The Contribution of Flux Transfer Events to Mercury's Dungey Cycle

Bursty dayside reconnection plays a proportionally larger role in the driving of Mercury's magnetosphere than it does at Earth. Individual bursts of reconnection, called flux transfer events (FTEs), are thought to open up to 5% of Mercury's polar cap; coupled with the much higher repetition rate of FTEs at Mercury and the short Dungey cycle timescale, this makes FTEs the major driver of Mercury's magnetosphere. However, comparison between spacecraft and ionospheric observations at Earth suggests that the terrestrial contribution of FTEs may have been severely underestimated, by making implicit assumptions about FTE structure. In this study, we consider the implications of removing these assumptions at Mercury; by considering FTE mechanisms based on longer reconnection lines, we find that the contribution of FTEs to Mercury's Dungey cycle could be 5 times greater than previously thought and that FTEs may be able to provide sufficient flux transport to drive Mercury's substorm cycle

Hot plasma in the magnetotail lobes shows characteristics consistent with closed field lines trapped in the lobes

We examine the magnetotail using data from the Hot Ion Analyzer on Cluster 1 during 2001–2009. We develop and utilise an algorithm in order to identify times during which Cluster 1 is in the magnetotail lobe but observes plasma which is hotter than our expectations of the lobe. We analyze the prevailing Interplanetary Magnetic Field (IMF) Bz conditions for our algorithm and a reference algorithm (with no particle energy criteria) and find that the periods we select are, on average, ~2 nT more towards northward IMF. Examining the temperature in the magnetotail for our periods shows that the morphology of the average temperature is consistent with the Milan et al. (2005) model of magnetotail structure during Northward IMF, in which closed field lines are prevented from convecting to the dayside, causing them and the plasma trapped on them to protrude into the magnetotail lobes. We also find evidence that ~0.5% of our identified periods may be driven by direct entry into the magnetosphere from the solar wind

Distributions of Birkeland current density observed by AMPERE are heavy‐tailed or long‐tailed

We analyze probability distributions of Birkeland current densities measured by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). We find that the distributions are leptokurtic rather than normal and they are sometimes heavy-tailed. We fit q-exponential functions to the distributions and use these to estimate where the largest currents are likely to occur. The shape and scale parameters of the fitted q-exponential distribution vary with location: The scale parameter maximises for current densities with the same polarity and in the same location as the average Region 1 current, whereas the shape parameter maximises for current densities with the same polarity and in the same location as the average Region 2 current. We find that current densities |J|≥ 0.2 μA m−2 are most likely to occur in the average Region 1 current region, and second most likely to occur in the average Region 2 current region. However, for extreme currents (|J|≥ 4.0 μA m−2), we find that the most likely location is colocated with the average Region 2 current region on the dayside, at a colatitude of 18° − 22°

Cold collisions of OH and Rb. I: the free collision

We have calculated elastic and state-resolved inelastic cross sections for cold and ultracold collisions in the Rb($^1 S$) + OH($^2 \Pi_{3/2}$) system, including fine-structure and hyperfine effects. We have developed a new set of five potential energy surfaces for Rb-OH($^2 \Pi$) from high-level {\em ab initio} electronic structure calculations, which exhibit conical intersections between covalent and ion-pair states. The surfaces are transformed to a quasidiabatic representation. The collision problem is expanded in a set of channels suitable for handling the system in the presence of electric and/or magnetic fields, although we consider the zero-field limit in this work. Because of the large number of scattering channels involved, we propose and make use of suitable approximations. To account for the hyperfine structure of both collision partners in the short-range region we develop a frame-transformation procedure which includes most of the hyperfine Hamiltonian. Scattering cross sections on the order of $10^{-13}$ cm$^2$ are predicted for temperatures typical of Stark decelerators. We also conclude that spin orientation of the partners is completely disrupted during the collision. Implications for both sympathetic cooling of OH molecules in an environment of ultracold Rb atoms and experimental observability of the collisions are discussed.Comment: 20 pages, 16 figure

Tips for writing a good recommendation letter

Bias exists in letters of recommendation, and it is reflected in the language used to describe and evaluate different candidates for countless opportunities in academia. Professional organizations are becoming more aware of this issue, and are pursuing avenues to address it. This paper discusses the type of information, that is, useful to have on hand when writing a recommendation letter, the structure of the letter, a process to follow for proof reading, when to say no, a compilation of additional resources, and tips for people asking for recommendation letters. Specifically, we discuss common grammar mistakes, the purpose of each portion of the letter, and ways conscious and unconscious bias can influence wording and structure. This paper is intended to provide a single place where people can go to learn all of the basics needed to write a strong recommendation letter, as currently available letter writing resources in the space physics community tend to focus on one aspect of letter writing

Tailward propagation of magnetic energy density variations With respect to substorm onset times

