On the location of dayside magnetic reconnection during an interval of duskward oriented IMF

We present space- and ground-based observations of the signatures of magnetic reconnection during an interval of duskward-oriented interplanetary magnetic field on 25 March 2004. In situ field and plasma measurements are drawn from the Double Star and Cluster satellites during traversals of the pre-noon sector dayside magnetopause at low and high latitudes, respectively. These reveal the typical signatures of flux transfer events (FTEs), namely bipolar perturbations in the magnetic field component normal to the local magnetopause, enhancements in the local magnetic field strength and mixing of magnetospheric and magnetosheath plasmas. Further evidence of magnetic reconnection is inferred from the ground-based signatures of pulsed ionospheric flow observed over an extended interval. In order to ascertain the location of the reconnection site responsible for the FTEs, a simple model of open flux tube motion over the surface of the magnetopause is employed. A comparison of the modelled and observed motion of open flux tubes (i.e. FTEs) and plasma flow in the magnetopause boundary layer indicates that the FTEs observed at both low and high latitudes were consistence with the existence of a tilted X-line passing through the sub-solar region, as suggested by the component reconnection paradigm. While a high latitude X-line (as predicted by the anti-parallel description of reconnection) may have been present, we find it unlikely that it could have been responsible for the FTEs observed in the pre-noon sector under the observed IMF conditions. Finally, we note that throughout the interval, the magnetosphere was bathed in ULF oscillations within the solar wind electric field. While no one-to-one correspondence with the pulsed reconnection rate suggested by the ground-based observation of pulsed ionospheric flow has been demonstrated, we note that similar periodicity oscillations were observed throughout the solar wind-magnetosphere-ionosphere system. These findings are consistent with previously proposed mechanisms of solar wind modulation of the dayside reconnection rate

A statistical study of the observed and modeled global thermosphere response to magnetic activity at middle and low latitudes

International audience[1] From one year (2004) of thermosphere total density data inferred from CHAMP/ STAR accelerometer measurements, we calculate the global thermosphere response to auroral magnetic activity forcing at middle and low latitudes using a method based on a singular value decomposition of the satellite data. This method allows separating the large-scale spatial variations in the density, mostly related to altitude/latitude variations and captured by the first singular component, from the time variations, down to timescales on the order of the orbital period, which are captured by the associated projection coefficient. This projection coefficient is used to define a disturbance coefficient that characterizes the global thermospheric density response to auroral forcing. For quiet to moderate magnetic activity levels (Kp < 6), we show that the disturbance coefficient is better correlated with the magnetic am indices than with the magnetic ap indices. The latter index is used in all empirical thermosphere models to quantify the auroral forcing. It is found that the NRLMSISE-00 model correctly estimates the main features of the thermosphere density response to geomagnetic activity, i.e., the morphology of Universal Time variations and the larger relative increase during nighttime than during daytime. However, it statistically underestimates the amplitude of the thermosphere density response by about 50%. This underestimation reaches 200% for specific disturbed periods. It is also found that the difference between daytime and nighttime responses to auroral forcing can statistically be explained by local differences in magnetic activity as described by the longitude sector magnetic indices. Citation: Lathuillère, C., M. Menvielle, A. Marchaudon, and S. Bruinsma (2008), A statistical study of the observed and modeled global thermosphere response to magnetic activity at middle and low latitudes

Electrodynamics of a flux transfer event: Experimental test of the Southwood model.

On 12 September 1999, a conjunction between two SuperDARN radars and the Ørsted satellite gave, for the first time, simultaneous access to the ionospheric convection enhancement and the field-aligned currents (FACs) associated with a Flux Transfer Event. The radars observed an azimuthally elongated convection flow burst and the Ørsted satellite observed a series of successive small-scale parallel currents alternating between downward and upward. The most poleward pair of currents, whose directions were in agreement with the Southwood model, was observed when Ørsted crossed the front edge of the flow burst. A quantitative comparison of the current density of each FAC and of the Pedersen current density indicates that the closure current for this FACs pair occurred inside the flow burst, confirming the validity of the Southwood model. The Poynting flux carried by the parallel currents was less than 1% of the power carried by the solar wind plasma

Mechanism for the formation of the high-altitude stagnant cusp: Cluster and SuperDARN observations

On 16 March 2002, Cluster moved from nightside to dayside, across the high-altitude northern cusp during an extended period of relatively steady positive IMF BY and BZ. Combined Cluster and SuperDARN data imply the existence of two reconnection sites: in the high- latitude northern hemisphere dusk and southern hemisphere dawn sectors. Within the cusp, Cluster encounters 3 distinct plasma regions. First, injections of magnetosheath-like plasma associated with dawnward and sunward convection suggest Cluster crosses newly- reconnected field lines related to the dusk reconnection site. Second, Cluster observes a Stagnant Exterior Cusp (SEC), characterized by nearly isotropic and stagnant plasma. Finally, Cluster crosses a region with significant antifield-aligned flows. We suggest the observed SEC may be located on newly re-closed field lines, reconnected first poleward of the northern hemisphere cusp and later reconnected again poleward of the southern hemisphere cusp. We discuss how the Cluster observations correspond to expectations of ’double reconnection’ model

