When studying magnetospheric convection, it is often necessary to map the steady-state electric field, measured at some point on a magnetic field line, to a magnetically conjugate point in the other hemisphere, or the equatorial plane, or at the position of a satellite. Such mapping is relatively easy in a dipole field although the appropriate formulae are not easily accessible. They are derived and reviewed here with some examples. It is not possible to derive such formulae in more realistic geomagnetic field models. A new method is described in this paper for accurate mapping of electric fields along field lines, which can be used for any field model in which the magnetic field and its spatial derivatives can be computed. From the spatial derivatives of the magnetic field three first order differential equations are derived for the components of the normalized element of separation of two closely spaced field lines. These can be integrated along with the magnetic field tracing equations and Faraday's law used to obtain the electric field as a function of distance measured along the magnetic field line. The method is tested in a simple model consisting of a dipole field plus a magnetotail model. The method is shown to be accurate, convenient, and suitable for use with more realistic geomagnetic field models.

The Substorm Current Wedge （SCW） occurrence in the late growth and onset phases of substorms was proposed as the current system which disrupts cross-tail current by diverting it to the ionosphere. The closure current for the SCW originally was suggested to be the strong westward auroral electrojet （WEJ）. However, the SCW-WEJ system has no viable generator current. Similarly, the asymmetric or Partial Ring Current （PRC） increases in strength during the growth phase, and is sometimes associated with an enhanced Region 2 field-aligned current （FAC） closing to the ionosphere, but specifics of that closure have been lacking. Here we present a unifying picture which includes the SCW post-and pre-midnight （AM and PM, respectively） currents and a generator current in the midnight portion of the PRC system, with these currents based upon a model of the nightside magnetotail magnetic geometry. That geometry consists of open north and south lobe regions surrounding a plasmasheet with two types of closed field line regions-stretched lines in the central part of the plasmasheet （SPS） and dipolar lines （DPS） between the low latitude boundary layer （LLBL） regions and the SPS. There is also an important plasmasheet transition region （TPS） in which the dipolar field near the plasmapause gradually transforms to stretched lines near the earthward edge of the SPS, and in which the midnight part of the PRC flows. We propose that our proposed near-onset current system consists of a central current which becomes part of the midnight sector PRC and which is the generator, to which are linked two three-part current systems, one on the dawnside and one on the duskside. The three-part systems consist of up and down FACs closing as Pedersen currents in the ionosphere. These 3-part systems are not activated until near-onset is reached, because of a lack of ionospheric conductivity in the appropriate locations where the Pedersen current closure occurs. The initial downward FAC of the 3-part dawnside system and the final upward FAC of the 3-part duskside system correspond to the AM and PM current segments, respectively, of the originally proposed SCW.

The subject of dynamics is fundamental to engineering programs in general, and can be challenging to teach. In particular, the topic of Coriolis acceleration and the “Coriolis effect” may present the greatest challenge of all. Although mathematical formulations and physical explanations are available in many textbooks and technical articles, undergraduate students still have trouble making meaningful and long-lasting connections between the mathematical expressions and the physical manifestations of the phenomenon. In our opinion, a significant part of the problem can be attributed to the traditional formulation of kinematic equations using rotating frames of reference. Normally, the Coriolis acceleration is defined, derived, and explained using at least one noninertial frame. At the University of Saskatchewan, our approach is to express the equations of rigid body dynamics solely with respect to a single inertial frame of reference, and to thereby solve all the problems that would otherwise be solved using rotating frames. We also use this approach for deriving and explaining the Coriolis effect. Although our approach is, in a sense, equivalent to the traditional method, we have found that students are better able to understand and apply the principles of dynamics when working within a single inertial frame. In this paper, our pedagogical approach will be presented and then applied to the question of “what’s going on” with the Coriolis effect

