◽

pp. 1559-1570 ◽

Cited By ~ 12

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.

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Magnetic interconnection of Saturn's polar regions: comparison of modelling results with Hubble Space Telescope UV auroral images

Annales Geophysicae ◽

10.5194/angeo-31-1447-2013 ◽

2013 ◽

Vol 31 (8) ◽

pp. 1447-1458 ◽

Cited By ~ 3

Author(s):

E. S. Belenkaya ◽

S. W. H. Cowley ◽

V. V. Kalegaev ◽

O. G. Barinov ◽

W. O. Barinova

Keyword(s):

Magnetic Field ◽

Southern Hemisphere ◽

Hubble Space Telescope ◽

Interesting Case ◽

Equatorial Region ◽

Field Structure ◽

Polar Regions ◽

Quadrupole Field ◽

Space Telescope ◽

Magnetic Field Structure

Abstract. We consider the magnetic interconnection of Saturn's northern and southern polar regions controlled by the interplanetary magnetic field (IMF), studying in particular the more complex and interesting case of southward IMF, when the Kronian magnetospheric magnetic field structure is the most twisted. The simpler case of northward IMF is also discussed. Knowledge of the magnetospheric magnetic field structure is very significant, for example, for investigation of the electric fields and field-aligned currents in Saturn's environment, particularly those which cause the auroral emissions. Here we modify the paraboloid magnetospheric magnetic field model employed in previous related studies by including higher multipole terms in Saturn's internal magnetic field, required for more detailed considerations of inter-hemispheric conjugacy, together with inclusion of a spheroidal boundary at the ionospheric level. The model is employed to map Southern Hemisphere auroral regions observed by the Hubble Space Telescope (HST) in 2008 under known IMF conditions to both the equatorial plane and the northern ionosphere. It is shown that the brightest auroral features map typically to the equatorial region between the central ring current and the outer magnetosphere, and that auroral features should be largely symmetric between the two hemispheres, except for a small poleward displacement and latitudinal narrowing in the Northern Hemisphere compared with the Southern Hemisphere due to the quadrupole field asymmetry. The latter features are in agreement with the conjugate auroras observed under near-equinoctial conditions in early 2009, when IMF data are not available.

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The auroral and ionospheric flow signatures of dual lobe reconnection

Annales Geophysicae ◽

10.5194/angeo-24-3115-2006 ◽

2006 ◽

Vol 24 (11) ◽

pp. 3115-3129 ◽

Cited By ~ 37

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.

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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

Annales Geophysicae ◽

10.5194/angeo-26-159-2008 ◽

2008 ◽

Vol 26 (1) ◽

pp. 159-166 ◽

Cited By ~ 20

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.

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

Annales Geophysicae ◽

10.5194/angeo-33-39-2015 ◽

2015 ◽

Vol 33 (1) ◽

pp. 39-46 ◽

Cited By ~ 1

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.

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Interplanetary magnetic field control of Saturn's polar cusp aurora

Annales Geophysicae ◽

10.5194/angeo-23-1405-2005 ◽

2005 ◽

Vol 23 (4) ◽

pp. 1405-1431 ◽

Cited By ~ 48

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.

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

Journal of Geophysical Research Space Physics ◽

10.1002/2016ja022784 ◽

2016 ◽

Vol 121 (7) ◽

pp. 6341-6352 ◽

Cited By ~ 4

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

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

Physics of Plasmas ◽

10.1063/1.1690300 ◽

2004 ◽

Vol 11 (5) ◽

pp. 2318-2321 ◽

Cited By ~ 11

Author(s):

D. D. Ryutov ◽

J. Kesner ◽

M. E. Mauel

Keyword(s):

Magnetic Field ◽

Field Line ◽

Toroidal Magnetic Field ◽

Closed Field ◽

Closed Field Line

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Magnetospheric mapping of the dayside UV auroral oval at Saturn using simultaneous HST images, Cassini IMF data, and a global magnetic field model

Annales Geophysicae ◽

10.5194/angeo-29-1233-2011 ◽

2011 ◽

Vol 29 (7) ◽

pp. 1233-1246 ◽

Cited By ~ 20

Author(s):

E. S. Belenkaya ◽

S. W. H. Cowley ◽

J. D. Nichols ◽

M. S. Blokhina ◽

V. V. Kalegaev

Keyword(s):

Magnetic Field ◽

Ring Current ◽

Hubble Space Telescope ◽

Dynamic Pressure ◽

Outer Boundary ◽

Outer Part ◽

Field Structure ◽

Closed Field ◽

Data Set ◽

Outer Magnetosphere

Abstract. We determine the field-aligned mapping of Saturn's auroras into the magnetosphere by combining UV images of the southern dayside oval obtained by the Hubble Space Telescope (HST) with a global model of the magnetospheric magnetic field. The model is tailored to simulate prevailing conditions in the interplanetary medium, corresponding to high solar wind dynamic pressure and variable interplanetary magnetic field (IMF) strength and direction determined from suitably lagged field data observed just upstream of Saturn's dayside bow shock by the Cassini spacecraft. Two out of four images obtained in February 2008 when such simultaneous data are available are examined in detail, exemplifying conditions for northward and southward IMF. The model field structure in the outer magnetosphere and tail is found to be very different in these cases. Nevertheless, the dayside UV oval is found to have a consistent location relative to the field structure in each case. The poleward boundary of the oval is located close to the open-closed field boundary and thus maps to the vicinity of the magnetopause, consistent with previous results. The equatorward boundary of the oval then maps typically near the outer boundary of the equatorial ring current appropriate to the compressed conditions prevailing. Similar results are also found for related images from the January 2004 HST data set. These new results thus show that the mapped dayside UV oval typically spans the outer magnetosphere between the outer part of the ring current and the magnetopause. It does not encompass the region of primary corotation flow breakdown within the inner Enceladus torus.

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

Annales Geophysicae ◽

10.5194/angeo-23-123-2005 ◽

2005 ◽

Vol 23 (1) ◽

pp. 123-134 ◽

Cited By ~ 14

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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