M. O. Archer

Global impacts of a Foreshock Bubble: Magnetosheath, magnetopause and ground-based observations

Using multipoint observations we show, for the first time, that Foreshock Bubbles (FBs) have a global impact on Earth’s magnetosphere. We show that an FB, a transient kinetic phenomenon due to the interaction of backstreaming suprathermal ions with a discontinuity, modifies the total pressure upstream of the bow shock showing a decrease within the FB’s core and sheath regions. Magnetosheath plasma is accelerated towards the the intersection of the FB’s current sheet with the bow shock resulting in fast, sunward, flows as well as outward motion of the magnetopause. Ground-based magnetometers also show signatures of this magnetopause motion simultaneously across at least 7 hours of magnetic local time, corresponding to a distance of 21.5 RE transverse to the Sun-Earth line along the magnetopause. These observed global impacts of the FB are in agreement with previous simulations and in stark contrast to the known localised, smaller scale eects of Hot Flow Anomalies (HFAs).

The role of pressure gradients in driving sunward magnetosheath flows and magnetopause motion

While pressure balance can predict how far the magnetopause will move in response to an upstream pressure change, it cannot determine how fast the transient response will be. Using Time History of Events and Macroscale Interactions during Substorms (THEMIS), we present multipoint observations revealing, for the first time, strong (thermal + magnetic) pressure gradients in the magnetosheath due to a foreshock transient, most likely a hot flow anomaly, which decreased the total pressure upstream of the bow shock. By converting the spacecraft time series into a spatial picture, we quantitatively show that these pressure gradients caused the observed acceleration of the plasma, resulting in fast sunward magnetosheath flows ahead of a localized outward distortion of the magnetopause. The acceleration of the magnetosheath plasma was fast enough to keep the peak of the magnetopause bulge at approximately the equilibrium position, i.e., in pressure balance. Therefore, we show that pressure gradients in the magnetosheath due to transient changes in the total pressure upstream can directly drive anomalous flows and in turn are important in transmitting information from the bow shock to the magnetopause.

The global structure and time evolution of dayside magnetopause surface eigenmodes

Theoretical work and recent observations suggest that the dayside magnetopause may support its own eigenmode, consisting of propagating surface waves which reflect at the northern and southern ionospheres. These magnetopause surface eigenmodes (MSEs) are a potential source of magnetospheric ultralow-frequency (ULF) waves with frequencies less than 2 mHz. Here we use the Space Weather Modeling Framework to study the magnetospheric response to impulsive solar wind dynamic pressure increases. Waves with 1.8 mHz frequency are excited whose global properties are largely consistent with theoretical predictions for MSE and cannot be explained by other known ULF wave modes. These simulation results lead to two key findings: (1) MSE can be sustained in realistic magnetic field geometries with nonzero flow shear and finite current layer thickness at the magnetopause and (2) MSE can seed the growth of tailward propagating surface waves via the Kelvin-Helmholtz instability.

Space magnetometer based on an anisotropic magnetoresistive hybrid sensor

We report on the design and development of a low resource, dual sensor vector magnetometer for space science applications on very small spacecraft. It is based on a hybrid device combining an orthogonal triad of commercial anisotropic magnetoresistive (AMR) sensors with a totem pole H-Bridge drive on a ceramic substrate. The drive enables AMR operation in the more sensitive flipped mode and this is achieved without the need for current spike transmission down a sensor harness. The magnetometer has sensitivity of better than 3 nT in a 0-10 Hz band and a total mass of 104 g. Three instruments have been launched as part of the TRIO-CINEMA space weather mission, inter-calibration against the International Geomagnetic Reference Field model makes it possible to extract physical signals such as field-aligned current deflections of 20-60 nT within an approximately 45,000 nT ambient field.

Frequency variability of standing Alfvén waves excited by fast mode resonances in the outer magnetosphere

Coupled fast mode resonances (cFMRs) in the outer magnetosphere, between the magnetopause and a turning point, are often invoked to explain observed discrete frequency field line resonances. We quantify their frequency variability, applying cFMR theory to a realistic magnetic field model and magnetospheric density profiles observed over almost half a solar cycle. Our calculations show that cFMRs are most likely around dawn, since the plasmaspheric plumes and extended plasmaspheres often found at noon and dusk can preclude their occurrence. The relative spread (median absolute deviation divided by the median) in eigenfrequencies is estimated to be 28%, 72%, and 55% at dawn, noon, and dusk, respectively, with the latter two chiefly due to density. Finally, at dawn we show that the observed bimodal density distribution results in bimodal cFMR frequencies, whereby the secondary peaks are consistent with the so-called “CMS” frequencies that have previously been attributed to cFMRs.

