Claudia Stolle

Lunar‐dependent equatorial ionospheric electrodynamic effects during sudden stratospheric warmings

[1]xa0We have used plasma drift and magnetic field measurements during the 2001–2009 December solstices to study, for the first time, the longitudinal dependence of equatorial ionospheric electrodynamic perturbations during sudden stratospheric warmings. Jicamarca radar measurements during these events show large dayside downward drift (westward electric field) perturbations followed by large morning upward and afternoon downward drifts that systematically shift to later local times. Ground-based magnetometer measurements in the American, Indian, and Pacific equatorial regions show strongly enhanced electrojet currents in the morning sector and large reversed currents (i.e., counterelectrojets) in the afternoon sector with onsets near new and full moons during northern winter warming periods. CHAMP satellite and ground-based magnetic field observations indicate that the onset of these equatorial afternoon counterelectrojets is longitude dependent. Our results indicate that these large electrodynamic perturbations during stratospheric warming periods are due to strongly enhanced semidiurnal lunar wave effects. The results of our study can be used for forecasting the occurrence and evolution of these electrodynamic perturbations during arctic winter warmings.

Magnetic signatures of equatorial spread F as observed by the CHAMP satellite

[1]xa0Magnetic observations on board the CHAMP satellite are used for the first comprehensive study of magnetic signatures of the postsunset equatorial spread F (ESF) events. This is derived from a continuous database covering the years 2001–2004. On the basis of an extended survey, the global distribution of magnetic signatures is derived. We find a distinct seasonal/longitudinal variation of the occurrence rate of magnetic signatures that is consistent with that obtained from previous satellite observations of plasma depletions. The latitudinal distribution of the ESF magnetic signatures from CHAMP is symmetrical about the dip equator. It can be approximated by two Gaussian curves peaking at ±9.5° magnetic latitude, both exhibiting an 1σ-width of 4.5°. We further find a close relation between the occurrence frequency and the solar EUV flux. The global average of the occurrence rate is linearly proportional to solar activity attaining ∼0.1% times the F10.7 value. The response of the ESF magnetic signatures to geomagnetic activity is also investigated. However, only a weak relation between the signature occurrence rate and the Kp index is found. Using high-resolution magnetic field measurements of the ESF structures, we are able to identify very small spatial scales of spread F of only few tens of meters. The vector magnetic field observations provide experimental evidence of the electromagnetic characteristics of ESF, valuable for testing model predictions. Finally, we discuss the effect of the ESF phenomenon on magnetic field modeling efforts based on satellite data.

Resolution of direction of oceanic magnetic lineations by the sixth‐generation lithospheric magnetic field model from CHAMP satellite magnetic measurements

The CHAMP satellite continues to provide highly accurate magnetic field measurements from decreasing orbital altitudes (<350 km) at solar minimum conditions. Using the latest 4 years (2004–2007) of readings from the CHAMP fluxgate magnetometer, including an improved scalar data product, we have estimated the lithospheric magnetic field to spherical harmonic degree 120, corresponding to 333 km wavelength resolution. The data were found to be sensitive to crustal field variations up to degree 150 (down to 266 km wavelength), but a clean separation of the lithospheric signal from ionospheric and magnetospheric noise sources was achieved only to degree 120. This new MF6 model is the first satellite-based magnetic model to resolve the direction of oceanic magnetic lineations, revealing the age structure of oceanic crust.

Estimating the daytime Equatorial Ionization Anomaly strength from electric field proxies

