ALBERT

Description: [1]  We devise an approach to calculate the polarization electric field in the ionosphere, when the ionospheric conductances, the primary (modeled) or the total (measured) electric field, and the Cowling efficiency are given. In contrast to previous studies, our approach is a general solution which is not limited to specific geometrical setups, and all parameters may have any kind of spatial dependence. The solution technique is based on spherical elementary current (vector) systems (SECS). This way, we avoid the need to specify explicit boundary conditions for the searched polarization electric field of its potential which would be required if the problem was solved in a differential equation approach. Instead, we solve an algebraic matrix equation, and the implicit boundary condition that the divergence of the polarization electric field vanishes outside our analysis area is sufficient. In order to illustrate our theory, we then apply it to two simple models of auroral electrodynamic situations, the first being a mesoscale strong conductance enhancement in the early morning sector within a relatively weak southward primary electric field, and a morning sector auroral arc with only a weak conductance enhancement, but a large southward primary electric field at the poleward flank of the arc. While the significance of the polarization electric field for maximum Cowling efficiency is large for the first case, it is rather minor for the second one. Both models show that the polarization electric field effect may not only change the magnitude of the current systems, but also their overall geometry. Further, the polarization electric field may extend into regions where the primary electric field is small, thus even dominating the total electric field in these regions. For the first model case, the total Joule heating integrated over the analysis area decreases by a factor of about 4 for maximum Cowling efficiency as compared to the case of vanishing Cowling efficiency. Further, for this case the resulting total electric field structurally shows a strong resemblance to that frequently observed during auroral omega band events.

Description: The question to which extent the divergence of the Hall current can be connected to the Pedersen current or to the closure current in the magnetosphere through field-aligned currents (FACs), that is, the Cowling channel process in the polar region, has long been debated but not fully understood. The present study reformulates the Cowling channel by introducing a two-layer model consisting of Hall and Pedersen conductivity layers with channel boundaries not only in the direction perpendicular to the channel but also in the direction along it. This new model enables us to better and more physically understand the connection between the Hall current, Pedersen current, and FAC. In particular, the finiteness of the channel along its direction enables us to understand that the primary nonzero electric field along the channel and FACs at the channel boundaries that faced each other in the channel direction carries the necessary energy for the Hall current to set up the secondary electric field from the magnetosphere. A case for a possible connection between the Pedersen and Hall currents is shown based on a polar current system derived from the Kamide-Richmond-Matsushita method. A more comprehensive analysis based on data is presented in the companion paper.

Description: The Cowling channel mechanism describes the creation of a secondary polarization electric field at sharp conductance boundaries in the ionosphere due to excess charges for the case in which the release of these charges to the magnetosphere is fully or partially impeded. The secondary currents generated by the polarization electric field effectively modify the effective ionospheric conductivity inside the Cowling channel. While the Cowling mechanism is generally accepted for the equatorial electrojet, there is a long-standing discussion about the importance of this mechanism and its efficiency in the auroral electrojet. We present a statistical investigation that enables us to identify the most probable geospace conditions and MLT locations for a high Cowling efficiency. This investigation is based on more than 1600 meridional profiles of data from the Magnetometers-Ionospheric Radars-All-sky Cameras Large Experiment (MIRACLE) network in Scandinavia, in particular, ground magnetic field data from the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network and electric field data from the Scandinavian Twin Auroral Radar Experiment (STARE) radar, supported with pointwise ionospheric conductance measurements from the European Incoherent Scatter (EISCAT) radar. We analyze the data in the framework of a 3-D ionospheric model, but our data set is filtered so that only electrojet-type situations are included so that the gradients of all measured quantities in longitudinal direction can be neglected. The analysis results in a steep peak of high Cowling channel efficiency probability in the early morning sector (0245–0645 MLT), with the largest probability around 0500 MLT and for medium and high geomagnetic activity. In agreement with an earlier single-event study by Amm and Fujii (2008), this indicates that the Cowling mechanism may be most effective in the early morning part of the central substorm bulge. Further, our analysis results in an almost monotonic increase of the probability of high Cowling channel efficiency with increasing geomagnetic activity.

