Defense Meteorological Satellite Program Spacecraft Research Articles

Multispacecraft Observations of the Simultaneous Occurrence of Magnetic Reconnection at High and Low Latitudes During the Passage of a Solar Wind Rotational Discontinuity Embedded in the April 9‐11, 2015 ICME

AbstractNumerous questions remain about how solar wind directional discontinuities, for example, rotational discontinuities (RDs), affect energy and transport processes at the magnetopause. The impact on the dayside magnetosphere of a solar wind RD is studied. The study shows that the RD leads to complex structures at the magnetopause, boundary layer, mantle, and cusp even though the geomagnetic activity level remains low. At low altitudes, the Defense Meteorological Satellite Program spacecraft observe a double cusp that is a signature of magnetic reconnection occurring at both high and low latitudes due to the dominant IMF By, while Cluster C2, located at high‐latitude and high‐altitude in the southern hemisphere, observes velocity fluctuations and reversals with peak‐to‐peak amplitudes >800 km s−1 as it crosses the magnetopause. A global MHD simulation of the event shows that the C2 observations are consistent with the spacecraft crossing reconnection outflows while moving from one side of the X‐line to the other.

First 3D hybrid-Vlasov global simulation of auroral proton precipitation and comparison with satellite observations

The precipitation of charged particles from the magnetosphere into the ionosphere is one of the crucial coupling mechanisms between these two regions of geospace and is associated with multiple space weather effects, such as global navigation satellite system signal disruption and geomagnetically induced currents at ground level. While precipitating particle fluxes have been measured by numerous spacecraft missions over the past decades, it often remains difficult to obtain global precipitation patterns with a good time resolution during a substorm. Numerical simulations can help to bridge this gap and improve the understanding of mechanisms leading to particle precipitation at high latitudes through the global view they offer on the near-Earth space system. We present the first results on auroral (0.5–50 keV) proton precipitation within a 3-dimensional simulation of the Vlasiator hybrid-Vlasov model. The run is driven by southward interplanetary magnetic field conditions with constant solar wind parameters. We find that on the dayside, cusp proton precipitation exhibits the expected energy–latitude dispersion and takes place in the form of successive bursts associated with the transit of flux transfer events formed through dayside magnetopause reconnection. On the nightside, the precipitation takes place within the expected range of geomagnetic latitudes, and it appears clearly that the precipitating particle injection is taking place within a narrow magnetic local time span, associated with fast Earthward plasma flows in the near-Earth magnetotail. Finally, the simulated precipitating fluxes are compared to observations from Defense Meteorological Satellite Program spacecraft during driving conditions similar to those in the simulation and are found to be in good agreement with the measurements.

Occurrence Statistics of Horse Collar Aurora

AbstractHorse collar aurora (HCA) are an auroral feature where the dawn and dusk sector auroral oval moves polewards and the polar cap becomes teardrop shaped. They form during prolonged periods of northward interplanetary magnetic field (IMF), when the IMF clock angle is small. Their formation has been linked to dual‐lobe reconnection (DLR) closing magnetic flux at the dayside magnetopause. The conditions necessary for DLR are currently not well‐understood therefore understanding HCA statistics will allow DLR to be studied in more detail. We have identified over 600 HCA events between 2010 and 2016 in UV images captured by the Special Sensor Ultraviolet Spectrographic Imager instrument on‐board the Defense Meteorological Satellite Program spacecraft F16, F17 and F18. As expected, there is a clear preference for HCA occurring during northward IMF. We find no clear seasonal dependence in their occurrence, with an average of 8 HCA events per month. The occurrence of HCA events does not appear to depend on the Bx component of the IMF. Considering the average radiance intensity across the dusk‐dawn meridian shows the HCA as a separate bulge inside the auroral oval and that the dawn side arc of the HCA is usually brighter than the dusk in the Lyman‐Birge‐Hopfield short band. We relate this to the expected field aligned current pattern of HCA formation. We further suggest that transpolar arcs observed in the dawn sector simultaneously in both northern and southern hemispheres are misidentified HCA.

