U. Auster

THEMIS observations of an earthward‐propagating dipolarization front

[1] We report THEMIS observations of a dipolarization front, a sharp, large-amplitude increase in the Z-component of the magnetic field. The front was detected in the central plasma sheet sequentially at X = -20.1 R E (THEMIS P1 probe), at X = -16.7 R E (P2), and at X = -11.0 R E (P3/P4 pair), suggesting its earthward propagation as a coherent structure over a distance more than 10 R E at a velocity of 300 km/s. The front thickness was found to be as small as the ion inertial length. Comparison with simulations allows us to interpret the front as the leading edge of a plasma fast flow formed by a burst of magnetic reconnection in the midtail.

Tail Reconnection Triggering Substorm Onset

Magnetospheric substorms explosively release solar wind energy previously stored in Earths magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at ∼10 RE (RE: Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at ∼20 to 30 RE. We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 RE, at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.

Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft

[1] Whistler mode chorus waves are receiving increased scientific attention due to their important roles in both acceleration and loss processes of radiation belt electrons. A new global survey of whistler-mode chorus waves is performed using magnetic field filter bank data from the THEMIS spacecraft with 5 probes in near-equatorial orbits. Our results confirm earlier analyses of the strong dependence of wave amplitudes on geomagnetic activity, confinement of nightside emissions to low magnetic latitudes, and extension of dayside emissions to high latitudes. An important new finding is the strong occurrence rate of chorus on the dayside at L > 7, where moderate dayside chorus is present >10% of the time and can persist even during periods of low geomagnetic activity. Citation: Li, W., R. M. Thorne, V. Angelopoulos, J. Bortnik, C. M. Cully, B. Ni, O. LeContel, A. Roux, U. Auster, and W. Magnes (2009), Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft, Geophys. Res. Lett., 36, L09104, doi:10.1029/2009GL037595.

Identifying the Driver of Pulsating Aurora

Auroral Chorus Energetic particles that arrive from near-Earth space produce photon emissions—the aurora—as they bombard the atmosphere in the polar regions. The pulsating aurora, which is characterized by temporal intensity variations, is thought to be caused by modulations in electron precipitation possibly produced by resonance with electromagnetic waves in Earths magnetosphere. Nishimura et al. (p. 81) present a detailed study of an event that showed a good correlation between the temporal changes in auroral luminosity and chorus emission—a type of electromagnetic wave occurring in Earths magnetosphere. The results points to chorus waves as the driver of the pulsating aurora. Correlations are found between aurora light intensity and a type of electromagnetic wave in Earth’s magnetosphere. Pulsating aurora, a spectacular emission that appears as blinking of the upper atmosphere in the polar regions, is known to be excited by modulated, downward-streaming electrons. Despite its distinctive feature, identifying the driver of the electron precipitation has been a long-standing problem. Using coordinated satellite and ground-based all-sky imager observations from the THEMIS mission, we provide direct evidence that a naturally occurring electromagnetic wave, lower-band chorus, can drive pulsating aurora. Because the waves at a given equatorial location in space correlate with a single pulsating auroral patch in the upper atmosphere, our findings can also be used to constrain magnetic field models with much higher accuracy than has previously been possible.

THEMIS analysis of observed equatorial electron distributions responsible for the chorus excitation

[1] A statistical survey of plasma densities and electron distributions (0.5–100 keV) is performed using data obtained from the Time History of Events and Macroscale Interactions During Substorms spacecraft in near‐equatorial orbits from 1 July 2007 to 1 May 2009 in order to investigate optimum conditions for whistler mode chorus excitation. The plasma density calculated from the spacecraft potential, together with in situ magnetic field, is used to construct global maps of cyclotron and Landau resonant energies under quiet, moderate, and active geomagnetic conditions. Statistical results show that chorus intensity increases at higher AE index, with the strongest waves confined to regions where the ratio between the plasma frequency and gyrofrequency, fpe/fce, is less than 5. On the nightside, large electron anisotropies and intense chorus emissions indicate remarkable consistency with the confinement to 8 RE. Furthermore, as injected plasma sheet electrons drift from midnight through dawn toward the noon sector, their anisotropy increases and peaks on the dayside at 7 6) on the dayside. In addition, very isotropic distributions at a few keV, which may be produced by Landau resonance and contribute to the formation of the typical gap in the chorus spectrum near 0.5 fce, are commonly observed on the dayside. Citation: Li, W., et al. (2010), THEMIS analysis of observed equatorial electron distributions responsible for the chorus excitation, J. Geophys. Res., 115, A00F11, doi:10.1029/2009JA014845.

