172 research outputs found
Magnetospheric convection electric field dynamics and stormtime particle energization: Case study of the magnetic storm of 4 May 1998
No full textInternational audienceIt is shown that narrow channels of high electric field are an effective mechanism for injecting plasma into the inner magnetosphere. Analytical expressions for the electric field cannot produce these channels of intense plasma flow, and thus, result in less entry and adiabatic energization of the plasma sheet into near-Earth space. For the ions, omission of these channels leads to an underprediction of the strength of the stormtime ring current and therefore, an underestimation of the geoeffectiveness of the storm event. For the electrons, omission of these channels leads to the inability to create a seed population of 10-100 keV electrons deep in the inner magnetosphere. These electrons can eventually be accelerated into MeV radiation belt particles. To examine this, the 1-7 May 1998 magnetic storm is studied with a plasma transport model by using three different convection electric field models: Volland-Stern, Weimer, and AMIE. It is found that the AMIE model can produce particle fluxes that are several orders of magnitude higher in the L = 2 ? 4 range of the inner magnetosphere, even for a similar total cross-tail potential difference
Stormtime particle energization with high temporal resolution AMIE potentials
No full textPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95010/1/jgra17210.pd
Understanding storm-time ring current sources: Data analysis and global modeling.
No full textWith the availability of abundant, high-quality recent observations (ground-based and space-borne) and the newly-developed Space Weather Modeling Framework (SWMF), the storm-time ring current sources are examined via extensive data analyses and global storm simulations. The results of this dissertation work improve our understanding of upstream solar wind conditions, immediate plasma-sheet sources, and ionospheric outflow effect during storms. It is found in the magnetic cloud-storm study that ∼76% of magnetic clouds cause storms, but only ∼30% of storms are caused by clouds. A storm can be driven by a cloud's leading field, axial field, trailing field, or the combination thereof. It is shown that the leading field is the most geoeffective region and the sheath is equally effective at causing storms during solar maximum compared to solar minimum. A new name, quasicloud, is proposed for those cloud-like solar wind structures. Superposed epoch analyses are performed to make comparisons of typical behaviors of upstream solar wind plasma and interplanetary magnetic field (IMF), geosynchronous-orbit hot ions, and geomagnetic indices in four storm categories: moderate and in tense storms at solar minimum and maximum. The long-known control of geomagnetic activity mainly by southward IMF Bz is supported in the investigations. The averaged interplanetary causes of intense storms at solar minimum are against the well-known empirical criteria (Bz ≤ -10 nT for ≥ 3 hrs). Around storm minimum in each category, hot ions at geosynchronous orbit are denser near dawn than dusk. In the second part of the dissertation work, high-performance storm simulations are performed with the actual and superposed solar wind as input to the SWMF, respectively. It is shown that the SWMF is reaching a sophistication level for allowing quantitative comparison with observations. Major storm characteristics except for the recovery phase are successfully reproduced. It is demonstrated that ionospheric outflow plays an important role in causing a storm. IMF Bz ≤ -5 nT for ≥ 2 hrs without large oscillations in the solar wind and an increased inner boundary mass density to compensate is not geoeffective enough to cause a moderate storm. 24 M virtual geostationary satellites are introduced to make data-model comparisons straightforward.PhDAtmospheric sciencesEarth SciencesGeophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125936/2/3224794.pd
Yet another caveat to using the Dessler‐Parker‐Sckopke relation
No full textPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95475/1/jgra16950.pd
Bulk plasma properties at geosynchronous orbit.