Observational Properties of 15MLT‐PCA in the Southern Hemisphere and the Switching Effects of IMF By on 15MLT‐PCA Occurrence

AbstractBased on Special Sensor Ultraviolet Spectrographic Imager on Defense Meteorological Satellite Program spacecraft (DMSP/SSUSI) observations in the Northern Hemisphere, a particular polar cap arc observed at ∼1,500 magnetic local time (MLT) sector was identified and was named 15MLT‐PCA. A 15MLT‐PCA model proposed for the Northern Hemisphere suggests that the 15MLT‐PCA is caused by a lobe reconnection related to the tilt angle and lobe field line twisting due to a long‐lasting positive interplanetary magnetic field (IMF) By condition. However, it is unclear if the 15MLT‐PCA can be observed in the Southern Hemisphere and how the IMF By influences its occurrence and location. Using the long‐term DMSP/SSUSI observations in the Southern Hemisphere, we confirmed that the 15MLT‐PCA could be observed in the Southern Hemisphere in summer under long‐lasting negative IMF By and predominantly positive IMF Bx. The ion energy dispersion and some other statistical properties are also investigated. Further, we showed that the IMF By has a switching effect on the occurrence of the 15MLT‐PCA using multiple cases. We found that the 15MLT‐PCA disappear as soon as the IMF By changed to an unfavorable condition, but it seems that the favorable conditions need to be maintained for ∼1 hr for the occurrence of 15MLT‐PCA. We also found that the IMF By magnitude is well correlated to the MLT location of the 15MLT‐PCA. We suggest that all the results support the 15MLT‐PCA model and imply that a magnetotail lobe reconnection should be common in the summer hemisphere because the well‐known Parker Spiral favors meeting the model's IMF requirements.

Hemispheric Asymmetries in Poynting Flux Derived From DMSP Spacecraft

AbstractWe provide high‐resolution maps of quasi‐static Poynting flux (PF) in each hemisphere based on nine‐satellite years of Defense Meteorological Satellite Program (DMSP) data. Conjugate comparisons from ∼850 km reveal more quasi‐static PF arriving in the northern hemisphere (NH) than the southern hemisphere (SH). This tendency is clear in the dawn‐dusk sectors and during intervals when Kp < 3, which accounts for ∼80% of the study interval. Summer‐to‐summer comparisons indicate this asymmetry is partially associated with more NH solar illumination, which supports stronger NH field‐aligned currents (FAC). Differing hemispheric FAC configurations may also play a role. Our findings support and broaden earlier reports of similar NH preference for the deposition of Alfvenic PF. Regionally the NH has stronger dusk‐region PF, while the SH has stronger mid‐morning PF. We find PF deposition in the near‐cusp regions that rivals and often exceeds the PF intensity in the auroral zones.

Statistical Analysis of Joule Heating and Thermosphere Response During Geomagnetic Storms of Different Magnitudes

AbstractDuring a geomagnetic storm, the energy deposition from magnetosphere to thermosphere through Joule heating leads to variations in the total mass density of thermosphere. In this study, measurements obtained from the Challenging Minisatellite Payload and Gravity Recovery and Climate Experiment satellites between 2002 and 2008 are used to study the response of thermospheric density during geomagnetic storms of different magnitudes inferred from the Dst index. Based on the height‐integrated Joule heating with data derived from the Defense Meteorological Satellite Program spacecraft, we statistically analyze 265 geomagnetic storm cases to study the characteristics of Joule heating distribution and its influence on thermospheric density. The results show that between thermospheric density and Joule heating during weak and moderate geomagnetic storms, the time lag is only about 0–2 hr, while the same for intense storms is 3–5 hr. This indicates that the time lag is positively correlated with the intensity of geomagnetic storms. Besides, it is found that the peak duration of Joule heating increases for more intense geomagnetic storms.

