Hemisphere Summer Research Articles

Transition of Interhemispheric Asymmetry of Equatorial Ionization Anomaly During Solstices

AbstractThe magnitudes of the two crests of equatorial ionization anomaly (EIA) vary with local time. During the solstices, EIA crest in the winter hemisphere is larger than that in the summer hemisphere before noon/early afternoon. Whereafter, the crest in the summer hemisphere becomes intensified, and the stronger EIA crest transits to the summer hemisphere. Using Constellation Observing System for Meteorology, Ionosphere, and Climate ionospheric radio occultation data, we examine the longitudinal and altitudinal variations of this interhemispheric transition in four longitudinal sectors and at seven heights under low/high solar activity conditions. The results show that during the June solstice the transition of the stronger EIA peak from the winter to the summer hemisphere is earlier in the sectors where the geomagnetic equator is further away from the subsolar point and the geomagnetic field declination is larger, while during the December solstice the longitudinal variations generally show the opposite compared with that in the June solstice. The distance between the geomagnetic equator and subsolar point and the geomagnetic field configuration control the upward/downward plasma movements in the summer/winter hemisphere, leading to the different transition times in different longitudinal sectors. For both solstices, transition times emerge earlier as height increases, which is mainly caused by the larger effective scale height in the summer hemisphere than in the winter hemisphere, resulting in a smaller electron density difference at higher altitudes with a fast transition. Solar activity alters the transition time below 320 km, whereas it has no evident effect at higher altitudes.

On Ionic Contributions to Pedersen Conductance

AbstractIonospheric conductivity is a key parameter to understand the upper atmosphere electrodynamics, magnetosphere‐ionosphere‐thermosphere coupling and several geophysical processes. In the case of Pedersen conductivity, ions and electrons are moving in opposite direction, so that it results from the addition of the separate contributions. In this work, the role of the three main ions, that is, NO+, O2+, and O+, in the conductivity height profile and in the conductance global pattern is analyzed for different solar activity levels and seasons. NO+ is the is the main contributor for the maximum conductivity in all the cases in the E layer; however, O+ takes relevance with a significant percentage of the conductance at certain regions such as the South Atlantic Anomaly and the crests at the equatorial ionization anomaly, becoming the main conducting ion in these zones when the solar activity is high. Regarding seasonal effects, the role of NO+ in the conductivity is maximum on summer hemisphere, while O+ becomes important in winter and maximum with high solar activity. O2+ has maximum percentage close to magnetic poles, where the magnetic field strength is intense lowering the peak conducting level.

A complete solar cycle (2006–2016) studies of scale heights derived using COSMIC radio occultation retrieved electron density profiles

Various ionospheric vertical scale heights (VSH) including, the Chapman (Hm), VSH at upper ionosphere (VSHUP) and 500 km (VSH500) derived using COSMIC radio occultation (RO) retrieved electron density profiles during a complete solar cycle (2006–2016) are presented in this research. VSH show distinct latitudinal variations and solar-activity dependencies. VSH follow the geomagnetic equator with a longitudinal structure irrespective of the season and year, although mid and high latitudes show higher values and irregular variations in low (2006–2010) and moderate-to-high (2011–2016) sunspot years during the daytime hours (1300 LT-1500 LT). The longitudinal structure of VSH500 appears only in moderate-to-high sunspot years, which is an important observation reported for the first time in this communication. Diurnal variations of Hm at equatorial latitudes show higher values starting from ∼0400 LT to ∼2300 LT with a post-sunset peak. However, an absence of the post-sunset peak is seen in VSHUP and VSH500 which is probably due to the altitudinal gradients of vertical drifts that increase (decrease) below (above) the F-layer peak during evening pre-reversal enhancements. The reduced drifts above the F-layer peak thereby, suppresses the post-sunset peak in VSH. Concurrent and large-scale structures in VSH500 and VSHUP at mid and high latitudes in summer hemispheres during June and December solstice seasons are reported for the first time, which occurred possibly due to the long-duration exposure to the Sun's strong radiation during summer seasons. A one-to-one correlation exists between the Chapman scale heights and peak height of the F2 layer (hmF2) at equatorial latitudes in different seasons, while moderate correlation is seen between them away from the equatorial latitudes. These presented scale heights at a global scale are very significant, particularly, for future empirical modeling studies and the long-term availability of COSMIC RO retrieved electron density profiles allowed us to present these scale heights.

