Poleward Boundary Research Articles

Meridional transport of salt in the global ocean from an eddy-resolving model

Abstract. The meridional transport of salt is computed in a global eddy-resolving numerical model (1/12° resolution) in order to improve our understanding of the ocean salinity budget. A methodology is proposed that allows a global analysis of the salinity balance in relation to surface water fluxes, without defining a "freshwater anomaly" based on an arbitrary reference salinity. The method consists of a decomposition of the meridional transport into (i) the transport by the time–longitude–depth mean velocity, (ii) time–mean velocity recirculations and (iii) transient eddy perturbations. Water is added (rainfall and rivers) or removed (evaporation) at the ocean surface at different latitudes, which creates convergences and divergences of mass transport with maximum and minimum values close to ±1 Sv. The resulting meridional velocity effects a net transport of salt at each latitude (±30 Sv PSU), which is balanced by the time–mean recirculations and by the net effect of eddy salinity–velocity correlations. This balance ensures that the total meridional transport of salt is close to zero, a necessary condition for maintaining a quasi-stationary salinity distribution. Our model confirms that the eddy salt transport cannot be neglected: it is comparable to the transport by the time–mean recirculation (up to 15 Sv PSU) at the poleward and equatorial boundaries of the subtropical gyres. Two different mechanisms are found: eddy contributions are localized in intense currents such as the Kuroshio at the poleward boundary of the subtropical gyres, while they are distributed across the basins at the equatorward boundaries. Closer to the Equator, salinity–velocity correlations are mainly due to the seasonal cycle and large-scale perturbations such as tropical instability waves.

Systematic study of intermediate-scale structures of equatorial plasma irregularities in the ionosphere based on CHAMP observations

Equatorial spread-F ionospheric plasma irregularities on the night-side, commonly called equatorial plasma bubbles (EPB), include electron density variations over a wide range of spatial scales. Here we focus on intermediate-scale structures ranging from 100 m to 10 km, which play an important role in the evolution of EPBs. High-resolution CHAMP magnetic field measurements sampled along north-south track at 50 Hz are interpreted in terms of diamagnetic effect for illustrating the details of electron density variations. We provide the first comprehensive study on intermediate-scale density structures associated with EPBs, covering a whole solar cycle from 2000 to 2010. The large number of detected events, almost 9000, allows us to draw a detailed picture of the plasma fine structure. The occurrence of intermediate-scale events is strongly favoured by high solar flux. During times of F10.7 < 100 sfu practically no events were observed. The longitudinal distribution of our events with respect to season or local time agrees well with that of the EPBs, qualifying the fine structure as a common feature, but the occurrence rates are smaller by a factor of 4 during the period 2000-2005. Largest amplitude electron density variations appear at the poleward boundaries of plasma bubbles. Above the dip-equator recorded amplitudes are small and fall commonly below our resolution. Events can generally be found at local times between 19 and 24 LT, with a peak lasting from 20 to 22 LT. The signal spectrum can be approximated by a power law. Over the frequency range 1 – 25 Hz we observe spectral indices between -1.4 and -2.6 with peak occurrence rates around -1.9. There is a weak dependence observed of the spectral index on local time. Towards later hours the spectrum becomes shallower. Similarly for the latitude dependence, there is a preference of shallower spectra for latitudes poleward of the ionisation anomaly crest. Our data suggest that the generation of small plasma structure

Statistical relationships between enhanced polar cap flows and PBIs

AbstractPoleward boundary intensifications (PBIs) are auroral intensifications along the poleward boundary of the auroral oval and occur during all levels of geomagnetic activity. However, little is known about the triggering of PBIs. Recent case studies have indicated the existence of longitudinally localized flow channels in the polar cap near, and directed toward, the nightside open‐closed field line boundary just before PBIs. Motivated by these studies, we analyze 115 events of coordinated observations by the Time History of Events and Macroscale Interactions during Substorms all‐sky imager and Super Dual Auroral Radar Network HF radar at Rankin Inlet to determine if this polar cap flow‐PBI relationship is commonly observed. We start with isolated and intense PBIs and examine the probability of them being associated with equatorward directed polar cap flows. Our results show the association to be frequent (90%), with one‐to‐one correlations occurring in ~50% of events. Considering the limitations of the radar observations, this result indicates that PBIs are commonly correlated with polar cap flow channels directed toward, and then traversing, the open‐closed field line boundary. The flows statistically occur ~1–2 min before the PBI initiations, and the duration and width of the flows are comparable to those of the PBIs. We also perform a reverse study by starting with isolated polar cap flows and obtain similar results. The remarkably high occurrence of association between enhanced polar cap flows and PBIs indicates that enhanced mesoscale flows within the open field line region that traverse the open‐closed field line boundary are an important driver of PBI formation.

