Evidence for potential and inductive convection during intense geomagnetic events using normalized superposed epoch analysis

The relative contribution of storm‐time ring current development by convection driven by either potential or inductive electric fields has remained an unresolved question in geospace research. Studies have been published supporting each side of this debate, including views that ring current buildup is entirely one or the other. This study presents new insights into the relative roles of these storm main phase processes. We perform a superposed epoch study of 97 intense ( Dst Min    Dst Min  > –100 nT) storms using OMNI solar wind and ground‐based data. Instead of using a single reference time for the superpositioning of the events, we choose four reference times and expand or contract each phase of every event to the average length of this phase, creating a normalized timeline for the superposed epoch analysis. Using the bootstrap method, we statistically demonstrate that timeline normalization results in better reproduction of average storm dynamics than conventional methods. Examination of the Dst reveals an inflection point in the intense storm group consistent with two‐step main phase development, which is supported by results for the southward interplanetary magnetic field and various ground‐based magnetic indices. This two‐step main‐phase process is not seen in the moderate storm timeline and data sets. It is determined that the first step of Dst development is due to potential convective drift, during which an initial ring current is formed. The negative feedback of this hot ion population begins to limit further ring current growth. The second step of the main phase, however, is found to be a more even mix of potential and inductive convection. It is hypothesized that this is necessary to achieve intense storm Dst levels because the substorm dipolarizations are effective at breaking through the negative feedback barrier of the existing inner magnetospheric hot ion pressure peak. Key Points Moderate and intense geomagnetic storms Evidence for potential and inductive convection Normalized superposed epoch analysisPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97285/1/jgra50014.pd

A new solar windâ driven global dynamic plasmapause model: 2. Model and validation

A new solar windâ driven global dynamic plasmapause (NSWâ GDP) model has been constructed based on the largest currently available database containing 49,119 plasmapause crossing locations and 3957 plasmapause profiles (corresponding to 48,899 plasmapause locations), from 18 satellites during 1977â 2015 covering four solar cycles. This model is compiled by the Levenbergâ Marquardt method for nonlinear multiparameter fitting and parameterized by VSW, BZ, SYMâ H, and AE. Continuous and smooth magnetic local time dependence controlled mainly by the solar windâ driven convection electric field ESW is also embedded in this model. Compared with previous empirical models based on our database, this new model improves the forecasting accuracy and capability for the global plasmapause. The diurnal, seasonal, and solar cycle variations of the plasmapause can be captured by the new model. The NSWâ GDP model can potentially be used to forecast the global plasmapause shape with upstream solar wind and interplanetary magnetic field parameters and corresponding predicted values of SYMâ H and AE and can also be used as input parameters for other inner magnetospheric coupling models, such as dynamic radiation belt and ring current models and even MHD models.Key PointsA new solar windâ driven global dynamic plasmapause model based on multisatellite observations is constructedThis model is parameterized by VSW, interplanetary magnetic field BZ, SYMâ H, and AE and has continuous and smooth MLT dependenceThis model is potentially applicable to inner magnetospheric research studies and space weather forecastsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138428/1/jgra53619.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138428/2/jgra53619_am.pd

A new solar windâ driven global dynamic plasmapause model: 1. Database and statistics

A large database, possibly the largest plasmapause location database, with 49,119 plasmapause crossing events from the in situ observations and 3957 plasmapause profiles (corresponding to 48,899 plasmapause locations in 1 h magnetic local time (MLT) intervals) from optical remote sensing from 1977 to 2015 by 18 satellites is compiled. The responses of the global plasmapause to solar wind and geomagnetic changes and the diurnal, seasonal, solar cycle variations of the plasmapause are investigated based on this database. It is found that the plasmapause shrinks toward the Earth globally and a clear bulge appears in the afternoon to premidnight MLT sector as the solar wind or geomagnetic conditions change from quiet to disturbed. The bulge is clearer during storm times or southward interplanetary magnetic field. The diurnal variations of the plasmapause are most probably the result of the difference between the magnetic dipole tilt and the Earth’s spin axis. The seasonal variations of the plasmapause are characterized by equinox valleys and solstice peaks. It is also found that the plasmapause approaches the Earth during high solar activity and expands outward during low solar activity. This database will help us study and understand the evolution properties of the plasmapause shape and the interaction processes of the plasmasphere, the ring current, and the radiation belts in the magnetosphere.Key PointsThe largest currently available plasmapause location database is compiled based on observations from 18 satellites from 1977 to 2015This database reveals the responses of the global plasmapause locations to solar wind and geomagnetic changesThe plasmapause locations exhibit clear MLTâ dependent diurnal, seasonal, and solar cycle variationsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138358/1/jgra53617_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138358/2/jgra53617.pd

The Inner Magnetospheric Response to Different Solar Wind Driving Conditions.

