Two Wide-angle Imaging Neutral-atom Spectrometers Research Articles

Terrestrial exospheric dayside H-density profile at 3–15 <i>R</i><sub>E</sub> from UVIS/HDAC and TWINS Lyman-<i>α</i> data combined

Abstract. Terrestrial ecliptic dayside observations of the exospheric Lyman-α column intensity between 3–15 Earth radii (RE) by UVIS/HDAC (UVIS – ultraviolet imaging spectrograph; HDAC – hydrogen-deuterium absorption cell) Lyman-α photometer at CASSINI have been analyzed to derive the neutral exospheric H-density profile at the Earth's ecliptic dayside in this radial range. The data were measured during CASSINI's swing-by maneuver at the Earth on 18 August 1999 and are published by Werner et al. (2004). In this study the dayside HDAC Lyman-α observations published by Werner et al. (2004) are compared to calculated Lyman-α intensities based on the 3D H-density model derived from TWINS (Two Wide-angle Imaging Neutral-atom Spectrometers) Lyman-α observations between 2008–2010 (Zoennchen et al., 2015). It was found that both Lyman-α profiles show a very similar radial dependence in particular between 3–8 RE. Between 3.0–5.5 RE impact distance Lyman-α observations of both TWINS and UVIS/HDAC exist at the ecliptic dayside. In this overlapping region the cross-calibration of the HDAC profile against the calculated TWINS profile was done, assuming that the exosphere there was similar for both due to comparable space weather conditions. As a result of the cross-calibration the conversion factor between counts per second and rayleigh, fc=3.285 counts s−1 R−1, is determined for these HDAC observations. Using this factor the radial H-density profile for the Earth's ecliptic dayside was derived from the UVIS/HDAC observations, which constrained the neutral H density there at 10 RE to a value of 35 cm−3. Furthermore, a faster radial H-density decrease was found at distances above 8 RE (≈r-3) compared to the lower distances of 3–7 RE (≈r-2.37). This increased loss of neutral H above 8 RE might indicate a higher rate of H ionization in the vicinity of the magnetopause at 9–11 RE (near subsolar point) and beyond, because of increasing charge exchange interactions of exospheric H atoms with solar wind ions outside the magnetosphere.

Average Ring Current Response to Solar Wind Drivers: Statistical Analysis of 61 Days of ENA Images

AbstractThis paper presents the first determination of the average global and regional response of ring current (RC) flux intensity versus four key solar wind (SW) and interplanetary magnetic field (IMF) parameters: SW electric field (ESW), IMF polarity (BZ,IMF), SW pressure (PSW), and SW speed (VSW). We analyze energetic neutral atom (ENA) images from Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) to measure the RC response, and use 5‐min downstream‐propagated data from the Advanced Composition Explorer (ACE) and Wind to measure SW driving. Our imaging data comprises 61 events, including 1,838 global 2D equatorial maps of line‐of‐sight (LOS) averaged ion flux derived from background‐subtracted, 15 min TWINS ENA images spanning 1–30 keV. Average spatial and spectral distributions were organized into four driver strength groups for each SW parameter. ESW has the strongest correlation with RC ion flux (coefficients are [0.95, 0.94, 0.88] at [1, 16, 30] keV, respectively), confirming that magnetospheric convection is a primary driver. In case studies, the global RC responds rapidly (≤15 min) to changes in ESW ranging from strong to weak. PSW is correlated with RC intensity, though not as strongly as ESW, indicating the secondary importance of PSW‐driven compression in RC injections. Strong ESW increases lower‐energy ions farther eastward and higher‐energy ions closer to midnight, consistent with an energy‐dependent interplay between magnetic drift and convection. The strongest SW driving preferentially increases the least energetic ions, suggesting that quasi‐steady convection can load the nightside RC with cooler ions.

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.