During geomagnetic substorms, around 1015 J of energy is extracted from the solar wind and processed by the Earth's magnetosphere. Prior to the onset of substorm expansion phases, this energy is thought to be largely stored as an increase in the magnetic field in the magnetotail lobes. However, how, when, and where this energy is stored and released within the magnetotail is unclear. Using data from the Cluster spacecraft and substorm onsets from Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE), we examine the variation in the lobe magnetic energy density with respect to substorm onset for 541 isolated onsets. Based on a cross‐correlation analysis and a simple model, we deduce the following: On average, the magnetic energy density increases approximately linearly in the hour preceding onset and decreases at a similar rate after onset. The timing and magnitude of these changes varies with downtail distance, with observations from the mid‐tail ( urn:x-wiley:jgra:media:jgra54303:jgra54303-math-0001) showing larger changes in the magnetic energy density that occur ∼20 min after changes in the near‐tail ( urn:x-wiley:jgra:media:jgra54303:jgra54303-math-0002). The decrease in energy density in the near‐tail region is observed before the ground onset identified by SOPHIE, implying that the substorm is driven from the magnetotail and propagates into the ionosphere. The implication of these results is that energy in the near‐tail region is released first during the substorm expansion phase, with energy conversion propagating away from the Earth with time

Extreme Birkeland Currents Are More Likely During Geomagnetic Storms on the Dayside of the Earth

We examine the statistical distribution of large‐scale Birkeland currents measured by the Active Magnetosphere and Planetary Electrodynamics Response Experiment in four unique categories of geomagnetic activity for the first time: quiet times, storm times, quiet‐time substorms, and storm‐time substorms. A novel method is employed to sort data into one of these four categories, and the categorizations are provided for future research. The mean current density is largest during substorms and its standard deviation is largest during geomagnetic storms. Current densities which are above a low threshold are more likely during substorms, but extreme currents are far more likely during geomagnetic storms, consistent with a paradigm in which geomagnetic storms represent periods of enhanced variability over quiet times. We demonstrate that extreme currents are most likely to flow within the Region 2 current during geomagnetic storms. This is unexpected in a paradigm of the current systems in which Region 1 current is generally larger

Semi-annual, annual and universal time variations in the magnetosphere and in geomagnetic activity: 1. geomagnetic data

We study the semi-annual variation in geomagnetic activity, as detected in the geomagnetic indices am, aa_{H}, AL, Dst and the four a-{\sigma} indices derived for 6-hour MLT sectors (around noon, dawn, dusk and midnight). For each we compare the amplitude of the semi-annual variation, as a fraction of the overall mean, to that of the corresponding variation in power input to the magnetosphere, P_{\alpha} , estimated from interplanetary observations. We demonstrate that the semi-annual variation is amplified in the geomagnetic data compared to that in P_{\alpha}, by a factor that is different for each index. The largest amplification is for the Dst index (factor ~10) and the smallest is for the a-{\sigm} index for the noon MLT sector (a_{sigma} -noon, factor is approximately 1.1). By sorting the data by the prevailing polarity of the Y-component (dawn-dusk) of the Interplanetary Magnetic Field (IMF) in the Geocentric Solar Equatorial (GSEQ) reference frame, we demonstrate that the Russell-McPherron (R-M) effect, in which a small southward IMF component in GSEQ is converted into geoeffective field by Earth’s dipole tilt, is a key factor for the semi-annual variations in both P_{\alpha} and geomagnetic indices. However, the variability in the southward component in the IMF in the GSEQ frame causes more variability in power input to the magnetosphere P_{alpha} than does the R-M effect, by a factor of more than two. We show that for increasingly large geomagnetic disturbances, P_{\alpha} delivered by events of large southward field in GSEQ (known to often be associated with coronal mass ejections) becomes the dominant driver and the R-M effect declines in importance and often acts to reduce geoeffectiveness for the most southward IMF in GSEQ: the semi-annual variation in large storms therefore suggests either preconditioning of the magnetosphere by average conditions or an additional effect at the equinoxes. We confirm that the very large R-M effect in the Dst index is because of a large effect at small and moderate activity levels and not in large storms. We discuss the implications of the observed “equinoctial” time-of-year (F) - Universal Time (UT) pattern of geomagnetic response, the waveform and phase of the semi-annual variations, the differences between the responses at the June and December solstices and the ratio of the amplitudes of the March and September equinox peaks. We also confirm that the UT variation in geomagnetic activity is a genuine global response. Later papers will analyse the origins and implications of the effects described

New chiral organosulfur donors related to bis(ethylenedithio)tetrathiafulvalene

Six new enantiopure chiral organosulfur donors, with structures related to BEDT-TTF, have been synthesised for use in the preparation of organic metals, starting either by double nucleophilic substitutions on the bis-mesylate of 2R,4Rpentane-2,4-diol or by a cycloaddition with subsequent elimination of acetic acid on the enol acetate of (+)-nopinone. Crystal structures of some of their radical cation triiodides salts and TCNQ complexes are reported

Increases in plasma sheet temperature with solar wind driving during substorm growth phases

During the substorm growth phase, magnetic reconnection extracts ~10^15 J from the solar wind through magnetic reconnection at the magnetopause, which is then stored in the magnetotail lobes. Plasma sheet pressure then increases to balance magnetic flux density increases in the lobes. We examine plasma sheet pressure, density and temperature during substorm growth phases using nine years of Cluster data (>316,000 data points). We show that plasma sheet pressure and temperature are higher during growth phases with higher solar wind driving whereas the density is approximately constant. We also show a weak correlation between plasma sheet temperature before onset and the minimum SuperMAG SML auroral index in the subsequent substorm. We discuss how energization of the plasma sheet before onset may result from thermodynamically adiabatic processes; how hotter plasma sheets may result in magnetotail instabilities and how this relates to the onset and size of the subsequent substorm expansion phase