Coordinated SuperDARN THEMIS ASI observations of mesoscale flow bursts associated with auroral streamers

AbstractNightside auroral zone localized flow channels, typically associated with auroral poleward boundary intensifications and streamers, are an important component of high‐latitude ionospheric plasma dynamics. We investigate the structure of these flow channels using two‐dimensional line‐of‐sight flow observations from the Super Dual Auroral Radar Network (SuperDARN) radars and auroral images from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) ground‐based all‐sky imager (ASI) array. Radar echoes captured &lt;~500 km horizontal distance from the radars were mainly used to detect small‐scale flow structures that would otherwise be missed or poorly resolved in long‐range radar echoes. After identifying 135 auroral streamers in the ASI images at close‐radar capture locations, we examined the associated ionospheric flow data in the radar echoes. Flow bursts and streamers are invariably correlated in all events. The flow bursts are often directed equatorward and appear simultaneously with the streamers. Equatorward flows are located just to the east of the streamers. Less frequently (~10% of the time), a poleward flow enhancement was detected even when a streamer propagated equatorward, the poleward flow enhancement being located to the west of the auroral streamer, or to the east of the equatorward flow enhancement, consistently with the spatial relationship between flow shear and upward field‐aligned currents in plasma sheet flow bursts. The azimuthal width of the flow channel is, on average, ~75 km, and the azimuthal offset of the equatorward flow channel relative to the auroral streamer is ~57 km eastward. This study demonstrates the capability of radar‐imager pairs for identifying the 2‐D structure of localized flows associated with plasma sheet flow bursts.

Comparative characteristics of ion and electron precipitation in the dawn and dusk sectors

Characteristics of ion and electron precipitations in the dawn and dusk sectors are investigated by DMSP F6 and F7 satellite observations. It is shown that in the dusk sector the positions of electron and ion precipitation boundaries are nearly coincident for all levels of magnetic activity; however the latitudinal dis� tribution of energy fluxes indicates that the positions of electron and ion precipitation maxima are spatially separated. Maximum energy fluxes of ions is observed at the equatorial precipitation boundary, while those of electrons at the poleward one. In the dawn sector, the electron precipitation region is 3°-4° wider than that of ions. The isotropy boundary in the dusk sector is located in the region of diffuse precipitation (DAZ) near its poleward boundary for all levels of magnetic activity, while in the dawn sector it falls in the region of struc� tured precipitations (AOP). Electron precipitations are dominating in the dawn sector. Here in the region of diffuse precipitation (DAZ), the ion energy fluxes Fi make less than 5% as compared to the electron energy flux Fe. In the region of structured precipitations (AOP), the portion of Fi decreases with increasing magnetic activity from ~10-20% for AL ≈ -100 nT to <5% for AL ≈ -1000 nT. As for the dusk sector, in the AOP region, electron precipitations are dominating as well, while in the DAZ region the ion energy fluxes are significant. In the 1500-1800 MLT sector, the ratio Fi/Fe increases from ~0.7 to ~3.0 with AL changing from -100 nT to -1000 nT.