Latitudinal Double‐Peak Structure of Stationary Planetary Wave 1 in the Austral Winter Middle Atmosphere and Its Possible Generation Mechanism

AbstractWe report a hitherto unknown latitudinal double‐peak structure in the amplitude of stationary planetary wave 1 (SPW1) geopotential height in the stratosphere and lower mesosphere during austral winter. The primary peak is located at 60–70°S and 30–40 km, and the secondary peak is at 30–50°S and 40–60 km. According to 36 years (1981–2016) of the Modern‐Era Retrospective analysis for Research and Applications, version 2, reanalysis data, the double‐peak structure occurs with frequencies of ~61%, 97%, 53%, and 25% in May, June, July and August, respectively, while it rarely exists in other seasons. Significant downward Eliassen‐Palm fluxes suggestive of downward‐propagating waves are often found above the secondary peak, and phase progressions show opposite directions on its two sides. From the free‐running Whole Atmosphere Community Climate Model and linear mechanistic model simulations, the secondary peak is likely generated by the interference of primary upward‐ and secondary downward‐propagating SPW1 excited in situ by gravity wave forcing in the upper mesosphere. The strengths of the primary and secondary waves need to be comparable to form an effective interference pattern, which may explain the missing double‐peak feature in boreal winter as the primary waves are too dominant. Summer‐to‐winter interhemispheric wave coupling is identified in the austral midwinter and appears to originate from the secondary SPW1 generated in the summer hemisphere. Since the double‐peak structure of SPW1 is sensitive to the mean wind, wave‐mean flow, and wave‐wave interactions, this study provides a reference for general circulation and mechanistic models to simulate the middle atmosphere wave dynamics in austral winter.

Stronger Local Overturning in Convective‐Permitting Regional Climate Model Improves Simulation of the Subtropical Annual Cycle

AbstractGlobal climate models fail to represent the annual cycle of tropical‐extratropical cloud bands and produce too much summer rainfall over subtropical southern Africa. This study demonstrates that running a regional convective‐permitting climate simulation alleviates these biases, counteracting biases that are present in the parent model. The improvement emerges from stronger vertical mass flux in the tropics, which forces a stronger local Hadley overturning into the summer hemisphere. This enhanced overturning increases upper‐level subsidence in the subtropics and amplifies the forcing of the local subtropical jet. Together, these improvements halve the wet subtropical rainfall bias and are associated with a 50% increase to an 80% match between the simulated and observed annual cycle of crucial tropical‐extratropical cloud band rainfall systems. The results advocate for the increased use of convective‐permitting climate models with domains that include regional tropical convection hot spots, in order to fully benefit from the explicit representation of deep convection.

Northward excursion of the ITCZ across the inter-Americas during boreal summer

The study describes northward excursions of the ITCZ across the inter-Americas during boreal summer, using observational datasets at multi-year, annual and daily time-scales. The annual cycle is felt across the region via evenly distributed SST warming in the summer hemisphere. Low-level air is propelled northward from June to October by the southern Hadley cell. Discharges from the Amazon and Orinoco Rivers spread into the Atlantic Ocean at that time of year. A N–S dipole pattern in satellite outgoing longwave radiation (OLR) is calculated and its temporal character reveals northward ITCZ excursions at 4–6 years period corresponding with a warm east Pacific and intensified diabatic heating near Panama. Composite analysis reveals evidence of a standing atmospheric Rossby wave pattern embedded in westerly flow anomalies which bifurcate on passing the Andes and slant across the inter-Americas in Jun–Oct season: lowering SLP and energizing Caribbean hurricanes. Variability at intra-seasonal time scale is underpinned by the southern jet stream and a ridge over Brazil that pushes the ITCZ northward, as seen in statistical regression and case study analysis of satellite OLR and NCEP2 reanalysis fields. The southern Hadley cell plays a prominent role, transmitting signals from winter to summer hemisphere at all time scales.