No full textWe present a comprehensive study of plasma properties at geosynchronous orbit for electron and ion energies between ∼1 eV and ∼45 keV, between 1990 and 2001. The variations of temperature and density are analyzed as functions of local time, magnetospheric convection strength, and the strength of the ring current. Various parameters derived from temperature and density are calculated to elucidate the temporal and spatial location of delivery of plasma sheet material into the inner magnetosphere. We find that the electron and proton densities are greatest in the dawn region for the periods of highest convection and ring current strength. We perform a superposed epoch analysis of 283 geomagnetic storms which occurred between 1991 and 2001 and examine the temporal variation of the plasma at geosynchronous orbit as a function of storm phase. This analysis demonstrates the local time variability of delivery from the plasma sheet into the inner magnetosphere and the concurrent changes in temperature and pressure. We demonstrate that the density of electrons in the plasma sheet is strongly dependent upon the magnetospheric convection strength and, for the first time, upon solar activity. Electron density at geosynchronous orbit is strongly correlated with solar activity. The average plasma sheet electron density at solar maximum can be a factor of two or more higher than that at solar minimum. We also outline a method to estimate the composition of the plasma sheet from MPA measurements and calculate the O+ and H+ density variations with solar cycle as a function of Kp and local time. We show that the O+ and H+ plasma sheet densities increase with increasing solar activity, as does the O+/H+ density ratio. During times of high solar activity and strong convection, the O+ and H+ densities may be comparable
A statistical comparison of hot-ion properties at geosynchronous orbit during intense and moderate geomagnetic storms at solar maximum and minimum.
No full textHot-ion measurements at geosynchronous orbit from the Los Alamos Magnetospheric Plasma Analyzer (MPA) instrument during geomagnetic storms at solar maximum (July 1999–June 2002) and at solar minimum (July 1994–June 1997) are collected, categorized, and analyzed through the superposed epoch technique. To investigate this source of the storm-time ring current, the local time (LT) and universal time (UT) dependence of the average variations of hot-ion fluxes (at the energies of ∼30, ∼17, ∼8, and ∼1 keV), density, temperature, entropy, and temperature anisotropy are examined and compared among four storm categories, i.e., 44 intense storms and 120 moderate storms, defined by the pressure corrected Dst (Dst*), at the two solar extrema. All the hot-ion parameters are highly disturbed around Dst*min; they show distinct peaks or minima and display obvious increase or decrease regions, whose locations do not change much with levels of geomagnetic activity and solar activity. It is also found that intense storms at solar minimum always have the highest (lowest) average peak value (minimum) in each hot-ion parameter. Around Dst*min in each storm category, hot ions are clearly denser near dawn than those near dusk. On the nightside and in the afternoon sector, temperature and entropy during solar minimum storms are usually higher than those during solar maximum storms; there is actually no clear temperature and entropy enhancement during solar maximum storms. During each type of storm, hot ions are isotropic on the nightside but anisotropic (T per /T par > 1) close to noon
Geomagnetic storms driven by ICME- and CIR-dominated solar wind.
No full textThe interaction of the solar wind and the Earth's magnetosphere is complex and the phenomenology of the interaction is very different for solar wind dominated by interplanetary coronal mass ejections (ICMEs) compared to solar wind dominated by corotating interaction regions (CIRs). We perform a superposed epoch study of the effects of ICME- and CIR-dominated solar wind upon the storm-time plasma at geosynchronous orbit using data from the magnetospheric plasma analyzer (MPA) instruments on board seven Los Alamos National Laboratory (LANL) satellites. Using 78 ICME events and 32 CIR events, we examine the electron and ion plasma sheets that are formed during each type of solar wind driver, at energy-per-charge between ∼0.1 and 45 keV/q. The results demonstrate that CIR events produce a more significant modulation in the plasma sheet temperature than ICME events, whilst ICME events produce a more significant modulation in the plasma sheet density than CIR events. We attribute these differences to the average speed in the solar wind and a combination of the density of the solar wind and the ionospheric component of the plasma sheet, respectively. We also show that for CIR events, the magnitude of the spacecraft potential is, on average, significantly greater than during ICME-events, with consequent effects upon the performance of instrumentation within this environment
High‐Citation Papers in Space Physics: Examination of Gender, Country, and Paper Characteristics
