We present a new common stratigraphic timescale for the North Greenland Ice Core Project (NGRIP) and GRIP ice cores. The timescale covers the period 7.9–14.8 kyr before present and includes the ...Bølling, Allerød, Younger Dryas, and early Holocene periods. We use a combination of new and previously published data, the most prominent being new high‐resolution Continuous Flow Analysis (CFA) impurity records from the NGRIP ice core. Several investigators have identified and counted annual layers using a multiparameter approach, and the maximum counting error is estimated to be up to 2% in the Holocene part and about 3% for the older parts. These counting error estimates reflect the number of annual layers that were hard to interpret, but not a possible bias in the set of rules used for annual layer identification. As the GRIP and NGRIP ice cores are not optimal for annual layer counting in the middle and late Holocene, the timescale is tied to a prominent volcanic event inside the 8.2 kyr cold event, recently dated in the DYE‐3 ice core to 8236 years before A. D. 2000 (b2k) with a maximum counting error of 47 years. The new timescale dates the Younger Dryas‐Preboreal transition to 11,703 b2k, which is 100–150 years older than according to the present GRIP and NGRIP timescales. The age of the transition matches the GISP2 timescale within a few years, but viewed over the entire 7.9–14.8 kyr section, there are significant differences between the new timescale and the GISP2 timescale. The transition from the glacial into the Bølling interstadial is dated to 14,692 b2k. The presented timescale is a part of a new Greenland ice core chronology common to the DYE‐3, GRIP, and NGRIP ice cores, named the Greenland Ice Core Chronology 2005 (GICC05). The annual layer thicknesses are observed to be log‐normally distributed with good approximation, and compared to the early Holocene, the mean accumulation rates in the Younger Dryas and Bølling periods are found to be 47 ± 2% and 88 ± 2%, respectively.
We reduce measurements made by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to give the total Birkeland (field‐aligned) current flowing in both hemispheres in ...monthly and hourly bins. We analyze these totals using 6 years of data (2010–2015) to examine solar zenith angle‐driven variations in the total Birkeland current flowing in both hemispheres, simultaneously, for the first time. A diurnal variation is identified in the total Birkeland current flowing, consistent with variations in the solar zenith angle. A seasonal variation is also identified, with more current flowing in the Northern (Southern) Hemisphere during Bartels rotations in northern (southern) summer. For months close to equinox, more current is found to flow in the Northern Hemisphere, contrary to our expectations. We also conduct the first test of the Milan (2013) model for estimating Birkeland current magnitudes, with modifications made to account for solar contributions to ionospheric conductance based on the observed variation of the Birkeland currents with season and time of day. The modified model, using the value of ΦD averaged by Bartels rotation (scaled by 1.7), is found to agree with the observed AMPERE currents, with a correlation of 0.87 in the Northern Hemisphere and 0.86 in the Southern Hemisphere. The improvement over the correlation with dayside reconnection rate is demonstrated to be a significant improvement to the model. The correlation of the residuals is found to be consistent with more current flowing in the Northern Hemisphere. This new observation of systematically larger current flowing in the Northern Hemisphere is discussed in the context of previous results which suggest that the Northern Hemisphere may react more strongly to dayside reconnection than the Southern Hemisphere.
Key Points
There are clear seasonal and diurnal variations in AMPERE‐observed Birkeland current magnitudes
Currents are well described by a combination of dayside reconnection and ionospheric conductance
The magnitude of the currents in the Northern Hemisphere is higher than in the Southern Hemisphere, averaged over a year
The region 1 (R1) and region 2 current systems typically form concentric rings of field‐aligned currents in the polar ionospheres; we term the inner ring the R1 oval. We apply an automated fitting ...scheme to field‐aligned current densities provided by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) and identify the latitude of maximum R1 current at all magnetic local times to yield the size of the R1 oval. We investigate the dynamics of the R1 oval size in response to geomagnetic activity for two cases corresponding to: repeated substorm activations with a minimally enhanced ring current; a significant ring current enhancement with multiple substorms. During the first event the dynamics of the R1 oval size reflected an expanding‐contracting polar cap: during substorm growth phase dayside reconnection added open magnetic flux to the polar cap, expanding the R1 oval equatorward. Tail reconnection during the substorm expansion phase converted open into closed magnetic flux and the polar cap contracts as reflected by the poleward retreat of the R1 oval. During the period of enhanced ring current intensity the R1 oval grew to larger sizes during each substorm growth phase than it did during the other event, consistent with the suggestion that a stronger ring current stabilizes the magnetospheric tail to the onset of magnetic reconnection. The presented methodology allows AMPERE data to be condensed into a single parameter, the R1 oval size, which reflects magnetospheric dynamics and provides a convenient measure of the instantaneous magnetospheric system state in both hemispheres.