Confirmation of arboreal habits in Dromiciops gliroides: a key role in Chilean Temperate Rainforests

AbstractDromiciops gliroides is a small marsupial endemic to the South American temperate rainforest; it is considered a living fossil because it is the only living species of the order Microbiotheria, more related to Australian marsupials than those found in South America. Dromiciops gliroides has been considered a scansorial marsupial most frequently found in the understory. Nonetheless, several authors have hypothesized that this species could be arboreal because of its ability to climb through the vegetation. However, all previous studies on D. gliroides have been conducted from the ground, with no documentation of this species’ ability to climb trees, or how high they may reach. Here, we present the first evidence of arboreal habits in D. gliroides, and we analyze its functional importance for the biodiversity of forest canopies. In the Bosque Pehuén Park (39°25′ S, 71°45′ W), in six emergent Nothofagus dombeyi (Nothofagaceae) trees, we installed camera traps, one per tree, between 12 and 21 m aboveground, in the trees’ crowns. Camera traps were active from January to April of 2017, during the Southern Hemisphere's summer and fall. We recorded a total of 2319 photographs, including small mammals, birds, and lizards, in addition to several unidentified species. A total of 230 photographs of D. gliroides were recorded, across all six of the surveyed trees. In a more recent survey (February 2018), we found this species at 26 m, 2 m from the tree's highest point. Dromiciops gliroides is thus a frequent canopy user and could influence canopy biota and ecological processes. Dromiciops gliroides is considered a seed disperser of most vascular epiphytes and vines, and may therefore strongly influence epiphytic dynamics. It is also an active invertebrate predator and as such could significantly decrease tree herbivory. The roles that D. gliroides plays in this ecosystem need more clarification; this living fossil may be more important for this forest ecosystem than previously believed, modulating biodiversity and functions in unexpected, yet relevant ways.

The Momentum Budget in the Stratosphere, Mesosphere, and Lower Thermosphere. Part I: Contributions of Different Wave Types and In Situ Generation of Rossby Waves

A momentum budget is examined in the stratosphere, mesosphere, and lower thermosphere using simulation data over ~11 years from a whole-atmosphere model in terms of the respective contributions of gravity waves (GWs), Rossby waves (RWs), and tides. The GW forcing is dominant in the mesosphere and lower thermosphere (MLT), as indicated in previous studies. However, RWs also cause strong westward forcing, described by Eliassen–Palm flux divergence (EPFD), in all seasons in the MLT and in the winter stratosphere. Despite the relatively coarse model resolution, resolved GWs with large amplitudes appear in the MLT. The EPFD associated with the resolved GWs is eastward (westward) in the summer (winter) hemisphere, similar to the parameterized GW forcing. A pair of positive and negative EPFDs are associated with the RWs and GWs in the MLT. These results suggest that the RWs and resolved GWs are generated in situ in the MLT. Previous studies suggested that a possible mechanism of RW generation in the MLT is the barotropic/baroclinic instability. This study revisits this possibility and examines causes of the instability from a potential vorticity (PV) viewpoint. The instability condition is characterized as the PV maximum at middle latitudes on an isentropic surface. Positive EPFD for RWs is distributed slightly poleward of the PV maximum. Because the EPFD equals the PV flux, this feature indicates that the RW radiation acts to reduce the PV maximum. The PV maximum is climatologically maintained in both the winter and summer mesospheres, which is caused by parameterized GW forcing.