[1]xa0The Equatorial Ionization Anomaly (EIA) is a significant feature of the low-latitude ionosphere. During daytime, the eastward electric field drives a vertical plasma fountain at the magnetic equator creating the EIA. Since the eastward electric field is also the driving force for the Equatorial Electrojet (EEJ), the latter is positively correlated with the EIA strength. We investigate the correlation between the zonal electric field and the EIA in the Peruvian sector and compare the results with correlations of the EEJ versus EIA strength. Analyzing 5 years of Challenging Minisatellite Payload (CHAMP) electron density measurements, plasma drift readings from the Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere (JULIA) radar, and magnetic field observations at Huancayo and Piura, we find the EEJ strength and the zonal electric field to be suitable proxies for the EIA intensity. Both analyses reveal high correlation coefficients of cc > 0.8. A typical response time of the EIA to variations in the zonal electric field is ∼1–2 h, and it is ∼2–4 h after EEJ strength variations. Quantitative expressions are provided, which directly relate the EIA parameters to both proxies. From these relations, we infer that an EIA develops also during weak Counter Electrojets (CEJs), but no EIA forms when the vertical plasma drift is zero. For positive EEJ magnetic signatures to form, a minimum eastward electric field of 0.2 mV/m is required on average. The above-mentioned delay between EIA and EEJ variations of ∼3 h is further confirmed by the investigation of the EIA response to transitions from CEJ to EEJ, e.g., during late morning hours.

The Swarm satellite loss of GPS signal and its relation to ionospheric plasma irregularities

In this study we investigated conditions for loss of GPS signals observed by the Swarm satellites during a 2u2009year period, from December 2013 to November 2015. Our result shows that the Swarm satellites encountered most of the total loss of GPS signal at the ionization anomaly crests, between ±5° and ±20° magnetic latitude, forming two bands along the magnetic equator, and these low-latitude events mainly appear around postsunset hours from 19:00 to 22:00 local time. By further checking the in situ electron density measurements of Swarm, we found that practically, all the total loss of GPS signal events at low latitudes are related to equatorial plasma irregularities (EPIs) that show absolute density depletions larger than 10u2009×u20091011u2009m−3; then, the Swarm satellites encountered for up to 95% loss of GPS signal for at least one channel and up to 45% tracked less than four GPS satellites (making precise orbit determination impossible). For those EPIs with density depletions less than 10u2009×u20091011u2009m−3, the chance of tracked GPS signals less than four reduces to only 1.0%. Swarm also observed total loss of all GPS signal at high latitudes, mainly around local noon, and these events are related to large spatial density gradients due to polar patches or increased geomagnetic/auroral activities. We further found that the loss of GPS signals were less frequent after appropriate settings of the Swarm GPS receivers had been updated. However, the more recent period of the mission, e.g., after the GPS receiver settings have been updated, also coincides with less severe electron density depletions due to the declining solar cycle, making GPS loss events less likely. We conclude that both lower electron density gradients and appropriate GPS receiver settings reduce the probability for Swarm satellites loss of GPS signals.

Scale analysis of equatorial plasma irregularities derived from Swarm constellation

In this study, we investigated the scale sizes of equatorial plasma irregularities (EPIs) using measurements from the Swarm satellites during its early mission and final constellation phases. We found that with longitudinal separation between Swarm satellites larger than 0.4°, no significant correlation was found any more. This result suggests that EPI structures include plasma density scale sizes less than 44xa0km in the zonal direction. During the Swarm earlier mission phase, clearly better EPI correlations are obtained in the northern hemisphere, implying more fragmented irregularities in the southern hemisphere where the ambient magnetic field is low. The previously reported inverted-C shell structure of EPIs is generally confirmed by the Swarm observations in the northern hemisphere, but with various tilt angles. From the Swarm spacecrafts with zonal separations of about 150xa0km, we conclude that larger zonal scale sizes of irregularities exist during the early evening hours (around 1900 LT).

A dayside plasma depletion observed at midlatitudes during quiet geomagnetic conditions

In this study we investigate a dayside, midlatitude plasma depletion (DMLPD) encountered on 22 May 2014 by the Swarm and GRACE satellites, as well as ground-based instruments. The DMLPD was observed near Puerto Rico by Swarm near 10 LT under quiet geomagnetic conditions at altitudes of 475–520u2009km and magnetic latitudes of ∼25°–30°. The DMLPD was also revealed in total electron content observations by the Saint Croix station and by the GRACE satellites (430u2009km) near 16u2009LT and near the same geographic location. The unique Swarm constellation enables the horizontal tilt of the DMLPD to be measured (35° clockwise from the geomagnetic east-west direction). Ground-based airglow images at Arecibo showed no evidence for plasma density depletions during the night prior to this dayside event. The C/NOFS equatorial satellite showed evidence for very modest plasma density depletions that had rotated into the morningside from nightside. However, the equatorial depletions do not appear related to the DMLPD, for which the magnetic apex height is about 2500u2009km. The origins of the DMLPD are unknown, but may be related to gravity waves.