The inner magnetosphere is where communications, navigation, and military satellites are stationed and thus understanding the region is essential in everyday activities. Predicting the inner magnetospheric response to space weather events will help to protect satellites, power grids, and astronaut explorers. This study applies statistical techniques to quantitatively examine the ring current and plasmasphere dynamics using both models and data. These techniques include data analysis and data model comparisons using superposed epoch analysis along a normalized storm timeline, bootstrap analysis, descriptive statistics, and data processing. In general, this study addresses the following two questions: (1) how do different solar wind driving conditions affect storm time inner magnetospheric dynamics and (2) does the inner magnetospheric response vary depending upon storm phase and intensity. The first portion of this work applies advances statistical approaches to focus on determining the relative contribution of potential and inductive convection during storms. That study discusses the negative feedback of ring current growth and suggests substorm dipolarization as a mechanism for breaking up the hot ion population and thus reducing the negative feedback, allowing an even stronger storm to develop. The second portion of this investigation compares low-to-mid latitude geomagnetic indices. That study discusses the error between supposedly interchangeable geomagnetic indices and how the error can be used in inner magnetospheric model-to-data comparisons. The third portion of this research develops an automated method to extract the Magnetic Local Time (MLT) dependent plasmapause location from the Imager for Magnetospause-to-Aurora Global Exploration (IMAGE) satellite Extreme Ultraviolet (EUV) instrument. That work determines the location of the plasmapause throughout the intense storms that occurred during the IMAGE mission. The final portion of this research applies numerical results form the Hot Electron and Ion Drift Integrator (HEIDI) model to examine the inner magnetospheric response in terms of the solar wind driver of intense geomagnetic storms. That study shows that Interplanetary Coronal Mass Ejections (ICME) driven events are associated with a more structured electric field than Corotating Interation Regions (CIR) driven storms. Furthermore, the data and model results show that sheath-driven events behave more like magnetic cloud-driven events than CIR-driven storms.PhDAtmospheric, Oceanic and Space SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studie

The effect of storm driver and intensity on magnetospheric ion temperatures

Energy deposited in the magnetosphere during geomagnetic storms drives ion heating and convection. Ions are also heated and transported via internal processes throughout the magnetosphere. Injection of the plasma sheet ions to the inner magnetosphere drives the ring current and, thus, the storm intensity. Understanding the ion dynamics is important to improving our ability to predict storm evolution. In this study, we perform superposed epoch analyses of ion temperatures during storms, comparing ion temperature evolution by storm driver and storm intensity. The ion temperatures are calculated using energetic neutral atom measurements from the Two Wide-Angle Imaging Neutral-Atom Spectrometers (TWINS) mission. The global view of these measurements provide both spatial and temporal information. We find that storms driven by coronal mass ejections (CMEs) tend to have higher ion temperatures throughout the main phase than storms driven by corotating interaction regions (CIRs) but that the temperatures increase during the recovery phase of CIR-driven storms. Ion temperatures during intense CME-driven storms have brief intervals of higher ion temperatures than those during moderate CME-driven storms but have otherwise comparable ion temperatures. The highest temperatures during CIR-driven storms are centered at 18 magnetic local time and occur on the dayside for moderate CME-driven storms. During the second half of the main phase, ion temperatures tend to decrease in the postmidnight to dawn sector for CIR storms, but an increase is observed for CME storms. This increase begins with a sharp peak in ion temperatures for intense CME storms, likely a signature of substorm activity that drives the increased ring current