Sources, Sinks, and Propagation Characteristics of the Quasi 6‐Day Wave and Its Impact on the Residual Mean Circulation

AbstractThis study employs a troposphere to lower thermosphere assimilation model data set generated by the Whole Atmosphere Community Climate Model with data assimilation provided by the Data Assimilation Research Test Bed (WACCM + DART) to explore the sources, sinks, and propagation characteristics of the quasi 6‐day wave (Q6DW) in the year 2007. WACCM + DART reproduces the burst‐like Q6DW and compares well with Sounding of the Atmosphere using Broadband Emission Radiometry and Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics Doppler Interferometer observations. The most prominent Q6DW took place in later February and mid‐October, while the Q6DW was absent during solstice conditions in 2007. The occurrence of a large Q6DW in the equinoctial mesosphere and lower thermosphere is highly dependent on wave amplification and overreflection processes associated with barotropic/baroclinic instabilities and wave critical layers defined by the zonal mean zonal winds. During solstices, the winter hemisphere waveguide is negative and prevents the vertical wave propagation from the source region into the mesosphere and lower thermosphere. Meanwhile, the critical layer for the Q6DW encloses the unstable region in the summer hemisphere and thus blocks the energy conversion from the mean flow to the wave. The resulting circulation pattern due to the Q6DW momentum deposition is upward and poleward in both hemispheres and thus weakens the residual mean circulation in the summer hemisphere but strengthens it in the winter hemisphere. Also, the Q6DW impact on the residual mean circulation points to broader implications for the mean state of the upper atmosphere, for example, the thermospheric O/N2 ratio due to upward constituent transport and related changes in the ionospheric plasma.

Global estimates of changes in shortwave low-cloud albedo and fluxes due to variations in cloud droplet number concentration derived from CERES-MODIS satellite sensors.

15 years of Aqua CERES and MODIS observations are combined to derive nearly global maps of shortwave albedo (A) and flux (F) response to changes in cloud droplet number concentration (Nd ). Absolute and relative albedo susceptibilities are computed seasonally by exploiting the linear relationship between A and ln(Nd ) for shallow liquid clouds. Subtropical stratiform clouds (eastern Pacific, eastern Atlantic, and Australia) yield the highest Sr , followed by the extratropical oceans during their hemispheric summer. When Sr is cast in terms of F, the eastern Pacific clouds dominate Sr , with a secondary maximum offshore eastern Asia. Sa is mainly governed by Nd , with more pristine environments being more susceptible to change their albedo. While both Sa and Sr are advantageous for understanding radiative aspects of the aerosol indirect effect, Sr is more suitable for calculating changes in A and F due to the linear relationship between A and ln(Nd ).

Energetic Constraints on the ITCZ Position in Idealized Simulations With a Seasonal Cycle

AbstractThe atmospheric energy budget has recently been shown to provide powerful constraints on the position and shifts of the zonal and annual mean intertropical convergence zone (ITCZ), which lies close to the latitude of zero vertically integrated energy transport (energy flux equator, EFE). Relatively little work has, however, explored the applicability of the energetic framework to ITCZ shifts on shorter time scales. This study investigates to what extent the EFE tracks the ITCZ on subseasonal time scales in idealized aquaplanet simulations with different mixed layer depths. It is shown that the ITCZ always lags the EFE, even in the simulation with the shallowest mixed layer depth, making it possible for the EFE and the ITCZ to reside on opposite sides of the equator. At these times, which occur as the winter cross‐equatorial Hadley circulation retreats from the summer hemisphere, the required energy balance is achieved not through shifts of the Hadley cell's ascending branch and ITCZ to track the EFE but through changes in the cell's vertical structure into one of negative gross moist stability. For any given position of the ascending branch, the winter cell is much weaker as it retreats from than as it expands into the summer hemisphere and develops a shallow return flow at middle‐to‐lower tropospheric levels where the moist static energy reaches its minimum, hence favoring a negative gross moist stability. It is argued that the asymmetry between the expanding and retreating phases of the winter Hadley cell is linked to the nonlinear seasonal evolution of near‐surface temperatures.