Fundamental Climate Data Records of Microwave Brightness Temperatures

An intercalibrated Fundamental Climate Data Record (FCDR) of brightness temperatures (Tb) has been developed using data from a total of 14 research and operational conical-scanning microwave imagers. This dataset provides a consistent 30+ year data record of global observations that is well suited for retrieving estimates of precipitation, total precipitable water, cloud liquid water, ocean surface wind speed, sea ice extent and concentration, snow cover, soil moisture, and land surface emissivity. An initial FCDR was developed for a series of ten Special Sensor Microwave/Imager (SSM/I) and Special Sensor Microwave Imager Sounder (SSMIS) instruments on board the Defense Meteorological Satellite Program spacecraft. An updated version of this dataset, including additional NASA and Japanese sensors, has been developed as part of the Global Precipitation Measurement (GPM) mission. The FCDR development efforts involved quality control of the original data, geolocation corrections, calibration corrections to account for cross-track and time-dependent calibration errors, and intercalibration to ensure consistency with the calibration reference. Both the initial SSMI(S) and subsequent GPM Level 1C FCDR datasets are documented, updated in near real-time, and publicly distributed.

Advances in Space Weather Data Interpretation and Simulations

This editorial highlights recent data and model results for Space Weather readers. One of the ways that American Geophysical Union (AGU) features some of the most interesting journal articles is Editors' Highlights. As of late 2017 these are now hosted on the Eos website: https://eos.org/editor-highlights, giving them new prominence. Editors contribute a short summary of the highlighted article in language that will appeal to general Eos readers and include a “teaser” line and visually attractive figure from the manuscript. Doing so helps to share scientific content beyond a journal's readership to a broader audience. However, occasionally a situation arises, wherein an Editor sees a manuscript worthy of additional notoriety, but the summary simply needs more technical wording than might be appropriate for general readers. Below, I am highlighting two such manuscripts that illustrate advances in Space Weather (SWE). Based on analysis of total electron content estimated from Global Positioning System (GPS) and Defense Meteorological Satellite Program spacecraft measurements, Katamzi-Joseph et al. (2017) report the occurrence of ionospheric depletions (plasma bubbles) at anomalously high, European latitudes (42°N geographic) during two intense geomagnetic storms in 2000 and 2001. Somewhat counterintuitively, such depletions may develop in the postsunset sector where plasma density has been previously enhanced. GPS signals often scintillate passing through these bubbles. The authors suggest that conditions supporting rapid upward plasma drift near the equator can extend to the midlatitude ionosphere during storm time, making these regions susceptible to navigation signal scintillation during intense geomagnetic storms. They estimate a maximum apex height of the postsunset plasma depletions as ~4,000 km. The data for the two intense storms show that the evening enhancement of the equatorial ionization anomaly extended to middle-latitude Europe during the time of the plasma bubble occurrence. The authors propose an eastward penetrating electric field-associated with the interplanetary Bz southward turning as a possible mechanism for plasma enhancement and subsequent middle-latitude plasma bubbles. Although the midlatitude ionosphere is generally considered safe from radio signal scintillation effects, this study confirms that there may be no safe zone from space-based signal scintillation during intense storms. (See Mendillo et al., 2018; Wei et al., 2015, and references, within these for previous examples and further impacts.) The Space Weather Prediction Center (SWPC) now runs subsets of the models that comprise the Space Weather Modeling Framework (SWMF). See http://www.swpc.noaa.gov/news/announcing-geospace-model-version-15. Haiducek et al. (2017) simulated a month of geospace response for January 2005 using SWMF with the University of Michigan global magnetohydrodynamic code Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme (BATS-R-US) coupled to the Rice Convection Model (RCM) inner magnetosphere and the Ridley Ionosphere Model. The interval included three large, previously simulated, and well-studied geospace storms. The authors launched the simulation three times. The first launch was with a standard 0.25 Earth radii (RE) SWMF resolution, corresponding to the operational version at SWPC. Then a higher (0.125 RE) resolution grid was applied with and without the RCM inner magnetosphere model. Various indices of geomagnetic activity (Kp, Sym-H, and AL) and the cross polar cap potential (CPCP), which is a proxy for geospace interaction with the solar wind, were calculated from the simulation and compared with observations and an empirical model that estimates CPCP. The SWMF model predicted the Sym-H (proxy for ring current) index very well. The Kp geomagnetic index and the CPCP are well predicted during storm conditions, but these indicators are overpredicted during quiet time. The model tends to underpredict the magnitude of the auroral electrojet, AL index. The Generally, the grid resolution has limited influence for the accuracy of the predictions except for auroral activity. However, the use of the inner magnetosphere model is essential. For the CPCP there is little effect from turning off the RCM. The Sym-H, AL, and Kp indexes, however, are very different when the inner magnetosphere model was turned off. This study is a significant step in showing how well one of the new SWPC models performs during storm conditions and in detailing the sensitivity in model driving. Importantly, the authors provide access to the model output for others to use. As an applications journal, SWE content covers data, model description and validation, and aspects of engineering that challenge even our technically savvy readers. So there are times when technical terms and acronyms might be problematic for a short highlight aimed at a wider readership. You can help SWE Editors promote and publicize as much content as possible by providing illustrative graphics with easy-to-follow captions and by writing plain language summaries of your manuscripts. In turn your Editors will work to highlight the best work. No data were created for this editorial.

