Ionospheric and boundary contributions to the Dessler-Parker-Sckopke formula for <i>Dst</i>

The Dessler-Parker-Sckopke formula for the disturbance magnetic field averaged over the Earth's surface, universally used to interpret the geomagnetic <i>Dst</i> index, can be generalized, by using the well known method of deriving it from the virial theorem, to include the effects of ionospheric currents. There is an added term proportional to the global integral of the vertical mechanical force that balances the vertical component of the Lorentz force <i><b>J</b></i>&times;<i><b>B</b></i>/c in the ionosphere; a downward mechanical force reduces, and an upward increases, the depression of the magnetic field. If the vertical component of the ionospheric Ohm's law holds exactly, the relevant force on the plasma is the collisional friction between the neutral atmosphere and the vertically flowing plasma. An equal and opposite force is exerted on the neutral atmosphere and thus appears in <EM>its</EM> virial theorem. The ionospheric effect on <i>Dst</i> can then be related to the changes of kinetic and gravitational energy contents of the neutral atmosphere; since these changes are brought about by energy input from the magnetosphere, there is an implied upper limit to the effect on <i>Dst</i> which in general is relatively small in comparison to the contribution of the plasma energy content in the magnetosphere. Hence the Dessler-Parker-Sckopke formula can be applied without major modification, even in the case of strong partial ring currents; the ionospheric closure currents implied by the local time asymmetry have only a relatively small effect on the globally averaged disturbance field, comparable to other sources of uncertainty. When derived from the virial theorem applied to a bounded volume (e.g. the magnetosphere bounded by the magnetopause and a cross-section of the magnetotail), the Dessler-Parker-Sckopke formula contains also several boundary surface terms which can be identified as contributions of the magnetopause (Chapman-Ferraro) and of the magnetotail currents

Dipole tilt effects on the magnetosphere‐ionosphere convection system during interplanetary magnetic field B Y ‐dominated periods: MHD modeling

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95426/1/jgra20334.pd

Simulating the effect of centrifugal forces in Jupiter's magnetosphere

Jupiter's large scale size and rapid planetary rotation period combine to produce the strong centrifugal force responsible for many unique properties of its magnetosphere. It was previously proposed that this centrifugal force and nonadiabatic field line stretching could cause the observed dawn‐dusk asymmetry of Jupiter's plasma sheet, which is thickest near dusk. As flux tubes rotate and stretch between noon and dusk, particles bouncing along the field gain parallel energy and create pressure anisotropy. Because bounce times can be long compared with the outward expansion timescale, particles may respond nonadiabatically, and the resulting pressure anisotropy can drive the plasma sheet to instability. We used a large‐scale kinetic simulation to follow a collection of rotating particles as they move in a time‐varying, rotating magnetic field designed to represent flux tube expansion in Jupiter's magnetosphere. The analysis quantifies the response of trapped particles by characterizing the pressure anisotropy and energy changes. We compare results of nonadiabatic and adiabatic outward expansions and find that the nonadiabatic case leads to a large pitch angle anisotropy and higher total energy than for adiabatic expansion. Although the calculation was not handled fully self‐consistently, the results support the proposition that plasma pressure changes lead to changes in the magnetic field structure with local time. Our findings are consistent with the idea that nonadiabatic effects in Jupiter's magnetosphere contribute to field dipolarization and the observed plasma sheet thickening between noon and dusk. Key Points The centrifugal force accelerates particles during outward flux tube expansion Nonadiabatic flux tube expansion occurs at Jupiter between noon and dusk LT These effects can explain why Jupiter's plasma sheet thickens from 12 to 18 LTPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106869/1/jgra50908.pd

Candidates for detecting exoplanetary radio emissions generated by magnetosphere-ionosphere coupling

In this paper we consider the magnetosphere-ionosphere (M-I) coupling at Jupiter-like exoplanets with internal plasma sources such as volcanic moons, and we have determined the best candidates for detection of these radio emissions by estimating the maximum spectral flux density expected from planets orbiting stars within 25 pc using data listed in the NASA/IPAC/NExScI Star and Exoplanet Database (NStED). In total we identify 91 potential targets, of which 40 already host planets and 51 have stellar X-ray luminosity 100 times the solar value. In general, we find that stronger planetary field strength, combined with faster rotation rate, higher stellar XUV luminosity, and lower stellar wind dynamic pressure results in higher radio power. The top two targets for each category are $\epsilon$ Eri and HIP 85523, and CPD-28 332 and FF And.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society Letter