Comparison of CIMI Simulations and TWINS Observations on June 28 and 29, 2013

AbstractA moderate geomagnetic storm (Sym/H −100 nT) during June 28 and 29, 2013 was studied using Comprehensive Inner Magnetosphere‐Ionosphere (CIMI) simulations and results were compared with Two wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) observations of the spatial and temporal evolution of ring current (RC) ions including ion pressure, anisotropy, intensity, and median energy. At the onset of the storm (11:00 UT on June 28), TWINS ion pressure peaks were located radially outward and dusk ward compared to CIMI. During the main phase and the recovery phase (06:00 UT on June 28 to 16:00 UT on June 29), southward IMF Bz and rapid AE index fluctuations were observed indicating substorm ion injections and TWINS observed multiple ion pressure peaks at least one of which were found to be close to the locations of CIMI ion pressure peaks. TWINS also observed regions of parallel pressure anisotropy near pre‐midnight sectors. The RC ion energies in TWINS were found to be very low (10 keV) indicating convective transport of ions. When the storm had nearly recovered post 17:00 UT on June 29, TWINS and CIMI observed single pressure peaks but TWINS ion pressure peaks were radially outward compared to CIMI. Also, the spatial profiles of TWINS ion energies showed gradual removal of low‐energy ions. Over all, TWINS and CIMI results were found to agree on the general features of the RC. However, effects of shielding electric fields, particle injections, and presence of ions were more prominent in TWINS data.

Mesoscale Structures in Earth's Magnetotail Observed Using Energetic Neutral Atom Imaging.

Mesoscale structures in Earth's magnetotail are a primary feature of particle transport to the inner magnetosphere during storms and substorms. We demonstrate that such structures can be observed in energetic neutral atom (ENA) data which can provide remote, global images of the magnetosphere. In particular, we present localized regions of increased ion temperatures that appear in equatorial ion temperature maps calculated from Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) ENA data. These regions are associated with a dipolarization front with bursty ion flows measured by Magnetospheric MultiScale (MMS) and are concurrent with substorm features observed in field aligned currents (FAC) from Active Magnetosphere and Planetary Electrodynamics Response Experiment measurements. We conduct a magnetohydrodynamics simulation of the same event and show simulated ion temperatures, ion flows, and FACs that agree with the measurements. However, the observed plasma heating is less intense in the simulated results than in the TWINS and MMS data, indicating that some heating processes may be missing from the model.

TWINS Observations of the Dynamics of Ring Currents Ion Spectra on March 17 and October 7, 2015

AbstractDirect comparisons between RBSP (Van Allen Probes or Radiation Belt Storm Probes) and TWINS (Two Wide‐angle Imaging Neutral‐atom Spectrometers) for the main phase of two storms, March 17 and October 7, 2015, showed agreement between the in situ ion measurements and the ion spectra from the deconvolved energetic neutral‐atom (ENA) measurements, except when O+ ions were significant. Spatial evolution of individual energy peaks in the ion spectra is studied using TWINS data. O+ ions are seen to result in intense peaks at 5–10 keV/amu in the TWINS ion spectra. These ion populations are confined to low L shells (L < 5) and localized in the premidnight sector. When H+ ions are significant, the low energy peaks (<25 keV/amu) are found to be less intense than the high energy peaks (>25 keV/amu), located at L > 4 and localized within the premidnight sector. During times of rapidly varying AE indices, two spatially distinct peaks, between 3–5RE and 6–8RE, are observed for the ions with energies >25 keV/amu. The outer peak appears for a few hours and fades while the inner peak is more stable. These structures are found to be consistent with particle injections observed in the RBSP data. When double peaked structures are swept off, low energy ions accumulate in the premidnight to midnight sectors whereas high energy ions are located premidnight to postmidnight sectors. Faster drift orbits of >25 keV/amu ions may cause this kind of distribution.

Database of Storm Time Equatorial Ion Temperatures in Earth's Magnetosphere Calculated From Energetic Neutral Atom Data

AbstractIon temperature is a key parameter that influences dynamics in the magnetosphere, such as particle transport and wave‐particle interactions. Measurements of ion heating and energization yield information about phenomena such as magnetic reconnection, bursty bulk flows, and ion injections. Taking advantage of the global view provided by energetic neutral atom imaging, a database of ion temperature maps during geomagnetic storms occurring throughout the National Aeronautics and Space Administration (NASA) Two Wide‐angle Imaging Neutral atom Spectrometers (TWINS) mission has been created. These ion temperature maps and relevant metadata are publicly available on CDAWeb to facilitate comparison to in situ measurements and model output, for use as boundary conditions for simulations, and for other relevant studies. A preliminary study of average plasma sheet ion temperatures calculated from these maps has revealed a common occurrence of decreasing ion temperature, and a case study for one storm is presented.