The Cascade SmallSat and Ionospheric Polar Explorer (CASSIOPE) satellite is to be launched in late 2012. On board this satellite will be a suite of eight scientific instruments composing the enhanced Polar Outflow Probe (ePOP). The Radio Receiver Instrument (RRI) on ePOP will be used to receive high‐frequency (HF) (10–18 MHz) transmissions from ground transmitters such as the Super Dual Auroral Radar Network (SuperDARN) array. Modeling of the characteristics of the HF signal received at ePOP for various ionospheric conditions has been undertaken in preparation for this RRI‐SuperDARN experiment. The effect of ionospheric electron density enhancements and depletions on signal parameters such as polarization and mode delay difference has been modeled. It has been found that at HF the polarization state of the received signal is highly sensitive to regions of locally enhanced or depleted electron density in the ionosphere. In particular, analysis of the orientation angle of the received signal, which changes because of Faraday rotation as the spacecraft passes over a ground transmitter, will allow detection of small‐scale electron density structures (on the order of tens of kilometers) with electron densities as little as 10% different from background values. Because of the sensitivity of the polarization of HF transionospheric waves, the signatures of these small‐scale and relatively weak ionospheric density structures will be apparent. Larger and denser structures will also be detectable from both the polarization state and other signal parameters, such as signal delay. The modeling demonstrates that detailed analysis of the signal parameters received at the ePOP satellite will allow determination of the location, size, and density of structures in the ionosphere.

Ionospheric plasma drift velocities measured by High Frequency (HF) coherent scatter radars, such as the Super Dual Auroral Radar Network (SuperDARN), are typically underestimated, sometimes significantly, because the refractive index in the scattering volume is not known. Large‐scale or background estimates of ionospheric electron density and refractive index can be made by other instruments; however, these instruments both do not cover the large field‐of‐view of the SuperDARN radars and do not provide information about the small‐scale structures which may be very important for the scattering process. A method has been developed to use different operating frequencies of the SuperDARN radars to obtain the average scattering volume electron density. These electron density measurements provide an estimate of refractive index and allow for corrections to the SuperDARN velocity data to be made. A comprehensive analysis of all SuperDARN data since its inception almost 20 years ago has provided estimates of average electron density in the scattering volume of the radars for various magnetic latitudes, solar activities, local times, and seasons. The analysis indicates that the average electron density, and therefore refractive index, in the scattering volume can vary significantly with the various parameters. Densities ranging from less than 2 × 1011 m−3 to more than 8 × 1011 m−3, result in refractive index corrections from less than 5% (not very significant) to more than 50% (extremely significant). These results provide estimates of appropriate adjustments to the drift velocities assumed by SuperDARN for various conditions. Further, this research has provided substantial insight into the physics of the coherent scattering process and provides a method by which electron density of the scattering structures can be monitored. This will be tested using in situ high‐latitude ionospheric measurements from the upcoming enhanced Polar Outflow Probe (ePOP) satellite mission.

The Cascade Demonstrator Small-Sat and Ionospheric Polar Explorer (CASSIOPE) satellite is scheduled to be launched in 2011. The satellite will carry a suite of eight scientific instruments comprising the enhanced Polar Outflow Probe (ePOP). One instrument is the Radio Receiver Instrument (RRI) which will be used to receive HF transmissions from ground transmitters such as the Super Dual Auroral Radar Network (SuperDARN) array. Magnetoionic polarization and propagation theory has been used to model the relative power that SuperDARN delivers to the Ordinary (O) and Extraordinary (X) modes of propagation. The geometry of the radars and magnetic field results in the X-mode dominating the transmitted signal when the modelled wave propagates northward and is nearly perpendicular to the magnetic field lines. Other propagation directions (i.e., above or southwards of the radar) results in propagation which is anti-parallel to the magnetic field lines and an equal splitting of transmitted power between the O- and X-modes occurs. For either high transmitting frequencies or low ionospheric electron densities, the range of latitudes that signal will be received at the satellite is quite large (up to ^90° of latitude). Conversely, for lower transmitting frequencies or higher ionospheric electron densities, the latitudinal range that signal will be received over is smaller. These relative mode power calculations will be used to characterize the average electron density content in the ionosphere or to provide a measure of relative absorption in the D- and E-regions when the satellite passes through the field-of-view of a SuperDARN radar.