Nighttime magnetic field fluctuations in the topside ionosphere at midlatitudes and their relation to medium-scale traveling ionospheric disturbances: The spatial structure and scale sizes

Previous studies suggested that electric and/or magnetic field fluctuations observed in the nighttime topside ionosphere at midlatitudes generally originate from quiet time nocturnal medium-scale traveling ionospheric disturbances (MSTIDs). However, decisive evidences for the connection between the two have been missing. In this study we make use of the multispacecraft observations of midlatitude magnetic fluctuations (MMFs) in the nighttime topside ionosphere by the Swarm constellation. The analysis results show that the area hosting MMFs is elongated in the NW-SE (NE-SW) direction in the Northern (Southern) Hemisphere. The elongation direction and the magnetic field polarization support that the area hosting MMFs is nearly field aligned. All these properties of MMFs suggest that they have close relationship with MSTIDs. Expectation values of root-mean-square field-aligned currents associated with MMFs are up to about 4xa0nA/m2. MMF coherency significantly drops for longitudinal distances of ≥1∘.

Statistical survey of nighttime midlatitude magnetic fluctuations: Their source location and Poynting flux as derived from the Swarm constellation

This is the first statistical survey of field fluctuations related with medium-scale traveling ionospheric disturbances (MSTIDs), which considers magnetic field, electric field, and plasma density variations at the same time. Midlatitude electric fluctuations (MEFs) and midlatitude magnetic fluctuations (MMFs) observed in the nighttime topside ionosphere have generally been attributed to MSTIDs. Although the topic has been studied for several decades, statistical studies of the Poynting flux related with MEF/MMF/MSTID have not yet been conducted. In this study we make use of electric/magnetic field and plasma density observations by the European Space Agencys Swarm constellation to address the statistical behavior of the Poynting flux. We have found that (1) the Poynting flux is directed mainly from the summer to winter hemisphere, (2) its magnitude is larger before midnight than thereafter, and (3) the magnitude is not well correlated with fluctuation level of in situ plasma density. These results are discussed in the context of previous studies.

The role of high-resolution geomagnetic field models for investigating ionospheric currents at low Earth orbit satellites

Low Earth orbiting geomagnetic satellite missions, such as the Swarm satellite mission, are the only means to monitor and investigate ionospheric currents on a global scale and to make in situ measurements of F region currents. High-precision geomagnetic satellite missions are also able to detect ionospheric currents during quiet-time geomagnetic conditions that only have few nanotesla amplitudes in the magnetic field. An efficient method to isolate the ionospheric signals from satellite magnetic field measurements has been the use of residuals between the observations and predictions from empirical geomagnetic models for other geomagnetic sources, such as the core and lithospheric field or signals from the quiet-time magnetospheric currents. This study aims at highlighting the importance of high-resolution magnetic field models that are able to predict the lithospheric field and that consider the quiet-time magnetosphere for reliably isolating signatures from ionospheric currents during geomagnetically quiet times. The effects on the detection of ionospheric currents arising from neglecting the lithospheric and magnetospheric sources are discussed on the example of four Swarm orbits during very quiet times. The respective orbits show a broad range of typical scenarios, such as strong and weak ionospheric signal (during day- and nighttime, respectively) superimposed over strong and weak lithospheric signals. If predictions from the lithosphere or magnetosphere are not properly considered, the amplitude of the ionospheric currents, such as the midlatitude Sq currents or the equatorial electrojet (EEJ), is modulated by 10–15xa0% in the examples shown. An analysis from several orbits above the African sector, where the lithospheric field is significant, showed that the peak value of the signatures of the EEJ is in error by 5xa0% in average when lithospheric contributions are not considered, which is in the range of uncertainties of present empirical models of the EEJ.