Global ENA Imaging and In Situ Observations of Substorm Dipolarization on 10 August 2016

AbstractThis paper presents the first combined use of data from Magnetospheric Multiscale (MMS), Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS), and Van Allen Probes (RBSP) to study the 10 August 2016 magnetic dipolarization. We report the first correlation of MMS tail observations with TWINS energetic neutral atom (ENA) images of the ring current (RC). We analyze 15‐min, 1° TWINS 2 images in 1–50 keV energy bins. To characterize the high‐altitude RC we extract peak ENA flux from L= 2.5 to 5 in the postmidnight sector. We estimate peak low‐altitude ion flux from ENAs near the Earth's limb. For a local perspective, we use spin‐averaged proton fluxes from the RBSP A Helium Oxygen Proton Electron (HOPE) spectrometer. We find that the 1000 UT dipolarization triggered an abrupt and significant increase in low‐altitude ions and a gradual but modest increase in the high‐altitude RC. The relative strength and timing of the low versus high‐altitude flux indicate that the dipolarization isotropized the injected ions and initially filled the loss cone. The substorm injection brought cooler ions in from the magnetotail, reducing the peak energy at both low and high altitudes. The post‐dipolarization low‐altitude flux exhibited a decay rate dispersion favoring longer decay times at lower energies, possibly caused by growth of the low energy RC providing enhanced flux into the loss cone. A variety of finer scale local injection structures were observed in the high‐altitude RC both before and after the dipolarization, and the average system level RC intensity increased after 1000 UT.

Tomographic Estimation of Exospheric Hydrogen Density Distributions

AbstractFor the past decade, the Lyman‐alpha detectors on board National Aeronautics and Space Administration's Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) mission have obtained routine measurements of solar Lyman‐α photons (121.6 nm) resonantly scattered by atomic hydrogen (H) in the terrestrial exosphere. These data have been used to derive global three‐dimensional (3‐D) models of exospheric H density beyond 3 RE, which are needed to understand various aspects of aeronomy and heliophysics, such as atmospheric chemistry and energetics, magnetospheric energy dissipation, ion‐neutral coupling, and atmospheric evolution through gravitational escape. These empirical distributions are obtained through parametric fitting of assumed functional forms that have little observational justification, thus limiting confidence in conclusions drawn from analysis of the resulting exospheric structure. In this work, we present a new means of global 3‐D reconstruction of exospheric H density through tomographic inversion of the scattered H Lyman‐α emission. Our approach avoids the conventional dependence on ad hoc parametric formulations and, based on the case studies reported here, appears to enable a more accurate characterization of the global structure of the H density in the outer exosphere. We evaluate the bounds of technique feasibility using simulated TWINS data and report new geophysical insights gained from applying this promising new approach to an example of actual TWINS data.

Empirical Characterization of Low‐Altitude Ion Flux Derived from TWINS

AbstractIn this study we analyze ion differential flux from 10 events between 2008 and 2015. The ion fluxes are derived from low‐altitude emissions (LAEs) in energetic neutral atom (ENA) images obtained by Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS). The data set comprises 119.44 hr of observations, including 4,284 per energy images with 128,277 values of differential ENA flux from pixels near Earth's limb. Limb pixel data are extracted and mapped to a common polar ionospheric grid and associated with values of the Dst index. Statistical analysis is restricted to pixels within 10% of the LAE emissivity peak. For weak Dst conditions we find a premidnight peak in the average ion precipitation, whose flux and location are relatively insensitive to energy. For moderate Dst, elevated flux levels appear over a wider magnetic local time (MLT) range, with a separation of peak locations by energy. Strong disturbances bring a dramatic flux increase across the entire nightside at all energies but strongest for low energies in the postmidnight sector. The arrival of low‐energy ions can lower the average energy for strong Dst, even as it raises the total integral number flux. TWINS‐derived ion fluxes provide a macroscale measurement of the average precipitating ion distribution and confirm that convection, either quasi‐steady or bursty, is an important process controlling the spatial and spectral properties of precipitating ions. The premidnight peak (weak Dst), MLT widening and energy‐versus‐MLT dependence (moderate Dst), and postmidnight low‐energy ion enhancement (strong Dst) are consistent with observations and models of steady or bursty convective transport.

