skip to main content

Title: Dayside Polar Cap Density Enhancements Formed During Substorms

The formation of polar cap density enhancements, such as tongues‐of‐ionization (TOIs), are often attributed to enhanced dayside reconnection and convection due to solar wind changes. However, ionospheric poleward moving density enhancements can also form in the absence of changes in the solar wind. This study examines how TOI and patch events that are not triggered by solar wind changes relate to magnetospheric processes, specifically substorms. Based on total electron content and Super Dual Auroral Radar Network (SuperDARN) observations, we find substorms that occur at the same time as TOIs are associated with sudden enhancements in dayside poleward flows during the substorm expansion phase. Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations also show enhanced field‐aligned currents (FACs) that extend into the dayside ionosphere during this period. We suggest that the global enhancement of FACs and convection during these substorms are the drivers of these TOIs by enhancing dayside convection and transporting high‐density lower‐latitude plasma into the polar cap. However, we also find that not all substorms are coincident with polar cap density enhancements. A superposed epoch study showed that the AL index for TOIs during substorms is not particularly stronger than substorms without TOIs, but epoch studies of AMPERE observations do show events with TOIs to have a higher total FAC on both the dayside and nightside. Our results show the importance of TOI formation during substorms when solar wind drivers are absent, and the importance of considering substorms in the global current system. This work also shows the need to incorporate substorms into models of high‐latitude global convection and currents.

more » « less
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Space Physics
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    We present examples of high‐latitude field‐aligned current (FAC) and toroidal magnetic potential patterns in both hemispheres reconstructed at a 2‐min cadence using an updated optimal interpolation (OI) method that ingests magnetic perturbation data provided by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) program. A solstice and an equinoctial event are studied to demonstrate the reconstructed patterns and to provide scientific insights into FAC response to different solar wind drivers. For the 14 June 2011 high‐speed stream event with mostly northwardBzdriving, we found persistently stronger FACs in the Northern Hemisphere. Extreme interhemispheric asymmetry is associated with the interplanetary magnetic field (IMF) direction and large dipole tilt, consistent with earlier studies. FAC asymmetries seen during an isolated substorm can be attributed to dipole tilt. During relatively low geomagnetic activity, the FAC response to IMFBxchanges is identified. For the 17–18 March 2013 period, we provide global snapshots of rapid FAC changes related to an interplanetary shock passage. We further present comparisons between instantaneous and mean behaviors of FAC for the solar wind sheath passage and interplanetary coronal mass ejection southwardBzinterval and northwardBzintervals. We show that (1) sheath passage results in strong FAC and high variation in the dayside polar cap region and pre‐midnight region, different from the typical R1/R2 currents during prolonged southwardBz; (2) four‐cell reverse patterns appear during northwardBzbut are not stable; and (3) persistent dawn‐dusk asymmetry is seen throughout the storm, especially during an extreme substorm, likely associated with a dawnside current wedge.

    more » « less
  2. Abstract

    We propose a mechanism for the formation of the horse‐collar auroral configuration during periods of strongly northward interplanetary magnetic field (IMF), invoking the action of dual‐lobe reconnection (DLR). Auroral observations are provided by the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite and spacecraft of the Defense Meteorological Satellite Program (DMSP). We also use ionospheric flow measurements from DMSP and polar maps of field‐aligned currents (FACs) derived from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). Sunward convection is observed within the dark polar cap, with antisunward flows within the horse‐collar auroral region, together with the NBZ FAC distribution expected to be associated with DLR. We suggest that newly closed flux is transported antisunward and to dawn and dusk within the reverse lobe cell convection pattern associated with DLR, causing the polar cap to acquire a teardrop shape and weak auroras to form at high latitudes. Horse‐collar auroras are a common feature of the quiet magnetosphere, and this model provides a first understanding of their formation, resolving several outstanding questions regarding the nature of DLR and the magnetospheric structure and dynamics during northward IMF. The model can also provide insights into the trapping of solar wind plasma by the magnetosphere and the formation of a low‐latitude boundary layer and cold, dense plasma sheet. We speculate that prolonged DLR could lead to a fully closed magnetosphere, with the formation of horse‐collar auroras being an intermediate step.

