The Super Dual Auroral Radar Network (SuperDARN) was built to study ionospheric convection at Earth and has in recent years been expanded to lower latitudes to observe ionospheric flows over a larger ...latitude range. This enables us to study extreme space weather events, such as geomagnetic storms, which are a global phenomenon, on a large scale (from the pole to magnetic latitudes of 40°). We study the backscatter observations from the SuperDARN radars during all geomagnetic storm phases from the most recent solar cycle and compare them to other active times to understand radar backscatter and ionospheric convection characteristics during extreme conditions and to discern differences specific to geomagnetic storms and other geomagnetically active times. We show that there are clear differences in the number of measurements the radars make, the maximum flow speeds observed, and the locations where they are observed during the initial, main, and recovery phase. We show that these differences are linked to different levels of solar wind driving. We also show that when studying ionospheric convection during geomagnetically active times, it is crucial to consider data at midlatitudes, as we find that during 19% of storm time the equatorward boundary of the convection is located below 50° of magnetic latitude.
Plain Language Summary
During geomagnetic storms, electrical currents which flow in space around the Earth change the magnetic field we measure at Earth. We use this to identify storms and look at how measurements from radars during the storm phases compare to other times when the disturbance of the geomagnetic field is similar, as well as during times when the solar wind (which drives the storms) is high and behaves similarly as to storms. The radars we use are located at high latitudes and are purposely built to measure ionospheric convection.
We find that the ionospheric convection during the main phase of a storm spans to much lower latitudes than previously thought: 40 degrees of magnetic latitude.
We also show that the initial and recovery phases of a geomagnetic storm show similar ionospheric convection as periods when enhanced solar wind driving, but no geomagnetic storm occurs. Whereas the main phase of a storm shows a faster moving and more disturbed ionosphere.
The initial and recovery phase show similar behavior, whereas the main phase of a storm shows higher ionospheric convection due to higher solar wind driving.
Key Points
During geomagnetic storms and enhanced solar wind driving, ionospheric convection expands to latitudes as low as 40° magnetic latitude
Initial and recovery phases of geomagnetic storms show similar convection as enhanced solar wind driving when no geomagnetic storm occurs
Main phase shows most scatter, fastest flows (CPCP 80 kV instead of 40 kV during initial and recovery) due to higher solar wind driving
Previous work has shown that earthward convective flow bursts in the magnetotail have a dusk‐dawn (v⊥y ${v}_{\perp y}$) component that is controlled by the historical state of the Interplanetary ...Magnetic Field (IMF) By component. Here, we analyze 27 years of Cluster, THEMIS and Geotail plasma and magnetic field data and identify 1,639 magnetotail fast flow “detections” that demonstrate a dusk‐dawn asymmetry. We find that ∼70% has a dusk‐dawn direction consistent with that expected from the penetration of IMF By. Superposed epoch analysis suggests that the inconsistency of the remaining ∼30% is not due to a lack of the expected IMF By penetration. Instead, we find that on average, the expected sense of IMF By penetration is associated with flows irrespective of whether those flows agree or disagree with the expected dusk‐dawn asymmetry. IMF By, and the penetrated By do, however, tend to be stronger for “agree” flows. Detections which agree (disagree) tend to be accompanied by a localized perturbation to the By component of the magnetotail magnetic field in the same sign as (opposite to) the prevailing IMF By conditions, which temporarily enhances (overrides) the penetrated field. Agree (disagree) flows also appear to be observed further away from (closer to) the neutral sheet (Bx = 0) and are associated with weaker (stronger) magnetic field dipolarization. Finally, we find that the slower “background” convective flow has an average direction which is consistent with penetration of the expected IMF By, regardless of whether the fast flow itself agrees or disagrees.
Key Points
Magnetotail fast flows with a dusk‐dawn direction inconsistent with IMF By can occur irrespective of the sense of IMF By penetration
These inconsistent flows occur within a slower “background” convection that is consistent with the expected IMF By sense
Localized, transient dynamics can override or prevent IMF By control of the flow, notably when IMF By is weaker
In the polar ionosphere, variations in Joule heating are significantly controlled by changes in plasma convection, such as that brought about by changes in the interplanetary magnetic field. However, ...another important consideration when calculating Joule heating is the velocity difference between this plasma and the neutral thermosphere colocated with the ionosphere. Neutral wind data are often difficult to obtain on a global scale; thus, Joule heating has often previously been calculated assuming that neutral velocities are small and can therefore be neglected. Previous work has shown the effect of neutral winds on Joule heating estimations to be more significant than originally thought; however, the diurnal variations of the neutrals due to changes in solar pressure gradients and Coriolis forces have yet to have their impact on Joule heating assessed. We show this universal time effect to be significant in calculating Joule heating and thus can differ significantly from that calculated by neglecting the neutrals. In this study, we use empirical models for the neutral wind, conductivities, and magnetic field to create Northern Hemispheric patterns of Joule heating for approximately 800,000 individual plasma convection patterns generated using data from the Super Dual Auroral Radar Network. From this, a statistical analysis of how Joule heating varies in morphology and magnitude with universal time is shown for differing seasons and levels of geomagnetic activity. We find that neutral winds do play a significant role in the morphology and total energy output of Joule heating.