Composition of 1–128 keV Magnetospheric ENAs

AbstractThe Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) mission has observed the inner magnetosphere since 2008. TWINS flies energetic neutral atom (ENA) cameras aboard two spacecraft in separate Molniya orbits. TWINS images the ENA emissions from the inner magnetosphere across a broad range of energies (1 to 128 keV for H, 16 to 128 keV for O, and higher energies for total ENAs). This allows TWINS to observe the evolution, on minute timescales, of the trapped and precipitating particles that dominate storm time dynamics. Presented here are ENA observations over this broad energy range of the large storm of 17 March 2015 (Dst < −223)—the St. Patrick's Day storm. The ENA observations are presented with a 15‐min cadence for improved counting statistics. During the St. Patrick's Day storm the flux of ENAs and the concentration of O+/H+ increased significantly during the main phase. The concentration increased to a value of 1.0 for the 16 keV ions, and the temperature in the inner magnetosphere dropped during the time between the prestorm phase and the main phase. Comparing the results from this storm to a moderate storm on 22 July 2009, and a collection of nine other large storms, we find that the most energetic O+ ions in the ring current occur before the peak of the storm.

The Big Picture: Imaging of the Global Geospace Environment by the TWINS Mission

AbstractEncircling our planet at distances of 2.5 to 8 Earth radii is a dynamic plasma population known as the ring current (RC). During geomagnetic storms, the solar wind's interaction with Earth's magnetic field pumps petaJoules of energy into the RC, energizing and transporting particles. To measure the global geospace response, RC imaging is performed by capturing energetic neutral atoms (ENAs) created by charge exchange between geospace ions and the neutral exosphere. The H exosphere is itself imaged via its geocoronal Lyman‐α glow. Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) is a stereoscopic ENA and Lyman‐α imaging mission that has recorded the deep minimum of solar cycle (SC) 23 and the moderate maximum of SC 24, observing geospace conditions ranging from utterly quiet to major storms. This review covers TWINS studies of the geospace response published during 2013 to 2017. Stereo ENA imaging has revealed new dimensionality and structure of RC ions. Continuous coverage by two imagers has allowed monitoring storms from start to finish. Deconvolution of the low‐altitude signal has extended ENA analysis and revealed causal connections between the trapped and precipitating ion populations. ENA‐based temperature and composition analyses have been refined, validated, and applied to an unprecedented sequence of solar activity changes in SC 23 and SC 24. Geocoronal imaging has revealed a surprising amount of time variability and structure in the neutral H exosphere, driven by both Sun and solar wind. Global models have been measurably improved. Routine availability of simultaneous in situ measurements has fostered huge leaps forward in the areas of ENA validation and cross‐scale studies.

The Effect of Storm Driver and Intensity on Magnetospheric Ion Temperatures

AbstractEnergy 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.

Storm time equatorial magnetospheric ion temperature derived from TWINS ENA flux

AbstractThe plasma sheet plays an integral role in the transport of energy from the magnetotail to the ring current. We present a comprehensive study of the equatorial magnetospheric ion temperatures derived from Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) energetic neutral atom (ENA) measurements during moderate to intense (Dstpeak < −60 nT) storm times between 2009 and 2015. The results are validated using ion temperature data derived from the Geotail low‐energy particle energy analyzer and the Los Alamos National Laboratory magnetospheric plasma analyzer. The ion temperatures are analyzed as a function of storm time, local time, and L shell. We perform a normalized superposed epoch analysis of 48 geomagnetic storms and examine the spatial and temporal evolution of the plasma as a function of storm phase. This analysis illustrates the spatial and temporal variation of the ions from the plasma sheet into the inner magnetosphere. We find that the ion temperature increases approaching the storm peak. This enhancement has the largest magnetic local time extent near 12 RE distance downtail.

