Effects of sudden commencement on the ionosphere: PFISR observations and global MHD simulation

Sudden commencement (SC) induced by solar wind pressure enhancement can produce significant global impact on the coupled magnetosphere‐ionosphere (MI) system, and its effects have been studied extensively using ground magnetometers and coherent scatter radars. However, very limited observations have been reported about the effects of SC on the ionospheric plasma. Here we report detailed Poker Flat Incoherent Scatter Radar (PFISR) observations of the ionospheric response to SC during the 17 March 2015 storm. PFISR observed lifting of the F region ionosphere, transient field‐aligned ion upflow, prompt but short‐lived ion temperature increase, subsequent F region density decrease, and persistent electron temperature increase. A global magnetohydrodynamic (MHD) simulation has been carried out to characterize the SC‐induced current, convection, and magnetic perturbations. Simulated magnetic perturbations at Poker Flat show a satisfactory agreement with observations. The simulation provides a global context for linking localized PFISR observations to large‐scale dynamic processes in the MI system.Key PointsPFISR‐observed ionospheric plasma responses to field‐aligned currents and ionospheric convection vortices formed during sudden commencementResponses include F region plasma lifting, field‐aligned ion upflow, density decrease, short‐lived Ti increase and long‐lasting Te increaseGlobal MHD simulation reproduced the magnetic perturbation on the ground and revealed SC‐related FACs and convection evolutionsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136741/1/grl55728_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136741/2/grl55728.pd

Small‐scale structure of the midlatitude storm enhanced density plume during the 17 March 2015 St. Patrick’s Day storm

Kilometer‐scale density irregularities in the ionosphere can cause ionospheric scintillation—a phenomenon that degrades space‐based navigation and communication signals. During strong geomagnetic storms, the midlatitude ionosphere is primed to produce these ∼1–10 km small‐scale irregularities along the steep gradients between midlatitude storm enhanced density (SED) plumes and the adjacent low‐density trough. The length scales of irregularities on the order of 1–10 km are determined from a combination of spatial, temporal, and frequency analyses using single‐station ground‐based Global Positioning System total electron content (TEC) combined with radar plasma velocity measurements. Kilometer‐scale irregularities are detected along the boundaries of the SED plume and depleted density trough during the 17 March 2015 geomagnetic storm, but not equatorward of the plume or within the plume itself. Analysis using the fast Fourier transform of high‐pass filtered slant TEC suggests that the kilometer‐scale irregularities formed near the poleward gradients of SED plumes can have similar intensity and length scales to those typically found in the aurora but are shown to be distinct phenomena in spacecraft electron precipitation measurements.Key PointsKilometer‐scale density irregularities measured in single‐station GPS TEC data from the 17 March 2015 storm enhanced density plume systemLocation, intensity, and length scales are estimated from spatial, temporal, and frequency analyses of multiple instrument dataFormation regions for small‐scale irregularities with length scales of 3‐10 km are identified for plasma velocities of 500–1200 m s−1Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136745/1/jgra53295_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136745/2/jgra53295.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136745/3/jgra53295-sup-0001-supplementary.pd

Modeling subauroral polarization streams during the 17 March 2013 storm

The subauroral polarization streams (SAPS) are one of the most important features in representing magnetosphere‐ionosphere coupling processes. In this study, we use a state‐of‐the‐art modeling framework that couples an inner magnetospheric ring current model RAM‐SCB with a global MHD model Block‐Adaptive Tree Solar‐wind Roe Upwind Scheme (BATS‐R‐US) and an ionospheric potential solver to study the SAPS that occurred during the 17 March 2013 storm event as well as to assess the modeling capability. Both ionospheric and magnetospheric signatures associated with SAPS are analyzed to understand the spatial and temporal evolution of the electrodynamics in the midlatitude regions. Results show that the model captures the SAPS at subauroral latitudes, where Region 2 field‐aligned currents (FACs) flow down to the ionosphere and the conductance is lower than in the higher‐latitude auroral zone. Comparisons to observations such as FACs observed by Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), cross‐track ion drift from Defense Meteorological Satellite Program (DMSP), and in situ electric field observations from the Van Allen Probes indicate that the model generally reproduces the global dynamics of the Region 2 FACs, the position of SAPS along the DMSP, and the location of the SAPS electric field around L of 3.0 in the inner magnetosphere near the equator. The model also demonstrates double westward flow channels in the dusk sector (the higher‐latitude auroral convection and the subauroral SAPS) and captures the mechanism of the SAPS. However, the comparison with ion drifts along DMSP trajectories shows an underestimate of the magnitude of the SAPS and the sensitivity to the specific location and time. The comparison of the SAPS electric field with that measured from the Van Allen Probes shows that the simulated SAPS electric field penetrates deeper than in reality, implying that the shielding from the Region 2 FACs in the model is not well represented. Possible solutions in future studies to improve the modeling capability include implementing a self‐consistent ionospheric conductivity module from inner magnetosphere particle precipitation, coupling with the thermosphere‐ionosphere chemical processes, and connecting the ionosphere with the inner magnetosphere by the stronger Region 2 FACs calculated in the inner magnetosphere model.Key PointsSAPS simulation using BATS‐R‐US coupled with ring current model RAM‐SCBComparisons done with AMPERE, DMSP, and Van Allen Probes observationsCaptured the basic physics and mechanism of SAPSPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111134/1/jgra51638.pd

