J. E. Ouellette

Ionospheric control of magnetotail reconnection

How the ionosphere gains influence In Earths upper atmosphere, the reconnection of magnetic field lines converts latent magnetic energy into the thermal and kinetic energy of plasma flows. But reconnection appears to produce faster flows before midnight compared with after. To find out why, Lotko et al. simulated this energy exchange. Challenging common assumptions about our space weather environment, they conclude that the ionosphere plays an active role when coupled to the magnetosphere in driving the behavior of the magnetotail. Science, this issue p. 184 Asymmetric plasma flows on the night side of Earth are regulated by magnetosphere-ionosphere coupling. Observed distributions of high-speed plasma flows at distances of 10 to 30 Earth radii (RE) in Earth’s magnetotail neutral sheet are highly skewed toward the premidnight sector. The flows are a product of the magnetic reconnection process that converts magnetic energy stored in the magnetotail into plasma kinetic and thermal energy. We show, using global numerical simulations, that the electrodynamic interaction between Earth’s magnetosphere and ionosphere produces an asymmetry consistent with observed distributions in nightside reconnection and plasmasheet flows and in accompanying ionospheric convection. The primary causal agent is the meridional gradient in the ionospheric Hall conductance which, through the Cowling effect, regulates the distribution of electrical currents flowing within and between the ionosphere and magnetotail.

The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind–magnetosphere‐ionosphere coupling

Received 13 April 2012; revised 19 September 2012; accepted 10 October 2012; published 16 November 2012. [1] The angle m between the geomagnetic dipole axis and the geocentric solar magnetospheric (GSM) z axis, sometimes called the “dipole tilt,” varies as a function of UT and season. Observations have shown that the cross-polar cap potential tends to maximize near the equinoxes, when on average m = 0, with smaller values observed near the solstices. This is similar to the well-known semiannual variation in geomagnetic activity. We use numerical model simulations to investigate the role of two possible mechanisms that may be responsible for the influence of m on the magnetosphere-ionosphere system: variations in the coupling efficiency between the solar wind and the magnetosphere and variations in the ionospheric conductance over the polar caps. Under southward interplanetary magnetic field (IMF) conditions, variations in ionospheric conductance at high magnetic latitudes are responsible for 10–30% of the variations in the cross-polar cap potential associated with m, but variations in solar wind–magnetosphere coupling are more important and responsible for 70–90%. Variations in viscous processes contribute slightly to this, but variations in the reconnection rate with m are the dominant cause. The variation in the reconnection rate is primarily the result of a variation in the length of the section of the separator line along which relatively strong reconnection occurs. Changes in solar wind–magnetosphere coupling also affect the field-aligned currents, but these are influenced as well by variations in the conductance associated with variations in m, more so than the cross-polar cap potential. This may be the case for geomagnetic activity too.

How does mass loading impact local versus global control on dayside reconnection

This paper investigates the effects of magnetospheric mass loading on the control of dayside magnetic reconnection using global magnetospheric simulations. The study iys motivated by a recent debate on whether the integrated dayside magnetic reconnection rate is solely controlled by local processes (local-control theory) or global merging processes (global-control theory). The local-control theory suggests that the integrated dayside reconnection rate is controlled by the local plasma parameters. The global-control theory argues that the integrated rate is determined by the net force acting on the flow in the magnetosheath rather than the local microphysics. Controlled numerical simulations using idealized ionospheric outflow specifications suggest a possible mixed-control theory, that is, (1) a small amount of mass loading at the dayside magnetopause only redistributes local reconnection rate without a significant change in the integrated reconnection rate and (2) a large amount of mass loading reduces both local reconnection rates and the integrated reconnection rate on the dayside. The transition between global-controland local-control-dominated regimes depends on (but not limited to) the source region, the amount, the location, and the spatial extension of the mass loading at the dayside magnetopause.

A study of asymmetric reconnection scaling in the Lyon-Fedder-Mobarry code

Using a three-dimensional magnetospheric simulation code we have studied the properties of magnetic reconnection at the subsolar point on solar wind parameters for southward interplanetary magnetic field conditions and compared the results with the predictions of the Cassak-Shay theory. We find that this theory predicts reconnection rates on the order of our observations and produces reasonable predictions of the reconnection outflow speed. We have quantified the contributions that differences between the assumed and measured mass, energy, and outflow density scalings make to predictions of the reconnection rate and outflow speed. In general, the theory makes reasonable assumptions about the mass and energy flux into the reconnection layer, but their outflowing counterparts are overestimated due to the narrowness of the reconnection outflow jet. Lastly, we find that newly reconnected flux tubes exit the merging region before their mass density can equilibrate, requiring a correction to the predicted outflow density.

The role of ionospheric O+ outflow in the generation of earthward propagating plasmoids

Earthward propagating plasmoids in the Earths magnetotail have been observed by satellites. Using a multi-fluid global magnetosphere simulation, earthward propagating plamoids are reproduced when ionospheric O+ outflow is included in the global simulation. Controlled simulations show that without ionospheric outflow, the plasmoids generated in the magnetotail during a substorm-SMC cycle only propagate tailward. With ionospheric outflow, earthward plasmoids can be induced through the modification of magnetotail reconnection at multiple X-lines. When multiple X-lines form in the magnetotail, plasmoids may be trapped between multiple reconnection sites. When magnetic reconnection rate is reduced at the near-Earth X-line by the presence of ionospheric O+, the earthward exhaust flow of reconnection occurring at the mid-tail X-line forces the plasmoid to propagate earthward. The propagation speed and spatial size of the simulated earthward plasmoid are consistent with observations from the Cluster satellites.

The effects of plasmaspheric plumes on dayside reconnection

We summarize the results of a study on the impact of plasmaspheric plumes on dayside reconnection using a three-dimensional magnetospheric simulation code. We find that the mass loading of magnetospheric flux tubes slows local reconnection rates, though not as much as predicted by Borovsky et al. (2013) due to differences in how well the Cassak-Shay theory matches magnetospheric configurations with and without plasmaspheric plumes. Additionally, we find that in some circumstances reconnection activity is enhanced on either side of the plumes, which moderates its impact on the total dayside reconnection rate. These results provide evidence that plasmaspheric plumes have both localand global-scale effects on dayside reconnection.

Transition from global to local control of dayside reconnection from ionospheric-sourced mass loading

We have conducted a series of controlled numerical simulations to investigate the response of dayside reconnection to idealized, ionosphere-sourced mass loading processes to determine whether they affect the integrated dayside reconnection rate. Our simulation results show that the coupled solar wind-magnetosphere (SW-M) system may exhibit both local and global control behaviors depending on the amount of mass loading. With a small amount of mass loading, the changes in local reconnection rate affects magnetosheath properties only weakly and the geoeffective length in the upstream solar wind is essentially unchanged, resulting in the same integrated dayside reconnection rate. With a large amount of mass loading, however, the magnetosheath properties and the geoeffective length are significantly affected by slowing down the local reconnection rate, resulting in an increase of the magnetic pressure in the magnetosheath, with a significant reduction in the geoeffective length in the upstream solar wind and in the integrated dayside reconnection rate. In this controlled simulation setup, the behavior of dayside reconnection potential is determined by the role of the enhanced magnetic pressure in the magnetospheath due to magnetospheric mass loading. The reconnection potential starts to decrease significantly when the enhanced magnetic pressure alters the thickness of the magnetosheath.