The ARTEMIS Mission Angelopoulos, V.
Space science reviews,
12/2011, Letnik:
165, Številka:
1-4
Journal Article
Recenzirano
The Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) mission is a spin-off from NASA’s Medium-class Explorer (MIDEX) mission THEMIS, a five ...identical micro-satellite (hereafter termed “probe”) constellation in high altitude Earth-orbit since 17 February 2007. By repositioning two of the five THEMIS probes (P1 and P2) in coordinated, lunar equatorial orbits, at distances of ∼55–65
R
E
geocentric (∼1.1–12
R
L
selenocentric), ARTEMIS will perform the first systematic, two-point observations of the distant magnetotail, the solar wind, and the lunar space and planetary environment. The primary heliophysics science objectives of the mission are to study from such unprecedented vantage points and inter-probe separations how particles are accelerated at reconnection sites and shocks, and how turbulence develops and evolves in Earth’s magnetotail and in the solar wind. Additionally, the mission will determine the structure, formation, refilling, and downstream evolution of the lunar wake and explore particle acceleration processes within it. ARTEMIS’s orbits and instrumentation will also address key lunar planetary science objectives: the evolution of lunar exospheric and sputtered ions, the origin of electric fields contributing to dust charging and circulation, the structure of the lunar interior as inferred by electromagnetic sounding, and the lunar surface properties as revealed by studies of crustal magnetism. ARTEMIS is synergistic with concurrent NASA missions LRO and LADEE and the anticipated deployment of the International Lunar Network. It is expected to be a key element in the NASA Heliophysics Great Observatory and to play an important role in international plans for lunar exploration.
The THEMIS Mission Angelopoulos, V.
Space science reviews,
12/2008, Letnik:
141, Številka:
1-4
Journal Article
Recenzirano
The Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission is the fifth NASA Medium-class Explorer (MIDEX), launched on February 17, 2007 to determine the trigger and ...large-scale evolution of substorms. The mission employs five identical micro-satellites (hereafter termed “probes”) which line up along the Earth’s magnetotail to track the motion of particles, plasma and waves from one point to another and for the first time resolve space–time ambiguities in key regions of the magnetosphere on a global scale. The probes are equipped with comprehensive in-situ particles and fields instruments that measure the thermal and super-thermal ions and electrons, and electromagnetic fields from DC to beyond the electron cyclotron frequency in the regions of interest. The primary goal of THEMIS, which drove the mission design, is to elucidate which magnetotail process is responsible for substorm onset at the region where substorm auroras map (∼10 R
E
): (i) a local disruption of the plasma sheet current (current disruption) or (ii) the interaction of the current sheet with the rapid influx of plasma emanating from reconnection at ∼25 R
E
. However, the probes also traverse the radiation belts and the dayside magnetosphere, allowing THEMIS to address additional baseline objectives, namely: how the radiation belts are energized on time scales of 2–4 hours during the recovery phase of storms, and how the pristine solar wind’s interaction with upstream beams, waves and the bow shock affects Sun–Earth coupling. THEMIS’s open data policy, platform-independent dataset, open-source analysis software, automated plotting and dissemination of data within hours of receipt, dedicated ground-based observatory network and strong links to ancillary space-based and ground-based programs. promote a grass-roots integration of relevant NASA, NSF and international assets in the context of an international Heliophysics Observatory over the next decade. The mission has demonstrated spacecraft and mission design strategies ideal for Constellation-class missions and its science is complementary to Cluster and MMS. THEMIS, the first NASA micro-satellite constellation, is a technological pathfinder for future Sun-Earth Connections missions and a stepping stone towards understanding Space Weather.
Within dipolarization fronts (DFs) in the Earth's magnetotail, significant magnetic energy is converted to plasma energy, and a significant portion of the electrons and ions therein are accelerated ...to suprathermal energies. The mechanism that produces these suprathermal particles while simultaneously reducing magnetic field energy is poorly understood, however. We use two‐dimensional particle‐in‐cell simulations to explore this process in conventional flux bundle‐type DFs, which are formed by single X line reconnection and connected to the Earth, and in newly proposed flux rope‐type DFs, which are formed and bracketed by two X lines. In flux bundle‐type DFs, electrons are betatron accelerated near the Bz peak, and ions are energized through reflection at the front. In flux rope‐type DFs, most suprathermal electrons and ions are confined to the flux rope's magnetic structure and are accelerated through repeated reflections at the structure's two ends.