No full textRelated dataset is at https://doi.org/10.7302/Z21C1V2H and also listed in the dc.relation field of the Full Work Page in Deep Blue Documents (see below)The number of citations to a refereed journal article from other refereed journal articles is a measure of its impact. Papers, individuals, journals, departments, and institutions are increasingly judged by the impact they have in their disciplines, and citation counts are now a relatively easy (though not necessarily accurate or straightforward) way of attempting to quantify impact. This study examines papers published in the Journal of Geophysical Research—Space Physics in the year 2012 (n = 705) and analyzes the characteristics of high‐citation papers compared to low‐citation papers. We find that high‐citation papers generally have a large number of authors (>5) and cite significantly more articles in the reference section than low‐citation papers. We also examined the gender and country of institution of the first author and found that there is not a statistically significant gender bias, but there are some significant differences in citation statistics between articles based on the country of first‐author institution.Plain Language SummaryThe number of citations to a refereed journal article from other refereed journal articles is a measure of its impact. Papers, individuals, journals, departments, and institutions are increasingly judged by the impact they have in their disciplines, and citation counts are now a relatively easy (though not necessarily accurate) way of attempting to quantify impact. This study examines papers published in the Journal of Geophysical Research—Space Physics and analyzes the characteristics of high‐citation papers compared to low‐citation papers. We find that high‐citation papers generally have large number of authors (>5) and cite significantly more articles in the reference section than low‐citation papers. We also found that there is not a statistically significant gender bias in terms of citation counts, but there are some significant differences in citation statistics between articles based on the country of first‐author institution.Key PointsLarge collaborative and international teams that cite the literature extensively write high‐citation papersNo gender bias is found in terms of citation rates between female and male first‐author papers, and they submit first‐author papers proportionally to their representation in the disciplineA statistically significant small difference in citations is found for papers from U.S. institutions compared to the rest of the worldPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/144280/1/jgra54185_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/144280/2/jgra54185.pd
Ring current simulations of the 90 intense storms during solar cycle 23
No full textPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94717/1/jgra19539.pd
Mars' Energetic Plume Ion Escape Channel
No full textMars is losing its atmosphere. The planet’s small size results in relatively low energy requirements for atmospheric particles to escape into deep space, and its lack of a planetary magnetic field allows the solar wind to directly interact with the upper atmosphere, providing an additional source from which particles may obtain this requisite energy. The escape of particles from Mar’s atmosphere over the course of billions of years is not only a story of atmospheric evolution; it is a story of the evolution of a global climate. It is now thought that oceans worth of liquid water may have existed on a warmer ancient Mars, and atmospheric escape of hydrogen and oxygen is one explanation of how such an ocean may have vanished. The research presented here revolves around the examination of one particular "loss channel" for oxygen (and other "heavy" ions) from Mars. This loss channel, known as the "energetic plume," consists of pickup ions, electrically charged planetary particles that, finding themselves in the solar wind flow past Mars, are accelerated in the direction of the solar wind's convective electric field (ESW). In the spatially zoomed out view, the acceleration in this direction is just the initial part of the first gyration of an ESW-cross-B drift in the direction of solar wind flow. Zoomed in closer to Mars, where ion-observing satellites have orbited, a result of the huge gyroradius of these pickup ions is that, in addition to having high energies, energetic plume particles have flight directions distinct from other escaping particles and are observed at locations not reached by other escaping particles. This dissertation introduces the Mars space environment and the problem of atmospheric escape generally before presenting the search for this distinct phase space signature of the energetic plume in ion data from the Mars Express satellite. It was found that despite the presence of obstacles to observing the energetic plume using the Ion Mass Analyzer (IMA) onboard Mars Express, it is possible to both identify unambiguous instances of energetic plume observations in IMA data and to see signatures of the energetic plume in statistical maps of the Mars space environment made using IMA observations. Furthermore, it was found that accounting for “weathervaning” – the subsolarward bending of magnetic field lines draped around the ionosphere – can be used to improve estimates of the direction of ESW. The resulting more accurate estimate for the direction of ESW improves statistical representations of the energetic plume in IMA data, and significant quantities of energetic plume type ions are observed by IMA ~ 60% more frequently in the newly estimated direction of ESW than in the previously estimated direction of ESW. We conclude that the improved method of estimating the direction of ESW should be used in place of previously existing proxies in studies concerning the variation of energetic plume fluxes for different solar conditions during the time period between Jan. 2004 and Oct. 2006.PhDAtmospheric, Oceanic & Space ScienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/143899/1/blakecjo_1.pd
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