Key Points
We derive statistical patterns of high‐latitude Joule heating for varying universal times
Joule heating has a significant universal time dependence due to varying neutral winds
Season and level of geomagnetic activity affect how much of a contribution the neutrals make
We use a 20 years database of Super Dual Auroral Radar Network (SuperDARN) observations to investigate the two component model of ionospheric convection. A convection pattern is included in the ...database if it is derived from at least 250 radar vectors and has a distribution of electric potential consistent with Dungey‐cycle twin vortex flow (a negative potential peak in the dusk cell and a positive potential peak in the dawn cell). We extract the locations of the foci of the convection cells from the SuperDARN convection patterns, and compare their dependencies on the north‐south component of the interplanetary magnetic field, IMF BZ, and the SuperMAG auroral electrojet index, SML. We use these parameters to define intervals of expected dayside or nightside dominated reconnection. Our results show that, under conditions favorable for dominant dayside reconnection, the dawn and dusk foci are shifted toward the dayside and that, under conditions favorable for dominant nightside reconnection, the dawn and dusk foci are shifted toward the nightside.
Plain Language Summary
The Earth's upper atmosphere is coupled to the near‐Earth space environment—the magnetosphere—via the planet's magnetic field. This magnetic coupling drives a circulation of plasma—the electrically charged component of the atmosphere, called the ionosphere—from day to night across the poles and back again at lower latitudes. This circulation of plasma is a key component of the energy transport in the magnetosphere‐ionosphere system. The circulation is not steady, instead changing in strength whilst expanding and contracting due to the time‐dependence of the driving mechanisms. To understand these mechanisms we can model the ionospheric circulation and test the models with observations. In this paper we use a 20 years database of ionospheric radar observations of the plasma flow to test one such model—the expanding‐contracting polar cap model—and find evidence to support its predictions of separate dayside and nightside components of the flow.
Key Points
The separation of the dawn and dusk ionospheric convection cell foci is found to vary from 4 to 22 hr of magnetic local time
When the interplanetary magnetic field (IMF) is southwards and the auroral westward electrojet weak, the foci move to the dayside
When the IMF is strongly northwards and the auroral electrojet modestly active, the foci move toward the nightside
Dusk‐Dawn Asymmetries in SuperDARN Convection Maps Walach, M.‐T.; Grocott, A.; Thomas, E. G. ...
Journal of geophysical research. Space physics,
December 2022, 2022-Dec, 2022-12-00, 20221201, Letnik:
127, Številka:
12
Journal Article
Recenzirano
Odprti dostop
The Super Dual Auroral Radar Network (SuperDARN) is a collection of radars built to study ionospheric convection. We use a 7‐year archive of SuperDARN convection maps, processed in 3 different ways, ...to build a statistical understanding of dusk‐dawn asymmetries in the convection patterns. We find that the data set processing alone can introduce a bias which manifests itself in dusk‐dawn asymmetries. We find that the solar wind clock angle affects the balance in the strength of the convection cells. We further find that the location of the positive potential foci is most likely observed at latitudes of 78° for long periods (>300 min) of southward interplanetary magnetic field (IMF), as opposed to 74° for short periods (<20 min) of steady IMF. For long steady dawnward IMF the median is also at 78°. For long steady periods of duskward IMF, the positive potential foci tends to be at lower latitudes than the negative potential and vice versa during dawnward IMF. For long periods of steady Northward IMF, the positive and negative cells can swap sides in the convection pattern. We find that they move from ∼0–9 MLT to 15 MLT or ∼15–23 MLT to 10 MLT, which reduces asymmetry in the average convection cell locations for Northward IMF. We also investigate the width of the region in which the convection returns to the dayside, the return flow width. Asymmetries in this are not obvious, until we select by solar wind conditions, when the return flow region is widest for the negative convection cell during Southward IMF.