Cross‐scale observations of the 2015 St. Patrick's day storm: THEMIS, Van Allen Probes, and TWINS

AbstractWe present cross‐scale magnetospheric observations of the 17 March 2015 (St. Patrick's Day) storm, by Time History of Events and Macroscale Interactions during Substorms (THEMIS), Van Allen Probes (Radiation Belt Storm Probes), and Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS), plus upstream ACE/Wind solar wind data. THEMIS crossed the bow shock or magnetopause 22 times and observed the magnetospheric compression that initiated the storm. Empirical models reproduce these boundary locations within 0.7 RE. Van Allen Probes crossed the plasmapause 13 times; test particle simulations reproduce these encounters within 0.5 RE. Before the storm, Van Allen Probes measured quiet double‐nose proton spectra in the region of corotating cold plasma. About 15 min after a 0605 UT dayside southward turning, Van Allen Probes captured the onset of inner magnetospheric convection, as a density decrease at the moving corotation‐convection boundary (CCB) and a steep increase in ring current (RC) proton flux. During the first several hours of the storm, Van Allen Probes measured highly dynamic ion signatures (numerous injections and multiple spectral peaks). Sustained convection after ∼1200 UT initiated a major buildup of the midnight‐sector ring current (measured by RBSP A), with much weaker duskside fluxes (measured by RBSP B, THEMIS a and THEMIS d). A close conjunction of THEMIS d, RBSP A, and TWINS 1 at 1631 UT shows good three‐way agreement in the shapes of two‐peak spectra from the center of the partial RC. A midstorm injection, observed by Van Allen Probes and TWINS at 1740 UT, brought in fresh ions with lower average energies (leading to globally less energetic spectra in precipitating ions) but increased the total pressure. The cross‐scale measurements of 17 March 2015 contain significant spatial, spectral, and temporal structure.

Global images of trapped ring current ions during main phase of 17 March 2015 geomagnetic storm as observed by TWINS

AbstractA unique view of the trapped particles in the inner magnetosphere provided by energetic neutral atom (ENA) imaging is used to observe the dynamics of the spatial structure and the pitch angle anisotropy on a global scale during the last 6 h of the main phase of a large geomagnetic storm (minimum SYM‐H = −230 nT) that began on 17 March 2015. Ion flux and pressure anisotropy obtained from Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) ENA images are shown. The ion flux shows two peaks, an inner one at approximately radii = 3–4 RE in the dusk‐to‐midnight sector and an outer peak at radii = 8–9 RE prior to midnight. The inner peak is relatively stationary during the entire period with some intensification during the final steep decline in SYM‐H to its minimum. The outer peak shows the significant temporal variation brightening and dimming and finally disappearing at the end of the main phase. The pressure anisotropy shows the expected perpendicular pitch angles inside of L = 6 but shows parallel pitch angles at greater L values. This is interpreted as consistent with pitch angle‐dependent drift as modeled in the Tsy05 magnetic field and Comprehensive Inner Magnetosphere‐Ionosphere simulations. The TWINS results are compared directly with Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE)‐A measurements. Using 15 min snapshots of flux and pressure anisotropy from TWINS along the path of RBSPICE‐A during the 6 h focused upon in this study, the essential features displayed in the TWINS global images are supported.

Analytical estimate for low‐altitude ENA emissivity

AbstractWe formulate the first analytical model for energetic neutral atom (ENA) emissivity that partially corrects for the global viewing geometry dependence of low‐altitude emissions (LAEs) observed by Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS). The emissivity correction requires the pitch angle distribution (PAD) and geophysical location of low‐altitude ENAs. To estimate PAD, we create an energy‐dependent analytical model, based on a Monte Carlo simulation. We account for energy binning by integrating model PAD over each energy bin. We account for finite angular pixels by computing emissivity as an integral over the pitch angle range sampled by the pixel. We investigate location uncertainty in TWINS pixels by performing nine variations of the emissivity calculation. Using TWINS 2 ENA imaging data from 1131 to 1145 UT on 6 April 2010, we derive emissivity‐corrected ion fluxes for two angular pixel sizes: 4° and 1°. To evaluate the method, we compare TWINS‐derived ion fluxes to simultaneous in situ data from the National Oceanic and Atmospheric Administration (NOAA) 17 satellite. The TWINS‐NOAA agreement for emissivity‐corrected flux is improved by up to a factor of 7, compared to uncorrected flux. The highest 1° pixel fluxes are a factor of 2 higher than for 4° pixels, consistent with pixel‐derived fluxes that are artificially low because subpixel structures are smoothed out, and indicating a possible slight advantage to oversampling the instrument‐measured LAE signal. Both TWINS and NOAA ion fluxes decrease westward of 2000 magnetic local time. The TWINS‐NOAA comparison indicates that the global ion precipitation oval comprises multiple smaller‐scale (3–5° of latitude) structures.