Description: We formulate the evolution of ionospheric conductivity in the framework of 3-D M-I coupling. Two important physical processes are taken into account. One is the ionization process by precipitating mono-energetic particles, which are accelerated by parallel-potential drops in the auroral acceleration region. The other process reflects the fact that part of field-aligned current (FAC) carried by electrons is closed with a perpendicular ionic current. Here, whereas the electric current is divergence-free, the divergence of electron current is finite. Therefore, the ionospheric electron density changes, and so does the conductivity. If the energy of electron precipitation is below ∼10 eV, this second process plays an important role in plasma transportation, production, and evacuation processes. In this case the density variation does not extend in space at the perpendicular electron velocity, but it rather moves at the ion perpendicular velocity. If the energy of electron precipitation is above ∼1 keV, in contrast, the precipitation has a nonlinear effect on plasma evolution. That is, the propagation speed of the density variation increases with increasing upward-FAC density, and the propagation takes place in the direction of the converging current into the upward FAC region. The Cowling effect on the plasma evolution process is crucially important. Our formulation is more general than the previous studies and is not limited to certain geometries, current component or interaction modes between the ionosphere and magnetosphere. It is therefore better-suited for describing the self-organized M-I coupling system, which evolves with current systems, conductivity, and magnetospheric processes interacting with each other.

Description: [1]  A physical process for the latitudinal motion of an auroral arc based on the four-side bound Cowling channel model is proposed. Assuming that an upward field-aligned current (FAC) is associated with the auroral arc that forms a Cowling channel with finite lengths not only latitudinally but also longitudinally and that the upward FAC region is primarily embedded in a purely northward electric field, the primary Hall current driven by the northward electric field accumulates positive excess charges at the eastern edge of the channel and negative charges at the western edge for a perfect or partial Cowling channel with a nonzero Cowling efficiency. The charges produce a westward secondary electric field, indicating that a westward electric field can thus be produced by a purely northward primary electric field. This secondary electric field moves the arc with its magnetospheric source drifting together with the magnetospheric plasmas equatorward and simultaneously produces the electric field outside the channel that moves the downward FAC equatorward of the upward FAC region equatorward together with the upward FAC. Thus, the whole 3-D current system is expected to move equatorward as often observed in the afternoon auroral zone.

Description: In this study, we examined middle- and low-latitude Pi 2 events to address the following two issues regarding the well-known substorm current wedge (SCW) model for Pi 2 pulsations: (1) the center of the SCW, which is estimated using the Pi 2 polarization pattern, is not always collocated with that determined using the magnetic bay pattern, and (2) although ideally Pi 2 hodograms would be linear, they tend to become circular. In this study, auroral breakup events were identified from Polar Ultra Violet Imager data. We assumed that the ionospheric footprint of the upward field-aligned current (FAC) in each event was located at the position of the auroral breakup and subsequently calculated the signature of the magnetic variation at the middle-latitude station Zyryanka (ZYK; GMLAT=59.6°) that was generated by the upward FAC. In order to examine the magnetic effects of the upward FAC, we selected Pi 2 events that were observed when ZYK was located on the duskward side of the auroral breakup location. A total of 112 events were selected and analyzed in this study. It was found that the location of the upward FAC of the SCW could be estimated more accurately by using an azimuth value predicted based on the initial deflection of the middle-latitude Pi 2. Our results suggest that the circular shapes of Pi 2 polarization curves are caused by the delayed driven Alfvénic waves that are superimposed on the geomagnetic northward components of SCW oscillations.

Description: Abstract The present study shows that the ionospheric Hall polarization can deform the high‐latitude ionospheric convection field, which is widely considered to be a manifestation of the convection field in the magnetosphere. We perform the Hall polarization field separation with a potential solver by changing the conductance distribution step‐by‐step from a uniform one to a more realistic one. We adopt dawn‐dusk and north‐south symmetric distributions of conductance and region 1 (R1) field‐aligned current (FAC). The pair of the primary field of the R1 system and each gradient of Off‐diagonal component of conductance tensor (Hall conductance) generates the Hall polarization field and consequently causes potential deformations as follows. (a) The equatorward gradient causes clockwise rotation. (b) The gradient across the terminator, together with the effect of the equatorward gradient, causes the dawn‐dusk asymmetry. (c) The high conductance band in the auroral region causes kink‐type deformations. In particular, a nested structure at the equatorward edge of the band in the midnight sector well resembles the Harang Reversal. Result (a) can explain the clockwise bias inexplicable by the IMF‐By effect alone, the combination of (a) and (b) can explain the clearness and unclearness in the round or crescent shapes of the dawn‐dusk cells depending on the IMF‐By polarity, and (c) suggests that the ionosphere may not need the upward‐FAC for the formation of the Harang Reversal. We suggest that the final structure of the ionospheric potential is established by the combined effects of the magnetospheric requirements (external causes) and ionospheric polarization (internal effect).