Temporal evolution and electric potential structure of the auroral acceleration region from multispacecraft measurements

Bright aurorae can be excited by the acceleration of electrons into the atmosphere in violation of ideal magnetohydrodynamics. Modelling studies predict that the accelerating electric potential consists of electric double layers at the boundaries of an acceleration region but observations suggest that particle acceleration occurs throughout this region. Using multi-spacecraft observations from Cluster we have examined two upward current regions on 14 December 2009. Our observations show that the potential difference below C4 and C3 changed by up to 1.7 kV between their respective crossings, which were separated by 150 s. The field-aligned current density observed by C3 was also larger than that observed by C4. The potential drop above C3 and C4 was approximately the same in both crossings. Using a novel technique of quantitatively comparing the electron spectra measured by Cluster 1 and 3, which were separated in altitude, we determine when these spacecraft made effectively magnetically conjugate observations and use these conjugate observations to determine the instantaneous distribution of the potential drop in the AAR. Our observations show that an average of 15% of the potential drop in the AAR was located between C1 at 6235 km and C3 at 4685 km altitude, with a maximum potential drop between the spacecraft of 500~V and that the majority of the potential drop was below C3. By assuming a spatial invariance along the length of the upward current region, we discuss these observations in terms of temporal changes and the vertical structure of the electrostatic potential drop and in the context of existing models and previous observations single- and multi-spacecraft observations

Equatorial Ionosphere Characterization for Sub-Saharan Africa SBAS

Performance Based Navigation (PBN) is a concept developed by ICAO (International Civil Aviation Organization) that specifies the operational performance required in an airspace, route or approach procedure. A Satellite Based Augmentation System (SBAS) enhances the performances of the existing satellite navigation system. It is used to deploy Global Navigation Satellite System (GNSS) approach for PBN procedures. The required performance level for vertical guidance is directly linked to approach category criteria. The real performance provided by an SBAS for a single-frequency user depends on the physical characteristics of the ionospheric layer. As Sub-Saharan Africa corresponds to geomagnetic equator region, the question of ionosphere dynamics characterization in equatorial zone is central to gauge what SBAS performance level can be achievable. In the equatorial zone the dynamics of the ionosphere is subject to complex physical phenomena, involving rapid recombination of ion-electron pairs. Moreover these phenomena are transient with high local spatial and temporal gradients. These zones promote the occurrence of scintillation phenomena, bubbles (strong local fall of TEC (Total Electron Content)), and small scale gradients, which must be evaluated for the ionosphere modeling and integrity data generation. Based on a large volume of GNSS measurements covering more than four years of data collected, Thales Alenia Space associated with IRAP (Astrophysics and Planetology Research Institute, Toulouse, France), present a panorama of observed physical events through the ionosphere in Sub-Saharan Africa zone. The main purpose of this study is to establish a clear view on the physical mechanisms that drive the equatorial ionosphere dynamics and the effects on GNSS measurements. This study is supported by information coming from TEC values, TEC gradients amplitudes, and the nature of scintillation events as intensity, impact area and occurrence in time. Conclusion of these activities is to highlight that ionosphere conditions above sub-Saharan area are consistent with the performances level of SBAS approach with vertical guidance. Indeed scientific analyses show that a precise service level is possible on this zone with a very good level of availability above the main airports

EISCAT Observations of Depleted High-Latitude F-Region During an HSS/SIR-Driven Magnetic Storm

