Continuous magnetic reconnection at Earth's magnetopause

The Lunar Environment Heliophysics X-ray Imager (LEXI) Mission

The Lunar Environment heliospheric X-ray Imager (LEXI) is a wide field-of-view soft X-ray telescope developed to study solar wind-magnetosphere coupling. LEXI is part of the Blue Ghost 1 mission comprised of 10 payloads to be deployed on the lunar surface. LEXI monitors the dayside magnetopause position and shape as a function of time by observing soft X-rays (0.1-2 keV) emitted from solar wind charge-exchange between exospheric neutrals and high charge-state solar wind plasma in the dayside magnetosheath. Measurements of the shape and position of the magnetopause are used to test temporal models of meso- and macro-scale magnetic reconnection. To image the boundary, LEXI employs lobster-eye optics to focus X-rays to a microchannel plate detector with a 9.1× ∘ 9.1 ∘ field of view.

Unsteady Magnetopause Reconnection Under Quasi-Steady Solar Wind Driving

The intrinsic temporal nature of magnetic reconnection at the magnetopause has been an active area of research. Both temporally steady and intermittent reconnection have been reported. We examine the steadiness of reconnection using space-ground conjunctions under quasi-steady solar wind driving. The spacecraft suggests that reconnection is first inactive, and then activates. The radar further suggests that after activation, reconnection proceeds continuously but unsteadily. The reconnection electric field shows variations at frequencies below 10 mHz with peaks at 3 and 5 mHz. The variation amplitudes are ∼10-30 mV/m in the ionosphere, and 0.3-0.8 mV/m at the equatorial magnetopause. Such amplitudes represent 30%-60% of the peak reconnection electric field. The unsteadiness of reconnection can be plausibly explained by the fluctuating magnetic field in the turbulent magnetosheath. A comparison with a previous global hybrid simulation suggests that it is the foreshock waves that drive the magnetosheath fluctuations, and hence modulate the reconnection.

Linking solar minimum, space weather, and night sky brightness

This paper presents time-series observations and analysis of broadband night sky airglow intensity 4 September 2018 through 30 April 2020. Data were obtained at 5 sites spanning more than 8500 km during the historically deep minimum of Solar Cycle 24 into the beginning of Solar Cycle 25. New time-series observations indicate previously unrecognized significant sources of broadband night sky brightness variations, not involving corresponding changes in the Sun's 10.7 cm solar flux, occur during deep solar minimum. New data show; (1) Even during a deep solar minimum the natural night sky is rarely, if ever, constant in brightness. Changes with time-scales of minutes, hours, days, and months are observed. (2) Semi-annual night sky brightness variations are coincident with changes in the orientation of Earth's magnetic field relative to the interplanetary magnetic field. (3) Solar wind plasma streams from solar coronal holes arriving at Earth's bow shock nose are coincident with major night sky brightness increase events. (4) Sites more than 8500 km along the Earth's surface experience nights in common with either very bright or very faint night sky airglow emissions. The reason for this observational fact remains an open question. (5) It is plausible, terrestrial night airglow and geomagnetic indices have similar responses to the solar energy input into Earth's magnetosphere. Our empirical results contribute to a quantitative basis for understanding and predicting broadband night sky brightness variations. They are applicable in astronomical, planetary science, space weather, light pollution, biological, and recreational studies.