What frequencies of standing surface waves can the subsolar magnetopause support

It is has been proposed that the subsolar magnetopause may support its own eigenmode, consisting of propagating surface waves which reflect at the northern/southern ionospheres forming a standing wave. While the eigenfrequencies of these so-called Kruskal-Schwarzschild (KS) modes have been estimated under typical conditions, the potential distribution of frequencies over the full range of solar wind conditions is not known. Using models of the magnetosphere and magnetosheath applied to an entire solar cycles worth of solar wind data, we perform time-of-flight calculations yielding a database of KS mode frequencies. Under nonstorm times or northward interplanetary magnetic field (IMF), the most likely fundamental frequency is calculated to be 0.64(-0.12)(+0.03) mHz, consistent with previous estimates and indirect observational evidence for such standing surface waves of the subsolar magnetopause. However, the distributions exhibit significant spread (of order +/- 0.3 mHz) demonstrating that KS mode frequencies, especially higher harmonics, should vary considerably depending on the solar wind conditions. The implications of such large spread on observational statistics are discussed. The subsolar magnetopause eigenfrequencies are found to be most dependent on the solar wind speed, southward component of the IMF, and the Dst index, with the latter two being due to the erosion of the magnetosphere by reconnection and the former an effect of the expression for the surface wave phase speed. Finally, the possible occurrence of KS modes is shown to be controlled by the dipole tilt angle.

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

Magnetosheath high-speed jets-localized dynamic pressure enhancements typically of similar to 1 Earth radius in size-impact the dayside magnetopause several times per hour. Here we present the firs ...

Magnetosheath High‐Speed Jets: Internal Structure and Interaction With Ambient Plasma

For the first time, we have studied the rich internal structure of a magnetosheath high speed jet. Measurements by the Magnetospheric Multiscale (MMS) spacecraft reveal large-amplitude density, temperature, and magnetic field variations inside the jet. The propagation velocity and normal direction of planar magnetic field structures (i.e., current sheets and waves) are investigated via four-spacecraft timing. We find structures to mainly convect with the jet plasma. There are indications of the presence of a tangential discontinuity. At other times, there are small cross-structure flows. Where this is the case, current sheets and waves overtake the plasma in the jets core region; ahead and behind that core region, along the jets path, current sheets are overtaken by the plasma, i.e., they move in opposite direction to the jet in the plasma rest frame. Jet structures are found to be mainly thermal and magnetic pressure-balance structures, notwithstanding that the dynamic pressure dominates by far. Although the jet is super-magnetosonic in the Earths frame of reference, it is sub-magnetosonic with respect to the plasma ahead. Consequently, we find no fast shock. Instead, we find some evidence for (a series of) jets pushing ambient plasma out of their way, thereby stirring the magnetosheath and causing anomalous sunward flows in the subsolar magnetosheath. Furthermore, we find that jets modify the magnetic field in the magnetosheath, aligning it with their propagation direction.

Magnetospheric and solar wind dependences of coupled fast-mode resonances outside the plasmasphere

We investigate the magnetospheric and solar wind factors that control the occurrence probabilities, locations, and frequencies of standing Alfven waves excited via coupled fast-mode resonances (cFMRs) in the outer magnetospheres dawn and dusk sectors. The variation of these cFMR properties with the observed magnetospheric plasma density profiles and inputs to the semiempirically modeled magnetic field from the numerical cFMR calculations of Archer et al. (2015) are studied. The probability of cFMR occurrence increases with distance between the magnetopause and the Alfven speeds local maximum. The latters location depends on magnetospheric activity: during high activity it is situated slightly outside the plasmapause, whereas at low activity it is found at much larger radial distances. The frequencies of cFMR are proportional to the Alfven speed near the magnetopause, which is affected by both density and magnetic field variations. The location of the excited resonance, however, depends on the relative steepness of the Alfven speed radial profile. The steeper this is, the closer the resonance is to the outer boundary and vice versa. The variation of the density profiles with solar wind conditions and activity is also shown.