[1] Measurements of ionospheric drift velocities using HF coherent scatter radars, such as SuperDARN, are generally underestimated because the refractive index in the scattering volume has not been taken into account. Refractive index values evaluated from electron density measurements, international reference ionosphere predictions, or elevation angle measurements have been applied to SuperDARN velocities in past studies. However, the SuperDARN velocities so obtained were, on average, statistically lower than velocities measured by other instruments. One possible explanation for this underestimation is that HF coherent scatter preferentially occurs in regions of the ionosphere where the scattering cross section is largest, and such regions are characterized by small-scale structures which have higher-than-average electron densities. This was not accounted for in past studies because the refractive index estimates used were from large scale and therefore smoothed estimates of electron density. In this paper, a new method of estimating the actual electron density (or plasma frequency) at the location of SuperDARN scatter (instead of the larger-scale background electron density) is presented. This method takes advantage of the frequency shifts which occur in normal SuperDARN operations. If it is assumed that, on average, the actual ionospheric drift velocity and plasma frequency are roughly constant before and after a shift in frequency, any change in measured velocity as SuperDARN changes frequency is due to a change in refractive index. An analysis of the change in the measured velocity resulting from each shift in frequency gives an experimentally based estimate of the electron density in the scattering volume. A statistical analysis of essentially all frequency shifts by SuperDARN and the estimated electron densities in the scattering volume has been performed. The resulting electron densities are appreciably higher than previous methods to estimate electron density predict. Application of this new method to velocity comparisons between SuperDARN and other instruments results in agreement between the HF radar and non-HF radar velocities for the first time. This new method allows for the first direct measurements of electron densities in the exact locations where the cross section for SuperDARN scatter maximizes.

Using numerical magnetohydrodynamic simulations, we examine the dipole tilt effects on the magnetosphere‐ionosphere convection system when the interplanetary magnetic field is oblique northward (BY = 4 nT and BZ = 2 nT). In particular, we clarify the relationship between viscous‐driven convection and reconnection‐driven convection. The azimuthal locations of the two viscous cell centers in the equatorial plane rotate eastward (westward) when the dipole tilt increases as the Northern Hemisphere turns toward (away from) the Sun. This rotation is associated with nearly the same amount of eastward (westward) rotation of the equatorial crossing point of the dayside separator. The reason for this association is that the viscous cell is spatially confined within the Dungey‐type merging cell whose position is controlled by the separator location. The ionospheric convection is basically a round/crescent cell pattern, but the round cell in the winter hemisphere is significantly deformed. Between its central lobe cell portion and its outer Dungey‐type merging cell portion, the round cell streamlines are deformed owing to the combined effects of the viscous cell and the hybrid merging cell, the latter of which is driven by both Dungey‐type reconnection and lobe‐closed reconnection.

[1] On the night of December 20, 2006, 630 nm airglow images obtained by an all-sky camera at Resolute Bay, Canada (74.73°N, 265.07°E; altitude adjusted corrected geomagnetic (AACGM) latitude 82.9°) showed the passage of successive polar cap patches. Shortly after convection came to a temporary halt, one of the patches was reorganized into two substructures in approximately 8 min. The two-dimensional background ionospheric convection pattern measured using the newly deployed PolarDARN radar at Rankin Inlet (62.82°N, 93.11°W; AACGM latitude 72.96°) showed that a velocity shear of approximately 120 m s−1/340 km suddenly appeared in the vicinity of the patch at the time of reorganization. A qualitative analysis of the relationship between the magnitude of the velocity shear and the distance between the divided patches indicates that the shear in the background plasma convection velocity significantly contributed to the reorganization of the patch. This shear structure appeared soon after a southward turning of the interplanetary magnetic field (IMF) and was probably associated with the reconfiguration of the convection pattern from a pre-existing northward-oriented IMF pattern to a southward-oriented one. The present observations indicate that the reconfiguration/deformation of patches because of a shear in the background convection field, especially reorganization of patches into smaller substructures, may play an important role in the rapid structuring of patches.