Mesoscale features in the global geospace response to the March 12, 2012 storm

The geospace response to coronal mass ejections includes phenomena across many regions, from reconnection at the dayside and magnetotail, through the inner magnetosphere, to the ionosphere, and even to the ground. Phenomena occurring in each region are often connected to each other through the magnetic field, but that field undergoes dynamic changes during storms and substorms. Improving our understanding of the geospace response to storms requires a global picture that enables us to observe all the regions simultaneously with both spatial and temporal resolution. Using the Energetic Neutral Atom (ENA) imager on the Two Wide-Angle Imaging Neutral-Atom Spectrometers (TWINS) mission, a temperature map can be calculated to provide a global view of the magnetotail. These maps are combined with in situ measurements at geosynchronous orbit from GOES 13 and 15, auroral images from all sky imagers (ASIs), and ground magnetometer measurements to examine the global geospace response of a coronal mass ejection (CME) driven event on March 12th, 2012. Mesoscale features in the magnetotail are observed throughout the interval, including prior to the storm commencement and during the main phase, which has implications for the dominant processes that lead to pressure buildup in the inner magnetosphere. Auroral enhancements that can be associated with these magnetotail features through magnetosphere-ionosphere coupling are observed to appear only after global reconfigurations of the magnetic field

Postmidnight depletion of the high‐energy tail of the quiet plasmasphere

The Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument measures the high‐energy tail of the thermal plasmasphere allowing study of topside ionosphere and inner magnetosphere coupling. We statistically analyze a 22 month period of HOPE data, looking at quiet times with a Kp index of less than 3. We investigate the high‐energy range of the plasmasphere, which consists of ions at energies between 1 and 10 eV and contains approximately 5% of total plasmaspheric density. Both the fluxes and partial plasma densities over this energy range show H+ is depleted the most in the postmidnight sector (1–4 magnetic local time), followed by O+ and then He+. The relative depletion of each species across the postmidnight sector is not ordered by mass, which reveals ionospheric influence. We compare our results with keV energy electron data from HOPE and the Van Allen Probes Electric Fields and Waves instrument spacecraft potential to rule out spacecraft charging. Our conclusion is that the postmidnight ion disappearance is due to diurnal ionospheric temperature variation and charge exchange processes.Key PointsOne to ten eV ion depletion in quiet time postmidnight sectorDepletion varies by ion species not ordered by massStrong diurnal variation in high‐energy tail of plasmaspherePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111226/1/jgra51633.pd

Local time variations of highâ energy plasmaspheric ion pitch angle distributions

Recent observations from the Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument revealed a persistent depletion in the 1â 10 eV ion population in the postmidnight sector during quiet times in the 2 < L < 3 region. This study explores the source of this ion depletion by developing an algorithm to classify 26 months of pitch angle distributions measured by the HOPE instrument. We correct the HOPE low energy fluxes for spacecraft potential using measurements from the Electric Field and Waves (EFW) instrument. A high percentage of low count pitch angle distributions is found in the postmidnight sector coupled with a low percentage of ion distributions peaked perpendicular to the field line. A peak in loss cone distributions in the dusk sector is also observed. These results characterize the nature of the dearth of the near 90° pitch angle 1â 10 eV ion population in the nearâ Earth postmidnight sector. This study also shows, for the first time, lowâ energy HOPE differential number fluxes corrected for spacecraft potential and 1â 10 eV H+ fluxes at different levels of geomagnetic activity.Key PointsDeveloped new pitch angle sorting algorithm for Van Allen ProbesFound 90 degree pitch angle population depletion in nearâ Earth postmidnight sectorCorrected low energy HOPE ion fluxes for spacecraft potentialPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134226/1/jgra52723_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134226/2/jgra52723.pd

Hiss or equatorial noise? Ambiguities in analyzing suprathermal ion plasma wave resonance

Previous studies have shown that lowâ energy ion heating occurs in the magnetosphere due to strong equatorial noise emission. Observations from the Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument recently determined that there was a depletion in the 1â 10 eV ion population in the postmidnight sector of Earth during quiet times at L < 3. The diurnal variation of equatorially mirroring 1â 10 eV H+ ions at 2 < L < 3 is connected with similar diurnal variation in the electric field component of plasma waves ranging between 150 and 600 Hz. Measurements from the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) data set are used to analyze waves of this frequency in nearâ Earth space. However, when we examine the polarization of the waves in the 150 to 600 Hz range in the equatorial plane, the majority are rightâ hand polarized plasmaspheric hiss waves. The 1â 10 eV H+ equatorially mirroring population does not interact with rightâ hand waves, despite a strong statistical relationship suggesting that the two are linked. We present evidence supporting the relationship, both in our own work and the literature, but we ultimately conclude that the 1â 10 eV H+ heating is not related to the strong enhancement of 150 to 600 Hz waves.Key PointsA 1â 10 eV ion loss from plasma wave interactionHighâ amplitude plasma waves seem like probable candidatePolarization analysis reveals that the waves are hissPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134763/1/jgra52995.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134763/2/jgra52995_am.pd