Windsock memory conditioned RAM (Co-Ram) pressure effect: forced reconnection in the Earth's magnetotail

Magnetic reconnection (MR) is a key physical concept explaining the addition of magnetic flux to the magnetotail and closed flux lines back-motion to the dayside magnetosphere. This scenario elaborated by \citet{dung63}, can explain many aspects of solar wind-magnetosphere interaction processes, including substorms. However, neither the Dungey model nor its numerous modifications were able to explain fully the onset conditions for MR in the tail. In this paper, we introduce new onset conditions for forced MR in the tail. We call our scenario the "windsock memory conditioned ram pressure effect". Our non-flux-transfer associated forcing is introduced by a combination of large-scale windsock motions exhibiting memory effects and solar wind dynamic pressure actions on the nightside magnetopause during northward oriented IMF. Using global MHD GUMICS-4 simulation results, upstream data from WIND, magnetosheath data from Cluster-1 and distant-tail data from the two-probe ARTEMIS mission, we show that the simultaneous occurrence of vertical windsock motions of the magnetotail and enhanced solar wind dynamic pressure introduces strong nightside disturbances, including enhanced electric fields and persistent vertical cross-tail shear flows. These perturbations, associated with a stream interaction region in the solar wind, drive MR in the tail during episodes of northward oriented interplanetary magnetic field (IMF). We detect MR indirectly, observing plasmoids in the tail and ground based signatures of Earthward moving fast flows. We also consider the application to solar system planets and close-in exoplanets, where the proposed scenario can elucidate some new aspects of solar/stellar wind - magnetosphere interactions.Comment: 16 pages, 12 figure

Understanding GIC in the UK and French high-voltage transmission systems during severe magnetic storms

The measurement and collection of digital magnetic field data in Europe extends back to the 1970s, providing over 30 years of data for the analysis of severe space weather. Although paper records can potentially extend these data sets back by over a century, few digitized records are currently available for use in extreme studies. Therefore, we rely on theoretical arguments and modeling to elucidate the largest likely variations of the magnetic field. We assess the relationship, during the three largest storms in the digital era, between variations in the horizontal magnetic field and the largest measured Dst index to estimate likely magnetic variations for more extreme storms in northern and midlatitude Europe. We examine how geomagnetically induced currents (GIC) flow in the UK and French networks during recent severe storms and analyze the sensitivity of these flows to changes in grid parameters. The maximum GIC computed at any one node in the French and UK grids are 44 A and 208 A, respectively. Sensitivity tests show that while gross changes of the whole network structure, such as disconnecting parts of the network, reduces the mean GIC per node, changes in GIC at individual nodes have distinct behaviors implying that local effects are network dependent and require detailed modeling to sufficiently characterize GIC. In addition, the scale factors we have derived allow GIC results from recent storms to be upscaled to estimate the potential risk to the system from more extreme events, such as the Carrington storm in 1859

Magnetosphere-ionosphere coupling at Jupiter-like exoplanets with internal plasma sources: implications for detectability of auroral radio emissions

In this paper we provide the first consideration of magnetosphere-ionosphere coupling at Jupiter-like exoplanets with internal plasma sources such as volcanic moons. We estimate the radio power emitted by such systems under the condition of near-rigid corotation throughout the closed magnetosphere, in order to examine the behaviour of the best candidates for detection with next generation radio telescopes. We thus estimate for different stellar X-ray-UV (XUV) luminosity cases the orbital distances within which the ionospheric Pedersen conductance would be high enough to maintain near-rigid corotation, and we then consider the magnitudes of the large-scale magnetosphere-ionosphere currents flowing within the systems, and the resulting radio powers, at such distances. We also examine the effects of two key system parameters, i.e. the planetary angular velocity and the plasma mass outflow rate from sources internal to the magnetosphere. In all XUV luminosity cases studied, a significant number of parameter combinations within an order of magnitude of the jovian values are capable of producing emissions observable beyond 1 pc, in most cases requiring exoplanets orbiting at distances between ~1 and 50 AU, and for the higher XUV luminosity cases these observable distances can reach beyond ~50 pc for massive, rapidly rotating planets. The implication of these results is that the best candidates for detection of such internally-generated radio emissions are rapidly rotating Jupiter-like exoplanets orbiting stars with high XUV luminosity at orbital distances beyond ~1 AU, and searching for such emissions may offer a new method of detection of more distant-orbiting exoplanets.Comment: 15 pages, 9 figures. In press at Mon. Not. R. Astron. So