An official website of the United States government
Abstract In the present study we investigate the response of the dayside ground magnetic field to the sequence of interplanetary magnetic field (IMF)BYchanges during the May 2024 geomagnetic storm. We pay particular attention to its extraordinarily large (>120 nT) and abrupt flip, and use GOES‐18 (G18) magnetic field measurements in the dayside magnetosheath as a time reference. In the dayside auroral zone, the northward magnetic component changed by as much as 4,300 nT from negative to positive indicating that the direction of the auroral electrojet changed from westward to eastward. The overall sequence was consistent with the conventional understanding of the IMFBYdriving of zonal ionospheric flows and Hall currents, which is also confirmed by a global simulation conducted for this storm. Surprisingly, however, the time delay from G18 to the ground increased significantly in time. The delay was 2–3 min for a sharpBYreduction ∼30 min prior to theBYflip, but it became as long as 10 min for the zero‐crossing of theBYflip. It is suggested that the prolonged time delay reflected the travel time from G18 to the reconnection site, which sensitively depends on the final velocity at the magnetopause, that is, the inflow velocity of the magnetic reconnection. Around theBYflip, the solar wind number density transiently exceeded 100 cm−3, and should have increased further through the bow shock crossing. It is suggested that this unusually dense plasma reduced the reconnection rate, and therefore, the solar wind‐magnetosphere energy coupling due to the extraordinary IMF.
more »
« less
Quantifying the Lobe Reconnection Rate During Dominant IMF B y Periods and Different Dipole Tilt Orientations
Abstract Lobe reconnection is usually thought to play an important role in geospace dynamics only when the Interplanetary Magnetic Field (IMF) is mainly northward. This is because the most common and unambiguous signature of lobe reconnection is the strong sunward convection in the polar cap ionosphere observed during these conditions. During more typical conditions, when the IMF is mainly oriented in a dawn‐dusk direction, plasma flows initiated by dayside and lobe reconnection both map to high‐latitude ionospheric locations in close proximity to each other on the dayside. This makes the distinction of the source of the observed dayside polar cap convection ambiguous, as the flow magnitude and direction are similar from the two topologically different source regions. We here overcome this challenge by normalizing the ionospheric convection observed by the Super Dual Aurora Radar Network (SuperDARN) to the polar cap boundary, inferred from simultaneous observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). This new method enable us to separate and quantify the relative contribution of both lobe reconnection and dayside/nightside (Dungey cycle) reconnection during periods of dominating IMFBy. Our main findings are twofold. First, the lobe reconnection rate can typically account for 20% of the Dungey cycle flux transport during local summer when IMFByis dominating and IMFBz ≥ 0. Second, the dayside convection relative to the open/closed boundary is vastly different in local summer versus local winter, as defined by the dipole tilt angle.
more »
« less
- Award ID(s):
- 2002574
- PAR ID:
- 10361674
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 126
- Issue:
- 11
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Specific polar cap auroras, such as 15MLT‐PCA, linked to lobe reconnection due to the influence of the interplanetary magnetic field (IMF) Bycomponent, were only observed in the summer. Although the variance in ionospheric conductivity between winter and summer has been proposed as a potential explanation for this seasonal dependency, it has also been argued that the differences in lobe reconnection between the winter and summer hemispheres could be the cause. To address this debate, we examined two data periods with similar IMF conditions when the northern hemisphere was in summer and winter, respectively. Using DMSP/SSUSI and AMPERE observations, we detected clear 15MLT‐PCA and associated field‐aligned currents in the summer, but not in the winter. These observations were compared with global MHD simulations from OpenGGCM. Lobe reconnection signatures were identified for both winter and summer in the simulation results. However, a detailed analysis showed that the pattern of lobe reconnection in the winter hemisphere was different from that in the summer. Based on the combined observation and simulation results, we suggest that particular lobe reconnection in summer is critical for generating 15MLT‐PCA, while the winter's reconnection may lead to transient or small‐scale auroral responses that were not easily identified by DMSP/SSUSI observations as a 15MLT‐PCA event.more » « less
-
Abstract Magnetic reconnection occurring between the interplanetary magnetic field (IMF) and the dayside magnetopause causes a circulation of magnetic flux and plasma within the magnetosphere, known as the Dungey cycle. This circulation is transmitted to the ionosphere via field‐aligned currents (FACs). The magnetic flux transport within the Dungey cycle is quantified by the cross‐polar cap potential (CPCP or transpolar voltage). Previous studies have suggested that under strong driving conditions the CPCP can saturate near a value of 250 kV. In this study we investigate whether an analogous saturation occurs in the magnitudes of the FACs, using observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment. The solar wind speed, density and pressure, theBzcomponent of the IMF, and combinations of these, were compared to the concurrent integrated current magnitude, across each hemisphere. We find that FAC magnitudes are controlled most strongly by solar wind speed and the orientation and strength of the IMF. FAC magnitude increases monotonically with solar wind driving but there is a distinct knee in the variation around IMFBz = −10 nT, above which the increase slows.more » « less
-
Winds in the nighttime upper thermosphere are often observed to mimic the ionospheric plasma convection at polar latitudes, and whether the same is true for the daytime winds remains unclear. The dayside sector is subject to large temperature gradient set up by solar irradiance and it also contains the cusp, which is a hotspot of Poynting flux and a region with the strongest soft particle precipitation. We examine daytime winds using a Scanning Doppler Imager (SDI) located at the South Pole, and investigate their distribution under steadily positive and negative IMF Byconditions. The results show that daytime winds exhibit significant differences from the plasma convection. Under negative IMF Byconditions, winds flow in the same direction as the plasma zonally, but have a meridional component that is strongest in the auroral zone. As a result, winds are more poleward-directed than the plasma convection within the auroral zone, and more westward-directed in the polar cap. Under positive IMF Byconditions, winds can flow zonally against the plasma in certain regions. For instance, they flow westward in the polar cap despite the eastward plasma convection there, forming a large angle relative to the plasma convection. The results indicate that ion drag may not be the most dominant force for daytime winds. Although the importance of various forcing terms cannot be resolved with the utilized dataset, we speculate that the pressure gradient force in the presence of cusp heating serves as one important contributor.more » « less
-
Abstract One of the most significant observations associated with a sharp enhancement in solar wind dynamic pressure,, is the poleward expansion of the auroral oval and the closing of the polar cap. The polar cap shrinking over a wide range of magnetic local times (MLTs), in connection with an observed increase in ionospheric convection and the transpolar potential, led to the conclusion that the nightside reconnection rate is significantly enhanced after a pressure front impact. However, this enhanced tail reconnection has never been directly measured. We demonstrate the effect of a solar wind dynamic pressure front on the polar cap closure, and for the first time, measure the enhanced reconnection rate in the magnetotail, for a case occurring during southward background Interplanetary Magnetic Field (IMF) conditions. We use Polar Ultra‐Violet Imager (UVI) measurements to detect the location of the open‐closed field line boundary, and combine them with Assimilative Mapping of Ionospheric Electrodynamics (AMIE) potentials to calculate the ionospheric electric field along the polar cap boundary, and thus evaluate the variation of the dayside/nightside reconnection rates. We find a strong response of the polar cap boundary at all available MLTs, exhibiting a significant reduction of the open flux content. We also observe an immediate response of the dayside reconnection rate, plus a phased response, delayed by ∼15–20 min, of the nightside reconnection rate. Finally, we provide comparison of the observations with the results of the Open Geospace General Circulation Model (OpenGGCM), elucidating significant agreements and disagreements.more » « less