New DMSP Database of Precipitating Auroral Electrons and Ions.

Since the mid 1970's, the Defense Meteorological Satellite Program (DMSP) spacecraft have operated instruments for monitoring the space environment from low earth orbit. As the program evolved, so to have the measurement capabilities such that modern DMSP spacecraft include a comprehensive suite of instruments providing estimates of precipitating electron and ion fluxes, cold/bulk plasma composition and moments, the geomagnetic field, and optical emissions in the far and extreme ultraviolet. We describe the creation of a new public database of precipitating electrons and ions from the Special Sensor J (SSJ) instrument, complete with original counts, calibrated differential fluxes adjusted for penetrating radiation, estimates of the total kinetic energy flux and characteristic energy, uncertainty estimates, and accurate ephemerides. These are provided in a common and self-describing format that covers 30+ years of DMSP spacecraft from F06 (launched in 1982) through F18 (launched in 2009). This new database is accessible at the National Centers for Environmental Information (NCEI) and the Coordinated Data Analysis Web (CDAWeb). We describe how the new database is being applied to high latitude studies of: the co-location of kinetic and electromagnetic energy inputs, ionospheric conductivity variability, field aligned currents and auroral boundary identification. We anticipate that this new database will support a broad range of space science endeavors from single observatory studies to coordinated system science investigations.

Regional differences of the ionospheric response to the July 2012 geomagnetic storm

AbstractThe July 2012 geomagnetic storm is an extreme space weather event in solar cycle 24, which is characterized by a southward interplanetary geomagnetic field lasting for about 30 h below −10 nT. In this work, multiple instrumental observations, including electron density from ionosondes, total electron content (TEC) from Global Positioning System, Jason‐2, and Gravity Recovery and Climate Experiment, and the topside ion concentration observed by the Defense Meteorological Satellite Program spacecraft are used to comprehensively present the regional differences of the ionospheric response to this event. In the Asian‐Australian sector, an intensive negative storm is detected near longitude ~120°E on 16 July, and in the topside ionosphere the negative phase is mainly existed in the equatorial region. The topside and bottomside TEC contribute equally to the depletion in TEC, and the disturbed electric fields make a reasonable contribution. On 15 July, the positive storm effects are stronger in the Eastside than in the Westside. The topside TEC make a major contribution to the enhancement in TEC for the positive phases, showing the important role of the equatorward neutral winds. For the American sector, the equatorial ionization anomaly intensification is stronger in the Westside than in the Eastside and shows the strongest feature in the longitude ~110°W. The combined effects of the disturbed electric fields, composition disturbances, and neutral winds cause the complex storm time features. Both the topside ion concentrations and TEC reveal the remarkable hemispheric asymmetry, which is mainly resulted from the asymmetry in neutral winds and composition disturbances.