Climate, tectonics and beach erosion: the case of Espinho (NW Portuguese coast)

Sea level is a very changeable surface. Furthermore, the land may also be moving, in a slower rate,generating relative sea level changes. The causes of relative sea level changes are variable, but the onesthat cause more intense variations are related to climate.During Little Ice Age (LIA) Northern Hemisphere's summer temperatures fell significantly below theAD 1961–1990 range. This climate situation was responsible for a greater discharge of rivers, whichcould lead to a greater transportation of sediments to the coastline. During these cold periods, sea levelwas lower than in present time. All this could imply a coastline progradation, with the successiveabandon of older beach ridges, reinforcing the sandy supply for dune building. The coastal situationshould be, in some sense, the opposite of the situations that we face today.In present warm period, rivers carry less sediment than during LIA. Moreover, the recent sea level risecontributes to a coastal migration inlands and the erosion of previous beaches and dunes.Our investigation on ancient marine levels and Holocene cemented dunes suggests that the area nearEsmoriz (20 km south of Porto, NW Portuguese coast) is probably subsiding. This possible subsidence,together with recent sea level rise, induced by the end of LIA, could explain the severe coastal erosionthat is taking place at Espinho area (15 km south of Porto) since the middle of the XIX century.This example shows clearly the complexity of relative sea-level changes. Because of this complexity,sea level curves are not similar worldwide, as they depend on the interference of multiple time-scalesphenomena.

Titan's Meteorology Over the Cassini Mission: Evidence for Extensive Subsurface Methane Reservoirs

AbstractCassini observations of Titan's weather patterns over >13 years, almost half a Saturnian year, provide insight into seasonal circulation patterns and the methane cycle. The Imaging Science Subsystem and the Visual and Infrared Mapping Spectrometer documented cloud locations, characteristics, morphologies, and behavior. Clouds were generally more prevalent in the summer hemisphere, but there were surprises in locations and timing of activity: Southern clouds were common at midlatitudes, northern clouds initially appeared much sooner than model predictions, and north polar summer convective systems did not appear before the mission ended. Differences from expectations constrain atmospheric circulation models, revealing factors that best match observations, including the roles of surface and subsurface reservoirs. The preference for clouds at mid‐northern latitudes rather than near the pole is consistent with models that include widespread polar near‐surface methane reservoirs in addition to the lakes and seas, suggesting a broader subsurface methane table is accessible to the atmosphere.

Effects of Non-orographic Gravity Wave Drag on Seasonal and Medium-range Predictions in a Global Forecast Model

This study implements the parameterizations of convective and frontal gravity wave drag (GWD) with wide phase speed spectra into a global forecast model with a model top near 0.3 hPa. The new convective GWD scheme replaces the existing one that considers only a stationary convective GW, and the frontal GWD scheme is newly introduced. When the new GWD schemes are used, the Rayleigh friction, applied above 2 hPa to mimic the effects of missing GWD, is removed. The convective (frontal) GWs are generated mainly in the Intertropical Convergence Zone and winter extratropical storm track regions (extratropics where strong baroclinicity exists). The convective and frontal GWD derived from the new schemes are significant near the model top, with maxima of ~2-4 and ~26-58 m s−1 day−1, respectively. The differences in convective GWD between the stationary and non-stationary schemes appear mainly in the tropics and summer hemisphere, where stationary GWs cannot propagate upward. The new schemes improve the seasonal representation of stratospheric wind, through changes in both the GWD and the resolved wave forcing, which is modulated by the changed large-scale wind due to the GWD. The downward influence, in response to the changed GWD, is also positive in the tropospheric fields, such as subtropical jet and planetary-scale disturbances. For the medium-range forecasts, improved skill scores on wind speed are achieved globally with the new schemes. The improvements mostly appear only in the stratosphere during the early forecast period (~3 days) but expand to the troposphere as forecast time increases.