Description: We have used chains of Magnetic Data Acquisition System (MAGDAS) magnetometers records of the horizontal (H) and vertical (Z) magnetic field intensity during September, 2008-August, 2009 (year of deep minimum) across Africa to study their variability during the quietest international days, which coincidently associated with the sudden stratospheric warming (SSW) event in January 2009. This selection of the most international quiet days is indicative of 80% that are strongly associated with days when unusually strong and prolonged sudden SSW event occur in January 2009. Interestingly, in January, significant magnitude depletion of solar quiet ( S q ) equivalent current was observed near noon hours around the magnetic equator (Addis-Ababa, ABB) compared to any other months along with a consistent significantly reduced value across the northern hemisphere and moderate decrease at the southern hemisphere. Also, we found that Nairobi (NAB) and Dares Salaam (DES) at the southern hemisphere, which are close to ABB (dip equator) are strongly prone to westward electric field compared to the magnetic equator and Khartoum (KRT) at the northern hemisphere. Significant negative values of MS q (Z) magnitudes observed near noon hours at Hermanus (HER) indicate the presence of induced currents that suggest ocean effects along with reversal to significant positive values in the afternoon, which subsided before 1800 LT in almost all the months indicate stronger influence of ionospheric currents. On seasonal variability of S q (H) , slight depression at ABB during September equinox is one of the evidences of seasonal S q focus shift. Latitudinal variability of S q near noon hours was also investigated.

Description: We present solar quiet ( S q ) variation of the horizontal ( H ) magnetic field intensity deduced from Magnetic Data Acquisition System (MAGDAS) records over Africa during an unusual strong and prolonged 2009 sudden stratospheric warming (SSW) event. A reduction in the S q H magnitude that enveloped the geomagnetic latitudes between 21.13 o N (Fayum FYM) in Egypt and 39.51 o S (Durban DRB) in South Africa was observed while the stratospheric polar temperature was increasing and got strengthened when the stratospheric temperature reached its maximum. Another novel feature associated with the hemispheric reduction is the reversal in the north-south asymmetry of the S q H , which is indicative of higher S q H magnitude in the Northern hemisphere compared to the Southern hemisphere during SSW peak phase. The reversal of the equatorial electrojet (EEJ) or the counter electrojet (CEJ), was observed after the polar stratospheric temperature reached its maximum. The effect of additional currents associated with CEJ was observed in the Southern hemisphere at middle latitude. Similar changes were observed in the EEJ at the South America, Pacific Ocean and Central Asia sectors. The effect of the SSW is largest in the South American sector and smallest in the Central Asian sector.

Description: In this report we present a temporal relationship between ground Pi2 and auroral kilometric radiation (AKR). We analyzed six isolated substorm events, which were observed by the MAGDAS/CPMN ground magnetometer network and the plasma wave instrument onboard the Polar satellite. We found that the time derivative of the height-integrated AKR power and the ground Pi2 D component had the same periodicity and that the two were synchronized with each other. When the D component fluctuated with the same (opposite) polarity as the magnetic bay variation, the AKR power tended to increase (decrease) during the corresponding interval. An isolated substorm event (AE ∼ 40 nT), which occurred around 10:19 UT on 24 January1997, was selected for a detailed study. The behavior of the Pi2 event can be interpreted by the substorm current wedge (SCW) and Pi2 propagation models. It is confirmed that the midlatitude and high-latitude D component oscillations can be treated as a proxy of the SCW oscillations, whereas the H component oscillations exhibited some phase shifts by the propagation delay of the Pi2 waves. That is, the temporal relation between the time derivative of the AKR power and the ground Pi2 suggests that the height-integrated AKR power was modulated coherently with the SCW oscillations.