The Location of Magnetic Reconnection at Earth's Magnetopause

One of the major questions about magnetic reconnection is how specific solar wind and interplanetary magnetic field conditions influence where reconnection occurs at the Earth's magnetopause. There are two reconnection scenarios discussed in the literature: a) anti-parallel reconnection and b) component reconnection. Early spacecraft observations were limited to the detection of accelerated ion beams in the magnetopause boundary layer to determine the general direction of the reconnection X-line location with respect to the spacecraft. An improved view of the reconnection location at the magnetopause evolved from ionospheric emissions observed by polar-orbiting imagers. These observations and the observations of accelerated ion beams revealed that both scenarios occur at the magnetopause. Improved methodology using the time-of-flight effect of precipitating ions in the cusp regions and the cutoff velocity of the precipitating and mirroring ion populations was used to pinpoint magnetopause reconnection locations for a wide range of solar wind conditions. The results from these methodologies have been used to construct an empirical reconnection X-line model known as the Maximum Magnetic Shear model. Since this model's inception, several tests have confirmed its validity and have resulted in modifications to the model for certain solar wind conditions. This review article summarizes the observational evidence for the location of magnetic reconnection at the Earth's magnetopause, emphasizing the properties and efficacy of the Maximum Magnetic Shear Model.

Evolution of Turbulence in the Kelvin–Helmholtz Instability in the Terrestrial Magnetopause

The dynamics occurring at the terrestrial magnetopause are investigated by using Geotail and THEMIS spacecraft data of magnetopause crossings during ongoing Kelvin–Helmholtz instability. Properties of plasma turbulence and intermittency are presented, with the aim of understanding the evolution of the turbulence as a result of the development of Kelvin–Helmholtz instability. The data have been tested against standard diagnostics for intermittent turbulence, such as the autocorrelation function, the spectral analysis and the scale-dependent statistics of the magnetic field increments. A quasi-periodic modulation of different scaling exponents may exist along the direction of propagation of the Kelvin–Helmholtz waves along the Geocentric Solar Magnetosphere coordinate system (GSM), and it is visible as a quasi-periodic modulation of the scaling exponents we have studied. The wave period associated with such oscillation was estimated to be approximately 6.4 Earth Radii ( R E ). Furthermore, the amplitude of such modulation seems to decrease as the measurements are taken further away from the Earth along the magnetopause, in particular after X ( G S M ) ≲ − 15 R E . The observed modulation seems to persist for most of the parameters considered in this analysis. This suggests that a kind of signature related to the development of the Kelvin–Helmholtz instabilities could be present in the statistical properties of the magnetic turbulence.

Near quantum-noise limited and absolute frequency stabilized 1083 nm single-frequency fiber laser

The Earth's magnetic field has significant effects that protect us from cosmic radiation and provide navigation for biological migration. However, slow temporal variations originating in the liquid outer core invariably exist. To understand the working mechanism of the geomagnetic field and improve accuracy of navigation systems, a high-precision magnetometer is essential to measure the absolute magnetic field. A helium optically pumping magnetometer is an advanced approach, but its sensitivity and accuracy are directly limited by the low-frequency relative intensity noise and frequency stability characteristics of a light source. Here, we demonstrate a near quantum-noise limited and absolute frequency stabilized 1083 nm single-frequency fiber laser. The relative intensity noise is only 5 dB higher than the quantum-noise limit, and the root mean square of frequency fluctuation is ∼17  kHz after locked. This fiber laser could suppress the fluctuation of magnetic resonant frequency and improve the signal-to-noise ratio of the magnetic resonance signal detection.

Transient Weakening of Earth's Magnetic Shield Probed by a Cosmic Ray Burst

The GRAPES-3 tracking muon telescope in Ooty, India measures muon intensity at high cutoff rigidities (15-24 GV) along nine independent directions covering 2.3 sr. The arrival of a coronal mass ejection on 22 June 2015 18:40 UT had triggered a severe G4-class geomagnetic storm (storm). Starting 19:00 UT, the GRAPES-3 muon telescope recorded a 2 h high-energy (∼20  GeV) burst of galactic cosmic rays (GCRs) that was strongly correlated with a 40 nT surge in the interplanetary magnetic field (IMF). Simulations have shown that a large (17×) compression of the IMF to 680 nT, followed by reconnection with the geomagnetic field (GMF) leading to lower cutoff rigidities could generate this burst. Here, 680 nT represents a short-term change in GMF around Earth, averaged over 7 times its volume. The GCRs, due to lowering of cutoff rigidities, were deflected from Earth's day side by ∼210° in longitude, offering a natural explanation of its night-time detection by the GRAPES-3. The simultaneous occurrence of the burst in all nine directions suggests its origin close to Earth. It also indicates a transient weakening of Earth's magnetic shield, and may hold clues for a better understanding of future superstorms that could cripple modern technological infrastructure on Earth, and endanger the lives of the astronauts in space.