Response of the Geospace System to the Solar Wind Dynamic Pressure Decrease on 11 June 2017: Numerical Models and Observations

On 11 June 2017, a sudden solar wind dynamic pressure decrease occurred at 1437 UT according to the OMNI solar wind data. The solar wind velocity did not change significantly, while the density dropped from 42 to 10 cm−3 in a minute. The interplanetary magnetic field BZ was weakly northward during the event, while the BY changed from positive to negative. Using the University of Michigan Block Adaptive Tree Solarwind Roe Upwind Scheme global magnetohydrodynamic code, the global responses to the decrease in the solar wind dynamic pressure were studied. The simulation revealed that the magnetospheric expansion consisted of two phases similar to the responses during magnetospheric compression, namely, a negative preliminary impulse and a negative main impulse phase. The simulated plasma flow and magnetic fields reasonably reproduced the Time History of Events and Macroscale Interactions during Substorms and Magnetospheric Multiscale spacecraft in situ observations. Two separate pairs of dawn‐dusk vortices formed during the expansion of the magnetosphere, leading to two separate pairs of field‐aligned current cells. The effects of the flow and auroral precipitation on the ionosphere‐thermosphere (I‐T) system were investigated using the Global Ionosphere Thermosphere Model driven by simulated ionospheric electrodynamics. The perturbations in the convection electric fields caused enhancements in the ion and electron temperatures. This study shows that, like the well‐studied sudden solar wind pressure increases, sudden pressure decreases can have large impacts in the coupled I‐T system. In addition, the responses of the I‐T system depend on the initial convection flows and field‐aligned current profiles before the solar wind pressure perturbations.Key PointsThe decrease in the solar wind dynamic pressure led to two separate pairs of oppositely rotating vortices in the dawn and duskFACs accompanied each magnetospheric vortex and altered the ionosphere convection patternsJoule heating increased in the regions sandwiched by the perturbation FACs, leading to increased ion temperaturesPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149314/1/jgra54868.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149314/2/jgra54868_am.pd

Segmentation of SED by Boundary Flows Associated With Westward Drifting Partial Ring current

The segmentation mechanism of polar cap patches is agreed to be related to temporal changes of interplanetary magnetic field or transient reconnection. In this letter, using Global Ionosphere Thermosphere Model driven by two‐way coupled Block‐Adaptive‐Tree‐Solarwind‐Roe‐Upwind‐Scheme and Rice Convection Model, a new segmentation mechanism is proposed. This mechanism works as follows: A strong boundary flow between the Region 1 and Region 2 field‐aligned currents develops, while a shielding process develops in the inner magnetosphere. As the partial ring current drifts westward, the peak of the boundary flow also moves westward. This strong boundary flow raises the ion temperature through enhanced frictional heating, enhances the chemical recombination reaction rate, and reduces the electron density. When this boundary flow crosses the storm‐enhanced density (SED) plume, the plume will be segmented into patches. No external interplanetary magnetic field variations or transient reconnections are required in this mechanism.Key PointsBoundary flows between Region 1 and Region 2 FACs segment SED plume into patchesLocalized plasma loss is due to enhanced frictional heating within boundary flowsNo external IMF direction change is needed in this segmentation scenarioPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151263/1/grl59354.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151263/2/grl59354-sup-0006-Text_SI-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151263/3/grl59354_am.pd

Statistical Analysis of the Main Ionospheric Trough Using Swarm in Situ Measurements

A statistical analysis of the topside main ionospheric trough is implemented by using the Swarm constellation in situ plasma density measurements from December 2013 to November 2019. The key features of the main trough, such as the occurrence rate, minimum position, width, and depth, are characterized and quantified. The distribution patterns of these parameters are investigated with respect to magnetic local time, season, longitude, solar activity, and geomagnetic activity levels, respectively. The main results are as follows: (1) The diurnal variation of the trough occurrence rate usually exhibits a primary peak in the early morning, a subsidiary peak in the late evening, and a slight reduction around midnight especially in the Northern Hemisphere. (2) The seasonal variation of the nighttime trough has maximum occurrence rates around equinoxes, higher than those in local winter. (3) The trough distribution has an evident hemispherical asymmetry. It is more pronounced in the Northern Hemisphere during the winter and equinoctial seasons, with its average nighttime occurrence rate being 20â 30% higher than that in the Southern Hemisphere. The trough minimum position and the trough width also exhibit more significant fluctuation in the Northern Hemisphere. (4) The longitudinal pattern of the trough shows clear eastâ west preferences, which has a higher occurrence rate in eastern (western) longitudes around the December (June) solstice. (5) Conditions for the trough occurrence are more favored in low solar activity and high geomagnetic activity periods.Key PointsThe occurrence rate of the main ionospheric trough at 450â 550 km exhibits a slight midnight reduction comparing with evening/morning peaksThe trough has a longitudinal preference with higher occurrence rate in the eastern (western) longitudes around the December (June) solsticeConditions for the trough occurrence are more favored in equinoxes than local winter and in Northern Hemisphere than Southern HemispherePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154408/1/jgra55592.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154408/2/jgra55592_am.pd