Open flux in Saturn’s magnetosphere

We characterise the interaction between the solar wind and Saturn’s magnetosphere by evaluating the amount of ‘open’ magnetic flux connected to the solar wind. This is deduced from a large set of Hubble Space Telescope images of the ultraviolet aurora, using the poleward boundary of the main aurora as a proxy for the open-closed field line boundary in the ionosphere. The amount of open flux is found to be 10–50GWb, with a mean of 35GWb. The typical change in open flux between consecutive observations separated by 10–60h is -5 or +7GWb. These changes are a result of imbalance between open flux creation at the dayside magnetopause and its closure in the magnetotail. The 5GWb typical decrease in open flux is consistent with in situ measurements of the flux transported following a reconnection event. Estimates of average, net reconnection rates are found to be typically a few tens of kV, with some extreme examples of unbalanced magnetopause or tail reconnection occurring at ∼300kV. The range of values determined suggest that Saturn’s magnetosphere does not generally achieve a steady state between flux opening at the magnetopause and flux closure in the magnetotail. The percentage of magnetic flux which is open in Saturn’s magnetosphere is similar to that measured at the Earth (2–11%), but the typical percentage that is closed between observations is significantly lower (13% compared to 40–70%). Therefore, open flux is usually closed in smaller (few GWb) events in Saturn’s magnetosphere. The exception to this behaviour is large, rapid flux closure events which are associated with solar wind compressions. While the rates of flux opening and closure should be equal over long timescales, they are evidently different on shorter (up to tens of hours) timescales. The relative independence of the magnetopause and tail reconnection rates can be attributed to the long loading timescales required to transport open field lines into the tail.

Westward traveling surges: Sliding along boundary arcs and distinction from onset arc brightening

After onset, auroral brightening rapidly spreads azimuthally along the onset arc, typically, but not always, extending substantially farther westward than eastward. Expansion‐phase auroras also expand poleward and can contact an arc along the auroral poleward boundary. For such events, we find that poleward expansion of the region of active aurora accelerates after contact, forming a bulge region that protrudes into the preexisting polar cap and expands westward as the westward traveling surge. This westward motion is slower than, and initiates later than, the azimuthal spreading of the onset arc brightening. The poleward boundary arc remains connected to the surge as the surge slides along the equatorward moving edge of the boundary arc, and this connection appears to set off the westward traveling, poleward protruding activity of the surge via localized polar cap flow adjacent to the boundary arc. Ground magnetic signatures show a relatively weak H component depression moving westward with the onset arc brightening, and a delayed substantially larger depression moving with the westward propagating surge that must be associated with a substantially larger westward ionospheric current. The clear distinction between onset arc brightening and the westward traveling surge seen in auroral and magnetometer current signatures implies that their underlying processes are very different. Evidence suggests that flow channels adjacent to the boundary arc possibly feed surge development. These results are consistent with current wedge formation being more associated with post‐onset activity than with onset, and with flow channels from the polar cap being important for surge/bulge development.

Tail reconnection region versus auroral activity inferred from conjugate ARTEMIS plasma sheet flow and auroral observations

AbstractPlasma sheet flow bursts have been suggested to correspond to different types of auroral activity, such as poleward boundary intensifications (PBIs), ensuing auroral streamers, and substorms. The flow‐aurora association leads to the important question of identifying the magnetotail source region for the flow bursts and how this region depends on magnetic activity. The present study uses the ARTEMIS spacecraft coordinated with conjugate ground‐based auroral imager observations to identify flow bursts beyond 45 RE downtail and corresponding auroral forms. We find that quiet‐time flows are directed dominantly earthward with a one‐to‐one correspondence with PBIs. Flow bursts during the substorm recovery phase and during steady magnetospheric convection (SMC) periods are also directed earthward, and these flows are associated with a series of PBIs/streamers lasting for tens of minutes with similar durations to that of the series of earthward flows. Presubstorm onset flows are also earthward and associated with PBIs/streamers. The earthward flows during those magnetic conditions suggest that the flow bursts, which lead to PBIs and streamers, originate from further downtail of ARTEMIS, possibly from the distant‐tail neutral line (DNL) or tailward‐retreated near‐Earth neutral line (NENL) rather than from the nominal NENL location in the midtail. We find that tailward flows are limited primarily to the substorm expansion phase. They continue throughout the period of auroral poleward expansion, indicating that the expansion‐phase flows originate from the NENL and that NENL activity is closely related to the auroral expansion of the substorm expansion phase.