Distinguishing between pulsed and continuous reconnection at the dayside magnetopause

Magnetic reconnection has been established as the dominant mechanism by which magnetic fields in different regions change topology to create open magnetic field lines that allow energy and momentum to flow into the magnetosphere. One of the persistent problems of magnetic reconnection is the question of whether the process is continuous or intermittent and what input condition(s) might favor one type of reconnection over the other. Observations from imagers that record FUV emissions caused by precipitating cusp ions demonstrate the global nature of magnetic reconnection. Those images show continuous ionospheric emissions even during changing interplanetary magnetic field conditions. On the other hand, in situ observations from polar-orbiting satellites show distinctive cusp structures in flux distributions of precipitating ions, which are interpreted as the telltale signature of intermittent reconnection. This study uses a modification of the low-velocity cutoff method, which was previously successfully used to determine the location of the reconnection site, to calculate for the cusp ion distributions the "time since reconnection occurred." The "time since reconnection" is used to determine the "reconnection time" for the cusp magnetic field lines where these distributions have been observed. The profile of the reconnection time, either continuous or stepped, is a direct measurement of the nature of magnetic reconnection at the reconnection site. This paper will discuss a continuous and pulsed reconnection event from the Polar spacecraft to illustrate the methodology.

Solar wind entry into the high-latitude terrestrial magnetosphere during geomagnetically quiet times

An understanding of the transport of solar wind plasma into and throughout the terrestrial magnetosphere is crucial to space science and space weather. For non-active periods, there is little agreement on where and how plasma entry into the magnetosphere might occur. Moreover, behaviour in the high-latitude region behind the magnetospheric cusps, for example, the lobes, is poorly understood, partly because of lack of coverage by previous space missions. Here, using Cluster multi-spacecraft data, we report an unexpected discovery of regions of solar wind entry into the Earth's high-latitude magnetosphere tailward of the cusps. From statistical observational facts and simulation analysis we suggest that these regions are most likely produced by magnetic reconnection at the high-latitude magnetopause, although other processes, such as impulsive penetration, may not be ruled out entirely. We find that the degree of entry can be significant for solar wind transport into the magnetosphere during such quiet times.

A magnetic reconnection X-line extending more than 390 Earth radii in the solar wind

Magnetic reconnection in a current sheet converts magnetic energy into particle energy, a process that is important in many laboratory, space and astrophysical contexts. It is not known at present whether reconnection is fundamentally a process that can occur over an extended region in space or whether it is patchy and unpredictable in nature. Frequent reports of small-scale flux ropes and flow channels associated with reconnection in the Earth's magnetosphere raise the possibility that reconnection is intrinsically patchy, with each reconnection X-line (the line along which oppositely directed magnetic field lines reconnect) extending at most a few Earth radii (R(E)), even though the associated current sheets span many tens or hundreds of R(E). Here we report three-spacecraft observations of accelerated flow associated with reconnection in a current sheet embedded in the solar wind flow, where the reconnection X-line extended at least 390R(E) (or 2.5 x 10(6) km). Observations of this and 27 similar events imply that reconnection is fundamentally a large-scale process. Patchy reconnection observed in the Earth's magnetosphere is therefore likely to be a geophysical effect associated with fluctuating boundary conditions, rather than a fundamental property of reconnection. Our observations also reveal, surprisingly, that reconnection can operate in a quasi-steady-state manner even when undriven by the external flow.