The purpose of this paper is to show a “proof of the existence” of the ionospheric situation that is expected for the interchange cycle, during periods of favorable interplanetary magnetic field (IMF) and dipole tilt conditions. To do so, we present three case studies of dayside high‐latitude ionospheric convection that is observed around the equinoxes (near‐zero dipole tilt) and at small IMF clock angles (one θ c ∼ −30° event and two θ c ∼ 30° events, where θ c ≡ Arg( B Z + iB Y )), using Super Dual Auroral Radar Network (SuperDARN)/Defense Meteorological Satellite Program (DMSP)/National Oceanic and Atmospheric Administration (NOAA) data in the Northern Hemisphere and, when available, DMSP data in the Southern Hemisphere. The convection pattern exhibits twin reverse cells in both hemispheres, but the constituents of each cell are different. In the Northern Hemisphere, for θ c ∼ 30° (θ c ∼ −30°), the center of the dawnside (duskside) cell is located poleward of the polar cap boundary, while the center of the duskside (dawnside) cell is located equatorward of the polar cap boundary. For θ c ∼ 30°, we confirmed that the above‐mentioned dawn‐dusk relation reverses in the Southern Hemisphere. The north‐south asymmetric behavior of the conjugate reverse cells, on the dawnside and duskside each, is consistent with two independent interchange cycles that result from the coupling of IMF‐lobe reconnection in one hemisphere with lobe‐closed reconnection in the opposite hemisphere.

The Cascade Demonstrator Small-Sat and Ionospheric Polar Explorer (CASSIOPE) satellite is scheduled to be launched in 2010. On board this satellite will be a suite of eight scientific instruments comprising the enhanced Polar Outflow Probe (ePOP). One instrument is the Radio Receiver Instrument (RRI) which will be used to receive HF transmissions from various ground transmitters such as the Super Dual Auroral Radar Network (SuperDARN) array. Magnetoionic polarization and propagation theory have been used to model the relative power that SuperDARN delivers to the Ordinary (O) and Extraordinary (X) modes of propagation. These calculations have been performed for various frequencies in the SuperDARN transmitting band and for all five Canadian based SuperDARN radars. The geometry of the radars with respect to the background magnetic field results in the X-mode dominating the transmitted signal when the modelled wave propagates northward and is nearly perpendicular to the magnetic field lines. Other propagation directions (i.e., above or southwards of the radar) results in propagation which is anti-parallel to the magnetic field lines and an equal splitting of transmitted power between the O- and X-modes occurs. The modelling analysis shows that for either high transmitting frequencies or low ionospheric electron densities, the range of latitudes that signal will be received is quite large (up to ~90° of latitude). Also for these conditions, the range of elevations where the X-mode signal strongly dominates the O-mode signal will be apparent in the received signal. Conversely, for lower transmitting frequencies or higher ionospheric electron densities, the latitudinal range that signal will be received over is smaller. Here the X-mode-only band is not apparent in the received signal as both modes will be received with roughly equal power. These relative mode power calculations can be used to characterize the average electron density content in the ionosphere or provide a measure of relative absorption in the D- and E-regions when the satellite passes through the field-of-view of a SuperDARN radar.

Gillies et al. (2009) proposed the use of interferometric measurements of the angle of arrival as a proxy for the scattering region refractive index ns needed to estimate the line‐of‐sight Doppler velocity of the ionospheric plasma from HF [Super Dual Auroral Radar Network (SuperDARN)] radar observations. This study continues this work by comparing measurements of line‐of‐sight velocities by SuperDARN with tristatic velocity measurements by the EISCAT incoherent scatter radar from 1995 to 1999. From a statistical viewpoint, velocities measured by SuperDARN were lower than velocities measured by EISCAT. This can, at least partially, be explained by the neglect in the SuperDARN analysis of the lower‐than‐unity refractive index of the scattering structures. The elevation angle measured by SuperDARN was used as a proxy estimate of ns and this improved the comparison, but the velocities measured by SuperDARN were still lower. Other estimates of ns using electron densities Ne based on both EISCAT measurements and International Reference Ionosphere model values did not increase the SuperDARN velocities enough to attain the EISCAT values. It is proposed that dense structures that were of comparable size to the SuperDARN scattering volume partially help resolve the low‐velocity issue. These dense, localized structures would provide the Ne gradients required for generation of the coherent irregularities from which the SuperDARN radar waves scatter, whereas EISCAT incoherent radar measurements provide only the background Ne and not the density of the small‐scale structures. The low‐velocity SuperDARN results suggest that small‐scale dense structures with refractive indices well below unity must exist within the SuperDARN scattering volume and may contribute greatly to the scattering process.