    more » « less
  3. Abstract

    We report the observation of solar wind‐magnetosphere‐ionosphere interactions using a series of flux transfer events (FTEs) observed by Magnetospheric MultiScale (MMS) mission located near the dayside magnetopause on 18 December 2017. The FTEs were observed to propagate duskward and either southward or slightly northward, as predicted under duskward and southward interplanetary magnetic field (IMF). The Cooling model also predicted a significant dawnward propagation of northward‐moving FTEs. Near the MMS footprint, a series of poleward‐moving auroral forms (PMAFs) occurred almost simultaneously with those FTEs. They propagated poleward and westward, consistent with the modeled FTE propagation. The intervals between FTEs, relatively consistent with those between PMAFs, strongly suggest a one‐to‐one correspondence between the dayside transients and ionospheric responses. The FTEs embedded in continuous reconnection observed by MMS and corresponding PMAFs individually occurred during persistent auroral activity recorded by an all‐sky imager strongly indicate that those FTEs/PMAFs resulted from the temporal modulation of the reconnection rate during continuous reconnection. With the decay of the PMAFs associated with the FTEs, patch‐like plasma density enhancements were detected to form and propagate poleward and then dawnward. Propagation to the dawn was also suggested by the Super Dual Auroral Radar Network (SuperDARN) convection and Global Positioning System (GPS) total electron content data. We relate the temporal variation of the driving solar‐wind and magnetospheric mechanism to that of the high‐latitude and polar ionospheric responses and estimate the response time.

    more » « less
  4. On the bow shock in front of Earth’s magnetosphere flows a current due to the curl of the interplanetary magnetic field across the shock. The closure of this current remains uncertain; it is unknown whether the bow shock current closes with the Chapman-Ferraro current system on the magnetopause, along magnetic field lines into the ionosphere, through the magnetosheath, or some combination thereof. We present simultaneous observations from Magnetosphere Multiscale (MMS), AMPERE, and Defense Meteorological Satellite Program (DMSP) during a period of strong B y , weakly negative B z , and very small B x . This IMF orientation should lead to a bow shock current flowing mostly south to north on the shock. AMPERE shows a current poleward of the Region 1 and Region 2 Birkeland currents flowing into the northern polar cap and out of the south, the correct polarity for bow shock current to be closing along open field lines. A southern Defense Meteorological Satellite Program F18 flyover confirms that this current is poleward of the convection reversal boundary. Additionally, we investigate the bow shock current closure for the above-mentioned solar wind conditions using an MHD simulation of the event. We compare the magnitude of the modeled bow shock current due to the IMF B y component to the magnitude of the modeled high-latitude current that corresponds to the real current observed in AMPERE and by Defense Meteorological Satellite Program. In the simulation, the current poleward of the Region 1 currents is about 37% as large as the bow shock I z in the northern ionosphere and 60% in the south. We conclude that the evidence points to at least a partial closure of the bow shock current through the ionosphere. 
    more » « less
  5. Abstract

    We present the observational and modeling study focused on the major factors determining the spatiotemporal structure of the high‐latitude ionospheric plasma density enhancement—the tongue of ionization (TOI) structure—during the 2015 St. Patrick's Day geomagnetic storm. We use the Global Self‐consistent Model of the Thermosphere, Ionosphere, Protonosphere (GSM TIP) to reproduce the plasma density distribution, and the results are compared with the observational data as deduced from the ground‐based global positioning system total electron content and in situ plasma probe measurements at different altitudes. Both the simulation and observation results show that a large‐scale TOI‐like structure of enhanced plasma density extends from the dayside midlatitude region toward the central polar cap along the antisunward cross‐polar convection flow. We reveal an important role of the clockwise convection cell rotation for the modification of TOI structure. According to model results during the storm main phase, the neutral thermospheric composition, particularly the “tongue” in n(N2), modifies the spatial structure of TOI in such a way that (1) the near‐pole region of enhanced plasma density is shifted to the duskside and, (2) atFregion heights, the TOI is split into the dusk and dawn branches. The signature of TOI in the topside ionosphere considerably differs from that in theFregion because of a lesser influence of the neutral composition changes at higher altitudes. Model results revealed that at plasmaspheric heights, the TOI structure appears in both the dawn and dusk convection cells.

    more » « less