Identifying the magnetotail source region leading to preonset poleward boundary intensifications

AbstractSubstorm auroral onset is often preceded by plasma sheet flow bursts, poleward boundary intensifications (PBIs), and subsequent auroral streamers, suggesting that enhanced transient flows in the plasma sheet propagating toward the near‐Earth plasma sheet play an important role in leading to substorm onset. This study aims at determining the magnetotail source region of preonset PBIs and plasma sheet flows by using the ARTEMIS spacecraft coordinated with the THEMIS all‐sky imager array. ARTEMIS has an advantage of measuring flows between the nominal near‐Earth neutral line (NENL) and distant neutral line (DNL) locations, and thus, the flow direction allows identifying which source region is dominant. We found three preonset conjunction events, and all cases show earthward flow bursts ~10–30 min before substorm onset when PBIs and streamers are detected in the conjugate ionosphere. This time lag is sufficiently long to bring plasma from the ARTEMIS location toward the near‐Earth plasma sheet before onset. The earthward flow direction indicates that preonset enhanced flow channels leading to precursor PBIs originate from further downtail of ARTEMIS, possibly from DNL rather than from NENL. The flow direction changes to tailward after the substorm onset, suggesting that NENL is not related to preonset flow and auroral activity but becomes active during the expansion phase.

New evidence links changing shelf phytoplankton communities to boundary currents in southeast Tasmania

Southern Tasmanian shelf waters are host to the seasonal interplay of Australia’s two poleward boundary currents; the East Australian Current (EAC) and the Leeuwin Current (LC). While the behaviour and properties of the LC remain underexplored, strong research focus has allowed insight into how an intensifying EAC has created greater subtropical influence, leading to changes in the physical and biological oceanography of the region. In this cool temperate setting seven species of dinoflagellates, all in the genus Ceratium, which are more typically associated with warm waters of eastern Australia, were observed. This coincided with the seasonal increase in the EAC’s southward penetration beginning in October. Despite the seasonal peak in EAC activity, temperature-salinity plots, nutrient, chlorophyll a and phytoplankton concentrations all indicate the presence of subantarctic waters on the shelf and in coastal waters in summer. Our results are consistent with the description of the EAC as an erratic, eddy-driven current; this itself allowing the periodic influx of subantarctic waters across the shelf. In winter, temperature-salinity plots and nutrient concentrations indicate that the LC was present in southern shelf waters. In addition to its high nitrate signature, the LC displayed low silicate properties in southern Tasmania. Chlorophyll a concentrations revealed a distinct spring bloom event and an extended, productive summer, typical of temperate and subantarctic systems, respectively. This suggests the region is a transitional state between classic seasonal primary production cycles for temperate and subantarctic waters. This paper links changes in southern Tasmanian microphytoplankton communities to shelf ventilation by the EAC, the LC and subantarctic waters, and provides new insight into the oceanography of the region. Consequently, this study provides an awareness of potential phytoplankton perturbations that may be applied to other coastal cool temperate marine environments.

Coordinated THEMIS spacecraft and all‐sky imager observations of interplanetary shock effects on plasma sheet flow bursts, poleward boundary intensifications, and streamers

AbstractIn order to characterize plasma sheet and nightside auroral disturbances in response to interplanetary shocks, we have examined three interplanetary shock events that occurred when multiple Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft were located in the plasma sheet near midnight while ground‐based aurora data were available near the spacecraft footprints. Large‐scale responses we found are that the magnetotail magnetic pressure started to increase within ~2 min of the SYM‐H jump, and the diffuse aurora near the auroral equatorward boundary intensified over a wide magnetic local time range, due to the shock compressional effect, on average 3 min after the shock arrival. In addition, we also identified plasma sheet and auroral disturbances that are more transient and localized. Earthward or equatorward flow bursts are observed in the near‐Earth plasma sheet on average 5 min after the SYM‐H increase. We find that these fast flows, originating downtail of the near‐Earth spacecraft, form a localized channel, since only some of the spacecraft detected the flow bursts. Poleward boundary intensifications (PBIs) and subsequent north‐south directed auroral streamers are then formed, while no substorm activity was detected. Those auroral forms are also localized in space near midnight and around the footprint of the spacecraft. These results indicate that the fast flows are azimuthally localized channels and are the magnetotail counterpart of the PBIs and streamers and that such localized disturbances are triggered by the interplanetary shocks in addition to the large‐scale compression of the magnetosphere.