Cases of mesoscale cloud bands in extratropical cyclones are observed a few hours after atmospheric gravity waves (AGWs) are launched from the auroral ionosphere. It is suggested that the solar-wind-generated auroral AGWs contribute to processes that release instabilities and initiate slantwise convection thus leading to cloud bands and growth of extratropical cyclones. Also, if the AGWs are ducted to low latitudes, they could influence the development of tropical cyclones. The gravity-wave-induced vertical lift may modulate the slantwise convection by releasing the moist symmetric instability at near-threshold conditions in the warm frontal zone of extratropical cyclones. Latent heat release associated with the mesoscale slantwise convection has been linked to explosive cyclogenesis and severe weather. The circumstantial and statistical evidence of the solar wind influence on extratropical cyclones is further supported by a statistical analysis of high-level clouds (<440 mb) extracted from the International Satellite Cloud Climatology Project (ISCCP) D1 dataset. A statistically significant response of the high-level cloud area index (HCAI) to fast solar wind from coronal holes is found in mid-to-high latitudes during autumn-winter and in low latitudes during spring-summer. In the extratropics, this response of the HCAI to solar wind forcing is consistent with the effect on tropospheric vorticity found by Wilcox et al. (1974) and verified by Prikryl et al. (2009). In the tropics, the observed HCAI response, namely a decrease in HCAI at the arrival of solar wind stream followed by an increase a few days later, is similar to that in the northern and southern mid-to-high latitudes. The amplitude of the response nearly doubles for stream interfaces associated with the interplanetary magnetic field BZ component shifting southward. When the IMF BZ after the stream interface shifts northward, the autumn-winter effect weakens or shifts to lower (mid) latitudes and no statistically significant response is found at low latitudes in spring-summer. The observed effect persists through years of low and high volcanic aerosol loading. The similarity of the response in mid-to-high and low latitudes, the lack of dependence upon aerosol loading, and the enhanced amplitude of the effect when IMF BZ component shifts southward, favor the proposed AGW link over the atmospheric electric circuit (AEC) mechanism (Tinsley et al., 1994). The latter requires the presence of stratospheric aerosols for a significant effect and should produce negative and positive cloud anomalies in mid-to-high and low latitudes, respectively. However, if the requirement of aerosols for the AEC mechanism can be relaxed, the AGW and AEC mechanisms should work in synergy at least in mid-to-high latitudes.

Cases of mesoscale cloud bands in extratropical cyclones are observed a few hours after atmospheric gravity waves (AGWs) are launched from the auroral ionosphere. It is suggested that the solar-wind-generated auroral AGWs contribute to processes that release instabilities and initiate slantwise convection thus leading to cloud bands and growth of extratropical cyclones. Also, if the AGWs are ducted to low latitudes, they could influence the development of tropical cyclones. The gravity-wave-induced vertical lift may modulate the slantwise convection by releasing the moist symmetric instability at near-threshold conditions in the warm frontal zone of extratropical cyclones. Latent heat release associated with the mesoscale slantwise convection has been linked to explosive cyclogenesis and severe weather. The circumstantial and statistical evidence of the solar wind influence on extratropical cyclones is further supported by a statistical analysis of high-level clouds (< 440 mb) extracted from the International Satellite Cloud Climatology Project (ISCCP) D1 dataset. A statistically significant response of the high-level cloud area index (HCAI) to fast solar wind from coronal holes is found in mid-to-high latitudes during autumn-winter and in low latitudes during spring-summer. In the extratropics, this response of the HCAI to solar wind forcing is consistent with the effect on tropospheric vorticity found by Wilcox et al. (1974) and verified by Prikryl et al. (2009). In the tropics, the observed HCAI response, namely a decrease in HCAI at the arrival of solar wind stream followed by an increase a few days later, is similar to that in the northern and southern mid-to-high latitudes. The amplitude of the response nearly doubles for stream interfaces associated with the interplanetary magnetic field BZ component shifting southward. When the IMF BZ after the stream interface shifts northward, the autumn-winter effect weakens or shifts to lower (mid) latitudes and no statistically significant response is found at low latitudes in spring-summer. The observed effect persists through years of low and high volcanic aerosol loading. The similarity of the response in mid-to-high and low latitudes, the lack of dependence upon aerosol loading, and the enhanced amplitude of the effect when IMF BZ component shifts southward, favor the proposed AGW link over the atmospheric electric circuit (AEC) mechanism (Tinsley et al., 1994). The latter requires the presence of stratospheric aerosols for a significant effect and should produce negative and positive cloud anomalies in mid-to-high and low latitudes, respectively. However, if the requirement of aerosols for the AEC mechanism can be relaxed, the AGW and AEC mechanisms should work in synergy at least in mid-to-high latitudes.