On the role of precipitation latent heating in modulating the strength and width of the Hadley circulation

The latent heat released in the clouds over the tropics plays a vital role in driving the Hadley circulation (HC). The present study discusses the influence of latent heating (LH) on the HC parameters viz., centre, strength and total width by using precipitation LH profiles derived from the space-borne observations of the Precipitation Radar (PR) onboard Tropical Rain Measuring Mission (TRMM) and meridional stream function (MSF) derived from ECMWF-Interim reanalysis. The latitude of peak latent heating, width of the latent heating distribution and the total LH released within the ascending limb of the HC are estimated and their influence on the HC centre, strength and width is quantified, for the first time. The present results show that the latitude of peak LH significantly influences the position of the HC centre with correlation coefficient of ~ 0.90. This high correlation between these two quantities seems to be due to their co-variability with the apparent motion of the Sun across the latitudes. The intensity of the HC in the NH as well as SH shows high correlation with the latitude of peak LH with coefficients − 0.85 and − 0.78, respectively. These results indicate that farther the latitude of peak LH from the equator in the summer hemisphere, stronger is the HC intensity in the winter hemisphere. The present analysis also reveals that the total LH released within the ascending limb of HC substantially influence the total width of the HC, with correlation coefficient ~ 0.52, as compared to the other two LH parameters. This observation can be attributed to the fact that the HC is sensitive to the latent heat release in the mid-tropospheric levels in the tropics. An attempt is also made to investigate the degree of variability of these parameters after deseasonalization and results are discussed in the light of present understanding. The significance of the present study lies in providing the observational evidence for the influence of latent heating on the HC strength/width variability, quantitatively, for the first time using TRMM observations of precipitation latent heating.

Interferometric Imaging of Titan’s HC3N, H13CCCN, and HCCC15N

Abstract We present the first maps of cyanoacetylene isotopologues in Titan’s atmosphere, including H13CCCN and HCCC15N, detected in the 0.9 mm band using the Atacama Large Millimeter/submillimeter array (ALMA) around the time of Titan’s (southern winter) solstice in 2017 May. The first high-resolution map of HC3N in its v 7 = 1 vibrationally excited state is also presented, revealing a unique snapshot of the global HC3N distribution, free from the strong optical depth effects that adversely impact the ground-state (v = 0) map. The HC3N emission is found to be strongly enhanced over Titan’s south pole (by a factor of 5.7 compared to the north pole), consistent with rapid photochemical loss of HC3N from the summer hemisphere combined with production and transport to the winter pole since the 2015 April ALMA observations. The H13CCCN/HCCC15N flux ratio is derived at the southern HC3N peak, and implies an HC3N/HCCC15N ratio of 67 ± 14. This represents a significant enrichment in 15N compared with Titan’s main molecular nitrogen reservoir, which has a 14N/15N ratio of 167, and confirms the importance of photochemistry in determining the nitrogen isotopic ratio in Titan’s organic inventory.

Global Atmospheric CO2 Concentrations Simulated by GEOS-Chem: Comparison with GOSAT, Carbon Tracker and Ground-Based Measurements

Accurate quantification of the distribution and variability of atmospheric CO2 is crucial for a better understanding of global carbon cycle characteristics and climate change. Model simulation and observations are only two ways to globally estimate CO2 concentrations and fluxes. However, large uncertainties still exist. Therefore, quantifying the differences between model and observations is rather helpful for reducing their uncertainties and further improving model estimations of global CO2 sources and sinks. In this paper, the GEOS-Chem model was selected to simulate CO2 concentration and then compared with the Greenhouse Gases Observing Satellite (GOSAT) observations, CarbonTracker (CT) and the Total Carbon Column Observing Network (TCCON) measurements during 2009–2011 for quantitatively evaluating the uncertainties of CO2 simulation. The results revealed that the CO2 simulated from GEOS-Chem is in good agreement with other CO2 data sources, but some discrepancies exist including: (1) compared with GOSAT retrievals, modeled XCO2 from GEOS-Chem is somewhat overestimated, with 0.78 ppm on average; (2) compared with CT, the simulated XCO2 from GEOS-Chem is slightly underestimated at most regions, although their time series and correlation show pretty good consistency; (3) compared with the TCCON sites, modeled XCO2 is also underestimated within 1 ppm at most sites, except at Garmisch, Karlsruhe, Sodankylä and Ny-Ålesund. Overall, the results demonstrate that the modeled XCO2 is underestimated on average, however, obviously overestimated XCO2 from GEOS-Chem were found at high latitudes of the Northern Hemisphere in summer. These results are helpful for understanding the model uncertainties as well as to further improve the CO2 estimation.