The Polar Marine Climate Revisited

Abstract As an additional classification to Köppen’s climate classification for polar (E) climates, the Polar Marine (EM) climate was presented nearly five decades ago and is revisited in this paper. The EM climate was traced to the North Atlantic, North Pacific, and Southern Ocean and recognized as wet, cloudy, and windy, especially during winter. These areas by definition are encompassed by monthly mean air temperatures of −6.7°C (20°F) and 10°C (50°F) in the coldest and warmest months of the annual cycle, respectively. Here three global reanalyses [ECMWF Interim Re-Analysis (ERA-Interim), Climate Forecast System Reanalysis (CFSR), and Japan Meteorological Agency (JMA) 25-yr reanalysis (JRA-25)] are used to produce a modern depiction of EM climate. General agreement is found between original and new EM boundaries, for which the poleward boundary can be approximated by the winter sea ice maximum and the equatorward boundary by the warmest month SSTs. Variability of these parameters is shown to largely dictate the EM area. A downward trend in global EM areal extent for 1979–2010 (−42.4 × 109 m2 yr−1) is dominated by the negative Northern Hemisphere (NH) EM trend (−45.7 × 109 m2 yr−1), whereas the Southern Hemisphere (SH) EM areal trend is insignificant. This observed reduction in NH EM areal extent of roughly 20% over the past three decades, largely from losses at the equatorward boundaries of these biologically rich EM zones, may not be fully compensated by poleward shifts in the EM environment due to projected warming and sea ice decline in the twenty-first century.

Substorm onset and expansion phase intensification precursors seen in polar cap patches and arcs

Localized, transient polar cap flows impinging on the poleward boundary of the nightside auroral oval have been suggested to precede poleward boundary intensifications (PBIs), which are often followed by auroral streamers, some of which propagate equatorward and appear to lead to substorm auroral onset. While echo coverage often limits radar flow measurements, imaging of polar cap patches and arcs has the potential to monitor localized polar cap flows by tracing emission structures, previously associated with enhanced flows, over long distances. We use such imaging to examine if polar cap patches and arcs moving over large distances can be seen as possible precursors to presubstorm‐onset PBIs. We find evidence that such features, which are longitudinally narrow, propagate from the dayside polar region toward the nightside polar cap boundary prior to pre‐onset PBIs. This sequence indicates that transient activity in the dayside polar region may initiate polar cap patches and arcs that propagate across the polar cap and are followed by pre‐onset PBIs. Furthermore, we find evidence that expansion‐phase active aurora expanded poleward soon after additional polar cap patches and arcs reached close to the nightside polar cap boundary. The expansion phase auroral activity significantly weakened when polar cap patches/arcs disappeared. Our findings suggest that plasma transport originating from the dayside and reaching the nightside open‐closed boundary may trigger in plasma sheet flow bursts and play a crucial role in pre‐ and post‐onset auroral activity. Polar cap imaging offers the possibility for monitoring such localized, transient plasma transport over large distances.

Observations of polar cap flow channel and plasma sheet flow bursts during substorm expansion

AbstractWe present the first simultaneous observations of an enhanced polar cap flow impinging on the nightside polar cap boundary (PCB), two flow bursts in the plasma sheet and a conjugate ionospheric flow burst within the auroral oval. The ionospheric measurements on 3 September 2006 were made by the European Incoherent Scatter (EISCAT) radars and the magnetospheric measurements by the four Cluster spacecraft. In the end of a substorm growth phase, EISCAT measured a channel of enhanced equatorward plasma flow within the polar cap, which was about 1° wide in latitude and drifted slowly equatorward. During the substorm expansion phase, the PCB started to contract poleward. The interaction between the equatorward drifting polar cap flow channel and the poleward contracting PCB took 2–3 min. During this time, the F‐region electron temperature was elevated at the PCB, which is interpreted as a possible signature of an auroral poleward boundary intensification (PBI). After that, enhanced equatorward flows were measured inside the auroral oval by EISCAT. During this period, the Cluster satellites measured two fast earthward flow bursts in the plasma sheet, which were associated with dipolarizations of the magnetic field, depletions in plasma density, and return flows. We suggest that the second flow burst in the plasma sheet represents the same flow burst that is seen in the ionosphere by EISCAT and propose that the plasma sheet flow bursts were triggered by the enhanced flow structure on open polar cap field lines. The suggestion is in line with Lyons et al. (2011).

