Ion Larmor radius effects near a reconnection X line at the magnetopause: THEMIS observations and simulation comparison

We report a Time History of Events and Macroscale Interactions during Substorms (THEMIS-D) spacecraft crossing of a magnetopause reconnection exhaust ~9 ion skin depths (di) downstream of an X line. The crossing was characterized by ion jetting at speeds substantially below the predicted reconnection outflow speed. In the magnetospheric inflow region THEMIS detected (a) penetration of magnetosheath ions and the resulting flows perpendicular to the reconnection plane, (b) ion outflow extending into the magnetosphere, and (c) enhanced electron parallel temperature. Comparison with a simulation suggests that these signatures are associated with the gyration of magnetosheath ions onto magnetospheric field lines due to the shift of the flow stagnation point toward the low-density magnetosphere. Our observations indicate that these effects, ~2–3 di in width, extend at least 9 di downstream of the X line. The detection of these signatures could indicate large-scale proximity of the X line but do not imply that the spacecraft was upstream of the electron diffusion region

ARTEMIS Science Objectives

NASA's two spacecraft ARTEMIS mission will address both heliospheric and planetary research questions, first while in orbit about the Earth with the Moon and subsequently while in orbit about the Moon. Heliospheric topics include the structure of the Earth's magnetotail; reconnection, particle acceleration, and turbulence in the Earth's magnetosphere, at the bow shock, and in the solar wind; and the formation and structure of the lunar wake. Planetary topics include the lunar exosphere and its relationship to the composition of the lunar surface, the effects of electric fields on dust in the exosphere, internal structure of the Moon, and the lunar crustal magnetic field. This paper describes the expected contributions of ARTEMIS to these baseline scientific objectives

In Situ Observations of a Magnetosheath High-Speed Jet Triggering Magnetopause Reconnection

Magnetosheath high‐speed jets—localized dynamic pressure enhancements typically of ∼1 Earth radius in size—impact the dayside magnetopause several times per hour. Here we present the first in situ measurements suggesting that such an impact triggered magnetopause reconnection. We use observations from the five Time History of Events and Macroscale Interactions during Substorms spacecraft in a string‐of‐pearls configuration on 7 August 2007. The spacecraft recorded magnetopause in‐and‐out motion during an impact of a magnetosheath jet (VN∼−300 km/s along the magnetopause normal direction). There was no evidence for reconnection for the preimpact crossing, yet three probes observed reconnection after the impact. We infer that the jet impact compressed the originally thick (60–70 di), high magnetic shear (140–160° magnetopause until it was thin enough for reconnection to occur. Magnetosheath high‐speed jets could therefore act as a driver for bursty dayside reconnection

The Scientific Foundations of Forecasting Magnetospheric Space Weather

The magnetosphere is the lens through which solar space weather phenomena are focused and directed towards the Earth. In particular, the non-linear interaction of the solar wind with the Earth's magnetic field leads to the formation of highly inhomogenous electrical currents in the ionosphere which can ultimately result in damage to and problems with the operation of power distribution networks. Since electric power is the fundamental cornerstone of modern life, the interruption of power is the primary pathway by which space weather has impact on human activity and technology. Consequently, in the context of space weather, it is the ability to predict geomagnetic activity that is of key importance. This is usually stated in terms of geomagnetic storms, but we argue that in fact it is the substorm phenomenon which contains the crucial physics, and therefore prediction of substorm occurrence, severity and duration, either within the context of a longer-lasting geomagnetic storm, but potentially also as an isolated event, is of critical importance. Here we review the physics of the magnetosphere in the frame of space weather forecasting, focusing on recent results, current understanding, and an assessment of probable future developments.Peer reviewe

Auroral activity associated with unsteady magnetospheric erosion: Observations on December 18, 1990

We report auroral observations made on December 18, 1990, when interplanetary conditions should lead to large-scale erosion of the dayside magnetosphere during a substorm growth phase. A long interval of strongly northward pointing interplanetary magnetic field (IMF) was succeeded by several hours of strongly southward pointing IMF. The interval of southward pointing IMF was punctuated by a number of IMF directional discontinuities during which the IMF north-south component, Bz changed polarity abruptly. The auroral responses, monitored at Ny Ålesund (75° magnetic latitude) by meridian scanning photometers and all-sky cameras, were as follows: The interval of negative IMF Bz was characterized by a net equatorward migration of the equatorward boundary of the dayside cusp/cleft aurora, as expected from previous studies. On this occasion, however, we find that the latitudinal shift occurred in steps which consisted of an initial brightening of individual auroral events at ∼0.5° MLAT equatorward of the preexisting luminosity, followed by a steady poleward retreat lasting typically 4–5 min. The net effect over the first hour of IMF Bz \u3c 0 conditions was to move the equatorward boundary toward the geomagnetic equator by ∼2.7° MLAT. The auroral data suggest that in this instance dayside magnetosphere erosion took place intermittently: bursts of reconnection (initial brightenings) are followed by a switch-off of the reconnection electric field (subsequent poleward retreat). The bursts of reconnection may be identified with flux transfer events or, equivalently, flux erosion events