ALBERT

Beschreibung: [1]  We investigate the storm-scale morphology of the magnetospheric magnetic field as well as underlying distributions of electric currents, equatorial plasma pressure and entropy for four Steady Magnetospheric Convection (SMC) events that occurred during the May 2000 and October 2011 magnetic storms. The analysis is made using the empirical geomagnetic field model TS07D, in which the structure of equatorial currents is not predefined and it is dictated by data. The model also combines the strengths of statistical and event-oriented approaches in mining data for the reconstruction of the magnetic field. The formation of a near-Earth minimum of the equatorial magnetic field in the midnight sector is inferred from data without ad hoc assumptions of a special current system postulated in earlier empirical reconstructions. In addition, a new SMC class is discovered where the minimum equatorial field is substantially larger and located closer to Earth. The magnetic field tailward of the minimumis also much larger, and the corresponding area of accumulated magnetic flux may occupy a very short tail region. The equatorial current and plasma pressure are found to be strongly enhanced far beyond geosynchronous orbit and in a broad local time interval covering the whole nightside region. This picture is consistent with independent recent statistical studies of the SMC pressure distributions, global MHD and kinetic RCM-E simulations. Distributions of the flux tube volume and entropy inferred from data reveal different mechanisms of the magnetotail convection crisis resolution for two classes of SMC events.

Beschreibung: We show that ~1 Hz magnetic compressional waves observed in Mercury's inner magnetosphere could be interpreted as ion-Bernstein waves in a moderate proton beta ~0.1 plasma. An observation of a proton distribution with a large planetary loss-cone is presented, and we show that this type of distribution is highly unstable to the generation of ion-Bernstein waves with low magnetic compression. Ray tracing shows that as these waves propagate back and forth about the magnetic equator they cycle between a state of low and high magnetic compression. The group velocity decreases during the high compression state leading to a pile up of compressional wave energy, which could explain the observed dominance of the highly compressional waves. This bi-modal nature is due to the complexity of the index of refraction surface in a warm plasma whose upper branch has high growth rate with low compression, and its lower branch has low growth/damping rate with strong compression. Two different cycles are found, one where the compression maximum occurs at the magnetic equator and one where the compression maximum straddles the magnetic equator. The later cycle could explain observations where the maximum in compression straddles the equator. Ray tracing shows that this mode is confined within ±12° magnetic latitude which can account for the bulk of the observations. We show that the Doppler shift can account for the difference between the observed and model wave frequency, if the wave vector direction is in opposition to the plasma flow direction. We note that the WKB approximation breaks down during the pile up of compressional energy and that a study involving full wave solutions is required.

Beschreibung: Magnetotail processes and structures related to substorm growth phase/onset auroral arcs remain poorly understood mostly due to the lack of adequate observations. In this study we make a comparison between ground-based optical measurements of the premidnight growth phase/onset arcs at subauroral latitudes and magnetically conjugate measurements made by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) at ~780 km in altitude and by the Van Allen Probe-B (RBSP-B) spacecraft crossing L values of ~5.0–5.6 in the premidnight inner tail region. The conjugate observations offer a unique opportunity to examine the detailed features of the arc location relative to large-scale Birkeland currents and of the magnetospheric counterpart. Our main findings include (1) At the early stage of the growth phase the quiet auroral arc emerged ~4.3° equatorward of the boundary between the downward Region-2 (R2) and upward Region-1 (R1) currents; (2) Shortly before the auroral breakup (poleward auroral expansion) the latitudinal separation between the arc and the R1/R2 demarcation narrowed to ~1.0°; (3) RBSP-B observed a magnetic field signature of a local upward field-aligned current (FAC) connecting the arc with the near-Earth tail when the spacecraft footprint was very close to the arc; and (4) The upward FAC signature was located on the tailward side of a local plasma pressure increase confined near L ~5.2–5.4. These findings strongly suggest that the premidnight arc is connected to highly localized pressure gradients embedded in the near-tail R2 source region via the local upward FAC.