Structure and dynamics of the nightside poleward boundary: Sounding rocket and ground‐based observations of auroral electron precipitation in a rayed curtain

The Cascades2 auroral sounding rocket provides a case study for comparing multipoint in situ ionospheric observations of a nightside auroral poleward boundary intensification with ground‐based optical observations of the same event. Cascades2 was launched northward from Poker Flat Alaska on 20 March 2009 at 11:04 UT. The 13 min flight reached an apogee of 564 km over the northern coast of Alaska. The experiment included a five‐payload array of in situ instrumentation, ground cameras at three different points under the trajectory, multiple ground magnetometers, the Poker Flat Incoherent Scatter Radar (PFISR) radar, and the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in the magnetotail. The rays of the poleward boundary intensification (PBI) curtain have along‐arc motions of 8.5 km/s and along‐arc spacings of 16 km. Modulated maximum energy envelopes and energy fluxes of the associated electron precipitation correspond to this spatial structure of the visible rays. The electron precipitation is additionally modulated at a higher frequency, and velocity dispersion analysis of these 8 Hz signatures implies Alfvénic wave‐particle acceleration of an ambient ionospheric electron source occurring a few hundred km above the observation point. These observations parameterize the curtain of Alfvénic activity above the PBI event, both in the dispersive ionosphere and in the magnetotail reconnection region. The along‐arc variations in brightness correspond to variations in precipitating electron energy flux interpreted as an along‐arc modulation of the maximum energy of the Alfvénic wave‐particle acceleration process; this is a new interpretation of the formation of rayed structures in auroral curtains. We consider the various possible magnetospheric and ionospheric drivers for the control of the observed along‐arc structuring and motions.

Auroral streamers implication for the substorm progression on September 14, 2004

Magnetospheric substorms manifest a fundamental disturbance in geospace. Understanding of substorm progression is essential for solar terrestrial physics and space weather research. The auroral streamer and the bursty bulk flows (BBFs) were simultaneously observed by the IMAGE/WIC and the Cluster in the midtail (X∼−16RE), whose footprint was near the auroral streamer, respectively, during a substorm on September 14, 2004. The auroral poleward boundaries (APBs) are 2∼3° MLAT lower than the open–closed field line boundaries (OCBs) during the growth phase. After the expansion onset, streamer elongated poleward, and the location of APBs became coincident with OCBs. These observations are consistent with the scenario that substorm activations start from plasma sheet reconnection of closed field lines during growth phase; in the expansion phase the open field line in the lobes are involved in reconnection in tail.

Simultaneous ground‐satellite observations of meso‐scale auroral arc undulations

We present simultaneous ground‐based and in situ measurements of a train of meso‐scale (about 100–300 km) auroral arc undulations, occurring in the postmidnight sector (∼1 MLT) between 0040 UT and 0054 UT on September 21, 2009. The undulations appeared at the auroral poleward boundary, and then moved eastward with a speed of 0.9–2.2 km s−1. Dynamic behaviors of the associated meso‐scale ionospheric plasma flows and current systems were also detected with the ground‐based magnetometer and radar measurements within the all‐sky camera field‐of‐view. During the interval of interest, simultaneous Cluster observations in the central near tail region (11–14 RE down tail) were available, and especially the ionospheric footprint of Cluster 2 (CL2) was close to the optical auroral forms. CL2 observed strong fluctuations in the in situ magnetic field with amplitude of 5–10 nT whenever a bright arc area, and its trailing adjacent area, of the auroral undulations passed its ionospheric footprint. Such in situ magnetic field changes at CL2 could be considered as a manifestation of localized upward and downward field‐aligned current sheets moving eastward at the central near‐Earth tail boundary, linked to the meso‐scale auroral undulation structures.

