Ionosphere-magnetosphere coupling and field-aligned currents

Raeder

2015

JGR Space Physics

We present a survey of interplanetary (IP) shocks using Wind and ACE satellite data from January 1995 to December 2013 to study how IP shock geoeffectiveness is controlled by IP shock impact angles. A shock list covering one and a half solar cycle is compiled. The yearly number of IP shocks is found to correlate well with the monthly sunspot number. We use data from SuperMAG, a large chain with more than 300 geomagnetic stations, to study geoeffectiveness triggered by IP shocks. The SuperMAG SML index, an enhanced version of the familiar AL index, is used in our statistical analysis. The jumps of the SML index triggered by IP shock impacts on the Earth's magnetosphere are investigated in terms of IP shock orientation and speed. We find that, in general, strong (high speed) and almost frontal (small impact angle) shocks are more geoeffective than inclined shocks with low speed. The strongest correlation (correlation coefficient R = 0.78) occurs for fixed IP shock speed and for varied IP shock impact angle. We attribute this result, predicted previously with simulations, to the fact that frontal shocks compress the magnetosphere symmetrically from all sides, which is a favorable condition for the release of magnetic energy stored in the magnetotail, which in turn can produce moderate to strong auroral substorms, which are then observed by ground‐based magnetometers.

Section: Geomagnetic Activitysupporting

confidence: 73%

Section: Introductionmentioning

confidence: 99%

Impact angle control of interplanetary shock geoeffectiveness: A statistical study

Raeder

2015

JGR Space Physics

“…In addition to the global magnetospheric compression, another immediate response to IP shock impacts is an enhancement of large scale FACs (field-aligned currents) between the magnetosphere and ionosphere (Cowley, 2000;Zesta et al, 2000;Oliveira, 2014). Since the electric conductivity along the field lines is high, the magnetospheric currents can easily close through the ionosphere, at about 120 km altitude, forming a joint MI current system.…”

Section: Modeling Impacts Of Head-on Shocksmentioning

confidence: 99%

Geoeffectiveness of interplanetary shocks controlled by impact angles: A review

Advances in Space Research

Samsonov

2018

The high variability of the Sun's magnetic field is responsible for the generation of perturbations that propagate throughout the heliosphere. Such disturbances often drive interplanetary shocks in front of their leading regions. Strong shocks transfer momentum and energy into the solar wind ahead of them which in turn enhance the solar wind interaction with magnetic fields in its way. Shocks then eventually strike the Earth's magnetosphere and trigger a myriad of geomagnetic effects observed not only by spacecraft in space, but also by magnetometers on the ground. Recently, it has been revealed that shocks can show different geoeffectiveness depending closely on the angle of impact. Generally, frontal shocks are more geoeffective than inclined shocks, even if the former are weaker than the latter. This review is focused on results obtained from modeling and experimental efforts in the last 15 years. Some theoretical and observational background are also provided.

“…The MHD equations are solved to within ∼3 RE of Earth. The region within 3 RE is treated as a magnetosphere-ionosphere (MI) coupling region [Oliveira, 2014] where physical processes that couple the magnetosphere to the ionosphere-thermosphere system are parameterized using simple models and relationships. The ionosphere-thermosphere system is modeled using the NOAA CTIM (Coupled Thermosphere Ionosphere Model [Fuller-Rowell et al, 1996;Raeder et al, 2001a]).…”

Section: Openggcm Modelmentioning

confidence: 99%

Impact angle control of interplanetary shock geoeffectiveness

Raeder

2014

JGR Space Physics

We use Open Geospace General Circulation Model global MHD simulations to study the nightside magnetospheric, magnetotail, and ionospheric responses to interplanetary (IP) fast forward shocks. Three cases are presented in this study: two inclined oblique shocks, hereafter IOS-1 and IOS-2, where the latter has a Mach number twice stronger than the former. Both shocks have impact angles of 30• in relation to the Sun-Earth line. Lastly, we choose a frontal perpendicular shock, FPS, whose shock normal is along the Sun-Earth line, with the same Mach number as IOS-1. We find that, in the IOS-1 case, due to the north-south asymmetry, the magnetotail is deflected southward, leading to a mild compression. The geomagnetic activity observed in the nightside ionosphere is then weak. On the other hand, in the head-on case, the FPS compresses the magnetotail from both sides symmetrically. This compression triggers a substorm allowing a larger amount of stored energy in the magnetotail to be released to the nightside ionosphere, resulting in stronger geomagnetic activity. By comparing IOS-2 and FPS, we find that, despite the IOS-2 having a larger Mach number, the FPS leads to a larger geomagnetic response in the nightside ionosphere. As a result, we conclude that IP shocks with similar upstream conditions, such as magnetic field, speed, density, and Mach number, can have different geoeffectiveness, depending on their shock normal orientation.