Key Points
Global FAC data can be condensed into one parameter, the R1 oval size
The parameter captures magnetospheric dynamics due to day/nightside reconnection
The R1 oval size is a measure of the instantaneous magnetospheric state
The Swarm satellites offer an unprecedented opportunity for improving our knowledge about polar cap patches, which are regarded as the main space weather issue in the polar caps. We present a new ...robust algorithm that automatically detects polar cap patches using in situ plasma density data from Swarm. For both hemispheres, we compute the spatial and seasonal distributions of the patches identified separately by Swarm A and Swarm B between December 2013 and August 2016. We show a clear seasonal dependency of patch occurrence. In the Northern Hemisphere (NH), patches are essentially a winter phenomenon, as their occurrence rate is enhanced during local winter and very low during local summer. Although not as pronounced as in the NH, the same pattern is seen for the Southern Hemisphere (SH). Furthermore, the rate of polar cap patch detection is generally higher in the SH than in the NH, especially on the dayside at about 77° magnetic latitude. Additionally, we show that in the NH the number of patches is higher in the postnoon and prenoon sectors for interplanetary magnetic field (IMF) By<0 and IMF By>0, respectively, and that this trend is mirrored in the SH, consistent with the ionospheric flow convection. Overall, our results confirm previous studies in the NH, shed more light regarding the SH, and provide further insight into polar cap patch climatology. Along with this algorithm, we provide a large data set of patches automatically detected with in situ measurements, which opens new horizons in studies of polar cap phenomena.
Key Points
New polar cap patch detection method based on Swarm in situ data provides an unprecedented data set for polar cap patch statistical studies
Polar cap patch occurrence rate is highest during local winter in both hemispheres; in the south it is also significant during local summer
There is a clear IMF By dependency in the spatial distribution of polar cap patches, consistent with the ionospheric flow pattern
In this paper we examine how polar cap patches, which have been frozen into the antisolar flow over the polar cap, are transported into the nighttime auroral oval. First we present a detailed case ...study from 12 January 2002, with continuous observations of polar cap patches exiting into the nighttime auroral oval in the Scandinavian sector. Satellite images of the auroral oval and all‐sky camera observations of 630.0 nm airglow patches are superimposed onto Super Dual Auroral Radar Network convection maps. These composite plots reveal that polar cap patches exit on both the dusk and on the dawn convection cells. Then we present statistics based on 8 years of data from the meridian scanning photometer at Ny‐Aalesund, Svalbard, to investigate the possible interplanetary magnetic field (IMF) By influence on the distribution of patch exits around magnetic midnight. The magnetic local time distribution of patch exits is almost symmetric around magnetic midnight, independent of IMF By polarity. Synthesizing these observations with previous results, we propose a three‐step mechanism for why patch material exits symmetrically around midnight. First, intake of patch material occurs on both convection cells for both IMF By polarities. Second, plasma intake by transient magnetopause reconnection stretches the newly cut polar cap patches into dawn‐dusk elongated forms during their transport into the polar cap. And finally at exit, dawn‐dusk elongated patches are split and diverted toward both the dawn and dusk flanks when grabbed by transient tail reconnection.
Key Points
Polar cap patches populate both the dawn and the dusk cells symmetrically
The symmetry in patch exits at night is marginally affected by IMF By
East‐west elongated patches are torn apart when grabbed by tail reconnection
High‐resolution in situ measurements from the three Swarm spacecraft, in a string‐of‐pearls configuration, provide new insights about the combined role of flow channel events and particle impact ...ionization in creating F region electron density structures in the northern Scandinavian dayside cusp. We present a case of polar cap patch formation where a reconnection‐driven low‐density relative westward flow channel is eroding the dayside solar‐ionized plasma but where particle impact ionization in the cusp dominates the initial plasma structuring. In the cusp, density features are observed which are twice as dense as the solar‐ionized background. These features then follow the polar cap convection and become less structured and lower in amplitude. These are the first in situ observations tracking polar cap patch evolution from creation by plasma transport and enhancement by cusp precipitation, through entrainment in the polar cap flow and relaxation into smooth patches as they approach the nightside auroral oval.
Key Points
Novel technique using string‐of‐pearls spacecraft to resolve plasma dynamics
In situ Swarm observations of the creation and evolution of polar cap patches
Cusp ionization can dominate the creation and initial structuring of patches
The electron density data from the ICI‐2 sounding rocket experiment in the high‐latitude F region ionosphere are analyzed using the higher‐order spectra and higher‐order statistics. Two regions of ...enhanced fluctuations are chosen for detailed analysis: the trailing edge of a polar cap patch and an electron density enhancement associated with particle precipitation. While these two regions exhibit similar power spectra, our analysis reveals that their internal structures are significantly different. The structures on the edge of the polar cap patch are likely due to nonlinear wave interactions since this region is characterized by intermittency and significant coherent mode coupling. The plasma enhancement subjected to precipitation, however, exhibits stronger random characteristics with uncorrelated phases of density fluctuations. These results suggest that particle precipitation plays a fundamental role in ionospheric plasma structuring creating turbulent‐like structures. We discuss the physical mechanisms that cause plasma structuring as well as the possible processes for the low‐frequency part of the spectrum in terms of plasma instabilities.