Beschreibung: Principal Component Analysis is performed on Birkeland or field-aligned current (FAC) measurements from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). PCA identifies the patterns in the FACs that respond coherently to different aspects of geomagnetic activity. The region 1 and 2 current system is shown to be the most reproducible feature of the currents, followed by cusp currents associated with magnetic tension forces on newly-reconnected field lines. The cusp currents are strongly modulated by season, indicating that their strength is regulated by the ionospheric conductance at the foot of the field lines. PCA does not identify a pattern that is clearly characteristic of a substorm current wedge. Rather, a superposed epoch analysis of the currents associated with substorms demonstrates that there is not a single mode of response, but a complicated and subtle mixture of different patterns.

Beschreibung: MESSENGER magnetometer data was surveyed between 0.3 and 0.7 AU from June 6th, 2007 to March 23rd, 2011 for Low Frequency Wave (LFW) storms, when the magnetometer was sampling at a rate of at least 2 s −1 . A total of 12197 LFW events were identified, of which 5506 lasted 10 minutes or longer. The events have a high degree of polarization, are circularly polarized, with wave vectors nearly aligned or anti-aligned with the interplanetary magnetic field (IMF) at frequencies in the vicinity of the proton cyclotron frequency. These events are observed about 6% of the time, preferentially associated with radially directed inward or outward IMF. Their occurrence rate and median duration do not change much with R , where R is the heliocentric radial distance. For a narrow frequency window in the solar wind frame, left-handed storms in the spacecraft frame have a power drop off that is roughly proportional to R −3 which is consistent with a source close the Sun, while right-handed storms have a power drop off roughly proportional R −1 which is not consistent with a source close to the Sun. The power in the left handed LFW storms is on average greater than the right handed ones by a factor of 3. In the solar wind frame the wave frequency decreases from 0.13 to 0.04 Hz moving from 0.3 to 0.7 AU, but the frequency normalized by the local proton cyclotron frequency does not change much with the running median varying from 0.35 to 0.5. The normalized frequency band widths of the wave power spectra increase slightly with R , possibly associated with energy dissipation.

Beschreibung: During June 16–21, 2010, an Earth-directed coronal mass ejection (CME) event was observed by instruments onboard STEREO, SOHO, MESSENGER and Wind. This event was the first direct detection of a rotating CME in the middle and outer corona. Here, we carry out a comprehensive analysis of the evolution of the CME in the interplanetary medium comparing in situ and remote observations, with analytical models and three-dimensional reconstructions. In particular, we investigate the parallel and perpendicular cross section expansion of the CME from the corona through the heliosphere up to 1 AU. We use height-time measurements and the Gradual Cylindrical Shell (GCS) technique to model the imaging observations, remove the projection effects, and derive the 3-dimensional extent of the event. Then, we compare the results with in situ analytical Magnetic Cloud (MC) models, and with geometrical predictions from past works. We find that the parallel (along the propagation plane) cross section expansion agrees well with the in situ model and with the Bothmer and Schwenn (1998) empirical relationship based on in situ observations between 0.3 and 1 AU. Our results effectively extend this empirical relationship to about 5 solar radii. The expansion of the perpendicular diameter agrees very well with the in situ results at MESSENGER (∼0.5 AU) but not at 1 AU. We also find a slightly different, from Bothmer and Schwenn (1998), empirical relationship for the perpendicular expansion. More importantly, we find no evidence that the CME undergoes a significant latitudinal over-expansion as it is commonly assumed. Instead, we find evidence that effects due to CME rotation and expansion can be easily confused in the images leading to a severe overestimation of the proper 3D size of the event. Finally, we find that the reconstructions of the CME morphology from the in situ observations at 1 AU are in agreement with the remote sensing observations but they show a big discrepancy at MESSENGER. We attribute this discrepancy to the ambiguity of selecting the proper boundaries due to the lack of accompanying plasma measurements.