The MAGIC of CINEMA: first in-flight science results from a miniaturised anisotropic magnetoresistive magnetometer

Abstract. We present the first in-flight results from a novel miniaturised anisotropic magnetoresistive space magnetometer, MAGIC (MAGnetometer from Imperial College), aboard the first CINEMA (CubeSat for Ions, Neutrals, Electrons and MAgnetic fields) spacecraft in low Earth orbit. An attitude-independent calibration technique is detailed using the International Geomagnetic Reference Field (IGRF), which is temperature dependent in the case of the outboard sensor. We show that the sensors accurately measure the expected absolute field to within 2% in attitude mode and 1% in science mode. Using a simple method we are able to estimate the spacecraft's attitude using the magnetometer only, thus characterising CINEMA's spin, precession and nutation. Finally, we show that the outboard sensor is capable of detecting transient physical signals with amplitudes of ~ 20–60 nT. These include field-aligned currents at the auroral oval, qualitatively similar to previous observations, which agree in location with measurements from the DMSP (Defense Meteorological Satellite Program) and POES (Polar-orbiting Operational Environmental Satellites) spacecraft. Thus, we demonstrate and discuss the potential science capabilities of the MAGIC instrument onboard a CubeSat platform.

The relation between transpolar potential and reconnection rates during sudden enhancement of solar wind dynamic pressure: OpenGGCM‐CTIM results

AbstractThis study investigates how solar wind energy is deposited into the magnetosphere‐ionosphere system during sudden enhancements of solar wind dynamic pressure (Psw), using the coupled Open Geospace General Circulation Model–Coupled Ionosphere Thermosphere Model (OpenGGCM‐CTIM) 3‐D global magnetosphere‐ionosphere‐thermosphere model. We simulate three unique events of solar wind pressure enhancements that occurred during negative, near‐zero, and positive interplanetary magnetic field (IMF) Bz. Then, we examine the behavior of the dayside and nightside reconnection rates and quantify their respective contributions to cross polar cap potential (CPCP), a proxy of ionospheric plasma convection strength. The modeled CPCP increases after a Psw enhancement in all three cases, which agrees well with observations from the Defense Meteorological Satellite Program spacecraft and predictions from the assimilative mapping of ionospheric electrodynamics technique. In the OpenGGCM‐CTIM model, dayside reconnection increases within 9–13 min of the pressure impact, while nightside reconnection intensifies about 13–25 min after the pressure increase. As the strong Psw compresses the dayside magnetosheath and, subsequently, the magnetotail, their magnetic fields intensify and activate stronger antiparallel reconnection on the dayside magnetopause first and near the central plasma sheet second. For southward IMF, dayside reconnection contributes to the CPCP enhancement 2–4 times more than nightside reconnection. For northward IMF, the dayside contribution weakens, and nightside reconnection contributes more to the CPCP enhancement. We find that high‐latitude magnetopause reconnection during northward IMF produces sunward ionospheric plasma convection, which decreases the typical dawn‐to‐dusk ionosphere electric field. This results in a weaker dayside reconnection contribution to the CPCP during northward IMF.

Comparison of magnetic perturbation data from LEO satellite constellations: Statistics of DMSP and AMPERE

AbstractDuring 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 29–30 May 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.

Field‐aligned current reconfiguration and magnetospheric response to an impulse in the interplanetary magnetic field BY component

AbstractWhen the interplanetary magnetic field (IMF) is dawnward or duskward, magnetic merging between the IMF and the geomagnetic field occurs near the cusp on the dayside flanks of the magnetosphere. During these intervals, flow channels in the ionosphere with velocities in excess of 2 km/s have been observed, which can deposit large amounts of energy into the high‐latitude thermosphere. In this study, we analyze an interval on 5 April 2010 where there was a strong dawnward impulse in the IMF, followed by a gradual decay in IMF magnitude at constant clock angle. Data from the Sondrestrom incoherent scatter radar and the Defense Meteorological Satellite Program spacecraft were used to investigate ionospheric convection during this interval, and data from the Active Magnetospheric and Planetary Electrodynamics Response Experiment (AMPERE) were used to investigate the associated Field‐Aligned Current (FAC) system. Additionally, data from AMPERE were used to investigate the time response of the dawnside FAC pair. We find there is a delay of approximately 1.25 h between the arrival of the dawnward IMF impulse at the magnetopause and strength of the dawnward FAC pair, which is comparable to substorm growth and expansion time scales under southward IMF. Additionally, we find at the time of the peak FAC, there is evidence of a reconfiguring four‐sheet FAC system in the morning local time sector of the ionosphere. Additionally, we find an inverse correlation between the dawn FAC strength and both the solar wind Alfvénic Mach number and the SYM‐H index. No statistically significant correlation between the FAC strength and the solar wind dynamic pressure was found.