Current carriers near dipolarization fronts in the magnetotail: A THEMIS event study

[1] We study current carriers observed within thin current sheets ahead of and during the passage of earthward moving dipolarization fronts in the near‐Earth plasma sheet using Time History of Events and Macroscale Interactions During Substorms (THEMIS) multipoint measurements. The fronts are embedded within flow bursts at the initial stage of bursty bulk flow events. Simultaneous north‐south and radial separations between probes P3, P4, and P5 and the planar current sheet approximation enable estimation of cross‐tail current density in the current sheet ahead of and within the fronts, respectively. The cross‐tail current density increase ahead of the fronts, a substorm growth phase signature, is predominantly due to the ion diamagnetic current; at times, however, the electron pressure gradient may contribute up to 60% of the total current density. Note that in this paper we refer to the horizontal (vertical) current sheet as the cross‐tail current sheet (current sheet associated with dipolarization fronts). At the dipolarization fronts, the horizontal cross‐tail current sheet (with a current density of several nA/m 2 ) relaxes, and a vertical current sheet (with a current density of several tens of nA/m 2 ), consistent with the thin interface of the front, appears. Thus, the cross‐tail current at longitudes adjacent to the flow burst feeds into the dipolarization front’s current sheet and may be extended to higher latitudes. The vertical current density also decreases after passage of the front. The pressure gradient of 1–10 keV electrons is a dominant contributor to the current in the dipolarization fronts. In the event studied, probes P1 and P2, which were several Earth radii downtail, reveal a tailward expansion of the current reduction process at a propagation velocity ∼50 km/s, even as the bulk flow carrying the magnetic flux remains earthward. This study shows how dipolarization fronts and their current systems are building blocks of the large‐scale substorm current wedge.

Structure of plasmaspheric plumes and their participation in magnetopause reconnection: First results from THEMIS

[1] New observations by the THEMIS spacecraft have revealed dense (>10 cm- 3 ) plasmaspheric plumes extending to the magnetopause. The large scale radial structure of these plumes is revealed by multi-spacecraft measurements. Temporal variations in the radial distribution of plume plasma, caused by azimuthal density gradients coupled with azimuthal flow, are also shown to contribute to plume structure. In addition, flux tubes with cold plume plasma are shown to participate in reconnection, with simultaneous observations of cold ions and reconnection flow jets on open flux tubes as revealed by the loss of hot magnetospheric electrons.

Estimation of magnetic field mapping accuracy using the pulsating aurora‐chorus connection

Although magnetic field models are widely used in magnetosphere-ionosphere coupling studies to perform field-line mapping, their accuracy has been difficult to estimate experimentally. Taking advan ...