The newly installed Rankin Inlet HF radar is very similar to other SuperDARN radars but uses a new type of antennae with its back lobe overlooking the auroral zone where ionospheric irregularities occur very frequently. Despite the fact that a special screen has been installed, there is a chance to receive echoes from the back/side lobe, which can affect the observed velocities. In this study, Rankin Inlet HF radar (RKN) velocities are compared with measurements from three independent instruments: the HF radar in Saskatoon, the CADI ionosonde at Resolute Bay, and drift meters on board DMSP satellites passing the RKN field of view. Although data spread and the degree of agreement vary from one comparison to another, the overall conclusion is that even if echoes are received from the back/side lobe, their effect is statistically insignificant. RKN velocities were found to be comparable to those inferred from other instrument outputs; the slope of the best fit line and the correlation coefficient can be as high as 0.7 and 0.8, respectively. The majority of inconsistencies are related to the difference in the spatial and temporal resolutions of the instruments involved in the comparison.

When the dawn‐dusk component of the interplanetary magnetic field (IMF BY) is dominant, in particular, for northward IMF, there often appears a field‐aligned current (FAC) system consisting of four latitudinally separated and longitudinally elongated current sheets in the prenoon or postnoon sector, depending on the polarity of IMF BY. A new explanation is proposed for the four‐sheet FAC system in terms of ionospheric convection which includes an exchange cell. In this configuration, in addition to the usual round and crescent convection cells seen during IMF BY‐dominated periods, a small convection cell called the primary exchange cell and having the same circulation direction as the round cell appears in the Northern Hemisphere prenoon (for BY < 0) or postnoon (for BY > 0) sector, equatorward of the round cell. A case study is presented in which a four‐sheet FAC system is successfully explained using the exchange cell configuration. In the observations, however, it often is the case that the primary exchange cell may lose its identity because it is incorporated into the round cell to form one large round cell. We suggest that this incorporation arises from diffusive processes in the magnetosphere‐ionosphere coupling.

In past calculations of convective velocities from Super Dual Auroral Radar Network (SuperDARN) HF radar observations, the refractive index in the scattering region has not been taken into account, and therefore the inferred ionospheric velocities may be underestimated. In light of the significant contribution by SuperDARN to ionospheric and magnetospheric research, it is important to refine the velocity determination. The refractive index in the ionosphere at SuperDARN observation F region altitudes has typical values between 0.8 and close to unity. In the scattering region, where conditions are more extreme, the index of refraction may be much lower. A simple application of Snell's law in spherical coordinates (Bouguer's law) suggests that a proxy for the index of refraction at the scattering location can be determined by measuring the elevation angle of the returned ionospheric radar signal. Using this approximation for refractive index, the Doppler velocity calculation can be refined for each SuperDARN ionospheric echo, using the elevation angles obtained from the SuperDARN interferometer data. A velocity comparison of DMSP and SuperDARN observations has revealed that the SuperDARN speeds were systematically lower than the DMSP speeds. A linear regression analysis of the velocity comparisons found a best fit slope of 0.74. When the elevation angle data were used to estimate refractive index, the best fit slope rose 12% to 0.83. As most SuperDARN radars employ an interferometer antenna array for elevation angle measurements, the improvement in velocity estimates can be done routinely using the method outlined in this paper.