Plain Language Summary
At high latitudes, near the Earth's magnetic pole, the ionosphere moves around in a dual‐cell pattern: The convection moves from the dayside, over the magnetic pole toward the nightside and then flows return back to the dayside at lower latitudes. Both cells tend to be centered away from the pole, one toward the dusk side and one toward the dawn side. The two cells have a tendency to be asymmetric with the dusk cell typically larger and stronger. Asymmetries in the two convection cells are often attributed to changes in the solar wind as there is a physical connection between the ionosphere and the solar wind. The mechanisms which describe this interaction are well known but some of the data sets with which we measure ionospheric convection have unquantified uncertainties associated with them. One of the longest running measurement systems of the ionospheric convection is the Super Dual Auroral Radar Network (SuperDARN). This ground‐based system was built specifically to measure ionospheric convection and it is often used to make convection maps of the ionosphere. Over the years, more radars have been added to the network and the software used to process the data has been updated. In this study we use different versions of the convection maps to statistically investigate 7 years of ionospheric convection asymmetries and understand which of the asymmetries were introduced by a change in the data set and which by the solar wind. We look at the location and strength of the cells and the width of the return flow region, which constrains the size of the cells.
Key Points
We study dusk‐dawn asymmetries in 7 years of Super Dual Auroral Radar Network convection maps which are introduced by solar wind orientations, or data processing
Asymmetries due to interplanetary magnetic field By can occur in the strength and location of the convection cells, and the return flow width
Asymmetries due to the background model are likely to occur in the locations of the convection cells
We exploit a database of high‐latitude ionospheric electric potential patterns, derived from radar observations of plasma convection in the Northern Hemisphere from the years 2000–2006, to ...investigate the timescales of interplanetary magnetic field (IMF) control of ionospheric convection and associated magnetospheric dynamics. We parameterize the convection observations by IMF clock angle, θ (the angle between geocentric solar magnetic (GSM) north and the projection of the IMF vector onto the GSM Y‐Z plane), and by an IMF timescale, τB (the length of time that a similar clock angle has been maintained prior to the convection observations being made). We find that the nature of the ionospheric convection changes with IMF clock angle, as expected from previous time‐averaged studies, and that for τB∼30 min, the convection patterns closely resemble their time‐averaged counterparts. However, as τB increases we find that the convection evolves away from the time‐averaged patterns to reveal modified characteristic flow features. We discuss these findings in terms of solar wind‐magnetosphere‐ionosphere coupling and consider their implications for understanding the time‐dependent nature of magnetospheric dynamics.
Key Points
Ordering the ionospheric convection by static IMF parameters is inadequate
IMF history is important in governing coupled magnetosphere‐ionosphere dynamics
Long, steady intervals of IMF continue to modify the ionospheric convection
We utilize principal component analysis to identify and quantify the primary electric potential morphologies during geomagnetic storms. Ordering data from the Super Dual Auroral Radar Network ...(SuperDARN) by geomagnetic storm phase, we are able to discern changes that occur in association with the development of the storm phases. Along with information on the size of the patterns, the first six eigenvectors provide over ∼80% of the variability in the morphology, providing us with a robust analysis tool to quantify the main changes in the patterns. Studying the first six eigenvectors and their eigenvalues shows that the primary changes in the morphologies with respect to storm phase are the convection potential enhancing and the dayside throat rotating from pointing toward the early afternoon sector to being more sunward aligned during the main phase of the storm. We find that the ionospheric electric potential increases through the main phase and then decreases once the storm phase begins. The dayside convection throat points toward the afternoon sector before the main phase and then as the potential increases throughout the main phase, the dayside throat rotates toward magnetic noon. Furthermore, we find that a two‐cell convection pattern is dominant throughout and that the dusk cell is overall stronger than the dawn cell.
Plain Language Summary
During geomagnetic storms, we see extreme changes to Earth's magnetic field structure. This is mainly due to an enhancement of electrical currents in geospace. This changes the Earth's magnetic environment, due to which we also see changes in the ionosphere, the layer of charged particles making up the top of the atmosphere where the current systems close. A geomagnetic storm has three phases: the initial phase, which is a precursor to the storm, the main phase where the current systems enhance abruptly, and a recovery phase. Here, we use a technique commonly used for pattern recognition to radar data to work out the changes to the average ionospheric flows. We find that most of the changes happen on the dayside. We suggest this means the average storm dynamics are driven directly by the solar wind.
Key Points
Using principal component analysis on SuperDARN data, we identify primary contributing basis convection patterns during geomagnetic storms
The first six eigenvectors of the analysis provide over 80% of the total variance, excluding expansions and contractions of the pattern
Main changes in the electric field are an enhancement and a motion toward later local times of the dayside convection throat
Previous studies have shown that there is a correlation between the By component of the interplanetary magnetic field (IMF) and the By component observed in the magnetotail lobe and in the plasma ...sheet. However, studies of the effect of IMF By on several magnetospheric processes have indicated that the By component in the tail should depend more strongly on the recent history of the IMF By rather than on the simultaneous measurements of the IMF. Estimates of this timescale vary from ∼25 min to ∼4 h. We present a statistical study of how promptly the IMF By component is transferred into the neutral sheet, based on Cluster observations of the neutral sheet from 2001 to 2008, and solar wind data from the OMNI database. Five thousand nine hundred eighty‐two neutral sheet crossings during this interval were identified, and starting with the correlation between instantaneous measurements of the IMF and the magnetotail (recently reported by Cao et al. (2014)), we vary the time delay applied to the solar wind data. Our results suggest a bimodal distribution with peaks at ∼1.5 and ∼3 h. The relative strength of each peak appears to be well controlled by the sign of the IMF Bz component with peaks being observed at 1 h of lag time for southward IMF and up to 5 h for northward IMF conditions, and the magnitude of the solar wind velocity with peaks at 2 h of lag time for fast solar wind and 4 h for slow solar wind conditions.