Philae's first days on the comet

On 12 November 2014, Philae landed on the surface of comet 67P/Churyumov-Gerasimenko (67P), making an almost 30-year dream a reality. The pioneering flybys of 1P/Halley in 1986 revealed that despite being made primarily of ice, it was covered in highly absorbing carbonrich molecules. What is their composition? When did they form, and through which chemical routes? Might they have constituted prebiotic molecules necessary for life? At a larger scale, what can one learn from comets that has relevance to the evolution of the solar system and planets? ![Figure][1] 12 NOVEMBER 2014: PHILAE LANDED ON THE NUCLEUS OF COMET 67P CREDIT: ESA/ROSETTA/MPS FOR OSIRIS TEAM MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA To address such questions, the Rosetta mission sought to perform a broad range of in-depth structural, physical, and chemical measurements from remote, in situ, and landed vantages. The candidate payload opened for a competitive selection included an instrumented Surface Science Platform (SSP). The initial two that were selected later merged into what is known as Philae, instrumented by 10 principal investigators selected by the SSP providers. The Philae platform and payloads were developed and operated by a highly integrated consortium of institutes, agencies, and industries. Philaes scientific objectives were to provide ground-truth information and complement remote measurements performed from the Rosetta orbiter ( Science 347 , 23 January 2015) and to offer a self-standing suite of in situ measurements never before performed on a comet. This issue presents a first set of results acquired aboard Philae in the first 63 hours after it separated from Rosetta, descended, initially touched down on the comet at the site known as Agilkia, and finally came to rest at the site known as Abydos. The release and descent happened as planned, precisely documented by imaging (Mottola et al. ), ranging (Kofman et al. ), thermal mapping (Spohn et al. ), and the evolution of the magnetic properties (Auster et al. ). The prospect of landing on such an alien body, at 515 million km from Earth and 3 astronomical units (AU) from the Sun, was far more challenging than imagined. The unexpected bounce at touchdown required a major reshuffling and adaptation of the first sequence of science operations. It also provided the opportunity for additional measurements, whereas the bouncing and traversing constrained the mechanical (Biele et al. ) and magnetic properties of the surface. ROLIS imagery at its highest resolution (1 cm per pixel) showed the surface of the comet near Agilkia to be dominated by the presence of granular material free of any dust deposits (Mottola et al. ). Regolith mobilization processes appear to be involved with the formation of these features. Once Philae came to rest at Abydos, the revised first science sequence began. CIVA panoramic images characterized the surrounding cometary material down to the millimeter scale and the attitude of Philae at rest (Bibring et al. ). The MUPUS package measured and constrained the thermal and mechanical properties of the near-surface material of the comet surface at Abydos (Spohn et al. ), indicating that the near-surface layers consist of a hard dust-rich sintered ice, possibly covered by a thin dust layer. The CONSERT bistatic radar provided an opportunity to investigate the comets internal structure (Kofman et al. ). The upper “head” of 67P is fairly homogeneous on a spatial scale of tens of meters. The average permittivity provides ranges of the volumetric dust/ice ratio and the internal porosity. The dust component may be comparable, from the dielectric properties, to that of carbonaceous chondritic meteorites. COSAC and Ptolemy independently measured the composition of the volatile constituents of the grains lifted at touchdown and of the species outgassed at the final landing site (Goesmann et al. and Wright et al. ). The grains are primarily made of carbon-rich species in a complex suite of molecules, including precursors to some biomolecules and other compounds never before identified in comets. Taken together, these first measurements performed at the surface of 67P profoundly modify our view of comets. 67P is nonmagnetized on a scale of less than a meter, with its surface layers composed of both sintered ices, which are hard in nature, and fluffy grains and pebbles of organic materials, possible remnants from the era of comet formation itself. Although it remains to be seen whether these observations hold true for all comets, the discoveries made by Philae—including these initial results—will continue to shape our view of the history of the solar system. [1]: pending:yes

THEMIS observations of consecutive bursts of Pi2 pulsations: The 20 April 2007 event

[1] On 20 April 2007, four Pi2 pulsation bursts occurred successively and simultaneously in the premidnight sector at the E spacecraft and ground-based observatories for the THEMIS mission, while the AE index was less than 100 nT. Especially for the last three onsets, both ground-based and GOES 12 magnetometers sensed magnetic perturbations, as expected from the formation of a substorm current wedge (SCW). Moreover, LANL 1994-084 detected an enhancement of energetic particle flux. Spectral analysis shows a matched wave frequency ∼6―8 mHz and another harmonic frequency ∼17 mHz for the fourth burst. The orientation of the major axis of the wave polarization hodogram points toward the SCW location. The first burst has both latitudinal and longitudinal polarization changes from counterclockwise (CCW) to clockwise (CW), in contrast to the other three that have a latitudinal reversal only. The longitudinal CW to CCW change at low latitudes signifies that hydromagnetic waves propagate westward and eastward from the longitude of the impulsive source responsible for SCW. The latitudinal CW to CCW reversal is consistent with induction by a westward moving upward field-aligned current carried by Alfven waves leading to field line oscillations. Consequently, they can be explained by the coupling of a fast magnetospheric cavity mode driven by fast compressional waves to field line resonances as expected from braking bursty bulk flows, resulting from magnetotail reconnection, triggered by a preceding northward interplanetary magnetic field turning. This event shows that the source mechanism of consecutive Pi2 onsets at times of weak geomagnetic activity is the same as during substorm times.