Abstract The effect of storms driven by solar wind high-speed streams (HSSs) on the high-latitude ionosphere is inadequately understood. We study the ionospheric F-region during a moderate magnetic storm on 14 March 2016 using the EISCAT Tromsø and Svalbard radar latitude scans. AMPERE field-aligned current (FAC) measurements are also utilized. Long-duration 5-day electron density depletions (20%–80%) are the dominant feature outside of precipitation-dominated midnight and morning sectors. Depletions are found in two major regions. In the afternoon to evening sector (12–21 magnetic local time, MLT) the depleted region is 10°–18° magnetic latitude (MLAT) in width, with the largest latitudinal extent 62°–80° MLAT in the afternoon. The second region is in the morning to pre-noon sector (04–10 MLT), where the depletion region occurs at 72°–80° MLAT within the auroral oval and extends to the polar cap. Using EISCAT ion temperature and ion velocity data, we show that local ion-frictional heating is observed roughly in 50% of the depleted regions with ion temperature increase by 200 K or more. For the rest of the depletions, we suggest that the mechanism is composition changes due to ion-neutral frictional heating transported by neutral winds. Even though depleted F-regions may occur within any of the large-scale FAC regions or outside of them, the downward FAC regions (R2 in the afternoon and evening, R0 in the afternoon, and R1 in the morning) are favored, suggesting that downward currents carried by upward moving ionospheric electrons may provide a small additional effect for depletion.Abstract The effect of storms driven by solar wind high-speed streams (HSSs) on the high-latitude ionosphere is inadequately understood. We study the ionospheric F-region during a moderate magnetic storm on 14 March 2016 using the EISCAT Tromsø and Svalbard radar latitude scans. AMPERE field-aligned current (FAC) measurements are also utilized. Long-duration 5-day electron density depletions (20%–80%) are the dominant feature outside of precipitation-dominated midnight and morning sectors. Depletions are found in two major regions. In the afternoon to evening sector (12–21 magnetic local time, MLT) the depleted region is 10°–18° magnetic latitude (MLAT) in width, with the largest latitudinal extent 62°–80° MLAT in the afternoon. The second region is in the morning to pre-noon sector (04–10 MLT), where the depletion region occurs at 72°–80° MLAT within the auroral oval and extends to the polar cap. Using EISCAT ion temperature and ion velocity data, we show that local ion-frictional heating is observed roughly in 50% of the depleted regions with ion temperature increase by 200 K or more. For the rest of the depletions, we suggest that the mechanism is composition changes due to ion-neutral frictional heating transported by neutral winds. Even though depleted F-regions may occur within any of the large-scale FAC regions or outside of them, the downward FAC regions (R2 in the afternoon and evening, R0 in the afternoon, and R1 in the morning) are favored, suggesting that downward currents carried by upward moving ionospheric electrons may provide a small additional effect for depletion

Total Electron Content Variations During an HSS/SIR-Driven Geomagnetic Storm at High and Mid Latitudes

Abstract Two interacting high‐speed solar wind streams (HSSs) and associated stream interaction regions (SIR) caused a moderate geomagnetic storm during 14–20 March 2016. The spatio‐temporal evolution of the total electron content (TEC) during the storm is studied by using Global Navigation Satellite System (GNSS) data. The moderate storm caused significant and long‐lasting changes on TEC within the polar cap (70°–90° MLAT), at auroral and sub‐auroral latitudes (60°–70° MLAT), and at mid‐latitudes (40°–60° MLAT). A 25%–50% depletion in TEC was observed for six days in the day, dusk and dawn sectors in the polar cap region and in the day and dusk sectors at the auroral and sub‐auroral latitudes. Sub‐auroral polarization streams observed by the Defense Meteorological Satellite Program satellite contributed to the sub‐auroral dusk TEC decreases. At mid‐latitudes, TEC depletion was observed in all local time sectors 21 hr after the storm onset. It is suggested that ion‐neutral frictional heating causes the TEC depletions, which is further supported by the observed spatial correlation between TEC depletions and ∑O/N2 decreases at mid‐latitudes observed by TIMED/GUVI. The storm induced a prolonged positive phase at mid‐latitudes lasting 9 hr. In the polar cap, enhancements of TEC up to 200% were caused by polar cap patches. TEC increases were the dominant feature in the night and morning sectors within the auroral oval because of particle precipitation and resulted up to regionally averaged 6 TECU (200%) increases.Abstract Two interacting high‐speed solar wind streams (HSSs) and associated stream interaction regions (SIR) caused a moderate geomagnetic storm during 14–20 March 2016. The spatio‐temporal evolution of the total electron content (TEC) during the storm is studied by using Global Navigation Satellite System (GNSS) data. The moderate storm caused significant and long‐lasting changes on TEC within the polar cap (70°–90° MLAT), at auroral and sub‐auroral latitudes (60°–70° MLAT), and at mid‐latitudes (40°–60° MLAT). A 25%–50% depletion in TEC was observed for six days in the day, dusk and dawn sectors in the polar cap region and in the day and dusk sectors at the auroral and sub‐auroral latitudes. Sub‐auroral polarization streams observed by the Defense Meteorological Satellite Program satellite contributed to the sub‐auroral dusk TEC decreases. At mid‐latitudes, TEC depletion was observed in all local time sectors 21 hr after the storm onset. It is suggested that ion‐neutral frictional heating causes the TEC depletions, which is further supported by the observed spatial correlation between TEC depletions and ∑O/N2 decreases at mid‐latitudes observed by TIMED/GUVI. The storm induced a prolonged positive phase at mid‐latitudes lasting 9 hr. In the polar cap, enhancements of TEC up to 200% were caused by polar cap patches. TEC increases were the dominant feature in the night and morning sectors within the auroral oval because of particle precipitation and resulted up to regionally averaged 6 TECU (200%) increases