Observations of He+concentrations in the topside ionosphere under equinox conditions near solar maximum: Morningside morphology

[1] Average He+ concentrations are examined for solar maximum conditions at equinox using data from Defense Meteorological Satellite Program (DMSP) flights F13 and F15. The DMSP spacecraft have Sun-synchronous orbits at 800 km altitude. F13 data were taken under presunrise conditions, while F15 data were taken at midmorning local times. Data are averaged for 31 days centered on the northern spring and fall equinoxes between ±40° magnetic latitude. Both the He+ concentration and He+ fraction show significant longitudinal and latitudinal variation, which can be accounted for by neutral wind induced ion drag. The He+ fraction can reach as much as 40% of the total plasma density in the presunrise hours, but 10% or less of the total plasma density at midmorning. While the He+ concentrations before sunrise and at midmorning have comparable magnitudes, the midmorning values of the He+ fraction are much more uniform than in the presunrise data. Examination of the F10.7 cm solar flux and geomagnetic activity implies solar flux has a much greater influence on He+ concentrations. The level of solar flux as well as the amount of variability in the solar flux appear to influence the average He+ concentrations and fractions.

Large magnetic storm‐induced nighttime ionospheric flows at midlatitudes and their impacts on GPS‐based navigation systems

Analysis of GPS phase fluctuations in conjunction with regional total electron content (TEC) maps, in situ measurements of subauroral polarization streams (SAPS) and auroral convection from several Defense Meteorological Satellite Program spacecraft, and dynasonde measurements at the Bear Lake Observatory obtained during the intense magnetic storm of 7–8 November 2004 have indicated the serious impact of large ionospheric velocities on GPS‐based navigation systems within the midlatitude region in the North American sector. The major difference between this superstorm and the others observed during the earlier October‐November 2003 events is the absence of appreciable storm‐enhanced density gradients, with the midlatitude region being enveloped by either the auroral oval or the ionospheric trough within which the SAPS were confined during the local dusk to nighttime hours. This shows that it is possible to disable GPS‐based navigation systems for many hours even in the absence of appreciable TEC gradients, provided an intense flow channel is present in the ionosphere during nighttime hours. The competing effects of irregularity amplitude ΔN/N, the background F region density, and the magnitude of SAPS or auroral convection are discussed in establishing the extent of the region of impact on such systems.

Separation of spatial and temporal structure of auroral particle precipitation

Knowledge of the dominant temporal and spatial scales of auroral features is instrumental in understanding the various mechanisms responsible for auroral particle precipitation. Single spacecraft data always suffer from temporal/spatial ambiguity. In an effort to separate the temporal and spatial variations of the aurora, we use electron and ion precipitation data from two co‐orbiting satellites, F6 and F8 of the Defense Meteorological Satellite Program (DMSP). The two spacecraft have almost identical polar orbits with a small difference in period. As a result the time difference between the two measurements varies with time. We use two statistical tools in order to determine the most probable lifetimes and spatial dimensions of the prevalent auroral features. The first tool is cross‐correlation analysis between the magnetic latitude series of electron and ion, number and energy fluxes measured by the two DMSP spacecraft. As one spacecraft overtakes the other, the variable time lag between the two measurements results in different cross‐correlation of the two series. We explore the dependence of this variation on the time lag between the satellites. We find that the electron precipitation exhibits a decreasing correlation between the two spacecraft with increasing time lag, whereas there is only a small similar effect for the ion precipitation data. The second statistical tool is cross‐spectral analysis, for which we compute the so‐called coherence function as a function of frequency (or inverse wavelength) and hence size of the auroral features. The coherence function is a measure of the stability of auroral features of different sizes. We investigate its variation as a function of the time separation between the two measurements. We show that the coherence function of both electrons and ions remains high for up to 1.5 min spacecraft separations for all features larger than about 100 km in width. For smaller features the coherence is lower even for time lags of a few seconds. The results are discussed in the context of characteristic temporal and spatial auroral scales deduced from complementary studies and expected from theory.