Sea-level fall, carbonate production, rainy days: How do they relate? Insight from Triassic carbonate platforms (Western Tethys, Southern Alps, Italy)

The interplay of sea-level fall, climate, and sedimentological changes is recorded across two sequence boundaries at the top of two Triassic carbonate platform systems in the Western Tethys (earliest Carnian and Norian-Rhaetian boundary in age, paleolatitude 18°–25° N). The sea-level falls caused subaerial exposure of the platform top and decreased carbonate production, leading to starvation in the intraplatform basins, followed by deposition of shale. Evidence of freshwater input indicates that the change in sedimentation was driven by increased rainfall on the previously arid European hinterland and Tethys coast. A uniformitarian approach (Holocene sea-level changes and global warming are coupled with changes in the distribution of precipitation) implicates global cooling as the probable cause of the observed sea-level, climate, and sedimentological changes. Global cooling likely triggered the sea-level fall by increased storage of fresh water in continental settings and change in seawater density, probably coupled with ephemeral ice sheet development, possible even during greenhouse intervals such as the Triassic. Furthermore, global cooling caused a shift toward the equator of the poleward boundary of the arid belt. This model is supported by the traceability of the sequence boundaries and climate-sensitive facies from the Tethys shelf up to the European continent. The observed association of global climate changes, sea-level fall, sedimentological changes, and shift toward humid climate documents how climate-sensitive facies record the control exerted by global changes on local sedimentation.

Periodic aurora surge propagating eastward/westward at poleward boundary of aurora zone during the first 10 min intervals of Pi2 onset

Using magnetometer data acquired by geosynchronous satellite and all-sky images from a ground optical station in the conjugate area, we show propagation of aurora surge repeating in a thin and narrow aurora arc at the poleward boundary of the aurora zone. The surge repetition correlated with the Pi2s at the geosynchronous altitudes. One-to-one conjunctions of the surge and Pi2 pulse were not always observed. The westward or eastward propagation directions of the surge corresponded to the clockwise or counterclockwise wave polarizations in the equatorial plane. Propagation velocities of the poleward surge were in the range of 0.2–0.5°/s or 12–30km/s at 100km altitudes. We discuss the propagating surge at poleward boundary in conjunction with bi-directional flows in the first 10min intervals of Pi2 onset that were inferred by Saka et al. (2010) from the Pi2 epoch analyses of magnetometer data at geosynchronous altitudes.

Quiet-time low latitude ionospheric electrodynamics in the non-hydrostatic Global Ionosphere–Thermosphere Model

Recently the University of Michigan's Global Ionosphere–Thermosphere Model (GITM) has been equipped with new potential dynamo solver. GITM explicitly solves the full three-dimensional dynamo equations of the thermosphere and ionosphere self-consistently with realistic forcing. Unlike the other coupled global ionospheric models, GITM relaxes the hydrostatic assumption. It includes the IGRF magnetic field with the modified APEX coordinate system, set for the date of the simulation. The model runs are initiated with MSIS model for the neutral atmosphere and with IRI model values for the ionospheric charged particles, but eventually computes these quantities self consistently. The model is also updated to include lower boundary tidal forcing. During geomagnetic quiet conditions, the poleward boundary of the ionospheric dynamo in the model is fixed at ±70° geomagnetic latitude. Two-dimensional electrostatic potential patterns are constructed assuming the equipotential field lines. The model is run for 10 days to ensure quasi-equilibrium state. The GITM simulations reproduce the stronger dynamo processes near dusk during solar maximum. The seasonal dependence is investigated during spring equinox, June and December solstices. GITM results indicate a weaker evening pre-reversal enhancement (PRE) during winter and a stronger PRE during equinoctial months. It is found that the time of PRE occurrence in the Philippine sector is independent of the solar activity and season. Also the seasonal effects are prominent near dawn and dusk sectors. The morning reversal of the vertical drifts occurs late during March and the evening reversals are late during month of December. The effects of different lower boundary tidal inputs have been studied, which reveal the significance of E-region wind dynamo in determining F-region plasma drifts. The model is capable of capturing the salient features of the low to mid-latitude ionosphere, and should be a helpful tool for the investigation of the upper atmosphere and ionosphere.