The seasonal distribution of sporadic E layers observed from radio occultation measurements and its relation with wind shear measured by TIMED/TIDI

By using the 2002–2016 dataset of COSMIC, CHAMP and GRACE radio occultation (RO) and TIMED/TIDI, we investigate the morphology of global sporadic E (ES) layers and its relation with wind shear. The occurrence rate of ES layers depends on temporal and spatial variation. The occurrence rate of ES layers has a maximum at around 103 km in middle latitude regions in summer hemisphere. The results show that the global distribution of ES layers is relatively consistent with the wind shear distribution. The height-local time distribution of ES layers occurrences are primary modulated by diurnal and semidiurnal atmospheric tides. However, the distribution of ES layers occurrence rate cannot be explained completely according to the wind shear theory, especially at equatorial and auroral regions. More observations are required to further investigate the formation of ES layers and its relation with wind shear.

Field‐Aligned Currents in Saturn's Nightside Magnetosphere: Subcorotation and Planetary Period Oscillation Components During Northern Spring

AbstractWe newly analyze Cassini magnetic field data from the 2012/2013 Saturn northern spring interval of highly inclined orbits and compare them with similar data from late southern summer in 2008, thus providing unique information on the seasonality of the currents that couple momentum between Saturn's ionosphere and magnetosphere. Inferred meridional ionospheric currents in both cases consist of a steady component related to plasma subcorotation, together with the rotating current systems of the northern and southern planetary period oscillations (PPOs). Subcorotation currents during the two intervals show opposite north‐south polar region asymmetries, with strong equatorward currents flowing in the summer hemispheres but only weak currents flowing to within a few degrees of the open‐closed boundary (OCB) in the winter hemispheres, inferred due to weak polar ionospheric conductivities. Currents peak at ~1 MA rad−1 in both hemispheres just equatorward of the open‐closed boundary, associated with total downward polar currents ~6 MA, then fall across the narrow auroral upward current region to small values at subauroral latitudes. PPO‐related currents have a similar form in both summer and winter with principal upward and downward field‐aligned currents peaking at ~1.25 MA rad−1 being essentially collocated with the auroral upward current and approximately equal in strength. Though northern and southern PPO currents were approximately equal during both intervals, the currents in both hemispheres were dual modulated by both systems during 2012/2013, with approximately half the main current closing in the opposite ionosphere and half cross field in the magnetosphere, while only the northern hemisphere currents were similarly dual modulated in 2008.

The Association of High‐Latitude Dayside Aurora With NBZ Field‐Aligned Currents

AbstractThe relationship between auroral emissions in the polar ionosphere and the large‐scale flow of current within the Earth's magnetosphere has yet to be comprehensively established. Under northward interplanetary magnetic field (IMF) conditions, magnetic reconnection occurs at the high‐latitude magnetopause, exciting two reverse lobe convection cells in the dayside polar ionosphere and allowing ingress of solar wind plasma to form an auroral “cusp spot” by direct impact on the atmosphere. It has been hypothesized that a second class of NBZ auroras, High‐latitude Dayside Aurora, are produced by upward field‐aligned currents associated with lobe convection. Here we present data from the Special Sensor Ultraviolet Spectrographic Imager instrument and from the Active Magnetosphere and Planetary Electrodynamics Response Experiment, from January 2010 to September 2013, in a large statistical study. We reveal a northward IMF auroral phenomenon that is located adjacent to the cusp spot and that is colocated with a region of upward electrical current in the clockwise‐rotating lobe cell. The emission only occurs in the sunlit summer hemisphere, demonstrating the influence of the conductance of the ionosphere on current closure. In addition, fast solar wind speed is required for this emission to be bright. The results show that dayside auroral emission is produced by IMF‐magnetosphere electrodynamic coupling, as well as by direct impact of the atmosphere by the solar wind, confirming the association of High‐latitude Dayside Aurora with NBZ currents.