Plain Language Summary
As the solar wind radiates away from the Sun it carries with it a magnetic field from the Sun. This magnetic field interacts with the magnetic field of the Earth in a large scale cycle called the Dungey cycle. The interaction between the solar wind and terrestrial magnetic field allows energetic particles (that make up the solar wind) to enter the Earth's environment which cause the aurora or, in the case of extreme solar storms, surges in power grids. How long it takes for energetic particles to fully penetrate into the Earth's magnetic field is dependent on the timescales associated with the Dungey cycle. In this article we correlate measurements of the magnetic field on the night side of Earth with lagged measurements from the solar wind in order to find at what time the configuration of the magnetotail is most closely related to that of the solar wind. We find that the timescales vary depending on the solar wind conditions but it typically takes on the order of a few hours (1‐5 hours).
Key Points
Dayside reconnection can introduce a By component into the magnetosphere, in the same sense as the IMF By
The Dungey cycle transfers field lines with this induced By component into the magnetotail
The timescale for this process is found to be between 1‐5 h, depending on a few contributing factors
It has previously been shown that in the high‐latitude thermosphere, sudden changes in plasma velocity (such as those due to changes in interplanetary magnetic field) are not immediately propagated ...into the neutral gas via the ion‐drag force. This is due to the neutral particles (O, O2, and N2) constituting the bulk mass of the thermospheric altitude range and thus holding on to residual inertia from a previous level of geomagnetic forcing. This means that consistent forcing (or dragging) from the ionospheric plasma is required, over a period of time, long enough for the neutrals to reach an equilibrium with regard to ion drag. Furthermore, mesoscale variations in the plasma convection morphology, solar pressure gradients, and other forces indicate that the thermosphere‐ionosphere coupling mechanism will also vary in strength across small spatial scales. Using data from the Super Dual Auroral Radar Network and a Scanning Doppler Imager, a geomagnetically active event was identified, which showed plasma flows clearly imparting momentum to the neutrals. A cross‐correlation analysis determined that the average time for the neutral winds to accelerate fully into the direction of ion drag was 75 min, but crucially, this time varied by up to 30 min (between 67 and 97 min) within a 1,000‐km field of view at an altitude of around 250 km. It is clear from this that the mesoscale structure of both the plasma and neutrals has a significant effect on ion‐neutral coupling strength and thus energy transfer in the thermosphere.
Key Points
The delay of thermospheric neutral winds fully responding to changes in ion‐drag is examined for locations separated by about 100 km
In this study, neutrals took 67–97 min to fully change velocity after changes in the ionospheric plasma, for regions within a 1,000‐km FOV
The neutral wind flywheel effect is significant when the neutral velocity begins to overtake that of the plasma
Past studies have demonstrated that the interplanetary magnetic field (IMF) By component introduces asymmetries in the magnetosphere‐ionosphere (M‐I) system, though the exact timings involved are ...still unclear with two distinct mechanisms proposed. In this study, we statistically analyze convective flows from three regions of the M‐I system: the magnetospheric lobes, the plasma sheet, and the ionosphere. We perform superposed epoch analyses on the convective flows in response to reversals in the IMF By orientation, to determine the flow response timescales of these regions. We find that the lobes respond quickly and reconfigure to the new IMF By state within 30–40 min. The plasma sheet flows, however, do not show a clear response to the IMF By reversal, at least within 4 hr postreversal. The ionospheric data, measured by the Super Dual Auroral Radar Network (SuperDARN), match their counterpart magnetospheric flows, with clear and prompt responses at ≥75° magnetic latitude (MLAT) but a less pronounced response at 60–70 MLAT. We discuss the potential implication of these results on the mechanisms for introducing the IMF By component into the M‐I system.
Key Points
Flows in the magnetotail lobes respond promptly to changes in the IMF By orientation, reaching a new state within 30–40 min
No clear flow response is detected on timescales of up to 4 hr in the plasma sheet
Ionospheric flows exhibit clear responses at higher latitudes and a less pronounced responses at lower latitudes
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