Key Points
Steep density gradients in the polar cap reveal nonlinear wave interactions and intermittency
Irregularities at the edge of polar cap patches are characterized by coherent mode coupling
Particle precipitation on plasma gradients gives turbulent‐like structures in high‐latitude F layer
Large thermospheric neutral density enhancements in the cusp region have been examined for many years. The Challenging Minisatellite Payload (CHAMP) satellite for example has enabled many ...observations of the perturbation, showing that it is mesoscale in size and exists statistically over solar cycle timescales. Further studies examining the relationship with magnetospheric energy input have shown that fine‐scale Poynting fluxes are associated with the density perturbations on a case‐by‐case basis, whilst others have found that mesoscale downward fluxes also exist in the cusp region statistically. In this study, we use nearly 8 years of the overlapping Super Dual Auroral Radar Network and Active Magnetosphere and Planetary Electrodynamics Response Experiment datasets to generate global‐scale patterns of the high‐latitude and height‐integrated Poynting flux into the ionosphere, with a time resolution of 2 min. From these, average patterns are generated based on the interplanetary magnetic field orientation. We show the cusp is indeed an important feature in the Poynting flux maps, but the magnitude does not correlate well with statistical neutral mass density perturbations observed by the CHAMP satellite on similar spatial scales. Importantly, the lack of correlation between mesoscale height‐integrated Poynting fluxes and the cusp neutral mass density enhancement gives possible insight into other processes that may account for the discrepancy, such as energy deposition at finer scale sizes or at higher altitudes than captured.
Key Points
Statistical patterns of the total downward Poynting flux into the atmosphere have been derived using Super Dual Auroral Radar Network and Active Magnetosphere and Planetary Electrodynamics Response Experiment data
Statistical patterns of neutral mass density perturbations as a percentage of the background density have been derived using Challenging Minisatellite Payload data
Mesoscale downward Poynting flux in the cusp region do not correlate very well with neutral mass density enhancements at a similar scale
We present Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations of a “Spontaneous Hot Flow Anomaly” (SHFA) upstream from the prenoon bow shock at 0431 UT on 12 ...August 2007. Although the SHFA exhibited the greatly heated and deflected solar wind plasmas used to identify hot flow anomalies (HFAs), it did not result from the standard mechanism invoked for the formation of HFAs, namely the interaction of an interplanetary magnetic field (IMF) discontinuity with the bow shock. We employ THEMIS A, B, C, and D observations to describe the evolution of the event from a proto‐SHFA exhibiting regions of depressed magnetic field strength and density but little evidence for plasma heating or flow deflection, to a well‐developed SHFA further downstream. These observations show that SHFA can be generated without the presence of an IMF discontinuity and are therefore a new category of HFAs.
Key Points
Hot Flow Anomalies can be generated spontaneously at quasi‐parallel shocks.
Hot Flow Anomalies can be generated without the presence of an IMF discontinuity
Multiple spacecraft observations were employed to study the evolution of an HFA.
We examine the relationship of convection electric fields to the formation of a polar cap tongue of ionization (TOI) from midlatitude plumes of storm enhanced density (SED). Observations from the ...geomagnetic storm on 26–27 September 2011 are presented for two distinct SED events. During an hour‐long period of geomagnetic activity driven by a coronal mass ejection, a channel of high‐density F region plasma was transported from the dayside subauroral ionosphere and into the polar cap by enhanced convection electric fields extending to middle latitudes. This TOI feature was associated with enhanced HF backscatter, indicating that it was the seat of active formation of small‐scale irregularities. After the solar wind interplanetary magnetic field conditions quieted and the dayside convection electric fields retreated to higher latitudes, an SED plume was observed extending to, but not entering, the dayside cusp region. This prominent feature in the distribution of total electron content (TEC) persisted for several hours and elongated in magnetic local time with the rotation of the Earth. No ionospheric scatter from SuperDARN radars was observed within this SED region. The source mechanism (enhanced electric fields) previously drawing the plasma from midlatitudes and into the polar cap as a TOI was no longer active, resulting in a fossil feature. We thus demonstrate the controlling role exercised by the convection electric field in generating a TOI from midlatitude SED.
Key Points
Present observations of ionospheric electric field controlling SED/TOI formationAnalyze extent of small‐scale irregularity backscatter within high‐TEC featuresDiscuss characteristics of fossil SED feature which persists for several hours
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