Beschreibung: [1]  Hot flow anomalies (HFAs) represent a subset of solar wind discontinuities interacting with collisionless bow shocks. They are typically formed when the normal component of the motional (convective) electric field points toward the embedded current sheet on at least one of its sides. The core region of an HFA contains hot and highly deflected ion flows and rather low and turbulent magnetic field. In this paper, we report observations of possible HFA-like events at Mercury identified over a course of two planetary years. Using data from the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, we identify a representative ensemble of active current sheets magnetically connected to Mercury's bow shock. We show that some of these events exhibit magnetic and particle signatures of HFAs similar to those observed at other planets, and present their key physical characteristics. Our analysis suggests that Mercury's bow shock does not only mediate the flow of supersonic solar wind plasma but also provides conditions for local particle acceleration and heating as predicted by previous numerical simulations. Together with earlier observations of HFA activity at Earth, Venus, Mars, and Saturn, our results suggest that hot flow anomalies could be a common property of planetary bow shocks, and show that the characteristic size of these events is controlled by the bow shock standoff distance and/or local solar wind conditions.

Beschreibung: [1]  During the past decade engineering-grade magnetic field measurements from the low Earth orbiting (LEO) Iridium constellation of communication satellites have been available to the geospace science community as a tool to map field-aligned currents. The Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) applied to Iridium measurements markedly improved the temporal and spatial resolution of these data. We developed new methods to compare data from the latest improvement to AMPERE with those from a constellation of four LEO Defense Meteorological Satellite Program (DMSP) spacecraft that carry high-resolution magnetometers. To perform the comparisons we transformed all data to a common coordinate frame and altitude (110 km) and developed a means of computing spacecraft magnetic conjunctions. These conjunctions yield discrepancies in the magnetic field perturbations measured at each proximate spacecraft. During the geomagnetic disturbance of May 29–30 2010, the vector differences in the horizontal perturbations at closest approach (typically a few tens of kilometers) had mean, median, and standard deviation values of 132 nT, 112 nT, and 90 nT, respectively. The DMSP spacecraft tend to report larger perturbations in the northern polar cap and cusp regions, especially during active intervals. We attribute some of the differences to limitations of spacecraft-attitude knowledge that propagate into AMPERE data. Overall, for the magnetic storm, we provide clear evidence that AMPERE data can provide high-resolution auroral zone data in good agreement with DMSP data for use in data assimilation algorithms. Such dual-use commercial data can provide important global augmentation to the nation's space weather monitoring capabilities.

Beschreibung: We use a nonstationary generalization of the higher-order structure function technique to investigate statistical properties of the magnetic field fluctuations recorded by MESSENGER spacecraft during its first flyby (01/14/2008) through the near-Mercury space environment, with the emphasis on key boundary regions participating in the solar wind – magnetosphere interaction. Our analysis shows, for the first time, that kinetic-scale fluctuations play a significant role in the Mercury's magnetosphere up to the largest resolvable timescale (∼20 s) imposed by the signal nonstationarity, suggesting that turbulence at this planet is largely controlled by finite Larmor radius effects. In particular, we report the presence of a highly turbulent and extended foreshock system filled with packets of ULF oscillations, broad-band intermittent fluctuations in the magnetosheath, ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail, and kinetic-scale fluctuations in the inner current sheet encountered at the outbound (dawn-side) magnetopause. Overall, our measurements indicate that the Hermean magnetosphere, as well as the surrounding region, are strongly affected by non-MHD effects introduced by finite sizes of cyclotron orbits of the constituting ion species. Physical mechanisms of these effects and their potentially critical impact on the structure and dynamics of Mercury's magnetic field remain to be understood.

Beschreibung: The Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) uses magnetic field data from the Iridium constellation to derive the global Birkeland current distribution every ten minutes. We examine cases in which the interplanetary magnetic field (IMF) rotated from northward to southward resulting in onsets of the Birkeland currents. Dayside Region-1/2 currents, totaling ~25% of the final current, appear within 20 minutes of the IMF southward turning and remain steady. Onset of nightside currents occurs 40 to 70 minutes after the dayside currents appear. Thereafter, the currents intensify at dawn, dusk, and on the dayside, yielding a fully formed Region 1/2 system ~30 minutes after the nightside onset. The results imply that the dayside Birkeland currents are driven by magnetopause reconnection, and the remainder of the system forms as magnetospheric return flows start and progress sunward, ultimately closing the Dungey convection cycle.