Key Points
Flux bundle‐type and flux rope‐type DFs are studied using 2‐D PIC simulations
A significant portion of electrons and ions are accelerated by DFs to suprathermal energies
Suprathermal particle energization mechanisms are revealed through particle tracing
Energetic particle injections are critical for supplying particles and energy to the inner magnetosphere. Recent case studies have demonstrated a good correlation between injections and transient, ...narrow, fast flow channels as well as earthward reconnection (dipolarization) fronts in the magnetotail, but statistical observations beyond geosynchronous orbit (GEO) to verify the findings were lacking. By surveying trans‐geosynchronous injections using Time History of Events and Macroscale Interactions during Substorms (THEMIS), we show that their likely origin is the earthward traveling, dipolarizing flux bundles following near‐Earth reconnection. The good correlation between injections and fast flows, reconnection fronts and impulsive, dawn‐dusk electric field increases is not limited to within 12 RE but extends out to 30 RE. Like near‐Earth reconnection, both ion and electron injections are most probable in the premidnight sector. Similar to bursty bulk flows (BBFs), injection‐time flow speeds are faster farther from Earth. With faster flows, injection intensity generally increases and extends to higher energy channels. With increased geomagnetic activity, injection occurrence rate increases (akin to that of BBFs) and spectral hardening occurs (κ decreases). The occurrence rate increase within the inner magnetosphere suggests that injections populate the radiation belts more effectively under enhanced activity. Our results are inconsistent with the classical concept of an azimuthally wide injection boundary moving earthward from ~9 to 12 RE to GEO under an enhanced cross‐tail electric field. Rather, particle injection and transport occur along a large range of radial distances due to effects from earthward penetrating, azimuthally localized, transient, strong electric fields of recently reconnected, dipolarizing flux bundles.
Key Points
Injections are correlated to reconnection‐related phenomena like fast flows
Injection occurrence rates increase with geomagnetic activity
Injection occurrence rate has dawn‐dusk asymmetry preferring premidnight
A dipolarizing flux bundle (DFB) is a small magnetotail flux tube (typically < ~3 RE in XGSM and YGSM) with a significantly more dipolar magnetic field than its background. Dipolarizing flux bundles ...typically propagate earthward at a high speed from the near‐Earth reconnection region. Knowledge of a DFB's flux transport properties leads to better understanding of near‐Earth (X = −6 to −30 RE) magnetotail flux transport and thus conversion of magnetic energy to kinetic and thermal plasma energy following magnetic reconnection. We explore DFB properties with a statistical study using data from the Time History of Events and Macroscale Interactions during Substorms mission. To establish the importance of DFB flux transport, we compare it with transport by bursty bulk flows (BBFs) that typically envelop DFBs. Because DFBs coexist with flow bursts inside BBFs, they contribute >65% of BBF flux transport, even though they last only ~30% as long as BBFs. The rate of DFB flux transport increases with proximity to Earth and to the premidnight sector, as well as with geomagnetic activity and distance from the neutral sheet. Under the latter two conditions, the total flux transport by a typical DFB also increases. Dipolarizing flux bundles appear more often during increased geomagnetic activity. Since BBFs have been previously shown to be the major flux transporters in the tail, we conclude that DFBs are the dominant drivers of this transport. The occurrence rate of DFBs as a function of location and geomagnetic activity informs us about processes that shape global convection and energy conversion.
Key Points
Dipolarizing flux bundles are the major flux carrier of bursty bulk flows
DFBs transport flux faster closer to Earth and in tail's premidnight sector
DFBs transport more flux during higher substorm activity
The ring current, an equatorial near‐Earth current, fluctuates in response to solar wind plasma interactions with Earth's magnetosphere. Despite extensive research on storm‐time ring current ...energization, direct evidence of the energy transport into the inner‐magnetosphere that powers this current remains scarce. Recent observations revealing that very‐near‐Earth reconnection (VNERX, occurring at geocentric distance <14 RE) can occur during storms suggest that such reconnection could play an important role in ring current development. Here we address how common VNERX is. We use inner‐magnetosphere and plasma sheet observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites spanning 13 years. During this period, THEMIS observed 512 storms and 7 VNERX events. All VNERX events occurred during storm main‐phase at or near the pre‐midnight sector (None were observed during storm recovery‐phase.). The events occurred within 1 RE of the modeled neutral sheet, suggesting that VNERX events are elusive because they lie near the neutral sheet. Since THEMIS spent 5,253 hr within 1 RE of the modeled neutral sheet during storm main‐phase, the inferred observational VNERX occurrence rate is 1.3 per 1,000 hr of storm main‐phase. This rate is lower than published ion‐diffusion‐region occurrence rates seen in the near‐Earth plasma sheet during non‐storm times (likely substorms). These results suggest that while VNERX events might be significant for the storm‐time ring current's initial buildup (during storm main‐phase), other transport mechanisms, like enhanced global convection, may be responsible for maintaining the strength of the ring current during storm recovery‐phase.