Interplanetary control of thermospheric densities during large magnetic storms

During the main phase of large magnetic storms significant energy can be deposited in the ionosphere but produce no commensurate magnetic perturbations on the ground. Consequently, models designed to predict and specify thermospheric energy budgets based on ground magnetic data are negatively impacted. To quantify these effects we compare thermospheric densities predicted by the MSIS model with those inferred from accelerometer measurements by the Gravity Recovery and Climate Experiment (GRACE) satellites during two magnetic storm periods in 2004. Although predictions and measurements are in substantial agreement during quiet times, the model significantly underpredicts densities during storms. Also, the model's maxima occur several hours after observed stormtime peaks. We show that polar cap potentials and magnetospheric electric fields derived from interplanetary parameters measured by the Advanced Composition Explorer satellite are roughly proportional to neutral densities observed by GRACE with lead times of ∼4 h. Finally, ion drift meter data from Defense Meteorological Satellite Program spacecraft suggest that unpredicted positive and negative spikes found in high latitude accelerometer data reflect encounters with strong head and tail thermospheric winds driven by anti-sunward convecting plasma.

Polynya Signature Simulation Method polynya area in comparison to AMSR-E 89GHz sea-ice concentrations in the Ross Sea and off the Adélie Coast, Antarctica, for 2002–05: first results

AbstractThe Polynya Signature Simulation Method (PSSM) is applied to Special Sensor Microwave/Imager observations from different Defense Meteorological Satellite Program spacecraft for 2002–05 to analyze the polynya area in the Ross Sea (Ross Ice Shelf polynya (RISP) and Terra Nova Bay polynya (TNBP)) and off the Adélie Coast (Mertz Glacier polynya (MGP)), Antarctica, on a sub-daily scale. The RISP and the MGP exhibit similar average total polynya areas. Major area changes (>10000km2; TNPB: >2000km2) occur over a range of 2–3 to 20 days in all regions. Sub-daily area changes are largest for the MGP (5800km2) and smallest for the TNBP (800km2), underlining the persistence of the forcing of the latter. ARTIST sea-ice (ASI) algorithm concentration maps obtained using 89 GHz Advanced Microwave Scanning Radiometer (AMSR-E) data are compared to PSSM maps, yielding convincing agreement in the average, similarly detailed winter polynya distribution. Average ASI algorithm ice concentrations take values of 25–40% and 65–80% for the PSSM open-water and thin-ice class, respectively. The discrepancy with expected values (0% and 100%) can be explained by the different spatial resolution and frequency used by the methods. A new land mask and a mask to flag icebergs are introduced. Comparison of PSSM maps with thermal ice thickness based on AVHRR infrared temperature and ECMWF ERA-40 data suggests an upper thickness limit for the PSSM thin-ice class of 20–25 cm.

First tomographic image of ionospheric outflows

An image of the dayside low‐energy ion outflow event that occurred on 16 December 2003 was constructed with ground‐ and space‐based GPS (Global Positioning System) Total Electron Content (TEC) data and ion drift meter data from the DMSP (Defense Meteorological Satellite Program). A tomographic reconstruction technique has been applied to the GPS TEC data obtained from the GPS receiver on the Low Earth Orbit (LEO) satellite FedSat. The two dimensional tomographic image of the topside ionosphere and plasmasphere reveals a spectacular beam‐like dayside ion outflow emanating from the cusp region. The transverse components of the magnetic field in FedSat's NewMag data show the presence of field aligned current (FAC) sheets, indicating the existence of low‐energy electron precipitation in the cusp region. The DMSP ion drift data show upward ion drift velocities and upward fluxes of low‐energy ions and electrons at the orbiting height of the DMSP spacecraft in the cusp region. This study presents the first tomographic image of the flux tube structure of ionospheric ion outflows from 0.13 Re up to 3.17 Re altitude.