Direct Observations of a Polar Cap Patch Formation Associated With Dayside Reconnection Driven Fast Flow

Dayside solar‐produced concentrated F region plasma can be transported from the midlatitude region into the polar cap during geomagnetically disturbed period, creating plasma density irregularities like polar cap patches, which can cause scintillation and degrade performance of satellite communication and navigation at polar latitudes. In this paper, we observed and investigated a dynamic formation process of a polar cap patch during the 13 October 2016 intense geomagnetic storm. During the storm main phase, storm‐enhanced density (SED) was formed within an extended period of strong southward interplanetary magnetic field (IMF) Bz condition. Total electron content (TEC) map shows that a polar cap patch was segmented from the SED plume. The Sondrestrom Incoherent Scatter Radar (ISR) was right underneath the segmentation region and captured the dynamic process. It shows that the patch segmentation was related with a sudden northeastward flow enhancement reaching ~2 km/s near the dayside cusp inflow region. The flow surge was observed along with abrupt E region electron temperature increase, F region ion temperature increase, and density decrease. The upstream solar wind and IMF observations suggest that the flow enhancement was associated with dayside magnetic reconnection triggered by a sudden and short period of IMF By negative excursion. Quantitative estimation suggests that plasma density loss due to enhanced frictional heating was insufficient for the patch segmentation because the elevated F region density peaking at ~500 km made dissociative recombination inefficient. Instead, the patch was segmented from the SED by low‐density plasma transported by the fast flow channel from earlier local time.Key PointsFormation of a polar cap patch was directly observed by the GPS TEC maps and Sondrestrom ISRDayside magnetic reconnection‐driven fast flow near cusp carrying low‐density cold plasma segmented the SED plume into polar cap patchesThe F layer height within SED before it enters the cusp is important in determining the most efficient segmentation mechanismPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154899/1/jgra55613_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154899/2/jgra55613.pd

GPS TEC observations of dynamics of the mid‐latitude trough during substorms

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95166/1/grl28288.pd

The Effect of F‐Layer Zonal Neutral Wind on the Monthly and Longitudinal Variability of Equatorial Ionosphere Irregularity and Drift Velocity

The effect of eastward zonal wind speed (EZWS) on vertical drift velocity (E × Bdrift) that mainly controls the equatorial ionospheric irregularities has been explained theoretically and through numerical models. However, its effect on the seasonal and longitudinal variations of E × B and the accompanying irregularities has not yet been investigated experimentally due to lack of F‐layer wind speed measurements. Observations of EZWS from GOCE and ion density and E × B from C/NOFS satellites for years 2011 and 2012 during quite times are used in this study. Monthly and longitudinal variations of the irregularity occurrence, E × B, and EZWS show similar patterns. We find that at most 50.85% of longitudinal variations of E × B can be explained by the longitudinal variability of EZWS only. When the EZWS exceeds 150 m/s, the longitudinal variation of EZWS, geomagnetic field strength, and Pedersen conductivity explain 56.40–69.20% of the longitudinal variation of E × B. In Atlantic, Africa, and Indian sectors, from 42.63% to 79.80% of the monthly variations of the E × B can be explained by the monthly variations of EZWS only. It is found also that EZWS and E × B may be linearly correlated during fall equinox and December solstice. The peak occurrence of irregularity in the Atlantic sector during November and December is due to the combined effect of large wind speed, solar terminator‐geomagnetic field alignment, and small geomagnetic field strength and Pedersen conductivity. Moreover, during June solstices, small EZWS corresponds to vertically downward E × B, which suggests that other factors dominate the E × B drift rather than the EZWS during these periods.Key PointsZonal neutral wind controls more the seasonal variations of E × B drift than the longitudinal variations of E × B driftAt most 50.85% of the longitudinal variations of E × B drift are accounted for by the eastward zonal neutral wind speed onlyZonal neutral wind speed and E × B drift may be linearly correlated during fall equinox and December solsticePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155994/1/jgra55709.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/155994/2/jgra55709_am.pd