Key Points
Very‐Near‐Earth reconnection (VNERX) events have an observed occurrence rate of 1.3 events per 1,000 hr of storm main phase
VNERX may be important for the initial buildup of the main‐phase ring current
VNERX is predominately observed in the pre‐midnight sector
Using Time History of Events and Macroscale Interactions during Substorms observations from four tail seasons, we study the three‐dimensional structure of the dipolarization front current sheet ...(DFCS), which demarcates the magnetic boundary of a dipolarizing flux bundle (DFB, the strong magnetic field region led by a dipolarization front) in Earth's magnetotail. An equatorial cross section of the DFCS is convex; a meridional cross section is consistent with a dipolarized field line. The equatorial flow pattern in the ambient plasma ahead of the DFCS exhibits diversions of opposite sense on its evening and morning sides. The magnetic field perturbations are consistent with local field‐aligned current generation of region‐2 sense ahead of the front and region‐1 sense at the front. The median thickness of the DFCS increases from 800 to 2000 km with increasing distance from the neutral sheet, indicating bundle compression near the neutral sheet. On a meridional cross section, DFCS's linear current density (1.2–1.8 nA/m) peaks ~±0.55 l from the neutral sheet (where l is the ambient cross‐tail current sheet half‐thickness, l ~1.5 RE in our database). This peak, reminiscent of active‐time cross‐tail current sheet bifurcation noted in past studies, suggests that the intense but thin DFCS (10 to 20 nA/m2) may be produced by redistribution (diversion) of the extended but weaker cross‐tail current (~1 nA/m2). Near the neutral sheet, the average DFCS current over the dipolarization front (DF) thickness is perpendicular to both the magnetic field interior to the DFB and the average field direction over the DF thickness. Away from the neutral sheet, the average current becomes progressively parallel to the internal field and the average field direction. The average current directions are indicative of region‐1‐sense field‐aligned current on the DF. As few as approximately three DFBs can carry sufficient total current that, if redirected into the auroral ionosphere, can account for the substorm current wedge's peak current for a sizable substorm (~1 MA). A collapsing DFB could thus be an elemental substorm current wedge, or “wedgelet,” that can divert a sizable portion of the cross‐tail current into the auroral ionosphere.
Key Points
The dipolarization front has the shape of a saddleThe dipolarization front current has field‐aligned components in region‐1 senseThe dipolarization front thickness and current density varies with location
The global distribution of chorus wave amplitudes and their wave normal angles is investigated using high‐resolution wave spectra and waveform data from THEMIS for lower‐band and upper‐band chorus ...separately. Statistical results show that large amplitude chorus (>300 pT) occurs predominantly from premidnight to postdawn and is preferentially observed at lower L shells (<8) near the magnetic equator. However, strong or moderate chorus extends further into the afternoon sector and to higher L shells. For lower‐band chorus, strong waves (>50 pT) tend to have wave normal angles of <20° and their wave normal angles become even smaller with increasing wave amplitudes. For modest waves, the wave normal angles are distributed over a broad range with a major peak at <20° and a secondary peak at 60°–80°. Wave normal angles of lower‐band chorus are generally smaller on the dayside than on the nightside possibly due to the more uniform and more compressed magnetic field configuration on the dayside. Lower‐band chorus becomes more oblique with increasing latitude on the dayside, whereas on the nightside the probability of observing oblique chorus decreases at higher latitudes. Compared to lower‐band chorus, the properties of upper‐band chorus are somewhat different. Upper‐band chorus is considerably weaker in magnetic wave amplitudes, shows tighter confinement to the magnetic equator (<10°), and occurs at smaller L shells (<8). Furthermore, wave normal angles of upper‐band chorus are generally larger than those of lower‐band chorus, but the occurrence rate still peaks at wave normal angles of <20°, particularly for strong upper‐band chorus.
Key Points
Large amplitude chorus occurs from premidnight to dawn near the equator
Strong lower‐band chorus typically has small wave normal angles (less than 20 deg)
Upper‐band chorus is more oblique and much weaker than lower‐band chorus
The terrestrial magnetopause is the boundary that shields the Earth's magnetosphere on one side from the shocked solar wind and its embedded interplanetary magnetic field on the other side. In this ...paper, we show observations from two of the Time History of Events and Macroscales Interactions during Substorms (THEMIS) satellites, comparing dayside magnetopause crossings with flank crossings near the terminator. Macroscopic properties such as current sheet thickness, motion, and current density are examined for a large number of magnetopause crossings. The results show that the flank magnetopause is typically thicker than the dayside magnetopause and has a lower current density. Consistent with earlier results from Cluster observations, we also find a persistent dawn‐dusk asymmetry with a thicker and more dynamic magnetopause at dawn than at dusk.
Key Points
Observations from THEMIS are used to characterize the magnetopause
The flank magnetopause is thicker than the dayside magnetopause
A dawn‐dusk asymmetries exists in many magnetopause parameters
The abrupt boundary between a magnetosphere and the surrounding plasma, the magnetopause, has long been known to support surface waves. It was proposed that impulses acting on the boundary might lead ...to a trapping of these waves on the dayside by the ionosphere, resulting in a standing wave or eigenmode of the magnetopause surface. No direct observational evidence of this has been found to date and searches for indirect evidence have proved inconclusive, leading to speculation that this mechanism might not occur. By using fortuitous multipoint spacecraft observations during a rare isolated fast plasma jet impinging on the boundary, here we show that the resulting magnetopause motion and magnetospheric ultra-low frequency waves at well-defined frequencies are in agreement with and can only be explained by the magnetopause surface eigenmode. We therefore show through direct observations that this mechanism, which should impact upon the magnetospheric system globally, does in fact occur.
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