TWINS stereoscopic imaging of multiple peaks in the ring current

AbstractGlobal, ion equatorial flux distributions and energy spectra are presented from stereoscopic Two Wide‐Angle Imaging Neutral‐Atom Spectrometers (TWINS) 1 and TWINS 2 energetic neutral atom (ENA) images for two time periods, 29 May 2010, 1330–1430 UT and 26 May 2011, 1645–1715 UT. The first is just after the main phase of a weak (minimum SYM/H ≈ −70 to −80 nT) corotating interaction region‐driven geomagnetic storm. The second is during a relatively quiet period. The global ion distributions show multiple spatial peaks that are coincident with peaks in the AE index. The energy spectra have a primary maximum in the 15–20 keV range. Below the energy maximum, the flux is Maxwellian. Above the main maximum, the flux is either significantly below that of a Maxwellian or has a second component with a maximum in the 40–50 keV range. For the 29 May 2010, 1330–1430 UT time period, the flux from the TWINS stereoscopic images is compared to the results from TWINS 1 and TWINS 2 alone illustrating the advantage of stereoscopic viewing. The flux deconvolved from the TWINS images also shows spatial and temporal correlations with Time History of Events and Macroscale Interactions during Substorms (THEMIS) in situ measurements. Magnetic field dipolarizations observed by GOES support the existence of a peak in the ion flux in the midnight/dawn sector. In summary, increased spatial resolution from TWINS stereoscopic ENA images is demonstrated. Multiple peaks in the ion flux of trapped particles in the ring current are observed. THEMIS electrostatic analyzer in situ ion flux measurements and GOES geosynchronous magnetic field measurements are consistent with the spatial and temporal structure obtained.

Regions of ion energization observed during the Galaxy‐15 substorm with TWINS

AbstractIons in the plasma sheet are measured by the energetic neutral atom imagers on the Two Wide‐Angle Imaging Neutral‐Atom Spectrometers (TWINS) spacecraft. A line of sight (LOS) projection is used to determine the location of the ions that dominate the measured spectrum, assuming that the equatorial plane is the hottest region along the LOS. We verify reasonable agreement between ion spectral shapes measured using this remote measurement technique and in situ measurements from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft during two moderate geomagnetic storms. Conditions for reliable use of this technique to determine ion spectra and effective ion temperatures are identified. The technique is applied to the substorm interval associated with the loss of communication with the Galaxy‐15 satellite. During this interval, localized and broad regions of ion energization are observed, demonstrating that magnetic reconnection and current disruption may have played a role during this very extreme event. These observations demonstrate the ability of this technique to cast local ion spectra measurements in a global context.

First results using TWINS‐derived ion temperature boundary conditions in CRCM

AbstractWe have integrated dynamic, spatiotemporally resolved ion temperature boundary conditions into the Comprehensive Ring Current Model (CRCM), which are based on 2‐D equatorial maps derived from the Two Wide‐Angle Imaging Neutral‐Atom Spectrometers (TWINS) energetic neutral atom (ENA) data. The high‐speed stream‐driven event on 22 July 2009 is simulated and compared against an identical simulation using a statistically derived boundary condition model. ENA‐derived temperatures allow users to include event‐specific observations associated with a dynamic plasma sheet. This method also provides temperatures in the important region between geosynchronous orbit and the plasma sheet, a region which existing empirical models exclude. We find that the spatial and energy distributions of ring current flux and pressure have sensitive dependence on boundary conditions during this event. The coupling of boundary conditions to the time history of the convection field strength also plays an important role by throttling the influence of the boundary plasma on the inner magnetosphere. Simulated moments and spectra from our simulations are compared with remotely imaged ion temperatures from TWINS and also in situ energy spectra and temperature moments from Time History of Events and Macroscale Interactions during Substorms‐D. Storm time dusk‐dawn asymmetries consistent with observational data, such as Zhang et al. (2006), are reproduced well when CRCM is provided with the event‐specific boundary model. A hot localized structure observed by TWINS at geosynchronous midnight during a strong northward interplanetary magnetic field interval is also reproduced with this boundary model, whereas the empirical boundary model fails to yield this feature.